AlN films with thicknesses from 53 to 79 nm were deposited on 6H-SiC substrates via Plasma Source Molecular Beam Epitaxy (PSMBE). The influence of deposition temperature on the growth mode and film roughness was assessed. The optical constants of the films in the range 0.73-8.75 eV were determined using spectroscopic ellipsometry. Pt/AlN/6H-SiC MIS structures were created and current-voltage (I-V) and capacitance-voltage (C-V) measurements were performed at room temperature and at 250°C. Most of the MIS structures showed rectifying I-V characteristics regardless of growth temperature. A 120-nm-thick AlN film was deposited at 500°C. MIS structures created on this film showed a very low leakage current densities of 6×10−8 A/cm2. The dielectric constant of the film was estimated at approximately 9. The relation between film structure and electrical properties of the films is discussed.

We present Monte Carlo based calculations of transport parameters useful in the simulation of III-nitride and SiC based devices. The calculations are performed using a full band ensemble Monte Carlo model that includes numerical formulations of the phonon scattering rates and impact ionization transition rates. Calculations are made for the wurtzite and zincblende phases of GaN, the wurtzite phase of InN, and the 3C (cubic) and 4H phases of SiC. The basic transport parameters determined are saturation drift velocity, and the ionization coefficients as a function of applied electric field. Results from the various materials are finally compared.

We report on the dc performance of the first GaN pnp bipolar junction transistor. The structure was grown by MOCVD on c-plane sapphire substrates and mesas formed by low damage Inductively Coupled Plasma etching with a Cl2/Ar chemistry. The dc characteristics were measured up to VBC of 65 V in the common base mode and at temperatures up to 250°C. Under all conditions, IC ∼ IE indicated higher emitter injection efficiency. The offset voltage was ≤ 2 V and the devices were operated up to power densities of 13.9 kW·cm−2.

Based on an extended electrothermal drift-diffusion model formulated within the framework of phenomenological transport theory, a consistent set of material parameters for 4H- and 6H-SiC is presented. Furthermore we report on detailed numerical studies of the coupled effect between transient impurity kinetics and impact ionization, which alters the reverse blocking characteristics of power devices under short switching conditions.

Co on AlGaN is expected to form a large barrier Schottky contact due to its high work function. We have used this material combination with 18 % of Al in AlxGaN for the study of transient photoresponse in the photovoltaic mode and in secondary photocurrent measurements after pulsed laser excitation. In reverse bias and in short- circuit mode a fast decay with a characteristic time of a few microseconds is dominant at room temperature. This mode is appropriate for UV detector operation. At elevated temperature, a much slower tail extending to several milliseconds is also observed. In forward bias operation the slow tail is dominating at any temperature. We discuss this asymmetry with respect to fast minority carrier collection within the space charge region for primary photocurrents and the slower majority carrier transport in forward bias.

We characterized oxides formed directly on n-GaN surfaces. The methods used for oxide layer formation were both photoanodic oxidation and thermal oxidation. The photoanodic oxidation took place in aqueous solutions of potassium hydroxide with pH values lower than 13. Homogenous oxide films were obtained in the voltage range from -0.6 V to 0.4 V vs the saturated calomel electrode (SCE). The characterization of the oxide layers was performed primarily by Auger electron spectroscopy (AES). First the surface chemistry was determined, proving that Ga-oxide is formed with an attributed stoichiometry of Ga2O3. Secondly, depth profiling shows the oxide thickness to be dependent on the photoanodic voltage and oxidation time. Complementary X-ray diffraction (XRD) studies suggest an amorphous state of the formed layers. Annealing GaN in O2-atmospheres above 900°C also lead to surfaces fully covered with gallium oxide. We found that N-polar surfaces oxidize faster than Ga-polar surfaces, which is in agreement to the theoretical work of Zywietz et al [1]. Furthermore, we report on the electrical properties of the anodized oxide layers by analyzing MOS structures.

This paper describes the characteristics of a variety of AlGaN-based photodetectors (photoconductors, metal-semiconductor-metal photodiodes, Schottky photodiodes and p-i-n photodiodes), grown on sapphire by metalorganic vapor phase epitaxy. The features that determine their performance are analyzed. Results using other substrates (epitaxial lateral overgrown GaN, Si) are also presented.

AlGaN High Electron Mobility Transistors (HEMTs) have made great progress for solid state power amplifiers with the demonstration of an X-band power density up to 9.8 W/mm. This high power density is the result of the high current and voltage capability of this material system. Recently, it has also been shown that these devices can achieve low microwave added noise figures (NF = 0.6 dB at 10 GHz) while maintaining a large breakdown voltage (>60 V) and hence a large dynamic range. These results imply that AlGaN HEMTs can be used to perform the active transmit and receive functions in more robust, higher dynamic range modules. In this paper, the progress in AlGaN microwave HEMTs is reviewed and the issues related to AlGaN transmit and receive (T/R) modules are described.

In this study thin AlxGa1−xN nucleation layers on sapphire were patterned and overgrown by plasma-induced molecular beam epitaxy (PIMBE) and metalorganic chemical vapor deposition (MOCVD) to obtain adjacent regions of GaN and AlGaN/GaN heterostructures with different polarities. The role of polarity on the structural and electrical properties of epitaxial layers and AlGaN/GaN heterostructures was investigated for samples grown on patterned AlN or GaN nucleation layers. Epitaxial GaN and AlGaN/GaN heterostructures grown on Al-face AlN or N- face GaN nucleation layers were found to be Ga-face or N-face, respectively, independent of the technique used for the overgrowth.

Together with sapphire, SiC is the most common substrate material for GaN epitaxial growth. In fact, SiC has advantages over sapphire because of its better thermal conductivity and lower film substrate lattice mismatch (∼3.5%). However, nucleation of GaN on SiC is rather difficult because of the low surface energy of SiC and the sensitivity of substrate preparation. This latter point makes it essential to use a very careful cleaning step, and also to pre-treat the substrate surface by growing a thick buffer layer of AlN at a relatively high temperature. In this study, several pre-treatment steps of SiC for GaN deposition were tested including (a) nitration with NH3 for 0.5-20 minutes, (b) pre-adsorption of trimethyl gallium (TMG) or trimethyl aluminum (TMA) for 0.5-5 minutes, and (c) deposition of an AlN buffer layer at ∼1150°C. After each pre-treatment, GaN was deposited by MOCVD using dilute H2(Ar+12%H2), NH3 and TMG. All the films were characterized by XRD and cross-sectional TEM. After nitration of SiC, the deposited GaN film was found to be polycrystalline. In case of pre-adsorption of TMG, epitaxial but island-like GaN formed on the substrate. In the third case, with an ultra-thin (∼1.5nm) coverage of AlN on SiC (by pre-adsorption of TMA or by 50 seconds deposition of AlN), GaN epilayers were successfully deposited on SiC. However, when AlN was deposited for longer than 3 minutes (up to 10 minutes), only polycrystalline GaN was obtained. With this technique of covering the surface with an ultra-thin layer of AlN, epitaxial GaN has been successfully deposited on 6H-SiC (0001), on 4H-SiC(0001), and on 3C-SiC/Si(111) substrates. The effect of the different pre-treatments of SiC on the quality of the deposited GaN films will be discussed and compared, and the optimal conditions for GaN deposition for each substrate will be presented.

The use of standard bulk semiconductor crystal growth processes for the production of GaN is prohibited by both the high melt temperature of GaN and thermal decomposition of the compound into Ga metal and N2 gas. We have employed a novel hydrostatic pressure system to grow GaN crystals. A high temperature, ultra-high pressure process was developed using a solid-phase nitrogen source to form GaN crystals in a Ga metal melt. Using a thermal gradient diffusion process, in which nitrogen dissolves in the high temperature region of the metal melt and diffuses to the lower temperature, lower solubility region, high quality crystals up to ∼1 mm in size were formed, as determined by SEM, X-ray diffraction and micro-Raman analysis.

Results are reported for the passivation of interface states near the conduction band edge in n-4H-SiC using post-oxidation anneals in nitric oxide, ammonia and forming gas (N2/5%H2). Anneals in nitric oxide and ammonia reduce the interface state density significantly, while forming gas anneals are largely ineffective. Results suggest that interface states in SiO2/SiC and SiO2/Si have different origins, and a model is described for interface state passivation by nitrogen in the SiO2/SiC system. The inversion channel mobility of 4H-SiC MOSFETs increases with the NO annealing.

Recent advances in developing process modules for GaN power devices are reviewed. These processes include damage removal in dry etched n- and p-GaN, implant doping and isolation, novel gate dielectrics, improved Schottky and ohmic contacts and deep via etching of SiC for hybrid GaN/SiC structures.

The effects of Ar+ ion beam etching (IBE) of AlGaN/GaN heterostructures and GaN/InGaN/GaN quantum well structures were investigated dependent on different ion incidence angles. The AlGaN/GaN heterostructure was measured before and after etching with respect to mobility and sheet resistance. The InGaN quantum well structure was measured with PL to determine the PL intensity and the energy shift, respectively. This experiments show that ion channeling is a significant defect generation phenomena in group- III nitrides at vertical ion incidence angle and can be minimized by tilting the sample against the ion beam.

Epitaxial TiC Ohmic and Schottky contacts to 4H-SiC were formed by a new deposition method, UHV co-evaporation with Ti and C60, at low temperature (< 500°C). We achieved a contact resistivity of 2 × 10−5Δcm2 at 25°C for as deposited Ohmic contacts on Al ion implanted 4H-Silicon carbide. The rectifying behavior of TiC Schottky contacts was also investigated using I-V and C-V. The measured Schottky barrier height (SBH) was 1.26 eV for n-type and 1.65 eV for p-type 4H-SiC using C-V measurements for frequencies ranging from 1kHz to 1MHz. LEED, RBS, XPS, and XRD measurements were performed to analyze composition ratio, interface reaction, and structural properties of the TiC epitaxial layer.

Two dimensional hole and electron gases in wurtzite GaN/AlxGa1-xN/GaN heterostructures are induced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness and doping. We have investigated the structural quality, the carrier concentration profiles and electrical transport properties by a combination of high resolution x- ray diffraction, Hall effect and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail

We achieved high hole concentrations above 1019 cm−3 at room temperature in Mg-doped InxGa1-xN/GaN (4 nm/4 nm) superlattices grown by metalorganic vapor phase epitaxy. The hole concentrations for the InxGa1-xN/GaN superlattices increased with the In mole fraction, and the maximum hole concentration reached 2.8 Å 1019 cm−3 for the In0.22Ga0.78N/GaN superlattice. The hole concentrations for the superlattices strongly depend on the structural parameters of the superlattices. The band bending due to the strain-induced piezoelectric field and the valence band structures of the InGaN/GaN heterostructures affect the hole generation in the superlattices. The weak temperature dependence of the resistivities for the InGaN/GaN superlattices with higher In mole fractions indicates highly efficient hole generation in the superlattice.

AlN layers were grown on 6H-SiC(0001) by molecular beam epitaxy using ammonia as the nitrogen source. Clean (√3×√3)R30° SiC surfaces was prepared by in-situ annealing alone and also by in situ annealing consisted of followed by Si deposition and subsequent annealing. The surface morphology of the AlN films observed by AFM was significantly changed by the nucleation procedure. When the AlN growth was initiated with Al flux exposure on a SiC surface prepared by thermal annealing, the surface roughness of the AlN was significantly reduced. Two-dimensional growth of AlN was observed with reflection high-energy electron diffraction from the very beginning. Atomically flat AlN surfaces with a RMS-roughness of ∼0.3 nm were obtained. On the other hand, when film growth was initiated with an ammonia flux exposure on a Si rich SiC surface, a high density of bumps was observed. The bumps seemed to originate from SiNx formation at the heteroepitaxial interface. It was found that control of the Si composition and the V/III ratio at the growth interface is crucial for the AlN film quality.

The breakdown voltages of 6H- and 4H-SiC rectifiers as function of temperature were modeled analytically in both non-reachthrough diode and reachthrough diode. The breakdown voltage was derived by the ionization integral employing accurate hole impact ionization coefficient. The breakdown voltage of SiC rectifiers was increased with increasing temperature and the positive temperature coefficient of breakdown voltage indicates that SiC rectifiers are suitable for high temperature applications. The breakdown voltages of both 6H- and 4H-SiC diodes were increased by M(T)-1/4 in NRDs and M(T)-1/8 in RDs.

Novel Ni/WSi/Ti/Pt composite Ohmic contacts to n-SiC were investigated as a function of annealing temperatures up to 1000°C. The onset of Ohmic behavior occurred at annealing temperatures of 900°C. Annealing at temperatures between 950°and 1000°C yielded excellent Ohmic behavior. At these temperatures the contact-SiC interface was smooth, defect free and characterized by a narrow Ni2Si reaction region. The annealed contacts possessed atomically smooth surface morphologies and exhibited minimal contact expansion. The residual carbon, resultant from SiC decomposition, was constrained by reaction with the WSi and Ti metallization layers forming carbide phases of W and Ti. The locations of the carbide phases were spatially distant from the metal semiconductor interface. Our results demonstrate that the Ni/WSi/Ti/Pt composite Ohmic contact maintains the desirable electrical properties associated with Ni contacts and possess excellent interfacial, compositional and surface properties which are required for reliable high power and high temperature device operation.