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For undoped 6H-SiC boules grown by physical vapor transport the variations of resistivity, of the type and density of deep electron and hole traps, and of the concentration of nitrogen and boron were studied as a function of position in the cross section normal to the growth axis and along the growth direction. It was observed that the concentrations of all deep electron and hole traps decreased when moving from seed to tail of the boule and from the center to the edge of the wafers. Modeling of the growth process suggests that the C/Si ratio increases in a similar fashion and could be responsible for observed changes. We also discuss the implications of such stoichiometry changes on compensation mechanisms rendering the crystals semi-insulating and on electrical uniformity of SI-SiC wafers.

Semi-insulating silicon carbide (SiC) wafers are important as substrates for high frequency devices such as AlGaN-GaN HEMT’s. A nondestructive characterization technique has been developed to measure the dielectric properties of SiC wafers in the GHz frequency range where the devices will operate in order to validate wafers for high yield working devices. The dielectric loss is measured at approximately 16 GHz in a split microwave cavity. Initial results show a correlation where the dielectric loss decreases as the resistivity increases, where the resistivity was measured using a Contactless Resistivity Mapping system (COREMA). The uniformity of dielectric loss across SiC wafers was evaluated using a split post dielectric resonator cavity fixed at 5.5GHz to measure the dielectric loss at five points on a wafer. Dielectric loss as a function of temperature from room temperature to 400°C was also studied.

Trimethylboron (TMB) was investigated as a p-type dopant source for the vapor–liquid–solid growth of boron-doped silicon nanowires (SiNWs). The boron concentration in the nanowires was measured using secondary ion mass spectrometry and results were compared for boron-doping using TMB and diborane (B2H6) sources. Boron concentrations ranging from 1×1018 to 4×1019cm−3 were obtained by varying the inlet dopant∕SiH4 gas ratio. TEM characterization revealed that the B2H6-doped SiNWs consisted of a crystalline core with a thick amorphous Si coating, while the TMB-doped SiNWs were predominantly single crystal even at high boron concentrations. The difference in structural properties was attributed to the higher thermal stability and reduced reactivity of TMB compared to B2H6. Four-point resistivity and gate-dependent conductance measurements were used to confirm p-type conductivity in the TMB-doped nanowires and to investigate the effect of dopant concentration on nanowire resistivity.

The fabrication of semiconductor nanowires, in which composition, size and conductivity can be controlled in both the radial and axial direction of the wire is of interest for fundamental studies of carrier confinement as well as nanoscale device development. In this study, group IV semiconductor nanowires, including Si, Ge and SixGe1-x alloy nanowires were fabricated by vapor-liquid-solid (VLS) growth using gaseous precursors. In the VLS process, gold is used to form a liquid alloy with Si and Ge which, upon supersaturation, precipitates a semiconductor nanowire. Nanoporous alumina membranes were used as templates for the VLS growth process, in order to control the diameter of the nanowires over the range from 45 nm to 200 nm. Intentional p-type and n-type doping was achieved through the addition of either trimethylboron, diborane or phosphine gas during nanowire growth. The electrical properties of undoped and intentionally doped silicon nanowires were characterized using field-assisted assembly to align and position the wires onto pre-patterned test bed structures. The depletion characteristics of back-gated nanowire structures were used to determine conductivity type and qualitatively compare dopant concentration. SiGe and SiGe/Si axial heterostructure nanowires were also prepared through the addition of germane gas during VLS growth. The Ge concentration in the wires was controllable over the range from 12 % to 25% by varying the inlet GeH4/SiH4 ratio.

Chemical elemental analysis, temperature-dependent Hall measurements, deep-level transient spectroscopy, and contactless resistivity mapping were performed on undoped semi-insulating (SI) and lightly nitrogen-doped conducting 6H-SiC crystals grown by physical vapor transport (PVT). Resistivity maps of commercial semi-insulating SiC wafers revealed resistivity variations across the wafers between one and two orders of magnitude. Two major types of variations were identified. First is the U-shape distribution with low resistivity in the center and high in the periphery of the wafer. The second type had an inverted U-shape distribution. Secondary-ion-mass spectrometry measurements of the distribution of nitrogen concentration along the growth axis and across the wafers sliced from different locations of lightly nitrogen-doped 6H–SiC boules were conducted. The measured nitrogen concentration gradually decreased along the growth direction and from the center to the periphery of the wafers. This change gives rise to the U-like distribution of resistivity in wafers of undoped SI-SiC. The concentrations of deep electron traps exhibited similar dependence. Compensation of nitrogen donors by these traps can result in the inverted U-like distribution of resistivity. Possible reasons for the observed nonuniformities include formation of a (0001) facet in PVT growth coupled with orientation-dependent nitrogen incorporation, systematic changes of the gas phase composition, and increase of the deposition temperature during boule growth.

Total insulation values of seven commercial variable volume dry diving suits, and of four of these suits worn in combination with various commercial and Navy insulating undergarments, were measured on an electrically heated copper man standing in air or immersed to the neck in water. Values in air ranged from 1.27 clo to 1.92 clo for the suits alone, and from 1.89 to 2.67 clo for the suit-undergarment combinations. These values decreased by from 0.73 clo to 1.29 clo in water, of which 0.66 clo represented reduction in the amount of film insulation at the suit surface Block 20, with immersion (0.84 clo in air versus 0.18 clo in water); the remainder decreased intrinsic insulation of the ensemble due to water pressure. Extension of these results to a diver working in water (metabolic rate M = 400 watts) indicated that none of the combinations would protect adequately for two hours at 0 C, although four were adequate at 5 C and above. However, these combinations would cause serious overheating after two hours of moderate activity (M = 200 watts) in air at 15 C or below unless the suit was unzipped or hood and gloves removed to increase cooling.