The growth of a-Si and nc-Si by PECVD is determined by electron density and energy distribution in the plasma, gas phase reaction chemistry, precursor transport to the growth surface, and surface reactions.
A mixture of SiH4 and H2 is adjusted into a chamber with power from an RF supply. The gas pressure is adjusted for the given RF voltage to initiate the plasma, that ionizes and decomposes the gas. The a-Si:H film grows on a substrate that may be mounted on one or both of the electrodes that are heated to 150-300 °C.
Thermally activated surface diffusion of ad-atoms is used for optimum film quality (Chapman 1980; Luft and Tsuo 1993; Kushner 1988). At lower substrate temperature, more H is incorporated that increases the band gap of a-Si:H slightly. At higher substrate temperature, less hydrogen is incorporated and the band-gap is reduced.
The pressure range is usually between 0.5 and 1 Torr. Lower pressure permits more uniform deposition, higher pressure permits higher growth rates.
The RF power usually is set to 10-100 mW/cm2. Power above 100 mW/cm2 causes rapid reactions in the gas and can create silicon polyhydride powder that contaminates the growing Si film.
Optimum spacing (d) between the RF electrode and the substrate is usually between 1 and 5 cm for a-Si deposition, with smaller spacing for a uniform deposition, however, larger spacing is easier for maintaining the plasma.
Important for the growth of high-quality a-Si films is the reduction of contaminants, such as oxygen, carbon, nitrogen, or metals. But, because of the flexibility of the bonding network, the tolerance for contaminants in a-Si is much higher than in its crystalline counterpart. However, when the contaminants in the i – layer exceed for O < 1019, C < 1018, and N < 1017/cm3, the fill factor of the resulting solar cell is reduced because of the reduced lifetime of photo-generated carriers (Kinoshita et al. 1996).