Magnetointersubband oscillations of the two-dimensional electron gas in AlxGa1-xN/GaN heterostructures

Shubmkov-de Haas (SdH) measurements are performed over a temperature range of 1.5-20K in AL(0.22)Ga(0.78)N/GaN heterostructures with two subbands occupied. In addition to an intermodulation between two sets of SdH oscillations from the first and second subbands, a beating in oscillatory magnetoresistance at 12K is observed, due to the mixing of the first subband SdH oscillations and 'magnetointersubband' (MIS) oscillations. A phase shift of pi between the SdH and MIS oscillations is also clearly identified. Our experimental results, i.e. that the SdH oscillations dominate at low temperature and MIS oscillations dominate at high temperature, fully comply with the expected behaviour of MIS oscillations.

Amorphous silicon carbide films prepared by H-2 diluted silane-methane plasma

This paper reports on the preparation and characterization of hydrogenated amorphous silicon carbide films prepared by H-2 diluted silane-methane plasma. Carbon-rich a-SiC:H film with band gap of up to 3.3 eV has been achieved. IR and UV Vis spectra were employed to characterize the chemical bonding and optical properties of as-prepared films. It is shown that hydrogen dilution is crucial in obtaining these wide band gap carbon-rich films. Raman and PL measurements were performed to probe the microstructure and photoelectronic properties of these films before and after annealing. Films with intermediate carbon concentration seem more defective and exhibit stronger photoluminescence and subband absorption than others. Films with different compositions exhibit different annealing behaviours. For silicon rich and carbon rich films, high temperature annealing results in the formation of silicon crystallites and graphite clusters, respectively. (C) 2003 Elsevier B.V. All rights reserved.

Theoretical investigation of intersubband transition in AlxGa1-xN/GaN/AlyGa1-yN step quantum well

A modified self-consistent method is introduced for the design of AlxGa1-xN/GaN step quantum well (SQW) with the position and energy-dependent effective mass. The effects of nonparabolicity are included. It is shown that the nonparabolicity effect is minute for the lowest subband energy level and grows in size for the higher subband states. The effects of nonparabolicity have significant influence on the transition energies and the oscillator strengths and should be taken into account in the investigation of the optical transitions. The strong asymmetric property introduced by the step quantum well magnifies the weak intersubband transition from the ground state to the third state (1 -> 3). It is shown that in an appropriate scope, the intersubband transition (1 -> 3) has the comparable oscillator strength with transition from the ground state to the second one (1 -> 2), which suggests the possible application of the two-color photodetectors. The results of this work should provide useful guidance for the design of optically pumped asymmetric quantum well lasers and quantum well infrared photodetectors (QWIPs). (c) 2005 Elsevier B.V. All rights reserved.

Effect of spontaneous and piezoelectric polarization on intersubband transition in AlxGa1-xN-GaN quantum well

A self-consistent solution of conduction band profile and subband energies for AlxGa1-xN-GaN quantum well is presented by solving the Schrodinger and Poisson equations. A new method is introduced to deal with the accumulation of the immobile charges at the AlxGa1-xN-GaN interface caused by spontaneous and piezoelectric polarization in the process of solving the Poisson equation. The effect of spontaneous and piezoelectric polarization is taken into account in the calculation. It also includes the effect of exchange-correlation to the one electron potential on the Coulomb interaction. Our analysis is based on the one electron effective-mass approximation and charge conservation condition. Based on this model, the electron wave functions and the conduction band structure are derived. We calculate the intersubband transition wavelength lambda(21) for different Al molar fraction of barrier and thickness of well. The calculated result can fit to the experimental data well. The dependence of the absorption coefficient a on the well width and the doping density is also investigated theoretically. (C) 2004 American Vacuum Society.

Origin of the blueshift in the infrared absorbance of intersubband transitions in AlxGa1-x/GaN multiple quantum wells

A self-consistent calculation of the subband energy levels of n-doped quantum wells is studied. A comparison is made between theoretical results and experimental data. In order to account for the deviations between them, the ground-state electron-electron exchange interactions, the ground-state direct Coulomb interactions, the depolarization effect, and the exciton-like effect are considered in the simulations. The agreement between theory and experiment is greatly improved when all these aspects are taken into account. The ground-to-excited-state energy difference increases by 8 meV from its self-consistent value if one considers the depolarization effect and the exciton-like effect only. It appears that the electron-electron exchange interactions account for most of the observed residual blueshift for the infrared intersubband absorbance in AlxGa1-xN/GaN multiple quantum wells. It seems that electrons on the surface of the k-space Fermi gas make the main contribution to the electron-electron exchange interactions, while for electrons further inside the Fermi gas it is difficult to exchange their positions. (C) 2004 Elsevier B.V. All rights reserved.

High and electric field tunable Curie temperature in diluted magnetic semiconductor nanowires and nanoslabs

The Curie temperature of diluted magnetic semiconductor (DMS) nanowires and nanoslabs is investigated using the mean-field model. The Curie temperature in DMS nanowires can be much larger than that in corresponding bulk material due to the density of states of one-dimensional quantum wires, and when only one conduction subband is filled, the Curie temperature is inversely proportional to the carrier density. The T-C in DMS nanoslabs is dependent on the carrier density through the number of the occupied subbands. A transverse electric field can change the DMS nanowires from the paramagnet to ferromagnet, or vice versae. (c) 2007 American Institute of Physics.

Energy band design for Si/SiGe quantum cascade laser

This paper introduces in detail the working principle of Si/SiGe Quantum cascade laser(QCL). Appropriate parameters are used to calculate the hole subband structure of Si/Si1-xGex quantum well using a six-band k center dot p method. The dispersion relation and energy band for different layer thickness and compositions are investigated. Meanwhile, the energy separations between hole subbands in Si/Si1-xGex/Si quantum wells are also analyzed. Finally the calculated results are used for the Si/SiGe QCL design, which will be beneficial to the structure optimization of Si/SiGe QCL.

Charge-density and spin-density excitations in a two-dimensional electron gas with Rashba spin-orbit coupling

We study theoretically the charge-density and spin-density excitations in a two-dimensional electron gas in the presence of a perpendicular magnetic field and a Rashba type spin-orbit coupling. The dispersion and the corresponding intensity of excitations in the vicinity of cyclotron resonance frequency are calculated within the framework of random phase approximation. The dependence of excitation dispersion on various system parameters, i.e., the Rashba spin-orbit interaction strength, the electron density, the Zeeman spin splitting, and the Coulomb interaction strength is investigated.

Correlation between optical and electrical properties in In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor structures on GaAs substrates

4.2 K photoluminescence (PL) and 77 K standard Hall-effect measurements were performed for In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor (HEMT) structures grown on GaAs substrates with different indium contents in the InxGa1-xAs well or different Si delta-doping concentrations. It was found that electron concentrations increased with increasing PL intensity ratio of the "forbidden" transition (the second electron subband to the first heavy-hole subband) to the sum of the "allowed" transition (the first electron subband to the first heavy-hole subband) and the forbidden transition. And electron mobilities decreased with increasing product of the average full width at half maximum of allowed and forbidden transitions and the electron effective mass in the InxGa1-xAs quantum well. These results show that PL measurements are a good supplemental tool to Hall-effect measurements in optimization of the HEMT layer structure. (c) 2006 American Institute of Physics.

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.

Rashba spin splitting in biased semiconductor quantum wells

Rashba spin splitting (RSS) in biased semiconductor quantum wells is investigated theoretically based on eight-band k center dot p theory. We find that at large wave vectors, RSS is both nonmonotonic and anisotropic as a function of in-plane wave vector, in contrast to the widely used isotropic linear model. We derive an analytical expression for RSS, which can qualitatively reproduce such nonmonotonic behavior at large wave vectors. We also investigate numerically the dependence of RSS on the various band parameters and find that RSS increases with decreasing band gap and subband index, increasing valence band offset, external electric field, and well width. All these dependences can be qualitatively described by our analytical model.

Electron transport properties of MM-HEMT with varied channel indium contents

Transport properties of two-dimensional electron gas (2DEG) are crucial to metamorphic high-electron-mobility transistors (MM-HEMT). We have investigated the variations of subband electron mobility and concentration versus temperature from Shubnikov-de Hass oscillations., and variable temperature Hall measurements. The results indicate that the electrical performance is the best when the In content is 0.65 in the channel for MM-HEMT. When the In content exceeds 0.65, a large lattice mismatch will cause dislocations and result in the decrease of mobility and the fall of performance in materials and devices.

Zero-field spin splitting in In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor structures on GaAs substrates using Shubnikov-de Haas measurements

Shubnikov-de Haas measurements were carried out for In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor structures grown on GaAs substrates with different indium contents and/or different Si delta-doping concentrations. Zero-field (B-->0) spin splitting was found in samples with stronger conduction band bending in the InGaAs well. It was shown that the dominant spin splitting mechanism is attributed to the contribution by the Rashba term. We found that zero-field spin splitting not only occurs in the ground electron subband, but also in the first excited electron subband for a sample with Si delta-doping concentration of 6x10(12) cm(-2). We propose that this In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor structure grown on GaAs may be a promising candidate spin-polarized field-effect transistors. (C) 2002 American Institute of Physics.

Valence band structures of the InAs/GaAs quantum ring

In the framework of effective-mass envelope function theory, the valence energy subbands and optical transitions of the InAs/GaAs quantum ring are calculated by using a four-band valence band model. Our model can be used to calculate the hole states of quantum wells, quantum wires, and quantum dots. The effect of finite offset and valence band mixing are taken into account. The energy levels of the hole are calculated in the different shapes of rings. Our calculations show that the effect of the difference between effective masses of holes in different materials on the valence subband structures is significant. Our theoretical results are consistent with the conclusion of the recent experimental measurements and should be useful for researching and making low-dimensional semiconductor optoelectronic devices. (C) 2002 American Institute of Physics.

Electron effective mass and resonant polaron effect in CdTe/CdMgTe quantum wells

Cyclotron resonance in CdTe/CdMgTe quantum wells (QWs) was studied. Due to the polaron effect the zero-field effective mass is strongly influenced by the QW width. The experimental data have been described theoretically by taking into account electron-phonon coupling and the nonparabolicity of the conduction band. The subband structure was calculated self-consistently. The best fit was obtained for an electron-phonon coupling constant alpha = 0.3 and bare electron mass of m(b) = 0.092m(0).