C. C. Chi

Carrier lifetime versus ion‐implantation dose in silicon on sapphire

We have measured the dependence of the free‐carrier lifetime on O+ ion‐implantation dose in silicon‐on‐sapphire. At low implant doses, the carrier trapping rate increased linearly with the trap density introduced by ion implantation. At doses above 3×1014 cm−2 the measured carrier lifetime reached a limit of 600 fs.

Generation of subpicosecond electrical pulses on coplanar transmission lines

Electrical pulses shorter than 0.6 ps were generated by photoconductively shorting a charged coplanar transmission line with 80 fs laser pulses. After propagating 8 mm on the line the electrical pulses broadened to only 2.6 ps.

Very high-density (23 fF//spl mu/m/sup 2/) RF MIM capacitors using high-/spl kappa/ TaTiO as the dielectric

A very high density of 23 fF//spl mu/m/sup 2/ has been measured in RF metal-insulator-metal (MIM) capacitors which use high-/spl kappa/ TaTiO as the dielectric. In addition, the devices show a small reduction of 1.8% in the capacitance, from 100 kHz to 10 GHz. Together with these characteristics the MIM capacitors show low leakage currents and a small voltage-dependence of capacitance at 1 GHz. These TaTiO MIM capacitors should be useful for precision RF circuits.

Capacitance free generation and detection of subpicosecond electrical pulses on coplanar transmission lines

By reanalyzing an earlier experiment to generate subpicosecond pulses using photoconductive switches (M.B. Ketchen, et al., Appl. Phys. Lett., vol.48, pp.751-753, 1986), it is shown that to first order, the sliding-contact generation site has no capacitance. This conclusion is further supported by a double sliding-contact experiment where, to first order, neither the generation nor the detection site has any capacitance. This result removes the parasitic capacitance of the electrical circuit as one of the major difficulties to short electrical pulse generation using photoconductive switches. >

Subpicosecond optoelectronic study of resistive and superconductive transmission lines

We have studed the propagation of subpicosecond electrical pulses on coplanar resistive and superconductive Nb transmission lines. Pulses with 0.9 ps full width at half maximum were generated and detected by shorting fast photoconductive switches with 80 fs laser pulses. Dramatic improvements in propagation characteristics were achieved when the Nb was superconductive. We observed the strong dispersion and attenuation predicted to occur for frequency components near the superconducting enery gap frequency.

Magnetron sputtering and laser patterning of high transition temperature Cu oxide films

High Tc Y‐Ba‐Cu‐O films have been prepared by dc magnetron sputtering of metal alloy targets. To circumvent the negative ion effect, two alloy targets, YCu and BaCu, are sputtered in an argon atmosphere with an oxygen spray near the substrate. Films deposited on sapphire with onsets at 92 K and a 6° transition width (10–90%) have been achieved using this technique. These films have been successfully patterned with the technique of laser ablation.

Micropatterning of high Tc films with an excimer laser

Micron‐wide lines of high Tc Y‐Ba‐Cu‐O have been successfully patterned by ablating the films with a pulsed excimer laser. The high Tc films are mounted onto a computer‐controlled stepping stage and irradiated with a demagnified image of a variable‐size rectangular aperture. This technique has been used for fabricating features ranging from several centimeters in length to submicron in width without any degradation in Tc. For example, a superconducting microstructure of Y‐Ba‐Cu‐O, nominally 1 μm wide and 2.5 μm long, with a Tc (R=0) of 88 K and a Jc of 5×104 A/cm2 at 4.2 K was fabricated.

RF loss and crosstalk on extremely high resistivity (10 k-1 M/spl Omega/-cm) Si fabricated by ion implantation

We have achieved 1.6 M/spl Omega/-cm resistivity using ion implantation that has little negative effect on MOS devices. Extremely low loss and cross coupling of 6.3 and -79 dB/cm (10 /spl mu/m gap) at 20 GHz are measured with 1 /spl mu/m Al, respectively, which is due to implant induced trap with /spl sim/1 ps carrier lifetime and stable to 400/spl deg/C.

Fabrication of very high resistivity Si with low loss and cross talk

We have used proton and As/sup +/ implantation to increase the resistivity of conventional Si (10 /spl Omega/-cm) and Si-on-quartz substrates, respectively. A high resistivity of 1.6 M/spl Omega/-cm is measured that is close to intrinsic Si and semi-insulating GaAs. Very low loss and cross coupling of 6.3 dB/cm and -79 dB/cm (10 /spl mu/m gap) at 20 GHz are measured on these samples, respectively. The very high resistivity and improved rf performance are due to the extremely fast /spl sim/1 ps carrier lifetime stable even after a 400/spl deg/C annealing for 1 h. Little negative effect on gate oxide integrity is also observed as evidenced by the comparable stress-induced leakage current and charge-to-breakdown for 30 /spl Aring/ oxides.

Thin-film YBaCuO superconductors formed by Cu/BaO/Y2O3 layer structures

Thin‐film superconductors of the YBaCuO family have been formed using layer structures of Cu, BaO, and Y2O3 deposited on different substrates by electron beam evaporation. Using a thickness ratio of Cu, BaO, and Y2O3 layers off that of the stoichiometric YBa2Cu3O7 phase, superconducting films were formed on MgO and SrTiO3 substrates, with a Tc for zero resistance exceeding 77 K. The films were found to be composed of the superconducting YBa2Cu3O7 phase and the green Y2BaCuO7 phase, with the excessive Cu repelled to the surface in an oxidized form. The studies also provide information on the dependence of the superconducting transition on the annealing temperatures and the substrates used.