Photoinduced optical changes in amorphous Se and Ge–Se films

Films of amorphous Se, Ge₁₅Se₈₅, and Ge₂₅Se₇₅ were prepared by thermal evaporation. Measurements of photodarkening at 78 K showed that the optical gap (E_g) and the slope of optical absorption edge (B) are intercorrelated. The correlation between the photoinduced changes of E_g and B is discussed within both the model of Davis and Mott and Tauc's model for optical transitions in amorphous solids.     

The effect of nitrogen doping on amorphous germanium

The effects of nitrogen doping on the electrical and optical properties of amorphous germanium are investigated. It is found that within the low nitrogen concentrations that cause no appreciable change in the optical energy gap, the room temperature conductivity and the B coefficient in the optical absorption show a maximum at the same nitrogen concentration. This behavior is interpreted by a delocalization of the electronic states in the conduction band due to the nitrogen incorporation.     

Deviations from square-root distributions of electronic states in hydrogenated amorphous silicon and their impact upon the resultant optical properties

We study the distributions of conduction band and valence band electronic states associated with hydrogenated amorphous silicon. We find that there are substantial deviations from square-root distributions, particularly deep within the bands and within the gap region. The impact of these deviations is assessed through a determination of the spectral dependence of both the joint density of states function and the imaginary part of the dielectric function. These deviations are found to have a considerable effect upon the determination of the corresponding Tauc optical gap, the optical gap obtained for the case of hydrogenated amorphous silicon being 220 meV lower than the energy difference between the valence band and conduction band band edges. We suggest that the standard interpretation for the Tauc optical gap, as the energy difference between these band edges, should be reconsidered in light of these results.     

Compositional dependence of the optical properties of amorphous Sb2Se3−xSx thin films

Thin films of Sb₂Se_3−xS_x solid solutions (x = 0, 1, 2, and 3) were deposited by thermal evaporation of presynthesized materials on glass substrates held at room temperature. The films compositions were confirmed by using energy dispersive analysis of X-rays (EDAX). X-ray diffraction studies revealed that all the as-deposited films as well as those annealed at T_a < 423 K have amorphous phase. The optical constants (n, k) and the thickness (t) of the films were determined from optical transmittance data, in the spectral range 500-2500 nm, using the Swanepoel method. The dispersion parameters were determined from the analysis of the refractive index. An analysis of the optical absorption spectra revealed an Urbach’s tail in the low absorption region, while in the high absorption region an indirect band gap characterizes the films with different compositions. It was found that the optical band gap energy increases quadratically as the S content increases.     

Preparation and characterization of p-type hydrogenated amorphous silicon oxide film and its application to solar cell

Thin film wide band gap p-type hydrogenated amorphous silicon (a-Si) oxide (p-a-SiOx:H) materials were prepared at 175 °C substrate temperature in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and applied to the window layer of a-Si solar cell. We used nitrous oxide (N₂O), hydrogen (H₂), silane (SiH₄), and diborane (B₂H₆) as source gases. Optical band gap of the 1% diborane doped films is in the range of 1.71 eV to 2.0 eV for films with increased oxygen content. Dark conductivity of these films is in the range of 8.7 × 10^− 5 S/cm to 5.1 × 10^− 7 S/cm. The fall in conductivity, that is nearly two orders of magnitude, for about 0.3 eV increase in the optical gap can be understood with the help of Arrhenius relation of conductivity and activation energy, and may not be significantly dependant on defects associated to oxygen incorporation. Defect density, estimated from spectroscopic ellipsometry data, is found to decrease for samples with higher oxygen content and wider optical gap. Few of these p-type samples were used to fabricate p-i-n type solar cells. Measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (V_oc) = 800 mV, short circuit current density (J_sc) = 16.3 mA/cm², fill-factor (FF) = 72%, and photovoltaic conversion efficiency (η) = 9.4%, which may be due to improved band gap matching between p-a-SiOx:H and intrinsic layer. J_sc, FF and V_oc of the cell can further be improved at optimized cell structure and with intrinsic layer having a lower number of defects.     

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)     

Structure and electronic properties of hydrogenated amorphous GaP

Hydrogenated amorphous GaP films have been prepared by reactive rf sputtering. Infrared spectroscopy, optical transmission and reflection, photothermal deflection spectroscopy and dc conductivity have been studied to investigate the local bonding configurations, the optical absorption edges and the temperature dependence of the conductivity as a function of hydrogen content. The results are discussed and compared with the effects of hydrogenation on amorphous Si.     

Substrate temperature effect on the optical properties of amorphous Sb2S3 thin films

Sb₂S₃ amorphous thin films were prepared by thermal evaporation of corresponding powder on thoroughly cleaned glass substrates held at temperature in the range 300‐473 K. X‐ray diffraction and atomic force microscopy have been used to order to identify the structure and morphology of surface thin films. The optical constants of the deposition films were obtained from the analysis of the experimental recorded transmission data over the wavelength range 400‐1400 nm. An analysis of the absorption coefficient values revealed an optical indirect transition with the estimation of the corresponding band gap values. It was found that the optical band gap energy decrease with substrate temperature from 1.67 eV at 300 K to 1.48 eV at 473K. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties and X-ray diffraction of amorphous systems

By appealing to the persistence of short range order in amorphous materials a simple estimate of the form of the static structure factor can be obtained. It is also possible to derive an expression for the optical absorption of amorphous materials involving a convolution of the corresponding crystalline expression and a simple function.     

Comparison of photocurrent spectra measured by FTPS and CPM for amorphous silicon layers and solar cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently introduced as a fast and highly sensitive method for the evaluation of the optical absorption coefficient of photoconductive thin films such as microcrystalline silicon layers. This contribution represents the first study of FTPS utilization for amorphous silicon layers and cells. FTPS spectra are compared with results of Constant Photocurrent Method (CPM) and Dual Beam Photoconductivity (DBP) measured at different chopping frequencies. We will concentrate to highlight the appropriate measuring conditions and evaluation procedures for correct data interpretation. Moreover, we will present our novel approach for the interference free determination of absorption coefficients of thin films grown on transparent substrates, which is mainly important for very thin layers where broad interference fringes do not allow correct evaluation of parameters such as a slope of the Urbach tail and the defect density.     

Luminescence gap in hydrogenated amorphous silicon

The “luminescence gap” is used instead of the thermalization gap and the hopping-gap because the gap is obtained from the luminescence measurement. The luminescence gaps in hydrogenated amorphous silicon (a-Si:H) are observed in the temperature range from 4.2 to 225 K for the films prepared at different substrate temperatures 170 to 300 °C by plasma CVD. It is shown from the temperature dependence of the luminescence gap that the luminescence edges are at the localized band tail states at which the waiting time for the hopping is equal to the life time of the luminescence. The excitation energy dependence of the luminescence peak energy similar to that of the porous Si has been observed.     

Temperature dependent formation of microcrystal and amorphous silicon by vacuum evaporation

In silicon films deposited in vacuum by electron beam evaporation of solid silicon, the substrate temperature dependence of the formation temperature of amorphous silicon and crystalline silicon is determined. On steel and alumina substrates, below 520°C, the silicon film is amorphous. Above this temperature, the film is crystalline with a pronounced optical band gap of 1.7 eV. With thermal treatment in vacuum, a transformation from an amorphous to a crystalline state is observed at 650°C.     

Extracting accurate optical parameters from glasses usingterahertz time-domain spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) can be used to characterize the optical properties of glasses in a frequency regime that is barely accessible by other techniques. It can be used to measure the electric field of a pulse of broadband terahertz radiation with frequency components between 0.1 and 4.5 THz propagating through the material of interest. Data processing is required to transform the information from the raw time-domain waveform data into the absorption spectrum. While most crystalline materials exhibit a number of distinct spectral features, amorphous materials show an almost featureless frequency response in the terahertz range and very accurate measurements of the absorption coefficient are required to characterize the materials. The accuracy of existing data processing techniques is limited as a result of truncating the time-domain data to remove multiple reflections of the terahertz pulse within the sample material and by neglecting scattering losses at the surface of the sample material. Using the experimental THz-TDS data of As₂S₃ we present a new approach to extract accurate optical parameters from THz-TDS experimental data.     

Influence of temperature on the microcrystalline structure of thermally evaporated Sb2S3 thin films

Thin films of antimony trisulfide (Sb₂S₃) were prepared by thermal evaporation under vacuum (p=5×10^–5 torr) on glass substrates maintained at various temperatures between 293 K and 523 K. Their microstructural properties have obtained by transmission electron microscopy (TEM). The electron diffraction analysis showed the occurrence of amorphous to polycrystalline transition in the films deposited at higher temperature of substrates (523 K). The polycrystalline thin films were found to have an orthorhombic structure. The interplanar distances and unit‐cell parameters were determined by high‐resolution transmission electron microscopy (HRTEM) and compared with the standard values for Sb₂S₃. The surface morphology of Sb₂S₃ thin films was investigated by scanning electron microscopy (SEM). The optical transmission spectra at normal incidence of Sb₂S₃ thin films have been measured in the spectral range of 400–1400 nm. The analysis of the absorption spectra revealed indirect energy gaps, characterizing of amorphous films, while the polycrystalline films exhibited direct energy gap. From the photon energy dependence of absorption coefficient, the optical band gap energy, E_g, were calculated for each thin films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectral properties of PbO-P2O5 glasses

The optical absorption spectra of xPbO-(100 − x) P₂O₅ glasses where x = 5, 10, 15, 20, 25, and 30 is reported. The spectral absorption of these glasses was measured in the spectral range 300-900 nm at room temperature. Optical absorption spectra show that the absorption edge has a tail extending towards lower energies. The edge shifts nearly linearly towards higher energies with increasing PbO content. The degree of the edge shift was found to depend on the PbO content and is mostly related to the structural rearrangement and the relative concentrations of the glass basic units. The optical energy gap increases, from 2.55 to 3.05 eV by increasing PbO content from 5 to 30 mol%. The width of the localized states is decreased by increasing PbO content.     

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.     

Voids in amorphous semiconductors

Small angle X-ray scattering (SAXS) has been used to investigate density fluctuations occurring in amorphous semiconductors prepared by sputtering, evaporation and electrodeposition. Correlations of the total number of dangling bonds determined by SAXS, optical absorption, and ESR signals have been made. The density deficits from the density of the FC-2 crystal are in some cases accounted for by the voids. It is argued that models based on domains 10–15 Å in size are not supported by the SAXS data.     

Thermopower of amorphous Si(Al)

Thermopower data for amorphous Si(Al) alloys from 230 to 500 K are reported and shown to be consistent with a two conduction mechanism model. Complementary optical absorption and electrical conductivity data are reported.     

Er-doped hydrogenated amorphous silicon: structural and optical properties

The effect of erbium-doping on the structural and optical properties of hydrogenated amorphous silicon (a-Si:H) is investigated. Optical absorption and Raman spectra indicate that erbium doping introduces defect states, and that above a concentration of 0.27 at.%, induces strong structural disorder. The photoluminescence measurements show that erbium doping introduces non-radiative decay paths for carriers in a-Si:H, leading to decrease in both the Er³⁺ and intrinsic a-Si:H luminescence intensity when the Er concentration is increased to more than 0.04 at.%. The results are compared to that of Er-doped crystalline Si, and the possible excitation mechanisms of Er in a-Si:H are discussed.     

Optical and electrical properties of chlorinated and hydrogenated amorphous silicon prepared by glow discharge

Chlorinated and hydrogenated amorphous silicon films were prepared by glow discharge of a SiCl₄/H₂ mixture. Infrared spectra of these films show that, in addition to the hydrogen induced bands, two new modes appear at 545 cm^?1 (SiCl stretching) and 500 cm^?1 (Si TO modes induced by chlorine). Observation of the 545 cm^?1 band proves that chlorine is able to act as a dangling bond terminator in an amorphous silicon matrix. A good agreement is found between the total amount of chlorine determined by electron microprobe analysis and the value estimated from the integrated strength of the SiCl stretching mode. The relatively high value of the optical band gap (1.80 eV) of our material containing only 5 at.% bonded hydrogen shows that chlorine plays a major role in the optical gap value. Electrical conductivity, photoconductivity and luminescence properties are qualitatively similar to that of a: SiH films.