W.J. Goedheer

Influence of pressure and plasma potential on high growth rate microcrystalline silicon grown by very high frequency plasma enhanced chemical vapour deposition

Microcrystalline silicon (?c-Si) based single junction solar cells have been deposited by very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a showerhead cathode at high pressures in depletion conditions. The i-layers are made near the transition from amorphous to crystalline. An energy conversion efficiency of 9.9% is obtained with a single junction solar cell that is deposited on a texture-etched ZnO:Al front contact. The ?c-Si i-layer is 1.5 ?m thick, deposited at a rate of 0.5 nm/s. In order to control the material properties in the growth direction, the hydrogen dilution of silane in the gas phase is graded following different profiles with a parabolic shape. Materials with higher deposition rates were developed by increasing the RF power and the total gas flow such that the depletion condition is constant. At a deposition rate of 4.5 nm/s, a stabilized conversion efficiency of 6.7% is obtained for a single junction solar cell with a ?c-Si i-layer of 1 ?m. It is found that the defect density increases one order of magnitude upon the increase in deposition rate from 0.45 to 4.5 nm/s. This increase in defect density is partially attributed to the increased energy of the ion bombardment during the plasma deposition. We have introduced an additional method to limit the ion energy by controlling the DC self bias voltage using an external power source. In this way, the defect density in the ?c-Si layers is decreased and the performance of the solar cells is further improved. It is observed that the performance of solar cells deposited at high rate improves under light soaking conditions at 50 ?C, which we attribute to post deposition equilibration of a fast deposited transition material.

Deposition rate in modulated radio-frequency silane plasmas

Plasma-enhanced chemical-vapor deposition of amorphous silicon by a square-wave amplitude-modulated radio-frequency excitation has been studied by optical emission spectroscopy and plasma modeling. By the modulation, the deposition rate is increased or reduced, depending on the plasma parameters. The increase in the deposition rate in powder-free (α-regime) plasmas is explained by the behavior of the electrons. High-energy electrons cause a large production of radicals at the onset of the plasma, as evidenced by an overshoot in optical emission. This is confirmed by a one-dimensional fluid model. An optimum in the deposition rate at a modulation frequency of about 100 kHz is determined by the decay time of the electron density.

Mechanism of argon treatment on a growing surface in layer-by-layer deposition of hydrogenated amorphous silicon

Abstract Hydrogenated amorphous silicon (a-Si:H) samples were deposited by plasma-enhanced chemical vapor deposition at 50 MHz, using the layer-by-layer (LBL) technique. Thin sub-layer deposition at a high deposition rate of ∼0.9 nm/s ( γ ′ – or dust regime) was alternated by an Ar-treatment period. The density of deposited a-Si:H can be improved by Ar treatment. Some Ar is incorporated into the material and the band gap varies between 1.66 and 1.84 eV, by changing the Ar-treatment time from 0 to 50 s. This article demonstrates that changes in the band gap in a LBL process have no correlation with the total hydrogen content.

Gas-efficient deposition of device-quality hydrogenated amorphous silicon using low gas flows and power modulated radio-frequency discharges

Hydrogenated amorphous silicon samples have been deposited by plasma-enhanced chemical-vapor deposition, using a square-wave amplitude-modulated radio-frequency excitation. In this article it will be shown that a combination of amplitude modulation and low gas flows improves the gas-utilization efficiency by a considerable amount. Using a conventional 50 MHz SiH4/H2 plasma with gas flows of 30 sccm, both for SiH4 and H2 at a pressure of 20 Pa, the gas-utilization efficiency is about 8%. It increases up to 50%, by modulating the amplitude of the radio-frequency excitation signal and reducing both gas flows to 10 sccm, keeping the pressure constant. In this case, the deposition rate amounted to 0.55 nm/s. The combination of amplitude modulation and gas flow reduction gives rise to sufficient ion bombardment and hydrogen dilution at low flows. Device-quality optoelectronic properties are obtained under these conditions. The refractive index at 2 eV is about 4.25 and the microstructure parameter has a value around 0.02. The electrical properties were also appropriate for solar cell application. The photo-to-dark-conductivity ratio varied between 105 and 107. The material exhibited a low defect density which is in the order of 1015–1016 cm−3. The Urbach energy amounted to 52 meV on the average.Hydrogenated amorphous silicon samples have been deposited by plasma-enhanced chemical-vapor deposition, using a square-wave amplitude-modulated radio-frequency excitation. In this article it will be shown that a combination of amplitude modulation and low gas flows improves the gas-utilization efficiency by a considerable amount. Using a conventional 50 MHz SiH4/H2 plasma with gas flows of 30 sccm, both for SiH4 and H2 at a pressure of 20 Pa, the gas-utilization efficiency is about 8%. It increases up to 50%, by modulating the amplitude of the radio-frequency excitation signal and reducing both gas flows to 10 sccm, keeping the pressure constant. In this case, the deposition rate amounted to 0.55 nm/s. The combination of amplitude modulation and gas flow reduction gives rise to sufficient ion bombardment and hydrogen dilution at low flows. Device-quality optoelectronic properties are obtained under these conditions. The refractive index at 2 eV is about 4.25 and the microstructure parameter has a value a...

Fast growth of amorphous silicon layers by amplitude modulation PECVD

Hydrogenated amorphous silicon has been deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) with a square wave amplitude modulated rf signal of 50 MHz. We studied the dependence of the deposition rate on the modulation frequency, using optical emission spectroscopy and plasma modeling. We observed an enhancement in deposition rate by modulating the plasma. This behavior is explained by the characteristics of the electronenergy distribution during the periodical onset of the plasma. According to a one-dimensional fluid model, high-energy electrons cause a large production of radicals at the onset. The heating occurs over the whole plasma volume, leading to an increase of the homogeneity of the layers. The discharge structure is changed completely. A comparison is made between results obtained at 13.56 MHz and at 50 MHz deposition voltages.