Localized Electron States Near a Metal-Semiconductor Nanocontact

The electronic structure of nanowires in contact with metallic electrodes of experimentally relevant sizes is calculated by incorporating the electrostatic polarization potential into the atomistic single particle Schr\"odinger equation. We show that the presence of an electrode produces localized electron/hole states near the electrode, a phenomenon only exhibited in nanostructures and overlooked in the past. This phenomenon will have profound implications on electron transport in such nanosystems. We calculate several electrode/nanowire geometries, with varying contact depths and nanowire radii. We demonstrate the change in the band gap of up to 0.5 eV in 3 nm diameter CdSe nanowires and calculate the magnitude of the applied electric field necessary to overcome the localization.Comment: 11 pages 4 figure

Identification of point defects in HVPE-grown GaN by steady-state and time-resolved photoluminescence

We have investigated point defects in GaN grown by HVPE by using steady-state and time-resolved photoluminescence (PL). Among the most common PL bands in this material are the red luminescence band with a maximum at 1.8 eV and a zero-phonon line (ZPL) at 2.36 eV (attributed to an unknown acceptor having an energy level 1.130 eV above the valence band), the blue luminescence band with a maximum at 2.9 eV (attributed to ZnGa), and the ultraviolet luminescence band with the main peak at 3.27 eV (related to an unknown shallow acceptor). In GaN with the highest quality, the dominant defect-related PL band at high excitation intensity is the green luminescence band with a maximum at about 2.4 eV. We attribute this band to transitions of electrons from the conduction band to the 0/+ level of the isolated CN defect. The yellow luminescence (YL) band, related to transitions via the −/0 level of the same defect, has a maximum at 2.1 eV. Another yellow luminescence band, which has similar shape but peaks at about 2.2 eV, is observed in less pure GaN samples and is attributed to the CNON complex. In semi-insulating GaN, the GL2 band with a maximum at 2.35 eV (attributed to VN) and the BL2 band with a maximum at 3.0 eV and the ZPL at 3.33 eV (attributed to a defect complex involving hydrogen) are observed. We also conclude that the gallium vacancy-related defects act as centers of nonradiative recombination

Physics of acceptors in GaN: Koopmans tuned HSE hybrid functional calculations and experiment

The Heyd-Scuseria-Ernzerhof (HSE) hybrid functional has become a widely used tool for theoretical calculations of point defects in semiconductors. It generally offers a satisfactory qualitative description of defect properties, including the donor/acceptor nature of defects, lowest energy charge states, thermodynamic and optical transition levels, Franck-Condon shifts, photoluminescence (PL) band shapes, and carrier capture cross sections. However, there are noticeable quantitative discrepancies in these properties when compared to experimental results. Some of these discrepancies arise from the presence of self-interaction in various parametrizations of the HSE. Other errors are due to the use of the periodic boundary conditions. In this study, we demonstrate that the error corrections scheme based on extrapolation to the dilute limit effectively eliminates the errors due to artificial electrostatic interactions of periodic images and interactions due to the defect state delocalization. This yields parametrizations of HSE that satisfy the generalized Koopmans' condition, essentially eliminating self-interaction from defect state orbitals. We apply this HSE Koopmans tuning individually to a range of cation site acceptors in GaN (Be\textsubscript{Ga}, Mg\textsubscript{Ga}, Zn\textsubscript{Ga}, Ca\textsubscript{Ga}, Cd\textsubscript{Ga}, and Hg\textsubscript{Ga}) and compare the HSE results with experimental data from PL spectra. The Koopmans-compliant HSE calculations show a significantly improved quantitative agreement with the experiment.Comment: 18 pages, 17 figure

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures forphotovoltaic applications

Although ZnO and ZnS are abundant, stable, environmentallybenign, their band gap energies (3.44 eV, 3.72 eV) are too large foroptimal photovoltaic efficiency. By using band-corrected pseudopotentialdensity-functional theory calculations, we study how the band gap,opticalabsorption, and carrier localization canbe controlled by formingquantum-well like and nanowire-based heterostructures ofZnO/ZnS andZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can besynthesized using existing methods, we obtain a band gap of 2.07 eV,which corresponds to a Shockley-Quiesser efficiency limitof 23 percent.Based on these nanowire results, we propose that ZnO/ZnScore/shellnanowires can be used as photovoltaic devices with organic polymersemiconductors as p-channel contacts

Formation mechanism and properties of CdS-Ag2S nanorod superlattices

The mechanism of formation of recently fabricated CdS-Ag{sub 2}S nanorod superlattices is considered and their elastic properties are predicted theoretically based on experimental structural data. We consider different possible mechanisms for the spontaneous ordering observed in these 1D nanostructures, such as diffusion-limited growth and ordering due to epitaxial strain. A simplified model suggests that diffusion-limited growth partially contributes to the observed ordering, but cannot account for the full extent of the ordering alone. The elastic properties of bulk Ag{sub 2}S are predicted using a first principles method and are fed into a classical valence force field (VFF) model of the nanostructure. The VFF results show significant repulsion between Ag{sub 2}S segments, strongly suggesting that the interplay between the chemical interface energy and strain due to the lattice mismatch between the two materials drives the spontaneous pattern formation

Determining factors of thermoelectric properties of semiconductor nanowires

It is widely accepted that low dimensionality of semiconductor heterostructures and nanostructures can significantly improve their thermoelectric efficiency. However, what is less well understood is the precise role of electronic and lattice transport coefficients in the improvement. We differentiate and analyze the electronic and lattice contributions to the enhancement by using a nearly parameter-free theory of the thermoelectric properties of semiconductor nanowires. By combining molecular dynamics, density functional theory, and Boltzmann transport theory methods, we provide a complete picture for the competing factors of thermoelectric figure of merit. As an example, we study the thermoelectric properties of ZnO and Si nanowires. We find that the figure of merit can be increased as much as 30 times in 8-Å-diameter ZnO nanowires and 20 times in 12-Å-diameter Si nanowires, compared with the bulk. Decoupling of thermoelectric contributions reveals that the reduction of lattice thermal conductivity is the predominant factor in the improvement of thermoelectric properties in nanowires. While the lattice contribution to the efficiency enhancement consistently becomes larger with decreasing size of nanowires, the electronic contribution is relatively small in ZnO and disadvantageous in Si