Reinhard Helbig

SiC devices: physics and numerical simulation

The important material parameters for 6H silicon carbide (6H-SiC) are extracted from the literature and implemented into the 2-D device simulation programs PISCES and BREAKDOWN and into the 1-D program OSSI Simulations of 6H-SiC p-n junctions show the possibility to operate corresponding devices at temperatures up to 1000 K thanks to their low reverse current densities. Comparison of a 6H-SiC 1200 V p-n/sup -/-n/sup +/ diode with a corresponding silicon (Si) diode shows the higher switching performance of the 6H-SiC diode, while the forward power loss is somewhat higher than in Si due to the higher built-in voltage of the 6H-SiC p-n junction. This disadvantage can be avoided by a 6H-SiC Schottky diode. The on-resistances of Si, 3C-SiC, and 6H-SiC vertical power MOSFETs are compared by analytical calculations. At room temperature, such SiC MOSFETs can operate up to blocking capabilities of 5000 V with an on-resistance below 0.1 /spl Omega/cm/sup 2/, while Si MOSFETs are limited to below 500 V. This is checked by calculating the characteristics of a 6H-SiC 1200 V MOSFET with PISCES. In the voltage region below 200 V, Si is superior due to its higher mobility and lower threshold voltage. Electric fields in the order of 4/spl times/10/sup 6/ V/cm occur in the gate oxide of the mentioned 6H-SiC MOSFET as well as in a field plate oxide used to passivate its planar junction. To investigate the high frequency performance of SiC devices, a heterobipolartransistor with a 6H-SiC emitter is considered. Base and collector are assumed to be out of 3C-SiC. Frequencies up to 10 GHz with a very high output power are obtained on the basis of analytical considerations. >

Über die züchtung von grösseren reinen und dotierten ZnO-kristallen aus der gasphase

Abstract Large single crystals of ZnO (up to 20 g weight) have been grown by oxidation of Zn-vapour. The crystals contain foreign impurities in a concentration of about 1–3 ppm. Furthermore, there have been grown also Li-, Na-, Cu-, Ga-, In- and Mn-doped crystals using this method.

Growth of SnO2 single crystals by a vapour phase reaction method

Abstract SnO 2 single crystals have been grown isothermally by a method which uses the dissociative sublimation of SnO 2 in SnO and O 2 supported by H 2 and reoxidation of the transported SnO. The grown SnO 2 crystals are mostly needles with a square cross section having sizes of 3 × 3 × 15 mm to 7 × 7 × 20 mm. Sometimes twins and hollow crystals are observed. The refractive index for E ⊥ c and E ‖ c was measured in the wavelength range of 300 ⩽ λ ⩽ 650 nm with an isotropic point at λ = 308 nm .

Chemical vapor deposition and characterization of undoped and nitrogen‐doped single crystalline 6H‐SiC

Homoepitaxial growth of single crystalline 6H‐SiC layers is performed by chemical vapor deposition (CVD). 6H‐SiC substrates are grown by a sublimation technique. They have vicinal surfaces inclined 1.5° to 2° from the (0001) plane towards the [1100] direction. We report CVD growth at 1600 °C in the hydrogen‐silane‐propane gas system with nitrogen as a dopant. High quality films are achieved with growth rates of about 1.8 μm per hour. The layers are examined by optical microscopy, infrared reflection, photoluminescence, and Rutherford backscattering. For electrical characterization capacitance‐voltage and Hall measurements are performed. Unintentionally doped layers have donor concentrations in the upper 1015 cm−3 range. Electron mobilities of 370 cm2/V s at room temperature and about 104 cm2/V s at 45 K are observed. To the authors’ knowledge this is the highest mobility so far reported for 6H silicon carbide.

Piezoelectric properties and elastic constants of 4H and 6H SiC at temperatures 4–320 K

4H and 6H silicon carbide (SiC) crystals are piezoelectric and can be excited to resonant vibrations with quality factors up to Q≊100 000 and an electromechanical coupling factor of k31≊0.03 for 6H material. The resonance frequencies are used for the determination of elastic constants from 4 to 320 K. One finds a Poisson’s ratio σ=0.212 and low‐temperature values s11=2.035×10−12 m2/N and c33=55.12×1010 N/m2 for 6H, s11=2.114×10−12 m2/N, and c33=60.52×1010 N/m2 for 4H crystals. All elastic constants show a nonmonotonic dependence from the temperature at about 50 K.

The spectral distribution of the intrinsic radiative recombination in silicon

Microscopic calculations of the intrinsic radiative recombination probability B(ω,T) of Si have been performed to obtain the absolute values of the spectral distribution and of its temperature dependence. In these calculations we use the concepts of k⋅p theory, consider excitonic effects, and take into account the electron‐phonon interaction. The calculated spectra are compared with measured absolute values of the intrinsic radiative recombination spectra obtained from forward biased Si p‐i‐n diodes and also with spectra obtained from the detailed balance theory of van Roosbroeck and Shockley [W. van Roosbroeck and W. Shockley, Phys. Rev. 94, 1558 (1954)]. Quantitative agreement is obtained for higher temperatures (about 300 K) and deviations for lower temperatures are critically discussed.

Electrical and optical properties of heterostructures made from diamond-like carbon layers on crystalline silicon

Abstract Thin films of amorphous, diamond-like carbon (DLC) were deposited in a capacitively coupled RF-plasma discharge on n- or p-type silicon substrates with different bias voltages, pressures and flow rates. The produced DLC/Si heterostructures show a broad variety of electrical and optical properties depending on the deposition parameters. The investigations concentrated on C—V measurements and on the photo-electrical properties of the DLC/Si heterostructure. The quantum efficiency of the heterostructure DLC on crystalline p-type silicon (specific resistance 10–20 Ω · cm) is 0.5 at a wavelength of 980 nm and a reverse bias of 2.7 V.

Photothermal ionization spectroscopy of shallow nitrogen donor states in 4H–SiC

Photothermal ionization spectroscopy (PTIS) measurements were carried out on a free-standing, high purity and high quality 4H–SiC epitaxial layer at various temperatures. The two step photothermal ionization process is clearly reflected in the temperature dependence of the photoconductivity. The PTI spectrum at a temperature of 25.6 K exhibits one order of magnitude higher energy resolution than the infrared absorption spectra of 4H–SiC bulk material. It reveals five strong, well resolved electronic transition lines associated with the shallow nitrogen donor. The ionization energy of the shallow nitrogen donor is deduced to be 60.2±0.5 meV based on experimental results. Furthermore, PTI magnetospectroscopy measurements were performed to investigate the symmetry properties of these transitions in Faraday configuration. No linear Zeeman splitting is observed, however, these lines show a diamagnetic shift. It indicates that the excited states of the shallow nitrogen donor are nondegenerate at zero magnetic f...

CHARACTERIZATION OF PHOSPHORUS DOPED N-TYPE 6H-SILICON CARBIDE EPITAXIAL LAYERS PRODUCED BY NUCLEAR TRANSMUTATION DOPING

Aluminum doped 6H-SiC epitaxial layers (p-type net doping: NA−ND=5.2×1015 cm−3, thickness 3 μm) on a p+ doped 6H-SiC substrate (NA−ND=1018 cm−3) were irradiated with neutrons in a nuclear reactor. The neutron fluences applied were 9.4×1019, 3.5×1020, and 6.4×1020 cm−2. The phosphorus impurity in SiC is produced by a nuclear (n,γ) reaction with the decay of 31Si to 31P. The irradiated samples were subsequently annealed at temperatures between 800 and 1850 °C. The annealing behavior was studied by low temperature photoluminescence, Fourier transform infrared spectroscopy, and Hall effect and I–V and C–V measurements. After the annealing process the 6H-SiC p-type epilayer changed to n type resulting in a pn junction within the material. The properties of the pn junction were characterized.

SiC-UV-Photodetectors

UV-photodiodes were fabricated by N-implantation in p-type 6H-SiC epitaxial layers grown on monocrystalline substrates. I-V characteristics (at 296-826 K) and spectral quantum efficiencies (at 295-673 K) are measured to characterize the photodiodes. Maximum quantum efficiences of 75% are observed at wavelengths around 280 nm. This means, that the diffusion length of the electron must be greater than 1 μm. From an analysis of the long wavelength cut-off, the band-gap energy and the temperature coefficient of the band-gap energy are determined.