Electronic_Structure

Additional Authors: Walter R. L. Lambrecht
Takao Kotani
Mark van Schilfgaarde

Poster Location: Veale
Presentation ID: 139
Presentation Group: Emerging Technology

Poster Title: Structural and Electronic Properties
of Copper Sulfide and Copper Bromide

Poster Abstract:

Chalcosite Cu2S and digenite Cu1.8S are possibly interesting semiconductors for photovoltaic applications. Their electronic structure is poorly understood because their crystal structure is complex. It consists of a close-packed lattice of S with mobile Cu occupying various types of interstitial sites with a statistical distribution depending on temperature. As a starting point for understanding these materials, we investigated the simpler antifluorite structure. Both local density approximation (LDA) and self-consistent quasiparticle GW calculations with the full-potential linearized muffin-tin orbital method give a semimetallic band structure with the Fermi level pinned at a degenerate Cu-d band state at Gamma point. A random distortion of the Cu atoms from the perfect antifluorite positions inside each S cage is found to break the degeneracy of the d state at Gamma point and thus opens up a small gap of about 0.1 eV in LDA. The experimental evidence for a semiconducting gap of about 1 eV is critically examined. To gain further insight into the Cu d and s-band shifts beyond LDA, we considered other Cu compounds such as Cu2O and CuBr. We compare their LDA and GW band structures and determined the effective masses and Kohn-Luttinger Hamiltonian parameters for CuBr.

Tula R. Paudel, Graduate Student

Additional Authors: Walter R.L. Lambrecht

Poster Location: Veale
Presentation ID: 455
Presentation Group: Basic/Applied Science

Poster Title: Electronic Band Structure, Structure
and Phonons of Zinc Silicon Nitride.

Poster Abstract:

Zinc Silicon Nitride is an interesting alternative to Gallium Nitride. Its crystal structure is derived from the wurtzite structure of Gallium Nitride by a particular ordered substation of the Gallium atoms by Zinc and Silicon in such as way that each Nitrogen is coordinated with two Silicon and two Zinc atoms. Electronic structure calculations were performed with two different approaches, the plane-wave ultra-soft pseudopotential approach and the full-potential linearized muffin-tin orbital method(FP-LMTO) both using the local density approximation (LDA). The structure was fully optimized. The relaxation consists primarily of the Nitrogen atom finding its optimum position inside its nearest neighbor tetrahedron by making a shorter Silicon-Nitrogen and longer Zinc-Nitrogen bond. An indirect LDA band gap of about 3.4 eV is obtained. Thus a gap larger than that of Gallium Nitride is expected. Phonons at the center of the Brillouin zone is calculated using the linear response approach. The results are compared with that of Zinc Germanium Nitride, which was studied earlier, [W. R. L. Lambrecht et al. Phys Rev. B 72, 155202 (2005)] . Work supported by AFOSR.

Maosheng Miao,
Senior Research Associate

Additional Authors: Walter R.L. Lambrecht

Poster Location: Veale
Presentation ID: 454
Presentation Group: Basic/Applied Science

Poster Title: Stacking Faults and 3C Quantum
Wells in Hexagonal SiC Polytypes

Poster Abstract:

Recently, there has been considerable interest in the expansion of stacking faults (SF) in SiC polytypes. SF expansion was observed to occur in PiN diode devices under forward bias and leads to a degradation of the devices over time. Mostly single SFs (SSF) are observed during this process. SF growth and specifically formation of mostly double SFs (DSF) and small 3C-SiC inclusions were also observed under oxidation and annealing of n-type SiC. We demonstrated that trapping of electrons in stacking fault (SF) interface states may lower the energy of a SF more than it costs to form the SF. This ``electronic stress'' driving force for SF expansion is evaluated for single and double stacking faults in 4H-SiC in terms of a 2D free electron density of states model based on first-principles calculations. In contrast with previous work, which claimed that the number of electrons that can be trapped in the SF is severely limited by the potential barrier arising from the space charge region adjacent to the SF, we find that the potential barrier is strongly reduced by screening and its effect is negligible. The electron driving force remains valid for both SSF and DSF but is significantly stronger for DSF. Using the same method, we also studied the band gap, the polarization and the quantum well states for 3C inclusions in 4H SiC in a systematic way with inclusions of 2 to 10 cubic layers in 4H SiC. The polarization is strongly reduced by screening and correspondingly the effective band gap of the 3C quantum well in 4H system is never smaller than that of pure 3C. To explain the observation of below 3C gap luminescence in such systems, an increase in exciton binding energy must be invoked.

Paul Larson, Research Associate

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Additional Authors: Aditi Herwadkar
Walter R.L. Lambrecht

Poster Location: Veale
Presentation ID: 453
Presentation Group: Basic/Applied Science

Poster Title: Electronic structure calculations
of transition metal and rare earth
nitrides using LSDA+U

Poster Abstract:

Electronic structure calculations within the local spin density approximation (LSDA) have limitations dealing with highly localized orbitals, such as d and f states. LSDA+U calculations allow to add a Hubbard U correction self-consistently to obtain the correct position of these localized states. A series of calculations have been performed with LSDA+U on rocksalt nitrides using the linearized muffin tin orbital (LMTO) method. CrN has an observed optical gap of 0.7 eV though LSDA predicts a metal. Addition of LSDA+U in the fully localized limit opens a gap while changing the order of the states near the Fermi level which makes it a charge transfer type Mott-insulator. Similar calculations were performed on the rare-earth (Ce-Lu) nitrides. The f orbitals are highly localized and become pinned at the Fermi level. Addition of LSDA+U moves the states away from the Fermi level. Within the rocksalt symmetry, the f orbitals split into two triply degenerate (t_1u and t_2u) states and a nondegenerate state (a_2u). The f orbitals prefer to remain completely empty or completely filled. In cases with 2 or 5 extra electrons, such as PrN, SmN, DyN, and TmN, partial filling is only possible, leading to heavy-Fermion behavior. This work is supported by ONR and NSF.

Please send comments and suggestions to paul.larson@case.edu