Amorphization induced by subnanosecond laser pulses in phase-change optical recording media

We have investigated the dynamics of amorphization induced in phase-change optical recording media by focused laser pulses of subnanosecond duration. We initiated localized amorphism by using a focused laser beam to melt the phase-change material and completed the change by rapid cooling by means of thermal diffusion. These studies were conducted by use of real-time reflectivity measurements with a pump-and-probe technique in which both pump and probe pulses had a duration of approximately 510 ps. Our transient-reflectivity measurements indicate that the process that leads to amorphism has three distinct stages, namely, rapid melting, solidification, and slow relaxation.     

Patterned structures fabricated on chalcogenide phase-change thin films by laser direct writing

In laser direct writing technology, the pattern is usually written in a photoresist. In this work, we use the chalcogenide phase change thin films as the laser direct writing materials, and patterned structures with different shapes and sizes were directly written with different laser wavelengths. Compared with traditional photoresist materials, the patterned structures can be directly formed in the chalcogenide phase change thin films without developing and etching procedures, and also can be directly written with different laser wavelengths. By tuning the laser parameters precisely, patterned structures with different sizes and shapes could be obtained as well. The analysis indicates that the formation mechanism of the patterned structure is mainly due to the volume expansion caused by material vaporization and the interior of the patterned structure is hollow with some solid leavings, and the chalcogenide phase change thin films are very good candidate materials for patterned structure formation.     

KrF excimer laser crystallization of silicon thin films

A KrF excimer laser operating at 249 nm has been employed to crystallize silicon thin films deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) and by RF magnetron sputtering on Corning glass and SiO₂. All films display a substantial improvement in crystallinity after ELC with the optimum laser fluence for as-deposited ECR films being higher than for sputtered films. This is probably related to the presence of Si-H_x bonds in the former. A pronounced bimodality in the Raman spectra of some amorphous, as-deposited ECR samples has been observed after laser crystallization where, in addition to the peak at 520cm^-1, a strong peak at 509cm^-1 is also present. Such behavior has not been reported previously to our knowledge in ELC silicon films. Interestingly, the XRD spectra of these samples do not exhibit any peaks suggesting the films are composed of nano-grain material. The dehydrogenation of ECR films by ELC has been demonstrated to be substantial, the hydrogen content typically decreasing from ~30 at % in an as-deposited film to ~10 at % after a single low fluence laser shot. Raman spectroscopy has shown that the film bonding changes from predominantly Si-H₂ to Si-H after ELC. Electrical resistivity measurements of phosphorus-doped films show a controllable and repeatable change with laser fluence. The results in this paper show that it is possible to crystallize and controllably change the electrical characteristics of ECR PECVD produced silicon thin films by ELC.     

Investigation of the crystallization process and crystal structure of Si-incorporated GeSb phase-change films

The effect of Si incorporation on the crystallization process and crystal structure of Te-free Sb-rich GeSb was investigated in this study. Si concentrations were controlled to 0, 5.1, 9.3, and 12.8 at.% by controlling the sputtering power of the GeSb alloy target (20:80 at.% for Ge:Sb) and Si target. After film deposition, the crystallization process and crystal structure were investigated. Crystallization temperature increased from 320 to 400 °C and the overall crystallinity was decreased with increasing Si concentration. These were analyzed by sheet resistance measurements after thermal annealing and optical contrast measurements by optical static testing. Glass transition temperatures were calculated and increased from 240 to 285 °C with increasing Si concentration. Considering the proportional relation between the glass transition temperature and crystallization temperature, it is thought that more energy is required for crystallization with increased Si concentration. A study of the crystallization process kinetics was conducted by applying the Johnson–Mehl–Avrami model to the optical static test results, which were carried out under a pseudo-isothermal process. The Avrami coefficient was 4.10 and decreased to 3.18 when the crystallization was generated with increased Si concentration from 0 to 12.8 at.%. Therefore, crystallization speed was thought to decrease with increased Si concentration. Based on the results of crystal structure analysis by XRD and HRTEM, the crystal structure of our Sb-rich GeSb PCM was revealed to be a typical Sb structure, i.e., an A7 hexagonal structure with lattice parameters of a = 4.26 Å and c = 11.45 Å. No crystal phase of Ge or Si was observed and no evidence of the structure change in Sb crystals due to Ge or Si incorporation was observed.     

Investigation of crystallization behavior of CIG-Se bi-layer thin films

Copper indium gallium diselenide (CIGSe) thin film was fabricated via a thermal treatment of GIG-Se bi-layer thin films. A CIG layer was prepared first, by a chemical solution deposition (CSD) process. The Se layer was deposited separately on the CIG layer by evaporation. The GIG-Se bi-layer then underwent a thermal treatment to cause a reaction between the two layers. In order to investigate the mechanism of CIG-Se bi-layer crystallization, the thermal treatment temperature was varied. The properties of the prepared CIGSe2 thin films were analyzed using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometry (EDS), and UV-visible spectrophotometry.     

Behavior of GeSbTeBi phase-change optical recording media under subnanosecond pulsed laser irradiation

We investigated the variations in reflectivity during the phase transition between amorphous and crystalline states of a Bi-doped GeTe-Sb2Te3 pseudobinary compound film with subnanosecond laser pulses, using a pump-and-probe technique. We also used a two-laser static tester to estimate the onset time of crystallization under 2.0-micros pulse excitation. Experimental results indicate that the formation of a melt-quenched amorphous mark is completed in approximately 1 ns, but that crystalline mark formation on an as-deposited amorphous region requires several hundred nanoseconds. Simple arguments based on heat diffusion are used to explain the time scale of amorphization and the threshold for creation of a burned-out hole in the phase-change film.     

非晶硅薄膜晶化过程的研究

多晶硅薄膜具有较高的电迁移率和稳定的光电性能,是制备微电子器件、薄膜晶体管、大面积平板液晶显示的优质材料.多晶硅薄膜被公认为是制备高效、低耗、最理想的薄膜太阳能电池的材料.因此,如何制备多晶硅薄膜是一个非常有意义的研究课题.固相法是制备多晶硅薄膜的一种常用方法,它是在高温退火的条件下,使非晶硅薄膜通过固相相变而成为多晶硅薄膜.本文采用固相法,利用X-ray衍射及拉曼光谱,对用不同方法制备的非晶硅薄膜的晶化过程进行了系统地研究.     

Features of the mechanoluminescence of thin metal films excited by long and short laser pulses

We have studied the strain-induced luminescence from thin fine-grained films of various metals under the action of short (subnanosecond) and long (millisecond) laser pulses. The possible mechanism of luminescence excitation is considered, according to which the emission is due to the interaction between grain boundary dislocations and impurities occurring in the intergranular region.     

Preparation and optical properties of phase-change VO2 thin films

In this work, VO₂ thin films were prepared on three kinds of substrates by the sol-gel dipcoating method followed by heat treatment under vacuum. These thin films were analysed by x-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The infrared and ultraviolet-visible spectra of the VO₂ thin films were also recorded during heating and cooling between room temperature and 100°C. The experimental results show that VO₂ thin films thus prepared exhibit thermally induced reversible phase transition, and the largest changes in transmittance and reflectivity are approximately 58 and 25%, respectively, in the case of vacuum heat treatment at 400°C and silica glass substrates. The refractive index (n) decreases and the absorption coefficient (k) increases when heating these thin films from room temperature to 100°C, and vice versa for cooling. The reasons why the optical constants and infrared absorption spectra change so remarkably are discussed.     

Encryption of nonlinear optical signals in ZnO:Al thin films by ultrashort laser pulses

Described herein is the effect of optical annealing on the third-order non-linear optical properties exhibited by nanostructured Al-doped ZnO thin films. The samples were synthetized by an ultrasonic spray pyrolysis method. The optical annealing process was carried out by laser pulses at 532, 835 and 1064 nm wavelengths with, ps, fs and ps pulse duration, respectively. The optical non-linearity of the films was measured by the z-scan method with three different irradiations of excitation: 100 fs at 835 nm, 120 ps at 532 nm, and 150 ps at 1064 nm. The as-grown samples showed a saturable optical absorption that evolves into two-photon absorption transitions by a picosecond optical annealing phenomenon induced at 532 nm wavelength. Potential applications for developing optical encryption functions were considered.     

Polycrystalline silicon thin films obtained by metal-induced crystallization

The formation of high-quality polycrystalline silicon (poly-Si) on different substrates has important applications in the development of thin-film transistors, solar cells, image sensors, etc. In this study, we present the results of an investigation of poly-Si films on glass, formed by aluminum-induced crystallization. The process is based on the isothermal annealing for 3 h at 500 °C of co-sputtered Al+Si or sputtered a-Si films on glass, with and without thermally evaporated Al. The poly-Si films were investigated by Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy.     

Picosecond laser pulse-driven crystallization behavior of SiSb phase change memory thin films

Transient phase change crystallization process of SiSb phase change thin films under the irradiation of picosecond (ps) laser pulse was studied using time-resolved reflectivity measurements. The ps laser pulse-crystallized domains were characterized by atomic force microscope, Raman spectra and ellipsometrical spectra measurements. A reflectivity contrast of about 15% can be achieved by ps laser pulse-induced crystallization. A minimum crystallization time of 11 ns was achieved by a low-fluence single ps laser pulse after pre-irradiation. SiSb was shown to be very promising for fast phase change memory applications.     

Fabrication of polycrystalline silicon thin films on glass substrates using fiber laser crystallization

Laser crystallization of amorphous silicon (a-Si), using a fiber laser of λ = 1064 nm wavelength, was investigated. a-Si films with 50 nm thickness deposited on glass were prepared by a plasma enhanced chemical vapor deposition. The infrared fundamental wave (λ = 1064 nm) is not absorbed by amorphous silicon (a-Si) films. Thus, different types of capping layers (a-CeO_x, a-SiN_x, and a-SiO_x) with a desired refractive index, n and thickness, d were deposited on the a-Si surface. Crystallization was a function of laser energy density, and was performed using a fiber laser. The structural properties of the crystallized films were measured via Raman spectra, a scanning electron microscope (SEM), and an atomic force microscope (AFM). The relationship between film transmittance and crystallinity was discussed. As the laser energy density increased from 10-40 W, crystallinity increased from 0-90%. However, the higher laser density adversely affected surface roughness and uniformity of the grain size. We found that favorable crystallization and uniformity could be accomplished at the lower energy density of 30 W with a-SiO_x as the capping layer.     

Optical limiting studies in a carbon-black suspension for subnanosecond and subpicosecond laser pulses

This paper presents results of measurements of short (0.3 ps, 0.2 ns, and 10 ns) laser pulse transmissions through a liquid suspension of fine carbon particles (named CBS for "carbon-black suspension") at input-pulse energies up to 10 mJ. The 10-ns pulses came from a Nd:YAG laser at 1064 nm, and the shorter pulses came from a Ti:sapphire laser at 800 nm. Limiting was observed with the 10-ns and the 0.2-ns laser pulses, but the 0.3-ps pulses produced white light and underwent the same level of attenuation in the solvent and in the CBS.     

Observation of carrier dynamics in CdO thin films by excitation with femtosecond laser pulse

Non-equilibrium dynamics of photoexcited carrier in cadmium oxide thin films were examined by the pump-probe transient optical transmission measurement technique using the second harmonics (400 nm) of femtosecond pulses as a pump light. Time-dependent blueshift of the absorption edge was clearly observed and it reached a maximum at ∼1 ps after the pumping. The magnitude of the blueshift increases with pump light power, which was well fitted to Burstein-Moss formula. The relaxation time of excited electrons from the bottom of the conduction band was obtained ∼350 ps and an effective mass for the electron carrier was estimated as ∼0.3m_e.     

Modification of AlN thin films morphology and structure by temporally shaping of fs laser pulses used for deposition

We studied the effect of temporally pulse shaping upon the properties of thin layers synthesized by pulsed laser deposition with fs laser pulses generated by a Ti-sapphire laser source. We showed that the film morphology and structure can be gradually modified when applying mono-pulses of different duration or passing to a sequence of two pulses of different intensities.     

Annealing and crystallization of amorphous germanium thin films

The isochronal and isothermal annealing of the room temperature electrical resistivity of vacuum-deposited a-e thin films was studied for different annealing temperatures up to about 500°C. By analysing the annealing kinetics, it is possible to show that the crystallization process can be characterized by a single activation energy of 3.5 eV and the nucleation process by an activation energy of 3 eV for different coating conditions. It is proposed that the crystal growth takes place by diffusion of extended divacancies to the crystalline-amorphous interface and by subsequent rearrangement of the relaxed atoms fit the crystalline matrix.     

Picosecond thermal pulses in thin gold films

In this paper it has been shown that, with the advent of lasers with a very short pulse duration, the effect of thermal wave propagation becomes important. To consider this effect, hyperbolic heat conduction in thin gold films was studied. It was shown that for heat fluxes of the order 10⁸ W·cm^–2, a thermal wave is generated in thin gold films. The consideration of the hyperbolicity of heat transfer enables one to describe the temperature profile with one value of fluence.     

Deposition of superconductive thin films by laser PVD

Thin films of the superconductive YBa₂ Cu₃O_7–k have been deposited on SrTiO₃ substrates by means of a pulse laser ablation technique, with variable substrate temperature, number of pulses and repetition rate of the laser beam. High substrate temperatures and low rates of repetition resulted in good quality superconducting films.     

Plasma enhanced germanium nitride dielectric thin films for phase-change recording systems

The more efficient dielectric material and structure have been developed for fast phase-change optical recording systems. The germanium nitride thin film was reactively sputtered using a d.c. chamber, operated at 3 kW with a mixed gas plasma of argon and nitrogen. The dielectric thin film of 11–13 nm has been designed as the upper dielectric layer between the Ge–In–Sb–Te active layer and the silver reflective layer. The optical storage disks were initialized by a Hitachi POP120-8E device to provide crystalline active layers before the dynamic tests. The initialization was conducted using 810 nm laser at 2300 mW while the disks were rotated at a constant linear velocity of 10 m/s. The optical properties were analyzed using a Steag ETA-RT optical measuring system at 410–1010 nm wavelength. The power margin window was shown to be effectively improved by 60% to 18–26 mW when compared with the traditional ZnS–SiO₂ dielectric system. TEM (transmission electron microscope) micrograph showed clear profile for the standard 3T laser marks on the active layer. The plasma enhanced germanium nitride dielectric thin film could improve digital versatile disk's (DVD) production yield and efficiency.