Optical and electrical properties of optimized thin gold films as top layer of MIS solar cells

The optical spectral response between 340–860 nm and electrical conductivity measurements were made on thermally evaporated gold films in the thickness range 50–130 Å in order to identify optimal properties for their use as the top layer of MIS solar cells. Films of lower evaporation rate and thickness range 70–100 Å were found to have high transmittance and low reflectance, which is desirable for the above purpose. Films thinner than 70 Å had poor electrical conductivity and thicker than 100 Å had poor solar transmittance, hence were rejected. Improved transmittance and conductivity were obtained upon annealing the film at 250°C for about two hours.     

Preparation and characterization of highly oriented,photoconducting WS2 thin films

A novel combination of methods is shown to produce semiconducting WS₂ thin films with properties close to those of a single crystal. The first step requires the deposition of a very thin Ni layer on a quartz substrate. On top of it an amorphous, sulphur rich, (WS_{3 +x} ) thin film is deposited by reactive rf sputtering. The final annealing step in an argon atmosphere yields 200 nm thick WS₂ films. X-ray diffraction shows that the films crystallize in the 2H-WS₂ phase and are perfectly oriented with the (002) basal planes parallel to the substrate. Residual W₁₈O₄₉ needles and-NiS grains are detected by transmission electron microscopy. The dc conductivity and its activation energy have values typical of bulk crystals. The optical absorption spectrum measured at Room Temperature (RT) shows excitonic peaks at the same energies as in a single crystal. RT photoconductivity measured as a function of wavelength is shown to result from interband transitions.     

Changes produced in the electrical resistivity of ErH2 thin films when converted to ErH3 due to hydrogen treatment

The resistivity of thin films (80–200 Å) of ErH₂ increases sharply when heated for 2 h at 300 °C in vacuum in the presence of hydrogen gas at 10^–2 Torr. This confirms that the films, originally metallic conductor, have become converted to semiconducting ErH₃ which is in conformity with the structural studies. The negative values of TCR also indicate the semiconducting nature of the hydrogen treated films. Activation energies of the films have been evaluated.     

Controlled doping of rf sputtered germanium films

Germanium films have been rf sputter deposited on a variety of substrates. A new techniques has been developed to control doping concentrations of the films at predetermined levels for bothp-type as well asn-type films. The hole concentrations of these films could be varied from 10¹⁵ to 2×10¹⁸/cm³ while the electron concentrations could be varied from 10¹⁵ to 5×10¹⁷/cm³ using this technique. Transmission electron microscope studies have been made to study the crystalline quality of the films.     

A simple technique to deposit molybdenum thin films

A simple technique is proposed for metallizing nonconductive objects. The technique, which is based on a plasma-induced dissociation of Mo(CO)₆ molecules, is inexpensive with no sophisticated instrument needed, and allows us to deposit fine grained, adherent thin films of molybdenum on nonrefractory substrates. It is believed that the technique possesses a variety of potential applications in laboratory works.     

Chemical bath-deposited ZnS thin films: Preparation and characterization

Chemical bath deposition of ZnS thin films from NH₃/SC(NH₂)₂/ZnSO₄ solutions has been studied. The effect of various process parameters on the growth and the film quality are presented. The influence on the growth rate of solution composition and the structural, optical properties of the ZnS thin films deposited by this method have been studied. The XRF analysis confirmed that volume of oxygen of the as-deposited film is very high. The XRD analysis of as-deposited films shows that the films are cubic ZnS structure. The XRD analysis of annealed films shows the annealed films are cubic ZnS and ZnO mixture structure. Those results confirmed that the as-deposited films have amorphous Zn(OH)₂. SEM studies of the ZnS thin films grown on various growth phases show that ZnS film formed in the none-film phase is discontinuous. ZnS film formed in quasi-linear phase shows a compact and a granular structure with the grain size about 100 nm. There are adsorbed particles on films formed in the saturation phase. Transmission measurement shows that an optical transmittance is about 90% when the wavelength over 500 nm. The band gap (E_g) value of the deposited film is about 3.51 eV.     

Electron mobility in compensated VPE GaAs films

Electron Hall mobilities were measured on a series of intentionally compensated vapor phase epitaxy (VPE) GaAs layers. Using Sn and Zn as dopants, compensation ratiosK=(N_D+N_A)/(N_D-N_A) as high as 50 were obtained. Already for samples with the lowestK values the 300 K mobilities are higher than the 77 K values. In the range 20<T<100 [K] the data may be represented by μ∼T ^α with α increasing from 0.6 to 1.1 with compensation. The experimental μ values are smaller than those predicted from current models in all cases. It appears that scattering at ionized impurities is the dominant process also at temperatures well above 77 K, and that this scattering process is quantitatively underestimated in current models.     

Low-temperature fabrication of silicon nitride films by ArF excimer laser irradiation

The low-temperature fabrication of silicon nitride films by ArF excimer laser irradiation has been studied. Two fabrication methods are presented. One is photoenhanced direct nitridation of a silicon surface with NH₃ for very thin gate insulators, and the other is photo-enhanced deposition of silicon nitride films with Si₂H₆ and NH₃ gases for stable passivation films. The ArF excimer laser irradiation dissociates the NH3 gas producing NH and NH₂ radicals which proved effective in instigating the nitridation reaction. The quality of both films has been much improved and the growth temperature has been lowered by using laser irradiation. These photo-enhanced processes seem to be promising ULSI techniques because they do not depend on high temperatures and are free from possible reactor contamination.     

Binding of gas atoms to extended crystal defects in molybdenum studied by positrons

Evidence for a reaction between aluminium and SiO₂ film is presented using Auger electron spectroscopy (AES) and low-energy electron-loss spectroscopy (ELS) techniques. This reaction is studied in situ during the manufacture of metal insulator semiconductor devices (MIS), under ultra-high vacuum conditions (UHV). A reduction of the SiO₂ film upon aluminization occurs, even at room temperature, giving rise to a complex interface.ERA CNRS n° 899ERA CNRS n° 373     

Spatially-resolved characterization of HTSC-films near T c by laser raster microscopy with thermal excitation

A method (laser raster microscopy with thermal excitation, LRMTE) for characterizing high-T _c thin-film superconductors (HTSC) with microscopic resolution is described. By means of spatially resolved laser excitation and subsequent monitoring of the time dependence of the film resistivity at a base temperature near the transition temperature T _c, spatial variations of the transition temperature (T _c), of the temperature coefficient of the resistivity (dQ/dT), of the heat conductivity and heat capacity of the film and of the heat conductivity between film and substrate can be detected with high spatial resolution (15 m have been achieved so far).     

Pulsed laser deposition of oxide thin films

The pulsed laser deposition (PLD) process was studied in detail during oxide thin film growth by constructing a sophisticated PLD chamber combined with an energy-stable excimer laser. It was revealed that the transmitting laser energy at the entrance view port decreased exponentially with deposition runs, roughly by A e^−0.02x%, where A is the original transmission and x is the run number, and transmission loss could become considerable (up to 90%) in 100 deposition runs. In addition, area of the focused spot on the target was found a function of charging high voltage, which is a parameter of excimer laser operation, even through a slit forming rectangular sharp-edge beam profile. Laser energy density is one of the most important parameter governing grown film properties, and therefore accurate laser energy and spot area calibration is vital for reproducible film growth. In course of this study, the importance of spot area as well as the energy density is discussed in the view of deposition rate.     

Surface characterization and microstructure of ITO thin films at different annealing temperatures

In this study, the electron beam evaporation method is used to generate an indium tin oxide (ITO) thin film on a glass substrate at room temperature. The surface characteristics of this ITO thin film are then investigated by means of an AFM (atomic force microscopy) method. The influence of postgrowth thermal annealing on the microstructure and surface morphology of ITO thin films are also examined. The results demonstrate that the film annealed at higher annealing temperature (300 °C) has higher surface roughness, which is due to the aggregation of the native grains into larger clusters upon annealing. The fractal analysis reveals that the value of fractal dimension D_f falls within the range 2.16-2.20 depending upon the annealing temperatures and is calculated by the height-height correlation function.     

Deposition and characterization of diamond epitaxial thin films on silicon substrates

Heteroepitaxial diamond growth has been attempted on mirror-polished monocrystalline (001), (111), and (110) silicon substrates by microwave plasma CVD. The surface morphology and the crystallographic properties of the films were characterized by means of Scanning Electron Microscopy (SEM), Raman spectroscopy, X-ray diffraction, and X-ray and Raman pole-figure analysis. The results demonstrate epitaxial growth of diamond on both (001) and (111) oriented silicon substrates. Preliminary results give strong evidence for substrate-induced orientation of the diamond crystallites also on (110) oriented silicon substrate. The heteroepitaxy can be assigned to the oriented covalent bonding across the interface between diamond and silicon.     

Structural, optical and electrical properties of ZnO and ZnO-Al2O3 films prepared by dc magnetron sputtering

ZnO and ZnO-Al₂O₃ thin films were prepared by dc magnetron sputtering and their structural, optical and electrical properties were studied comparatively. It is discovered that the ZnO-Al₂O₃ thin films remain transparent in a shorter wavelength range than the ZnO films, resulting from the increase of their band gap. Their resistivity decreases by seven orders of magnitude, which is caused by doping of Al to ZnO grains in the film. Preferential orientation of ZnO grains in the ZnO-Al₂O₃ thin films deteriorates because of the existence of Al₂O₃ impurity phase in the film. Received: 5 January 1999 / Accepted: 11 October 1999 / Published online: 8 March 2000     

The effect of CO adsorption on the resistivity of thin Pd films

Thin Pd films of 5–50nm thickness are deposited on glass substrates under UHV conditions and are subsequently covered step by step with CO at 77 or 293 K. The resistivity increases linearly with the number of adsorbed CO molecules in the initial stages of adsorption while a saturation value develops at higher coverages. The thickness dependence of the linear increase can be quantitatively interpreted with the help of the scattering hypothesis. The scattering cross section is calculated to beA=1.6Ų in good agreement with the experimental evidence.Paper in part presented at the 9th Int'l Conf. on Thin Films, Vienna (1993)     

Growth and characteristics of reactive pulsed laser deposited molybdenum trioxide thin films

Molybdenum trioxide thin films were prepared by reactive pulsed laser deposition on Corning 7059 glass substrates. The influence of oxygen partial pressure and deposition temperature on the structure, surface morphology and optical properties of these films was studied to understand the growth mechanism of MoO₃ thin films. The films formed at 473 K in an oxygen partial pressure of 100 mTorr exhibited predominantly a (0k0) orientation, corresponding to an orthorhombic layered structure of α-MoO₃. The evaluated optical band gap of the films was 3.24 eV. The crystallite size increased with increase of deposition temperature. The films formed at an oxygen partial pressure of pO₂=100 mTorr and at a deposition temperature greater than 700 K exhibited both (0k0) and (0kl) orientations, representing α-β mixed phases of MoO₃. The films formed at an oxygen partial pressure less than 100 mTorr were found to be sub-stoichiometric with α-β mixed phases. The investigation revealed the growth of polycrystalline and single-phase orthorhombic-layered-structure α-MoO₃ thin films with composition nearly approaching the nominal stoichiometry at moderate substrate temperatures in an oxygen partial pressure of 100 mTorr. Received: 9 April 2001 / Accepted: 6 August 2001 / Published online: 17 October 2001     

The effect of gas adsorption on the electrical resistivity of thin silver films

Silver films in the thickness range 20–60 nm deposited on glass substrates in UHV conditions were covered stepwise with oxygen, carbon monoxide and ethylene at 77 K. Resistivity was measured in situ. The evaluation of the thickness dependence of resistivity on the basis of a scattering hypothesis leads to scattering cross sections decreasing from O₂ to CO and C₂H₄. The results are explained by considering the energetic structure of the broadened molecular orbitals of the adsorbates near the Fermi level.     

Formation of a metallic layer from individual atoms

Adsorption of copper, gold and beryllium on (110), (100), and (211) single-crystal planes of tungsten leads to essentially different work-function changes. It is known from numerous investigations of alkali-metal adsorption on metal substrates that the work-function variation reflects the electronic processes occurring during the formation of the adlayer. It is obvious that copper, gold and beryllium adsorption is accompanied by a wide variety of physical processes different from those appearing in alkali-metal adsorption. The existing experimental data concerning work-function changes induced by copper, gold and beryllium adsorption are compiled. A model is developed, which may explain these changes.     

Hydrogen sulphide doping of GaAs and AlxGa1−xAs grown by molecular beam epitaxy (MBE)

H₂S gas has been used during molecular beam epitaxy (MBE) growth of GaAs and Al _x Ga_1–x As as sulphur vector forn-type doping. Doping efficiencies are less than 10^–3 at usual growth temperatures, and are limited by an incorporation competitive surface process, probably 2Ga+H₂SGa₂S+H₂. In Al_xGa_1–x As forx0.2 the doping efficiency is further reduced by carrier freeze-out at deep levels. Measured thermal activation energies depend on growth conditions and remain relatively low even up to the direct-indirect bandgap crossover for substrate temperatures in the 585–645 C range.     

The effect of surface roughness on the resistivity increase of thin metal films during gas adsorption

The resistivity increase of very thin metal films during gas adsorption is stronger than predicted by the commond ^–1 law. This effect is contributed to surface roughness. A quantitative theory is presented which leads to an additionald ^–3 term. The theory is checked using literature data obtained for the Ag/O₂ system.