A.E. Hill

Structural and optical investigations on CdS thin films grown by chemical bath technique

Cadmium sulphide (CdS) thin films have been prepared by using a chemical bath deposition (CBD) method. Structural analysis (XRD and TEM) revealed that the films showed hexagonal structure with lattice constants a=0.4015 nm and c=0.6545 nm. On annealing, the films exhibited secondary phase of beta -CdS (cubic) along with hexagonal phase, but density of dislocations decreased. The stacking faults have been observed in the crystalline arrangement by high resolution transmission electron microscopy (HRTEM). The direct band gaps of as-grown and annealed films were found to be 2.42 and 2.62 eV with sub band gap of 2.35 eV, respectively, which were confirmed by optical analysis. On annealing, the resistivity of the films decreased from 3 x 10(6) to 1 x 10(3) Omega cm. Multiple Cd and S defects were observed by employing photoluminescence (PL) method. The intensities of emission peaks for annealed films differed with that of as-grown films. As was discussed with giving reasons.

Optical, structural and electrical properties of tin doped indium oxide thin films prepared by spray-pyrolysis technique

Tin doped indium oxide (In2O3:Sn) or indium tin oxide (ITO) thin films have been successfully deposited by the low cost spray-pyrolysis method. Low sheet resistance and high mobility films were obtained when the films were deposited at the substrate temperature of 793 K. The direct optical bandgaps for the films deposited at 793 (a) and 753 K (b) were found to be 3.46 and 3.40 eV, respectively. Similarly, the indirect bandgaps for a- and b-type films were found to be 3.0 and 2.75 eV, respectively. The Burstein-Moss shift was observed in the films. The refractive index (n) and extinction coefficient (k) were found to be in the range of 2.1 to 1.1 and 0.6 to 0.01, respectively. The various scattering mechanisms such as lattice, ionized impurity, neutral impurity, grain boundary and alloy scattering due to variation of theoretical mobilities with temperature are discussed, in order to compare experimental results. In the lattice scattering mechanism, the quantum size effect phenomena were employed to estimate the energy dilation (EI). The a-type films exhibited SnO2 as secondary phase whereas b-type films showed single phase In2O3:Sn with high sheet resistance. The lattice constants were found to be 10.16 and 10.09 A for a- and b-type films, respectively.

Electrical conduction in silicon monoxide films

The permittivity and loss tangent of vacuum-deposited Al-SiO-Al capacitors have been measured in the frequency range 102–106 Hz at temperatures in the range 77–573 °K together with d.c. characteristics in the range 77–293 °K. The measurements were made in the original vacuum in which the films were deposited in order to determine the effect of atmospheric exposure. The substrate temperature during deposition was varied in the range 320–573 °K. A d.c. activation energy of 0.38 eV was found, while at high frequencies the activation energy was 0.004 eV. The results are, in general, in good agreement with those previously reported for specimens exposed to the atmosphere before investigation.

Optical properties of high-quality CuInSe2 single crystals

CuInSe2 single crystals have been studied employing photoluminescence (PL), optical reflection (OR), optical absorption (OA) and wavelength derivative reflection (WDR) techniques at temperatures from 4.2 to 300 K. Exciton-related peaks were observed in the near-band-edge region of the PL spectra: several narrow lines, with full width at half maximum (FWHM) of about 0.3 meV, and two wider peaks (FWHM about 0.7 meV) at 1.0414 (A) and 1.0449 eV (B). The A and B peaks were also observed in the OR and OA spectra and identified as A and B free excitonic states. The narrow lines were attributed to bound exciton recombination on intrinsic defects. A third exciton resonance (C) was observed in the WDR spectra at 1.2779 eV. The crystal-field and spin-orbit splittings were derived to be 5.3 and 234.7 meV, respectively.

Conduction mechanisms in thin vacuum-deposited cadmium selenide films

Abstract Thin cadmium selenide films were deposited on Corning 7059 glass substrates under controlled deposition rates, source temperatures and substrate temperatures. The conductivities and Hall coefficients of these films were measured over temperatures ranging from — 100 °C to 80 °C. Ion beam backscattering experiments showed that the films were deficient in selenium when deposited at low rates and deficient in cadmium when deposited at high rates. The structure of the films is polycrystalline with a crystallite size in the region of 200 A. Conduction above about 273 °K is attributed to compensated donors with an ionisation energy at infinite dilution of 0.186 eV and a concentration of about 10 20 cm -3 . The predominant scattering mechanism at these temperatures is that of intercrystalline potential barrier scattering. At temperatures below about 273 °K, conduction is attributed to impurity conduction processes.

Instability in the conductivity of cadmium selenide films

Abstract The electrical properties of thin cadmium selenide films deposited with controlled conditions were found to be unstable when the films were exposed to the atmosphere. This paper gives a detailed report on this instability and discusses the phenomenon in terms of an existing oxygen adsorption model which has been further extended. The model is shown to be satisfactory in accounting for experimental data gathered from films deposited under different conditions. Values of ionized donor concentration estimated using this model were consistent with those obtained from Hall measurements.

The effect of sulfur concentration on the properties of chemical bath deposited CdS thin films

We study the structural, surface morphology and optical properties of chemical bath deposited (CBD) cadmium sulfide (CdS) thin films under the effect of variation of S/Cd ratio. CdS thin films have been successfully deposited by CBD technique with solutions containing S/Cd ionic concentration ratio of 5.0, 2.5, 1.0, 0.5 and 0.25. Single phase CdS, with a hexagonal structure, is observed for the concentration of S/Cd = 5.0, 2.5, 1.0 and 0.5 films while for the ratio of 0.25, the films exhibited a partially amorphous nature. These have been confirmed by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM) analyses. The band gaps of the films obtained by transmission and photoacoustic spectra are found to be in the range of 2.40 to 3.26 eV. The large variation of band gaps of the films with composition is discussed by employing quantum size effect phenomena. The transition levels of CdS are also studied using photoacoustic spectroscopy.

Changes in the opto-electronic properties of CuInSe 2 following ion implantation

The development of efficient thin-film solar cells based on CuInSe2 absorber layers has encouraged fundamental research on both thin films and single crystals of this chalcopyrite semiconducting compound. The resistance to radiation and ion bombardment is of technical importance particularly for a material which could find future applications in space photovoltaic power systems. In this paper results are described for an ion implantation study using CuInSe2 single crystal substrates. Oxygen, helium and neon implantations have produced significant changes in surface resistivity and photoconductivity. Also the near-surface regions ofn-type crystals have been type-converted top-type following ion implantation. It is apparent that the ion implantation process creates defects which affect surface state densities and recombination probabilities. In the case of oxygen there is an additional doping effect caused either by the introduction of acceptor states or by the reduction of the existing donor state population. Following implantation there appears to be an overall decrease in carrier recombination at the surface which leads to an enhanced photoconductive response.

Switching of deep levels in CuInSe2 due to electric field-induced Cu ion migration

To search for the origin of the 0.26 eV deep hole trap, which is well established in p-type CuInSe2 single crystals and thin films, we investigated the influence of electric field-induced Cu ion migration on this level. We show that this hole trap can be removed from the space charge region of a reverse-biased Schottky barrier in p-type CuInSe2 single crystals by annealing at 490 K for 10 min. Simultaneously, a new hole trap at 0.49 eV from the valence band is created in this region. Conversely, the application of a forward bias under the same annealing conditions causes a decrease in the concentration of the 0.49 eV hole trap and a recreation of the 0.26 eV hole trap to the original state. These two processes were found to be reversible. Our results are completely consistent with recent evidence of an electric field-induced Cu ion migration via a vacancy mechanism in CuInSe2. According to a recent theoretical paper on the defect physics of CuInSe2, a candidate for experimentally observed 0.26 eV deep hol...

Impact of annealing processes on the properties of CuIn0.75Ga0.25Se2 thin films

Abstract The efficiency of a solar cells utilizing polycrystalline Cu(In,Ga)Se 2 is to a large extent limited by crystalline defects of the semiconductor. Depending on the fabrication process the density of grain boundaries and dislocations, can vary considerably. However, the material properties can be improved significantly by the subsequent processing steps. In this paper results obtained using various post deposition methods to improve the structural and electro-optical properties of CuIn 0.75 Ga 0.25 Se 2 (CIGS) thin films have been presented and discussed. Films deposited by the evaporation of pre-reacted polycrystalline CIGS onto glass substrate were subsequently processed under several sets of conditions including vacuum, selenium, inert and forming gas ambients at different temperature and times. The structural and electro-optical properties of both as-deposited and annealed films were studied using a variety of analytical techniques. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies of the films showed a columnar structure with strong orientation, which after heat treatments was relaxed to form a chalcopyrite structure. Raman analysis showed that full wave half maximum (FWHM) value reduced from 20 to 10 cm −1 with the annealing process indicating a change in both film composition and microstructure. In addition, investigations using energy dispersive X-ray analysis (EDAX), X-ray fluorescence (XRF) and Rutherford backscattering spectroscopy (RBS) revealed that the composition was approaching that of the starting polycrystalline material. Both n- and p-type conductivities were observed and gave resistivity values in the range 10 −1 to 10 6 Ω cm. Annealing in selenium changed the observed n-type conductivity of the as-deposited films to p-type. Photoacoustic spectroscopy (PAS) have also been applied to verify the improvement in the optical properties of annealed films.