G. Augustine

Semi‐insulating 6H–SiC grown by physical vapor transport

Semi‐insulating 6H–SiC crystals have been achieved by using controlled doping with deep‐level vanadium impurities. High resistivity undoped and semi‐insulating vanadium‐doped single‐crystals with diameters up to 50 mm were grown by physical vapor transport using an induction‐heated, cold‐wall system in which high purity graphite materials constituted the hot zone of the furnace. Undoped crystals were p‐type due to the presence of residual acceptor impurities, mainly boron, and exhibited resistivities ranging up to 3000 Ω cm. The semi‐insulating behavior of the vanadium‐doped crystals is attributed to compensation of residual acceptors by the deep‐level vanadium V4+(3d1) donor located near the middle of the band gap.

DEEP LEVEL TRANSIENT SPECTROSCOPIC AND HALL EFFECT INVESTIGATION OF THE POSITION OF THE VANADIUM ACCEPTOR LEVEL IN 4H AND 6H SIC

Hall effect, deep level transient spectroscopy (DLTS) and optical absorption measurements were employed in concert to determine the position of the vanadium acceptor level in vanadium and nitrogen doped 6H and 4H SiC. Hall effect results indicate that the acceptor position in 4H SiC is at 0.80 eV beneath the conduction band edge, and 0.66 eV for the 6H polytype. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 0.80 eV and a capture cross section of 1.8×10−16 cm−2. The optical absorption measurements proved that the levels investigated are related to isolated vanadium, and therefore the vanadium acceptor level. Based on the DLTS measurements and secondary ion mass spectroscopy data, the maximum solubility of vanadium in SiC was determined to be 3.0×1017 cm−3. At these incorporation limits and with the depth of the level, the vanadium acceptor level could be used in the creation of semi‐insulating silicon carbide.

On the compensation mechanism in high‐resistivity 6H–SiC doped with vanadium

A model is presented which describes the compensation mechanism resulting in semi‐insulating 6H silicon carbide by vanadium doping. Undoped 6H–SiC crystals grown by physical vapor transport methods frequently contain between 1×1017 and 5×1018 cm−3 uncompensated boron acceptors. Upon addition of vanadium, the 3d1 electron of the vanadium donor compensates the holes of the boron centers. It is shown that when vanadium is present in concentrations greater than that of boron, the Fermi level is pinned to the vanadium donor level. From temperature dependent Hall effect measurements, this donor level has been determined to reside 1.35 eV below the conduction band minimum. Thermally stimulated current measurements on V‐doped SiC crystals show that boron is the major compensating center for the vanadium impurity.

Characterization of Polishing‐Related Surface Damage in (0001) Silicon Carbide Substrates

Because of its wide bandgap and physical stability silicon carbide (SiC) is currently being viewed as a potentially important semiconductor material for high power and high temperature solid-state devices. The nature and extent of surface damage in 6H-SiC substrates prepared by mechanical polishing have been studied using backscattering of ultraviolet light and cross-sectional transmission electron microscopy. When the basal plane surface is prepared by lapping or polishing with large size diamond abrasives, the surface roughness is about one-fifth the particle size, while the subsurface damage extends to a depth of about half the abrasive size. Under optimum conditions of particle size, vertical load, and relative rotation speed, the extent of subsurface damage can be minimized resulting in a nominally defect-free specular surface exhibiting a uniform strained layer of less than 8 nm.

An atomic force microscopy study of super-dislocation/micropipe complexes on the 6H-SiC(0 0 0 1) growth surface

We have used atomic force microscopy (AFM) to study the (0 0 0 1) growth surface of a 6H-SiC single crystal at the points where micropipes emerge on the growth surface. All of the micropipes examined are origins of spiral steps, indicating that dislocations intersect the surface at these points. The dislocations observed at the surface/micropipe intersections have Burgers vectors of at least 4b0, where b0 is the Burgers vector of a unit screw dislocation aligned along the c-axis (b0 = 15.19A). Single and double unit dislocations were also observed, but they are not associated with micropipes. Micron-scale deposits of a heterogeneous phase were observed in the vicinity of the micropipes. The curvature of growth steps around these heterogeneities indicates that they impeded step motion while the crystal was growing. Based on our observations, we propose a model for the formation of super-dislocation/micropipe complexes that involves the coalescence of unit screw dislocations that are forced towards one another as large steps grow around heterogeneous material on the surface.

OPTICAL AND ELECTRICAL CHARACTERIZATION OF BORON IMPURITIES IN SILICON CARBIDE GROWN BY PHYSICAL VAPOR TRANSPORT

Undoped SiC crystals grown by physical vapor transport have been characterized by temperature dependent Hall effect and near infrared optical absorption measurements. Crystals with reduced nitrogen content were found to exhibit p‐type conductivity with carrier concentrations in the 5×1014–1×1016 cm−3 range at room temperature. The Fermi level position determined from Hall effect measurements at elevated temperatures was 0.35 eV above valence band. The primary acceptor‐type impurity was identified as substitutional boron with total concentration of uncompensated acceptors in the 1×1017–5×1018 range. This interpretation was confirmed by near infrared absorption spectra, which were dominated by a broad photoionization band with a threshold at 0.7 eV and a maximum at 1.75 eV. The shape of the band was fitted, and the thermal ionization energy of the defect was found to be in the 0.3–0.4 eV range. A correlation between the photoionization band intensity, and the uncompensated boron content was used to determin...

Vanadium related near‐band‐edge absorption bands in three SiC polytypes

Low‐temperature optical absorption experiments have been performed on a variety of n‐type, p‐type, and high‐resistivity silicon carbide samples, including the polytypes: 4H, 6H, and 15R. These experiments reveal a set of absorption band close to the band edge with a fine structure depending upon the polytype. Each sample exhibits a spectrum with the number of lines corresponding to the number of inequivalent substitutional lattice sites contained in the polytype. A correlation of these lines with the neutral vanadium 2E→2T2 intracenter transition indicates that the initial state for the near‐band‐gap absorption lines is the 2E state of the 3d1 configuration of vanadium. The near‐band‐edge absorption lines were interpreted as due to an exciton bound to a vanadium donor with an electron occupying an atomic‐like d state. The position of the vanadium acceptor level was estimated to be, at most, 250 meV from the conduction band for the cubic site in 6H SiC.