Effect of Filament Bias on the Properties of Amorphous and Nanocrystalline Silicon from Hot-Wire Chemical Vapor Depositi
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ABSTRACT At the high temperatures during hot-wire assisted chemical vapor deposition, thermal emission of electrons from the filament occurs. We studied the effect of filament bias, and thus the filament-to-substrate current, on the structural, electronic and optical properties of amorphous and nanocrystalline silicon deposited by this method. The current drawn by the substrate can be varied by many orders of magnitude as thermally emitted electrons are increasingly collected with applied bias voltage. The crystallinity of the nanocrystalline samples is not affected by the bias voltage. The defect density in amorphous silicon is affected by the electron bombardment at high bias voltage only, for which we also find a significant reduction in the mobility-lifetime product from steady-state photoconductivity.
INTRODUCTION At higher temperatures tungsten thermally emits electrons which at 2500'C results 2 in a current density of 0.8 A cm- [1]. Heintze et al. [2] showed that the electron current emitted from the hot tungsten filament reaches the substrate in a hot-wire chemical vapor deposition (HWCVD) system. The authors varied the current by applying such a voltage bias between the filament and the substrate, which allowed them also to change the sign of the current. Having reported this effect they focus on a variety of properties of hot-wire deposited amorphous silicon (a-Si:H) and microcrystalline silicon without application of a bias. We report here on the effect of such a voltage bias between filament and substrate and the effect of the current, drawn by the substrate, on the properties of a-Si:H and microcrystalline silicon, which we call nanocrystalline silicon (nc-Si) here with reference to the crystallite size.
EXPERIMENTAL The voltage bias Vb is defined as the potential of the substrate, which was grounded by the substrate holder, with respect to the filament, i. e., a positive Vb directs an enhanced electron flow to the substrate. Varying Vb between -100 V and +100 V, a series of amorphous and nanocrystalline samples with thicknesses between 700 nm and 1000 nm was deposited on Corning glass 7059 in a high vacuum (HV) hot-wire CVD deposition system. This system, also employed in Ref. [2], had a filament-to-substrate distance of 2 cm and a 10 cm distance between the 4 cm long filament and the walls. The deposition temperature was 4000C for both a-Si:H and nc-Si samples. The filament temperature was adjusted to 1650'C by a DC current. 'email: [email protected] 915 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
The hydrogen gas flow rate was set to 60 sccm with the addition of 9 sccm (2 sccm) silane gas flow rate without use of a gas purifier. The pressure was 15 (20) Pa for amorphous (nanocrystalline) samples. Spectroscopic ellipsometry was measured in situ as described in Ref. [3]. RESULTS Filament-to-substrate current Figure 1 shows the large variation in the filament-to-substrate current If upon application of a positve Vb. We find a positive current at zero bias (at fixe
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