Session V51: Semiconductors (Theory)

\textbf{Accurate Electronic, Transport, and Bulk Properties of Wurtzite Beryllium Oxide (BeO)}

We present \textit{ab-initio}, self-consistent density functional theory (DFT) description of electronic, transport, and bulk properties of wurtzite Beryllium oxide (w-BeO). We used a local density approximation potential (LDA) and the linear combination of atomic orbitals (LCOA) formalism. Our implementation of the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF), ensures the full, physical content of our local density approximation (LDA) calculations -- as per the derivation of DFT [AIP Advances, 4, 127104 (2014) We report the band gap, density of states, partial density of state, effective masses, and the bulk modulus. Our calculated band gap of 10.29 eV, using an experimental, room temperature lattice constant of 2.6979 A at room temperature is in agreement with the experimental value of 10.6 eV. Acknowledgments\textbf{: }This work was funded in part the US National Science Foundation [NSF, Award Nos. EPS-1003897, NSF (2010-2015)-RII-SUBR, and HRD-1002541], the US Department of Energy, National Nuclear Security Administration (NNSA, Award No. DE-NA0002630), LaSPACE, and LONI-SUBR.   

First principles studies of the stability and Shottky barriers of metal/CdTe(111) interfaces

CdZnTe and CdTe based semiconductor X-Ray and Gamma-Ray detectors have been intensively studied recently due to their promising potentials for achieving high-resolution, high signal-to-noise ratios and low leakage current, all are desirable features in applications ranging from medical diagnostics to homeland security. Using density functional calculations, we systematically studied the stability, the atomic and electronic structures of the interfaces between CdTe (111) surfaces (Cd- and Te-terminated) and the selected metals (Cu, Al Ni, Pd and Pt). We also calculated the Schottky barrier height (SBH) by aligning the electrostatic potentials in semiconductor and metal regions. Our calculations revealed significant differences between the Cd- and Te- terminated interfaces. While metals tend to deposit directly on reconstructed Te-terminated surfaces, they form a Te-metal alloy layer at the Cd-Terminated metal/CdTe interface. For both Te- and Cd- terminated interfaces, the Schottky barrier heights do not depend much on the choice of metals despite the large variation of the work functions. On the other hand, the interface structure is found to have large effect on the SBH, which is attributed to the metal induced states in the gap.   

Ab-initio Electronic, Transport and Related Properties of Zinc Blende Boron Arsenide (zb-BAs)

We present results from~\textit{ab-initio}, self-consistent~density functional theory~(DFT) calculations of electronic, transport, and bulk properties~of~\textit{zinc~blende}~boron~arsenide (zb-BAs). We utilized a local density approximation (LDA) potential and the linear combination of atomic orbital (LCAO) formalism. Our computational technique follows the Bagayoko, Zhao, and Williams method, as enhanced by Ekuma and Franklin. Our results include electronic~energy~bands, densities of states, and~effective masses. We explain the agreement between these findings, including the indirect band gap, and available, corresponding, experimental ones. This work confirms the capability of DFT to describe accurately properties of materials, provided the computations adhere to the conditions of validity of DFT [AIP Advances, 4, 127104 (2014)]. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- National, Nuclear Security Administration (NNSA) (Award No. DE- NA0002630), LaSPACE, and LONI-SUBR.   

Ab-initio Density Functional Theory (DFT) Studies of Electronic, Transport, and Bulk Properties of Sodium Oxide (Na$_{2}$O).

We present the findings of \textit{ab-initio} calculations of electronic, transport, and structural properties of cubic sodium oxide (Na$_{2}$O). These results were obtained using density functional theory (DFT), specifically a local density approximation (LDA) potential, and the linear combination of Gaussian orbitals (LCGO). Our implementation of LCGO followed the Bagayoko, Zhao, and Williams method as enhanced by the work of Ekuma and Franklin (BZW-EF). We describe the electronic band structure of Na$_{2}$O with a direct band gap of 2.22 eV. Our results include predicted values for the electronic band structure and associated energy eigenvalues, the total and partial density of states (DOS and pDOS), the equilibrium lattice constant of Na$_{2}$O, and the bulk modulus. We have also calculated the electron and holes effective masses in the $\Gamma $ to L, $\Gamma $ to X, and $\Gamma $ to K directions. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- National, Nuclear Security Administration (NNSA) (Award No. DE- NA0002630), LaSPACE, and LONI-SUBR.   

Hybrid DFT calculations of the band structure of alpha-Sn

The electronic properties of bulk alpha-tin were revisited using first principles. The band structure, in addition to other properties, such as the absorption spectrum and density of states, were calculated using Density Functional Theory and the HSE06 hybrid functional. The direct and indirect band gaps obtained from these calculations are in better agreement with experimental results than previously reported calculations.   

Ab-initio Calculation of Optoelectronic and Structural Properties of Cubic Lithium Oxide (Li$_{2}$O).

Using the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF), we performed ab-initio, density functional theory (DFT) calculations of optoelectronic, transport, and bulk properties of Li$_{2}$S. In so doing, we avoid ``band gap'' and problems plaguing many DET calculations [AIP Advances 4, 127104 (2014)]. We employed a local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). With the BZW-EF method, our results possess the full, physical content of DFT and agree with available, corresponding experimental ones. In particular, we found a room temperature indirect band gap of 6.659 eV that compares favorably with experimental values ranging from 5 to 7.99 eV. We also calculated total and partial density of states (DOS and PDOS), effective masses of charge carriers, the equilibrium lattice constant, and the bulk modulus. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- National, Nuclear Security Administration (NNSA) (Award Nos. DE-NA0001861 and DE- NA0002630), LaSPACE, and LONI-SUBR.   

\textbf{Comparisons}\textbf{\textit{ of Accurate }}\textbf{ Electronic, Transport, and Bulk Properties of XP (X}$=$\textbf{B, Al, Ga, In).}

We present comparisons of results from \textit{ab-initio, }self-consistent local density approximation (LDA) calculations of accurate, electronic and related properties of zinc blende XP (X$=$B, Al, Ga, In) phosphides. We implemented the linear combination of atomic orbitals following the Bagayoko, Zhao, and Williams (BZW) method as enhanced by Ekuma and Franklin (BZW-EF). Consequently, our results have the full physical content of DFT and agree very well with corresponding experimental ones [AIP Advances, 4, 127104 (2014)]. Our calculated, indirect band gap of 2.02 eV for BP, 2.56 eV for AlP, and of 2.29 eV for GaP, from $\Gamma $ to X-point, are in excellent agreement with experimental values. Our calculated direct band gap of 1.43 eV, at $\Gamma $, for InP is also in an excellent agreement with experimental value. We discuss calculated electron and hole effective masses, total (DOS) and partial (pDOS) densities of states, and the bulk modulus of these phosphides. Acknowledgments: NSF and the Louisiana Board of Regents, LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, DOE -- National, Nuclear Security Administration (NNSA) (Award Nos. DE-NA0001861 and DE- NA0002630), LaSPACE, and LONI-SUBR.   

\textbf{\textit{Ab-initio}}\textbf{ Calculations of Electronic Properties of Calcium Fluoride (CaF}$_{\mathbf{2}}$\textbf{)}

We have performed first principle, local density approximation (LDA) calculations of electronic and related properties of cubic calcium fluorite (CaF$_{2})$. Our non-relativistic computations employed the Ceperley and Alder LDA potential and the linear combination of atomic orbitals (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). We discuss the electronic energy bands, including the large band gap, total and partial density of states, electron and hole effective masses, and the bulk modulus. Our calculated, indirect (X-$\Gamma )$ band gap is 12.98 eV; it is 1 eV above an experimental value of 11.8 eV. The calculated bulk modulus (82.89 GPA) is excellent agreement with the experimental result of 82.0 \textpm 0.7. Our predicted equilibrium lattice constant is 5.42{\AA}. Acknowledgments\textbf{:} This work is funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR], and NSF HRD-1002541, the US Department of Energy, National, Nuclear Security Administration (NNSA) (Award No. DE-NA-0002630), LaSPACE, and LONI-SUBR.   

Direct detection of dark matter via single-electron excitations in semiconductors

Over the last several decades, there has been an enormous experimental effort to search for dark matter (DM). Traditionally, semiconductors have been used to detect DM via scattering with nuclei and the subsequent relaxation of the crystal. However, if DM has mass below order 10 GeV these methods lose detection sensitivity. This is because the DM is lighter than a typical nucleus and, since DM particles move at non-relativistic speeds, they cannot transfer enough energy and momentum to the crystal to produce observable signals. In our work [arXiv:1509.01598], we demonstrate that DM-electron scattering in semiconductors increases the sensitivity of DM detection in this mass regime by several orders of magnitude and is a viable avenue for the direct detection of sub-GeV DM. We use density functional theory (DFT) to calculate the crystal wavefunctions and the band energies, which we correct with an empirical scissor operator. These wavefunctions are used to do perturbation theory, which allows us to calculate the DM-electron scattering rates. In this talk we will focus on the computational and theoretical challenges, discuss future directions and present new expected limits for DM-electron scattering.   

Raman spectra calculations for Si-Ge core-shell nanocrystals using \textit{ab initio} real-space methods

We use a real-space pseudopotential method within density functional theory to calculate Raman spectra for Si-Ge core-shell nanocrystals. We examine the lattice strain induced by the interface of the core and the shell. We calculate how this strain affects the vibrational modes and Raman spectra. We also find that the relative size of the Si and Ge peaks in the Raman spectrum is proportional to the size of the Si core and Ge shell regions, which suggests that Raman spectroscopy can be used to experimentally determine the relative size of the core and the outer shell in these nanocrystals.   

k.p Parameters with Accuracy Control from Preexistent First-Principles Band Structure Calculations

The \textbf{k.p} method is a successful approach to obtain band structure, optical and transport properties of semiconductors. It overtakes the \textit{ab initio} methods in confined systems due to its low computational cost since it is a continuum method that does not require all the atoms' orbital information. From an effective one-electron Hamiltonian, the \textbf{k.p} matrix representation can be calculated using perturbation theory and the parameters identified by symmetry arguments. The parameters determination, however, needs a complementary approach. In this paper, we developed a general method to extract the \textbf{k.p} parameters from preexistent band structures of bulk materials that is not limited by the crystal symmetry or by the model. To demonstrate our approach, we applied it to zinc blende GaAs band structure calculated by hybrid density functional theory within the Heyd-Scuseria-Ernzerhof functional (DFT-HSE), for the usual 8$\times$8 \textbf{k.p} Hamiltonian. Our parameters reproduced the DFT-HSE band structure with great accuracy up to 20\% of the first Brillouin zone (FBZ). Furthermore, for fitting regions ranging from 7-20\% of FBZ, the parameters lie inside the range of values reported by the most reliable studies in the literature.   

Transferable tight binding model for strained group IV and III-V heterostructures

Modern semiconductor devices have reached critical device dimensions in the range of several nanometers. For reliable prediction of device performance, it is critical to have a numerical efficient model that are transferable to material interfaces. In this work, we present an empirical tight binding (ETB) model with transferable parameters for strained IV and III-V group semiconductors. The ETB model is numerically highly efficient as it make use of an orthogonal sp3d5s* basis set with nearest neighbor inter-atomic interactions. The ETB parameters are generated from HSE06 hybrid functional calculations. Band structures of strained group IV and III-V materials by ETB model are in good agreement with corresponding HSE06 calculations. Furthermore, the ETB model is applied to strained superlattices which consist of group IV and III-V elements. The ETB model turns out to be transferable to nano-scale hetero-structure. The ETB band structures agree with the corresponding HSE06 results in the whole Brillouin zone. The ETB band gaps of superlattices with common cations or common anions have discrepancies within 0.05eV.   

Valley Physics in Tin (II) Sulfide

The field of 2D physics has experienced a rapid growth in recent years. Improved manipulation and growth techniques have resulted in isolation and characterization of novel materials with fascinating properties. A family of compounds that was described recently is transition metal monochalcogenides. Due to their non-trivial crystal structure, studying these materials is a challenging task. Using a combination of density functional theory (DFT) and analytical methods, we investigate the band structure of tin (II) sulfide, a naturally occurring material, to discover that SnS has two pairs of valleys positioned in perpendicular orientation to each other. DFT is employed to construct $\mathbf{k}\cdot\mathbf{p}$ Hamiltonians around each of the valleys. Finally, we show that these individual valley pairs can be separated using linearly polarized light or by utilizing the nonlinear current response, making SnS a candidate for valleytronic applications.   

Reduced-Density-Matrix Description of Single-Photon and Multi-Photon Processes in Quantized Many-Electron Systems

The frequency-dependent transition rates for single-photon and multi-photon processes in quantized many-electron systems are evaluated using a reduced-density-matrix approach. We provide a fundamental quantum-mechanical foundation for systematic spectral simulations. A perturbation expansion of the frequency-domain Liouville-space self-energy operator is introduced for detailed evaluations of the spectral-line shapes. In the diagonal-resolvent (isolated-line) and short-memory-time (Markov) approximations, the lowest-order contributions to the spectral-line widths and shifts associated with environmental electron-photon and electron-phonon interactions are systematically evaluated. Our description is directly applicable to electromagnetic processes in a wide variety of many-electron systems, without premature approximations. In particular, our approach can be applied to investigate quantum optical phenomena involving electrons in both bulk and nanoscale semiconductor materials entirely from first principles, using a single-electron basis set obtained from density functional theory as a starting point for a many-electron description.   

Lifshitz Transitions in Bias-Resonant Twisted Bilayer Graphene

Topological transitions of the Fermi surface (Lifshitz transitions) have been shown to cause discontinuities in materials properties. One such transition has been predicted to occur in AB-stacked bilayer graphene.$^{[1]}$ In this talk we discuss incommensurately twisted bilayer graphene with an interlayer bias energy.$^{[2]}$ New physics emerges when the bias energy is tuned into resonance with the kinetic energy cost of interlayer hopping due to the mismatch between the Dirac points of the twisted layers. We show that the system near resonance is described by relatively simple low-energy theories that nevertheless produce a vast number of Lifshitz transitions. An exhaustive description of the topological transitions in a universal regime at weak interlayer coupling will be presented.$\qquad$ [1] Y. Lemonik et al. Phys. Rev. B, 82:201408 (2010). [2] H.K. Pal et al. arXiv:1409.1971 (2014).