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Oxygen is a common impurity in AlN samples. Using hybrid density functional calculations, we investigate the role of substitutional oxygen (ON) in the optical absorption. We construct configuration coordination diagrams for ON and related complexes. Our results indicate that an optical transition involving ON− (a DX center) gives rise to an absorption band peaked at 2.22 eV, suggesting it is a source of the absorption band with an onset at ∼ 2 eV observed in oxygen-containing samples. We also propose that neutral ON–DX complexes can form, which would give rise to absorption peaking at 3.06 eV. In addition, we find that oxygen, in spite of its DX character, may behave as an “optically shallow donor” and be involved in optical transitions from deep defect states to the conduction band. This observation provides an alternative physical mechanism for the optical absorption bands observed in AlN samples in the visible and ultraviolet (UV) region. 

First-principles calculations of defects and electron–phonon interactions play a critical role in the design and optimization of materials for electronic and optoelectronic devices. The late Audrius Alkauskas made seminal contributions to developing rigorous first-principles methodologies for the computation of defects and electron–phonon interactions, especially in the context of understanding the fundamental mechanisms of carrier recombination in semiconductors. Alkauskas was also a pioneer in the field of quantum defects, helping to build a first-principles understanding of the prototype nitrogen-vacancy center in diamond, as well as identifying novel defects. Here, we describe the important contributions made by Alkauskas and his collaborators and outline fruitful research directions that Alkauskas would have been keen to pursue. Audrius Alkauskas’ scientific achievements and insights highlighted in this article will inspire and guide future developments and advances in the field. 

The ability to achieve highly resistive beta-phase gallium oxide (β-Ga2O3) layers and substrates is critical for β-Ga2O3 high voltage and RF devices. To date, the most common approach involves doping with iron (Fe), which generates a moderately deep acceptor-like defect state located at EC-0.8 eV in the β-Ga2O3 bandgap. Recently, there has been growing interest in alternative acceptors, such as magnesium (Mg) and nitrogen (N), due to their predicted deeper energy levels, which could avoid inadvertent charge modulation during device operation. In this work, a systematic study that makes direct correlations between the introduction of N using ion implantation and the observation of a newly observed deep level at EC-2.9 eV detected by deep-level optical spectroscopy (DLOS) is presented. The concentration of this state displayed a monotonic dependence with N concentration over a range of implant conditions, as confirmed by secondary ion mass spectrometry (SIMS). With a near 1:1 match in absolute N and EC-2.9 eV trap concentrations from SIMS and DLOS, respectively, which also matched the measured removal of free electrons from capacitance-voltage studies, this indicates that N contributes a very efficiently incorporated compensating defect. Density functional theory calculations confirm the assignment of this state to be an N (0/−1) acceptor with a configuration of N occupying the oxygen site III [NO(III)]. The near ideal efficiency for this state to compensate free electrons and its location toward the midgap region of the β-Ga2O3 bandgap demonstrates the potential of N doping as a promising approach for producing semi-insulating β-Ga2O3.