33 research outputs found
Managing the supercell approximation for charged defects in semiconductors: finite size scaling, charge correction factors, the bandgap problem and the ab initio dielectric constant
The errors arising in ab initio density functional theory studies of
semiconductor point defects using the supercell approximation are analyzed. It
is demonstrated that a) the leading finite size errors are inverse linear and
inverse cubic in the supercell size, and b) finite size scaling over a series
of supercells gives reliable isolated charged defect formation energies to
around +-0.05 eV. The scaled results are used to test three correction methods.
The Makov-Payne method is insufficient, but combined with the scaling
parameters yields an ab initio dielectric constant of 11.6+-4.1 for InP. Gamma
point corrections for defect level dispersion are completely incorrect, even
for shallow levels, but re-aligning the total potential in real-space between
defect and bulk cells actually corrects the electrostatic defect-defect
interaction errors as well. Isolated defect energies to +-0.1 eV are then
obtained using a 64 atom supercell, though this does not improve for larger
cells. Finally, finite size scaling of known dopant levels shows how to treat
the band gap problem: in less than about 200 atom supercells with no
corrections, continuing to consider levels into the theoretical conduction band
(extended gap) comes closest to experiment. However, for larger cells or when
supercell approximation errors are removed, a scissors scheme stretching the
theoretical band gap onto the experimental one is in fact correct.Comment: 11 pages, 3 figures (6 figure files). Accepted for Phys Rev
The Structure of the [Zn_In - V_P] Defect Complex in Zn Doped InP
We study the structure, the formation and binding energies and the transfer
levels of the zinc-phosphorus vacancy complex [Zn_In - V_P] in Zn doped p-type
InP, as a function of the charge, using plane wave ab initio DFT-LDA
calculations in a 64 atom supercell. We find a binding energy of 0.39 eV for
the complex, which is neutral in p-type material, the 0/-1 transfer level lying
0.50 eV above the valence band edge, all in agreement with recent positron
annihilation experiments. This indicates that, whilst the formation of
phosphorus vacancies (V_P) may be involved in carrier compensation in heavily
Zn doped material, the formation of Zn-vacancy complexes is not.
Regarding the structure: for charge states Q=+6 to -4 the Zn atom is in an
sp^2 bonded DX position and electrons added/removed go to/come from the
remaining dangling bonds on the triangle of In atoms. This reduces the
effective vacancy volume monatonically as electrons are added to the complex,
also in agreement with experiment. The reduction occurs through a combination
of increased In-In bonding and increased Zn-In electrostatic attraction. In
addition, for certain charge states we find complex Jahn-Teller behaviour in
which up to three different structures, (with the In triangle dimerised,
antidimerised or symmetric) are stable and are close to degenerate. We are able
to predict and successfully explain the structural behaviour of this complex
using a simple tight binding model.Comment: 10 pages text (postscript) plus 8 figures (jpeg). Submitted to Phys.
Rev.
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.
For dilute Mn concentration, states of Mn form an impurity band completely
separated from the valence band states of the host GaN. This is in contrast to
the Mn doped GaAs system where Mn states in the gap lie very close to the
valence band edge and hybridizes 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 in 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.Comment: 6 figures, submitted to Phys. Rev.
Relative concentration and structure of native defects in GaP
The native defects in the compound semiconductor GaP have been studied using a pseudopotential density functional theory method in order to determine their relative concentrations and the most stable charge states. The electronic and atomic structures are presented and the defect concentrations are estimated using calculated formation energies. Relaxation effects are taken into account fully and produce negative-U charge transfer levels for VP and PGa. The concentration of VGa is in good agreement with the results of positron annihilation experiments. The charge transfer levels presented compare qualitatively well with experiments where available. The effect of stoichiometry on the defect concentrations is also described and is shown to be considerable. The lowest formation energies are found for PGa +2 in p-type and VGa â3 in n-type GaP under P-rich conditions, and for GaP â2 in n-type GaP under Ga-rich conditions. Finally, the finite size errors arising from the use of supercells with periodic boundary conditions are examined
Calculated oscillation periods of the interlayer coupling in Fe/Cr/Fe and Fe/Mo/Fe sandwiches
Semiconductor Surface Studies
Contains an introduction, reports on two research projects and a list of publications.Joint Services Electronics Program Grant DAAH04-95-1-003
Structural and magnetic properties of Fe/ZnSe(001) interfaces
We have performed first principles electronic structure calculations to
investigate the structural and magnetic properties of Fe/ZnSe(001) interfaces.
Calculations involving full geometry optimizations have been carried out for a
broad range of thickness of Fe layers(0.5 monolayer to 10 monolayers) on top of
a ZnSe(001) substrate. Both Zn and Se terminated interfaces have been explored.
Total energy calculations show that Se segregates at the surface which is in
agreement with recent experiments.
For both Zn and Se terminations, the interface Fe magnetic moments are higher
than the bulk bcc Fe moment.
We have also investigated the effect of adding Fe atoms on top of a
reconstructed ZnSe surface to explore the role of reconstruction of
semiconductor surfaces in determining properties of metal-semiconductor
interfaces. Fe breaks the Se dimer bond formed for a Se-rich (2x1)
reconstructed surface. Finally, we looked at the reverse growth i.e. growth of
Zn and Se atoms on a bcc Fe(001) substrate to investigate the properties of the
second interface of a magnetotunnel junction. The results are in good agreement
with the theoretical and experimental results, wherever available.Comment: 7 pages, 8 figures, accepted for publication in PR