DOE/JPL-956046/86-1 

9950-1 202 

RECEIVED 

MAR 12 1986 

PATENTS AND TU OFFlC6 

Cornel1 University 

Department of Materials Science and Engineering 

Bard Hall 

Ithaca, NY 14853 

ELECTRICAL, STRUCTURAL AND CHEMICAL CHARACTERIZATION OF SI SHEET MATERIAL 

Annual Report - Contract No. 956046 

Principal Investigator: Dieter G. Ast 

Covering Period: September 1, 1984 - August 31,1985 

"The JPL Flat Plate Solar Array Pro,act is sponsored by the U.S. Department of 

Energy and forms part of the Solar Photovoltaic Conversion Program to initiate 

a major effort toward the development of flat plate solar arrays. This work 

was performed for the Jet Propulsion Laboratory, California Institute of 

Technology by agreement between NASA and DOE.' 

( NAS A-Cfi- 179 9 49 ) 

CEjEMICAt CHARACTEhIZBHiCN CF 5i 2HEET 

MATERIAL Annual Eeport, 1 S€F. 1984 - 31 

Au9. 1985 (Cornell Univ.) 65 F CSCL 1OA Unclas 

ELE CTR I: C BL , S 'I r( U CTUR AL AND N87-13155 

G3/44 43635 

1 

? 
I I 

ABSTRACT 

Study of Web Dendritic Si ribbons under four-point bending stress was 

undertaken to lead to an understanding of how Web Si deforms under a 

well-defined applied stress. Since the mechanical properties of Si are known 

to depend on the 0 content of the material, 0 measurements using Fourier 

Transform Infrared Spectroscopy were done to determine a proper control sample 

of non-defected single crystal Si. 

Four point bending revealed a unique two step bending behavior for the Web Si 

ribbons. initial theory for this behavior involves the interaction of the 

dislocations generated by the deformation with the central twin planes of the 

ribbons. 0 measurements showed a uniformly high 0 content for the Web Si 

ribbons, approximately 1018 atoms/cma for all the samples measured. All the 

Web samples had a much broader absorption peak at 9 km than is usually seen 

for well-annealed single crystal Si. This broadening is thought to be related 

to stress in the Web Si ribbons. Two samples containing a known amount of 

residual stress support this hypothesis. Also, a shoulder on the infrared 

absorption peak associated with interstitial 0 in Si appears in the 

transmission vs. wavenumber plots for some of the Web samples. In the 

literature, this shoulder has been associated with 0-vacancy complexes or with 

0 at dislocations. The 0 content and configuration do not seem to correlate 

with the growth configuration of the Web Si ribbon. 

An 

ACKNOWLEDGEMENTS 

We wish to thank Dr. J. 

Corporation for the use of 

helpful discussions. We 

Kalejs and Mr. A. Menna at Mobil Solar Energy 

their four point bending set-up and for many 

also wish to thank Dr. A. Banhdadi at the National 

Bureau of Standards for the use of the Fourier Transf ormVInf rared Spectroscope 

and for his help in interpretation of our data. 

"This report was prepared as an account of work sponsored by the United States 

Government. Neither the United States nor the U.S. Department of Energy, nor 

any of their employees, nor any of their contractors, subcontractos, of their 

employees, makes any warranty express or implied, or assumes any legal 

liability or responsibility for the accuracy, completeness or usefulness of 

any information, apparatus, product or process disclosed, or represents that 

its use would not infringe privately owned rights." 

1 
Table of Contents 

List of Figures 

Introduction 

Four Point Bending of Web Silicon Ribbons 

Oxygen Measurements in Web Silicon Ribbons 

Surface Profiles of Web Silicon Ribbons 

Conclusions 

References 

Page 2 

Page 4 

Page 7 

page 20 

page 32 

page 36 

page 39 
1 a 

Figures 

Figure 1: Schematic of Four-Point Bending Set-Up. 

Figure 2: Four-Point Bending Data for Web Samples 5372-1.5 and 5372-2.4, 

Figure 3: Four-Point Bending Data for Web Sample 2071-7.3. 

and a Float Zone Sample. 

Figure 4: Four-Point Bending Data for Web Sample N140-10.3. 

Figure 5: Four-Point Bending Data for Web Sample R485-7.5-1. 

Figure 6: Four-Point Bending Data for Web Sample R485-7.5-2. 

Figure 7: IR Transmission vs. Wavenumber for Web Sample 5376-4.3. 

Figure 8: IB Transmission vs. Wavenumber for Web Sample 5376-4.3. 

Figure 9: IB Transmission vs. Wavenumber for a Cz Sample. 

Figure 10: IR Transmission vs. Wavenumber for Various Heat Treatments 

(Courtesy: I. Koguchi) 

Figure 11: Separation of Two Peaks Seen in Figure 8. 

Figure 12: IR Transmission vs. Wavenumber for Web Sample N128-19.1, 

Grown in 2058 Configuration. 

Figure 13: IR Transmission vs. Wavenumber for Web Samples 2284-15.8 and 

2-305-5.11, Grown in the 5435 Configuration. 

Figure 14: IR Transmission YS. Wavenumber for Web Sample R479-13.2, Grown 

in the 5460 Configuration. 

Figure 15: IR Transmission YS. Wavenumber for Web Sample R479-13.2, 

Before and After an HF Clean. 

Figure 16: IR Transmission vs. Wavenumber for Two Areas along Web 

Sample N128-19.1. 

Figure 17: IR Transmission vs. Wavenumber for Three Areas across the Width 

of Web Sample 2025-3.7A. 

Figure 18: IR Transmission YS. Wavenumber for Two Web Samples with a Known 

Amount of Residual Stress. 

2 
Figure 19: Surface Profile of Web Sample 5376-4.3. 

Figure 20: Surface Profile of Web Sample 5376-4.3. 

Figure 21: Surface Profile of Web Sample RE343-3.3. 

Figure 22: Expanded Surface Profile of Web Sample RE343-3.3. 

3 
Introduction 

During the reporting period of September, 1984 to August, 1985, we studied the 

deformation behavior of Dendritic Web Ribbon silicon under four point bending 

stress. Four-point bending experiments were carried out at Mobil Solar Energy 

Corporation on samples supplied by JPL. 

Dendritic Web silicon has a unique crystalline structure. The ribbon is a 

single crystal but 

planes parallel to the ribbon surface[l] . 

its mid-plane contains one or several closely spaced twin 

In addition, Web material contains 

bulk dislocations, some of which appear at the ribbon surface. Surface 

etching shows a high dislocation density at the ribbon center and near the 

dendrites, separated by regions of low dislocation density. Angle lapping and 

etching reveals dislocations at the twin plane, probably trapped there as a 

result of stress during ribbon growth [2]. The stresses experienced during 

ribbon growth are not well understood, but the plastic deformation as a result 

of these stresses in the form of dislocations intersecting the ribbon surface 

and dislocations trapped at the twin planes can be studied in an attempt to 

understand the stresses generated during the crystal growth. In addition, 

elastic residual stresses are present which can lead to lower mobility for 

4 
minority carriers and, consequently, to lower efficiencies in finished solar 

cells, and can also limit the rate at which the silicon ribbons can be grown. 

It has become clear that an understanding of the growth-rate and efficiency 

limiting stresses must be attained to optimize the growth of Dendritic Web 

silicon. 

For this reason, we studied the behavior of Web silicon under four-point 

bending, because understanding of how Web silicon deforms under a well-defined 

applied stress lead to a model of the stresses experienced during ribbon 

growth. 

can 

Previous work on Edge-Def ined Film-Fed Growth (EFG) ribbons under this 

Contract has determined that twins in the material interfere with the motions 

of dislocations [3]. This interference should lead to differences in the 

creep behavior of the Web material and an untwinned single crystal of 

identical dimensions. And indeed, our experiments have shown that Web silicon 

deforms very differently from single crystal silicon, either Float-Zone or 

Czochralski, and from polycrystalline material such as EFG silicon. In fact, 

Web silicon has shown a unique two-stage deformation behavior. The mechanisms 

responsible for these two stages are not clear and further study is needed to 

relate this behavior to the ribbon growth conditions and quality. 

Since oxygen hardens the silicon lattice, the amount of oxygen and the way the 

oxygen is bound in the silicon lattice is a major factor in the behavior of 

silicon under stress [4]. As shown in the following section, Web silicon 

contains nearly the maximum amount of oxygen soluble in silicon at the growth 

temperature. Part of the oxygen appears in the infrared absorption peak 

associated with oxygen in interstitial sites (Si-0-Si) in the silicon lattice, 

5 
. 

and part of the oxygen appears in an infrared absorption line that forms a 

shoulder on the standard interstitial oxygen peak. Various interpretations of 

this shoulder have been given in the literature, and the peak has been 

ascribed variously to homogeneously distributed precipitates of the coesite 

phase of SiOa in the bulk [5], to oxygen segregated to dislocation cores 

where, according to Bourret, it forms a coesite-like arrangement [6], or to 

oxygen involved in a complex with silicon interstitials and vacancies [7]. In 

the case of as-grown Web silicon, we believe that the coesite phase is likely 

to nucleate heterogeneously at the grown-in dislocations, since dislocations 

are present in the growing ribbon and heterogeneous nucleation is the prefered 

nucleation mechanism for any system. Whether oxygen does or does not 

precipitate on deformation induced dislocations generated in the cool-down 

phase of ribbon growth (T<750°C) is an open question. The oxygen decoration 

of these dislocations will depend critically on the ratio of dislocation 

mobility to oxygen mobility. Because decorated dislocations, as a rule, are 

less mobile than undecorated ones,the residual stress will be higher if the 

oxygen can move sufficiently fast to keep up with the dislocations. If the 

oxygen is bound to dislocations in the twin planes, the motion of dislocations 

through the twin planes is likely to be further impeded because the 

interaction of dislocations with the intrinsic dislocations in the twin 

planes will be hindered by dislocation pinning at oxygen precipitates. The 

amount and condition of the oxygen in the Web silicon, and its behavior with 

temperature and growth conditions, needs to be studied to determine the effect 

of the oxygen on the growing ribbon and on the solar cells fabricated on these 

ribbons. 

bulk 

6 
Four-Point Bending of Web Silicon Ribbons 

Introduction 

To study the effects of stress on Web ribbons, the ribbons had to be stressed 

in a controlled manner. Tensile testing of silicon is very difficult because 

the material is brittle at room temperature and it is hard to grip the 

specimen properly for a tensile test without damage to the specimen. We 

therefore decided to introduce deformation by bending. There are two possible 

ways to introduce deformation by bending; three point bending or four point 

bending. The four point bending is the simplest form of bending to analyze 

because the sample experiences a constant bending moment between the inner 

load pins. In three point bending, the moment increases linearly towards the 

center pin and is therefore not constant over any appreciable area. We 

decided to deform the Web ribbons in four point bending for simplicity 

in sample preparation and control in sample deformation. 

creep 

Four-point bending of Web silicon ribbons was carried out on an at 

Mobil Solar Energy Corporation (MSEC) with the cooperation of Dr. Juris 

Kalejs [8]. A schematic diagram of the bending apparatus is shown in Figure 

7 

apparatus 
1, showing the location of the sample, the thermocouples, the deflection 

probe, and the load-applying rollers. about 

an inch and are fixed, and the bottom rollers are attached to the applied load 

through a pulley system so that a bending moment is applied to the sample by 

moving the bottom rollers upward against the sample bottom. The deflection of 

the sample is determined simply by the downward motion of a deflection probe 

which rests The deflection of the sample center 

has been shown to agree well with the amount of curvature measured in the 

sample after cool-down and removal from the bending apparatus. The samples 

tested were all cut to a length of four inches, but the width of the sample 

was determined by the width that the particular ribbon was grown. 

The top rollers are separated by 

in the center of the sample. 

The samples were brought to the test temperature and a load, W, 

The sample was 

the ribbon center was measured with time. 

was applied. 

allowed to creep under the applied load and the deflection of 

Creep is the general name applied to plastic deformation which occurs under 

conditions of constant temperature and constant stress, or time-dependent 

plastic deformation. Creep can occur by a variety of mechanisms [9], 

including dislocation climb and glide (where the strain rate is proportional 

to some power of the applied stress, sometimes called power-law creep), and 

direct vacancy transport (where the strain rate is directly proportional to 

the applied stress. creep 

is called Nabarro-Herring creep; when controlled by vacancy transport through 

the grain Which mechanism controls the 

observed creep in a sample depends on the temperature and stress level that 

the sample experiences during the test. 

When the vacancy transport is bulk controlled, the 

boundaries it is called Coble creep.) 

8 
The stress levels in the Web samples during four-point bending are 

by simple beam-bending theory [lo]. 

calculated 

The Web sample can be modeled as a bar of rectangular cross-section, b x t, 

where b is the width of the ribbon and t is the ribbon thickness. Using 

standard mechanics, the bending moment, M, applied to the sample is 

determined by the spacing between the inner and outer pins, a, and the applied 

weight, W. The stress in the outer fiber of a rectangular cross-section 

ribbon is 

I 

where c/I is a geometrical factor called the inverse section modulus and is 

given by 

c6 

I bt3 

--- 

The deflection in the center of the ribbon due to the applied weight is given 

by 

s= [Wa(h-2a) 2] 

max 

16EI 

9 
where E is the Young's modulus of the silicon, 1.9 x 10l2 dyn/cm 2 in the [lll] 

direction, and I is the moment of inertia of the rectangular bar. C is the 

distance from the fiber where the maximum stress occurs (in this case, the 

ribbon surface) from the neutral stress bending axis. I and c are given by 

bt3 I= - 

12 

In the system at MSEC 

h=5cm 

a = 1.25 cm 

so the relationships become 

M = 0.625 W 

12 w 6 = 3.08 x 10- - 

bt3 

w 

bt2 

Q = 3.75 - 

These elastic solutions are also good for inelastic creep provided that the 

strain rate is proportional to the first power of the stress. If the sample 

is deforming by a power law creep then the problem is more complex and 

10 
numerical solutions must be applied [ll]. 

Initially, we tried to bend the ribbons at very high temperatures (about 

135OOC) to avoid the temperature region where the mechanisms for creep in 

silicon are controversial (around 900OC). However, because the Web ribbons 

are thin, they began to creep under the load of the deflection measuring probe 

(about 7 gm) when the temperature reached about 118OoC. It was decided to 

test the samples at 1000°C to avoid an uncontrolled "three-point" bending 

under the weight of the probe, even though this temperature range is 

undesirable from the standpoint of interpretation of the deformation 

mechanisms. All ribbon tests reported here were carried out at 1000°C. 

One Web ribbon was tested with the dendrites still attached to the ribbon. 

There was little bending even under a large applied load. We believe that 

this is a result of the stiffness of the dendrites and not the strength of the 

actual Web ribbon. The curve of deflection vs. time is shown in Figure 2, 

and it can be seen that the Web with the dendrites on did not bend appreciably 

even under twice the load experienced by the single-crystal Float Zone (FZ) 

sample tested. It appears from this result that the dendrites are a 

significant help in strengthening the Web ribbons during any kind of handling. 

All other samples tested had the dendrites removed before testing so that the 

deflection measured resulted from plastic deformation of the actual Web ribbon 

and not from the support that the ribbon receives from the dendrites. 

Samples obtained from Westinghouse and from JPL were from three growth 

configurations; 5435, generally regarded as producing the ribbons with the 

highest amount of stress, 5460, an improvement on the 5435 configuration, and 

2058, the growth configuration producing ribbons with little or no measurable 

11 
residual stress. Only samples from growth configurations 5435 and 2058 were 

tested. The growth configuration of each sample will be identified in 

parentheses after the sample number. 

Four-Point Bending Results 

Sample 5372-1.5 was tested with an applied stress of 27 MPa. The resulting 

deflection vs. time curve is shown in Figure 2. The results for the Web 

ribbon are obviously very different from the curve obtained from the 

single crystal FZ sample. First, the Web sample is seen to begin to creep as 

soon as the load is applied. In single crystal material, there is a delay 

period after the load is applied where dislocations, which are necessary for 

plastic deformation, are generated. In Web silicon, dislocations are present 

as a result of the growth, and hence there is no time lag between plastic 

deformation and loading. In the single crystal FZ material, once the 

deformation begins, the sample continues to creep at a fairly constant rate 

until further deflection is no longer measurable in the bending apparatus. 

One reason that the FZ material creeps at a rapid rate is that the material 

contains no oxygen, which solute solution hardens the lattice and increases 

the friction opposing dislocation motion. The Web sample shows initially a 

high creep rate, but after approximately 1 minute the creep rate decreases to 

almost zero. Then another period of rapid creep commences, followed by a 

period of slow creep. This sequence repeats one more time before the test was 

ended. This unusual fast creep-slow creep behavior is not commonly observed 

in bending samples and was cause for a great deal of speculation as to the 

mechanism causing it. Since this behavior was not seen on any sample other 

than Web silicon tested in this apparatus, we believe that it is not an 

artifact of the set-up or the measuring devices. Also, other Web samples 

12 

testing 
tested (to be discussed below) showed indications of the same type of 

behavior, which supports the idea that the oscillations in the creep rate are 

related in some way to the structure of the Web silicon ribbons. 

The model we are currently considering is that of dislocations initially 

moving on (111) planes inclined 540 to the ribbon surface, starting at the 

surface, which is the most highly stressed region in the ribbon. When the 

dislocations move towards the ribbon center, they initially experience a 

decreasing shear stress. This is so because the initial stress distribution 

is elastic and the stress increases linearly with the distance from the 

neutral bending axis. However, after some time the stress will be almost 

constant throughout the ribbon if power law conditions apply. In this case, a 

classical dislocation pile-up against the twin plane is formed, resulting in a 

decrease in the observed deflection rate. When N dislocations in a constant 

stress field, a, are piled up at the twin planes, the local stress, al, 

generated on the leading dislocation is equal to N'a. At some stress level, 

the local stress becomes sufficient to cause the dislocations nearest the twin 

planes to interact with the twin planes and cause reaction products of this 

interaction to travel to the opposite surface of the ribbon, resulting in a 

sudden large amount of deformation. This theory of dislocation punch-through 

fits all the observations, but much work needs to be done to determine that 

this is indeed the mechanism causing the two-stage creep behavior. Other 

ribbons tested showed a similar two-stage creep behavior, some to a lesser 

degree than that seen in sample 5372-1.5. 

Sample 2071-7.3(2058) was loaded with 31 gm, resulting in an applied stress of 

16.3 MPa. The sample crept in a manner similar to that of sample 5372-1.5, as 

can be seen in Figure 3. A second piece from this sample was loaded in the 

13 
same manner, but the load was removed during the "slow creep" region of the 

curve. On a third specimen from sample 2071-7.3, we removed the load in the 

"fast creep" portion of the curve. These two samples will be compared for 

dislocation density and configuration by optical microscopy and Transmission 

Electron Microscopy (TEM) to determine if the model of dislocation pile up at 

the twin planes and punching through to give a very rapid creep behavior is 

accurate. 

Figure 4 shows the deflection YS. time curve obtained for sample 

N140-10.3(2058). This sample was loaded with 49.2 gm, resulting in a stress 

of 35.4 MPa, approximately twice that of the previous samples. From the 

Figure, it can be seen that the sample crept very quickly to a deflection of 

approximately 2 mm and then stayed at that level, without displaying the 

stepped character of the previous samples. 

Two specimens from sample R485-7.5(2058) were loaded with the same to 

confirm the accuracy of comparing the behavior of two specimens from the same 

sample under different loading conditions. As can be seen in Figure 5, the 

effect of loading the two samples with 16.8 MPa resulted in curves of similar 

shape with regions of high creep rate at a deflection of 0.3 mm in both 

samples. One sample reached this deflection in less time than the other 

weight 

sample, sample #1 taking just over two minutes while sample #2 reached this 

point in just over one minute. Sample #2 from this ribbon also reaches a 

second higher creep rate after approximately two minutes. The beginning of 

this higher creep rate are seen in sample #1 from this ribbon for times 

greater than approximately five minutes. Study of these two curves leads us 

to believe qualitative comparisons can be made between samples from the 

same ribbons under different bending loads, but quantitative comparisons will 

14 

that 
be inaccurate. 

Discussion 

To distinguish between power law creep (strain rate proportional to an) and 

linear creep (strain rate proportional to a), we made the following 

calculations. 

If power law creep is dominating, the initial stress distribution will 

as 

appear 

but the final stress distribution will be 

The final stress distribution can be modelled, to a good approximation as 

15 
Such a simple 

test, the creep curve should look like 

model predicts that if the material does not change during the 

where Sf = [1.5n]-1 Si. In general, the creep exponent is about 2.5-3, so the 

final slope should be 

Sf = 0.3 to 0.36' Si 

The experimental data from Figure 2 show 

Si = 2 mm/min 

Sf = 0.1 mm/min 

which is less than the 0.6-0.72 mm/min predicted by the theory. This 

indicates that the material does change during the test, that is, the material 

hardens as dislocations become entangled and immobilized. 

The initial slope of the deflection YS. time curve varies roughly like 

16 
be inaccurate. 

Discussion 

To distinguish between power law creep (strain rate proportional to a") and 

linear creep (strain rate proportional to a), we made the following 

calculations. 

If power law creep is dominating, the initial stress distribution will appear 

\ as 

ribbon surface 

ribbon surface - 

but the final stress distribution will be 

The final 

ribbon surface 

I t 

I I 

c-- 

ribbon surface 

stress distribution can be modelled, to a good approximation as 

.. 

-. 

ribbon surface 

I 

I I 

ribbon surface 15 

I .. .. .. . . ." . ... :: 

: i. *' . . . , :- J 
Such a simple model predicts that if the material does not change during the 

test, the creep curve should look 

#*e-- 

like 

/ quasi steady dd state 

/) final slope = Sf .- 

/ 0' transient 

0 

/ 

rc / 

time 

where Sf = [l .Sn]-l S.. In general, the creep exponent is about 2.5-3, so the 

1 

\ final slope should be 

Sf = 0.3 to 0.36' Si 

The experimental data from Figure 2 show 

Si = 2 mm/min 

Sf = 0.1 mm/min 

which is less than the 0.6-0.72 mm/min predicted by he 'heory. Th .s 

indicates that the material does change during the test, that is, the material 

hardens as dislocations become entangled and immobilized. 

The initial slope of the deflection vs. time curve varies roughly like 

16 
Experiment Applied Shear Stress 

2 mm/min 

0.5 mm/min 

6 mm/min 

0.5 mm/min 

0.5 mm/min 

27 Wa 

16.3 MPa 

35.4 MPa 

16.8 MPa 

16.8 MPa 

This follows a law of the form 

1000 7000 

with Q in MPa and Si in mm/min. This law then predicts 

Experimental Power Law Fit 

Si (mm/min) 

Si (mm/min) Q (Wa) n=3 n=2.5 

2 27 

0.5 16.3 

6 35.4 

0.5 16.8 

0.5 16.8 

2.8 3.8 

0.62 1.1 

6.3 7.5 

0.68 1.15 

0.68 1.15 

The fit to a power law type creep is not perfect, but the trend indicates that 

the creep we observe can be modelled in this way. As a final check, we can 

compare the data obtained from as sample with the dendrites on to those 

17 
obtained after 

and we can model the behavior of the Web ribbons under an 

bending using simple beam bending theory. 

removal of the dendrites to confirm that power law creep holds 

applied four-point 

The initial slope of the curve obtained on the sample with the dendrites on 

was about 0.1 mm/min with an applied stress of 52.4 MPa. Modelling the 

ribbon/dendrite system as 

0.15 mm 

Without 

Dendrites With Dendrites 

it can be shown that the initial stress is almost totally carried by the 

dendrites. The maximum stress in the dendrite is, with the above geometry, 

only about one-quarter that in the non-dendrite part of the .ribbon for the 

same loading. The effective stress in the dendrite is then about 13 MPa. 

From this stress, the strain rate is predicted to be approximately 0.3 mm/min 

using the power law derived above and n=3. Considering the experimental error 

and the very much simplified analysis, the predicted creep rate is in 

satisfactory agreement with the 0.1 mm)min observed for the sample. It 

therefore appears certain that the Web specimens creepp according to a 

power law and that simple elastic or linear viscoelastic theory is inadequate. 

quite 

Our future measurements will employ numerical modeling. 

8 

Conclusions 

Testing of the behavior of Web silicon under applied four point bending loads 

18 
was carried out at Mobil Solar Energy Corporation. Testing was done at 1000°C 

because the thin Web ribbons crept uncontrollably under the weight of the 

deflection probe at higher temperatures. The dendrites were seen to 

strengthen the ribbons considerably and testing was done on all subsequent 

samples after removal of the dendrites. The Web samples were seen to 

plastically deform as soon as the load was applied, as compared to Ca and FZ 

samples which have an "incubation time" for the formation of mobile 

dislocations. The Web ribbons also showed an unusual high creep-low creep 

behavior which we believe to be linked to the pile up and punch through of 

dislocations at the central twin boundaries. 

Three samples from a single Web ribbon were bent under the same load the 

test was stopped at various times to allow TEM investigation of the 

microstructure during the slow creep rate period and during the high creep 

rate period. Two samples from a different ribbon were tested under identical 

loads and lead us to believe that qualitative comparisons can be made between 

the samples One sample was subjected to a higher level 

of stress. It crept rapidly to a deflection of approximately 80 mils and then 

there was no further deformation. 

but 

from the same ribbon. 

These samples, as well as Cz and FZ samples subjected to four point bending, 

will be used for TEM studies of the microstructure of Web ribbons under 

stress. 

19 
Oxygen Measurements of Web Silicon 

Experimental Design 

A Nicolet 8000 model Fourier Transform Infra-red Spectrometer (FTIR) was used 

to measure the concentration of oxygen in Web Silicon. The beam size was 

approximately 4mm x 5mm. After the measurement the sample was removed from 

the beam The spectrum obtained from the 

sample was divided by the background spectrum to remove any spurious 

absorption peaks due to the atmosphere or the detector. In ASTM calculation 

standard F 120-75, this method is called the Sample In-Sample Out method [12]. 

The Air Reference method, F 121-83, was used to calculate concentrations 

of interstitial oxygen present in the samples. According to this reference 

the absorption coefficent, a, at the wavenumber of the absorption by 

interstitial silicon, 1106 cm-' is multiplied by 4.9 to obtain the level of 

interstitial oxygen in ppma, and it is multiplied by 2.45~10~~ to obtain the 

concentration of interstitial oxygen in atoms/cm3 at 300°K. 

and a background spectrum was taken. 

the 

Since the Web ribbons are very thin, typically 100-150 microns, multiple 

reflections of the IR beam in the sample presented a great deal of difficulty. 

Multiple reflections cause a periodic variation in the transmitted intensity 

20 
vs. wavelength. All samples measured by infrared transmission show this 

effect of multiple internal reflections, but in thick samples the intensity 

varies so rapidly with wavelength that the variations are not resolved by the 

detection system. However, in thin samples, such as Web ribbons, the 

intensity variations from the multiple reflections are sufficiently gradual to 

be resolved by the detection system. variations 

caused by multiple reflections in the infrared transmission signal. To 

suppress these oscillations, we applied a 2.25 micron thick coating of ZnS to 

the ribbon surface to act as an anti-reflection coating [13], but the coating 

proved too uneven over the areas being measured to be of any benefit. In 

fact, the addition of the ZnS coating created more modes of internal 

reflection and made the measurement even more difficult. We believe that if a 

good ZnS coating was applied to the surface of a Web ribbon, oxygen 

measurements would be more reliable. We will continue work on developing a 

suitable coating method. 

Figure 7 shows the intensity 

One benefit of using FTIR instead of standard single scan spectrometry is that 

the signal from the sample is collected and stored as an interferogram in 

Fourier Transform space, and can be edited to remove some of the effects of 

the multiple reflections . Specifically , the frequency corresponding to 

multiple internal reflections can be removed from the interferogram and hence 

edited out in the plot of transmission vs. wavenumber. 

Initially, the samples were mounted at right angles to the incident infrared 

beam resulting In Figure 7, multiple 

reflections cause the strong sine wave modulating the transmission signal. 

The multiple reflections were so strong that their removal reduced the 

remaining transmission data to the point where the resulting transmission 

21 

in a great deal of multiple reflection. 
versus wavenumber However, tilting the samples a 

few degrees away from perpendicular incidence removed many of the multiple 

reflection peaks and the edited results could thus be analyzed with 

confidence. With the exception of the sample in Figure 7, all samples 

measured were held a few degrees off the vertical. The resulting 

interferogram was edited by deleting all contributions to the signal above 

1300 cm-l. 

plot could not be analyzed. 

The thinness of the Web ribbons also complicates the infrared measurements 

because the strength of the transmitted signal is related to the volume 

sampled. That is, the strength of the absorption, and therefore the height of 

the absorption peak, is proportional to how many bonds of a particular type 

are encountered in the volume tested. Since the Web ribbons are thin, the 

volume sampled by the 4mm x 5mm beam is small, and many scans sample 

had to be superimposed to raise the signal above the level of the noise. In 

our measurements, the samples were each scanned 1000 times for a single set of 

data, whereas a typical infrared sample would require only about 32 scans for 

an acceptable signal level to be obtained. Measurement at low temperature 

would also enhance the signal and allow us to use fewer scans for a reliable 

signal, but this approach was not available on the system used and so is not 

included in this report. 

of the 

Experimental Results 

Figure 7 is the transmission spectrum of sample J 376-4.3 from 800 to 1300 

wavenumbers (cm-') . This region of the spectrum contains information about the 

interstitial oxygen that is present, SO 

22 

as well as any information about Si0 

X 
all spectra presented will be restricted to this range. 

As can be seen from the Figure, a distinct sine wave is contained in the 

transmission signal. The envelope of this spectrum was entered into our 

computer and the arithmetic average signal was computed in an attempt to 

uncouple the sine wave from the information-containing transmission signal. 

The thickness of the Web sample was determined from the frequency of the sine 

wave to be about 150 microns in good agreement with the direct measurement. 

Figure 8 was obtained after removing multiple reflections from Figure 7. 

Comparison of the Web spectrum in Figure 8 with the spectrum shown in Figure 9 

obtained from a Ceochralski (Cz) grown sample shows the following features. 

Firstly, the average transmission level of the Web ribbon is higher than that 

of the sample, approximately 57% for the Web as compared to approximately 

48% for the C5 in the flatter parts of the curves. Theory predicts a 

transmission of 51% for Si the thickness of the Cz sample, approximately 

400 microns, and 53% for Si the thickness of the Web sample, approximately 

150 microns. Cz sample transmission is less than predicted, probably due 

to free carrier absorption not accounted for in the theory. However, the Web 

sample transmits more than theory predicts. This increase in transmission 

could be due to an oxide on the ribbon surface acting as an anti-reflective 

coating, or perhaps due to the sample shape acting to reduce reflection over 

the value predicted by a theory based on two parallel surfaces. It is not 

likely that there is a surface oxide on these samples thick enough to act as 

an anti-reflective coating, so we will study the effect of sample shape on the 

IR transmission in future experiments. Even though the Web sample is much 

thinner than the Cz sample, the height of the peak associated with 

interstitial oxygen at 1106 cm-' is seen to be larger for the Web ribbon than 

23 

C5 

The 
for the C1; sample, a height of 3.6% for the Web and 2.7% for the Cz. This 

larger peak indicates a much higher oxygen content for the Web ribbon. 

Calculations show an oxygen level of 25.3 ppma (1.3~10 cm ) for the Web and 18 -3 

17 -3 3.8 ppma (1.9~10 cm ) for the Ce sample. Since the maximum solubility of 

18 -3 oxygen in silicon at the melting temperature of silicon is about 3x10 cm 

the Web material is nearly saturated with oxygen at the growth temperature. 

The most striking difference between the spectrum for the Web ribbon and the 

spectrum for the Cz sample is, however, the noticeable shoulder at about 

1060 cm-' on the interstitial oxygen peak in the Web ribbon. This shoulder 

has been attributed in the literature to a 

coesite, which forms in single crystal 

treatment at approximately 60OoC. This 

high pressure phase of Si02 called 

non-dislocated silicon on heat 

phase is found to be unstable at 

higher temperatures except when dislocations are present, in which case the 

coesite is assumed to remain stable in the dislocation core. Figure 10 is 

taken from Koguchi, et al. [5], and shows the distinct shoulder on the 

interstitial oxygen peak formed both by annealing non-dislocated silicon at 

6OO0C and by deforming silicon to create dislocations at high temperature. It 

can also be seen that the shoulder disappears from the non-dislocated sample 

at heat treatments at 900°C as the peak associated with the more stable phase 

of amorphous SiOa at 1225 cm-' appears. In the dislocated crystal, however, 

the shoulder associated with coesite continues to grow with heat treatments at 

900°C. Because the Web ribbons contain a great many dislocations, some 

intersecting the surface and some pinned at the central twin planes, it is 

likely that the behavior of oxygen in Web ribbons follows that of the 

dislocated sample investigated by Koguchi et al. 

These two peaks can be separated as shown in Figure 11 for the sample in 

24 
Figure 8 into two distinct peaks, one associated only with the interstitial 

oxygen at 1106 cm , and the other reflecting only the absorption by the 

coesite phase centered at approximately 1060 cm . It can be seen in the 

Figure that the peak associated with the interstitial oxygen in spectrum 

taken on the Web sample is broader than on the spectrum taken on the Cz 

material, shown in Figure 9. The Full Width at Half Maximum (FWHM) is 

48.5 an-' for the Web interstitial oxygen peak and about 33 cm-l for the Cz 

interstitial oxygen peak. The broadening of the interstitial oxygen peak is 

assumed to be related to stress in the silicon, but no quantitative data 

relating sample stress to peak width has been located. Although the internal 

stress of this Web ribbon is not known, the broad interstitial oxygen peak 

indicates that the internal stress is greater in the Web ribbon than in the Cz 

sample. We will investigate further whether the level of stress in the Web 

ribbons correlates Since the 

ASTM standard requires that the FWBM be less than 32 cm-' for accuracy in the 

use of standard, the Web Silicon peaks cannot be strictly interpreted by 

the ASTM standard. Another method, such as Secondary Ion Mass Spectrometry 

(SIMS), will have to be used to determine the accuracy of the ASTM standard 

calculations for use in measuring the oxygen content of Web silicon. Since 

SIBS and other atomic counting methods of measuring oxygen in silicon measure 

the total number of oxygen atoms and give no information about the binding 

states of the oxygen in the silicon, direct comparison of the size of the 

interstitial oxygen peak measured by IR absorption to the number of oxygen 

atoms counted by another method will not be straightforward. We are 

considering the possibility of relating the area under the IR absorption curve 

to the total number of oxygen atoms for direct comparison with other methods 

such as SIMS. The peak associated with the coesite phase of Si0 is also 

wide, for sample J 376-4.3 the FWHM is 57.6 cm-l with a peak height 2.2%. 

25 

-1 

-1 

the 

with the width of the interstitial oxygen peak. 

the 

2 

of 
The amount of oxygen present as coesite cannot be calculated in a 

straightforward manner, since there is no ASTM standard for the measurement of 

coesite in silicon. 

Figures 12, 13 and 14 show the spectra for Web ribbons grown in various growth 

configurations. Figure 12 depicts the spectra of two different samples grown 

in the low stress 2058 configuration. As can be seen, sample N128-19.1 shows 

the distinct shoulder seen in the previous sample. However, another 2058 

sample, N140-10.3, did not exhibit any shoulder at all. The interstitial 

oxygen content of these two ribbons is 19.8 ppma (9.9~10 17 cm -3 ) for sample 

N128-19.1, and 18.6 ppma ( 9 . 3 ~ 1 0 l ~ c m - ~ ) f o r sample N140-10.3. The FWHM for 

sample N128-19.1 is 43.3 cm-'. Figure 13 shows the spectra obtained from two 

samples grown in the relatively high stress configuration 5435. Again, the 

shoulder can be seen distinctly on one sample, 2284-15.8, and is nearly absent 

in the other sample, 2-305-5.11. The interstitial oxygen content calculated 

for these samples was 17.3 ppma (8.7~10 ~m-~) for sample 2284-15.8, and 18 

17 

ppma (9x10 for sample 2-305-5.11. Their FWHM are 43.3 cm-' for sample 

17 

2284-15.8, and 37 an-' for sample 2-305-5.11. Two samples from the third 

growth configuration for which samples were available, 5460, did not show any 

development of a shoulder on the interstitial oxygen peak as can be seen in 

Figure 14 for sample R479-13.2. This sample was measured to have an 

interstitial oxygen content of 20 ppma (1x10 18 cm -3 ) , and a FWHM of 36 cm-l. 

Another measurement taken approximately 10 cm away from the area described by 

the spectrum in Figure 14 showed the same oxygen content, but had a FWHM of 

39.7 cm-'. a 

coesite phase the Web material does not seem to correlate with the growth 

configuration. Since only a small number of 5460 samples were tested, the 

fact that the coesite shoulder did not appear in the spectra from these 

26 

The appearance of the shoulder associated with the presence of 

in 
samples can not be considered to prove that the coesite phase does not exist 

in the ribbons grown in the 5460 growth configuration. The FWBM of the 

interstitial oxygen peak cannot be said to correlate with the growth 

configuration of ribbon, but we need to study more samples to be sure of 

this. 

the 

Figure 15 shows the spectra for the same sample as shown in Figure 14, 

R479-13.2(5460), but now also includes the spectrum taken before a 

hydrofluoric acid etch was used to remove the dark blue surface oxide or 

oxy-nitride resulting from the growth ambient. This oxide is approximately 

lo00 A. It can be seen that this heavy surface oxide creates a shoulder on 

the interstitial oxygen peak 

The surface oxide signal also 

interstitial oxygen content, 

with the oxide on the surface 

indistiguishable from that of the coesite phase. 

interferes with accurate measurement of the 

which was calculated as 32.7 ppma (1.6~10 cm ) 

and 20.3 ppma (1x10 cm ) after removal of the 

18 -3 

18 -3 

oxide layer. This demonstrates that in these samples the surface oxide needs 

to be thoroughly removed before any IR measurements are performed to determine 

the oxygen content and oxygen binding. All samples were HF etched before IR 

measurements to preclude the confusion over interpretation of the IR 

transmission YS. wavenumber data. 

A scan along the length of sample N128-19.1(2058) showed little change in the 

oxygen content Figure 16 

shows two spectra taken on this sample approximately 20 cm apart. The 

or oxygen binding state along the growth direction. 

interstitial oxygen content changes by about 1.1~10 17 cm -3 , or 2.4 ppma, but 

the coesite shoulder appears more distinct in the sample with the lower 

interstitial oxygen content. We speculate that there is an approximately 

constant amount of oxygen in the Web ribbons, close to the maximum solubility 

27 
of oxygen in silicon, and that the oxygen is present either as interstitial 

oxygen or as oxygen in the Si02 coesite phase, perhaps associated with 

dislocations in the material. 

A scan was also made across the width of sample 3025-3.76(5435), and three of 

the spectra The resolution obtained with a 4 mm beam 

scanning across a 2 cm ribbon is low, but some trends can be discerned from 

the data. 0-10a and 0-1Oc are near the two dendrites at the edge of 

the sample and Figure 0-lob was taken near the center of the sample. The 

interstitial oxygen content appears to decrease slightly in the center of the 

ribbon compared to the edges near the dendrites. For this Figure, the 

interstitial oxygen content near one dendrite is calculated to be 19 ppma 

(9.5~10 ~m-~), 18 ppma (9x10 cm ) near the ribbon center, and 22 ppma 

(1.1~10 ~m-~) near the other dendrite. It is not clear whether this is a 

significant change in the measured interstitial oxygen content and more scans 

will be We will also attempt to 

improve the resolution of this measurement, but using a smaller beam will 

decrease the signal, making the detection of the oxygen peak more difficult. 

are shown in Figure 17. 

Figures 

17 17 -3 

18 

carried out to confirm or reject this trend. 

Finally, oxygen measurements were performed on samples with a known amount of 

internal stress. These samples were obtained from Westinghouse where adjacent 

pieces of material had been etched to determine the amount of residual stress 

in the ribbon. Figure 18 shows the spectra from samples 2025-3.74(5435) and 

R461-5.7-A(J460L). The interstitial oxygen content appears slightly lower in 

17 sample 2025-3.71, 18.8 ppma (9.4~10 ~m-~), versus 20.5 ppma (l~lo~~cm-~) for 

sample R461-5.7-A. Sample 3025-3.711 is also the sample with the higher 

residual stress, measured as 40 Mdyn/cm as opposed to -3 Mdyn/cm for sample 

R461-5.7-A by the etch-pit counting technique developed by Westinghouse. The 

28 
shapes of the two spectra are also similar, indicating that there is no 

significant difference in the configuration of the oxygen, that is, the 

ribbons with the higher residual stress do not seem to contain the coesite 

phase at higher concentrations than ribbons with a low residual stress. The 

FWBM for the two samples are; 43.3 cm-l for the high stress sample, 

2025-3.7A, and 35.4 an-' for the sample with the lower residual stress, 

R461-5.74. This agrees with the supposition that the width of the 

interstitial oxygen peak increases with residual stress in the sample. It is 

not known whether the difference in interstitial oxygen content between these 

two ribbons significant or if it is related to the residual stress in the 

ribbons. Further work will be carried to determine if there is any 

correlation between the oxygen content and configuration, and the stress in 

the ribbon. 

is 

Conclusions on Oxygen Measurements 

The interstitial oxygen content of the Web samples measured by FTIR is 

and 

1.1~10 cm ) , where the maximum solubility of oxygen in silicon near the 

consistently high, ranging between 18 and 22.5 ppma (9x10 17 cm -3 

18 -3 

melting point of silicon is about 3x10 18 cm -3 . Some Web samples display a 

shoulder on the interstitial oxygen absorption peak which coincides with the 

IR absorption of coesite, a high pressure phase of Si0 According to the 

literature, this phase is present in non-dislocated single crystal silicon 

after heat treatments in the vicinity of 6OO0C, as well as in single crystal 

silicon with dislocations. non-dislocated 

single crystal silicon at temperatures above approximately 80OoC. Further 

tests including etching and optical microscopy, TEM and FTIR will carried 

29 

2' 

The coesite phase is not stable in 

be 
out on the Web ribbons to determine if the coesite in the Web ribbons is 

present as free precipitates that dissolve at high temperatures or if it is 

associated with dislocations, in which case the coesite will be stable up to 

temperatures at which the oxygen can redissolve into the silicon matrix, about 

1200°C [SI. 

The interstitial oxygen content and the appearance of the coesite shoulder do 

not seem to correlate with the growth configurations studied, 5435, 5460 and 

2058. Furthermore, some samples grown in either the 5435 or the 2058 

configuration lacked the shoulder associated with coesite, while other samples 

from the same growth configuration did display this shoulder. Neither of the 

5460 samples tested showed the coesite shoulder, but we do not feel that this 

small sampling indicates that the coesite phase does not form during the 

ribbon growth in the 5460 configuration. 

The peaks associated with interstitial oxygen were seen to be wider for the 

Web samples than for the Ca sample. In fact, these peaks are wider than the 

ASTM allows for proper use of the calculation from absorption to 

the interstitial oxygen content. The peak broadening is thought to be related 

to the internal stresses in the Web ribbons, but no trends with growth 

configuration have been discerned. 

coefficient 

The surface oxide present on some samples as a result of temperature and 

ambient seen by the ribbons during growth was found to interfere with both the 

identification of the coesite shoulder and the quantitative measurement of the 

interstitial oxygen content. It was concluded that samples must be etched in 

HF prior to FTIR to measure oxygen reliably. 

30 
The oxygen content was not seen to change significantly along the growth 

direction in the sample measured. There seemed to be some indication that the 

interstitial oxygen content to be higher near the dendrites than in the ribbon 

center in the one sample tested, but the change was so slight that more data 

are needed to confirm this behavior and determine if the change is 

significant. 

For the two samples obtained from Westinghouse with a known amount of residual 

stress there seemed to be no correlation between the amount of stress in the 

ribbon and the interstitial oxygen content or the presence of the coesite Si02 

phase. peak width for the sample with a higher amount of residual stress 

was greater than that for the sample with a lower amount of residual stress, 

correlating with the theory that the peak width is increased with increasing 

sample stress. 

The 

Further measurements will be carried out to determine any correlation between 

the amount and state of oxygen in the Web ribbons and the stress in the 

ribbon, the growth configuration of the ribbon and the position within the 

ribbon, i.e., in the center of the ribbon as opposed to near the dendrites. 

Although the use of a ZnS anti-reflective coating on the surface of the Web 

ribbons was not successful on the first attempt, we believe that the use of an 

anti-ref lective coating may make data manipulation in the Fourier Transform 

space unnecessary, and make the curves of transmission vs. wavenumber more 

reliable. We will continue work on this aspect of the oxygen measurements. 

31 
Surface Profiles of Web Ribbons 

Lateral profiles of the surfaces of Web ribbons were determined using a 

Tallysurf 5-120 Surface Profilometer. The profiles were taken to check the 

accuracy of using the thickness of the Web ribbons calculated from the Fourier 

Transform Inf rued (FTIR) interferograms in the calculations of the 

interstitial oxygen content of the ribbons [14]. 

The transmission of the IR beam depends on the thickness of the silicon by 

I = expc ) (1-R)2(l-R2exp(-2x/L))-1 

L 

0 

T 

where 

and 

n, = 1.003 (air) 

% = 3.4181 (silicon at 9 microns), 

32 
I is the incident beam intensity, x is the distance in the silicon, and L is 

a characteristic absorption length, or optical depth, for light in the 

silicon. The value of L is dependent on the wavelength of the incident light. 

0 

For wavelengths near that of the interstitial oxygen absorption peak, 9 

microns, L is approximately 1 cm , but the exact value depends on how the -1 

silicon was grown. Strictly speaking, the average thickness of the sample in 

the beam cannot be used since the thicker parts of the sample will dominate 

the absorption. However, when x << L, as in the case of Web ribbons, then the 

exponential term can be expanded as 

0 

exp(< = 1 - + -2 XY -.... 

L 2L 

which, at about 9 microns, 

I 

- = 0.5406 (1 

I 

0 

yields for oxygen free silicon 

- 1.46~ + ...) (x in cm) 

and the higher order terms can be neglected. Then, the thickness of the 

sample determines the absorption of the IB linearly. In Web ribbons, x is 

approximately 150 microns, much less than L and the absorption can be 

considered a linear function of the sample thickness. 

Figure 18 shows the lateral surface profile of Web ribbon 5376-4.3. The 

surfaces of the ribbon are smooth, but there is a large variation in the 

thickness across the ribbon from dendrite to dendrite. In this case, the 

center of the ribbon is approximately 30 microns thicker than the edges of the 

ribbon. The Web ribbon was measured to be 130 microns thick in the 

approximate ribbon center. The thinnest part of the ribbon is about 1 mm in 

33 
from the edge of the dendrite. This change in thickness of the ribbon over 

the area of the infrared beam in the FTIR makes determination of the 

interstitial oxygen content using an average thickness of the ribbon less 

accurate. The fact that the ribbon is significantly thinner at the edges will 

also complicate the measurement of oxygen content from dendrite to dendrite, 

making it difficult to determine if the oxygen content changes across the 

ribbon width or if the variation in ribbon thickness causes some error in the 

calculations. 

Figure 19 shows a front surface lateral profile of the same sample as that in 

Figure 18, 5376-4.3, but offset approximately 10 cm from the previous profile. 

On one side, the surface is similar to that shown previously, that is, narrow 

near the dendrite and increasing smoothly in thickness toward the ribbon 

center. On the other side, however, the ribbon does not get thinner toward 

the dendrite, but stays about the same thickness except within 2 mm of the 

dendrite, where the ribbon narrows drastically to join the dendrite as before. 

This wide variation longitudinal in surface profile over 10 cm of growth 

suggests that it would be impractical to try to determine the exact surface 

profile of an area measured by FTIR since a small error in the location of the 

IR beam results in a large error in the surface profile. 

Figure 20 is an example of one of the widest ribbon thickness variations seen 

in the surface profile measurements. Sample RE343-3.3 has a 120 micron "humpA 

on one side of the ribbon and then tapers smootly down to the other side. 

This ribbon was measured to be 150 microns thick at the approximate ribbon 

center, so the variation in thickness across the ribbon is a significant 

fraction of the measured ribbon thickness. Figure 21 is of the same area of 

RE343-3.3 as in the previous Figure at a lower magnification. In this Figure, 

34 
the thin part of the ribbon and the profile of the dendrite are shown. The 

thinnest part of the ribbon is again about 1 m in from the dendrite and the 

ribbon joins The profile of the dendrite is also 

quite smooth on this scale, while visually the dendrite is very jagged in 

appearance. 

smoothly into the dendrite. 

Conclusions 

Surface profiles of the Web ribbons revealed that the ribbons are generally 

thicker in the center than at the edges, although this was not always seen to 

be the case. grows 

and can develop thickness variations from dendrite to dendrite on the order of 

the measured central ribbon thickness. This thickness variation will affect 

the accuracy of the calculations to determine the interstitial oxygen content, 

but since the surface profile changes rapidly we feel that using average 

thickness over the IR measurement area as measured from the FTIR interferogram 

will provide the highest accuracy for the oxygen calculations. 

The lateral surface profile changes rapidly as the ribbon 

the 

35 
Conclusions 

The study of Dendritic Web silicon ribbons under applied stresses was begun 

during the reporting period September, 1984 to August, 1985. 

The Web ribbons were found to behave unusually under four-point bending at 

1000°C. The ribbons exhibited a fast creep-slow-creep behavior where slow 

creep began immediately on loading, followed by a short period of rapid creep. 

In some ribbons this pattern was seen to repeat more than once during the 

test. One ribbon tested under a larger stress did not exhibit this fast 

creep-slow creep behavior, but crept rapidly to a deflection of approximately 

2 mm and remained at that level for the duration of the test. Our current 

model for this unusual creep behavior is one of initial easy movement of 

dislocations on until the dislocations encounter the obstacle of the 

central twin planes. The dislocations pile up at the twin planes until the 

stresses generated on the leading dislocation is great enough to cause the 

dislocations closest to the twin planes to punch through and travel to the 

other side of the ribbons, resulting in a short period of fast creep. 

Specimens prepared for investigation by Transmission Electron Microscopy will 

be studied in the next reporting period. 

loading 

Measurements of the oxygen content of the Web ribbons also revealed unusual 

36 
behavior. interstitial oxygen content was found to be consistently high, 

around 10'' ~rn-~, near the solubility limit of oxygen in silicon at the 

melting point of silicon. The oxygen content seemed to change little along a 

10 cm length of ribbon or across the width of one ribbon. The peak associated 

with interstitial oxygen in the Infrared Transmission spectrum was seen to be 

broad. This peak broadening has been associated with stress in silicon. In 

the two samples where the stress in the ribbon was known, the ribbon with the 

higher residual stress did have a broader interstitial oxygen absorption peak. 

The relationship between the measured stress in Web ribbons and the width of 

the interstitial oxygen peak will be studied in the upcoming year. In 

addition to the broad interstitial oxygen peak seen in the Web ribbons, a 

shoulder on this peak was seen in some ribbons. The appearance of the 

shoulder and its height could not be correlated with the ribbon growth 

configuration. This shoulder was not observed in the two ribbons from the 

5460 growth configuration studied, but we cannot assume that ribbons grown in 

this growth configuration do not ever exhibit this shoulder. The appearance 

of the shoulder did change slightly along a 10 cm length of ribbon. We 

currently believe that this shoulder is caused by the formation of SiOa 

coesite precipitates along the grown-in dislocations during the crystal 

growth. Further IB studies on samples that have been heat-treated and TEM 

investigation of the dislocations at the twin boundaries will be performed 

during the coming year to clarify the state of the oxygen in the Web ribbons. 

The 

Surface profiles of Web ribbons showed that the ribbons are generally thicker 

in the ribbon center by about 30 microns. The shape of the surface changes 

rapidly as the ribbon grows. surface 

morphology would make any attempt to measure the exact area of a ribbon where 

oxygen measurements had been obtained to confirm the thickness of the ribbon 

37 

We determined that the rapid changes in 
as measured Therefore, we will continue to use the 

average thickness of the ribbon determined from the IR measurement for the 

calculation of the interstitial oxygen content. 

by the IR beam fruitless. 

38 
References 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

O'Hara, S. and G. H. Schwuttlce, J.Appl.Phys. - 36 (1965), 2475. 

Cunningham, B., H. Strunk and D. Ast, Defect Structure of Web Silicon, 

DOE/JPL Report 954852-5, October, 1980. 

Gleichmann, R., M.D. Vaudin and D.G. Ast, "Recovery of Edge-Defined 

Film-Fed Grown Silicon", Phil .Mag.A - 51 (3) (1985) , 449. 

Doerschel, J. and F.G. Kirscht, "Differences in Plastic Deformation 

Behavior of Cz and FZ Silicon Crystals", Phys.Stat.So1. A64 (1981), K85. 

Koguchi, M., I. Yonenaga and K. Sumino, "Oxygen Donors Developed 

Around Dislocations in Silicon", Jap.J.Appl.Phys. - 21 (1982), L411. 

Ourmazd, A., W. Schroeter and A. Bourret, "Oxygen Related Thermal Donors 

in Si: A New Structural and Kinetic Model", J.Appl.Phys. 56 (1984),1670. 

Pajot, B. and B Cales, "Infrared Spectroscopy of Interstitial 0 in Sin, 

Materials Research Society Symposium Proceedings, Vol. 59, Oxygen, Carbon, 

Hydrogen and Nitrogen in Crystalline Silicon, Fall, 1985, to be published. 

Kalejs, J.P., "Stress Studies in EFGn, DOE/JPL Report 956312/83/04, 

April-June 1983. 

- 

- 

Re'ed-Hill, R.E., Physical Metallurgy Principles, Second Edition, 

University Series in Basic Engineering, Litton Educational Publishing, 

Inc., Monterey, CA, 1973. 

10. Beer, F.P., and E.R. Johnson, Jr., Vector Mechanics for Engineers:Statics, 

11. Ast, D.G. and D. J. Krenitsky, %tress Relaxation in Coiled Ribbons of 

12. Annual Book of ASTM Standards, Volume 10.05. 

13. Ohsawa, A, K. Honda, S. Ohkawa and R. Ueda, "Determination of 0 

Second Edition, McGraw-Hill Book Company, New York, NY, 1972. 

Fe40Ni40P14Bs and Fe29Ni49P14B6S2", J. Mat. Sci . - 14 (1979) , 287. 

Concentration Profiles in Si Crystals Observed by Scanning IR Absorption 

Using Semiconductor Laser", Appl .Phys .Lett. - 36 (2) ,15 January, 1980, 147. 

14. Runyan, W.R., Silicon Semiconductor Technology, McGraw-Hill Book Co., 

New York, NY, 1965. 

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.- 

Figure 19: Surface profile of top and bottom of Web ample 5376-4.3 

showing a large variation in sample thickness from dendrite 

to dendrite. This ribbon was measures to be approximately 

130 microns thick in the ribbon center. 
.. J376 -4.3 

ribbon front 

I 

Figure 20: Surface profile of the front surface of the ribbon 

shown in Figure 18, but approximately 10 cm away 

from the previous profile. There is a significant 

change in the ribbon profile over this short distance. 
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Figure 22: Expansion of the trace shown in Figure 20 for Web sample 

RE343-3.3 showing how the crystalline ribbon joins smoothly 

into the supporting dendrite.