Theoretical analysis of influence of random alloy fluctuations on the optoelectronic properties of site-controlled (111)-oriented InGaAs/GaAs quantum dots

We use an sp(3)d(5)s* tight-binding model to investigate the electronic and optical properties of realistic sitecontrolled (111)-oriented InGaAs/GaAs quantum dots. Special attention is paid to the impact of random alloy fluctuations on several factors that determine the fine-structure splitting in these systems. Using a pure InAs/GaAs quantum dot as a reference system, we show that the combination of spin-orbit coupling and biaxial strain effects can lead to sizable spin-splitting effects in these systems. Then, a realistic alloyed InGaAs/GaAs quantum dot with 25% InAs content is studied. Our analysis reveals that the impact of random alloy fluctuations on the electronic and optical properties of (111)-oriented InGaAs/GaAs quantum dots reduces strongly as the lateral size of the dot increases and approaches realistic sizes. For instance the optical matrix element shows an almost vanishing anisotropy in the (111)-growth plane. Furthermore, conduction and valence band mixing effects in the system under consideration are strongly reduced compared to standard (100)-oriented InGaAs/GaAs systems. All these factors indicate a reduced fine-structure splitting in site-controlled (111)-oriented InGaAs/GaAs quantum dots. Thus, we conclude that quantum dots with realistic (50-80 nm) base length represent promising candidates for polarization-entangled-photon generation, consistent with recent experimental data.