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Argon ions were implanted into n-type 6H-SiC epitaxial layers at 600°C. Postimplantation annealing was carried out at 1,600°C for 5 min in an Ar ambient. Four implantation-induced defect levels were observed at EC-0.28 eV, EC-0.34 eV, EC-0.46 eV, and EC-0.62 eV by deep level transient spectroscopy. The defect center at EC-0.28 eV is correlated with ED1/ED2 and with ID5. The defect at EC-0.46 eV with a capture cross section of 7.8 × 10−16 cm2 is correlated with E1/E2, while the defect at EC-0.62 eV with a capture cross section of 2.6 × 10−14 cm2 is correlated with Z1/Z2. Photo deep level transient spectroscopy was also used to study these defects. Upon illumination, the amplitudes of the deep level transient spectroscopy (DLTS) peaks increased considerably. Two emission components of Z1/Z2 were revealed: one fast and the other slow. The fast component could only be observed with a narrow rate window. In addition, a new defect was observed on the low-temperature side of the defect at EC-0.28 eV when the sample was illuminated.

Nitrogen doped 4H–SiC and 6H–SiC epitaxial layers with net doping concentration of 1.5×1015 cm−3 were implanted with either Al, B, or Ar ions at 600 °C. The energy of the ions was 160 keV and at a dose of 2×1016 cm−2. After annealing at 1600 °C for 5–60 min, Schottky diodes were fabricated on the samples. The measured junction capacitance of ion implanted samples varies with both temperature and frequency. The thermal activation energy determined from the frequency break ωB ranges from 183 to 202 meV. The Ar implanted samples remain n-type and show no capacitive dispersion. Similarly, the junction capacitance of samples doped with Al does not exhibit dispersion. It is suggested that the implantation induced defects play a role in the observed dispersion. The data show that the series resistance is insignificant and does not account for the observed dispersion.

Layered oxide materials having alternating repeated layer thicknesses of 10 nm or less are difficult to make, especially with sharp interfaces. Nanostructured thin films having repeated layers of two different oxide materials were obtained by using pulsed laser deposition and two independent stationary targets consisting of Al 2 O 3 and BaTiO 3 . Desired thicknesses were achieved by using a specific number of pulses from a 248-nm KrF excimer laser, at an energy of 450 mJ / pulse , a galvanometer mirror system, and a background pressure of oxygen. Trends in material properties were identified by systematically varying the number of pulses for multiple nanostructured thin films and comparing the resulting properties measured using in-situ spectroscopic ellipsometry and ex-situ capacitance measurements, including relative permittivity and loss. Four films were deposited with a goal of having 0.25-, 1-, 4-, and 10-nm thick layers, and each ∼ 220 nm thick. Ellipsometry data were modeled in situ to calculate thickness, n and k . A representative transmission electron microscopy measurement was also collected for the 10-nm sample with corresponding x-ray photoelectron spectroscopy and energy disperive x-ray spectroscopy. Ellipsometry and capacitance measurements were all performed on each of the samples, with one sample having calculated impedance greater than 30 GOhm at 0.001 Hz.

By analyzing the temperature dependence of the capacitance and AC conductance of a zero-biased Schottky diode one obtains the activation energy of the primary shallow dopant in a semiconductor. This technique has become known as Thermal Admittance Spectroscopy. Thermal admittance spectroscopy has been used to characterize nitrogen donors in 4H-, 6H-, and 15R-SiC. p-type dopants (Al, B) have also been characterized in 6H-SiC. In addition, an activation energy attributable to hopping conduction has been obtained in n-type specimens of all three polytypes. The change in the admittance of a Schottky diode caused by illumination, measured as a function of wavelength, is called Optical Admittance Spectroscopy. Optical admittance spectroscopy has been used to study deep centers in silicon carbide where traditional electrical detection is made difficult by the requirement for very high measurement temperatures. Using this technique, the energies of the midgap donor-like levels attributed to vanadium atoms substitutionally occupying the inequivalent lattice sites have been determined. The bandgaps of 6H- and 4H-SiC polytypes have been measured, and the phonon spectra associated with the indirect transitions from the valence band to the conduction band have been determined.

The nitrogen levels in 4H‐SiC have been determined using thermal admittance spectroscopy. The values of Ec−0.053 eV for nitrogen at the hexagonal site and Ec−0.10 eV for nitrogen at the quasicubic site agree with those reported using other techniques. The deep levels in 4H‐SiC were studied using optical admittance spectroscopy. The optical admittance spectrum showed, besides the conductance peak corresponding to band to band transitions, four other conductance peaks. These peaks correspond to photoexcitation of carriers from the defect levels to the conduction band. It is inferred from a comparison with 6H‐SiC that the conductance peak b4 is due to excitation of electrons from the vanadium donor at Ec−1.73 eV. The photoconductance build up transients of the Ec−1.73 eV level are described fully by one exponential term. This suggests that only one center contributed to the observed conductance. The decay kinetics of persistent photoconductance due to the Ec−1.73 eV level follow the stretched exponential for...

The vanadium donor level in high‐resistivity p‐type 6H‐SiC has been studied using optical admittance spectroscopy. Besides the conductance peak due to the band to band transitions, there are three conductance peaks in the spectra of most of the samples. These peaks correspond to photoexcitation of electrons from the valence band to the defect levels. The conductance peak due to the vanadium donor [V4+(3d1)] level at Ev+1.55 eV is identified. The build up of the photoconductance at this peak was studied and it was found that the conductance transients are completely described by a sum of two exponential expressions. The relevant parameters, α1, α2, Gmax(1) and Gmax(2), were determined as functions of temperature. The persistent photoconductance (PPC) due to this defect was also studied. The decay kinetics of the PPC follow the stretched exponential form. The potential barrier against recapture of carriers was determined to be 220 meV for the vanadium donor level.

We have examined the temperature dependence of the barrier height of Al, Ag, Au, and W metal‐semiconductor contacts on n‐type 6H‐SiC, and Al and Ag metal‐semiconductor contacts on p‐type 6H‐SiC. The barrier height was determined from the reverse biased capacitance‐voltage characteristics of the contacts at temperatures ranging from 300 to 670 K. The measurements were made at 1 MHz. These measurements were compared to the behavior predicted by standard models. All the diodes displayed a negative temperature dependence on n‐type SiC, and a positive temperature dependence on p‐type SiC. The temperature coefficient is related to the electronegativity of the metal by linear expression.

Defects in n‐type 6H‐SiC have been studied using optical admittance spectroscopy. Six conductance peaks, which correspond to photoexcitation of electrons into the conduction band from defects and the valence band at different wavelengths, were clearly identified. Persistent photoconductance (PPC) due to a defect 1.07 eV below the conduction band was studied. The decay kinetics of the PPC follow the stretched exponential form. The relaxation time constant τ and the stretching factor β were systematically measured as functions of temperature. The thermal capture barrier of 61 meV was determined from these results. It was also found that the PPC can be quenched optically by illumination with sub band gap radiation. This is the first reported observation of optical quenching of PPC in n‐type 6H‐SiC. The lattice relaxation model is used to qualitatively explain these experimental results.

The potential of bio-dielectrics for thin film transistor applications was explored via the incorporation of titanium dioxide (TiO2) nanoparticles, rutile form, a high dielectric constant (ε) ceramic, in the deoxyribonucleic acid (DNA) bio-polymer. The DNA-ceramic hybrid films were fabricated from stable suspensions of the TiO2 nanoparticles in viscous, aqueous DNA solutions. Dielectric characterization revealed that the incorporation of TiO2 in DNA resulted in enhanced dielectric constant (14.3 at 1 kHz for 40 wt % TiO2) relative to that of DNA in the entire frequency range of 1 kHz-1 MHz. Variable temperature dielectric measurements, in the 20-80°C range, of the DNA-TiO2 films revealed that the ceramic additive stabilizes DNA against large temperature dependent variations in both ε and the dielectric loss factor tan δ. The bulk resistivity of the DNA-TiO2 hybrid films was measured to be two to three orders of magnitude higher than that of the control DNA films, indicating their potential for utilization as insulating dielectrics in transistor and capacitor applications.

Polymeric materials are widely used in power generation and energy storage applications. Deoxyribonucleic acid (DNA) biopolymer-based hybrids have been found to display interesting electrical characteristics, such as a relatively high dielectric constant, good resistivity and dielectric breakdown behavior, and are promising as insulating dielectrics for capacitor applications. This research describes the processing, test structure design, and electrical characterization of DNA-sol-gel hybrids for energy storage applications.

Deep level transient spectroscopy (DLTS) has been used to characterize deep impurity levels in n‐type 6H‐SiC single crystals. A defect level at Ec−0.71 eV with an electron capture cross section σ=5.63×10−20 cm2 was observed. Defect concentration profiles confirm that the defect is a bulk defect. Infrared absorption measurements in the spectral range of 7000–7700 cm−1 were made using these samples. The infrared absorption spectrum characteristic of vanadium in silicon carbide is composed of a group of three absorption lines in the spectral range of 7000–7700 cm−1. This infrared signature was seen in the specimens in which the DLTS spectrum revealed the presence of deep traps. This signature was absent in those specimens where no deep traps were indicated by DLTS. Correlating these facts, we have concluded that the observed peak at Ec−0.71 eV was due to vanadium atoms in silicon carbide.

Admittance spectroscopy has been used to study shallow levels in n‐type 6H‐SiC single crystals. A total of eight unintentionally doped n‐type samples obtained from three different sources were used in this study. Two of the samples were grown by the Lely method, while the others were grown by physical vapor transport. Two electron traps at EC−0.04 eV and EC−0.03 eV were detected in the more heavily n‐type (ND−NA=1018 cm−3) samples. These defects may be due to contaminants other than nitrogen. A defect level at EC−0.08 eV as detected in a sample with ND−NA=8.9×1017 cm−3. This level is associated with nitrogen at the hexagonal site (h). An electron trap at EC−0.11 eV was detected and is associated with nitrogen at the quasicubic sites (k1k2). This level was observed only in the lightly n‐type samples (ND−NA=4.7 ×1015–6.4×1017 cm−3).

The potential of DNA-based dielectrics for energy storage applications was explored via the incorporation of high dielectric constant (e) ceramics such as TiO2 (rutile) and BaTiO3 in the DNA bio-polymer. The DNA-Ceramic hybrid films were fabricated from stable suspensions of the nanoparticles in aqueous DNA solutions. Dielectric characterization revealed that the incorporation of TiO2 (rutile) in DNA resulted in enhanced dielectric constant (14.3 at 1 kHz for 40 wt % TiO2) relative to that of DNA in the entire frequency range of 1 kHz-1 MHz. Variable temperature dielectric measurements, in the 20-80°C range, of both DNA-TiO2 and DNA-BaTiO3 films, revealed that the ceramic additive stabilizes DNA against large temperature-dependent variations in both e and the dielectric loss factor tan δ. The bulk resistivity of the DNA-Ceramic hybrid films, in the case of both TiO2 and BaTiO3 additives in DNA, was measured to be two to three orders of magnitude higher than that of the control DNA films, indicating their potential for utilization as insulating dielectrics in capacitor applications. As a part of a baseline study, results based on a comparison of the temperature-dependent dielectric behavior of DNA and DNA-CTMA complex films as well as their frequency-dependent polarization behavior, are also discussed.

The allotropic transformation of cobalt was studied with a differential scanning calorimeter. The temperatures of the fcc to hep transformation, As, and fcc to hcp transformation, Ms, exhibit a linear dependence on the heating and cooling rates: As(K) = 723.2 + 0.28R and Ms(K) = 671.4-0.28R whereR is the rate in K/min. As, Ms, and the associated enthalpies of the phase changes were observed to increase with increasing number of cycles through the transformation. The Ms reaction was observed to proceed slowly isothermally while exhibiting athermal behavior when cooled rapidly.

Oxide materials of desired stoichiometry are challenging to make in small quantities. Nanostructured thin films of multiple oxide materials were obtained by using pulsed laser deposition and multiple independent targets consisting of Si, BaTiO3, and B. Programmable stoichiometry of nanostructured thin films was achieved by synchronizing a 248-nm krypton fluoride excimer laser at an energy of 300 mJ/pulse, a galvanometer mirror system, and the three independent target materials with a background pressure of oxygen. Island growth occurred on a per pulse basis; some 500 pulses are required to deposit 1 nm of material. The number of pulses on each target was programmed with a high degree of precision. Trends in material properties were systematically identified by varying the stoichiometry of multiple nanostructured thin films and comparing the resulting properties measured using in situ spectroscopic ellipsometry, capacitance measurements including relative permittivity and loss, and energy dispersive spectroscopy (EDS). Films were deposited ∼150 to 907 nm thickness, and in situ ellipsometry data were modeled to calculate thickness n and k. A representative atomic force microscopy measurement was also collected. EDS, ellipsometry, and capacitance measurements were all performed on each of the samples, with one sample having a calculated permittivity greater than 20,000 at 1 kHz.

Layers of GaN deposited on Si substrates have been studied using Thermal Admittance Spectroscopy (TAS) and Optical Admittance Spectroscopy (OAS). Transparent front-side contacts were used to facilitate the optical measurements. Six specimens were cut from the same location in two different wafers, and three samples were randomly chosen from other growths. A shallow level at EC − 0.051 eV was found in all the specimens using TAS. In some specimens this peak was asymmetric, indicating more than one level near this energy. Deeper levels were also seen in the high-temperature portion of the spectra, but were poorly resolved in most specimens. Illuminating the specimen with UV light at 25 K resulted in the thermal position of the peak shifting to a lower temperature, and hence, the calculated energy, of the peak shifting lower. The amplitude of the peak also decreased. Transient OAS measurements revealed the interesting phenomenon of negative persistent photo conductance at room temperature in some of the specimens when the illumination photon energy was less than the bandgap. The negative response time was very short. At lower temperatures, below 100 K, the negative response diminished, but the response time was still short. At wavelengths above the bandgap energy, normal transient response was seen, with a longer time constant.

The transient photo response of 4H- and 6H-SiC specimens at various wavelengths has been studied using Optical Admittance Spectroscopy. Differences in the transient response were found for excitation and recombination for different specimens in both materials. The results indicate that optical excitation of charge carriers to the conduction band is a single transition, but recombination of free carriers from the conduction band is a process involving multiple transitions to the ground state mediated by deep centers in the material. Similarities in the persistent photoconductance after excitation from deep centers, and after excitation at above bandgap energies, suggest that carriers from the conduction band drop through deep centers on the way to the valence band.

Thermal admittance spectroscopy has been used to determine the ground-state energies of the boron impurity in 6H-SiC. The background doping, NA−ND, of the samples used in this study ranged from 3×1016 to 1×1018 cm−3. From electron spin resonance studies, it is known that boron substitutes for silicon in the silicon carbide lattice occupying three inequivalent sites. Using admittance spectroscopy the ground state energies of Ev+0.27 eV, Ev+0.31 eV, and Ev+0.38 eV were determined for the shallow boron acceptor in 6H-SiC. The free carrier concentration does not appear to be the only determining factor for which the boron acceptor level is observed.

Impurity conduction (or hopping conduction) has been observed in the more heavily n‐type 4H–SiC samples by both temperature dependent resistivity measurements and thermal admittance spectroscopy. The measured activation energies e3 for hopping were 4–5 meV and 2.3–3.0 meV, respectively. No evidence of hopping conduction was seen by either method in the sample where ND–NA

Argon ions (Ar+) were implanted into n-type 4H-SiC epitaxial layers at 600 °C. The energy of the ions was 160 keV and at a dose of 2 × 1016 cm−2. After post-implantation annealing at 1600 °C, Schottky diodes were fabricated on the ion implanted samples. Bulk n-type 4H-SiC samples were irradiated at room temperature with 1 MeV electrons at doses of 1 × 1016 to 5.1 × 1017 el/cm2. The current density of the beam was 0.91 μA/cm2. Deep Level Transient Spectroscopy (DLTS) was used to characterize the induced defects. DLTS studies of Ar+ implanted samples showed six defect levels at EC – 0.18 eV, EC – 0.23eV, EC – 0.31eV, EC – 0.38 eV, EC – 0.72 eV, and EC – 0.81 eV. Z1/Z2 defect is the dominant defect in the electron irradiated sample and anneals out completely after 10 minutes at 1000 °C. However, Z1/Z2 defect in Ar+ implanted samples was stable up to 1600 °C. It is suggested that the annealing behavior of Z1/Z2 depends on the source of its formation.

Optical admittance spectroscopy has been used to study defects in 4H–SiC; the vanadium donor level at EC‐1.73 eV has been identified. The optical admittance spectrum of 4H–SiC is similar to that of 6H–SiC, where the vanadium donor level is at EC‐1.59 eV. The band gaps of 6H–SiC and 4H–SiC were measured. The values of 3.10±0.03 eV for 6H–SiC and 3.41±0.03 eV for 4H–SiC are in reasonable agreement with reported values. Using the vanadium donor level in both 4H–SiC and 6H–SiC as a common reference, the band offsets for 6H–SiC/4H–SiC heterojunction are estimated to be ΔEC=0.14 eV and ΔEV=0.17 eV.

The slow buildup of photoconductance due to a peak centered at 1.13 eV below the conduction band in n‐type 6H–SiC samples has been studied using optical admittance spectroscopy. The background doping of the samples ranges from 3.2×1015 cm−3 to 2×1018 cm−3. The results indicate that the carriers responsible for the observed conductance are excited from defect levels that are energetically close. The photoconductance transients are completely described by a sum of two exponential expressions. The relevant parameters, α1,α2,Gmax(1), and Gmax(2), are determined as functions of temperatures. The background doping ND‐NA has no effect on the conductance buildup process, this indicates that the nitrogen donors play no role either in the formation of the defect or in the buildup process of the photoconductance.

Aluminum (Al) and argon (Ar) ions were implanted into n-type 4H-SiC epitaxial layers at 600 °C. The energy of the ions was 160 keV at a dose of 2 × 1016 cm-2. After annealing at 1600 °C for 5–60 minutes, Schottky diodes were fabricated on the ion implanted samples. Deep Level Transient Spectroscopy (DLTS) was used to characterize ion implantation induced defects. A defect at EC-0.18 eV was observed in the Al+ implanted devices annealed for five and fifteen minutes. However, annealing for 30 minutes produced an additional deeper defect at EC -0.24 eV. This defect annealed out after a sixty minute anneal. DLTS studies of Ar+ implanted devices showed six defect levels at EC -0.18 eV, EC -0.23 eV, EC -0.31 eV, EC -0.38eV, EC -0.72 eV, and EC -0.81eV. These defects are attributed to intrinsic-related defects. It is suggested that “hot” implantation of Al+ inhibits the formation of intrinsic-related defects. While “hot” implantation of Ar+ into 4H-SiC does not reduce the concentration of the vacancies and interstitials.

We have examined specimens of high-resistivity, and semi-insulating, 4H-SiC before and after thermal annealing at 1600 °C, using Optical Admittance Spectroscopy. We have found enhanced ultraviolet response in most specimens. Enhanced activation of previously undetected V impurities has also been observed. Peaks believed to be attributable to complex V-related defects were greatly reduced by annealing. The annealing was in addition to a thermal oxidation at 1150 °C for 4 hours. The purpose of the oxidation was to remove surface-related deep levels known to be present in polished SiC. Transition metal impurities in these bulk specimens were quantified by SIMS. In specimens where Ti was not detected by SIMS, no further activation of Ti centers was detected by Optical Admittance Spectroscopy.

The anisotropy of Young's modulus in human cortical bone was determined for all spatial directions by performing coordinate rotations of a 6 by 6 elastic stiffness matrix. The elastic stiffness coefficients were determined experimentally from ultrasonic velocity measurements on 96 samples of normal cortical bone removed from the right tibia of eight human cadavers. The following measured values were used for our analysis: c11 = 19.5 GPa, c22 = 20.1 GPa, c33 = 30.9 GPa, c44 = 5.72 GPa, c55 = 5.17 GPa, c66 = 4.05 GPa, c23 = 12.5 GPa. The remaining coefficients were determined by assuming that the specimens possessed at least an orthorhombic elastic symmetry, and further assuming that c13 = c23 c12 = c11 - 2c66. Our analysis revealed a substantial anisotropy in Young's modulus in the plane containing the long axis of the tibia, with maxima of 20.9 GPa parallel to the long axis, and minima of 11.8 GPa perpendicular to this axis. A less pronounced anisotropy was observed in the plane perpendicular to the long axis of the tibia. To display our results for the full three-dimensional anisotropy of cortical bone, a closed surface was used to represent Young's modulus in all spatial directions.

The optical and electrical properties of chrome-doped CdGeAs2 (CGA), an important non-linear optical material, are reported. CGA, a chalcopyrite semiconductor of the pseudo- III-V type, is a close ternary analog to GaAs, possessing significant differences. To date, the electrical and optical properties of as-grown undoped CGA have been controlled by a somewhat shallow dominant residual acceptor which it is the source of significant undesirable optical absorption. Highly transparent semi-insulating CGA should be attainable using compensation and counterdoping schemes similar to those used for GaAs. However, identifying suitable deep and shallow n-type and p-type dopants will require extensive empirical studies. As a starting point of survey to find deep levels, the properties of CGA:Cr have been investigated. Cr is a reasonable choice as it has been used extensively to provide a deep level in GaAs. Thermal Admittance Spectroscopy was used to examine the electrically active levels in this material. These measurements were correlated with temperature dependent Hall effect measurements, and IR absorption measurements. SIMS analysis was utilized to estimate the Cr concentration as the segregation coefficient for Cr in CGA has not been reported.. The results show that there is a p-type level introduced into the band gap at about 0.16 eV above the valence band, a value nominally 50% deeper than that of the native acceptor. The background doping as measured by Capacitance-Voltage measurements was determined to be 8 × 1016 cm−3 near the surface, and 1.0 × 1017 cm−3 in the bulk. These results are compared to similar measurements in undoped material.

Brudnyi, et al., and Zwieback, et al., have shown that introducing damage by irradiation with MeV electrons can alter the electrical and optical properties of undoped p-type CdGeAs2(CGA) crystals. Brudnyi's studies of the electrical transport properties of isochronally annealed samples demonstrated type conversion and identified at least two new centers, one a stable donor. Zwieback used multi-MeV electrons to introduce compensating donors, thereby, significantly improving the optical transparency of CGA crystals. However, at the present little is known about these centers. Therefore, we have studied these centers by observing the properties of electron-irradiated specimens using Thermal Admittance Spectroscopy (TAS) and correlated the results of these measurements with capacitance-voltage measurements and Hall effect measurements. Measurements before an after irradiation are compared. The as-grown native acceptor concentrations in our samples varied from a low in the mid 1016 cm−3to nearly 1019 cm−3. Significant changes in the electrically active states in the band gap were seen as a result of a single irradiation with 2 MeV electrons to a total dose of 5 × 105cm−2. The samples appear to respond more strongly than Brudnyi's samples. The thermal activation energies have been determined using TAS and they will be reported.

Thermal admittance spectroscopy was used to characterize the shallow dopants in chemical vapor deposition ( CVD) grown thin films and in sublimation sandwich method ( SSM) grown 4H-SiC layers. The values of the activation energy levels of EC − 0.054 eV for Nitrogen at the hexagonal site and of EC − 0.10 eV for Nitrogen at the cubic site were indices of comparison. The net carrier concentrations ( ND − NV ) of the films were determined by capacitance-voltage measurements. The net carrier concentrations for the SSM films ranged from 2 × 1017 to 7 × 1017 cm−3. The two Nitrogen levels were observed in the CVD films. Hopping conduction with an activation energy of EC −0.0058 eV was observed in one SSM sample having ND − NV = 7 × 1017 cm−3.

P-type conductivity is normally produced in silicon carbide by doping with Al or B. The aluminum atom substitutes for silicon in the SiC lattice. Several studies have shown that nitrogen, which substitutes for carbon atoms and produces n-type conductivity in SiC, sits at the two inequivalent lattice sites in 4H-SiC, and two N activation energies of 53 meV (h) and 100 meV (k) have been reported.Hall effect measurements and infrared absorption studies of SiC doped with Al showed only one Al activation energy, however, and the published Al activation energy in SiC (3C, 4H, and 6H) ranges from 191 meV to 280 meV. This has lead to the general conclusion that the activation energy of Al acceptors in SiC does not depend on the polytype or on the lattice site occupied.Thermal Admittance Spectroscopy (TAS) has been used to determine the activation energy of Al in 4H-SiC specimens where NA-ND ranged from 7.2 × 1015 cm2to 5.6 × 1018 cm−2. By analogy to B in 6H-SiC, we propose two levels for Al in 4H-SiC at Ev + 0.212 eV and Ev + 0.266 eV.

Thermal admittance spectroscopy has been used to study the thermal activation energy of nitrogen at the hexagonal and cubic sites in 4H-SiC as function of net doping concentration. The net doping concentration- of te samples, which was determined from 1/C2 vs. V plots, ranges from 1.5 × 1014 cm−3 to 4 × 1018 cm−3. The thermal activation energy of nitrogen was determined to be Ee O.054 eV and Ee O.101 eV for nitrogen at hexagonal and cubic sites respectively for ND - NA ≤ 1016 cm−3. As the free carrier concentration increases from 1016 cm−3 to 1.0 × 1018 cm3, the thermal activation energy of nitrogen at the hexagonal site decreases from 54 meV to 24 meV. At ND - NA ≥1.0 × 10 cm−3 hopping conduction is the only conduction mechanism and has an activation energy of 3-9 meV.

Nominally undoped n-type GaN layers grown by metalorganic chemical vapor deposition on silicon substrates were investigated using Thermal Admittance Spectroscopy and Optical Admittance Spectroscopy (OAS). A defect level was observed at Ec – 0.051 eV, and it is correlated with the nitrogen vacancy (NV) which is a donor in GaN. Illuminating the samples with a monochromatic light with wavelengths ranging from 200 nm to 450 nm, the OAS spectrum was measured at different temperatures and with different excitation light intensities. A dominant peak was observed in the OAS spectrum at λ = 365 nm (3.40 eV); this is attributed to transitions from the valence band to the donor level. Our results show that the saturation level, Gm, of the photoconductance is a function of both light intensity and temperature. The photoconductance decay, after the illumination has been terminated, is non-exponential but it is fully described by the stretched exponential function. The value of β ranges from 0.78 to 0.86. The analysis suggests that the observed photoconductance decay is due to thermal emission of photo-excited carriers from the donor level.

This paper describes an optical fiber based sensor for detecting groundwater and for monitoring the concentration of particulate material which may be entrained in that water. The sensing system uses white light from a multi-wavelength spectral source which is transmitted through a 200 micrometer diameter multimode fiber to a sensing head. The infra-red portion of the light acts as a reference while the visible part is transmitted through a sampling area and is reflected back in to a receiving fiber. The reference and modulated spectra are detected by a dual epitaxial diode. Electronic processing results in compression of the information from an intensity versus wavelength format to a single number called dominant wavelength. This number is different for air and water, and for water with and without particulates. The discrimination between air and water is possible because of differences in the absorption spectra and refractive indices, and between water with and without particulates through Mie scattering.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Thermal Admittance Spectroscopy(TAS) has been used to detect energy levels in the bandgap of CdGeAs2 specimens. Capacitance-Voltage(CV) measurements were used to determine the net free carrier concentration of the specimens as well as the conductivity type. All specimens were found to be p-type. CV measurements determined that the free carrier densities ranged from 1.2 × 1017 cm−3 to 8 × 1018 cm−3. Usually one peak (but in some cases two) was observed in the thermal admittance spectra. One peak present in two samples indicates an acceptor with a thermal activation energy of Ev+(0.10–0.13) eV which corresponds closely to the value of 0.10–0.12 eV found from Hall effect measurements on these specimens. The additional peak observed could correspond to a second deeper acceptor at Ev+0.346 eV, however, the energy could not be accurately determined because the peak was not fully resolved. Evidence for the existence of two native acceptors from electron paramagnetic resonance has recently been reported which tends to support a two acceptor model.

Impurity conduction (or hopping conduction) has been observed in the more heavily n-type 4H-SiC samples by both temperature dependent resistivity measurements and thermal admittance spectroscopy. The measured activation energies ɛ 3 for hopping were 4–5 meV and 2.3–3.0 meV respectively. No evidence of hopping conduction was seen by either method in the sample where ND-NA < 1018 cm-3. The thermal admittance spectrum of the lightly n-type sample showed the two nitrogen levels at 53 and 100 meV.

Nitrogen is the common n-type dopant of the various polytypes of silicon carbide. The nitrogen levels in 4H-SiC (at EC-53 meV and EC-100 meV) and in 6H-SiC (at EC-89 meV, EC- 100 meV, and EC-125 meV) have been studied in detail by temperature dependent Hall effect measurements, electron spin resonance (ESR), and thermal admittance spectroscopy. Until now, such detailed studies of the nitrogen levels in 15R-SiC have not been carried out.Lely-grown 15R samples were used in these studies. The net carrier concentrations (ND-NA), determined by room temperature CV measurements, ranged from 1 × 1018 to 3 × 1018 cm−3. The nitrogen levels in 15R-SiC were studied using thermal admittance spectroscopy. Optical admittance spectroscopy (OAdS) was used to study the deeper defects in this polytype. It was found that optical transitions to the conduction band were inhibited in the heavily doped material.

Hall effect, deep level transient spectroscopy, optical absorption, and optical admittance spectroscopy were employed to determine the position of the vanadium acceptor and vanadiumnitrogen complex in vanadium- and nitrogen-doped 4H and 6H SiC. Hall effect results indicate that the acceptor position in 4H(6H) SiC is 0.80(0.66) eV beneath the conduction band edge. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 806 meV and a capture cross section of 1.8×10−16 cmr−2 The optical absorption measurements proved that the acceptor level investigated is related to isolated vanadium, and therefore the vanadium acceptor level. Based upon DLTS and SIMS measurements, the maximum solubility of vanadium in SiC was determined to the 3×10−17 crn3. An examination of polarized light experiments indicates that vanadium also complexes with another element to form electronic(at 5000 cm−1) and vibrational absorption(at 683 cm−1) bands. While the other constituent cannot be identified, evidence suggests that nitrogen is a likely candidate. This complex introduces a deep level at Ec−0.78 eV as determined using optical admittance spectroscopy.

Deep levels in high resistivity p-type 6H-SiC has been studied using optical admittance spectroscopy ( OAS ). Besides the conductance peak due to the band to band transitions, there are three conductance peaks in the spectra of most of the samples. The conductance peak due to the vanadium donor (0/+) level at EV+ 1.55 eV is identified. The persistent photoconductance (PPC) at this defect was also studied. The decay kinetics of the PPC follow the stretched exponential form. The potential barrier against recapture of carriers was determined to be 220 meV for the vanadium donor level.

We have examined the temperature dependence of the barrier height of Au, Ag, Ni, and Al, metal-semiconductor contacts on n-type 6H-SiC, and Al metal-semiconductor contacts on p-type 6H-SiC. The barrier height was determined from the (1/C2) vs VRcharacteristics of the contacts at temperatures ranging from 300K to 670K. The measurements were made at 1 MHz. These measurements were compared to I-V measurements at various temperatures, and to the behavior predicted by standard models.

Several n-type 4H-SiC samples grown by the physical vapor transport technique were studied using both thermal and optical admittance spectroscopy. The ionization energy ED for nitrogen donors in 4H-SiC was determined to be 0.053 eV and 0.10 eV below the conduction band. This agrees reasonably well with the values of 0.052 eV and 0.092 eV determined by IR absorption measurements. It is believed that EC-0.053 eV is the ionization energy of the nitrogen atom occupying the hexagonal site (h). The 0.10 eV activation energy has been attributed to the nitrogen atoms occupying the cubic sites(k).The optical admittance studies reveal five conductance peaks. One of these corresponds to the band-to-band transitions from which the bandgap of 3.41 eV was determined at 40K. The other four conductance peaks correspond to transitions from defect levels to the conduction band. These defects are located at EC-1.73 eV, EC-1.18 eV, EC-0.87 eV, and EC-0.72 eV.

An optical fibre sensing system is assessed for radiation performance. The optical fibres are irradiated withgamma radiation from a cobalt 60 source. The sensing system consists of a white light source, silicon detectors and multimode optical fibres. The optical system measures the chromaticity of the light in dominant wavelength derived from either a distimulus or tristimulus measurement. Both measurement systems are generic and can operate with up to eightmodulating sensors simultaneously and are designed to form part of an existing monitoring or control system. Allcomponents are commercially available and the optical fibre system is undergoing trails in the nuclear, chemical and electrical power distribution industries. 1. INTRODUCTIONFibre optic cables have been used for communication purposes for 15 years. The bandwidth benefits offered bysuch systems has improved the speed and quality of information services. Optical sensors using fibres developed forcommunication purposes were also developed in parallel. The technology used for such sensing systems was, in thebeginning, much reliant upon the technology available in the communication sector and therefore these sensing systemshad inhereted limitations due to fibres, sources and detectors. The application of the communication technology formilitary purposes lead to much work in the development of not only radiation tolerant fibres but various optical sourcesand detectors. This advance coupled with the growth in optical sensing systems has lead to much interest in the potentialapplication of this new generation of optical fibre sensing systems to nuclear environments. The number of modulation

Optical admittance spectroscopy is a technique for measuring the conductance and capacitance of a junction under illumination as a function of the wavelength of the light and the frequency of the measuring AC signal. This technique has been applied to characterize deep defect levels in 6H-SiC:N. Nitrogen is a donor atom in 6H-SiC which substitutes for carbon in three inequivalent sites (h, k1, k2). giving rise to n-type conduction. Deep defect levels attributable to transition metal impurities have also been identified in 6H-SiC:N. We have examined persistent photoconductance in this material by optical admittance spectroscopy.

The chemical state of (NH4)2S treated (100)GaAs surfaces exposed to ambient conditions for several days was correlated with barrier height measurements. Surface chemistry was characterized by Auger electron spectroscopy and barrier heights were calculated from C-V measurements obtained with a Hg probe. Results show that surfaces are chemically and electrically unstable for several hours following the sulfide treatment. The chemical and electrical states continually changed during ambient exposure up to 300 hours. Although strictly speaking, the surfaces were not passivated, the presence of sulfur did inhibit the formation of Ga and As oxides and the incorporation of carbon. In addition, stable, low barrier heights were observed after ambient exposure for several hours. Barrier heights from C-V measurements using deposited Au and Al contacts were compared to the barrier heights obtained with a Hg probe.

Transition metal-germanide/silicide based ohmic contact systems to n-type GaAs have been studied using current-voltage (I-V) and transmission line measurements (TLM). The effects of substituting various metals from groups IVB and VIB into the metallization stack have been compared, as well as the effects of using germanium or silicon. Rapid Thermal Annealing (RTA) was used to facilitate the reactions. Titanium and molybdenum were shown to be more effective metallizations than chromium, even though chromium readily forms a silicide or germanide. Germanium has been found to produce an ohmic contact of low resistivity, while metallization with silicon alone does not show ohmic behavior.

Z1/2 defect centers were produced by irradiating 4H-SiC bulk samples with 1 MeV electrons at room temperature. The emission rate dependence on the electric field in the depletion region was measured using deep level transient spectroscopy and double-correlation deep level transient spectroscopy. It is found that the Z1/2 defect level shows a strong electric field dependence with activation energy decreasing from Ec−0.72 eV at zero field to Ec−0.47 eV at 6.91×105 V/cm. The phonon assisted tunneling model of Karpus and Perel [Sov. Phys. JETP 64, 1376 (1986)] completely describes the experimental data. This model describes the dependence of the emission rate on electric field F as en(F)=eno exp(F2/Fc2), where Fc is the characteristic field that depends on the phonon assisted tunneling time τ2. The values of Fc and τ2 were determined and the analysis of the data leads to the suggestion that Z1/2 may be a substitutional point defect.

Contact metallizations for space-based GaAs solar cells must survive at high temperatures for several minutes. Which metallizations will survive can be predicted by properly calculated phase diagrams instead of performing hundreds of reactions. A method for calculating the ternary temperature constitution phase diagrams Is briefly explained and the phase diagrams are calculated for two Ga-As-X alloy systems. The free energy functions of the liquid and solid phases are approximated by the regular solution theory. The phase diagrams of the three binary alloy systems which form the boundaries of the ternary alloy system are utilized to calculate the binary regular solution parameters. The free energy functions for the ternary liquid and solid phases are expressed In terms of the binary regular solution parameters. The temperature and composition of the liquidus and solidus boundaries for the ternary alloy systems are calculated from these free energy functions. Calculated results are presented for the Ga-As-Ag and the Ga-As-Ge systems.

Temperature dependent Hall effect (TDH), low temperature photoluminescence (LTPL), secondary ion mass spectrometry (SIMS), optical admittance spectroscopy (OAS), and thermally stimulated current (TSC) measurements have been made on 6H-SiC grown by the physical vapor transport technique without intentional doping. n- and p-type as well semi-insulating samples were studied to explore the compensation mechanism in semi-insulating high purity SiC. Nitrogen and boron were found from TDH and SIMS measurements to be the dominant impurities that must be compensated to produce semi-insulating properties. The electrical activation energy of the semi-insulating sample determined from the dependence of the resistivity was 1.0eV. LTPL lines near 1.00 and 1.34eV, identified with the defects designated as UD-1 and UD-3, were observed in all three samples but the intensity of the UD-1 line was almost a factor of 10 more in the n-type sample than in the the p-type sample with that in the semi-insulating sample being inter...

We reply to the comment by Frojdh and Petersson concerning the measurements of the barrier heights of Schottky diodes on 6H‐SiC. We present the argument that the temperature range over which Frojdh and Petersson present data does not allow accurate comparison with our results. The problem of interaction of deep levels in the capacitance‐voltage measurements is refuted by deep‐level transient spectroscopy data. We conclude that our data are accurate and our methods reliable.

We demonstrate ohmic contacts to SiC using carbon films after thermal annealing. Carbon films are deposited on 4H-SiC and 6H-SiC substrates using a radio frequency sputtering method. The carbon/SiC samples convert from Schottky behavior to ohmic behavior after annealing in the temperature range from 1150 to 1350°C. Nanosize graphitic flakes are identified after annealing by Raman spectroscopy and are associated with the ohmic behavior of the contacts. The contact behavior of carbon films on 4H-SiC and 6H-SiC after annealing are compared and the polytype of the SiC has no effects on the structural evolution of the carbon films during annealing. This study reveals that the structural evolution of carbon is associated with formation of ohmic contacts on SiC and that nanosize graphitic flakes play a determinative role in the formation of ohmic contacts. © 2003 The Electrochemical Society. All rights reserved.

We present the results of low‐temperature Hall measurements on 1‐MeV‐electron‐irradiated In‐doped Si. It is observed that the concentrations of the acceptor In‐X, Nx, and the compensating donor ND increase immediately after irradiation with fluences ranging from 2×1015 to 1016 e/cm2. It is suggested that ND increases after irradiation due to the creation of divacancies, interstitial indium, and interstitial indium–substitutional indium pairs, where the interstitial indium atoms occupy the tetrahedral site. To explain the increase in Nx we propose three possible models for In‐X in terms of indium‐vacancy pairs, indium interstitials in the split configuration, or indium interstitials in the bond‐centered configuration.