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摘要:
飞秒激光利用其超快的作用时间、超强的峰值功率的特性,可以与半导体材料表面发生瞬态光化学反应,从而对材料进行有效的掺杂,且可以实现超过材料固溶度极限的过饱和掺杂,同时能在材料表面形成准周期的微纳结构。导致半导体表面性质发生改变,产生超宽光谱高吸收的特性,从而突破传统物理限制,并由此产生了一系列全新的应用。本文总结了飞秒激光与硅相互作用的基本理论和几种物理模型,介绍了其在相关领域的应用,并对飞秒激光过饱和掺杂及改性硅的发展前景作出展望。
Abstract:Femtosecond laser pulses induce intriguing transient photochemical reactions with semiconductors at the sample surface, due to its ultrashort duration and ultrahigh peak power. Taking advantage of these characteristics, material can be effectively doped. The doping level is likely far beyond the solid solubility limit (so called supersaturated doping), meanwhile quasi-periodic structures with micro/nano-scales are created at the material surface as well. As a result, surface properties are strikingly changed, e.g. ultra-high absorption over a broad range from near ultraviolet to infrared emerges, which breaks the limit of traditional physics and brings novel applications. In this review, we summarize the basic theories and several physical models of femtosecond laser-silicon interaction, introduce its applications in relevant areas, and depict future prospects of femtosecond laser hyperdoped and processed silicon.
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Key words:
- hyperdoping /
- femtosecond laser doping /
- femtosecond laser processing /
- silicon
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Abstract: Silicon one of the most abundant elements in the earth's crust has a large impact on modern industry. It is an extremely versatile material with various applications ranging from solar energy to electronic devices. After decades of process, the crystalline silicon solar cell has been successfully commercialized all over the world due to its low cost and high efficiency. As the fundamental component of integrated circuits, silicon-based chips have shown outstanding performance in computers and cell phones. Surprisingly silicon is also a promising host material for the new generation of quantum devices owing to its excellent properties of spin. Definitely it is the core material and classical platform among various materials in the world. However, there are still some blocks which limit its applications, eg. The bandgap of crystalline silicon is only 1.12 eV (~1100 nm), which prohibited the usage in far-infrared range. The carrier mobility of silicon is not high enough, which limited performance of electronic devices.
Benefiting from the rapid development of ultrafast laser, those constraints mentioned above could be resolved by interaction between femtosecond laser and silicon. Femtosecond laser pulses induce intriguing transient photochemical reactions with semiconductors at the surface, owing to its ultrafast duration and ultra-high intensity. Taking advantage of these characteristics, material is effectively doped. The doping level is likely far beyond the solid solubility limit (so called supersaturated doping), in the meanwhile quasi-periodic structures with micro-/nano- scales are created at the surface of material as well. As a result, surface properties are strikingly changed, e.g. ultra-high absorption over a broad range from near ultraviolet to infrared emerges, which breaks the limit of traditional physics and brings novel applications. The excellent properties of this modified silicon material tap more potentials in the silicon-based semiconductor industry. In addition, compared with ion implantation, femtosecond hyperdoping can reduce lattice defects, and improve supersaturated substitution doping.
In this review, we summarized the ultrafast dynamics of the interaction between femtosecond laser and silicon and the physical mechanism of supersaturated doping. Based on the typical TTM (two-temperature model), the Drude model was introduced to correct the surface reflectivity, and a two-dimensional TTM-Drude model was proposed to explain the ultrafast energy transfer process. Subsequently, a competitive model was established that led to LIPSS (laser-induced periodic surface structures) formation on the silicon surface. Meanwhile, the microscopic process and physical mechanism of femtosecond laser supersaturated silicon doping had been improved through the researches of the plasma plume in silicon etching. Finally, we also introduced its applications in relevant areas, such as photoluminescence and photodetectors and depicted future prospects of femtosecond laser hyperdoped and modified silicon.
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图 3 飞秒激光辐照下硅表面载流子温度(蓝色实线)、晶格温度(红色短线)以及载流子浓度(绿色点划线)随时间变化曲线,紫色点线为飞秒激光示意图[20].
Figure 3. Evolution of carrier temperature (blue solid curve), lattice temperature (red dash curve) and carrier density (green dash dot curve) under the irradiating of femtosecond. The purple dot curve shows the femtosecond laser pulse[20].
图 8 在SF6中10 kJ/m2的激光通量条件下,羽流的光谱谱线390.5 nm Si Ⅰ和505.6 nm Si Ⅱ的强度衰减曲线.图中点为实验数据,曲线为理论拟合[34].
Figure 8. The decay curves of the 390.5 nm Si Ⅰ line and of the 505.6 nm Si Ⅱ line under a laser fluence of 10 kJ/m2 in SF6. The dots represent experimental data and the solid curves represent bi-exponential fits[34].
表 1 六氟化硫中不同激光通量下,谱线505.6 nm的强度衰减曲线的双指数拟合参数及连续谱在700 nm处的光谱强度衰减的单指数拟合参数[34].
Table 1. Lifetimes of the two decay components of the 505.6 nm line and the lifetime of the continuum spectrum at 700 nm in SF6[34].
Laser fluence/(kJ/m2) τ1/ns A1/(%) τ2/ns A2/(%) τ0/ns 3.5 0.54 87.0 2.49 13.0 0.64 10 0.83 79.3 4.13 20.7 0.87 20 1.10 77.8 5.49 22.2 1.21 40 2.43 69.5 11.3 30.5 2.67 -
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