Indices of refraction for Mg x Zn 1Ϫx O epitaxial films grown by pulsed-laser deposition on sapphire substrates with x up to 0.36 were determined in the range of wavelength 457-968 nm by analysis of optical transmission spectra and prism-coupled waveguide measurements. The dispersion follows the first-order Sellmeier dispersion equation. Absorption coefficients, exciton energy gaps, and binding energies of Mg x Zn 1Ϫx O alloys were determined by transmission spectroscopy. The excitonic absorption features were clearly visible at room temperature despite alloy broadening. These results provide important information for the design and modeling of ZnO/MgZnO heterostructure optoelectronic devices.

Optical-field profiles in wide-band-gap AlxGa1−xN/InxGa1−xN multiple-quantum well (MQW) separate-confinement heterostructure (SCH) laser diodes (LDs) were calculated using a 2×2 transfer-matrix approach that accommodates complex refractive indices. The refractive indices of AlxGa1−xN and InxGa1−xN were approximated by shifting the refractive index of GaN according to the band-gap energy of the solid solution. Current LDs were analyzed and show reasonable optical confinement. Optimization of the SCH waveguide for a three MQW active region was performed by varying the waveguide and cladding layer thicknesses. For 0.8μm thick Al0.10Ga0.90N cladding layers, waveguides on sapphire and SiC substrates had a maximum confinement factor of ∼3.3%. Layers outside of the waveguide strongly affected the optical field for thin (∼0.4 μm) cladding layer thicknesses and resulted in resonant coupling of the light out of the waveguide. Sapphire substrates were found to enhance light confinement, while SiC substrates were found to reduce optical confinement as the cladding layer thickness is reduced.

Dispersion of the ordinary and extraordinary indices of refraction for wurtzite Al x Ga 1Ϫx N epitaxial layers with xϭ0.00, 0.04, 0.08, 0.11, and 0.20 in the range of wavelengths 457ϽϽ980 nm were measured via a prism-coupled waveguide technique. The quantitative accuracy of x is Ϯ10% and the accuracy of the refractive indices is ϳϮ0.01. The dispersion is found to be well described by a 1st-order Sellmeier dispersion formula. A simple functional form is presented that allows calculation of the refractive indices as functions of x and .

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We present the results of numerical simulations of a drift-diffusion model-including electric-fielddependent generation-recombination processes-for closely compensated p-type Ge at low temperature and under dcϩac and dcϩnoise voltage biases, with an Ohmic boundary condition. We observe frequency locking and quasiperiodicity under dcϩac bias, but do not find chaotic behavior for a uniform impurity profile. Noise-induced intermittent switching near the onset of solitary-wave conduction is compared to experimentally observed intermittency, type-III intermittency, and on-off intermittency. For a linearly increasing acceptor concentration, we find that the size of the solitary waves diminishes as they advance across the sample.FIG. 1. The homogeneous stationary current density j sh (E) for ␣ϭ1.21, illustrating the NDR in our model.