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SOME ASPECTS OF THE MEASUREMENTS OF ELECTRICAL EFFECTS OF DISLOCATIONS IN SILICON USING A COMPUTERISED EBIC SYSTEM
P. Wilshaw, A. Ourmazd, G. Booker
To cite this version:
P. Wilshaw, A. Ourmazd, G. Booker. SOME ASPECTS OF THE MEASUREMENTS OF ELECTRI- CAL EFFECTS OF DISLOCATIONS IN SILICON USING A COMPUTERISED EBIC SYSTEM.
Journal de Physique Colloques, 1983, 44 (C4), pp.C4-445-C4-450. �10.1051/jphyscol:1983452�. �jpa- 00223072�
P H Y S I Q U E
Colloque C4, suppl6ment au n09, Tome 44, s e p t e m b r e 1983 page C4-445
SOME A S P E C T S O F T H E MEASUREMENTS O F E L E C T R I C A L E F F E C T S OF D I S L O C A T I O N S I N S I L I C O N U S I N G A C O M P U T E R I S E D E B I C S Y S T E M
P.R. Wilshaw, A . Ourmazd and G . R . Booker
Department of Metallurgy and Science o f Materials, &ford University, U . K .
Resume - On d d c r i t un systdme comprenant un m i c r o s c o p e 3 b a l a y a g e JSM-35X e t un m i n i - o r d i n a t e u r PDP11/03 s e r v a n t a u c o n t r d l e e t 3 l ' a c q u i s i t i o n d e d o n n e e s d ' u n d e t e c t e u r s y n c h r o n e u t i l i s e en EBIC. A l ' o r i g i n e c e systdme
a 6 t e m i s e n o e u v r e p o u r d e s m e s u r e s d e c o n t r a s t e E B I C d e d i s l o c a t i o a s i n d i v i d u e l l e s i n t r o d u i t e s p a r d e f o r m a t i o n d a n s S i .
D e fagon 2 i n t e r p r e t e r p l u s compl@temenc i e s n e s u r e s a e c o n t r a s t e , il e s t n e c e s s a i r e d e d e t e r m i n e r l a l o n g u e u r d e d i f f u s i o n d e s p r t e u r s mino- r i t a i r e s . C e l l e - c i a B t e m e s u r d e e n t r e llOK e t 300K s u r d e s d c h a n t i l l o n s d e S i d e t y p e n c o n t e n a n t d e s r e s e a u x d e d i s l o c a t i o n s i n t r o d u i t s p a r d e f o r m a t i o n .
C e s r d s u l t a t s p e r m e t t e n t d 1 i n t e r p r 8 t e r l e s m e s u r e s e e c o n t r a s t e d e s d i s l o c a t i o n s .
A b s t r a c t - A s y s t e m i s d e s c r i b e d i n c o r p o r a t i n g a PDP11/03 minicomputer f o r c o n t r o l and o n - l i n e d a t a c o l l e c t i o n of a l o c k - i n EBIC s e t up b a s e d on a JSM-35X SEM. The s y s t e m h a s been p r i m a r i l y u s e d f o r t h e measurement of EBIC c o n t r a s t from i n d i v i d u a l d e f o r m a t i o n - i n d u c e d d i s l o c a t i o n s i n S i . I n o r d e r t o i n t e r p r e t t h e c o n t r a s t r e s u l t s more f u l l y , i t i s n e c e s s a r y t o know t h e m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h . T h i s h a s b e e n measured a t llOK and 300K f o r an n-type S i specimen c o n t a i n i n g d e f o r m a t i o n i n d u c e d d i s l o c a t i o n networks.
The s i g n i f i c a n c e o f t h e r e s u l t s o n t h e i n t e r p r e t a t i o n o f t h e d i s l o c a t i o n c o n t r a s t measurements i s d e s c r i b e d .
1. INTRODUCTION.
The EBIC mode of t h e S.E.M. i s o f g r e a t v a l u e f o r s t u d y i n g t h e e l e c t r i c a l p r o p e r t i e s of s e m i c o n d u c t o r s w i t h h i g h s p a t i a l r e s o l u t i o n . EBIC t e c h n i q u e s u s e an e l e c t r o n beam t o g e n e r a t e m i n o r i t y c a r r i e r s w i t h i n t h e s e m i c o n d u c t o r specimen.
These d i f f u s e t o a p-n j u n c t i o n o r S c h o t t k y b a r r i e r where t h e r e s u l t i n g c u r r e n t i s c o l l e c t e d . F o r t h e specimen geometry shown i n F i g . 1 t h e EBIC s i g n a l p r o d u c e d i s g i v e n by -
a ( r ) dxdy IEBIC
z=o
where D i s t h e c o e f f i c i e n t of d i f f u s i o n and P ( r ) i s t h e c o n c e n t r a t i o n of m i n o r i t y c a r r i e r s . Thus i t i s p o s s i b l e t o u s e EBIC t e c h n i q u e s t o s t u d y specimen p a r a m e t e r s which a f f e c t a P ( r ) a t t h e c o l l e c t i n g j u n c t i o n . T h e s e i n c l u d e measurements of .
m i n o r i t y c a r r i e r l i f e t i m e s 1 and d i f f u s i o n l e n g t h s 2 ' and i n v e s t i g a t i o n s i n t o t h e r e c o m b i n a t i o n of m i n o r i t y c a r r i e r s a t i n d i v i d u a l d e f e c t s . The s p a t i a l r e s o l u t i o n a v a i l a b l e f o r t h e l a t t e r i s a p p r o x i m a t e l y t h e same a s t h e s i z e of t h e g e n e r a t i o n volume c r e a t e d by t h e i n c i d e n t beam4. T h i s depends on t h e e n e r g y of t h e e l e c t r o n s
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983452
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as E 1 . 7 5 and by using energies less than lOkV spatial resolutions better than lum
may be achieved. This high spatial resolution enables individual, well character- ised dislocation segments to be studied on a sub-micron scale, thus making the EBIC mode of an SEM an excellent tool for investigation of recombination at dislocations.
it is the purpose of the present work to describe an EBIC system which has been developed principally for such an investigation. Details are also given of work undertaken to measure minority carrier diffusion lengths in the specimens containing dislocatiorsand why this is of relevance to the interpretation of the EBIC contrast produced by these dislocations.
2 - A COMPUTERISED LOCK-IN EBIC SYSTEM
( I . ) Requirements
This system has been developed to study recombination at dislocations. This is achieved by monitoring the EBIC signal generated at a Schottky barrier as the elec- tron beam of an SEM is scanned across a dislocation (Fig. 1). Recombination reduces the number of minority carriers reaching the barrier whilst the beam is in the vicinity of a dislocation and so reduces the EBIC signal collected. Thus by measur- ing the EBIC contrast produced by a dislocation defined as C = ( I B - I ~ ) / ~ B where ID and IB are the current at and away from the defect, it is possible to monitor the recombination of carriers at the defect.
For these measurements to be performed efficiently the following requirements should be met:
1. Good spatial resolution
2. Operation in the low excitation regime 3. Production of accurate data
4. Temperature control 5. Minimum contamination
6. Easy operation - should include "on-line" data analysis.
To achieve good spatial resolution accelerating voltages of 5-10kV must be used and this together with the small beam currents < 1 0 - " ~ , employed to maintain low exci- tation conditions, results in the production of very small EBIC signals. Some form of signal processing is therefore essential to reduce the noise present in the EBIC signal to be recorded. The dislocation should be scanned rapidly to minimise the effect on the measured contrast of drift in the EBIC signal. For this reason it is better, for example, to take five fast line-traces than one slow one. Digital recording of the signal is then advantageous as this can be both very fast and accurate compared with a chart recorder. Fas: scan speeds also help to minlmise contamination which is a problem at low temperatures if thevacuumat the specimen is
not < torr. Finally the system must be easy to operate if consistant results
are to be produced by a single operator. In particular when measurements are being made at varying temperatures computer control has been found particularly helpful.
(ii) System
The system developed is shown in Fig.2. The EBIC signal is encoded ready for phase sensitive detection by repetitively chopping the beam incident on the specimen typi- cally at 10kHz. Preliminary amplification takes place at a head amplifier whilst inside the microscope column. Final amplification is performed by a Brookdeal lock- in amplifier to further reduce the noise present in the signal to be recorded. The specimen is mounted on a liquid nitrogen coldstage which allows measurements to be made at temperatures between llOK and 400K. Overall control of the system and data co1;ection are performed by a PDP11/03 minicomputer.
The computer programs written to handle EBIC data colkction and processing include a number of features designed to enable easy user operation throughout. Each experi- ment is given a unique identifier and all data associated with that experiment con- tains that identifier as part of its key. Each experiment has a control file which contains details entered bythe user concerning the specimen, type of experiment, date and conditions. This file is updated as the experiment progresses. When an experiment is running, details of each run are collected. These include the data
late the x-coordinate of any data field in a data file. The sequence of dislocations being examined is also entered so that the analysis programs will recognise which dislocation are present on file. When the operator has the specimen in the correct position at the correct temperature it is only necessary to switch the microscope to line scan and press the photo button. This triggers data colkction from the out- put of the lock-in amplifier. The conputer sets the phase sensitive detection frequency and measures the specimen temperature and amplifier offset. Typical con- ditions would be to record 2000 data points at a collection rate of 700Hz whilst the microscope scans a given set of dislocations five times. The data colected is displayed and a preliminary measurement of EBIC contrast can be made. If the data is acceptable it is written to file - the first 10 fields containing the parameters measured or enetered by the operator, ready for access when the data is processed.
Only experimental details which have changed need to be entered for the next run and so in many cases it is unnecessary for the user to do anything more than wait for the specimen to reach the correct temperature, reposition it and trigger data collec- tion again.
Data analysis is automatic. To measure the complete set of contrast data for a par- ticular dislocation, the computer flrst determines which files contain EBIC scans of that dislocation - it does this using the dislocation sequence number entered at the time of the experiment. It then locates the position of the dislocation in these files. Rolling averages are calculated for the data fields corresponding to regions at and away from the effect. The maximum and minimum values of these averages are used to calculate the contrast for that scan - this is repeated for the other scans recorded in that run and these are then averaged. This has the effect of further reducing the noise and hence scatter on 'the EBIC contrast measurements calculated.
Files are created to hold these measurements and the temperature and frequency at whzch they were recorded. These contrast data files are then accessed by plot and display programs which present the data to the user in the form required. Examples are shown by Ourmazc / 5 / .
3 - DIFFUSION LENGTH MEASUREMENTS IN SILICON CONTAINING DEFORMATION INDUCED DISLOCATION NETWORKS
The system described has mostly been used for accurate measurement of the.EBIC con- trast from Individual dislocations. By investigating the way in which this contrast depends on temperature it is hoped to gain new information about the recombination of minority carriers at well characterlsed defects. However to be able to do this it is necessary to show that any changes in the measured EBIC contrast are due to real changes in the minority carrier lifetime at the defect and that they are not dependant on other specimen parameters which may be changing.
An approximate model for the EBIC contrast produced by a point defect has been developed by Donolato / 4 / . This model assumes a uniform, spherical generation volume and lf the effect of the surface Schottky barrier and resulting depletion region are neglected it can be used to describe the experiments we have performed, though for a point defect rather than a dislocatlon. Subject to these approximation Donolato has shown that the EBIC contrast produced is of the form
where T ' and T are minority carrier lifetimes at and away from the defect y is the strength of the defect
LD is the bulk diffusion length
r is the radius of the generation volume a is depth of the defect.
Tl!us it is necessary to know how LD, r and a behave durlng an experiment in order to attribute any change in C to a change in y . It is assumed here that r and a are constant for a given accelerating voltage and dislocatlon.
F19.3 shows the results of calculations based on Donolato's model for the dependance of EBIC contrast on bulk diffusion length for various representative defect depths
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and accelerating voltages. Clearly for changes in C to be attributed to changes in y it is necessary for either (i) the value of thediffusion length to remain large in comparison with r or (ii) for the diffusion length to remain constant over the temperature range investigated. Experiments were therefore performed to deter- mine the minority carrier diffusion length.
(i) Experimental
The diffusion length measurements were performed on one of the specimens used for the EBIC contrast experiments. This specimen was phosphorus doped, 10'' cm-' silicon
(kindly supplied by Prof H Alexander, University of Cologne). It contained disloca- tion networks produced by a two-stage compressive deformation along [213]. Pre- deformation at 850° was followed by further deformation at 420° resulting in a strain of 0.24%. The sample was cooled under load. The specimen was made from a (111) slice by polishing and cleaning followed by vacuum deposition of a thin layer of Au/Pd to form a Schottky diode. This specimen contained straight, well characterised screw and 60° dislocations.
The method used to determine the minority carrier diffusion lengths was that due to Wu and Wittry /3/. The beam of the JSM 35X SEM was used to generate minority carriers in the specimen as shown in Fig. 1. The efficiency of collection of the carriers was determined by measuring the beam current incident on the specimen and the EBIC signal it generated. A Keithly 602 electrometer was used for this purpose.
Measurements were made for accelerating voltages ranging from 5 to 39kV. The inci- dent electron beam was defocussed to 40 or 80pm and the beam power was kept to 0.5mW or less to ensure that the low excitation regime was maintained. Incident power was kept constant for all measurements in a given experiment.
Comparison with the theoretical efficiency of the barrier for various values of metal thickness, depletion region width and diffusion length was made. The parameters were chosen to give the best fit to the experimental data. The value of n, the ideality factor, for the Schottky barrier in the equation
I = ~!exp(=) nkt - 11
was also measured (ii) Results
Measurements were made in areas of low and high dislocation density at 300 and llOK.
The data produced is shown In figures 4 and 5 with the relevant theoretical curves superimposed. It is clear that the agreement between the values of measured EBIC efficiency and those predicted theoretically using a calculated value for depletion layer width is not good. However the theoretical treatment used assumes no recombina- tion within the depletion region, that is to say that the Schottky barriers should have an ideality factor of close to unity at room temperature. Measurement of n for the specimen investigated showed it to be 1.4 at 300K. This indicates that the Schottky barrier is far from ideal and that recombination will be taking place inside the depletion region. The limiting case of this would be when recombination inside the depletion region was the same as in the rest of the specimen. This is shown on.
the diagrams by the curve with depletion region width zero. It can be seen that the experimental data either lies on or between this curve and the ideal case.
Despite the non-ideal behaviour of the Schottky barrier, a value for the diffusion length can be obtained by considering the higher accelerating voltage portion of the curve where the effect, on the gradient of line, of recombination within the deple- tioq region is much less significant. In this way the results shown in table 1 have been obtained.
The low dislocation density area corresponds to a region approximately 70pm across where no dislocations could be detected using an accelerating voltage of 39kV to generate an EBIC image. The centre area 40pm across was used to make the diffusion length measurement. In the high dislocation density region it was difficult to make an estimate of the density but this was judged to be between 10' and lo8 cmeZ over the 70pm region measured.
beam Schottky barrier
\ Generation Volume (
Fig.1-Schematic diagram of specimen geometry used f o r E B I C c o n t r a s t and d i f f u s i o n l e n g t h measurements.
0 2 'i B
omus&lenem(~lm) Fig.3-Theoretical curves f o r EBIC c o n t r a s t v e r s u s d i f f u s i o n l e n g t h f o r a p o i n t d e f e c t . Curves c a l c u l a t e d f o r d i f f e r e n t d e f e c t depths.
o 10 20 a YO Y]
Accelerating voltage (KV]
Fig.5-As Fig.4 b u t f o r high d i s l o c a t i o n :!ensity region. A l l curves show L =
1 . 2 ~ m . D
Fig.2-Schematic diagram of t h e E B I C system.
! Depletrm reglm width (d 0
Fig.6-Curves showing t h e e f f e c t of a 15X change i n LD on t h e EBIC c o l l e c t i o n e f f i c i e n c y .
1 ' + . d
0 10 20 30
~cceleratina %Itage (Kt8 Fig.4-EBIC c o l l e c t i o n e f f i c i e n c y v e r s u s a c c e l e r a t i n g v o l t a g e f o r a low
d i s l o c a t i o n d e n s i t y region. The curves correspond t o LD = 7pm.
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Table 1 - Summary of the results produced by the diffusion length experiments.
The absolute error of the diffusion length measurements is probably greater than 25%, mainly due to the non-ideal nature of the collecting junction. However the inaccuracy involved in determining a small change in diffusion length is much less because most of the errors are the same for each measurement. It is thus posslble to say that any change in diffusion length between lOOK and 300K is less than 15% for both areas measured.
Dislocation density
Low Low High High
The measurements show that both in a region of high and of low dislocation density the minority carrier diffusion length, LD, did not change by more than 15% between llOK and 300K. It is reasonable to apply Donolato's model for contrast from a point defect to this result in order to determine, approximately, the effect that a 15%
change in LD might have on the dislocation contrast measured. The value of LD is close to 7pm in the low dislocation density regions of the specimen used for EBIC contrast and here according to the model a change of 15% would produce a change in EBIC contrast of 2. 2% for the imaging conditions used. The implication of this for the EBIC contrast measurements made on dislocations as a function of temperature is that the changes in contrast detected, s 5 0 % , are not due to changes in bulk diffu- sion length.
L, (pm)
7.0 7.0 1.2 1.2 Temperature
(K)
300 110 300 110
The diffusion length measurements also show, as might be expected, that the value of LD is smalkr for regions of high dislocation density than for regions of low disloca- tion density. It may be deduced that as LD is independent of temperature, whilst the EBIC contrast of dislocations does show a strong temperature dependence, that LD is not determined by the dislocations themselves but rather by some other recombina- tion centre. It also follows that as LD is smaller in regions of high dislocation density the concentration of these centres is higher here also. This is not surpris- ing as these areas are likely to contain a greater concentration of point defects and deformation debris which will behave as centres for recombination.
Thickness .
of banier(A1 130 130 70 70
ACKNOWLEDGEMENTS
The authors express their gratitude to Professor H Alexander, DrS E Weber and
H Gottschalk of Cologne University, FRG, for the supply of deformed samples and their interest in this work. Financial support from the Science and Engineering Research Council of Great Britain is acknowledged.
REFERENCES
/1/ HUNTER, D.R., PAXMAN, D.H., BURGERSS, M. and BOOKER, G.R., 1nst.Phys.Conf.Ser.
18 (1973) 208.
-
/2/ DAVIDSON, S.M and DIMITRIADIS, C.A., J.MlcroSc. 118 lii (1980) 275.
/3/ WU, C. J. and WITT-ZY, !;.3., J.Appl.Phys. 49 (1978) 2827.
/4/ DONOLATO, C., Opti:; 52 (1978) 19.
/5/ OURMAZD, A. , WILSHAW,'P.R. and BOOKER, G.R. , this conference.
