Thin-film poly-Si solar cells on glass substrate fabricated at low temperature

The performances of thin-film poly-Si solar cells with a thickness of less than 5 μm on a glass substrate have been investigated. The cell of glass/back reflector/n-i-p type Si/ITO is well characterized by the structure of naturally surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si layer was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 μm demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), an open-circuit voltage of 0.539 V and a short-current density of 25.8 mA/cm² as independently confirmed by Japan Quality Assurance. No light-induced degradation is observed. The optical and transport properties of poly-Si cells are summarized. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 7 July 1999     

Holographic and photoelectric characterization of a novel photorefractive organic glass

We present a detailed study of the photoelectric as well as the holographic properties of a novel organic photorefractive glass based on triphenylamine. We studied the quantum efficiency Φ of the photogeneration of charges by means of photoinduced discharge measurements. The photoconductivity σ and the charge carrier mobility μ were obtained via dc photoconduction and pulsed time-of-flight experiments, respectively. The holographic characterization was performed by two-wave and degenerate four-wave mixing experiments allowing for the determination of properties such as diffraction efficiency η, modulation of the refractive index Δn, gain coefficient Γ, and phase-shift φ_p of the investigated system. The experimental data for Φ could be successfully described by the Onsager formalism with a thermalization radius of r₀=24 ? and a primary quantum yield of Φ₀=40%. We evaluated the E field and temperature-dependent measurements of μ using the B?ssler formalism yielding a width of the density of states of σ=0.13 eV and a disorder parameter Σ=3.6. On this basis the lifetime and the average drift length of the charge carriers could be estimated from the dc photoconduction experiments. From the photoelectric measurements we also calculated the holographic response time that matched very well to the measured response time and described the E-field dependence satisfactorily. The presented photorefractive system shows outstanding optical properties and stability with respect to degradation. We measured a gain coefficient of Γ=90 cm^-1, and a diffraction efficiency of η=27% at a response time of 30 ms for only 40-μm-thick samples. Orientational enhancement was observed and evaluated quantitatively. To our knowledge, this work presents the first determination of each of the above quantities all in one single organic photorefractive material. Received: 16 November 1998 / Revised version: 4 January 1999 / Published online: 31 March 1999     

Crystalline Si thin-film solar cells: a review

The present review summarizes the results of research efforts in the field of crystalline silicon thin-film solar cells on foreign substrates. The large number of competing approaches can be broadly classified according to the grain size of the crystalline Si films and the doping of the crystalline absorber. Currently, solar cells based on microcrystalline Si films on glass with an intrinsic or moderately doped absorber film achieve efficiencies around 10%, whereas thin-film cells fabricated from large-grained polycrystalline Si on high-temperature-resistant substrates have efficiencies in the range of 15%. The paper discusses the limitations of various approaches and describes recent developments in the area of thin, monocrystalline Si films that may open the way towards 20% efficient thin-film Si solar cells. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Quasi-monocrystalline silicon for thin-film devices

Thermal crystallization of a double layer porous Si film creates a monocrystalline Si film with a thin separation layer between the Si film and the reusable starting wafer. The process enables transfer of thin monocrystalline Si films to foreign substrates, whereby devices may be formed before or after separation of the film. Sub-micrometer thick films are almost compact, while films with a thickness of several μm contain voids, and are therefore termed “quasi-monocrystalline”. Internal voids strongly enhance optical absorption by light scattering. The hole mobility is 78 cm² V^-1 s^-1 at a p-type starting wafer resistivity of 0.05 Ω cm. Received: 24 March 1999 / Accepted: 29 March 1999 / Published online: 5 May 1999     

High-temperature processing of crystalline silicon thin-film solar cells

The crystalline silicon thin-film solar cell combines, in principle, the advantages of crystalline silicon wafer-based solar cells and of thin-film solar cell technologies. Its efficiency potential is the highest of all thin-film cells. In the “high-temperature approach” thin silicon layers are deposited on substrates that withstand processing temperatures higher than 1000 °C. The basic features of the high-temperature crystalline silicon thin-film cell technology are described and some important results are discussed. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Study of optical waveguide of barium sodium niobate films on potassium titanyl phosphate

Barium sodium niobate (BSN) optical waveguide films were grown on potassium titanyl phosphate (KTP) substrate by pulsed laser deposition (PLD). X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray pole spectroscopy, and X-ray photoelectron spectroscopy (XPS) showed that the as-grown BSN films are epitaxially smooth, fine-particled and show small constituent deviation. Both m lines of the waveguided TE and TM modes were recorded. Lattice match between the BSN(110) and both the KTP(001) and KTP(100) planes is examined with mismatch less than 3%. Effective refractive indices as a function of incident wavelength and order of the waveguided modes are studied. A phase matching relationship of k ⁽¹⁾−2k ⁽⁰⁾=0 is established between the first-order and fundamental modes in the BSN/KTP waveguided system. Effective thicknesses for TE and TM modes were obtained to be equal to 0.76 and 0.72 μm, respectively, larger than the thickness of the as-grown BSN film of 0.60 μm. A constituent ratio of Ba:Na:Nb in the BSN film measured was equal to 2:0.82:5.14, slightly deviating from the stoichiometric ratio of 2:1:5, which is attributed to higher relative asymptotic velocity for Nb species and higher evaporation pressure for Na species. Finally, the main factors affecting quality of the as-grown BSN films are also discussed. Received: 26 March 1999 / Accepted: 30 March 1999 / Published online: 19 August 1999     

Film structure and ferroelectric properties of in situ grown SrBi2Ta2O9 films

Ferroelectric SrBi₂Ta₂O₉ (SBT) films were grown by pulsed-laser deposition (PLD) at different substrate temperatures and fluences. A correlation between film structure and ferroelectric properties is established. The dielectric function ε^′ of thin SBT films shows a Curie–Weiss behavior well below the peak temperature T_max and relaxor-like behavior in the vicinity of T_max. Domain walls have a strong influence on the dielectric and ferroelectric properties and on the polarization fatigue of SBT films below 100 °C. The formation of ferroelectric phases is favored at lower substrate temperatures by incorporating Bi₂O₃ template layers into the structure. Received: 18 March 1999 / Accepted: 19 March 1999 / Published online: 5 May 1999     

Improvement of transport properties for polycrystalline silicon prepared by plasma-enhanced chemical vapor deposition

The carrier transport property of polycrystalline silicon (poly-Si:H:F) thin films was studied in relation to film microstructure, impurity, in situ or post-annealing treatments to obtain better carrier transport properties. Poly-Si:H:F films were prepared from SiF₄ and H₂ gas mixtures at temperatures <300 °C. Dark conductivity of the films prepared at high SiF₄/H₂ gas flow ratio (e.g., 60/3 sccm) exhibits a high value for intrinsic silicon and its Fermi level is located near the conduction band edge. The carrier incorporation is suppressed well, either by in situ hydrogen plasma treatment or by post-annealing with high-pressure hot-H₂O vapor. It is confirmed that weak-bonded hydrogen atoms are removed by the hot-H₂O vapor annealing. In addition, evident correlation between impurity concentrations and dark conductivity is not found for these films. It is thought that the carrier incorporation in the films prepared at high SiF₄/H₂ gas flow ratios is related to grain-boundary defects such as weak-bonded hydrogen. By applying hot-H₂O vapor annealing at 310 °C to a 1-μm-thick p-doped (400)-oriented poly-Si:H:F film, Hall mobility was improved from 10 cm²/Vs to 17 cm²/Vs. Received: 7 August 2000 / Accepted: 2 March 2001 / Published online: 20 June 2001     

The effect of silicon substrate on thickness of SiO2 thin film analysed by spectral reflectometry and interferometry

Techniques of spectral reflectometry and interferometry are used for measuring small changes in thickness of SiO₂ thin film grown by thermal oxidation on different silicon substrates. A slightly dispersive Michelson interferometer with one of its mirrors replaced by a thin-film structure is used to measure the reflectance and interferometric phase of the thin-film structure at the same time. The experimental data are used to determine precisely the thickness of the SiO₂ thin film on silicon wafers of two crystallographic orientations and different dopant concentrations. We confirmed very good agreement between the experimental data and theory and revealed that the thin-film thickness, which varies with the type of silicon substrate, depends linearly on the wavelength at which minimum in the spectral reflectance occurs. Similar behaviour was revealed for the interferometric phase.     

Formation of gold nanowires through self-assembly during scanning force microscopy

Gold films with a nominal thickness of 5–40 monolayers were grown on dielectric substrates and imaged by scanning force microscopy (SFM). The films originally consisted of well-separated or densely packed clusters. During imaging in contact mode, the morphology of the films changed drastically. At low coverage, i.e. Θ<10 monolayers, the well-known stripes originating from mobile clusters, eventually accumulated into larger aggregates, were observed. In contrast, at larger coverage, highly ordered structures consisting of one-dimensional wires evolved during scanning. They often were parallel with equal separation, i.e. well-defined periodicity, over distances of several μm. Typically, the wires were 5–10 nm high and 50–100 nm wide. Investigations of Au films prepared at varying temperature on different dielectric substrates allow us to suggest a self-assembling mechanism for wire formation in which gold is periodically collected by the SFM tip and redeposited as soon as a critical amount is reached. Received: 22 February 1999 / Accepted: 2 March 1999 / Published online: 7 April 1999     

Texture control and its impact on the properties of SrBi2Ta2O9 thin films prepared by pulsed laser deposition

SrBi₂Ta₂O₉ (SBT) ferroelectric thin films with different preferred orientations were deposited by pulsed laser deposition (PLD). Several methods have been developed to control the preferred orientation of SBT thin films. For SBT films deposited directly on Pt/TiO₂/SiO₂/Si substrates and in situ crystallized at the deposition temperature, the substrate temperature has a significant impact on the orientation and the remnant polarization (Pr) of the films; a higher substrate temperature benefits the formation of (115) texture and larger grain size. The films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C are (115)-oriented and exhibit 2Pr of 6 μC/cm². (115)- and (200)-predominant films can be formed by using a La_0.85Sr_0.15CoO₃ (LSCO) buffer layer or by annealing amorphous SBT films deposited on Pt/TiO₂/SiO₂/Si substrates at 450 °C using rapid thermal annealing (RTA). These films exhibit good electric properties; 2Pr of the films are up to 12 μC/cm² and 17 μC/cm², respectively. The much larger 2Pr of the films deposited on the LSCO buffer layer and of the films obtained by RTA than 2Pr of the films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C is attributed to a stronger (200) texture. Received: 30 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Electronic behaviour of thin-film CdTe solar cells

New support is given for one of the controversial models about the electronic consequences of the CdCl₂ treatment of a thin-film CdTe solar cell: the assumption that deep acceptor states are introduced in the bulk of the CdTe layer as a result of the CdCl₂ treatment. A detailed study of the doping profile using capacitance–voltage (C-V) measurements is performed as a first step. The above assumption is numerically simulated with our simulation programme SCAPS. In this way, anomalous features of the C-V measurements are fully explained, and further correspondence between calculated and measurable quantities is found. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

Microstructure, microhardness, composition, and corrosive properties of stainless steel 304 I. Laser surface alloying with silicon by beam-oscillating method.

Laser surface alloying (LSA) with silicon was conducted on austenitic stainless steel 304. Silicon slurry composed of silicon particle of 5 μm in average diameter was made and a uniform layer was supplied on the substrate stainless steel. The surface was melted with beam-oscillated carbon dioxide laser and then LSA layers of 0.4–1.2 mm in thickness were obtained. When an impinged energy density was adjusted to be equal to or lower than 100 W mm⁻², LSA layers retained rapidly solidified microstructure with dispersed cracks. In these samples, Fe₃Si was detected and the concentration of Si in LSA layer was estimated to be 10.5 wt.% maximum. When the energy density was equal to or greater than 147 W mm⁻², cellular grained structure with no crack was formed. No iron silicate was observed and alpha iron content in LSA layers increased. Si concentration within LSA layers was estimated to be 5 to 9 wt.% on average. Crack-free as-deposited samples exhibited no distinct corrosion resistance. The segregation of Si was confirmed along the grain boundaries and inside the grains. The microstructure of these samples changed with solution-annealing and the corrosion resistance was fairly improved with the time period of solution-annealing. Received: 2 September 1999 / Accepted: 6 September 1999 / Published online: 1 March 2000     

Electrochromic materials from the visible to the infrared region: an example WO<Subscript>3</Subscript>

Herein, the example of the most typical electrochromic material, namely WO₃, is used to illustrate the potential of electrochromic materials for controlling infrared reflectance and hence, emissivity. Playing with various growth parameters, contrast in reflectance between the inserted H _xWO₃ and deinserted WO₃ states as high as 73% in mid-wavelength band (MW, 3–5 μm) was achieved for 320 nm WO₃ films. The latter electrochromic materials were radio frequency sputtered on Au substrate at ambient temperature in 6 Pa of chamber pressure. In comparison, for long wavelength band (LW, 8–12 μm), the contrast in reflectance did not exceed 30%. The origins of the various electrochromic behaviours are correlated to the film structure, morphology and composition, indicating better properties for porous, nonstoichiometric films. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Diode-pumped passively Q-switched Nd:GdVO4 lasers operating at 1.06 μm wavelength

With a 10-W diode laser to pump Nd:GdVO₄ crystal in a folded cavity, we demonstrated Cr⁴⁺:YAG passively Q-switched Nd:GdVO₄ lasers at 1.06 μm. The maximum average output power of 2.1 W and the highest peak power of 625 W were, respectively, obtained when the initial transmissions of the Cr⁴⁺:YAG crystals were 90% and 80%. Received: 8 September 1999 / Revised version: 30 December 1999 / Published online: 8 March 2000     

CdTe epilayers for uses in optical waveguides

CdTe epilayers have been grown by vapor phase epitaxy (VPE) on glass, MgO, sapphire, LiNbO₃ and mica substrates. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies show the good structural quality of the epilayers. In these epilayers, a few optical modes were excited with a 1.33-μm laser. The measured propagation losses were in the range between 5 dB/cm and less than 0.5 dB/cm. From dark-mode m-lines, the epilayer thickness was found to be in the 1–3 μm range, in good accord with that obtained by SEM measurements. The refractive index obtained from the fitting is also in good accord with that of bulk CdTe. Received: 7 October 1999 / Accepted: 13 March 2000 / Published online: 5 July 2000     

Novel optical film sensor design based on p-polarized reflectance

A new scheme of optical film sensor is presented. The sensor is based on p-polarized reflectance, consisting of a sensing coated substrate, is easily optimized for maximum sensitivity in different applications. The resolutions of refractive index n_f, extinction coefficient k_f and thickness h_f of the sensitive films are predicted to be 10⁻⁷, 10⁻⁵ and 10⁻³ nm, respectively. Experimentally, we selected the sol–gel derived SnO₂ films as gas-sensitive films and conducted preliminary gas-sensing test. The results indicate that novel optical film sensor scheme has higher sensitivity, and the detection sensitivity is available to 10⁻¹ ppm on the condition of optimum optical parameters and incident angle.     

Laser polishing of diamond plates

Results are reported on laser polishing of 150–400-μm-thick free-standing diamond films with either a copper vapor laser (510 nm wavelength) or an ArF excimer laser (193 nm wavelength). Studies were focused on three particular goals. First, we aimed at a choice of optimum conditions for laser polishing of thick diamond films. It was shown that the laser polishing conditions and the resulting surface roughness were controlled by varying the angle of incidence of a scanning laser beam and by polishing time. Second, the laser ablation technique was applied to remove a defective layer from the “substrate” side of the diamond plates in order to reduce optical losses due to absorption in this layer. Third, the structure of the laser-graphitized diamond surface was studied using UV, visible, and IR optical spectroscopy techniques in the course of the “step-by-step” oxidative removal of the graphitic layer with increasing temperature of the oxidation in ambient air. Once the graphitic layer was removed, the optical transmission in the UV-visible-IR spectral range of the diamond films polished under optimum conditions was measured and compared with the optical transmission of the mechanically polished diamond films. It was shown that the optical quality (in the long-wave infrared region) of the laser-polished diamond plates was sufficient to reach the transmittance value very close to the theoretical limit. Received: 20 October 1998 / Accepted: 8 March 1999 / Published online: 5 May 1999     

Structural evolution and metastable phase formation in Ni-W multilayers upon ion irradiation

A crystal-to-amorphous structural transition was induced in the Ni₂₅W₇₅ and Ni₃₅W₆₅ multilayers by ion irradiation at room temperature. More interestingly, prior to complete amorphization, a sequential disordering of first Ni and then W crystalline lattices was observed in the Ni₂₅W₇₅ sample with increasing ion dose. Such sequence in disordering is attributed to the difference in melting points between the two constituent metals. In another two multilayered samples with overall compositions of Ni₆₀W₄₀ and Ni₇₈Nb₂₂, ion irradiation under similar conditions resulted in the formation of two Ni-based fcc solid solutions, respectively. In comparison, the same Ni-based fcc solid solution was formed in the Ni₃₅W₆₅ multilayered sample upon solid-state reaction at 500 °C. Solid-state reaction at 550 °C resulted in the formation of a new W-rich metastable hcp phase in the Ni₂₅W₇₅ multilayered sample and the bcc–hcp transition was thought to be realized through a shearing mechanism. A Gibbs free-energy diagram, including the free-energy curves of the newly formed metastable crystalline phases, of the Ni-W system was calculated based on Miedema’s model and it can give a reasonable explanation of the observed sequential disordering. The calculated results also showed that the free-energy difference between the amorphous and metastable crystalline phases was quite small, leading to a situation that the phase selection, namely which phase was more favored to be formed eventually than its competitors, was influenced or even determined by the kinetics involved in the respective processes. Besides, the growth kinetics of the MX phases was also discussed. Received: 26 January 1999 / Accepted: 8 March 1999 / Published online: 14 June 1999