Self-compensation due to point defects in Mg-doped GaN

Abstract

Using hybrid density functional theory, we address point defects susceptible to cause charge compensation upon Mg doping of GaN. We determine the free energy of formation of the nitrogen vacancy and of several Mg-related defects. The entropic contribution as a function of temperature is determined within the quasiharmonic approximation. We find that the Mg interstitial shows a noticeably lower free energy of formation than the Mg substitutional to Ga in p-type conditions. Therefore, the Mg impurity is amphoteric behaving like an acceptor when substitutional to Ga and like a double donor when accommodated in an interstitial position. The hybrid-functional results are then linked to experimental observations by solving the charge neutrality equations for semiconductor dominated by impurities. We show that a thermodynamic equilibrium model is unable to account for the experimental hole concentration as a function of Mg doping density, due to nitrogen vacancies and Mg interstitials acting as compensating donors. To explain the experimental result, which includes a dropoff of the hole concentration at high Mg densities, we thus resort to nonequilibrium models. We show that either nitrogen vacancies or Mg interstitials could be at the origin of the self-compensation mechanism. However, only the model based on interstitial Mg donors provides a natural mechanism to account for the sudden appearance of self-compensation. Indeed, the amphoteric nature of the Mg impurity leads to Fermi-level pinning and accounts for the observed dropoff of the hole concentration of GaN samples at high Mg doping. Our work suggests that current limitations in p-type doping of GaN could be overcome by extrinsically controlling the Fermi energy during growth

Computationally predicted energies and properties of defects in GaN

“…Results similar to the HSE calculations were found in other studies employing hybrid functionals. 77,78 However, V 0 N was not found to be stable in ref. 78, and the (0/−) transition level was found to occur 4 meV below the CBM in ref.…”

“…77,78 However, V 0 N was not found to be stable in ref. 78, and the (0/−) transition level was found to occur 4 meV below the CBM in ref. 77.…”

Lattice Location of Mg in GaN: A Fresh Look at Doping Limitations

“…Recently this controversial discussion was revived by the results of hybrid density functional calculations of Miceli and Pasquarello [16] who concluded that "…the amphoteric nature of the Mg impurity is critical to explain the dropoff in the hole density observed experimentally": once the doping limit has been reached, additional Mg atoms are not incorporated on substitutional Ga sites any more but on interstitial sites where they form compensating double donors, thus pinning the Fermi level. The reader is referred to Ref.…”

Oxygen DX center in In0.17Al0.83N: Nonradiative recombination and persistent photoconductivity

“…51 and 52). From the charge neutrality equations for semiconductors dominated by impurities, 54,55 we inferred that F lies at 0.16 eV below the conduction band edge. The energy profiles of the two charge states along the distortion paths are illustrated in Fig.…”

“…Temperature-dependent entropic effects on the formation energies are negligible and can be neglected. 55 Hence, the variation of the Fermi energy destabilizes (D s 4 ) − through a rigid upwards shift of its minimum energy path. Our calculations indicate that it is sufficient to assume a downshift of the Fermi energy lower than 0.1 eV to destabilize (D s 4 ) − with respect to the (D s 4 ) 0 [cf.…”