Microstructure and strain analysis of GaN epitaxial films using in-plane grazing incidence x-ray diffraction

This paper investigates the major structural parameters, such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition, using an in-plane grazing incidence x-ray diffraction technique. The results are analysed and compared with a complementary out-of-plane x-ray diffraction technique. The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method. The method for directly determining the in-plane lattice parameters of the GaN layers is also presented. Combined with the biaxial strain model, it derives the lattice parameters corresponding to fully relaxed GaN films. The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established, reaching to a maximum level of -0.89 GPa.

Influence of AlN Buffer Thickness on GaN Grown on Si(111) by Gas Source Molecular Beam Epitaxy with Ammonia

Hexagonal GaN is grown on a Si(111) substrate with AlN as a buffer layer by gas source molecular beam epitaxy (GSMBE) with ammonia. The thickness of AlN buffer is changed from 9 to 72 nm. When the thickness of AlN buffer is 36 nm, the surface morphology and crystal quality of GaN is optimal. The in-situ reflection high energy electron diffraction (RHEED) reveals that the transition to a two-dimensional growth mode of AlN is the key to the quality of GaN. However, the thickness of AlN buffer is not so critical to the residual in-plane tensile stress in GaN grown on Si(111) by GSMBE for AlN thickness between 9 to 72 nm.

Spin precession induced by an effective magnetic field in a two-dimensional electron gas

We theoretically study the spatial behaviors of the spin precession in a two-dimensional electron system with spin-orbit interaction. Through analysis of interaction between the spin and the effective magnetic field in the system, we obtain the general conditions to generate a persistent spin helix and predict a persistent spin helix pattern in [001]-grown quantum wells. Particularly, we demonstrate that the phase of spin can be locked to propagate in a quantum well with SU(2) symmetry.

Theoretical analysis of gate voltage-controlled subband states in an AlxGa1-xN/GaN heterostructure

The subband structure and inter-subband transition as a function of gate voltage are determined by solving the Schrodinger and Poisson equations self-consistently in an AlxGa1-xN/GaN heterostructure. Different aluminum mole fraction and thickness of AlxGa1-xN barrier are considered. Calculation results show that energy difference between the first and second subband covers a wide range (from several tens to hundreds milli-electron volt) by applying different gate voltage, which corresponds to the midinfrared and long-wave infrared wavelength scope. Furthermore, such a modulation on the subband transition energy is much more pronounced for the structure with thin barrier. When the applied positive gate voltage is increased, the triangle well formed at the interface turns to be deeper and narrower, which enhances the confinement for electrons. As a result, the overlap between electron wave function at two subbands increases, and thus the optical intersubband transition also enhances its intensity. This tendency is in good agreement with the available data in the literature. (c) 2005 Elsevier B.V. All rights reserved.

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.

Self-consistent analysis of double-delta-doped InAlAs/InGaAs/InP HEMTs

We have carried out a theoretical study of double-delta-doped InAlAs/InGaAs/InP high electron mobility transistor (HEMT) by means of the finite differential method. The electronic states in the quantum well of the HEMT are calculated self-consistently. Instead of boundary conditions, initial conditions are used to solve the Poisson equation. The concentration of two-dimensional electron gas (2DEG) and its distribution in the HEMT have been obtained. By changing the doping density of upper and lower impurity layers we find that the 2DEG concentration confined in the channel is greatly affected by these two doping layers. But the electrons depleted by the Schottky contact are hardly affected by the lower impurity layer. It is only related to the doping density of upper impurity layer. This means that we can deal with the doping concentrations of the two impurity layers and optimize them separately. Considering the sheet concentration and the mobility of the electrons in the channel, the optimized doping densities are found to be 5 x 10(12) and 3 x 10(12) cm(-2) for the upper and lower impurity layers, respectively, in the double-delta-doped InAlAs/InGaAs/InP HEMTs.

Magneto-transport characteristics of two-dimensional electron gas for Si delta-doped InAlAs/InGaAs single quantum well

Magneto-transport measurements have been carried out on a Si heavily delta-doped In0.52Al0.48As/In(0.53)G(0.47)As single quantum well in the temperature range between 1.5 and 60 K under magnetic field up to 10 T. We studied the Shubnikov-de Haas(SdH) effect and the Hall effect for the In0.52Al0.48As/In(0.53)G(0.47)As single quantum well occupied by two subbands, and have obtained the electron concentration, mobility, effective mass and energy levels respectively. The electron concentrations of the two subbands derived from mobility spectrum combined with multi-carrier fitting analysis are well consistent with the result from the SdH oscillation. From fast Fourier transform analysis for d(2)rho/dB(2)-1/B, it is observed that there is a frequency of f(1)-f(2) insensitive to the temperature, besides the frequencies f(1), f(2) for the two subbands and the frequency doubling 2f(1), both dependent on the temperature. This is because That the electrons occupying the two different subbands almost have the same effective mass in the quantum well and the magneto-intersubband scattering between the two subbands is strong.

Quasi-thermo dynamic analysis of MOVPE growth of GaxAlyIn1-x-yN

A quasi-thermodynamic model of metalorganic vapor phase epitaxy (MOVPE) growth of GaxAlyIn1-x-yN alloys has been proposed. In view of the complex growth behavior of GaxAlyIn1-x-yN, we focus our attention on the galliumrich quaternary alloys that are lattice matched to GaN, In0.15Ga0.85N or Al0.15Ga0.85N, which are widely used in the GaN-based optoelectronic devices. The relationship between GaAlInN alloy composition and input molar ratio of group III metalorganic compounds at various growth conditions has been calculated. The influence of growth temperature, nitrogen fraction in the carrier gas, input partial pressure of group III metalorganics, reactor pressure, V/III ratio and the decomposition rate of ammonia on the composition of deposited alloys are studied systematically. Based on these calculated results, we can find out the appropriate growth conditions for the MOVPE growth of GaxAlyIn1-x-yN alloy lattice matched to GaN, In0.15Ga0.85N or Al0.15Ga0.85N. (C) 2002 Elsevier Science B.V. All rights reserved.

Quasi-thermodynamic analysis of MOVPE of AlGaN

A quasi-thermodynamic analysis of the MOVPE growth of AlxGa1-xN alloy using TMGa, TMA1 and ammonia has been proposed. The effect of varying growth conditions (growth temperature, reactor pressure, input V/III ratio, hydrogen pressure fraction in the carrier gas and the decomposed fraction of ammonia) on the distribution coefficient of Al has been calculated. In the case of AlxGa1-xN, preferential incorporation of Al is predicted. The calculated relationship between input vapour and deposited solid composition has been compared with data in the literature. A good agreement between the calculated and the experimental composition shows that our improved model is suitable for applying to the AlxGa1-xN alloy grown by MOVPE. (C) 2000 Elsevier Science B.V. All rights reserved.

Doping during low-temperature growth of materials for n-p-n Si/SiGe/Si heterojuction bipolar transistor by gas source molecular beam epitaxy

In situ doping for growth of n-p-n Si/SiGe/Si heterojuction bipolar transistor (HBT) structural materials in Si gas source molecular beam epitaxy is investigated. We studied high n-type doping kinetics in Si growth using disilane and phosphine, and p-type doping in SiGe growth using disilane, soild-Ge, and diborane with an emphasis on the effect of Ge on B incorporation. Based on these results, in situ growth of n-p-n Si/SiGe/Si HBT device structure is demonstrated with designed structural and carrier profiles, as verified from characterizations by X-ray diffraction, and spreading resistance profiling analysis. (C) 2000 Elsevier Science B.V. All rights reserved.

Influence of rapid thermal annealing on the optical properties of gallium nitride grown by gas-source molecular-beam epitaxy

Raman scattering, photoluminescence (PL), and nuclear reaction analysis (MA) have been employed to investigate the effects of rapid thermal annealing (RTA) on GaN films grown on sapphire (0001) substrates by gas-source molecular-beam epitaxy, The Raman spectra showed the presence of the E-2 (high) mode of GaN and shift of this mode from 572 to 568 cm(-1) caused by annealing. The results showed that RTA has a significant effect on the strain relaxation caused by the lattice and thermal expansion misfit between the GaN epilayer and the substrate. The PL peak exhibited a blueshift in its energy position and a decrease in the full width at half maximum after annealing, indicating an improvement in the optical quality of the film. Furthermore, a green luminescence appeared after annealing and increased in intensity with increasing annealing time. This effect was attributed to H concentration variation in the GaN film, which was measured by NRA. A high H concentration exists in as-grown GaN, which can neutralize the deep level, and the H-bonded complex dissociates during RTA, This leads to the appearance of a luminescent peak in the PL spectrum. (C) 1998 American Institute of Physics.

Self-consistent analysis of AlSb/InAs high electron mobility transistor structures

The influences of channel layer width, spacer layer width, and delta-doping density on the electron density and its distribution in the AlSb/InAs high electron mobility transistors (HEMTs) have been studied based on the self-consistent calculation of the Schrodinger and Poisson equations with both the strain and nonparabolicity effects being taken into account. The results show that, having little influence on the total two dimensional electron gas (2DEG) concentration in the channel, the HEMT's channel layer width has some influence on the electron mobility, with a channel as narrow as 100-130 angstrom being more beneficial. For the AlSb/InAs HEMT with a Te delta-doped layer, the 2DEG concentration as high as 9.1 X 10(12) cm(-2) can be achieved in the channel by enhancing the delta-doping concentration without the occurrence of the parallel conduction. When utilizing a Si delta-doped InAs layer as the electron-supplying layer of the AlSb/InAs HEMT, the effect of the InAs donor layer thickness is studied on the 2DEG concentration. To obtain a higher 2DEG concentration in the channel, it is necessary to use an InAs donor layer as thin as 4 monolayer. To test the validity of our calculation, we have compared our theoretical results (2DEG concentration and its distribution in different sub-bands of the channel) with the experimental ones done by other groups and show that our theoretical calculation is consistent with the experimental results.

Synthesis Gas Generation by Chemical-Looping Reforming Using Ce-Based Oxygen Carriers Modified with Fe, Cu, and Mn Oxides

Chemical-looping reforming (CLR) is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. It involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from combustion air to the fuel. Composite oxygen carriers of cerium oxide added with Fe, Cu, and Mn oxides were prepared by co-precipitation and investigated in a thermogravimetric analyzer and a fixed-bed reactor using methane as fuel and air as oxidizing gas. It was revealed that the addition of transition-metal oxides into cerium oxide can improve the reactivity of the Ce-based oxygen carrier. The three kinds of mixed oxides showed high CO and H-2 selectivity at above 800 degrees C. As for the Ce-Fe-O oxygen carrier, methane was converted to synthesis gas at a H-2/CO molar ratio close to 2:1 at a temperature of 800-900 degrees C; however, the methane thermolysis reaction was found on Ce-Cu-O and Ce-Mn-O oxygen carriers at 850-900 degrees C. Among the three kinds of oxygen carriers, Ce-Fe-O presented the best performance for methane CLR. On Ce-Fe-O oxygen carriers, the CO and H-2 selectivity decreased as the Fe content increased in the carrier particles. An optimal range of the Ce/Fe molar ratio is Ce/Fe > 1 for Ce-Fe-O oxygen carriers. Scanning electron microscopy (SEM) analysis revealed that the microstructure of the Ce-Fe-O oxides was not dramatically changed before and after 20 cyclic reactions. A small amount of Fe3C was found in the reacted Ce-Fe-O oxides by X-ray diffraction (XRD) analysis.

Some properties of gallium nitride films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy

The Raman and photoreflectivity spectra of gallium nitride (GaN) films grown on (0001) oriented sapphire substrates by gas source molecular beam epitaxy (GSMBE) have been investigated. The Raman spectra showed the presence of the E-2(high) mode and a shift in the wavenumber of this mode with respect to the GaN epilayer thickness. The Raman scattering results suggest the presence of stress due to lattice and thermal expansion misfit in the films, and also indicate that the buffer layer play an important role in the deposition of high quality GaN layers. The residual stress changes from tensile to compressive as the epilayer thickness increases. Samples subjected to anneal cycles showed an increase in the mobility due probably to stress relaxation as suggested by an observed shift in the E-2(high) mode in the Raman spectra after annealing.

INFRARED-ABSORPTION DUE TO 2-DIMENSIONAL-ELECTRON-GAS COLLECTIVE EXCITATION IN GAAS/ALXGA1-XAS MULTIPLE-QUANTUM-WELL STRUCTURES

Infrared absorption due to a collective excitation of a two-dimensional electronic gas was observed in GaAs/AlxGa1-xAs multiple-quantum wells when the incident light is polarized parallel to the quantum-well plane. We attribute this phenomenon to a plasma oscillation in the quantum wells. The measured wavelength of the absorption peak due to the plasma oscillation agrees with our theoretical analysis. In addition, in this study the plasma-phonon coupling effect is also fitted to the experimental result. We show that the absorption is not related to the intersubband transitions but to the intrasubband transition, which originates from a plasma oscillation.