Cathodoluminescence atomic scale images of monolayer islands at GaAs/GaAlAs interfaces

Abstract

Direct images of growth islands differing by 2.8 Å [1 monolayer (ML)] height at GaAs/AlGaAs heterointerfaces and of the columnar structure of quantum wells are reported for the first time. The structures are grown by molecular-beam epitaxy (MBE) with interruptions of the growth of ≊2 min at the interfaces. The method used to obtain these images is scanning cathodoluminescence. The dependence of the lateral extension of these islands on growth conditions is investigated. For fixed growth rate rs≊0.5 ML/s the mean island size decreases from 6–7 μm to 2 μm upon an increase of growth temperature from Tg=600 to 660 °C. Apparently the growth process changes from a planar to a three-dimensional one. For low-growth temperature and rate the lateral extension of such islands can be larger than the carrier diffusion length. Under these conditions interisland thermalization of carriers is largely suppressed. Quantitative information on the reduction of roughness of the quantum well interfaces with increasing growth interruption time is obtained from detailed analysis of luminescence line shapes. A full theory of quantum well luminescence line shapes is given. For heterostructures grown at rs=2.8 Å/s, Ts=620 °C, the interface roughness is observed to decrease from σ=0.44 to σ=0.20 Å for an increase of the interruption time from 1 to 100 s.