Defects in strained In0.2Ga0.8As/GaAs multiple quantum wells on patterned and unpatterned substrates: A near-infrared cathodoluminescence study

Abstract

The spatial distribution of the long-wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) grown on patterned and unpatterned substrates has been investigated using cathodoluminescence (CL) imaging and spectroscopy. By spatially correlating the luminescence arising from the MQW exciton recombination (λ≊950 nm) with the longer wavelength (1000≲λ≲1200 nm) luminescence arising from the defect-induced recombination, we demonstrate that it is possible to determine the regions of highest film quality in both the mesa and valley regions for growth on patterned GaAs substrates. The present approach enables a judicious determination of the optimal regions to be used for active pixels in InGaAs/GaAs devices. For growth on unpatterned substrates, the CL spectra show defect-induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy showed that [110]-oriented misfit dislocations occur primarily at the substrate-to-MQW interface. The large spatial variation of the luminescence intensities indicates that the DLDs observed in CL images are caused by the presence of nonradiative recombination centers occurring in the MQW region located above the interface dislocations. This study provides new information describing the origin and nature of DLDs and differs from previous models, which have regarded the electronic nature of dislocation cores as the primary mechanism for inducing DLD radiative contrast in luminescence imaging of strained InGaAs/GaAs.