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Characterization of macroscopic defects in silicon after processing for CMOS and bipolar circuits
MateriaLs Science and Engineering, B4 (1989) 373-376
373
Characterization of Macroscopic Defects in Silicon After Processing for CMOS and Bipolar Circuits J. W. STEEDS and F. J O H N S O N
H. H. Wills Physics Laboralorv, University of Bristol, Bristol B,%' 1 FL (U. K.) M. B. SIMPSON
(;E(' Hirst Research ('entre, East Lane, Wembh,y, Middh'sex HA ~) 71'P (U. K.)
P. D. AUGUSTUS PlessQ' Research (Caswell) lSrnited, Allen Clark Research ('entre, Fowcester, Northants NNI2 8EQ dS.K.)
',Received May 3(1, 1989)
Abstract
We have been engaged in a systematic and carefitl study of the early stages of oxygen precipitation at temperatures up to 850 °C which involved transmission electron microscopy correlated with diffraction, optical and 1R spectroscopy. We have simultaneously made a detailed study of macroscopic defects induced in Czochralski silicon wafers q/?er repeated heat cycfing during the fi~brication of circuits. Even though there have been a number of such studies there remain many questions about the effects which are observed, such as why do smoking ]'aults sometimes occur while" on other occasions prismatic' loop punching predominates ? Variable precipitate morphologies are also observed such as/tO0~ plates or octahedra without any general agreement on the reasons for such differences. The question of how precipitates nucleated at lower temperatures grow attd develop after repeated high temperature cycles has also not ()]'ten been addressed. Further questions exist over the heterogeneous or homogeneous nucleation of precipitates, the involvement of transition metals, the un]'aulting of stacking fau# loops. Answers to many of these questions arise out of our work which seeks to provide an over-view of the variety ()]'oxygen precipitation routes in silicon.
apparent discrepancies in the literature attest to the complexity of this problem. In the present study we have taken a somewhat different point of view from many and concentrated our efforts on two wafers which have been through complete processing schedules, one for bipolar technology, the other for complementary metal-oxide-semiconductor (CMOS). The samples were chosen for their rich precipitation behaviour and because they exhibited predominantly different types of defects: the bipolar processed sample contained large numbers of stacking fault loops, whereas the CMOS sample contained high densities of prismatic punched out loops. We have attempted to draw general conclusions from the rich variety of complex precipitation effects which we observed and to rationalize our results. In this respect, we have been assisted by close involvement with parallel, more basic, studies of oxygen precipitation at lower temperatures, and undertaken with much greater experimental control. In this work, early stages of precipitation were monitored by Fourier transform IR spectroscopy, neutron diffraction and high resolution electron microscopy. Results of these investigations have appeared in the literature [1-4]. Some of our work on the CMOS-processed material has already been accepted for publication [5].
1. Introduction
2. Experimental background
There have been many previous studies on the nature of oxygen precipitation in bulk silicon heat treated at temperatures in the region 9501200 °C. The lack of clear conclusions and the
The bipolar sample was an n-type (Sb-doped) {001} Czochralski silicon wafer with a low carbon content ( C < S x l 0 1 5 cm -3) and an oxygen content of about 8 x 1() j: cm ~. The CMOS
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374
sample was an n-type (2 ~ cm--P-doped){001} Czochralski silicon wafer with a low carbon content ([C,] < 0.15 ppma) and an oxygen content of about 7 x 1()~7 cm -~. The interstitial oxygen concentrations were calculated either from the absorption coefficient values at 300 K according to the current ASTM standard El 21-83, or using the relationship given by Pajot [6] for those measured at 4.2 K. The carbon concentration in the unprocessed wafers was determined both at the centre and close to the wafer edge by Fourier transform absorption measurement at 77 K, using ASTM F123-83. The wafers were from different suppliers. Samples were prepared from these wafers for examination by transmission electron microscopy and examined in a 300 kV instrument equipped with facilities for energydispersive X-ray analysis and electron energy loss spectroscopy.
Fig. I. ltansmission clccmm mtc~t~g~aph oi a i l l : >cctiolh ol the bipolar processed ~atcr sh,~wmg the complex cemrc ~1 a large stacking fault loop ~diamc~cJ -- 10 tan). In particular. the annular ring of precipitate wi!h about 2/_,rll diameter and 1he hexagonal nature ~f thc core sh~,~uldbe noted.
3. Experimental results Both the bipolar and CMOS-processed wafers contained high densities of bulk defects. The bipolar wafer contained predominantly large extrinsic stacking fault loops across the whole wafer, while the CMOS wafer contained a high density of prismatic punched-out loops in a ring 5 mm wide at its edge with a low density at its centre. In spite of this predominance of one type of precipitation there were considerable numbers of punched-out loops in the bipolar wafer, often in close proximity to the stacking fault loops, and a few stacking fault loops in the CMOS wafer. Repeated heat cycling led to complex defect structures, the details depending on whether stacking fault loops or prismatic loops were the first to form. The heat treatment of the bipolar wafer commenced with two periods of 2 h, first at 1000°C and next at 1025°C. This gave rise to stacking fault loops with central precipitates encircled by thick annular rings of oxide. This oxide was nucleated on the terminating Frank partials at lower temperatures, later in the processing cycle (Fig. 1 ). The single largest period of loop growth occurred during 1 h at 1200 °C, and the Frank loop thus formed also became quite heavily decorated with oxygen precipitates. The later behaviour of the loop growth was variable. As a result of the pinning of the loop sometimes only small segments broke away on further high temperature treatment. The resulting curved segments themselves became decorated at lower
Fig. 2. lransmission electron microgvaph ol a qandard I()01 section of the bipolar wafer showing the delicate tracery formed by oxygen precipitation of successive positions of a growing Frank dish)cation terminating an exU-insic stacking fau]l loop.
temperatures, leading to a beautiful record of the progress of the loop growth (Fig. 2). In other cases the inner oxide annulus acted as a nucleus for new stacking fault loops which grew on top of the original fault. In yet other cases the 1200 °C loop perimeter acted as a nucleus for further loops, while in others still the original Frank loop continued to expand without nucleation of new faults. This variety of behaviour led to the richness of the details of the observations. In the case of prismatic punched-out loops, the loops themselves became decorated with oxygen precipitates. This precipitation took a variety of forms, either forming simple annular rings on the dislocation loops or forming small {100} platelets which distorted the shape of the loop. Occasionally the {110} plane of the loop was converted to a complete plate-like precipitate or a set of loops was converted to a set of regular extended octahedra (Fig. 3). At later high temperature excursions the original dislocation loop could break away from the decorating precipitates or else the precipitated loops became a source of secondary prismatic punching.
375
Fig. 3. Weak beam dark field transmission electron micrograph of a set of elongated octahedra formed on a line of prismaticallypunched-out dislocationloops. The micrograph was obtained in a {100} orientation of the bipolar wafer and the elongatedoctahedra are aligned along ~011). Occasional unusual observations were made which were of considerable interest. A few stacking fault loops were found where the nucleating central precipitate was also the origin of a line of prismatic loops. Conversely, a decorated prismatic loop occasionally became the origin of a stacking fault loop rather than secondary prismatic loops. On the occasions when this observation was made it was the last loop (farthest from the origin) in a sequence which generated the stacking fault loop and the stacking faults intersected the plane of the wafer perpendicular to the direction of prismatic punching. 4. Discussion of results
It appears that a rather delicate balance exists between stacking fault loop creation and prismatic punching. In any particular region of a wafer one of these processes initiates and then persists except in the very few exceptions which have been cited. The question of the choice between these two mechanisms does not appear to have been addressed in the previous literature but is nevertheless of some interest. Traditionally, prismatic punching has been considered by analogy with the related situation in metals where it is concerned with stress relief at a misfitting precipitate. In contrast, stacking fault nucleation occurs to relieve the supersaturation of excess interstitial atoms. However, the situation under consideration here is somewhat different in that both the stress and the excess interstitials exist and both of the observed mechanisms can be seen as a response to them. A loop nucleated under the influence of local stress and interstitial atom supersaturation can be either a glide or a climb loop, the climb loop can be perfect or
imperfect (Frank loop). Screw segments of a glide loop are likely to cross slip and lead to prismatic punching according to a well-known mechanism; a perfect climb loop would glide along its glide cylinder away from its nucleation point under the influence of local stresses. The Frank loop is however the loop which has the lowest elastic energy of nucleation on account of the small Burgers vector of the associated dislocations. Its growth produces the stacking fault loop. In considering the question of the choice of mechanism there are a number of important points to take into account. First, silica is a marvellous solvent and it is, as may be deduced from reference to standard texts [7 l, one of the best solvents known. There is abundant:evidence that it absorbs metal contaminant vigorously (the most recent work to come to our attention is that of Falster [8]). No doubt many other impurities are also absorbed. When we also take into account the well-known ability for metal contaminants to catalyse oxygen precipitation in the first place (see ref. 5 for example) we see that there exists good evidence for high impurity level in the precipitates. The second important point to bear in mind is that the glass transition temperature is critically dependent on the impurities contained in the glass. It is therefore reasonable to suppose that individual oxygen precipitates will pass through glass transitions at different temperatures dependent on the impurities which they have acquired. Once above the glass transition much greater expansion of the material within the precipitate will occur, creating internal stresses in the lattice large enough to generate dislocation loops. The critical temperature region for loop punching or stacking fault creation is therefore in the range 900-1100°C. Moreover, the higher the impurity content within, the greater is likely to be its expansion coefficient. The impurity level and type in the precipitate can also be expected to have an important influence on the precipitate morphology. This morphology will in turn determine the nature of the stress fields generated in the silicon lattice and the nature of the dislocations created to relieve them. Dislocation mobility is itself affected by the impurity level and type in the surrounding matrix. For example, recent work has revealed that the brittle-ductile transition temperature is dependent on the oxygen and hydrogen level in the silicon and the state of oxygen aggregation [9].
376
It would seem from the factors outlined above that there is ample scope to explain the variability evident in our observations on precipitation in silicon. The envisaged situation during high temperature heat treatment may be summarized as fob lows. Oxygen precipitation at lower temperatures has produced a large excess of interstitial silicon atoms. As the temperature rises above 900 °C the amorphous oxide in the precipitates passes through its glass transition temperature and consequently into a region where its expansion is considerably greater than that of the surrounding silicon matrix. The actual stress fields generated by the expansion will be critically dependent upon the precipitate morphology and, at some point, dislocations are nucleated. These relieve the associated stress fields and absorb some of the excess interstitial atoms. Extrinsic stacking fault loops, once nucleated, grow by climb, while prismatic punched-out loops carry interstitial atoms away from their source by glide. As a broad generalization it can therefore be anticipated that higher processing temperatures will favour the climb process relative to glide which is less temperature dependent. We therefore conclude that although the precipitate morphology will play an important role in deciding whether stacking faults loops or prismatic loops are formed, the effect of high processing temperatures will tend to facilitate the former. On cooling again, the oxygen solubility will decrease and the newly created dislocations will provide nucleating centres for the formation of decorating precipitates. Our view that the precipitate morphology has an important effect on the later choice between
stacking fault loop creation or prismatic punching was formed as a result of studying the shape of the nucleating precipitates. Although irregular, it was noted that the stacking fault loops often had a flat {l 11} plate at their centres. This prompted the study of the 85(1°C annealed samples from Reading University. Much to our surprise, !111 plates were also discovered in these samples, an observation which does not appear to have been reported previously. While it is not claimed here that these {11 l} plates arc tile' sole source ol extrinsic stacking fault loops, i! does seem x,erv likely that they are one very good reason fol favouring their formation.
References I M. Claybourn and R. (7. Newman, Appl. Phys. Lett., 5,2 (1988)2139-2141. 2 R.C. Newman, Fall Meeting, Boston. MA, December 1987. Proc. Mater. Res. Soc. Sy'mp., 1(t4. 3 F.M. Livingston, S. Messoloras, R. (2. Newman, B. C. Pike, R. J. Stewart. M. J. Binns, W. R Brown and J. G. Wilkes, J. Phys. C 17 (1984)6253. 4 W. Bergholz, M. J. Binns, G. R. Booker, J. C. Hutchison, S. H. Kinder, S. Messoloras, R. C. Newman, R. J. Stewart and J. G. Wilkes, Philos. Mag., B59 (1989) 499. 5 M. B. Simpson, R J. Halfpenny, M. A. Emmou, J. Brown, J. W. Steeds, F. Johnson and A Brinklow, Sernicond. Sci. Fechnol., in the press. 6 B. Pajot, Analysis, 5 (7) (1977) 293. 7 }-br example, C. T. Lynch (cd.;. Handbook of Materials Science, Vol. II, Metals ('omposites and Rej)'actory Materials, CRC Press, Cleveland. OH, 1975. 8 R. Falster, Proc. 6th ltll. ,~,rttp. ovt the Structure and l'ropenies of Dislocations m Semiconductors, Oxfi>rd, April 1989, Institute of Physics, in the press. 9 R. Behrensmeier, M. Brede and R Haasen, Scr. Metall., 2t 1 9 8 7 ) 1581.
373
Characterization of Macroscopic Defects in Silicon After Processing for CMOS and Bipolar Circuits J. W. STEEDS and F. J O H N S O N
H. H. Wills Physics Laboralorv, University of Bristol, Bristol B,%' 1 FL (U. K.) M. B. SIMPSON
(;E(' Hirst Research ('entre, East Lane, Wembh,y, Middh'sex HA ~) 71'P (U. K.)
P. D. AUGUSTUS PlessQ' Research (Caswell) lSrnited, Allen Clark Research ('entre, Fowcester, Northants NNI2 8EQ dS.K.)
',Received May 3(1, 1989)
Abstract
We have been engaged in a systematic and carefitl study of the early stages of oxygen precipitation at temperatures up to 850 °C which involved transmission electron microscopy correlated with diffraction, optical and 1R spectroscopy. We have simultaneously made a detailed study of macroscopic defects induced in Czochralski silicon wafers q/?er repeated heat cycfing during the fi~brication of circuits. Even though there have been a number of such studies there remain many questions about the effects which are observed, such as why do smoking ]'aults sometimes occur while" on other occasions prismatic' loop punching predominates ? Variable precipitate morphologies are also observed such as/tO0~ plates or octahedra without any general agreement on the reasons for such differences. The question of how precipitates nucleated at lower temperatures grow attd develop after repeated high temperature cycles has also not ()]'ten been addressed. Further questions exist over the heterogeneous or homogeneous nucleation of precipitates, the involvement of transition metals, the un]'aulting of stacking fau# loops. Answers to many of these questions arise out of our work which seeks to provide an over-view of the variety ()]'oxygen precipitation routes in silicon.
apparent discrepancies in the literature attest to the complexity of this problem. In the present study we have taken a somewhat different point of view from many and concentrated our efforts on two wafers which have been through complete processing schedules, one for bipolar technology, the other for complementary metal-oxide-semiconductor (CMOS). The samples were chosen for their rich precipitation behaviour and because they exhibited predominantly different types of defects: the bipolar processed sample contained large numbers of stacking fault loops, whereas the CMOS sample contained high densities of prismatic punched out loops. We have attempted to draw general conclusions from the rich variety of complex precipitation effects which we observed and to rationalize our results. In this respect, we have been assisted by close involvement with parallel, more basic, studies of oxygen precipitation at lower temperatures, and undertaken with much greater experimental control. In this work, early stages of precipitation were monitored by Fourier transform IR spectroscopy, neutron diffraction and high resolution electron microscopy. Results of these investigations have appeared in the literature [1-4]. Some of our work on the CMOS-processed material has already been accepted for publication [5].
1. Introduction
2. Experimental background
There have been many previous studies on the nature of oxygen precipitation in bulk silicon heat treated at temperatures in the region 9501200 °C. The lack of clear conclusions and the
The bipolar sample was an n-type (Sb-doped) {001} Czochralski silicon wafer with a low carbon content ( C < S x l 0 1 5 cm -3) and an oxygen content of about 8 x 1() j: cm ~. The CMOS
0921-5107/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
374
sample was an n-type (2 ~ cm--P-doped){001} Czochralski silicon wafer with a low carbon content ([C,] < 0.15 ppma) and an oxygen content of about 7 x 1()~7 cm -~. The interstitial oxygen concentrations were calculated either from the absorption coefficient values at 300 K according to the current ASTM standard El 21-83, or using the relationship given by Pajot [6] for those measured at 4.2 K. The carbon concentration in the unprocessed wafers was determined both at the centre and close to the wafer edge by Fourier transform absorption measurement at 77 K, using ASTM F123-83. The wafers were from different suppliers. Samples were prepared from these wafers for examination by transmission electron microscopy and examined in a 300 kV instrument equipped with facilities for energydispersive X-ray analysis and electron energy loss spectroscopy.
Fig. I. ltansmission clccmm mtc~t~g~aph oi a i l l : >cctiolh ol the bipolar processed ~atcr sh,~wmg the complex cemrc ~1 a large stacking fault loop ~diamc~cJ -- 10 tan). In particular. the annular ring of precipitate wi!h about 2/_,rll diameter and 1he hexagonal nature ~f thc core sh~,~uldbe noted.
3. Experimental results Both the bipolar and CMOS-processed wafers contained high densities of bulk defects. The bipolar wafer contained predominantly large extrinsic stacking fault loops across the whole wafer, while the CMOS wafer contained a high density of prismatic punched-out loops in a ring 5 mm wide at its edge with a low density at its centre. In spite of this predominance of one type of precipitation there were considerable numbers of punched-out loops in the bipolar wafer, often in close proximity to the stacking fault loops, and a few stacking fault loops in the CMOS wafer. Repeated heat cycling led to complex defect structures, the details depending on whether stacking fault loops or prismatic loops were the first to form. The heat treatment of the bipolar wafer commenced with two periods of 2 h, first at 1000°C and next at 1025°C. This gave rise to stacking fault loops with central precipitates encircled by thick annular rings of oxide. This oxide was nucleated on the terminating Frank partials at lower temperatures, later in the processing cycle (Fig. 1 ). The single largest period of loop growth occurred during 1 h at 1200 °C, and the Frank loop thus formed also became quite heavily decorated with oxygen precipitates. The later behaviour of the loop growth was variable. As a result of the pinning of the loop sometimes only small segments broke away on further high temperature treatment. The resulting curved segments themselves became decorated at lower
Fig. 2. lransmission electron microgvaph ol a qandard I()01 section of the bipolar wafer showing the delicate tracery formed by oxygen precipitation of successive positions of a growing Frank dish)cation terminating an exU-insic stacking fau]l loop.
temperatures, leading to a beautiful record of the progress of the loop growth (Fig. 2). In other cases the inner oxide annulus acted as a nucleus for new stacking fault loops which grew on top of the original fault. In yet other cases the 1200 °C loop perimeter acted as a nucleus for further loops, while in others still the original Frank loop continued to expand without nucleation of new faults. This variety of behaviour led to the richness of the details of the observations. In the case of prismatic punched-out loops, the loops themselves became decorated with oxygen precipitates. This precipitation took a variety of forms, either forming simple annular rings on the dislocation loops or forming small {100} platelets which distorted the shape of the loop. Occasionally the {110} plane of the loop was converted to a complete plate-like precipitate or a set of loops was converted to a set of regular extended octahedra (Fig. 3). At later high temperature excursions the original dislocation loop could break away from the decorating precipitates or else the precipitated loops became a source of secondary prismatic punching.
375
Fig. 3. Weak beam dark field transmission electron micrograph of a set of elongated octahedra formed on a line of prismaticallypunched-out dislocationloops. The micrograph was obtained in a {100} orientation of the bipolar wafer and the elongatedoctahedra are aligned along ~011). Occasional unusual observations were made which were of considerable interest. A few stacking fault loops were found where the nucleating central precipitate was also the origin of a line of prismatic loops. Conversely, a decorated prismatic loop occasionally became the origin of a stacking fault loop rather than secondary prismatic loops. On the occasions when this observation was made it was the last loop (farthest from the origin) in a sequence which generated the stacking fault loop and the stacking faults intersected the plane of the wafer perpendicular to the direction of prismatic punching. 4. Discussion of results
It appears that a rather delicate balance exists between stacking fault loop creation and prismatic punching. In any particular region of a wafer one of these processes initiates and then persists except in the very few exceptions which have been cited. The question of the choice between these two mechanisms does not appear to have been addressed in the previous literature but is nevertheless of some interest. Traditionally, prismatic punching has been considered by analogy with the related situation in metals where it is concerned with stress relief at a misfitting precipitate. In contrast, stacking fault nucleation occurs to relieve the supersaturation of excess interstitial atoms. However, the situation under consideration here is somewhat different in that both the stress and the excess interstitials exist and both of the observed mechanisms can be seen as a response to them. A loop nucleated under the influence of local stress and interstitial atom supersaturation can be either a glide or a climb loop, the climb loop can be perfect or
imperfect (Frank loop). Screw segments of a glide loop are likely to cross slip and lead to prismatic punching according to a well-known mechanism; a perfect climb loop would glide along its glide cylinder away from its nucleation point under the influence of local stresses. The Frank loop is however the loop which has the lowest elastic energy of nucleation on account of the small Burgers vector of the associated dislocations. Its growth produces the stacking fault loop. In considering the question of the choice of mechanism there are a number of important points to take into account. First, silica is a marvellous solvent and it is, as may be deduced from reference to standard texts [7 l, one of the best solvents known. There is abundant:evidence that it absorbs metal contaminant vigorously (the most recent work to come to our attention is that of Falster [8]). No doubt many other impurities are also absorbed. When we also take into account the well-known ability for metal contaminants to catalyse oxygen precipitation in the first place (see ref. 5 for example) we see that there exists good evidence for high impurity level in the precipitates. The second important point to bear in mind is that the glass transition temperature is critically dependent on the impurities contained in the glass. It is therefore reasonable to suppose that individual oxygen precipitates will pass through glass transitions at different temperatures dependent on the impurities which they have acquired. Once above the glass transition much greater expansion of the material within the precipitate will occur, creating internal stresses in the lattice large enough to generate dislocation loops. The critical temperature region for loop punching or stacking fault creation is therefore in the range 900-1100°C. Moreover, the higher the impurity content within, the greater is likely to be its expansion coefficient. The impurity level and type in the precipitate can also be expected to have an important influence on the precipitate morphology. This morphology will in turn determine the nature of the stress fields generated in the silicon lattice and the nature of the dislocations created to relieve them. Dislocation mobility is itself affected by the impurity level and type in the surrounding matrix. For example, recent work has revealed that the brittle-ductile transition temperature is dependent on the oxygen and hydrogen level in the silicon and the state of oxygen aggregation [9].
376
It would seem from the factors outlined above that there is ample scope to explain the variability evident in our observations on precipitation in silicon. The envisaged situation during high temperature heat treatment may be summarized as fob lows. Oxygen precipitation at lower temperatures has produced a large excess of interstitial silicon atoms. As the temperature rises above 900 °C the amorphous oxide in the precipitates passes through its glass transition temperature and consequently into a region where its expansion is considerably greater than that of the surrounding silicon matrix. The actual stress fields generated by the expansion will be critically dependent upon the precipitate morphology and, at some point, dislocations are nucleated. These relieve the associated stress fields and absorb some of the excess interstitial atoms. Extrinsic stacking fault loops, once nucleated, grow by climb, while prismatic punched-out loops carry interstitial atoms away from their source by glide. As a broad generalization it can therefore be anticipated that higher processing temperatures will favour the climb process relative to glide which is less temperature dependent. We therefore conclude that although the precipitate morphology will play an important role in deciding whether stacking faults loops or prismatic loops are formed, the effect of high processing temperatures will tend to facilitate the former. On cooling again, the oxygen solubility will decrease and the newly created dislocations will provide nucleating centres for the formation of decorating precipitates. Our view that the precipitate morphology has an important effect on the later choice between
stacking fault loop creation or prismatic punching was formed as a result of studying the shape of the nucleating precipitates. Although irregular, it was noted that the stacking fault loops often had a flat {l 11} plate at their centres. This prompted the study of the 85(1°C annealed samples from Reading University. Much to our surprise, !111 plates were also discovered in these samples, an observation which does not appear to have been reported previously. While it is not claimed here that these {11 l} plates arc tile' sole source ol extrinsic stacking fault loops, i! does seem x,erv likely that they are one very good reason fol favouring their formation.
References I M. Claybourn and R. (7. Newman, Appl. Phys. Lett., 5,2 (1988)2139-2141. 2 R.C. Newman, Fall Meeting, Boston. MA, December 1987. Proc. Mater. Res. Soc. Sy'mp., 1(t4. 3 F.M. Livingston, S. Messoloras, R. (2. Newman, B. C. Pike, R. J. Stewart. M. J. Binns, W. R Brown and J. G. Wilkes, J. Phys. C 17 (1984)6253. 4 W. Bergholz, M. J. Binns, G. R. Booker, J. C. Hutchison, S. H. Kinder, S. Messoloras, R. C. Newman, R. J. Stewart and J. G. Wilkes, Philos. Mag., B59 (1989) 499. 5 M. B. Simpson, R J. Halfpenny, M. A. Emmou, J. Brown, J. W. Steeds, F. Johnson and A Brinklow, Sernicond. Sci. Fechnol., in the press. 6 B. Pajot, Analysis, 5 (7) (1977) 293. 7 }-br example, C. T. Lynch (cd.;. Handbook of Materials Science, Vol. II, Metals ('omposites and Rej)'actory Materials, CRC Press, Cleveland. OH, 1975. 8 R. Falster, Proc. 6th ltll. ,~,rttp. ovt the Structure and l'ropenies of Dislocations m Semiconductors, Oxfi>rd, April 1989, Institute of Physics, in the press. 9 R. Behrensmeier, M. Brede and R Haasen, Scr. Metall., 2t 1 9 8 7 ) 1581.