Structural characterization of GaInNAs/GaAs double quantum well structures

Abstract

We performed a structural analysis of strained GaInNAs/GaAs double quantum well samples (about 2% N content) having various well-layer thicknesses and investigated the relaxation process beyond the critical thickness. We used photoluminescence (PL) measurements, x-ray diffraction measurements, and transmission electron microscopy (TEM) analysis. PL measurement reveals deterioration of crystal quality as the well thickness increases. X-ray diffraction reveals that no serious relaxation occurs even when the deterioration proceeds. Cross-sectional and plan-view TEM analysis clarify the deterioration process in detail. In a sample with thin well layers, local dark spots are first generated only at the first well layer, whereas no such dark spots are observed in the second well layer. Through careful observation, the dark spots are identified as local defects (dislocation loop), not the quantum dot structures reported for other materials. For samples with thicker wells, large dark spots penetrating both well layers are observed. Furthermore, threading dislocations and/or stacking faults, which derive from the dark spots, are also observed in the upper GaAs cladding layers. However, no misfit dislocations are observed in any samples, although two-dimensional growth is confirmed. These results indicate a different type of deterioration from that conventionally reported in InGaAs/GaAs material systems. The relationships between the deterioration process and growth process are discussed based on the interface smoothness. The discussion clarifies that dynamical processes play essential roles in the deterioration.