The ultra-wide bandgap of gallium oxide provides a rich plethora of electrically active defects. Understanding and controlling such defects is of crucial importance in mature device processing. Deep-level transient spectroscopy is one of the most sensitive techniques for measuring electrically active defects in semiconductors and, hence, a key technique for progress toward gallium oxide-based components, including Schottky barrier diodes and field-effect transistors. However, deep-level transient spectroscopy does not provide chemical or configurational information about the defect signature and must, therefore, be combined with other experimental techniques or theoretical modeling to gain a deeper understanding of the defect physics. Here, we discuss the current status regarding the identification of electrically active defects in beta-phase gallium oxide, as observed by deep-level transient spectroscopy and supported by first-principles defect calculations based on the density functional theory. We also discuss the coordinated use of the experiment and theory as a powerful approach for studying electrically active defects and highlight some of the interesting but challenging issues related to the characterization and control of defects in this fascinating material.

Perspective on electrically active defects in β-Ga2O3 from deep-level transient spectroscopy and first-principles calculations

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Doping and defect control in semiconductors are essential prerequisites for their practical applications. First-principles defect calculations based on density functional theory offer crucial theoretical guidance for doping and defect control. This paper introduces the developments in the theoretical methods of first-principles semiconductor defect calculations. We will firstly introduce the method of the defect formation energy calculations, as well as finite-size errors to defect formation energies caused by the supercell method. We then present corresponding image charge correction schemes, which includes the widely used post-hoc corrections (such as Makov-Payne (MP), Lany-Zunger (LZ), Freysoldt-Neugebauer-Van de Walle(FNV) schemes), the recently developed self-consistent potential correction which performs the image charge correction in the self-consistent loop for solving Kohn-Sham equations, and the self-consistent charge correction scheme which does not require an input of macroscopic dielectric constants. Furthermore, we extend our discussions to charged defect calculations in low-dimensional semiconductors, elucidating the issue of charged defect formation energy divergence with increasing vacuum thickness under the jellium model, and introducing our theoretical model which fixes this energy divergence issue by placing the ionized electrons or holes in the realistic host band-edge states instead of the virtual jellium state. Then, we further provide a brief overview on defect calculation correction methods due to the DFT band gap error, including the scissors operator, LDA+U and hybrid functionals. Finally, to describe the defects formation energy calculations under illumination, we present our self-consistent two-Fermi-reservoirs model, which predicts well the defects and carrier concentrations under illumination in the Mg doped GaN system. This work summarizes the recent developments regarding first-principles defect calculations in semiconductors and might offer guidance for defect calculations in semiconductors especially low-dimensional ones, under both equilibrium and non-equilibrium conditions, aiding further advancements in doping and defect control within semiconductors.

Review of the developments of first-principles defect calculations in semiconductors

4H Silicon carbide (4H-SiC) is widely recognized as a highly promising material for high-voltage and high-power electronic applications due to its exceptional properties. The performance of devices based on 4H-SiC is often weakened by the presence of carbon-related point defects, particularly carbon vacancies (VC). The defects of VC introduce deep-level traps (e.g., Z1/2 and EH6/7) that deteriorate device functionality. Experimental and theoretical studies on VC have led to some conflicting results about the charge states of VC, especially for the charge state ordering of EH6/7. We now employ ab initio metadynamics (META) to systematically investigate configuration space including the direction and magnitude of bond distortion and identify the most stable structures of VC. Eventually, the charge states of VC in 4H-SiC are identified. The Z1 (EH6) and Z2 (EH7) indicate transitions from acceptor (donor) levels of VC, located on the h and k sublattice sites, respectively. Z1 and Z2 demonstrate negative-U ordering, characterized by U values of −0.16 and −0.37 eV, respectively. Conversely, EH6 and EH7 display positive-U ordering, with U values of 0.16 and 0.08 eV, respectively. The current results provide insights into the properties of VC in 4H-SiC, highlighting the effectiveness of META in the exploration of complex potential energy surfaces associated with point defects in solids.

Identifying the charge states of carbon vacancies in 4H-SiC by ab initio metadynamics

The emerging wide bandgap semiconductor -Ga2O3 has attracted great interest due to its promising applications for high-power electronic devices and solar-blind ultraviolet photodetectors. Deep-level defects in -Ga2O3 have been intensively studied towards improving device performance. Deep-level signatures E 1, E 2, and E 3 with energy positions of 0.55–0.63, 0.74–0.81, and 1.01–1.10 eV below the conduction band minimum have frequently been observed and extensively investigated, but their atomic origins are still under debate. In this work, we attempt to clarify these deep-level signatures from the comparison of theoretically predicted electron capture cross-sections of suggested candidates, Ti and Fe substituting Ga on a tetrahedral site (TiGaI and FeGaI) and an octahedral site (TiGaII and FeGaII), to experimentally measured results. The first-principles approach predicted electron capture cross-sections of TiGaI and TiGaII defects are 8.56 × 10–14 and 2.97 × 10–13 cm2, in good agreement with the experimental values of E 1 and E 3 centers, respectively. We, therefore, confirmed that E 1 and E 3 centers are indeed associated with TiGaI and TiGaII defects, respectively. Whereas the predicted electron capture cross-sections of FeGa defect are two orders of magnitude larger than the experimental value of the E 2, indicating E 2 may have other origins like CGa and Gai, rather than common believed FeGa.

Clarifying the atomic origin of electron killers in β-Ga2O3 from the first-principles study of electron capture rates

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Clarifying the atomic origin of electron killers in β-Ga2O3 from the first-principles study of electron capture rates