Single‐crystal layers of AlN have been grown on sapphire substrates between 1000 and 1100 °C by vapor‐phase reaction of aluminum chlorides with ammonia. The purity, color, crystallinity, growth morphology, and electrical resistivity of the epitaxial layers have been investigated. Infrared specular reflection measurements showed the presence of an appreciable strain at the AlN‐sapphire epitaxy interface. Optical absorption data strongly suggest the AlN is a direct band‐gap material with a value of about 6.2 eV at room temperature.

Epitaxially grown AlN and its optical band gap

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

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

The refractive index of AlAs at 300 °K has been measured in the 0.5‐ to 2.2‐eV range of photon energy. The long‐wavelength dielectric constant with lattice vibrations neglected e∞ is determined to be 8.16 ± 0.02, and the resulting static dielectric constant e0 is 10.06 ± 0.04.

G.D. Pettit

Papers

The fundamental absorption edge of AlAs and AlP

Optical properties of the Ge donor and acceptor in GaP