Computation of the Spatial Distribution of Charge-Carrier Density in Disordered Media.

The space- and temperature-dependent electron distribution n(r,T) determines optoelectronic properties of disordered semiconductors. It is a challenging task to get access to n(r,T) in random potentials, while avoiding the time-consuming numerical solution of the Schrödinger equation. We present several numerical techniques targeted to fulfill this task. For a degenerate system with Fermi statistics, a numerical approach based on a matrix inversion and one based on a system of linear equations are developed. For a non-degenerate system with Boltzmann statistics, a numerical technique based on a universal low-pass filter and one based on random wave functions are introduced. The high accuracy of the approximate calculations are checked by comparison with the exact quantum-mechanical solutions.

Energy Scales of Compositional Disorder in Alloy Semiconductors.

The study of semiconductor alloys is currently experiencing a renaissance. Alloying is often used to tune the material properties desired for device applications. It allows, for instance, to vary in broad ranges the band gaps responsible for the light absorption and light emission spectra of the materials. The price for this tunability is the extra disorder caused by alloying. In this mini-review, we address the features of the unavoidable disorder caused by statistical fluctuations of the alloy composition along the device. Combinations of material parameters responsible for the alloy disorder are revealed, based solely on the physical dimensions of the input parameters. Theoretical estimates for the energy scales of the disorder landscape are given separately for several kinds of alloys desired for applications in modern optoelectronics. Among these are perovskites, transition-metal dichalcogenide monolayers, and organic semiconductor blends. While theoretical estimates for perovskites and inorganic monolayers are compatible with experimental data, such a comparison is rather controversial for organic blends, indicating that more research is needed in the latter case.

Tunneling current modulation in atomically precise graphene nanoribbon heterojunctions.

Lateral heterojunctions of atomically precise graphene nanoribbons (GNRs) hold promise for applications in nanotechnology, yet their charge transport and most of the spectroscopic properties have not been investigated. Here, we synthesize a monolayer of multiple aligned heterojunctions consisting of quasi-metallic and wide-bandgap GNRs, and report characterization by scanning tunneling microscopy, angle-resolved photoemission, Raman spectroscopy, and charge transport. Comprehensive transport measurements as a function of bias and gate voltages, channel length, and temperature reveal that charge transport is dictated by tunneling through the potential barriers formed by wide-bandgap GNR segments. The current-voltage characteristics are in agreement with calculations of tunneling conductance through asymmetric barriers. We fabricate a GNR heterojunctions based sensor and demonstrate greatly improved sensitivity to adsorbates compared to graphene based sensors. This is achieved via modulation of the GNR heterojunction tunneling barriers by adsorbates.

Formation energies of antiphase boundaries in GaAs and GaP: an ab Initio study.

Electronic and structural properties of antiphase boundaries in group III-V semiconductor compounds have been receiving increased attention due to the potential to integration of optically-active III-V heterostructures on silicon or germanium substrates. The formation energies of {110}, {111}, {112}, and {113} antiphase boundaries in GaAs and GaP were studied theoretically using a full-potential linearized augmented plane-wave density-functional approach. Results of the study reveal that the stoichiometric {110} boundaries are the most energetically favorable in both compounds. The specific formation energy gamma of the remaining antiphase boundaries increases in the order of gamma({113}) approximately gamma({112}) < gamma({111}), which suggests {113} and {112} as possible planes for faceting and annihilation of antiphase boundaries in GaAs and GaP.

STEMsalabim: A high-performance computing cluster friendly code for scanning transmission electron microscopy image simulations of thin specimens.

We present a new multislice code for the computer simulation of scanning transmission electron microscope (STEM) images based on the frozen lattice approximation. Unlike existing software packages, the code is optimized to perform well on highly parallelized computing clusters, combining distributed and shared memory architectures. This enables efficient calculation of large lateral scanning areas of the specimen within the frozen lattice approximation and fine-grained sweeps of parameter space.