K. A. Khan

Quantum efficiency of Cu2O electrode in photoelectrochemical cells with and without bias

The photoresponse of a p-type Cu2O film on copper is presented. The current-voltage characteristics exhibit a large photocurrent with an onset potential at ca. −0.2 V (S.C.E.). The electrode in the Cu2O/Pt biased cell shows high quantum efficiency which is approaching 100% for wavelength λ ≈ 300 nm. Studies show that even for potentials just slightly more negative than Von, the quantum efficiency is relatively large. This result may be explained either by a current doubling phenomena or could be due to a contribution to additional band gaps which may exist at higher energies. The overall high quantum efficiencies, moreover, can be obtained without the use of any external bias if a TiO2 counter electrode is used.

DC Conduction and Switching Mechanisms in Electroformed Al/ZnTe:V/Cu Devices at Atmospheric Pressure

Vanadium-doped zinc telluride (ZnTe:V) thin film sandwiched by two different metal electrodes, that is, Al/ZnTe:V/Cu structure, was deposited onto the glass substrate by e-beam deposition technique in vacuum at a pressure of ~8 × 10−4 Pa. The deposition rate of the film was maintained at 2.052 nms−1. Circulation current was measured through this device as a function of potential difference applied across the structure. The Al/ZnTe:V/Cu structures exhibit memory switching characteristics at atmospheric pressure in room temperature. Switching characteristics of deposited Al/ZnTe:V/Cu structure as a memory device have been investigated in detail for various vanadium compositions, thicknesses of ZnTe:V films as well as various film temperatures, respectively. In all cases, it is seen that the metal/insulator/metal (Al/ZnTe:V/Cu) structures based on ZnTe:V can undergo an electroforming process and exhibit voltage-controlled negative resistance (VCNR) or a new switching process. It is also observed that the electric field, temperature, thickness, and dopant composition have important role in the switching characteristics. Switching characteristics have been interpreted by using a filamentary model. The switching effects of Al/ZnTe:V/Cu device may have important applications in the energy-oriented devices.

Photoelectrochemical studies of Cu2O Photoelectrode coated with Au and SiO thin films

An attempt has been made to stabilize the photocurrent in a photoelectrochemical cell by depositing thin Au and SiO films onto the Cu2O photoelectrode. In case of Au deposition, the photocurrent was either quenched or reduced. This may be the cause of insufficient formation of surface states in the electrode-electrolyte interfaces. In a Sio deposited photoelectrode, its effect was to decrease the quantum efficiency of a fresh sample, however, this deposition does not affect the value obtained for the band gap at 2.11 eV for an uncoated sample. It may be interpreted that the observed deterioration is not due strictly to surface effects or chemical reaction at the surface.

Optical properties of vanadium-doped ZnTe thin cermet films for selective surface applications

ZnTe:V thin cermet films (containing 0 to 10wt% V in ZnTe matrix) were prepared onto glass substrate by e-beam evaporation in vacuum at ~10-6 torr. The deposition rate of the films was at about 2.05 nms-1. The effects of various deposition conditions on the electrical and optical properties of the cermet films have been studied in detail. The structure analysis of the film was performed by X-ray diffraction technique and it was found that the films are amorphous in nature. The optical properties of both the as-deposited and annealed films were studied in the wavelength range 300<&lgr;<2500 nm, respectively. The special feature of transmittance spectra is that as the doping vanadium is increased to a concentration of 2.5wt% V, the transmittance value is increased in the entire visible & infra-red up to &lgr;=1600 nm and beyond that concentration value, the transmittance is decreased. Similar behavior has also been observed in annealed films. For both types of cermet sample, the values of Urbach tail, optical band gap, refractive index and dielectric constants were evaluated for different compositions and thicknesses, respectively. Evaluation of these parameters may help in view of their technological applications in selective surface as well as in optoelectronic devices.