A m ( c r a m roc 75 121 295-5Oi (1992) The electrical conductivity of high-purity single-crystal alumina is determined in a temperature range from 400" to 1300°C. By applying an advanced fully guarded threeterminal measurement technique, reliable conductivity measurements are performed to as low as R-'*cm-'. Gas and surface conduction are measured separately and shown to be negligible. High-purity sapphire exhibits a conductivity of W' ocm-' at 400"C, two characteristic activation energies of 0.4 and 4.8 eV with increasing temperature, and a conductivity of 3 X lo-' W'-cm-' at 1300°C. The fraction of the current carried by ions is determined by electron probe analysis of the electrodes following a 640-h transference test at 1200°C with 4 kV/cm field applied. Only 0.3% of the current at 1200°C is carried by ions. A mathematical model of electrical conduction in sapphire is developed which describes sapphire as a wide-bandgap semiconductor, doped with one dominant donor and one dominant acceptor. The observed conductivity is well described by the model over the entire temperature range from 400" to 1300°C. [
An experimental setup and novel measurement technique are described which allow reliable conductivity measurements to be made at conductivities as low as C!-'*cm-' and temperatures up to at least 1300°C. This capability is of particular interest for conductivity measurements on highresistivity insulators over a large range of temperatures. This technique has been used to measure the conductivity of single-crystal alumina from 400" to 1300°C in a torr (-1.3 x Pa) vacuum, equivalent to an oxygen partial pressure of about lo-' torr (-1.3 X lo-" atm or -1.3 X lo-(' Pa). Surface and gas-phase conductance are determined as a function of temperature, and the requirements for their minimization are described. A key requirement is a very low voltage between the volume guard and the guarded electrode. The effect of leakage currents due to the sample fixture, electrical feedthroughs, and electronic instrumentation are also evaluated, and proper design features to make these effects negligible are outlined.
Photovoltaic cells require deposition of a platinum layer at the cathode to serve as a catalyst for reduction of redox carriers in PV cells. Current dye-sensitized solar cells (DSSC) employ high temperature decomposition of chloroplatinic acid to give platinum islands. In order to produce DSSCs with plastic substrates, a low temperature platinum deposition process was developed. Initial experiments showed that platinum was deposited if Karstedt platinum catalyst solution in hexamethyldisilazane (HMDZ) was coated onto a substrate followed by heating under150°C. PV cell performance of Karstedt-HMDZ-containing platinum was inferior to cells made with high temperature platinum. However, CODPtMe 2 (COD = 1,5-cyclooctadiene) was found to be a platinum precursor that led to PV cell performance equivalent to that obtained from high temperature platinum. Other precursors were evaluated as well including MeCpPtMe 3 that permitted platinum deposition via UV irradiation. Kelvin Probe analysis was also performed on several platinum films prepared from a variety of precursors on several substrates under a variety of conditions. CPD values of < -0.6eV appeared to predict good PV cell performance. Further application of the low temperature-derived platinum films was made for organic light emitting diodes.
Aluminum corrosion in a polymer-coated circuit model was studied using in-situ microscopy coupled with time-lapse video recording.Pinholes in the polymer coating and exposure to water with bias voltage of 40V applied between adjacent aluminum tracks resulted in fast anode corrosion, accompanied by gas evolution. A mathematical model is developed that is in good agreement with the observed current-time relationship. The rate of corrosion is controlled by the resistance of a 6000A thick water film occupying the space of the corroded Al film between wafer and polymer coating.A tentative corrosion mechanism is proposed.
The electrical conductivity of high-purity, single-crystal alumina is determined parallel to and perpendicular to the c-axis. The mean conductivity of four samples of each orientation is a factor 3.3 higher parallel to the c-axis than perpendicular to it. The conductivity as a function of temperature is attributed to extrinsic electron conduction at temperatures from 400" to 900°C and intrinsic semiconduction at temperatures from 900" to 1300°C. In the hightemperature regime, the slope on all eight specimens is 4.7 f 0.1 eV. Hence, the thermal bandgap at 0 K is 9.4 * 0.2 eV.
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