Series Of AlxGa1 Research Articles

Structural and optical analyses of AlxGa1−xN thin films grown by metal organic chemical vapor deposition

A series of AlxGa1−xN thin films with x = 0.20–0.60 were grown by metal organic chemical vapor deposition (MOCVD) on sapphire (0001) substrate using AlN buffer layer. High resolution X-ray diffraction (HRXRD) was performed for (0002), (0004), and (0006) reflections to investigate the threading dislocation density in variation with Al composition by X-ray analysis technique; Williamson–Hall (WH) plot. A symmetric high resolution 2θ–ω scans exhibit high crystal quality for all the AlGaN samples. A room temperature deep ultraviolet (DUV) photoluminescence (PL) spectroscopy (excitation at 248 nm) has also been employed to investigate the effect of various Al compositions on crystal structure of the thin film layers. It was observed that the band edge transition peak energy blueshifts from 3.87 eV for x = 0.23 to 4.55 eV for x = 0.47. In addition to the band edge transition, each spectrum also shows deep impurity transitions.

Subband structure and transport properties of two‐dimensional electron gas in Al x Ga 1–x N/GaN heterostructures

We report on Hall mobility and Shubnikov-de Haas (SdH) measurements of the two dimensional electron gas (2DEG) formed in a series of AlxGa1–xN/GaN heterostructures with different Al composition grown on sapphire substrates. In samples of x = 0.15 and 0.20, we found single periodical oscillations. In a sample of x = 0.30 a double periodical one was clearly resolved, which indicates that more than one subband has been occupied by electrons. Fast Fourier transform (FFT) analysis yielded information about the electron density and subband structure of the 2DEGs. The oscillatory part of the SdH signal was analyzed to extract the quantum scattering time at 4.2 K. It was found that as the increase of the Al content, quantum scattering time of the ground subband decreases gradually. We attributed it to the enhanced interface roughness scattering and the onset of the inter-subband scattering in the sample with higher Al content. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transport properties of the two-dimensional electron gas inAlxGa1−xN/GaN heterostructures

Magnetotransport measurements were performed on a series ofAlxGa1−xN/GaN heterostructures with different Al compositions(x = 0.15,0.20 and 0.30) at 4.2 K. Adopting a fast Fourier transform method, we analysed the Shubnikov–deHass oscillations due to the two-dimensional electron gas to derive the quantum scattering time(τq). It was found that the quantum scattering time in the groundsubband decreases with increasing Al composition: 0.194 ps(x = 0.15), 0.174 ps(x = 0.20) and0.123 ps (x = 0.30), respectively. To discern the predominant scattering process, the scattering time limited byinterface roughness, the residual impurity and the alloy disorder were investigatednumerically by including inter-subband scattering. We found that enhanced interfaceroughness scattering dominates both the transport and quantum scattering time in theground subband.

Temperature dependence of breakdown voltage in AlxGa1−xAs

Avalanche breakdown voltage has been measured in a series of AlxGa1−xAs(0<x<0.8)p+in+ and n+ip+ diodes at temperatures between 20 and 500K. The temperature sensitivity falls with decreasing avalanche region width for all alloy compositions, with a minimum sensitivity at x=0.6, for which we offer a physical explanation. While Monte Carlo simulations show that alloy scattering reduces the temperature dependence of breakdown voltage, the effect is too small to account for the observations.

Effects of the DX center multiplicity on the transport properties of AlxGa1−xAs:Si

Magnetotransport measurements were performed on a series of AlxGa1−xAs:Si samples having different alloy compositions in the temperature range 77–300 K. Multiple-peak structures were observed in the temperature-dependent resistivity, and free carrier concentration spectra during the heating process. A detailed analysis is made to explain the physical origin of these structures using the large lattice relaxation model. Two main features are revealed: the multicomponent structure depends on the alloy composition; and in contrast to the deep level transient spectroscopy data, the peaks of this structure appear in the same order than the thermal activation of the DX center capture barriers. An attempt to assign this multiple-peak structure to the nonresonant energy levels related to the DX configurations will be presented.

Phonon response ofAlxGa1−xSb/GaSbepitaxial layers by Fourier-transform infrared-reflectance and Raman spectroscopies

Far-infrared reflectance and firstand second-order Raman spectra were carefully measured at room temperature on a series of Al xGa1???xSb layers epitaxially grown on GaSb ???with 0.0???x???0.5). For all the compositions the clean ‘‘two-mode’’ behavior of lattice vibrations was confirmed. The fit of the reflectance curves with classical dielectric functions yielded the compositional variation of the TO and LO phonon frequencies, which are in very good agreement with the Raman results, as well as the oscillator strengths and the line broadenings. The effects of cation disorder were evidenced in both the reflectance and Raman spectra. A comparison was made with previous data, which show some scattering and discrepancies, having been measured by a single technique ???reflectance or Raman??? on samples with different characteristics. ???S0163-1829???97???06236-X???

Impact ionization in thin AlxGa1−xAs (x=0.15 and 0.30) p-i-n diodes

The electron and hole multiplication characteristics, Me and Mh, respectively, have been measured in two series of AlxGa1−xAs (x=0.15 and 0.30) p-i-n diodes where the i-region thicknesses, w, vary from 1.0 down to 0.025 μm. From these, the effective electron and hole ionization coefficients, α and β, respectively, have been determined and in the thicker structures agreement is found with data published previously in the literature. However, in the devices where w⩽0.1 μm, the dead space effect reduces multiplication below their bulk values at lower values of bias. As the bias is increased, α and β increase very rapidly suggesting that overshoot effects are compensating for the dead space.

Stimulated emission in ultrathin (20 Å) AlxGa1−xAs-GaAs single quantum well heterostructures

Spontaneous and stimulated emission spectra from a series of AlxGa1−xAs-GaAs single quantum well heterostructures are demonstrated for well widths as thin as 20 Å. These undoped samples, grown by molecular beam epitaxy, are the thinnest single quantum wells ever reported to support stimulated emission. Laser thresholds are generally quite low (1.2 kW/cm2) despite the fact that the single well is undoped and of dimensions which were previously thought to be too small to effectively collect excess carriers (Lz≪scattering path length). A simple model based on the spatial extent of the wave function, rather than the well width, is proposed to explain the experimental results.