Structural and chemical study of AlGaAs/InGaAs based QWIPs

Abstract

AlGaAs/InGaAs on GaAs substrate is the most commonly used material for QWIPs. Using these compounds, we have already demonstrated focal plane arrays from 4.5 to 15 μm. Furthermore, detection below the CO 2 absorption band (4.2 μm) can be very useful for various applications. However, going to lower wavelengths requires a higher indium content in the active layer. As a consequence, the layers are strained and can possibly exhibit dislocations. Moreover, indium atoms present a high probability of segregation during the growth and the composition profile may be quite different from the nominal one. Due to localized states, the quantum well properties are highly sensitive to the shape of the confining potential and the quality of the surrounding materials. Thus, the composition profile changes induced by indium segregation or other processes need to be monitored. This paper presents a structural and chemical study of AlInGaAs based QWIPs. The study is performed on samples grew at different surface temperatures. We perform structural analysis by high resolution TEM (Transmission Electron Microscopy) and by aberration-corrected STEM (Scanning Transmission Electron Microscopy). The accumulation of strain over the active layer is higher than the pseudomorphic limit (Matthews–Blakeslee model). Yet, the samples do no present any sign of defect, even at low growth temperature. The spectroscopic measurement is performed by EELS–STEM (Electron Energy Loss Spectroscopy–STEM). The limitation of the segregation processes induced by the temperature reduction is observed on the HAADF (High Angular Annular Dark Field) images and on the EELS data. The changes are numerically evaluated with a kinetic model. The experimental and the simulated absorption spectra exhibit a blue shift with the temperature growth reduction.