特定气氛中的脉冲激光烧蚀及其在薄膜制备上的应用

【作者】 凌浩；

【导师】 李富铭；

【作者基本信息】 复旦大学 ， 光学， 2004， 博士

【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(pulsedlaser 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<WP=6>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(atombeam 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更多还原

【Abstract】 The interacting between laser beam and materials has been the focus of bothfundamental research and technology applications in related fields during the pastforty years since T.H. Maiman produced the first ruby laser in 1960. Scientificresearch and technological applications based on pulsed laser ablation (PLA) has beendeveloped. In this thesis, processes of PLA and characteristics of the plasma producedin vacuum and different background ambient were investigated. Methods anddynamics of pulsed laser deposition (PLD) assisted with active gas resources based onPLA in specific ambient gas were also examined and utilized on synthesis of severalkinds of thin films. By observing the temporal and spatial resolved optical emission spectra fromcopper plasma induced by laser ablation of a solid copper target, the dynamics of thegeneration of the plasma and the temporal revolution and spatial distribution of theplasma and the species within were examined. The results indicated that the plasmawas ignited by laser ablation of the copper target, and then the internal interactionsevolved from the optical breakdown and collision breakdown by hot electrons of thecopper vapor to the collisions between copper species and low-energy electrons. Atthe initial stage, the plasma was composed of large amounts of copper ions and highlyexcited copper atoms near the target surface, and then gradually dominated bylow-energy copper atoms and expanded away from the target. In addition to theslowdown and the confinement of the plasma expansion by the background gas, theionization and excitation of the ablated copper species were also facilitated bycollision with ambient gas molecules so that the optical emission lifetime wasprolonged. It is also found that the influence of the background gas on the ablatedspecies varies depending on their energy states. Methods of PLD combined with electron cyclotron resonance (ECR) plasma oratomic beam from a dc-plasma source were developed and utilized in deposition ofIII-nitrides films and B-C-N compound system films. Some compound films, such asAlN、GaN、BC、BCN and CNx were synthesized at low temperature. Aluminum nitride films have been prepared through two methods: basic pulsedlaser deposition from sintered aluminum nitride ceramics and ECR plasma-aidedreactive pulsed laser deposition from elemental aluminum metal. The band gap of thefilms obtained from basic PLD was found to be 5.6 eV, while that of the ECR-PLDprepared 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<WP=8>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 byECR-PLD at low temperature. The band gap of the GaN film was determined to be3.4 eV and it luminesced from 350nm to 550nm in PL test. It was convinced that theexcitation of nitrogen by ECR microwave discharge could provide highly active gasambient for nitride films preparation and ECR-PLD provided a new method toproduce Group III nitrides films. BC and BCN films have been produced from a sintered B4C target by PLD andECR-PLD respectively. The average atomic ratio of B:C was 3:1 with 10% of oxygenimpurity in the BC film. Heating the substrate was beneficial to the structure of theBC film. The average atomic ratio of B:C:N was 3:1:3.8 with 46.1% nitrogen and5.6% oxygen in the BCN film, which mainly contained h-BN and other amorphousstructures. 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 ablationof graphite target combined with ECR discharge in nitrogen gas. Effects of differentDC biases applied to the substrate were studied while the film with nitrogen contentof 53% was achieved at a substrate bias of –50V. The results indicated that under anappropriate negative DC bias, bombardment of the growing更多还原