Low Temperature Electrodeposition of Epitaxial Silicon Thin Films

Abstract

This thesis explores the applicability of Si electrodeposition to the synthesis of crystalline Si at low temperatures. Electrodeposition has been long perceived as an ideal route for the synthesis of semiconductor materials owing to the technical simplicity, scalability and intrinsically low cost of the method. However, to date, Si electrodeposition has found limited success for crystalline Si synthesis. Recently, a new method, called the electrochemical liquid liquid solid (ec-LLS) strategy, has been shown to synthesize crystalline Si at low temperatures by electrodeposition with SiCl4 at liquid metal electrodes. While crystalline Si growth by ec-LLS was demonstrated, there was deemed an immutable temporal limitation to the electrolyte bath lifetime. Accordingly, the expansion of the ec-LLS method for crystalline Si film synthesis was hypothesized to require a better optimized Si electrodeposition electrolyte bath. On this front, this thesis has two major advances. First, two previously unexplored molecular classes of silanes are explored as precursors for Si electrodeposition. Here, the electrochemical activity of the perchlorinated oligochlorosilanes hexachlorodisilane and tetrakis(trichlorosilyl)silane, which contain the Si-Si bonding moiety, and tetraisocyanatosilane, which contains the pseduohalide NCO- ligand, were investigated in comparison to tetrachlorosilane as a function of electrolyte composition and bath temperature. For both the oligochlorosilanes and tetraisocyanatosilane the voltammetric responses were similar to tetrachlorosilane, suggesting that at ambient temperatures the electroreductive mechanism proceeds through a similar route. Following amperometric deposition of all of these precursors at ambient temperature, amorphous Si was obtained. Despite their utility under ambient conditions, the chemical stability and electrochemical activity of both precursor classes were significantly attenuated at elevated temperatures, demonstrating the limited thermal range of their use therein. Further, when electrodeposition of these precursors at Ga-based electrodes was performed, under no conditions explored was the synthesis of crystalline Si observed. Cumulatively, while both precursor classes may be useful for Si electrodeposition at ambient conditions, further work is required to apply them for the low temperature synthesis of crystalline Si by ec-LLS. Second, the application of Si electrodeposition to the electrochemical Liquid Phase Epitaxy (ec-LPE) method, a macroscopic thin film variant of ec-LLS, was demonstrated for the synthesis of thick epitaxial Si films at ambient temperature. Previously, using electrolyte baths containing SiCl4 ec-LPE could be used to grow epitaxial Si thin films limited to a few hundred nanometers in thickness before the inevitable cessation of further growth. Here, the electrodeposition waveform was demonstrated to be critical to both the electrolyte bath lifetime and also for the general synthesis of thick films. Specifically, using a pulsed electrodeposition waveform in which a brief pulse generating Si0 was followed by a longer pulse at a potential absenting any redox process, the electrodeposition of Si was shown to be sustained for hours rather than minutes. The presented electron microscopy, electron diffraction, and x-ray diffraction data demonstrate that even down to T = 23 ºC, the synthesis of thick epitaxial Si films by ec-LPE is possible. Further efforts discussing methods for optimizing materials quality in Si ec-LPE and the application of as-grown thin films are discussed, with final conclusions on the cumulative work shown here.