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Carefully optimized low-pressure metalorganic vapor phase epitaxy is used for homoepitaxial growth on distinctively pretreated GaN bulk single crystal substrates. Thereby, outstanding structural and optical qualities of the material have been achieved, exhibiting photoluminescence linewidths for bound excitons as narrow as 95 μeV. These extremely sharp lines reveal fine structures, not reported for GaN. Additionally, all three free excitons as well as their excited states are visible in low-temperature photoluminescence at 2 K. These transitions are clearly identified by reflectance measurements. X-ray diffraction analysis of these layers reveal about 20 arcsec linewidth for the (0004) reflex using CuKα radiation.

Piezoelectric AlGaN/GaN FETs on SiC with high carrier mobility have been fabricated yielding I DS =450 mA/mm and g m =200 mS/mm. In the on-state, under UV-illumination, the devices sustain a drain voltage of V DS =49 V, corresponding to a power dissipation of 26.5 W/mm. On turn-on of the device from the pinch-off state, a significant delay in the drain current build-up is observed. This effect depends on the pinch-off time and the pinch-off voltage and can be removed by either a brief UV-illumination or a V DS >25 V applied in the on-state. The drain current transients are characterized by a relaxation time τ , which is in the order of several hundred seconds. From the temperature dependence of τ , an activation energy of about 280 meV and a capture cross section of 4.4·10 -18 cm 2 were determined. The devices show pronounced persistent photoconductivity (PPC) and the drain current I D is sensitive to illumination.

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