P-type 4H Research Articles

P-type 4H and 6H-SiC high-voltage Schottky barrier diodes

High-voltage Schottky barrier diodes have been successfully fabricated for the first time on p-type 4H- and 6H-SiC using Ti as the barrier metal. Good rectification was confirmed at temperatures as high as 250/spl deg/C. The barrier heights were estimated to be 1.8-2.0 eV for 6H-SiC and 1.1-1.5 eV for 4H-SiC at room temperature using both I-V and C-V measurements. The specific on resistance (R/sub on,sp/) for 4H- and 6H-SiC were found to be 25 m/spl Omega/ cm/sup -2/ and 70 m/spl Omega/ cm/sup -2/ at room temperature. A monotonic decrease in resistance occurs with increasing temperature for both polytypes due to increased ionization of dopants. An analytical model is presented to explain the decrease of R/sub on,sp/ with temperature for both 4H and 6H-SiC which fits the experimental data. Critical electric field strength for breakdown was extracted for the first time in both p-type 4H and 6H-SiC using the breakdown voltage and was found to be 2.9/spl times/10/sup 6/ V/cm and 3.3/spl times/10/sup 6/ V/cm, respectively. The breakdown voltage remained fairly constant with temperature for 4H-SiC while it was found to decrease with temperature for 6H-SiC.

Effect Of No On The Electrical Characteristics Of SiO2 Grown On P-Type 4H SiC

ABSTRACTIn this paper the effect of NO nitridation on the electrical characteristics of the oxides grown on p-type 4H SiC has been investigated. The results show that NO nitrided oxide has a lower interfacial trap density and a lower net oxide charge compared to N2 annealed oxides.

Compensation implants in 6H–SiC

In this work, we have performed Si and C isoelectronic implantations in n-type and vanadium (V) implantations in p-type 6H–SiC to obtain highly resistive regions. The compensation is achieved by the lattice damage created by the Si and C implantations and the chemically active nature of the V implant. For the Si and C implantations, the as-implanted resistivity initially increased with increasing implant fluence due to the introduction of compensating levels caused by the lattice damage, then decreased at higher fluences due to hopping conduction of the trapped carriers. The resistivity of the Si and C implanted material has been measured after isochronal heat treatments over the temperature range of 400–1000 °C. The maximum resistivity values measured for Si and C implanted and heat treated material were ∼1012 Ω cm. For the 700 °C V implantation in p-type SiC, resistivities of >1012 Ω cm were measured after 1500 or 1600 °C annealing to activate the V implant. Redistribution of the V implant is observed after annealing.

Thin oxide growth on 6H-silicon carbide

A marked difference is observed for the initial oxidation rates of 6HSiC (silicon face) obtained with oxygen and argon as carrier gases in a wet oxidation process. A modified Deal-Grove model with surface and bulk reaction constants is presented to model the early oxidation process at the SiO 2SiC interface in the case of oxygen as the carrier gas. In the early phase of oxidation there is rapid growth with the oxygen carrier gas, in contrast with the argon carrier gas. This is a surprising result, since silicon oxidation does not demonstrate a dependence on the carrier gas. After a time of approximately 150 minutes, a linear growth rate of 1.2Ȧ/min. occurs for both carrier gases. Thermal oxides on p-type (doping=4×10 16/cm 3) 6HSiC using oxygen and argon as the carrier gases have interface trap densities (D it) of 2.5×10 11/cm 2eV and 5×10 11/cm 2eV respectively. Fixed charge (Q f ) was determined to be 4×10 11/cm 2 and 8×10 11/cm 2 for the oxygen and argon carrier gas case respectively. Electrical breakdown measurements indicated oxide breakdown field strengths on fabricated MOS capacitors to be 8.5 MV/cm.

Surface barrier height in metal-SiC structures of 6H, 4H and 3C polytypes

We have studied metal-silicon carbide barrier height for various structures. Material of both n- and p-type conductivity 3C, 6H and 4H varieties have been used with Mo or Au Schottky barriers. In some cases, we observe a dependence of the barrier height on the uncompensated impurity concentration which cannot be explained in terms of the simple image force barrier lowering. It demands to take into account the total charge balance at the metal-semiconductor interface. The analysis of these data, together with other data on the surface barrier height, gives useful information for the development of high temperature devices.

Electrical properties of metal-diamond-like-nanocomposite (Me-DLN) contacts to 6H SiC

We have fabricated tungsten-diamond-like-nanocomposite (W-DLN) Schottky contacts on n-type and p-type 6H SiC (Si-face). The as-deposited n-type and p-type contacts are rectifying and measurement results suggest that the electrical characteristics are dominated by the properties of the tungsten SiC interface. The n-type contacts have a reverse leakage current density of 4.1 × 10−3 Acm−2 and the p-type contacts have a reverse leakage current density of 1.4 × 10−7 Acm−2 at −10 V. The n-type contacts have an current-voltage (I-V) extracted effective ϕBn of 0.7 eV with an ideality factor of 1.2 and a capacitance-voltage (C-V) extracted ϕBn of 1.2 e V. The p-type contacts have an I-V extracted effective ϕBp of 1.8 eV with an ideality factor of 1.7. Non-ideal I-V and C-V characteristics may be due to surface damage during W-DLN deposition.

Infrared reflectance of thick p-type porous SiC layers

Thick p-type porous 6H SiC layers were fabricated by anodization of p-type 6H SiC bulk crystals in dilute HF. Striking differences are observed in the reststrahl region room-temperature reflectance of these porous layers compared to that of bulk 6H SiC crystals. Instead of the single broad band reflectance spectrum typically observed in bulk 6H SiC, a two-band reflectance spectrum is observed. Several effective medium models, based on different morphologies of the component materials, 6H SiC and air, are used to obtain the frequency-dependent dielectric function of porous SiC from which calculated reflectance spectra are generated. The best match between measured and calculated spectra is obtained for a Maxwell–Garnett model with SiC acting as the host material and air cavities acting as the inclusion material. The model reproduces the two reflectance band structure observed in the measured reflectance of the porous SiC layers. The differences in the reststrahl region reflectance spectra of the porous SiC layers, compared to bulk SiC, are associated with polarization effects introduced by the cavities combined with a mean field average of interactions among the cavities.

Effects of Ar and H 2 annealing on the electrical properties of oxides on 6H SiC

We report on the influence of post-oxidation annealing (POA) on the electrical properties of metal-oxide-semiconductor capacitors fabricated on n- and p-type 6H SiC (Si face). All samples were oxidized in dry or wet oxygen at 1150 °C. POA at this temperature reduces the densities of fixed oxide charges and interface states. Negative fixed oxide charges on n-type SiC were found to be located at the SiO 2SiC interface. High temperature H 2 annealing experiments show that hydrogen plays a critical role in the SiO 2SiC system. Oxide defects on p-type SiC, which are believed to result from dopant (Al) incorporation in SiO 2, could be passivated by annealing in forming gas.

Plasma-Assisted MBE of GaN and AlGaN on 6H SiC(0001)

ABSTRACTThe growth of undoped and doped GaN and AlGaN films on off-axis 6H SiC substrates was investigated using plasma-assisted molecular beam epitaxy (MBE). Smooth and crack-free GaN and AlGaN films were obtained; the best results occurred at the highest growth temperature studied (800°C) and with a 40 to 50 nm A1N buffer layer grown at the same temperature. Carrier concentrations of up to n = 4 × 1020 cm−3 were accomplished with silicon, with a 40 to 50% activation rate as determined by secondary ion mass spectrometry (SIMS). Unintentionally doped AlxGa,.xN (x≈0.1) was n-type with a carrier concentration of 7 × 1018 cm−3. N-type AlGaN (x≈0.1)/p-type 6H SiC (0001) heterostructures showed excellent junction characteristics with leakage currents of less than 0.1 nA at 5 V reverse bias at room temperature and 0.5 nA at 200°C operating temperature.

Temperature stability of cobalt Schottky contacts on n- and p-type 6H silicon carbide

Rectifying Schottky contacts have been manufactured on n- and p-type 6H silicon carbide using e-beam evaporation of cobalt. Heat treatments in the 300 to 1100°C temperature range have been made to study the feasibility of high temperature contacts in this material system. Rutherford backscattering spectrometry and X-ray diffraction have revealed the formation of different cobalt silicides (Co 2Si, CoSi, and CoSi 2) at higher temperatures than for the Co/Si system. No evidence of silicidation was found below 600°C and SEM micrographs revealed carbon agglomerates at the surface after silicidation. Electrical properties have been examined using I–V and C–V measurements, and the barrier heights of cobalt and Co 2Si were evaluated. The contacts displayed excellent forward I–V characteristics with good linearity over 3–6 decades and were rectifying even after heat treatments at 800°C.

High-transconductance beta -SiC buried-gate JFETs

An improved performance buried-gate SiC junction field-effect transistor (JFET) has been fabricated and evaluated. This structure uses an n-type beta -SiC film epitaxially grown by chemical vapor deposition on the Si(0001) face of a p-type 6H alpha -SiC single crystal. The current in the n-type channel was modulated using the p-type alpha -SiC layer as a gate. Electron-beam-evaporated Ti/Au was utilized as an ohmic contact to the n-type beta -SiC layer, and thermally evaporated Al was used to contact the p-type gate. A maximum DC transconductance of 20 mS/mm was obtained, which is the highest reported for a beta -SiC FET structure. The experimental data are analyzed using a charge-control model. Calculated drain current versus drain voltage characteristics for a buried-gate JFET are in good agreement with experimental data. >

Application of wide-gap semiconductors to surface ionization: Work functions of AlN and SiC single crystals

For surface ionization purposes, a study of the work functions of SiC and AlN, both refractory wide-gap semiconductors, has been undertaken. Work function measurements have been performed in the 300–1600-K range using the Shelton retarding field method. Surface cleaning was carried out by heating in uhv to a high temperature using of a cw CO2 laser. Both n- and p-type 6H SiC single crystals with extreme bulk doping levels were investigated. For each doping type, the work functions have been found to be temperature independent. They exhibit only a slight variation from the n to the p type (4.75 and 4.85 eV, respectively) giving evidence, as in the case of Ge and Si, of Fermi level pinning at the surface by the intrinsic surface states. For an n-type AlN single crystal, the work function, measured in the high temperature range, remains constant as the temperature varies. The value of 5.35 eV obtained for AlN makes it a most attractive material for positive surface ionization applications.

ESR in irradiated silicon carbide

Thirty samples of n- and p-type 6H and n-type 3C silicon carbide single crystals were irradiated at room temperature with either 0.8 or 1.0 MeV electrons or reactor neutrons. Seven different radiation- induced e.s.r. spectra were found. Some of these have been described before, but further investigations of production rates and annealing behaviour have necessitated changes in the previously proposed models. The e.s.r. parameters of these centres are given. Three of the spectra are tentatively ascribed to (1) the uncharged carbon vacancy, (2) the negatively charged carbon divacancy, and (3) the uncharged silicon vacancy trapped at a boron site. Tentative models for three other centres are discussed.