AVS 58th Annual International Symposium and Exhibition
    Energy Frontiers Focus Topic Tuesday Sessions
       Session EN+TF-TuA

Paper EN+TF-TuA4
Deposition of Microcrystalline Silicon Thin Films by Radio Frequency PECVD using Voltage Waveform Tailoring

Tuesday, November 1, 2011, 3:00 pm, Room 108

Session: Thin Films for Solar Cells
Presenter: Sylvain Pouliquen, Ecole Polytechnique (Palaiseau), France
Authors: S. Pouliquen, Ecole Polytechnique (Palaiseau), France
P.-A. Delattre, Ecole Polytechnique (Palaiseau), France
E.V. Johnson, Ecole Polytechnique (Palaiseau), France
J.-P. Booth, Ecole Polytechnique (Palaiseau), France
Correspondent: Click to Email
This paper shows the feasibility of depositing thin films of hydrogenated microcrystalline silicon (µc-Si:H) for photovoltaic applications by radiofrequency discharge using voltage waveform tailoring. Contrary to typically used sinusoidal waveforms, the films were deposited using asymmetric voltage waveforms, resembling “peaks” and “valleys”, composed of a fundamental frequency of 15 MHz and three harmonics. Such waveforms lead to an electrical asymmetry effect in the plasma, and a different ion bombardment energy on each electrode. For these experiments, we used a gas mixture of silane diluted in hydrogen (from 1-4%), a total flow rate less than 100 sccm, and a range of pressure from 100 up to 1000 mTorr. We show that the chemical, optical and structural properties - as measured by FTIR, spectroscopic ellipsometry, Raman scattering and profilometry – are strongly determined by the shape of the voltage waveform, and therefore the ion bombardment energy. We observe this effect on the Si-Si bond configuration (microcrystalline vs amorphous peaks in Raman scattering between 480 and 520 cm-1) as well as on the Si-H bond configuration (FTIR peaks at 2000-2100 cm-1). We examine the strength of the effect for varying process parameters, including pressure, total gas flow, voltage amplitude, and SiH4/H2 dilution ratio. For the range of parameters used, a growth rate for highly microcrystalline silicon of up to 3 Å/s was achieved without exceeding a pressure of 100 mTorr.