The properties of bismuth-doped vacuum-evaporated CdS films

The effect of doping with bismuth on various properties of vacuum-evaporated CdS films was studied. The properties specifically studied were (1) the dark conductivity and photoconductivity as functions of the temperature and doping concentration, (2) the Hall mobility and carrier concentration at room temperature, (3) the thermally stimulated current and (4) the optical absorption and spectral response. It was found that on doping with bismuth the dark conductivity decreases and the photosensitivity increases significantly. These phenomena probably arise from the formation of cadmium vacancies due to bismuth doping. Studies of the Hall effect show that the carrier concentration is reduced by doping with bismuth. Investigations of thermally stimulated currents for both pure and doped films show that the trap concentrations are reduced by doping.     

Electrical properties of vacuum annealed CdS films

Cadmium sulphide (CdS) films were evaporated in vacuum on glass substrates maintained at room temperature. These films were later annealed in vacuum at temperatures in the range 30 to 300° C. The variation of d.c. electrical conductivity was studied in the temperature range 100 to 300 K. While the conductivity data in the range 100 to 150 K were observed to follow Mott's variable range hopping process, the conductivity in the high-temperature region (150 to 300 K) could be explained by Seto's model.     

Electrical properties of In doped CdS thin films

Indium doped polycrystalline cadmium sulphide CdS:In thin films have been prepared by the spray pyrolysis technique on glass substrates in an enclosed dome. The different scattering mechanisms such as lattice, impurity and grain boundary scattering for CdS:In films are observed at low temperature, in the range of 303 to 120 K. The experimentally determined mobilities due to these scatterings are well interpreted with those of theoretically calculated mobilities. The d.c. conductivity for CdS:In films has also been studied in the same temperature region. The Mott variable range hopping conduction process followed below the temperature of 150 K. The Mott parameters such as N(E _F ), R, W and are found to be 1.26 × 10¹⁹ eV^–1cm^–3, 9.8 × 10^–-7cm, 0.02 eV^–1 and 2.38 × 10⁶cm^–1, respectively from the conductivity data.     

The optical properties of sprayed CdS thin films

In the study presented in this paper we attempted to interpret the reflectance and the transmittance of sprayed CdS films. Assuming a model based on multilayer film theory we showed that sprayed CdS film is a combination of multilayer stacks of crystallites and gaseous inclusions.     

Electrical and optical properties of phthalocyanine films

Thin films of various phthalocyanines (Pcs) were sublimed onto quartz glass in ultrahigh vacuum. The electrical and optical properties were studied without breaking the ultrahigh vacuum. The dyes were purified by repeated sublimation in vacuum.The conductivity varied with pretreatment (modification) and temperature. Exposure to oxygen, iodine and bromine increased the p-type conductivity by orders of magnitude and lithium doping caused n-type conductivity. Heavy exposure to halogens caused not only broadening of the optical absorption band but also bleaching. Annealing in vacuum removed the electrical and optical effects of doping nearly completely.Field effect studies were used to identify p- and n-type conductivity, to determine the drift mobility and to estimate the trap densities. The field effect and the photoconductivity of p- and n-type films increased by several orders of magnitude with the dark conductivity, i.e. with doping. This correlation is attributed either to traps with a continuous distribution in energy or to trapping by at least two discrete defect levels crossing the Fermi level within the surface-induced accumulation layer. Possible consequences for the sensitivity of semiconductor gas sensors are discussed.     

Electrical and optical properties of rf-sputtered CdTe films

In this paper some electrical and optical properties ofn-type CdTe films prepared by rf sputtering at 180 W power have been reported. For doping the films a number of pellets of pure Cd placed on the CdTe target were simultaneously sputtered with the target material to get Cd-doped CdTe films. The films after doping were foundn-type. Maximum doping concentration obtained this way was of the order of 10¹⁴ cm⁻³. XRF spectra of target material and the rf-sputtered films were found to be more or less similar. All the films were found to have large number of defects indicated by profound aging effect in the initial stages of aging. The films became stable for measurements after about 8–10 days. Activation energy and band gap found from the temperature dependence of dark conductivity were 0–5 eV and 1.43 eV respectively. Photoconductivity of the films was studied and the photoconductive rise time, decay time and the decay constants were determined from the photoconductive rise and decay curves at 500 Lx and 1000 Lx of intensity of illumination.     

Electrical and optical properties of TiN thin films

TiN thin films have been grown by reactive magnetron sputtering. It has been shown that an Ohmic contact to TiN thin-film can be made from indium. The TiN thin films have been shown to be n-type semiconductors with a carrier concentration of 2.88 × 10²² cm^?3 and resistivity of ρ = 0.4 Ω cm at room temperature. The activation energy for conduction in the TiN films at temperatures in the range 295 K < T < 420 K is 0.15 eV. The optical properties of the TiN thin films have been investigated. The material of the TiN thin films has been shown to be a direct gap semiconductor with a band gap E _g = 3.4 eV.     

Dielectric properties of vacuum-evaporated films of tungsten oxide

The dielectric constant of vacuum-evaporated films of tungsten oxide has been measured in the frequency range 300 Hz–10 MHz at various temperatures (25°C–155°C) and with various thickness (300–-5000Å). The frequency response exhibits a dispersion region between 10 and 100 kHz at room temperature (25°C) which shifts to higher frequencies as the temperature rises. The thickness dependence of the dielectric constant shows a rapid rise at low thicknesses but gradually attains a saturation value at thicknesses at which the density of the films is seen to approach its bulk value. Electron diffraction patterns show that these films are stoichiometric, possibly due to the very slow rate of evaporation. The films have a dielectric strength of the order of 5.5 × 10⁶ V cm⁻¹.     

Electrical transport properties of heavily indium- doped polycrystalline CdS films

CdS was doped with up to 1.5 at.% In by means of a chemical method. Films were produced from the chemically prepared charge by evaporation onto glass. Measurements of the d.c. conductivity and the Hall effect were made on these films. It was observed that the carrier concentration in the films increases by two orders of magnitude on the addition of 1.0 at.% In. The carrier concentration remains almost constant, however, on further addition of indium. In contrast, the mobility increases on the addition of indium up to 1.0 at.% but it decreases when further indium is added.     

Electrical and optical properties of molybdenum trioxide thin films

Infrared spectra of vacuum-deposited molybdenum trioxide thin films have been studied. The variation of electrical conductivity with temperature for different thicknesses of films has been investigated. Electrical conductivity of the films as a function of time of UV irradiation was found to increase initially, then decreased rapidly and reached a steady value. It increased and reached a steady value with time when irradiation was cut-off.     

Formation and properties of vacuum-evaporated high conductivity n-type CdTe films

High dark conductivity CdTe films have been prepared by co-evaporating CdTe and cadmium. The structural, electrical and optical properties were investigated. The dark conductivity of the film increased monotonically with an increase in the amount of co-evaporated cadmium. The highest dark conductivity of the films obtained in this experiment was 1.4x10^-2Ω^-1 cm^-1. The film structure was of the zinc blende type with a preferential orientation of the (111) planes parallel to the substrate and fibrous. The crystallinity of the films was similar to that of films without cadmium doping. The dark conductivity vs. the reciprocal temperature characteristics showed regular aspects. High dark conductivity films will be useful for CdTe thin film device applications.     

Electrical and optical properties of vitreous chalcogenide semiconductor films

Nitrogen-doped tantalum films were deposited by d.c. sputtering in a semicontinuous d.c. diode system. Their electrical properties varied with nitrogen in a similar manner to that generally reported; e.g. the resistivity first decreased from 200 μω cm to a minimum value of 140 μω cm and then increased monotonically. The film density also decreased with increasing nitrogen flow. The films were oxidized both by anodization and by the thermal oxidation in an oxygen atmosphere at 550°C. For a given anodization voltage, the capacitance decreased by approximately 50% when the nitrogen content in the sputtered film reached an estimated 30 at.%. The thermal oxides acted as waveguides for He-Ne light and their refractive indices were strongly dependent on nitrogen concentration. For Ta₂O₅, the indices for TE and TM modes were 2.21 but both decreased by different amounts as the nitrogen concentration increased. For the highest nitrogen concentration, the values for the TE and TM modes were 1.91 and 1.82 respectively. The birefringence must be due to local asymmetry around the Ta atom which is caused by the presence of nitrogen but the mechanism responsible for the ordering in amorphous or randomly oriented micropolycrystalline films is not clear.     

Electrical and photoconducting properties of CdS (Cu,Cl) thin films

Dielectric and photoconductive properties of thin films of cadmium sulphide activated with copper and chlorine have been studied. Films are shown to have good dielectric properties over the frequency range 10²–10⁴ Hz and to have high photosensitivity over the whole visible radiation range.     

Electrical and optical properties of Zn-In-Sn-O transparent conducting thin films

Indium tin oxide (ITO) is one of the widely used transparent conductive oxides (TCO) for application as transparent electrode in thin film silicon solar cells or thin film transistors owing to its low resistivity and high transparency. Nevertheless, indium is a scarce and expensive element and ITO films require high deposition temperature to achieve good electrical and optical properties. On the other hand, although not competing as ITO, doped Zinc Oxide (ZnO) is a promising and cheaper alternative. Therefore, our strategy has been to deposit ITO and ZnO multicomponent thin films at room temperature by radiofrequency (RF) magnetron co-sputtering in order to achieve TCOs with reduced indium content. Thin films of the quaternary system Zn-In-Sn-O (ZITO) with improved electrical and optical properties have been achieved.The samples were deposited by applying different RF powers to ZnO target while keeping a constant RF power to ITO target. This led to ZITO films with zinc content ratio varying between 0 and 67%. The optical, electrical and morphological properties have been thoroughly studied. The film composition was analysed by X-ray Photoelectron Spectroscopy. The films with 17% zinc content ratio showed the lowest resistivity (6.6 × 10^− 4 Ω cm) and the highest transmittance (above 80% in the visible range). Though X-ray Diffraction studies showed amorphous nature for the films, using High Resolution Transmission Electron Microscopy we found that the microstructure of the films consisted of nanometric crystals embedded in a compact amorphous matrix. The effect of post deposition annealing on the films in both reducing and oxidizing atmospheres were studied. The changes were found to strongly depend on the zinc content ratio in the films.     

Electrical and optical properties of silicon-doped gallium nitride polycrystalline films

Si-doped GaN films in polycrystalline form were deposited on quartz substrates at deposition temperatures ranging from 300–623 K using r.f. sputtering technique. Electrical, optical and microstructural properties were studied for these films. It was observed that films deposited at room temperature contained mainly hexagonal gallium nitride (h-GaN) while films deposited at 623 K were predominantly cubic (c-GaN) in nature. The films deposited at intermediate temperatures were found to contain both the hexagonal and cubic phases of GaN. Studies on the variation of conductivity with temperature indicated Mott’s hopping for films containing c-GaN while Efros and Shklovskii (E-S) hopping within the Coulomb gap was found to dominate the carrier transport mechanism in the films containing h-GaN. A crossover from Mott’s hopping to E-S hopping in the ‘soft’ Coulomb gap was noticed with lowering of temperature for films containing mixed phases of GaN. The relative intensity of the PL peak at ∼2·73 eV to that for peak at ∼3·11 eV appearing due to transitions from deep donor to valence band or shallow acceptors decreased significantly at higher temperature. Variation of band gap showed a bowing behaviour with the amount of cubic phase present in the films.