Susanne Mirbt

Electronic structure and magnetism of Mn-doped GaN

Mn-doped semiconductors are extremely interesting systems due to their novel magnetic properties suitable for the spintronics applications. It has been shown recently by both theory and experiment that Mn-doped GaN systems have a very high Curie temperature compared to that of Mn-doped GaAs systems. To understand the electronic and magnetic properties, we have studied Mn-doped GaN system in detail by a first-principles plane-wave method. We show here the effect of varying Mn concentration on the electronic and magnetic properties. In agreement with previous studies, d states of Mn form an impurity band completely separated from the valence-band states of the host GaN for dilute Mn concentration. This is in contrast to the Mn-doped GaAs system where Mn d states in the gap lie very close to the valence-band edge and hybridize strongly with the delocalized valence-band states. To study the effects of electron correlation, LSDA+U calculations have been performed. Calculated exchange interaction in (Mn,Ga)N is short ranged contrary to that in (Mn,Ga)As where the strength of the ferromagnetic coupling between Mn spins is not decreased substantially for large Mn-Mn separation. Also, the exchange interactions are anisotropic in different crystallographic directions due to the presence or absence of connectivity between Mn atoms through As bonds.

Density functional theory calculations of defect energies using supercells

Reliable calculations of defect properties may be obtained with density functional theory (DFT) using the supercell approximation. We systematically review the known sources of error and suggest ho ...

Improved electronic structure and optical properties of sp-hybridized semiconductors using LDA+U SIC

We propose the local density approximation (LDA) plus an on-site Coulomb self-interaction-like correction (SIC) potential for describing sp-hybridized bonds in semiconductors and insulators. We motivate the present LDA+USIC scheme by comparing the exact exchange (EXX) hole with the LDA exchange hole. The LDA+USIC method yields good band-gap energies Eg and dielectric constants e(w » 0) of Si, Ge, GaAs, and ZnSe. We also show that LDA consistently underestimates the G-point effective electron mc and light-hole mlh masses, and the underlying reason for this is a too strong light-hole-electron coupling within LDA. The advantages of the LDA+USIC approach are a computational time of the same order as the ordinary LDA, the orbital dependent LDA+USIC exchange-correlation interaction is asymmetric analogously to the EXX potential, and the method can be used for materials and compounds involving localized d- and f-orbitals.

A systematic study of polarons due to oxygen vacancy formation at the rutile TiO2(110) surface by GGA + U and HSE06 methods.

The polaronic nature of excess electrons accompanying an oxygen vacancy in a TiO(2)(110) surface has been studied by several theoretical approaches. According to previous studies, DFT + U and hybrid functional methods predict different sites of localization of the polarons. In this paper, we conducted a thorough comparison of the results obtained by GGA + U (generalized gradient approximation + Hubbard U) and HSE06 (Heyd-Scuseria-Ernzerhof hybrid functional) approximations. Considering initial symmetry breaking in the geometry optimization process, we show that regardless of the approximations used, electrons localize at two particular subsurface Ti sites in a state with mixed d(x(2)-y(2))/d(z(2)) character in the global coordinate frame with a spatial extent of the order of 7 Å. The lowest state of the polarons is a singlet, but the triplet is only about 0.1 meV higher in energy. Our results agree with previous experiments and calculations, wherever available. We stress that the hybrid functional has been first applied on this surface with a realistic coverage of oxygen vacancies corresponding to the experimental situation (~12.5%).

Finite-size scaling as a cure for supercell approximation errors in calculations of neutral native defects in InP

The relaxed and unrelaxed formation energies of neutral antisites and interstitial defects in InP are calculated using ab initio density functional theory and simple cubic supercells of up to 512 atoms. The finite-size errors in the formation energies of all the neutral defects arising from the supercell approximation are examined and corrected for using finite-size scaling methods, which are shown to be a very promising approach to the problem. Elastic errors scale linearly, while the errors arising from charge multipole interactions between the defect and its images in the periodic boundary conditions have a linear plus a higher order term, for which a cubic provides the best fit. These latter errors are shown to be significant even for neutral defects. Instances are also presented where even the 512 atom supercell is not sufficiently converged. Instead, physically relevant results can be obtained only by finite-size scaling the results of calculations in several supercells, up to and including the 512 atom cell and in extreme cases possibly even including the 1000 atom supercell.

First-principles calculations of Fe on GaAs(100)

We have calculated from first principles the electronic structure of 0.5-ML upto 5-ML-thick Fe layers on top of a GaAs(100) surface. We find the Fe magnetic moment to be determined by the Fe-As distance. As segregates to the top of the Fe film, whereas Ga most likely is found within the Fe film. Moreover, we find an asymmetric in-plane contraction of our unit cell along with an expansion perpendicular to the surface. We predict the number of Fe

Magnetic properties of 3d-impurities substituted in GaAs

3d

Small hole polaron in CdTe: Cd-vacancy revisited

holes to increase with increasing Fe thickness on p-doped GaAs.

High resistivity in undoped CdTe: carrier compensation of Te antisites and Cd vacancies

We have calculated the magnetic properties of substituted 3d impurities (Cr-Ni) in a GaAs host by means of first-principles electronic structure calculations. We provide a novel model explaining the ferromagnetic long-range order of III-V dilute magnetic semiconductors. The origin of the ferromagnetism is shown to be delocalized spin-uncompensated As dangling-bond electrons. Besides the quantitative prediction of the magnetic moments, our model provides an understanding of the half-metallicity, and the rise of the critical temperature with the impurity concentration.

Cl-doping of Te-rich CdTe: Complex formation, self-compensation and self-purification from first principles

The characteristics of electronic states of Cd-vacancies in CdTe, an important semiconductor for various technological applications, are under debate both from theoretical and experimental points of view. Experimentally, the Cd-vacancy in its negative charge state is found to have C3v symmetry and a (−1/−2) transition level at 0.4 eV. Our first principles density functional calculations with hybrid functionals confirm for the first time these experimental findings. Additionally, we find that the C3v symmetry and the position of the (−1/−2) transition level are caused by the formation of a hole polaron localised at an anionic site around the vacancy.