P. Craig Taylor

Excitation spectroscopy of thin-film disordered semiconductors

In this paper we examine the use of a high-power, tunable free-electron laser (FEL) source to measure photoluminescence (PL) and photoluminescence excitation (PLE) spectra in two classes of disordered semiconductors, amorphous semiconductors and partially ordered III-V ternary semiconductors. The source must be tunable to follow the absorption continuously across the region of the optical energy gap, and the source must be of high power to provide enough absorbed photons in this relatively transparent spectral region so that PL processes can be measured. The usefulness of PLE spectroscopy in these semiconducting thin films lies in the fact that if the quantum efficiency (eta) for the PL process is independent of energy, then the PLE spectrum is a measure of the optical absorption. In addition, disordered semiconductors often exhibit enhanced absorption below the optical gap due to the disorder itself. PLE measurements that probe regions where the absorption coefficient (alpha) is small ((alpha) < < 103 cm-1) are most important because in these regions (alpha) is dominated by the electronic states introduced by the disorder.

Triplet excitons in selected chalcogenide crystals and glasses

Optically detected magnetic resonance (ODMR) experiments in crystalline As2Se3 provide detailed evidence for the existence of triplet excitonic states in some layered chalcogenide crystals. Comparisons with analogous optical absorption, luminescence, and ODMR measurements in glassy chalcogenides, such as As2Se3, As2S3, and alloys of these two glasses with copper suggest the presence of similar excitonic states in these materials.

Effects of P4 Annealing on Ordered Ga0.52In0.48P

We present results of an investigation of the details of the disordering process die to P4 annealing for Ga0.52In0.48P grown in ordered configurations with large and small macro-domain sizes. The effects of P4 annealing (710 degree(s)C) were monitored as a function of isothermal annealing time by transmission electron microscopy (TEM), photoreflectance (PR), and photoluminescence (PL). During the annealing process the band gap and PL emission energies characteristic of the disordered phase of the sample grow while those of the ordered phase decay, rather than undergoing a continuous energy change. The disordering, indicated by TEM photographs, is found to proceed downward from the free surface of the epilayer and to propagate into the bulk with a well-defined boundary rather than occurring simultaneously throughout the bulk of the layer. This disordering process is consistent with the combined results of PR and PL. The rate at which the disordering occurs is found to be much greater for samples initially having larger domains.

Microwave-modulated photoluminescence: technique and application to III-Vs (Poster Paper)

A new modulation spectroscopy, microwave modulated photoluminescence (MMPL), is described. Application of the technique to a well characterized semiconductor system, InP:Zn, in which the radiative recombination processes are not understood, allows interpretation of the resulting spectra. In the ordered ternary Ga0.52In0.48P MMPL provides information about both carrier transport properties and the extent of ordering. When applied to new materials, MMPL can aid identifying radiative recombination processes.