Phase Separated Microstructure and Its Stability in InGaAs Epitaxial Layers Grown by LPE
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ABSTRACT In 0 .53Ga 0 .47As layers were grown by LPE on (001) InP substrates in the temperature range of 480-7800C. The fine speckle contrast, which is attributed to two-dimensional phase separation, was observed in layers grown at temperatures as high as 780 0 C. Since the critical temperature for bulk miscibility gap is predicted to be around 4500C, this suggests that in the presence of the surface the critical temperature for the two-dimensional surface decomposition is higher than that for the bulk. The wavelength of the fine modulations does not change with the growth temperature. This could be due to the fast diffusion kinetics in the solid-liquid interface and the balance between the driving force and the gradient energy. To examine the stability of the above microstructures, zinc diffusion experiments were carried out in the temperature range of 390 to 540 0C using the ampoule technique. The diffused layers exhibit homogenous microstructures. This demonstrates that critical temperature for phase separation in bulk is below 390 0 C and is comparable to that predicted by theory. INTRODUCTION The III-V alloy semiconductors, such as InGaAs and InGaAsP, are of particular interest for opto-electronics and high speed devices. It has been reported that these ternary and quaternary epitaxial materials undergo pbase separation. The evidence of composition modulation in InGaAs or InGaAsP grown by LPE on InP and GaAs substrates has been reported by various authors[I-5]. Depending on the growth condition and layer composition, two types of modulations have been observed. The typical microstructures seen in transmission electron microscopy(TEM) have a quasi-periodic fine contrast modulations with a wavelength of -1Onm and a coarse basket-weave pattern with a periodicity of 100-200nm oriented along the two orthogonal [100] and [010] directions in (001) surface. Diffraction contrast experiments using two-beam conditions indicate that the principal strain is perpendicular to the linear direction of the contrast. The fine speckle type contrasts develop by two-dimensional phase separation and occurs at the surface on which the layer is growing[6]. The coarse modulations can be attributed to the artifacts in thin foil due to the accommodation of two-dimensional strain that results from the fine scale modulations[7]. Once the layers are grown, their microstructure is frozen due to limited diffusion kinetics of the group Ifi and V atoms. Thermodynamic calculations of In-Ga-As system have been investigated extensively. The critical temperature for chemical spinodal in InGaAs alloy has been predicted around 450-500OC[8, 9]. However, when the coherency strain energy term is considered, the critical temperature for the coherent spinodal is expected to be below room temperature. This implies that phase separated microstructure should be unstable. 109 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
Fig. I Typical SAD pattern and DF micrographs obtained from plan view sample of as-grown layer. A) [0011 SADP, sho
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