A. E. Rakhshani

Electrodeposition and characterization of cuprous oxide

Abstract Films of cuprous oxide were electrodeposited potentiostatically and galvanostatically on a variety of substrates. The usable range of deposition parameters was determined and discussed. Uniform and adherent films of thickness up to 6 μm could be deposited galvanostatically and to higher values potentiostatically. The films were polycrystalline with grains of a few micrometers in size when deposited at 60 °C and one order of magnitude smaller when deposited at 49 °C. Regardless of the mode of deposition, the temperature of deposition and also the type of substrate, deposited films were highly oriented along the (100) plane parallel to the substrate surface. The composition of the films was found to be Cu2O with an optical band gap of 1.95 eV.

Charge Transport Mechanisms in Au–CdTe Space-Charge-Limited Schottky Diodes

Space-charge-limited Schottky diodes are fabricated by evaporation of gold on electrodeposited CdTe films. The charge transport mechanisms, in both polarities, are studied. Thermionic emission and space-charge-limited conduction theories can adequately describe the transport properties. The latter mechanism is controlled by a donor-type deep level whose ionization energy varies with the applied voltage according to the Poole-Frenkel effect. Bardeens model in which the Fermi level is pinned to the interface states, can explain the voltage dependence of the barrier height in these devices. Some of the important parameters of CdTe films are measured from the characteristics of these diodes.

Potentiostatic electrodeposition of cuprous oxide

Abstract Details of the potentiostatic electrodeposition of Cu 2 O onto stainless steel at 60°C are discussed. It is demonstrated that monitoring the deposition parameters during the growth of films can be useful in extracting further knowledge about the type of deposition process and the electrolyte. Uniform films of Cu 2 O can be deposited at a working electrode potential in the range from +0.1 to -0.3 V vs. the saturated hydrogen electrode. For uniform films a reaction of the two-electron transfer type is likely to be responsible for the deposition. All deposits consist of Cu 2 O grains with a preferential orientation of (100) planes parallel to the surface of the substrates. The grain size is of the order of 1 μm. The substrate does not have a major effect on the orientation and the size of the grains.

Deep energy levels and photoelectrical properties of thin cuprous oxide films

Thin films of cuprous oxide have been electrodeposited on molybdenum substrates. Some preliminary photoelectrical properties of two devices with semitransparent aluminum and gold top-electrodes have been measured. The device with a gold top-electrode exhibits a weak photovoltaic effect and a space charge limited current conduction mechanism in dark. The other device which had been annealed before the deposition of the aluminum top-electrode, exhibits a three-fold increase in resistivity and a marked difference in its temperature dependence of conductivity. From the temperature dependence of conductivity, two activation energies of 0.09 and 0.87 eV could be measured compared to 0.11 and 0.26 eV for the device with a gold electrode. Photoinduced current transient spectroscopy with a novel approach to the post-capture data analysis (additive double gate analysis) has been used in detection and the measurements of the parameters of several deep energy levels. These levels have activation energies in the range of 0.12 to 0.63 eV and capture cross-sections in the range of 10−22 to 10−12 cm2. Using the role of the sample bias polarity on the identification of the type of the traps, one electron trap (surface state) and one hole trap (bulk state) could be identified among the five detected deep levels.

Photo-Induced Current Transient Spectroscopy (PICTS) of Deep Levels in Electrodeposited CdTe Films

The PICTS technique in conjunction with a highly absorptive excitation wavelength is used to characterize and to distinguish the electron and hole traps in electrodeposited CdTe films. All the deep levels measured are within two deep (donor-type and acceptor-type) energy bands and are related to only native defects and their complexes with chlorine. Except chlorine, no other trace-impurity levels could be detected, implying the effectiveness of electropurification of the bath solution prior to deposition of CdTe. Some results related to the effect of different annealing processes on the spectrum of deep levels are also discussed.

The Poole-Frenkel conduction mechanismin Mo-Cu2O-Au thin film structures

Thin films of cuprous oxide (4.6 μm) were electrodeposited on molybdenum. Gold contacts were vacuum evaporated on the films to form devices. These films showed relatively low electrical resistivities at around 106 Ω cm and a charge transport mechanism which is different from the space charge limited current conduction previously reported for the 1011 Ω cm films. The charge transport mechanism in these films was determined by isothermal measurements of the devices current-voltage (I–V) characteristics at some selected temperatures in the range of 78–321 K. In this temperature range the dominant transport mechanism can be explained by the Poole-Frenkel effect through the relation I = VG0exp(−φ0L/kT)exp(BLV1/2)+I0exp(−φ0H/kT)exp(BHV1/2) where the numerical values of the parameters are measured. φ0L = 0.12 eV is the zero-field ionization energy of a shallow acceptor-type level (measured from the edge of the valence band) which has the dominant effect in the range of 78–230 K. Similarly φ0H = 0.70 eV corresponds to a deep level dominant in the high-temperature range 230–321 K. In the high-temperature region a 2.7 μm thick hole accumulation layer forms beneath the oxide-gold interface, assuming the ionized deep level is doubly charged.

Diffusion‐Controlled Electrodeposition and Characteristics of CdTe Films

Cadmium telluride was deposited on various conducting substrates from an acidic bath of CdSO 4 (1 M) containing up to 200 ppm of TcO 2 . The deposition process which is a six-electron-transfer reaction, is described satisfactorily by a model developed based on the diffusion of HTeO 2 + cations. The film structure, composition and the effect of annealing were studied. The optical dispersion of films which agrees well with that for the single crystal can be described by the classical bound-electron dispersion model. The number of bound electrons per CdTe molecule which participate in the dispersion process was determined to be 3.0. Annealing in air at 350°C has no influence on dispersion.

Electrodeposited Films of CdxHg1—xTe with Bandgap Energy in the Range 1.2–1.5 eV

Electrodeposition of Cd x Hg 1-x Te films with a thickness of about 1 μm and a bandgap energy in the range 1.2-1.5 eV (0.87 < x < 1) is investigated. Deposition is performed on stainless steel substrates at 80-90 °C from a bath containing CdSO 4 , TeO 2 and HgCl 2 . Film composition and bandgap energy are evaluated, respectively, from the variation of the deposition current with time and from the spectral response of photocurrent in Schottky devices formed on these films. In addition, other characterization techniques such as XRD, SEM and EDAX are also used. Films grow with a graded composition unless HgCl 2 is added periodically into the solution during deposition. Films consisting of several layers with different thickness and composition can also be deposited. Films consist of grains with an average size of about 100 nm and are less oriented than grains in identically deposited CdTe films.