特定气氛中的脉冲激光烧蚀及其在薄膜制备上的应用
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摘要
1960 年,T.H. Maiman 成功地制造了世界上第一台红宝石激光器。此后的 40
多年里,激光与物质的相互作用一直是相关领域基础研究的热点和技术应用的基
础,并且作为其中的重要内容和方向之一,逐渐形成了以脉冲激光烧蚀(pulsed
laser ablation, PLA)为主题的研究方向和应用技术。本文的研究对象是真空和多
种背景气氛中的 PLA 的过程和由此引发的 PLA 等离子体特性,以及以特定气氛
中 PLA 为基础的、活性源辅助的脉冲激光沉积(pulsed laser deposition, PLD)的
方法和机理,并尝试将此方法应用于若干种薄膜材料的合成制备。
采用空间和时间分辨的光谱测量和分析方法研究了脉冲激光对铜靶烧蚀所
引发的铜等离子体的时空特性和动力学过程,包括对 PLA 铜等离子体的产生、
等离子体及其中激光烧蚀产物的时间演变和空间分布的观察和探讨。实验观察和
分析研究表明:铜等离子体始于激光脉冲对铜靶的烧蚀,继而从铜蒸汽的光学击
穿和热电子碰撞击穿向低能电子碰撞过程演化;在初始阶段,位于靶面附近的等
离子体中含有大量的铜离子和高激发态铜原子,之后逐步演变为以低激发态铜原
子为主要成份,并且可以延展到较广的空间范围;背景气氛的存在不仅限制激光
烧蚀等离子体的膨胀速度和范围,也可以通过碰撞延长等离子体的发光时间;背
景气氛对不同能量状态的粒子的影响也不尽相同。
通过活性气氛中的 PLA、利用激光烧蚀产物易于与背景气氛中的活性气相
物质发生反应的特点,开展了电子回旋共振(electron cyclotron resonance, ECR)
微波放电等离子体和辉光放电原子束这两种活性源辅助的脉冲激光沉积方法的
技术探索,并选择多种 III 族氮化物和 B-C-N 系化合物薄膜为对象材料进行了应
用尝试,实现了常温下 AlN、GaN、BC、BCN 和 CNx等多种薄膜材料的制备。
分别用真空中的 PLD 和 ECR 微波放电等离子体辅助的脉冲激光沉积
(ECR-PLD)方法、以烧结 AlN 和高纯金属铝为靶材料,合成制备了 AlN 薄膜,
样品的能隙宽度分别为 5.6eV 和 5.7eV,光致荧光的发射范围在 325nm-550nm。
以多晶 GaAs 为靶材料,用 ECR-PLD 方法在常温条件下合成制备了 GaN 薄膜,
得到的薄膜的能隙宽度约为 3.4eV,光致荧光的发射范围在 350nm-550nm,主要
发射带位于近紫外到 450nm 范围。这部分的工作表明对氮气实施 ECR 微波放电
可以为氮化物薄膜的合成制备提供高度活性的化学气相环境,还为 III 族氮化物
薄膜的合成制备摸索了一种新的途径。
以 B4C 烧结靶为原材料、分别以 PLD 和 ECR-PLD 方法开展了 BC 薄膜和
BCN 薄膜的制备。得到的 BC 薄膜中 B 和 C 的原子比约 3:1,并含有约 10%的 O
杂质,衬底加温可以改善 BC 薄膜的结构。在 ECR 氮等离子体的辅助下得到的
复旦大学博士学位论文 1
特定气氛中的脉冲激光烧蚀及其在薄膜制备上的应用
BCN 薄膜中的含 N 量高达 46.1%,O 杂质减少到 5.6%,B、C、N 三元素的组
份比 B:C:N 约为 3:1:3.8,并形成了以 sp2杂化的六角氮化硼(h-BN)为主体、包
含其他大量无定型结构的三元化合物,在 1.4μm?1.7μm 近红外波段还具有较高
的透射率。
以石墨为靶材料,利用 ECR-PLD 方法合成制备了高 N 含量的 CNx薄膜,当
衬底加有-50V 的偏置电压时得到的 CNx薄膜中 N 的含量高达 53%,并考察了
不同衬底偏压、也即不同能量的 N 等离子体束流的影响。研究表明,适当能量
的 N 等离子体束对衬底和生长中的膜层的轰击可以促进表面反应和膜层生长,
有利于 N 含量和沉积速率的提高;但是如果 N 等离子体束流的能量过高,则等
离子体束流对膜层有明显的溅射作用,从而使沉积速率下降,并且对 N 的择优
溅射也使膜层中的 N 含量降低。采用辉光放电原子束辅助脉冲激光沉积(atom
beam assisted pulsed laser deposition, AB-PLD)方法、同样以石墨为靶材料,在有
Co/Ni 过渡层的衬底上制备了 CNx 薄膜。实验测得的 Raman 散射结果与β-C3N4
的理论计算频率有较多的相近,结合 TEM 的测试结果,表明薄膜中形成的晶粒
比较多,Co/Ni 过渡层在薄膜制备过程中起了重要的催化作用。
基于强激光与物质相互作用的脉冲激光沉积和由此引发的等离子体在材料
的处理制备和元器件的制作方面有广泛而成功的应用,也具有明显的优越之处,
而对其中的过程机理的研究相对滞后。本论文对特定气氛特别是活性气氛中的脉
冲激光烧蚀以及以此为基础的活性载能源辅助的脉冲激光沉积的过程机理作了
一些探讨。具有特定功能的薄膜材料是制作先进元器件的基础,而性能优异的功
能薄膜的制备有赖于先进的薄膜制备技术。随着元器件向着微型化、集成化方向
的发展趋势,材料的薄膜化以及对薄膜制备技术的要求也越来越高。近年,为适
应制备新型功能薄膜的需要,人们进行了多种新技术、新工艺的尝试。其中,PLD
和等离子体的应用颇为引人关注,并且收到了独特的效果。化合物薄膜的合成制
备还要求在可以控制的具有化学活性的环境中进行,合适的活性气相物质的有效
提供是成功制备化合物薄膜的关键之一。本论文结合过程机理的研究探讨,利用
PLD 可以与不同的辅助手段灵活结合的特
The interacting between laser beam and materials has been the focus of both
fundamental research and technology applications in related fields during the past
forty years since T.H. Maiman produced the first ruby laser in 1960. Scientific
research and technological applications based on pulsed laser ablation (PLA) has been
developed. In this thesis, processes of PLA and characteristics of the plasma produced
in vacuum and different background ambient were investigated. Methods and
dynamics of pulsed laser deposition (PLD) assisted with active gas resources based on
PLA in specific ambient gas were also examined and utilized on synthesis of several
kinds of thin films.
By observing the temporal and spatial resolved optical emission spectra from
copper plasma induced by laser ablation of a solid copper target, the dynamics of the
generation of the plasma and the temporal revolution and spatial distribution of the
plasma and the species within were examined. The results indicated that the plasma
was ignited by laser ablation of the copper target, and then the internal interactions
evolved from the optical breakdown and collision breakdown by hot electrons of the
copper vapor to the collisions between copper species and low-energy electrons. At
the initial stage, the plasma was composed of large amounts of copper ions and highly
excited copper atoms near the target surface, and then gradually dominated by
low-energy copper atoms and expanded away from the target. In addition to the
slowdown and the confinement of the plasma expansion by the background gas, the
ionization and excitation of the ablated copper species were also facilitated by
collision with ambient gas molecules so that the optical emission lifetime was
prolonged. It is also found that the influence of the background gas on the ablated
species varies depending on their energy states.
Methods of PLD combined with electron cyclotron resonance (ECR) plasma or
atomic beam from a dc-plasma source were developed and utilized in deposition of
III-nitrides films and B-C-N compound system films. Some compound films, such as
AlN、GaN、BC、BCN and CNx were synthesized at low temperature.
Aluminum nitride films have been prepared through two methods: basic pulsed
laser deposition from sintered aluminum nitride ceramics and ECR plasma-aided
reactive pulsed laser deposition from elemental aluminum metal. The band gap of the
films obtained from basic PLD was found to be 5.6 eV, while that of the ECR-PLD
prepared films to be 5.7 eV. Both of the AlN thin films luminesced from 325nm to
复旦大学博士学位论文 3
特定气氛中的脉冲激光烧蚀及其在薄膜制备上的应用
550nm in PL test. GaN thin film from polycrystalline GaAs has been prepared by
ECR-PLD at low temperature. The band gap of the GaN film was determined to be
3.4 eV and it luminesced from 350nm to 550nm in PL test. It was convinced that the
excitation of nitrogen by ECR microwave discharge could provide highly active gas
ambient for nitride films preparation and ECR-PLD provided a new method to
produce Group III nitrides films.
BC and BCN films have been produced from a sintered B4C target by PLD and
ECR-PLD respectively. The average atomic ratio of B:C was 3:1 with 10% of oxygen
impurity in the BC film. Heating the substrate was beneficial to the structure of the
BC film. The average atomic ratio of B:C:N was 3:1:3.8 with 46.1% nitrogen and
5.6% oxygen in the BCN film, which mainly contained h-BN and other amorphous
structures. The BCN film had a high optical transmission from 1.4μm to 1.7μm.
CNx films with rich nitrogen content have been prepared by pulsed laser ablation
of graphite target combined with ECR discharge in nitrogen gas. Effects of different
DC biases applied to the substrate were studied while the film with nitrogen content
of 53% was achieved at a substrate bias of –50V. The results indicated that under an
appropriate negative DC bias, bombardment of the growing
多年里,激光与物质的相互作用一直是相关领域基础研究的热点和技术应用的基
础,并且作为其中的重要内容和方向之一,逐渐形成了以脉冲激光烧蚀(pulsed
laser ablation, PLA)为主题的研究方向和应用技术。本文的研究对象是真空和多
种背景气氛中的 PLA 的过程和由此引发的 PLA 等离子体特性,以及以特定气氛
中 PLA 为基础的、活性源辅助的脉冲激光沉积(pulsed laser deposition, PLD)的
方法和机理,并尝试将此方法应用于若干种薄膜材料的合成制备。
采用空间和时间分辨的光谱测量和分析方法研究了脉冲激光对铜靶烧蚀所
引发的铜等离子体的时空特性和动力学过程,包括对 PLA 铜等离子体的产生、
等离子体及其中激光烧蚀产物的时间演变和空间分布的观察和探讨。实验观察和
分析研究表明:铜等离子体始于激光脉冲对铜靶的烧蚀,继而从铜蒸汽的光学击
穿和热电子碰撞击穿向低能电子碰撞过程演化;在初始阶段,位于靶面附近的等
离子体中含有大量的铜离子和高激发态铜原子,之后逐步演变为以低激发态铜原
子为主要成份,并且可以延展到较广的空间范围;背景气氛的存在不仅限制激光
烧蚀等离子体的膨胀速度和范围,也可以通过碰撞延长等离子体的发光时间;背
景气氛对不同能量状态的粒子的影响也不尽相同。
通过活性气氛中的 PLA、利用激光烧蚀产物易于与背景气氛中的活性气相
物质发生反应的特点,开展了电子回旋共振(electron cyclotron resonance, ECR)
微波放电等离子体和辉光放电原子束这两种活性源辅助的脉冲激光沉积方法的
技术探索,并选择多种 III 族氮化物和 B-C-N 系化合物薄膜为对象材料进行了应
用尝试,实现了常温下 AlN、GaN、BC、BCN 和 CNx等多种薄膜材料的制备。
分别用真空中的 PLD 和 ECR 微波放电等离子体辅助的脉冲激光沉积
(ECR-PLD)方法、以烧结 AlN 和高纯金属铝为靶材料,合成制备了 AlN 薄膜,
样品的能隙宽度分别为 5.6eV 和 5.7eV,光致荧光的发射范围在 325nm-550nm。
以多晶 GaAs 为靶材料,用 ECR-PLD 方法在常温条件下合成制备了 GaN 薄膜,
得到的薄膜的能隙宽度约为 3.4eV,光致荧光的发射范围在 350nm-550nm,主要
发射带位于近紫外到 450nm 范围。这部分的工作表明对氮气实施 ECR 微波放电
可以为氮化物薄膜的合成制备提供高度活性的化学气相环境,还为 III 族氮化物
薄膜的合成制备摸索了一种新的途径。
以 B4C 烧结靶为原材料、分别以 PLD 和 ECR-PLD 方法开展了 BC 薄膜和
BCN 薄膜的制备。得到的 BC 薄膜中 B 和 C 的原子比约 3:1,并含有约 10%的 O
杂质,衬底加温可以改善 BC 薄膜的结构。在 ECR 氮等离子体的辅助下得到的
复旦大学博士学位论文 1
特定气氛中的脉冲激光烧蚀及其在薄膜制备上的应用
BCN 薄膜中的含 N 量高达 46.1%,O 杂质减少到 5.6%,B、C、N 三元素的组
份比 B:C:N 约为 3:1:3.8,并形成了以 sp2杂化的六角氮化硼(h-BN)为主体、包
含其他大量无定型结构的三元化合物,在 1.4μm?1.7μm 近红外波段还具有较高
的透射率。
以石墨为靶材料,利用 ECR-PLD 方法合成制备了高 N 含量的 CNx薄膜,当
衬底加有-50V 的偏置电压时得到的 CNx薄膜中 N 的含量高达 53%,并考察了
不同衬底偏压、也即不同能量的 N 等离子体束流的影响。研究表明,适当能量
的 N 等离子体束对衬底和生长中的膜层的轰击可以促进表面反应和膜层生长,
有利于 N 含量和沉积速率的提高;但是如果 N 等离子体束流的能量过高,则等
离子体束流对膜层有明显的溅射作用,从而使沉积速率下降,并且对 N 的择优
溅射也使膜层中的 N 含量降低。采用辉光放电原子束辅助脉冲激光沉积(atom
beam assisted pulsed laser deposition, AB-PLD)方法、同样以石墨为靶材料,在有
Co/Ni 过渡层的衬底上制备了 CNx 薄膜。实验测得的 Raman 散射结果与β-C3N4
的理论计算频率有较多的相近,结合 TEM 的测试结果,表明薄膜中形成的晶粒
比较多,Co/Ni 过渡层在薄膜制备过程中起了重要的催化作用。
基于强激光与物质相互作用的脉冲激光沉积和由此引发的等离子体在材料
的处理制备和元器件的制作方面有广泛而成功的应用,也具有明显的优越之处,
而对其中的过程机理的研究相对滞后。本论文对特定气氛特别是活性气氛中的脉
冲激光烧蚀以及以此为基础的活性载能源辅助的脉冲激光沉积的过程机理作了
一些探讨。具有特定功能的薄膜材料是制作先进元器件的基础,而性能优异的功
能薄膜的制备有赖于先进的薄膜制备技术。随着元器件向着微型化、集成化方向
的发展趋势,材料的薄膜化以及对薄膜制备技术的要求也越来越高。近年,为适
应制备新型功能薄膜的需要,人们进行了多种新技术、新工艺的尝试。其中,PLD
和等离子体的应用颇为引人关注,并且收到了独特的效果。化合物薄膜的合成制
备还要求在可以控制的具有化学活性的环境中进行,合适的活性气相物质的有效
提供是成功制备化合物薄膜的关键之一。本论文结合过程机理的研究探讨,利用
PLD 可以与不同的辅助手段灵活结合的特
The interacting between laser beam and materials has been the focus of both
fundamental research and technology applications in related fields during the past
forty years since T.H. Maiman produced the first ruby laser in 1960. Scientific
research and technological applications based on pulsed laser ablation (PLA) has been
developed. In this thesis, processes of PLA and characteristics of the plasma produced
in vacuum and different background ambient were investigated. Methods and
dynamics of pulsed laser deposition (PLD) assisted with active gas resources based on
PLA in specific ambient gas were also examined and utilized on synthesis of several
kinds of thin films.
By observing the temporal and spatial resolved optical emission spectra from
copper plasma induced by laser ablation of a solid copper target, the dynamics of the
generation of the plasma and the temporal revolution and spatial distribution of the
plasma and the species within were examined. The results indicated that the plasma
was ignited by laser ablation of the copper target, and then the internal interactions
evolved from the optical breakdown and collision breakdown by hot electrons of the
copper vapor to the collisions between copper species and low-energy electrons. At
the initial stage, the plasma was composed of large amounts of copper ions and highly
excited copper atoms near the target surface, and then gradually dominated by
low-energy copper atoms and expanded away from the target. In addition to the
slowdown and the confinement of the plasma expansion by the background gas, the
ionization and excitation of the ablated copper species were also facilitated by
collision with ambient gas molecules so that the optical emission lifetime was
prolonged. It is also found that the influence of the background gas on the ablated
species varies depending on their energy states.
Methods of PLD combined with electron cyclotron resonance (ECR) plasma or
atomic beam from a dc-plasma source were developed and utilized in deposition of
III-nitrides films and B-C-N compound system films. Some compound films, such as
AlN、GaN、BC、BCN and CNx were synthesized at low temperature.
Aluminum nitride films have been prepared through two methods: basic pulsed
laser deposition from sintered aluminum nitride ceramics and ECR plasma-aided
reactive pulsed laser deposition from elemental aluminum metal. The band gap of the
films obtained from basic PLD was found to be 5.6 eV, while that of the ECR-PLD
prepared films to be 5.7 eV. Both of the AlN thin films luminesced from 325nm to
复旦大学博士学位论文 3
特定气氛中的脉冲激光烧蚀及其在薄膜制备上的应用
550nm in PL test. GaN thin film from polycrystalline GaAs has been prepared by
ECR-PLD at low temperature. The band gap of the GaN film was determined to be
3.4 eV and it luminesced from 350nm to 550nm in PL test. It was convinced that the
excitation of nitrogen by ECR microwave discharge could provide highly active gas
ambient for nitride films preparation and ECR-PLD provided a new method to
produce Group III nitrides films.
BC and BCN films have been produced from a sintered B4C target by PLD and
ECR-PLD respectively. The average atomic ratio of B:C was 3:1 with 10% of oxygen
impurity in the BC film. Heating the substrate was beneficial to the structure of the
BC film. The average atomic ratio of B:C:N was 3:1:3.8 with 46.1% nitrogen and
5.6% oxygen in the BCN film, which mainly contained h-BN and other amorphous
structures. The BCN film had a high optical transmission from 1.4μm to 1.7μm.
CNx films with rich nitrogen content have been prepared by pulsed laser ablation
of graphite target combined with ECR discharge in nitrogen gas. Effects of different
DC biases applied to the substrate were studied while the film with nitrogen content
of 53% was achieved at a substrate bias of –50V. The results indicated that under an
appropriate negative DC bias, bombardment of the growing
引文
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复旦大学博士学位论文 9
特定气氛中的脉冲激光烧蚀及其在薄膜制备上的应用
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复旦大学博士学位论文 10
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复旦大学博士学位论文 9
特定气氛中的脉冲激光烧蚀及其在薄膜制备上的应用
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复旦大学博士学位论文 10
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