Crystallization of Ge2Sb2Te5 films by amplified femtosecond optical pulses
Crystallization of Ge2Sb2Te5 films by amplified femtosecond optical pulses
复制标题
DOI：
10.1063/1.4770359
复制
发表时间：
2012-12-15
影响因子：
3.2
通讯作者：
Hicken, R. J.
中科院分区：
文献类型：
--
作者：
Liu, Y.;Aziz, M. M.;Hicken, R. J.
Ge 2Sb 2 Te 5的非晶态和晶态之间的相变已经通过暴露于一系列的60飞秒（fs）放大的激光脉冲的薄膜进行了研究。几十微米大小的标记的显微镜图像的分析提供了一个机会，以检查连续范围的光通量的效果。对于一个固定数量的脉冲，依赖性的结晶标记后的注量的面积是很好地描述了简单的代数结果，提供了有力的证据，热传输内的样品是一维的（垂直）。因此，结晶标记区域由入射fs激光束轮廓而不是由横向热扩散限定，如从显微镜图像的线扫描所确认的，在结晶和非晶材料之间具有急剧的过渡。一个简化的，一维模型，占光吸收，热传输和热激活结晶提供的光学反射率和标记面积的值是非常好的定量协议与实验数据，进一步证明了一维热流假设。通常，对于低于损伤阈值的注量，晶体标记具有环形形状，其中在照射标记的中心处的注量足以引起熔化。标记中心处的能量密度与从热模型得到的熔融深度相关，以正确地预测所观察到的熔融能量密度阈值，并解释作为激光能量密度和脉冲数的函数的环形结晶标记的闭合和持久性。对于较小的注量，可以获得实心椭圆形标记。对放大飞秒脉冲所做标记的分析提出了一种新的有效手段，用于观察相变材料在熔点附近的高温下的结晶动力学，其提供了7-9 m/s范围内的生长速度的估计。此外，可以通过控制脉冲的数量来获得对相变介质中的结晶过程的更精细的控制，所述脉冲的数量沿着激光能量密度一起可以被定制为对叠层中的层的热性质具有宽松限制的任何介质叠层。(c)2012年美国物理学会。[http：//dx.doi.org/10.1063/1.4770359]
The phase transition between the amorphous and crystalline states of Ge2Sb2Te5 has been studied by exposure of thin films to series of 60 femtosecond (fs) amplified laser pulses. The analysis of microscope images of marks of tens of microns in size provide an opportunity to examine the effect of a continuous range of optical fluence. For a fixed number of pulses, the dependence of the area of the crystalline mark upon the fluence is well described by simple algebraic results that provide strong evidence that thermal transport within the sample is one-dimensional (vertical). The crystalline mark area was thus defined by the incident fs laser beam profile rather than by lateral heat diffusion, with a sharp transition between the crystalline and amorphous materials as confirmed from line scans of the microscope images. A simplified, one-dimensional model that accounts for optical absorption, thermal transport and thermally activated crystallization provides values of the optical reflectivity and mark area that are in very good quantitative agreement with the experimental data, further justifying the one-dimensional heat flow assumption. Typically, for fluences below the damage threshold, the crystalline mark has annular shape, with the fluence at the centre of the irradiated mark being sufficient to induce melting. The fluence at the centre of the mark was correlated with the melt depth from the thermal model to correctly predict the observed melt fluence thresholds and to explain the closure and persistence of the annular crystalline marks as functions of laser fluence and pulse number. A solid elliptical mark may be obtained for smaller fluences. The analysis of marks made by amplified fs pulses present a new and effective means of observing the crystallization dynamics of phase-change material at elevated temperatures near the melting point, which provided estimates of the growth velocity in the range 7-9 m/s. Furthermore, finer control over the crystallization process in phase-change media can be obtained by controlling the number of pulses which, along with the laser fluence, can be tailored to any medium stack with relaxed restrictions on the thermal properties of the layers in the stack. (c) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770359]