Investigation of crystallization and amorphization dynamics of phase-change thin films by subnanosecond laser pulses

We report experimental results on amorphization and crystallization dynamics of reversible phase-change (PC) thin-film samples, GeSbTe and GeBiTe, for optical disk data storage. The investigation was conducted with subnanosecond laser pulses using a pump-and-probe configuration. Amorphization of the crystalline films could be achieved with a single subnanosecond laser pulse; the amorphization dynamics follow closely the temperature kinetics induced in the irradiated spot. As for crystallization of the samples initially in the amorphous state, a single subnanosecond pulse was found to be insufficient to fully crystallize the irradiated spot, but we could crystallize the PC film (in the area under the focused spot) by applying multiple short pulses. Our multipulse studies reveal that the GeSbTe crystallization is dominated by the growth of nuclei whose initial formation is slow but, once formed, their subsequent growth (under a sequence of subnanosecond pulses) happens quickly. In the case of GeBiTe samples, the crystalline nuclei appear to be present in the material initially, as they grow immediately upon illumination with laser pulses. Whereas our amorphous GeSbTe samples required approximately 200 pulses for full crystallization, for the GeBiTe samples approximately 15 pulses sufficed.     

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.     

Effects of Ag doping on the crystallization properties of Sb-rich GeSb thin films

Ag-doped and un-doped Sb-rich GeSb thin films were deposited by DC magnetron co-sputtering. The electrical, structural, and optical properties of the thin films phase change were investigated using 4-point probe measurement, X-ray diffraction (XRD), transmission electron microscopy (TEM), and a static tester. With increasing Ag doping content, the crystallization temperature and sheet resistance of crystalline state decreased from 325 °C to 283 °C and from 187.33 Ω/□ to 114.62 Ω/□, respectively. XRD patterns of the films showed a Sb hexagonal structure, and the calculated grain size increased from 13.9 nm to 17 nm as the Ag concentration increased. Grain sizes of the Ag-doped thin films were larger than the grain sizes of un-doped thin films, as determined by TEM images. A static tester verified the decreased crystallization speed and optical contrast. Un-doped GeSb crystallization took 160 ns and 16 at.% Ag-doped GeSb crystallization took 200 ns when the laser power was 13 mW. Based on a power-time-effect diagram, the 12.6 at.% Ag-doped GeSb showed good thermal stability in a crystalline state.     

Investigation of phase transition behaviors of the nitrogen-doped Sb-rich Si-Sb-Te films for phase-change memory

The phase transformation properties of the nitrogen-doped Sb-rich Si-Sb-Te films were investigated in detail. It was found that the addition of N atoms into the Si-Sb-Te films increases the temperature for phase transition from the amorphous phase to a stable hexagonal structure and enhances the sheet resistance of the films following grain refinement. The surface topography of the crystalline films was improved by doping nitrogen atoms. The activation energy for crystallization of the films was increased from 1.84 to 2.89 eV with the increased nitrogen content from 0 to 21 at.%, which promises an improved thermal stability. A prolonged data lifetime up to 10 years at 149.4 °C was realized. From the device performance point of view, the N-doped Si-Sb-Te film with a moderate nitrogen content was preferable for the phase-change memory applications due to its advantage of higher reliability.     

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.     

超声波对亚硫酸钙晶粒形成及其形态的影响

以饱和Ca（OH）2溶液与亚硫酸反应形成亚硫酸钙的反应过程为研究对象,对该过程进行超声处理,研究超声功率、超声作用时间和超声探头深入反应溶液距离对该反应及所形成的亚硫酸钙晶体形态的影响。结果表明：与磁力搅拌反应相比,应用超声处理可以明显减少溶液中Ca2＋和SO32-的残留量,并且超声波的引入影响了所形成的亚硫酸钙晶体的形态和大小。超声功率、超声作用时间和超声探头深入溶液的距离等三个因素都对反应溶液中的Ca2＋和SO32-的残留量有影响;与自然反应,磁力搅拌反应所形成的亚硫酸钙晶体相比,超声作用下所形成的亚硫酸钙晶体更细小、更均匀。     

Microscopic origin of the fast crystallization ability of Ge-Sb-Te phase-change memory materials

Ge-Sb-Te materials are used in optical DVDs and non-volatile electronic memories (phase-change random-access memory). In both, data storage is effected by fast, reversible phase changes between crystalline and amorphous states. Despite much experimental and theoretical effort to understand the phase-change mechanism, the detailed atomistic changes involved are still unknown. Here, we describe for the first time how the entire write/erase cycle for the Ge(2)Sb(2)Te(5) composition can be reproduced using ab initio molecular-dynamics simulations. Deep insight is gained into the phase-change process; very high densities of connected square rings, characteristic of the metastable rocksalt structure, form during melt cooling and are also quenched into the amorphous phase. Their presence strongly facilitates the homogeneous crystal nucleation of Ge(2)Sb(2)Te(5). As this simulation procedure is general, the microscopic insight provided on crystal nucleation should open up new ways to develop superior phase-change memory materials, for example, faster nucleation, different compositions, doping levels and so on.     

Investigation of the transport properties of InSb single-crystal films obtained by directional crystallization

Both n-type and p-type InSb films (with a wide range of carrier concentration) were obtained by the directional crystallization method from the melt on large areas of mica, quartz and sapphire substrates. The p-type films were doped with germanium. It is shown that, depending on the crystallization conditions, one can obtain films of different structure: dendritic films, films containing macrodefects and homogeneous single-crystal films. The optimal growth conditions of single-crystal films having transport properties close to those of bulk material are given. The investigation of some transport properties in single-crystal p-type InSb films was carried out in the temperature range 77–600 K.A hole mobility in the range from 180 cm² V^-1 s^-1 to 5×10³ cm² V^-1 s^-1 in films with a concentration p=1.2×10¹⁶?5×10¹⁸cm^-3 of uncompensated acceptors was observed.An investigation of the concentration and temperature dependence of the hole mobility was carried out. Experimental results are in good agreement with the theory if a combined impurity, acoustic and optical mode scattering is taken into account.The phonon drag effect in p-type InSb films was observed. The temperature dependence of the thermoelectric power shows fair agreement with Herring's theory.     

Composition and crystal structure of resorbable calcium phosphate thin films

Silicon stabilized tricalcium phosphate (Si-TCP) is formed, among other phases, as a result of sintering hydroxyapatite (HA) in the presence of silica (SiO₂) at >800°C. Calcium phosphate films sintered at 1000°C on quartz substrates are examined with and without additional SiO₂ added to the starting precipitate. Data from transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) separate the undoped film morphology into a surface layer with a monoclinic crystal structure P2₁/a characteristic of α or Si-tricalcium phosphate and grain size in the range 100–1000 nm and a substrate layer with a crystal structure which is predominantly apatitic P6₃/m and grain size in the range 30–100 nm. The silicon content is greatest in the substrate layer. The addition of SiO₂ to the film material during fabrication induces a more uniform grain size of 10–110 nm and a higher Si content. The structural and phase evolution of these films suggests the nucleation of α-TCP by the local formation of Si-TCP at a SiO₂-hydroxyapatite interface. The results are consistent with X-ray diffraction studies and are explained by a model of nucleation and growth developed for bulk powders.     

Kinetics of non-isothermal crystallization process and activation energy for crystal growth in amorphous materials

An equation expressing the volume fraction,x, of crystals precipitating in a glass heated at a constant rate, α, was derived. When crystal particles grow m-dimensionally,x is expressed as In [- ln(1 -x)] = -n (nα - 1.052mE/RT + Constant whereE is the activation energy for crystal growth andn is a numerical factor depending on the nucleation process. When the nuclei formed during the heating at the constant rate,α, are dominant,n is equal tom + 1, and when the nuclei formed in the previous heat-treatment before thermal analysis run are dominant,n is equal tom. The validity and usefulness of this equation was ascertained by applying it to a Li₂O·2SiO₂ glass. A method for determining the values ofn andm from DSC curves was proposed and it was concluded that the modified Ozawa-type plot is very useful and convenient to obtain the activation energy for crystal growth.     

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.     

Investigation into polishing process of CVD diamond films

A new technique used for polishing chemical vapor deposition (CVD) diamond films has been investigated, by which rough polishing of the CVD diamond films can be achieved efficiently. A CVD diamond film is coated with a thin layer of electrically conductive material in advance, and then electro-discharge machining (EDM) is used to machine the coated surface. As a result, peaks on the surface of the diamond film are removed rapidly. During machining, graphitization of diamond enables the EDM process to continue. The single pulse discharge shows that the material of the coated layer evidently affects removal behavior of the CVD diamond films. Compared with the machining of ordinary metal materials, the process of EDM CVD diamond films possesses a quite different characteristic. The removal mechanism of the CVD diamond films is discussed.     

Phase transition characteristics and electrical properties of nitrogen-doped GeSb thin films for PRAM applications

We have investigated the nitrogen doping effect on phase transition characteristics and electrical property of nitrogen-doped GeSb (N-doped GS) thin films. The nitrogen gas flow rate changed from 0 sccm (GS(0)) to 6 sccm (GS(6)) during the deposition. The sheet resistance of crystalline state was increased from 2.6 to 5.1 kΩ/□ and thermal stability of amorphous was increased as nitrogen gas flow rate increased due to nitrogen doping effect. Moreover, the average grain size was decreased from 9.7 to 6.6 nm at 400 °C as nitrogen gas flow rate increased. However, the crystallization threshold time and laser power of GS(6) were shorter and lower than GS(0) caused by lower optical reflectivity. Nitrogen-doped GeSb showed the possibility of low RESET power and high speed PRAM operation.     

Measurement of the thermal coefficients of nonreversible phase-change optical recording films

We have developed a procedure to obtain the critical temperature for the amorphous-to-crystalline phase transition as well as the thermal conductivity and the specific heat of the phase-change media of optical recording. The procedure involves estimating the thermal conductivity from the data obtained by measuring the threshold cw laser power required for inducing phase transition. Then, from the data obtained in short-pulse measurements, we estimate the specific heat. In principle this method can yield the thermal parameters of any number of layers, so long as one of the layers is made of a phase-change material having a well-defined transition temperature.     

Bismuth doping effect on the phase-change characteristics of nitrogen-doped GeTe films

The microstructures and electrical properties of 8.4% nitrogen-doped GeTe and GeBi(6 at.%)Te films thermally annealed in N₂ atmosphere were investigated. With the addition of Bi to N-doped GeTe films, the initial crystallization temperature was reduced and crystallization speed slowed. The N-doped GeBiTe films showed a rapid increase in crystallite size compared to the N-doped GeTe films. The formation energy of the nucleus may be lower due to the Bi atoms and the growth speed may be slower.     

Influence of crystal structure on the light scatter of zirconium oxide films

The relationship of light scatter by a thin film to thin-film morphology is examined. Light scatter by reactively evaporated ZrO(2) thin films is analyzed by using in situ total internal reflection microscopy and angle-resolved scatterometry. Film crystal structure is analyzed by transmission electron microscopy and x-ray diffraction. Relations between film crystal structure and film scatter are established by using this information. Surface topography is analyzed by the use of scanning force microscopy. Results of a spectrophotometric determination of the film refractive index are reported. The film scatter is found to be sensitive to the crystal phase of the film, which is a function of substrate deposition temperature. A simple method of separating bulk from surface scatter is described.     

Investigation on the growth and structure of Fe-doped near-stoichiometric LiNbO3 crystal

The Fe-doped near-stoichiometric LiNbO₃ crystal has been grown from the Li-rich melt by Czochralski method. The UV-Vis absorption spectra, IR transmittance spectra and X-ray diffraction were measured and used to study the defect structure of the crystal. Compared with that of congruent Fe:LiNbO₃, in near-stoichiometric Fe:LiNbO₃, the lattice parameter reduces, the basal absorption edge shifts towards shorter wavelength, and the OH stretching vibration absorption band shifts towards the lower wavenumber side (3466 cm^–1) in the Fe-doped near-stoichiometric LiNbO₃.     

The crystal structure of epitaxially grown SnO2 thin films

It is shown by computer simulation of the transmission electron diffraction patterns and by structural considerations that the recently reported thin film form β-SnO₂ is equivalent to the high pressure form SnO₂-II. It crystallizes in a columbite-like structure either under the influence of the substrate in the case of epitaxial oxidation of α-SnO to SnO₂ or under the influence of volume contraction conditioned by enhanced pressure and/or temperature during the cassiterite- columbite transformation. The columbite-brookite-cassiterite structural relationship is given and the possibility of the existence of a thin film fluorite-like polymorph is discussed.     

Synthesis of photocatalytic TiO2 thin films via the high-pressure crystallization process at low temperatures

TiO2 thin films with a monophasic anatase structure were synthesized via a high-pressure crystallization (HPC) process which successfully lowered the crystallization temperature of TiO2 films from 350 to 150 degrees C. The thermal budget and energy consumption during the crystallization process were markedly reduced and dense films without cracks were obtained. During the HPC process, crystallization took place throughout the films and TiO2 films with uniform crystallinity were obtained. The HPC process also led to an enhancement in the wettability of TiO2 thin films. The hydrophilicity of the films increased with heating temperatures via high-pressure annealing. In comparison with the conventional annealing, the HPC process not only produced TiO2 films with superior photo-induced super-hydrophilicity, but also led to higher photocatalytic activity of the films. The HPC process was confirmed to provide a new route for synthesizing well-crystallized anatase TiO2 thin films with high photocatalytic activity and good wettability at low temperatures.