Bakos Tamas, Valipa Mayur S, Maroudas Dimitrios
Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA.
J Chem Phys. 2005 Feb 1;122(5):54703. doi: 10.1063/1.1839556.
Hydrogen abstraction by growth precursors is the dominant process responsible for reducing the hydrogen content of amorphous silicon thin films grown from SiH(4) discharges at low temperatures. Besides direct (Eley-Rideal) abstraction, gas-phase radicals may first adsorb on the growth surface and abstract hydrogen in a subsequent process, giving rise to thermally activated precursor-mediated (PM) and Langmuir-Hinshelwood (LH) abstraction mechanisms. Using results of first-principles density functional theory (DFT) calculations on the interaction of SiH(3) radicals with the hydrogen-terminated Si(001)-(2x1) surface, we show that precursor-mediated abstraction mechanisms can be described by a chemisorbed SiH(3) radical hopping between overcoordinated surface Si atoms while being weakly bonded to the surface before encountering a favorable site for hydrogen abstraction. The calculated energy barrier of 0.39 eV for the PM abstraction reaction is commensurate with the calculated barrier of 0.43-0.47 eV for diffusion of SiH(3) on the hydrogen-terminated Si(001)-(2x1) surface, which allows the radical to sample the entire surface for hydrogen atoms to abstract. In addition, using the same type of DFT analysis we have found that LH reaction pathways involve bond breaking between the silicon atoms of the chemisorbed SiH(3) radical and the film prior to hydrogen abstraction. The LH reaction pathways exhibit energy barriers of 0.76 eV or higher, confining the abstraction only to nearest-neighbor hydrogens. Furthermore, we have found that LH processes compete with radical desorption from the hydrogen-terminated Si(001)-(2x1) surface and may be suppressed by the dissociation of chemisorbed SiH(3) radicals into lower surface hydrides. Analysis of molecular-dynamics simulations of the growth process of plasma deposited silicon films have revealed that qualitatively similar pathways for thermally activated hydrogen abstraction also occur commonly on the amorphous silicon growth surface.
生长前驱体夺取氢是低温下从SiH₄放电生长非晶硅薄膜时降低氢含量的主要过程。除了直接（埃利-里德）夺取外，气相自由基可能首先吸附在生长表面，然后在后续过程中夺取氢，从而产生热激活前驱体介导（PM）和朗缪尔-欣谢尔伍德（LH）夺取机制。利用第一性原理密度泛函理论（DFT）对SiH₃自由基与氢终止的Si(001)-(2x1)表面相互作用的计算结果，我们表明前驱体介导的夺取机制可以通过化学吸附的SiH₃自由基在过配位表面Si原子之间跳跃来描述，同时在遇到有利于氢夺取的位点之前与表面弱键合。计算得到的PM夺取反应的能垒为0.39 eV，与计算得到的SiH₃在氢终止的Si(001)-(2x1)表面扩散的能垒0.43 - 0.47 eV相当，这使得自由基能够在整个表面上寻找氢原子进行夺取。此外，使用相同类型的DFT分析，我们发现LH反应途径涉及化学吸附的SiH₃自由基的硅原子与薄膜之间在氢夺取之前的键断裂。LH反应途径的能垒为0.76 eV或更高，将夺取限制在最近邻氢原子。此外，我们发现LH过程与氢终止的Si(001)-(2x1)表面的自由基解吸竞争，并且可能被化学吸附的SiH₃自由基分解为较低表面氢化物所抑制。对等离子体沉积硅膜生长过程的分子动力学模拟分析表明，热激活氢夺取的定性相似途径也普遍发生在非晶硅生长表面。
