Photoluminescence investigation of two-dimensional electron gas in an undoped AlxGa1-xN/GaN heterostructure

Low-temperature photoluminescence measurement is performed on an undoped AlxGa1-xN/GaN heterostructure. Temperature-dependent Hall mobility confirms the formation of two-dimensional electron gas (2DEG) near the heterointerface. A weak photoluminescence (PL) peak with the energy of similar to 79meV lower than the free exciton (FE) emission of bulk GaN is related to the radiative recombination between electrons confined in the triangular well and the holes near the flat-band region of GaN. Its identification is supported by the solution of coupled one-dimensional Poisson and Schrodinger equations. When the temperature increases, the red shift of the 2DEG related emission peak is slower than that of the FE peak. The enhanced screening effect coming from the increasing 2DEG concentration and the varying electron distribution at two lowest subbands as a function of temperature account for such behaviour.

Spin-polarized transport of two-dimensional electron gas embedded in a diluted magnetic semiconductor

The spin-polarized transport property of a diluted magnetic semiconductor two-dimensional electron gas is investigated theoretically at low temperature. A large current polarization can be found in this system even at small magnetic fields and oscillates with increasing magnetic field while the carrier polarization is vanishingly small. The magnitude as well as the sign of the current polarization can be tuned by varying magnetic field, the electron density and the Mn concentration. (c) 2005 American Institute of Physics.

Circular photogalvanic effect of the two-dimensional electron gas in AlXGa1-XN/GaN heterostructures under uniaxial strain

The circular photogalvanic effect (CPGE) of the two-dimensional electron gas (2DEG) in Al0.25Ga0.75N/GaN heterostructures induced by infrared radiation has been investigated under uniaxial strain. The observed photocurrent consists of the superposition of the CPGE and the linear photogalvanic effect currents, both of which are up to 10(-2) nA. The amplitude of the CPGE current increases linearly with additional strain and is enhanced by 18.6% with a strain of 2.2x10(-3). Based on the experimental results, the contribution of bulk-inversion asymmetry (BIA) and structure-inversion asymmetry (SIA) spin splitting of the 2DEG to the CPGE current in the heterostructures is separated, and the ratio of SIA and BIA terms is estimated to be about 13.2, indicating that the SIA is the dominant mechanism to induce the k-linear spin splitting of the subbands in the triangular quantum well at AlxGa1-xN/GaN heterointerfaces. (C) 2007 American Institute of Physics.

Weak anti-localization in InAlAs/InGaAs/InAlAs high mobility two-dimensional electron gas systems

Magneto-transport measurements have been carried out on three heavily Si delta-doped In-0.52 Al-0.48 As/In-0.53 Ga-0.47 As/In-0.52 A(10.48) As single quantum well samples in which two subbands were occupied by electrons. The weak anti-localization (WAL) has been found in such high electron mobility systems. The strong Rashba spin-orbit (SO) coupling is due to the high structure inversion asymmetry (SIA) of the quantum wells. Since the WAL theory model is so complicated in fitting our experimental results, we obtained the Rashba SO coupling constant alpha and the zero-field spin splitting Delta(0) by an approximate approach. The results are consistent with that obtained by the Shubnikov-de Haas (SdH) oscillation analysis. The WAL effect in high electron mobility system suggests that finding a useful approach for deducing alpha and Delta(0) is important in designing future spintronics devices that utilize the Rashba SO coupling.

Theoretical analysis about the influence of channel layer thickness on the 2D electron gas and its distribution in InP-based high-electron-mobility transistors

The principle of high-electron-mobility transistor (HEMT) and the property of two-dimensional electron gas (2DEG) have been analyzed theoretically. The concentration and distribution of 2DEG in various channel layers are calculated by numerical method. Variation of 2DEG concentration in different subband of the quantum well is discussed in detail. Calculated results show that sheet electron concentration of 2DEG in the channel is affected slightly by the thickness of the channel. But the proportion of electrons inhabited in different subbands can be affected by the thickness of the channel. When the size of channel lies between 20-25 nm, the number of electrons occupying the second subband reaches the maximum. This result can be used in parameter design of materials and devices.

Blocking of the polaron effect and spin-split cyclotron resonance in a two-dimensional electron gas

Cyclotron resonance (CR) of high density GaAs quantum wells exhibits well-resolved spin splitting above the LO-phonon frequency. The spin-up and spin-down CR frequencies are reversed relative to the order expected from simple band nonparabolicity. We demonstrate that this is a consequence of the blocking of the polaron interaction which is a sensitive function of the filling of the Landau levels.

High-field cyclotron resonance and electron-phonon interaction in modulation-doped multiple quantum well structures

A systematic study of electron cyclotron resonance (CR) in two sets of GaAs/Al0.3Ga0.7As modulation-doped quantum-well samples (well widths between 12 and 24 nm) has been carried out in magnetic fields up to 30 T. Polaron CR is the dominant transition in the region of GaAs optical phonons for the set of lightly doped samples, and the results are in good agreement with calculations that include the interaction with interface optical phonons. The results from the heavily doped set are markedly different. At low magnetic fields (below the GaAs reststrahlen region), all three samples exhibit almost identical CR which shows little effect of the polaron interaction due to screening and Pauli-principle effects. Above the GaAs LO-phonon region (B > similar to 23 T), the three samples behave very differently. For the most lightly doped sample (3 x 10(11) cm(-2)) only one transition minimum is observed, which can be explained as screened polaron CR. A sample of intermediate density (6 x 10(11) cm(-2)) shows two lines above 23 T; the higher frequency branch is indistinguishable from the positions of the single line of the low density sample. For the most heavily, doped sample (1.2 x 10(12) cm(-2)) there is no evidence of high frequency resonance, and the strong, single line observed is indistinguishable from the lower branch observed from sample with intermediate doping density. We suggest that the low frequency branch in our experiment is a magnetoplasmon resonance red-shifted by disorder, and the upper branch is single-particle-like screened polaron CR. (C) 1998 Elsevier Science B.V. All rights reserved.

Photoluminescence investigation of GaInP/GaAs multiple quantum wells grown on (001) and (311) B GaAs surfaces by gas source molecular beam epitaxy

Photoluminescence (PL) investigation was carried out on GaInP/GaAs multiple quantum wells structures grown on (001) and (311) B surfaces of GaAs by gas source molecular beam epitaxy. Superlattice structures of GaAs/GaInP grown on (001) GaAs substrate were also studied in comparison. Deep-level luminescence was seen to dominate the PL spectra from the quantum wells and superlattice structures that were grown on (001) GaAs substrate. In contrast, superior optical properties were exhibited in the same structures grown on (311) B GaAs surfaces. The results suggested that GaAs/GaInP quantum well structures on (311) B oriented substrates could efficiently suppress the deep-level emissions, result in narrower PL peaks indicating smooth interfaces. (C) 1998 American Institute of Physics.

Structural identification of a cubic phase in hexagonal GaN films grown on sapphire by gas-source molecular beam epitaxy

The structural characteristics of gallium nitride (GaN) films grown on sapphire(0001) substrates by gas source molecular beam epitaxy (GSMBE) have been investigated using high-resolution synchrotron irradiation X-ray diffraction and cathodoluminescence with a variable energy electron beam. Besides the well-known GaN hexagonal structure, a small portion of cubic phase GaN was observed. The X-ray measurements provide an essential means for the structural identification of the GaN layers. Arising from the variable penetration depth of the electron beam in the cathodoluminescence measurements, it was found that the fraction of the GaN cubic-phase typically increased as the probing depth was increased. The results suggest that the GaN cubic phase is mostly located near the interface between the substrate and GaN layer due to the initial nucleation.

Study on the Dissociation Behavior of Gas Hydrate in Porous Sediment Based on Fractal Theory

The dissociation process of gas hydrate was regarded as a gas-solid reaction without solid production layer when the temperature was above the zero centigrade. Based on the shrinking core model and the fractal theory, a fractional dimension dynamical model for gas hydrate dissociation in porous sediment was established. The new approach of evaluating the fractal dimension of the porous media was also presented. The fractional dimension dynamical model for gas hydrate dissociation was examined with the previous experimental data of methane hydrate and carbon dioxide hydrate dissociations, respectively. The calculated results indicate that the fractal dimensions of porous media acquired with this method agree well with the previous study. With the absolute average deviation (AAD) below 10%, the present model provided satisfactory predictions for the dissociation process of methane hydrate and carbon dioxide hydrate.

Transient flow patterns and bubble slug lengths in parallel microchannels with oxygen gas bubbles produced by catalytic chemical reactions

Transient flow patterns and bubble slug lengths were investigated with oxygen gas (O-2) bubbles produced by catalytic chemical reactions using a high speed camera bonded with a microscope. The microreactor consists of an inlet liquid plenum, nine parallel rectangular microchannels followed by a micronozzle, using the MEMS fabrication technique. The etched surface was deposited by the thin platinum film, which is acted as the catalyst. Experiments were performed with the inlet mass concentration of the hydrogen peroxide from 50% to 90% and the pressure drop across the silicon chip from 2.5 to 20.0 kPa. The silicon chip is directly exposed in the environment thus the heat released via the catalytic chemical reactions is dissipated into the environment and the experiment was performed at the room temperature level. It is found that the two-phase flow with the catalytic chemical reactions display the cyclic behavior. A full cycle consists of a short fresh liquid refilling stage, a liquid decomposition stage followed by the bubble slug flow stage. At the beginning of the bubble slug flow stage, the liquid slug number reaches maximum, while at the end of the bubble slug flow stage the liquid slugs are quickly flushed out of the microchannels. Two or three large bubbles are observed in the inlet liquid plenum, affecting the two-phase distributions in microchannels. The bubble slug lengths, cycle periods as well as the mass flow rates are analyzed with different mass concentrations of hydrogen peroxide and pressure drops. The bubble slug length is helpful for the selection of the future microreactor length ensuring the complete hydrogen peroxide decomposition. Future studies on the temperature effect on the transient two-phase flow with chemical reactions are recommended.

Resonant subband Landau level coupling in symmetric quantum well

Subband structure and depolarization shifts in an ultrahigh mobility GaAs/Al0.24Ga0.76As quantum well are studied using magnetoinfrared spectroscopy via resonant subband Landau level coupling. Resonant couplings between the first and up to the fourth subbands are identified by well-separated antilevel-crossing split resonance, while the hy-lying subbands were identified by the cyclotron resonance linewidth broadening in the literature. In addition, a forbidden intersubband transition (first to third) has been observed. With the precise determination of the subband structure, we find that the depolarization shift can be well described by the semiclassical slab plasma model and the possible origins for the forbidden transition are discussed.

DENSITY-OF-STATES OF THE 2-DIMENSIONAL ELECTRON-GAS STUDIED BY MAGNETOCAPACITANCES OF BIASED DOUBLE-BARRIER STRUCTURES

The magnetocapacitive response of a double-barrier structure (DBS), biased beyond resonances, has been employed to determine the density of states (DOS) of the two-dimensional electron gas residing in the accumulation layer on the incident side of the DBS. An adequate procedure is developed to compare the model calculation of the magnetocapacitance with the experimental C vs B curves measured at different temperatures and biases. The results show that the fitting is not only self-consistent but also remarkably good even in well-defined quantum Hall regimes. As a result, information about the DOS in strong magnetic fields could reliably be extracted.

DISTRIBUTION FUNCTION OF A DRIFTING ELECTRON-GAS FOR GENERAL ENERGY-BAND STRUCTURES

The usual application of the Lei-Ting balance equation method for treating electron transport problems makes use of a Fermi distribution function for the electron motion relative to the center of mass. It is pointed out that this presumes the existence of a moving frame of reference that is dynamically equivalent to the rest frame of reference, and this is only true for electrons with a constant effective mass. The method is thus inapplicable to problems where electrons governed by a general energy-band dispersion E(k) are important (such as in miniband conduction). It is demonstrated that this difficulty can be overcome by introducing a distribution function for a drifting electron gas by maximizing the entropy subject to a prescribed average drift velocity. The distribution function reduces directly to the usual Fermi distribution for electron motion relative to the center of mass in the special case of E(k)=($) over bar h(2)\k\(2)/2m*. This maximum entropy treatment of a drifting electron gas provides a physically more direct as well as a more general basis for the application of the balance equation method.

SCATTERING AND RESONANT-TUNNELING THEORY FOR A CYLINDRICAL WELL IN PLANAR STRUCTURES

A two-dimensional atomic scattering theory is developed for scattering of electrons by a circularly symmetric quantum structure in the two-dimensional electron gas. It is found that the scattering cross section oscillates as a function of ka where k is the electron wave vector and a is the radius of the cylindrical potential barrier. If there is a quantum well inside the potential barrier, there appears a series of sharp resonant-tunneling peaks superposed on the original scattering-cross-section curves. The width of the resonant-tunneling peak depends sensitively on the thickness, the height of the potential barrier, and the electron energy.