Carrier drift mobility in porous silicon carbide

The time-of-flight technique was used to determine the drift mobilities of electrons and holes in porous silicon carbide produced by surface anodization of n-type 4H-SiC wafers. The electron and hole mobilities at 300 K in an electric field of 10⁴ V/cm were μ_e=6×10^?3 and μ_h=3×10^?3 cm² V^?1 s^?1, respectively. The low values of the mobilities are accounted for by carrier capture in localized states.     

Conductivity compensation in n-4H-SiC (CVD) under irradiation with 0.9-MeV electrons

The effect of electron irradiation on n-4H-SiC is studied by the methods of capacitance-voltage characteristics and photoluminescence. The carrier-removal rate is found to be V _d ≈ 0.25 cm^?1. Total conductivity compensation in samples with an initial carrier concentration of (1–2) × 10¹⁵ cm^?2 is observed at irradiation doses of ～5 × 10¹⁵ cm^?2. Simultaneously with an increase in the compensation, a rise in the intensity of defect-related luminescence characteristic of 4H-SiC is observed. The sample parameters before irradiation and after irradiation and annealing are compared. The physical mechanisms of compensation in the samples under study are analyzed.     

Electrical conduction in implanted polycrystalline silicon

Undoped polycrystalline silicon (poly-Si) films, 0.5 μm thick, have been prepared at 700‡C by chemical vapor deposition (CVD) onto thermally oxidized n⁺-Si substrates. The impurity concentration was varied by implanting with As, P, and Sb ions, accelerated to 30 keV; total doses ranged from 2×10¹¹ to 3×10¹⁵ ions/cm². Sheet resistance measurements, spanning 8 orders of magnitude, were made as a function of implantation dose. A reduction of 6 orders of magntiude in poly-Si sheet resistance took place within the implantation dose range between 10¹² and 10¹⁴ ions/cm². Some samples also exhibited large reductions in sheet resistance following the standard heat treatment for Al contact sintering and surface state reduction, which is normally at 450‡C for 0.5 hr in H₂. Sheet resistance measurements were also made as a function of temperature in the range 0 to 315‡C. The effective activation energy for electrical conduction depends upon implantation dose. At low doses (2×10¹¹cm₋₂) the poly-Si is intrinsic, withE _A = 0.65 eV. At a dose of 10¹⁵ cm⁻², electrical conduction is a weak function of temperature, withE _A = 0.027 eV.     

Effect of irradiation on the properties of nanocrystalline silicon carbide films

The effect of irradiation with electrons on the optical and electrical properties of the nanocrystalline rhombohedral 21R-SiC and 27R-SiC films is studied. The cycles of irradiation of nanocrystalline SiC films on sapphire substrates with electrons with the energy 10 MeV are conducted in the range of fluences from 5 × 10¹⁴ to 9 × 10¹⁹ cm^?2. It is established that, at the irradiation doses above 10¹⁹ cm^?2, the optical absorption of the films at the photon energies E > E _g becomes less efficient than the optical absorption of unirradiated films. It is established that the Urbach energy as a function of the irradiation dose exhibits a minimum at the dose ～10¹⁷ cm^?2 for the 21R-SiC films and ～5 × 10¹⁷ cm^?2 for the 27R-SiC films, suggesting that radiation induces some ordering in the films. As the dose is increased from 5 × 10¹⁷ up to 9 × 10¹⁹ cm^?2, an increase in the Urbach energy and a decrease in the optical band gap are observed. The effect is attributed to an increase in the concentration of radiation defects in the films.     

Optical and electrical properties of 4H-SiC irradiated with Xe ions

Structures with aluminum-ion-implanted p ⁺-n junctions formed in 26-μm-thick chemicalvapor-deposited-epitaxial 4H-SiC layers with an uncompensated donor concentration N _d ?N _a = (1–3) × 10¹⁵ cm^?3 are irradiated with 167-MeV Xe ions at fluences of 4 × 10⁹ to 1 × 10¹¹ cm^?2 and temperatures of 25 and 500°C. Then as-grown and irradiated structures are thermally annealed at a temperature of 500°C for 30 min. The as-grown, irradiated, and annealed samples are analyzed by means of cathodoluminescence, including the cross-sectional local cathodoluminescence technique, and electrical methods. According to the experimental data, radiation defects penetrate to a depth in excess of several tens of times the range of Xe ions. Irradiation of the structures at 500°C is accompanied by “dynamic annealing” of some low-temperature radiation defects, which increases the radiation resource of 4H-SiC devices operating at elevated temperatures.     

The influence of hydrogen gas on the characteristics of amorphous silicon deposited by RF sputtering

The electrical and optical properties of a-Si depend greatly on the fill gas and substrate temperature during RF sputtering. From the measured absorption coefficient, it was estimated that after the introduction of H₂ gas during sputtering the gap state density reduces from 3.2 × 10¹⁹ cm^?3 to 4.8 × 10¹⁷ cm^?3. As a consequence the optical band gap was found to increase from 1.74 to 1.82 eV. The d.c. conductivity measurement shows three distinct conducting mechanisms at different temperature regions. The SiH bonds in RF sputtered samples are persistant to higher temperature treatment than the CVD prepared ones.     

A 4<Emphasis Type="Italic">H</Emphasis>-SiC<Emphasis Type="Italic">p-i-n</Emphasis> diode fabricated by a combination of sublimation epitaxy and CVD

The possibility of fabricating heavily doped (N _a ?N _d ≥ 1 × 10¹⁹ cm_?3) p⁺-4H-SiC layers on CVD-grown lightly doped n-4H-SiC layers by sublimation epitaxy has been demonstrated. It is shown that a Au/Pd/Ti/Pd contact, which combines a low specific contact resistance (～2 × 10^?5 Ω cm²) with high thermal stability (up to 700°C), is the optimal contact to p-4H-SiC. The p-n structures obtained are used to fabricate packaged diodes with a breakdown voltage of up to 1400 V.     

Semiempirical model of carrier mobility in silicon carbide for analyzing its dependence on temperature and doping level

Experimental data on electron and hole mobility in three silicon carbide polytypes, 4H-SiC, 6H-SiC, and 3C-SiC, are analyzed. A semiempirical model is proposed for describing the dependence of the majority carrier mobility on temperature and doping level. The model describes well the accumulated body of experimental data and can be applied to model characteristics of multilayer silicon carbide structures.     

Carbon in silicon: Properties and impact on devices

The properties, origin and analysis of carbon in silicon and its influence on the electrical characteristics of devices are investigated and reviewed. The typical carbon concentrations in electronic-grade silicon are still some 10¹⁶ cm^?3. The small distribution coefficient (k₀ = 0.058) causes an inhomogeneous incorporation of carbon along the crystal axis and across the crystal diameter during crystal growth. Carbon concentrations exceeding about 5 × 10¹⁶ cm^?3 in float-zoned silicon can lead to the formation of process-induced defects in the fabrication of power rectifiers and thyristors. These defects which are frequently arranged in a swirl-like pattern strongly deteriorate the electrical characteristics of these devices. It is shown that carbon is involved primarily in the generation of the defect nuclei whereas the defects finally observed form via precipitation of oxygen and agglomeration of silicon interstitials. Reasons for the benign behavior of high carbon concentrations in the processing of integrated circuits are discussed. In powder device processing the formation of carbon-induced defects is safely avoided by application of silicon containing carbon less than 5 × 10¹⁶ cm^?3.     

The problem of uniformity of properties of 4H-SiC CVD films

Nonuniformities of electrical properties of 4H-SiC CVD films have been revealed using physicochemical reactions occurring upon introduction of radiation-induced structural defects. Primary knocked-on atoms and vacancies actively interact with impurities and defects of the starting material and thereby form the final system of radiation centers. The samples were irradiated with 900-keV electrons and 8-MeV protons at doses not leading to conductivity compensation (<7.5 × 10¹² cm⁻²) and a dose of 6 × 10¹⁴ cm⁻² causing deep compensation. Despite their area-averaging nature, capacitance methods demonstrated that characteristics of samples ∼3 mm in size are not identical. The nuclear spectrometry technique, which enables microprobing of samples, demonstrated individual behavior of separate parts of a film with areas of tens of square micrometers.     

Spectrometric properties of SiC detectors based on ion-implanted p +-n junctions

Results of spectrometric studies of nuclear radiation detectors based on p ⁺-n junctions formed in 4H-SiC films are presented for the first time. The junctions were fabricated by ion implantation of aluminum into 26-μm-thick CVD-grown epitaxial 4H-SiC layers with an uncompensated donor concentration of (3–5) × 10¹⁵ cm^?3. The detector characteristics were measured in testing with natural-decay alpha particles with energies of 3.35 and 5.4 MeV. The collection efficiency of charge generated by 3.35 MeV alpha particles was as high as 100% at an energy resolution of ? 2%.     

A study of deep traps at the SiO2/6H-SiC interface relying upon the nonequilibrium field effect

The nonequilibrium field effect associated with deep surface states at the SiO₂/6H-SiC interface has been observed and studied in a 6H-SiC MOSFET of depletion-accumulation type. An analysis of the relaxation of channel conductance at elevated temperatures upon filling of the surface traps with nonequilibrium carriers has shown that the energy distribution of the surface traps has the form of a narrow Gaussian peak in the upper half of the 6H-SiC band gap, with a peak energy E _C ?E _tm = 1.19eV, peak width ΔE _t ≈85 meV, and electron capture cross section σ_n≈10^?14 cm². These surface states are believed to have the fundamental nature of “oxidation defects” similar to P _b centers in the SiO₂-Si system (of dangling silicon bonds).     

Paramagnetic structural defects and conductivity in hydrogenated nanocrystalline carbon-doped silicon films

The behavior of paramagnetic defects and dark conductivity in heterogeneous samples of hydrogenated nanocrystalline carbon-containing silicon (nc-SiC:H) prepared by the photo-CVD method is studied. It is shown that, with increasing carbon content in nc-SiC:H, a phase transition from a nanocrystalline to an amorphous structure occurs, producing a reduction in paramagnetic defect density and a significant decrease in the dark conductivity.     

Carrier transport in porous silicon

Carrier transport in porous silicon layers has been studied by the time-of-flight method in the strong injection mode at temperatures T=290–350 K and electric field strengths F=(1.5–7)×10⁴ V cm^?1. The electron and hole drift mobilities μ_e≈2×10^?3 cm² V^?1 s^?1 and μ_h≈6×10^?4 cm² V^?1 s^?1 were obtained at T=292 K and F=4×10⁴ V cm^?1. An exponential temperature dependence of drift mobility with activation energy of ～0.38 and ～0.41 eV for, respectively, electrons and holes was established. It is shown that the type of time dependences of the photocurrent associated with carrier drift and the superlinear dependence of the transit time on the reciprocal of the voltage applied to a sample allow use of the concept of space-charge-limited currents under the conditions of anomalous dispersive transport. The experimental data are accounted for in terms of the model of transport controlled by carrier trapping into localized states with energy distribution near the conduction and valence band edges described by an exponential function with a characteristic energy of ～0.03 eV.     

Capture cross sections of the gold donor and acceptor states in n-type Czochralski silicon

The hole and electron capture cross sections of the gold donor and acceptor have been measured directly in n-type silicon. The samples have been grown by the Czochralski technique and have originated from several different suppliers. They have been diffused with gold so that N_T ? 0.1 (N_D-N_A). Measurements have been made on both Schottky diodes and diffused junctions and similar results obtained from all samples. The electron cross section of the acceptor level was found to be (0.85±0.2) × 10^?16cm² and the hole cross section of the donor (3.5±0.8) × 10^?15cm², both were essentially temperature independent. The hole cross section of the acceptor was (0.9±0.2) × 10^?14cm² at 300 K and showed a T^?1.3 temperature dependence. The electron cross section of the donor was (0.9±0.2) × 10^?15cm² at 180 K with a T^?2 dependence.     

Au schottky barrier diodes on β-SiC thin films deposited on silicon substrates by reactive magnetron sputtering technique

β-SiC thin films have been grown on (100) silicon substrates using reactive magnetron sputtering of a silicon target in an Ar/CH₄ mixed plasma. For the first time it has been possible to make gold Schottky diodes on β-SiC grown by reactive magnetron sputtering. Current-voltage measurements showed an ideality factor of 1.27 and a leakage current density of 4 μA/cm². Capacitance-voltage measurements gave a barrier height of 1.04 eV. The static dielectric constant for β-SiC was determined to be 9.     

Spectrometry of short-range ions using detectors based on 4<Emphasis Type="Italic">H</Emphasis>-SiC films grown by chemical vapor deposition

Schottky barriers, 10^?2 cm² in area, have been prepared by thermal deposition of Cr in vacuum on 50-μm-thick 4H-SiC epitaxial layers grown by chemical vapor deposition. The uncompensated donor concentration in these films is (4–6) × 10¹⁴ cm^?3, which makes it possible to extend the depletion region of the detector to ≈30 μm by applying a reverse bias of 400 V. The spectrometric characteristics of the detectors are determined using α particles in the energy range 4.8–7.7 MeV. The energy resolution attained for the 5.0-to 5.5-MeV lines is higher than 20 keV (0.34%), which, by a factor of 2, is second only to precision silicon detectors fabricated by specialized technology. The maximum signal amplitude corresponds, in SiC, to a mean electron-hole pair creation energy of 7.70 eV.     

Silicon nitride for the improvement of silicon inversion layer solar cells

It is demonstrated that CVD Si-nitride films as transparent dielectric satisfy all requirements for achieving MIS/IL solar cells with high efficiency and long term stability. Deposited on silicon by the SiH₄/NH₃ reaction at temperatures lower than usual (between 600° and 650°C) fixed positive interface charge densities Q_N/q up to 7 × 10¹² cm^?2 with excellent stability have been obtained. Utilizing the Si-nitride charge storage effect, the highest known Q_N/q values (> 10¹³cm^?2) combined with low values of N_it have been achieved. The charge distribution is discussed and an energy band diagram modified according to new analytical results is presented. MIS/IL solar cells with AM1 efficiencies of 15% (active area) and high UV sensitivity have been obtained.     

The contact of metal with silicon carbide: Schottky barrier height in relation to SiC polytype

The results obtained in measuring the Schottky barrier height Φ _b ⁿ for a chromium contact with 8H-, 6H-, 15R-, 27R-and 4H-SiC silicon carbide polytypes with n-type conduction are analyzed in terms of a simple model. It is shown that the Φ _b ⁿ value is proportional to the concentration of silicon vacancies in the polytypes. The results of Φ _b ⁿ measurements for palladium and platinum contacts to silicon carbide polytypes are discussed.     

The effects of incomplete annealing on the temperature dependence of sheet resistance and gage factor in aluminum and phosphorus implanted silicon on sapphire

The temperature coefficient of resistivity (TCR) of ion implanted silicon can be significantly reduced by partially annealing the crystal damage produced during implantation. The extent to which this method can be used to temperature compensate the resistivity and the gage factor has been determined for 300 ohm-cm silicon on sapphire implanted with either 100 keV Al²⁷ or P³¹ ions. The implantations were made at room temperature parallel to the 〈100〉 axis and in four fluences ranging from 1 × 10¹³cm^?2 to 1·25 × 10¹⁵ cm^?2. Sheet resistance, Hall coefficient, and effective mobility were measured from ?150°C to 150°C for various anneal temperatures. It was possible to obtain very low temperature dependences of sheet resistance at 300°K for all dopant fluences by appropriate partial annealing. On samples having the lowest temperature dependence of sheet resistance, the gage factor was measured from ?75°C to 75°C. The measurements were made along the 〈100〉 direction for phosphorus doped samples, and along the 〈110〉 direction for aluminum doped samples for all four fluences. The gage factor and its temperature dependence for these crystal orientations are not drastically affected by the crystal damage. These results are interpreted in terms of a model previously developed to explain the effect of electron damage on the temperature dependence of the resistivity and the piezoresistance of silicon.