Exciton-plasma Mott transition in CdSe at 77K

Photoluminescence spectra of CdSe obtained in conditions of moderate excitation densities show anomalies in the behavior of peak position and intensity. These anomalies, on the basis of the optical gain and in the shift of peak position to lower energy side, are ascribed to the onset of an exciton-plasma Mott-transition in the photoexcited semiconductor.     

Biexciton gain and the Mott transition in GaAs quantum wires

Optical gain and the Mott transition in GaAs quantum wires were studied via simultaneous measurements of absorption and photoluminescence (PL). We observed well-separated PL peaks assigned to excitons (X) and biexcitons (XX) even at densities where optical gain existed. A sharp optical gain first appeared when the XX peak overtook the X peak, indicating the gain origin of biexciton-exciton population inversion. The XX peak eventually changed to a broad peak of plasma, and a broad gain due to plasma was observed as the Mott transition was completed.     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Metastability of defects,potential fluctuations,and percolation transition in GaAs

Potential fluctuations due to donor–acceptor compensation have been used to observe localization–delocalization transition in semi-insulating GaAs. Photoinduced transients, resulting from relaxation of stored charges in potential valleys, have two components. The long-lived power law decay at low temperature signifies a microscopically inhomogeneous disordered phase, and single exponential decay at a higher temperature signifies a homogeneous ordered phase. Temperature dependence of steady state photocurrent and kinetics of photocurrent decay suggest percolation as a possible mechanism for photoinduced transition in semi-insulating GaAs.     

Quantum Hall effect-insulator transition in the InAs/GaAs system with quantum dots

The InAs/GaAs structures consisting of quantum-dot layers with electronic properties typical of two-dimensional systems are investigated. It is found that, at a low concentration of charge carriers, the variable-range-hopping conductivity is observed at low temperatures. The localization length corresponds to characteristic quantum-dot cluster sizes determined using atomic-force microscopy (AFM). The quantum Hall effect-insulator transition induced by a magnetic field occurs in InAs/GaAs quantum-dot layers with metallic conductivity. The resistivities at the transition point exceed the resistivities characteristic of electrons in heterostructures and quantum wells. This can be explained by the large-scale fluctuations of the potential and, hence, the electron density.     

Nature of the Stranski-Krastanow transition during epitaxy of InGaAs on GaAs

We report first quantitative measurements by energy-selected imaging in a transmission electron microscope of In segregation within an uncapped islanded In0.25Ga0.75As layer grown epitaxially on GaAs. This layer has the lowest In concentration at which islanding occurs and, then, only after a flat approximately 3 nm alloy layer has been formed. In buildup by segregation at the surface of this initial flat layer is considered the driving force for islanding and, importantly, the segregation process introduces the characteristic delay seen before the Stranski-Krastanow transition. We observe strong inhomogeneous In enrichment within the islands (up to x(In) approximately 0.6 at the apex) and a simultaneous In depletion in the remaining flat layer.     

Parallel magnetic field induced transition in transport in the dilute two-dimensional hole system in GaAs

A magnetic field applied parallel to the two-dimensional hole system in the GaAs/AlGaAs heterostructure, which is metallic in the absence of an external magnetic field, can drive the system into insulating at a finite field through a well-defined transition. The value of resistivity at the transition is found to depend strongly on density.     

Features of the semiconductor-metal transition in GaAs at ultrahigh pressures: New intermediate phases

Using the thermopower method (Seebeck effect), the semiconductor-metal transition that occurs in gallium arsenide single crystals of n and p types at ultrahigh pressures P above ～11–18 GPa has been studied. It has been found that the transition in n-type samples begins at lower pressures. In the region of the semiconductor-metal phase transition, features have been observed on the thermopower dependences S(P). These features indicate that lattices intermediate between the initial semiconductor structure of zinc blende and the Cmcm high-pressure orthorhombic metallic phase are formed. By analogy with ZnTe, one intermediate phase (semiconductor with hole conductivity) is suggested to have the cinnabar structure and the second intermediate phase (semimetallic with electron conductivity) possibly has the SC16 structure. A model of the semiconductor-metal transition is discussed. The behavior of the thermoelectric properties in GaAs under pressure is compared with the behavior of these properties in other A_NB_8?N semiconductors, which also undergo the transition to the metallic state.     

Positive magnetoresistance peaked at a ferromagnetic transition in Mn-doped GaAs/AlGaAs quantum wells

A large positive magnetoresistance peaked at the Curie temperature has been observed in quantum well structures GaAs/AlGaAs doped by Mn. We suggest a new mechanism of magnetoresistance within low T _c ferromagnets resulting from a pronounced dependence of spin polarization at the vicinity of T _c on the external magnetic field. As a result, any contribution to resistance dependent on the Zeeman splitting of the spin subbands is amplified with respect to the direct effect of the external field. In our case we believe that the corresponding contribution is related to the upper Hubbard band. We propose that the mechanism considered here can be exploited as the mark of ferromagnetic transition.     

X ray absorption spectroscopy investigation of phase transition in Ge,GaAs and GaP

Abstract

The phase transitions of GaAs, GaP and Ge under pressure have been investigated by x-ray absorption spectroscopy (XAS). At the onset of the transition the Debye-Waller factor increases and the x-ray absorption near edge structure (XANES) is progressively modified. A mixing of the low and high pressure phase can be determined by XAS as well as amorphization of the sample on pressure release.     

Experimental investigation of the oscillator strength of the exciton transition in GaAs single quantum wells

A technique that makes it possible to investigate the mechanisms of phase relaxation of excitons in GaAs single quantum wells has been developed using resonant reflection spectroscopy. The dependence of the oscillator strength of the exciton transition on the quantum well thickness has been measured in the thickness range 9.1–30.0 nm. It has been demonstrated that the oscillator strength with a high accuracy does not depend on the temperature in the range 8–90 K. The temperature dependence of the homogeneous broadening has been measured, and the inhomogeneous broadening of the resonance exciton line has been determined. A nonmonotonic dependence of the spectral broadening of the exciton line on the intensity of the resonant excitation at a temperature of 8 K has been revealed for the sample with a high-quality quantum well. It has been established that an increase in the excitation level by five orders of magnitude above the linear limit leads to an insignificant change in the oscillator strength of the exciton transition and to a multiple broadening of the spectral line profile.     

Thermodynamic density of states of two-dimensional GaAs systems near the apparent metal-insulator transition

We perform combined resistivity and compressibility studies of two-dimensional hole and electron systems which show the apparent metal-insulator transition--a crossover in the sign of deltaR/deltaT with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a tenfold difference in r(s), the compressibility of both electrons and holes is well described by the theory of nonlinear screening of the random potential. We show that the resistivity exhibits a scaling behavior near the percolation threshold found from analysis of the compressibility. Notably, the percolation transition occurs at a much lower density than the crossover.     

First-principles combinatorial design of transition temperatures in multicomponent systems: the case of Mn in GaAs

The transition temperature TC of multicomponent systems--ferromagnetic, superconducting, or ferroelectric--depends strongly on the atomic arrangement, but an exhaustive search of all configurations for those that optimize TC is difficult, due to the astronomically large number of possibilities. Here we address this problem by parametrizing the TC of a set of approximately 50 input configurations, calculated from first principles, in terms of configuration variables ("cluster expansion"). Once established, this expansion allows us to search almost effortlessly the transition temperature of arbitrary configurations. We apply this approach to search for the configuration of Mn dopants in GaAs having the highest ferromagnetic Curie temperature. Our general approach of cluster expanding physical properties opens the way to design based on exploring a large space of configurations.