Correlation Between GaInAsSb Surface Step Structure and Phase Separation
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ABSTRACT A strong correlation between the surface step structure and phase separation in metastable GaInAsSb epitaxial layers grown by organometallic vapor phase epitaxy has been identified. The full width at half maximum (FWHM) of 4-K photoluminescence (PL) peak energy is used as a semi-quantitative measure of the degree of phase separation. The step structure of GaInAsSb grown at 525 'C is vicinal, while it is step-bunched for layers grown at 575 'C. The corresponding 4-K PL FWHM data indicate that the degree of phase separation is minimized when the layers are grown at the lower growth temperature. It is proposed that the longer terrace lengths of a stepbunched surface are associated with a longer adatom lifetime compared to a vicinal surface, and thus the adatoms have more time to cluster and phase separate, which is the preferred equilibrium state. Increasing the growth rate, which reduces the adatom lifetime, also reduces the PL FWHM, and thus, the degree of phase separation.
INTRODUCTION Various approaches have been used to theoretically predict the thermodynamic stability of bulk multi-component rn-V semiconductors, and a general result is that ternary and quaternary alloys exhibit a miscibility gap [1,2]. Although it is difficult to establish precise temperatures and compositions for the onset of phase separation, especially for epitaxial materials that are grown using nonequilibrium techniques such as organometallic vapor phase epitaxy (OMYPE) or molecular beam epitaxy, these models are useful in predicting general tendencies. Indeed, phase separation has been reported to occur in numerous ternary and quaternary rn1-V systems and has been recently reviewed [3]. It is associated with contrast modulation in transmission electron microscopy (TEM) images. Phase separation has also been related to degradation in materials properties. It results in reduced electron mobility [4,5] and broadening of both photoluminescence (PL) spectra [6-81 and x-ray diffraction (XRD) curves [7-10]. Since these Ill-V materials are technologically important for both electronic and optoelectronic devices, it is of interest to understand the mechanism by which phase separation occurs. There is general consensus that phase separation evolves at the epitaxial surface [11-131, which is reasonable because bulk diffusion coefficients of the constituent elements of the alloy are too low to account for the length scales associated with phase separation observed in TEM. Thus, surface diffusion and adatom lifetime before incorporation into the lattice are expected to influence phase separation. In fact, growth temperature, growth rate, V/II ratio, and substrate misorientation, all factors that are expected to influence growth kinetics, influence TEM contrast modulation [7,12,14]. It has been reported that various growth parameters can affect the surface step structure of III-V epitaxial layers [15-17]. Furthermore, the step structure has been shown to play a role in CuPt ordering of GaInP [18]. Consequently, it might also be expected that t
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