S. R. Smith

Compensation mechanism in high purity semi-insulating 4H-SiC

A study of deep levels in high purity semi-insulating 4H-SiC has been made using temperature dependent Hall effect (TDH), thermal and optical admittance spectroscopies, and secondary ion mass spectrometry (SIMS). Thermal activation energies from TDH varied from a low of 0.55eV to a high of 1.65eV. All samples studied showed n-type conduction with the Fermi level in the upper half of the band gap. Fits of the TDH data to different charge balance equations and comparison of the fitting results with SIMS measurements indicated that the deep levels are acceptorlike even though they are in the upper half of the band gap. Carrier concentration measurements indicated that the deep levels are present in concentrations in the low 1015cm−3 range, while SIMS results demonstrate nitrogen and boron concentrations in the low to mid-1015-cm−3 range. The results suggest that compensation in this material is a complex process involving multiple deep levels.

Determination of the band offsets of the 4H–SiC/6H–SiC heterojunction using the vanadium donor (0/+) level as a reference

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 effect of electron irradiation on the In‐X acceptor in In‐doped silicon

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.

Electrical properties of unintentionally doped semi-insulating and conducting 6H-SiC

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...

Deoxyribonucleic acid-ceramic hybrid dielectrics for potential application as gate insulators in organic field effect transistors

Hybrid films incorporating high dielectric constant k ceramics (BaTiO3 and TiO2) in deoxyribonucleic acid (DNA) were fabricated from highly stable dispersions of the ceramic nanoparticles in viscous, aqueous DNA solutions. Dielectric and electrical properties of the as-prepared nanocomposite films were investigated for potential use as gate insulators in organic field effect transistors. A k value as high as 14 was achieved with a 40 wt. % loading of ceramic nanoparticles in DNA. Electrical resistivities on the order of 1014 Ω cm with leakage current densities on the order of 10−9 A/cm2 were measured from current-voltage experiments under electric field biases up to 50 V/μm.

Admittance Spectroscopy of 6H, 4H, and 15R Silicon Carbide

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.

Deoxyribonucleic acid-based hybrid thin films for potential application as high energy density capacitors

Deoxyribonucleic acid (DNA) based hybrid films incorporating sol-gel-derived ceramics have shown strong promise as insulating dielectrics for high voltage capacitor applications. Our studies of DNA-CTMA (cetyltrimethylammonium) complex/sol-gel ceramic hybrid thin film devices have demonstrated reproducibility and stability in temperature- and frequency-dependent dielectric properties with dielectric constant k ∼ 5.0 (1 kHz), as well as reliability in DC voltage breakdown measurements, attaining values consistently in the range of 300–350 V/μm. The electrical/dielectric characteristics of DNA-CTMA films with sol-gel-derived ceramics were examined to determine the critical energy storage parameters such as voltage breakdown and dielectric constant.

Deep levels in undoped horizontal Bridgman GaAs by Fourier transform photoconductivity and Hall effect

Deep levels between 0.1 and 1.0 eV in semi‐insulating and high resistivity undoped horizontal Bridgman GaAs have been studied by temperature‐dependent Hall effect (TDH) and Fourier transform photoconductivity (FTPC). Activation energies at 0.77, 0.426, and 0.15 eV have been observed by TDH. Photoionization thresholds at 1.0, 0.8, 0.56, 0.44, and 0.25 are reported. The photoconductivity thresholds at 0.56 and 0.25 eV are reported for the first time. New features in the 0.44 eV threshold suggest that the defect responsible for this level has a small lattice relaxation and Frank–Condon shift. Possible associations of the FTPC and TDH energies with the deep‐level transient spectroscopy levels EL2, EL3, and EL6 are presented.

Bio-dielectrics based on DNA-Ceramic hybrid films for potential energy storage applications

The potential of DNA-based dielectrics for energy storage applications was explored via the incorporation of high dielectric constant (ε) 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 ε 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.

Tunable Stoichiometry of SiOx-BaTiOy-BOz Fabricated by Multitarget Pulsed Laser Deposition

Abstract. 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.