We report measurements of the Schottky barrier heights of Ni/Au contacts on Ga-polarity and N-polarity n-GaN under hydrostatic pressure and applied in-plane uniaxial stress. Under hydrostatic pressure the two different polarities of GaN yield significantly different rates of Schottky barrier height increase with increasing pressure. Uniaxial stress parallel to the surface affects the Schottky barrier height only minimally. The observed changes in barrier height under stress are attributed to a combination of band structure and piezoelectric effects.

We report the effect of compressive hydrostatic pressure on the current-voltage characteristics of AlGaN/ GaN heterojunction field effect transistors ͑HFETs͒ on a sapphire substrate. The drain current increases with hydrostatic pressure and the maximum relative increase occurs when the gate bias is near threshold and drain bias is slightly larger than saturation bias. The increase of the drain current is associated with a pressure induced shift of the threshold voltage by −8.0 mV/ kbar that is attributed to an increase of the polarization charge density at the AlGaN/ GaN interface due to the piezoelectric effect. The results demonstrate the considerable potential of AlGaN/ GaN HFETs for strain sensor applications.

We report measurements of the Schottky barrier height of Ni contacts on Ga-polarity n-Al0.08Ga0.92N as a function of pressure. With applied hydrostatic pressure, Al0.08Ga0.92N Schottky diodes show a decrease in the forward bias current, and correspondingly an increase in the barrier height, which is approximately twice as large as that previously reported for Schottky contacts on Ga-polarity n-GaN. The observed change in barrier height with pressure is attributed to a combination of band structure and piezoelectric effects. The larger change of barrier height for Al0.08Ga0.92N can be explained by its larger piezoelectric constants and smaller density of interface states at the metal-semiconductor interface compared to GaN.

We investigate the magnetic properties of a strongly n-type GaMnN alloy grown by molecular beam epitaxy. Transport, X-ray diffraction, and magnetic characterizations are presented. The carrier concentration and mobility obtained by Hall effect measurements are nearly constant with respect to temperature over the entire temperature range from 4 K to 300 K. Magnetometry data indicates a transition temperature at approximately 170 K with hysteresis measurements indicating magnetic behavior at least to 300 K. The role of alloy microstructure is investigated using X-ray diffraction and shows the possible presence of alternate phases giving rise to magnetization. In transport, we observe a linear Hall effect dependence on magnetic field strength.