We theoretically investigate the thermal boundary resistance and heat diffusion in AlGaN/GaN heterostructure field-effect transistors. Our calculations based on the diffuse mismatch model show that the thermal boundary resistance at the interface between GaN and SiC can strongly influence the temperature rise in the device channel.

Atomic resolution high-voltage transmission electron microscopy and electron energy loss spectroscopy were performed on grain boundaries of boron-doped diamond, cooperated with the ab-initio calculation. Segregated boron in the {112}∑3 boundary was caught by the EELS spectra. The change in atomic structure of the segregated boundary was successfully observed from the image by ARHVTEM. Based on the ARHVTEM image, a segregted structure model was proposed.

Ohmic Ta/Ti/Ni/Au contacts to n-GaN have been studied using high resolution electron microscopy (HREM), energy dispersive X-ray spectrometry (EDX) and electron energy loss spectrometry (EELS). Two different samples were used: A - annealed at 7500C withcontact resistance 5×10-6 Ω cm2 and B-annealed at 7750C with contact resistance 6×10-5 Ω cm2. Both samples revealed extensive in- and out-diffusion between deposited layers with some consumption ofGaNlayerand formation of TixTa1-xN50 (0<x<25) at the GaN interface. Almost an order of magnitude difference in contact resistances can be attributed to structure and chemical bonding of Ti-O layers formed on the contact surfaces.

We present preliminary results of a theoretical investigation of the thermal conductivity of polycrystalline GaN films. It is assumed that grain boundaries play a major role in limiting the thermal conductivity, which is calculated using the phonon-hopping transport approach. The effect of the grain size, size dispersion, and inter-grain interface structure on the thermal conductivity values is analyzed. The obtained results are compared with available experimental data for polycrystalline films and model predictions for crystalline GaN films.

(1010) GaN wafers grown on (100) face of γ-LiAlO2 were studied using transmission electron microscopy. Despite good lattice matching in this heteroepitaxial system, high densities of planar structural defects in the form of stacking faults on the basal plane and networks of boundaries located on prism planes inclined to the layer/substrate interface were present in these GaN layers. In addition, significant numbers of threading dislocations were observed. High-resolution electron microscopy indicates that stacking faults present on the basal plane in these layers are of low-energy intrinsic I1 type. This is consistent with diffraction contrast experiments.

We report on observation of stimulated emission at 258 nm in AlN/AlGaN multiple quantum wells. The structures were grown over Al-face single crystal bulk AlN substrates. AlN/AlGaN structures with 50% of Al in the well material were grown using low-pressure metalorganic chemical vapour deposition. Characterization by using X-ray, AFM, SEM, and photoluminescence techniques indicated high structural quality of the structures. The stimulated emission was measured using the variable stripe length method under excitation by 4-ns-long pulses of the fifth harmonic of Nd:YAG laser radiation at 213 nm (5.82 eV). The stimulated emission exhibited a characteristic superlinear dependence of emission intensity on the pump intensity as well as an exponential increase of the sample-edge emission intensity with increasing stripe length up to ∼430 μm and the intensity saturation beyond this range. The observation of stimulated emission at 258 nm is very promising for the future development of III-nitride-based deep-UV laser diodes on bulk AlN substrates.

GaN layers were grown on AlGaN with nanopipes by molecular beam epitaxy (MBE) and analyzed. AlGaN films were grown by MBE using rf-plasma nitrogen source under metal-rich condition. Within the Al composition range of 0.5-0.6, open-end nanopipes were formed at the surface of AlGaN films with a density of ∼6×109 cm-2 and a size ranging from 10 to 20 nm. These nanopipes, observed within ∼300 nm of the surface, served as a nanoporous AlGaN template for re-growth of GaN epilayers. GaN epilayers grown to different thickness by MBE were studied for their microstructural and optical properties. For an AlGaN buffer layer with dislocation density of 3×1010 cm-2 near its surface, the overlaying GaN layers with thickness ranging from 0.1 μm to ∼2μm were grown and analyzed by transmission electron microscopy for dislocation density. The GaN layer started with hexagonal islands on the nanopiped AlGaN and began to coalesce at about 0.1μm thickness. At a thickness of 2.0 μm, the dislocation density reduced to ∼1×109 cm-2. Low temperature photoluminescence data demonstrate the improved optical quality of GaN epilayer grown on the porous AlGaN buffer layer.

Both MgO and Sc2O3 are shown to provide low interface state densities (in the 1011 eV-1 cm-2 range) on n- and p-GaN, making them useful for surface passivation layers to mitigate current collapse in GaN/AlGaN high electron mobility transistors(HEMTs) and also gate dielectrics for metal-oxide semiconductor(MOS) devices. Clear evidence of inversion has been demonstrated in gate-controlled MOS p-GaN diodes using both types of oxide. Charge pumping measurements on diodes undergoing a high temperature implant activation anneal show a total surface state density of ∼3 × 1012 cm-2. On HEMT structures, both oxides provide effective passivation of surface states and these devices show improved output power. The MgO/GaN diodes and Sc2O3 passivated HEMT are also found to be quite radiation-resistant, making them attractive for satellite and terrestrial communication systems requiring a high tolerance to high energy(40MeV) protons.

Double heterostructure diodes were widely studied in the 80's, based on the use of “classical” III-V materials, i.e. based on (Ga, Al, In, As, P). Negative differential conductivities were predicted in these systems, making them highly interesting for hyperfrequency applications, but experimental results were always far from theoretical predictions, which resulted in a loss of interest for these systems. Recently, there has been a lot of efforts devoted to Nitride semiconductors, based on (Al,Ga,In)N alloys. These materials have been regarded as potential candidates for high frequency, high power applications, since they both exhibit high electron saturation velocities and high thermal and chemical stability. Moreover, although they usually contain a lot of growth defects, device properties were surprisingly good with regards to defect densities. In this work, we have modeled double heterostructure tunnel diodes based on AlGaN/GaN system. The extremely high conduction band offset in these materials, along with the quite low dielectric constants, and built-in electric fields (originating from both the spontaneous and piezoelectric polarization), are extremely favorable parameters, which may renew the interest for such devices in this material system. It will be shown that the built-in electric field leads to a symmetric potential profile under a given external applied bias, which optimizes the transmission coefficient in the structure. We have calculated extremely high peak/valley ratios, which suggests that even mid quality samples could still exhibit interesting device properties.

This paper provides a brief summary of the structural and optical study of ZnO and ZnO:Mn materials fabricated from simple process: oxidation of ZnS and ZnS:Mn thin films grown by LPMOCVD. Both structural analysis and optical characterization have confirmed the good quality ZnO material formation. XRD results suggest that the fabricated ZnO film is composed of ∼30 nm ZnO nanocrystals with a hexagonal wurtzite structure. Raman spectra show a strong resonant multiphonon effect. In all cases, a UV emission is observed at ∼ 380 nm. A green emission is also observed in the ZnO nanocrystalline film. On the other hand, it is demonstrated that the Mn doping in ZnS film can dramatically affect the luminescence properties of end products: nanocrystalline ZnO thin films. The photoluminescence spectra of the films with an optimized Mn doping concentration only showed the ultraviolet emission, while the green emission was fully quenched. It is argued that the presence of the Mn can effectively passivate the electron and hole trapping sites at the surface and interface of the ZnO nanocrystals.

ZnO is a wide bandgap (3.2 eV) semiconductor with potential application in LEDs, lasers, and transparent transistors, among other uses. These applications require uniform thickness, high quality materials (amorphous, poly- or single crystal), pinhole- and defect-free-single-and multilayer-conformal coatings. These attributes are generally best achievable by MOCVD. We have mounted a significant effort to develop automated MOCVD systems and process technologies for single and multicomponent oxides. The reactors use high speed rotation and are of a vertical orientation built to all metal UHV standards. We have demonstrated reactor scaled performance from 3” to 12” diameter depositions planes with modeling scales through 24” diameter. Metalorganics are used for zinc and dopant sources as well as dopant gases to optimize performance at low pressures. In this paper we will discuss our most recent results with epitaxial ZnO films, achievements in p-type doping, multilayer structures, and polycrystalline doped ZnO films.

The chemical vapor deposition of icosahedral boron arsenide, B12As2, on 6H-SiC (0001) (on and off-axis) substrates was studied using hydrides as the reactants. The effects of temperature and reactant flow rates on the phases deposited and the crystal quality were determined. The growth rate increased with temperature from 1.5μm/h at 1100°C to 5 μm/h at 1400°C and decreased thereafter. X-ray diffraction revealed that the deposits were amorphous when the deposition temperature is below 1150° C. Above 1150°C, smooth B12As2 films were formed on 6H-SiC substrates with an orientation of (0001) B12As2 parallel to 6H-SiC (0001). Raman spectroscopy confirmed the strongly c-axis oriented nature of B12As2 film on 6H-SiC.

Ordinary and extraordinary indices of refraction, film thickness and waveguide mode information of zinc oxide, zinc oxide doped with nitrogen and zinc oxide doped with tellurium were measured by using a prism coupling waveguide technique. The films were grown on c-axis sapphire substrates by pulsed laser deposition (PLD). High accuracy waveguide measurements show that the ordinary and extraordinary indices of refraction of ZnO samples change with the introduction of nitrogen or tellurium. The densification of films with annealing could also be tracked with precision refractive index measurement. The crystal structure and the optical properties of the films were also characterized by using x-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM) and cathodoluminescence (CL).

A complete analysis of leakage currents in AlGaN/GaN Current Aperture Vertical Electron Transistors (CAVETs) with regrown aperture and source regions was carried out. The total observed leakage current was found to be a combination of both gate leakage and source leakage. Two paths for source leakage have been identified; electrons passing directly through the insulating layer to the drain region as well as electrons traveling through the aperture but underneath the 2DEG at the AlGaN/GaN interface. Source leakage through the insulating layer resulted from pits formed at the onset of regrowth, as the sample was heated to growth temperature, and was successfully eliminated by optimizing regrowth conditions. Source leakage underneath the 2DEG occurred when the unintentionally doped (UID) GaN layer above the insulating layer was not fully depleted and could be eliminated by reducing the thickness of the UID GaN layer. Gate leakage has been attributed to the enhanced incorporation of n-type impurities inside as well as above the aperture region during regrowth, resulting in a narrowing ofthegateSchottkybarrier.

This letter reports on multi-layer submicron epitaxial device structures grown by hydride vapor phase epitaxy (HVPE). This is the first demonstration of both high electron mobility transistor (HEMT) devices and ultraviolet light emitting diodes (LED) emitting in the wavelength range from 305 to 340 nm based on AlGaN/GaN and AlGaN/AlGaN heterostructures grown by HVPE. Two unique aspects of this technological approach are the growth of Al-containing epitaxial material by HVPE and use of HVPE to form submicron multi-layer epitaxial structures. The high performance of HVPE grown devices presented in this report demonstrates the significant potential that exists for HVPE growth technology for mass production of device epitaxial wafers.

In this work we report on solar blind (Al,Ga)N photovoltaic metal-semiconductor-metal (MSM) detectors with a cutoff wavelength as short as ∼270 nm. (Al,Ga)N heterostructures, that allow backside illumination, were grown on sapphire substrates by both metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). We observed that both the interdigitated electrodes and the contact areas contribute to the overall photocurrent. In order to avoid the parasitic current due to the contact pads, we developed a new process where the electrodes are deposited on the (Al,Ga)N surface whereas the contact pads are deposited on an insulator (dielectric) layer besides the electrodes. Some layers develop microcracks related to excess stress. In that case, we showed that both the dark current and the responsivity strongly depend on the crack density. By using our two-level process, we could reduce the parasitic effects of cracks on the dark current. Several dielectrics were tested and our results are reported; values of dark current < 10 fA have been measured at 10 V bias voltage. An extremely high performance can then be reached in these ultraviolet solar blind detectors.

An overview of silicon carbide (SiC) power device technology is given with an emphasis on processing issues and commercial applications. Schottky Barrier Diodes (SBDs) were the first to be made commercially available in 2001, with power switch and RF amplifiers soon to follow. This paper discusses the performance of current available rectifiers and published power switch development and identifies key issues in processing and device structures which have influenced past and will impact future SiC product development.

Single crystal AlGaN bulk materials have been fabricated, for the first time. AlGaN thick (up to 0.6 mm) layers were grown by hydride vapor phase epitaxy on SiC substrates. The substrates were removed resulting in free-standing AlGaN wafers up to 0.5 inch in diameter. Fabricated AlGaN wafers were investigated by x-ray diffraction, transmission electron microscopy (TEM), and cathodoluminescence. X-ray diffraction and TEM studies confirmed single crystal structure of grown material. Based on x-ray diffraction results, AlN concentration in grown material was estimated of about 35 mol.%. Cathodoluminescence measurements demonstrated a number of peaks in UV spectral region with the most intense luminescence at a wavelength of about 325 nm (100 K). The wafers demonstrated n-type conductivity with electron concentration in the 1017 cm-3 range at room temperature. Development of AlGaN substrates with controlled alloy composition may lead to stress-free device epitaxial structures for AlGaN-based transistors and UV emitters and sensors.

We have designed, fabricated and tested resonant cavity enhanced visible-blind AlGaN-based Schottky photodiodes. The bottom mirror of the resonant cavity was formed with a 20 pair AlN/AlGaN Bragg mirror. The devices were fabricated using a microwave compatible fabrication process. Au and indium-tin-oxide (ITO) thin films were used for Schottky contact formation. ITO and Au-Schottky devices exhibited resonant peaks with 0.153 A/W and 0.046 A/W responsivity values at 337 nm and 350 nm respectively. Temporal high-speed measurements at 357 nm resulted in fast pulse responses with pulse widths as short as 77 ps. The fastest UV detector had a 3-dB bandwidth of 780 MHz.

The current conduction in GaN is very topical and is the topic of a vast amount of research. By simultaneously mapping the topography and the current distribution, conductive atomic force microscopy (C-AFM) has the potential to establish a correlation between topological features and localized current paths. In this study, this technique was applied to image the conduction properties of as-grown and post-growth chemically etched samples GaN epitaxial layers on a microscopic scale. Our results show that prismatic planes have a significantly higher conductivity than the surrounding areas of the sample surface. A large and stable local current was mainly observed from the walls of the etched pits, under forward and reverse bias of the metallized AFM tip/semiconductor junction.