A linear perturbation theory is developed to investigate the interface instabilities of a radially-expanding, liquid jet in cylindrical geometries. The theory is applied to rapidly spreading droplets upon collision with solid surfaces as the fundamental mechanism behind splashing. The analysis is based on the observation that the instability of the liquid sheet, i.e., the formation of the fingers at the spreading front, develops in the extremely early stages of droplet impact. The shape evolution of the interface in the very early stages of spreading is numerically simulated based on the axisymmetric solutions obtained by a theoretical model. The effects that factors such as the transient profile of an interface radius, the perturbation onset time, and the Weber number have on the analysis results are examined. This study shows that a large impact inertia, associated with a high Weber number, promotes interface instability, and prefers high wave number for maximum instability. The numbers of fingers at the spreading front of droplets predicted by the model agree well with those experimentally observed.

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First-order Raman scattering of hexagonal GaN single crystal films deposited on sapphire substrate by low pressure metal organic chemical vapor deposition is studied between 78 and 870 K. The temperature dependence of the five GaN Raman modes is obtained. Both the linewidth and Raman shift exhibit a quadratic dependence on temperature in our measured temperature range. Excellent agreement was found between the experiment data and calculated results based on a model involving three- and four-phonon coupling. Our results indicate it is necessary to include the contributions of both the thermal expansion and four-phonon terms in the four-phonon anharmonic processes to explain the change of Raman shift and linewidth with temperature. In addition, a decrease in the splitting between the longitudinal optical and transverse optical phonons with increasing temperature was also observed. From these data a weak nonlinear decrease of the transverse effective charge with increasing temperature is derived. The comparison of the transverse effective charge eT* at room temperature was made between experimental data and theoretical calculations by a pseudopotential expression and bond orbital model. Good agreement between theory and experiment is achieved.

Hafnium oxide (HfO 2) thin films have been made by atomic vapor deposition (AVD) onto Si substrates under different growth temperature and oxygen flow. The effect of different growth conditions on the structure and optical characteristics of deposited HfO 2 film has been studied using X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), grazing incidence X-ray diffraction (GIXRD) and variable angle spectroscopic ellipsometry (VASE). The XPS measurements and analyses revealed the insufficient chemical reaction at the lower oxygen flow rate and the film quality improved at higher oxygen flow rate. Via GIXRD, it was found that the HfO 2 films on Si were amorphous in nature, as deposited at lower deposition temperature, while being polycrystalline at higher deposition temperature. The structural phase changes from interface to surface were demonstrated. The values of optical constants and bandgaps and their variations with the growth conditions were determined accurately from VASE and XPS. All analyses indicate that appropriate substrate temperature and oxygen flow are essential to achieve high quality of the AVD-grown HfO 2 films.

Effects on diamond-diamond friction of ultra-high vacuum (UHV) ( approximately 4*10-9 Torr) and low pressure ( approximately 1*10-5 Torr) gases including hydrogen, oxygen and nitrogen in their molecular and atomic states have been investigated. The friction coefficient for diamond sliding on diamond in UHV was found to be between 0.6 and 1.0, which is about ten times that measured in air. Among molecular gases studied at pressures of approximately 1*10-5 Torr, only oxygen caused a significant reduction of the friction coefficient. When a heated filament was used to dissociate the gases in the vacuum chamber, atomic hydrogen produced the largest decrease in friction coefficient. The reduction in friction coefficient increased with increasing exposure time before sliding started. These results suggest that passivation of diamond surfaces by atomic hydrogen and oxygen as well as molecular oxygen is effective in reducing the carbon-carbon bond formation during sliding and consequently the friction coefficient.