i

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1
NBS SPECIAL PUBLICATION 400-

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Semiconductor uremen

Improved Infrared Response

Technique for Detecting Defects and

Impurities in Germanium and

Silicon P-/-A7 Diodes

too

. us?

no. 400-13
NATIONAL BUREAU OF STANDARDS

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Burosu cf Standards 7 1975

0

Semiconductor Measurement Technology:

0O-)3

Improved Infrared Response Technique

tor Detecting Defects and Impurities

in Germanium and Silicon p-i-n Diodes

A. H. Sher

[AS.

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Sponsored by: U.S. Atomic Energy Commission

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secrefory

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Direcfor

Issued February 1975
Library of Congress Cataloging in Publication Data

Sher, Alvin H

Improved Infrared Response Technique for Detecting Defects and Impurities in Germanium and Silicon p-i-n Diodes.

(Semiconductor Measurement Technology) (National Bureau of Standard Special Publication; 400-13)

Supt. of Docs. No.: €13.10:400-13

1. Germanium Diodes—Defects. 2. Silicon Diodes—Defects. 3. Infra-red technology. I. United States. Atomic Energy Commission.

II. Title. III. Series. IV. Series: United States. National Bureau

of Standards. Special Publication; 400-13. QC100.U57 No. 400-13 [TK7871.86] 389'.08s 621.3815'22 75-1210

National Bureau of Standards Special Publication 400-13

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-13, 26 pages (Feb. 1975)

rODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.10:400-13). Price 75 cents
Semiconduator Measurement Technology

:

Improved Infrared Response Technique for Detecting Defects

and Impurities in Germanium and Silicon p-i-n Diodes

A. H. Sher

Table of Contents

Page

Preface iv

1. Introduction

.

2

2. Initial Measurements 2

3. Mechanism for Diode Impurity Photovoltage (IRR) 3

4. IRR Measurements on Germanium 4

4.1 IRR Spectrum Types 5

5. IRR Measurements on Silicon 8

6. Identification of Energy Levels 9

6.1 Energy Levels in Germanium 10

6.2 Energy Levels in Silicon 11

7. Conclusions 11

8. References 12

iii
PREFACE

The Semiconductor Technology Program serves to focus NBS efforts to

enhance the performance, interchangeability , and reliability of discrete

semiconductor devices and integrated circuits through improvements in

measurement technology for use in controlling device fabrication processes

and in specifying materials and devices in national and international

commerce. Its major thrusts are the development of carefully evaluated

and well documented test procedures and associated technology, for use on production lines and in the exchange of devices and materials, and the

dissemination of such information to the electronics community. Application of the output by industry is expected to contribute to higher yields,

lower cost, and higher reliability of semiconductor devices. In addition,

the improvements in measurement technology will lead to greater economy

in government procurement and will provide a basis for controlled improvements in fabrication processes and in essential device characteristics.

Cooperation with industrial users and suppliers of semiconductor

devices is achieved through NBS participation in standardizing organizations; through direct consultations with device and material suppliers,

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symposia and workshops. In addition, progress reports are regularly prepared for issuance in the NBS Special Publication 400 sub-series. More

detailed reports such as state-of-the-art reviews, literature compilations,

and summaries of technical efforts conducted within the Program are Issued

as these activities are completed. Reports of this type which are publishe

by NBS also appear in the Special Publication 400 sub-series. Announcements of availability of all publications in this sub-series are sent by

the Government Printing Office to those who have requested this service.

A request form for this purpose may be found at the end of this report.

iv
SEMICONDUCTOR MEASUREMENT TECHNOLOGY : IMPROVED INFRARED RESPONSE TECHNIQUE FOR DETECTING DEFECTS

AND IMPURITIES IN GER24ANIUM AND SILICON p-i-n DIODES*

A. H. Sher

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

An infrared response (IRR) technique was evaluated for its utility in qualifying germanium

for radiation detector use. Because of several improvements in the sensitivity and interpretation of

the technique made during the evaluation, it was possible to observe a number of discrete energy

levels lying within the forbidden energy gap of

germanium which had passed unobserved in previous

studies. These levels correlate with the type of

defects and vacancies introduced by radiation

damage into germanium as measured using such

techniques as photoconductivity and Hall effect

measurements after irradiation. Furthermore,

the improved infrared response measurement method

was used to identify impurities, such as copper,

gold, and iron, and dislocations resulting from

heat treatments in germanium. A major advance was made when it was determined that the IRR spectra

could be grouped into five distinct types on the

basis of spectral features observed in the energy

range from 0.6 to 0.7 eV. One of the spectrum

types represented crystals from which good quality

detectors could be fabricated; the other four

represented crystals that yielded poorer quality

detectors due to carrier trapping, or crystals that

presented problems such as low lithium drift mobility

in detector fabrication. Three of the four spectrum

types representative of poor crystal quality could

be duplicated by suitably degrading specimens of a good quality crystal. The material and detector

characteristics of crystals within each spectrum

type were found to be similar.

Key Words: Carrier trapping; gamma-ray detector; germanium; Ge(Li)

detector; infrared response; silicon.

Work supported by the AEC Division of Biomedical and Environmental

Research

1
1. INTRODUCTION

Armantrout introduced a method that made use of the response of p-i-n

diodes to monochromatic infrared radiation to identify trapping centers

in material intended for use in fabricating Ge(Li) detectors [1]. Further

investigation of this infrared response (IRR) technique was undertaken

in order to corroborate the previously published findings and determine

the applicability of the method. In the process, modifications were

made that enhanced the sensitivity and scope of the measurement. This

is manifested by increased observation of details in the IRR spectra

and led to a better interpretation of energy levels arising from

impurities and defects in both germanium and silicon crystals.

2. INITIAL MEASUREMENTS

In the IRR measurement, photons incident on a reverse-biased p-i-n

diode induce the formation of free carriers in the depleted region of

the device by excitation of carriers into or out of energy levels within

the band gap. The current due to the generation of free carriers and the

subsequent transport of these carriers to the electrodes of the diode

is measured as a function of the incident photon energy. The diodes used

in this study were, in most cases, lithium-drifted germanium gamma-ray

spectrometer structures. These structures are produced by first thermally

diffusing lithium, a donor impurity, into single crystal p-type germanium

doped with an acceptor impurity, usually gallium. The resulting p-n

junction is reverse-biased so that the mobile lithium ions drift into the

crystal under the influence of the applied field. The lithium-ion

donors compensate the charge of the acceptors present and produce a com- pensated region that contains few free charge carriers. This compensated

or depleted region, sandwiched between the n-type diffused layer and

remaining bulk p-type crystal, is sensitive to radiation. Figure 1

shows IRR spectra obtained from four lithium-compensated p-i-n germanium

diodes using apparatus similar to that used in the original work [1]

except that a grating (rather than a prism) monochromator and a phasesensitive amplifier were employed [2]. The results are similar to those

previously observed. Diodes NBS 83-3 and NBS 83-4 were fabricated from

specimens of the same p-type germanium single crystal; the former was subjected only to the typical lithium-drifted detector fabrication process

while the latter was, in addition, heated to 800°C and quenched to room

temperature. Diodes NBS 13 and NBS 13Cu were fabricated from specimens

of a germanium crystal with an initially high dislocation density

(10^ cm-3). Diode NBS 13Cu was also doped with electrically active

copper to a density of approximately 1 x 10^^ cm~^. All that could

be inferred from the IRR spectra of the four diodes on the basis of the

previous studies [1] is that all the diodes except diode NBS 83-3

would show some degree of carrier trapping due to their observed response

below approximately 0.7 eV and that diode NBS 13 exhibits the lithiumdefect interaction as manifested by the shelf like response at 0.50 eV.

Considerably more information was obtained by improving the apparatus

used to measure diode IRR by using 1-mm thick germanium filter windows
rather than the 3 to A-iran thickness previously used. Figure 2 shows the

IRR spectrum of diode NBS 83-3 obtained using the improved IRR technique;

for the first time in this type of measurement the spectrum exhibits

features which can be attributed to discrete levels rather than a con- tinuum of states within the band gap as previously observed [1]. Assuming

that the features indicated in figure 2 correspond to actual transitions

into or out of energy levels within the band gap, the level scheme that

can be constructed is in general agreement with those obtained from

measurements of photoconductivity on radiation-damaged germanium [3].

The IRR spectra for the four diodes described above obtained using

a 1-mm-thick germanium filter window are shown in figure 3. In contrast

to the spectra in figure 1, those shown in figure 3 permit more positive

identification of the origins of the spectral features. For example,

the features in the spectrum of diode NBS 13Cu at 0.38 and 0.49 eV,

corresponding to the known energy levels related to the presence of copper

at 0.33 eV above the valence band and 0.22 eV below the conduction band,

respectively [4], confirm the presence of that impurity in the test diode.

The spectrum exhibited by diode NBS 83-4, the heat-treated specimen, is

in agreement with a recent photoconductivity study [5] of the acceptor

levels in germanium caused by plastic deformation which showed a broad

spectral response corresponding to a continuum of states extending from

approximately 0.4 to 0.6 eV, with a peaked response at 0.58 eV.

The peak at 0.36 eV (one-half the bandgap energy) shown in all the

spectra in figure 3 is thought to arise from higher-order diffracted

radiation present in the monochromator ; peaks also observed at energies

of 0.24 and 0.18 eV corresponding to one-third and one-quarter of the

bandgap, respectively, seem to bear this out.

3. MECHANISM FOR DIODE IMPURITY PHOTOVOLTAGE (IRR)

A review of the literature on measurements of energy levels arising

from the presence of defects and impurities in semiconductors by photoconductivity and related techniques has suggested that the steady-state

photo-EMF, or photovoltage, established when minority carriers are generated by illumination is probably the basic mechanism for the IRR

in p-i-n diodes with wide t-regions [6]. Figure 4 illustrates possible

models whereby a photovoltage due to light in the impurity region (with

energy less than the bandgap energy) can be observed in a diode near

liquid nitrogen temperature, 77 K.

As shown in figure 4a, excitation by light in the impurity region

can lead to optical transitions from the valence band, Ey, to an energy

state which is empty or only partially occupied by electrons at equilibrium [7]. Double-optical transitions can also occur, as shown in

figure 4b, where light transfers electrons from filled states to the

conduction band, E^,. The equilibrium between these energy states and

the valence band is upset and can be reestablished by subsequent opticallyinduced transitions from E-^ to these states. Double-optical transitions

are possible only if the energy of the exciting light is greater than

one-half of the forbidden energy gap [7]. Furthermore, excitation

with light of energy near the bandgap energy can produce "overpopulation"

(charge exchange) of impurity or defect states which may lead to the

3
generation of a photovoltage. This is illustrated in figure 4c which

represents the equilibrium energy level scheme of a semiconductor

containing three levels in the forbidden gap. Upon illumination with

light of energy equal to or greater than the bandgap energy (figure 4d)

the electrons and holes produced by such excitation are captured by

different states. Nonequilibrium electrons from the conduction band

may be captured by level III establishing a steady-state population.

Then holes may be captured by both level II and level III [8].

That the above photovoltage effects are responsible for the diode

IRR is supported by the following observations made on germanium diodes

[6,9] and confirmed in work with silicon. Firstly, no. spectral features

that can be linked to impurity or defect states have been seen at incident

photon energy less than one-half the bandgap energy. Secondly, the IRR

spectrum exhibits evidence of many discrete levels when a thin semiconductor filter is used. Thinner filters transmit more radiation of

energy near the bandgap energy than do thicker filters.

Even though the photovoltage is only observed at energies greater

than one-half the bandgap energy, it is possible to observe energy states

which lie in both halves of the forbidden gap. The photovoltage is

observed if the photon energy exceeds the energy difference between the

state through which excitation is occurring and the edge of the conduction or valence band, whichever is greater [8].

4. IRR MEASUREMENTS ON GERMANIUM

Infrared response measurements were carried out on two high-purity

germanium detectors in which lithium-compensation was not used.

Figure 5 shows IRR spectra obtained from these two high-purity diodes

[10] and also from lithium drifted diode NBS 83-3. Diode NBS 112 was fabricated from a crystal with an initial net donor concentration of

approximately 7 x 10^° cm'^. Relative to diode NBS 83-3, diode NBS 112

exhibits a conspicuous lack of structure in the spectrum between 0.50

and 0.54 eV, presumably because of the absence of lithium.

Diode NBS 113, the other high purity specimen, was intentionally

doped with gold to a density of approximately 10-^^ cm~^. Gold is

known to introduce three acceptor levels in germanium located 0.05 eV

and 0.15 eV above the valence band and 0.20 eV below the conduction

band, and a donor level 0.04 eV below the conduction band [4]. The

spectrum of diode NBS 113 shown in figure 5 shows evidence for the

presence of levels at 0.57 eV (0.15 eV above the valence band),

0.66 eV (0.05 eV above the valence band), and 0.52 eV (0.20 below

the conduction band)

.

Diode NBS 301 was fabricated from a crystal pulled from a melt which

was contaminated with a chunk of stainless steel [11]. Levels in germanium

arising from iron impurity are at energies of 0.34 eV above the valence

band and 0.27 eV below the conduction band [4], and chromium introduces

levels extending from 0.07 to 0.12 eV above the valence band [4]. The

IRR spectrum of diode NBS 301 shown in figure 6 is interpreted to

confirm the presence of these impurities by virtue of the incompletely

resolved peak at 0.37 eV (iron level at 0.34 eV above the valence band)

4
shown in the inset and the "smearing" of the spectrum in the region

extending from about 0.64 to 0.59 eV (chromium levels) relative to the

spectrum of diode NBS 83-3. Diode NBS 83-6, whose spectrum is also shown

in figure 6, was fabricated from a specimen of the same crystal as diode

NBS 83-3, but after the lithium diffusion it was stored at room temperature for several months, resulting in lithium precipitation during the

storage period. Lithium precipitate clusters would induce strain in the

crystal lattice giving rise to a band of energy levels [1]. This is

believed to account for the smoothing out of the IRR spectrum of diode

NBS 83-6 relative to that of diode NBS 83-3.

While IRR spectra for lithium-drifted diodes with similar measured

detector trapping characteristics may not be completely identical, the

following comments can be made from a study of the spectra. In diodes

which exhibit preferential electron trapping, the level at 0.50 eV is

always observed in the IRR spectrum. This level corresponds to the

lithium-defect trap located 0.21 eV below the conduction band which

has been found to be an electron trap [1]. Figure 7 shows IRR spectra

obtained from four lithium-drifted diodes two of which (diodes NBS 88

and NBS 303) exhibit marked electron trapping.

In diodes which exhibit preferential hole trapping (diodes NBS 74

and NBS 76), the level at 0.50 eV is not observed. The IRR spectra

of this type of diode show a response that increases smoothly from

0.40 eV, peaks at about 0.56 eV and decreases to a valley near 0.59 eV.

The IRR spectrum resulting from neutron-irradiation of germanium

was also studied for two diodes fabricated from specimens of the same crystal [12]. The portion of the crystal from which diode NBS 83-8

was fabricated had been irradiated with neutrons to a fluence of approximately 1.3 X 10^° cm~2. The IRR spectra of the two diodes, NBS 83-3

and NBS 83-8, are shown in figure 8. It is evident that features in

the spectrum of diode NBS 83-8 at energies of 0.55, 0.61, and 0.63 eV

are enhanced over those in the spectrum of diode NBS 83-3. The other

features of the spectrum of diode NBS 83-3 appear to be preserved in

the spectrum of diode NBS 83-8.

In studying radiation-damaged material, possible annealing effects

must be considered. A published study of high-temperature annealing of

defects produced by neutron irradiation of germanium [13] at fluences in

the range 10^^ to 10^^ cm~^ is particularly appropriate to the present

case as the temperature range studied includes the temperatures employed

in the fabrication process for lithium-drifted diodes. After irradiation, four energy states were reported, located at 0.01, 0.07, and

0.17 eV above the valence band and 0.20 eV below the conduction band.

After annealing, a new level 0.10 eV below the conduction band was

observed. With the exception of the state located 0.01 eV above the valence

band which is not within the energy range shown in figure 8, these states

correspond to the features labeled in the figure.

4.1 IRR SPECTRUM TYPES

A study of infrared response spectra obtained from lithium-drifted

germanium detectors showed that spectra observed to date can be grouped

5
into five types: one representative of good quality detectors and four

representative of poor quality detectors [14].

Figure 9 shows a comparison of IRR spectra obtained from five Ge(Li)

detectors fabricated from crystals from five different suppliers [curves

(1) through (5)] with spectra obtained from four specimens of one crystal,

each treated in a different manner [curves (A) through (D) ] . From previous

studies and a review of pertinent literature, it has been concluded that,

in general, the energy region covered in figure 9, 0.57 to 0.70 eV, is that

in which crystalline imperfections are detected; most of the features

of these spectra arising from impurities fall below 0.57 eV in the IRR

measurements. The spectra of these detectors exhibit distinct features

that serve as the basis for dividing IRR spectra into the five types.

The characteristics of each type of IRR spectrum are given in table 1.

Table 1 - Characteristics of Infrared Response Spectrum Types

Spectrum Energy Position Energy Position of Comments

of Minimum (eV) Major Features (eV)

(1) , (A) 0.66 0.65, 0.64, 0,63, Rises sharply into bandedge

0.61, 0.59, 0.58 above 0.66 eV.

(2) , (B) 0.65 0.63, 0.61 Rises sharply into bandedge

above 0.65 eV.

(3) , (C) 0.68 0.66, 0,65, 0.64 Rises into bandedge above 0,68 eV; spectrum decreases below 0,66 eV.

(4) , (D) 0.67, 0.66, 0.65 No prominent minimum; this type

shows the least number of spectrum

features (fairly featureless

from 0.58 to 0,65 eV)

,

(5) 0.66 0.65, 0.63, 0.61 Small minimum; sharply rises

into bandedge above 0.66 eV.

as shown in figure 9

Table 2 summarizes some of the measured material and detector characteristics of the diodes studied. Of particular interest are the four

reference diodes, whose IRR spectra are shown on the left side of figure 9.

Each reference diode spectrum is set opposite the spectrum type on the

right for which similar features are shown. The reference diodes were all

fabricated from specimens of germanium crystal NBS 83, and the treatments

to which they had been subjected, previously detailed, are summarized in

table 2. Diode 83-4 (Spectrum D in figure 9) was heated to 800°C,

quenched to room temperature, and is thought to contain approximately

10^^ cm-3 thermal defects [15]. That the IRR spectrum of a germanium

specimen of known high quality can be modified by specific treatment of

6
a crystal to yield spectra similar to that of lesser quality crystals

as shown in figure 9, indicates for the first time that the energy region

from about 0.57 to 0.70 eV can be used to yield qualitative information

on crystalline imperfections in germanium.

Table 2 - Summary of Crystal and Diode Characteristics of Germanium Specimens

Spectrum Diode

No.

(1) NBS 619

(2) NBS 76

(3) NBS 91

(4) NBS 13

(5) NBS 302

Material

Properties

Oxygen cone.

< lOl** cm3

High Li mobility

Low Li mobility,

oxygen cone.

High Li mobility,

high dislocation

density

Detector

Properties

Good diode, slight

electron trapping

Good diode, severe

hole trapping

Very unstable,

diode could not be

maintained

Poor diode,

electron trapping

Corresponding

Reference

Diode

NBS 83-3:

lithium drift

only , good

defictor

NBS 83-8:

neutronirradiated

NBS 83-6:

lithium

precipitates

NBS 83-4:

thermal

defects

Spectrum

(A)

(B)

(C)

(D)

as shown in figure 9.

Of 48 diodes studied, 25 percent exhibited spectra corresponding to

that of a good quality gamma-ray detector, type (1). The remainder

exhibited spectra of types thought to correspond to detectors of poorer

quality: 38 percent like type (2); 12 percent like type (3); 4 percent

like type (4); and 21 percent like type (5). Nearly four-fifths of the

poorer quality diodes examined fell into types (2) and (5) [16].

Table 3 summarizes the measured material characteristics common

to most of the diodes found in each representative group. These characteristics are generally similar to those of the diodes listed in table 2.

It is interesting to note that the crystals yielding the poor quality

detectors can be grouped, on the basis of the IRR spectra, into groups

representative of two of the major detector problem areas: trapping of one of the charge carriers or poor lithium drift mobility. The problem

of poor diode characteristics (reverse leakage current as a function of

bias voltage) does not appear to be separately differentiated by IRR.

Of course, carrier trapping and poor diode characteristics are

7
interrelated; if a sufficient field cannot be applied to the device, charge

carriers may be trapped before being collected.

Problems manifested by crystals that exhibit IRR spectrum types (3) or (4) seem to be related to either the lithium-oxygen interaction or high dislocation density, respectively. With regard to carrier trapping,

identified with spectrum types (2) and (5), the problem is more complex.

The origin of spectrum type (5) has not been identified. Spectrum type (2)

is characteristic of the neutron-irradiated reference diode, but it is

not possible at present to link the defects known to be produced in

germanium by neutron irradiation directly with the defects that produce

IRR spectra of this type in the test crystals.

Table 3 - Material Properties of Lithium-Drifted Germanium Detectors

Spectrum

Type

Dislocation

Density

Oxygen

Density

Lithium

Mobility

1 <2500 cm-2 <2 X 10^'* cm-3 High

2 <3500 cm"

2

<3 X lO^** cm-3 High

3 >5000 cm"2 >5 X lOl** cm-3 Low

4

>10'* cm-2 >1 X 10^3 cm-3 High

5 2500-8000 cm-2 <2 X lO^'* cm-3 High

5. IRR MEASUREMENTS ON SILICON

Concurrent with the IRR study of germanium, IRR work on silicon nuclear

radiation detectors was carried out [6,12]. The results of these measurements, mainly from radiation-damage experiments, were used to support the

interpretation of IRR results on germanium.

Figure 10 shows IRR spectra obtained for a lithium-drifted silicon

detector before and after irradiation with fast neutrons through the

p-contact to a fluence of 5 x 10^ cm~^ [12], As might be expected for

such a fluence the two spectra are nearly the same. However, the peaks

at 0.86 and 0.77 eV are somewhat enhanced, and the feature at 0.93 eV

is much enhanced after irradiation. The two lower energy peaks are associated with levels located 0.40 eV below the conduction band (0.77eV)

and 0.31 eV above the valence band (0.86 eV) . Such levels have been previously reported from observations of photoconductivity on silicon

specimens irradiated with neutrons at fluences from 1 x 10^^ to

5 X 10-^^ cm~^ and are attributed to divacancies [17].

Figure 11 shows IRR spectra obtained from three commercial lithiumdrifted silicon detectors presumed to be fabricated from specimens of

the same silicon crystal. Diode NBS-3S was not irradiated, diode NBS-4S

was irradiated with 1.9-MeV protons to a fluence of 1 x 10^"^ cm"^ incident

on the p-contact, and diode NBS-5S was irradiated with 1.5-MeV electrons

to a fluence of approximately 3 x 10^^ cm~^ incident on the p-contact.

8
The spectra of diodes NBS-3S and NBS-4S are similar as might be expected

since damage caused by 1.9-MeV protons should be localized in the contact

region and not in the compensated region of the device. As in the case of

NBS-6S (figure 10), the main features are observed at 0.77 and 0.86 eV,

and 1.03 eV. The IRR spectrum of the electron-irradiated specimen,

diode NBS-5S, shows features at 0.99, and 0.90 eV as well as those at

0.93, 0.86, and 0.77 eV observed in the other silicon diodes, and

attributed to divancies after electron irradiation [18] . In the literature, a level located 0.18 eV below the conduction band (0.99 eV)

arising from the vacancy-oxygen complex has been reported after neutronand gamma-irradiation [19,20], while a level 0.27 eV above the valence

(0.90 eV) may be associated with lithium precipitates [21].

Since the commercial lithium-drifted silicon detectors used in this

study had gold p-contacts, approximately 15 to 20 nm thick, through

which the infrared radiation must pass, it was necessary to determine

what effects, if any, this thin contact had on the measurement of IRR.

The infrared transmission of a 0.14-mm thick silicon filter, the thickness used in the measurement of the IRR of silicon detectors, was measured

using a thermocouple detector both before and after evaporation of

approximately 15 nm of gold onto one side of the filter. No effect was

observed in the spectral distribution of the radiation; however, the

overall transmission was reduced by a factor of about three as compared

to the uncoated filter.

6. IDENTIFICATION OF ENERGY LEVELS

In connection with a comprehensive study of methods for the

evaluation of germanium suitable for use in Ge(Li) detectors,

85 germanium specimens were collected between 1967 and 1973. Most

of these had been rejected for use in the fabrication of high-quality

Ge(Li) detectors. Infrared response spectra from 55 of these specimens

were identified as to spectrum type; the remaining specimens were of

insufficient thickness to permit the fabrication of diodes, were

damaged during processing, or yielded IRR spectra whose type could not

be identified due to excess noise.

In early IRR measurements, good agreement had been found between

energy levels determined from the energies of IRR spectral features

and energy levels arising from radiation damage experiments in germanium [3]. However, at that time no attempt was made to distinguish

whether features seen in IRR measurements were associated with levels

in the upper or in the lower half of the forbidden energy gap. If the

various levels observed in IRR spectra could be uniquely identified

based on data in the technical literature, and thus linked to a specific

crystalline defect, then it might be possible for the crystal grower to

Identify the source of the defect in the crystal growth procedure. If

corrective action could be taken, the quality of the crystal could be

upgraded based on the IRR data.

9
6.1 ENERGY LEVELS IN GERMANIUM

After a review of the literature concerning energy levels resulting

from radiation or thermal damage in germanium as measured either by

photoconductivity or Hall effect [13,23-46], an attempt was made to link

features in the IRR spectra specifically with energy levels observed in

the literature [22]. The results are shown in figure 12. From the data

published on fast neutron, proton, gamma ray, fast electron, and thermal

damage, etc., the summary of energy levels shown in column B was obtained.

The shaded bars indicate the energy range of levels for which there were some discrepancy in reported energy but which appeared to

arise from the same center. In column A are shown energy levels

resulting from IRR measurements of diode NBS 83-3. In this case each

energy level appears only once in the scheme; the level was placed at the

appropriate energy either below the conduction band edge or above the

valence band edge using the levels shown in column B as the basis for

comparison. In the majority of cases, a feature observed in the IRR

spectrum can be linked uniquely on the basis of energy with a level or band of levels obtained from the summary of the results reported in the

literature.

The energy level located 0.18 eV above the valence band (observed

at an energy of about 0.54 eV in IRR spectra) is reported to arise

specifically from the divacancy-lithium (VVLi) complex and thus should

not be observed in lithium-free germanium [38,46]. Comparison of the IRR

spectra obtained f rom a lithium—drifted diode, NBS 83—3, and from a diode

fabricated from high-purity germanium (without lithium compensation)

,

NBS-112, confirms this expectation. The spectrum of NBS 83-3 exhibits

a feature at 0.54 eV, but the spectrum of NBS-112 does not.

It is possible to make some specific statements, on the basis of IRR

measurements, with regard to the nature of trapping centers in Ge(Li)

detectors. Table 4 summarizes the data on the two IRR spectrum types

covering detectors that exhibit hole or electron trapping.

Table 4 - Energy Levels in Ge(Li) Detectors Exhibiting Trapping

Spectrum Carrier Level Nature of

^ Reference

Type^ Trapped (eV) Energy Level

(2) hole E - 0.09

c VO complex [33]

E^ + 0.11 WLiO complex

(5) electron Ev + 0.07 WLi complex [41.51]

Ec - 0.09 VO complex [33]

Ev + 0.11 WLiO complex [44] Ec - 0.21 WLiO complex [44]

as shown in figure 9 V - vacancy, W - divancy, Li - lithium, 0 - oxygen

10
Work on growing high purity germanium indicates that residual oxygen

creates charge trapping centers in this material [54]. Thus the assignment of LiO"*" as the donor species in those levels thought to arise from

WD complexes (Ey + 0.11 eV and Ec - 0.21 eV [46]) seems reasonable.

That the main electron trap in Ge(Li) detectors must lie deeper than

0.175 eV [55], appears to point to the Ec - 0.21 eV level which is not

observed in hole-trapping crystals

.

6.2 ENERGY LEVELS IN SILICON

The energy level scheme in figure 13 is a summary of the state of

the IRR measurements on radiation-damaged, lithium-drifted silicon nuclear

radiation detectors. As in the case of germanium, energy levels reported

in the literature [17,18,20,47,50-53] are listed in column B, while energy

levels observed in IRR measurements are listed in column A. Tentative

identification of IRR-detected levels with corresponding levels reported

in the literature has been made previously [6].

7. CONCLUSIONS

It has been demonstrated that the IRR technique can be used to

qualitatively detect and identify impurities and defects in germanium and

silicon with a high degree of sensitivity. In the case of lithium-drifted

nuclear radiation detectors, the technique can be used as a predictor of

crystal suitability for detector use early in the fabrication process.

Similarly, the results of such measurements regarding the presence

of certain types of defects in germanium and silicon single crystals

should be useful to the crystal grower to better control the growing

process.

Given suitable sources of optical radiation, the IRR technique

should be applicable to a wide range of semiconductor materials from

which diodes can be fabricated. In the case of discrete microelectronic

devices (transistors and diodes) the technique might be extended to

studies of impurities and defects. This would be useful in analyzing

device failure and, in particular, radiation damage effects.

11
8. REFERENCES

I. Armantrout, G. A., Infrared Evaluation Techniques for Ge(Li) Detectors, IEEE Trans.

Nucl. Sai. NS-17 , No. 1, 16-23 (1970).

2. Sher, A. H.jand Keery, W. J,, Improved Infrared-Response Technique for Determining

Impurity and Defect Levels in Semiconductors, Appl. Phys. Lett. 20_, No. 3,

120-122 (1972).

3. Methods of Measurement for Semiconductor Materials, Process Control, and Devices,

W. M. Bullis, ed., NBS Tech. Note 571, pp. 16-18 (April 1971).

^. Neuberger, M. , Electronic Properties Information Center, Report DS-143 (1965);

available from NTIS, Springfield, VA. , Accession Number AD 610828.

5. Miles, M. H. , Extrinsic Photoconductivity from Edge Dislocations in Germanium,

J. Appl. Phys. 40, 2720-2724 (1969).

6. Sher, A. H., Liu, Y. M. , and Keery, W. J., Infrared Response Measurements on Radiation-Damaged Si (Li) Detectors, IEEE Trans. Nual. Sai. NS-19 , No. 3,

312-317 (1972).

7. Ryvkin, S. M., Photoelectric Effects in Semiconductors , pp. 369-373 (Consultants

Bureau, New York, 1964).

8. Berkovskii, F. M. , and Ryvkin, S. M. , Sensitivity of Germanium and Silicon Photocells

in the Impurity Excitation Region, Sov. Phys . -Solid State 4, 263-269 (1962).

9. Sher, A. H., Keery, W. J., and Dyson, H. E., Improved Infrared Response Measurements

in Semiconductor Nuclear Radiation Detectors, IEEE Trans. Nucl. Sai. NS-19 , 341-344 (1972).

10. Diodes supplied by R. D. Baertsch, General Electric Company, Schenectady, NY.^ 12301,

II. Crystal Supplied by I. L. Fowler, AECL, Chalk River Nuclear Laboratories, Chalk River,

Ontario, Canada.

12. Methods of Measurement for Semiconductor Materials, Process Control, and Devices,

W. M. Bullis, ed., NBS Tech. Note 727, pp. 16-21 (June 1972).

13. Konopleva, R. F., Novikov, S. R. , and Ryvkin, S. M. , High-Temperature Annealing of

Defects Produced by Fast Neutrons in Germanium, Sov . Phys, -Solid State 6,

2610-2612 (1965).

14. Methods of Measurement for Semiconductor Materials, Process Control, and Devices,

W. M. Bullis, ed., NBS Tech. Note 733, pp. 17-23 (September 1972).

15. Logan, R. A., Thermally Induced Acceptors in Germanium, Phys. Bev. 101 , 1455-1459

(September 1972)

.

16. Methods of Measurement for Semiconductor Materials, Process Control, and Devices,

W. M. Bullis, ed., NBS Tech. Note. 743, pp. 17-22 (December 1972).

17. Lappo. M. T., and Tkachev, V. D., Divacancies in Silicon Irradiated with Fast

Neutrons, Sov. Phys.-Semiaonduators ^, 1882-1884 (1971).

18. Young, R. C. , and Corelli, J. C. , Infrared Photoconductivity of Electron-Irradiated

Phosphorous-Doped Silicon, Air Force Cambridge Research Laboratories Final Report

(August 1,;1971). Available from National Technical Information Service, Springfield,

Virginia 22151, Accession Number AD 729477.

19. Daly D. F., and Hoffke, H. E.. An EPR Study of Fast Neutron T.adiation Damage i Silicon, Bad. Effeats S_. 203--211 (1971).

12
20. Konozenko, I. D. , Semenyak, A. K., and Khivrich, V. I., Radiation Defects

Created by Co^'' y-Rays in p- and n-Type Silicon of High Purity, Phys. Stat. Sol.

35, 1043-1052 (1969).

21. Smirnova, I. V., Chapnin, V. A., and Vavilov, V. S., Radiation Defects in LithiumDoped Silicon, Sov. Phys. -Solid State 4, 2469-2474 (1962-63).

22. Methods of Measurement for Semiconductor Materials, Process Control, and Devices,

W. M. Bullis, ed., NBS Tech. Note 773, pp. 14-20 (June 1973).

23. Konopleva, R. F., Novikov, S. R. , and Ryvkin, S. M. , Energy Levels in Germanium

Induced by Fast-Neutron Bombardment, Sov . Phys. -Solid State 5, 1345-1351 (1964).

24. Novikov, S. R., Rubinova, E. E., and Ryvkin, S. M., Photoconductivity of Germanium

Irradiated with Fast Neutrons, Sov . Phys .-Solid State 6, 690-691 (1964).

25. Novikov, S. R. , and Rubinova, E. E., Photoconductivity of Germanium Irradiated by Fast Neutrons at 77°K, Sov . Phys. -Solid State 6, 1037-1049 (1964).

26. Konopleva, R. F. , et al.. Radiation Defects in Germanium Irradiated with High-Energy

Protons, Sov. Phys .-Senriconduators 3^, 948-951 (1970).

27. Evseev, V. A., Konopleva, R. F. , and Yuferev, A. A., Recombination Properties of

p- and n-Type Germanium Irradiated with 660-MeV Protons, Sov . Phys . -Semiconductors

4^, 1323-1327 (1971).

28. Evseev, V. A., Konopleva, R. F. , and Yuferev, A. A., Investigation of Trapping

Phenomena in p- and n-Type Germanium Irradiated with 660-MeV Protons, SoV . Phys,-

Semiaonductors 5^, 625-628 (1971).

29. Vitovskii, N. A., Maksimov, M. , and Mashovets, T. V., Investigation of yRadiation

Defects in High Purity Germanium, Sov. Phys . -Semiconductors 4^, 880-886 (1970).

30. Vitovskii, N. A., Mashovets, T. V., and Ryvkin, S.-M., High Temperature Annealing of

Y-Radiation Defects in n-Type Germanium, Sov. Phys. -Solid State 6^, 148101487 (1964).

31. Gerasimov, A. B. , et al. , Impurity Photoconductivity in Germanium Due to Gamma-Ray

Irradiation, Sov. Phys. -Solid State 6, 2527-2528 (1965).

32. Mashovets, T. V., and Khanserarov, R. Yu., The Energy Structure of y-Radiation Defects

in Germanium, Sov. Phys. -Solid State 7_, 245-246 (1965).

33. Akimchenkov, I. P., Vavilov, V. S., and Plotnikov, A. F. , Photoconductivity Spectra

of Germanium Doped with Oxygen and Irradiated with Fast Electrons, Sov. Phys . -Solid

State 8, 1577-1579 (1967).

34. Gerasimov, A. B., Ryvkin, S. M. , and Yaroshetskii , I. D., Impurity Photoconductivity

in Germanium Irradiated by Fast Electrons, Sov. Phys. -Solid State 6^, 543-550 (1964).

35. Akimchenkov, I. P., Ginzburg, M. I., and Plotnikov, A. F. , Spectra and Kinetics of the

Photoconductivity of p- and n-Type Germanium Crystals Irradiated with Fast Electrons

at 100 and 5.2°K, Sov. Phys. -Solid State 8, 932-937 (1966).

36. Tkachev, V. D., and Urenev, V. L., Thermal Defects in Crystals of Pure Germanium,

Sov. Phys. -Semiconductors ^, 1880-1881 (1971).

37.- Abdurakhamonava , S. N., et al. , Investigation of Thermal Defects in High-Purity

Germanium, Sov . Phys .-Semiconductors 4., 1979-1985 (1971).

38. Sinishchuk, I. K. , Tkachev, V. K. , and Urenev, V. I., Interaction of Lithium with

Radiation Defects in Pure n-Type Germanium, Sov. Phys .-Semiconductors _5, 1910-1911 (1972).

13
39. Goncharov, L. A., et at., Divacancy Donor Complexes in Oxygen-Free y-Irradiated

Germanium, Sov . Phys . -Semiaonduators _6, 369-370 (1972).

40. Emtzev, V. V., et at.. On the Donor-Vacancy Type Complexes in Germanium, Bad. Effects

9, 181-187 (1971).

41. Sinishchuk, P. K, and Abiev, A. K. , Changes in the Energy-Level Structure of LithiumDoped Germanium Due to Irradiation with 10^^ Y~Rays/cm2 Annealing, Sov . Phys.-

Semioonduotor ]_, 148-149 (1973).

42. Smirnov, G. A., Tkachev, V. D., and Urenev, V. I., Mechanism of the Precipitation of

Lithium in Germanium, Sov. Phys .-Semiconductors ]_, 1556-1557 (1974).

43. Tkachev, V. D., Urenev, V. I., and Yavid, V. Yu., Recombination Centers in n-Type

Germanium, Sov. Phys .-Semiconductors 6_, 1549-1550 (1973).

44. Konopleva, R. F. , and Novikov, S. R. , Formation of Defects in Lithium-Doped

Germanium Due to Bombardment with Fast Neutrons, Sov. Phys . -Semiconductors ]_, 906-908 (1974).

45. Basman, A. R. , et at.. Influence of Donor Impurities in the Kinetics of Annealing of

Radiation Defects in Germanium, Sov. Phys .-Semiconductors _7, 922-924 (1974).

46. Sinishchuk, I. K., and Reznikov, M. Ya . , Interaction of Lithium with Disordered

Regions Formed by Fast .Neutrons in n-Type Germanium, Sov. Phys. -Semiconductors

]_, 1131-1132 (1974).

47. Vavilov, V. S., Effects of Radiation on Semiconductors, p. 178 (Consultants Bureau,

New York, 1965); contains an energy level scheme compiled from 13 references

published up to 1963)

.

A8. Johnson, E. S., and Compton, W. D. , Recombination Luminescence in Irradiated Silicon - Effects of Thermal Annealing and Lithium Impurity, Bad, Effects 9_, 89-92 (1971).

49. Smirnoya, I. V., Chapnin, V. A., and Vavilov, V. S., Radiation Defects in LithiumDoped Silicon, Sov. Phys.-Sotid State 4, 2469-2474 (1963).

50. Vorobeichik, V. A., Konopleva, R. F,, Novikov, S. R. , and Rubinova, E. E. , Photoelectric Properties of Si Irradiated with 660-MeV Protons, Sac. Phys .-Semiconductors

5^, 397-401 (1971).

51. Urli, N. B., Annealing of y-Ray Irradiated Lithium Compensated p-Type Silicon,

Bad. Effects % 93-97 (1971).

52. Kozlov, I. P., et at., Divancy-Oxygen Complexes in Silicon, Sov. Phys . -Semiaonduators

6, 1743-1744 (1973).

53. Baranova, E. K. , Gusev, V. M. , and Strel'tsov, L. N. , Infrared-Absorption Study of

Radiation Defects in Silicon Bombarded with Lithium Ions, Sov. Phys . -Semiconductors

6, 2009-2010 (1973).

54. Pehl, R. H., Cordi, R. C, and Goulding, F. S., High-Purity Germanium: Detector

Fabrication and Performance, IEEE Trans. Nuat. Sci. NS-19 , No. 1, 265-269 (1972).

55. McMath, T. A. and Sakai, E. , A Search for Deep Trap Levels in Ge(Li) Detectors,

IEEE Trans. Nuat. Sci. NS-19 , No. 1, 289-294 (1972).

14
lOi 1 1 1 r

045 0.55 065 0.75 ENERGY (eV)

Figure 1. Infrared response obtained using a 3-inm- thick germanium filter.

The spectra have been plotted so that the bandedge response peaks of each

specimen (at approximately 0.71 eV) overlap. In order to observe a measurable infrared response below the bandedge peak from NBS 83-3, the

signal sensitivity level would had to have been increased by a factor of

approximately 200. Specimen temperature was approximately 100 K [2],

0.45 0.55 0.65 ENERGY (eV)

Figure 2. Infrared-response spectrum of NBS 83-3 obtained using a 1-mmthick germanium filter. The dashed curve is that obtained using the

3-mm- thick germanium filter at an enhanced signal sensitivity. Specimen

temperature is approximately 100 K [2].

15
O 1.0 0.

0.50 ENERGY (eV)0.70

Figure 3. Intrared-response spectra obtained using a l-mm- thick filter

window In the Incident photon energy range 0.30 to 0.70 eV. (The spectra

are plotted so that the levels of the minimum detectable signal coincide.

To the right of the dashed line a 640-llne/mm grating was employed; to

the left 240 llne/mm.) Specimen temperature was about 100 K [2].

m

n

©--©•-»-©--©--G--G--©--e- I

-•^--6-e--©--©-— —

n

-e- -i- -B- -e- i- -& -••-•-I

Figure 4. Representations of mechanisms that might yield a photovoltage

In p-i-n diode: a) transition from valence band to unfilled level;

b) double-optical transition; c) the equilibrium conditions for the charge

exchange process, and d) the transitions for the charge exchange process

[6].

16
0.50 0.60 0.70

I ENERGY (eV)

lire 5. IRR spectra of two high purity ger-

_um diodes and NBS 83-3, shown for comparison,

jiined using a l-mm thick germanium filter [9].

0.34 0.36 u.Jil

J I 1 0.50 0.60 0.70 ENERGY (eV)

Figure 6. IRR spectra of three lithium-drifted

germanium diodes obtained using a l-mm thick

germanium filter. Inset shows a portion of the

spectrum of NBS 301 [9].

70

[re 7. IRR spectra obtained using a l-mm thick Figure 8. Infrared response spectra of two

iianium filter from two lithium-drifted ger- germanium gamma-ray detectors [12].

Ijium diodes that exhibit electron trapping (NBS

|.nd NBS 303) and two diodes that exhibit hole

Iping (NBS 74 and NBS 76) [9].

17
0.60 0.65 0.70 0.60 0.65 0.70

ENERGY (eV)

Figure 9. Infrared response spectra obtained from five lithium-drifted germanium diodes

fabricated from crystals from five different sources [(1) - (5)] and four diodes fabricated from specimens of another crystal treated in known manners [(A) - (D) ] . (Spectral

features of interest as indicated by short vertical lines are identified in table 1. The diodes are identified in table 2.) [14].

18
UJ

q:< NBS-6S

0.70 0.80 0.90 ENERGY (eV)

1.00 1.10

Figure 10. Infrared response spectra of a commercial lithium-drifted silicon detector

obtained before (A) and after (B) irradiation with fast neutrons [9].

T

ENERGY (eV)

Figure 11. Infrared response spectra of three commercial lithium-drifted silicon

detectors. (Large peaks, similar to that in the spectrum of NBS-6S shown in figure 10, occur at 1.03 eV in the spectra of NBS-3S and NBS-4S. These are not shown because they

exceed the scale of the figure.) [9].

19
CONDUCTION BAND

0 I—

0.1

^-0.2

>

>

CD

iil

0.3

^ +0.3

+ 0.2 -

+0.1

B

VALENCE BAND

-[13,29,33,34,36]

-[26,28,30]

-[13,23-25,34,36,38,44]

[26-28,31 ,34,43]

[26,30-32,34,38,46]

[30,32,34,35]

[13,23-25,38,46]

[26,39,41 ,42]

[30,32,34,37,44-46]

[13,23-25,41 ,42]

[13,23-25,30]

NATURE OF ENERGY LEVEL

VO COMPLEX [33]

(no lithium)

VD COMPLEX [32,38]; VVLiO [44]

OD COMPLEX [43]

VD COMPLEX [32,45]

VVLi COMPLEX [46]

VVLi COMPLEX [41,42]

WD COMPLEX [40,45,46]; VVLiO [44]

VVLi COMPLEX [41 ,42]

ACCEPTOR [13,23,24,30]

I Figure 12. Energy level scheme comparing features detected -in IRR measurements on germanium diod

(A) with levels resulting from radiation or thermal damage (B) . References are given in brackets

Symbols used: V-vacancy, W-divacancy , 0-oxygen, D-donor, Li-lithium [26].

CONDUCTION BAND

0

NATURE OF ENERGY LEVEL

VALENCE BAND

[50]

[18,19,47,52] VO COMPLEX (A-CENTER) [18,47,52]

[50]

[21]

[18,47,48] VD COMPLEX AT LATTICE SITE [47,48]

[18,47]

[50] VO COMPLEX (A-CENTER) [50]

[50]

[19,50,53] VV [19]; VVLi [53]

[47,49,53] LITHIUM PRECIPITATE [49], at VV [53]

[48] K-CENTER [48]

[17,47,50,51] LiV or LiO COMPLEX [47,51]

[47,49,53] LITHIUM PRECIPITATE [9], at VV [53]

[21,48] VV [21]

[48] VV [48]

[48] VV [48]

Figure 13. Energy level scheme comparing features detected in IRR measurements on silicon diodes'

(A) with levels resulting from radiation damage (B) . References are given in brackets. Symbols

used: V-vacancy, W-divacancy, 0-oxygen, D-donor, Li-lithium [26]. '

20
S.114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS-SP-400-13

2. Gov't Accession

No.

3. Recipient's Accession No.

TITLE AND SUBTITLE Semioonductor Measurement Technology:

Improved Infrared Response Technique for Detecting Defects

and Impurities in Germanium and Silicon p-i-n Diodes

5. Publication Date

6. Performing Organization Code

AUTHOR(S)

A. H. Sher

6. Performing Organ. Report No.

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

]U. S. Atomic Energy Commission

jDivision of Biomedical and Environmental Research

(Washington, D.C. 20545

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

SUPPLEMENTARY NOTES

Library of Congress Catalog Number: 75~1210

ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

I bibliography or literature survey, mention it here.) An infrared response (IRR) technique was evaluated for its utility in

qualifying germanium for radiation detector use. Because of several improve-

^ments in the sensitivity and interpretation of the technique made during the

I evaluation, it was possible to observe a number of discrete energy levels lying

I within the forbidden energy gap of germanium which had passed unobserved in

j previous studies. These levels correlate with the type of defects and vacancies

introduced by radiation damage into germanium as measured using such techniques

las photoconductivity and Hall effect measurements after irradiation. Furthermore,

the improved infrared response measurement method was used to identify impurlities, such as copper, gold, and iron, and dislocations resulting from heat treatments in germanium. A major advance was made when it was determined that

the IRR spectra could be grouped into five distinct types on the basis of spectral

features observed in the energy range from 0. 6 to 0. 7 eV. One of the spectrum

types represented crystals from which good quality detectors could be fabricated;

[ the other four represented crystals that yielded poorer quality detectors due to

' carrier trapping, or crystals that presented problems such as low lithium drift

mobility in detector fabrication. Three of the four spectrum types representative

of poor crystal quality could be duplicated by suitably degrading specimens of a good quality crystal. The material and detector characteristics of crystals

within each spectrum type were found to be similar.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Carrier trapping; gamma-ray detector; germanium; GeCLi) detector; infrared response

silicon.

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these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints, and supplements available from ACS, 1155 Sixteenth

St. N. W., Wash. D. C. 20056.

Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials, components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under pro- cedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose of the standards is to establish nationally

recognized requirements for products, and to provide

all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations. Federal Information Processing Standards Publications (FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations). Consumer Information Series—Practical information, based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background

knowledge for shopping in today's technological

marketplace.

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for

outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Service (Springfield, Va. 22161) in paper

copy or microfiche form.

Order NBS publications (except NBSIR's and Biblio- graphic Subscription Services) from: Superintendent of Documents, Government Printing Office, Washington,

D.C. 20402. BIBLIOGRAPHIC SUBSCRIPTION SERVICES

le following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service A literature survey issued weekly. Annual subscription: Domestic, $20.00; foreign, $25.00.

Liquefied Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00. Superconducting Devices and Materials. A literature

survey issued quarterly. Annual subscription: $20.00. Send subscription orders and remittances for the pre- ceding bibliographic services to National Technical Information Service, Springfield, Va. 22161.

Electromagnetic Metrology Current Awareness Service Issued monthly. Annual subscription: $100.00 (Special rates for multi-subscriptions). Send subscription order and remittance to Electromagnetics Division, National Bureau oi Standards, Boulder, Colo. 80302.
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