Journal of Crystal Growth 198/199 (1999) P.S. Dutta, A.G. Ostrogorsky*

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1 Journal of Crystal Growth 198/199 (1999) Melt growth of quasi-binary (GaSb) crystals P.S. Dutta, A.G. Ostrogorsky* Center for Integrated Electronics and Electronics Manufacturing, Department of Mechanical Engineering, Aeronautical Engineering and Mechanics, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Abstract A new class of III V quasi-binary [A B ] [C D ] semiconductor alloys has been synthesized and bulk crystals grown from the melt for the first time. The present investigation is focused on (GaSb) (0(x(0.05) due to its importance for thermophotovoltaic applications. The structural properties of this melt-grown quasi-binary alloy are found to be significantly different from the conventional quaternary compound Ga As Sb with composition x"y. Synthesis and growth procedures are discussed Elsevier Science B.V. All rights reserved. 1. Introduction Recently, anomalous band gap narrowing in quasi-binary (GaSb) (0(x(0.05) alloys synthesized from melt was demonstrated for the first time [1,12]. Possible explanations for this anomalous property in these crystals include chemical and structural relaxation and coherent length ordering, in an otherwise disordered alloy [2,13]. This arises from the physical properties of GaSb and InAs, and the growth conditions. Apart from band gap narrowing, ordering would also result in long minority carrier lifetimes and higher electron mobility due to the presence of ordered domains in a disordered matrix [3]. The band gap demonstrated by the GaSb InAs alloy system extends * Corresponding author. Fax: # ; ostroa@rpi.edu. beyond 2 μm [1] which is important for several technological applications [4], including thermophotovoltaic (TPV) generation of electricity [5,6]. The choice of GaSb InAs quasi-binary systems for this study was motivated by its close lattice constant, tunability of band gap and electronic properties, type II band alignment for GaSb InAs, etc. The quasi-binary alloys were found to exhibit band gaps [1] and structural quality (this work) significantly different from that of a conventional quaternary alloy Ga As Sb with x"y. This paper describes the synthesis and growth procedures for this alloy system. The need for bulk substrates of III V compounds with tunable band gaps and lattice constants is increasing. Novel device architectures on tunable substrates and production of low-cost, high-performance devices are the two motivating factors. The simplest approach to obtain tunable substrates would be through ternary alloys (A B CorAB C ) between two binaries (AC /99/$ see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S ( 9 8 )

2 385 and BC or AB and AC). In practice, growing bulk ternary alloys is a formidable task; due to the wide separation between the solidus liquidus curves [7] of the pseudo-binary phase diagram. This arises mainly from the difference in lattice constants and the melting points of the constituents binaries forming the alloy. Recent work by Nakajima et al. [8,9] on compositionally graded Ga As bulk crystal growth seems encouraging. To add an additional degree of freedom for band gap and lattice parameter tuning, quaternary alloy A B C D with adjustable x and y can be lattice matched to binary substrates. However, miscibility gaps in the psuedo-quaternary plane and phase separation are the main obstacles for the solidification of quaternary alloys from melts [7,10]. These arise mainly from the difference in chemical interaction between the constituent elements in the melt. Currently, devices based on ternaries and quaternaries alloys are fabricated on thin epilayers of the respective alloys grown on binary substrates. For large volume, low cost production, solidification from the melt is by far the most desired technique. In view of the thermodynamic limitations for the bulk crystal growth of ternary and quaternary alloys, new exotic materials systems need to be suitably designed and tested. In the present investigation, it was experimentally demonstrated that large crystals of an alloy semiconductor having four components (tetra-atomic) can be synthesized and grown from a melt preferentially consisting of two binaries having close physical (lattice constant, density, melting points), but dissimilar optical and electronic properties. Similar physical parameters enable the growth of high crystalline quality material, while tuning the optical and electronic properties by composition. The growth behavior and the properties of these grown alloys justify them to be designated as tetra-atomic quasi-binaries. 2. Experimental details The quasi-binary (GaSb) alloys were synthesized from pre-compounded InAs and GaSb. Synthesis was carried out in a multi-zone Mellen furnace [11] (vertical Bridgman set-up) in 20 and Fig. 1. Quasi-binary crystal of (GaSb). 32 mm diameter silica crucibles from pre-synthesized GaSb and InAs freshly etched with CP4 etchant (CH COOH : HF : HNO in 3:3:5 by volume). Melt encapsulation was provided by boric oxide (B O ) or alkali halide salts (LiCl KCl and NaCl KCl). Boric oxide encapsulation was found to be more satisfactory and suitable (due to extremely low-vapor pressure and high viscosity) for inhibiting volatilization from the melt surface. The growth chamber was usually pressurized to slightly more than 1 atm by argon gas to prevent decomposition of the InAs. If melt is not pressurized properly, the compounds decompose, and consequently form GaAs and InSb rich phases upon solidification. GaAs and InSb have a significantly different lattice constant. Therefore, the quality of the grown crystals will be poor. Synthesis was done at various temperatures in the range of C, corresponding to the melting points of GaSb and InAs, respectively. After synthesis, the crucible was lowered at a constant rate of 3.3 mm/h through a temperature of K/cm. At the end of solidification, the furnace was cooled down slowly to room temperature over a period of several hours. Crystal growth was performed without seed, either in flat bottom or conical tipped crucibles. After the growth, the ingots were sliced parallel to the growth axis to evaluate the structural and compositional properties. Fig. 1 shows a longitudinally sliced quasi-binary crystal of (GaSb)

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4 387 Fig. 3. Microstructures (SEM photomicrograph) of (a) (GaSb) quasi-binary alloy synthesized from GaSb and InAs, (b) (GaSb) (InSb) ternary alloy synthesized from GaSb and InSb. grown in a pyrolytic boron nitride (pbn) crucible of 50 mm in diameter. To reveal the microscopic grain structure, the following sequential chemical treatments were found to be suitable: HCl : 30% H O :H O (1 : 1 : 1) followed by de-ionized (DI) H O dip and then CrO (34.2 gm) : HF (5 ml) : H O 120 ml) and H O dip and rinsed in soap solution. The composition of the grown crystals (Ga, In, Sb, and As) were evaluated by the electron probe micro-analysis (EPMA) measurements in a JEOL 733 electron microprobe set-up. The standards used were InAs and GaSb single crystal substrates. Corrections for atomic number (Z), self-absorption (A) and fluorescence (F) effects (ZAF corrections) were performed by employing the commercial software SCOTT-I. The error in determining the composition was in the order of 1 2% of the measured values. The microstructures of the crystals were studied through secondary electron microscopy (SEM) in the EPMA set-up. Fig. 2. Microstructures (SEM photomicrograph) of GaInAsSb quaternary alloys synthesized from (a) Ga, In, Sb and InAs at 950 C, (b) Ga, In, Sb and GaAs at 950 C, (c) Ga, In, Sb and GaAs at 800 C. The mol% for In, Ga, As, and Sb in the melt were 20, 80, 13, and 87, respectively. 3. Results and discussion In the absence of any phase diagrams, the initial studies were focused towards studying the microstructure of quaternary Ga As Sb

5 388 synthesized at various temperatures and charge preparation cycles from melts containing 20 mol% In, 80 mol% Ga, 13 mol% As and 87 mol% Sb. This alloy composition was attempted with the aim of obtaining band gap &0.55 ev necessary for specific TPV applications [5]. In preliminary studies, spatial inhomogeneity in the crystal composition was observed due to the multi-phase formation as shown in Fig. 2a Fig. 2c. Phase separation is thermodynamically expected in quaternary systems grown from a regular solution [7]. The multi-phase formation is attributed to the presence of elemental sources. Fig. 2a shows the microstructure of Ga In As Sb synthesized at 950 C from melt containing elemental Ga, In, Sb, and compounded InAs (arsenic source). The multiple phases are formed due to the decomposition of InAs and subsequent formation of random mixed alloys with elemental Ga, In and Sb. Fig. 2b shows typical microscopic phases observed in Ga In As Sb synthesized from a melt containing elemental Ga, In, Sb, and compounded GaAs (arsenic source) at 950 C. Low synthesis temperature improves the compositional homogeneity. However, it does not fully avoid the formation of multiple phases. Fig. 2c shows inclusions in the quaternary crystals synthesized at 800 C from the same melt composition as the crystal in Fig. 2b. By correlating the microstructure of the crystals with the melt constituents, it can be concluded that the multiple phase formation arises from chemical interaction between elemental sources in the melt. Hence, they can be suppressed by synthesizing charge from compounded sources as demonstrated in later studies discussed. Typical microstructure of the quasi-binary (GaSb) (InAs) synthesized in the C temperature range from compounded GaSb and InAs is depicted in Fig. 3a. From SEM studies, it is concluded that the quasi-binary crystals are completely single phase in nature, unlike the quaternary alloys. Moreover, synthesis from compounded sources significantly reduces the probability of multi-phase formation. It is also evident from the comparison of Fig. 3a and Fig. 3b, that the quasibinary GaSb InAs crystals do not exhibit cracks, unlike their ternary GaSb InSb counterpart. This is due to 10 times less lattice mismatch of InAs and GaSb as compared to InSb and GaSb. Fig. 4a shows the axial profile of InAs in the crystal grown from an initial homogenized melt consisting of 97 mol% GaSb and 3 mol% InAs. This uniform InAs content indicates virtually no segregation during growth. To determine the segregation coefficient k, lnc /C was plotted as a function of ln(1!f), (see Fig. 4b). Here C is the InAs content in the solid, C is the InAs concentration in the initial homogenized melt, and f is the fraction of melt solidified. The slope of the plot gives the value of k, which has been estimated from different experiment sets to be in the range. The segregation coefficient approaching unity is an important advantage of the quasi-binary alloys. Hence, crystals with uniform alloy composition can be grown by simple directional solidification. Fig. 4. (a) Axial profile of InAs concentration in the crystal with initial homogeneized melt composition (C ) of 97 mol% GaSb and 3 mol% InAs, (b) ln C /C versus ln(1!f) plot for determining segregation coefficient k of InAs in (GaSb) (InAs) quasi-binary alloy.

6 Conclusion In conclusion, large polycrystals of semiconductor quasi-binary alloys of GaSb InAs have been grown from melt. These alloys possess better crystalline perfection (crack free and are single phase) and compositional homogeneity (close to unity segregation) than melt grown bulk ternary and quaternary alloys. Acknowledgements The authors would like to thank Dr. Ronald Gutmann (Rensselaer Polytechnic Institute, Troy, NY) and, Dr. Greg Charache and Mr. Greg Nichols (Lockheed Martin Inc., Schenectady, NY) for invaluable scientific information and discussions. We are indebted to Dr. David Wark (Rensselaer Polytechnic Institute, Troy, NY) for the assistance in the EPMA measurements. The growth equipment used in the present work was provided by the Lucent Technologies, NJ. References [2] S.H. Wei, A. Zunger, Phys. Rev. B 39 (1989) [3] R.K. Ahrenkiel, S.P. Ahrenkiel, D.J. Arent, in: J.P. Benner, T.J. Coutts (Eds.), D.S. Ginley, Proc. 2nd NREL Conf. on Thermophotovoltaic Generation of Electricity, Colorado Springs, CO, AIP Conf. Proc. 358, NY, 1996, p [4] P.S. Dutta, H.L. Bhat, V. Kumar, J. Appl. Phys. 81 (1997) [5] T.J. Coutts, C.S. Allman (Eds.), J.P. Benner, Proc. 3rd NREL Conf. on Thermophotovoltaic Generation of Electricity, Colorado Springs, CO, AIP Conf. Proc. 401, NY, [6] A. Zunger, S. Wagner, P.M. Petroff, J. Electron. Mater. 22 (1993) 3. [7] K.J. Bachmann, F.A. Thiel, H. Schrieber Jr., Progr. Crystal Growth Characterization 2 (1979) 171. [8] K. Nakajima, T. Kusunoki, K. Otsubo, J. Crystal Growth 173 (1997) 42. [9] K. Nakajima, T. Kusunoki, J. Crystal Growth 169 (1996) 217. [10] K. Nakajima, K. Osamura, K. Yasuda, Y. Murakami, J. Crystal Growth 41 (1977) 87. [11] P.S. Dutta, A.G. Ostrogorsky, R.J. Gutmann, in: T.J. Coutts, C.S. Allman, J.P. Benner (Eds.), Proc. 3rd NREL Conf. on Thermophotovoltaic Generation of Electricity, Colorado Springs, CO, AIP Conf. Proc. 401, New York, 1997, p [12] P.S. Dutta, A.G. Ostrogorsky, J. Crystal Growth 197 (1999) 1. [13] S.H. Wei, A. Zunger, Phys. Rev. B 43 (1991) [1] P.S. Dutta, A.G. Ostrogorsky, Alloys and methods for their preparation, US Patent, pending.