We report pseudodielectric function 〈e〉 data for AlxGa1−xAs alloys of target compositions x=0.00–0.80 in steps of 0.10 grown by liquid‐phase epitaxy and measured by spectroellipsometry. Cleaning procedures that produce abrupt interfaces between the technologically relevant alloys x≤0.5 and the ambient are described. The 〈e2〉 data are corrected near the fundamental direct absorption edge by a Kramers–Kronig analysis of the 〈e1〉 data to circumvent a limitation of the rotating‐analyzer ellipsometric technique. The results and the associated pseudooptical functions 〈n〉, 〈R〉, and 〈α〉 are listed in tabular form. Accurate values of the E0 and E1 threshold energies are determined from these spectra by Fourier methods. From these values, and from similar values for a GaAs‐capped AlAs sample grown by organometallic chemical vapor deposition, the dependencies of the E0 and E1 interband critical point energies on nominal composition are obtained. Cubic polynomial representations of these dependences are determined to...

Optical properties of AlxGa1−x As

A thorough discussion of the various features of the photoluminescence spectra of undoped, p‐doped and n‐doped AlxGa1−xAs (0≤x≤1) alloys is given. This review covers spectral features in the energy region ranging from the energy band gap down to ≂0.8 eV, doping densities from isolated impurities to strongly interacting impurities (heavy‐doping effects) and lattice temperatures from 2 to 300 K. The relevance of photoluminescence as a simple but very powerful characterization technique is stressed also in comparison with other experimental methods. The most recent determinations of the Al concentration dependence of some physical properties of the alloy (energy gaps, carrier effective masses, dielectric constants, phonon energies, donor and acceptor binding energies, etc.) are given. The main physical mechanisms of the radiative recombination process in semiconductors are summarized with particular emphasis on the experimental data available for AlxGa1−xAs. The effects of the nature of the band gap (direct ...

Photoluminescence of AlxGa1−xAs alloys

Wigner-function model of a resonant-tunneling semiconductor device.

Thick layers of Ga1−x Al x As with uniform composition were grown by an isothermal liquid‐phaseepitaxial technique. Infrared reflectivity and Raman‐scattering spectra were measured with the same samples. The clean ’’two mode’’ behavior was confirmed in the reststrahlen band region, and the infrared reflectivityspectra were analyzed by using a Kramers‐Kronig dispersion relation and by curve fitting to two forms for the classical dielectric function, one an additive form and the other a factorized form. The experimental data are in good agreement with the calculated TO and LO frequencies, when the Chang‐Mitra model has been modified with the assumption of a nonlinear dependence of the oscillator strengths on composition. In the Raman spectra, the disorder‐activated LA mode near 200 cm−1, a mode near 250 cm−1, and a background‐level change were observed and attributed to atomic disorder. In the infrared reflectivity measurements the major disorder‐induced effect was the increase in the TO and LO damping constants.

Infrared reflectivity spectra and Raman spectra of Ga1−xAlxAs mixed crystals

The Tauc plot is a method originally developed to derive the optical gap of amorphous semiconductors such as amorphous germanium or silicon. By measuring the absorption coefficient α(hν) and plotting versus photon energy hν, a value for the optical gap (Tauc gap) is determined. In this way non‐direct optical transitions between approximately parabolic bands can be examined. In the last decades, a modification of this method for (poly‐) crystalline semiconductors has become popular to study direct and indirect interband transitions. For this purpose, (ahν)n (n = , 2) is plotted against hν to determine a value of the electronic bandgap. Due to the ease of performing UV–vis measurements, this method has nowadays become a standard to analyze various (poly‐) crystalline solids, regardless of their different electronic structure. Although this leads partially to widely varying values of the respective bandgap of nominally identical materials, there is still no study that critically questions which peculiarities in the electronic structure prevent a use of the Tauc plot for (poly‐) crystalline solids and to which material classes this applies. This study aims to close this gap by discussing the Tauc plot and its limiting factors for exemplary (poly‐) crystalline solids with different electronic structures.

Limitations of the Tauc Plot Method

Optical transmission data covering the Γ15V–Γ1C absorption edge are presented, together with photoluminescence (PL) results, for AlxGa1−xAs crystals of high purity [n (293 K) <1017 cm−3] for 0<x<0.9. The results indicate that the fundamental absorption edge is domnnated by electron‐hole interaction even in an an alloy. In the direct‐gap region the bound‐exciton ground‐state peak is clearly resolved. In the indirect‐gap region the relaxation of crystal momentum conservation in optical transitions for an alloy apparently has significant effects on the Γ15V–Γ1C absorption edge, and the discrete peak in absorption spectra disappears. From the variation of the absorption coefficient αth at the Γ15V–Γ1C edge with x, we can deduce the corresponding variation of the excitonic binding energy Eex, excitonic effective mass m*, and electron mass mc for the Γ1C minimum. Extrapolated values for these quantities in AlAs are Eex=5 meV, m*=0.060 m0, and mc=0.11 m0 for a value αth = 2.3×104 cm−1. Further, there is a shift of the Γ15V‐Γ1C absorption edge towards lower energies only in the indirect‐gap region. Thus the usually employed quadratic expression for the bowing of band gaps in alloys is inadequate for the Γ15V‐Γ1C gap in AlxGa1−xAs. We observe no bowing of band gaps in alloys is inadequate for the Γ15V‐Γ1C gap in AlxGa1−xAs. We observe no bowing at all (<0.01 eV) in the direct‐gap region, in sharp disagreement with earlier experiments employing other methods. Our PL data for the indirect‐gap region indicate no significant bowing in the Γ15VC gap either (<0.01 eV). The PL results for the direct‐gap region establish significant deviations in near‐ban‐gap luminescence peak positions from the band gap established in absorption, in agreement with detailed balance predictions.

Some optical properties of the AlxGa1−xAs alloys system

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Some optical properties of the AlxGa1−xAs alloys system