Hydrogen Injection in ETP Plasma Jet for Fast-Deposition of High-Quality a-Si:H
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Hydrogen Injection in ETP Plasma Jet for Fast-Deposition of High-Quality a-Si:H A.M.H.N. Petit1,2, R.A.C.M.M. van Swaaij1 and M.C.M. van de Sanden2 1 Delft University of Technology, DIMES-ECTM, P.O. Box 5053, 2600 GB Delft, The Netherlands. 2 Eindhoven University of Technology, Department of Applied Physics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. ABSTRACT We have used a cascaded-arc expanding thermal plasma (ETP) to produce thin films of amorphous silicon at high growth rates (> 3 nm/s). Here, we present a study of the effect on material properties of hydrogen injection in the nozzle, i.e., at the exit of the arc where the plasma expands into the reactor chamber. The advantage of using extra H2 in the nozzle is that the plasma chemistry and pressure in the arc remain unchanged, whilst higher growth rates and a material with low defect densities can be obtained. We observe that with an increase of substrate temperature the growth rate decreases due to densification of the material. This densification is accompanied by a reduction of the hydrogen content and of the microstructure parameter. Further we observe that hydrogen content decreases with higher growth rate. A strong relation is found between the light conductivity and the microstructure parameter indicating a large void fraction in samples grown at low temperature. We have been able to grow a-Si:H material, with H2 in the nozzle, at 350°C and 3 nm/s with a light conductivity of 1.2 × 10-5 Ω-1cm-1, which can be suitable for solar-cell application. INTRODUCTION Expanding Thermal Plasma chemical vapor deposition (ETP-CVD) is a very promising technique to deposit hydrogenated amorphous silicon (a-Si:H) at high growth rates. A gas mixture of argon and hydrogen is used in the arc and silane is injected downstream in the reactor chamber. The material properties are strongly influenced by the ions emanating from the arc as silicon clusters are created when ion-induced dissociation of SiH4 takes place. It was shown that the ion density decreases significantly for increasing H2 flow due to charge transfer and dissociative recombination reactions [1]. The ion density saturates for a H2/Ar ratio of about 0.15 for a 4-mm arc and even for a lower ratio (0.05) for a 2-mm arc [2]. In order to reduce the ion density further, hydrogen has been injected directly into the plasma jet at the exit of the arc. The addition of H2 in the nozzle permits to reach higher growth rates while keeping the pressure, voltage and gas mixture in the arc constant, leaving the plasma chemistry in the arc unchanged. In this article, we present the results of an investigation of the material properties when hydrogen is added in the nozzle. We will show the influence of substrate temperature on the optical, structural and electric parameters of the material. In particular, we will point out the correlation between the microstructure parameter, R*, and light conductivity, as well as with the defect density.
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EXPERIMENTS The amorphous silicon depositions were carried out in
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