RESUMEN
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.
RESUMEN
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.
RESUMEN
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.
RESUMEN
A new approach to improve the light-emitting efficiency of Ge(Si) quantum dots (QDs) by the formation of an ordered array of QDs on a pit-patterned silicon-on-insulator (SOI) substrate is presented. This approach makes it possible to use the same pre-patterned substrate both for the growth of spatially ordered QDs and for the formation of photonic crystal (PhC) in which QDs are embedded. The periodic array of deep pits on the SOI substrate simultaneously serves as a template for spatially ordering of QDs and the basis for two-dimensional PhCs. As a result of theoretical and experimental studies, the main regularities of the QD nucleation on the pre-patterned surface with deep pits were revealed. The parameters of the pit-patterned substrate (the period of the location of the pits, the pit shape, and depth) providing a significant increase of the QD luminescence intensity due to the effective interaction of QD emission with the PhC modes are found.
