Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L45: Semiconductor Electronic Structure: Theory & Spectra I |
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Sponsoring Units: FIAP Chair: Gael Nardin, NIST/University of Colorado Room: Mile High Ballroom 4D |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L45.00001: Differences in the surface electronic structure of Ge(001) and Si(001) from angle-resolved photoemission spectroscopy and \textit{ab-initio} theory Richard C. Hatch, Hosung Seo, Patrick Ponath, Miri Choi, Agham B. Posadas, Alexander A. Demkov Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we compare the surface electronic structure of both Ge(001) and Si(001) surfaces. Unlike previous ARPES experiments, where the Ge(001) surfaces were prepared using cycles of ion sputtering and annealing, our Ge(001) surfaces were prepared using a combination of wet etching and oxygen plasma cleaning. This new technique has the advantage that it avoids the incomplete healing of surface roughening associated with sputtering and annealing cycles. The ARPES data show that the dimer-derived surface state that determines the charge neutrality level, and thus the Schottky barrier height in Si, is actually a surface resonance in Ge, and the highest occupied state is a bulk state. In order to avoid theory predicting an overlap of the valence and conduction bands, we employed first-principles, hybrid density functional theory (DFT). This theory effectively explains the presence of a number of photoemission features in both Si and Ge. We found it is necessary to incorporate spin-orbit interaction in the hybrid DFT calculations for Ge in order to model ARPES data, and we found a spin-orbit splitting of 0.28 eV both experimentally and theoretically. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L45.00002: Differences in the surface electronic structure of Ge(001) and Si(001) from hybrid density functional theory and angle-resolved photoemission spectroscopy Hosung Seo, Richard C. Hatch, Patrick Ponath, Miri Choi, Agham B. Posadas, Alexander A. Demkov Even with renewed interest in Ge as a competitor to Si in field effect transistors, several key features of the surface electronic structure of Ge(001) have remained controversial. Notably, the character of the valence band top of Ge(001) has been heavily debated. Using first-principles hybrid density functional theory and angle-resolved photoemission spectroscopy, we unambiguously establish the critical differences between the electronic structure of the Si and Ge (001) surfaces. In order to avoid the problems associated with the band gap underestimation in LDA and GGA, we utilized the screened Hartree-Fock hybrid exchange correlation density functional due to Heyd, Scuseria, and Ernzerhof (HSE06). We explicitly show that the surface state that determines the charge neutrality level, and thus the Schottky barrier height in Si, is actually a surface resonance in Ge. Our results strongly suggest that the recently observed strong Fermi level pining in Ge/metal junctions comes from the evanescent states. Additionally, using surface resonance calculations and bulk HSE06 calculations with the spin-orbit coupling, we identify the origin of a number of highly debated ARPES features for Ge(001) and Si(001). [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L45.00003: Optoelectronics, Theory and Defect Physics of Zn-IV Nitride Semiconductors Prineha Narang, Shiyou Chen, Aashrita Mangu, Jason Cooper, Sheraz Gul, Junko Yano, Lin-Wang Wang, Nathan Lewis, Harry Atwater ZnSn$_{\mathrm{x}}$Ge$_{\mathrm{1-x}}$N$_{\mathrm{2}}$ alloys with optical band gaps ranging from 2-3.1eV can be tuned to span a large portion of the solar spectrum, and could therefore be a viable earth-abundant light absorber and replacement for InGaN in nitride optoelectronic devices. They exhibit local order as demonstrated via X-ray absorption fine structure spectroscopy (EXAFS) and a linear relationship between the (002) peak position and composition in XRD studies. The bowing parameter is 0.29 eV for the measured band gaps of ZnSn$_{\mathrm{1-x}}$Ge$_{\mathrm{x}}$N$_{\mathrm{2}}$, significantly smaller than that of In$_{\mathrm{1-x}}$Ga$_{\mathrm{x}}$N, indicating that the ZnSn$_{\mathrm{1-x}}$Ge$_{\mathrm{x}}$N$_{\mathrm{2}}$ alloy band gaps can be tuned almost linearly by controlling the Sn/Ge composition. In this presentation we show theoretical studies of the optoelectronic behavior and defect physics of Zn(Sn,Ge)N$_{\mathrm{2\thinspace }}$series and experimental investigations via X-ray absorption and emission spectroscopy to probe the conduction and valence-band partial density of states. Band structure calculations from different methods will be shown in comparison with the experimental optical properties. Resonant inelastic scattering studies of the Zn(Sn,Ge)N$_{\mathrm{2}}$ lattice will be presented with their carrier dynamics obtained from pump-probe spectroscopy. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L45.00004: Quantum Electron-Hole Droplets in Gallium Arsenide Quantum Wells Andrew Hunter, Hebin Li, Steven Cundiff, Martin Mootz, Mackillo Kira, Stephan Koch In a solid, the choice of appropriate quasiparticles greatly simplifies our understanding of the system. For example, (quasi)electrons allow one to disregard the interaction between an electron and the macroscopic number of ionic potentials in a solid, and instead treat the system as a free quasielectron with an effective mass. Similarly, we improve our understanding of the complex electronic many-body system in a solid if we can identify the stable many-body quasiparticles of the system, such as excitons, biexcitons, and trions. We present experimental and theoretical evidence for the existence of a new quasiparticle that we call a quantum droplet, a charge-neutral bound state of a few electrons and holes. Unlike the macroscopic electron-hole droplets observed in indirect-gap semiconductors, quantum droplets contain only a small number of particles, leading to quantization of binding energy, but with a two-particle correlation function characteristic of a liquid. Using transient-absorption spectroscopy with ultrafast pulses, we show that we can create quantum droplets in gallium arsenide quantum wells. Projection onto a correlated quantum optical state allows us to separate the effects of the droplet state from other multiple-particle states, such as biexcitons. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L45.00005: Auger recombination in sodium iodide Andrew McAllister, Emmanouil Kioupakis, Daniel \r{A}berg, Andr\'e Schleife Scintillators are an important tool used to detect high energy radiation - both in the interest of national security and in medicine. However, scintillator detectors currently suffer from lower energy resolutions than expected from basic counting statistics. This has been attributed to non-proportional light yield compared to incoming radiation, but the specific mechanism for this non-proportionality has not been identified. Auger recombination is a non-radiative process that could be contributing to the non-proportionality of scintillating materials. Auger recombination comes in two types - direct and phonon-assisted. We have used first-principles calculations to study Auger recombination in sodium iodide, a well characterized scintillating material. Our findings indicate that phonon-assisted Auger recombination is stronger in sodium iodide than direct Auger recombination. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L45.00006: \textit{Ab-initio} Calculations of Electronic Properties of AlP, GaP and InP Yuriy Malozovsky, Azizjon Saliev, Lashaunda Franklin, Chinedu Ekuma, Guang-Lin Zhao, Diola Bagayoko We present results from \textit{ab-initio}, self consistent local density approximation (LDA) calculations of electronic and related properties of zinc blende aluminum, gallium and indium phosphides (AlP, GaP {\&} InP). We employed a local density approximation (LDA) potential and implemented the linear combination of atomic orbitals (LCAO) formalism. This implementation followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). Our calculated, indirect band gap of 2.56 eV for AlP, and of 2.14 eV for GaP, from $\Gamma $ to X, are in excellent agreement with experimental values. Our calculated direct band gap of 1.40 eV, at $\Gamma $ -point for InP is also in excellent agreement with experimental value. We also report calculated electron and hole effective masses for AlP, GaP and InP and total (DOS) and partial (pDOS) densities of states. This research 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-NA0001861), LaSPACE, and LONI-SUBR. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L45.00007: \textit{Ab-initio} Calculations of Electronic Properties of Boron Phosphide (BP) John Ejembi, Lashaunda Franklin, Yuriy Malozovsky, Diola Bagayoko We present results from \textit{ab-initio}, self consistent local density approximation (LDA) calculations of electronic and related properties of zinc blende boron phosphide (BP). We employed a local density approximation (LDA) potential and implemented the linear combination of atomic orbitals (LCAO) formalism. This implementation followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). We discuss our preliminary results for the indirect band gap, from $\Gamma $ to X, of Boron Phosphide. We also report calculated electron and hole effective masses for Boron Phosphide and total (DOS) and partial (pDOS) density of states. Acknowledgments: This research 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-NA0001861), LaSPACE, and LONI-SUBR. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L45.00008: \textit{Ab-initio} Calculations of Accurate Electronic Properties of Wurzite AlN Ifeanyi Nwigboji, Yuriy Malozovsky, Diola Bagayoko We present results from \textit{ab-initio}, self consistent local density approximation (LDA) calculations of electronic and related properties of wurtzite Aluminum Nitride (w-AlN). Our non-relativistic computations employed the Ceperley and Alder LDA potential and the linear combination of atomic orbital (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams' method as enhanced by Ekuma and Franklin (BZW-EF). The BZW-EF method verifiably obtains the minima of the occupied energies; these minima provide the most variationally and physically valid density functional theory (DFT) description of the ground states of materials under study. Our preliminary results for w-AlN show that w-AlN has a direct band gap of 5.82 eV at the $\Gamma $ point. The preliminary energy bands were obtained with a basis set comprising 48 functions. None of the several, larger basis sets tested to date led to occupied energies lower than those obtained with the above 48. While most previous LDA calculations are 2 eV smaller or more than the experimental value of 5.9 eV that is in excellent agreement with our finding, considering the typical experimental uncertainty of 0.2 eV for absorption measurements on AlN. We also discuss our calculated density of states (DOS) and partial densities of states (pDOS). [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L45.00009: Density Functional Theory Revisited: The Mathematical and Physical Conditions for the Physical Content of the Eigenvalues Diola Bagayoko, Lashounda Franklin, Yuriy Malozovsky, Bethuel Khamala, Chinedu Ekuma, Yacouba Diakite, Azizjon Saliev We briefly recall the derivation of density functional theory (DFT) and of its local density approximation (LDA). From this derivation, we show that eigenvalues resulting from self consistent DFT calculations utilizing a single input basis set do not necessarily have much physical content. We subsequently present \textit{the necessary conditions for obtaining eigenvalues with a physical content, for the ground state and low energy excited states.} These conditions include the verifiable attainment of the minima of the occupied energies, on the one hand, and the avoidance of a mathematical artifact stemming from the Rayleigh theorem, on the other. We show a few new results, obtained with DFT potentials, that agree very well with corresponding experimental ones. These results include band gaps, effective masses, and structural properties of selected semiconductors. Our calculations utilized the Bagayoko, Zhao, and Williams (BZW) method as enhanced by Ekuma and Franklin (BZW-EF). The distinctive feature of the method includes its strict adherence to the necessary conditions described noted above. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L45.00010: Electron-Phonon Renormalization of Band Gap of BN and Si nanowires Andrew Franson We compute the electron-phonon renormalization of band gap of BN with three different crystal structures from first-principles using the linear-response theory and the Allen-Heine theory. We find that the zero-point renormalization of band gap in BN depends strongly on crystal structures, varying from -222 meV to -434 meV. We also calculate the temperature-dependent band gaps and electronic band structures of BN. In addition, we investigate the quantum confinement effect on electron-phonon renormalization by comparing band gap shifts due to electron-phonon coupling in bulk Si and Si nanowires. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L45.00011: Effect of Interface Roughness in Ultra-Thin Semiconductor Quantum Wells Yu Song, Rajaram Bhat, Chung-En Zah, Claire Gmachl Ultra-thin, few monolayer semiconductor quantum well (QW) structures are extensively used in optoelectronic devices, especially when the effective mass is high ($\geq 0.1 m_e$). Traditionally, interfaces roughness in QWs are either ignored, or treated as a 2D scattering potential ideally localized on the interface plane. This treatment is valid when roughness is small compared to the layer thickness. But in situations of ultra-thin QWs, a more systematic model is needed. In this work, we model the potential associated with the interface roughness as a 3D function with dependence on the actual interface position. With the help of Green's function we show that this potential, when averaged in-plane, produces an effective grading potential out of the plane which significantly alters the energy spectrum. This effect is reaffirmed by the experimental results from the measurement of intersubband (ISB) optical transitions in III-Nitride thin QWs. The general expression of the scattering matrix element for carrier transport is also derived, which requires full 3D calculation and significantly differs from the traditional treatment. The scattering lifetimes are calculated for the example of III-nitride ISB devices, and the results are being compared to that from the traditional formulas. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L45.00012: Intermediate band in type-II silicon clathrate with Cu/Ag guest atoms ZhaoHui Huang, HuaShan Li, ZhiGang Wu, Mark T. Lusk We investigate the structural and electronic properties of type-II clathrate with guests of Cu and Ag atoms, and our first-principles calculations demonstrate that an intermediate band (IB) would exist in the originally forbidden gap if one or two isolated Cu or Ag atoms located in a cage. These IBs have nearly ideal energy separations to VBM and CBM of the host clathrate, thus they would be useful for making highly-efficient solar cells. However, Cu and Ag atoms tend to form clusters larger than two atoms, which lead to a heavily doped semiconductor instead of generating useful IBs. We will discuss possible approaches to overcome this severe problem. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L45.00013: Valence Band Alignment at (111) / (0001) ScN/SiC and ScN/GaN Interfaces as Determined by Photoemission Sean King, Robert Nemanich, Robert Davis Scandium nitride (ScN) is a transition metal nitride material that over the past decade has garnered significant interest for nano-electronic, spin-tronic, optoelectronic, electro-acoustic, and thermoelectric applications. This is due to the reasonably close lattice matching exhibited between the (111) plane of ScN (0.3139 nm) and the (111) / (0001) planes of SiC and GaN (0.3073 and 0.3189 nm respectively). For these specific applications, the valence and conduction band alignment of ScN to SiC and GaN will play a significant role. In this regard, we have utilized x-ray photoelectron spectroscopy (XPS) to investigate the growth and interfacial valence band alignment for gas-source molecular beam epitaxy (GSMBE) of ScN on (111) 3C-SiC / (0001) 6H-SiC substrates. Using a detailed analysis of the attenuation of the Si2p core level from multiple ScN growths and XPS measurements, we find that ScN grows on (111) 3C-SiC in a layer by layer fashion. UPS measurements (Figure 1) show the ScN valence band to be 1.6-2.1 eV below the system Fermi level indicating a minimum band gap on this order. Detailed XPS/UPS measurements indicate the ScN/3C-SiC valence band offset is small ($\le $ 0.3 eV). Additional measurements for GSMBE GaN on ScN show a larger interfacial valence band discontinuity of $\sim $ 0.8 eV. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L45.00014: Spectral Description of Multi-Photon Processes in Quantized Many-Electron Systems Based on a Reduced-Density-Matrix Approach Verne Jacobs, Alex Kutana The frequency-dependent transition rates for multi-photon processes in quantized many-electron systems are evaluated using a reduced-density-matrix approach. We provide a fundamental foundation for systematic spectral simulations for atomic, molecular, and solid-state systems. A perturbation expansion of the frequency-domain Liouville-space self-energy operator is employed in detailed evaluations of the spectral-line widths and shifts in the isolated-line and short-memory-time (Markov) approximations. The lowest-order contributions associated with environmental electron-photon and electron-phonon interactions are systematically taken into account. Our description is directly applicable to electromagnetic processes in a wide variety of semiconductor, photochemical, and biological systems, without premature approximations. In particular, our approach can be applied to investigate optical phenomena involving electrons in both bulk and nanoscale semiconductor materials entirely from first principles, using the density functional formalism and existing electronic structure codes. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L45.00015: Strain-engineered Surface Transport in Si(001): Complete Isolation of the Surface State via Tensile Strain Miao Zhou, Zheng Liu, Zhengfei Wang, Zhaoqiang Bai, Yuanping Feng, Max G. Lagally, Feng Liu When silicon channel layer grows increasingly thinner in microelectronics, surface conductance becomes increasingly dominant, opening an opportunity to use surface states for novel devices. By combining density functional theory, non-equilibrium Green's function formulism, and effective-Hamiltonian approaches, we demonstrate strain-engineered surface transport in Si(001), with the complete isolation of the Si surface states from the bulk bands. Our results show that sufficient tensile strain can effectively remove the overlap between the surface valence state and the bulk valence band, because of the drastically different deformation potentials. Isolation of the surface valence state is possible with a tensile strain of $\sim$1.5\%, a value that is accessible experimentally. Quantum transport simulations of a chemical sensing device based on strained Si(001) surface confirm the isolation of dominating surface conductance, giving rise to an enhanced molecular sensitivity. Our results show the promise for combining surface engineering with strain engineering to further our ability to manipulate surface states for quantum information processing and surface-state based devices. [Preview Abstract] |
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