Photoluminescence ͑PL͒, Raman spectroscopy, and x-ray diffraction are employed to demonstrate the coexistence of a biaxial and a hydrostatic strain that can be present in GaN thin films. The biaxial strain originates from growth on lattice-mismatched substrates and from post-growth cooling. An additional hydrostatic strain is shown to be introduced by the presence of point defects. A consistent description of the experimental results is derived within the limits of the linear and isotropic elastic theory using a Poisson ratio ϭ0.23Ϯ0.06 and a bulk modulus Bϭ200Ϯ20 GPa. These isotropic elastic constants help to judge the validity of published anisotropic elastic constants that vary greatly. Calibration constants for strain-induced shifts of the near-bandedge PL lines with respect to the E 2 Raman mode are given for strain-free, biaxially strained, and hydrostatically contracted or expanded thin films. They allow us to extract differences between hydrostatic and biaxial stress components if present. In particular, we determine that a biaxial stress of one GPa would shift the near-band-edge PL lines by 27Ϯ2 meV and the E 2 Raman mode by 4.2Ϯ0.3 cm Ϫ1 by use of the listed isotropic elastic constants. It is expected from the analyses that stoichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom. ͓S0163-1829͑96͒03148-7͔

Faraday rotation spectrum has been measured at room temperature in a magnetic nanocomposite of ␥-Fe 2 O 3 /SiO 2. The material consists of isolated ␥-Fe 2 O 3 nanoparticles dispersed in a silica matrix, and it was prepared through a sol-gel method. The composite contains 18% of ␥-Fe 2 O 3 in weight with an average particle size of 20 nm. It has a coercitivity of 30 Oe and an M S of 6 emu/g. The specific Faraday rotation spectrum exhibits a narrow peak centered around 765 nm, reaching a value of 110°/cm and an absorption coefficient of 64 cm Ϫ1. Faraday rotation versus applied field has also been measured, and a cycle similar to the one described by the magnetization has been found.

Reverse bias current-voltage measurements of ϳ100-m-diameter gold Schottky contacts deposited on as-received, n-type ZnO(0001) wafers and those exposed for 30 min to a remote 20% O 2 /80% He plasma at 525Ϯ20°C and cooled either in vacuum from 425°C or the unignited plasma gas have been determined. Plasma cleaning resulted in highly ordered, stoichiometric, and smooth surfaces. Contacts on as-received material showed A leakage currents and ideality factors Ͼ2. Contacts on plasma-cleaned wafers cooled in vacuum showed ϳ36Ϯ1 nA leakage current to Ϫ4 V, a barrier height of 0.67Ϯ0.05 eV, and an ideality factor of 1.86Ϯ0.05. Cooling in the unignited plasma gas coupled with a 30 s exposure to the plasma at room temperature resulted in decreases in these parameters to ϳ20 pA to Ϫ7 V, 0.60Ϯ0.05 eV, and 1.03Ϯ0.05, respectively. Differences in the measured and theoretical barrier heights indicate interface states. ͑0001͒ and (0001) are used in this letter to designate the polar zinc-and oxygen-terminated surfaces, respectively.

The microstructure and the lateral epitaxy mechanism of formation of homoepitaxially and selectively grown GaN structures within windows in SiO2 masks have been investigated by transmission electron microscopy (TEM) and scanning electron microscopy. The structures were produced by organometallic vapor phase epitaxy for field emission studies. A GaN layer underlying the SiO2 mask provided the crystallographic template for the initial vertical growth of the GaN hexagonal pyramids or striped pattern. The SiO2 film provided an amorphous stage on which lateral growth of the GaN occurred and possibly very limited compliancy in terms of atomic arrangement during the lateral growth and in the accommodation of the mismatch in the coefficients of thermal expansion during cooling. Observations with TEM show a substantial reduction in the dislocation density in the areas of lateral growth of the GaN deposited on the SiO2 mask. In many of these areas no dislocations were observed.

Tellurium-modified silicon nanowires with a large negative temperature coefficient of resistance Appl. Phys. Lett. 101, 133111 (2012) Tapered and aperiodic silicon nanostructures with very low reflectance for solar hydrogen evolution Appl. Phys. Lett. 101, 133906 (2012) Minimizing scattering from antireflective surfaces replicated from low-aspect-ratio black silicon Appl. Phys. Lett. 101, 131902 (2012) Robust hydrophobic Fe-based amorphous coating by thermal spraying Appl. Phys. Lett. 101, 121603 (2012) Influence of high temperature on solid state nuclear track detector parameters Rev. Sci. Instrum. 83, 093502 (2012) Additional information on J. Appl. Phys.Successful ex situ and in situ cleaning procedures for AlN and GaN surfaces have been investigated and achieved. Exposure to HF and HCl solutions produced the lowest coverages of oxygen on AlN and GaN surfaces, respectively. However, significant amounts of residual F and Cl were detected. These halogens tie up dangling bonds at the nitride surfaces hindering reoxidation. The desorption of F required temperatures Ͼ850°C. Remote H plasma exposure was effective for removing halogens and hydrocarbons from the surfaces of both nitrides at 450°C, but was not efficient for oxide removal. Annealing GaN in NH 3 at 700-800°C produced atomically clean as well as stoichiometric GaN surfaces.

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Thermal conductivity in non-metallic crystalline materials results from cumulative contributions of phonons that have a broad range of mean free paths. Here we use high frequency surface temperature modulation that generates non-diffusive phonon transport to probe the phonon mean free path spectra of GaAs, GaN, AlN, and 4H-SiC at temperatures near 80 K, 150 K, 300 K, and 400 K. We find that phonons with MFPs greater than 230 ± 120 nm, 1000 ± 200 nm, 2500 ± 800 nm, and 4200 ± 850 nm contribute 50% of the bulk thermal conductivity of GaAs, GaN, AlN, and 4H-SiC near room temperature. By non-dimensionalizing the data based on Umklapp scattering rates of phonons, we identified a universal phonon mean free path spectrum in small unit cell crystalline semiconductors at high temperature.