与薄膜生长和加工相关的长度尺度的Laser产生的等离子体：模拟和实验
Laser-generated plasmas in length scales relevant for thin film growth and processing: simulation and experiment
摘要：
在脉冲激光沉积中，薄膜的生长是由激光产生的等离子体介导的，其性质对于控制膜的微结构是关键的。2D材料的出现重新引起了人们对如何使用这种烧蚀等离子体来操纵原子级薄系统的生长和加工的兴趣。为此目的，需要在等离子体成分对材料加工做出贡献的位置处对等离子体成分的密度、电荷状态和动能进行定量理解。在这里，我们研究了激光诱导等离子体从烧蚀目标，这是材料生长和修改的相关长度尺度的几厘米的扩展距离。该研究是通过使用自适应笛卡尔网格求解器的激光烧蚀/等离子体膨胀模型的快速实现。KrF准分子激光烧蚀Cu的模拟结果与朗缪尔探针和光学发射光谱测量进行了比较。等离子体屏蔽阈值、等离子体中物质的电离状态和离子的动能的模拟预测与实验数据符合良好。对于1-4 J/cm $^2$的激光能量密度，羽流由Cu $^0$主导，在膨胀前沿具有小浓度的Cu $^{+}$和电子。更高的激光能量密度（例如，7 J/cm $^2$）导致富含Cu $^{+}$的等离子体，具有完全电离的前缘，其中Cu $^{2+}$是主要物质。在这两种制度中，模拟表明存在低密度，高温等离子体膨胀前沿，具有高电离度，可能在2D材料中的掺杂，退火和动力学驱动的相变中发挥重要作用。
英文摘要：
In pulsed laser deposition, thin film growth is mediated by a laser-generated plasma, whose properties are critical for controlling the film microstructure. The advent of 2D materials has renewed the interest in how this ablation plasma can be used to manipulate the growth and processing of atomically thin systems. For such purpose, a quantitative understanding of the density, charge state, and kinetic energy of plasma constituents is needed at the location where they contribute to materials processes. Here we study laser-induced plasmas over expansion distances of several centimeters from the ablation target, which is the relevant length scale for materials growth and modification. The study is enabled by a fast implementation of a laser ablation/plasma expansion model using an adaptive Cartesian mesh solver. Simulation outcomes for KrF excimer laser ablation of Cu are compared with Langmuir probe and optical emission spectroscopy measurements. Simulation predictions for the plasma-shielding threshold, the ionization state of species in the plasma, and the kinetic energy of ions, are in good correspondence with experimental data. For laser fluences of 1-4 J/cm$^2$, the plume is dominated by Cu$^0$, with small concentrations of Cu$^{+}$ and electrons at the expansion front. Higher laser fluences (e.g., 7 J/cm$^2$) lead to a Cu$^{+}$-rich plasma, with a fully ionized leading edge where Cu$^{2+}$ is the dominant species. In both regimes, simulations indicate the presence of a low-density, high-temperature plasma expansion front with a high degree of ionization that may play a significant role in doping, annealing, and kinetically-driven phase transformations in 2D materials.
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