- Email: [email protected]
Bridgman solidification of GaSb in space
Journal of Crystal Growth 192 (1998) 63—72
Bridgman solidification of GaSb in space T. Duffar!,*, M.D. Serrano", C.D. Moore#, J. Camassel$, S. Contreras$, P. Dusserre!, A. Rivoallant!, B.K. Tanner# ! CEA/CEREM/DEM, 17, Avenue des Martyrs, 38054 Grenoble, France " Dpto. Fisica de Materiales, University of Autonoma de Madrid, 28049 Madrid, Spain # Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK $ Groupe d+Etude des Semiconducteurs, University of Montpellier, France Received 2 January 1998; accepted 20 March 1998
Abstract Two samples of GaSb and Ga In Sb (x"0.001) have been grown in rough crucibles in space and full detachment of 1~x x the sample from the crucible was obtained over a large part of the crystals. Due to problems with the control system of the space furnace both seeds were molten and the growth began with polycrystals. As the growth proceeded, one grain was selected in each sample and the growth finished with a single crystal. X-ray analysis has shown that, in the zone where there was no contact with the crucible, the structural quality continuously improved. Chemical segregation obtained in space is representative of a diffusive regime of transport in the melt, in spite of the low growth rate used. This differs from ground experiments which are under convective conditions. Measurements of electrical properties have given results comparable to that of standard samples obtained on earth, but with a rather lower electrical resistivity. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Microgravity; Bridgman growth; GaSb; Crucible; Segregation
1. Introduction It is well known that crucible-sample interactions have a strong effect on crystals growing from the melt, especially in the case of the Bridgman growth process where the sample is in contact with
* Corresponding author. Fax: #33 376 88 51 17: e-mail: [email protected].
the crucible all the time. The interactions are of several kinds: chemical at the liquid-crucible interface, thermal (particularly at the crystal—melt— crucible triple interface) and mechanical at the crystal—crucible interface. We have studied the effect of rough crucibles on the growth of semiconductors in space [1] and it was earlier shown that, if the roughness of the crucible satisfies some geometrical conditions, it is possible to reduce strongly chemical and thermal [2] interactions and to fully cancel the mechanical contact between the crystal and the
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 2 1 - 7
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T. Duffar et al. / Journal of Crystal Growth 192 (1998) 63–72
crucible. It can then be expected that the structural quality of the crystal should be improved and such rough crucibles were used for experiment described in this paper. This was devoted to the growth of GaSb related crystals, taken as a model material, on board the automatic satellite EURECA. Another field of interest is related to chemical segregation during solidification. It is well known that the solutal boundary layer present on the liquid side of the solid—liquid interface is disturbed by convection in the melt and that this leads to heterogeneity of the solute distribution in the crystal [3]. The thickness of this layer, and its sensibility to the liquid flow, depends inversely on the growth rate. Due to the large solidification time available, the experiment was focusing on growth rates one order of magnitude less than what has generally been investigated during other semiconductor Bridgman growth experiments in space: 0.1 lm/s rather than a few lm/s. The EURECA AMF-118 experiment was focused on investigating these effects during the growth of two pure GaSb crystals, as reference, and of two Ga In Sb (x"0.001) crystals, to study 1~x x segregation and its effect on the crystal structure, one on earth and the other one under microgravity conditions. The proposed experiment was accepted by the European Space Agency (ESA) to fly onboard the EURECA platform in the automatic mirror furnace (AMF).
2. Experimental set-up and procedure A drawing of the experimental ampoule is shown in Fig. 1. The sample was made of a stoichiometric GaSb or Ga In Sb feed material, 35 mm long, 1~x x prepared from 6N pure elements, and a single crystal seed, 40 mm long, made of pure GaSb. In order to prevent the occurrence of bubbles during melting in space, the feed and seed were joined by melting, by HF heating, with a thin layer of GaSb in between. The dislocation density in the seeds was lower than 100 cm~2 before integration. Both sides of the cylindrical sample were drilled in order to place a 1 cm long silica capillary at the seed end, to receive two thermocouples for temperature control, and a graphite rod plunged in the liquid, on the hotter side. The rough crucible was made of silica and machined with a screw thread with a 1 mm step and a cutting angle of 52°, in order to meet the geometrical conditions of de-wetting, taking into account the wetting angle of GaSb on silica [1,4]. The imposed internal diameter of the ampoule and the thickness of the crucible led to samples of 10 mm diameter. Some insulating (alumina wool) and conducting (graphite) parts were added to adjust the thermal gradient and a getter was placed in contact with the seed in order to pump any remaining or degassing atmosphere during heating. The purpose of the experiment was to get crystals as perfect as possible. To avoid the generation of
Fig. 1. Drawing of the experimental ampoule.
T. Duffar et al. / Journal of Crystal Growth 192 (1998) 63–72
dislocations of thermal origin, it is necessary to have an axial thermal gradient of a few °C/cm and radial thermal gradients as low as possible. The AMF is a mirror furnace designed to perform zone melting experiments, that means that heat is provided radially to the middle of the sample. This is of course totally inconsistent using our experiment and we solved the problem using a gold coating on the external wall of the ampoule which reflected the heating light. The only possibility for the heat to leave the furnace was then to enter the top (noncoated) of the ampoule and flow axially along the sample. In this way, we obtained an axial thermal gradient of 20°C/cm, increasing slightly all along the experiment up to 30°C/cm, as measured during preliminary thermal tests on earth with a thermocouple sliding in a capillary situated along-side the axis of the sample. The ampoule was moved along the furnace axis at a speed of 0.2 lm/s over 104 h and the resulting displacement rate of the interface, deduced from the same preliminary thermal tests, was found to be between 0.1 and 0.15 lm/s from the beginning to the end of the growth. This was found to be acceptable to solidify the 40 mm of molten sample. Due to delays in the launch of the satellite, the first sample (pure GaSb) was processed two years after the delivery of the experimental ampoule. Because of a problem with the regulation system of the furnace, it was not possible to get a stable temperature in the time alloted and the furnace was automatically cut-off. It was shown later that temperature oscillations during the attempts to reach the steady state caused the single crystal seed to melt. This sample was processed again at the end of AMF operations without regulation, at a fixed power approximated from the results of the first attempt. The second sample (Ga In Sb) was pro1~x x cessed in between, also without regulation to avoid the problems encountered with the first ampoule. Due to the still unexplained irreproductibility of the furnace, the approximated power was too high and a large part of the seed was also melted, as seen after opening the ampoule. The terrestrial reference samples were processed with the same thermal behaviour, i.e. melting of both seeds. The crucibles for terrestrial reference tests were smooth because otherwise hydrostatic pressure forces the liquid in the screw thread and the resulting crystals are bro-
65
ken due to the huge mechanical stresses suffered during the cooling process.
3. De-wetting and crystal structure The ground samples were polycrystals, with grain sizes of the order of a few millimeters. This is not surprising due to the practically full melting of the seeds. Fig. 2 shows an X-ray picture of both flight ampoules as received. There is evidence that de-wetting occurred, with some necking of the crystal diameter and no penetration of the material in the screw thread. Also the melting of the seeds can be observed, total for the first sample and only 1 mm remaining for the second one. The samples were not sticking on the ampoules and were easily removed, Fig. 3 shows their external aspect. The contact of the sample with the inner part of the crucible occurred only on the hot side but the periodic waves predicted by the theory [1] were not present, in contradiction to observations made in a previous space growth of GaSb during the D2 Spacelab mission, using the same kind of crucible [4]. This is attributed to the oxidation of the external surface of the sample which can easily be seen in the picture. Either the getter, which has been heated up to 700°C during the experiment, was not efficient enough, or there had been too much degassing or diffusion through the silica wall during the two year storage of the ampoules. Observation of the samples showed grains at the periphery, so that no special orientation was chosen for cutting them longitudinally. Fig. 4 is the result of their metallographic analysis by etching for 1 min with (1 HCl, 1 H O , 2 H O) after 2 2 2 lapping and polishing. They both began to grow as polycrystals but after some distance one grain was selected which occupied the whole diameter. An X-ray analysis by the Laue back-scattering method shows a good homogeneity of the grain orientation, with misorientations of the order of 1° between different places in the selected grains. The Ga In Sb sample inherited its orientation from 1~x x the S1 1 1T seed and the dislocation density in this plane revealed by etching for 2 min with (5 HNO , 3 5 HF, 3 CH COOH, 11 H O), at the position 3 2
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Fig. 3. External view of the flight samples, (a) GaSb and (b) Ga In Sb. 1~x x
Fig. 2. X-ray pictures of the flight ampoules as received, (a) GaSb and (b) Ga In Sb. 1~x x
where the structural quality is the best, was between 104 and 105 cmv2. It was not possible to find a simple crystallographic direction of growth for the pure GaSb crystal.
X-ray transmission topographs (Laue images) were taken through the samples, thanks to the high flux and low wavelengths available at the European synchrotron radiation facility. The exposure time was about 100 s and the sample was translated in the beam in order to get a picture every cm. In order to image the whole sample at once, reflection topography was also performed, using a small angle of incidence and tilting the film in order to expand the image. For the alloyed sample, examination of the topographs shows that a slightly strained layer exists at
T. Duffar et al. / Journal of Crystal Growth 192 (1998) 63–72
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the sample, a tendency towards polycrystallinity is observed, with an absence of uniform diffraction images, showing a high degree of strain. This marked deterioration at the end of the sample could be caused by the increase of the In concentration, uncontrolled conditions of growth or contact with the crucible. An interesting observation is that no strain was seen associated with the silica capillary trapped in the crystal at the seed side. The topographs of the second, pure GaSb, sample show that the polycrystalline structure improves gradually away from the seed as can be seen in the two transmission topographs of Fig. 6, taken in the middle and at the top of the sample. The size of the diffracted spots increases and it can be concluded that the structure improved during the growth to the extent that a single crystal begins to grow from the polycrystalline material. However, in spite of this constant improvement, the quality of the sample remains poor.
4. Chemical segregation and properties
Fig. 4. Metallography of a longitudinal cut of the flight samples, (a) GaSb and (b) Ga In Sb. 1~x x
the junction between the crystal grown and the small remaining part of the seed, probably because of the step in chemical composition. On the topograph taken above the seed, it is observed that the crystal is growing in the S1 1 1T direction with twins and the side of the crystal is only slightly strained, where some individual dislocations are visible. The quality of the crystal increased during the growth as far as the growth conditions were controlled and no interaction existed with the crucible. Fig. 5 shows enlargements of reflection topographs below and above the point were the sample began to contact the teeth of the crucible. At this position the structure gradually deteriorates, with a high dislocation density at the edges. At the top of
Figs. 7 and 8 show the longitudinal and radial chemical analyses performed by a Secondary ion mass spectroscopy (SIMS) on the flight and ground alloyed samples respectively. The apparatus was calibrated with GaSb samples containing known amounts of In introduced by ion implantation. The experiment was designed to melt 5 mm of the seed and solidify a 40 mm long sample, after which the power of the heating lamp was decreased slowly, while pulling at a constant speed continued. Due to the overheating, much more sample was molten and perturbations of the longitudinal chemical profile can be seen on both samples after about 40 mm of solidification. It will then be considered that only this length has been solidified under controlled conditions. The segregation curve of the terrestrial sample fits well with the theoretical Scheill law for full mixing in the melt. The radial segregations are not symmetric; these might be due to misalignment of the sample with respect to the furnace axis or to the gravity vector. In spite of the fact that the longitudinal segregation for the flight sample looks like a diffusion controlled growth, it does not fit with the curve
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Fig. 5. Enlargements of the reflection X-ray topograph of the Ga In Sb sample, (a) in the region where there was no contact with the 1~x x crucible (position A on Fig. 4b) and (b) where this contact began (position B on Fig. 4b).
T. Duffar et al. / Journal of Crystal Growth 192 (1998) 63–72
Fig. 6. Transmission topographs of the GaSb sample taken (a) at the middle (position A on Fig. 4a) and (b) at the top of the sample (position C on Fig. 4a).
corresponding to diffusive conditions in a homogeneous liquid. As 35 mm of pure GaSb seed was melted prior to solidification and mixing of the liquid by diffusion was poor during the melting and stabilisation phase of the experiment, the characteristic diffusion length during these 8 h was less than 2 cm. We therefore used a simple one-dimensional finite-difference software, with implicit time resolution [5], to solve the diffusion equation in the frame of the moving interface, D
d2C dC L#» L"0, I dx2 dx
with the following boundary conditions: x'10D/» : dC /dx"0, I L x"0: D dC /dx"» (1!k)C . L I L,I
(1)
69
D is the diffusion coefficient of InSb in GaSb (1.2]10~8 m2/s), » the growth rate (0.1 lm/s), I k the segregation coefficient (0.15) and C the L,I concentration in the liquid at the interface. The initial composition of the liquid prior to solidification was taken as 1]1019 at/cm3 at 25 mm, corresponding to the seed, and the remaining at the feed composition was 7]1019 at/cm3. We obtain the dotted line represented in Fig. 7, which fits well with the experimental observation. It can then be reasonably estimated that the transport of solute in the liquid during the experiment (melting, stabilisation and solidification phases) was essentially diffusive. The fact that an oxide layer covered the sample probably helped by preventing any kind of Marangoni convection. The radial segregation curves suffer an asymmetry depending on the position; sometimes the highest concentration is on the left and sometimes on the right. This might be associated with low-frequency fluctuations in the direction of the gravity vector, which can disturb the solutal boundary layer without significantly decreasing its thickness [6]. Nevertheless, all attempts to find a periodic fluctuation of the chemical composition along the sides of the sample failed. The controlled solidification was stopped after 40 mm of pulling and the bump observed on Fig. 7 at 45 mm is attributed to quenching of the solutal boundary layer. Spark spectrographic chemical analysis was performed and compared to the composition of the initial material. Si, Al and K, elements typically associated with pollution by silica, were measured at the ppm level but without significant difference between the samples processed on earth and those in space, so there is no evidence of an improvement in the chemical pollution by the de-wetting phenomena observed in space. Finally six square wafers, two from GaSb sample and four from Ga In Sb, were cut in the metal1~x x lography plane, at the positions denoted B and C in Fig. 4a and Fig. 4b, respectively. They had typical dimensions of 5]6]1 mm3 and, on the six different pieces, both photoluminescence (PL) at 2 K and electronic transport properties have been investigated. Concerning first photoluminescence and the space GaSb sample, we have found that two
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Fig. 7. Longitudinal and radial (inlets, with the same at/cc scale) chemical analysis of the flight sample.
Fig. 8. Longitudinal and radial (inlets, with the same at/cc scale) chemical analysis of the ground sample.
transitions resolve predominantly in the spectral range 730—810 meV. They correspond with the well-known transitions BE and A, already dis4 cussed in the literature (see, for instance, [7] and references therein). The first structure comes from the radiative recombination of excitons bound to residual neutral acceptors (line BE ). The second 4 structure comes from donor—acceptor pair transitions (line A). This very characteristic PL
signature corresponds with weakly compensated material, the electrical properties being essentially governed by the double acceptor state associated with the classical lattice defect of nominally undoped p-type material. In this respect there is no strong difference between the space GaSb sample and similar material grown on earth. Weaker transitions are also resolve at &810, 766, 757 and 748 meV. They correspond now with the free
T. Duffar et al. / Journal of Crystal Growth 192 (1998) 63–72
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Fig. 9. PL spectra collected at 2 K for pure GaSb (at position B on Fig. 4a) and Ga In Sb (at position C in Fig. 4b) samples. 1~x x
exciton recombination line (denoted FE), the phonon replica of the BE line (denoted BE-LO), 4 an additional acceptor (denoted B) and phonon replica of the A line. This is shown in Fig. 9. When performing a deconvolution with standard Lorentzian functions, we find relatively small values (2 meV) for the full-width at half-maximum of the different components. When taken at several positions on the same sample, the amplitude and relative intensities of the different components vary slightly which indicates nonhomogeneous distribution of defects, residual impurities and nonradiative centers like dislocations, for instance. For the alloyed samples, the PL spectra appear very similar to the one collected on pure GaSb (see Fig. 9). The most striking feature is a shift of about 2 meV towards low energy. The similarity of pure and alloyed spectra shows that the overall growth conditions were very close while the magnitude of the low-energy shift allows independent check of the alloy composition. Using for the nonlinear variation of band gap energy versus composition the bowing parameter of [8], we find about 3.5]1019 indium atoms per cm3. Within experimental uncertainty, this appears in reasonably good agreement with the results of SIMS measurements. After completion of the PL experiments, indium dots were diffused at 300°C for 2—3 min in order to
form ohmic contacts. Then, resistivity and Hall effect measurements were conducted in the temperature range 80—300 K. Results indicated that all flight samples are less resistive (more conductive) than a typical reference sample cut from a Czochralski wafer by a typical factor of 0.7 for GaSb and 0.6 for the alloys. Analysis of the variation of carrier concentration and mobility as a function of temperature show that this is due to a higher number of residual carriers (typically by a factor of 4) compensated by a lower mobility (typically by a factor of 3). From wafer to wafer slight heterogeneity is observed, in good agreement with the PL data.
5. Conclusions The principle of using a rough crucible to get de-wetting of a semiconductor crystal in space has been demonstrated. The presence of an oxide layer on the sample, due to a very long storage of the ampoule before use, probably helped to obtain this behaviour. In spite of the fact that the single-crystal seeds were molten during the experiment, the samples finally grew as single crystals, after initial nucleation of polycrystals. Furthermore, the quality of the selected grain improved gradually during the
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T. Duffar et al. / Journal of Crystal Growth 192 (1998) 63–72
growth, at least when there was no contact with the crucible. It is noticeable that some of the authors have been growing antimonide crystals for more than a decade but have never before observed such behaviour. It is thus undoubtedly proven that the de-wetting obtained in space has a dramatic effect on the structural quality of semiconductor crystals, not only on stresses, twins and dislocations as already described by Larson in the case of CdTe [9], but also by allowing the sample to develop freely into a single crystal and improving its quality. On the other hand, due to the melting of the seeds, one aim of the experiment, namely looking at the structural effects of chemical segregation, was not achieved. Chemical segregations obtained in space are representative of diffusion solute transport in the melt. Thanks to the very low residual gravity levels on board EURECA, less than or equal to 10~7g in 0 the low-frequency range [10], this result has been achieved in spite of a growth rate one order of magnitude lower than that generally used in space Bridgman growth. Electrical properties measurements have given results comparable to the standard samples obtained on the earth, but with a rather lower electrical resistivity. We have thus demonstrated the benefit which can be obtained from growing crystals under microgravity, namely better structural quality, better homogeneity and without significant loss of electronic properties.
Acknowledgements This experiment has been performed in the framework of the GRAMME agreement between the CNES and the CEA and received support from ESA. M.D. Serrano would like to thank the Euro-
pean Community for his support in the frame of the HCM program, network no. CHRX-CT930106 and C.D. Moore acknowledges support from the UK EPSRC. The EURECA experiment was prepared with the help of the Dornier AMF team and the preliminary and ground reference tests have been performed in the DLR-MUSC, Ko¨ln. Great thanks are due to these very long-suffering and understanding people, especially to Mme. B. Pa¨tz for her great support and assistance. M Pernin (CEA/CEREM) helped with the SIMS analysis and C. Raffy (ENSEEG Grenoble) with their analysis. The experiments in ESRF were performed with the help of J. Baruchel and J. Hartwig and the spark emission chemical analysis was performed by M. Tabarant (CEA-Saclay).
References [1] T. Duffar, I. Harter, P. Dusserre, J. Crystal Growth 100 (1990) 171. [2] T. Duffar, M. Bal, J. Crystal Growth 151 (1995) 213. [3] J.P. Garandet, J.J. Favier, D. Camel, in: D.T.J. Hurle (Ed.), Handbook of Crystal Growth, vol. 2b, North-Holland, Amsterdam, 1994, p. 659. [4] T. Duffar, P. Dusserre, M.D. Serrano, Adv. Space Res. 16 (8) (1995) 101. [5] S. Corre, T. Duffar, M. Bernard, M. Espezel, J. Crystal Growth 180 (1997) 604. [6] J.P. Garandet, J. Crystal Growth 125 (1992) 112. [7] F. Meinardi, A. Parisini, L. Tarricone, Semicond. Science and Technol. 8 (1993) 1985. [8] D. Auvergne, J. Camassel, H. Mathieu, A. Joullie´, J. Phys. Chem. Solids 35 (1974) 133. [9] D.J. Larson, J.I.D. Alexander, D. Gillies, F.M. Carlson, J. Wu, D. Black, NASA Conf. Public. No. 3272, vol. 1, 1994, 129. [10] R. Danner, H. Hamacher, Mikrogravitationsbedingungen von EURECA-1, Technische Universita¨t Mu¨nchen, Ref. RT-DA 94/10, July 1994.
Bridgman solidification of GaSb in space T. Duffar!,*, M.D. Serrano", C.D. Moore#, J. Camassel$, S. Contreras$, P. Dusserre!, A. Rivoallant!, B.K. Tanner# ! CEA/CEREM/DEM, 17, Avenue des Martyrs, 38054 Grenoble, France " Dpto. Fisica de Materiales, University of Autonoma de Madrid, 28049 Madrid, Spain # Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK $ Groupe d+Etude des Semiconducteurs, University of Montpellier, France Received 2 January 1998; accepted 20 March 1998
Abstract Two samples of GaSb and Ga In Sb (x"0.001) have been grown in rough crucibles in space and full detachment of 1~x x the sample from the crucible was obtained over a large part of the crystals. Due to problems with the control system of the space furnace both seeds were molten and the growth began with polycrystals. As the growth proceeded, one grain was selected in each sample and the growth finished with a single crystal. X-ray analysis has shown that, in the zone where there was no contact with the crucible, the structural quality continuously improved. Chemical segregation obtained in space is representative of a diffusive regime of transport in the melt, in spite of the low growth rate used. This differs from ground experiments which are under convective conditions. Measurements of electrical properties have given results comparable to that of standard samples obtained on earth, but with a rather lower electrical resistivity. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Microgravity; Bridgman growth; GaSb; Crucible; Segregation
1. Introduction It is well known that crucible-sample interactions have a strong effect on crystals growing from the melt, especially in the case of the Bridgman growth process where the sample is in contact with
* Corresponding author. Fax: #33 376 88 51 17: e-mail: [email protected].
the crucible all the time. The interactions are of several kinds: chemical at the liquid-crucible interface, thermal (particularly at the crystal—melt— crucible triple interface) and mechanical at the crystal—crucible interface. We have studied the effect of rough crucibles on the growth of semiconductors in space [1] and it was earlier shown that, if the roughness of the crucible satisfies some geometrical conditions, it is possible to reduce strongly chemical and thermal [2] interactions and to fully cancel the mechanical contact between the crystal and the
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 2 1 - 7
64
T. Duffar et al. / Journal of Crystal Growth 192 (1998) 63–72
crucible. It can then be expected that the structural quality of the crystal should be improved and such rough crucibles were used for experiment described in this paper. This was devoted to the growth of GaSb related crystals, taken as a model material, on board the automatic satellite EURECA. Another field of interest is related to chemical segregation during solidification. It is well known that the solutal boundary layer present on the liquid side of the solid—liquid interface is disturbed by convection in the melt and that this leads to heterogeneity of the solute distribution in the crystal [3]. The thickness of this layer, and its sensibility to the liquid flow, depends inversely on the growth rate. Due to the large solidification time available, the experiment was focusing on growth rates one order of magnitude less than what has generally been investigated during other semiconductor Bridgman growth experiments in space: 0.1 lm/s rather than a few lm/s. The EURECA AMF-118 experiment was focused on investigating these effects during the growth of two pure GaSb crystals, as reference, and of two Ga In Sb (x"0.001) crystals, to study 1~x x segregation and its effect on the crystal structure, one on earth and the other one under microgravity conditions. The proposed experiment was accepted by the European Space Agency (ESA) to fly onboard the EURECA platform in the automatic mirror furnace (AMF).
2. Experimental set-up and procedure A drawing of the experimental ampoule is shown in Fig. 1. The sample was made of a stoichiometric GaSb or Ga In Sb feed material, 35 mm long, 1~x x prepared from 6N pure elements, and a single crystal seed, 40 mm long, made of pure GaSb. In order to prevent the occurrence of bubbles during melting in space, the feed and seed were joined by melting, by HF heating, with a thin layer of GaSb in between. The dislocation density in the seeds was lower than 100 cm~2 before integration. Both sides of the cylindrical sample were drilled in order to place a 1 cm long silica capillary at the seed end, to receive two thermocouples for temperature control, and a graphite rod plunged in the liquid, on the hotter side. The rough crucible was made of silica and machined with a screw thread with a 1 mm step and a cutting angle of 52°, in order to meet the geometrical conditions of de-wetting, taking into account the wetting angle of GaSb on silica [1,4]. The imposed internal diameter of the ampoule and the thickness of the crucible led to samples of 10 mm diameter. Some insulating (alumina wool) and conducting (graphite) parts were added to adjust the thermal gradient and a getter was placed in contact with the seed in order to pump any remaining or degassing atmosphere during heating. The purpose of the experiment was to get crystals as perfect as possible. To avoid the generation of
Fig. 1. Drawing of the experimental ampoule.
T. Duffar et al. / Journal of Crystal Growth 192 (1998) 63–72
dislocations of thermal origin, it is necessary to have an axial thermal gradient of a few °C/cm and radial thermal gradients as low as possible. The AMF is a mirror furnace designed to perform zone melting experiments, that means that heat is provided radially to the middle of the sample. This is of course totally inconsistent using our experiment and we solved the problem using a gold coating on the external wall of the ampoule which reflected the heating light. The only possibility for the heat to leave the furnace was then to enter the top (noncoated) of the ampoule and flow axially along the sample. In this way, we obtained an axial thermal gradient of 20°C/cm, increasing slightly all along the experiment up to 30°C/cm, as measured during preliminary thermal tests on earth with a thermocouple sliding in a capillary situated along-side the axis of the sample. The ampoule was moved along the furnace axis at a speed of 0.2 lm/s over 104 h and the resulting displacement rate of the interface, deduced from the same preliminary thermal tests, was found to be between 0.1 and 0.15 lm/s from the beginning to the end of the growth. This was found to be acceptable to solidify the 40 mm of molten sample. Due to delays in the launch of the satellite, the first sample (pure GaSb) was processed two years after the delivery of the experimental ampoule. Because of a problem with the regulation system of the furnace, it was not possible to get a stable temperature in the time alloted and the furnace was automatically cut-off. It was shown later that temperature oscillations during the attempts to reach the steady state caused the single crystal seed to melt. This sample was processed again at the end of AMF operations without regulation, at a fixed power approximated from the results of the first attempt. The second sample (Ga In Sb) was pro1~x x cessed in between, also without regulation to avoid the problems encountered with the first ampoule. Due to the still unexplained irreproductibility of the furnace, the approximated power was too high and a large part of the seed was also melted, as seen after opening the ampoule. The terrestrial reference samples were processed with the same thermal behaviour, i.e. melting of both seeds. The crucibles for terrestrial reference tests were smooth because otherwise hydrostatic pressure forces the liquid in the screw thread and the resulting crystals are bro-
65
ken due to the huge mechanical stresses suffered during the cooling process.
3. De-wetting and crystal structure The ground samples were polycrystals, with grain sizes of the order of a few millimeters. This is not surprising due to the practically full melting of the seeds. Fig. 2 shows an X-ray picture of both flight ampoules as received. There is evidence that de-wetting occurred, with some necking of the crystal diameter and no penetration of the material in the screw thread. Also the melting of the seeds can be observed, total for the first sample and only 1 mm remaining for the second one. The samples were not sticking on the ampoules and were easily removed, Fig. 3 shows their external aspect. The contact of the sample with the inner part of the crucible occurred only on the hot side but the periodic waves predicted by the theory [1] were not present, in contradiction to observations made in a previous space growth of GaSb during the D2 Spacelab mission, using the same kind of crucible [4]. This is attributed to the oxidation of the external surface of the sample which can easily be seen in the picture. Either the getter, which has been heated up to 700°C during the experiment, was not efficient enough, or there had been too much degassing or diffusion through the silica wall during the two year storage of the ampoules. Observation of the samples showed grains at the periphery, so that no special orientation was chosen for cutting them longitudinally. Fig. 4 is the result of their metallographic analysis by etching for 1 min with (1 HCl, 1 H O , 2 H O) after 2 2 2 lapping and polishing. They both began to grow as polycrystals but after some distance one grain was selected which occupied the whole diameter. An X-ray analysis by the Laue back-scattering method shows a good homogeneity of the grain orientation, with misorientations of the order of 1° between different places in the selected grains. The Ga In Sb sample inherited its orientation from 1~x x the S1 1 1T seed and the dislocation density in this plane revealed by etching for 2 min with (5 HNO , 3 5 HF, 3 CH COOH, 11 H O), at the position 3 2
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Fig. 3. External view of the flight samples, (a) GaSb and (b) Ga In Sb. 1~x x
Fig. 2. X-ray pictures of the flight ampoules as received, (a) GaSb and (b) Ga In Sb. 1~x x
where the structural quality is the best, was between 104 and 105 cmv2. It was not possible to find a simple crystallographic direction of growth for the pure GaSb crystal.
X-ray transmission topographs (Laue images) were taken through the samples, thanks to the high flux and low wavelengths available at the European synchrotron radiation facility. The exposure time was about 100 s and the sample was translated in the beam in order to get a picture every cm. In order to image the whole sample at once, reflection topography was also performed, using a small angle of incidence and tilting the film in order to expand the image. For the alloyed sample, examination of the topographs shows that a slightly strained layer exists at
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the sample, a tendency towards polycrystallinity is observed, with an absence of uniform diffraction images, showing a high degree of strain. This marked deterioration at the end of the sample could be caused by the increase of the In concentration, uncontrolled conditions of growth or contact with the crucible. An interesting observation is that no strain was seen associated with the silica capillary trapped in the crystal at the seed side. The topographs of the second, pure GaSb, sample show that the polycrystalline structure improves gradually away from the seed as can be seen in the two transmission topographs of Fig. 6, taken in the middle and at the top of the sample. The size of the diffracted spots increases and it can be concluded that the structure improved during the growth to the extent that a single crystal begins to grow from the polycrystalline material. However, in spite of this constant improvement, the quality of the sample remains poor.
4. Chemical segregation and properties
Fig. 4. Metallography of a longitudinal cut of the flight samples, (a) GaSb and (b) Ga In Sb. 1~x x
the junction between the crystal grown and the small remaining part of the seed, probably because of the step in chemical composition. On the topograph taken above the seed, it is observed that the crystal is growing in the S1 1 1T direction with twins and the side of the crystal is only slightly strained, where some individual dislocations are visible. The quality of the crystal increased during the growth as far as the growth conditions were controlled and no interaction existed with the crucible. Fig. 5 shows enlargements of reflection topographs below and above the point were the sample began to contact the teeth of the crucible. At this position the structure gradually deteriorates, with a high dislocation density at the edges. At the top of
Figs. 7 and 8 show the longitudinal and radial chemical analyses performed by a Secondary ion mass spectroscopy (SIMS) on the flight and ground alloyed samples respectively. The apparatus was calibrated with GaSb samples containing known amounts of In introduced by ion implantation. The experiment was designed to melt 5 mm of the seed and solidify a 40 mm long sample, after which the power of the heating lamp was decreased slowly, while pulling at a constant speed continued. Due to the overheating, much more sample was molten and perturbations of the longitudinal chemical profile can be seen on both samples after about 40 mm of solidification. It will then be considered that only this length has been solidified under controlled conditions. The segregation curve of the terrestrial sample fits well with the theoretical Scheill law for full mixing in the melt. The radial segregations are not symmetric; these might be due to misalignment of the sample with respect to the furnace axis or to the gravity vector. In spite of the fact that the longitudinal segregation for the flight sample looks like a diffusion controlled growth, it does not fit with the curve
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Fig. 5. Enlargements of the reflection X-ray topograph of the Ga In Sb sample, (a) in the region where there was no contact with the 1~x x crucible (position A on Fig. 4b) and (b) where this contact began (position B on Fig. 4b).
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Fig. 6. Transmission topographs of the GaSb sample taken (a) at the middle (position A on Fig. 4a) and (b) at the top of the sample (position C on Fig. 4a).
corresponding to diffusive conditions in a homogeneous liquid. As 35 mm of pure GaSb seed was melted prior to solidification and mixing of the liquid by diffusion was poor during the melting and stabilisation phase of the experiment, the characteristic diffusion length during these 8 h was less than 2 cm. We therefore used a simple one-dimensional finite-difference software, with implicit time resolution [5], to solve the diffusion equation in the frame of the moving interface, D
d2C dC L#» L"0, I dx2 dx
with the following boundary conditions: x'10D/» : dC /dx"0, I L x"0: D dC /dx"» (1!k)C . L I L,I
(1)
69
D is the diffusion coefficient of InSb in GaSb (1.2]10~8 m2/s), » the growth rate (0.1 lm/s), I k the segregation coefficient (0.15) and C the L,I concentration in the liquid at the interface. The initial composition of the liquid prior to solidification was taken as 1]1019 at/cm3 at 25 mm, corresponding to the seed, and the remaining at the feed composition was 7]1019 at/cm3. We obtain the dotted line represented in Fig. 7, which fits well with the experimental observation. It can then be reasonably estimated that the transport of solute in the liquid during the experiment (melting, stabilisation and solidification phases) was essentially diffusive. The fact that an oxide layer covered the sample probably helped by preventing any kind of Marangoni convection. The radial segregation curves suffer an asymmetry depending on the position; sometimes the highest concentration is on the left and sometimes on the right. This might be associated with low-frequency fluctuations in the direction of the gravity vector, which can disturb the solutal boundary layer without significantly decreasing its thickness [6]. Nevertheless, all attempts to find a periodic fluctuation of the chemical composition along the sides of the sample failed. The controlled solidification was stopped after 40 mm of pulling and the bump observed on Fig. 7 at 45 mm is attributed to quenching of the solutal boundary layer. Spark spectrographic chemical analysis was performed and compared to the composition of the initial material. Si, Al and K, elements typically associated with pollution by silica, were measured at the ppm level but without significant difference between the samples processed on earth and those in space, so there is no evidence of an improvement in the chemical pollution by the de-wetting phenomena observed in space. Finally six square wafers, two from GaSb sample and four from Ga In Sb, were cut in the metal1~x x lography plane, at the positions denoted B and C in Fig. 4a and Fig. 4b, respectively. They had typical dimensions of 5]6]1 mm3 and, on the six different pieces, both photoluminescence (PL) at 2 K and electronic transport properties have been investigated. Concerning first photoluminescence and the space GaSb sample, we have found that two
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Fig. 7. Longitudinal and radial (inlets, with the same at/cc scale) chemical analysis of the flight sample.
Fig. 8. Longitudinal and radial (inlets, with the same at/cc scale) chemical analysis of the ground sample.
transitions resolve predominantly in the spectral range 730—810 meV. They correspond with the well-known transitions BE and A, already dis4 cussed in the literature (see, for instance, [7] and references therein). The first structure comes from the radiative recombination of excitons bound to residual neutral acceptors (line BE ). The second 4 structure comes from donor—acceptor pair transitions (line A). This very characteristic PL
signature corresponds with weakly compensated material, the electrical properties being essentially governed by the double acceptor state associated with the classical lattice defect of nominally undoped p-type material. In this respect there is no strong difference between the space GaSb sample and similar material grown on earth. Weaker transitions are also resolve at &810, 766, 757 and 748 meV. They correspond now with the free
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Fig. 9. PL spectra collected at 2 K for pure GaSb (at position B on Fig. 4a) and Ga In Sb (at position C in Fig. 4b) samples. 1~x x
exciton recombination line (denoted FE), the phonon replica of the BE line (denoted BE-LO), 4 an additional acceptor (denoted B) and phonon replica of the A line. This is shown in Fig. 9. When performing a deconvolution with standard Lorentzian functions, we find relatively small values (2 meV) for the full-width at half-maximum of the different components. When taken at several positions on the same sample, the amplitude and relative intensities of the different components vary slightly which indicates nonhomogeneous distribution of defects, residual impurities and nonradiative centers like dislocations, for instance. For the alloyed samples, the PL spectra appear very similar to the one collected on pure GaSb (see Fig. 9). The most striking feature is a shift of about 2 meV towards low energy. The similarity of pure and alloyed spectra shows that the overall growth conditions were very close while the magnitude of the low-energy shift allows independent check of the alloy composition. Using for the nonlinear variation of band gap energy versus composition the bowing parameter of [8], we find about 3.5]1019 indium atoms per cm3. Within experimental uncertainty, this appears in reasonably good agreement with the results of SIMS measurements. After completion of the PL experiments, indium dots were diffused at 300°C for 2—3 min in order to
form ohmic contacts. Then, resistivity and Hall effect measurements were conducted in the temperature range 80—300 K. Results indicated that all flight samples are less resistive (more conductive) than a typical reference sample cut from a Czochralski wafer by a typical factor of 0.7 for GaSb and 0.6 for the alloys. Analysis of the variation of carrier concentration and mobility as a function of temperature show that this is due to a higher number of residual carriers (typically by a factor of 4) compensated by a lower mobility (typically by a factor of 3). From wafer to wafer slight heterogeneity is observed, in good agreement with the PL data.
5. Conclusions The principle of using a rough crucible to get de-wetting of a semiconductor crystal in space has been demonstrated. The presence of an oxide layer on the sample, due to a very long storage of the ampoule before use, probably helped to obtain this behaviour. In spite of the fact that the single-crystal seeds were molten during the experiment, the samples finally grew as single crystals, after initial nucleation of polycrystals. Furthermore, the quality of the selected grain improved gradually during the
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growth, at least when there was no contact with the crucible. It is noticeable that some of the authors have been growing antimonide crystals for more than a decade but have never before observed such behaviour. It is thus undoubtedly proven that the de-wetting obtained in space has a dramatic effect on the structural quality of semiconductor crystals, not only on stresses, twins and dislocations as already described by Larson in the case of CdTe [9], but also by allowing the sample to develop freely into a single crystal and improving its quality. On the other hand, due to the melting of the seeds, one aim of the experiment, namely looking at the structural effects of chemical segregation, was not achieved. Chemical segregations obtained in space are representative of diffusion solute transport in the melt. Thanks to the very low residual gravity levels on board EURECA, less than or equal to 10~7g in 0 the low-frequency range [10], this result has been achieved in spite of a growth rate one order of magnitude lower than that generally used in space Bridgman growth. Electrical properties measurements have given results comparable to the standard samples obtained on the earth, but with a rather lower electrical resistivity. We have thus demonstrated the benefit which can be obtained from growing crystals under microgravity, namely better structural quality, better homogeneity and without significant loss of electronic properties.
Acknowledgements This experiment has been performed in the framework of the GRAMME agreement between the CNES and the CEA and received support from ESA. M.D. Serrano would like to thank the Euro-
pean Community for his support in the frame of the HCM program, network no. CHRX-CT930106 and C.D. Moore acknowledges support from the UK EPSRC. The EURECA experiment was prepared with the help of the Dornier AMF team and the preliminary and ground reference tests have been performed in the DLR-MUSC, Ko¨ln. Great thanks are due to these very long-suffering and understanding people, especially to Mme. B. Pa¨tz for her great support and assistance. M Pernin (CEA/CEREM) helped with the SIMS analysis and C. Raffy (ENSEEG Grenoble) with their analysis. The experiments in ESRF were performed with the help of J. Baruchel and J. Hartwig and the spark emission chemical analysis was performed by M. Tabarant (CEA-Saclay).
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