Investigation of Barrier Inhomogeneities and Electronic Transport on Al-Foil/p-Type-4H-SiC Schottky Barrier Diodes Using Diffusion Welding

The diffusion welding (DW) is a comprehensive mechanism that can be extensively used to develop silicon carbide (SiC) Schottky rectifiers as a cheaper alternative to existing mainstream contact forming technologies. In this work, the Schottky barrier diode (SBD) fabricated by depositing Al-Foil on the p-type 4H-SiC substrate with a novel technology; DW. The electrical properties of physically fabricated Al-Foil/4H-SiC SBD have been investigated. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics based on the thermionic emission model in the temperature range (300 K–450 K) are investigated. It has been found that the ideality factor and barrier heights of identically manufactured Al-Foil/p-type-4H-SiC SBDs showing distinct deviation in their electrical characteristics. An improvement in the ideality factor of Al-Foil/p-type-4H-SiC SBD has been noticed with an increase in temperature. An increase in barrier height in fabricated SBD is also observed with an increase in temperature. We also found that these increases in barrier height, improve ideality factors and abnormalities in their electrical characteristics are due to structural defects initiation, discrete energy level formation, interfacial native oxide layer formation, inhomogenous doping profile distribution and tunneling current formation at the SiC sufaces.

Electron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models

n-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron–phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter- and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1→3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3.

Linear and nonlinear Fano resonance in the main chain-structure of additional defects with an isolated ring composed of defects

As is well known, Fano resonance originates from the interference between a continuum energy band and an embedded discrete energy level. We study transmission properties of the discrete chain-structure of additional defects with an isolated ring composed of N defect states, and obtain the analytical transmission coefficient of similar Fano formula. Using the formula, we reveal conditions for perfect reflections and transmissions due to either destructive or constructive interferences. It is found that a nonlinear Kerr-like response leads to bistable transmission, and for either linear cases or nonlinear ones, the defects in main arrays have a major impact on perfect reflections, but has no effect on perfect transmission.

Carrier Hopping and Relaxation in InAs/GaAs Quantum Dot Heterostructures

We investigate the effect of carrier dynamics on the temperature dependence of photoluminescence spectra from InAs/GaAs quantum dot heterostructures with different dot size uniformity. Intersublevel relaxation lifetimes and carrier transferring mechanisms are simulated using a model based on carriers relaxing and thermal emission of each discrete energy level in the quantum dot system. Calculated relaxation lifetimes are decreasing with temperature and have larger values for sample with lower dot size uniformity. In the quantitative discussion of carrier dynamics, the influence of thermal redistribution on carriers relaxing process of quantum dot system is demonstrated by our model.

PHOTOCURRENT ABSORPTION SPECTROSCOPIC STUDY OF Si0.6Ge0.4/Si QUANTUM WELLS

Absorption spectra of Si 0.6 Ge 0.4/ Si quantum wells are characterized by photocurrent measurements. The absorption coefficients of two different transitions, namely the transition between the Si band states and the discrete energy level in quantum wells, and the interlevel transition in quantum wells are deduced. They are directly proportional to (ℏω-ΔE)3/2 and δ(ℏω-Eeh), respectively. The valence band offsets of Si 0.6 Ge 0.4/ Si interface are 297 meV. The ground state energy levels in valence band and conduction band Si 0.6 Ge 0.4/ Si quantum wells are 37 meV and 23 meV, respectively.

QUANTUM ORIGIN OF A HOT UNIVERSE

We present the results on the quantum birth of a hot FRW universe in de Sitter vacuum from a quantum fluctuation which contains radiation and strings or some quintessence with the equation of state p=-ε/3. The presence of radiation results in quantum tunnelling from a discrete energy level with a non-zero quantized temperature. Energy levels have non-zero width corresponding to temperature fluctuations. The observational constraint on the CMB anisotropy selects the admissible range of the model parameters. For the GUT scale initial de Sitter vacuum, the lower limit on temperature at the start of classical evolution is close to the values predicted by theories of reheating, while an upper limit is far from the threshold for a monopole rest mass. The probability of quantum birth from a level of non-zero energy is much bigger than the probability of quantum birth from nothing. The presence of material with p=-ε/3 mimics a positive curvature term which makes possible quantum tunnelling for an open and a flat universe. Most plausible case is a flat universe arising from an initial fluctuation with a small admixture of radiation and strings with the negative deficit angle.