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Articles you may be interested inDispersion properties and low infrared optical losses in epitaxial AlN on sapphire substrate in the visible and infrared range A prism coupling method was used to measure the ordinary (n o ) and extraordinary (n e ) refractive indices of Al x Ga 1Ϫx N films, grown by hydride vapor phase epitaxy ͑HVPE͒ and metalorganic chemical vapor deposition ͑MOCVD͒ on sapphire, at several discrete wavelengths from 442 nm to 1064 nm. In addition, spectroscopic transmittance and reflectance, correlated with the prism coupling results, were used to measure n o as a continuous function of wavelength between the band gap of each sample ͑255 nm to 364 nm, depending on Al fraction͒ and 2500 nm. The Al mole fractions ͑x͒, determined by energy dispersive x-ray spectroscopy ͑EDS͒, were xϭ0.144, 0.234, 0.279, 0.363, 0.593, and 0.657 for the HVPE-grown samples, and xϭ0.000, 0.419, 0.507, 0.618, 0.660, and 0.666 for the MOCVD-grown samples. The maximum standard uncertainty in the EDS-determined value of x was Ϯ0.02. The maximum standard uncertainty in the refractive indices measured by prism coupling was Ϯ0.005 and a one-Sellmeier-term equation was adequate to fit the wavelength dependence of n e from 442 nm to 1064 nm. Due to the spectral proximity of the absorption edge, the wavelength dependence of n o measured by spectroscopic transmittance/ reflectance ͑correlated with the prism-coupling results͒, from the band gap of each sample to 2500 nm, was fit with a two-Sellmeier-term equation.

Basic physical ideas and methods that are used to improve the quality of modern thermoelectric materials and to increase the thermoelectric figure-of-merit are reviewed, with special emphasis on how nanostructure affects the thermoelectric properties of materials.

Piezoelectric field-enhanced second-order nonlinear optical susceptibilities in wurtzite GaN/AlGaN quantum wells Rotational Maker fringes, scaled with respect to 11 ͑2͒ of crystalline quartz, were used to determine the second order susceptibilities 31 ͑2͒ and 33 ͑2͒ for samples of thin Al x Ga 1−x N films, a thicker GaN film, and a free-standing GaN platelets. The pump wavelength was 1064 nm. The Al x Ga 1−x N samples, ranging in thickness from roughly 0.5 to 4.4 m, were grown by metalorganic chemical vapor deposition ͑MOCVD͒ and hydride vapor-phase epitaxy ͑HVPE͒ on ͑0001͒ sapphire substrates. The Al mole fractions x were 0, 0.419, 0.507, 0.618, 0.660, and 0.666, for the MOCVD-grown samples, and x = 0, 0.279, 0.363, and 0.593 for the HVPE-grown samples. An additional HVPE-grown GaN sample ϳ70 m thick was also examined. The free-standing bulk GaN platelets consisted of an HVPE grown film ϳ226 m thick removed from its growth substrate, and a crystal ϳ160 m thick grown by high-pressure techniques. For the Al x Ga 1−x N samples, the magnitudes of 31 ͑2͒ and 33 ͑2͒ decrease roughly linearly with increasing x and extrapolate to ϳ0 for x = 1. Furthermore, the constraint expected for a perfect wurtzite structure, namely 33 ͑2͒ =−2 31 ͑2͒ , was seldom observed, and the samples with x = 0.660 and x = 0.666 showed 31 ͑2͒ and 33 ͑2͒ having the same sign. These results are consistent with the theoretical studies of nonlinear susceptibilities for AlN and GaN performed by Chen et al. ͓Appl. Phys. Lett. 66, 1129 ͑1995͔͒. The thicker bulk GaN samples displayed a complex superposition of high-and low-frequency Maker fringes due to the multiple-pass interference of the pump and second-harmonic generation beams, and the nonlinear coefficients were approximately consistent with those measured for the thin-film GaN sample.

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