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Chemical and thermal etching of Se-Te whisker crystals
Journal of Crystal Growth 37 (1977) 23 28 © North-Holland Publishing Company
CHEMICAL AND THERMAL ETCHING OF Se—Te WHISKER CRYSTALS V.P. BHATT and S.B. TRIVEDI Physics Department, Faculty of Science, MS. University of Baroda, Baroda, India Received 14 July 1976
A new chemical etchant (saturated solution of iodine in methanol) capable of revealing the sites of emergence of dislocations on the prism faces of Se 90 Te10 whisker crystals grown from the vapour phase is reported. A systematic detailed study otthermal etching of whisker crystals at various temperatures and the period of etching is also reported. The results of thermal etching are compared with those of chemical etching. Various experiments are performed and it has been shown that the thermal pits are not formed at dislocation sites but are probably due to impurities in the crystal.
1. Introduction
the present paper a new chemical etchant capable of revealing dislocations intersecting the prism planes of Se Te whisker crystals and a detailed study of therma! etching of these crystals, are reported. The results of thermal etching are compared with those of chemical etching. It has been shown that the thermal pits are not formed at dislocation sites but are probably due to impurities.
The wide range applications of semiconductor devices have increased the demand of semiconducting crystals of very high perfection. Various methods are used to assess the perfection of crystals. Chemical etching is an established technique and because of its simplicity is widely used to determine the dislocation density in the crystals. Formation of thermal etch pits and a one-to-one correspondence between these pits and dislocations have become a subject of great interest and importance in recent years. Many workers have reported the study of thermal pits on metals, alloys, semiconducting and ionic crystals. Only in limited cases the correspondence between thermal pits and dislocations is reasonably established. In some cases [1 8] the thermal pits are correlated with dislocations whereas in others [9 12], no correspondence is observed between thermal pits and dislocations, Doherthy et al. [13] have shown in the case of aluminium, that the thermal pits are formed due to the vacancy condensation mechanism. Fisher et al. [14], in the case of indium antimonide, correlated thermal pits with the gettering action of indium used as a solvent and diluent for the cadmium diffusion source, Little information [15] is available in the literature on chemical etching of Se—Te crystals. The present authors have not come across any reference in literature on thermal etching of Se—Te crystals. In
2. Experimental Selenium and tellurium of 99.999% purity, supplied by Special Materials Plant, Hyderabad, India, were used for the preparation of Se Te alloy containing 90 wt% of selenium. The whisker crystals used in this study were grown from this alloy using the technique reported earlier by the present authors [15]. Saturated solution of iodine in methanol was found to be an etchant capable of revealing dislocations on the prism faces of Se90 Te10 whisker crystals. Freshly grown crystals with well developed, smooth faces were etched in this solution for 30 35 sec. This was followed by first washing the crystal in methanol, then washing in ethanol and finally drying in air. For thermal etching, freshly grown crystals were vacuum sealed (~wl0 ~ Torr) in a small thin-walled pyrex tube. The tube was then inserted in the precalibrated resistance furnace kept at the desired tem23
24
I
.!~
B/,att, 5.11. Trivsdz
/ 1’ rehing oJ 5e
L
Hg. 1. Chemical etch pattern on the prism face of a Se Te whisker crystal (X920).
perature. The crystals were thermally etched for a specific time and then the current from the furnace was switched off and the furnace was allowed to cool slowly to the room temperature. The etched samples were then carefully removed from the tube and ob. servations were made on a Vicker’s projection microscope.
Ic whis/~r rTstal,v
The etchant pioduces well defined pits which tetam their shape while growing largei on further etching, figs. 2a and 2b. This clearly shows that the pits are at the sties of emergence of dislocations. A further test for the reliability of an etchant was made by indenting the crystal by a sharp needle followed by etching. A cluster of pits near the indentation mark and the alignment of pits along slip tiaces are clearly visible (fig. 3). Further evidence for the ability of the etchant to reveal dislocations was obtained by etching the crystal first in the saturated solution of iodine in methanol, fig. 4a, and then in the dislocation etchant [1
part cone,
HNO3 (70%) + 4
parts conc.
112S04
(70%)] reported earlier by the present authors [151,
fig. 4b. Comparing the two photographs, it is clearly seen that there is no increase in the number of pits
and a one-to-one correspondence in the position of pits is observed. It is also clearly visible that the pits shown in fig. 4a have grown larger in size and have
taken the shape of a truncated triangle. The results of pit density and distribution obtained for the whisker crystals etched in the present etchant are in good agreement with those etched using previously ye ported etchant. This shows that the etchant is capable of revealing the sites of emergence of dislocations.
3. Results and discussions
3.2. Thermal etching
3.1. Chemical etching
Freshly grown whisker crystals with well developed plane prism faces were thermally etched in the temperature range 180 200°C for 10 40 nun and the results are reported below. When the crystals were etched at 180°Cfor 10 mm. very small etch pits of undefined shape were ohserved. On increasing the etching time to 20 mm di-
The chemical etch produces point bottomed triangular pits on the prism faces of whisker crystals. Fig. 1 shows the random distribution and a row of etch pits. One of the sides of the pit is almost parallel to the c-axis.
-~~--
.‘‘‘-~
,..---. -
—
—.
k
~1,
I
S
‘1 (a)
(b)
Fig. 2. (a) Ftch pits on the prism face of a Se Te whisker crystal (X660). (b) The same pits on further etching (X660).
V.P. Bhatt, S.B. Trivedi/Etching of Se Te whisker crystals
_
Hg 3. 1 mcli pattern near indentation mark (/160).
hedral pits with rectangular out lines were observed as shown in fig. 5. The shape of the thermal pits at higher magnification (fig. 6) clearly shows the development of the extra triangular facets at the ends of the pits. On further etching for an additional 10 mm, the dihedral pits were found to be converted into boat shaped pits. Etching at 190°Cfor 10 mm resulted in the development of small dihedral pits. On increasing the etching time to 20 mm, large boat shaped pits randomly distributed on the surface were observed as shown in fig. 7a. It is interesting to note that one edge of a pit is almost parallel to the c-axis, and the pits are of assorted size. When the same crystal face was etched for 30 mm, practically all the pits grew larger in size and some new pits were also formed as shown in fig. 7b. When the crystals were etched at 200°Cfor 10 mm, boat shaped, egg shaped and pits bounded by four rectilinear edges were observed as shown in fig. 8a. On prolonged etching the pits grew larger in size but became more and more shallow, The details of thermal etching at various temperatures and etching period are summarized in table 1. The salient features of the above observations are the following,
~
~
(a)
25
~
-~
.~
11g. 5. Thermal etch pits oii the prism whisker crystal etched at 180°C(X925).
face
of a Sc
Ie
(i) No visible pit formation is observed on crystals etched at temperatures below 180°Cfor a long time. (ii) Etching the crystals at 180 and 190°Cresult mitiahly in dihedral pits which are converted into boat shape on prolonged etching. At 200°Cinitially boat shaped, egg shaped and pits bounded by four rectilinear edges are observed which on further etching are converted in td boat shaped pits. (iii) In every run, on prolonged etching, the size of boat shaped pits is found to increase, predominantly along the c-axis and the pits become more and more shallow. To ascertain whether the pits are formed at dislocations or not, a thermally etched crystal, fig. 8a, was chemically etched in the dislocation etchant (saturated solution of iodine in methanol), fig. 8b. Comparing the two photographs it is clearly observed that the density of the etch pits produced on chemical etching is very large as compared to thermal etching and only at a few places the chemical etch pits are
~
f~j;~f
(b)
Fig. 4. (a) Etch pits on the lrism face of a Se Te whisker crystal etched in a solution of iodine in methanol (X855). (b) The same face after etching with the previously reported etchant (X855).
1 .1’ Bhatt. SB. Tm cdi P t hing of Sc’ Tm whisker crystals
26 -
-
~
~
,....!
.~,.
-
.,~
.
. -
______________________
-~
~.
_________________ ~
,~.
________ ______
I ig. 6. \1,ieni!icd
e~s of thermal pits oht,on~dat 180°C
(X2805).
formed at the sites of thernual etch pits, The prism face of a crystal was indented with a sharp needle and subsequently thermally etched at 190°C.On observing the surface it is found that no pits are formed near the indentation mark and along the slip traces. A crystal was slightly bent and etched
thermally. It was observed that the etch pit density after bending and etching did not correspond to the dislocation density expected from the plastic deformation.
While studying the perfection of Se Te whisker crystals using a chemical dislocation etchant, the present authors [15] reported that the pit density was greatest near the root and lower in the central part and near the tip. Contrary to this, the thermal pits are randomly distributed on the surface and show no special preference of the place for pit formation. It
(a)
(b)
lig. 7. (a) Prism face of a Se Ic ushisker crystal etched at 190 C for 20 mm mm (X890).
(‘<890). (b) The same face etched for additional 10
___
(a)
-
.
was further observed that the chemical etch pit density in the whisker crystals decreased with decreasing growth temperature. During the course of the present study, whiskers grown at various growth temperatures were thermally etched and it was found that the pit density was independent of the growth temperature, which contradicts our previous observations of chemical etching. From the above observations and the lack of correspondence between thermal and chemical pits
-
Oil
1mg. 8. (a) Thermal etch pits on the prism face of the whisker crystal etched at 200°C (x595). (b) The same face after chemical etching (XS95).
V.P. Bhatt, SB. Trivedi / Etching of Se Te whisker crystals
27
Table 1 Details of thermal etching of the prism plane of Se Te whisker crystals at various temperature and period of etching Ftching temperature ( C)
Etching time (mm)
Remarks
170
90
No visible pits
180
10 20
Very small pits of undefined shape Randomly distributed dihedral pits of assorted size (fig. 5); few shallow pits are seen in the back ground Boat shaped pits; in some cases few dihedral pits arc also observed Pits grow in size predominantly along the c-axis; pits become more and more shallow
30 I urther etching 190
10 20 30
Further etching 200
10 Further etching
Randomly distributed dihedral pits of assorted size Boat shaped pits (fig. 7a) Pits grow in size predominantly along caxis, and become shallow (fig. 7b) Same as above Boat shaped, egg shaped and pits bounded by four rectilinear edges; most of the pits are shallow (fig. 8a) Pits grow in size predominantly along the c-axis and become more and more shallow.
it may be concluded that the thermal pits dislocation sites.
If the pits
are not at
are not formed at dislocation sites, then
they may be due to vacancy condensation, precipitates or impurities in the crystal. If the pits are due to a vacancy condensation mechanism, then the rate of cooling will definitely have its effect on the pit density. In the present study, to check this point crystals after thermal etching were suddenly cooled to room temperature in one case, and in another they were cooled very slowly after the desired time of etching. On observing the surfaces, It was found in both cases that there was no significant change in the pit density. This rules out the possibility of pit formation due to a vacancy condensation mechanism.
Some crystals were grown from doped with thallium as an impurity. On
Se Te alloy etching these
crystals thermally, it was observed that the pit den-
sity had increased considerably, fig. 9. It is therefore reasonable to believe that the thermal pits are probably due to impurities in the crystals. 4. Conclusions (1) Saturated solution of iodine in methanol is Capable of revealing the sites of emergence of dislocations on the prism face of Se90—Te10 whisker crystals. (2) Thermal etching is not capable of revealing the sites of dislocations on the prism face of Se90 Te10 whisker crystals. (3) Thermal pits on the prism face of whisker crys-
tals are not formed due
to
vacancy condensation
mechanism, but are probably due to impurities in the crystals. Acknowledgements 11g. 9. fhermal etclm pits on the whisker crystals (loped with thallium 0<1023).
One of the authors (S.B.T.) is thankful to The Council of Scientific and Industrial Research, New
28
1 P Bhatt, SB. Tmo’edi / 1’ tehinc’ ol Sc
Dellu, India, for the award of a Junior Research Eellowship. The authors are thankful to Professor MM. Pate! for his keen interest in the work.
Tm
it
hiskem cit coils
[6] Anthony I .R . Acta. \let. 18 t1970) 471.
[71 1 U Wen Ling amid I .A. Starke. Jr., Metallogiaph~ S (1972)
399.
81 AR. Patel and S.K. Ar~’ra,J. Ph~s. t~7(1°74) 23(11. [91 J.P. 1-Iirth and L. \ assrnillet, 1. AppI. Ph~s 29 (19514) 595.
References [1] MB. Ives and J.P. llirth, J. Oiem. l’hys. 33 (1960) 517. [2] A.W. Ruff. Jr., J. Chem. Phys. 41(1964)1204. [3] G.M. Rosenhlatt, P.K. Lee and MB. 1)owell, I. Chem. Phys. 45 (1966) 3454, 14] J.l . Lester and (1.1 Somorjai, 3. Chem. Ph5s. 49 (1968) 2940 [5] 1. 1 jima, \\ U Robinson and il’. Hirth. I. Crystal Grossth 7 (1970) 155.
[10] A.A. Hendrickson and I .S. MachUn, Acta Met. 64.
(19S~)
[1~ J.P. HItth and J. Budke. J. AppI. Phys, 40 (1969) 64 t. 1121 D. Saran and AS. Parasnis. Indian 1. Pure AppI. Ph~s. 13(1975) 725. [131 1)ohertlm~P.1 . and 1)avis R.S., Acta Met. 7 (l9~9)118. [141 C. I ischer and I . H~asell,Surface Sci. 1(1972) ~i92. ~1S I SB. frivedi and V.P. Bhatt, J. C~stal (,rowth 32 (1976) 227.
CHEMICAL AND THERMAL ETCHING OF Se—Te WHISKER CRYSTALS V.P. BHATT and S.B. TRIVEDI Physics Department, Faculty of Science, MS. University of Baroda, Baroda, India Received 14 July 1976
A new chemical etchant (saturated solution of iodine in methanol) capable of revealing the sites of emergence of dislocations on the prism faces of Se 90 Te10 whisker crystals grown from the vapour phase is reported. A systematic detailed study otthermal etching of whisker crystals at various temperatures and the period of etching is also reported. The results of thermal etching are compared with those of chemical etching. Various experiments are performed and it has been shown that the thermal pits are not formed at dislocation sites but are probably due to impurities in the crystal.
1. Introduction
the present paper a new chemical etchant capable of revealing dislocations intersecting the prism planes of Se Te whisker crystals and a detailed study of therma! etching of these crystals, are reported. The results of thermal etching are compared with those of chemical etching. It has been shown that the thermal pits are not formed at dislocation sites but are probably due to impurities.
The wide range applications of semiconductor devices have increased the demand of semiconducting crystals of very high perfection. Various methods are used to assess the perfection of crystals. Chemical etching is an established technique and because of its simplicity is widely used to determine the dislocation density in the crystals. Formation of thermal etch pits and a one-to-one correspondence between these pits and dislocations have become a subject of great interest and importance in recent years. Many workers have reported the study of thermal pits on metals, alloys, semiconducting and ionic crystals. Only in limited cases the correspondence between thermal pits and dislocations is reasonably established. In some cases [1 8] the thermal pits are correlated with dislocations whereas in others [9 12], no correspondence is observed between thermal pits and dislocations, Doherthy et al. [13] have shown in the case of aluminium, that the thermal pits are formed due to the vacancy condensation mechanism. Fisher et al. [14], in the case of indium antimonide, correlated thermal pits with the gettering action of indium used as a solvent and diluent for the cadmium diffusion source, Little information [15] is available in the literature on chemical etching of Se—Te crystals. The present authors have not come across any reference in literature on thermal etching of Se—Te crystals. In
2. Experimental Selenium and tellurium of 99.999% purity, supplied by Special Materials Plant, Hyderabad, India, were used for the preparation of Se Te alloy containing 90 wt% of selenium. The whisker crystals used in this study were grown from this alloy using the technique reported earlier by the present authors [15]. Saturated solution of iodine in methanol was found to be an etchant capable of revealing dislocations on the prism faces of Se90 Te10 whisker crystals. Freshly grown crystals with well developed, smooth faces were etched in this solution for 30 35 sec. This was followed by first washing the crystal in methanol, then washing in ethanol and finally drying in air. For thermal etching, freshly grown crystals were vacuum sealed (~wl0 ~ Torr) in a small thin-walled pyrex tube. The tube was then inserted in the precalibrated resistance furnace kept at the desired tem23
24
I
.!~
B/,att, 5.11. Trivsdz
/ 1’ rehing oJ 5e
L
Hg. 1. Chemical etch pattern on the prism face of a Se Te whisker crystal (X920).
perature. The crystals were thermally etched for a specific time and then the current from the furnace was switched off and the furnace was allowed to cool slowly to the room temperature. The etched samples were then carefully removed from the tube and ob. servations were made on a Vicker’s projection microscope.
Ic whis/~r rTstal,v
The etchant pioduces well defined pits which tetam their shape while growing largei on further etching, figs. 2a and 2b. This clearly shows that the pits are at the sties of emergence of dislocations. A further test for the reliability of an etchant was made by indenting the crystal by a sharp needle followed by etching. A cluster of pits near the indentation mark and the alignment of pits along slip tiaces are clearly visible (fig. 3). Further evidence for the ability of the etchant to reveal dislocations was obtained by etching the crystal first in the saturated solution of iodine in methanol, fig. 4a, and then in the dislocation etchant [1
part cone,
HNO3 (70%) + 4
parts conc.
112S04
(70%)] reported earlier by the present authors [151,
fig. 4b. Comparing the two photographs, it is clearly seen that there is no increase in the number of pits
and a one-to-one correspondence in the position of pits is observed. It is also clearly visible that the pits shown in fig. 4a have grown larger in size and have
taken the shape of a truncated triangle. The results of pit density and distribution obtained for the whisker crystals etched in the present etchant are in good agreement with those etched using previously ye ported etchant. This shows that the etchant is capable of revealing the sites of emergence of dislocations.
3. Results and discussions
3.2. Thermal etching
3.1. Chemical etching
Freshly grown whisker crystals with well developed plane prism faces were thermally etched in the temperature range 180 200°C for 10 40 nun and the results are reported below. When the crystals were etched at 180°Cfor 10 mm. very small etch pits of undefined shape were ohserved. On increasing the etching time to 20 mm di-
The chemical etch produces point bottomed triangular pits on the prism faces of whisker crystals. Fig. 1 shows the random distribution and a row of etch pits. One of the sides of the pit is almost parallel to the c-axis.
-~~--
.‘‘‘-~
,..---. -
—
—.
k
~1,
I
S
‘1 (a)
(b)
Fig. 2. (a) Ftch pits on the prism face of a Se Te whisker crystal (X660). (b) The same pits on further etching (X660).
V.P. Bhatt, S.B. Trivedi/Etching of Se Te whisker crystals
_
Hg 3. 1 mcli pattern near indentation mark (/160).
hedral pits with rectangular out lines were observed as shown in fig. 5. The shape of the thermal pits at higher magnification (fig. 6) clearly shows the development of the extra triangular facets at the ends of the pits. On further etching for an additional 10 mm, the dihedral pits were found to be converted into boat shaped pits. Etching at 190°Cfor 10 mm resulted in the development of small dihedral pits. On increasing the etching time to 20 mm, large boat shaped pits randomly distributed on the surface were observed as shown in fig. 7a. It is interesting to note that one edge of a pit is almost parallel to the c-axis, and the pits are of assorted size. When the same crystal face was etched for 30 mm, practically all the pits grew larger in size and some new pits were also formed as shown in fig. 7b. When the crystals were etched at 200°Cfor 10 mm, boat shaped, egg shaped and pits bounded by four rectilinear edges were observed as shown in fig. 8a. On prolonged etching the pits grew larger in size but became more and more shallow, The details of thermal etching at various temperatures and etching period are summarized in table 1. The salient features of the above observations are the following,
~
~
(a)
25
~
-~
.~
11g. 5. Thermal etch pits oii the prism whisker crystal etched at 180°C(X925).
face
of a Sc
Ie
(i) No visible pit formation is observed on crystals etched at temperatures below 180°Cfor a long time. (ii) Etching the crystals at 180 and 190°Cresult mitiahly in dihedral pits which are converted into boat shape on prolonged etching. At 200°Cinitially boat shaped, egg shaped and pits bounded by four rectilinear edges are observed which on further etching are converted in td boat shaped pits. (iii) In every run, on prolonged etching, the size of boat shaped pits is found to increase, predominantly along the c-axis and the pits become more and more shallow. To ascertain whether the pits are formed at dislocations or not, a thermally etched crystal, fig. 8a, was chemically etched in the dislocation etchant (saturated solution of iodine in methanol), fig. 8b. Comparing the two photographs it is clearly observed that the density of the etch pits produced on chemical etching is very large as compared to thermal etching and only at a few places the chemical etch pits are
~
f~j;~f
(b)
Fig. 4. (a) Etch pits on the lrism face of a Se Te whisker crystal etched in a solution of iodine in methanol (X855). (b) The same face after etching with the previously reported etchant (X855).
1 .1’ Bhatt. SB. Tm cdi P t hing of Sc’ Tm whisker crystals
26 -
-
~
~
,....!
.~,.
-
.,~
.
. -
______________________
-~
~.
_________________ ~
,~.
________ ______
I ig. 6. \1,ieni!icd
e~s of thermal pits oht,on~dat 180°C
(X2805).
formed at the sites of thernual etch pits, The prism face of a crystal was indented with a sharp needle and subsequently thermally etched at 190°C.On observing the surface it is found that no pits are formed near the indentation mark and along the slip traces. A crystal was slightly bent and etched
thermally. It was observed that the etch pit density after bending and etching did not correspond to the dislocation density expected from the plastic deformation.
While studying the perfection of Se Te whisker crystals using a chemical dislocation etchant, the present authors [15] reported that the pit density was greatest near the root and lower in the central part and near the tip. Contrary to this, the thermal pits are randomly distributed on the surface and show no special preference of the place for pit formation. It
(a)
(b)
lig. 7. (a) Prism face of a Se Ic ushisker crystal etched at 190 C for 20 mm mm (X890).
(‘<890). (b) The same face etched for additional 10
___
(a)
-
.
was further observed that the chemical etch pit density in the whisker crystals decreased with decreasing growth temperature. During the course of the present study, whiskers grown at various growth temperatures were thermally etched and it was found that the pit density was independent of the growth temperature, which contradicts our previous observations of chemical etching. From the above observations and the lack of correspondence between thermal and chemical pits
-
Oil
1mg. 8. (a) Thermal etch pits on the prism face of the whisker crystal etched at 200°C (x595). (b) The same face after chemical etching (XS95).
V.P. Bhatt, SB. Trivedi / Etching of Se Te whisker crystals
27
Table 1 Details of thermal etching of the prism plane of Se Te whisker crystals at various temperature and period of etching Ftching temperature ( C)
Etching time (mm)
Remarks
170
90
No visible pits
180
10 20
Very small pits of undefined shape Randomly distributed dihedral pits of assorted size (fig. 5); few shallow pits are seen in the back ground Boat shaped pits; in some cases few dihedral pits arc also observed Pits grow in size predominantly along the c-axis; pits become more and more shallow
30 I urther etching 190
10 20 30
Further etching 200
10 Further etching
Randomly distributed dihedral pits of assorted size Boat shaped pits (fig. 7a) Pits grow in size predominantly along caxis, and become shallow (fig. 7b) Same as above Boat shaped, egg shaped and pits bounded by four rectilinear edges; most of the pits are shallow (fig. 8a) Pits grow in size predominantly along the c-axis and become more and more shallow.
it may be concluded that the thermal pits dislocation sites.
If the pits
are not at
are not formed at dislocation sites, then
they may be due to vacancy condensation, precipitates or impurities in the crystal. If the pits are due to a vacancy condensation mechanism, then the rate of cooling will definitely have its effect on the pit density. In the present study, to check this point crystals after thermal etching were suddenly cooled to room temperature in one case, and in another they were cooled very slowly after the desired time of etching. On observing the surfaces, It was found in both cases that there was no significant change in the pit density. This rules out the possibility of pit formation due to a vacancy condensation mechanism.
Some crystals were grown from doped with thallium as an impurity. On
Se Te alloy etching these
crystals thermally, it was observed that the pit den-
sity had increased considerably, fig. 9. It is therefore reasonable to believe that the thermal pits are probably due to impurities in the crystals. 4. Conclusions (1) Saturated solution of iodine in methanol is Capable of revealing the sites of emergence of dislocations on the prism face of Se90—Te10 whisker crystals. (2) Thermal etching is not capable of revealing the sites of dislocations on the prism face of Se90 Te10 whisker crystals. (3) Thermal pits on the prism face of whisker crys-
tals are not formed due
to
vacancy condensation
mechanism, but are probably due to impurities in the crystals. Acknowledgements 11g. 9. fhermal etclm pits on the whisker crystals (loped with thallium 0<1023).
One of the authors (S.B.T.) is thankful to The Council of Scientific and Industrial Research, New
28
1 P Bhatt, SB. Tmo’edi / 1’ tehinc’ ol Sc
Dellu, India, for the award of a Junior Research Eellowship. The authors are thankful to Professor MM. Pate! for his keen interest in the work.
Tm
it
hiskem cit coils
[6] Anthony I .R . Acta. \let. 18 t1970) 471.
[71 1 U Wen Ling amid I .A. Starke. Jr., Metallogiaph~ S (1972)
399.
81 AR. Patel and S.K. Ar~’ra,J. Ph~s. t~7(1°74) 23(11. [91 J.P. 1-Iirth and L. \ assrnillet, 1. AppI. Ph~s 29 (19514) 595.
References [1] MB. Ives and J.P. llirth, J. Oiem. l’hys. 33 (1960) 517. [2] A.W. Ruff. Jr., J. Chem. Phys. 41(1964)1204. [3] G.M. Rosenhlatt, P.K. Lee and MB. 1)owell, I. Chem. Phys. 45 (1966) 3454, 14] J.l . Lester and (1.1 Somorjai, 3. Chem. Ph5s. 49 (1968) 2940 [5] 1. 1 jima, \\ U Robinson and il’. Hirth. I. Crystal Grossth 7 (1970) 155.
[10] A.A. Hendrickson and I .S. MachUn, Acta Met. 64.
(19S~)
[1~ J.P. HItth and J. Budke. J. AppI. Phys, 40 (1969) 64 t. 1121 D. Saran and AS. Parasnis. Indian 1. Pure AppI. Ph~s. 13(1975) 725. [131 1)ohertlm~P.1 . and 1)avis R.S., Acta Met. 7 (l9~9)118. [141 C. I ischer and I . H~asell,Surface Sci. 1(1972) ~i92. ~1S I SB. frivedi and V.P. Bhatt, J. C~stal (,rowth 32 (1976) 227.