Effect Of Localized States Research Articles

Effect of localized states on the optical properties in InGaAs/GaAs multiple quantum wells grown by MOCVD

The InGaAs/GaAs multiple quantum wells (MQWs), as the most commonly used semiconductor laser epitaxial material, have important optical properties and wide-spread applications. However, the segregation of In atoms in the InGaAs/GaAs MQWs with the high In component has always been a serious problem. In this paper, the In0.29Ga0.71As/GaAs MQWs were prepared by metal-organic chemical vapor deposition (MOCVD), and then the temperature and excitation power-dependent photoluminescence spectra of MQWs were performed and studied. In the variable temperature measurement, the position peak of PL and full width at half maxima (FWHM) showed an "S"-shaped change from 20 to 300 K. Our systematic theoretical and experimental studies were performed that the peak before 50 K was considered to be the lasering caused by the localized states which play a leading role. And the α parameter value obtained by fitting the theoretical formula was less than 1, which also proved the existence of the localized states at low temperature in the variable power measurement at a fixed temperature. Through this research, it is of great significance to comprehensively understand the radiation emission mechanism in InGaAs materials and further develop the epitaxial growth of InGaAs quantum wells, and can deepen the understanding of the luminescence properties of InGaAs quantum well materials.

Exciton condensation in quantum wells. Exciton hydrodynamics. The effect of localized states

The hydrodynamic equations for indirect excitons in the double quantum wells are studied taking into account 1) a possibility of an exciton condensed phase formation, 2) the presence of pumping, 3) finite value of the exciton lifetime, 4) exciton scattering by defects. The threshold pumping emergence of the periodical exciton density distribution is found. The role of localized and free exciton states is analyzed in the formation of emission spectra.

Effect of localized states on internal quantum efficiency of III‐nitride LEDs

AbstractA model is suggested accounting for effects of localized electron and hole states formed by composition fluctuations in the InGaN active region of a III‐nitride LED on non‐radiative carrier recombination at threading dislocations. The model enables explanation of the abnormal temperature dependence of internal quantum efficiency (IQE) of a green LED structure recently observed at low current densities. The theoretical predictions are in quantitative agreement with experiment in the temperature range between 200 K and 453 K. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of InGaN/GaN SQD nanocolumn and InGaN nanocolumn

We have studied the emission mechanism of InGaN/GaN nanocolumns. We extracted only the effect of localized states and investigated the difference between InGaN/GaN single-quantum-disc (SQD) and InGaN nanocolumns by means of photoluminescence and photoluminescence excitation measurements. The difference between the localized states was interpreted by the band tail model. We conclude that the InGaN nanocolumn has widely distributed localized states, while the InGaN/GaN SQD has a single localized state.

Effects of localization on the excitonic linewidth in partially orderedGaxIn1−xPalloys

Spontaneous ordering in the ternary alloy GaxIn1−xP naturally leads to a reduction in alloy scattering and thus a reduction in the excitonic linewidth with increasing order parameter h. Concomitantly, the effect of localized states associated with various defects tends to increase the exciton linewidth, and it was observed that for hu0.45, the linewidth actually increases with order parameter [Phys. Rev. B 61, 9910 (2000)]. This was attributed to the increase in native defects which occurs for higher order parameters due to limitations in current growth technology. We show, using spatially resolved PL imaging with confocal and near-field microscopy, that this trend is reproduced in all orderedGaxIn1−xP samples, within the micro-ensemble formed by the collection of all spectra in a spatially and spectrally resolved PL image, directly imaging the effect of exciton localization. This reversed trend is a result of the interplay between the effects of localization and spontaneous ordering, and the spatial averaging which occurs when examining areas many times larger than the exciton Bohr radius. Agreement with the trend observed in macro-PL is obtained via micro-ensemble averaging of each data set, and by examination of the spatial correlation of exciton linewidths. The implications of these results on the origin of the exciton linewidth in ordered alloys and the effect of ordering on the spatial distribution of exciton linewidths are discussed.

The quasineutrality condition in amorphous semiconductors: Reformulation of the ‘lifetime/relaxation’ criterion

The concepts of lifetime and relaxation semiconductors introduced by van Roosbroeck and Casey are reconsidered for amorphous semiconductors and the effect of localized states on the lifetime/relaxation criterion specified: The quantity to be considered is τ ρ/ T d, where T d is (as before) the dielectric relaxation time, but τ ρ is now the lifetime of the total charge including charge stored in localized states. τ ρ is equal to the free carrier lifetime times a correction factor that is much larger than unity for amorphous semiconductors. Bandtail states and dangling bonds act differently on the criterion. The correction factor is frequency-dependent and decreases at higher frequencies. Two illustrative experimental examples (low substrate temperature a-Si:H, a-SiC:H alloys) are given, showing borderline cases.

Effect of Localized States on the Electrical Characteristics of Metal–Semiconductor Field-Effect Transistors

The electrical characteristics of MESFETS are studied by considering the effect of localized states at the gate-substrate interface. It is seen that in presence of a high density of localized states, the Id-Ud characteristics become independent of gate bias. This signifies the importance of Fermi level pinning in these structures. An expression for the threshold voltage is derived which is a function of the doping concentration, drain voltage, and the interface parameters, namely, the density of localized states, the neutral level energy, and the thickness of the interfacial oxide layer.

Effect of localized states on the current-voltage characteristics of metal-semiconductor contacts with thin interfacial layer

The role of discrete localized states on the current-voltage characteristics of metal-semiconductor contact is examined. It is seen that, because of these localized states, the logarithmic current vs voltage characteristics become nonlinear. Such nonlinearity is found sensitive to the temperature, and the energy and density of the localized states. The predicted temperature dependence of barrier height and the current-voltage characteristics are in agreement with the experimental results of Aboelfotoh [ Phys. Rev. B 39, 5070 (1989)].

Properties of free-carrier transport in a-Se and a-Si:H

The existence of the localized tail states, typical of amorphous semiconductors, makes the study of the mobility of free carriers (μo) very difficult. Here we first review a few ways how to exclude the effect of localized states, which have been successfully used in a-Se. It is shown how the subnanosecond TOF (Time of Flight) method can be used for the study of trapping kinetics into the tail states and for the finding of the temperature dependence of the free mobility of holes in a-Se. Deactivation of the drift mobility (μ) and the impact ionization at high electric fields allowed us to describe the properties of hot holes in a-Se and experimentally construct the mobility gap. Although the properties of drift mobilities in a-Se and a-Si:H (related with the tail states) are very similar, the free carrier transport related properties are completely different. The Onsager photogeneration, Langevin-like recombination and trapping (the proof of the existence of which is discussed in detail), and field induced impact ionization, the properties typical of a-Se, have not been observed in a-Si:H.

Phonon-drag thermoelectric power in high magnetic fields in heterojunctions

A theory is presented for the low-temperature phonon-drag thermopower S xx in a strong magnetic field. Gigantic quantum oscillations (much larger than electron-diffusion contributions) are obtained. The temperature and field dependences of S xx agree well with recent data in GaAs/Al x Ga 1− x As heterojunctions. Effects of localized states on S xx are discussed.

Band Structure and Electronic Excitations in Cdl-xMnxTe

AbstractWe have performed spin-polarized, self-consistent local spin density total energy and band structure calculations for the prototype semimagnetic semiconductor alloy Cd1-xMnxTe. Based on the calculated band structures and taking into account the many body effects of localized states, we propose a schematic energy level diagram to interpret the d→d*, p→d, and photoemission transitions in Cd1-xMnxTe.

Localized electrons in molecular aggregates

Electron localization in molecular aggregates is related to two independent physical concepts. One is the non-periodicity of the system and the other is the dynamic concept of electron-phonon interaction. These two concepts are separately investigated in terms of one-dimensional models. The effect of localized states on the quasi-free state, V0, is also discussed.

The effect of localized states on the impurity soft x-ray spectrum of dilute alloys

The soft x-ray emission from solute atoms in dilute alloys is discussed. Using a simple model for an attractive impurity in a metal, the qualitative effects on the intensity of virtual bound and bound states round the impurity are fully investigated. The effect of localized s states is found to be different in character from that due to localized states with non-zero angular momentum. The simple model used enables the transition probabilities to be evaluated analytically; it is these quantities rather than the density of states which are responsible for the large distortions of the solute intensity profiles when there are virtual bound states.