C. B. Fleddermann

Characterization of a semi-insulating GaAs photoconductive semiconductor switch for ultrawide band high power microwave applications

Simulation results depicting physical conditions in a photoconductive semiconductor switch in the pulse charging state, prior to high power switching, are analyzed. Results show that surface conditions and EL2 traps in semi-insulating GaAs influence the conduction process, specifically at high bias. Formation of trap-filled regions renders the device inhomogeneous for stable conduction and premature breakdown occurs, due to a large extent on unstable current filamentation within the device. Unlike insulators, the breakdown of desorbed gas from the surface (surface flashover) does not contribute to premature breakdown.Simulation results depicting physical conditions in a photoconductive semiconductor switch in the pulse charging state, prior to high power switching, are analyzed. Results show that surface conditions and EL2 traps in semi-insulating GaAs influence the conduction process, specifically at high bias. Formation of trap-filled regions renders the device inhomogeneous for stable conduction and premature breakdown occurs, due to a large extent on unstable current filamentation within the device. Unlike insulators, the breakdown of desorbed gas from the surface (surface flashover) does not contribute to premature breakdown.

Measurements of etch rate and film stoichiometry variations during plasma etching of lead‐lanthanum‐zirconium‐titanate thin films

Plasma etching of lead‐lanthanum‐zirconium‐titanate (PLZT) ferroelectric thin films using a dc hollow‐cathode discharge in CF4 and dilute HCl has been investigated. The etch rate and film stoichiometry variations of sputter‐deposited lead‐lanthanum‐titanate (PLT), and solution‐deposited PLT, lead‐zirconium‐titanate (PZT), and PLZT have been measured over a wide range of etching conditions. Etching does not take place at room temperature, but requires substrate heating above 200 °C. Etch rate and film stoichiometry varied strongly as a function of substrate temperature, gas mixture, and film preparation method. For sputter‐deposited films, the fastest etch rates of 6500 A/h were obtained using a mixture of CF4 and dilute HCl. Etch rates for solution‐deposited films were higher than for sputter‐deposited films, reaching 2000 A/min in CF4.

Repeatable electron emission from (Pb,La)(Zr,Ti)O3 ferroelectric cathodes using direct‐current reset

The use of ferroelectric (Pb,La)(Zr,Ti)O3 as a cathode material is investigated. It has been suggested in the literature that by rapidly changing or reversing the spontaneous polarization in the ferroelectric, a large charge imbalance can be produced at the surface of the material. The field from this charge imbalance extracts bound carriers from the cathode. An approach to achieving this polarization change by using a negative bias field to preset the material prior to switching is discussed in this letter. This bias field can also be used to control the bound surface charge on the material. Emission currents on the order of 100 mA are observed and the emitted charge is on the order of 30 nC. Measurable emission occurs only when the material is driven into saturation.

Analysis of high voltage operation of gallium arsenide photoconductive switches used in high power applications

An opposed-contact photoconductive semiconductor switch, with a n+ region next to the cathode electrode has been simulated. Physical conditions during the pulse charging state, prior to high power switching, are analyzed in order to explain the increased hold-off characteristic of such devices. Results show that the introduction of the n+ region near the cathode inhibits the flow of electrons at the n+/semi-insulating interface until very high fields are reached. The formation of trap-filled regions near the contacts and the resultant inhomogeneous device characteristics that lead to breakdown are thereby shifted to higher voltages. Thus, for switches with a n+ region next to the cathode, the breakdown voltage due to unstable filamentary conduction is also increased beyond those achieved previously, allowing for higher power operation.

Optical emission spectroscopy during sputtering of Y‐Ba‐CU‐oxide targets

The sputter deposition of high‐temperature superconducting thin films was studied using optical emission spectroscopy. Argon or oxygen ions generated by a Kaufman ion gun were used to sputter material from a composite target containing yttrium, barium, and copper which had been oxygen annealed. The impact of ions onto the target generates a plume of sputtered material which includes various excited‐state atoms and molecules. In these studies, optical emission is detected for all the metallic components of the film as well as for metallic oxides ejected from the target. No emission due to atomic or molecular oxygen was detected, however. Variations in sputter conditions such as changes in sputter ion energy, oxygen content of the beam, and target temperature are shown to greatly affect the emission intensity, which may correlate to the characteristics of the sputtering and the quality of the films deposited. The results suggest that optical emission from the sputtered material may be useful for real‐time monitoring and control of the sputter deposition process.The sputter deposition of high‐temperature superconducting thin films was studied using optical emission spectroscopy. Argon or oxygen ions generated by a Kaufman ion gun were used to sputter material from a composite target containing yttrium, barium, and copper which had been oxygen annealed. The impact of ions onto the target generates a plume of sputtered material which includes various excited‐state atoms and molecules. In these studies, optical emission is detected for all the metallic components of the film as well as for metallic oxides ejected from the target. No emission due to atomic or molecular oxygen was detected, however. Variations in sputter conditions such as changes in sputter ion energy, oxygen content of the beam, and target temperature are shown to greatly affect the emission intensity, which may correlate to the characteristics of the sputtering and the quality of the films deposited. The results suggest that optical emission from the sputtered material may be useful for real‐time m...

Electron emission from thin-film ferroelectric cathodes

Electron emission from thin-film (<1 μm thick) ferroelectric cathodes has been investigated. The cathodes were made using sol-gel deposition and standard microelectronic patterning techniques and were excited using either dc or pulsed bias. Repeatable emission current densities up to 10 μA/cm2 were measured from 0.8-μm-thick lead-niobium-zirconium-titanate cathodes driven in the pulsed mode with switch voltages up to 22 V. Intermittent emission up to 20 mA/cm2 was measured for higher switch voltages. The dependence of emission current on switch voltage, grid dimensions, and extraction voltage will be presented.

SUPPRESSION OF VACUUM BREAKDOWN USING THIN-FILM COATINGS

Thin‐film metal and metal oxide coatings ion sputter deposited onto the cathode electrode of a parallel‐plate high‐voltage gap were found to suppress electron emission in high vacuum. Electric fields as high as 60 kV/mm have been sustained across a 1 mm gap for pulse durations of 10 μs. Lesser electric fields were sustained for pulse durations exceeding 10 ms. The behavior of 500 nm thick coatings was independent of the type of coating used, whereas the breakdown voltage for thinner coatings depended upon the material and the deposition conditions. The breakdown properties of various coatings, as well as a discussion of the suppression of electron emission is presented.

Simulation studies of persistent photoconductivity and filamentary conduction in opposed contact semi-insulating GaAs high power switches

A self-consistent, two-dimensional, time-dependent, drift-diffusion model is developed to simulate the response of high power photoconductive switches. Effects of spatial inhomogeneities associated with the contact barrier potential are incorporated and shown to foster filamentation. Results of the dark current match the available experiment data. Persistent photoconductivity is shown to arise at a high bias even under the conditions of spatial uniformity. Filamentary currents require an inherent spatial inhomogeneity, and are more likely to occur for low optical excitation. Under strong uniform illumination, the spatial nonuniformities were quenched as a result of a polarization-induced collapse in the internal fields. However, strong electric fields resulting at the contacts create a bipolar plasma, and hence, a virtual “double injection.”

Optical‐emission‐spectroscopic studies of the sputtering of Y‐Ba‐Cu‐oxide thin films

Optical emission spectroscopy is used to determine the identity of species ejected during sputtering of a composite Y‐Ba‐Cu‐oxide target. In this experiment, Y, Ba, Cu, Ba+, and YO were detected near the surface of the target, indicating that oxygen is transported to the substrate during sputtering. Varying of the sputter ion energy caused some variation in relative peak heights, but changing the gas mixture in the system did not significantly alter the spectra.

Chemical plasma etching of Y-Ba-Cu-oxide thin films

The use of chemical plasma etching for patterning thin films of superconducting Y‐Ba‐Cu‐oxide thin films is reported. Etch rates as high as 10 μm/min were measured, and were found to be highly dependent on substrate temperature and annealing of the film. Energy dispersive spectroscopy measurements showed significant variations in film stoichiometry as substrate temperatures increased, with copper being the most volatile element, followed by barium and yttrium. Although the etching is isotropic, this study indicates that chemical plasma etching can be a viable technology for high‐Tc superconducting thin‐film patterning.