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Anomalous growth behavior of GaAs dendritic web
Journal of Crystal Growth 62 (1983) 309—3 16 North-Holland Publishing Company
309
ANOMALOUS GROWTH BEHAVIOR OF GaAs DENDRITIC WEB
*
T.A. GOULD and AM. STEWART Westinghouse R&D Center, 1310 Beulah Road, Pittsburgh, Pennsylvania 15235, USA
Received 22 October 1982; manuscript received in final form 18 January 1983
We have grown single-crystal GaAs ribbons by a liquid-encapsulated dendritic web technique. Undoped GaAs webs exhibited multiple dendrites and heavy surface facets despite careful optimization of the growth system. GaAs ribbons of more conventional web morphology were obtained from Ge-doped melts. The doping results indicate that atypical GaAs web morphology arises from fundamental attachment kinetics which are modified by incorporation of Ge. The observed relationships between facetting and growth behavior of both doped and undoped GaAs webs is consistent with the facetted interface model of web growth.
1. Introduction The dendritic web technique is a ribbon growth process which has been highly successful for the growth of single crystal Si ribbons for solar cells [1]. We investigated the application of this technique to GaAs as a cost-effective method for growing high efficiency solar cell base material. Dendritic web growth techniques and thermal modelling were combined with a B203-encapsulated growth system [2]. We obtained single crystal GaAs ribbons; however, they exhibited an unconventional multidendrite, textured morphology. Analyses of chemical modification, facetting behavior and twin orientation relationships in the context of web interface stability suggest that the inherent atomic attachment kinetics of GaAs produce atypical web morphology under what otherwise would be conventional web growth conditions.
ated with the thermal environment induce lateral growth to form a thickened “button”. When the button is pulled from the melt two dendrites propagate from the ends, and a film of liquid is drawn up between them. In Si web growth this film freezes into a single crystal ribbon which is smooth and well-suited for subsequent device fabrication. However, the GaAs webs were heavily textured with multiple dendrites and surface facets. 2. Experimental procedures The GaAs web pulling system and crystal growth procedures are described in detail elsewhere f.~••~________~~ DendrIte Seed Button ~Bounding
Typical Si web morphology is illustrated schematically in fig. 1. A dendrite seed with the required twin structure is brought into contact with the melt at the “hold” temperature. At this ternperature the seed neither melts nor nucleates growth. As the melt is undercooled, the twin structure of the seed and the attachment kinetics associ-
Dendrites Web
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1oi’ 1~ ~/ LiquId
Dendrite Tip *
This work supported in part by the Aerospace Power Division, Wright Air Force Aeronautical Laboratories, Contract F33615-78-C-2031.
0022-0248/83/0000—0000/$03.OO
©
Fig. I. Schematic section of conventional web growth.
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Anomalous growth behavior of GaAs dendritie web
[2,3]. Several parameters changed, however, as growth conditions were optimized throughout the experiments. Pull speeds for seed dendrites and webs varied mostly from 1.5—3.0 cm/mm. The range of apparent undercooling required for stable web propagation (measured by thermocouples inserted next to the crucible) was reduced to 8—l2°C from 15—20°C by optimization of the thermal geometry. Doping the GaAs melt with 0.3—3.0 at% Ge produced a further reduction of this parameter to the 2—6°Crange, values typical of the undercooling range for conventional Si web growth. We determined the crystallographic orientation of web surface facets by a 4 mm etch in tartaric acid: HNO3, 3: 1 [4], or by first smoothing the surface with a 1—3 mm AB etch [5] and reducing the time in the subsequent tartaric acid etch. Doped samples were more sensitive to the etchants and required shorter etching times. Both methods produced triangular etch pits on the Ga face of the web pointing in the (211) directions [6]. The twin structures of over 70 GaAs dendrites and webs were analyzed in cross section after mounting them in Quickmount and polishing. Etching these cross sections for 5—lO s with HF: H2O: Cr03 (saturated solution), 20:20: 1, or for 3—5 s with HNO3: H20, 4: 3, revealed the twin planes. The nitric acid etch was also useful for determining orientation relationships from the
Fig. 2. Cross section of an odd-ts~in-planedendrite etched with HNO~:H20; 4:3. The [211] direction at the top of the crystal responded to the etch faster than the [211] Section at the bottom,
cross sections since the [2111 direction was more sensitive to the etch than the [2111 direction (fig. 2).
3. Web morphology 3.1. Undoped GaAs web In contrast to Si web, which grows as a single. smooth (111) facet bounded by edge dendrites (fig. 1), the GaAs webs grew as multidendrite crystals with narrow web areas textured by small, individual (111) facets (fig. 3). A total of 48 GaAs webs were grown ranging in length from 1.2 cm to over 30.0 cm, the pulling limit of the apparatus. The width was typically 1.5 cm for undoped crystals and 1.0 cm for Ge-doped crystals. (The narrow width of the Ge-doped crystals reflects the small undercooling at which they grew.) The web areas between the dendrites were 100—300 ~sm thick,
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Fig. 4. ( omparison ol Gal Ill) face I top) and .As 1111 face (bottom) of a GaAs dendrite. The Ga face is rough with vertically developed facets, while the As face is smoother, (Droplet features are an artifact of surface As loss during growth.)
structure, one ribbon face is composed of (11 1)Ga planes, while the opposite face is composed of (lll)As planes [6]. Both seed dendrites and web crystals exhibited a marked difference in morphology on these faces. The Ga face was rough with vertically developed facets, while the As face was flatter and smoother (fig. 4). 3.2. Ge doped GaAs web
which is comparable to typical Si web thickness. The crystals grew in [211] direction and developed (111) faces. The rough, textured surfaces of the multidendrite crystals suggested that the web was polycrystalline. However, several randomly selected samples yielded single crystal (111) Laue patterns. Also, surface etching with the tartaric acid etchant which was performed regularly throughout the experiments, consistently produced triangular etch pits which remained aligned across facet boundaries. Therefore, the facetted morphology was a surface texture of single-crystal ribbons. Because GaAs crystallizes in the zinc blende
The growth behavior and resultant morphology of web crystals are controlled by the interaction of the system thermal geometry with the atomic attachment kinetics of the material. There exists a “growth window” of suitable parameter ranges for which stable web propagation will occur. (For example, Si web degenerates to a multidendrite structure if the thermal geometry is incorrect, or if the pull speed or undercooling are too great.) Therefore, we adjusted many system parameters in a partially successful attempt to improve web morphology and eliminate surface texturing. It was chemical modification of the melt, however, which
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had the greatest impact on the GaAs web growth habit, Doping the melt with 3.0—0.3 at% Ge changed the web dramatically, producing narrow crystals of more conventional morphology (fig. 5). A large percentage of the Ge-doped crystals contained more extensive web areas (in proportion to the overall crystal size) and fewer dendrites than other crystals. On close examination the flat, textured web areas were composed of small facets as iilustrated in fig. 5. These morphological improvements are consistent with the reduced undercooling (2—6°C) at which the doped crystals grew. (Conventional web morphology apparently arises from anisotropic attachment kinetics which are effective only if the undercooling is not excessive see following section). One might surmise that the only direct result of the Ge doping was the reduction of the difference between the “hold” and “buttoning” temperatures in the system so that growth was mitiated at reduced undercoolings. In this case, the improved web morphology could be attributed solely to propagation at these reduced undercoolings and not directly related to the Ge doping. However, when the melt doping level was reduced to ~ 0.5 at% Ge to eliminate Ge inclusions which formed at higher doping levels, the tendency for multidendrite morphology increased but the undercooling remained in the same range (fig. 6). Therefore, we conclude that the Ge dopant (1) produced a direct improvement of the GaAs mor—
phology which decreased with decreasing Ge content between 3.0 and 0.3 at% melt doping; and (2) produced a reduction in undercooling which was relatively insensitive to changes in doping level within the stated range. (Since the most heavily doped experiments were performed first and later experiments were performed with decreasing Ge doping levels, we believe the slight decrease in undercooling from left to right in fig. 6 reflects only the increasing skill of the operator in successive experiments with the doped system). Prior to the doping experiments we had assumed that the unconventional GaAs web morphology resulted from excessive undercooling and that this undercooling requirement itself was due to improper thermal geometry or incorrect growth parameters. However, the results of chemical modification of the melt indicate that inherent growth kinetics of the GaAs itself generate the atypical web morphology, even in the conventional undercooling regime. Therefore, we considered in detail
the facetting behavior, twin structure, and orientation relationships of GaAs web to determine if these characteristics reflect attachment kinetics which can account for the over-all growth hehavior. These analyses are presented in the context of the facetted interface model for stable web growth whtch is outlined briefly below. Interface stability A rigorous theory of web interface stability has not been developed; however, plausible growth
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TA. Gould, A - M. Stewart
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Anomalous growth behavior of GaAs dendritic web
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(lIlt Facets
Twin Planes Layer Nucleation
Fig. 7. Schematic section of web growth illustrating the facetted interface. Layer spreading is initiated by corner nucleation at the intersection of the dendrite and the web.
models of the Si web interface suggest that atomic attachment processes play a crucial role in stable web propagation. It is assumed that the Si web interface is stable against dendritic breakdown in the presence of an undercooled melt because it is bounded by (111) facets. Pronounced (ill) facetting in Si indicates that atomic attachment is difficult on these planes. Thus nucleation on the facetted interface is difficult, while corner nucleation at the intersection of the web and bounding dendrite is energetically more favourable. Forward propagation then occurs by rapid lateral growth across the interface once a layer is nucleated, as illustrated in fig. 7 [7]. One might expect the twin structure of the web to provide a mechanism for spontaneous dendrite growth from the interface by the reentrant corner mechanism that is responsible for the propagation of single dendrites [8]. The stability of the web interface against dendrite breakdown is attributed to the fact that any interface protuberance is immediately eliminated by rapid layer spreading. The
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latent heat flow from the flat growth front further inhibits dendritic propagation. However, if the undercooling is too large, these nucleation barriers are overcome and random dendrite growth occurs [7]. Although the microscopic features of the growth interface cannot be observed, the web crystal itself provides clues to interface stability in terms of the foregoing model. Si web crystals are mirror smooth, consisting of essentially a single (111) facet. The GaAs web surfaces consisted of many small (ill) facets rather than a smooth ribbon face. This implies that (1) nucleation on the (111) faces was not as difficult for GaAs as it is for Si, and/or (2) steps or perturbations on the (111) GaAs ribbon face were not removed by layer spreading. If these conditions apply to the (111) planes on the crystal—liquid interface as well as the (111) ribbon faces, we would expect the GaAs web interface to be less stable against dendritic breakdown than the Si web interface. The persistent multidendrite morphology of our GaAs webs is consistent with this model. The rough facets and steps on the (111) GaAs web faces were smoothed as a result of Ge doping. This observable change in the (Ill) facetting of the web faces implies a concurrent change in interface facetting. In the context of our model we would expect increased interface stability against dendritic breakdown in these crystals. This was borne out experimentally by the reduction of multidendrite morphology in the Ge-doped crystals. Also, both of these morphological improvements simultaneously degenerated as the Ge content was reduced. Therefore, the morphology and growth behavior of both doped and undoped GaAs web crystals are consistent with the facetted interface model if we assume that the Ge doping stabilizes the facetting on (111 )GaAs planes. There is no obvious mechanism by which we might expect Ge to stabilize the (111) facetting in GaAs. However, since the web interface is maintamed by rapid layer spreading, the effect of doping on GaAs growth terraces may be relevant to this question. In this context, Toyoda et a!. [9] found that the width and height of growth terraces on (100) LPE GaAs decreased with increasing concentrations of Ge and Sn dopants. While these
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A nomalous growth behavior of GaAs dendritic web Web
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Fig. 8. Facetted interface model of GaAs web. The [211] growth direction is hounded h~an As(lI1) plane while the [211] growth direction is bounded by a Ga(1l I) plane. (Dewald’s notion [11] for ~l 1 l} planes.)
results cannot be interpreted directly in terms of melt growth on the (111) planes, they corroborate a Ge-doping effect on layer spreading mechanisms in GaAs. A second factor which influences the facetting of (111) planes and may impact interface stability is the polar nature of GaAs. A difference in facet-
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Fig. 9. Facetted interface for three-twin-plane webs. For both interface shapes half of the interface is bounded by a Ga(11l) and half is bounded by an As( ITT).
ting between (lll)Ga and the (lll)As surfaces has been observed in Czochralski growth 110] and was also apparent on our dendrite and web surfaces (fig. 4). This characteristic influences interface stability because both (Ill )Ga and (111 )As planes outcrop at the growth interface of a twinned web crystal. For example. in webs with two twin planes the interface is bounded primarily by an As plane for growth in the [211] direction. Conversely, growth (fig. 8). A three-twin-plane structure requires that halfthe in of
[~l the I]crystal direction growsis inbounded the [211]bydirection, a Ga plane and
half in the [~l I]as direction. shapes result, shown inTwo fig. possible 9, with interface half the interface bounded by an As plane. and half by a Ga plane. Si web typically grows with three twin planes, but for GaAs the three twin plane structure was very difficult to obtain and propagate. Two-twinplane crystals grew with equal ease in the [211] or [211] directions, but only 12 of 75 webs and dendrites that were examined in cross section con-
tained three twin planes. Of these, only two maintamed that structure throughout the entire length of the crystal.
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form across a twinned interface. In this regard, it is interesting to note that Ge can be incorporated on both Ga and As sites during melt growth [12]. The improved growth behavior produced by the Ge doping may have been due in part to a modification of the polar nature of the tnterface How
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growth behavior of GaAs dendritic web
.
.
ever the data are too few to indicate if stabiltza tion of three twin plane structures occurred in Ge-doped crystals.
ioo~.. 5. Conclusions We have grown single-crystal GaAs ribbons by a liquid-encapsulated dendritic web technique. In Fig. 10. cross section of dendrite and ~eh showing degeneration of three-twin-plane structure. A thin [211]lamella (heavily etched) is widening into the main [211] section (unetched) of the dendrite,
Five crystals in which the three-twin-plane structure degenerated were examined in detail. Four of these degenerated to a_ two-twin-plane structure by adopting an overall [211] orientation. In terms of fig. 9 the thin lamella propagating in the [211] direction grew at the expense of the surrounding [211] matrix. The outer twin plane bounding this lamella became incoherent and moved progressively toward the crystal face in successive cross sections until an overall [211] growth direction (fig. 8) resulted. This occurred for both interface shapes shown in fig. 9. Fig. 10 illustrates this process in cross section. The heavily_etched areas of the dendrite and web denote the [211] direction and delineate the widening of a [211] lamella into the [211] (unetched) portion of the dendrite. Note that the three-twinplane structure was lost from the web area before it was completely lost from the bounding dendrite. This is a typical pattern in the degeneration of the three-twin-plane structures and indicates that the reentrant edge mechanism of dendrite growth maintained the structure more easily than did the layer spreading mechanism of web growth. Clearly, the polar nature of GaAs impacts the stability of three-twin-plane structures. More subtle polar effects may influence interface stability in general since the attachment kinetics are not uni-
contrast to conventional Si web crystals, the GaAs webs exhibited multiple dendrites and heavy surface facets despite careful optimization of the thermal geometry and growth parameters. However, a significant improvement in GaAs web morphology resulted from doping the melt with Ge. This response to chemical modification indicates that fundamental attachment kinetics are responsible for the anomalous morphology of GaAs web. The mechanism by which Ge influences the growth kinetics of GaAs is not known; however, the doping results are self-consistent in the context of the facetted interface model of web growth. The Ge doping clearly influenced the facetting of the GaAs(l 11) planes which stabilize the web interface against dendritic breakdown. Also, Ge incorporation may have modified the polar nature of the GaAs web interface. The results show that web facetting hence, web interface stability can be influenced and improved by chemical modification. The details of dopant effects on web growth mechanisms have yet to be determined. —
—
Acknowledgements The authors are indebted to T.J. Isaacs for the insightful Ge doping recommendation. Our thanks also to R.G. Seidensticker, R. Mazelsky, RH. Hopkins, and J.P. McHugh for helpful discussions. This work was supported in part by the Aerospace Power Division, Wright Air Force
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Aeronautical Laboratories, Contract F33615-78-C2031. References [1] R.G. Seidensticker and RH. Hopkins. J. Crystal Growth 50 (1980) 221.
[2] TA. Gould and AM. Stewart, J. Crystal Growth 54 (1981) 399. [3] TA. Gould, R.G. Seidensticker and R. Mazelsky, Ribbon Growth of Single-Crystal GaAs For Solar Cell Application, Air Force Contract F33615-78-C-2031, Final Report No. AFWAL-TR-81-21 15. Westinghouse Electric Corporation (1981). [4] J.W. Faust, in: Compound Semiconductors, Vol. 1, Preparation of 111—V Compounds, Eds. R.K. Willardson and H.L. Goering (Reinhold, New York, 1962).
growth behavior of GaAs dendritic web
[5] MS. Abrahams and C.J. Buiocchi, J. AppI. Phys. 36 (1965) 2855. [6] 0. Lindberg and J.W. Faust, in: Compound Semiconductors, Vol. 1, Preparation of Ill—V Compounds, Eds. R.K. Willardson and H.L. Goering (Reinhold, New York, 1962). [7] R.G. Seidensticker, in: Crystals — Growth, Properties. Applications, Vol. 8; Silicon, Chemical Etching, Ed. J. Grabmaier (Springer, New York, 1982). [8] D.R. Hamilton and R.G. Seidensticker, J. Appi. Phys. 31 (1960) 1165. [9] N. Toyoda, M. Mihara and T. Hara. Phys. Status Solidi (a) 54 (1979) 225. [10] A. Steinemann and U. Zimmerli, Solid State Electron 6 (1963) 597. [11] J.F. Dewald, J. Electrochem. Soc. 104 (1957) 244. [12] D.T.J. Hurle, J. Phys. Chem. Solids 40 (1979) 647.
309
ANOMALOUS GROWTH BEHAVIOR OF GaAs DENDRITIC WEB
*
T.A. GOULD and AM. STEWART Westinghouse R&D Center, 1310 Beulah Road, Pittsburgh, Pennsylvania 15235, USA
Received 22 October 1982; manuscript received in final form 18 January 1983
We have grown single-crystal GaAs ribbons by a liquid-encapsulated dendritic web technique. Undoped GaAs webs exhibited multiple dendrites and heavy surface facets despite careful optimization of the growth system. GaAs ribbons of more conventional web morphology were obtained from Ge-doped melts. The doping results indicate that atypical GaAs web morphology arises from fundamental attachment kinetics which are modified by incorporation of Ge. The observed relationships between facetting and growth behavior of both doped and undoped GaAs webs is consistent with the facetted interface model of web growth.
1. Introduction The dendritic web technique is a ribbon growth process which has been highly successful for the growth of single crystal Si ribbons for solar cells [1]. We investigated the application of this technique to GaAs as a cost-effective method for growing high efficiency solar cell base material. Dendritic web growth techniques and thermal modelling were combined with a B203-encapsulated growth system [2]. We obtained single crystal GaAs ribbons; however, they exhibited an unconventional multidendrite, textured morphology. Analyses of chemical modification, facetting behavior and twin orientation relationships in the context of web interface stability suggest that the inherent atomic attachment kinetics of GaAs produce atypical web morphology under what otherwise would be conventional web growth conditions.
ated with the thermal environment induce lateral growth to form a thickened “button”. When the button is pulled from the melt two dendrites propagate from the ends, and a film of liquid is drawn up between them. In Si web growth this film freezes into a single crystal ribbon which is smooth and well-suited for subsequent device fabrication. However, the GaAs webs were heavily textured with multiple dendrites and surface facets. 2. Experimental procedures The GaAs web pulling system and crystal growth procedures are described in detail elsewhere f.~••~________~~ DendrIte Seed Button ~Bounding
Typical Si web morphology is illustrated schematically in fig. 1. A dendrite seed with the required twin structure is brought into contact with the melt at the “hold” temperature. At this ternperature the seed neither melts nor nucleates growth. As the melt is undercooled, the twin structure of the seed and the attachment kinetics associ-
Dendrites Web
~anes
1oi’ 1~ ~/ LiquId
Dendrite Tip *
This work supported in part by the Aerospace Power Division, Wright Air Force Aeronautical Laboratories, Contract F33615-78-C-2031.
0022-0248/83/0000—0000/$03.OO
©
Fig. I. Schematic section of conventional web growth.
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Anomalous growth behavior of GaAs dendritie web
[2,3]. Several parameters changed, however, as growth conditions were optimized throughout the experiments. Pull speeds for seed dendrites and webs varied mostly from 1.5—3.0 cm/mm. The range of apparent undercooling required for stable web propagation (measured by thermocouples inserted next to the crucible) was reduced to 8—l2°C from 15—20°C by optimization of the thermal geometry. Doping the GaAs melt with 0.3—3.0 at% Ge produced a further reduction of this parameter to the 2—6°Crange, values typical of the undercooling range for conventional Si web growth. We determined the crystallographic orientation of web surface facets by a 4 mm etch in tartaric acid: HNO3, 3: 1 [4], or by first smoothing the surface with a 1—3 mm AB etch [5] and reducing the time in the subsequent tartaric acid etch. Doped samples were more sensitive to the etchants and required shorter etching times. Both methods produced triangular etch pits on the Ga face of the web pointing in the (211) directions [6]. The twin structures of over 70 GaAs dendrites and webs were analyzed in cross section after mounting them in Quickmount and polishing. Etching these cross sections for 5—lO s with HF: H2O: Cr03 (saturated solution), 20:20: 1, or for 3—5 s with HNO3: H20, 4: 3, revealed the twin planes. The nitric acid etch was also useful for determining orientation relationships from the
Fig. 2. Cross section of an odd-ts~in-planedendrite etched with HNO~:H20; 4:3. The [211] direction at the top of the crystal responded to the etch faster than the [211] Section at the bottom,
cross sections since the [2111 direction was more sensitive to the etch than the [2111 direction (fig. 2).
3. Web morphology 3.1. Undoped GaAs web In contrast to Si web, which grows as a single. smooth (111) facet bounded by edge dendrites (fig. 1), the GaAs webs grew as multidendrite crystals with narrow web areas textured by small, individual (111) facets (fig. 3). A total of 48 GaAs webs were grown ranging in length from 1.2 cm to over 30.0 cm, the pulling limit of the apparatus. The width was typically 1.5 cm for undoped crystals and 1.0 cm for Ge-doped crystals. (The narrow width of the Ge-doped crystals reflects the small undercooling at which they grew.) The web areas between the dendrites were 100—300 ~sm thick,
‘ -
\ Fig. ~. GaAs ~~chse~hihitingniultidendrite morphology and heavily facetted s;eh areas.
TA. Gould, AM. Stewart
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Anomalous growth behavior
of GaAs dendrlzk
311
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Fig. 4. ( omparison ol Gal Ill) face I top) and .As 1111 face (bottom) of a GaAs dendrite. The Ga face is rough with vertically developed facets, while the As face is smoother, (Droplet features are an artifact of surface As loss during growth.)
structure, one ribbon face is composed of (11 1)Ga planes, while the opposite face is composed of (lll)As planes [6]. Both seed dendrites and web crystals exhibited a marked difference in morphology on these faces. The Ga face was rough with vertically developed facets, while the As face was flatter and smoother (fig. 4). 3.2. Ge doped GaAs web
which is comparable to typical Si web thickness. The crystals grew in [211] direction and developed (111) faces. The rough, textured surfaces of the multidendrite crystals suggested that the web was polycrystalline. However, several randomly selected samples yielded single crystal (111) Laue patterns. Also, surface etching with the tartaric acid etchant which was performed regularly throughout the experiments, consistently produced triangular etch pits which remained aligned across facet boundaries. Therefore, the facetted morphology was a surface texture of single-crystal ribbons. Because GaAs crystallizes in the zinc blende
The growth behavior and resultant morphology of web crystals are controlled by the interaction of the system thermal geometry with the atomic attachment kinetics of the material. There exists a “growth window” of suitable parameter ranges for which stable web propagation will occur. (For example, Si web degenerates to a multidendrite structure if the thermal geometry is incorrect, or if the pull speed or undercooling are too great.) Therefore, we adjusted many system parameters in a partially successful attempt to improve web morphology and eliminate surface texturing. It was chemical modification of the melt, however, which
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had the greatest impact on the GaAs web growth habit, Doping the melt with 3.0—0.3 at% Ge changed the web dramatically, producing narrow crystals of more conventional morphology (fig. 5). A large percentage of the Ge-doped crystals contained more extensive web areas (in proportion to the overall crystal size) and fewer dendrites than other crystals. On close examination the flat, textured web areas were composed of small facets as iilustrated in fig. 5. These morphological improvements are consistent with the reduced undercooling (2—6°C) at which the doped crystals grew. (Conventional web morphology apparently arises from anisotropic attachment kinetics which are effective only if the undercooling is not excessive see following section). One might surmise that the only direct result of the Ge doping was the reduction of the difference between the “hold” and “buttoning” temperatures in the system so that growth was mitiated at reduced undercoolings. In this case, the improved web morphology could be attributed solely to propagation at these reduced undercoolings and not directly related to the Ge doping. However, when the melt doping level was reduced to ~ 0.5 at% Ge to eliminate Ge inclusions which formed at higher doping levels, the tendency for multidendrite morphology increased but the undercooling remained in the same range (fig. 6). Therefore, we conclude that the Ge dopant (1) produced a direct improvement of the GaAs mor—
phology which decreased with decreasing Ge content between 3.0 and 0.3 at% melt doping; and (2) produced a reduction in undercooling which was relatively insensitive to changes in doping level within the stated range. (Since the most heavily doped experiments were performed first and later experiments were performed with decreasing Ge doping levels, we believe the slight decrease in undercooling from left to right in fig. 6 reflects only the increasing skill of the operator in successive experiments with the doped system). Prior to the doping experiments we had assumed that the unconventional GaAs web morphology resulted from excessive undercooling and that this undercooling requirement itself was due to improper thermal geometry or incorrect growth parameters. However, the results of chemical modification of the melt indicate that inherent growth kinetics of the GaAs itself generate the atypical web morphology, even in the conventional undercooling regime. Therefore, we considered in detail
the facetting behavior, twin structure, and orientation relationships of GaAs web to determine if these characteristics reflect attachment kinetics which can account for the over-all growth hehavior. These analyses are presented in the context of the facetted interface model for stable web growth whtch is outlined briefly below. Interface stability A rigorous theory of web interface stability has not been developed; however, plausible growth
~•
TA. Gould, A - M. Stewart
/
Anomalous growth behavior of GaAs dendritic web
~
(lIlt Facets
Twin Planes Layer Nucleation
Fig. 7. Schematic section of web growth illustrating the facetted interface. Layer spreading is initiated by corner nucleation at the intersection of the dendrite and the web.
models of the Si web interface suggest that atomic attachment processes play a crucial role in stable web propagation. It is assumed that the Si web interface is stable against dendritic breakdown in the presence of an undercooled melt because it is bounded by (111) facets. Pronounced (ill) facetting in Si indicates that atomic attachment is difficult on these planes. Thus nucleation on the facetted interface is difficult, while corner nucleation at the intersection of the web and bounding dendrite is energetically more favourable. Forward propagation then occurs by rapid lateral growth across the interface once a layer is nucleated, as illustrated in fig. 7 [7]. One might expect the twin structure of the web to provide a mechanism for spontaneous dendrite growth from the interface by the reentrant corner mechanism that is responsible for the propagation of single dendrites [8]. The stability of the web interface against dendrite breakdown is attributed to the fact that any interface protuberance is immediately eliminated by rapid layer spreading. The
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latent heat flow from the flat growth front further inhibits dendritic propagation. However, if the undercooling is too large, these nucleation barriers are overcome and random dendrite growth occurs [7]. Although the microscopic features of the growth interface cannot be observed, the web crystal itself provides clues to interface stability in terms of the foregoing model. Si web crystals are mirror smooth, consisting of essentially a single (111) facet. The GaAs web surfaces consisted of many small (ill) facets rather than a smooth ribbon face. This implies that (1) nucleation on the (111) faces was not as difficult for GaAs as it is for Si, and/or (2) steps or perturbations on the (111) GaAs ribbon face were not removed by layer spreading. If these conditions apply to the (111) planes on the crystal—liquid interface as well as the (111) ribbon faces, we would expect the GaAs web interface to be less stable against dendritic breakdown than the Si web interface. The persistent multidendrite morphology of our GaAs webs is consistent with this model. The rough facets and steps on the (111) GaAs web faces were smoothed as a result of Ge doping. This observable change in the (Ill) facetting of the web faces implies a concurrent change in interface facetting. In the context of our model we would expect increased interface stability against dendritic breakdown in these crystals. This was borne out experimentally by the reduction of multidendrite morphology in the Ge-doped crystals. Also, both of these morphological improvements simultaneously degenerated as the Ge content was reduced. Therefore, the morphology and growth behavior of both doped and undoped GaAs web crystals are consistent with the facetted interface model if we assume that the Ge doping stabilizes the facetting on (111 )GaAs planes. There is no obvious mechanism by which we might expect Ge to stabilize the (111) facetting in GaAs. However, since the web interface is maintamed by rapid layer spreading, the effect of doping on GaAs growth terraces may be relevant to this question. In this context, Toyoda et a!. [9] found that the width and height of growth terraces on (100) LPE GaAs decreased with increasing concentrations of Ge and Sn dopants. While these
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TA. Gould. A - M. Stewart
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A nomalous growth behavior of GaAs dendritic web Web
Web Fac~twt~
~
/
Faces
Faces
/
Direction
11111
11111 ~ As
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10111
11111 WEB Growth SIDE VIEW As
Ga
10111
~
(211] Growth Direction
~
12111 Growth Direction
(1111
Facetted~’N
[1111 Ga
Ga
Fig. 8. Facetted interface model of GaAs web. The [211] growth direction is hounded h~an As(lI1) plane while the [211] growth direction is bounded by a Ga(1l I) plane. (Dewald’s notion [11] for ~l 1 l} planes.)
results cannot be interpreted directly in terms of melt growth on the (111) planes, they corroborate a Ge-doping effect on layer spreading mechanisms in GaAs. A second factor which influences the facetting of (111) planes and may impact interface stability is the polar nature of GaAs. A difference in facet-
J
Direction As I~l
Ga
[Ii~j ~IIl
As
Ga
~
Growth As
11111
10111
~ ~
G
12111 Growth Direction 12111 Growth Direction
Fig. 9. Facetted interface for three-twin-plane webs. For both interface shapes half of the interface is bounded by a Ga(11l) and half is bounded by an As( ITT).
ting between (lll)Ga and the (lll)As surfaces has been observed in Czochralski growth 110] and was also apparent on our dendrite and web surfaces (fig. 4). This characteristic influences interface stability because both (Ill )Ga and (111 )As planes outcrop at the growth interface of a twinned web crystal. For example. in webs with two twin planes the interface is bounded primarily by an As plane for growth in the [211] direction. Conversely, growth (fig. 8). A three-twin-plane structure requires that halfthe in of
[~l the I]crystal direction growsis inbounded the [211]bydirection, a Ga plane and
half in the [~l I]as direction. shapes result, shown inTwo fig. possible 9, with interface half the interface bounded by an As plane. and half by a Ga plane. Si web typically grows with three twin planes, but for GaAs the three twin plane structure was very difficult to obtain and propagate. Two-twinplane crystals grew with equal ease in the [211] or [211] directions, but only 12 of 75 webs and dendrites that were examined in cross section con-
tained three twin planes. Of these, only two maintamed that structure throughout the entire length of the crystal.
TA. Goul4 A.M Stewart
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form across a twinned interface. In this regard, it is interesting to note that Ge can be incorporated on both Ga and As sites during melt growth [12]. The improved growth behavior produced by the Ge doping may have been due in part to a modification of the polar nature of the tnterface How
-
-!
•
growth behavior of GaAs dendritic web
.
.
ever the data are too few to indicate if stabiltza tion of three twin plane structures occurred in Ge-doped crystals.
ioo~.. 5. Conclusions We have grown single-crystal GaAs ribbons by a liquid-encapsulated dendritic web technique. In Fig. 10. cross section of dendrite and ~eh showing degeneration of three-twin-plane structure. A thin [211]lamella (heavily etched) is widening into the main [211] section (unetched) of the dendrite,
Five crystals in which the three-twin-plane structure degenerated were examined in detail. Four of these degenerated to a_ two-twin-plane structure by adopting an overall [211] orientation. In terms of fig. 9 the thin lamella propagating in the [211] direction grew at the expense of the surrounding [211] matrix. The outer twin plane bounding this lamella became incoherent and moved progressively toward the crystal face in successive cross sections until an overall [211] growth direction (fig. 8) resulted. This occurred for both interface shapes shown in fig. 9. Fig. 10 illustrates this process in cross section. The heavily_etched areas of the dendrite and web denote the [211] direction and delineate the widening of a [211] lamella into the [211] (unetched) portion of the dendrite. Note that the three-twinplane structure was lost from the web area before it was completely lost from the bounding dendrite. This is a typical pattern in the degeneration of the three-twin-plane structures and indicates that the reentrant edge mechanism of dendrite growth maintained the structure more easily than did the layer spreading mechanism of web growth. Clearly, the polar nature of GaAs impacts the stability of three-twin-plane structures. More subtle polar effects may influence interface stability in general since the attachment kinetics are not uni-
contrast to conventional Si web crystals, the GaAs webs exhibited multiple dendrites and heavy surface facets despite careful optimization of the thermal geometry and growth parameters. However, a significant improvement in GaAs web morphology resulted from doping the melt with Ge. This response to chemical modification indicates that fundamental attachment kinetics are responsible for the anomalous morphology of GaAs web. The mechanism by which Ge influences the growth kinetics of GaAs is not known; however, the doping results are self-consistent in the context of the facetted interface model of web growth. The Ge doping clearly influenced the facetting of the GaAs(l 11) planes which stabilize the web interface against dendritic breakdown. Also, Ge incorporation may have modified the polar nature of the GaAs web interface. The results show that web facetting hence, web interface stability can be influenced and improved by chemical modification. The details of dopant effects on web growth mechanisms have yet to be determined. —
—
Acknowledgements The authors are indebted to T.J. Isaacs for the insightful Ge doping recommendation. Our thanks also to R.G. Seidensticker, R. Mazelsky, RH. Hopkins, and J.P. McHugh for helpful discussions. This work was supported in part by the Aerospace Power Division, Wright Air Force
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Aeronautical Laboratories, Contract F33615-78-C2031. References [1] R.G. Seidensticker and RH. Hopkins. J. Crystal Growth 50 (1980) 221.
[2] TA. Gould and AM. Stewart, J. Crystal Growth 54 (1981) 399. [3] TA. Gould, R.G. Seidensticker and R. Mazelsky, Ribbon Growth of Single-Crystal GaAs For Solar Cell Application, Air Force Contract F33615-78-C-2031, Final Report No. AFWAL-TR-81-21 15. Westinghouse Electric Corporation (1981). [4] J.W. Faust, in: Compound Semiconductors, Vol. 1, Preparation of 111—V Compounds, Eds. R.K. Willardson and H.L. Goering (Reinhold, New York, 1962).
growth behavior of GaAs dendritic web
[5] MS. Abrahams and C.J. Buiocchi, J. AppI. Phys. 36 (1965) 2855. [6] 0. Lindberg and J.W. Faust, in: Compound Semiconductors, Vol. 1, Preparation of Ill—V Compounds, Eds. R.K. Willardson and H.L. Goering (Reinhold, New York, 1962). [7] R.G. Seidensticker, in: Crystals — Growth, Properties. Applications, Vol. 8; Silicon, Chemical Etching, Ed. J. Grabmaier (Springer, New York, 1982). [8] D.R. Hamilton and R.G. Seidensticker, J. Appi. Phys. 31 (1960) 1165. [9] N. Toyoda, M. Mihara and T. Hara. Phys. Status Solidi (a) 54 (1979) 225. [10] A. Steinemann and U. Zimmerli, Solid State Electron 6 (1963) 597. [11] J.F. Dewald, J. Electrochem. Soc. 104 (1957) 244. [12] D.T.J. Hurle, J. Phys. Chem. Solids 40 (1979) 647.