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A synchrotron radiation and theoretical study of the UV reflectivity of wurtzite GaN films in the range 0 -30 eV is presented. First-principles calculations of the reHectivity and related optical constants are reported and used to analyze the experimental results. The calculations are performed assuming the local-density approximation and using a mufBn-tin orbital basis set. The calculated peak positions are found to be in good agreement with the experiment to within a few tenths of an eV except for a constant shift of 0.98 eV which simultaneously corrects the minimum band gap. This constant upward shift of the conduction bands of the Kohn-Sham eigenvalue band structure is in good agreement with a recently calculated self-energy gap correction of 0.95 eV. A discrepancy in absolute intensities increasing with increasing energy is found between theory and experiment, the origin of which is discussed. The imaginary part of the dielectric function e2(w) is decomposed into its most important interband contributions and the relation between the features in the optical response functions and the band structure is described. The transitions from the upper N 2p-like valence bands occur in the range 3.5 -15 eV. The transitions from the rather localized Ga 3d and N 2s bands whose energy is expected to need additional self-energy corrections are presented separately. The spectrum of the transitions from Ga 3d is closely related to the Ga 4p-like density of states in the conduction band.

The effect of ion-implantation-induced damage on the resistivity of n-type GaN has been investigated. H, He, and N ions were studied. The resistivity as a function of temperature, implant concentration, and post-implant annealing temperature has been examined. Helium implantation produced material with an as-implanted resistivity of 1010 Ω-cm. He-implanted material remained highly resistive after an 800 °C furnace anneal. The damage associated with H implantation had a significant anneal stage at 250 °C and the details of the as-implanted resistivity were sample dependent. N implants had to be annealed at 400 °C to optimize the resulting resistivity but were then thermally stable to over 800 °C. The 300 °C resistivity of thermally stabilized He- and N- implanted layers was 104 Ω-cm, whereas for H-implanted layers the 300 °C resistivity was less than 10 Ω-cm.

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Extended x-ray absorption fine structure above the Ga–K edge has been used to study the local structure of AlxGa1−xN films grown by metal organic chemical vapor deposition. With increasing Al content, x, the Ga–N bond length decreases, but much less than the average bond length. On the other hand, the x dependence of the Ga–Ga and Ga–Al distances does follow the variation of the average cation–cation distance. We conclude that bond angle distortions accommodate the differences between the Ga–N and Al–N bond lengths.

The principles of operation of a microelectromechanical (MEMS)-based magnetometer designed on the magnetoelastic effect are described. The active transduction element is a commercial (001) silicon microcantilever coated with an amorphous thin film of the giant magnetostrictive alloy Terfenol-D [(Dy0.7Te0.3)Fe2]. In addition to the magnetostrictive transducer, basic components of the magnetometer include: (a) mechanical resonance of the coated-microcantilever through coupling to an ac magnetic field; and (b) detection by optical beam deflection of the microcantilever motion utilizing a laser diode source and a position-sensitive detector. Currently, the sensitivity of this MEMS-based magnetostrictive magnetometer is ∼1μT.

It has been shown by optical and x-ray measurements that GaN nucleation layers deposited at 540 °C on (0001)-oriented sapphire substrates have a measurable crystalline component, although the x-ray data and the lack of absorbance features near the direct band gap of GaN suggest that the crystallite size is very small. Upon annealing to higher temperatures, the crystallite size increases and the crystal perfection improves markedly, until at temperatures near those empirically determined to be optimum for growth of an epitaxial overlayer, it approaches that of good quality single-crystal material. Most of the recrystallization of the nucleation layer occurs during the ramp from its deposition temperature to the growth temperature of the GaN overlayer, and there appears to be no advantage to prolonged annealing at high temperatures prior to epitaxial growth. In fact, x-ray diffractometer results suggest that the nucleation layer deteriorates after 20 min at temperatures above 1015 °C, under the conditions used in this study.

A comprehensive study of the nitrogen K edge and gallium M 2,3 edge in gallium nitride is presented. Results of two different experimental techniques, x-ray absorption by total photocurrent measurements and glancingangle x-ray reflectivity, are presented and compared with each other. First-principles calculations of the ͑polarization averaged͒ dielectric response ⑀ 2 () contributions from the relevant core-level to conduction-band transitions and derived spectral functions are used to interpret the data. These calculations are based on the local density approximation ͑LDA͒ and use a muffin-tin orbital basis for the band structure and matrix elements. The angular dependence of the x-ray reflectivity is studied and shown to be in good agreement with the theoretical predictions based on Fresnel theory and the magnitude of the calculated x-ray optical response functions. The main peaks in the calculated and measured spectra are compared with those in the relevant partial density of conduction-band states. Assignments are made to particular band transitions and corrections to the LDA are discussed. From the analysis of the N K and Ga M 2,3 edges the latter are found to be essentially constant up to ϳ10 eV above the conduction-band minimum. The differences in spectral shape found between the various measurements were shown to be a result of polarization dependence. Since the c axis in all the measurements was normal to the sample surface, p-polarized radiation at glancing angles corresponds to Eʈc while s polarization corresponds to EЌc at all incident angles. Thus, this polarization dependence is a result of the intrinsic anisotropy of the wurtzite structure. Spectra on powders which provide an average of both polarizations as well as separate measurements of reflectivity with s polarization and p polarization were used to arrive at this conclusion.