Analysis of Plasma Properties and Deposition of Amorphous Silicon Alloy Solar Cells Using Very High Frequency Glow Disch
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EXPERIMENTAL A parallel plate capacitance reaction chamber (MVHF chamber), which was connected to a multi-chamber glow discharge system, was designed to have the capability to couple RF (13.56 MHz) and VHF (75 MHz) frequencies to the plasma. The electrode space between the cathode and anode was 3 cm. The substrate can be grounded, externally biased, or floated, giving us flexibility to study the effect of ion bombardment. A retarding field analyzer was installed in the plasma chamber to measure the energy distribution of positive ions. The structure of the analyzer is similar to the one used by Ingram and Braithwaite [7]. A hole with a diameter of 4 mm on a 13 [tm thick stainless steel substrate allowed positive ions and electrons to flow into the analyzer. A fine grid screen was attached at the front of the hole to prevent the penetration of plasma sheath field to the analyzer. Behind the hole, a second grid screen was negatively biased to block electrons from reaching the collector. Two 5 jim thick Kapton spacers were used to insulate the substrate, the screen, and the collector. The screen voltage, V,, was set at -40 V for most measurements, which was sufficient to block electrons from reaching the collector. The collector voltage, Vc, (retarding potential) was ramped from -100 V to +100 V with a relatively low speed to ensure a steady state collector current, Jc. Obtaining precise ionic energy distribution from measured J,-Vc characteristic is a very complicated procedure [7]. But as a first order approximation, the derivative of the Jc-Vc characteristic is proportional to the ionic energy distribution. Experimentally, the ion energy distributions of H2 , Ar, Sill4 , and their mixtures were studied as functions of excitation frequency, power density, and pressure. In order to study the effect of ion bombardment, a-Si and a-SiGe alloy component cells were deposited at different pressures and/or with varied external biases. In addition, other deposition conditions such as gas flow rate and substrate temperature were optimized to improve the performance of solar cells. RESULTS AND DISCUSSION Ion enerv distribution Figure 1 shows the ionic energy distribution for a H 2 plasma with 75 MHz excitation, where the VHF power was 10 W. At a low pressure (0.1 torr), a sharp peak appears at 22 eV with a half-width of about 6 eV. The cut-off at the high energy side is very steep. The corresponding value could represent the plasma potential. However, at a high pressure (1.0 torr), the peak of the ionic energy distribution shifts to zero and the width becomes significantly broader. Similar measurements were carried out for an RF plasma. Figure 2 shows the results of an RF plasma with the same conditions as in Fig. 1 except the excitation frequency was 13.56 MHz. The peak position is much higher (37 eV) than that shown in Fig. 1, and the distribution is also much broader (18 eV of half-width). Similar to the MVHF plasma, there are no high energy ions reaching the substrates at 1.0 torr. In addition, the positive ion current tha
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