Donor and acceptor characteristics of native point defects in GaN

Abstract

The semiconducting behaviour and optoelectronic response of gallium nitride is governed by point defect processes, which, despite many years of research, remain poorly understood. The key difficulty in the description of the dominant charged defects is determining a consistent position of the corresponding defect levels, which is difficult to derive using standard supercell calculations. In a complementary approach, we take advantage of the embedded cluster methodology that provides direct access to a common zero of the electrostatic potential for all point defects in all charge states. Charged defects polarise a host dielectric material with long-range forces that strongly affect the outcome of defect simulations; to account for the polarisation we couple embedding with the hybrid quantum mechanical/molecular mechanical (QM/MM) approach and investigate the structure, formation and ionisation energies, and equilibrium concentrations of native point defects in wurtzite GaN at a chemically accurate hybrid-density-functional-theory level. N vacancies are the most thermodynamically favourable native defects in GaN, which contribute to the n-type character of as-grown GaN but are not the main source, a result that is consistent with experiment. Our calculations show no native point defects can form thermodynamically stable acceptor states. GaN can be easily doped n-type, but, in equilibrium conditions at moderate temperatures acceptor dopants will be compensated by N vacancies and no significant hole concentrations will be observed, indicating non-equilibrium processes must dominate in p-type GaN. We identify spectroscopic signatures of native defects in the infrared, visible and ultraviolet luminescence ranges and complementary spectroscopies. Crucially, we calculate the effective-mass-like-state levels associated with electrons and holes bound in diffuse orbitals. These levels may be accessible in competition with more stronglylocalised states in luminescence processes and allow the attribution of the observed 3.46 and 3.27 eV UV peaks in a broad range of GaN samples to the presence of N vacancies.

Enhancement of Hydrogen Productions by Accelerating Electron-Transfers of Sulfur Defects in the CuS@CuGaS2 Heterojunction Photocatalysts

“…In addition, the electrons trapped by the dopant cannot generate excitons because they are bound by surface oxygen defects and other defects, thereby reducing the overall PL intensity. The blue-green emission peak at about 480 nm corresponds to the radiative transition of an electron to the deep donor level of the metal interstitials to an acceptor level of neutral V metal [35]. In contrast to photocurrent measurement, the photoluminescence intensity decreased in the order of CuS > 1.5CuS@0.5CuGaS 2 > CuGaS 2 > 1.0CuS@1.0CuGaS 2 > 0.5CuS@1.5CuGaS 2 .…”

Deep‐Level Defects and Impurities in InGaN Alloys

“…Although a number of theoretical studies have been devoted to the properties of defects in the parent compounds, GaN and InN, less is known about the properties of defects in InGaN as a function of indium content. The experimental low‐temperature bandgaps of GaN and InN are 3.51 and 0.70 eV; both gaps are direct .…”

Equilibrium point defect and charge carrier concentrations in a material determined through calculation of the self-consistent Fermi energy

“…A comprehensive description of how these energies have been computed is given in Ref. [61]; here it suffices to say that the hybrid quantum mechanical/molecular mechanical (QM/MM) embedded cluster technique was employed, [62,63,64,61,65] using DFT with the hybrid functional B97-2. [66] The results are given for "anion-poor" conditions, i.e.…”

“…The set of defect formation energies from Ref. [61] has been chosen, not necessarily for accuracy, but for the availability of all defect charge states over a Fermi energy range greater than the band gap, which is required to demonstrate the full analytical power of the codes described here. It is shown below, however, that the current set in fact is quite similar to others determined using standard supercell techniques [58,59].…”

“…The values given in Table 1, along with ρ(E) (shown in Fig. 2), E g , T and the unit cell parameters of wurtzite GaN (a = 3.181Å, c = 5.185Å) [61] Table 1: The formation energies E f (X q ) of the five lowest energy defects X in GaN in their various charge states q (each with degeneracy g X q ), determined at E F = 0 eV in anion-poor conditions. The values are taken from Ref.…”