A. Ellison

Influence of epitaxial growth and substrate-induced defects on the breakdown of 4H-SiC Schottky diodes

Morphological defects and elementary screw dislocations in 4H-SiC were studied by high voltage Ni Schottky diodes. Micropipes were found to severely limit the performance of 4H-SiC power devices, w ...

In situ substrate preparation for high-quality SiC chemical vapour deposition

Abstract In situ preparation of 4H and 6H silicon carbide substrate surfaces in hydrogen and hydrogen-propane etching systems has been studied. The etching of on-axis(0001) 6H-SiC substrates resulted in regular straight terraces and one unit high steps. The etching of on-axis (0001) 4H-SiC substrates resulted in broad terraces interrupted by large step formations. The 4H- and 6H-SiC (0001) off-axis substrates (3.5° towards 〈112¯0〉 yield smooth etched surfaces with the exception of stripe-like defects on the 4H polytype which are shown to be related to stacking-faults. The stacking faults are suggested to be a cause for step-bunching and surface roughening. Hydrogen-etching prior to growth has been shown to improve the epitaxial layer quality both concerning defect formation and step-bunching.

Growth of SiC by "Hot-Wall" CVD and HTCVD

A reactor concept for the growth of high-quality epitaxial SiC films has been investigated. The reactor concept is based on a hot-wall type susceptor which, due to the unique design, is very power efficient. Four different susceptors are discussed in terms of quality and uniformity of the grown material. The films are grown using the silane–propane–hydrogen system on off-axis (0001) 6H- and 4H-SiC substrates. Layers with doping levels in the low 1014 cm—3 showing strong free exciton emission in the photoluminescence spectra may readily be grown reproducibly in this system. The quality of the grown layers is also confirmed by the room temperature minority carrier lifetimes in the microsecond range and the optically detected cyclotron resonance data which give mobilities in excess of 100000 cm2/Vs at 6 K. Finally, a brief description will be given of the HTCVD technique which shows promising results in terms of high quality material grown at high growth rates.

High temperature chemical vapor deposition of SiC

A growth process has been investigated for the epitaxial growth of silicon carbide. The technique can simply be described as chemical vapor deposition (CVD) at high temperatures, hence the name high temperature CVD (HTCVD). The growth process however, differs greatly from that of the CVD process due to the significant sublimation and etch rates at the extreme growth temperatures (1800–2300°C). The grown rates obtained with the HTCVD are in the order of several tens of μm/h to 0.5 mm/h. The purity and crystallinity of the growth layers are outstanding showing strong free exciton related photoluminescence.

Dislocation evolution in 4H-SiC epitaxial layers

4H-SiC commercial wafers and sublimation grown epitaxial layers with a thickness of 100 μm have been studied concerning crystalline structure. The substrates and the epitaxial layers have been separately investigated by high-resolution x-ray diffraction and synchrotron white beam x-ray topography. The results show that the structural quality was improved in the epitaxial layers in the [1120] and [1100] directions, concerning domain distribution, lattice plane misorientation, mosaicity, and strain, compared with the substrates. Misoriented domains have merged together to form larger domains while the tilt between the domains was reduced, which resulted in nonsplitting in diffraction curves. If the misorientation in the substrate is large, we can only see a slight decrease in the misorientation in the epilayer. At some positions on the substrates block structures (mosaicity) were observed. ω-rocking curves showed smaller full width at half maximum values and more uniform and narrow peaks, while the curvat...

High temperature CVD growth of SiC

Two high temperature CVD techniques, respectively optimised for epitaxial and crystal growth, are presented. A chimney reactor has been developed for fast epitaxy, carried out at 1700‐1900°C, with growth rates ranging from 10 to 25 m mh 1 , and a material quality close to conventional CVD processes. The growth of 4H-SiC epilayers with low n-type doping (10 14 ‐10 15 cm 3 ) and carrier lifetimes up to 0.4 ms is described, while the feasibility of high voltage Schottky rectifiers (1.8 kV) is demonstrated. On the other side, developments of the stagnant flow HTCVD process, where growth is carried out at 2000‐2300°C, are shown to enable growth rates ranging from 0.3 up to 0.8 mm h 1 . The main characteristics of HTCVD grown SiC crystals (up to nearly 7 mm thick) are described.

A 3 kV Schottky barrier diode in 4H-SiC

High-voltage Schottky barrier diodes with low reverse leakage current were processed on hot-wall chemical vapor deposition grown 4H-SiC films. A metal overlap onto the oxide layer was employed to reduce electric field crowding at the contact periphery. By utilizing a 42–47 μm thick, high-quality epitaxial layers with doping in the range of 7×1014–2×1015 cm−3, a record blocking voltage of above 3 kV was achieved. The large diodes with 1.0 mm diameter showed breakdown at 2.1 kV. The reverse leakage current density at 1.0 kV was measured to be 7.0×10−7 A cm−2. Specific on-resistance of the diode with breakdown voltage at 3 kV was 34 mΩ cm2.

Epitaxial growth of SiC in a chimney CVD reactor

A high growth rate (>10mm/h) Chemical Vapour Deposition (CVD) process is investigated in a vertical hot-wall, or ‘‘chimney’’, reactor. By the use of increased temperatures (1650–18501C) and concentrations of reactants, this process is shown to enable growth rates up to 50mm/h and demonstrates a material quality comparable to established CVD techniques until growth rates of 25mm/h. The gas flow dynamics, the growth rate and the thickness uniformity determining steps are investigated, and the role of homogenous nucleation is analysed. The growth rate is shown to be influenced by two competing processes: the supply of growth species and the etching of the hydrogen carrier gas. The exponential increase of the growth rate with temperature is related to a Si-vapour release from clusters homogeneously nucleated in the inlet of the susceptor and acting as a growth species reservoir. r 2002 Elsevier Science B.V. All rights reserved.

HTCVD Grown Semi-Insulating SiC Substrates

The low residual doping of HTCVD grown semi-insulating SiC crystals enables the use of decreased concentrations of compensating deep levels, thereby providing new material solutions for microwave devices. Depending on the growth conditions, high resistivity crystals with either a dominating Si-vacancy absorption or with an EPR signature of intrinsic defects such as the C-vacancy and the Si-antisite are obtained. The electrical properties of substrates with resistivities above 10(11) Omega-cm are shown to be stable upon annealing during SiC epitaxy conditions. Micropipe closing at the initial growth stage enables the demonstration of low defect density off- and on-axis 2 2-inch semi-insulating 4H SiC substrates with micropipe densities down to 1.2 cm(-2).

Defects in High-Purity Semi-Insulating SiC

Defects and impurities in high-purity semi-insulating (HPSI) SiC substrates grown by high temperature chemical vapour deposition (HTCVD) and physical vapour transport (PVT) are studied using electron paramagnetic resonance (EPR) and photoluminescence (PL). The carbon vacancy in the positive charge state ( + C V ) is observed in all HTCVD and PVT HPSI substrates. EPR signals of (CSi–VC) pairs are often detected in HPSI samples. The TV2a, which was previously attributed to 0 Si V , is often observed with different concentrations in HTCVD material. The (+/0) donor level of VC at 1.47 eV above the valence band is suggested to be important for the SIproperties of HPSI 4H-SiC substrates with the activation energies Ea~1.4-1.5 eV. The SI-5 center may be the vacancy pair in the negative charge state (VC-VSi) – and its acceptor level (-1/-2) is in the region ~1.24-1.51 eV below the conduction band. This center is stable at annealing temperature of 1600 oC. After annealing, + C V and VSi-related signals decrease but can still be observed, whereas the (CSi–VC) pairs completely disappear.