The temperature dependence of the optical-absorption edge (Urbach edge) of GaAs is measured in semi-insulating and n-type GaAs (n=2X 10" cmm3) over the temperature range from room temperature to 700 "C. Both the optical absorption and the temperature are measured using a diffuse reflectance technique. The characteristic energy of the exponential absorption edge is found to increase linearly with temperature, from 7.5 meV at room temperature to 12.4 meV at 700 "C, for semi-insulating GaAs. The temperature dependent part of the width of the Urbach edge for semi-insulating GaAs is six times smaller than predicted by the standard theory where the edge width is proportional to the phonon population.

An elementary empirical model for the distribution of electronic states of an amorphous semiconductor is presented. Using this model, we determine the functional form of the optical absorption spectrum, focusing our analysis on the joint density of states function, which dominates the absorption spectrum over the range of photon energies we consider. Applying our optical absorption results, we then determine how the empirical measures commonly used to characterize the absorption edge of an amorphous semiconductor, such as the Tauc gap and the absorption tail breadth, are related to the parameters that characterize the underlying distribution of electronic states. We, thus, provide the experimentalist with a quantitative means of interpreting the physical significance of their optical absorption data.

The photoluminescence from a Ga͑AsBi͒ sample is investigated as a function of pump power and lattice temperature. The disorder-related features are analyzed using a Monte Carlo simulation technique. A two-scale approach is introduced to separately account for cluster localization and alloy disorder effects. The corresponding characteristic energy scales of 11 and 45 meV are deduced from the detailed comparison between experiment and simulation.

The temperature dependences of the optical absorption edges of semi-insulating GaAs and InP have been measured from room temperature to 905°C and 748°C, respectively, with accuracies of Ϯ1°C at room temperature and Ϯ5°C at 900°C. The temperature dependence of the optical absorption edge is adequately reproduced by an Einstein model although the Varshni model gives an improved fit to the band gap. Finally, the widths of the absorption edges are correlated with ionicity.

The real and imaginary photonic band structure of modes attached to two-dimensionally textured semiconductor membranes is determined experimentally and theoretically. These porous waveguides exhibit large ͑1000 cm Ϫ1 at 9500 cm Ϫ1 ) second-order optical gaps, highly dispersive lifetimes, and bands with well-defined polarization along directions of high symmetry.