Comparison of Chemical, Electronic, and Optical Properties of Mg-Doped AlGaN

Abstract

Hydrogen, carbon, and oxygen are common unintentional impurities of Al(x)Ga(1−x)N crystals. This impurity structure and its interplay with Mg impurities in Al(x)Ga(1−x)N semiconductors are relevant to develop the p-type nitride crystals for various devices (e.g, LEDs, transistors, gas sensors) but are still unclear. Here we have investigated Mg-doped Al0.5Ga0.5N before and after postgrowth annealing with valence-band and core-level photoelectron spectroscopy, photoluminescence, and resistivity measurements. First, it is found that a surface part of the Al0.5Ga0.5N crystal is surprisingly inert with air and stable against air exposure-induced changes. Thus, the relatively surface-sensitive photoelectron spectroscopy measurements reflect in this case also the bulk crystal characteristics. The measurements reveal the presence of deep states up to 1 eV above valence-band maximum before and after the annealing and that oxygen and carbon occupy N lattice sites (i.e., ON and CN). The model where CN-induced acceptor states in the band gap participate in the blue emission (photoluminescence) is supported. Furthermore, the presented Mg 2p core-level spectra demonstrate that part of Mg atoms forms direct bond(s) with oxygen in the bulklike structure of Al0.5Ga0.5N and that the chemical environment of Mg atoms is much richer than was expected previously. (Less)