Surface Segregation and Composition Fluctuations in ammonia MBE and MOVPE of InGaN
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Surface Segregation and Composition Fluctuations in ammonia MBE and MOVPE of InGaN Sergey Yu. Karpov, Roman A. Talalaev, Eugene V. Yakovlev and Yuri N. Makarov1 Soft Impact Ltd, P.O. Box 33, 194156, St.Petersburg, Russia 1 Fluid Mechanics Institute, University of Erlangen-Nürnberg, Cauerstraβe 4, 91058 Erlangen, Germany ABSTRACT Surface segregation and phase separation are investigated as processes limiting the indium incorporation in InGaN grown by ammonia Molecular Beam Epitaxy (MBE) and MetalOrganic Vapor Phase Epitaxy (MOVPE). It is shown that a significant concentration of indium on the growing surface may prevent the adsorption of ammonia via site blocking mechanism and result in appearance of In droplets instead of InGaN growth. Another conclusion is that the composition fluctuations in InGaN are related to coexistence of strained and relaxed InGaN islands rather than to the phase separation as commonly assumed. INTRODUCTION Despite the significant progress achieved in development of III-nitride optoelectronic devices, the control of indium composition as well as growth of device-quality InGaN epilayers with high In content still remain critical issues. The aim of this work is to analyze the effects of indium surface segregation and phase separation on growth of InGaN. The segregation is known to be a factor affecting the composition profile in the ternary and quaternary compound heterostructures with very different volatility of the atomic species. Normally, segregation results in a delayed incorporation of a volatile species into the crystal, in appearance of the ternary compound tail in a binary “cap” layer, and, eventually, in formation of liquid droplets on the growth surface when the coverage of group-III volatile species becomes greater than about one monolayer. The strain in the epilayer due to lattice constant mismatch is found to be a major factor affecting segregation. In this paper, the steady state In surface segregation in the InGaN/GaN structures is investigated with reference to their growth by ammonia MBE and MOVPE. The role of segregation in limitation of In incorporation into the solid is discussed. We also analyze the factors favoring appearance of double-peaks in the InGaN luminescence spectra that are frequently attributed to phase separation. From the analysis, we suggest the mechanisms responsible for this phenomenon. INDIUM SURFACE SEGREGATION In this section we discuss the effect of surface segregation on In incorporation into InGaN. We discuss both ammonia MBE and MOVPE as the growth techniques having common features in surface chemistry. For the MBE conditions the quasi-thermodynamic model developed in our previous studies [1,2] is used to predict indium solid composition as a function of group-III and group-V incident fluxes and growth temperature. The basic assumption of the model is that the rates of all microscopic processes occurring on the growth surface are much higher than those of the reactive species adsorption/desorption. This assumption allows us to express the species
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