B.E. Weiland

Characterization of Graphene Films and Transistors Grown on Sapphire by Metal-Free Chemical Vapor Deposition

We present a novel method for the direct metal-free growth of graphene on sapphire that yields high quality films comparable to that of graphene grown on SiC by sublimation. Graphene is synthesized on sapphire via the simple decomposition of methane at 1425-1600 °C. Film quality was found to be a strong function of growth temperature. The thickness, structure, interface characteristics, and electrical transport properties were characterized in order to understand the utility of this material for electronic devices. Graphene synthesized on sapphire is found to be strain relieved, with no evidence of an interfacial buffer layer. There is a strong correlation between the graphene structural quality and carrier mobility. Room temperature Hall effect mobility values were as high as 3000 cm(2)/(V s), while measurements at 2 K reached values of 10,500 cm(2)/(V s). These films also display evidence of the quantum Hall effect. Field effect transistors fabricated from this material had a typical current density of 200 mA/mm and transconductance of 40 mS/mm indicating that material performance may be comparable to graphene on SiC.

Bulk growth of high-purity 6H-SiC single crystals by halide chemical-vapor deposition

High-purity 6H-SiC single crystals were grown by the halide chemical-vapor deposition process. Growth was performed in a vertical hot-wall reactor with a separate injection of a silicon precursor (silicon tetrachloride) and a carbon precursor (propane). Typical growth rates were between 100 and 300μm∕h. The crystals contain very low concentrations of residual impurities. The main contaminants, namely, nitrogen and boron, are in the 1014atomscm−3 range. Crystals grown under Si-rich conditions were n type with low room temperature electron concentrations in the 1014–1015atomscm3 range and with room-temperature electron mobilities approaching 400cm2∕Vs. The resistivity of the material increased up to 1010Ωcm with increasing C∕Si ratio. Deep levels spectra show that the electron traps density decreases with increasing C∕Si ratio.

Growth of Bulk SiC by Halide Chemical Vapor Deposition

A novel halide chemical vapor deposition (HCVD) process has been developed for bulk growth of high purity, single crystal 6H SiC. The effects of major process parameters including furnace temperature over the range of 1900-2150°C, reactor pressure over the range of 20-400 torr, reactant concentrations, and flow rates on the growth rate, crystallinity, and electrical properties of the single-crystal 6H boules grown by HCVD are described. Typical growth rates for the 6H polytype are on the order of 100-125 μm/h with a maximum observed rate of 180 μm/h. Thicknesses up to 1 mm have been demonstrated. GDMS analyses of the purity of HCVD grown material is discussed and compared to 6H SiC produced by commercial PVT and HTCVD processes. Boron and aluminum concentrations less than 1.8 E 15 atoms/cm 3 were demonstrated. Introduction The HCVD process was developed for growth of bulk, high purity, 6H SiC. This process has significant advantages over conventional physical vapor transport (PVT) processes [1] for manufacturing semi-insulating SiC. Foremost is the ability to maintain a constant gas phase chemistry at the growth surface. During PVT growth the solid source material sublimes incongruently leading to variations in the Si/C ratio during the growth process. In addition, impurities in the source material and furnace components evaporate at different rates resulting in a transient flux of impurities to the growth surface. Variations in the gas phase chemistry lead to variations in electrical properties along the length of the boule. CVD-based processes provide a means for carefully controlling the chemical composition of the gas phase and the growing crystal over time. This is accomplished by using high purity source gases and through independent control of the Si and C precursor flow rates. For example, in the HTCVD process [2] SiH4 and C3H8 are mixed and reacted at temperatures above 2000°C to grow crystals with very low impurity concentrations and high electrical resistivity. The highly reactive precursors used in this process can result in deposition of SiC in the gas inlet and outlet ports if gas flows and thermal gradients are not optimized. This can lead to reduced process times and shorter boules. The geometry of the HCVD reactor and the use of thermally stable precursors results in significant reductions of parasitic deposits in the gas inlet and outlet ports. Use of semiconductor grade precursors and dilution of contaminants from the furnace by the carrier gases results in the growth of high purity material.

Properties of 6H–SiC crystals grown by hydrogen-assisted physical vapor transport

Effects of hydrogen addition to the growth ambient during physical vapor transport (PVT) growth of 6H–SiC were investigated using secondary ion mass spectrometry, deep level transient spectroscopy, and Hall effect measurement. The background nitrogen concentration and the free electron density decrease with increasing hydrogen content. The formation of electron traps (activation energies of 0.4eV, 0.6eV, 0.7eV, 0.9eV, and 1eV) was also strongly suppressed. The above results are interpreted as a consequence of hydrocarbon formation produced by the reaction of hydrogen with the SiC source and the graphite parts of the furnace. This leads to more congruent evaporation of SiC and the shift of the gas phase and the SiC deposit stoichiometry due to less Si-rich conditions than in standard PVT growth.

Thermodynamic equilibrium limitations on the growth of SiC by halide chemical vapor deposition

Single crystal SiC for semiconductor applications is commonly produced by physical vapor transport (PVT). Incongruent sublimation of SiC causes the gas phase composition in the PVT growth cell to drift from Si-rich to C-rich as growth proceeds. The change in C/Si ratio in the gas phase causes significant variations in deep center and dopant concentrations along the growth axis of the crystal. Growth of SiC by halide chemical vapor deposition (HCVD) provides direct control over the C/Si ratio by independently metering C and Si precursor gases to the growth environment. Thermally stable Si sources, such as SiCl4, are used instead of SiH4 to eliminate premature decomposition of the Si source. Use of chlorinated precursors, combined with the high precursor concentrations required for growth rates of 50–250 μm/h, impose thermodynamic limits on the maximum C/Si ratio that can be used for deposition of single crystal SiC. A thermodynamic model is provided for predicting the boundary between deposition of SiC and...

Growth of SiC Boules with Low Boron Concentration

The effects of growth conditions, diffusion barrier coatings, and hot zone materials on B incorporation in 6H-SiC crystals grown by physical vapor transport (PVT) were evaluated. Development of high purity source material with a B concentration less than 1.8x1015 atoms/cm3, was critical to the growth of boules with a B concentration less than 3.0x1016 atoms/cm3. Application of refractory metal carbide coatings to commercial graphite to serve as boron diffusion barriers and the use of very high purity pyrolytic graphite components ultimately led to the growth of SiC boules with boron concentrations as low as 2.4x1015 atoms/cm3. The effect of growth temperature and pressure were closely examined over a range from 2100°C to 2300°C and 5 to 13.5 Torr. This range of growth conditions and growth rates had no effect on B incorporation. Attempts to alter the gas phase stoichiometry through addition of hydrogen gas to the growth environment also had no impact on B incorporation. These results are explained by considering site competition effects and the ability of B to diffuse through the graphite growth cell components.

Hybrid Physical-Chemical Vapor Transport Growth of SiC Bulk Crystals

The effects of H2 addition to the growth ambient during physical vapor transport (PVT) growth of 6H and 4H SiC were investigated using SIMS, DLTS and Hall effect measurements. Using this hybrid physical-chemical vapor transport (HPVT) approach, boules were grown using Ar-H2 and He-H2 mixtures with H2 concentrations up to 50 at%. Thermodynamic modeling suggests that addition of H2 improves the carbon transport in HPVT compared to standard PVT. This should lead to a substantial decrease in the concentration of residual N donors and the concentration of electron traps. This is confirmed by the experimental results. As expected, the source transport rate increased as H2 was added to the growth environment due to increased C transport. The background nitrogen concentration and the free electron density decreased significantly with increasing H2 concentration. The formation of electron traps (activation energies of 0.4 eV, 0.6-0.65 eV, 0.7 eV, 0.9 eV and 1 eV) was also strongly suppressed. These changes were observed for H2 concentrations as low as 4 at%. The decreased N concentration improves the ability to produce high resistivity SiC material, and for H2 concentrations as high as 10-25%, the very first wafers cut from the seed end of the boules have a resistivity exceeding 106 cm.

Growth Kinetics and Polytype Stability in Halide Chemical Vapor Deposition of SiC

Growth rates and relative stability of 6H- and 4H-SiC have been studied as a function of growth conditions during Halide Chemical Vapor Deposition (HCVD) process using silicon tetrachloride, propane and hydrogen as reactants. The growth temperature ranged from 2000 to 2150 oC. Silicon carbide crystals were deposited at growth rates in the 100-300 μm/hr range in both silicon- and carbon-supply limited regimes by adjusting flows of all three reactants. High resolution x-ray diffraction measurements show that the growth on Si-face of 6H- and C-face of 4H-SiC substrates resulted in single crystal 6H- and 4H-SiC polytype, respectively. The growth rate results have been interpreted using thermodynamic equilibrium calculations.