Surface Phase Separation and Ordering in Compound Semiconductor Alloys
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SURFACE PHASE SEPARATION AND ORDERING IN COMPOUND SEMICONDUCTOR ALLOYS T.L. MCDEVITT*, S. MAHAJAN°, D.E. LAUGHLIN', W.A. BONNER** and V.G. KERAMIDAS**
*Department of Metallurgical Engineering and Materials Science Carnegie Mellon University, Pittsburgh, PA 15213 **Bellcore Red Bank, NJ 07701
ABSTRACT The orientation dependence of phase separation has been examined in detail in InGaAsP layers grown by liquid phase epitaxy on (001), (110), (111)In and (123) InP substrates. It is shown that phase separation is two-dimensional in nature and does not occur along the growth direction for the cases examined. Further, phase separation takes place along the soft directions lying in the growth plane. These results very strongly suggest that phase separation evolves at the surface while the layer is growing. CuPt-type ordering characteristics of InGaAsP layers are presented. In addition, the Influence of growth temperature and growth rate on domain sizes have been investigated in GaInP 2 layers. A model has been proposed to rationalize the formation of domains and involves steps present on the surface. Results suggest that ordering like phase separation occurs at the surface while the layers is being deposited. It is inferred that the two microstructural features evolve concomitantly at the surface during layer growth. INTRODUCTION
It is now well accepted that atoms In epitaxial layers of ternary and quaternary III-V compound semiconductors are not distributed at random within their respective sublattices. These deviations from randomness are manifested in two ways: (I) phase separation, and (11)long range atomic ordering. Based on bulk thermodynamic calculations, de Cremoux et al. [1] were the first to suggest that InGaAsP materials phase separate. Subsequent calculations by Stringfellow [21 and Onabe [31 are consistent with the work of de Cremoux et al. [1]. Henoc et al. [4] were the first to observe by transmission electron microscopy (TEM) phase separation in InGaAsP epitaxial layers grown by liquid phase epitaxy (LPE) on (001) InP substrates. They have seen two types of contrast modulations: (i) coarse modulations with a periodicity of 150nm, oriented along the directions lying in the (001) growth plane, and (ii) fine scale speckle structure, also oriented along the directions with a period of - 15nm. In the presence of the two types of overlapping microstructural features It Is very difficult to discern compositional differences associated with the coarse contrast modulations. Since the wavelength of the coarse modulations is too large for it to evolve by bulk diffusion, Launois et al. [5] have suggested that they develop by surface diffusion, tacitly Ignoring the presence of the fine scale structure. Following the work of Henoc at al. [4], Mahajan et al. [6] have examined the microstructures of Mat. Res. Soc. Symp. Proc. Vol. 198. @1990 Materials Research Society
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InGaAsP layers of different compositions grown on (001) InP substrates by LPE. They have found that the wavelength of the fine scale structure is in
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