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     

Structure and magnetic properties of Co nanoclusters fabricated by ion beam synthesis in SiO2 films

Co nanoparticles fabricated by ion beam synthesis in SiO₂ films were investigated with transmission electron microscopy and superconducting quantum interference device technique. Variation of the thermal treatment enables the formation of Co nanoclusters of different sizes ranging from 2 to 40 nm. Small nanoclusters of about 2–3 nm are amorphous, whereas clusters above 7 nm show the configuration of cubic Co nanocrystals. Measurements of magnetisation at temperatures between 2 K and 360 K reveal superparamagnetic behaviour for the small nanoclusters up to 3 nm and ferromagnetism for clusters above 7 nm. Received: 12 February 2001 / Accepted: 3 May 2001 / Published online: 27 June 2001     

硅基硒纳米颗粒的发光特性研究

由于尺寸缩小引起的量子效应, 硒(Se) 材料的低维纳米结构具有更高的光响应和低的阈值激射等特性, 因此成为纳米电子与纳米光电子器件领域一个重要的研究方向. 本文通过对非晶硒薄膜的快速热退火来制备硒纳米颗粒, 退火温度在100–180℃之间时, 结晶后的硒纳米颗粒均为三角晶体结构, 其颗粒尺寸随退火温度的增加而线性增大. 光致发光谱测试发现三个发光峰, 分别位于1.4eV, 1.7eV和1.83eV. 研究发现位于1.4eV处的发光峰来源于非晶硒缺陷发光, 位于1.83eV处的发光峰来源于晶体硒的带带跃迁发光; 而位于1.7eV处的发光峰强度随激发功率增强而指数增大, 且向短波长移动, 该发光峰应该来源于非晶硒与硒纳米颗粒界面处的施主-受主对复合发光. 关键词： 硅基 硒纳米颗粒 光致发光 施主-受主对     

Effect of laser fluence on the deposition and hardnessof boron carbide thin films

We deposited amorphous thin films of boron carbide by pulsed laser deposition using a B₄C target at room temperature. As the laser fluence increased from 1 to 3 J/cm², the number of 0.25–5 μm particulates embedded in the films decreased, and the B/C atomic ratio of the films increased from 1.8 to 3.2. The arrival of melt droplets, atoms, and small molecular species depending on laser fluence appeared to be involved in the film formation. In addition, with increasing fluence the nanoindentation hardness of the films increased from 14 to 32 GPa. We believe that the dominant factor in the observed increase in the films’ hardness is the arrival of highly energetic ions and atoms that results in the formation of denser films. Received: 23 March 2001 / Accepted: 1 July 2001 / Published online: 2 October 2001     

Structure, refractive-index dispersion and the optical absorption edge of chemically deposited ZnxCd(1-x)S thin films

Zinc cadmium sulfide, Zn_xCd_(1-x)S, thin films have been deposited by a simple and inexpensive chemical bath deposition method from an aqueous medium using thiourea as a sulfide-ion source. The structure of the deposited films has been characterized by X-ray diffraction and transmission electron microscopy. It was observed from X-ray diffraction that the as-deposited films were amorphous in nature. However Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h show a crystalline structure with a small scattering volume. The obtained results were confirmed throughout the transmission electron microscopy and the corresponding electron-diffraction patterns. The optical constants of Zn_xCd_(1-x)S films annealed at 423 K for 1.5 h in the compositional range 0≤x≤1 were estimated using transmission and reflection spectra in the wavelength range 300–2500 nm. The band gap varies non-linearly with the value of x. The dependence of the refractive index on the wavelength obeys the single-oscillation model, from which the dispersion parameters and the high-frequency dielectric constant were determined. A graphical representation of the surface and volume energy-loss functions was also given. Received: 23 February 2001 / Accepted: 26 February 2001 / Published online: 27 June 2001     

Mixing kinetics and write-once optical recording characteristics of Sb/Se bilayer films

Bilayer Sb/Se films are irradiated with 12 ns pulses from an ArF laser (extended areas) and from a focused Ar⁺ laser (micron-sized areas). Real-time reflectivity measurements are used to determine if the process occurs within the solid or liquid phase and the transformation time, in addition to measure the optical contrast and the medium sensitivity. Transmission electron microscopy is used to analyze the structure of the transformed areas and the medium resolution. The results show that mixing is initiated by preferential melting at the grain boundaries and an amorphous phase is produced upon irradiation at high energy densities. Finally, the characteristics of the mixing process in Sb/Se films as a write-once optical recording mechanism are discussed in terms of the sensitivity and resolution of the recorded spots and the time required for recording.     

Carbon nanotubes and nanostructures grown from diamond-like carbon and polyethylene

We report a simple way to synthesize carbon nanotubes and nanostructures from the solid phase. Vacuum annealing of diamond-like carbon (DLC) films or polyethylene mixed with catalyst in argon atmosphere leads to the formation of nanotubes and nanostructures. High-resolution transmission electron microscopy studies reveal highly graphitized multi-walled nanotubes (MWNTs) or amorphous fibre-like structures, depending on the catalyst amount. This synthesis process may give a new approach to understanding the phase transition of different carbon allotropes into nanotubes or nanostructures. Received: 3 July 2001 / Accepted: 3 July 2001 / Published online: 2 October 2001     

On the yellow-band emission in CdS films

CdS polycrystalline thin films were prepared by the chemical bath deposition (CBD) method on glass substrates. X-ray diffraction (XRD) studies show that the films grow in the cubic zinc-blende crystalline phase. Upon thermal annealing (TA) in Ar+S₂ flux at normal pressure in the temperature range 240–510 °C, the evolution of the transformation into the hexagonal wurtzite phase is observed. This hexagonal crystalline structure is the stable phase. From XRD diagrams the phase transition can be appreciated to occur upon TA at approximately 300 °C. Photoluminescence (PL) data prove that the green-emission band is present for well-defined phases – cubic or hexagonal ones. A second band located at 2.2 eV appears for samples near the transition region. This band at 2.2 eV, called the yellow band, has already been reported to be associated with interstitial Cd atoms. A model for this yellow-band-mechanism formation, arising during the phase transformation, has been proposed based on Frenkel-pair creation. Received: 27 June 2000 / Accepted: 19 December 2000 / Published online: 23 March 2001     

Surface relief measurements in side-chain azobenzene polyesters with different substituents

Light-induced surface modification of a series of liquid-crystalline side-chain azobenzene polyesters which have the same main- and side-chain structure but eleven different substituents on the azobenzene is investigated. Using a transmission mask as well as single focused beams we show that the formation of the surface relief is dependent on the substituents. In both experiments irradiation with p-polarized light generates peaks for the CN, CF₃, CH₃ and F substituents, while for a Cl substituent valleys are observed. Also s-polarized light is found to produce surface deformations. An amorphous azobenzene polyester was included in the study for comparison. The results point to a mechanism of the surface relief phenomenon, in which the architecture of the polyesters plays a crucial role. Received: 11 September 2000 / Revised version: 4 December 2000 / Published online: 21 February 2001     

Photoluminescence in amorphous TiO2-PbO systems

Photoluminescence (PL) at room temperature has been achieved in amorphous thin films and powders of the TiO₂–PbO system. They were prepared by the polymeric precursor method with [PbO]/[TiO₂] molar ratios ranging from 0.0 to 1.0. The energy position of maximum PL emission and the PL intensity showed dependence on Pb concentration. The Pb addition suggests an increase in the number of non-bridging oxygens (NBO) in the amorphous TiO₂ network. These results support the relationship between photoluminescence and structure in TiO₂-based amorphous materials. Received: 7 February 2001 / Accepted: 8 February 2001 / Published online: 27 June 2001     

Diffractive structures holographically recorded in amorphous hydrogenated carbon (a-C:H) films

We propose and demonstrate the direct recording of submicrometer relief gratings in amorphous hydrogenated carbon (a -C:H) films by reactive ion etching (RIE) for use as diffractive optical components. The high refractive index of this film and its transparency in the IR make such structures promising candidates for IR-transmission diffractive optical components. The structures are holographically recorded in photoresist and then transferred to a thin aluminum layer that is used as a mask for RIE of the a -C:H films. The diffraction measurements of the structures recorded in these films demonstrated the feasibility of using the materials as diffractive optical components.     

Holographic recording mechanisms of gratings in indium oxide films using 325 nm helium–cadmium laser irradiation

UV (325 nm) holographic recording of gratings in indium oxide films fabricated by reactive pulsed laser deposition has been investigated as a function of growth temperature, oxygen pressure and angle of incidence of the plasma plume on the substrate. The influence of the ambient environment (air or vacuum) and the film temperature during recording has also been studied. Large steady state refractive index changes up to 6×10^-3 were observed in layers grown at an oblique angle of 75°. About 77% of the magnitude of these changes residues after thermal annealing and is attributed to UV-induced permanent structural rearrangements. In contrast, refractive index changes in films grown at normal incidence were smaller in magnitude and completely reversible. Received: 30 July 2001 / Accepted: 20 November 2001 / Published online: 23 January 2002     

Phase matching of second-harmonic generation in birefringent porous silicon

Optical second-harmonic generation is investigated in free-standing (110) porous silicon films having strong in-plane birefringence. Based on the measured values of the refractive indices, the conditions of phase matching are calculated and verified in the experiments on angular and polarization dependences of second-harmonic generation. Filling pores with a transparent dielectric liquid allows phase matching to be achieved and the second-harmonic signal to be increased by two orders of magnitude. The results of these experiments suggest an opportunity to form new-type nonlinear-optical media consisting of birefringent porous silicon as a phase-matching matrix for the materials embedded in the pores. Received: 30 April 2001 / Published online: 7 June 2001     

Size evolution and photoluminescence of silicon nanocrystallites in evaporated SiOx thin films upon thermal processing

Silicon suboxide thin films have been fabricated by physical vapor deposition of silicon monoxide in vacuum at controlled oxygen partial pressure. These films undergo a phase separation into silicon- and oxygen-enriched regions upon thermal processing. At temperatures around 900 °C, the onset of Si nanocrystallite formation is observed, regardless of film stoichiometry. With increasing initial oxygen content of the films, the mean size of created nanocrystallites decreases whereas the corresponding photoluminescence emission blueshifts. The photoluminescence intensity increases with increasing annealing temperature up to 1050 °C. Upon resonant excitation at low temperatures, the photoluminescence exhibits phonon replica signature. Therefore, the emission may be attributed to excitonic recombination in the nanocrystallites. Received: 3 July 2001 / Accepted: 6 August 2001 / Published online: 17 October 2001     

The effects of dc bias voltage on the crystal size and crystal quality. of cBN films

For the deposition of cubic boron nitride thin films in Ar–N₂–BF₃–H₂ system by dc jet plasma chemical vapor deposition, the role of dc substrate bias ranging from -70 V to -150 V was investigated. A critical bias voltage was observed for the formation of cBN phase. The cBN content in the film increased with bias voltage and reached a maximum at the bias voltage of -85 V. Increasing the bias voltage further caused a decrease in cBN content and peeling of the films from the substrate. By combining the results of infrared spectroscopy, Raman spectroscopy and X-ray diffraction, the bias voltage was also found to strongly affect the crystal size, crystal quality and residual stress of the deposited films. A bias voltage a little higher than the critical value was demonstrated to be favorable for the deposition of a high-quality cBN film with large crystal size and low residual stress. Received: 13 June 2000 / Accepted: 21 June 2000 / Published online: 23 August 2000     

Magnetic properties and magnetic entropy change of amorphous and crystalline GdNiAl ribbons

The structure and magnetic properties of amorphous melt-spun and subsequently crystallized GdNiAl ribbons were investigated. An amorphous phase was formed after the quenching process by melt spinning with a copper wheel having a surface speed of 30 m/s. A hexagonal phase with lattice parameters a=7.023 ? and c=3.916 ? was formed in the GdNiAl ribbon after annealing above its crystallization temperature. Magnetic entropy change was calculated directly from isothermal magnetic measurements. The results show that both the amorphous and annealed samples have a high magnetocaloric effect, indicating that these alloys can be considered as candidates for magnetic refrigeration applications. Received: 14 August 2001 / Accepted: 18 September 2001 / Published online: 23 January 2002     

Metastable phase formation and structural change characteristics of vapor deposited semiconductor films

Reported here are studies about the thermal and laser induced metastable phase formation in amorphous GeSb₂Te₄ thin films prepared by RF-magnetron sputtering. The general structural properties of this most promising optical phase change recording material are discussed from the point of view of fast structural phase transformation.     

Pressure-controlled preparation of nanocrystalline complex oxides using pulsed-laser ablation at room temperature

Nanocrystalline thin films of complex oxides such as BaTiO₃ and LaFeO₃ were prepared by pulsed laser ablation without substrate heating. Targets under various Ar pressures were irradiated using an ArF excimer laser. The off-axis configuration of targets and substrates was used to synthesize the films. The crystallinity and chemical composition of the deposited films were strongly dependent on the processing Ar gas pressure. In case of BaTiO₃, the film deposited at 10 Pa was a single phase of BaTiO₃ with a crystallite size around 7.2 nm. With increasing Ar pressure to 200 Pa, XRD peaks of BaTiO₃ as well as BaCO₃ were observed. The by-products could be due to reaction with carbon dioxide in air after taking the sample out of the chamber. For LaFeO₃, the films deposited under 50 to 200 Pa had a single phase with a crystallite size below 10 nm. When the Ar pressure exceeded 100 Pa, the crystallite size tended to decrease for both BaTiO₃ and LaFeO₃, which could be due to formation of aggregated nanoparticles. Below 10 Pa, oxygen deficiency was observed. Over 50 Pa, the atomic concentration of all the constituent elements was almost constant, especially the [Ba]/[Ti] and [La]/[Fe] ratios, which were nearly unity. Received: 19 June 2002 / Accepted: 24 June 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +81-298/61-6355, E-mail: yoon-jw@aist.go.jp     

Structure and reflective properties of Al-Cu-Fe quasicrystalline thin film prepared by laser induced arc method

Al-Cu-Fe thin films were prepared by laser induced arc (laser-arc) method from a single source-Al63Cu25Fe12 alloy, which was proved to consist of quasicrystalline phase together with approximant phase. The composition of the deposited films meets the requirement for formation of icosahedral symmetry phase. Quasicrystalline phase was obtained after annealing the amorphous as-deposit film samples. The optical properties of the samples were investigated. Thin film samples of Al, Cu and Fe deposited under the same condition were employed for comparison. The results showed specific reflective properties of Al-Cu-Fe quasicrystal thin film in some wavelength range. The optical conductivity of the films exhibited a negative peak, centered about 440 nm in range of 190to 800 nm. The Al-Cu-Fe quasicrystal thin films could absorb almost all the ray in the wavelength range from 420nm to 450 nm. The ratio of absorption was greater than 99%.     

钛酸锶钡薄膜的室温光学性能研究

采用溶胶-凝胶法制备了(Ba0.75Sr0.25)TiO3薄膜,研究了不同退火温度下样品的物相结构、薄膜的光致发光性能和光学透过率。结果表明:室温下非晶钛酸锶钡薄膜在蓝光激发下具有明显的发光现象,发光波长范围是500～650 nm,峰值在525 nm附近。延长非晶态薄膜的退火时间能够显著提高样品的发光强度,且发光强度随薄膜厚度增加而增大。晶态薄膜有微弱的发光现象。透射谱测试结果表明,钛酸锶钡薄膜在可见光范围内具有良好的光学透过率。