I

United States Department of Commerce

National Institute of Standards and Technology

NIST Technical Note 1287

High Resolution Diffraction Imaging

of Crystals Grown in Microgravity

and Closely Related Terrestrial Crystals

B. Steiner

R. Dobbyn

D. Black

H. Burdette

M. Kuriyama

R. Spal

National Institute of Standards and Technology

Gaithersburg, MD 20899

August 1991

^'^TES O*

"^^

L. van den Berg

EG&G Energy Measurements

A. Fripp

R. Simchick*

NASA Langley Research Center

R. Lai A. Batra

Alabama A&M University

D. Matthiesen

B. Ditchek

GTE Laboratories

* Lockheed Engineering and Sciences Co. Hampton, VA

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards U.S. Government Printing Office For sale by the Superintendent

and Technology Washington: 1991 of Documents

Technical Note 1287 U.S. Government Printing Office

Natl. Inst. Stand. Techno!. Washington, DC 20402

Tech. Note 1287

162 pages (Aug. 1991)

CODEN: NTNOEF
EXECUTIVE SUMMARY

This paper reports on irregularities found in three crystals

grown in space, in four crystals grown entirely on the ground, and

compares the two sets. Irregularities have been observed in

mercuric iodide, lead tin telluride, triclycine sulfate, and

gallium arsenide by high resolution synchrotron x-radiation

diffraction imaging.

Radiation detectors made from mercuric iodide crystals grown

in microgravity have been reported to perform far better than

conventional detectors grown from the same material under full

gravity. Effort is now underway to reproduce these "space" crystals, optimize their properties, and extend comparable superiority

to other types of material.

Interest in space crystals arises in another way as well.

Increase in the local regularity of protein crystals through growth

in space would lead to more precise knowledge of their chemical

structure. This, in turn, would provide greatly increased efficiency in the tailoring of drugs specifically designed to interact

with particular chemical groups.

For advances both in superior devices and in tailored drugs,

observation of crystal irregularity is essential, first to provide

fundamental scientific insight, then to optimize expensive space

growth, and ultimately to establish superior growth conditions in

far less extreme and more economical environments.

Determination of the specific factors responsible for the

extraordinary character of space crystals thus should have far-reaching scientific, industrial, and health implications. Insight

into characteristic irregularities is essential to the predictable

growth of optimum crystals, both in space and on the ground. What

is required is determination of the specific nature of the

mesoscopic irregularities in these crystals, their level of

incidence, and their distribution.

Recent advances in diffraction imaging with highly parallel

monochromatic synchrotron x-radiation now offer the chance to take

major steps toward these ends. We find ourselves in a position,

given suitable samples, to develop comprehensive understanding both

of the nature of the special structures associated with crystals

grown in space and of their genesis and control.

In order to accelerate the realization of these opportunities,

long term collaboration has been initiated between investigators

iii
associated with the NASA Microgravity Sciences and Applications

Division and synchrotron radiation scientists in the NIST Materials

Science and Engineering Laboratory. This collaboration is designed

to accomplish three specific goals:

1) Establishment of a base line of mesoscopic structural

information on those crystals of greatest importance to

the space program;

2) Correlation of observed variation in characteristics with

observed features in the mesoscopic structure;

3) Determination of conditions optimum for growth, both on

the ground and in microgravity.

At the request of the Microgravity Sciences and Applications

Division, the effort the first year focused on three distinct

activities. The first of these established a systematic approach

to the achievement of these ends through dialogue with each of 49

principal investigators in the NASA Microgravity Program directly

related to crystal growth. The second activity inaugurated

collaborative research on four materials of particular urgency.

The third activity advanced the capability of quasi-real-time

diffraction imaging.

Systematic interaction with the various principal investigators was required because of the large number of crystal growth

factors to which diffraction imaging might contribute. As a result

of this dialogue, areas that should benefit most from information

on mesoscopic structural irregularity have been delineated and

planning initiated. Through this dialogue, we have confirmed the

promise of high-resolution mesoscopic structural information for

each of the 13 projects in which the successful growth of extensive

high-quality monolithic crystals is an immediate goal. Another 15

investigators want to explore the applicability of high resolution

diffraction imaging to the novel systems on which they are working;

these would require extension of high resolution diffraction

imaging techniques, which to date have addressed relatively large,

monolithic, high-quality crystals.

These 28 projects and their requirements for mesoscopic

structural information are identified here. Most of the members of

this large group consider the information of such central value to

their research programs that their commitment extends beyond the

offer of characteristic samples to active participation in the

diffraction imaging experiments. This interest should ensure that

full advantage of the diffraction imaging is achieved. However,

the number of technical questions on which work is contemplated is

clearly larger than can be accommodated within the scope of the

current program.

IV
As a result of the initial dialogue with NASA and its crystal

growers, seven crystals were selected from four different materials

for immediate imaging. Important, unanticipated observations have

been made on each of the four. For two of these, mercuric iodide

and lead tin telluride, more than one phase (an array of non

diffracting inclusions) was observed in terrestrial samples; but

the formation of these multiple phases appears to have been

strongly suppressed in the comparable crystals grown in microgravity. The third terrestrial material examined, triglycine sulfate,

displayed an unexpected layered structure. The fourth terrestrial

material, gallium arsenide, revealed a mesoscopic structure substantially different from that of the seed.

These differences, varying in fundamental ways from material

to material, are described in section VII. The reduction in multiphase formation in microgravity appears to be associated in one of

the two cases with gradual changes in the lattice over a distance

of millimeters. These results together support independent

observations that extreme lattice regularity may be less important

in influencing device performance than other factors: multiple

phases may limit this performance severely. The passage of 5 years

since the growth of the space crystals that have been observed and

the fabrication of a device from one of these may have influenced

some of these observations, however. Important thrusts of further

investigation clearly would address questions of aging and the role

of device fabrication.

In addition to our observation of crystals grown 5 years ago,

we have observed also two recently grown crystals for subsequent

comparison with crystals to be grown in flight experiments

scheduled for this fall. Results on one of these coincide with the

recent observation that increases in purity can dramatically

improve device performance.

Immediate examination of all seven crystals that we have

examined has proved to be especially valuable for the guidance that

it provides for the suitable treatment of materials to be grown in

subsequent flights as well as for the fundamental insights into the

nature, the genesis, and the importance of various irregularities.

The wisdom of an accelerated pace for the first experimental phase

of this collaboration thus has been borne out.

CONTENTS

Executive SLimitiary iii

Introduction 1

A. The Work Covered in this Study 1

B. Crystal Growth in Microgravity 1

CTypes of Interest 2

D. Mesoscopic Structure 3

E. Initial Program Foci 4

F. Guidance for Future Work 5

II . Imaging Requirements 6

A. General Considerations 6

B. Lattice Orientation and Strain 7

C. Grain and Subgrain Boundaries 8

D. Dislocations 8

E. Phase Domain Boundaries 9

F. Additional Phases 9

G. Surface Scratches 10

III. Resulting Capability 11

A. Storage Ring 11

B. Optics 11

C. Resulting Performance 11

D. High Spatial Resolution in Real-time 12

IV. Principal Areas for Investigation 13

A. Survey 13

B. Traditional Materials 14

1 . Binary Inorganic Crystals 14

a. Gallium Arsenide 14

b. Gallium Antimonide 15

c. Mercuric Iodide 16

d. Cadmium Telluride, Cadmium Sulfide, and Lead Selenide 16

Vll
2. Ternary Inorganic Crystals 16

a. Cadmium Telluride Ternaries 16

b. Mercury Zinc Telluride 17

c. Lead Tin Telluride 17

d. Indium Gallium Antimonide 17

3. Low Molecular Weight Organic Crystals.. 18

a. Triglycine Sulfate 18

b. 1-Arginine Phosphate 18

c. Benzil 18

C. Novel Materials 18

1. Epitaxial Systems 18

a. General Considerations 18

b. Mercury Cadmium Telluride on Cadmium Telluride 19

c. Zinc Germanium Phosphide on Gallium Phosphide 19

d. Silicon on Germanium 19

2. Proteins 20

a. General Considerations 20

b. Interest in the Imaging of Proteins 22

3

.

Other Systems 2 2 a. Whiskers 22

b. Small Particles 22

c. Polygrain Growth by Physical Vapor

Transport 2 3

d. Rapid Solidification 23

e. Liquid Encapsulated Melt Zone 23

f

.

Convective Effects 2 3

D. Materials Not Relevant to this Study 24

1. Glass Formation 23

a. Oxide Glasses 23

b. Metglasses 24

2. Fine Particles 24

a. Rapid Solidification 24

b. Diamond Films 24

3

.

Specific Stages of Growth 24

a. Seeding 24

b. Transport in Alloys 24

c. Ostwald Ripening 25

d. Magnetostriction 25

e. Proteins 25

4

.

Model Systems 2 5 a. Ammonium Chloride 2 5

b. Organic Analogues of Metal Systems. 25

5. Diagnostic Techniques 25

a. Acoustic Techniques 25

b. Dynamic Measurements 2 5

Vlll
V. Materials In the Current Effort 2 6

A. Selection Considerations 26

B. Mercuric Iodide 28

C. Lead Tin Telluride 29

D. Triglycine Sulfate 29

E. Gallium Arsenide 30

VI. The Diffraction Images 31

A. Mercuric Iodide 31

1. Terrestrial Crystal Compared with

Spacelab III Crystal 31

2

Spacelab III Crystal 59

3. Terrestrial Crystal to Be Compared to

a Future Flight Crystal 66

B. Lead Tin Telluride 71

1. Terrestrial Crystal Comparable to

STS 61A Crystal 71

2STS 61A Crystal 93

C. Triglycine Sulfate 96

D. Gallium Arsenide Ill

VII. Summary of Initial Observations 124

A. General Effects of Microgravity 124

B. Mercuric Iodide 124

C. Lead Tin Telluride 125

D. Triglycine Sulfate 125

E. Gallium Arsenide 126

VIII. Current Limitations and Improvement in

Diffraction Imaging Techniques 127

A. Advanced X-Ray Image Detectors 127

B. Higher Sensitivity Requirements 130

C. Phase Contrast Microscopy — Interface Problems 136

IX. Future Activity 141

A. Interest in Collaboration 141

B. The Prospects for Proteins 141

C. Crystal Preparation 142

D. Correlation with Auxiliary Data 143

E. Crystal Selection Criteria 143

IX
X. Acknowledgment 145

XI . References 147

Appendix 151
I. INTRODUCTION

A. The Work Covered in this Study

This paper reports on irregularities found in three crystals

grown in space, in four crystals grown entirely on the ground, and

compares the two sets. Irregularities have been observed in

mercuric iodide, lead tin telluride, triclycine sulfate, and

gallium arsenide by high resolution synchrotron x-radiation

diffraction imaging.

B. Crystal Growth in Microgravity

The performance of materials is determined by their structure;

in this performance, irregularity typically plays a leading role.

The growth of materials in microgravity has long been of interest

because of the anticipation that reduction in gravitational forces

would strongly affect crystal growth and therefore the nature of

the resulting irregularity. Both buoyancy-driven convection and

loading on the warm boule are of potential importance [ 1] [2 ] . Reports of early space crystal growth experiments indeed bore out

this expectation; mercuric iodide crystals grown in microgravity

were found to be so superior to comparable crystals grown on the

ground [1][2] that reproduction and optimization of the properties

of these crystals and the extension of these advantages to other

types have stimulated great interest.

However, because many factors affect crystal growth, and

because these can interact strongly with one another, the understanding of the structural variation necessary for its effective

exploitation has not yet been achieved. Determination of the

specific factors responsible for the reported superiority of some

crystals grown in space remains an exciting challenge, with farreaching scientific, industrial, and health implications. Knowledge of the irregularities in these materials, developed in

conjunction with an understanding of their genesis and detailed

affects on properties, would be an important intellectual

achievement. Such knowledge is expected also to contribute to

dramatically improved single crystal production, both in space and

on the ground.
C. Types of Interest

Interest in "space" crystals arises in two distinct ways. In

some instances, the performance of devices made from space-grown

crystals has been reported to be far superior to similar devices

made from similar ground-grown material. One example has just been

noted. In x-ray and gamma-ray detectors made from mercuric iodide,

the mobility of charge carriers in detectors made from space-grown

crystals was at least six times as high as that for similar detectors made from ground-grown crystals [1][2]. This is expected to

lead to increased energy resolution in radiation detectors made

from this material.

In other instances, crystallographers have found dramatic

increases in the regularity and in the morphology and size of

crystals grown in space [3], which promise more precise knowledge of the chemical structure of complex proteins. This, in turn,

should provide greatly increased efficiency in the fabrication of

drugs designed to interact with particular chemical groups. In

principle, the requisite knowledge of structural chemistry can be

achieved through crystallographic structural determination of highquality protein specimens. However, this work has been seriously

impeded by the absence of specimens with sufficient regularity for

effective structural analysis. Growth of proteins in space

promises to address this difficulty and thus to accelerate substantially the realization of effective drugs.

Two events have led to widespread optimism that this promise

will be realized. Several years ago, liquid-liquid diffusion

experiments carried out in Spacelab I produced larger crystals than

crystals grown on the ground by the same technique [4]. More

recently, vapor diffusion growth of isocitrate lyase on space

shuttle flight STS 2 6 achieved a dramatic increase in the regularity of the crystal morphology. This flight produced prism-like

crystals, while material grown in a similar manner on the ground

forms only dendritic clusters [3]. Moreover, two of the crystals

obtained on this flight were substantially larger than those

produced in any ground experiments. Most important, the quality of

crystals from this flight is such that they diffract "to higher

resolution than the best crystals obtained under similar conditions

on Earth" [3]. Improvement in the regularity of such proteins

through growth in microgravity thus is expected to facilitate

greatly the determination of their chemical structure. The saving

in time and effort in the testing of potential new drugs that can

be achieved by precise structural chemical knowledge of the active

protein sites has led to experimentation in the growth of exceptionally regular proteins in space.

In summary, the design both of better devices and of more

effective drugs requires knowledge of crystal regularity. Irregularity in these materials is of great interest, first for the
scientific insight that it presages, then for the optimization of

expensive space growth, and ultimately for the establishment of

desirable growth conditions in far less extreme and expensive

environments

.

D. Mesoscopic Structure

Determination of the specific nature of the mesoscopic

irregularities in these materials, their level of incidence, and

their distribution in space-grown and comparable terrestrial

materials is now required. Fortunately, recent technical advances

in diffraction imaging with high resolution monochromatic synchrotron x-radiation present a timely opportunity to achieve all of

these ends [5] [6] [7].

The appeal of high-resolution synchrotron radiation diffraction imaging of proteins is further enhanced by the fact that

the "performance parameter" of particular interest in these

proteins is their structural diffraction pattern in synchrotron xray beams. The question naturally arises, "Can high-resolution

synchrotron diffraction imaging of x-radiation contribute to the

improvement of the best methods of chemical structure determination, which also utilize diffraction of synchrotron radiation?"

Structural irregularity in all of these materials need to be

observed, interpreted, and understood in terms of the various

stages of crystal growth so that growth can be controlled in a more

reliable fashion. These structural variations must then be

correlated with observed anomalies in the characteristics of the

crystals so that the new control that is achieved can be guided in

an effective way.

The requirements for obtaining such structural information are

reviewed in section II. The recently developed ability to meet

these requirements is reviewed in section III. As a result of our

current capability, we are in a position, given suitable samples,

to develop a comprehensive understanding of the nature of the special structures associated with materials grown in space and their

genesis and control.

The current collaboration between investigators associated

with the National Aeronautics and Space Administration (NASA)

Microgravity Sciences and Applications Division and synchrotron

radiation scientists in the National Institute of Standards and

Technology (NIST) Materials Science and Engineering Laboratory has

been initiated in order to provide this information in a timely and

systematic way. Specific areas for this collaboration have been

defined and effort in several of these inaugurated. This work is

geared to the following goals [8]:
1) Establishment of a reference base line of structural

information on crystals of particular importance to the

space program;

2) Correlation of observed differences in performance with

mesoscopic structure;

3) Determination of conditions optimum for growth, both on

the ground and in microgravity

.

Initial Program Foci

Under the direction of the Microgravity Sciences and Applications Division, the collaborative effort in its first year has

been focused on three activities. The first of these established

a systematic approach to the ends just enumerated, through dialogue

with each of NASA's principal investigators working on crystal

growth. These scientists are addressing a comprehensive variety of

crystal growth factors, including both space growth and terrestrial

growth. As a result they have broad interest in the determination

of mesoscopic structural irregularity. Following the dialogue that

has now been established, specific areas for the exploration of

irregularity have been defined (sec. IV) ; planning and the preparation of suitable samples have been initiated.

The second activity initiated imaging of four materials for

which samples were already available and which were considered most

urgent by the NASA Microgravity Sciences and Applications Division.

These four materials are described in section V; they include early

space-grown crystals and closely related crystals of particular

interest, which had been retained in anticipation of just such an

opportunity.

The first two inorganic crystals grown in space and comparable

ground controls present interesting mesoscopic differences, shown

in section VI. These differences are profound in the case of

mercuric iodide, and more subtle, but perhaps equally important, in

the case of lead tin telluride. One space-grown organic crystal,

triglycine sulfate, was also examined and found to have been seeded

by a nominally monolithic crystal layered in a manner not previously observed. The passage of time since the growth of these

crystals, about 5 years, and the fabrication of a device from one

of these, may have played a crucial role in some of these observations. Implications of the diffraction images are discussed in

section VII. Further investigation is clearly required in order to

separate questions on aging and the role of device fabrication from

those of initial crystal growth and subsequent handling.
In preparation for flights scheduled for this fall, we have

observed also two crystals for comparison with crystals to be grown

then: one, mercuric iodide; and the other, gallium arsenide.

Images of these crystals are included also in sections VI and VII,

where the implications of their observed structure are discussed.

Another crystal of importance for the fall flight schedule,

triglycine sulfate, was made available by the crystal grower, but

arrived too late for examination in the current series of experiments.

The effectiveness of future work in high resolution

diffraction imaging will be substantially enhanced by the application of recently available image magnifiers and detectors.

Increase in spatial resolution below 1/xm is now feasible in quasireal-time and would permit greatly increased effectiveness in the

study of specific details of particular interest to the crystal

grower. This capability would also greatly accelerate various

stages of the analysis and permit electronic image manipulation as

well. Realization of these goals requires more sensitive sample

control. These improvements are described in section VIII.

F. Guidance for Future Work

Issues central to future activity are discussed in section IX.

Our initial examination of crystals at this particular time has

proven to be especially valuable because of the important guidance

that it provides for the suitable treatment of crystals to be grown

this fall and in subsequent flights.
II. IMAGING REQUIREMENTS

A. General Considerations

X-Ray topography has long promised the type of information on

structural irregularities that is required both for fundamental

insight into the nature of performance anomalies and for development of understanding of the origins of these irregularities in

crystal growth. However, if this information had indeed been

accessible it would already have permitted optimization of the

various performance parameters that are accorded high priority.

A principal impediment has been x-ray beam divergence.

Individual irregularities and the immediately surrounding matrix in

a typical crystal are illuminated by laboratory x-ray sources over

an angular range measured in arc minutes. The divergence of this

incoming beam (at a given point on the sample) unfortunately supports diffraction simultaneously from many irregularities along

with that from the surrounding regular regions. The spread in

wavelength in white beam synchrotron radiation also supports

diffraction simultaneously from irregular and regular regions. In

both instances, contrast that would otherwise be present is

severely reduced or eliminated entirely. Even where some contrast

remains, the spatial information contained is convoluted in a way

that will not permit unfolding and subsequent detailed analysis.

Lattice deviations influencing diffraction by seconds of arc,

which are frequently critical to satisfactory interpretation and

understanding of mesoscopic irregularity, are rendered visible in

diffraction only by a source of monochromatic radiation parallel

within an arc second. In such a beam, spatial fidelity also is

preserved at the micrometer level. The recent availability of such

a source of x-radiation thus now permits the realization of the

long term promise of x-ray topography.

Where such a beam is available, the complete analysis of a

crystal depends on the unobstructed observation of diffraction in

Bragg and Laue geometries in all critical diffraction orientations.

Such observation, in turn, places several requirements on the

sample itself:

* The thickness of the crystal may limit observation in

Laue geometry. Successful diffraction from a typical,

optically thick crystal in Laue geometry depends on

anomalous transmission by means of dynamical diffraction
which is determined by the crystal quality. For highquality crystals, thickness of the order of a half

millimeter is suitable.

The orientation of the sample surfaces may restrict

observation in either Bragg of Laue geometry. If the

surface orientation is not close to a low index crystallographic plane, it can severely restrict the accessibility of diffraction critical to effective analysis and

thus the detailed strain visibility analysis that can be

carried out.

The condition of the surface can limit the analysis of

growth related irregularity even when diffraction is

observed. While the residual strain from scratches can

provide useful orientation in the comparison of images,

where such strains dominate an image only a restricted

analysis of growth-related features is possible. Crystal

growers are sometimes surprised and distressed to observe

the amount of residual strain from such scratches that is

revealed by high-resolution diffraction imaging.

The principal remaining requirement is that the various

irregularities not interfere severely with one another.

This requirement places a limit on the density of

irregularities , although this limit depends strongly on

the particular nature of the irregularities that are

present, to which we turn now.

B. Lattice Orientation and Strain

Of the various types of irregularity in high-quality crystals,

perhaps the most pervasive is gradual change in the lattice. The

orientation of the lattice or the magnitude of the lattice

parameter, or both, may vary. For any one orientation of the

crystal with respect to the diffracting beam, such variation

results in diffraction only from a portion of a single grain.

In some systems, such as the Czochralski growth of doped material, this variation is oscillatory, leading to striations in

diffraction images of crystals cut obliquely to the local growth

direction and oriented slightly off of the Bragg condition.

Contrast is inverted on opposite sides of the Bragg peak. Like

tree rings, these striations record, not only variation in chemical

composition but, taken together, also the shape of the crystal at

various stages in its growth; they can be deciphered in a somewhat

similar if more complex and sophisticated manner [5] [9].
Diffraction from grains whose lattice orientation or parameter

varies monotonically or aperiodically yields images of restricted

regions of a single grain. The part of such a grain that is in

diffraction shifts as the crystal is rotated in the Bragg direction. The moving edge of this image is characteristically soft and

relatively indistinct.

C. Grain and Subgrain Boundaries

Sharp contrast in the image of a crystal can delineate

homogeneous grains or subgrains. In contrast to the preceding

case, the boundaries of such an image do not move as the crystal is

rotated in the Bragg direction. Where the lattice orientation of

a pair of such homogeneous grains differs by rotation in the

diffraction plane by more than the acceptance angle, only one of

these grains (or subgrains) will diffract at a time; and, if it is

not strained, it does so in its entirety. Variations in real-time

images of such a crystal permit rapid and detailed assessment of

the relative misorientation (in the Bragg direction) of the various

grains and subgrains with respect to one another.

Where the lattice orientation of a pair of such contiguous

grains differs in a direction orthogonal to the plane of diffraction, both grains may appear in diffraction simultaneously, but the

resulting images are displaced with respect to one another in this

direction. The pair of these images is either separated or

overlapped, depending on the relative inclination of the two

lattices.

The boundary of an individual grain or subgrain may end up

strained with respect to the interior of the grain. In this case,

diffraction from the boundary will generally contrast with that

from the bulk grain, permitting the boundary image to be distinguished from that of the grain.

D. Dislocations

Dislocations typically appear in diffraction images taken in

Laue geometry (transmission) as linear features that are broader at

one end than the other. The broadening of one end of such a

feature arises from scattering deep within the crystal, while the

sharp end locates the intersection of the dislocation with the xray exit surface of the crystal. The orientation of a dislocation

can be determined with high precision for those cases in which the

intersection of the dislocation with both entrance and exit faces

is distinct in the diffraction image.
Variation in the visibility of such a line feature in successive diffraction images indicates the direction of atomic displacement associated with the dislocation, which is parallel to its

Burgers vector. However, since the visibility of such a dislocation varies relatively slowly, that is, as the cosine of the angle

between the Burgers vector and the diffraction vector, the

determination of this direction is most precise when contrast can

be observed to disappear. When the direction of diffraction is

oriented normal to the atomic displacement, such a feature vanishes

from the diffraction image.

E. Phase Domain Boundaries

Twins are distinguished normally by absence of diffraction

from regions between sharp, parallel boundaries visible in some

diffraction directions but not in others. The contrast in the

latter, when observed by high-resolution beams, may be affected by

very slight differences in lattice alignment.

With angular collimation of the order of an arc second, the

images of other boundaries recorded in Laue geometry may also

become visible when the diffraction vector falls along the boundary. Such boundaries are visible even under these restrictive

conditions only when they separate atomically coherent regions

differing by an atomic phase shift [10] [11]. Those

boundaries that have been observed to date to fulfill these

conditions appear to separate antiphase domains. Radiation with a divergence of the order of a second of arc or less is necessary to

image such boundaries.

F. Additional Phases

The absence of diffraction from particular regions of a crystal under all diffraction conditions supporting diffraction

from the rest of the crystal strongly suggests the presence of a second phase, although in principle the non diffracting regions may

simply be misoriented with respect to the rest. In stoichiometric

materials, the boundaries of two phases are sharply delineated. In

alloys, this sharpness is vitiated by the gradual changes in

composition that may be permitted.
6. Surface Scratches

The strain associated with surface scratches is linear, but

sometimes gently curved, and typically non crystallographic in

orientation. The images of scratches typically are distinguished

also by three other characteristics: 1) uniform width, 2) sharp

edges, and 3) contrast reversal either laterally, longitudinally,

or both, particularly as the Bragg peak is scanned.

10
III. RESULTING CAPABILITY

A. Storage Ring

Suitable synchrotron radiation sources now offer opportunities

to fulfill the long awaited promise of x-ray topography; but the

degree of success in their realization depends upon the particular

parameters of the storage ring and its beam lines. Since the

precise orientation of the x-radiation at individual points on the

sample is crucial to the analysis, the vertical source size,

together with the distance of the sample from the tangent point on

the ring, may limit the potential utility of the images produced.

The x-ray storage ring of the National Synchrotron Light

Source at Brookhaven National Laboratory offers the most suitable

combination of characteristics of any existing ring, providing an

unusually bright beam whose degree of vertical convergence on a

point on a sample mounted in front of the monochromator on beam

line X-23A3 is 1.5 arc seconds.

B. Optics

Although this 1.5 arc second beam provides a considerable

improvement over other sources, it is not yet sufficient for

diffraction imaging with maximum sensitivity to defects. For

maximum useful sensitivity to irregularities, which requires

further improvement in beam divergence by another order of magnitude, i.e., 0.1 arc second, the optics of the monochromator are

crucial. Such a beam is necessary for rendering critical features

visible, for preservation of the spatial information in the image

within the plane of diffraction, and for displaying essential clues

to the strains upon which the success or failure of detailed

analysis can depend [6].

C. Resulting Performance

With such a resulting, dedicated 0.1 arc second monochromatic

capability, however, which is available on a routine basis only on

Beam Line X-2 3A3 at the National Synchrotron Light Source, the

detection and interpretation of irregularities are limited

11
principally by the quality of the crystals undergoing study.

Irregularities can be recorded photographically with a spatial

resolution of 1 iim. Observation with an x-ray vidicon and charge

coupled device (ccd) cameras readily provides complementary

information with a spatial resolution of 35 /xm with intermediate

sensitivity in real-time, and 20 iim with shot noise limited

sensitivity in quasi real-time.

D. High Spatial Resolution in Real-time

Data collection and optimization would be facilitated by

reduction of the 35 fim spatial resolution of real-time or quasi

real-time observation toward the one micron resolution of film. An

extremely promising approach, which would greatly facilitate the

application of diffraction imaging, is the use of an x-ray image

magnifier in conjunction with a sensitive ccd. We have thus

devoted considerable effort to the application and demonstration of

this capability.

12
IV. PRINCIPAL AREAS FOR INVESTIGATION

A. Survey

Since there are 49 principal investigators in the crystal

growth program of the NASA Microgravity Sciences and Applications

Division, important tasks for us at the outset were to formulate

those questions that might be addressed, establish a systematic

program for addressing them, and determine initial priorities.

The process to carry out these tasks was initiated by Robert

A. Schmitz , who wrote to each of the principal investigators

identified in the Appendix on 8 March 1990 describing the collaborative effort and soliciting expressions of interest and other

comment [8]. NIST was then asked to confer directly with each of

the investigators to determine the nature of individual interests

and the availability of suitable samples. Although the survey was

to include only NASA Microgravity funded activity, the investigators included a major fraction of the American crystal growth

community. The results of this survey should therefore be of more

than routine interest.

The investigators fall into three groups. Thirteen investigators are pursuing the growth of highly regular but otherwise

traditional macroscopic monolithic crystals and an understanding of

the factors affecting crystal regularity. All of these investigators are interested in high-resolution diffraction imaging of

their crystals. Most want also to take an active role in the

experimentation and analysis, which demonstrates the seriousness of

their commitment to the growth of the highest quality crystals.

Moreover, this interest illustrates the promise of high-resolution

diffraction imaging to realization of this end.

Fifteen investigators are pursuing the growth of novel

materials that would extend the utility of high-resolution diffraction imaging beyond the traditional macroscopic monolithic

crystals, for which it has already proved to be of value. All of

these investigators are interested in the successful application of

these techniques to the systems on which they are working. In

addition, many of these investigators also want to participate actively in the imaging and detailed analysis. The degree of success

that can be anticipated in each of these exploratory cases is

discussed below.

13
The remaining 21 investigators are working in areas to which

diffraction imaging is irrelevant, either because the systems that

they are addressing are intrinsically unsuitable or because they

are working with a single, restrictive aspect of the growth to

which diffraction imaging would not contribute in a substantial

way.

The interest of 28 investigators in collaboration is much

larger than can be accommodated in the immediate future. Thus

priorities must be weighed very carefully. The following synopsis

summarizes the information that provides a basis for these

priorities.

B. Traditional Materials

1. Binary Inorganic Crystals

a. Gallium Arsenide

Two pairs of closely related experiments are being carried out

on the growth of gallium arsenide. Among other issues, these experiments seek to determine the influence of gravity on the morphology of the solid/liquid interface during solidification. Each

pair of experiments will compare growth involving periodic Peltier

marking with comparable growth without marking. Brian Ditchek has

designed one pair of "Get Away Special" (GAS can) experiments for

flight STS 40, originally scheduled for August 1990. David Matthiesen is designing a pair of USML 1 experiments, scheduled

initially for March 1992, which will be based on the results of the

STS 4 experiments and complementary terrestrial experiments. Both

flight experiments involve selenium doped growth from a Czochralski

seed.

Twenty-six terrestrial experiments related to the GAS can

experiments are scheduled for the study of growth under various

combinations of gravitational and magnetic fields as part of this

effort. An extensive array of characterization experiments is also

planned with which the diffraction imaging can be very usefully

compared.

Dr. Matthiesen has posed a variety of fundamental questions to

be addressed by diffraction imaging: propagation of defects from

the seed; influence of interface shape; extent of crystallographic

matching between seed and new growth; nucleation of defects at the

growth interface; influence of the Peltier demarcation; inauguration of polycrystalline breakdown; and the influence of gravity on

each of these. He looks forward to active participation in the

experiments providing the answers to these questions.

14
Although a number of terrestrial samples comparable to those

expected from the next STS experiment were available for immediate

analysis, only one of these was thin enough to present the hope for

observation of diffraction in Laue geometry. We have obtained

images of this sample, presented in section VI, which contain an

immediate answer to one of the questions that Dr. Matthiesen had

posed. Implications for growth in microgravity are discussed in

section VII.

b. Galliiun Antimonide

August Witt is conducting two matrices of terrestrial Czochralski growth experiments under NASA auspices and is prepared to

conduct heat pipe experiments in space when suitable flight

hardware becomes available. The first matrix of experiments

involves the growth of gallium antimonide. In this matrix, four

material variations are planned, one undoped, and three doped

respectively with tellurium, zinc, and tin. Experiments are

planned both with and without the imposition of magnetic fields.

The second matrix is similar to the first but uses a ternary including indium rather than gallium antimonide. The flight experiments

involve the growth of germanium in heat pipes. In these experiments the appearance of line defects, clusters, point defects,

precipitates, and other more gradual strains is anticipated.

The rapid scheduling of many such experiments arranged in a large matrix and the interrelationships among such successive,

closely spaced experiments challenge in the following way the

extensive time typically taken for a comprehensive analysis with

diffraction imaging. Because of the number of steps involved in

such an analysis: orientation of the sample, the recording and

enlargement of visual data, complex subsequent analysis frequently

encompassing further enlargement and detailed comparison and study

of the results, weeks or months elapse from the onset of a series

of experiments to its completion. This delay is incompatible with

timely feedback into an elaborate series of experiments scheduled

a week or two apart.

In order to provide essential information on a time scale of

a week or so, substantially different operating procedures would

have to be developed. First, very specific questions would have to

be posed, so that the analysis could be restricted to a few highly

specific issues. Second, utilization of the real-time, high

spatial resolution imaging capability that has been developed

recently would permit one to obtain the structural information in

essentially real-time rather than after a long wait for travel

between Brookhaven and Washington, several stages of photographic

development, and the exploration of various models in order to

describe in a comprehensive manner what is observed. Finally,

active participation of crystal growers in the data acquisition on

the beam line would assume even more importance than is normally

the case.

15
c. Mercuric Iodide

The results of mercuric iodide growth on Spacelab III by

physical vapor transport [1][2] have already proved to be of major

interest, and another set of microgravity experiments has been

scheduled for December 1990 on XML 1. Laboratory topography has

shown some correlation between grain structure and the performance

of detectors, but only in the relatively extreme cases [12].

Unexplained cellular structure with a characteristic dimension of

2-6 jum has been observed in all mercuric iodide by scanning

cathodoluminescence microscopy [13]. Previous exploratory

high-resolution diffraction imaging of terrestrial mercuric iodide

on the beam line [7] suggested that high-resolution imaging of

additional crystals might prove to be extraordinarily interesting

and fruitful in wrestling with these questions.

Therefore, Lodewijk van den Berg provided immediately three

crystals: one grown in Spacelab III, a comparable terrestrial

crystal grown about the same time, and a new, higher purity crystal

grown from material to be used on IML 1. He planned also to

participate in the imaging on the beam line, which would have been

highly desirable. However, the extremely tight beam line schedule

and Dr. van den Berg's travel schedule proved at the last minute to

be incompatible.

Our previous examination had led us to anticipate that the

extreme softness of this material, its high atomic number, and its

extremely interesting defect structure together would prevent

immediate observation in Laue geometry. In the current set of

experiments, therefore, we have restricted ourselves to observation

in Bragg geometry, shown in section VI. The results of this

imaging, described in section VII, are so interesting that extraordinary effort to thin samples of this material and to support them

rigidly in innovative ways in future work seems now to be justified.

d. Cadmiiim Telluride, Cadmium Sulfide, and Lead Selenide

Bridgman and vapor growth of cadmium telluride and cadmium

sulfide are under study in terrestrial experiments by Sandor

Lehoczky. A discussion of diffraction imaging of these materials

is covered under closely related ternary materials in space,

described in the next two sections.

2. Ternary Inorganic Crystals

a. Cadmium Telluride Ternaries

The growth of ternaries of cadmium telluride is being studied

by four principal investigators. Sandor Lehoczky and Frank Szofran

are undertaking growth of mercury cadmium telluride on USML 1,

scheduled for March of 1992. David Larson is planning experiments

16
on zinc and selenium ternaries of cadmium telluride, while Ratnakar

Neurgaonkar is planning experiments on the vanadium ternary.

Growth of some or all of these latter materials is also being

scheduled for USML 1. We expect that Dr. Larson and his staff

would join us in work on the beam line, as they have in the past,

and would participate actively in the analysis.

These investigators clearly recognize the fundamental challenges presented by the growth of these ternaries, a principal one

of which is a steady shift in chemical composition during growth.

A variety of characterization approaches are being contemplated,

and investigators are intrigued by the opportunity to push the

limits of diffraction imaging with several samples of these

materials and related binaries and ternaries.

b. Mercury Zinc Telluride

Sandor Lehoczky is planning growth of mercury zinc telluride

on USMP 2, scheduled for February 1993. Diffraction imaging of

this and related material is covered in the preceding section.

c. Lead Tin Telluride

Roger Crouch and Archibald Fripp grew lead tin telluride on

STS 61A, and have been awaiting for several years the opportunity

for synchrotron examination of this and related terrestrial

samples. Several examples of each were provided by Richard

Simchick, who participated in the imaging on the beam line and

initial analysis, sections VI and VII. He has also provided

extremely useful auxiliary chemical data central to that analysis.

Archibald Fripp is designing a second flight experiment, which

he would like to carry out on USMP 2, scheduled for February 1993.

Examination of crystals of this and related terrestrial growth

should prove to be particularly valuable after the collaborative

imaging experiments on the beam line reported below.

d. Indium Gallium Antimonide

William Wilcox is studying the directional solidification of

alloys of indium antimonide and gallium antimonide. He would like

to use high-resolution diffraction imaging to observe compositional

inhomogeneities as well as grain and twin orientation. Professor

Wilcox and a student have spent substantial time on the beam line

with us in the past. We anticipate that they would continue to

participate actively.

17
3. Low Molecular Weight Organic Crystals

a. Triglycine Sulfate

Ravindra Lai grew triglycine sulfate from solution on Spacelab

III and has been scheduled for a second growth experiment on IML 1

in December 1990. He has supplied a sample from the Spacelab III

growth, which we examined at the very end of the current series of

experiments, section VI. The extraordinarily interesting results

are in summarized in section VII.

Professor Lai is particularly anxious that we examine potential seed materials, thus helping him to select the most promising

seed crystals for the IML 1 experiments. This would simultaneously

provide comparable terrestrially grown material for later comparison with that grown in microgravity . These experiments will

include both (010)- and (001) -seeded growth, and we all anticipate

that diffraction imaging would be particularly helpful in a comprehensive comparison of the two types of growth.

b. 1-Arginine Phosphate

Marcus Vlasse is growing highly regular and optically featureless crystals of 1-arginine phosphate, the study of which by

diffraction imaging would be of great interest. Dr. Vlasse would

like to participate actively in the imaging experiments.

c. Benzil

Sindo Kou is in the early stages of growing benzil. He would

be interested both in determining the extent of defect structure in

his material and in detailed correlation of this information with

his strategy for crystal improvement.

C. Novel Materials

1. Epitaxial Systems

a. General Considerations

Epitaxial systems, in a manner fundamentally similar to more

conventional seeded crystal growth, present opportunities for

diffraction imaging in the examination of the degree of lattice

matching and the propagation of strains within a layer (or new

growth) from the substrate (or seed) . Such effects are frequently

magnified in epitaxial systems by the thinness of the epitaxial

layer and by deliberate changes in the chemistry between successive

layers. One will observe irregularity at the interface, however,

only to the extent permitted by the internal regularity of the

various layers of the completed crystal. Although diffraction from

18
various layers has been observed, the analysis of previous highresolution images has been severely limited by the regularity of

the layers [5] .

b. Mercury Cadmium Telluride on Cadmium Telluride

Heribert Wiedemeier is scheduled for vapor growth of mercury

cadmium telluride, epitaxially on cadmium telluride, on USML 1 in

March 1992. He would like to use diffraction imaging in order to

optimize growth conditions. He would participate both in data

acquisition and in subsequent analysis.

This prospective intensive collaboration, and its explicit

goal of optimization of the flight program appear to make such a study unusually productive. Past diffraction imaging of cadmium

telluride grown by Bridgman techniques has shown a high degree of

irregularity. A principal challenge in the imaging of epitaxial

samples, therefore, will be to locate substrates that are good

enough not to obscure information on structure of the interface.

c. Zinc Germanium Phosphide on Gallium Phosphide

Klaus Bachmann is motivated more by the cleanliness of space

processing than by the reduction in gravity to grow high-purity

multiple quantum well structures by organometallic molecular beam

epitaxy. The effects of this purity on the strain structure should

be interesting to observe in the 0.1% lattice matched systems that

he is contemplating. The opportunity for close collaboration with

those using transmission electron microscopy that is presented here

would make such diffraction imaging experiments unusually interesting.

d. Silicon on Germanium

Randall German and Krishna Raj an are focusing on the detailed

nature of the interface structure in dissimilar, non latticematched systems. Their interests span a range from germanium/silicon bicrystals, through solidification, to sintering. They

are focusing on local strain gradients and their effects on solute

diffusion. Thus their interest in crystal regularity derives from

the sophisticated knowledge of interfaces that they want to

acquire, rather than the formation and study of generally more

regular crystals. The use of complementary electron microscopy,

which is characteristic of their work, would make collaboration

with them in high-resolution diffraction imaging unusually

valuable.

19
2. Proteins

a. General Considerations

The strong interest in protein crystal regularity arising from

the promised improvement of crystal lographic structural determination was summarized in section I. Diffraction imaging of proteins

has not been attempted, however; and suggestions that it be tried

lead to several important questions, which we now address. Both

those issues noted by potential collaborators and additional issues

that we view with concern are identified here and examined.

i. Relevance of the mesoscopic regularity observed in

diffraction imaging to the order required for structural determination. A perfect single crystal is not required for structural

determination. Indeed, dynamical diffraction from a perfect

crystal can complicate the analysis of kinematic scattering used in

structural determination. Nevertheless, crystals that diffract

more effectively have been observed to display superior morphology

as well [3]. If this proves to be true of proteins in general,

then any guidance from diffraction imaging that would assist in the

achievement of superior morphology may well contribute to success

in the determination of crystal structure.

ii. Substantial mesoscopic irregularity in the crystallization of organic molecules in general and of protein molecules in

particular. One of the conditions already identified for successful diffraction imaging is the presence of sufficiently small

numbers of irregularities that their interpretation is not obscured

by scattering from adjacent features. Pervasive irregularity even

that characteristic of a specimen that could prove ideal for the

determination of crystal structure, may well constrain the amount

of useful information in a diffraction image. This is perhaps the

most serious source of concern in anticipating the utility of

diffraction imaging of protein systems. The relative importance of

this factor can be determined only empirically. This means that

the ultimate extent of high-resolution diffraction imaging may be

established only by trial. Initial attempts clearly should focus

on the most promising, that is the most regular, protein crystals.

iii. Change in regularity of crystals grown in space on

return to earth. Changes in the regularity observed in space-grown

crystals in the days following return to earth [14] raise some

question about the long-term stability of their structure.

However, rapid diffraction imaging during the period of instability

might prove to be exceptionally interesting.

iv. The necessity to keep good protein crystals in contact

with their mother liquor. The presence of a liquor around the

crystal to be analyzed presents two challenges. The first is the

necessity to surround the crystal by a vial to contain the liquor.

Diffraction through such a vial could prove extremely challenging

20
in practice. The presence of the liquor, moreover, could well

interfere with the firm orientation of the sample as it is tilted

for diffraction in various directions. Since useful crystallographic patterns have been obtained of such systems, this would not

appear to be an absolute limitation. But successful imaging of a

crystal swimming in its liquor could prove to be very difficult.

V. The destructive power of x rays in systems with low atomic

number. Substantial crystal damage has been observed in proteins

after crystallographic study with high intensity x-ray beams, which

often contain a full white radiation spectral distribution.

Indeed, a principal advantage of synchrotrons for this analysis is

the high flux that they deliver. The resulting useful exposure

time is sufficiently short that relatively slow damage processes

are completed only after the exposure. The focused beams used in

these crystallography experiments deposit orders of magnitude more

energy on the sample than do high-resolution diffraction imaging

beams, for which the desired improvement in collimation and

monochromatization severely reduces the irradiance of the magnified

probe beam. The hope that proteins will withstand the lower

irradiance levels characteristic of high-resolution diffraction

imaging is difficult to evaluate quantatively . The extent of

damage will depend partly on the length of time required for

diffraction imaging, which depends on many factors.

vi. The low x-ray scattering power of organic molecules in

general and of protein molecules in particular: possible low

contrast in the images. The kinematic scattering responsible for

the diffraction spots used for crystallography is certainly strong

enough to support structural analysis. Individual spots indeed

contain structure as well [15]. We report in section VI on the

diffraction imaging of triglycine sulfate. Contrast thus does not

appear to be an insurmountable problem in the diffraction imaging

of proteins.

vii. Complexity in the analysis of diffraction images because

of the mixture of kinematic as well as dynamical scattering

expected from the small optical thickness of such materials.

Crystallography depends on kinematic scattering from adjacent atoms

or groups of atoms. Diffraction imaging of the characteristic

optically thick crystals to which it is applied is most successful

under conditions that suppress such scattering in favor of perfect

crystal dynamical scattering. In proteins and other organic

materials consisting largely of "low Z" atoms, however, we

anticipate that both kinematic and dynamical scattering will

coexist, as is observed later in this report in triglycine sulfate.

The interpretation of such mixed images may well be complicated and

possibly limited by the simultaneous occurrence of both types of

scattering.

21
b. Interest in the Imaging of Proteins

Six of the nine principal investigators working with protein

growth: Robert Feigelson, Daniel Carter, Charles Bugg, Larry

DeLucas, Franz Rosenberger, and Donald Voet, are interested in

exploring the relevance of diffraction imaging to the growth of

better crystals for crystallography. Of these, Charles Bugg and

Larry DeLucas have performed a number of space experiments and are

actively planning a number of new ones, with particular emphasis on

USML 1 scheduled for March 1992. The views of the other three are

reviewed below in section D.

All nine investigators would very much like to succeed in the

growth of more regular crystals for crystallographic analysis, but

are sensitive to the preceding set of issues that may restrict the

utility of the results of diffraction imaging for protein crystal

growth. It seems fair to say that all would approach the imaging

of protein crystals with preparation that is at least as thorough

as that which should precede the diffraction imaging of any

prospective system. Perhaps the principal initial practical issue

is the establishment of a consensus on a promising system for exploratory experiments.

3. Other Systems

a. Whiskers

Herman Hobbs is studying the evolution of whiskers with

characteristic dimensions of 1 square micrometer by 1 centimeter.

He observes that critical changes in the structure occur when the

whisker thickness has grown to about 5 jum, so that the imaging of

whiskers as they approach this size might prove to be unusually

interesting. He would like to collaborate actively in such a project; and this collaboration should contribute to the chances

for success.

However, the current spatial resolution in real-time is 30 /im;

the comparable resolution is 1 iim for nuclear emulsion plates.

Thus whiskers of the size that is most interesting would strain

these spatial resolution limits severely. Extension of the realtime capability toward 1 /xm should improve the chances for success

in such experiments.

b. Small Particles

William Hofmeister is working on the containerless growth of

high temperature 1-2-3 superconductors in a similar size range, 20-

100 jLxm. The challenge in the diffraction imaging of such materials

is increased by the known propensity of these materials to twin.

As with the whiskers, the prospects for success in working with

micrometer size materials should be increased substantially by the

extension of the real-time capability toward 1 micrometer.

22
c. Polygrain Growth by Physical Vapor Transport

Elmer Anderson is investigating the polygrain growth of zinc

selenide by physical vapor transport. He achieves clusters of

single crystals and large twinned boules, and would like to

minimize the irregularity in optical materials. He is interested

in exploring the applicability of diffraction imaging to such

polygrained systems and would join us actively in such experiments,

which would contribute to the prospects for success. The principal

question here is whether the crystals have sufficient regularity

and size to lend themselves to successful high-resolution analysis.

d. Rapid Solidification

Robert Schiffman is studying the establishment of properties

in microgravity in real-time, for example following rapid solidification by quenching from very high temperature. Although he is

interested in collaboration with us, the utility of high-resolution

diffraction imaging for these systems may depend ultimately on the

observation of longer duration changes, changes in properties at

least over several days following return to full gravity.

e. Liquid Encapsulated Melt Zone

Reza Abbaschian is planning the liquid encapsulated melt zone

growth of bismuth doped with tin on USMP 2 , scheduled for February

1993. He anticipates analysis by a variety of techniques among

which he will consider diffraction imaging.

f. Convective Effects

Singh is planning the growth of lead bromide on USML 2,

scheduled for March 1994, and IML 3, scheduled for April 1995. His

primary interest is in the study of convective effects. The

utility of high-resolution diffraction imaging will depend on the

particular nature of the convective effects.

23
D. Materials Not Relevant to this Study

1. Glass Formation

a. Oxide Glasses

George Neilson, Chandra Ray, and Michael Weinberg are each

working in the area of glass formation. Without a crystal structure, glasses are not candidates for diffraction imaging, high or

low resolution.

b. Metglasses

William Johnson is working on metglasses, which also, by

definition, are not candidates for diffraction imaging.

2. Fine Particles

a. Rapid Solidification

Merton Flemings and Carl Koch are studying rapid solidification leading to submicrometer crystallites. The required resolution would be substantially less than their size, say of the order

of a nanometer. This is below the best current spatial resolution

available with diffraction imaging.

b. Diamond Films

John Margrave is studying the formation of diamond films. The

crystallographic perfection of individual diamond crystallites is

not of immediate interest in this work. Moreover, the submicrometer particle size would require resolution of the order of a

nanometer, which is below that accessible with current diffraction

imaging.

3. Specific Stages of Growth

a. Seeding

Frederick Carlson is studying the role of seeding, particularly in conjunction with modeling. Although diffraction imaging

is able to shed light on some aspects of seeding, other analytic

information is likely to be more useful to this modeling effort.

b. Transport in Alloys

Martin Glicksman, John Perepzko, Robert Bayuzick, and Eugene

Trinh are each engaged in the study of various effects of transport

on the solidification of alloys. The information provided by

diffraction imaging is not immediately relevant to these studies

focussing on dendritic growth.

24
c. Ostwald Ripening

Peter Voorhees is focusing on the Ostwald ripening of alloys.

Although he and NIST scientists have carried out white beam

diffraction imaging and radiography on such systems [16],

diffraction imaging is not immediately relevant to his current

activity.

d. Magnetostriction

Julian Szekely is studying the magnetostrictive levitation of

liquid metal droplets. They are clearly not accessible to diffraction imaging.

e. Proteins

Our survey indicates that three principal investigators, Ponzy

Lu, Alexander McPherson, and Marc Pusey are focusing on aspects of

protein science to which diffraction imaging is not likely to

contribute.

4. Model Systems

a. Ammonium Chloride

Mary McCay is planning experiments on the holographic observation of diffusion of ammonium chloride on IML 1, scheduled for

March 1992. These experiments do not produce "returnable" crystals.

b. Organic Analogues of Metal Systems

Barry Andrews is observing the solidification of transparent

organic analogues of immiscible alloys. The degree of perfection

of these materials is not a principal factor in this work.

5. Diagnostic Techniques

a. Acoustic Techniques

Martin Barnatz is applying acoustic techniques to the sintering of ceramics. Crystal perfection is irrelevant.

b. Dynamic Measurements

Ared Cezairliyan is making dynamic measurements on systems

that do not result in the growth of high-quality crystals.

25
V. MATERIALS IN THE CURRENT EFFORT

A. Selection Considerations

The large number of systems and questions identified in the

preceding section, suggests both the importance of careful sample

selection and the complexity of carrying it out. The proximity of

space flights scheduled for crystal growth (table 1) introduces

urgency as an additional factor. Realization of the three goals of

the imaging program identified in section IV: establishment of a baseline of image data, correlation with performance, and optimization of growth experiments, required an immediate start for

this work.

In work toward each of these goals, optimization of the

scientific value of diffraction images requires that samples have

the four characteristics identified in section II. However, the

desirability of starting on the structural base line before the

next scheduled flights dictated that work this first year be

limited to samples already at hand. The definition of our initial

activity thus required some compromises in the crystals observed.

The first crystals selected included three early examples of

space growth, i.e. mercuric iodide, lead tin telluride, and triglycine sulfate, and comparable terrestrial samples of each. The

comparison crystal for the space-grown triglycine sulfate was

actually the seed used for the space growth, so that a separate

sample was not required. We also observed two terrestrial samples

for later comparison with crystals to be grown in space this fall:

mercuric iodide and gallium arsenide.

The resulting images and what we have learned from them,

although not optimized in terms of the detailed interpretation that

might come from specially prepared samples, are of such far-reaching consequence to the future program that they support the wisdom

of the decision to observe immediately those high-priority crystals

already available.

26
TABLE 1: Flight PI samples anticipated

89-90 90-91 91-92

Many flights : Bugg (protein)

STS 40 (August 1990)

Ditchek (GaAs) 3-6 2-4 2-4

IML 1 (December 1990)

van den Berg (Hglj) 3-6 6-8 6-8

Lai (TGS)

' 2 4 2

McPherson (protein)

USML 1 (March 1992):

Matthiesen (GaAs) 2 2

Wiedemeier (HgCdTe) 2 2

Larson (CdXTe) 3 3

Carlson (CdTe)

Neurgaonkar (CdVTe) 3-4 3-4

Lehoczky (HgCdTe) 3 3

DeLucas (protein)

USMP 2 (February 1993):

Fripp (PbSnTe incl. sg)

Abbaschian (Bi(Sn))

Lehoczky (HgZnTe)

USML 2 (March 1994):

Singh (PbBr,) X

IML 3 (April 1995)

Singh (PbBr,) .0

TOTALS 10-16 31-36 32-37

2 2 4

1

2 2

27
B. Mercuric Iodide

Mercuric iodide was given high priority for several reasons.

The extraordinary charge carrier mobility displayed by a mercuric

iodide crystal grown by physical vapor transport in Spacelab III

has been noted in the Introduction. This improvement over

comparable terrestrially grown crystals is particularly noteworthy

because the intrinsic advantages of mercuric iodide radiation

detectors, high quantum efficiency with high energy resolution

[17], have typically been compromised by marked variation in

energy resolution within a single detector and by low yield [18].

Crystalline imperfections, chemical and crystallographic, have been

shown to degrade the energy resolution [ 12 [ 19] [20] [21]

,

and these can arise either during crystal growth or in subsequent

processing into a device [22 ] [23 ] [24 ] . Previous collaboration in high-resolution diffraction imaging

of terrestrial mercuric iodide with Lodewijk van den Berg [7], who

had grown this material himself in space, had verified the prominence in this material of individual grains, as reported following

neutron and gamma-ray diffraction [25] and x-ray topography

[12] [26]. A 2-6 jLtm cellular structure had also been reported

[13]. However, neither the origin of the grains nor that of the

cellular structure had been identified.

The unusual performance of space-grown material, the active

collaboration of the grower, and the opportunity to incorporate

what was to be learned in scheduled space growth all ensured that

progress on these questions would exert maximum scientific leverage. Only conflicting schedules at the last minute prevented Dr.

van den Berg's joining us on the diffraction imaging synchrotron

beam line for the imaging itself.

We observed three mercuric iodide crystals: 1) the crystal

grown on Spacelab III, which had been made into a detector by the

application of graphite electrodes, which were not expected to

interfere directly with the observation; 2) a terrestrial control

grown about the same time and from similar material; and 3) a terrestrial sample recently grown from new, high-purity material

comparable to that to be used in the IML 1 flight scheduled for

December 1990. The terrestrial samples were selected on the basis

of the regularity of their Laue spot patterns from two samples of

each that were made available.

The high atomic number of the constituents and the interesting

structure observed in earlier work indicated that samples of

mercuric iodide much thinner than a millimeter would be required

for observation in Laue geometry. The extraordinary softness of

the material, however, suggested that the achievement of sufficient

thinning without deformation would require a special research

project that would seriously delay the imaging. In any event, it

28
seemed inadvisable to carry out thinning research on a rare space

sample. We therefore elected to focus these initial experiments on

an evaluation of the general nature of the crystals by imaging them

in Bragg geometry, solidly mounted so that deformation on thinning

would not interfere, and to save until later experiments with extreme thinning and mounting procedures if the present results of

the imaging in Bragg geometry indicated that a special effort might

prove worthwhile.

C. Lead Tin Telluride

A crystal of lead tin telluride grown by Bridgman techniques

on STS 61A [27] and a comparable terrestrial sample were studied in order to explore any additional effects that previous space

growth had on a ternary and to provide a foundation for the analyis

of new growth experiments on USMP 2, scheduled for February 1993.

This ternary is of particular interest for growth in microgravity

because its growth is thermo-solutally unstable in a terrestrial

environment [27].

The presence for the imaging on the beam line of Richard Simchick, who participated in the growth of these crystals and had

carried out chemical analyses of them, ensured that the results of

these experiments would be optimized and would exert maximum

influence on subsequent growth of this material. Both samples were

selected on the basis of the regularity of their Laue spot patterns

from two samples of each type.

In planning these imaging experiments, we recognized that

existing samples of this material also were too thick to support

diffraction in Laue geometry. The irregularity that we anticipated

by itself would require severe thinning for successful transmission. This requirement for thinning would be amplified by the

high atomic number of the constituents. Because of the undesirability of unnecessary handling of a space-grown crystal before

examining it, we elected not to thin them at this juncture. We

thus minimized the effect of surface treatment in this initial

analysis.

D. Triglycine Sulfate

Triglycine sulfate (NH2CH2COOH) 3H2SO4, a ferroelectric material,

is a good model crystal for solution crystal growth. It is

pyroelectric below 49 °C. It has a monoclinic crystal structure

with a (010) cleavage plane normal to the pyroelectric axis.

Interest in TGS for thermal imaging and detection devices, spaceborne radiometry, and sensing high energy pulse lasers has led to

a search for crystals of high optical quality and improved pyro29
electric properties. In Spacelab III, two TGS growth experiments

were carried out. In both, disc-shaped (001) -oriented seed

crystals were used. In one of these (FES-3, cell # 204) the growth

conditions were T^^ = T^, < T^^, where T^,, T^,, and T^^ are the temperatures of the sting (crystal) , solution, and saturation, respectively. For characterization, slices perpendicular to the [001] growth

direction were cut, lapped, and polished.

One of these slices was examined as part of the current

effort. In this case, the terrestrial seed acted as its own internal control. The regularity of this material has been shown to be

extremely sensitive to the growth rate, growth regularity including

temperature stability, and purity, provided that the seed is sufficiently regular [28] [29]. In addition to the opportunity

to survey marked anomalies in early space growth, the immediate

influence that results might have on the IML 1 growth, originally

scheduled for December 1990, was an important factor in the

selection of this crystal. Additional factors were the active

collaboration of Ravindra Lai and his associates and the anticipation, in light of earlier collaboration with Professor Lai, that

diffraction from these crystals in Laue geometry would be accessible.

E. Gallium Arsenide

Finally, although gallium arsenide has not yet been grown in

the NASA space program, two Bridgman-growth experiments are now

planned: one by Brian Ditchek in the STS 40 GAS can flight,

originally scheduled for August 1990, and one by David Matthiesen

on USML 1, originally scheduled for March 1992. One comparable

terrestrial crystal on hand was thin enough to offer a chance to

observe diffraction in Laue geometry, so this was selected.

Additional factors supporting its selection were the potential

technological importance of gallium arsenide, the availability of

infrared images that could be compared directly with diffraction

images, the active collaboration of David Matthiesen in the

selection of fundamental issues to be addressed and in planning the

experiments, and the anticipation that he would join us on the beam

line. Unfortunately that proved at the last minute not to be

possible.

30
VI. THE DIFFRACTION IMAGES

A. Mercuric Iodide

1. Terrestrial Crystal Compared with Spacelab III Crystal

While the terrestrial mercuric iodide crystal grown from the

same source material as the Spacelab III crystal diffracts over a range of one half degree, a large central portion is sufficiently

regular to diffract only within a few minutes of arc. Full highresolution diffraction images of this terrestrial crystal appear in

figures 1-6, and enlarged portions of these in figures 7-25.

Most of the central portion of the crystal is in diffraction

in the (1 1 12) and (1 1 10) images in figures 1 and 2, respectively, indicating lattice regularity with respect to rotation around

a [110] axis of the order of a few arc seconds. However, the

absence of diffraction in a wide [110] (vertical) stripe in the

center of figures 1 and 2 indicates that the lattice is deformed by

a sharp twist of about 10 minutes of arc around an axis defined by

this stripe. The extent of this twist is determined from the 100

^m width of the stripe in figures 1 & 2, and the knowledge that the

photographic plate was located about 3 . 5 centimeters from the

crystal. The twist of the crystal lattice is evident indirectly

also in the (0 1 10) and (0 1 11) diffraction images, figures 3-6,

for which the crystal was rotated azimuthally 45°. In this orientation, the misalignment of the two parts of the crystal precludes

bringing them simultaneously into diffraction. Examination of the

full images, figures 1-6, and of a sequence of real-time images of

the (008) diffraction, for which the acceptance angle is narrower

than for that in the printed images, indicates that the principal

lattice twist axis itself bends through several minutes of arc,

differing slightly in the two subgrains.

The other principal large feature of the full images of this

crystal in figures 1-6 is a set of textural stripes, which are

oriented in the [110] direction. The enlargements in figures 7-25

show these stripes to consist of a relatively high density of

discrete features, typically out of diffraction in these images and

therefore ascribed to one or more additional phases. Some of

these features take the form of thin { 100}-oriented stripes a few

micrometers wide; they are sometimes crossed. The others are more

irregular, globular particles, 1-60 ^um in diameter. These may

differ completely from the stripes, but may simply represent

similar stripes normal to the (001) image and projected on it.

31
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s*v

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Figure 7Enlargement of central portion of figure 1,

(1 1 12) diffraction. Darker areas diffract more

strongly.

38
fS'^./y'f¥\,\

i'-IW^ l^^.'^rt ^t*'"

r f>i#*^^^..#*t-'^^,•'^

Figure 8. Enlargement of lower central portion of figure 1,

(1 1 12) diffraction. Darker areas diffract more

strongly.

39
Figure 9 Enlargement of upper central portion of figure 1,

(1 1 12) diffraction. Darker areas diffract more

strongly.

40
*¥% '.<*

%'

:•.. ^

^

100pm

Figure 10 Enlargement of central portion of figure 2,

(1 1 10) diffraction. This image is to be

compared with the same crystal region imaged

in (1 1 12) diffraction, figure 7. Darker

areas diffract more strongly.

41
Figure 11. Enlargement of lower central portion of figure 2

(1 1 10) diffraction. This image is to be compared with the same crystal region imaged in (1 1

12) diffraction, figure 8. Darker areas diffract

more strongly.

42
100 010

Figure 12

Enlargement of upper central portion of figure 2,

(1 1 10) diffraction. This image is to be compared with the same crustal region imaged in

(1 1 12) diffraction, figure 9. Darker areas

diffract more strongly.

43
Figure 13 Enlargement of lower left-hand corner of figure 3,

(0 1 10) diffraction. This image is to be compared with an overlapping crystal region imaged in

(1 1 12) diffraction, figure 8, and in (1 1 10)

diffraction, figure 11. Darker areas diffract

more strongly.

44
Figure 14. Enlargement of upper left-hand corner of figure 3

(0 1 10) diffraction. Darker areas diffract more

strongly.

45
tl»

'Mf

4-*

1»

#«

I

'< «* n> X

^ -f^

< I

^ ^

-^f^^^

010

100 100 pm

Figure 15 Enlargement of the central portion of figure 3,

(0 1 10) diffraction. This image is to be compared with the same crystal region imaged in

(1 1 12) diffraction, figure 7, and in (1 1 10)

diffraction, figure 10. Darker areas diffract

more strongly.

46
A010

100

Figure 16 Enlargement of upper central portion of figure 3,

(0 1 10) diffraction. The crystal region in this

image is contiguous to the region imaged in (1 1

12) diffraction, figure 9, and in (1 1 10) diffraction, figure 12. Darker areas diffract more

strongly

47
Figure 17 Enlargement of central portion of fig^^e

(0 1 10)

strongly

diffraction. Darker areas diffract more

48
.010

I 1 100 100 ^im

.•I

* *.

Figure 18 Enlargement of central portion of figure 4, (0 1

10) diffraction, overlapping the region shown in

figure 17 . The image here is to be compared with

the same crystal region imaged in ( 1 1 12) diffraction, figure 7, and in (1 1 10) diffraction,

figure 10. Darker areas diffract more strongly.

49
Figure 19. Enlargement of central right hand portion of

figure 4, (0 1 10) diffraction. This crystal

region overlaps that shown in (1 1 12) diffraction, figure 9, and in (1 1 10) diffraction,

figure 12. Darker areas diffract more strongly,

50
Figure 20 Enlargement of central portion of figure 5, (0 1

11) diffraction. Darker areas diffract more

strongly.

51
' m

Figure 21 Enlargement of central portion of figure 5,

(0 1 11) diffraction, which overlaps the region

imaged in figure 20 and in (1 1 12) diffraction,

figure 7, and in (1 1 10) diffraction, figure 10

Darker areas diffract more strongly.

52
100

Figure 22 Enlargement of upper right central portion of

figure 5, (0 1 11) diffraction. This image is to

be compared with the same crystal region imaged in

(0 1 10) diffraction, figure 16, and contiguous to

the region imaged in (1 1 12) diffraction, figure

9, and in (1 1 10) diffraction, figure 12. Darker

areas diffract more strongly.

53
Figure 23 Enlargement of central portion of figure 6,

(0 1 11) diffraction. This image is to be compared with the same crystal region shown in

(0 1 10) diffraction, figure 17. Darker areas

diffract more strongly.

54
010

i^'^SC '

•iaf*'5^Site>..

Figure 24. Enlargement of the subgrain junction in the central portion of figure 6, (0 1 11) diffraction.

This image overlaps the region imaged in figure 23

and is to be compared with the same crystal region

imaged in (1 1 12) diffraction, figure 7, and in

(1 1 10) diffraction, figure 10.

55
Figure 25. Enlargement of central right hand region of figure

6, (1 1 11) diffraction. This image is to be

compared with the same crystal region imaged in

(1 1 12) diffraction, figure 9, in (1 1 10) diffraction, figure 12, and in (0 1 10) diffraction,

figure 19. Darker regions diffract more strongly.

56
In those regions characterized by a high density of discrete

features, diffraction appear to be restricted to small {^5 iim)

cells of the type observed in scanning cathodoluminescence microscopy [ 13 ] . The other areas of the crystal contain similar features

typically out of diffraction, but with a much lower density. In

addition, however, these regions contain thin, curved features

marked by varying sections of higher diffraction, lower diffraction, or alternating regions of higher and lower diffraction in

tandem. The inability to observe diffraction in Laue geometry in

the current series of experiments, because of the sample thickness,

precludes firm identification now of these features as dislocations.

The nature and arrangement of these various features in this

sample appear to make this crystal into a Rosetta stone in understanding the evolution of irregularity in mercuric iodide. Perhaps

the initial question that arises is associated with the origin of

the lattice twist. Does it arise during growth or only later

during subsequently handling of this very soft crystal?

Six distinct observations all indicate that this lattice twist

occurred during growth. The first two are that the twist axis does

not extend across the entire crystal, and that once started, the

magnitude of the apparent separation of the two parts of the images

does not increase. It is difficult to conceive of such a partial

lattice twist, one lying precisely in the (001) plane, developing

through inadvertent mishandling. _The third observation is that

this twist axis is normal to the [110] layered texture formed by a high density of additional phase features. These layers appear to

be broad striations formed during growth; the [110] lattice twist

axis thus appears to be aligned with the crystal growth direction.

Fourth, the gradually curved nature of some of the linear multiple

phase configurations in the vicinity of the lattice rotation is

more consistent with growth than with post-growth bending. Fifth,

the onset of the lattice twist immediately precedes a major

textural change that appears to be growth-related. The final

observation is the bending that has been noted in the lattice twist

axis, bending that differs in the two resulting subgrains.

Additional examination of the interfaces between the widest

stratum of high-density features and the adjacent low feature

density strata, Figures 9, 12, 19, and 7, 10, 15, 17, 18, 20, 21,

23, 24, respectively, confirm the growth orientation and the origin

of the lattice twist. The linear additional phase features in the

low-density layer immediately adjacent to the high-density region

near the center of the crystal appear correlated closely with

individual features in the high-density region. Growth thus took

place in this part of the crystal from the high-density stratum to

the low-density stratum, i.e., in a direction projecting onto the

(001) crystal surface in the [110] direction. Moreover, as just

57
noted, the sharp lattice twist appears to begin immediately

preceding the onset of the broad textural stratum of high density

of (precipitate) features, where it joins the preceding low feature

density textural stratum.

All of these observations are consistent with a growth model

in which growth begins in the extreme [110] corner of this crystal

(in figure 1, for example, this is the top corner) and proceeds

relatively uneventfully in the [110] direction (downward in figure

1, for example), or at least in a direction projected onto this

direction in the images, until just before the onset of the wide

swath of a high density of additional phase features, one of which

initiated the sharp lattice twist visible in figures 9, 12, 16, 19,

22, 25. This twist then propagated for the remainder of the

growth. During this subsequent growth, briefer periods of relatively high additional phase density alternate with periods of

relatively low additional phase density.

Several closely related questions now arise. What is the

nature of the additional phase(s)? What causes the additional

phase (s) to form? Why does one form appear oriented crystallographically in {100} directions of the matrix? What restricts

diffraction to small regions in some areas? And, finally, are the

features that are out of diffraction responsible for the much

higher degree of long range lattice perfection in lattice orientation (within a few seconds of arc over several millimeters) , with

which they appear to be associated?

An important observation is made in the evaporation of such

crystals. As material is removed, small specks of foreign material

similar in size to the additional phase features observed in this

study accumulate on the surface, at an irregular rate. Chemical

analysis indicates these specks are neither mercury nor iodine

precipitates but rather consist of metal organic impurities with a carbon content of the order of 70% and a wide variety of metals.

It is tempting to associate these observed impurity formations with

the additional phase (s) observed in diffraction and therefore to

conclude that these impurities reside in such crystals in discrete

form.

The morphology of the diffraction images permits us to develop

two alternative growth models, which tie together all of these

observations. Growth over a region of a few micrometers forms a crystal with a relatively high degree of purity and crystal perfection, creating small regions that diffract strongly. Impurities

are rejected from the crystal during this stage of the growth

process, in a manner similar to constitutional supercooling, and

accumulate near the growth surface.

In one model, the level of impurities after growth of a few

micrometers accumulates to such an extent that they precipitate

out, marking the local growth surface in {100} directions. At

58
reentrant corners of such {100} growth surfaces a globular precipitate possibly forms. In a second model, the rejection of

impurity stimulates dendritic growth, which leaves the linear

features observed in {100} directions. In this case, the globular

form of the precipitate may form in the reentrant dendritic locations. Alternatively, the features that appear to be globular may

simply represent the cross section of dendrites normal to the image

surface. None of our observations to date permit us to distinguish

absolutely between these two models.

Either model must be modulated by an as yet unidentified

process that controls the general impurity level and produces as a result textural stripes or striations delineated by the density of

precipitates. The resulting composite formed in either model

resists deformation. It consists of relatively pure and thus

relatively strain-free components.

2. Spacelab III Crystal

The crystal grown in Spacelab III diffracts over a wider

angular range, about one and one half degrees. Full high-resolution diffraction images appear in figures 26-28, with enlarged

regions of these in figures 29-31. It is clear from the appearance

of the full images as well as from the one and one half degree

acceptance angle for diffraction that the lattice orientation or

parameter of the space crystal in its entirety is less uniform than

the comparable terrestrial crystal shown in figures 1-6: that is,

less of the space crystal appears in diffraction at a given angle

of incidence than does the comparable terrestrial crystal,

indicating gradual variation either in lattice parameter or lattice

orientation, or both.

Perhaps closely related, but potentially far more important,

is absence in the enlargements of the images of the Spacelab III

crystal of arrays of features that are out of diffraction, the

textural arrays characteristic of the comparable terrestrial

crystal. A few irregular regions of the order of 50 /xm across that

are out of diffraction are observed, but they are much less

pervasive and sharply delineated than are those in the terrestrial

crystal. None of the crystallographically oriented regular regions

that are typically out of diffraction in the images of the comparable terrestrial crystal are observed in the Spacelab III

crystal. The formation of regions of additional phase (s) thus

appears to be almost completely suppressed in the crystal grown in

microgravity

One set of features visible in the enlarged images of the

Spacelab III crystal is not observed in the images of its terrestrial counterpart: small (10-30 ^m) highly diffracting spots in the

vicinity of the main highly diffracting regions. These features

suggest cells oriented in a direction differing from the direction

59
o <U o J-( ^

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tj>

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-H • CO c fHH -H

-p tH o (C

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-PWu -H

CM > H

(U tp X ffi

COHH

,—

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-iH Cu

-Pw

IK rH Uh • W >1

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fd

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x: '4-1 en 5-1 5-1 H p -P X w w

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61
r^OO QJ

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^2s

*^

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•H^j 5^

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62
100

^^» • ^

Figure 29 Enlargement of main region of figure 26, (008)

diffraction. Darker areas diffract more strongly

63
Figure 30. Enlargement of main region of figure 27, (11 10)

diffraction. Darker areas diffract more strongly,

64
1 00

Figure 31 Enlargement of main region of figure 28, (10 10)

diffraction. Darker areas diffract more strongly,

65
of the surrounding matrix by perhaps a few tens of arc seconds.

Such regions may also be present in the terrestrial crystal, but if

so they are obscured by the heavy incidence of additional phase (s)

.

The Spacelab III sample differed from the terrestrial sample

not only by its growth in microgravity , but also by the superposition of graphite electrodes; so that its performance as a neutron

and x-ray detector could be measured. Since graphite is relatively

transparent to x-rays, these electrodes were not expected to interfere with the imaging process itself. While they could in

principle have affected the surface strain, we found no evidence

for this.

However, this crystal was not encapsulated. With the passage

of the 5 years that this crystal has been in the laboratory, some

deterioration in electronic performance of the device made from it

actually has been observed, as is characteristic also of unencapsulated devices made in terrestrial environments.

The role of microgravity in the drastic reduction in the

formation of the additional phase (s) observed in the comparable

terrestrial crystal is not yet understood. The observed gradual

variation in lattice is consistent with varying retention within

the lattice of some foreign material. This analysis leads to increased interest in the results of the growth of a mercuric iodide

from the much purer material scheduled for a future flight.

3. Terrestrial Crystal to Be Compared to a Future Flight

Crystal

The third mercuric iodide crystal, grown from higher purity

material similar to that to be used on a future flight, diffracts

over a full two degrees. Complete high-resolution diffraction

images appear in figures 32-33, with enlarged regions of these in

figures 34-35. The extent and character of the diffraction in

these images, reflecting the general lattice uniformity, resembles

much more the diffraction from the Spacelab III crystal than that

from its terrestrially-grown counterpart. Moreover, the absence of

an array of small features that are out of diffraction also gives

these images much more the character of those from the Spacelab III

crystal than those from the terrestrial crystal grown about the

same time from similar material. The complete absence of evidence

for additional phases in this, high purity crystal is consistent

with the thesis that the features that are out of diffraction in

images of the earlier crystals are impurities, but this thesis

certainly is not proved conclusively by this correlation.

66
O

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'U

tJ^ o <D >i ^ u

•p

-P

g nH o

Q) (d P

tT> >1

fd o

CQ op

-H

(1)

fd >i

(d fd rH g tJI •HC oup

-H in -P n o ^^ 0)

fd <Hu

U

^-i rH

g

H-i fd •H 4J

p

1:5

w o >

>1 fdu

Q) Mh

4x^ <4H

rg

H

COH TJ

,-^ n: in o fd H 1—

Q)

fd !h o -H fd H p u — w Q)

Q) P c ^ x: u Cr^ •H Q) -H p p J

:2 -H M-l . w rH

0) 0)

^-1 o hJ 1 fd s

x^ 4-1

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tri s-i •H :3 c K m

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Q)U

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67
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-p -p e r-(fO -p

tP >1

fd

u

-p

•H

(U (d >i

fd

(d rH E Cn

•Hc g

-P •H W

-P

n

r-l Q)

fO

H

h ^

(+-4 ,—1 g

4-1 •H

fd -P

T!

w

u >

>1 fdu

Q) <4-l A^ M-i

(N •H

CO -0

,^-v

K

M

fd <H n-i dJ

(d

u

fH -H

^1

fd

r-{ +J 5q • U) 0)

QJ +J c u x: ^ D^ •H q; •H

-p 4-1 h:i p

.H M-l . w iH

0) Q)

J-l

U

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,C <+-!

H

Cn ^

•H :3 c K w

QiUP

-H

ft.

68
-V

100

Figure 34 Enlargement of lower left portion on figure 32, (1

10) diffraction. Darker areas diffract more

strongly.

69
Figure 35 Enlargemenl: of lower central portion of figure 33,

(1 1 10) diffraction. Darker areas diffract more

strongly.

70
We turn now to the extremely important comparison of detectors

made from the Spacelab III crystal and from terrestrial crystals

resembling those of this study. The performance of devices made

from the new high purity material approaches the original

performance of the device made from the Spacelab III crystal. The

improved performance of the Spacelab III crystal is traceable to

the higher mobility of its charge carriers. Recent improvements in

terrestrial crystal growth represented by the crystal examined here

have led to improved carrier lifetime. Although the electronic

improvements are quite distinct in these two cases, in neither of

them do we find the additional phase features that we have observed

in the first terrestrial crystal. Thus absence of a additional

phase precipitates appears to be much more important to device

performance than the generally higher level of lattice uniformity

that we observe in the first terrestrial crystal. The stiffening

provided by additional phase precipitates apparently comes at too

high a price in terms of charge carrier trapping.

Future space growth of this high purity material now assumes

particular interest. Will incorporation of residual impurities in

the final crystal even below their currently low level in the

charge material be achieved in space growth? And, if so, will

these lower impurity levels lead to greater general lattice

uniformity? And finally, will this new level of regularity result

in still further improvement in device performance?

B. Lead Tin Telluride

1. Terrestrial Crystal Comparable to STS 61A Crystal

Various regions of the terrestrially grown sample of lead tin

telluride, similar to one grown on STS 61 A, diffract as the

crystal is rocked over a full two degrees. Full high-resolution

diffraction images are shown in figures 36-43. Growth was in the

[001] direction, which is oriented "down" in all figures.

The sample was a regular half cylinder. The sharply delineated irregular outlines of the images in figures 36 and 38 thus

indicate immedi_ately that several grains are present: the curvature of the [110] (right hand) edge indicates that a subsidiary

grain started to grow almost simultaneously with the main grain.

Then, after 1 centimeter of growth, a third grain started to grow

between the center of the boule and the opposite edge of the main

grain. It grew laterally more rapidly than the nucleating grain,

however, displacing and, after another 2 centimeters, completely

overtaking the growth of the nucleating grain. The new grain is

brought into diffraction in figures 37 and 39 simply by rotation of

the sample about the boule (growth) axis.

71
Figure 3 6 High-resolution (220) 8 keV diffraction image in

Bragg geometry of approximately (22 0) surface of

terrestrial PbSnTe crystal 108. The growth direction is [001] . Lighter areas diffract more

strongly.

72
Figure 37. High-resolution (220) 8 keV diffraction image in

Bragg geometry of approximately (220) surface of

terrestrial PbSnTe crystal 108. The angle of incidence is 33 arc minutes larger than that for

figure 36; the growth direction is [001]. Lighter

areas diffract more strongly.

73
Figure 3i High-resolution (620) 8 keV diffraction image in

Bragg geometry of approximately (220) surface of

terrestrial PbSnTe crystal 108. The growth direction is [001]. Lighter areas diffract more

strongly.

74
Figure 39 High-resolution (620) 8 keV diffraction image in

Bragg geometry of approximately (220) surface of

terrestrial PbSnTe crystal 108. The angle of incidence is 36 arc minutes smaller than that for

figure 38; the growth direction is [001]. Lighter

areas diffract more strongly.

75
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76
x:

-p^ou

tp

(U

4-1 ^H

>iu • -p CO

OJ o g r-{ . QJ r-\ >i

Jji (0 rH

-P Cn

CP

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to

u 5^

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u •H D J

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'^

'^ ^-^ • "^ o 1—

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w

D 0) -H

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to

q

0]

g o

Q) -H -H

5-1

X -P 1 £ ^ Q) D^ D. S-i •H a•H

:i: fO TD

-H

77
Figure 42 Second high-resolution (444) 8 keV diffraction

image in Bragg geometry of approximately (220)

surface of terrestrial PbSnTe crystal 108. The

growth direction is [001]. Lighter areas diffract

more strongly.

78
Figure 43 Third high-resolution (444) 8 keV diffraction

image in Bragg geometry of approximately (220)

surface of terrestrial PbSnTe crystal 108. The

angle of incidence is 30 arc minutes smaller than

that for figure 42; the growth direction is [001]

Lighter areas diffract more strongly.

79
Subgrains within each of the main grains are clearly visible

through terraced variation in contrast. The generally strong diffraction from a 1.5 cm length of each of the two principal grains

observed is notable, however, in light of the increase in tin level

from 14% to 18% during the first 3 centimeters of growth visible in

these images [27]. The fractional change in lattice constant over

the 1 . 5 cm length of the grains is 4 X 10"^, which changes the Bragg

angle by 90 arc seconds. Nevertheless, diffraction is observed in

a single image of one of the grains through broadening by kinematic

scattering, which is difficult to quantify, as well as by local

compositional variation. Because of the mixture of these two

broadening mechanisms, unfortunately we can not use the broadening

to evaluate the degree of local compositional variation.

Nevertheless, other aspects of this variation are evident in

the enlargements in figures 44-55. Cellular regions of high

diffraction varying in size from 10 to several hundred micrometers

are observed. They are separated by lines of reduced diffraction

that are 10-50 /im wide.

Many of these lines at first glance appear to be scratches

because of their curvature and random orientation. However, three

characteristics typical of surface scratches, such as those visible

for example in the gallium arsenide images to which we turn later,

are not observed in these linear features. First, the lines vary

in width, both from line to line, and even over the length of a

given line. In reality these lines separate cellular regions of

high diffraction. Second, the boundaries of the lines are very

indistinct. And third, contrast reversal is never observed in

them. They are invariably out of diffraction over their entire

length and even as the crystal is rotated while it is observed by

video camera. Thus, while we cannot rule out scratches, their

images differ markedly in several respects from those of typical

scratches in other materials. Moreover, they are not observed in

the image of the space-grown sample, whose images follow.

We are thus left with the postulate that the highly diffracting cells are separated by material of other phase(s). The

indistinctness of the boundaries between these regions of differing

phase strongly suggest gradual change in chemical composition on a

scale of 1-10 /im or so, in contrast to the sharp delineation

between diffracting and non diffracting features in the images of

mercuric iodide discussed above. The intrusion of material of

different phase (s) is clearly important. The pseudobinary phase

diagram along the lead-tin axis predicts complete miscibility

[30]. However, the observation of similar structure following

electrolytic etching led twenty years ago to a series of experiments on the metal/tellurium ratio, which delineated its importance

in the growth of this material. This earlier work provides a

satisfactory model for the current observations as well [31] [30].

80
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Figure 52. Enlargement of upper right portion of figure 40,

(220) diffraction. Darker areas diffract more

strongly.

89
100|im

^

Figure 53 Enlargement of left central portion of figure 41,

(444) diffraction. Darker areas diffract more

strongly.

90
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91
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92
While the metal constituents are widely recognized to be interchangeable, a single phase is preserved only with tellurium

concentration in excess of 51%. Below this value, two phases are

formed, differing in metal/tellurium ratio. Since the tellurium

concentration of the current crystals is 50.1%, two phases are

actually to be expected. Constitutional supercooling may also play

an important role, depending on the the temperature gradients imposed [31] .

2. STS 61A Crystal

A full image of the space shuttle STS 61A crystal appears in

figure 56 and an enlargement of the central portion of this in

figure 57. The multigrain nature of the STS 61A crystal is superficially similar to that of the terrestrial crystal. But, while

these images appear qualitatively similar to the full images for

the corresponding terrestrial crystal, they differ in important

ways.

Each grain is more generally uniform than those of the

terrestrial crystal. This uniformity follows a drastic reduction

in the incidence of linear features and subgrains. As a result,

variation in diffraction on a scale of 10-100 micrometers is

greatly reduced. Thus, while the granular structure resembles that

for the terrestrial crystal, variation within individual grains

from the intrusion of a distinct second phase appears to be greatly

suppressed in microgravity . The absence of thermo-solutal

instability for this system in microgravity was noted in the

preceding section.

93
/

1

\ .J ; 1H|H|

1

\

i

If"

1

I jk'W^^^^^^^^^

iBHi

L Douoouo

Figure 56 High-resolution (220) 8 keV diffraction image in

Bragg geometry from approximately (220) surface of

PbSnTe crystal 111, grown on STS 61A. The growth

direction is [001]. Lighter areas diffract more

strongly.

94

i
U

fd QCO

-H

+Jo

fdu

4-1

4-1 -HO

LT)

^113O^ H

<W

<MOco

-H

-po

a, o M -p

fd

m

-P 0) c ^

Q) o o g

4-1 -p O O

fd -p U

Q) 4-1 e -H ^ W

(d fd

n-i Q) w fd

IT)

Q)up

en H

95
C. Triglycine Sulfate

A normal slice taken from the terrestrial seed crystal with

additional growth achieved on Spacelab III diffracts into images,

each of which appears over less than half of an arc minute. Full

high-resolution diffraction images appear in figures 58-63, with

enlargements in figures 64-71. The character of the diffraction

from this crystal is very different from that of the others. This

crystal was thin enough and low enough in atomic number to allow

diffraction in Laue geometry. Moreover, superimposed images of

this crystal taken as it was rotated about its [100] and [001] axes

appear in closely spaced groups, each associated with the diffraction directions expected for diffraction from one set of (hOO)

or (00£) planes, respectively. The various images have similar,

but not identical, shapes. Subsequent work, summarized in table 2,

indicates that the various members of a given group of images appearing at nearly similar diffraction angles come into diffraction

at differing sample orientation.

TABLE 2

Layer Figure No. (hkl) Orientation (°)

C2 58 300 +6.6

B3 59 200 +2.1

Bl 60 200 +2.5

A2 62 001 -19.2

A3 63 001 -36.8

The appearance of images in groups indicates that this crystal

consists of layered grains whose lattices are similar but rotated

with respect to one another by rotation about the [100] and [001]

axes. Since each image is ostensibly nearly "complete," the grain

boundaries are roughly parallel to the (010) crystal surface. In

an optically thick material, transmission through such a layered

crystal would be precluded by the misalignment of the successive

grains. However, this crystal is optically thin, permitting the

observation of symmetrical diffraction from each of the grains in

turn. From the occurrence of similar features in pairs of images,

which can be ascribed to features shared by adjacent grains at

their interface and the degree of clarity, we can assign a tentative order to the various grains as intersected by the x-ray beam.

This is the order in which their images are presented in the

figures and in table 2. Most of the features thus appear to be

96
Figure 58. High-resolution (300) 10 keV diffraction image in

Laue geometry from (001) surface of grain C2 of TGS

from Spacelab III. Lighter areas

strongly.

crystal 120,

diffract more

97
Figure 59. High-resolution (200) 10 kev diffraction image in

Laue geometry from (001) surface of grain B3 of TGS

crystal 120, from Spacelab III. Lighter areas

diffract more strongly.

98
Figure 60. High-resolution (200) 10 keV diffraction image in

Laue geometry from grain Bl of TGS crystal 120,

from Spacelab III. Lighter areas diffract more

strongly.

99
Figure 61 High-resolution (200) 10 keV diffraction image in

Laue geometry from grain Bl of TGS crystal 120,

from Spacelab III. Lighter areas diffract more

strongly.

100
Figure 62. High-resolution (001) 10 keV diffraction image in

Laue geometry from grain A2 of TGS crystal 120,

from Spacelab III. Lighter areas diffract more

strongly.

101
Figure 63. High-resolution (001) diffraction image in Laue

geometry from grain A2 of TGS crystal 120, from

Spacelab III. Lighter areas diffract more strongly.

102
Figure 64

Enlargement of middle

(300) diffraction,

strongly.

left-hand edge of figure 58,

Darker areas diffract more

103
Figure 65 Enlargement of central portion of figure 58, (300)

diffraction. Darker areas diffract more strongly.

104
Figure 66 Enlargement of lower left-hand edge of figure 59,

(200) diffraction. Darker areas diffract more

strongly.

105
1 \

1

Figure 67 Enlargement of lower right-hand edge of figure 59,

(200) diffraction. Darker areas diffract more

strongly.

106
Figure 68 Enlargement of middle left-hand edge of figure 60,

(200) diffraction. Darker areas diffract more

strongly.

107
<hk

1

100pm

Figure 69 Enlargement of lower left-hand edge of figure 61,

(200) diffraction. Darker areas diffract more

strongly.

108

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0)u e fd

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0)

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109
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110
associated with irregularities at the granular interfaces, although

radiographic effects from each layer are present.

The seed portion of this crystal takes up most of each image.

Space growth was in the [001] direction along that one edge of the

seed. The absence of a clear demarcation between the sseed and new

growth in this region in most of the images is in contrast to the

terrestrial growth of comparable material. The interface between

the seed and the new growth is visible only in figure 60; defects

in this grain do not appear to propagate into the new growth in the

central portion of the disc. In the other grains, defects from the

seed indeed appear to have propagated into the part grown in

microgravity . Toward the edge of the disc, the irregularity

observed in all of the grains is consistent both with rapid growth

anticipated from the higher concentration gradients there and with

the previous observation of defects before faceting becomes fully

developed later in growth.

The last image, fig. 63, differs in shape along the growth

edge from the others. It thus appears that new growth did not occur uniformly on all layers. On this one layer, growth appears to

have been much slower than on the others, although this may

represent initial meltback associated with the premature contact

with the solution.

D. Gallium Arsenide

The terrestrial crystal of selenium-doped Bridgman-grown

gallium arsenide diffracts over several degrees. A low resolution

diffraction image, achieved by rocking the crystal 4° around a [112]

axis during diffraction is shown in figure 72. An infrared image

of the same crystal is shown in figure 73. The similarity of these

two images is striking. The additional information in the full

high-resolution diffraction images of this crystal, figures 74-77,

is also striking by contrast. Enlargements of portions of these

images are shown in figures 78-82.

The demarcation of the Czochralski seed from the new Bridgman

growth is very clear in those images in which this region is in

diffraction. The seed/growth boundary is delineated in two ways.

First, toward the periphery of the boule it marks a smooth limit to

diffraction, past which the lattice does not diffract under the

same conditions. Thus, either the lattice constant, or orientation, or both differs in the new growth. Second, in the one region

of the seed interface supporting diffraction from both sides, the

mesoscopic structure of the growth is observed to be transformed at

the interface. The cellular structure of the seed is characteristic of diffraction images of Czochralski-grown undoped gallium

arsenide. In the new Bridgman growth, the formation of cells

appears to be completely suppressed. Freedom from other demarca111
Figure 72 Low resolution (220) 8 keV diffraction image of

approximately (220) surface of terrestrial GaAs

crystal 119, in Bragg geometry. The sample was

rocked through and angle of 4° during this exposure; the growth direction is [111]. Lighter areas

diffract more strongly.

112
Figure 73. Infrared image of same crystal

113
Figure 74 High-resolution (220) stationary diffraction image

from approximately (220) surface of terrestrial

GaAs crystal 119, in Bragg geometry. The growth

direction is [111]. Lighter areas diffract more

strongly.

114 i
I

Figure 75 High-resolution (220) 8 keV diffraction image of

approximately (220) surface of terrestrial GaAs

crystal 119, in Bragg geometry. The angle of

incidence is 3.9 arc minutes larger than in that

for figure 38; the growth direction is [111].

Lighter areas diffract more strongly.

115
Figure 7 6 High-resolution (242) 10 keV diffraction image of

approximately (220) surface of terrestrial GaAs

crystal 119, in Bragg geometry. The growth

direction is [111]. Lighter areas diffract more

strongly.

116
Figure 77. High-resolution (242) 10 keV diffraction image of

approximately (220) surface from GaAs crystal 199,

in Bragg geometry. The angle of incidence is 19

arc seconds smaller than in that in figure 40; the

growth direction is [111]. Lighter areas diffract

more strongly.

117
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122
tion, however, indicates that, to the extent permitted by the

lattice parameter match, the two lattices continue uninterrupted.

The lattice parameter mismatch appears to set up a gradual warping

of the crystal lattice. Further analysis of the features observed

is precluded by the inability to observe diffraction in Laue geometry.

123
VII. SUMMARY OF INITIAL OBSERVATIONS

These results are summarized in the following sections. The

three crystals of mercuric iodide, two of lead tin telluride, one

triglycine sulfate crystal, and one gallium arsenide crystal show

remarkable differences in their irregularities.

A. General Effects of Microgravity

Seven very different crystals do not provide a sample that is

sufficiently large to form definitive conclusions. Nevertheless,

our observations provide critical guidance for evaluation and

crystal growth. The formation of pervasive multiple phases

observed in two terrestrial crystals appears to have been greatly

suppressed on growth of two comparable crystals in microgravity.

The extent of space growth in a third crystal was too restricted to

permit reliable interpretation, and its analysis was further

complicated by the appearance of multiple grains in the seed.

Even more important, however, observation of the terrestrial

specimens has led to new insight on the nature, distribution, and

genesis of irregularity in each material. This insight does not

yet resolve all outstanding questions, but permits us to pose the

fundamental questions that we have identified in the preceding section. It provides guidance likely to lead to improved growth, both

in space and on the ground.

B. Mercuric Iodide

Observation of a terrestrial specimen grown about the time of

a Spacelab III crystal indicates that more than one phase is

present. The features containing the additional phase(s), which

appear to be globular in part and partly in the form of thin layers

oriented along the {100} crystallographic directions of the matrix,

dominating its mesoscopic structure, appear to have initiated a sharp lattice twist by 10 minutes of arc around an axis aligned

with the growth direction and to have stiffened the two resulting

subgrains. Formation of the additional phase (s) is suppressed both

in the comparable Spacelab III crystal and a recently grown high

purity terrestrial crystal. At the same time, the general

regularity of the lattice of these latter crystals is lower than in

the earlier terrestrial crystal. Superior performance of detectors

made from these materials thus appears to be limited far more by

124
I

sharp discontinuities associated with additional phase (s) than by

slow variation in the lattice.

New space growth experiments now assume even greater importance. Will use of new high purity material in space lead to still

further enhancement of device properties? If so, will this be

correlated finally with lattice uniformity on a scale of seconds of

arc over a range of millimeters, which to date has proved to be

less important than the formation of what now appear to be traps

provided by features consisting of additional phase (s)? We thus

find ourselves launched on a study of the fundamental questions,

"What determines charge carrier mobility and lifetime in mercuric

iodide?" and "How can they be improved through structural modification?"

C. Lead Tin Telluride

The mesoscopic structure of lead tin telluride appears also to

be influenced strongly by the intrusion of additional phase

material. But in this instance all available evidence points to

identification of the additional phase (s) with a different alloy of

the major constituents; the terrestrial sample displays subgrain

structure, consistent with identification with additional lead tin

telluride phase(s). Indeed, the presence of two phases has been

predicted for systems with tellurium concentration very close to

50%.

Although the STS 61A crystal has grain structure that appears

to be similar to that of the comparable terrestrial crystal, the

formation of the subgrain variation characteristic of the terrestrial sample is suppressed in microgravity . This subgrain

structure, associated with formation of a second lead tin telluride

phase in the terrestrial crystal, thus appears to have been

suppressed by reduction in gravity. This suppression is correlated

with the predicted thermo-solutal stability in microgravity.

D. Triglycine Sulfate

Interpretation of the space-growth of triglycine sulfate is

complicated by the inadvertent contact between the seed and the

solution prior to space growth and by the layered structure that we

observe in the seed. Defects in one of these layers appear not to

have propagated in the central portion of the disc in microgravity,

while defects in the other seed layers appear indeed to have

propagated into the new growth. In addition, one of the seed

layers appears to have grown at a rate slower than the others.

125
In the IML 1 mission scheduled, use of multifaceted natural

seeds is planned. They will be characterized not only for various

physical properties but also for defects and structural properties.

E. Gallixim Arsenide

Although gallium arsenide has not yet been grown in space, the

mesoscopic structure of a Bridgman-grown boule has been observed to

differ from that of the Czochralski-grown seed. The Bridgman-grown

lattice appears to be warped more than that of the Czochralskigrown seed. Neither observation is yet understood, and their

resolution will require a sample thin enough to sustain anomalous

transmission.

126
VIII. CURRENT LIMITATIONS

AND IMPROVEMENT IN DIFFRACTION IMAGING TECHNIQUES

A. Advanced X-Ray Image Detectors

The foregoing sections have described how the techniques of

synchrotron radiation diffraction imaging can be used to characterize the phenomena occurring in crystal growth. Most of the

techniques applied to the four materials studied have been adapted

from concepts developed in the classical laboratory experiments of

x-ray topography performed between 1930 and 1960. The capability

of laboratory techniques is rather limited. The relatively recent

availability of synchrotron x-radiation has provided a new

opportunity to improve the quality of the incident beam through

improved x-ray optics which, in turn, has provided the wealth of

microscopic detail now seen in the images as well as the realization of high resolution transmission video imaging in real time

[32].

Much of the effort in establishing these now-routine techniques has concentrated on improving the parallelism and enlarging

the size of the monochromatic incident beam to the point that the

NIST x-ray imaging beamline at NSLS is the best facility for

diffraction imaging in the world. However, as a direct result of

our interaction and collaboration with NASA crystal growers certain

areas of the imaging capability require improvement if the overall

goals of the collaboration are to be met.

A concern expressed in the past by a number of crystal growers

has been the turnaround time for acquiring the basic image data;

i.e., the time that elapses before the images are ready for

analysis. This can now be shortened by orders of magnitude.

Serious effort has been devoted to this opportunity; the development that corrects this deficiency provides not only for hard copy

images with the same resolution as images enlarged with ordinary

magnification (lOX) from film, but also speeds the acquisition of

the image by up to an order of magnitude.

A natural choice of this development is to use image detectors

currently available. There are three major types of such image

detectors [33] [34]. One is the combination of an image

intensifier stage and either a charge-injection-device (CID) or

charge-coupled-device (CCD) solid state image detector [35].

This type of detector is called an indirect image detector, because

a phosphor screen must be placed in front of the detector to

convert x-ray photons to visible photons. The second type is a

127
vidicon detector that receives x-ray photons directly [36] [37]

[38] [39]. This is called a direct image detector. The third is a CCD detector, which should belong to the class of direct imaging

detectors, although the majority of the applications to hard x-ray

radiation favors the use of phosphors in front of the detector.

For the spatial resolution necessary to observe structural

details even at the minimum level, the insertion of phosphors for

the conversion of x-ray photons to visible photons is not desirable. Therefore, indirect image detectors cannot be used. Although

vidicon detectors are used currently to follow image-changes as the

incident or glancing angle of an x-ray beam is changed [40],

they have never been tested in practice as a means of viewing

isolated dislocations. We have used a high-quality GaAs crystal to

test the practical applicability of a vidicon detector as a replacement for wet film technology. The vidicon detector produces

images of reasonable resolution, but is poor in sensitivity at low

photon flux levels. In other words, images from most crystals

cannot be viewed using the vidicon detector with reasonable

resolution, except for the images obtained in the reflection

geometry. The images in reflection geometry do not provide much

information on the microstructures in materials. We emphasize that

the vidicon detector has been used throughout this work to locate

diffraction images, as described in previous sections.

The next choice is obviously to adopt the CCD detector.

Direct conversion has been considered by many to be sedulously

avoided for fear of destroying the CCD array because it is subject

to radiation damage. Prior developmental work, however, has shown

that serious degradation does not ensue if the intensity of x-rays

striking the CCD array is kept below 10"* to 10^ photons/mm^/ sec.

We have applied a CCD area detector to image the GaAs test

crystal in transmission [41]. Figure 83 contains two forward

(0-beam) transmission diffraction images of the high-quality GaAs

crystal, using 220 diffraction planes and 13.5 keV radiation.

Figure 83a is a 100 second exposure of the CCD detector, while

Figure 83b is a 150 second exposure of a nuclear plate (25 /xm-thick

emulsion) . The same dislocations are seen in both images, but the

CCD image indicates a better dynamic range than the image on the

plate. The image size is 1 cm X 0.85 cm, and the pixel size is 20

lim X 2 jum.

This improvement not only eliminates film development but

provides digital data without densitometry. It has single photon

sensitivity, linear photometric response. The high sensitivity

greatly reduces the exposure time compared to x-ray film and the

linear photometric response gives higher contrast images than film

which has a logarithmic response. The digital image data may be

processed and enhanced by computer at the beamline. The unique

128
Figure 83. Transmission topographs of GaAs (a) with CCD

detector and (b) a nuclear plate.

129
processed and enhanced by computer at the beamline. The unique

feature of this CCD detector which accounts for its much higher

resolution compared with other CCD applications in the x-ray field

is the use of direct conversion of x-ray photons to charges

accumulated in each pixel [41]

.

B. Higher Sensitivity Requirements

Our experience through collaboration with crystal growers has

shown that there are three classes of sample crystals:

1. Samples which, because of growth conditions, preparation,

handling, etc., do not allow us to pursue a detailed analysis

of the microstructure in the raw data image. Transmission is

usually impossible for this class. The images, however, may

be helpful to the growers in their science.

2. Samples which are of a quality that requires many different

(hkl) images, especially in transmission, revealing the microstructure in sufficient detail for detailed analysis.

3. Samples which are so perfect as to "test" the resolution

limits of our instrumentation. This class of crystal has

great value for the pursuits of x-ray physics but little

additional analysis is usually required for the crystal

grower.

Rarely do we get a class 3 crystal; most of the crystals

provided have been class 1, as described in earlier sections. For

an understanding of crystal growth mechanisms, samples of class 2

are desirable so that the relationships between crystal growth

parameters and the resultant crystal imperfections can be studied

in detail within the body of crystal boules at different local

regions. As an example, a GaAs crystal which was grown in a laboratory is shown in Figure 84. The sample was sliced vertically

along the growth direction of a boule grown with varying growth

conditions in equal intervals. The changes in growth parameters

are hardly discernible in an overall stationary image of diffraction, even in transmission. This crystal is of very high quality

as evidenced by anomalous transmission images (the O and H-beam

images) through a thickness of almost 1 mm. The major reason why

such transient regions are not discernible is the lack of local

strain sensitivity in obtaining those images. Had a diffraction

image been obtained with more precise x-ray tuning in the specific

region, the region corresponding to the change in growth conditions

should have revealed a complex microstructure associated with such

a change. This need for higher local sensitivity, higher spatial

resolution and more precise sample manipulation, while viewing the

image, certainly implies the inadequacy of the current routine way

of recording images on film with the aid of a vidicon detector. We

130
a)

u

r-\ -p

(U o g

sx >1

(T3w

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-H

-p . W

<u iH

v^ fd H >

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x; -P

-p

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;=! Q) tr

XX 0)

4-1 c

tP-H C w . <-\ c >

fO Q) -H X

13 4-> 0) -H

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1—

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cy] fd

r-H Jd QJ fd -P tp

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>i ^ •H

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g

CP fd

w c QJ <c -H ja

fO >1 1 o s

U-l > c

x: •H

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w

a, -H

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rd 5 •H

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E-i tP-P

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0)

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131
need a new technique to "tune in" on local phenomena that are

normally missed using the routine diffraction imaging techniques.

The application of the direct imaging CCD combined with x-ray image

magnification, which constitute the Hard X-Ray Microscope

[6] [42], has allowed us to make substantial progress in imaging

these subtle details. This microscope provides a powerful tool for

imaging not only in microradiography, but also in diffraction

imaging, as described below. Figure 85 shows a microradiograph of

a rather special specimen crystal consisting of a 0.38 mm-thick Si

single crystal wafer upon which has been deposited patterns of

lines and contact pads of Pd with a thickness of 0.15 /im. The

image was obtained at a magnification of 79 with 12.25 keV x-rays

[41] [42 ] [43 ] . At this magnification one CCD pixel represents

0.25 jLtm. The eleven evenly-spaced lines are 1 jum wide and 50 /im

long, with a center-to-center spacing of 5 jum. The single line

above the eleven lines is 0.6 fim wide. A sub-micrometer feature is

clearly seen with this microscope. A blank image, taken before and

after without the sample, is subtracted from the raw image data to

produce Figure 85. This is another advantage of the CCD detector

over film; the digital data may be further processed and analyzed

quantitatively.

The substrate of the sample in Figure 85 is a single crystal

and the Pd pattern is epitaxially coherent; diffraction images can

be obtained and analyzed, as seen in previous sections. In

transmission, two diffracted images appear: one in the forward

direction of the incident beam, called the 0-beam image, and the

other in the Bragg diffracted direction, called the H-beam image.

Without moving the microscope from the position set for microradiography, the 0-beam image can be viewed on the monitor screen

merely by rotating the sample. Figure 8 6 shows a microradiographic

image of this sample to be compared with Figure 87 which is the Obeam image taken in transmission for the 220 diffraction with the

microscope set for the same magnification, 22 X at 8.3 keV

[41] [42 ] [43 ] . In dynamical diffraction, a strain field acts as a scattering center and produces a black and white image [6] . The

black/white contrast is expected to be inverted in the H-beam

image. This contrast inversion has been confirmed by moving the

microscope to view the H-beam image. Therefore, there are

interfacial strains around the features. The direction of the

atomic shifts in these strained areas can be analyzed in detail in

a quantitative way, since the mechanism of the x-ray microscope

automatically allows it to act as a precise scattering angle

analyzer.

132
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135
C. Phase Contrast Microscopy -- Interface Problems

For many of the materials of interest the ultimate goal of

crystal growth in microgravity as well as on earth is to provide

substrate materials required for advanced microelectronic devices.

This relationship and interaction of the substrate materials with

device construction, such as epilayers, quantum wells and dopants,

etc., is of great interest because device performance depends on

these interactions. Crystals grown for general purposes and for

the production of seed crystals, for example, have been observed to

consists of layers which mimic the growth of epilayers. Device

features used in microelectronics and photonics are prepared by a sequence of deposition of a few atoms at a time and doping in

appropriate time intervals. These features are almost perfectly

lattice-matched to each other and to the substrate. It is

difficult to create images with good contrast from mutually

coherent features consisting of similar materials.

Revealing these subtle features requires not only a higher

degree of sensitivity but also delicate control and manipulation of

the sample. To demonstrate such a need for crystal growth and

fabrication science and engineering we show some results of imaging

such device structures in a In(GaAs)P laser diode [41] [42], in

which many layers are produced by OMVPE (organometallic vapor phase

epitaxy) and subsequent doping of different atoms, and are required

to be highly lattice coherent (almost nonexistent mismatch in

lattices) among themselves and with their substrates. It is

extremely difficult to obtain images of the layers with good

contrast in such a material using microradiography. In particular,

most advanced microelectronic devices utilize materials composed of

elements of high atomic numbers and do not permit x-rays to

transmit through the unthinned sample. However, these materials

are often highly perfect single crystals and, hence, x-rays can

pass through more than one-half millimeter thick samples, almost

regardless of x-ray energy, when the sample is set for Bragg

diffraction. This penetration is possible due to one of those

dynamical effects called anomalous transmission or the Borrmann

effect [6] [44] [45]. Another feature of dynamical effects

guarantees that those transmitted images represent true transverse

cross sectional images of the layers [6].

Aided by micro-manipulation of angles and positions of the

sample and a zooming capability, the x-ray microscope can view the

diffraction image of_ particular layers or layer sections [41] [42].

Figure 88 shows a 220 diffraction image in transmission in the

Bragg diffracted direction (H-beam image) with a magnification of

136
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137
100 X at 12.3 keV, precisely tuned to local features of two 1 iim

thick layers near the surface of this device. Also visible are 2-3

lim. thick layers located 2 " 50 fira below the surface (within the

substrate crystal) . These thicker layer images are broadened due

to the interfacial strain observable under this diffraction

condition, where the diffracting vector is parallel to the

interfaces. These thicker images are "growth striations", wellknown to crystal growers, and are produced when the substrate

material was grown. Being magnified by the x-ray microscope, the

striations appear in the shape of bands.

Two thin layer images are created by either local strains or

lattice parameter changes, representing the 0.5-1 lira InP (or

AlGaAs) layers doped differently on the InP substrate. These doped

layers are part of the device mechanism. An enlarged portion of

Figure 88 is reproduced in Figure 89 where A's show the discontinuous first layer nearest to the surface and B's are the segments of

the second layer. The discontinuous and segmented images of these

layers are indicative of the local variations of the precise Bragg

angle for these layers. When the sample is rotated by less than 1

arc sec, new segments of the layers replace the existing images.

This type of operation requires precise micromanipulation while

magnified images are under view.

To summarize, recent experiments and accomplishments as part

of this effort have advanced the x-ray imaging capability for

crystal growth science and fabrication engineering. These improved

measurement capabilities and techniques are ready to meet the goals

and challenges of NASA's community of crystal growers when these

techniques are accepted as readily available instruments and are

accommodated into the routine practice of diffraction imaging.

These advancements include increased spatial resolution in the

real-time mode of operation, greatly improved turnaround time with

on-line diagnostic and analysis capability, improved control and

manipulation of the specimen crystal and the increased sensitivity,

spatial resolution and data analysis capability of direct CCD

imaging with image magnification, inherent in the hard x-ray

microscope. The power of the x-ray microscope, using it as a phase

contrast microscope, continues to be explored. One extremely

promising application in diffraction imaging as well as microradiography is its use in producing the basic data for microtomographic

reconstructions. We have shown its capability of producing twodimensional images with 1 /Lxm or less spatial resolution in

microradiography and diffraction imaging [41] [42 ] [43 ] . A set of

two-dimensional images is easily collected as a function of

rotational angle. A tomographic image can be reconstructed from

this set of high resolution two-dimensional images. To reconstruct

a three-dimensional image with 1 jum spatial resolution, the

increment of the sample rotational angle is, for example, roughly

0.5° for a cylindrical sample 100 jum in radius. Obviously, the

138

1
Figure 89

i

An enlarged portion of the image shown in Figure

74 where the A's show the discontinuous first

layer and the B's are segments of the second

layer.

139
larger the sample, the smaller the required angular increment. For

the 3D tomographic image construction in diffraction imaging, care

must be taken to use the direction of scattering vector, H, as a

rotational axis. The micro-rotator currently in use can easily

provide this capability with a well fixed rotational center

controlled by a set of submicrometer translators. Algorithms are

now available for tomographic reconstruction. A working microtomographic system capable of reaching one micrometer or less spatial

resolution has become a reality with the hard radiation microscope

described here.

140
IX. FUTURE ACTIVITY

A. Interest in Collaboration

The interest in high-resolution diffraction imaging among

these NASA investigators is remarkable; the broad participation

that they envision in the imaging experiments themselves is unprecedented. Direct participation of crystal growers in work on the

beam line as well as in the analysis promises to increase the

insight that is obtained over that possible with the ex post facto

analysis, which has been characteristic of most previous work in

high-resolution diffraction imaging.

B. The Prospects for Proteins

There appear to be strong grounds for the skepticism of many

protein crystal growers with respect to the prospects for utility

to them of high-resolution diffraction images. However, with

proper attention at the outset, some, if not all, of the sources of

concern identified may be circumvented.

Among the sources of concern most difficult to address are: 1)

the mixture of kinematic and dynamical diffraction that we expect,

and 2) the high level of irregularities that we also expect because

it is characteristic of crystals of many types. Kinematic

diffraction is likely to coexist with dynamical diffraction because

of the optical thinness expected both from the crystal size and

because of the small atomic number of the atoms in these crystals.

It is not clear without trial whether or not the interpretational

challenges presented by the irregularity and by the optical thinness can be met effectively.

Among the difficulties that may prove to be highly challenging, we anticipate the following: 1) the relevance of diffraction

imaging to the desired order in these crystals, 2) changes in

crystal regularity immediately following return to earth, 3) the

presence of a mother liquor, 4) destruction by the x-ray beam, and

5) the relatively low scattering power of the atoms in these

materials

Because none of these difficulties is frivolous, and at least

two of them may well prove to be insurmountable, we propose that

special attention be directed to the identification of a material

141
and a particular sample that is likely to give us the best

opportunity to circumvent the difficulties.

C. Crystal Preparation

We have noted the importance of four sample characteristics

for fully successful analysis: thickness, surface orientation, surface condition, and density of irregularities. The analysis

undertaken this first year was severely limited by the necessity to

work with immediately available samples, which we recognized at the

outset would not fulfill all of these conditions. It will clearly

be important to continued success in future analyses to place

special effort on the preparation and selection of suitable

crystals.

Anomalous transmission through optically thick crystals is a

function of their regularity. All six optically thick crystals

that we examined as part of the effort this year were sufficiently

thick that warping of the lattice during sample preparation was not

likely to be a problem. This indeed proved to be an extremely

important factor in the analysis that was carried out. Nevertheless, six of the seven crystals were too thick for observation of

diffraction in Laue geometry, and this precluded a complete analysis. On the basis of this year's work, we anticipate that the

information that would be provided by optimum analysis of comparable thin crystals would be worth the effort necessary to achieve

satisfactory thinning.

The challenge here is not trivial, however. One must be

careful not to distort the lattice in the thinning and subsequent

mounting for imaging, particularly in the thinning of soft materials such as mercuric iodide, [22] [26]. Because of the interesting

irregularities in these materials, it seems likely that sample

thickness less than a half millimeter, perhaps substantially less,

is desirable; and warping and support then also become serious

concerns

.

Orientation close to a low-index lattice plane is also

important to success in observing an optimum range of diffraction.

Suitable orientation is not particularly challenging, but does

require attention in the early stages of crystal preparation.

Observation of several of the samples this year was limited by

orientation that was far from ideal.

Feature-free surface treatment, both important and challenging, is not always achieved. The surfaces of all of the samples

observed this year, however, were exemplary. None interfered in

any way with the imaging analysis. This is strong testimony both

to the interest and to the diligence of our collaborators.

142
Finally, success in the interpretation of diffraction images

depends on the ability to distinguish the various irregularities,

which places a limit on their density. This is a challenge

particularly at the time of sample selection. Preliminary

selection of the most promising materials through auxiliary

structural information can dramatically increase efficiency in the

exploitation of diffraction imaging.

D. Correlation with Auxiliary Data

Since high-resolution diffraction imaging analysis involves

considerable effort, optimization of the knowledge gained through

utilization of auxiliary data is important. This not only

minimizes the special effort required for imaging, but increases

the effective use of the other data and can lead ultimately to the

deepest insight into the control of irregularity, which is a principal goal here. High-resolution diffraction imaging is not a suitable production line quality control tool. But the insight to

which it contributes, in conjunction with other more readily

obtainable data, can lead to the production of crystals with superior structure.

E. Crystal Selection Criteria

In summary, insight into the nature and genesis of irregularity through high-resolution diffraction imaging can be optimized

through crystal selection along several lines: the nature of the

crystal; collaboration with the crystal grower; and availability of

auxiliary data. Wisdom is most likely to follow when attention is

devoted to all of these areas.

In order to contribute to useful analysis, irregularities must

be observed to vary under a range of conditions. This requires

that crystals to be imaged are thin, suitably oriented, effectively

polished, and free enough of irregularities that those remaining

are isolated enough for analysis.

Since a high-resolution diffraction image is never unique and

frequently not predictable, but typically varies strongly as the

Bragg peak is traversed, imaging can be carried out so that the

relative visibility of selected features is optimized. Participation of the crystal grower, first in the imaging and then in

the interpretation, can contribute to the efficiency with which

understanding achieved. Thus active collaboration with the crystal

grower can play a central role in the success of the diffraction

imaging.

143
Finally, success in diffraction imaging analysis can also be

measured by the extent to which it ties together observations made

by a variety of other characterization approaches. The presence of

other data thus can contribute dimensions to the understanding

achieved that can arise in no other way. Such data can serve to

complement, to confirm, or to be confirmed by, the predictions

based on diffraction imaging.

144
X. ACKNOWLEDGMENT

The extraordinarily interesting results of the initial year of

collaboration described in this report would have not have been

possible without the multifaceted support of the NASA Microgravity

Sciences and Applications Division. Thanks to this strong NASA

support, great strides have been taken in the sophistication with

which several important crystal systems can be grown. Even more

dramatic advances can now be anticipated with confidence.

I

145

XI . REFERENCES

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[2.] L. van den Berg and W. F. Schnepple, "Mercuric Iodide Crystal Growth in Space," Nucl. Inst. Meth. Phys. Res. A283,

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[3.] Lawrence J. DeLucas, Craig D. Smith, H. Wilson Smith,

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[4.] W. Littke and C. John, Science 225, 203 (1984)

[5.] Bruce Steiner, Masao Kuriyama, and Ronald C. Dobbyn, "Insight into the Genesis of Irregularity during Crystal Growth

Achieved through High Sensitivity Monochromatic Synchrotron

X-Radiation Diffraction Imaging (Topography)," Prog. Crystal

Growth and Char., in press (1990)

[6.] M. Kuriyama, B. W. Steiner, and R. C. Dobbyn, "Dynamical

Diffraction Imaging (Topography) with X-Ray Synchrotron

Radiation," Ann. Rev. Mat. Sci. 19, 183-207 (1989)

[7.] Bruce Steiner, Masao Kuriyama, Ronald C. Dobbyn, and Uri

Laor, "Diffraction Imaging (Topography) with Monochromatic

Synchrotron Radiation," J. Res. NBS 93, 577-601 (1988)

[8.] Robert A. Schmitz , private communication (1990)

[9.] Bruce Steiner, Uri Laor, Masao Kuriyama, Gabrielle G. Long,

and Ronald C. Dobbyn, "Diffraction Imaging of High Quality

Bismuth Silicon Oxide with Monochromatic Synchrotron Radiation: Implication for Crystal Growth," J. Cryst. Growth 87,

79 (1988)

i

147
[10.] Masao Kuriyama, Bruce Steiner, Ronald C. Dobbyn, Uri

Laor, David Larson, and Margaret Brown, "Streaking

Images That Appear Only in the Plane of Diffraction in

Undoped GaAs Single Crystals: Diffraction Imaging

(Topography) by Monochromatic Synchrotron Radiation,"

Phys. Rev. B 38, 12421 (1988)

[11.] B. Steiner, M. Kuriyama, R. C. Dobbyn, U. Laor, D.

Larson, and M. Brown, "Structural Anomalies in Undoped

Gallium Arsenide Observed in High Resolution Diffraction Imaging with Monochromatic Synchrotron Radiation,"

J. Appl. Phys. 66, 559 (1989)

[12.] M. Schieber, C. Ortale, L. van den Berg, W. Schnepple,

L. Keller, C.N. J. Wagner, W. Yelon, F. Ross, G.

Georgeson, and F. Milstein, "Correlation between Mercuric Iodide Detector Performance and Crystalline Perfection," Nucl. Inst. Meth. Phys. Res. A283, 171-187

(1989)

[13.] Y. F. Nicolau and M. Dupuy, "Study of alpha Hglj Crystals Grown in Space by Differential Scanning Calorimetry. Scanning Cathodoluminescence Microscopy and Optical Microscopy," Nucl. Inst. Meth. Phys. Res. A283,

355-362 (1989)

[14.] Daniel Carter, private communication.

[15.] Robert Sweet, private communication.

[16.] W. J. Boettinger, P. W. Voorhees, R. C. Dobbyn, and H.

E. Burdette, "A study of the Coarsening of Liquid-Solid

Mixtures Using Synchrotron Radiation Microradiography,"

Met. Trans. A 18A, 481-490 (1987)

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[17.] G. R. Ricker, J. V. Vallerga, and D. R. Wood, "A Mercuric Iodide Detector System for X-Ray Astronomy," Nucl.

Inst. Methods 213, 133-144 (1983)

[18.] R. C. Whited and M. M. Schieber, "Cadmium Telluride and

Mercuric Iodide Gamma Radiation Detectors," Nucl. Inst.

Meth. 162, 113-123 (1979)

[19.] R. C. Whited and L. van den Berg, "Native Defect Compensation in Hgl2 Crystals," IEEE Trans. Nucl. Sci. 24

165-167 (1977)

[20.] M. Schieber, M. Roth, and W. F. Schnepple, "Crystal

Growth and Applications of Mercuric Iodide," J. Cryst.

Growth 65, 353-364 (1983)

148
[21.] I. F. Nicolau and G. Rolland, "Deviation from Stoichiometry in alpha-Hglj," Mat. Res. Bull. 16, 759-770

(1981)

[22.] S. Gits and A. Authier, "Plastic Defects in Alpha-Hgl2

Single Crystals," J. Cryst. Growth 58, 473-485 (1982)

[23.] T. E. Felter, R. H. Stulen, W. F. Schnepple, C. Ortale,

and L. van den Berg, "Rutherford Backscattering and

Auger Spectroscopy of Mercuric Iodide Detectors," J.

Nucl. Materials (1988)

[24.] D. Wong, X. J. Bao, T. E. Schlesinger, R. B. James, A.

Cheng, C. Ortale, and L. van den Berg, "Photoluminescence Variations Associated with the Deposition of

Palladium Electrical Contacts on Detector-Grade Mercuric Iodide," Appl. Phys. Lett. 53, 1536 (1988)

[25.] W. B. Yelon, R. W. Alkire, M. M. Schieber, and L. van

den Berg, "Crystal Perfection of Hgl2," J. Appl. Phys.

52, 4604 (1981)

[26.] S. Gits, "Characterization of Extended Defects in

alpha-Hgl2 Single Crystals," Nucl. Inst. Methods 213,

43-50 (1983)

[27.] Roger K. Crouch, Archibald L. Fripp, William J. Debnam,

Fredrick M. Carlson, and Richard T. Simchick, "Results

from a Compound Semiconductor Crystal Growth Experiment

in a Low Gravity Environment," SAMPE ELectronics Materials and Processes Conf., Santa Clara, June 22-25,

1987

[28.] V. F. Miuskov, V. P. Konstantinova, and A. I. Gusev,

"X-Ray Diffraction Investigation of Defects in Triglycine Sulfate Crystals," Kristallograf iya 13, 909-912

(1968); Soviet Phys. - Cryst. 13, 791-794 (1969)

[29.] A. Izrael, J. F. Petroff, A. Authier, and Z. Malek, "XRay Topographic Study of Growth Defects in Triglycine

Sulphate Crystals in Relation to their Growth Conditions," J. Cryst. Growth 16, 131-141 (1972)

[30.] T. C. Harman, "Control of Imperfections in Crystals of

Pbi_^Sn^Te, Pbj.^Sn^Se, and PbSi.^Se/' , J. Nonmetals 1, 183-

194 (1973) .

[31.] J. F. Butler and T. C. Harman, " Metallic Inclusions

and Cellular Substructure in Pbj.^Sn^Te Single Crystals,"

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149
[32.

[33.

[34.

[35.

[36.

[37.

[38.

[39.

[40.

[41.

[42.

[43.

[44.

[45.

M. Kuriyama, W.J. Boettinger and G.G. Cohen, Ann.

Rev. Mater. Sci 12 23 (1982)

R.E. Green Jr: Advances in X-Ray Analysis 14. 311

(1971) & 20 211 (1977)

A. Rose and P.K. Weimer: Physics Today, September p.

24 (1989)

W.J. Boettinger, H.E. Burdette, M. Kuriyama and R.E.

Green, Jr. Rev. Sci. Instrum. 47 906 (1976)

J. Chikawa and I. Fujimoto: Appl. Phys. Lett. 13 387

(1968)

J. Chikawa: "Real Time Radiologic Imaging: Medical

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Quaisser: Appl. Phys. Lett. 27 308 (1975)

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and Industrial Applications" p. 201 (1980) ASTM Special

Tech. Pub. 716, ASTM, Philadelphia

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Dobbyn: J. Cryst. Growth 87 79 (1988)

M. Kuriyama, Nuclear Instrum. & Method (1990)

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150
I

APPENDIX: NASA Principal Investigations

Engaged in Crystal Growth

Prof. G. J. Abbaschian

University of Florida, Gainesville

Prof. Elmer Anderson

University of Alabama, Huntsville

Prof. J. Barry Andrews

University of Alabama, Birmingham

Prof. Klaus J. Bachmann

North Carolina State University

Dr. Martin E. Barmatz

Jet Propulsion Laboratory

Prof. Robert J. Bayuzick

Vanderbilt University

Prof. Charles E. Bugg

University of Alabama, Birmingham

Prof. Frederick M. Carlson

Clarkson University

Dr. Daniel C. Carter

Marshall Space Flight Center

Dr. Ared Cezairliyan

National Institute of Standards and Technology

Dr. Larry DeLucas

The University of Alabama, Birmingham

Prof. Robert S. Feigelson

Stanford University

Prof. Merton C. Flemings

Massachusetts Institute of Technology

Dr. Archibald L. Fripp

Langley Research Center

Prof. Randall M. German

Rensselaer Polytechnic Institute

Mr. Thomas Glasgow

NASA Lewis Research Center

151
Prof. Martin E. Glicksman

Rensselaer Polytechnic Institute

Prof. Herman H. Hobbs

George Washington University

Prof. William Hofmeister

Vanderbilt University

Prof. William L. Johnson

California Institute of Technology

Prof. Carl C. Koch

North Carolina State University

Prof. Sindo Kou

University of Wisconsin

Prof. Ravindra B. Lai

Alabama A&M University

Dr. David J. Larson, Jr.

Grumman Aerospace Corporation

Dr. Sandor L. Lehoczky

Marshall Space Flight Center

Prof. Ponzy Lu

University of Pennsylvania

Prof. John L. Margrave

Rice University

Dr. David Matthiesen

GTE Laboratories

Dr. Mary Helen McCay

University of Tennessee Space Institute

Prof. Alexander McPherson, Jr.

University of California, Riverside

Prof. George F. Neilson

University of Arizona

Dr. R. R. Neurgaonkar

Rockwell International Science Center

Prof. John H. Perepezko

University of Wisconsin

152
Dr. Marc Lee Pusey

Marshall Space Flight Center

Prof. Chandra S. Ray

University of Missouri, Rolla

Dr. Franz Rosenberger

University of Alabama, Huntsville

Dr. Robert Schiffman

Intersonics

Dr. N. B. Singh

Westinghouse Research and Development Center

Prof. Julian Szekely

Massachusetts Institute of Technology

Dr. Frank R. Szofran

Marshall Space Flight Center

Dr. Eugene H. Trinh

Jet Propulsion Laboratory

Dr. Lodewijk van den Berg

EG&G Energy Measurements, Inc.

Dr. Marcus Vlasse

Marshall Space Flight Center

Prof. Donald Voet

University of Pennsylvania

Dr. Peter Voorhees

Northwestern University

Prof. Michael C. Weinberg

University of Arizona

Prof. Heribert Wiedemeier

Rensselaer Polytechnic Institute

Prof. William R. Wilcox

Clarkson University

Prof. August Witt

Massachusetts Institute of Technology

153

NIST-114A

(REV. 3-90) U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBUCATION OR REPORT NUMBER

NIST/TN-1287

PERFORMING ORGANIZATION REPORT NUMBER

3. PUBUCATION DATE August 19^1

4. TITLE AND SUBTITLE High Resolution Diffraction Imaging of Crystals Grown in

Microgravity and Closely Related Terrestrial Crystals

5. AUTHOR(S)

B. Steiner, R. Dobbyn, D. Black, H. Burdette, M. Kuriyama, R. Spal , L. van den Berg,

A. Fripp. R. Simchick, R. Lai, A. Batra, D. Matthiesen, B. Ditchek

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURQ, MD 20699

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP) National Aeronautics and Space Administration

Microgravity Science and Applications Division

Washington, DC 20546

10. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOGRAPHY OR

UTERATURE SURVEY, MENTION IT HERE.)

This paper reports on irregularities found in three crystals

grown in space, in four crystals grown entirely on the ground, and

compares the two sets. Irregularities have been observed in

mercuric iodide, lead tin telluride, triclycine sulfate, and

gallium arsenide by high resolution synchrotron x-radiation

diffraction imaging.

Radiation detectors made from mercuric iodide crystals grown

in microgravity have been reported to perform far better than

conventional detectors grown from the same material under full

gravity. Effort is now underway to reproduce these "space" crystals, optimize their properties, and extend comparable superiority

to other types of material.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

crystal growth; gallium arsenide; lead tin telluride; mercuric iodide; microgravity;

monochromatic synchrotron radiation diffraction imaging; TGS

13. AVAILABIUTY UNUMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). X

JL

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE, WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

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162

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