Ge2 Sb2 Te5相变薄膜光学及擦除性能研究

利用蓝绿激光对非晶态Ge2Sb2Te5 相变薄膜进行擦除性能的研究,分别用1000 ns,500 ns,100 ns,60 ns脉宽的蓝绿激光进行实验.结果表明,一定脉宽下,反射率对比度随擦除功率的增加而增大.并且,在1000 ns,500 ns,100 ns,60 ns的激光作用时间范围内,非晶态薄膜均可转变成晶态.对于脉宽为60 ns的蓝绿激光,擦除功率大于4.49 mW以后,薄膜的反射率对比度高于15％,这表明Ge2Sb2Te5相变薄膜在短脉宽、低擦除功率条件下,可具有较高的晶化速度.同时,分析了非晶态和晶态Ge2Sb2Te5相变薄膜的光谱特性,对比研究了780 nm,650 nm,514 nm和405 nm波长处的反射率和反射率对比度,提出了Ge2Sb2Te5相变薄膜用于蓝光光盘的改进方法.     

Different crystallization processes of as-deposited amorphous Ge2Sb2Te5 films on nano- and picosecond single laser pulse irradiation

The crystallization dynamics of as-deposited amorphous Ge₂Sb₂Te₅ films induced by nano- and picosecond single laser pulse irradiation is studied using in situ reflectivity measurements. Compared with nanosecond laser pulse, the typical recalescence phenomenon did not appear during the picosecond laser pulse-induced crystallization processes when the pulse fluence gradually increased from crystallization to ablation threshold. The absence of melting and recalescence phenomenon significantly decreased the crystallization time from hundreds to a few tens of nanoseconds. The role of pulse duration time scale on the crystallization process is qualitatively analyzed.     

新型AgInSbTe相变薄膜的光学及记录性能

利用直流磁控溅射法制备了一种新型AgInSbTe相变薄膜。示差扫描量热(DSC)实验测定的结晶峰温度为193．92℃。X射线衍射(XRD)表明未经热处理的沉积态薄膜是非晶态,而经过200℃热处理,X射线衍射图出现衍射峰,薄膜从非晶态转变到晶态。同时,研究了晶态和非晶态相变薄膜的吸收率、透射率和反射率随波长的变化。测定了650nm激光作用下的相变薄膜的记录性能,分析了记录功率、记录脉宽对薄膜反射率衬比度的影响,在同一记录脉宽条件下,记录功率越大,反射率衬比度也越大;在同一记录功率条件下,随记录脉宽的增加,反射率衬比度也增大。结果表明,新型AgInSbTe相变薄膜在激光作用下具有较高的反射率衬比度,可获得良好的记录性能。     

High Reflection Ge_(0.45)Te_(0.55) Recording Films

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Visualization of nanosecond laser-induced dewetting,ablation and crystallization processes in thin silicon films

In the present work, nanosecond pulsed laser crystallization, dewetting and ablation of thin amorphous silicon films are investigated by time-resolved imaging. Laser pulses of 532 nm wavelength and 7 ns temporal width are irradiated on silicon film. Below the dewetting threshold, crystallization process happens after 400 ns laser irradiation in the spot central region. With the increasing of laser fluence, it is observed that the dewetting process does not conclude until 300 ns after the laser irradiation, forming droplet-like particles in the spot central region. At higher laser intensities, ablative material removal occurs in the spot center. Cylindrical rims are formed in the peripheral dewetting zone due to solidification of transported matter at about 500 ns following the laser pulse exposure.     

Femtosecond laser pulse irradiation of Sb-rich AgInSbTe films: Scanning electron microscopy and atomic force microscopy investigations

The single-layer and multilayer Sb-rich AgInSbTe films were irradiated by a single femtosecond laser pulse with the duration of 120 fs. The morphological feature resulting from the laser irradiation have been investigated by scanning electron microscopy and atom force microscopy. For the single-layer film, the center of the irradiated spot is a dark depression and the border is a bright protrusion; however, for the multilayer film, the center morphology changes from a depression to a protrusion as the energy increases. The crystallization threshold fluence of the single-layer and the multilayer films is 46.36 mJ/cm², 63.74 mJ/cm², respectively. PACS 79.20.Ds; 78.55.Qr; 78.66.Jg; 68.37.Ef; 68.37.Ps     

Enhanced electron field emission from dense Si nano-dots prepared by laser crystallization of ultrathin amorphous Si films

Dense Si nano-dots with a surface area density of >10¹⁰ cm^?2 were fabricated by excimer laser induced crystallization of 15 nm-thick amorphous Si thin films. The enhanced electron field emission characteristics were found from laser irradiated samples. The threshold electric field is as low as 9.8V/μm and the field enhancement factor can reach as large as 719, which is compatible with the other good cold cathode materials. The improvements in field emission behavior can be associated with the change in the surface morphology after laser irradiation as well as the enhanced internal electric field due to the formation of Si nano-dots within the films.     

SbOx Thin Films Used for Write-Once Blue Laser Recording

SbOx thin films are deposited by reactive dc-magnetron sputtering from an antimony metal target in Ar＋O2 with the relative O2 content 7%. It is found that the as-deposited films can represent a two-component system comprising amorphous Sb and amorphous Sb2 O3. The crystallization of Sb is responsible for the changes of optical properties of the films. The results of the static test show that the SbOx thin films have good writing sensitivity for blue laser beams and the recording marks are very clear and circular. High reflectivity contrast of about 41% is obtained at a writing power 6mW and writing pulse width 300ns. In addition, the films show a good stability after reading 10000 times.     

Photoassisted chemical solution deposition method for fabricating uniformly epitaxial VO2 films

Epitaxial VO₂ films were prepared on the TiO₂ (001) substrates by the excimer-laser-assisted metal–organic deposition (ELAMOD). The quality of the epitaxial films obtained by irradiation with a KrF laser was found to be affected by the film structure obtained after preheating at 500 or 300°C. When the films containing crystal domains, which were obtained by preheating at 500°C, were irradiated with the laser at room temperature under a base pressure of 250 Pa, epitaxial and polycrystalline VO₂ phases were simultaneously formed. In contrast, when the amorphous films containing organic components, which were obtained by preheating at 300°C, were irradiated with the laser at room temperature in air, a single phase of epitaxial VO₂ was formed. By using thermal simulations, we determined that the formation of the epitaxial phase was affected both by the temperature distribution within the film during the laser irradiation and by the laser intensity at the interface between the substrate and the film. The latter factor is considered to play a role in the nucleation of crystallization, causing the epitaxial phase to form preferentially compared to the polycrystalline phase in the amorphous matrix of the films. These results indicate that the ELAMOD process is effective for the fabrication of epitaxial VO₂ films at low temperature.     

高温合金不同脉宽超快激光作用下多脉冲去除阈值

实验研究了激光脉冲宽度和脉冲个数对镍基高温合金材料去除阈值的影响,分别在290 fs,1 ps和7 ps脉宽的激光下,使用1,10,50,100,300,500和1000个不同能量的激光脉冲辐照高温合金样品表面。实验结果表明,烧蚀坑尺寸会随脉冲数的增加而增加,而脉冲宽度的增加会加大脉冲个数对烧蚀坑直径的影响。通过烧蚀坑直径的平方值与激光脉冲能量之间存在的对数关系,得到了不同脉冲宽度下镍基高温合金的多脉冲材料阈值。3种不同脉宽下的高温合金多脉冲去除阈值都存在显著的累积效应。根据去除阈值计算得到290 fs,1 ps和7 ps脉宽下的累积效应系数分别为0.88,0.86和0.78。     

Excimer laser crystallization of amorphous silicon on metallic substrate

An attempt has been made to achieve the crystallization of silicon thin film on metallic foils by long pulse duration excimer laser processing. Amorphous silicon thin films (100 nm) were deposited by radiofrequency magnetron sputtering on a commercial metallic alloy (N42-FeNi made of 41 % of Ni) coated by a tantalum nitride (TaN) layer. The TaN coating acts as a barrier layer, preventing the diffusion of metallic impurities in the silicon thin film during the laser annealing. An energy density threshold of 0.3 J?cm^?2, necessary for surface melting and crystallization of the amorphous silicon, was predicted by a numerical simulation of laser-induced phase transitions and witnessed by Raman analysis. Beyond this fluence, the melt depth increases with the intensification of energy density. A complete crystallization of the layer is achieved for an energy density of 0.9 J?cm^?2. Scanning electron microscopy unveils the nanostructuring of the silicon after laser irradiation, while cross-sectional transmission electron microscopy reveals the crystallites’ columnar growth.     

Partial crystallization of silicon by high intensity laser irradiation

Commercial single crystal silicon wafers and amorphous silicon films piled on single crystal silicon wafers were irradiated with a femtosecond pulsed laser and a nanosecond pulsed laser at irradiation intensities between 10¹⁷ W/cm² and 10⁹ W/cm². In the single crystal silicon substrate, the irradiated area was changed to polycrystalline silicon and the piled silicon around the irradiated area has spindly column structures constructed of polycrystalline and amorphous silicon. In particular, in the case of the higher irradiation intensity of 10¹⁶ W/cm², the irradiated area was oriented to the same crystal direction as the substrate. In the case of the lower irradiation intensity of 10⁸ W/cm², only amorphous silicon was observed around the irradiated area, even when the target was single crystal silicon. In contrast, only amorphous silicon particles were found to be piled on the amorphous silicon film, irrespective of the intensity and pulse duration.Three-dimensional thermal diffusion equation for the piled particles on the substrate was solved by using the finite difference methods. The results of our heat-flow simulation of the piled particles almost agree with the experimental results.     

Optical Recording Performance of In_（47）Sb_（14）Te_（39）Phase Change Thin Films using 514．5nm Wavelength Laser Beam

ＯｐｔｉｃａｌＲｅｃｏｒｄｉｎｇＰｅｒｆｏｒｍａｎｃｅｏｆＩｎ_（４７）Ｓｂ_（１４）Ｔｅ_（３９）ＰｈａｓｅＣｈａｎｇｅＴｈｉｎＦｉｌｍｓｕｓｉｎｇ５１４．５ｎｍＷａｖｅｌｅｎｇｔｈＬａｓｅｒＢｅａｍ￥ＭＥＮＬｉｑｉｕ；ＪＩＡＮＧＦｕｓｏｎｇ；ＧＡＮ?..     

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     

Femtosecond pulse crystallization of thin amorphous hydrogenated films on glass substrates using near ultraviolet laser radiation

Femtosecond laser treatments (second harmonic of Ti-sapphire laser, λ ≈ 400 nm wavelength, <30 fs pulse duration) were applied for crystallization of thin hydrogenated amorphous silicon films on glass substrates. The concentration of atomic hydrogen in the films was varied from 10 to ≈35%. The energy densities (laser fluences) for crystallization of the films with thicknesses from 20 to 130 nm were found. Assumedly, non-thermal processes (plasma annealing) take place in phase transition caused ultra-fast pulses. The developed approach can be used for creation of polycrystalline silicon films on non-refractory substrates.     

飞秒激光对高反膜的破坏及其超快动力学过程

研究了800nm飞秒激光照射下45°高反膜ZrO2-Si O2的破坏及其超快动力学过程。利用原子力显微镜和扫描电镜观察了材料的烧蚀形貌,测量了破坏阈值与脉冲宽度、烧蚀深度与脉冲能量的依赖关系。随着脉冲宽度从50fs增加到900fs,其烧蚀阈值从0.35J/cm2增加到1.78J/cm2。烧蚀深度与激光能流密度近似成对数关系。当激光强度略高于烧蚀阈值时,材料很快被烧蚀到几百纳米,烧蚀深度表现出明显的层状特性。同时,利用建立的抽运探针实验系统,测量了高强度抽运脉冲作用下材料对探针光的反射率随延迟时间的变化,揭示了薄膜烧蚀的超快动力学过程。实验结果表明高反膜表层的材料对烧蚀特性有重要影响。     

Damage performance of TiO2/SiO2 thin film components induced by a long-pulsed laser

In order to study the long-pulsed laser induced damage performance of optical thin films, damage experiments of TiO₂/SiO₂ films irradiated by a laser with 1 ms pulse duration and 1064 nm wavelength are performed. In the experiments, the damage threshold of the thin films is measured. The damages are observed to occur in isolated spots, which enlighten the inducement of the defects and impurities originated in the films. The threshold goes down when the laser spot size decreases. But there exists a minimum threshold, which cannot be further reduced by decreasing the laser spot size. Optical microscopy reveals a cone-shaped cavity in the film substrate. Changes of the damaged sizes in film components with laser fluence are also investigated. The results show that the damage efficiency increases with the laser fluence before the shielding effects start to act.     

Morphological characteristics of amorphous Ge2Sb2Te5 films after a single femtosecond laser pulse irradiation

The morphology of materials resulting from laser irradiation of the single-layer and the multilayer amorphous Ge₂Sb₂Te₅ films using 120 fs pulses at 800 nm was observed using scanning electron microscopy and atomic force microscopy. For the single-layer film, the center of the irradiated spot is depression and the border is protrusion, however, for the multilayer film, the center morphology changes from a depression to a protrusion as the increase of the energy. The crystallization threshold fluence of the single-layer and the multilayer film is 22 and 23 mJ/cm², respectively.     

高重频CO_2激光损伤HgCdTe晶体的数值分析

针对CO2激光作用下HgCdTe晶体的损伤问题进行了数值分析。首先,建立了高重频CO2激光损伤Hg0.784Cd0.216Te晶体的三维热传导物理模型;然后,利用有限元方法计算了单脉冲和高重频CO2激光作用下,Hg0.784Cd0.216Te晶体的损伤阈值;最后,分析了激光重频以及辐照时间对晶体损伤阈值的影响。研究结果表明:单脉冲激光辐照下,晶体的损伤阈值为64.5 J/cm2;高重频(f>1 kHz)激光辐照下,激光重频的改变对晶体损伤阈值的影响较小,损伤阈值应由平均功率密度表征,且与辐照时间密切相关;辐照时间的增加,可以有效地减小晶体的损伤阈值,当激光辐照功率密度<1.95 kW/cm2时,不会发生晶体损伤。研究结果对高重频CO2激光在激光加工以及激光防护的应用方面具有指导意义。     

Numerical and experimental analysis on green laser crystallization of amorphous silicon thin films

The effect of laser fluence on the crystallization of amorphous silicon irradiated by a frequency-doubled Nd:YAG laser is studied both theoretically and experimentally. An effective numerical model is set up to predict the melting threshold and the optimized laser fluence for the crystallization of 200-nm-thick amorphous silicon. The variation of the temperature distribution with time and the melt depth is analyzed. Besides the model, the Raman spectra of thin films treated with different fluences are measured to confirm the phase transition and to determine the optimized fluence. The calculating results accord well with those obtained from the experimental data in this research.