Growth of InxGa1-xSb alloy semiconductor at the International Space Station (ISS) and comparison with terrestrial experiments

Abstract

Background:In_x Ga_1-x Sb is an important material that has tunable properties in the infrared (IR) region and is suitable for IR-device applications. Since the quality of crystals relies on growth conditions, the growth process of alloy semiconductors can be examined better under microgravity (μG) conditions where convection is suppressed.Aims:To investigate the dissolution and growth process of In _x Ga_1-x Sb alloy semiconductors via a sandwiched structure of GaSb(seed)/InSb/GaSb(feed) under normal and μG conditions.Methods:In_x Ga_1-x Sb crystals were grown at the International Space Station (ISS) under μG conditions, and a similar experiment was conducted under terrestrial conditions (1G) using the vertical gradient freezing (VGF) method. The grown crystals were cut along the growth direction and its growth properties were studied. The indium composition and growth rate of grown crystals were calculated.Results:The shape of the growth interface was nearly flat under μG, whereas under 1G, it was highly concave with the initial seed interface being nearly flat and having facets at the peripheries. The quality of the μG crystals was better than that of the 1G samples, as the etch pit density was low in the μG sample. The growth rate was higher under μG compared with 1G. Moreover, the growth started at the peripheries under 1G, whereas it started throughout the seed interface under μG.Conclusions:Kinetics played a dominant role under 1G. The suppressed convection under μG affected the dissolution and growth process of the In _x Ga_1-x Sb alloy semiconductor.

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Growth of InxGa1 xSb alloy semiconductor at the International Space Station (ISS) and comparison with terrestrial experiments

Y Inatomi1,2,6, K Sakata1, M Arivanandhan3, G Rajesh3, V Nirmal Kumar3, T Koyama3, Y Momose3, T Ozawa4, Y Okano5,6 and Y Hayakawa3,6

BACKGROUND: InxGa1 xSb is an important material that has tunable properties in the infrared (IR) region and is suitable for IR-device applications. Since the quality of crystals relies on growth conditions, the growth process of alloy semiconductors can be examined better under microgravity (G) conditions where convection is suppressed.

AIMS: To investigate the dissolution and growth process of InxGa1 xSb alloy semiconductors via a sandwiched structure of GaSb (seed)/InSb/GaSb(feed) under normal and G conditions.

METHODS: InxGa1 xSb crystals were grown at the International Space Station (ISS) under G conditions, and a similar experiment was conducted under terrestrial conditions (1G) using the vertical gradient freezing (VGF) method. The grown crystals were cut along the growth direction and its growth properties were studied. The indium composition and growth rate of grown crystals were calculated.

RESULTS: The shape of the growth interface was nearly at under G, whereas under 1G, it was highly concave with the initial seed interface being nearly at and having facets at the peripheries. The quality of the G crystals was better than that of the 1G samples, as the etch pit density was low in the G sample. The growth rate was higher under G compared with 1G. Moreover, the growth started at the peripheries under 1G, whereas it started throughout the seed interface under G.

CONCLUSIONS: Kinetics played a dominant role under 1G. The suppressed convection under G affected the dissolution and

growth process of the InxGa1 xSb alloy semiconductor.

npj Microgravity (2015) 1, 15011; doi:http://dx.doi.org/10.1038/npjmgrav.2015.11

Web End =10.1038/npjmgrav.2015.11 ; published online 27 August 2015

INTRODUCTIONInxGa1 xSb is a promising narrow-band-gap alloy semiconductor useful for numerous applications that demand high-quality crystals as substrate materials.15 It has the advantage of having a tuneable lattice parameter and wavelength ranging from 6.096 to 6.479 and 1.7 to 6.8 m by just adjusting its composition. However, it is difcult to grow high-quality InxGa1 xSb alloy bulk crystals because the solute and heat transport are inuenced by convection, which affects the quality of the crystals.6,7 Micro-

gravity (G) is an appropriate environment for investigating the solute transport and growth kinetics.8,9 Under G conditions, it is quite feasible to suppress the complex convective heat and mass transports to gain deeper insight into the transport phenomena. Also, materials science plays a vital part in mankinds efforts towards deeper space and planetary exploration.10 The knowl

edge gained in the G experiments can help to better understand and improve applications on Earth. In this respect, the role of gravity in the dissolution and growth mechanism of various materials, including narrow-band-gap semiconductors such as Ge, SiGe, GaSb and InSb, has been investigated for the past three decades.1116 Various physical factors such as granular convection, surface tension of liquid melts, dendritic growth velocities,

thermophysical properties of alloy melts, and phase separation in critical uids have been examined in G experiments.1721 We

have also carried out several G experiments using the space shuttle,22 a drop tower23 and a Chinese recoverable satellite.24 The effect of diffusive transport and surface tension-induced Marangoni convection on the mixing of multi-component melts has been investigated using an In/GaSb/Sb sandwich sample.25,26

Recently, the dissolution process and solute transport in a GaSb (seed)/InSb/GaSb (feed) sandwich sample were observed in situ using an X-ray penetration method under terrestrial conditions.27

The results showed that gravity-induced solutal transport strongly inuenced the dissolution process, and the numerical results clearly supported the experimental observations.28 The above

mentioned results clearly suggest that convection affected not only the composition proles but also the shape of the solid liquid interface. Prior to the G experiments at the International Space Station (ISS), preliminary experiments were carried out to optimize the growth conditions and measure various thermo-physical properties of InxGa1 xSb alloy semiconductors.2931 The purpose of the Alloy Semiconductor crystal growth project was to elucidate the factors affecting the crystal growth of a high-quality bulk ternary alloy semiconductor under G conditions at the ISS.

1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan; 2School of Physical Sciences, SOKENDAI (The Graduate University for Advanced Studies), Sagamihara, Japan; 3Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan; 4Department of Electrical Engineering, Shizuoka Institute of Science and Technology, Fukuroi, Japan and 5Graduate School of Engineering Science, Osaka University, Toyonaka, Japan.

Correspondence: Y Inatomi (mailto:[email protected]

Web End [email protected])

6These authors contributed equally to this work.

Received 28 March 2015; revised 5 June 2015; accepted 22 July 2015

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This project was selected as one of the 14 candidate missions of the Kibo Second Phase Utilization in 2008. In the present article, we report our rst G experimental results for InxGa1 xSb alloy crystals grown at the ISS, and the results are compared with the terrestrial experiments.

MATERIALS AND METHODSA GaSb(111)A/Te-doped InSb/GaSb(111)A sandwich sample was used for the growth of the InxGa1 xSb bulk crystal under G at the ISS by the vertical gradient freezing method. GaSb single crystals along the o1114 orientation were grown by the Czochralski method for the preparation of sandwich samples. Figure 1ad shows a schematic diagram of the vertical temperature prole (a), initial stage of the sandwich sample (b), dissolution(c) and growth process (d) of the vertical gradient freezing method that was employed for the crystal growth. The ampoule with (GaSb(seed)/InSb/ GaSb(feed)) sandwich sample was kept inside the furnace in which the temperature gradient was established (Figure 1a). During the high-temperature growth process, InSb was melted at its melting point of 525 C. GaSb seed and feed crystals started to dissolve in InSb melt, and thereby InGaSb solution was formed (Figure 1c). Owing to the vertical temperature gradient, the solution at the seed interface gets super-saturated due to diffusive transport of solute from feed interface. Supersaturation provides the driving force to initiate the growth from un-dissolved seed crystal. Once crystal growth initiated at the seed

interface, seed dissolution was stopped and further growth was assisted by dissolution of feed crystal which is at relatively high temperature. Before the growth experiment, the temperature gradient of the furnace was calibrated at the ISS. The furnace was heated up to the reference temperature of 700 C at a suitable heating rate (0.06 C/min) after placing the growth ampoule in an appropriate position, and the temperature was kept constant for 107 h of growth. The temperature gradient in the furnace was xed at 0.6 C/mm. Based on the reference temperature (measured by the thermocouple) and the temperature gradient, the temperature at the GaSb seed interface was determined and controlled. To measure the growth rate of the crystal, several numbers of heat pulses were periodically introduced during the growth process after 3 h at 700 C. To apply a heat pulse, the furnace temperature was increased by 10 C from the growth temperature, held for 2 min and then cooled to the initial growth temperature. The increasing and decreasing times were xed at 1 min and 10 s, respectively. By applying this kind of heat pulse, the Te impurity concentration was modulated and striations were introduced in the grown crystal. A total of 50 heat pulses were applied during the growth period at intervals of 2 h. For clear identication of the striations that are created by heat pulses, the introduction time interval between pulse number 4 and 5 was changed to 1 h. After applying all of the pulses, the temperature was decreased to 400 C at a cooling rate of about 0.5 C/min to reduce the thermal stress in the grown crystals. Then, the temperature was decreased to room temperature at a cooling rate of about 1 C/min. A similar experiment was conducted under terrestrial conditions to study the effect of gravity on the dissolution and growth process (this sample is denoted the 1G sample). The grown crystals were cut along the growth direction and polished with alumina powder to obtain a mirrored surface. The polished sample was etched in a 1:3:1 solution of HF:KMnO4:CH3COOH for 30 min at room temperature to observe the Te impurity striations and etch pits. The composition proles, interface shapes and striations of the grown crystals were analyzed by electron probe micro analysis (EPMA), polarized optical microscopy and eld emission scanning electron microscopy.

For the numerical calculation, a two-dimensional symmetric rectangular model containing the GaSb seed and feed, an InSb melt, a boron nitride (BN) tube and a quartz tube was considered. In the simulation, the solution was assumed to be incompressible and Newtonian, and Boussinesq approximation was utilized. The NavierStokes and mass-continuity equations were used for the numerical analysis of the dissolution process. The boundary conditions in the solution/crystal interfaces used the heat balance equation and mass balance equation.

To relate these equations, the liquidus curve in the InSbGaSb pseudo-binary phase diagram was used. The governing equations and the associated boundary conditions were transformed to non-dimensional

Position

InSb (Te-doped)

Initial stage

InGaSb solution

Solution formation Crystal growth

InGaSb solution

InGaSb crystal

Temperature

Temperature Profile

GaSb (feed)

GaSb (seed)

Figure 1. Schematic diagram of the vertical temperature prole (a), initial stage of sandwich sample (b), dissolution (c) and growth process (d) of the vertical gradient freezing method that was employed for the crystal growth.

23 mm

a b

G

Feed interface after growth

GaSb (feed)

GaSb (seed)

GaSb (feed)

Feed interface before growth

InSb

InSb

Seed interface before growth

20.7 mm

Seed interface after growth

23 mm

1G

18.1 mm

GaSb (seed)

Figure 2. Sandwich samples before and after experiments (a) under G and (b) 1G. The interface positions of G (a) and 1G samples (b) before and after the experiments are indicated.

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p3

p2

p1

300m

70

200m

70

Figure 3. Polarized optical microscope images of the etched surfaces of (a) G and (b) 1G samples. (c) Optical micrograph and (d) eld emission scanning electron microscopy image of the periphery of the 1G sample. The striations are marked as p1, p2 and p3 in (c).

equations, and the boundary xing method was adapted to the equations to handle the difculties associated with curved interfaces. These equations were solved using the nite-volume method.

RESULTSFigure 2a, b shows the cross section of G and 1G samples. The dissolution lengths of GaSb seed and feed crystals under G were2.3 and 15 mm, respectively, whereas they were 4.9 and 11 mm in the 1G sample. The dissolution length of the seed crystal was larger in the 1G sample, whereas the dissolution length of the feed crystal was larger in the G sample. The strong convection in the solution causes a large amount of seed dissolution under 1G, whereas under G, the solute transport was diffusion dominant, decreasing the seed dissolution rate, as conrmed by our numerical simulation results.28 The length of the crystal grown under G (14.9 mm) was larger than that of the 1G sample(12.6 mm) because a larger amount of feed was supplied for growth under diffusive transport compared with the 1G experiment. The shape of the un-dissolved feed interface was similar for both samples, which ensured the same temperature distribution during both experiments. Figure 3ac shows an optical micro-graph of the un-dissolved seed interface of G and 1G samples. As can be seen from Figure 3a, the interface shape of the G sample was slightly concave toward the seed end and symmetric. However, the interface shape of the 1G sample was nearly at with steps at the peripheries (Figure 3b, c). Moreover, the angles of steps 1 and 2 (Figure 3d) are around 70, which conrmed that both steps were in the (111) plane. This shows that the kinetics played a more dominant role in the dissolution process under 1G when compared with G.

The shapes of striations near the seed interface in G and 1G samples are shown in Figure 3a, b. For clarity, the striations are marked as solid lines in the optical microscopic images of the etched samples. The striations formed by their corresponding heat pulses were marked as p1, p2 and so on. Initially, the growth started at the peripheries in the 1G samples, as striations 1 and 2 were observed at the peripheries and disappeared at the center (marked as p1 and p2 in Figure 3c). The initial striations were clearly observed at the center and periphery of the G sample,

and the shape of the striations resembled the interface shape, which indicated a at interface (Figure 3a). In the 1G sample, the striations were highly concave, while the initial seed interface was nearly at, with steps at the peripheries. Figure 4a and b shows the EPMA mapping of indium distribution in the G and 1G samples. The magnied images of indium mapping at the seed and feed interfaces of both samples illustrate the variations of the interface shape at the initial and nal stages of growth under G and 1G. From the mapping, it is clear that the shape of the growth interface of the G sample remained almost the same from the initial to the nal stage of growth (Figure 4a). However, the growth interface of the 1G sample was nearly at at the initial stage and highly concave toward the feed at the nal stage of growth, as shown in Figure 4b. Moreover, a V-shaped feed interface was observed in both samples beyond the residual solution region. To ascertain the quality of the grown crystals, the etch pit variations in crystals grown under G and 1G were comparatively analyzed. Figure 5a, b shows optical micrographs of the etched surfaces of G and 1G samples. The etch pit density (EPD) was calculated by counting the pits in both samples with respect to seed and grown crystals. The EPD was higher for the seed crystals in both samples because the seed crystals are grown by the Czochralski method at high temperature. The EPD was lower in the grown crystal under G (2.16 104/cm2) compared with that grown under 1G(2.72 104 /cm2).

DISCUSSIONThe strong convection and vortex at the center causes the solute transport from the center to the periphery under 1G. The velocity of the vortex ow is relatively low at the peripheries; thus, the dissolution is smaller at the peripheries. Moreover, the solutes accumulate at the peripheries while the solution at the center remains undersaturated because of strong convection. Therefore, the solution at the peripheries becomes supersaturated and the growth is initiated at the peripheries under 1G. Under G, the solute transport is diffusion dominant; the solute is evenly transported from feed to seed, the solution attains equilibrium at the seed interface, and growth is initiated at the peripheries and center at nearly the same time. The growth was initiated faster

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Final growth interface shape

Feed interface

Feed interface

Initial growth interface shape

Seed interface

Seed interface

0 1

In

Figure 4. Electron probe micro analysis mapping of indium distribution in the (a) G and (b) 1G samples. The magnied seed and feed interfaces of both samples illustrate the variations of the interface shape at the initial and nal stages of growth under G and 1G.

100 [afii9839]m

Seed interface

[afii9839]G

100 [afii9839]m

1G

Figure 5. Optical microscope images of the etched surfaces of (a) G and (b) 1G samples. The arrow shows the position of seed interface between GaSb and InGaSb crystals. The etch pits are shown as white arrows in the images.

0.10

Microgravity ([afii9839]G)

Normal gravity (1G)

Indium composition

0.18

0.16

0.08

Growth rate (mm/h)

0.14

0.06

0.12

0.1

0.04

0.08

0.02

0.06

0.04

0.00

Microgravity ([afii9839]G)

Normal gravity (1G)

0.02

0 10 20 30 40 50

Distance (mm)

Figure 6. (a) Indium composition prole of G and 1G samples and (b) growth rate variations of G and 1G samples.

0 0 5 10 15

Growth distance

under G compared to 1G, as all the striations were observed in the crystals (Figure 3a), possibly because of the absence of solutal convection. In the 1G sample, the growth was delayed because of the presence of convection, which hinders the solution from attaining equilibrium. As a result, the initial striations were not observed at the center of the sample (Figure 3b). The solution

exhibits two kinds of ows resulting from compositional and temperature variations in the solution. One is the dominant convective ow in the middle region from seed to feed interface that returns to seed interface through the periphery. Another weak ow is at the periphery region from the seed to feed interface that returns to the seed interface through the middle. As

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time elapses, the ow at the periphery region is suppressed by the ow from the central region. Therefore, higher amounts of solutes are accumulated at the peripheries. As a consequence, the solution at the periphery becomes supersaturated, leading to the initiation of growth under 1G condition.

The composition prole of the crystals was measured along the growth direction, as shown in Figure 6a. The composition prole clearly indicates that InxGa1 xSb crystal grew under G from 20.2 to 35.1 mm; thus, the length of the grown crystal was 14.9 mm. However, the crystal grew under 1G from 18.0 to 30.6 mm, with a grown length of 12.6 mm. As can be seen from Figures 2a, b and 6a, approximately 2.3 and 4.9 mm of GaSb seed was dissolved into the InSb melt under G and 1G, respectively. The amount of indium gradually decreased along the growth direction and uctuated after 35.1 and 30.6 mm in G and 1G samples, respectively. Because the crystal cooled after the growth period, the residual solution rapidly solidied, which caused the compositional uctuation at the end of growth. As growth proceeds, the growth interface moves towards the high-temperature region, leading to a gradual increase in growth temperature in accordance with the temperature gradient. As a result, the relative amount of indium decreased along the grown length. The indium composition of the G sample at the seed interface was 0.028 and gradually decreased along the growth direction. The indium composition of the 1G sample at the seed interface was 0.041 because of the large amount of seed dissolution, which shifted the interface position towards the low-temperature seed end. The EPMA mapping of the samples (Figure 4a) illustrates the homogeneous distribution of indium in the grown region. Moreover, the indium composition data were converted to growth temperature using an InSbGaSb binary phase diagram, and the temperature gradient was calculated. The calculated temperature gradient of the G sample(0.64 C/mm) was slightly lower than that of the 1G sample(0.58 C/mm), and the calculated values were almost the same as that xed during the growth.

By measuring the grown length between the striations, the growth rates of the crystals were determined. Figure 6b shows the growth rate variation along the growth direction of G and 1G samples. The growth rate of the crystal under G was higher than that under 1G. The growth rate of the G sample was 0.135 mm/h at 0.5 mm of grown length, increasing to a maximum of 0.16 mm/ h when the grown length reached 2.4 mm, and the growth rate became stable after the grown length of 3 mm. The diffusion-controlled steady-state growth continued for the next 6 mm of grown length. Under 1G, the initial growth rate was one order of magnitude lower than that under G, as a rate of 0.04 mm/h was measured at 0.5 mm of grown length of the 1G sample. The maximum growth rate of 0.11 mm/h was observed at 7.8 mm of grown length, and the diffusion-controlled steady-state growth period was only a few mm at the nal stage of growth because of the strong convection under 1G.

Under G the solutes dissolved at the seed interface were not able to move up to the feed interface unlike the 1G condition, due to the absence of convection, and thus the solution at the seed interface gets locally supersaturated. Therefore, under G, the growth started earlier than 1G condition. Moreover, the dissolved solutes from feed interface sturdily diffuse towards seed interfaces due to density gradient between the solute and solvent. Therefore, the growth rate is relatively high under G compared to 1G. On the contrary, under 1G, convection is dominant, which reduced the solute accumulation near the seed interface. Moreover, seed dissolution is stopped once crystal starts to grow from the seed interface. Hence the initiation of growth was delayed and growth rate is low compared to G.

InxGa1 xSb alloy crystal was grown under G at the ISS using a GaSb (111)A/Te-doped InSb/GaSb (111)A sandwich sample using a vertical gradient freezing method. A similar experiment was conducted under 1G on Earth. The dissolution and growth

processes of G and 1G samples were comparatively analyzed. The kinetics played a dominant role in the dissolution process under 1G, as steps were observed at the peripheries of the seed interface. The seed interface of the G sample was highly symmetric and slightly concave. The growth started at the peripheries under 1G, which can be explained using the calculated ow velocity of the solution at high temperature. The growth rate was higher under G compared with 1G. The quality of the G sample was better, as low EPD was observed compared with the 1G sample. The suppressed convection under G affected the dissolution and growth process of this alloy semiconductor.

ACKNOWLEDGMENTS

This work was nancially supported by (1) a Grant-in-Aid for Scientic Research (B) (no. 22360316, 25289270, 25289087) and a Grant-in-Aid for Young scientist C (no. 22760005) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, (2) JSPS and DST under the JapanIndia Science Cooperative Program (Joint Research Project), and (3) the cooperative research projects of the Research Institute of Electronics, Shizuoka University. The authors thank Prof T. Ishikawa, Prof S. Yoda and Mr. M. Takayanagi of the Institute of Space and Astronautical Science, and Japan Aerospace Exploration Agency (JAXA) for their support in the related research and in fruitful discussion. The authors also thank the GHF experiment operation team and the staff of the Space Environment Utilization Center, HSMD and JAXA for their technical help in microgravity experiments.

CONTRIBUTIONS

Y.I., Y.H. and Y.O. designed and carried out the G experiments. M.A., G.R., V.N.K., T.K., Y.M., T.O. and Y.H. prepared the ampoules and performed the crystal growth under normal gravity. Y.O. calculated the convective ow velocities by numerical simulation. M.A., V.N.K., Y.H., Y.I. and K.S. measured the composition of the crystal and analyzed the grown samples.

COMPETING INTERESTS

The authors declare no conict of interest.

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