Jian H. Zhao

4H-SiC UV photo detectors with large area and very high specific detectivity

Pt/4H-SiC Schottky photodiodes have been fabricated with the device areas up to 1 cm/sup 2/. The I-V characteristics and photoresponse spectra have been measured and analyzed. For a 5 mm/spl times/5 mm area device leakage current lower than 10/sup -15/ A at zero bias and 1.2/spl times/10/sup -14/ A at -1 V have been established. The quantum efficiency is over 30% from 240 to 320 nm. The specific detectivity, D/sup */, has been calculated from the directly measured leakage current and quantum efficiency are shown to be higher than 10/sup 15/ cmHz/sup 1/2//W from 210 to 350 nm with a peak D/sup */ of 3.6/spl times/10/sup 15/ cmHz/sup 1/2//W at 300 nm.

Monte Carlo study of electron transport in SiC

Temperature- and electric field-dependent electron transport in 3C–, 4H–, and 6H–SiC has been calculated by the Monte Carlo technique. Due to the freezeout of deep donor levels the role of ionized impurity scattering in 6H–SiC is suppressed and the role of phonon scattering is enhanced, compared to 3C– and 4H–SiC. There are indications of impurity band formation for impurity concentrations exceeding 1019 cm−3. It is found that ionized impurity scattering along with the deep donor ionization is responsible for the temperature dependence of mobility anisotropy ratio. Electron effective masses and electron-phonon coupling constants have been deduced from the comparison of Monte Carlo simulation results with available experimental data on low-field electron mobility. The extracted model parameters are used for high-field electron transport simulations. The calculated velocity-field dependencies agree with experimental results. The saturation velocities in all three polytypes are close, but the transient veloc...

A general planar waveguide autoregressive optical filter

A general architecture for an autoregressive (AR) planar waveguide optical filter is demonstrated for the first time. Its advantages are a flatter passband, sharper rolloff and better rejection in the stopband compared to finite impulse response (FIR) filters with the same number of stages. The architecture can be extended to an arbitrary number of stages. A modified Levinson algorithm is derived for filter synthesis and analysis which includes waveguide loss and phase errors between stages. The filter analysis algorithm allows the filters coupling ratios and phase errors for each stage to be determined from the filters spectral response. When combined with a postfabrication tuning process, this analysis method provides feedback for optimizing the response. Autoregressive lattice filters were designed and fabricated using Ge-doped silica waveguides. Measurements are reported which demonstrate the synthesis and analysis algorithms. The impact of fabrication tolerances on filter synthesis and of measurement uncertainties on filter analysis are investigated.

Demonstration of the first 10-kV 4H-SiC Schottky barrier diodes

This letter reports the demonstration of the first 4H-SiC Schottky barrier diode (SBD) blocking over 10 kV based on 115-/spl mu/m n-type epilayers doped to 5.6 /spl times/ 10/sup 14/ cm/sup -3/ through the use of a multistep junction termination extension. The blocking voltage substantially surpasses the former 4H-SiC SBD record of 4.9 kV. A current density of 48 A/cm/sup 2/ is achieved with a forward voltage drop of 6 V. The Schottky barrier height, ideality factor, and electron mobility for this very thick epilayer are reported. The SBDs specific-on resistance is also reported.

High density, ultrafine precipitates in YBa2Cu3O7−x thin films prepared by plasma‐enhanced metalorganic chemical vapor deposition

Thin films of yttrium‐rich YBa2Cu3O7−x with c‐axis orientation prepared by plasma‐enhanced metalorganic chemical vapor deposition have been examined by high‐resolution transmission electron microscopy. Yttria precipitates smaller than 50 (A) in size have been identified in the matrix. They are uniformly distributed, have a high density as large as 1024 per cubic meter and are oriented with respect to the matrix. The magnetic field dependence of the critical current density of the thin films indicates that the yttria precipitates are effective flux pinning centers.

4H-SiC normally-off vertical junction field-effect transistor with high current density

4H-silicon carbide (SiC) normally-off vertical junction field-effect transistor (JFET) is developed in a purely vertical configuration without internal lateral JFET gates. The 2.1-/spl mu/m vertical p/sup +/n junction gates are created on the side walls of deep trenches by tilted aluminum (Al) implantation. Normally-off operation with blocking voltage V/sub bl/ of 1 726 V is demonstrated with an on-state current density of 300 A/cm/sup 2/ at a drain voltage of 3 V. The low specific on-resistance R/sub on-sp/ of 3.6 m/spl Omega/cm/sup 2/ gives the V/sub bl//sup 2//R/sub on-sp/ value of 830 MW/cm/sup 2/, surpassing the past records of both unipolar and bipolar 4H-SiC power switches.

Study of interface state density and effective oxide charge in post-metallization annealed SiO/sub 2/-SiC structures

A systematic study of post-metallization annealing (PMA) effect on the quality of thermal SiO/sub 2/ on p-type 6H- and 4H-SiC has been carried out. A simultaneous quasi-static hi-lo frequency capacitance-voltage method has been employed to measure the total effective oxide charge (N/sub eff/) and interface state density (D/sub it/). To ensure accurate results, D/sub it/ was measured at 350/spl deg/C which, depending on the hole capture cross sections, should enable the measurement of interface states located in the band gap as deep as 1.3-1.5 eV from the valence band edge. The dependence of N/sub eff/ and D/sub it/ on annealing temperature and ambient as well as the effect of thermal and sputtered gate metal on the oxide quality are reported. It is shown that N/sub eff/ values close to the detection limit due to the uncertainty in SiC electron affinities and D/sub it/ values below 1/spl times/10/sup 11/ cm/sup -2//eV deep in the band gap can be reproducibly obtained for both p-type 6H- and 4H-SiC.

A high-current and high-temperature 6H-SiC thyristor

A 6H-SiC thyristor has been fabricated and characterized. A forward breakover voltage close to 100 V and a pulse switched current density of 5200 A/cm/sup 2/ have been demonstrated. The thyristor is shown to operate under pulse gate triggering for turn-on and turn-off, with a rise time of 43 ns and a fall time of less than 100 ns. The forward breakover voltage is found to decrease by only 4% when the operating temperature is increased from room temperature to 300/spl deg/C. It is found that anode ohmic contact resistance dominates the device forward drop at high current densities.

Nonlocal effects in thin 4H-SiC UV avalanche photodiodes

The avalanche multiplication and excess noise characteristics of 4H-SiC avalanche photodiodes with i-region widths of 0.105 and 0.285 /spl mu/m have been investigated using 230-365-nm light, while the responsivities of the photodiodes at unity gain were examined for wavelengths up to 375 nm. Peak unity gain responsivities of more than 130 mA/W at 265 nm, equivalent to quantum efficiencies of more than 60%, were obtained for both structures. The measured avalanche characteristics show that /spl beta/>/spl alpha/ and that the /spl beta///spl alpha/ ratio remains large even in thin 4H-SiC avalanche regions. Very low excess noise, corresponding to k/sub eff/<0.15 in the local noise model, where k/sub eff/=/spl alpha///spl beta/(/spl beta///spl alpha/) for hole (electron) injection, was measured with 365-nm light in both structures. Modeling the experimental results using a simple quantum efficiency model and a nonlocal description yields effective ionization threshold energies of 12 and 8 eV for electrons and holes, respectively, and suggests that the dead space in 4H-SiC is soft. Although dead space is important, pure hole injection is still required to ensure low excess noise in thin 4H-SiC APDs owing to /spl beta///spl alpha/ ratios that remain large, even at very high fields.

Smooth etching of single crystal 6H‐SiC in an electron cyclotron resonance plasma reactor

Single‐crystal 6H‐SiC has been etched using a CF4/O2 gas mixture in an electron cyclotron resonance (ECR) plasma reactor. ECR etching results in SiC surfaces which are extremely smooth, without the problematic micromasking effects which have been reported to result from reactive ion etching in capacitively coupled radio‐frequency plasma reactors. The effects of microwave power, total pressure, substrate temperature, and substrate bias on the etch rate, surface morphology, etch profile, and etch selectivity have been evaluated. The etch rate increases with increasing power and bias, and decreasing pressure. However, high biases lead to enhanced etching in regions adjacent to sidewall features. Improved etch profiles and selectivity are obtained with lower applied substrate bias.