The absorption coefficient for a 0.4-μm-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature. A strong, well defined exciton peak for the A and B excitons was obtained. The A, B, and C excitonic features are clearly defined at 77 K. At room temperature, an energy gap Eg=3.452±0.001 eV and an exciton binding energy ExA,B=20.4±0.5 meV for the A and B excitons and ExC=23.5±0.5 meV for the C exciton were determined by analysis of the absorption coefficient. From this measured absorption coefficient, together with the detailed balance approach of van Roosbroek and Shockley, the radiative constant B=1.1×10−8 cm3/s was obtained.

We report novel GaN detectors grown by molecular beam epitaxy on Si(111) substrates. Wurtzite structure epitaxial GaN exhibits room-temperature photoluminescence with a band-edge-related emission width as narrow as 7 nm and intensities comparable to high quality layers grown on sapphire by metalorganic chemical vapor deposition. Spectral response of lateral geometry Schottky detectors shows a sharp cutoff at 365 nm with peak responsivities of ∼0.05 A/W at 0 V, and ∼0.1 A/W with a −4 V bias. The dark current is ∼60 nA at −2 V bias. The noise equivalent power is estimated to be 3.7×10−9 W over the response bandwidth of 2.2 MHz.

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The photoluminescence ͑PL͒ in as-received and milled Si and SiO 2 powder is reported. The Si and SiO 2 powder is characterized by chemical analysis, Raman scattering, x-ray photoelectron spectra, infrared absorption, x-ray diffraction, and differential thermal analysis. The results indicate that the Si powder has amorphous Si oxide and suboxide surface layers. The milling of Si powder results in the formation of nanocrystalline/ amorphous Si components. An amorphous SiO 2 component is formed by milling crystalline SiO 2 . The PL spectra for as-received Si, milled Si, and SiO 2 powder exhibit similar peak shapes, peak maxima, and full width at half maximum values. For both the as-received and the milled Si powder, experimental results appear to exclude mechanisms for PL related to an amorphous Si component or Si-H or Si-OH bonds, or the quantum confinement effect. Similarly, for milled SiO 2 powder mechanisms for PL do not appear related to Si-H or Si-OH bonds. Instead the greatly increased intensity of PL for milled SiO 2 can be related to both the increased volume fraction of the amorphous SiO 2 component and the increased density of defects introduced in the amorphous SiO 2 upon milling. It is suggested that the PL for as-received Si, milling-induced nanocrystalline/ amorphous Si, and milled SiO 2 results from defects, such as the nonbridging oxygen hole center, in the amorphous Si suboxide and/or SiO 2 components existing in these powder samples. The PL measurement for milled SiO 2 is dependent on air pressure whereas that for as-received SiO 2 is not, suggesting that new emitting centers are formed by milling.

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