(0001)-oriented epitaxial wurtzite III-nitride layers grown on mismatched substrates have no resolved shear stress on the natural basal and prismatic slip planes; however, strained Ill-nitride layers may gradually relax. We report on the stress relaxation of Al0.49Ga0.51N layers grown on nominally relaxed Al0.62Ga0.38N buffer layers on sapphire. The reduction in elastic strain of the Al0.49Ga0.51N was enhanced by Si doping which caused an increased surface roughness. Despite the Si doping, the films always sustained step-flow growth. The extent of relaxation of the Al0.49Ga0.51N layer was determined by on-axis omega-2 theta scans of (0001) peaks and reciprocal space maps of inclined (off-axis) peaks. Cross-section and plan-view transmission electron microscopy studies showed that the threading dislocations in the Al(0.49)Gao(0.5)N layer inclined from the [0001] direction towards (1 (1) over bar 00) directions by similar to 15-25 degrees, perpendicular to their Burgers vector (1/3 ) These inclined threading dislocations have a misfit dislocation component and thus provide stress relief. The contribution of the dislocation inclination to the degree of relaxation has been formulated and the energy release has been determined for dislocation inclination in mismatched stressed layers. (c) 2005 American Institute of Physics.

N-p-n Al0.05GaN/GaN heterojunction bipolar transistors with a common emitter operation voltage higher than 330 V have been demonstrated using selectively regrown emitters. Devices were grown by metalorganic chemical vapor deposition on sapphire substrates. The n-type emitter was grown selectively on a 100-nm-thick p-base with an 8 mum n-collector structure using a dielectric mask. The shallow etch down to the collector mitigates damages induced in the dry etch, resulting a low leakage and a high breakdown. The graded AlGaN emitter results in a common emitter current gain of similar to18 at an average collector current density of up to 1 kA/cm(2) at room temperature.

High-voltage Al0.22Ga0.78N-GaN high-electron mobility transistors have been fabricated using multiple field plates over dielectric passivation layers. The device breakdown voltage was found to increase with the addition of the field plates. with two field plates, the device showed, a breakdown voltage as high as 900 V. This technique is easy to apply, based on the standard planar transistor fabrication, and especially attractive for the power switching applications.

The influence of AlGaN and GaN cap layer thickness on Hall sheet carrier density and mobility was investigated for Al0.32Ga0.68N/GaN and GaN/Al0.32Ga0.68N/GaN heterostructures deposited on sapphire substrates. The sheet carrier density was found to increase and saturate with the AlGaN layer thickness, while for the GaN-capped structures it decreased and saturated with the GaN cap layer thickness. A relatively close fit was achieved between the measured data and two-dimensional electron gas densities predicted from simulations of the band diagrams. The simulations also indicated the presence of a two-dimensional hole gas at the upper interface of GaN/AlGaN/GaN structures with sufficiently thick GaN cap layers. A surface Fermi-level pinning position of 1.7 eV for AlGaN and 0.9-1.0 eV for GaN, and an interface polarization charge density of 1.6x10(13)-1.7x10(13) cm(-2), were extracted from the simulations. (C) 2003 American Institute of Physics.

Recombination dynamics of the 268 nm photoluminescence (PL) peak in a quaternary Al0.53In0.11Ga0.36N/Al0.58In0.02Ga0.40N multiple quantum well (MQW) grown on relaxed AlGaN templates were studied. Although the polarization field in the compressively strained Al0.53In0.11Ga0.36N wells was as high as 1.6 MV/cm, the value of integrated PL intensity at 300 K divided by that at 8 K (eta(int)) was as high as 1.2%. The value was similar to that obtained for the 285 nm PL peak in an Al0.30Ga0.70N/Al0.70Ga0.30N MQW (1.3%), though the A1N molar fraction in the wells was higher by a factor of 1.7. According to these results and the fact that time-resolved PL signal exhibited a stretched exponential decay shape, the improved eta(int) of the AlInGaN wells was attributed to a beneficial effect of the exciton localization as is the case with InGaN alloys; doping or alloying with InN was confirmed to work also on AlGaN in improving eta(int) to realize deep UV optoelectronic devices.

Evaluation of the structural properties of 200-nm-thick Si-doped Al0.49Ga0.51N films, grown on nominally relaxed 1-mum-thick Al0.62Ga0.38N buffer layers on sapphire, revealed that increased Si doping promoted the relaxation of the compressively strained layers. The degree of strain relaxation R of the Al0.49Ga0.51N films, as determined by x-ray diffraction (XRD), increased from R=0.55 to R=0.94 with an increase in disilane injection from 1.25 nmol/min to 8.57 nmol/min. Transmission electron microscopy analysis showed that the edge threading dislocations (TDs) in the Al0.49Ga0.51N layers were inclined, such that the redirected TD lines had a misfit dislocation component. The calculated strain relaxation due to the inclined TDs was in close agreement with the values determined from XRD. We propose that the TD line redirection was promoted by the Si-induced surface roughness. (C) 2003 American Institute of Physics.

We report on the fabrication and high-frequency characterization of AlGaN/GaN high-electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). In devices with a gate length of 160 nm, a record power density of 10.5 W/mm with 34% power added efficiency (PAE) has been measured at 40 GHz in MOCVD-grown HEMTs biased at V-DS = 30 V. Under similar bias conditions, more than 8.6 W/mm, with 32% PAE, were obtained on the MBE-grown sample. The dependence of output power, gain, and PAE on gate and drain voltages, and frequency have also been analyzed.

In this letter, unpassivated high power deeply recessed GaN-based high electron mobility transistors (HEMTs) are reported. The introduction of a thick graded AlGaN cap layer and a novel fluorine-plasma surface treatment reduced the gate-leakage current and increased breakdown voltage significantly, enabling the application of much higher drain biases. Due to excellent dispersion suppression achieved at an epitaxial level, an output power density of more than 17 W/mm with an associated power added efficiency (PAE) of 50% was measured at 4 GHz and V-DS = 80 V without SiNx passivation. These results demonstrate the great potential of this novel epitaxial approach for passivation-free GaN-based HEMTs for high-power applications.

Cathodoluminescence technique combined with transmission electron microscopy (TEM-CL) has been used to characterize optical properties of dislocations in GaN epilayers. The dislocations act as nonradiative centers with different recombination rates. TEM-CL observation showed that even for the same Burgers vector of a, the dislocations show different electrical activity depending on the direction of dislocation line, i.e., the edge-type dislocation parallel to the c plane is very active, while the screw-type one is less active. The simulation of the CL images gives us the information of parameters such as carrier lifetime and diffusion length. (C) 2003 American Institute of Physics.