IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 37, NO. 9, SEPTEMBER 2009 1723 Evolution of Surface Morphology With Hydrogen Dilution During Silicon Epitaxy by Mesoplasma CVD Jose Mario A. Diaz, Makoto Kambara, and Toyonobu Yoshida Abstract—The influence of hydrogen in high-rate and low- temperature silicon epitaxy under mesoplasma conditions has been investigated from growth precursors and film structural evolution points of view. In situ small-angle X-ray scattering mea- surement has confirmed that silicon nanoclusters that are around 2 nm in size and having a loosely bound structure were formed as growth precursors, independent of the amount of hydrogen. Surface morphological analysis, on the other hand, has revealed that the deposition mechanism changes from surface diffusion to primarily step flow with hydrogen addition due potentially to the anisotropic etching of the silicon surface. Atomically smooth epitaxial films with Hall mobilities of up to 300 cm 2 /(V · s) were deposited accordingly at high partial pressures of hydro- gen (> 220 mtorr), while polycrystalline films were produced at lower hydrogen amounts and still retaining relatively high electric properties. Index Terms—Epitaxial growth, plasma CVD, silicon thin films, X-ray scattering. I. I NTRODUCTION T HE DEPOSITION of device grade epitaxial silicon films at high rates and low temperatures is economically bene- ficial in the fabrication of large-area devices, such as crystalline silicon thin film solar cells, as the potential of high conversion efficiencies around 24% has been recently demonstrated [1]. In developing such deposition technologies, it is essential to consider the effect of hydrogen as it is known to interact with silicon by recombination and abstraction of hydrogen on the silicon surface, whether it be dihydride or monohydride termi- nated [2]–[4], thus affecting the processing in many ways. In particular, epitaxial deposition at low temperatures is generally limited by low deposition rates and the presence of a critical epitaxial thickness beyond which a transition from epitaxial to amorphous and/or polycrystalline film growth occurs [5]–[9]. Several models describing this phenomenon have recognized the importance of atomic hydrogen in the breakdown of epitaxy, either by segregation to some critical surface coverage or by Manuscript received February 22, 2009. First published August 11, 2009; current version published September 10, 2009. This work was supported in part by the Grant-in-Aid for Scientific Research 20686049 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. J. M. A. Diaz was with The University of Tokyo, Tokyo 113-8656, Japan. He is now with the Department of Chemistry, Ateneo de Manila University, Quezon City 1108, Philippines (e-mail: jmdiaz@plasma.t.u-tokyo.ac.jp). M. Kambara and T. Yoshida are with the Department of Materials Engineer- ing, The University of Tokyo, Tokyo 113-8656, Japan. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2009.2024780 Fig. 1. Schematic illustration of the mesoplasma epitaxy from the silicon nanocluster precursors. inducing kinetic roughening which disrupts the growth of the crystalline phases [6], [10]. It has also been reported that, for the case of microcrystalline silicon deposition from hydrogen- diluted SiH 4 by conventional plasma-enhanced chemical vapor deposition (CVD), the addition of hydrogen is necessary to form the crystalline phase by enhancing the surface diffusion of the precursors (SiH 3 ) and/or etching Si–Si bonds [11]– [13], although the reported deposition rates are relatively low (< 1 nm/s). In the case of silicon deposition under mesoplasma condi- tions using SiH 4 with high hydrogen dilution, on the other hand, epitaxial growth has been achieved with no epitaxial breakdown at deposition rates that are as high as 33 nm/s and substrate temperatures that are as low as 360 ◦ C [14], [15]. Furthermore, the electric Hall mobility of these films has been found to be generally constant at 270 cm 2 /(V · s) irrespective of the deposition rate and the substrate temperature. In such a unique mesoplasma CVD, the silicon atoms from the de- composition of the source gas silane (SiH 4 ) undergo clustering and aggregation within the thermal boundary existing between the plasma and the substrate, as schematically illustrated in Fig. 1. In fact, the silicon nanocluster characteristics have been identified in situ by using small-angle X-ray scattering (SAXS), and, also, their strong correlation with the film structure and quality has been observed. For instance, when the epitax- ial films with high electrical mobility are deposited at high RF power, silicon nanoclusters having loosely bound atomic 0093-3813/$26.00 © 2009 IEEE