Here I will discuss first-principles calculations based on a hybrid functional that describe the properties of acceptor dopants in gallium oxide. These calculations now have predictive power, as will be demonstrated for the magnesium acceptor in gallium oxide. Acceptors are unlikely to lead to p-type conductivity in gallium oxide, they can compensate prevailing p-type conductivity, and I will compare the stability of acceptor impurities against native defects such as gallium vacancies. The stability of hole polarons in a set of ultrawide-bandgap oxides will also be examined and compared, both in pristine material and in the presence of acceptor impurities.
We determine the band alignment of ScxAl1-xN/GaN heterojunctions at ScN alloy fractions ranging from x = 0.04 to 0.20 using x-ray photoemission spectroscopy, and determine the band gap from spectroscopic ellipsometry. We find a transition from straddling to staggered gap as ScN alloy fraction increases, and show that the experimental results are consistent with first-principles calculations. This crossover from type-I to type-II band alignment shows a degree of freedom for engineering improved heterostructures in ScAlN/GaN-based electronic and optoelectronic devices.
Gallium oxide has emerged as a promising ultrawide-bandgap semiconductor for electronic applications. Part of the attraction of Ga2O3 is its ability to be alloyed with other materials (such as Al2O3) for band-gap engineering or doped with other elements (such as silicon) for modifying its electrical conductivity. But what is still unknown is how these alloying capabilities extend into the orthorhombic phases, or how well the ultrawide-bandgap AlGO alloys can be n-type doped. Here hybrid density functional theory calculations are used to determine the electronic structure of AlGO alloys. Conduction-band offsets of AlGO alloys in the orthorhombic phase are calculated, as are donor ionization energies as a function of Al content. In light of these results, we discuss band engineering and doping strategies in AlGO alloys for electronic device applications.
Optical spectroscopy is a powerful approach for detecting defects and impurities in ZnO, an important electronic material. However, knowledge of how common optical signals are linked with defects and impurities is still limited. The Cu-related green luminescence is among the best understood luminescence signals, but theoretical descriptions of Cu-related optical processes have not agreed with experiment. Regarding native defects, assigning observed lines to specific defects has proven very difficult. Using first-principles calculations, we calculate the properties of native defects and impurities in ZnO and their associated optical signals. Oxygen vacancies are predicted to give luminescence peaks lower than 1 eV; while related zinc dangling bonds can lead to luminescence near 2.4 eV. Zinc vacancies lead to luminescence peaks below 2 eV, as do the related oxygen dangling bonds. However, when complexed with hydrogen impurities, zinc vacancies can cause higher-energy transitions, up to 2.3 eV. We also find that the Cu-related green luminescence is related to a (+/0) deep donor transition level.
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