Effect of Si 2 H 6 as a gas phase additive to increase growth rate of a-Si:H films and solar cells
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Effect of Si2H6 as a gas phase additive to increase growth rate of a-Si:H films and solar cells Lala Zhu1,2, Ujjwal K Das1 , Chandan Das1 , Steven S Hegedus1 1. Institute of Energy Conversion, University of Delaware, Newark, Delaware, USA 19716; 2. Department of Physics and Astronomy, University of Delaware, Newark, Delaware, USA 19716. ABSTRACT We have studied the growth rate enhancement of a-Si:H films using Si2H6 as a gas phase additive to SiH4+ H2 gas mixtures using relatively low pressure and standard 13.56 MHz RF power. With the addition of 1.7% Si2H6 in the gas mixtures (10% more Si atoms into the chamber), the a-Si:H film growth rate increased by ~30% at 1.25 Torr and ~60% at 2.5 Torr. The optical emission spectroscopy (OES) exhibits reduction of SiH* intensity with addition of Si2H6, which indicates the reduction of high energy electron impacts with Si containing molecules. The microstructural defect parameter (fraction of dihydride bonding in the film) appears to decrease with increase of RF power (i.e. increase of growth rate). Similar a-Si:H p-i-n cell efficiency 7-8% is achieved with and without enhanced i layer growth rate with the Si2H6 additive. INTRODUCTION Enhancement of growth rate of amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) thin films has been a subject of extensive study for high throughput, large volume production of thin film Si tandem solar modules. Increased frequency (>40 MHz) and/or high deposition pressure (>5 Torr) in plasma enhanced chemical vapor deposition (PECVD) processing can achieve high efficiency solar cells at a high growth rate. However, both very high frequency (VHF) and high pressure approaches have limitations in terms of non-uniform deposition over large area for VHF and excessive dust formation and frequent chamber cleaning for high pressure conditions. As another approach to increase the growth rate, W. Futako et al. reported high growth rate (20 A/s) of a-Si:H using pure disilane (Si2H6) [1]. A. Hammad et al. reported that SiH4/H2 discharges with Si2H6 additive at low pressure had significant increase in growth rate [2]. However, little data was published characterizing the film properties and solar cell performance with enhanced growth rate by the gas phase additive. In this work, we have studied the effect of Si2H6 as an additive into SiH4/H2 mixtures and compare a-Si:H growth rates, film properties and cell performance with and without additive. Our ultimate goal is to evaluate additives for nc-Si deposition. This paper contains our first report of using Si2H6 additive to to establish a baseline understanding of its effect on a-Si:H film growth. No attempt was made to optimize growth rate or device performance. EXPERIMENT In this study, all the a-Si:H layers were deposited by a multi-chamber PECVD system with chamber volume around 6.7x104 cm3and electrode area 1100 cm2, which is larger than a typical research chamber. The intrinsic a-Si:H layer (i layer) was deposited by capacitively coupled 13.56 MHz RF plasma and the electrode to substrate g
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