Abstract
This study investigates how the prediction of the gallium nitride (GaN) bandgap is affected by treating semi-core d-electrons as either valence or core states in the pseudopotentials, which correspond to small-core and large-core approximations, respectively. To distinguish the effect of semi-core treatment from another bandgap problem recognized in density functional theory (DFT), that is, the underestimation related to the self-interaction problem, we perform diffusion Monte Carlo (DMC) evaluations under the fixed-node approximation and the optical gap scheme (where the evaluation uses N-electron many-body wavefunctions). A comparison to experimental measurements of bandgap energies indicates that DMC predictions are overestimated, whereas DFT simulations, which are used as a guiding function (DFT → DMC), are typically underestimated. This agrees with the trend reported in previous DMC studies on bandgap estimates. The large-core approximation results in a greater overestimation than the small-core treatment in both DFT and DMC. The bias in the overestimation is ∼30% for the DFT → DMC operation. Several possible causes of this bias are considered, such as pd-hybridization, core-polarization, and electronic screening effects. However, although these factors could qualitatively account for the overestimation caused by the large-core treatment, the estimated magnitude of the bias is too small to explain the evaluated difference between small-core and large-core approximations of the bandgap.
Highlights
The electronic structures of group III nitride semiconductors have been intensively studied due to their potential applications in Light Emitting Diodes (LEDs) and solar-cell batteries.1–5 For example, the bandgaps of AlN and gallium nitride (GaN) were reported in the 1970s as ∼6.28 eV6,7 and 3.4 eV,8–10 respectively
To distinguish the effect of semi-core treatment from another bandgap problem recognized in density functional theory (DFT), that is, the underestimation related to the self-interaction problem, we perform diffusion Monte Carlo (DMC) evaluations under the fixed-node approximation and the optical gap scheme
This study evaluated the bias in bandgap prediction according to the choice of valence range in pseudopotentials, taking GaN with Ga-d semi-core electrons as an example
Summary
The electronic structures of group III nitride semiconductors have been intensively studied due to their potential applications in Light Emitting Diodes (LEDs) and solar-cell batteries. For example, the bandgaps of AlN and gallium nitride (GaN) were reported in the 1970s as ∼6.28 eV6,7 and 3.4 eV, respectively. The electronic structures of group III nitride semiconductors have been intensively studied due to their potential applications in Light Emitting Diodes (LEDs) and solar-cell batteries.. The bandgaps of AlN and gallium nitride (GaN) were reported in the 1970s as ∼6.28 eV6,7 and 3.4 eV, respectively. Theoretical estimates of bandgaps are predominantly achieved through ab initio calculations based on density functional theory (DFT), which typically produces underestimates that are sensitive to the choice of exchange-correlation (XC) functional.. DFT calculations with XC functionals that more precisely consider the exchange part are useful for solving this underestimation problem; previous examples include B3LYP, HSE, TB-mBJ, and DFT + U.34,35. Several studies have attempted to understand the origin of bandgap underestimates for group III nitride semiconductors.. The incomplete cancellation of the self-interaction is a major reason for this underestimation, which prompted the development of the Self-Interaction Correction (SIC) scheme. DFT calculations with XC functionals that more precisely consider the exchange part are useful for solving this underestimation problem; previous examples include B3LYP, HSE, TB-mBJ, and DFT + U.34,35 the GW method is an important framework based on scitation.org/journal/adv the many-body theory that aims to achieve reliable estimates of the bandgap. GW evaluations are overestimated when the screened Coulomb potential W and the dielectric constants are both updated in the self-consistent cycle. Several studies have attempted to understand the origin of bandgap underestimates for group III nitride semiconductors. For example, previous research on a
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