Abstract
The effect of small disilane addition on the gas phase properties of silane-hydrogen plasmas and the microcrystalline silicon thin films growth is presented. The investigation was conducted in the high pressure regime and for constant power dissipation in the discharge with the support of plasma diagnostics, thin film studies and calculations of discharge microscopic parameters and gas dissociation rates. The experimental data and the calculations show a strong effect of disilane on the electrical properties of the discharge in the pressure window from 2 to 3 Torr that is followed by significant raise of the electron number density and the drop of the sheaths electric field intensity. Deposition rate measurements show an important four to six times increase even for disilane mole fractions as low as 0.3 %. The deposition rate enhancement was followed by a drop of the material crystalline volume fraction but films with crystallinity above 40 % were deposited with different combinations of total gas pressure, disilane and silane molar ratios. The enhancement was partly explained by the increase of the electron impact dissociation rate of silane which rises by 40% even for 0.1% disilane mole fraction. The calculations of the gas usage, the dissociation and the deposition efficiencies show that the beneficial effect on the growth rate is not just the result of the increase of Si-containing molecules density but significant changes on the species participating to the deposition and the mechanism of the film growth are caused by the disilane addition. The enhanced participation of the highly sticking to the surface radical such as disilylene, which is the main product of disilane dissociation, was considered as the most probable reason for the significant raise of the deposition efficiency. The catalytic effect of such type of radical on the surface reactivity of species with lower sticking probability is further discussed, while it is also used to explain the restricted and sensitive process window where the disilane effect appears.
Highlights
Hydrogenated microcrystalline silicon exhibits particular scientific interest due to its application as an intrinsic layer in thin film solar cells of single or tandem structure.[1,2,3] The method that is most commonly used for the fabrication of such a material is the Plasma Enhanced Chemical Vapor Deposition (PECVD) using highly diluted silane (SiH4) in hydrogen (H2)
In order to investigate the effect of Si2H6 on these microscopic parameters and their correlation with the film growth rate enhancement, we focused on the case of 2 Torr and 1% SiH4 fraction to the mixture, conditions that show the stronger Si2H6 effect on the growth rate
The effect of small Si2H6 addition to the SiH4/H2 mixture on the silicon thin film plasma deposition process has been thoroughly investigated with the support of plasma diagnostics, thin film studies and simple calculations of discharge microscopic parameters and gas dissociation rates
Summary
Hydrogenated microcrystalline silicon (μc-Si:H) exhibits particular scientific interest due to its application as an intrinsic layer in thin film solar cells of single or tandem structure.[1,2,3] The method that is most commonly used for the fabrication of such a material is the Plasma Enhanced Chemical Vapor Deposition (PECVD) using highly diluted silane (SiH4) in hydrogen (H2). One of the key obstacles for the production of cost-effective solar cells is the relatively low growth rate of the intrinsic μc-Si:H and the research is focused on the increase of the deposition rate while maintaining the thin film quality.[6,7]. Several discharge parameters influence the film quality and growth rate, the most important of which are the excitation frequency, the rf power, the discharge geometry, the total gas pressure and the feed gas composition.
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