Nanocrystalline Si Films and Devices Produced Using Chemical Annealing with Helium
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A20.6.1
Nanocrystalline Si Films and Devices Produced Using Chemical Annealing with Helium Nanlin Wang and Vikram L. Dalal Dept. of Electrical and Computer Engineering and Microelectronics Research Center Iowa State University Ames, Iowa 50011, USA
ABSTRACT We report on growth of nanocrystalline Si:H films and devices using a layer-by-layer growth technique, where the growth of a thin amorphous layer by PECVD is followed by chemical annealing in a Helium plasma. The films and devices were grown using a remote, low pressure ECR plasma process. It was found that the structure of the films grown using the layerby-layer technique depended critically upon whether the annealing was done with hydrogen or helium, and the time taken to do the annealing. When the annealing was done in a hydrogen plasma, the films remained amorphous; in contrast, when the annealing was done in helium, and the annealing time was increased to 20 seconds from 10 seconds, the films became crystalline. The crystallinity of the films was confirmed using Raman spectroscopy and x-ray diffraction. The result obtained here shows that it is not necessary to have a high hydrogen dilution to obtain nanocrystalline films. Rather, the amount of hydrogen already present in an amorphous film is enough to cause crystallization, provided that enough ion flux and perhaps energy are available for converting the amorphous structure to a crystalline structure. Proof - of - concept p+nn+ junction devices were fabricated in these chemically annealed materials, and they showed classical nanocrystalline Si solar cell type behavior. INTRODUCTION Chemical annealing technique, where one subjects very thin films of a-Si:H (2-3 nm) to various plasma treatments, and then grows a thicker film by doing multiple cycles of growth and annealing, has been used in the past to change the bandgap and H bonding of a-Si:H [1-4]. Chemical annealing of thin a-Si:H films has also been used to show that the amorphous films become more ordered when they are subjected to H plasma [5 ]. It has generally been assumed that one needs the reactive etching effect of H plasma in order to convert an amorphous film into a crystalline film, whether in layer-by-layer growth or by continuous deposition, where one achieves a similar effect by using high dilution ratios in hydrogen/silane mixtures. It is generally found that one needs a high dilution ratio (~20:1) to obtain good crystallinity in continuously grown Si films. However, recently we showed that at lower pressures, one can tolerate a lower hydrogen/silane ratio while still obtaining nanocrystalline films [6]. In this work, we carry the concept of controlled ion bombardment further, by using a non-reactive helium ion beam, to induce crystallinity in thin a-Si:H films, thus showing that it is not necessarily the reactive etching effect of hydrogen that is responsible for crystallinity.
A20.6.2
DESCRIPTION OF EXPERIMENT The film growth experiments were done in a low pressure, remote ECR reactor which has been described previously [7]. The film
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