Observation of Hexagonal AlGaAs Grown by OMCVD
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OBSERVATION OF HEXAGONAL AIGaAs GROWN BY OMCVD D. M. HWANG, T. S. RAVI, R. BHAT, S. SIMHONY, C. Y. CHEN, AND E. KAPON Bellcore, Red Bank, New Jersey 07701-7040, USA
ABSTRACT Extended regions of hexagonal zinc sulfide (wurtzite) structure are found to exist in AIGaAs grown by low-pressure OMCVD at 750"C. The specimen was prepared on a (001) GaAs substrate patterned with [1101-oriented V-grooves, intended for a quantum wire laser structure. A high density of planar faults was observed to originate in the A10.66Ga oAs cladding layers near the inner corners of the V-grooves and 0 propagate towards the surface along the {111} planes. Many of these faults are stacking faults and microtwins. However, there also exhibit extended regions of hexagonal structure, revealed in electron diffraction patterns and high resolution lattice images. The hexagonal phase shares the same close-packed layers with the cubic phase, i.e., (0001)h,,,gon // {111}cubic. The mechanism for the formation of hexagonal structure in this specimen is not yet fully understood.
INTRODUCTION There are two ways to arrange rigid spheres into close-packed structures: the face-centered cubic (fcc) structure and the hexagonal close-packed (hcp) structure. Covalent semiconductors can also exist in two crystal forms: the cubic structure (the cubic diamond or the zinc blende structure) which consists of two interpenetrating fcc sublattices, and the hexagonal structure (the hexagonal diamond or the wurtzite structure) which consists of two interpenetrating hcp sublattices. The hexagonal structure has been observed in crystals of Group IV elements, I-VI compounds, and III-V nitrides. The transformation between the cubic structure and the hexagonal structure has been studied in detail [1-5]. However, the hexagonal structure has not been previously reported in III-V semiconductor compounds with P, As, or Sb. In this article, we report the first experimental identification of the hexagonal structure in AIGaAs. The specimen was grown by low pressure organometallic chemical vapor deposition (OMCVD) [6]. Electron diffraction patterns of single-phase hexagonal structure were obtained with a 0.271im selected area aperture. High resolution lattice images indicated that the hexagonal phase is grown out from the cubic phase and extended towards the surface along the the common close-packed planes. We speculate that the thermodynamic conditions during growth favor the formation of the hexagonal phase. Details of the relevant growth conditions are yet to be established.
EXPERIMENTAL -The substrate used for the growth was a (001) oriented GaAs wafer with V-grooves prepared along the [110] direction. The V-grooves were prepared by using conventional photolithography and anisotropic wet chemical etching in H 2 SO 4 :H2 0 2 (30%):H 2 0 (1:8:40 by volume). The mask was removed and the wafer was cleaned in H 2 S0 4 :H2 0 2 (30%):H 2 0 (20:1:1 by volume) for 20 minutes. The resulting V-grooved substrate is delineated in the low-magnification TEM micrograph shown in Fig. I as the da
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