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We study thermal annealing effects on the size and composition variations of indium-aggregated clusters in two InGaN thin films with photoluminescence (PL) in the yellow and red ranges. The methods of investigation include optical measurement, nanoscale material analysis, and theoretical calculation. Such a study is important for determining the relation between the band gap and the average indium content of InGaN. In one of the samples, the major part of the PL spectrum is shifted from the yellow band into the blue range upon thermal annealing. In the other sample, after thermal annealing, a broad spectrum covering the whole visible range is observed. To obtain a basic physics picture behind the spectral blue shift upon thermal annealing in the yellow emission sample, we theoretically study the quantum-confinement effects of InGaN clusters based on a quantum box model. [ABSTRACT FROM AUTHOR]

The temperature-dependent Hall resistivity and carrier concentrations of Ga[sub 1-x]Mn[sub x]As epilayers grown on (100) semi-insulating GaAs substrates by molecular beam epitaxy have been investigated in the temperature range of 10–300 K. A Ga[sub 1-x]Mn[sub x]As sample with x≈4.4% shows typical insulator behavior and Ga[sub 1-x]Mn[sub x]As samples with x≈2.2 and 3.7 % show typical metallic behavior. A model taking into account ionized impurity and spin disorder scattering mechanisms was used to portray properly the observed features of the temperature-dependent Hall resistivity data. The value of the p-d exchange energy was J=59.4±0.5 and 71.9±0.5 eV Å3 for the samples with x≈2.2 and 3.7 %, respectively. Ionized impurity scattering dominates the entire temperature range, with a temperature-independent spin disorder scattering in the paramagnetic region. It was found that the spin disorder scattering mechanism had a strong temperature dependence on 1-T[sup 2] in the ferromagnetic region. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Highly conductive Si-doped n-type Al0.7Ga0.3N alloys were grown by metalorganic chemical vapor deposition on sapphire substrates. Variable temperature Hall-effect measurements have been employed to study the electrical properties for samples with nominal Si dopant concentration (NSi) from 2.6 to 6.8 × 1019 cm³. For the sample with NSi=6.0 × 1019 cm³, we have achieved n-type resistivity of 0.0075 Ω cm with an electron concentration of 3.3 × 1019 cm³ and mobility of 25 cm²/V s at room temperature. For the same sample, the effective donor (Si) activation energy E0 was determined to he as low as 10 meV. E0 increases to 25 meV as NSi is reduced to 2.6 × 1019 cm³, which can be explained by the handgap renormalization effect. This implies that heavy doping is necessary in high-Al-content AlGaN alloys to bring down the donor activation energy, therefore a higher conductivity. [ABSTRACT FROM AUTHOR]

III-nitride photonic crystal (PC) ultraviolet (UV) light-emitting diodes (LEDs) were fabricated. Triangular arrays of the PCs with different diameters/periodicities were patterned using electron-beam lithography and inductively coupled plasma dry etching. The optical power output of LEDs was enhanced by a factor of 2.5 due to PC formation. It was observed that the optical enhancement factor depends strongly on the lattice constant and hole size of the PCs. The achievement of nitride PCs is expected to benefit many applications of III-nitride optoelectronics, particularly for the improvement of extraction efficiency in III-nitride deep-UV emitters (λ<340 nm), which are crucial for many important applications, but presently have a very low quantum efficiency. [ABSTRACT FROM AUTHOR]

We compared the optical and material properties of an InGaN thin film with an average indium content at 0.31 between as-grown and postgrowth thermally annealed conditions. The major part of the photoluminescence spectrum was shifted from the original yellow band into the blue range upon thermal annealing. Cathodoluminescence (CL) spectra showed that the spectral shift occurred essentially in a shallow layer of the InGaN film. The deeper layer in the as-grown sample contributed blue emission because it had been thermally annealed during the growth of the shallow layer. The spectral change was attributed to the general trends of cluster size reduction and possibly quantum-confined Stark effect relaxation upon thermal annealing. The attribution was supported by the observations in the CL, x-ray diffraction, and high-resolution transmission electron microscopy results. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The present study is to enhance the critical heat flux (CHF) in pool boiling from a flat square heater immersed in nanofluid (water mixed with extremely small amount of nanosized particles). The test results show that the enhancement of CHF was drastic when nanofluid is used as a cooling liquid instead of pure water. The experiment was performed to measure and compare pool boiling curves of pure water and nanofluid at the pressure of 2.89 psia (T[sub sat]=60 °C) using 1×1 cm[sup 2] polished copper surfaces as a boiling surface. The tested nanofluid contains alumina (Al[sub 2]O[sub 3]) nanoparticles dispersed in distilled and deionized water. Tested concentrations of nanoparticles range from 0 g/l to 0.05 g/l. The measured pool boiling curves of nanofluids saturated at 60 °C have demonstrated that the CHF increases dramatically (∼200% increase) compared to pure water case; however, the nucleate boiling heat transfer coefficients appear to be about the same. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the epitaxial and preferentially oriented polycrystalline Bi[sub 4]Ti[sub 3]O[sub 12] thin films using La[sub 0.5]Sr[sub 0.5]CoO[sub 3] bottom electrodes. Explanation of growth behaviors using oxygen ion arrangements; Dependence of microstructures of the heterostructures on the substrates; Factors affecting leakage current behaviors the film.