An Investigation of the Gas Phase and Surface Chemistry Active During the PECVD of nc-Silicon: A Detailed Model of the Gas Phase and Surface Chemistry
An Investigation of the Gas Phase and Surface Chemistry Active During the PECVD of nc-Silicon: A Detailed Model of the Gas Phase and Surface Chemistry
复制标题
NC-硅 PECVD 过程中气相和表面化学活性的研究：气相和表面化学的详细模型
DOI：
10.1149/1.3207581
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发表时间：
2009
影响因子：
5.2
通讯作者：
G. Isella
中科院分区：
文献类型：
--
作者：
C. Cavallotti;M. Rondanini;T. Moiseev;D. Chrastina;G. Isella
近年来，纳米硅的等离子体沉积因其在光伏产业中的应用前景而成为众多实验和理论研究的对象。尽管如此，很难将生长材料的性质与其生长过程中活跃的气相和表面化学严格联系起来。在这项工作中，我们提出了一个详细的模型，该模型能够描述薄膜沉积过程中气相和表面上活跃的元素化学。该模型由两部分组成，第一部分侧重于等离子体放电的电磁特性(放电模型)，第二部分侧重于描述气体和表面化学(化学模型)。在放电模型中，泊松方程与离子和电子的质量守恒方程一起在二维中被求解，该方程显式地考虑了磁约束效应。通过与采用朗缪尔探头生长nc-Si薄膜的反应器中测量的实验数据的比较，验证了该模型的有效性。放电模型的输出是局部轴向电场、径向电场和电子密度，由中性和带电物质的质量守恒方程和中性物质的能量和动量平衡组成的化学模型作为输入。在反应器模型中嵌入了包含30个电子碰撞过程、19个离子反应和8个中性反应的气相动力学方案。虽然方案中只考虑了很少的中性反应，主要是因为在这些低压下可以形成的化学物种的数量有限，但它们在描述SiHx自由基的渐进脱氢过程中发挥了重要作用。特别是，这项工作的主要创新之一是采用了采用从头算-RRKM-Master方程计算的SiHx自由基的H抽提率，这表明这些反应的速度可以比之前认为的快5倍。在nc-Si生长过程中计算的SiH3浓度如图1所示。SiH3在衬底上方以高速率产生，并被表面反应消耗。尽管通常进行沉积的压力很低(1-2帕)，离子反应仍以相当快的速度进行，并导致通过电子碰撞电离Ar产生的Ar+快速转化为ArH+，然后转化为H3+，最后在与SiH4碰撞时转化为SiH3+。为了验证模型的有效性，采用质谱仪将计算的离子摩尔分数与在衬底高度现场测量的离子摩尔分数进行了比较。图2中报告了SiH3+和H+作为供给反应器的硅烷流量的函数的结果。高浓度的H+与生长表面上方的高摩尔分数的H原子配对，分别约为0.05。图1.在LE-PECVD反应器中计算了下列生长条件下的SiH3摩尔分数：SiH4流量：20sccm；Ar流量：50sccm；压力1.5Pa；衬底温度523K。
The plasma deposition of nano-crystalline silicon has been the object of much experimental and theoretical research in the last years because of its perspective application in the photovoltaic industry. Despite of this, it is difficult to relate strictly the properties of the grown material with the gas phase and surface chemistry active during its growth. In this work we present a detailed model that is able to describe the elementary chemistry active in the gas phase and on the surface during the film deposition. The model consists of two parts, a first focused on the electromagnetic properties of the plasma discharge (the discharge model) and a second on the description of the gas phase and surface chemistry (the chemistry model). In the discharge model the Poisson equation is solved in two dimensions together with the mass conservation equations of ions and electrons accounting explicitly for magnetic confining effects. The model is validated through comparison with experimental data measured in a reactor used to grow nc-Si films adopting a Langmuir probe, as described in detail in our previous publications. The discharge model outputs are the local axial and radial electric fields and the electronic density, which are taken as input by the chemistry model, which consists of the mass conservation equations for neutral and charged species and of the energy and momentum balance for neutral species. A gas phase kinetic scheme consisting in a set of 30 electron impact processes, 19 ionic reactions, and 8 neutral reactions is embedded in the reactor model. Though only few neutral reactions were considered in the scheme, mainly because of the limited number of chemical species that can be formed at these low pressures, they play an important role in describing the progressive de-hydrogenation of the SiHx radicals. In particular, one of the main novelties of this work is the adoption of H abstraction rates for SiHx radicals that were calculated adopting ab initio-RRKM-Master Equation calculations, which indicated that these reactions can be faster up to a factor of 5 than previously thought. The calculated SiH3 concentration during the nc-Si growth is reported in Fig. 1. SiH3 is produced at a high rate above the substrate and consumed by surface reactions. Ionic reactions proceed at a significant rate, despite the low pressure at which the deposition is usually conduced (1-2 Pa), and lead to the fast conversion of Ar+, generated through electron impact ionization of Ar, to ArH+, followed by its conversion to H3+ and finally, upon impact with SiH4, to SiH3+. The calculated ionic mole fractions were compared to those measured in situ at the substrate height adopting a mass spectrometer in order to validate the model. The results are reported in Fig. 2 for SiH3+ and H+ as a function of the silane flux fed to the reactor. The high concentration of H+ is paired by a high mole fraction of atomic H, about 0.05, respectively, above the growth surface. Fig. 1. SiH3 mole fraction calculated in the LE-PECVD reactor for the following growth conditions: SiH4 flux: 20 sccm; Ar flux: 50 sccm; pressure 1.5 Pa; substrate temperature 523 K.