Tuning carrier localization in In-rich InGaN alloys: Correlations between growth kinetics and optical properties

Abstract

Alloy clustering and consequent carrier localization in InGaN alloys are important aspects with large effect in materials’ optoelectronic properties and related device operation. Their importance is even higher for alloy compositions close to midrange where alloy compositional fluctuations get stronger. Such fluctuations are considered a result of thermodynamic immiscibility and indium segregation to structural defects. However, their dependence on the epitaxial growth process is unknown. Here, we examined the dependence of the degree of resulting carrier localization on growth surface kinetics in InGaN alloys developed by plasma-assisted molecular beam epitaxy. We deposited a large number of alloy thin films, heteroepitaxially on GaN(0001), under different growth temperatures and metallic surface coverage during growth but with similar compositions (∼40%), thicknesses, and growth rates. Their structure was examined by x-ray diffraction. Carrier localization phenomena were investigated employing temperature dependent photoluminescence, while spectroscopic ellipsometry was applied to determine the films’ complex dielectric function. A strong correlation of compositional uniformity and decreased carrier localization with growth conditions that reduce adatoms surface mobility and enhance “out-of-equilibrium” epitaxy character was revealed. This correlation was largely independent of the effects on films' structural properties. Thus, the importance of epitaxial surface kinetic effects on alloy clustering and localization is highlighted. These results are expected to be applicable to the entire alloy composition range and could pave the way for on demand optimal tuning of their optoelectronic properties depending on application requirements.