Plasma Enhanced Layer-by-layer Deposition and Nano-crystallization of Si:H films
Nano-crystalline Si (nc-Si) is a promising candidate for photovoltaic applications due to its better stability compared to amorphous Si, and relatively easy to manufacture at low cost, by plasma enhanced chemical vapor deposition (PECVD), compared to single crystal Si. The crystalline volume fraction of nc-Si films needs to be well controlled to prevent light-induced degradation of the otherwise amorphous hydrogenated Si (a-Si:H). A layer-by-layer technique using two separate plasma sources for a-Si:H deposition and nano-crystallization was developed. A capacitively-coupled plasma (CCP) with SiH4/He feed gas was used to deposit thin a-Si:H layers that were subsequently exposed to a H2 or D2 inductively-coupled plasma (ICP) to induce crystallization in the films. Deposition and nano-crystallization were performed sequentially and periodically to grow thin films. Raman spectroscopy was used to characterize the films and determine the fraction of crystalline. The crystalline volume fraction obtained in this work ranged from 0% to 72%. Many short exposures (20 s or 5 s) to the plasmas were more effective in producing nano-crystalline Si compared to one long exposure (40 min. or 4 min.). In addition, the fraction of nano-crystalline Si increased with increasing H2 ICP-to-SiH4/He CCP exposure time ratio (from 1/4 to 3/2). The crystallites had columnar structure along the film growth direction based on transmission electron microscopy (TEM). Etching of films by the D2 plasma was monitored by mass spectrometry. At 250 oC, the amorphous Si etching rate (0.25 nm/min) was much lower than the deposition rate (1.4 nm/min), and that etching did not occur exclusively on the surface or the near surface region. The blueshift (by about 1 eV) of the dielectric constants peak, found by spectroscopic ellipsometry (SE), suggested the formation of nano-crystallites in the bulk of the films. It is proposed that by tailoring the CCP deposition time as well as the H2 ICP exposure time per cycle, the crystalline fraction and crystallite size of the resulting films can be controlled for more stable solar cell materials. Further, by spatially separating film deposition and nano-crystallization, each of these processes can be individually optimized, providing flexibility in controlling film nanostructure and properties.