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The electrochemical reactivity and suitability of hexachlorodisilane and tetrakis(trichlorosilyl)silane as Si ec-LLS electrodeposition precursors in several electrolyte solutions have been investigated. Voltammetric data indicated that perchlorinated silanes exhibit mechanistically similar electrochemical responses as SiCl4, regardless of the Si–Si bond content in the precursor. The voltammetric responses were a strong function of the concentration of the precursor, indicating the participation of electrogenerated intermediates during the reduction and concomitant Si electrodeposition. Variation of the anion in the supporting electrolyte was found to be a critical factor for the thermal and chemical stability of the precursor bath. A combination of chronoamperometry and electron microscopy data were used to study the deposition efficiency specifically for hexachlorodisilane. The faradaic efficiency was low, regardless of overpotential or the composition of the electrolyte. Cumulatively, these data show that while larger chlorosilanes can be used for conventional Si electrodeposition over a wider range of conditions, their chemical instability and propensity for low faradaic efficiency limit their utility as reagents relative to SiCl4 for Si electrodeposition by ec-LLS.

Epitaxial films of Si have been prepared at room temperature by electrochemical liquid phase epitaxy (ec-LPE). Crystalline Si films were grown on both Si(111) and Si(100) substrates, demonstrating clear evidence of low-temperature homoepitaxy. The ec-LPE method was demonstrated as a hybrid approach that combined elements of conventional electrodeposition and traditional liquid phase epitaxy. Voltammetric and amperometric data were collected that indicated conditions where Si ec-LPE is possible with SiCl 4 in propylene carbonate electrolyte and eutectic gallium indium (e-GaIn) thin-film electrodes. Scanning electron micrographs, scanning transmission electron micrographs, and electron and X-ray diffraction data demonstrated that epitaxy extended throughout films that were several microns in thickness. Raman spectra, high-resolution X-ray diffraction data, time-of-flight secondary ion mass spectrometry, and energy-dispersive elemental mapping indicated that the asprepared films were uniformly hyperdoped with Ga at >10 at %. These cumulative results demonstrate a distinct new way to realize crystalline Si films suitable for optoelectronic applications.

The electrochemical reduction and concomitant suitability of tetraisocyanatosilane, Si(NCO)4, as a precursor for Si electrodeposition has been examined. At T = 25°C, voltammetric data indicated that the electroreductive pathway of Si(NCO)4 paralleled the electroreduction of SiCl4, albeit with attenuated current densities. At T = 150°C, the measured current density increased significantly due to coupled reaction likely involving an electropolymerization of the tetraisocyanato ligand. At either temperature, the electrochemical response of Si(NCO)4 did not strongly depend upon the identity of the anion in the supporting electrolyte. A combination of chronoamperometric, electron microscopy, and X-ray photoelectron spectroscopy data were used to study the morphology and composition of the electrodeposited Si films at room temperature. The composition and morphology of the as-electrodeposited films were consistent with the formation of amorphous Si when using solid n+-Si electrodes, similar to what is observed during the electroreduction of SiCl4. Attempts to use Si(NCO)4 for Si crystal growth by the electrochemical liquid-liquid-solid (ec-LLS) process with a liquid e¬-GaIn electrode resulted in no appreciable crystalline Si product. Although Si(NCO)4 can be used for electrodeposition of amorphous Si onto solid electrodes at room temperature, more work is needed to facilitate its use in ec-LLS

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