Si concentration dependence of structural inhomogeneities in Si-doped Al_xGa_(1-x)N/Al_yGa_(1-y)N multiple quantum well structures (x= 0.6) and its relationship with internal quantum efficiency

摘要

We investigated the distribution of luminescence in Si-doped Al_xGa_(1-x)N/Al_yGa_(1-y)N multiple quantum well (MQW) structures (x = 0.6) with different Si concentrations by cathodoluminescence (CL) mapping combined with scanning electron microscopy. The effects of surface morphology, dark spot density, and full width at half-maximum of spot CL spectra on internal quantum efficiency (IQE) were determined. A flat surface morphology and uniform CL map were observed for Si-doped AlGaN MQWs, in contrast to undoped AlGaN MQW and Si-doped AlGaN with relatively low Al content. The dark spot density in the Si-doped AlGaN MQWs increased exponentially as the Si concentration increased and did not explain the Si concentration dependence of IQE. In contrast, there was a clear correlation between the dark spot density and IQE of the AlGaN MQWs at a constant Si concentration. The emission energy distribution arising from the inhomogeneity of the relative Al content and the well layer thickness was estimated by monochromatic CL measurements, although there was almost no difference in the distribution for different Si concentrations. Therefore, the previously reported dependence of the defect complexes on Si concentration is reflected in the IQE of Si-doped AlGaN MQWs. Defect complexes composed of cation vacancies and impurities rather than dislocations and interfacial quality are the major contributor to the IQE of the Si-doped AlGaN MQWs with different Si concentrations.