Excimer-laser-assisted RF glow-discharge deposition of amorphous and microcrystalline silicon thin films

We combine the deposition of Hydrogenated amorphous Silicon (a-Si:H) by rf glow discharge with XeCl-excimer laser irradiation of the growing surface in order to obtain different kinds of silicon films in the same deposition system. In-situ UV-visible ellipsometry allows us to measure the optical properties of the films as the laser fluence is increased from 0 up to 180 mJ/cm² in separate depositions. For fixed glow-discharge conditions and a substrate temperature of 250° C we observe dramatic changes in the film structure as the laser fluence is increased. With respect to a reference a-Si:H film (no laser irradiation) we observe at low laser fluences (15–60 mJ/cm²) that the film remains amorphous but demonstrates enchanced surface roughness and bulk porosity. At intermediate fluences (80–165 m/Jcm²), we obtain an amorphous film with an enhanced density with respect to the reference film. Finally, at high fluences (165–180 mJ/cm²), we obtain microcrystalline films. The in-situ ellipsometry measurements are complemented by ex-situ measurements of the dark conductivity, X-ray diffraction, and Elastic Recoil Detection Analysis (ERDA). Simulation of the temperature profiles for different film thicknesses and for three laser fluences indicates that crystallization occurs if the surface temperature reaches the melting point of a-Si:H ( 1420 K). The effects of laser treatment on the film properties are discussed by taking into account the photonic and thermal effects of laser irradiation.Presented at LASERION 93, Munich, June 21–23, 1993     

Kinetics of CW laser-induced crystallization and oxidation of thin,Sb, Se,and Sb2Se3 films in air

Thin films of Sb, Se and Sb₂Se₃ are deposited onto glass and irradiated by a cw-Ar⁺ laser beam. The kinetics of crystallization and oxidation are traced via the time dependence of optical reflectivity and temperature, T, of the irradiated zone. For Sb₂Se₃, transformations start abruptly when T attains a critical value, T _c, independently of the laser beam power. These T _c values are comparable to the ones observed under furnace annealing conditions.     

Defect structure and optical damage resistance of Zn:Fe:LiNbO<Subscript>3</Subscript> crystals

A series of Zn:Fe:LiNbO₃ crystals were prepared by the Czochralski technique with 0.015 wt. % Fe₂O₃ content and various concentrations of ZnO. The ultraviolet-visible absorption spectra and the infrared absorption spectra of the Zn:Fe:LiNbO₃ crystals were detected in order to investigate their defect structure. Their optical damage resistance was characterized by the photoinduced birefringence change and transmission facula distortion method. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases remarkably when the concentration of ZnO is over its threshold concentration (more than 6.0 mol. %). The effects of defects on the optical damage resistance of the Zn:Fe:LiNbO₃ crystals are discussed in detail. Received: 25 October 2002 / Revised version: 6 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +86-451/2300-926, E-mail: zzxxhhdoctor@sina.com     

Ultrashort-laser-pulse-driven rewritable phase-change optical recording in Sb-based films

This paper reviews the work we have carried out over the last years on the development of ultrashort-laser-pulse-driven, rewritable, phase-change optical memories. The materials we have tailored for this application are non-stoichiometric, Sb-rich amorphous thin films, which can be crystallized upon irradiation with ultrashort laser pulses, showing a large optical contrast upon transformation. This result makes them very promising for the development of rewritable phase-change optical memories under ultrashort pulses, since the reversibility of the process has also been demonstrated. Adequate control of the heat-flow conditions has allowed us to achieve a full transformation time faster than 400 ps for crystallization/amorphization using 30-ps pulses. The crystallization threshold fluence has been found to decrease upon irradiation with pulses shorter than 800 fs, thus suggesting the relevance of high-electronic-excitation-induced processes in the amorphous-to-crystalline phase transition. This has been confirmed by the observation of an ultrafast, non-thermal phase transition occurring 200–300 fs after the arrival of the laser pulse at the surface, for fluences above the crystallization threshold. The presence of this transient phase conditions the final state induced therefore enabling the applicability of this material as a rewritable recording medium using femtosecond pulses. Received: 11 October 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/564-5557, E-mail: J.Solis@io.cfmac.csic.es     

ZnO nanowall networks grown on DiMPLA pre‐patterned thin gold films

ZnO nanowall networks grown by a high pressure pulsed laser deposition (PLD) technique on a pre‐patterned thin gold film are presented. The thin gold film on a c ‐plane oriented sapphire substrate was structured with diffraction mask projection laser ablation (DiMPLA). It is shown that only areas processed with the laser patterning technique reveal homogeneous growth of ZnO nanowall networks. Photoluminescence measurements prove the higher material quality of the pre‐patterned regions compared to the untreated ones. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of luminescent defects in hafnia thin films made with different deposition techniques

Hafnia thin films for high-power optical coatings have been characterized by photoluminescence pumped by 4.66 eV photons and photothermal deflection measurements. These data are compared to the statistical laser damage behavior in order to find correlations between destructive and non-destructive characterizations. Thin films have been produced at two thicknesses and using different thin-film deposition techniques typically employed for optical coating fabrication: EBD (HfO₂ target), EBD (Hf target), RLVIP and DIBS. The photoluminescence spectra show significant differences depending on the deposition techniques and thicknesses. EBD films show significant luminescence but the luminescence of ion-assisted films could not be distinguished from the uncoated substrate. All EBD coating spectra could be described by a linear combination of four bands. Further, XRD measurements show that the 255-nm-thick films had a relatively high crystallinity: EBD films contained the monoclinic phase and the ion-assisted films contained oriented nanocrystals of orthorhombic hafnia. The presence of orthorhombic phases indicates high compressive strain quenching the photoluminescence of these samples.     

Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC

We report observation of nanostructures formed on thin TiN and DLC films that were irradiated by 800- and 267-nm, femtosecond (fs) Ti:sapphire laser pulses at an energy fluence slightly above the ablation threshold. On the ablated thin-film surfaces, the linearly polarized fs pulses produce arrays of fine periodic structures that are almost oriented to the direction perpendicular to the laser polarization, while the circularly polarized light forms fine-dot structures. The size of these surface structures is 1/10–1/5 of the laser wavelength and decreases with a decrease in the laser wavelength. Received: 3 September 2002 / Accepted: 4 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-778/62-3306, E-mail: yasuma@fukui-nct.ac.jp     

Pulsed laser crystallization of hydrogen-free a-Si thin films for high-mobility poly-Si TFT fabrication

The possibility to fabricate high-mobility polysilicon TFTs by nanosecond pulsed laser crystallization of unhydrogenated amorphous Si thin films has been investigated. Two types of lasers have been used: a large area ( 1 cm²) single ArF excimer laser pulse and a small diameter ( 100 m) frequency-doubled Nd:YAG laser beam, working in the scanning regime. Processed films have been characterized in detail by different optical and microscopic techniques. Device performances indicate that the best results are achieved with the excimer laser leading to high mobility values (up to 140 cm²/Vs) which are much larger than in polysilicon TFTs fabricated onto the same quartz substrates by low-temperature thermal (630° C) crystallization of amorphous Si films (_fe55 cm²/Vs).     

Structure dependence of ultrafast third-order optical nonlinearity for GeS2-In2S3-CsI chalcohalide glasses

Ultrafast third-order nonlinear optical responses of GeS₂-In₂S₃-CsI chalcohalide glasses have been measured by using the femtosecond time-resolved optical Kerr effect (OKE) technique at a wavelength of 820 nm. The third-order nonlinear susceptibility was estimated to be as large as 5.12×10⁻¹³ esu. The full width at half maximum of the Kerr signal was 120 fs and its response was dominantly assigned to the ultrafast distortion of the electron cloud. The relationship between the structural units and the third-order nonlinear optical responses was analysed by Raman spectra. It is suggested that the covalent bonds of S-Ge or S-In constituting the tetrahedral units [GeS_4/2] or [InS_4−xI_x], respectively, play an important role in the ultrafast third-order nonlinear optical responses of these chalcohalide glasses.     

Dielectric properties of P2O5-Na2O-Li2O glasses containing WO3, CoO or Fe2O3

The aim of this work was to study some polarization mechanisms in phosphate glasses containing different transition metal oxides, performed at radio frequency (100 kHz) and in the microwave region (9 GHz). Cobalt, iron and tungsten oxides were chosen for the present investigation. The results show that the dielectric constant increases linearly with increasing Co concentration, whereas for iron and tungsten ions, the dielectric constant values were found to decrease up to a given concentration and further, increase for higher concentration of W and Fe. This anomaly was observed both at 100 kHz and 9.00 GHz and was attributed to the valence state of the transition metal ions in the glass structure. The dielectric constant is lower in the microwave region, due to structural relaxation mechanisms.     

XRD and XPS analysis of laser treated vanadium oxide thin films

In this work, we present X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis of laser treated vanadium oxide sols. The films were also observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to reveal how the original xerogel structure changes into irregular shaped, layer structured V₂O₅ due to the laser radiation. XRD revealed that above 102 W/cm² the original xerogel structure disappears and above 129 W/cm² the films become totally polycrystalline with an orthorhombic structure. XPS spectra showed O/V ratio increment by using higher laser intensities.     

Photo-induced dynamical changes in amorphous As2S3 films

Optical properties of amorphous As₂S₃ films, which have been illuminated well by bandgap light in advance, can be changed dynamically by exposing to less-bandgap light. This dynamical change has been studied in connection with its plausible relation to the reversible photo-induced change. It has been found that these changes have intimate connections with each other, and can be explained by a certain configurational diagram in a coherent fashion.     

Selective wet-etching of amorphous/crystallized Ag–As–S and Ag–As–S–Se chalcogenide thin films

The paper is focused on the possibilities of selective wet etching of optically and thermally crystallized/amorphous Ag-doped chalcogenide thin films, namely Ag_x(As_0.33S_0.67)_100−x and Ag_x(As_0.33S_0.335Se_0.335)_100−x. The selective etching of optically(thermally) crystallized Ag_x(As_0.33S_0.67)_100−x and thermally crystallized Ag_x(As_0.33S_0.335Se_0.335)_100−x thin films in water solution of NaCN is presented. The good surface quality is an important and crucial parameter for optical elements fabrication (e.g. grids, waveguides, etc.) especially in nanometer dimensions. The selective etching of undoped and Ag optically doped region was also carried out to observe surface roughness of doped region before and after selective etching. Characterization of the structure and surface of studied films by Raman spectroscopy, X-ray diffraction, AFM and SEM methods has been done and potential application suggested.     

Studies of the photoreduction of methylene blue in gelatin solutions and films with a view to improving the holographic recording process at 633 nm

The photoreduction of methylene blue (MB), in methylene blue sensitized gelatin (MBG) solutions and films, was studied for the first time with time-resolved laser spectroscopy. The dependence of the lifetime of the triplet state precursor was estimated by varying the pH, and the efficiency of this photoprocess was evaluated in solid thin films. This work constitutes the first step towards improving the holographic recording speed of MBG.     

Experimental study and configurational-coordinate model for photo-induced effects in amorphous As2Se3 films

The optical absorption of As₂Se₃ thin films is studied in the UV–VIS spectral range and the value of the optical energy gap is determined. The effect of photo-irradiation on the optical absorption and transmission of thin film samples is also investigated. The optical energy gap was found to decrease with photo-irradiation time. The results of photo-irradiation are discussed in correlation with the structural aspects of As₂Se₃. A model is proposed to account for the structural changes, resulting from photo-irradiation, causing the decrease of the energy gap. The effect of γ-radiation on the optical absorption of As₂Se₃ thin films was studied also and no detectable effect on the value of the optical energy gap was observed.     

Numerical analysis of Excimer laser assisted processing of multi-layers for the tailored dehydrogenation of amorphous and nano-crystalline silicon films

The application of the striking electrical and optical properties of amorphous and nano-crystalline silicon in photovoltaic, photonic and nano-electronic devices is attracting increasing attention. In particular, its use both on polymeric substrates and in Integrated Circuit technology for the development of enhanced new devices has shown that processing techniques to produce amorphous hydrogenated and nano-crystalline silicon films avoiding high substrate temperatures are of great importance. A promising strategy to achieve this purpose is the combination of Hot-Wire Chemical Vapor Deposition at 150 °C with Excimer Laser Annealing, thus maintaining the substrate at relatively low temperature during the complete process.In this work we present a numerical analysis of Excimer Laser Annealing, performed at room temperature, of a multilayer structure of thin alternating a-Si:H and nc-Si films deposited on glass and grown by Hot-Wire Chemical Vapor Deposition. A set of two different layer thicknesses a-Si:H (25 nm)/nc-Si (100 nm) and a-Si:H (30 nm)/nc-Si (60 nm) were analysed for a total structure dimension of 900 nm. The aim is to determine the probable temperature profile to achieve controlled localized in depth dehydrogenation.Temperature distribution has been calculated inside the multilayer during the irradiation by a 193 nm Excimer laser, 20 ns pulse length, with energy densities ranging from 50 to 300 mJ/cm². Calculations allowed us to estimate the dehydrogenation effect in the different layers as well as the structural modifications of the same layers as a function of the applied laser energy.The numerical results have been compared to the experimental ones obtained in similar multilayer structures that have been analysed through Raman spectroscopy and TOF-SIMS in depth profiling mode.     

Laser-induced voltage characteristics of Bi2Sr2Co2Oy thin films on LaAlO3 substrates

We report the laser-induced voltage (LIV) effects in c-axis oriented Bi₂Sr₂Co₂O_y thin films grown on (0 0 1) LaAlO₃ substrates with the title angle α of 0°, 3°, 5° and 10° by a simple chemical solution deposition method. A large open-circuit voltage with the sensitivity of 300 mV/mJ is observed for the film on 10° tilting LaAlO₃ under a 308 nm irradiation with the pulse duration of 25 ns. When the film surface is irradiated by a 355 nm pulsed laser of 25 ps duration, a fast response with the rise time of 700 ps and the full width at half maximum of 1.5 ns is achieved. In addition, the experimental results reveal that the amplitude of the voltage signal is approximately proportional to sin 2α and the signal polarity is reversed when the film is irradiated from the substrate side rather than the film side, which suggests the LIV effects in Bi₂Sr₂Co₂O_y thin films originate from the anisotropic Seebeck coefficient of this material.     

The trapping centers of holes in a-As2Se3

The density of the trapping centers and their capture radius for holes in a-As₂Se₃ are determined by analyzing the imaginary term of the modulated photocurrent. The light-induced changes in the imaginary term are also examined to study the influences of the photo structural changes to the density of states and to detect possible hole trapping at light created charged defects.     

Simulation of KrF laser ablation of Al2O3-TiC

Previous work by the authors on micromachining of Al₂O₃-TiC ceramics using excimer laser radiation revealed that a columnar surface topography forms under certain experimental conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations show that the columns develop from small globules of TiC, which appear at the surface of the material during the first laser pulses. To understand the mechanism of formation of these globules, a 2D finite element ablation model was developed and used to simulate the time evolution of the temperature field and of the surface topography when a sample of Al₂O₃-TiC composite is treated with KrF laser radiation. Application of the model showed that the surface temperature of TiC rises much faster than that of Al₂O₃, but since TiC has a very high boiling temperature, its vaporization is significant only for a short time. By contrast, the surface temperature of Al₂O₃ rises above its boiling temperature for a much longer period, leading to a greater ablation depth than TiC. As a result, a small TiC globule stands above the Al₂O₃ surface. The results of the model are compared with experimental measurements performed by AFM. After three pulses, the height of the globules predicted by the model is about 340 nm, in good agreement with the height measured experimentally, about 400 nm.