High-quality 4H-SiC has been epitaxially grown on (1120) substrates by chemical vapor deposition. The physical properties of epilayers and MOS interfaces on both (1120) and off-axis (0001) substrates are elucidated. An unintentionally doped 4H-SiC epilayer on (1120) shows a donor concentration of 1×1014 cm−3 with a total trap concentration as low as 3.8×1012 cm−3. Inversion-type planar MOSFETs fabricated on 4H-SiC (1120) exhibit a high channel mobility of 96 cm2/Vs. The channel mobility decreases according to the T−2.2 dependence above 200K, indicating reduced Coulomb scattering and/or electron trapping. The superior MOS interface on (1120) originates from the much lower interface state density near the conduction band edge.

Theoretical calculations are performed on the role of smooth interfaces in the Raman spectra of Si/3C-SiC superlattices. The dispersion relations were obtained using a linear chain model with the alloyed interface δ-(3C-SiC)0.5(Si)0.5 described in the virtual crystal approximation. A modified bond-polarizability model was used to calculate the Raman spectra. The main results are the enhancement of the Raman spectra and the appearance of new peaks in between those related to the Si quasi confined and the 3C-SiC-confined modes with increasing number δ?of interfacial monolayers (3C-SiC)0.5(Si)0.5. Some of the smooth interface related Raman peaks have intensity comparable with those of an abrupt Si/3C-SiC superlattice.

Growth of very thick 4H-SiC epilayers up to 215 μm has been demonstrated in a vertical radiant-heating reactor. Surface roughness is maintained as small as ˜0.2 nm even for epilayers over 150 μm in thickness, and a regular step structure without macro step bunching is observed from the very thick epilayers indicating a stable step-flow growth. Photoluminescence and secondary ion mass spectroscopy (SIMS) were performed for a 150 μm-thick epilayer. The Photoluminescence showed strong free excitons and comparatively small nitrogen bound excitons, while aluminum-, boron- and titanium-related lines were almost negligible. The SIMS analysis found no impurities exceeding 1 × 1014 cm−3. The influence of growth parameters on thickness uniformity will also be shown.

Ab initio calculations of the polar interfaces between thin films of titanium and cubic silicon-carbide (SiC) have been performed by using the first-principles molecular dynamics method. Stable configurations, adhesive energies and Schottky-barrier height (SBH) for the Si-terminated and the C-terminated interfaces are obtained. The C-terminated interface has covalent C-Ti bonds, while the Si-terminated interface has shown metallic nature. Adhesive energy/SBH of the C-terminated interface is larger/smaller than that of the Si-terminated one, respectively. In order to examine a conventional SBH model, work functions of SiC slab with Si and with C surface and Ti slab have been calculated and SBHs have been estimated from the difference of work functions. In estimated SBHs between the interfaces, the relationship depend on the crystal orientation as (111) and (001).

In the present work, we investigated sputtered titanium tungsten (TiW) contacts for Ohmic contacts to both n- and p-type 4H-SiC with long-term stability under high temperature (500°C).. Epitaxial layers with a doping concentration of 1.3×1019 and 6×1018 cm−3 were used. After high temperature annealing (>950°C) sputtered TiW contacts showed Ohmic behavior with good uniform distribution of the specific contact resistance. We obtained an average specific contact resistance (pc) of 4×10−5 Ωcm2 and 1.2 ∼ 1.7×10−4 Ωcm2 for p- and n-type, respectively from linear TLM measurement. We also found some variation of the specific contact resistance and the sheet resistance from our TLM measurement for p-type contacts. We will discuss this behavior with the measurement of SIMS. Long-term stability with a top-cap layer is also discussed

Using pure and alloyed silicon melt saturated with carbon we investigated systematically hollow defect elimination during SiC solution growth over a wide temperature range from 1500°C to 2100°C. In the process of solution growth all hollow defects present in a substrate demonstrate an evident tendency to act as growth centers and after an adequate period of treatment they were overgrown. Growth morphologies observed in the vicinity of hollow defects are rather different. A new visualization method is proposed, which reveals crystalline defects associated with hollow cores and allows to detect the quantity and the distribution of HD in a whole SiC wafer. Classification of hollow defects based on these observations is presented and the corresponding closing mechanisms are discussed.

While SiC devices are an attractive alternative to Si in high power applications, interface trap densities measured in SiC-based MOSFETs are significantly larger than in Si-based ones. Here, we study SiC MOSFETs using both spatial images and spectral analysis of light emission due to electron-hole recombination. The light emission is produced by alternately driving the channel between accumulation and inversion using what is essentially a charge-pumping set-up. Emission is due to interface trap and bulk electron-hole recombination. The spatial imaging studies suggest that recombination occurs at both interface traps and bulk defects. Spectral studies of the emission indicate the presence of a narrow band centered at 425 nm and a broad band extending from approximately 500 to 800nm. The former we suggest is due to bulk recombination and the latter to interface trap recombination. The spectral studies of the 500 to 800 nm band are timed to separate light emitted during the inversion-to-accumulation transition from that emitted during the accumulation-to-inversion transition and visa versa. Comparisons of the emission spectra collected during these specific periods are consistent with a larger Dit in the upper half of the bandgap than the lower half in both 4H and 6H devices.

This work deals with the structural, optical and chemical characterization of ion implanted and annealed crystalline SiC. N+ or Al+ ions with various MeV energy and fluence values were implanted at 300°C in 6H-SiC wafers. These processes produced about 1 μm thick doping profiles characterized by a sequence of peaks and valleys across an average plateau value. The annealing process was done in a radio frequency furnace at 1700°C for 30 min in high purity Ar atmosphere. The hot implanted layers show the presence of a weak damage amount, whose profile was estimated by confocal micro-Raman measurements. The annealed samples, in spite of a good crystal recovery, showed the formation of secondary defects as inferred by the Rutherford Back Scattering-Channeling spectrometry. The relative magnitude and position of peaks and valleys in the doping profiles were preserved by the high temperature annealing but the annealed doping profile was displaced towards the surface with respect to the as-implanted profile as shown by the Secondary Ion Mass Spectrometry measurements. No material loss but material densification due to the reordering of the crystalline structure can explain the trend of the chemical profiles.

Affordable, high quality SiC wafers are very desirable for a variety of new technologies including GaN based lighting, RF, and high-power electronics based on wide band gap materials. At Litton Airtron we have a major effort in the growth and characterization of SiC. We will present data on 35, 50 and 75-mm diameter crystals. We are growing both n-type, semiinsulating 4H, 6H, and 15R material. A variety of characterization techniques are being used at Litton Airtron to determine wafer quality. These include Raman microscopy, digital wafer photography, and crossed polarizer images. Raman spectroscopy is an excellent probe of polytype and carrier concentration for n-type materials; in addition it can be done at room temperature and is sufficiently fast that it can be used in an industrial environment. The use of digital photography allows for the collection of images that can be quantitatively analyzed and archived.

The effect of the seed surface finish on the dislocation density of sublimation grown silicon carbide was investigated. Growth on seeds that were polished down to 1 μm diamond paste resulted in the nucleation of threading screw dislocations in a density of 106 cm−2 and threading edge dislocations in densities of 107 cm−2. Following the mechanical polish of the seeds with a hydrogen etch or chemo-mechanical polish prior to growth resulted in the screw dislocation density decreasing by four orders of magnitude and the threading edge dislocation density dropping two orders of magnitude. Using the dislocations density and the hydrogen etch rate, the depth of damage in mechanically damaged seeds was determined to be between 400 and 1000 Å.

The status of SiC vapor growth technique (PVT) is reviewed and related innovative aspects are introduced. Problems of the preparation of SiC crystals with uniform electronic properties are addressed, especially the growth of semiinsulating SiC. An overview about the performance of numerical modeling is given as tool for the optimization of the PVT process. Development activities in the field of liquid phase processing for the preparation of SiC bulk crystals and micropipe healing are presented. Finally recent results on the present understanding of filamentary void formation/elimination (micropipes, macrodefects) are summarized.

This paper reviews the development of a multilayer, micromolding-based surface micromachining process for SiC microelectromechanical systems (MEMS). The micromolding process uses polysilicon and SiO2 thin films that are deposited onto polysilicon and SiO2 sacrificial layers, patterned into micromolds by reactive ion etching, filled with polycrystalline SiC (poly-SiC), planarized by mechanical polishing, and eventually dissolved and released in selective wet chemical etchants. In addition, a SiC lift-off technique that exploits the microstructural differences between SiC films deposited on Si, SiO2 and Si3N4 surfaces has been developed. The micromolding and lift-off techniques are being used as the basic patterning processes for a four-layer, poly-SiC surface micromachining process that we call the MUSiC (Multi-User SiC) process.

This paper describes the development of a nitrogen-based passivation technique for interface states near the conduction band edge [Dit(Ec)] in 4H-SiC/SiO2. These states have been observed and characterized in several laboratories for n- and p-SiC since their existence was first proposed by Schorner, et al. [1]. The origin of these states remains a point of discussion, but there is now general agreement that these states are largely responsible for the lower channel mobilities that are reported for n-channel, inversion mode 4H-SiC MOSFETs. Over the past year, much attention has been focused on finding methods by which these states can be passivated. The nitrogen passivation process that is described herein is based on post-oxidation, high temperature anneals in nitric oxide. An NO anneal at atmospheric pressure, 1175°C and 200–400sccm for 2hr reduces the interface state density at Ec-E ≅0.1eV in n-4H-SiC by more than one order of magnitude - from > 3×1013 to approximately 2×1012cm−2eV−1. Measurements for passivated MOSFETs yield effective channel mobilities of approximately 30–35cm2/V-s and low field mobilities of around 100cm2/V-s. These mobilities are the highest yet reported for MOSFETs fabricated with thermal oxides on standard 4H-SiC and represent a significant improvement compared to the single digit mobilities commonly reported for 4H inversion mode devices. The reduction in the interface state density is associated with the passivation of carbon cluster states that have energies near the conduction band edge. However, attempts to optimize the the passivation process for both dry and wet thermal oxides do not appear to reduce Dit(Ec) below about 2×1012cm−2eV−1 (compared to approximately 1010cm−2eV−1 for passivated Si/SiO2). This may be an indication that two types of interface states exist in the upper half of the SiC band gap – one type that is amenable to passivation by nitrogen and one that is not. Following NO passivation, the average breakdown field for dry oxides on p-4H-SiC is higher than the average field for wet oxides (7.6MV/cm compared to 7.1MV/cm at room temperature). However, both breakdown fields are lower than the average value of 8.2MV/cm measured for wet oxide layers that were not passivated. The lower breakdown fields can be attributed to donor-like states that appear near the valence band edge during passivation.

We present results of an oxidation study of a-Si1−xCx:H films prepared by the plasma enhanced chemical vapor deposition of silane and acetylene. The composition (i.e. x) of the samples was determined by the flow rates of silane and acetylene. Oxidation was carried out at 400 to 850°C in dry oxygen ambient. The infrared (IR) spectra of the as-prepared films showed the intensity of the Si-C peak decreases and the Si-CH3 peak increases as x increases. The Si-H peak shifts to higher frequency as x increases. Note that the incorporation of CH3 radicals in a-Si1−xCx:H films has shown to introduce voids and increased the porosity of the films. The IR spectra of the oxidized samples showed clear Si-O stretching and rocking/wagging modes for all films. We suggest that the growth of oxide on a-Si1−xCx:H is a result of voids that facilitate the diffusion of oxidants into the film. We shown that the activation energy, obtained from the linear rate region of the oxide growth, was far less than the dissociation energies of the Si-Si, Si-C and Si-H bonds. We suggest that this could be due to the amorphous nature of the samples that caused the various chemical bonds to be weaker during oxidation.

Single-crystal wafers of 6H-SiC were irradiated at 300 K with 2 MeV Au2+ ions over fluences ranging from 0.029 to 0.8 ions/nm2. The accumulated disorder on both the Si and C sublattices in the irradiated specimens has been studied in situ using 0.94 MeV D+ channeling along <0001>, <1102> and <1011> axes. At low doses, results show that some of the Si and C defects are well aligned with the <0001> axis with more C defects shielded by the <0001> atomic rows; a higher level of C disorder is observed, which is consistent with a smaller threshold displacement energy on the C sublattice. There is only a moderate recovery of disorder, produced at and below 0.058 Au2+/nm2, during the thermal annealing at 570 K; similar behavior is observed in the higher-dose samples annealed between 720 and 870 K. The results suggest the presence of defect clusters and amorphous domains formed during the Au2+ irradiation. Reordering processes at 570 K in the weakly damaged 6H-SiC (0.12 Au2+/nm2, 300 K) appear to occur closely along the <1011> direction.

The objective of this work is to study defect occurrence and appearance in thick sublimation epitaxial layers grown on top of LPE layers with different thickness and substrates with reduced micropipe density. Data from growth on C-terminated surfaces are also presented. The results were analyzed with the aid of optical microscopy, SEM images, HRXRD and synchrotron white beam X-ray topography. A pronounced effect of the LPE (buffer) layer surface roughness on formation of dislocations and micropipes is observed in the sublimation epitaxy layers. While with increasing the thickness of the buffer layer the efficiency of the micropipe reduction increases, the structure quality of the top sublimation epilayer degrades. LPE layers with a thickness of 0.1 μm appear to be the best compromise.

The development of a novel SiC crystal growth technique, generically described as High Temperature Chemical Vapor Deposition (HTCVD) is reviewed. The structural, optical and electrical properties of 4H-SiC semi-insulating substrates are investigated with the aim of providing optimal microwave device performances. In particular, alternative compensation mechanisms to vanadium doping in S.I substrates are investigated to eliminate substrate induced trapping effects. Carried out at temperatures above 2100°C, the HTCVD technique uses, as in CVD, gas precursors (silane and a hydrocarbon) as source materials. The growth process can be described as “Gas Fed Sublimation” and proceeds by the gas phase nucleation of Six-Cy clusters, followed by their sublimation into active species that are condensed on a seed. Crystals with diameters up to 45 mm have been obtained with growth rates of 0.6 mm/h. The use of specific process steps, such as in-situ seed surface preparation and micropipe closing are presented and high resistivity wafers with micropipe densities down to 10 cm−2 are demonstrated. 4H-SiC substrates prepared from undoped crystals (with vanadium concentration lower than 5×1014 cm−3) exhibit semi-insulating behavior with a room temperature resistivity of the order of 1010Ω?cm. Infrared absorption measurements show that two types of semi-insulating crystals can be grown, with a spectrum either dominated by the Si-vacancy, or by a previously unreported defect labeled UD-1. These two types of semi-insulating wafers are also differentiated by the temperature dependence of their resistivity, with activation energies of 0.85 and 1.4±0.1 eV, respectively, and by the stability of their resistivity upon an annealing at 1600°C. Initial MESFET devices processed on HTCVD grown substrates show better DC characteristics than devices processed on vanadium doped substrates.

We have systematically investigated the effect of C/B sequential implantation on the B-related acceptors and deep levels in 4H-SiC using thermal admittance spectroscopy. With increasing concentration of co-implanted C, the density of deep levels was found to start decreasing and to be completely suppressed for the C and B ratio of 1:1. Moreover, the density and ionization energy of B acceptors were seen to start increasing and lowering, respectively, with increasing C concentration. However, we found that excess C-content leads to the formation of a new complex defect. The C-V results also support the expected increase in the total hole concentration with increasing concentration of the co-implanted C-atoms, which is followed by a decrease in the concentration under C-rich condition. This is in reasonable agreements with the behavior of the B acceptors and deep defect levels. Therefore, the concentration of co-implanted C-atoms is considered to be very sensitive to the formation of the B acceptor levels.

We have observed electroluminescence from 4H-SiC Ni-Schottky diodes on 1015cm−3 nitrogen doped n-type epilayers. A high barrier Schottky contact will form an inversion layer close to it. This creates minority carriers that can be injected into the epi and recombine to emit light. The spectral composition and its temperature dependence have been investigated from liquid He temperatures to room temperature. Band edge luminescence, Al related luminescence and DI bound exciton have been observed. To study the electroluminescence from Schottky diodes provides an easy and additional technique for defect characterization of epitaxial layers.

Single crystal SiC is a wide band-gap semiconductor with material characteristics that make it quite suitable for high voltage and high current applications. However, these devices are currently limited by their passivation. Significant improvements have been made with oxides on SiC. The most notable oxide processes are the re-oxidation anneal, a stacked ONO dielectric, and nitridation using an NO or N2O anneal. Additional improvements in lateral MOSFET mobility have been achieved using a surface channel implant, and lower temperature implant activation anneals. However, the passivation remains a significant limitation for SiC power devices.