Metallurgical reactions between contacts and SiC can alter the electrical characteristics of the contacts, either beneficially or detrimentally. Simultaneously, consumption of the underlying SiC epilayer takes place. During prolonged operation at elevated temperature, contacts that are not in thermodynamic equilibrium with SiC may continue to react with it. For this reason, interest in thermally stable carbide and silicide contacts to SiC has been growing. To select appropriate carbides or silicides for further study, however, knowledge of the transition metal-silicon-carbon (TM-Si-C) phase equilibria is required. A significant body of literature on the TM-Si-C systems exists and should therefore be examined in the context of electronic applications. In this paper, phase equilibria for representative TM-Si-C systems are presented, trends in these systems with respect to temperature and position of the metal in the periodic table are discussed, and attractive carbide and silicide contacts and processing schemes for thermally stable contacts are highlighted.

Thin Zr films were deposited on natural single crystal diamond (100) substrates by ebeam evaporation in ultra-high vacuum (UHV). Before metal deposition the surfaces were cleaned by UHV anneals at either 500°C or 1150°C. Following either one of these treatments a positive electron affinity was determined by means of UV photoemission spectroscopy (UPS). Depositing 2Å of Zr induced a NEA on both surfaces. Field emission current - voltage measurements resulted in a threshold field (for a current of 0.1 µA) of 79 V/µm for positive electron affinity diamond surfaces and values as low as 20 V/µm for Zr on diamond.

We formed titanium Ohmic contacts to n-type 6H-SiC epitaxial layer byreducing the Schottky barrier heights. The barrier heights were reduced enough toform the Ohmic contacts by releasing the Fermi level from pinning through makingatomically-flat surfaces. The current transport by thermionic emission wasdominant at the Ti/SiC interface. Since the Ti contacts were formed without postannealing,surfaces of the Ti electrodes were flat and homogeneous maintaining asdepositedstructures. Contact resistivity was (6±1)×10−3 Ω-cm2, which is comparableto that of the annealed Ni contact formed on the SiC epitaxial layer with the samecarrier concentration.

Photoassisted dry etching of GaN in HC1 by 193 nm ArF excimer laser is developed as apotential alternative process to eliminate the ion damage and surface roughness which occur inetching techniques that involve an energetic ion beam impinging the surface. A directed stream ofHC1 etchant with background pressure of ∼ 5 × 10−4 Torr, sample surface temperature between 200 to 400°C, and laser fluence of 10 to 20 mJ/ pulse combine to produce etching. The photoassistedetching reaction under these process conditions is thermal in nature, with activation energy near 1.2kcal/ mol. Increases in laser fluence results in increase of etch rate, but the surface also becomesrougher. Distinct etch features can be produced with smooth surfaces at expense of etch rate.

Damage introduction in III-V nitrides during dry etching can be simulated by exposingthe samples to pure Ar plasmas for study of the physical (ion-bombardment) effects.Changes in conductivity of InN, In0.5Ga 0.5N and In0.5Al0.5N layers exposed to Ar plasmasunder both Electron Cyclotron Resonance and reactive ion etching conditions have beenmeasured as a function of rfpower, pressure and exposure time. The combination of highmicrowave and high rf powers produces large increases (10-_104 times) in sheet resistanceof the nitrides, but conditions more typical of real etching processes (rf power < 150W) donot change the bulk electrical properties. The nitrides are more resistant to damageintroduction than other III-V semiconductors. The removal of damage-related trapsoccurs with an activation energy of ∼2.7eV. High ion currents during ECR etching canproduce substantial conductivity changes, whereas the lower currents under RIEconditions do not affect the nitrides. It is difficult to avoid preferential loss of N in thenear-surface of these materials, which leads to leakage currents in rectifying metal contactsdeposited on these surfaces.

We have realized GaN wire gratings with periods down to 80 nm and with wire widths down to 26 nm- GaN layers of good structural and optical quality with thicknesses down to 100 nm were grown by electron cyclotron resonance assisted MBE (ECR-MBE). The grating structures were fabricated by high resolution electron beam lithography and electron cyclotron enhanced reactive ion etching (ECR-RIE) using CL2/Ar as etching gas. The optical properties of the GaN structures were investigated by photoluminescence (PL) spectroscopy in the temperature range between 40 K and 110 K. The wire patterns show intense excitonic photoluminescence and only a small dry etched induced degradation of the quantum efficiency even for the narrowest wires.

This novel technique involves the ultraviolet (UV) lamp-assisted photoelectrochemical etching of n-type 3C- and 6H-SiC for device processing. In order for this method to be effective, the UV light must be able to enhance the production of holes in the SiC during the etching process thus providing larger currents with light from the photocurrents generated than those currents with no light. Otherwise dark methods would be used as in the case of p-type 3C-SiC.

Experiments have shown that the I/V characteristics of the SiC-electrolyte interface reveal a minimum etch voltage of 3 V and 4 V for n- and p-type 3C-SiC, respectively. Hence, it is possible for etch-stops to occur. From Auger spectroscopy, an oxide formation is present on n-type 3C-SiC where after etching a yellowish layer corresponds to a low silicon to carbon (Si/C) ratio with large photocurrents and a white layer corresponds to a high Si/C ratio with small photocurrents. P-type 3C-SiC shows a grayish or silver layer with a high Si/C ratio and a green layer with a low Si/C ratio. Additionally, n-type 6H-SiC shows a brown or blue layer with a minimum etch voltage of 3 V.

The colors of the etched regions of SiC represent layers that have some degree of porousity formed by the electrochemical process. As a result, the photo-excitation allows control of the porousity and changes the electrical properties of SiC. The nature of these porous layers with its increased resistivity can lead to the formation of devices.

High dose Si has been implanted into MOCVD grown high resistivity and n-type GaN in the 26–500°C temperature range. The implant activation varies widely (30 −>100%) depending on, what energy level is assigned to the Si, the implantation and annealing temperatures, and the quality of the substrate. The usable maximum temperature for activation is limited by the severe decomposition of the GaN. After 1050°C 15s RTA Ga liquid droplet formation has been observed by SEM. This decomposition changes the surface morphology but did not introduce measurable change in the electrical properties up to 1150°C /120s RTA.

III-N photonic devices have made great advances in recent years following the demonstration of doping of GaN p-type with Mg and n-type with Si. However, the deep ionization energy level of Mg in GaN (∼160 meV) limits the ionized of acceptors at room temperature to less than 1.0% of the substitutional Mg. With this in mind, we used ion implantation to characterize the ionization level of Ca in GaN since Ca had been suggested by Strite [1] to be a shallow acceptor in GaN. Ca-implanted GaN converted from n-to-p type after a 1100°C activation anneal. Variable temperature Hall measurements give an ionization level at 169 meV. Although this level is equivalent to that of Mg, Ca-implantation may have advantages (shallower projected range and less straggle for a given energy) than Mg for electronic devices. In particular, we report the first GaN device using ion implantation doping. This is a GaN junction field effect transistor (JFET) which employed Ca-implantation. A 1.7 µm JFET had a transconductance of 7 mS/mm, a saturation current at 0 V gate bias of 33 mA/mm, a ft of 2.7 GHz, and a fmax of 9.4 GHz. 0-implantation was also studied and shown to create a shallow donor level (∼25 meV) that is similar to Si. SIMS profiles of as-implanted and annealed samples showed no measurable redistribution of either Ca or0inGaNat 1125°C.

We studied high dose implantation of tungsten into 6H-silicon carbide in order to synthesize an electrically conductive layer. Implantation was performed at 200 keV with a dose of 1×1017 W+cm−2 at temperatures of 90°C and 500°C. The samples were subsequently annealed either at 950°C or 1100°C. The influence of implantation and annealing temperatures on the reaction of W with SiC was investigated. Rutherford backscattering spectrometry (RBS), x-ray diffiraction (XRD) and Auger electron spectroscopy (AES) contributed to study the structure and composition of the implanted layer as well as the chemical state of the elements. The implantation temperature influences the depth distribution of C, Si and W as well as the damage production in SiC. The W depth profile exhibits a bimodal distribution for high temperature implantation and a customary gaussian distribution for room temperature implantation. Formation of tungsten carbide and silicide was observed in each sample already in the as-implanted state. Implantation at 90°C and annealing at 950°C lead to crystallization of W2C; tungsten silicide, however, remains amorphous. After implantation at 500°C and subsequent annealing at 11007deg;C crystalline W5Si3 forms, while tungsten carbide is amorphous.

Transmission electron microscopy has been applied to characterize the structure of Ti/Al and Ti/Al/Ni/Au ohmic contacts on n-type GaN (˜1017 cm−3 ) epitaxial layers. A thin polycrystalline cubic TiN layer epitaxially matched to the (0001) GaN surface was detected at the interface with the GaN substrate. This layer was studied in detail by electron diffraction and high resolution electron microscopy. The orientation relationship between the cubic TiN and the GaN was found to be: {111}TiN//{00.1}GaN, [110]TiN//[11.0]GaN, [112 ]TiN//[ 10.0]GaN. The formation of this cubic TiN layer results in an excess of N vacancies in the GaN close to the interface which is considered to be the reason for the low resistance of the contact.

Beta silicon carbide (3C-SiC) diodes have been fabricated using ion implantation as the selective doping technique. Previous work on 3C-SiC diodes have exhibited properties such as low reverse breakdown voltages and high ideality factors. Also, 6H and 4H SiC diodes have been reported. This paper studies a different procedure to produce better 3C-SiC diodes for use in the electronics industry. Current versus voltage, capacitance versus voltage and temperature versus voltage tests were conducted on the devices.

Isolation between devices is a prominent concern when building integrated circuits. Proton bombardment is the preferred planar process for forming isolation regions in gallium arsenide (GaAs) due to the lack of a stable native oxide. Hydrogen and boron in GaAs have exhibited good electrical isolation between devices. This paper investigates using proton bombardment to form isolation regions in 3C-SiC. Cubic SiC samples are implanted with a variety of implant doses, ranging from 1 × 1014 to 1 × 1015 ions / cm2 , and implant energies ranging from 150 to 300 keV. Hall measurement tests were performed to study the characteristics of the implanted material.

Experimental and numerical analysis have been performed on the sublimation growth process of SiC bulk crystals. Crystallographic, electrical and optical properties of the grown silicon carbide (SIC) crystals have been evaluated by various characterization techniques. Numerical models for the global simulation of SiC bulk growth including heat and mass transfer and chemical processes are applied and experimentally verified.

Gallium nitride (GaN) thick films (to 150 μm) have been deposited by hydride vapor phase epitaxy (HVPE). These films are unintentionally doped n-type (n = 1–2 × 1017 cm−3 at 300 K) and exhibit structural and electronic properties which are comparable with the best reported for GaN films grown by organometallic vapor phase epitaxy. Additionally, these properties are found to be uniform over 2-in diameter films grown on sapphire substrates. The use of either a GaCl or ZnO surface pretreatment has been found to substantially enhance the nucleation density, resulting in improved surface morphology and film properties, even though it appears that the ZnO film is thermochemically desorbed early on in the growth. Dislocation densities as low as ˜5×107 cm−2 have been attained for films 40 μtm thick. Homoepitaxial overgrowths both by electron-cyclotron-resonance plasma enhanced molecular beam epitaxy and OMVPE proceed in a straightforward manner, essentially replicating the defect structure of the HVPE GaN film.

A numerical model of an experimental gallium nitride horizontal vapor phase epitaxy reactor is presented. The model predicts the flow, concentration profiles, and growth rates. The effects of flowrate variation and geometry on the growth rate, growth uniformity and crystal quality were investigated. Numerical model predictions are compared to experimentally observed values. Parasitic gas phase reactions between group III and group V sources and deposition of material on the wall are shown to lead to reduced overall growth rates and inferior crystal quality. A low ammonia concentration is correlated to deposition of polycrystalline films. An optimum HVPE growth process requires selection of reactor geometry and operating conditions to minimize parasitic reactions and wall deposition while providing a uniform reactant distribution across the substrate.

Thermodynamic calculations were carried out on chloride transport vapor phase epitaxy of GaN using GaCl3/N2 and NH3/N2. At typical growth temperature and gas flow rates, both GaN formation and gas phase etching reactions of GaN are thermodynamically favored. Under thermodynamic equilibrium, most ammonia should decompose to nitrogen and hydrogen gases and gas phase etching of GaN occurs by HCI. From experimental measurements, less than 10% of the incoming ammonia decomposes and under this condition, GaN formation from GaCl3 /N2, and NH3 /N2 is thermodynamically favored. Higher V/III ratios give a larger driving force for GaN fromation. These calculations match our experimental results. Experimentally, we have optimized the growth conditions of GaN. High crystalline quality thick GaN films (10 ˜15μm) were grown on c-Al2O3. The GaN films show band edge emission dominated PL at both room temperature and 77 K. Only one set of diffraction peaks from (1012) planes with 60° spacing in the φ-scan of X-ray diffraction are observed. This indicates that the GaN films grown on c-Al2O3 are single crystalline. Typical growth rates were about 10 ˜ I5μm/hr and typical Hall mobility values of GaN films were in the range of 3 to 40 cm2/Vsec.

We report a structural analysis of GaN layers with thicknesses ranging from 10 μm to 250 μm which have been grown on sapphire substrates by halide vapor phase epitaxy (HVPE). The effect of growth rate during HVPE growth has also been examined. The growth was performed using GaCl and ammonia as reactants; growth rates in excess of 90 μm/hr have been achieved. The structural characteristics of these layers have been performed wit'i high resolution x-ray diffractometry. Longitudinal scans parallel to the GaN [0002] direction, transverse scans perpendicular to the [0002], and reciprocal space maps of the total diffracted intensity have been obtained from a variety of GaN layers. The transverse scans typically show broad rocking curves with peak breadths of several hundreds of arcseconds. In contrast, the longitudinal scans (or “θ/2θ scans”) which are sensitive only to strains in the GaN layers (and not their mosaic distributions) showed peak widths that were at least an order of magnitude smaller and in some cases were as narrow as 16 arcseconds. These results suggest that the defect structure of the GaN layers grown by HVPE is dominated by a dislocation-induced mosaic distribution, with the effects of strain in these materials being negligible in comparison.

The structural quality of GaN films grown by hydride vapor phase epitaxy (HVPE) was characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD), and atomic force microscopy (AFM). Films were grown up to 40μm on sapphire with either a GaC1 pretreatment prior to growth or on a ZnO buffer layer. Dislocation densities were found to decrease with increasing film thickness. This is attributed to the mixed nature of the defects present in the film which enabled dislocation annihilation. The thickest film had a defect density of 5×107 dislocations/cm2.

In this paper an overview is given on the current state of epitaxial growth of SiC with special regard to our work at SIEMENS CRD. Problems concerning impurity incorporation and ways to achieve background doping levels as low as 1014 cm−3 are discussed as well as the influence of high speed wafer rotation on the gas flow in our reactor and related effects on uniformity in thickness and doping. Precise control of the C/Si ratio in the gas phase, which is easily achieved in the described reactor, and the use of reduced pressure lead to a good control of dopant incorporation over more than 3 orders of magnitude while maintaining smooth surface morphology even at growth rates higher than 5 μm/h. Doping variations <±8 % across 30 mm wafers can routinely be obtained. The quality of the egilayers is proven by electrical breakdown fields as high as 2*106 V/cm at NA−ND=5* 10−15 cm−and an electron mobility greater than 700 cm2 /Vs at 300 K (4H-SiC).

Finally it is demonstrated that the gas composition at the end of the epitaxial growth process is an important step in order to get oxygen resistant surface properties for subsequent device processing.

We have deposited cubic BN films on silicon (100) using a novel ion-assisted RF-sputtering process. Our efforts over the past several years have enabled us to significantly reduce the ionenergy needed to form the cubic phase, with values now substantially less than 100 eV. Through a better understanding of the growth process we have been able to make improvements in film crystallinity and orientation, with an associated reduction of the high film stress which has severely limited film thickness in the past. The combination of low-energy nitrogen ions and high temperature growth has enabled us to deposit cubic BN films as thick as 1.9 μm. A nucleation study, performed using scanning force microscopy, shows that the cubic BN is nucleating as triangular crystallites, indicative of (111) growth. More specifically, the cubic BN appears to be nucleating on the edges of perpendicularly oriented hexagonal planes such that the cubic BN [ 111] is normal to the hexagonal BN [0002] (c-axis). The results suggest a pathway to “compliant” oriented growth on a variety of substrates.