First-principles study of self-trapped holes and acceptor impurities in Ga2O3 polymorphs

Abstract

We investigate the stability of self-trapped holes (STHs) and the acceptor levels of substitutional Mg and N impurities in α-, β-, δ-, and ε-Ga 2 O 3 using first-principles calculations based on the hybrid functional approach to assess their p-type dopability. When Fock-exchange and screening parameter values in the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional are optimized to satisfy the generalized Koopmans' theorem for a STH level in β-Ga 2 O 3 , the band gap is slightly overestimated, while functionals that well reproduce the band gap show slight convex behavior against the fractional electron number. However, the absolute position of the STH level at a fixed geometry is nearly independent of the parameter value, showing that the results are robust as long as the STH geometry and localized electronic nature are appropriately described and the band edges are well reproduced. In all of the polymorphs, holes localize with high self-trapping energies rather than being delocalized. Furthermore, both Mg and N impurities introduce polaronic acceptor states, and their acceptor levels lie far above the valence band maximum in all the polymorphs. Thus, the p-type doping of the four Ga 2 O 3 polymorphs seems unfeasible in terms of the STH formation and the related deep, polaronic acceptor nature of the Mg and N impurities.

Band alignment at surfaces and heterointerfaces ofAl2O3,Ga2O3,Hinuma1, Gake2, Oba3 2019Phys. Rev. MaterialsSelf Cite384210View full textAdd to dashboardCite

“…The nsc-dd hybrid functional approach thus shows a significant improvement over PBEsol in the estimation of the band gaps and such improvements have also been found for group-IV, III-V, and II-VI semiconductors [62] and group-II oxides [51]. β-Ga 2 O 3 shows a direct-type band structure both in our nsc-dd hybrid functional and previously reported G 0 W 0 @HSE03 results [81], while slightly indirect in our PBEsol result and previous reports using a tuned HSE hybrid functional [82,83]. In any case, the differences between the direct and indirect gaps are small, at most 40 meV.…”

Energetics and electronic structure of native point defects in α-Ga2O3

“…Since the experimental bandgap of α-Ga 2 O 3 varies in the literature (4.9 -5.6 eV 20,41-44) ), we determined the Fock-exchange mixing HSE functional to be 0.35 with the screening parameter kept fixed at a standard value of 0.208 Å -1 45) so that the calculated bandgap of β-Ga2O3 (4.89 eV) agrees well with the experimental one (4.9 eV46) ). This approach has been shown to reproduce the experimental ionization potential well and nearly satisfies generalized Koopmans' theorem for a self-trapped hole in β-Ga2O3 24). The bandgap of α-Ga2O3 obtained using this functional is 5.49 eV in the indirect-type band structure.…”

Role of long-range exact exchange in polaron charge transition levels: The case of MgO

“…Recently it has been proposed that the fraction of exact exchange can be tuned to satisfy Koopmans' theorem for a localized defect level in a supercell [28][29][30][31][32][33]. Koopmans' theorem in DFT [19,34], also known as the ionization potential theorem, says that the highest occupied Kohn-Sham eigenvalue (in this case the occupied defect level) should be equal to the ionization potential calculated by total energy differences (i.e., by the -SCF method).…”