Vibrational spectra of vapor-deposited binary phosphosilicate glasses

Room temperature infrared transmission spectra in the range 4000-250 cm^?1 of binary phosphosilicate glass (PSG) films deposited by reacting argon- or nitrogen-diluted PH₃SiH₄O₂ mixtures on heated silicon substrates at 300–400° C have been obtained across the whole composition range. In all the as-deposited binary films, an absorption at ≈1300 cm^?1 characteristics of the P=O vibration was found to persist, together with a couple of broad absorptions in the regions 1200-900 cm^?1 and 500 cm^?1. Using a differential infrared technique the broad feature in the higher frequency region has been resolved into two well-defined bands at ≈1100 and 970 cm^?1. A detailed analysis shows that the intensity variation of the differential band at ≈1100 cm^?1 conforms well, at least to 50 mol% P₂O₅, to a simple structural model that yields an analytic distribution of POSi linkages as a function of composition by assuming chemical mixing in the vapor-deposited P₂O₅SiO₂ system. Furthermore, the system may be written as (P=O)₂ O₃SiO₂ in order to emphasize the similarity of its coordination scheme with that of the B₂O₃SiO₂ system studied earlier. The nature of these CVD films has also been elucidated by thermal and water treatments.     

Raman and infrared spectra on silica gel evolving toward glass

Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC₂H₅)₄ (TEOS) which have been thermally treated in the 40–800°C temperature range, are reported and discussed with reference to the spectra of fused quartz.The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C.Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes.In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm^?1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature.     

Instabilitäten in anodischen Siliziumoxidschichten

Heat-treatment of anodic of silicon oxide films results in the shift of the SiO stretching band absorption peak in the IR-Spektrum from wave number 1055–1060 cm⁻¹ to 1095 cm⁻¹ characteristic for thermally grown silicon oxide films. Similar tendency can be observed by storage in atmosphere, too. This phenomenon can be explained by rearrangement of the SiO stretching bands.     

On the oxidation mechanism of microcrystalline silicon thin films studied by Fourier transform infrared spectroscopy

Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si-H_x stretching (2000-2100 cm⁻¹), Si-O-Si (1000-1200 cm⁻¹), and O_xSi-H_y modes (2130-2250 cm⁻¹). A short time scale (< 3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O-H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains.     

Raman study of the structure of glasses along the join SiO2GeO2

In order to better understand the distribution of tetrahedra in multicomponent tetrahedral network structures of melts and glasses, we have investigated the Raman spectra of binary SiO₂GeO₂ glasses. We compare the Raman spectral features of the end-member glasses and discuss their vibrational origins. The mixing of GeO₂ and SiO₂ melts results in a continuous random network structure of TO₄ tetrahedra (T  Si, Ge) in the glass. Raman bands corresponding to the asymmetric stretch (v_as) of oxygen in GeOGe, SiOSi and SiOGe bonds are observed in the glasses having intermediate compositions along the SiO₂GeO₂ join. The presence of three distinct v_as (TOT) bands in the spectrum of a glass having Si/Ge one reveals that a considerable degree of SiGe disorder exists in the glass. The presence of a single symmetric oxygen stretching band in the spectra of binary SiO₂GeO₂ glasses indicates that the symmetric stretch modes (v_s) of oxygen in SiOSi, SiOGe and GeOGe bonds are strongly coupled. An observed decrease in the halfwidth of the v_s (TOT) band in the spectra of SiO₂GeO₂ glasses with increasing concentration of GeO₂ may be attributed to a decrease in the average TOT bond angle and a predominance of six-membered ring structures. Results of the present study support the assignment of the bands in the 900–1200 cm^?1 region of the alumino-silicate glasses, spectra to the v_as(AlOSi) and v_as(SiOSi) modes. In contrast to the alumino-silicate glasses, however, the SiO₂GeO₂ glasses have a much higher degree of disorder of the network-forming cations.     

Effect of the hydrogen content on optical band gap in a-Si:H:N

The effect of the hydrogen content on the optical band gap in glow discharge a-Si:H:N produced from SiH₄ and N₂ mixtures is demonstrated. The hydrogen content in the film can be well controlled by making a non-plasma-excited region above the substrate surface during deposition. When the concentration of hydrogen is high, a large amount of nitrogen atoms included in the film does not cause a widening of the optical band gap. The effects of the hydrogen content on the infrared absorption peak wavelength relevant to SiN lattice vibration and the optical band gap are discussed.     

Effect of hydrogen on the intrinsic stress in ion beam sputtered amorphous silicon films

The results of measurements of the intrinsic stress in hydrogenated amorphous silicon (a-Si:H) films deposited by ion beam sputtering are presented. The data show that the total intrinsic stress is composed of growth stress and hydrogen induced stress. At high hydrogen concentration (> 24%), an abrupt decrease in stress is observed which correlates with a microstructural change as shown by SEM and spectrosoopic ellipsometry measurements. The change in microstructure is accompanied by a modification in the IR transmission spectrum at 2000 cm⁻¹ indicating a change in hydrogen bonding configuration or in local environment near SiH bonds. A model is proposed which reconciles the observation of both tensile and compressive stress in unhydrogenated silicon reported in the literature.     

The effects of annealing and rate of deposition on the electrical,optical and structural properties of amorphous GeSi alloy films

The electrical, optical and structural properties of rf-sputtered amorphous GeSi alloy films have been investigated as functions of the rate of deposition between 90 and 360 Å min^?1, and various annealing conditions. The stabilization evidenced in the electrical conductivity, optical absorption and density of these films deposited at higher rates and/or annealing temperatures may be explained in terms of the reduction of imperfections in the as-deposited films.     

Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio (R_H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3-5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.     

Laser produced amorphous phases in NbSi and VSi thin films

The binary systems NbSi and VSi are investigated by laser induced melting and quenching of vapor deposited thin films. Glassy phases of various compositions are produced. The established compositional glass forming ranges support that the equilibrium compounds Nb₄Si is stable only at high temperature. Thermal decomposition of the amorphous NbSi films proceeds via several intermediate stages. In particular, the AuCu₃-type configuration of Nb₃Si was found to be formed. In addition, amorphous NbSi films around 20 at.% si show the formation of an undentified metastable compounds upon post-irradiation annealing. Amorphous VSi films proved to be stable up to at least 500°C. Above this temperature they decompose directly into their respective equilibrium phases.     

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.     

Effect of Silane flow rate on microstructure of Silicon films deposited by HWCVD

Hydrogenated silicon films ranging from pure amorphous to those containing small crystallites in large crystalline fraction are prepared using the HWCVD technique without using any hydrogen dilution which is supposed to be necessary for the deposition of nanocrystalline Si films. The only parameter that is varied is Silane flow rate. The deposition rate ranges from 6–27 Å/s. The band gap of the films (1.8–2.0 eV) is high compared to the regular films, which is attributed to the improved short and medium range order as well as the presence of low density amorphous tissues in the grain boundary regions. The films show improved stability under long term light exposure due to more ordered structure and presence of hydrogen mostly as strong Si―H bonds.     

Improved method for preparation of anhydrous silica by microwave irradiation with spectroscopic characterization and toxicity assay

Amorphous anhydrous silica SiO_{2 (mw)} (99.99%) is successfully synthesized through microwave irradiation technique and time of reaction is reduced up to 1 h. The dehydration phase study of Si–water, Si–OH, Si–O–Si networking, elemental analysis and surface morphology was carried out by FTIR, FTNIR, SEM and EDAX spectroscopic techniques. The broad absorption stretching and bending of Si–OH and H₂O at 3695.38–2832.96 cm^? 1, 1638 cm^? 1 and 1191.20–1017.14 cm^? 1 completely disappeared and appearance of new bands at 946.93 and 797.63 cm^? 1 confirmed the amorphous anhydrous silica with Si–O–Si networking. The SEM images of SiO_{2 (mwc)} described the smooth and fine particle texture and confirmed 99.99% Si–O–Si networking of anhydrous silica. The 99.99% purity was verified by EDAX spectra which exhibited sharp signals only for oxygen and silicon. Toxicity against Monomorium minimum and Tribolium castaneum with 100% mortality and LT₅₀ 91 min and 7.5 h respectively is being reported. It can be used for long storage of grains in the future.     

Preparation and properties of microcrystalline silicon films using photochemical vapor deposition

microcrystalline silicon films have been prepared through mercury photosensitized decomposition of monosilane at low gas pressures. The dark and light conductivities of the silicon films tend to increase at reactant pressures lower than 65 Pa and become 10^?2Ω^?1· cm^?1 at 26 Pa. From the Raman scattering and x-ray diffraction, silicon films were found to consist of a mixed phase structure including both microcrystalline and amorphous regions.     

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.     

The effects of oxygen,hydrogen and fluorine on the conductivity of high purity evaporated amorphous silicon films

Electron bombardment evaporation was used to deposit amorphous silicon (α-Si) films in an evaporator with a base pressure of 2 × 10^?10 Torr. Rutherford backscattering analysis was used to establish the conditions necessary for deposition of pure films.The DC conductivity was measured as a function of temperature (? 150°C to + 140°C). Pure films, which were deposited between room temperature and 400°C, were found to have a room temperature conductivity (σ_RT) in the region of 10^?3μ^?1cm^?1 and a log $\text{σα}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ dependence. The value of σ_RT could be reduced by annealing reaching a minimum of 2 × 10^?7μ^?1 cm^?1 for an anneal temperature (T_A) of 520°C, but activated conduction was not observed.The implantation of hydrogen or fluorine (or contamination with oxygen) had the effect of reducing σ_RT, with a minimum value of less than 10^?8μ^?1cm^?1 (T_A = 400°C) for fluorine implantation to a dose of ≈ 10¹⁶ cm² (≈ 0.4 at% concentration). These films had high temperature (50°C) activation energies typical of activated conduction in extended states on the edge of the mobility gap. Implantation of fluorine to a dose of 1.5 × 10¹⁷ resulted in a rise of σ_RT (T_A = 400°C) to nearly 10⁴μ^?1 cm^?1 and log $\text{σα}\text{T}{}^{\mathrm{<mtext>?1</mtext><mtext>4</mtext>}}$ behaviour.X-ray analysis revealed that some crystallization occurred in films annealed at 600°C. This is correlated with a rise in σ_RT of the pure films and the disappearance of the effects of the introduced impurities.     

Electrical, structural and optical properties of SnO2 thin films prepared by spray pyrolysis

This study investigated the effect of the substrate temperature on the structural, optical, morphological, and electrical properties of undoped SnO₂ films prepared by a spray deposition method. The films were deposited at various substrate temperatures ranging from 300-500 °C in steps of 50 °C and characterized by different optical and structural techniques. X-ray diffraction studies showed that the crystallite size and preferential growth directions of the films were dependent on the substrate temperature. These studies also indicated that the films were amorphous at 300 °C and polycrystalline at the other substrate temperatures used. Infrared and visible spectroscopic studies revealed that a strong vibration band, characteristic of the SnO₂ stretching mode, was present around 630 cm⁻¹ and that the optical transmittance in the visible region varied over the range 75-95% with substrate temperature, respectively. The films deposited at 400 °C exhibited the highest electrical conductivity property.     

Ionic conductivity of Li2OB2O3 thin films

The ionic conductivity of evaporated Li₂OB₂O₃ thin films has been studied. These thin films were found to show a considerably high ionic conductivity of 1 × 10^?7 Ω^?1 cm^?1 at room temperature. The conductivity increases with increasing Li content and exhibits a maximum value near 3Li₂O·B₂O₃. The structure of these films was determined using infrared absorption and laser Raman scattering spectroscopy. Using the results, the correlation between structure and conductivity is also discussed.     

Trap-limited diffusion of hydrogen in precursor derived amorphous Si–B–C–N-ceramics

The tracer diffusion of hydrogen is studied in precursor derived amorphous Si–C–N and Si–B–C–N ceramics using deuterium as a tracer and secondary ion mass spectrometry (SIMS). Since the amorphous ceramics are separated in carbon rich phases (amorphous carbon and amorphous C(BN)_x, respectively) and silicon rich phases (amorphous Si₃N₄ and amorphous Si_3+(1/4)xC_xN_4−x, respectively) we additionally measured the diffusivities of hydrogen in amorphous carbon, in amorphous SiC and in amorphous C–B–N films. The silicon rich phases are identified as diffusion paths for hydrogen in the precursor derived ceramics. Diffusion of hydrogen in these materials is explained with a trap limited diffusion mechanism with a single trap level. We found activation enthalpies of about 2 eV for the precursor derived ceramics, where the activation enthalpy is the sum of a migration enthalpy and a binding enthalpy. The low values for the pre-exponential factors of less than 10⁻⁷ m²/s can be explained with an appropriate expression for the entropy factor.