4H-SiC was selectively etched in a Reactive Ion Etch (RIE) system using a nickel mask. The power, pressure, and electrode spacing were varied within a RF generated SF6:O2 (1:2) plasma. Peak etch rates of up to 2600 Aring;/min. were obtained at a pressure of 350 mT, power of 90 W (2 W/cm2), and electrode spacing of 3.180 cm. Etches were all residue-free, although power levels above 60 W (1.36 W/cm2) resulted in the SiC surface being roughened, which limited smooth surface etch capability to 2000 Aring;/min. When comparing electrode spacing from 3.180 cm to 1.270 cm, the 3.180 cm spacing was found to have the highest etch rate at pressures ranging from 250 mT to 500 mT.

An AlN (insulator) MIS Hydrogen Sensor was created using plasma source molecular beam epitaxy (PSMBE) deposition on Si (111) and 6H-SiC. A Pd layer was deposited on top of the AlN film via magnetron sputtering technique utilizing a hard mask. Pd was chosen since H2 readily diffuses within its bulk, thus Pd acts not only as a metal electrode of the MIS structure, but also as a catalyst for hydrogen dissociation. To optimize the design structure several sensors with different AlN and Pd thickness have been developed. RHEED and XRD measurements show that AlN film is epitaxial on both Si (111) and 6H-SiC substrates. The sensors were characterized using capacitance versus voltage C(V) and I(V) measurements, at different frequencies ranging from 1kHz to 1 MHz. Shifts in the C-V and I-V curves occurred with the introduction of hydrogen in the chamber. The temperature, hydrogen partial pressure, effects of oxygen and hydrocarbon gases, insulator and metal thicknesses on sensor response were analyzed.

Electron cyclotron resonance (ECR) excited N2 plasma exposure were performed to explore a possibility of a robust surface passivation on Ar+ ion beam etched GaN. We adopted Taguchi orthogonal design to optimize the ECR operating parameters with respect to reverse breakdown voltage (VB) and then used the results to determine the VB dependence on the parameters close to the optimum operating point. Rather consistent results were obtained by Taguchi and normal experimental methods. It is clearly indicated that the radio frequency (rf) chuck power play the most important role on the plasma passivation. After N2 plasma treatment, the diode reverse breakdown voltage is drastically restored from 15V to 95V. With the aid of N2 plasma exposure, the Schottky characteristics of ion beam etched GaN can be thus effectively improved.

This paper investigates the potential of Gallium Nitride (GaN) Gunn diodes for generating high power radiation at millimeter and sub-millimeter wave frequencies. Simulations were carried out based on an ensemble Monte Carlo computer model. This model accounts for thermal effects, series resistance, and device circuit interaction through the harmonic balance technique. The accuracy of the model has been validated in the case of InP Gunn devices at frequencies above 100 GHz[1]. Initially uniform and linearly graded doping profiles in the active region were considered. It is found that, similar to devices based on InP, the graded profile resulted in a much improved performance in terms of power, efficiency, and operating temperature. In particular, a GaN Gunn structure consisting of a 1 μm thick active region with a graded doping profile increasing from 6 × 1015 cm−3 at the cathode terminal to 4.6×1016cm−3 at the anode terminal yielded promising results. The DC bias voltage was estimated from the calculated velocity-electric field data to be about 25 V. With this bias, it was found that oscillations in the fundamental mode could be obtained over the frequency range from 220 GHz to 330 GHz subject to a load resistance of 1 Ohm and a maximum operating temperature of 800 K. The maximum output power was 30 mW at 290 GHz with a corresponding conversion efficiency of 0.25 %. Considerable improvement was obtained from a device with a heterojunction at the cathode. For a 1 μm device, an optimum power level close to 150 mW was predicted at 215 GHz with an efficiency of 2 %. This estimated power level is about an order of magnitude higher than what can be achieved from InP Gunn oscillators at the same frequency.

The arsenic dose dependence of electrical properties for implanted samples at 500°C and subsequently annealed at 1600°C for 30min has been investigated to derivate the activation energies of the arsenic donors in silicon carbide. Hall effect measurements were performed between 20K and 773K. Hall carrier concentration of implanted sample with high dose of 7×1015 cm−2 is independence of temperature, which indicates the formation of implanted layer with metallic conduction. For the sample with low dose of 1×1014 cm−2, the experimental Hall mobility varies directly as T3/2 below 80K and as T−3/2 above 150K. The activation energies of arsenic donors determined from the implanted sample with low dose using a least-squares fit of the charge neutrality equation are 66.8 meV for hexagonal site and 127.0 meV for cubic site, respectively.

The output characteristics of AlGaN/GaN semiconductor heterostructure field effect transistors are strongly dependent on the ambient temperature of the channel region due to relatively large variations in the electron drift velocity with temperature. A model to simulate the heat flow in AlGaN/GaN HFETs on sapphire and SiC substrates is presented. The non- linearity of the problem arising from the temperature dependencies of the III-nitride and substrate thermal conductivities is examined through the technique of self-consistent boundary conditions. It is found that the use of the linearizing Kirchhoff transformation is a good approximation for these systems. Alternative approaches of heatsinking the devices from the top of the wafer are explored also.

AlGaN/GaN is a promising system for high power electron devices. Quality of ohmic contacts is a critical parameter in determining the performance of the device. Although we have achieved a transfer resistance (Rc) of 0.35Δmm and ρc of 9.5×10−7 Δcm−2 the morphology and edge acuity of the contacts are poor. The standard ohmic contact recipes consist of a combination of Titanium and Aluminum with Nickel and/or Gold. This is annealed at 800°C-950°C [1-5]. In this work we study ohmic contacts on unintentionally doped Al0.3Ga0.7N/GaN system. We look at ratios of Ti/Al from 0 to 2 to determine which is the optimum ratio in terms of surface morphology and electrical characteristics. From our studies we conclude that morphology of a Ti/Al contact is good over a ratio of 0.3 and the contact resistance is minimized at a Ti/Al of 0.6. The ohmic contacts are improved electrically if a layer of gold is added on top. The best electrical contacts however were obtained with a four layer recipe of Ti/Al/Ti/Au, which gave contact resistance (Rc) around 0.45Δmm, but the morphology of the contacts was poor.

The initial stage of hydride vapor phase epitaxy GaN growth on ZnO-buffered sapphire is reported. A high supersaturation in the growth ambient was used to favor a rapid initial growth on the substrate. A subsequent step with high lateral growth rate was chosen to promote coalescence of the initial islands and provide optimal material properties. The specific mole fractions of the GaCl and NH3 control these vertical and lateral growth rates. The use of a two- step growth process in the GaN growth has led to improved and controlled morphology and high quality GaN materials have then been grown on sapphire substrate with and without ZnO buffer layers.

An enhancement-mode GaN metal-insulator-semiconductor field-effect transistor was successfully fabricated on a GaN/AlGaN/GaN double heterojunction structure with SiO2 as insulator layer. The enhancement mode DC characteristics have been achieved in the device with a gate length of 6 μm and a gate width of 100 μm. The device exhibited a DC transconductance of 0.6 mS/mm and a maximum drain-source current of 5 mA/mm. The gate leakage current is lower than 10−6 A at a bias of -10 V and the gate breakdown voltage is higher than 20 V. The channel stands a good chance of forming by hole accumulation between the top GaN layer and the AlGaN layer. The p-channel can be attributed to the presence of a piezoelectric field in the heterojunction, and the strongly asymmetric band bending and carriers distribution induced by the piezoelectric field. High-frequency capacitance-voltage measurement also gives a circumstantial evidence of the presence of a p-channel in the device structure.

To reduce the defect density inherent in conventional heteroepitaxial growth of SiC on Si, selective epitaxy followed by lateral epitaxial growth was performed in a conventional atmospheric pressure chemical vapor deposition (APCVD) system. The source gas was primarily hexamethyldisilane (HMDS). Hydrogen was used as the carrier gas and small amounts of hydrogen chloride (HCl) were added to improve the selectivity. Si(001) wafers, with an oxide layer (∼ 700 nm thick) as a mask, were used as substrates. The grown films were analyzed using optical microscopy and scanning electron microscopy (SEM). In earlier work, we had demonstrated the problems associated with the application of this technique – viz., oxide degradation and high growth temperature. Using HMDS, the growth temperature has been considerably reduced allowing the continued use of an oxide mask. Selective growth was demonstrated in films grown at 1250° and below.

Photoconductive detectors were fabricated on autodoped n-GaN films, with resistivity varying from 10 Ohm-cm to 107Ohm-cm, by molecular beam epitaxy. The mobility-lifetime product, determined from the measurement of photoconductive gain, was found to decrease monotonically from 10−2 cm2/V to 10−7 cm2/V as the dark resistivity was increased. This variation in the ντ products is attributed to changes in photocarrier lifetimes. In order to understand the recombination mechanisms responsible for this photoconductive behavior, the dependence of photoconductivity on excitation intensity (δμσ fλ) was investigated. The exponent λ was found to vary from 0.5 to 1.0, as the dark resistivity of the films increased. These results indicate the presence of exponential band tails extending from the conduction band edge. Furthermore, the dependence of photoconductivity on dark resistivity indicates that the photoconductive response is governed primarily by the location of the dark fermi level. A model accounting for these observations is presented.

We investigated the optical, electrical and structural properties of the layer which was implanted with sulfur ion(S+) in 4H-SiC. By using the high temperature ion implantation technique more less residual defects were observed compared with the room temperature ion implantation by Rutherford backscattering spectrometry and channeling(RBS-channeling). After annealing at 1700°C there was no significant difference between the implanted sample and virgin sample in crystallinity within the detection limit of RBS-channeling. From the result of low temperature photoluminescence(LTPL) we could see the photoluminescences, so-called D1 and D2center, originating in the defects formed by ion implantation and post-annealing(∼1700°C) processes and confirmed that their intensities decreased with the increasing of the total dose of S+. The result of Hall effect measurement suggested that the conduction type of S+-implanted layer is n-type and their activation energies were 275meV and 410meV by the fitting of neutrality equation assuming the two activation energies for the hexagonal and cubic lattice sites in 4H-SiC.

Reactive ion etching (RIE) of silicon carbide (SiC) to depths ranging from 10 μm to more than 100 μm is required for the fabrication of SiC power electronics and SiC MEMS. A deep RIE process using an inductively coupled plasma (ICP) etch system has been developed which provides anisotropic etch profiles and smooth etched surfaces, a high rate (3000 Å/min), and a high selectivity (80:1) to the etch mask. An etch depth of 100 μm is demonstrated.

Pendeo Epitaxy is a type of Lateral Epitaxial Overgrowth (LEO) that instead of using a dielectric buffer layer, uses an etched substrate to grow laterally without an interface layer. We report the first successful growth of 3C-SiC on Si using Pendeo epitaxy. Rectangular stripes of 3C-SiC on (100) Si substrates were fabricated, along both the [110] and [100] directions. Pendeo epi was only observed for columns parallel to [001], indicating a preferred growth facet for Pendeo epi of 3C-SiC on Si. SEM and TEM investigations were performed to assess the material quality of the Pendeo 3C-SiC material. Films were grown for 60 min at 1310°C and film coalescence was achieved without evidence of voids where the growth fronts joined. TEM data indicate not only the growth of vertical and lateral 3C-SiC on the 3C-SiC seed layer but direct nucleation of 3C-SiC on the exposed Si columns side wall and trench bottom, despite the lack of a carbonization procedure. The quality of the Pendeo 3C-SiC film appears to be of high quality indicating that Pendeo epi of 3C-SiC on low-cost, large-diameter Si substrates may prove to be a cost effective way to grow device-grade SiC layers on Si substrates for device applications.

Lattice polarities and film qualities of GaN grown by rf-MBE were investigated concentrating on the use of different buffer layer processes at the initial stage. Direct clarifying by coaxial impact collision ion scattering spectra technique, together with RHEED and chemical wet etching, were applied to identify the lattice polarity of GaN films. XRD rocking curve and photoluminescence results showed that the qualities of GaN films with Ga-polarity were dramatic improved compared to those with N-polarity. Hall effect measurement results indicated that the mobility of the Ga-face film was increased to one order higher (568 cm2/Vs in maximum at room temperature) than that of N-face one.

Current collapse effects in an Al0.25Ga0.75N/GaN HFET have been investigated under pulsed bias conditions, and a detailed investigation of current responses to changes in drain or gate bias voltage (drain-lag and gate-lag, respectively) has been performed. Three components of transient current response to changes in drain and gate bias voltages are distinguished. Surface treatment using KOH etching and the influence of pulsed bias conditions on threshold voltage are investigated to explore the origins of traps associated with each current transient component.

GaN and Al0.25Ga0.75N lateral Schottky rectifiers were fabricated either with (GaN) or without (AlGaN) edge termination. The reverse breakdown voltage VB (3.1 kV for GaN with both p+ guard rings and metal overlap edge terminations; 4.3 kV for Al0.25Ga0.75N without edge termination) displayed a negative temperature coefficient of −6.0 ± 0.4 V·K−1 for both types of rectifiers. The reverse current originated from contact periphery leakage at moderate bias, while the forward turn-on voltage at a current density of 100A·Cm−2 was ∼5 V for GaN and ∼7.5 V for AlGaN. The on-state resistances, RON, were 0.13 Δcm2 for GaN and 2.3 Δcm2 for AlGaN, producing figures-or-merit (VRB)2/RON of 73.9 and 8.2 MW.Cm−2, respectively. The activation energy of the reverse leakage was 0.13 eV at moderate bias.

We report a comprehensive study on the optical properties of GaN- and AlGaN-based lasing structures at high-levels of optical excitation (carrier densities of 1017−1020 cm−3) and identify critical issues necessary for the development of near- and deep-UV light emitting devices. We successfully achieved room temperature stimulated emission (SE) with emission wavelengths ranging from 351 nm to 373 nm in a variety of samples. Through an analysis of the temperature-dependent lasing characteristics, combined with absorption and time-resolved photoluminescence measurements, we estimated the carrier density required to achieve the SE threshold in GaN epilayers. We found that in AlGaN epilayers, the onset of SE (∼1019 cm−3) occurs at carrier densities one order of magnitude higher than in thick GaN epilayers, indicating that an electron-hole plasma is the dominant gain mechanism over the entire temperature range studied (10 K to 300 K). A remarkably low lasing threshold was observed in GaN/AlGaN heterostructures over the temperature range of 10 K to 300 K. Our experimental results indicate that GaN/AlGaN heterostructures could be used to efficiently generate laser emission with wavelengths shorter than 373 nm. The implications of this study on the development of UV laser diodes is discussed.

The fabrication of a wide variety of GaN-based photonic and electronic devices depends on dry etching, which typically requires ion-assisted removal of the substrate material. Under conditions of both high plasma flux and energetic ion bombardment, GaN etch rates greater than 0.5 νm/min and anisotropic etch profiles are readily achieved in Inductively Coupled Plasma (ICP) etch systems. Unfortunately, under these conditions plasma-induced damage often occurs. Attempts to minimize such damage by reducing the ion energy or increasing the chemical activity in the plasma often result in a loss of etch rate or profile control which can limit dimensional control and reduce the utility of the process for device applications requiring anisotropic etch profiles. It is therefore necessary to develop plasma etch processes which couple anisotropy for critical dimension and sidewall profile control and high etch rates with low-damage for optimum device performance. In this study we report changes in source resistance, reverse breakdown voltage, transconductance, and drain saturation current for GaN MESFET structures exposed to an Ar ICP plasma. In general, device performance was sensitive to ion bombardment energy and ion flux.

Single crystal silicon carbide (SiC) has such excellent physical, chemical, and electronic properties that SiC based semiconductor electronics can operate at temperatures in excess of 600°C well beyond the high temperature limit for Si based semiconductor devices. SiC semiconductor devices have been demonstrated to be operable at temperatures as high as 600°C, but only in a probe-station environment partially because suitable packaging technology for high temperature (500°C and beyond) devices is still in development. One of the core technologies necessary for high temperature electronic packaging is semiconductor die-attach with low and stable electrical resistance. This paper discusses a low resistance die-attach method and the results of testing carried out at both room temperature and 500°C in air. A 1 mm2 SiC Schottky diode die was attached to aluminum nitride (AlN) and 96% pure alumina ceramic substrates using precious metal based thick-film material. The attached test die using this scheme survived both electronically and mechanically performance and stability tests at 500°C in oxidizing environment of air for 550 hours. The upper limit of electrical resistance of the die-attach interface estimated by forward I-V curves of an attached diode before and during heat treatment indicated stable and low attach-resistance at both room-temperature and 500°C over the entire 550 hours test period. The future durability tests are also discussed.