Refractive index,band gap and oscillator parameters of amorphous GaSe thin films

GaSe thin films are obtained by evaporating GaSe crystals onto ultrasonically cleaned glass substrates kept at room temperature under a pressure of ∼10^–5 Torr. The X‐ray analysis revealed that these films are of amorphous nature. The reflectance and transmittance of the films are measured in the incident photon energy range of 1.1–3.0 eV. The absorption coefficient spectral analysis revealed the existence of long and wide band tails of the localized states in the low absorption region. The band tails width is calculated to be 0.42 eV. The analysis of the absorption coefficient in the high absorption region revealed an indirect forbidden band gap of 1.93 eV. The transmittance analysis in the incidence photon wavelength range of 500–1100 nm allowed the determination of refractive index as function of wave length. The refractive index–wavelength variation leads to the determination of dispersion and oscillator energies as 31.23 and 3.90 eV, respectively. The static refractive index and static dielectric constant were also calculated as a result of the later data and found to be 9.0 and 3.0, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and energy band gap of CaSeS thin films prepared by hot-wall epitaxy

This paper investigates preparation of CaSeS thin films using hot-wall epitaxy. These films can be grown epitaxially on cleaved BaF₂(1 1 1) at a substrate temperature of 873 K by tailoring the VI/II flux ratio vaporized from Ca and SeS resources. The optical absorption edge of these films thus tailored can be observed clearly, shifting toward higher photon energy with increasing S content. In particular, the energy band gap of CaSe_0.66S_0.34, capable of lattice-matching to InP was found to be 4.69 eV, producing considerably large band gap difference of 3.34 eV between the CaSe_0.66S_0.34 and InP.     

Structural investigation and electronic band transitions of nanostructured TiO2 thin films

Titanium dioxide (TiO₂) thin film was deposited on n‐Si (100) substrate by reactive DC magnetron sputtering system at 250 °C temperature. The deposited film was thermally treated for 3 h in the range of 400‐1000 °C by conventional thermal annealing (CTA) in air atmosphere. The effects of the annealing temperature on the structural and morphological properties of the films were investigated by X‐ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD measurements show that the rutile phase is the dominant crystalline phase for the film annealed at 800 °C. According to AFM results, the increased grain sizes indicate that the annealing improves the crystalline quality of the TiO₂ film. In addition, the formation of the interfacial SiO₂ layer between TiO₂ film and Si substrate was evaluated by the transmittance spectra obtained with FTIR spectrometer. The electronic band transitions of as‐deposited and annealed films were also studied by using photoluminescence (PL) spectroscopy at room temperature. The results show that the dislocation density and microstrain in the film were decreased by increasing annealing temperature for both anatase and rutile phases. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transient photoconductivity studies of band tails states in compensated a-Si:H

The density of conduction band tail states has been determined for a series of compensated a-Si:H films using transient photoconductivity measurements. Relative to undoped material, the energy width of shallow tail states lying within 0.45 eV of the conduction band edge are found to be insensitive to the level of compensated doping for doping levels up to 1000 vppm. An observed reduction in photocurrent magnitude as the doping is increased is consistent with a corresponding reduction in the electron extended state mobility. The magnitude of the mobility reduction is found to be quantitatively consistent with potential fluctuations which arise from ionized dopants for compensated doping levels up to about 100 vppm.     

Metastability effects in hydrogenated microcrystalline silicon thin films investigated by the dual beam photoconductivity method

Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark conductivity, σ_D, photoconductivity, σ_ph, and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O₂ gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si:H, films, other results were unaffected from the aging and annealing processes applied.     

Improvement of photoconductivity in Silicon Tin (SiSn) thin films

Hydrogenated-amorphous silicon tin alloy (a-SiSn (C:H)) films have been prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique. We present investigations of the absorption and photoconductivity characteristics of a-SiSn(C:H) thin films as a function of the tin concentration, which could be a pivotal feature for the bottom solar cell development. We demonstrate that the photoconductivity is significantly improved by the film growth under enhanced hydrogen dilution. The results suggest that the hydrogen dilution plays an important role in development of high quality a-SiSn(C:H) photosensitive thin films.     

Transient photoconductivity in amorphous Se70Sb20Ag10 thin films

Se₇₀Sb₂₀Ag₁₀ bulk material was obtained using a conventional melt quenching technique. Thin films of a-Se₇₀Sb₂₀Ag₁₀ were prepared by vacuum evaporation technique in a base pressure of 10⁻⁴ mbar onto well-cleaned glass substrates. Dark, light conductivity and transient photoconductivity have been studied. The photoconductivity increases initially and then saturates with time when illuminated with low intensity light (<400 Lux); however, when illuminated with higher intensity light (>600 Lux) the photoconductivity increases initially, attains a maximum and then decreases with time. This behavior of photoconductivity has been studied at various temperatures. The results have been explained on the basis of Dember voltage and the interaction between photoexcited holes and the trapped electrons on the surface.     

Examination of the electronic energy band model for one-dimensional disordered thin films

The nature of the electronic states in a disordered one-dimensional finite system subject to traveling wave boundary conditions is examined. A method is developed whereby these states can be characterized as either “localized” or extended. The concept of localization of states is modified so as to be applicable to amorphous thin films. The extent to which the disorder modifies the band structure of the ordered system is investigated as a function of disorder and the nature of the electronic states is related to the elastic tunneling transmission coefficient for a model metal-semiconductor-metal heterojunction. The results seem to corroborate the Mott-CFO model.     

Optical band gap and refractive index dispersion parameters of In-Se-Te amorphous films

Different compositions of In_x(Se_0.75Te_0.25)_100 − x (where 0 ≤ x ≤ 10 at.%) chalcogenide glasses were prepared by the usual melt quench technique. Chalcogenide thin films of these glasses were prepared by using thermal evaporation method. The film transmittance (T(λ)) at normal incidence for these films was measured in the wavelength range 400-2500 nm using a double beam spectrophotometer. Successfully applying Swanepoel's method helps us to determine the film thickness and the real (n) and imaginary (k) parts of the complex index of refraction with high accuracy. Optical absorption measurements show that, the fundamental absorption is due to the allowed non-direct transitions. It was found that, the addition of In content leads to the increase of the refractive index increases while the optical band gap decreases. The obtained results are well discussed in terms of the chemical bond approach and the cohesive energy.     

Optical band gap of zinc nitride films prepared by reactive rf magnetron sputtering

Polycrystalline Zn₃N₂ films are prepared on Si and quartz glass substrates by RF magnetron sputtering at room temperature. The structural and optical properties are studied by X‐ray diffraction and double beam spectrophotometer, respectively. X‐ray diffraction indicates that the Zn₃N₂ films deposited on Si and quartz glass substrates both have a preferred orientation in (321) and (442), also are cubic in structure with the lattice constant a=0.9847 and 0.9783 nm, respectively. The absorption coefficients as well as the film thickness are calculated from the transmission spectra, and their dependence on photon energy is examined to determine the optical band gap. Zn₃N₂ is determined to be an indirect‐gap semiconductor with the band gap of 2.11(2) eV. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nanoindentation investigation of amorphous hydrogenated carbon thin films deposited by ECR-MPCVD

Nanomechanical properties of amorphous hydrogenated carbon thin films are performed by nanoindentation technique. The amorphous hydrogenated carbon films are produced on silicon substrate by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD). The effect of negative bias voltage on amorphous hydrogenated carbon films is examined by Raman spectroscopy and the results showed that the intensity ratio of D-peak to G-peak (I_D/I_G) of amorphous hydrogenated carbon films at various bias voltages, increased as the bias voltage increased. The results also showed that Young’s modulus and hardness also increased as the bias voltage increased. In addition, Young’s modulus and hardness both decreased as the indentation depth increased.     

Distribution of electronic states in amorphous Cd-As thin films on the basis of optical measurements

Transmission and fundamental reflectivity studies, completed on amorphous Cd-As thin films, allowed us to obtain parameters describing the fundamental absorption edge, i.e. the optical pseudogap E_G^opt, Urbach energy E_U and exponential edge parameter E_T. All these data, together with the results of earlier transport measurements, have been utilized in developing simple models of electronic structure (distribution of electronic states) for amorphous Cd-As thin films of various compositions.     

Modulated photocurrent as a powerful method to reveal predominant transport by the majority carriers of disordered semiconductors and to resolve all the kinds of probed gap states

A basic difficulty in the interpretation of photoconductivity measurements may arise from possible mixed contributions of both carriers to the photocurrent. In this work it is demonstrated that a universal behavior in the simulated spectra of the out of phase modulated photocurrent signal is observed in cases where the majority carriers dominate. In these cases, a general formula can be used to evaluate the densities of various species of states using the data from all frequencies. Deviations from the universal behavior can be observed when there are contributions from both carriers. The applicability of our analysis is demonstrated in a-Si:H.     

Growth,structure and properties of thin monocrystalline InSb films grown by recrystallization in a narrow gap

The results of investigation of recrystallization kinetics of (≦ 500 Å) InSb films are described with the aid of a laser refractometer. The thermodynamic process characteristic considered to the gap thickness and substrate material are investigated. It was revealed that the method of producing a protective SiO_x coating greatly affects the electric properties of InSb films.     

Defect levels in the band gap of amorphous selenium

Energy locations in the band gap have been determined for the thermally accessible levels of the negative-U defect centers in amorphous selenium. Both the temperature dependence of the steady-state photocurrents, and an analysis of emission currents in the post-transit regime of a time-of-flight transient photoconductivity experiment on the same samples, agree on the presence of defect levels at (0.42 ± 0.04) eV above the valence band mobility edge and (0.53 ± 0.06) eV below the conduction band. Both measured current levels and the resolved energy positions of the defects are subject to the Poole-Frenkel effect.     

ESR investigation of graphite-like amorphous carbon films revealing itinerant states as the ones responsible for the signal

The origin of paramagnetic centers in graphite-like amorphous carbon is investigated. The films were deposited by the ion beam assisted deposition (IBAD) and have a concentration of sp² sites of about 90% and zero energy band gap. The density of the film and the electrical resistivity are close to these of crystalline graphite. However, the hardness and stress of the films are similar to those of diamond-like carbon. Electron spin resonance (ESR) performed at the X-band (9.4 GHz) revealed an unexpected low density of paramagnetic centers, ascribed to conduction electrons with a g-value of about 2.003.