Cataloged from PDF version of article.Semi-insulating character ( sheet resistivity of 3.26 x 10(11) ohm/sq ) of thick GaN layers was developed for AlGaN/GaN high electron mobility transistor ( HEMT ) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN/GaN HEMT structures on sapphire substrates. (c) 2006 American Institute of Physics

We investigated the behavior of the forward bias current-voltage-temperature ͑I-V-T͒ characteristics of inhomogeneous ͑Ni/ Au͒-Al 0.3 Ga 0.7 N / AlN / GaN heterostructures in the temperature range of 295-415 K. The experimental results show that all forward bias semilogarithmic I-V curves for the different temperatures have a nearly common cross point at a certain bias voltage, even with finite series resistance. At this cross point, the sample current is temperature independent. We also found that the values of series resistance ͑R s ͒ that were obtained from Cheung's method are strongly dependent on temperature and the values abnormally increased with increasing temperature. Moreover, the ideality factor ͑n͒, zero-bias barrier height ͑⌽ B0 ͒ obtained from I-V curves, and R s were found to be strongly temperature dependent and while ⌽ B0 increases, n decreases with increasing temperature. Such behavior of ⌽ B0 and n is attributed to Schottky barrier inhomogeneities by assuming a Gaussian distribution ͑GD͒ of the barrier heights ͑BHs͒ at the metal/semiconductor interface. We attempted to draw a ⌽ B0 versus q /2kT plot in order to obtain evidence of the GD of BHs, and the values of ⌽ B0 = 1.63 eV and 0 = 0.217 V for the mean barrier height and standard deviation at a zero bias, respectively, were obtained from this plot. Therefore, a modified ln͑I 0 / T 2 ͒ − q 2 0 2 /2͑kT͒ 2 versus q / kT plot gives ⌽ B0 and Richardson constant A * as 1.64 eV and 34.25 A / cm 2 K 2 , respectively, without using the temperature coefficient of the barrier height. The Richardson constant value of 34.25 A / cm 2 K 2 is very close to the theoretical value of 33.74 A / cm 2 K 2 for undoped Al 0,3 Ga 0,7 N. Therefore, it has been concluded that the temperature dependence of the forward I-V characteristics of the ͑Ni/ Au͒-Al 0.3 Ga 0.7 / AlN / GaN heterostructures can be successfully explained based on the thermionic emission mechanism with the GD of BHs.

The preparation conditions for Co doping process into the ZnO structure were studied by the ultrasonic spray pyrolysis technique. Structural and optical properties of the Co:ZnO thin films as a function of Co concentrations were examined. It was observed that hexagonal wurtzite structure of ZnO is dominant up to the critical value, and after the value, the cubic structural phase of the cobalt oxide appears in the X-ray diffraction patterns. Every band-edge of Co:ZnO films shifts to the lower energies and all are confirmed with the PL measurements. Co substitution in ZnO lattice has been proved by the optical transmittance measurement which is observed as the loss of transmission appearing in specific region due to Co 2+ characteristic transitions.

ZnO thin films are deposited by radio-frequency magnetron sputtering on thermally grown SiO2 on Si substrates. Pt/Au contacts are fabricated by standard photolithography and lift-off in order to form a metal-semiconductor-metal (MSM) photodetector. The dark current of the photodetector is measured as 1 pA at 100 V bias, corresponding to 100 pA/cm2 current density. Spectral photoresponse measurement showed the usual spectral behavior and 0.35 A/W responsivity at a 100 V bias. The rise and fall times for the photocurrent are measured as 22 ps and 8 ns, respectively, which are the lowest values to date. Scanning electron microscope image shows high aspect ratio and dense grains indicating high surface area. Low dark current density and high speed response are attributed to high number of recombination centers due to film morphology, deducing from photoluminescence measurements. These results show that as deposited ZnO thin film MSM photodetectors can be used for the applications needed for low light level detection and fast operation.