Fictive temperature measurement of amorphous SiO2 films by IR method

The structure and properties of amorphous materials, in general, change with their thermal history. This is usually explained using the concept of fictive temperature, i.e., the temperature at which the super-cooled liquid state turned into a glassy state. In earlier studies, a simple IR method was used to determine the fictive temperature of silica glasses, both bulk and fiber. In the present study the applicability of the same technique for thin amorphous silica films on silicon was examined. It was found that the IR absorption as well as reflection peak wavenumber of the silica structural band can be used to determine the fictive temperature of amorphous silica films on silicon with an unknown thermal history. Specifically, IR absorbance spectra of an amorphous silica film of thickness greater than 0.5 μm grown on silicon can be taken before and after etching a thin surface layer of 20–30 nm and the peak wavenumber of the difference signal can be compared with the pre-determined calibration curve to convert the peak wavenumber to the fictive temperature. For a film thicker than ∼2 μm, IR reflection peak wavenumber can be converted directly to the fictive temperature of the film by using the calibration curve.     

Transition from amorphous semiconductor to amorphous insulator in hydrogenated carbon–germanium films investigated by IR spectroscopy

Thin a-Ge_XC_1?X:H plasma polymerized films, depending on deposition conditions, can be produced in two very different structures, namely amorphous semiconductor and amorphous insulator. The transition from amorphous insulator to amorphous semiconductor is related to the formation of germanium nanoclusters due to ions bombarding the surface of the growing material. This paper concentrates on investigations of the transition by means of IR spectroscopy. To this end a quantitative analysis of IR spectra obtained for thin films deposited on silicon substrate has been described and used for estimation of hydrogen atom concentration and bonding in the investigated material. It was found that the probability that a given H atom is bonded to a germanium or to a carbon atom is almost the same. This conclusion is true both for a-S and a-I films. The average concentration of hydrogen in the investigated material was found to be about 2.4–3.4 × 10²² cm^?2 which means that there are two times more atoms of the carbon family than hydrogen atoms in the film structure.     

Time evolution of surface defect states in hydrogenated amorphous silicon studied by photothermal and photocurrent spectroscopy and optical simulation

The time evolution of surface defect density and width of space charge region in thin layer of amorphous silicon is observed experimentally by Fourier transform photocurrent spectroscopy. This work reviews the assumption that photocurrent is insensitive to surface defects for samples thinner than 1500 nm. We show that correct evaluation based on simple optical model comprising layers representing surface defects and layers representing space charge region with reduced collection allows obtaining the same results as from photothermal deflection spectroscopy. Our main approach is the comparison of photocurrent or photothermal deflection spectra measured in absorptance/transmittance mode from layer and substrate side of the thin film.     

Spectroscopic ellipsometry studies of reactively sputtered nitrogen-rich GaAsN films

Nitrogen-rich GaAsN thin films have been deposited at 500 °C by reactive rf sputtering of GaAs target in argon-nitrogen atmosphere. The arsenic content of the films was varied by changing the nitrogen percentage in the sputtering atmosphere and the As/Ga ratio in the films was estimated by X-ray fluorescence measurements. Spectroscopic ellipsometry measurements have been carried out on these films and the measured ellispometric spectra were fitted with theoretical spectra generated by using suitable model sample structures. From the best fit parameters of the dispersion model, band-gap values and variation of refractive index and extinction coefficient as a function of wavelength have been obtained for films deposited with different percentages of nitrogen in the sputtering atmosphere. The films deposited with 12% to 100% nitrogen in the sputtering atmosphere, which are of hexagonal GaN, exhibit GaN-like optical properties, though effects due to excess arsenic in amorphous phase are seen in the films deposited with less than 40% nitrogen. The films deposited with 5% to 12% nitrogen in sputtering atmosphere are dominantly polycrystalline GaAs_xN_1−x (x ≈ 0.01 to 0.08) and exhibit variations in optical parameters, which are consistent with their structure and composition. The films deposited with less than 5% nitrogen in sputtering atmosphere are arsenic-rich and amorphous.     

Transformation of microcrystalline state of hydrogenated silicon to amorphous one due to presence of more electronegative impurities

When a slight fraction (～ 2 mol %) of N₂ is added into H₂, sputtering atmosphere for microcrystalline hydrogenated Si films, without changing other fabrication parameters, the amorphous film rather than microcrystalline one forms. The stabilization mechanism of the amorphous film of Si is discussed through TEM and IR observations of this kind of transformation from microcrystal to amorphous state.     

Hydroxyl groups in phosphosilicate glasses for fibre optics

IR absorption spectra of phosphosilicate glasses containing 1–19 mol% of P₂O₅ are measured and analyzed. Interaction of hydroxyl groups with the phosphorus centres in the phosphosilicate glass is investigated in cluster models using PM3 and MNDO quantum-chemical methods. An explanation of the experimentally observed change of vibrational absorption bands caused by the hydroxyl groups is proposed.     

Studies on interface roughness scattering effects in molecular beam epitaxy grown resonant tunneling structures

Interface roughness (IR) scattering effects in the resonant tunneling (RT) process through molecular beam epitaxy (MBE) grown double barrier (DB) structures have been investigated both theoretically and experimentally. The time dependent perturbation theory was applied to an elastic scattering event which is caused by the IR potential within the dwelling time period of an incident electron inside a DB. The calculated result indicates that the peaked structure of the tunneling transmission coefficient can be degraded drastically when the IR has lateral Fourier components in a specific spatial frequency range, and that the upper and lower limits of the range are related to such structural parameters as the well width and the cavity finesse of the DB. It has been found that the above prediction is possibly supported by the experimental results obtained in tunneling spectroscopy measurements performed on AlAs-GaAs DB structures; measured peak widths of tunneling spectra in samples grown with growth interruption periods are broadened by a factor of more than 2.     

Study of in-gap defects in intrinsic and B-doped a-Si1−xCx:H by photo-induced optical absorption and photoluminescence

The infrared (IR) absorption dependence on visible light illumination has been measured in doped and undoped hydrogenated amorphous silicon carbide (a-Si_1−xC_x:H) films grown by plasma enhanced chemical vapour deposition. The measurements were made by a highly sensitive technique which exploits properly designed a-Si_1−xC_x:H/ZnO test waveguides for lengthening the interaction region between the IR and visible (VIS) radiations in the material. Experimental data show that boron doping strongly enhances the VIS light induced variation of the IR absorption, whereas the increase in carbon content has a quenching effect on the phenomenon. The a-SiC:H films have been also characterized by photoluminescence measurements. The spectra are dominated by a photoluminescence band, ranging between 1.4 eV and 1.9 eV. This band is enhanced by the increase in carbon content, while is strongly quenched with increasing B-doping level. On the basis of these results, a correlation is found between the measurements of optical absorption, photo-induced absorption and photoluminescence. The type and the density of defects induced in the films by the different growth conditions have been recognized as the origin of the different behaviors observed.     

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.     

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)     

Characterization of organic adsorbates on model glass surfaces

Model silicate glasses in planar thin film structures are prepared using sol/gel techniques. A typical structure consists of 3.0–12.0 nm of glass on vapor deposited Ag. In this study, a pure silica and a 50/50 binary alumina-silica films are examined. Poly(methyl methacrylate) (PMMA) is adsorbed from chlorobenzene solution onto the surfaces. Infrared reflection (IR) spectroscopy, ellipsometry and quartz oscillator microgravimetry (QOM) are applied to characterize the PMMA/silicate glass interfaces formed. The QOM and IR data show that PMMA adsorption on the pure silica surface is irreversible with respect to removal by pure solvent rinse while on the binary glass surface and on a pure alumina surface, solvent rinses remove the initially adsorbed polymer. These results are interpreted in terms of Bronsted acid-base interactions involving the basic properties of the PMMA C=O group, the acidic nature of the silica and the more basic nature of the alumina containing surfaces. Further evidence for this interpretation is given by the IR spectral data which show broadening of the C=O stretching mode to lower frequencies for the irreversible adsorbed polymer on silica as compared with simulated spectra of non-surface bonding PMMA thin films. This spectral perturbation is interpreted as evidence for a hydrogen bonding interaction between OH groups on the silica surface and the C=O groups. The overall conclusion is that the surface of a 50/50 binary alumina-silica composition is dominated by the basic nature of alumina.     

Application of the optical method for determining of the RMS roughness of porous glass surfaces

The application of an optical method for characterizing surface roughness is presented. This method was used for an examination of porous glass surfaces. The expressions relating the root mean square rms (σ) of a surface to its specular reflectance at normal incidence are used for σ ? λ, (λ - wavelength). For light of sufficiently long wavelength the decrease in the measured specular reflectance due to the surface roughness depends only on the root mean square (rms) height of the surface irregularities. On the basis of reflectance spectra, one can determine σ for the porous glass surfaces after technological processes. The measured reflectance spectra were compared with calculated ones for which the scattered component of light was taken into account. The parameters rms determined from the optical method are comparable to those obtained from atomic force microscopy examinations.     

Causal Voigt profile for modeling reflectivity spectra of glasses

A new dielectric function model appropriate for a quantitative analysis of infrared spectra of amorphous solids and glasses is introduced and validated by the study of the infrared reflectivity spectra of two different glasses, a calcium aluminosilicate and a borosilicate. Modeling results show its superiority to the classical sum model and confirm its efficiency in the reproduction of the absorption bands of glasses located in the far infrared range.     

Substrate engineering for high efficiency thin film solar cells

The design considerations for a spectra modifying, light scattering layer for amorphous silicon solar cells were investigated. Efficient commercially available phosphors absorb one near IR photon and one near UV photon and emit one photon in the visible spectrum. Thereby such phosphors offer the possibility to convert two poorly utilized portions of the solar spectrum to photons that are converted to electric energy with high quantum efficiency in amorphous silicon-based solar cells. Large band gap, conductive, a-SiC:H and a-SiN:H are attractive matrices for phosphors as scattered light and emitted photons are thereby directed towards the underlying solar cell structure.     

Advanced optical characterization of disordered semiconductors by Fourier transform photocurrent spectroscopy

The paper summarizes progress in the Fourier transform photocurrent spectroscopy (FTPS) since 2001 when it was introduced for the first time for evaluation of the spectral dependence of the optical absorption coefficient in microcrystalline silicon thin films. We concentrate on the appropriate measuring conditions and evaluation procedures for a correct data interpretation of various thin films and solar cell structures. We show how the measured and finally evaluated absorption spectra differ in case of single junction amorphous and microcrystalline silicon solar cells as well as for a micromorph tandem. The key issue of the tandem structure diagnostics is a separation of FTPS signals from amorphous and microcrystalline parts of a stacked structure and their correct interpretation. For an appropriate simulation of the light propagation within the structure and confirmation of our measuring approach the computer model cell has been used.     

Solid state amorphization in Cu---Hf multilayers studied by time-differential perturbed-angular correlation

The time differential perturbed angular correlation spectra of the Cu---Hf multilayers and the change in the spectra during the amorphous phase change have been measured. The mechanism for the phase change of the multilayers is discussed.     

Vibrational spectra and structure of chloro-fluorozirconate glasses

A series of barium chloro-fluorozirconate glasses have been prepared. Their IR absorption, IR reflectivity and Raman spectra have been measured down to 33 cm^?1. The glass transition and crystallization temperatures have also been measured. The high frequency IR absorption and Raman modes of the chloro-fluorozirconate glasses have been assigned as in fluorozirconate glasses. The IR reflection spectra of chloride-containing glasses differed from the fluorozirconates in that one band was clearly related to Cl atom motions. The structure of the glasses probably consists of zig-zag chains of ZrCl₂F₄ mixed halide octahedra plus a pure fluoride matrix whose structure is similar to that of a ZrF₄BaF₂ glass with the same composition.     

Characterization of mixed phase silicon by Raman spectroscopy

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar⁺ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.     

Investigation of the network structure of niobium borate glasses

Some structure parameters of niobium borate glasses have been determined with the analytical data of X-ray diffraction and IR spectra. On the basis of experimental results, the model of the Nb₂O₅---B₂O₃---K₂O (NBK) glass network was inferred to be framed by circular structural units which are composed of six-membered and four-membered groups of NbO₆, BO₄ and BO₃ polyhedra in the glasses.