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Growth of GaAs from a free surface melt under controlled arsenic pressure in a partially confined configuration
Acta Astronautica Vol. 20, pp. 179-183, 1989
0094-5765/89 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain GROWTH OF GaAs FROM A FREE SURFACE MELT UNDER CONTROLLED ARSENIC PRESSURE IN A PARTIALLY CONFINED CONFIGURATION H.C. Gatos, J. Lagowski and Y. Wu Massachusetts Institute of Technology Cambridge, Massachusetts 02139, USA
Abstract. A new partially confined configuration was recently developed for the growth of GaAs from the melt in space. This configuration consists of a triangular prism containing the seed crystal, and source material in the form of a rod. Upon melting, the liquid phase, due to surface tension forces, assumes an elongated cylindrical shape touching the prism approximately along three straight lines. This configuration is believed to overcome two major obstacles encountered in the growth of GaAs (or other compounds with a volatile constituent) in space, namely, total confinement in a quartz crucible, and lack of arsenic (or other volatile constituent) pressure control. It was successfully tested on earth in growing GaAs, and unexpectedly it was found that it provides a unique means, not only for crystal growth in space, hut also for studying the growth of GaAs from a free-surface melt even on earth. New phenomena were observed related to the surface tension and its interaction with compositional and/or thermal variations. They were manifested as oscillations of the crystal diameter. The resulting chemical composition and electrical property variations were recorded. Phenomenological models to account for the results are presented. Introduction The advantages of near-zero gravity conditions in materials processing in general and crystal growth in particular, have been demonstrated, at least qualitatively, in a limited number of experiments.I, 2 Valid questions, however, have been raised regarding the consequences of total confinoment of the melt prior to solidification (i.e. stresses resulting from volume expansion during solidification, contamlnatlon of the melt surface, and others). Total confinement could also obscure the study of effects of surface-driven convection (Maragoni effect) which continue to be a fundamental issue regarding materials processing in space. An additional issue became critically evident following the analysis of the results of experiments carried out in space. The effects of near-zero gravity conditions can be quantitatively assessed only if reliable approaches and techniques are available for characterization of the system in question on earth and following processing in space. Furthermore, it is imperative that ground research addresses the study and control of all experimental parameters and conditions which could possibly interfere with the identification and assessment of the zero gravity effects and/or lead to artifacts attributable to space processing. It was with the above questions in mind that we carried out this work. We gave critical consider, ation to the choice of the material. We chose GaAs in order to achieve optimization of the scientific benefits resulting from the elimination of convective interference. In addition, we believe it was important to consider the potential technological impact that these scientific benefits might have.
The Material The defect structure of GaAs presents immense challenges regarding the nature and origin of defects, their interactions, and their effects on the critical electronic properties. = The understanding of the effects of stoichiometric variations, co~nonly present in GaAs, is severely interfered with by gravity-induced convection during solldification. 4 In turn, convective effects on classical segregation kinetics are usually obscured by stoichiometry variations. The defect structure and segregation kinetics influenced by thermal convection are at the same time substantively affected by Fermi statlstics. 5 The Experimental Approach The experimental approach has been directed along ~wo main goals: (a) the implementation and study of a novel configuration for crystal growth under partial confinement, and (b) the investigation of native defects interactions and the key electronic characteristics (carrier concentration and carrier mobility) as related to the thermal cycle following solidification and to stoichiometry. Partlally Confined Configuration The novel partially confined configuration conceived for the growth of GaAs crystals from the melt in space is shown schematically in Fig. 1. The melt resultlng from a cylindrical charge is confined in a triangular prism. In the absence of gravity the surface energy of the m~it is minimized by acquiring a cylindrlcal shape.4,6'gThe key features of this new configuration are as follows: 1. The melt is only partially confined, i.e., the contact area bet~veen the walls of the container and the melt is minimized. 2. Large free surface of the melt and the large empty space assure efficient control of the melt composition through the melt interaction with arsenic vapor. 3. Large empty space in the container is beneficial for accbmmodation of the volume expansion (~ii%) taking place upon solidification of GaAs. The growth ampul is made of quartz or pyrolytic boron nitrlde. Upon proper surface preparation, both of these materials exhibit highly desired nonwetting characteristics. Utilization of quartz leads to Si contamination; however, this process can be effectively suppressed by oxygen added into the growth ampul. A Horizontal Bridgman (HB) apparatus_constructed for the growth of GaAs in standard boats I was adapted for our ground-based experiments with the partiallyconfined configuration (Fig. 2). The apparatus has Copyright 0 1 9 8 8 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Professor of Electronic Materials, Department of Materials Science and Engineering.
179
180
H.C. GATOSet al.
Quartz Confinment ~/%
/ Z ~
Necking Breakable GaAs Seed Source Seal As
/
I
~
/
Reoion I ~
GaAs
meltArsenicVopor
Constant Diameter Crystal ............... ~ii~...................~ ................... ~ :~:~;:~;;.'~.
/
Magnet
solid---,-' =
melt
1238°C
O.
TAs610,0 623°c
E I--
Fig. i.
1250°C
J
Schematic representation of the partially confined configuration in Horizontal Bridgman mode including the corresponding temperature profiles.
I
X~x
(1511K~/~
/ Forced-Gas Array
IWVMI|I
Fig. 2.
"-Guide Rails
Horizontal Brldgman growth apparatus and thermal profile (top), The quartz ampul contains the As source, i; a breakable seal,,2; and seal breaking weight, 3; the quartz diffusion barrier, 4; a quartz boat, 5; the GaAs seed crystal, 6; and polycrystalline GaAs, 7,
three temperature zones for achieving the desired temperatures and temperature gradients. The low temperature zone which determines the partial pressure of As over the melt is controlled by a heat pipe which leads to excellent temperature control. It should be pointed out that in this HB apparatus the viewing port to facilitate seeding has been eliminated to maximize thermal symmetry and thus minimize gradient-driven convection in the melt. RESULTS We carried out at first a series of experiments
using triangular prismatic containers of BN with the side of the triangular cross section being 3 m~ long. They were positioned vertically on a strip heater to minimize the effect of gravity on the lateral filling of the containers. Low melting point alloy wires (Pb-In-Sn) were inserted into the cavity. The wires were heated from the bottom (free ends). In all instances, upon solidification, the triangular cavity was not completely filled. These results demonstrated that the melt in a triangular prismatic container indeed assumes a shape dictated by the surface energy. We considered
Growth of GaAs from free surface melt
them as a confirmation of the validity of our "partial confinement" concept and we proceeded with the construction of triangular quartz pris~s suitable for actual GaAs growth experiments.
181
three different spacings: about 1 mm, 100 tm, and 10 ~ , and were manifested by changes in the crystal diameter. The morphology of the oscillations is shown in Figs. 4 and 5.
In the prismatic growth ampul a cylindrical charge of Gabs with a cross-section diameter of 4 mm could be inserted (Fig. 3a). Growth experiments were carried out in the HB apparatus by traversing the prismatic container through the appropriate thermal gradients. The results were gratifying. As shown in Fig. Sb and c, even in the Horizontal Brldgman configuration the GaAs melt does not fill the entire cavity,
Fig. 4. Oscillations on as-grown crystal.
(c) /
a
~
10/_20
•..
~
~
,
-
_,
,~\
..7/
Directionof
Fig. 5. Schematic representation of oscillations on crystals. Designation of (a), (b) and (c) is as in Table I. Fig. 3. (A) Quartz triangular prism containing GaAs cylindrical charge. (B) Portion of the GaAs crystal after growth; the cylindrical seed and the grown crystal are clearly delineated. (C) Cross section of crystal grown in the triangular prism container. On the left-hand side it is apparent that the melt wetted the container; on the right-hand side growth took place under clean conditions. provided the internal surfaces of the prismatic container are properly prepared. As a result, an equilibrium As pressure is maintained over the mel~ through the three unfilled segments of the triangular pri~m. 4 We found that the electronic characteristics (carrier concentration, mobility, deep levels) and defect structure (dislocation density, native traps, and recombination centers) measured on a macroscale were as good as those achieved in our HB apparatus using conventional boats. In addition, however, new phenomena of an oscillatory nature were revealed by microscale measurements in crystals grown in the triangular prism. The oscillations had
Further analysis of these oscillations was carried out by chemical etching, (followed by depth profiling of the etch pattern) and by photoluminescents scanning measurements. Typical results, obtained with differential etching of the sample sliced parallel to the growth direction are shown in Fig. 6. The photomicrograph illustrates a microscale striation pattern with periodicity of about 10-20!~m superimposed on larger variations with periodicity of about 70-100 ~m. A depth profile of the etched sample (Fig. 7) reveals clear oscillatlons of higher and lower frequencies. It is important to note that the bulk striation pattern revealed by the differential etching corresponds very well to crystal diameter fluctuations shown in Figs. 4 and 5. It is of further importance to note that the same pattern is revealed by 4.2 K photoluminescence scanning measurements of the free exciton line (see Fig. 8). A clear relationship is thus evident between the crystal growth morphology (i.e., crystal shape), the defect structure (i.e., differential etching pattern), and the electronic properties (i.e., photolumlnescence
182
H.C. G'ATOSet al. modeling shows that even small changes in surface tension can lead to substantive changes in the contact angle, and thus to a cross section area of the melt. In fact, in Table I results of estimated changes in surface tenslonwhlch can lead to oscillation of the type observed in the crystals are shown (see also Figs. 9, 10, and ii). L / A - LIQUID/AMBIENT
S/L - SOLID/LIQUID S / A - SOLID/AMBIENT qUARTZ AMBIENT Fig. 6. Oscillations (with about i0 ~m spacing) revealed by chemical etching.
12oo tOO0
GoAs
800
o~
(c)
(b,
~.~
.
60C '*" 4.00
7s/, 7S/A Fig. 9. Representation of contact angle of melt with quartz container.
0 -2~
11~
200 500 400 Distance (/~rn)
500 (e
,•
~,A> Ys,~Ys,L
.......... ~
8 - 180o
Fig. 7. Oscillations revealed by profilometer scanning. Designation of (b) and (c) is as in Table I.
~
~'~o>90" > ~'- ~'~
(°) ~
'~
~"
GoAs
Fig. i0. Representation of various contact angles of melt with quartz container.
i
~R
I 0
+ 87
,,:~'~!~
~I 1
I
I
2 :5 Distance ( mm]
I
I
4
5
Fig. 8. Photoluminescence scanning revealing oscillations in crystals grown in partially confined configuration. Designation of (a) and (b) is as in Table I.
'~'~:
A smell change in y will change the contact eng., 8 end the cross section of the llquld column
=-
cosO
intensity). Periodical instabilities of the crystal diameter are not present in crystals grown in a standard growth ampul. It appears that they are most likely surface tension-driven' from variations in surface tension which may originate in variations in melt composition and/or thermal gradients. Simplifying
Fig. ii. Illustration of surface tension changes and their effects. The required surface tension fluctuations, ~7/7,~ 5Z are well within the range expected from variations in melt/gas phase composition. Surface
Growth of OaAs from free surface melt
183
Table I INHOMOGENEITIES IN GaAs GROWN L~DER PARTIAL CONFINEMENT
Inhomogeneitles Magnitude ~R/R Spacin~ Macroscopic a Intermediate b Microscopic c
5%
1 ~-,
Estimated Surface Tension Fluctuations Magnitude ~Y17 Period 5%
I0 mln
<0.3%
50-100 ~m
<1%
0.5-1 min
--
10-20 ~m
--
4-15 s
aseen as changes in crystal diameter and as photolumlnesence fluctuations. bseen as changes in crystal diameter, photolumlnescenee fluctuations, and in differential etching. Cseen in dlfferentlal etching. tension of molten GaAs depends strongly on the melt stolchlometry and/or doping. Thus, it exhibits a maximum value of about 440 dyne/cm for stoichlometric melt and decreases to 370 dyne/cm and 400 dyne/cm with increasing and decreasing arsenic ambient pressure (by about 0.5 atm), respectively~ Te and Zn doping also decreases the surface tension; however, the doping level required to obtain mea~ingf~ changes of 67/y of about 10% exceeds I01 cm - . Presently grown crystals were not intentionally doped, and thus we believe that surface tension changes were brought about by the stoichiometry fluctuations. In s , ~ r y , it was found that surface tension variations in the melt lead to crystal diameter oseillatlon~ and to corresponding oscillations of the electronic and defect characteristics. ACKNOWLEDGMENT The authors are grateful to the National Aeronautics and Space Administration for financial support. REFERENCES i. A.F. Witt, H.C. Gatos, M. Lichtenstelger, M.C. Lavlne, and C.J. Herman, J. Electrochem. Soc. 122, 276 (1975); A.F. Witt, H.C. Gatos, M. Lichtenstelger, and C.J. Herman, J. Electrochem. Soc. 125, 1832 (1978). 2. H.C. Gatos, in "Materials Processing in the Reduced Gravity Environment of Space," Guy E. Rindone, Editor, Elsevier Science Publishing Company, Inc., 1982, p. 355. 3. See, for example, Defects in Semiconductors, edited by H.J. yon Bardeleben, Mat. Science Forum vol. 10-12 (1986), Trans. Tech. Publications, Ltd., Switzerland, 1986. 4. H.C. Gatos, J. Lagowskl, L.M. Pawlowlcz, F. Dabkowski, and C.-J. Li, Proc. 5th European Symp. on Mat. Sciences under Microgravlty, Schloss Elmau, 5-7 Nov. 1984 (ESA SP-222). 5. T. Aoyama, J. Lagowski, D.G. Lin, K.Y. Ko and O. Ueda, Inst. Phys. Conf. Set. 79, 19 (1985). 6. J. Lagowski, H.C. Gatos and F. Dabkowskl, J. Cryst. Growth 72, 595 (1985).
AA ~--M
7. J. Parsey, Y. Nanishi, J. Lagowskl and H.C. Gatos, J. Electrochem. Soc. 129, 388 (1982). 8. V.V. Korataev, M.G. Mil'vidskii, and N. Ya. Zakhorova, Inorg. Materials 2, 833 (1966). 9. An independent prediction of such behavior was also provided by R, Sen and W,R. Wilcox, J. Crystal Growth78, 129 (1986) in their analysis of the behavior of a nonwetting melt in free fall,
0094-5765/89 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain GROWTH OF GaAs FROM A FREE SURFACE MELT UNDER CONTROLLED ARSENIC PRESSURE IN A PARTIALLY CONFINED CONFIGURATION H.C. Gatos, J. Lagowski and Y. Wu Massachusetts Institute of Technology Cambridge, Massachusetts 02139, USA
Abstract. A new partially confined configuration was recently developed for the growth of GaAs from the melt in space. This configuration consists of a triangular prism containing the seed crystal, and source material in the form of a rod. Upon melting, the liquid phase, due to surface tension forces, assumes an elongated cylindrical shape touching the prism approximately along three straight lines. This configuration is believed to overcome two major obstacles encountered in the growth of GaAs (or other compounds with a volatile constituent) in space, namely, total confinement in a quartz crucible, and lack of arsenic (or other volatile constituent) pressure control. It was successfully tested on earth in growing GaAs, and unexpectedly it was found that it provides a unique means, not only for crystal growth in space, hut also for studying the growth of GaAs from a free-surface melt even on earth. New phenomena were observed related to the surface tension and its interaction with compositional and/or thermal variations. They were manifested as oscillations of the crystal diameter. The resulting chemical composition and electrical property variations were recorded. Phenomenological models to account for the results are presented. Introduction The advantages of near-zero gravity conditions in materials processing in general and crystal growth in particular, have been demonstrated, at least qualitatively, in a limited number of experiments.I, 2 Valid questions, however, have been raised regarding the consequences of total confinoment of the melt prior to solidification (i.e. stresses resulting from volume expansion during solidification, contamlnatlon of the melt surface, and others). Total confinement could also obscure the study of effects of surface-driven convection (Maragoni effect) which continue to be a fundamental issue regarding materials processing in space. An additional issue became critically evident following the analysis of the results of experiments carried out in space. The effects of near-zero gravity conditions can be quantitatively assessed only if reliable approaches and techniques are available for characterization of the system in question on earth and following processing in space. Furthermore, it is imperative that ground research addresses the study and control of all experimental parameters and conditions which could possibly interfere with the identification and assessment of the zero gravity effects and/or lead to artifacts attributable to space processing. It was with the above questions in mind that we carried out this work. We gave critical consider, ation to the choice of the material. We chose GaAs in order to achieve optimization of the scientific benefits resulting from the elimination of convective interference. In addition, we believe it was important to consider the potential technological impact that these scientific benefits might have.
The Material The defect structure of GaAs presents immense challenges regarding the nature and origin of defects, their interactions, and their effects on the critical electronic properties. = The understanding of the effects of stoichiometric variations, co~nonly present in GaAs, is severely interfered with by gravity-induced convection during solldification. 4 In turn, convective effects on classical segregation kinetics are usually obscured by stoichiometry variations. The defect structure and segregation kinetics influenced by thermal convection are at the same time substantively affected by Fermi statlstics. 5 The Experimental Approach The experimental approach has been directed along ~wo main goals: (a) the implementation and study of a novel configuration for crystal growth under partial confinement, and (b) the investigation of native defects interactions and the key electronic characteristics (carrier concentration and carrier mobility) as related to the thermal cycle following solidification and to stoichiometry. Partlally Confined Configuration The novel partially confined configuration conceived for the growth of GaAs crystals from the melt in space is shown schematically in Fig. 1. The melt resultlng from a cylindrical charge is confined in a triangular prism. In the absence of gravity the surface energy of the m~it is minimized by acquiring a cylindrlcal shape.4,6'gThe key features of this new configuration are as follows: 1. The melt is only partially confined, i.e., the contact area bet~veen the walls of the container and the melt is minimized. 2. Large free surface of the melt and the large empty space assure efficient control of the melt composition through the melt interaction with arsenic vapor. 3. Large empty space in the container is beneficial for accbmmodation of the volume expansion (~ii%) taking place upon solidification of GaAs. The growth ampul is made of quartz or pyrolytic boron nitrlde. Upon proper surface preparation, both of these materials exhibit highly desired nonwetting characteristics. Utilization of quartz leads to Si contamination; however, this process can be effectively suppressed by oxygen added into the growth ampul. A Horizontal Bridgman (HB) apparatus_constructed for the growth of GaAs in standard boats I was adapted for our ground-based experiments with the partiallyconfined configuration (Fig. 2). The apparatus has Copyright 0 1 9 8 8 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Professor of Electronic Materials, Department of Materials Science and Engineering.
179
180
H.C. GATOSet al.
Quartz Confinment ~/%
/ Z ~
Necking Breakable GaAs Seed Source Seal As
/
I
~
/
Reoion I ~
GaAs
meltArsenicVopor
Constant Diameter Crystal ............... ~ii~...................~ ................... ~ :~:~;:~;;.'~.
/
Magnet
solid---,-' =
melt
1238°C
O.
TAs610,0 623°c
E I--
Fig. i.
1250°C
J
Schematic representation of the partially confined configuration in Horizontal Bridgman mode including the corresponding temperature profiles.
I
X~x
(1511K~/~
/ Forced-Gas Array
IWVMI|I
Fig. 2.
"-Guide Rails
Horizontal Brldgman growth apparatus and thermal profile (top), The quartz ampul contains the As source, i; a breakable seal,,2; and seal breaking weight, 3; the quartz diffusion barrier, 4; a quartz boat, 5; the GaAs seed crystal, 6; and polycrystalline GaAs, 7,
three temperature zones for achieving the desired temperatures and temperature gradients. The low temperature zone which determines the partial pressure of As over the melt is controlled by a heat pipe which leads to excellent temperature control. It should be pointed out that in this HB apparatus the viewing port to facilitate seeding has been eliminated to maximize thermal symmetry and thus minimize gradient-driven convection in the melt. RESULTS We carried out at first a series of experiments
using triangular prismatic containers of BN with the side of the triangular cross section being 3 m~ long. They were positioned vertically on a strip heater to minimize the effect of gravity on the lateral filling of the containers. Low melting point alloy wires (Pb-In-Sn) were inserted into the cavity. The wires were heated from the bottom (free ends). In all instances, upon solidification, the triangular cavity was not completely filled. These results demonstrated that the melt in a triangular prismatic container indeed assumes a shape dictated by the surface energy. We considered
Growth of GaAs from free surface melt
them as a confirmation of the validity of our "partial confinement" concept and we proceeded with the construction of triangular quartz pris~s suitable for actual GaAs growth experiments.
181
three different spacings: about 1 mm, 100 tm, and 10 ~ , and were manifested by changes in the crystal diameter. The morphology of the oscillations is shown in Figs. 4 and 5.
In the prismatic growth ampul a cylindrical charge of Gabs with a cross-section diameter of 4 mm could be inserted (Fig. 3a). Growth experiments were carried out in the HB apparatus by traversing the prismatic container through the appropriate thermal gradients. The results were gratifying. As shown in Fig. Sb and c, even in the Horizontal Brldgman configuration the GaAs melt does not fill the entire cavity,
Fig. 4. Oscillations on as-grown crystal.
(c) /
a
~
10/_20
•..
~
~
,
-
_,
,~\
..7/
Directionof
Fig. 5. Schematic representation of oscillations on crystals. Designation of (a), (b) and (c) is as in Table I. Fig. 3. (A) Quartz triangular prism containing GaAs cylindrical charge. (B) Portion of the GaAs crystal after growth; the cylindrical seed and the grown crystal are clearly delineated. (C) Cross section of crystal grown in the triangular prism container. On the left-hand side it is apparent that the melt wetted the container; on the right-hand side growth took place under clean conditions. provided the internal surfaces of the prismatic container are properly prepared. As a result, an equilibrium As pressure is maintained over the mel~ through the three unfilled segments of the triangular pri~m. 4 We found that the electronic characteristics (carrier concentration, mobility, deep levels) and defect structure (dislocation density, native traps, and recombination centers) measured on a macroscale were as good as those achieved in our HB apparatus using conventional boats. In addition, however, new phenomena of an oscillatory nature were revealed by microscale measurements in crystals grown in the triangular prism. The oscillations had
Further analysis of these oscillations was carried out by chemical etching, (followed by depth profiling of the etch pattern) and by photoluminescents scanning measurements. Typical results, obtained with differential etching of the sample sliced parallel to the growth direction are shown in Fig. 6. The photomicrograph illustrates a microscale striation pattern with periodicity of about 10-20!~m superimposed on larger variations with periodicity of about 70-100 ~m. A depth profile of the etched sample (Fig. 7) reveals clear oscillatlons of higher and lower frequencies. It is important to note that the bulk striation pattern revealed by the differential etching corresponds very well to crystal diameter fluctuations shown in Figs. 4 and 5. It is of further importance to note that the same pattern is revealed by 4.2 K photoluminescence scanning measurements of the free exciton line (see Fig. 8). A clear relationship is thus evident between the crystal growth morphology (i.e., crystal shape), the defect structure (i.e., differential etching pattern), and the electronic properties (i.e., photolumlnescence
182
H.C. G'ATOSet al. modeling shows that even small changes in surface tension can lead to substantive changes in the contact angle, and thus to a cross section area of the melt. In fact, in Table I results of estimated changes in surface tenslonwhlch can lead to oscillation of the type observed in the crystals are shown (see also Figs. 9, 10, and ii). L / A - LIQUID/AMBIENT
S/L - SOLID/LIQUID S / A - SOLID/AMBIENT qUARTZ AMBIENT Fig. 6. Oscillations (with about i0 ~m spacing) revealed by chemical etching.
12oo tOO0
GoAs
800
o~
(c)
(b,
~.~
.
60C '*" 4.00
7s/, 7S/A Fig. 9. Representation of contact angle of melt with quartz container.
0 -2~
11~
200 500 400 Distance (/~rn)
500 (e
,•
~,A> Ys,~Ys,L
.......... ~
8 - 180o
Fig. 7. Oscillations revealed by profilometer scanning. Designation of (b) and (c) is as in Table I.
~
~'~o>90" > ~'- ~'~
(°) ~
'~
~"
GoAs
Fig. i0. Representation of various contact angles of melt with quartz container.
i
~R
I 0
+ 87
,,:~'~!~
~I 1
I
I
2 :5 Distance ( mm]
I
I
4
5
Fig. 8. Photoluminescence scanning revealing oscillations in crystals grown in partially confined configuration. Designation of (a) and (b) is as in Table I.
'~'~:
A smell change in y will change the contact eng., 8 end the cross section of the llquld column
=-
cosO
intensity). Periodical instabilities of the crystal diameter are not present in crystals grown in a standard growth ampul. It appears that they are most likely surface tension-driven' from variations in surface tension which may originate in variations in melt composition and/or thermal gradients. Simplifying
Fig. ii. Illustration of surface tension changes and their effects. The required surface tension fluctuations, ~7/7,~ 5Z are well within the range expected from variations in melt/gas phase composition. Surface
Growth of OaAs from free surface melt
183
Table I INHOMOGENEITIES IN GaAs GROWN L~DER PARTIAL CONFINEMENT
Inhomogeneitles Magnitude ~R/R Spacin~ Macroscopic a Intermediate b Microscopic c
5%
1 ~-,
Estimated Surface Tension Fluctuations Magnitude ~Y17 Period 5%
I0 mln
<0.3%
50-100 ~m
<1%
0.5-1 min
--
10-20 ~m
--
4-15 s
aseen as changes in crystal diameter and as photolumlnesence fluctuations. bseen as changes in crystal diameter, photolumlnescenee fluctuations, and in differential etching. Cseen in dlfferentlal etching. tension of molten GaAs depends strongly on the melt stolchlometry and/or doping. Thus, it exhibits a maximum value of about 440 dyne/cm for stoichlometric melt and decreases to 370 dyne/cm and 400 dyne/cm with increasing and decreasing arsenic ambient pressure (by about 0.5 atm), respectively~ Te and Zn doping also decreases the surface tension; however, the doping level required to obtain mea~ingf~ changes of 67/y of about 10% exceeds I01 cm - . Presently grown crystals were not intentionally doped, and thus we believe that surface tension changes were brought about by the stoichiometry fluctuations. In s , ~ r y , it was found that surface tension variations in the melt lead to crystal diameter oseillatlon~ and to corresponding oscillations of the electronic and defect characteristics. ACKNOWLEDGMENT The authors are grateful to the National Aeronautics and Space Administration for financial support. REFERENCES i. A.F. Witt, H.C. Gatos, M. Lichtenstelger, M.C. Lavlne, and C.J. Herman, J. Electrochem. Soc. 122, 276 (1975); A.F. Witt, H.C. Gatos, M. Lichtenstelger, and C.J. Herman, J. Electrochem. Soc. 125, 1832 (1978). 2. H.C. Gatos, in "Materials Processing in the Reduced Gravity Environment of Space," Guy E. Rindone, Editor, Elsevier Science Publishing Company, Inc., 1982, p. 355. 3. See, for example, Defects in Semiconductors, edited by H.J. yon Bardeleben, Mat. Science Forum vol. 10-12 (1986), Trans. Tech. Publications, Ltd., Switzerland, 1986. 4. H.C. Gatos, J. Lagowskl, L.M. Pawlowlcz, F. Dabkowski, and C.-J. Li, Proc. 5th European Symp. on Mat. Sciences under Microgravlty, Schloss Elmau, 5-7 Nov. 1984 (ESA SP-222). 5. T. Aoyama, J. Lagowski, D.G. Lin, K.Y. Ko and O. Ueda, Inst. Phys. Conf. Set. 79, 19 (1985). 6. J. Lagowski, H.C. Gatos and F. Dabkowskl, J. Cryst. Growth 72, 595 (1985).
AA ~--M
7. J. Parsey, Y. Nanishi, J. Lagowskl and H.C. Gatos, J. Electrochem. Soc. 129, 388 (1982). 8. V.V. Korataev, M.G. Mil'vidskii, and N. Ya. Zakhorova, Inorg. Materials 2, 833 (1966). 9. An independent prediction of such behavior was also provided by R, Sen and W,R. Wilcox, J. Crystal Growth78, 129 (1986) in their analysis of the behavior of a nonwetting melt in free fall,