(697e) High Rate Deposition of Nanocrystalline Silicon By Thermal Plasma Enhanced CVD Process

Hydrogenated nanocrystalline silicon (nc-Si:H) is a heterogeneous mixture consisting of a crystalline phase and an amorphous phase [1]. nc-Si:H has attracted lots of attention in both academic and industry communities during the last three decades for its outstanding properties such as higher electrical conductivity and greater doping efficiency compared to hydrogenated amorphous silicon (a-Si:H). The material’s stability under the Steabler–Wronski effect leads to its broad use in photovoltaic cells. Good electron and hole mobility makes it suitable for the application in thin film transistors (TFT).

The nc-Si:H material is usually fabricated by PECVD process in vacuum [2]. The cold plasma is mostly used with the power less than 1 kW. Highly diluted SiH₄ in H₂ is employed as deposition precursor. SiH₄input rate is very low because of the low energy input of cold plasma leading to a relatively low deposition rate. The maximum deposition rate was reported as 0.6 nm/s [3].

In this work, we proposed an atmospheric pressure thermal plasma enhanced CVD process for nc-Si:H fabrication with improved deposition rate. An 13 MHz RF plasma generator with a power up to 10 kW was built. Thermal plasma offered us a much higher energy density, temperature field and reactive particle concentration than cold plasma, which accordingly gave us the probability to adopt a higher raw material input rate to improve the deposition rate, and a thermodynamically more stable deposition precursor to improve the process safety.

SiCl₄ diluted in H₂and Ar was used as deposition precursor. Deposition rate was analyzed using scanning electron microscope (SEM). As a result, we implemented a deposition rate up to 9.8 nm/s, 15 times faster than the maximum rate reported. The structural and optical properties of the deposited nc-Si:H films were characterized. The nc-Si:H product had a grain size less than 10 nm and a crystallinity in the range of 14~28%. XRD spectra indicated that the nc-Si:H films had a preferential crystal orientation in the (111) direction. Photoluminescence analysis showed that the nc-Si:H films had a visible luminescence at room temperature. Moreover, the material’s properties were flexible, which could be controlled by deposition parameters. In this presentation, we will provide the experimental results on the influence of deposition parameters on deposition rate and material properties.

References:

[1] E. BUSTARRET, M.A. HACHICHA, M. BRUNEL, EXPERIMENTAL-DETERMINATION OF THE NANOCRYSTALLINE VOLUME FRACTION IN SILICON THIN-FILMS FROM RAMAN-SPECTROSCOPY,Appl Phys Lett, 52(1988)1675-1677.

[2] S. VEPREK, V. MARECEK, PREPARATION OF THIN LAYERS OF GE AND SI BY CHEMICAL HYDROGEN PLASMA TRANSPORT,Solid State Electron, 11(1968)683.

[3] W.S. Yan, D.Y. Wei, S. Xu, H.P. Zhou, Highly doped p-type nanocrystalline silicon thin films fabricated by low-frequency inductively coupled plasma without H-2 dilution,J Appl Phys, 110(2011).