Characterisation and application of weakly coordinating solvents for the electrodeposition of semiconductors

Abstract

Weakly coordinating solvents, such as dichloromethane, have been shown to be attractive for the electrodeposition of functional p-block alloy and compound semiconductors for application to electronic devices. Weakly coordinating solvents are of interest for the electrodeposition of p-block elements since they do not interact strongly with solute molecules and will not disrupt the speciation of labile p-block element complexes. Dichloromethane is commonly used as a weakly coordinating solvent but is volatile. In this thesis, alternative weakly coordinating solvents were identified and electrochemically characterised in order to improve understanding of electrochemistry in weakly coordinating solvents. They were then applied to the electrodeposition of metals and semiconductors at elevated temperatures to achieve electrodeposits with improved material properties. Weakly coordinating solvents can be defined and identified using solvent descriptors. Here, a set of solvent selection criteria were identified using Kamlet and Taft’s π∗, α and β parameters. Suitable solvents should be polar (π∗ ≥ 0.55), aprotic (α ≤ 0.2) and weakly coordinating (β ≤ 0.2). With this criteria, five candidate solvents were identified: trifluorotoluene, o-dichlorobenzene, p-fluorotoluene, chlorobenzene and 1,2-dichloroethane. The solvents were characterised and compared to dichloromethane with a suite of measurements including electrolyte potential window, conductivity, and double-layer capacitance, as well as the electrochemistry of the model redox couples decamethylferrocene and cobaltocenium hexafluorophosphate. The measurements indicated that ion pairing was a determining feature in weakly coordinating solvents. o-dichlorobenzene (oDCB) and 1,2-dichloroethane (DCE) were chosen as the most promising solvents for application to electrodeposition because of their polarity. In order to understand the nature of electrodeposition in oDCB and DCE, the electrochemistry of the metal precursors [NⁿBu₄ ][SbCl₄ ], [NⁿBu₄ ][BiCl₄ ] and [NⁿBu₄ ]2 [TeCl₆ ] were then studied using macro- and microelectrodes, and the electrochemical quartz crystal microbalance. The voltammograms indicated that the Sb^3+/0 and Bi^3+/0 broadly displayed simple deposition and stripping processes. Similarly the quartz crystal microbalance showed mass gain and ensuing mass loss with a high Faradaic efficiency. The microelectrodes were used to measure the redox potential and diffusion coefficient of the precursors. The electrochemistry of [NⁿBu₄ ]2 [TeCl₆ ] was found to be more complex. The deposition of Te appeared kinetically limited in DCE and there was also evidence to suggest the cathodic stripping of deposited Te to form Te^2–, along with subsequent chemical reactions. Sb, Bi and Te were successfully electrodeposited onto TiN substrates from oDCB and DCE. The resulting deposits were characterised using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Raman spectroscopy. It was found that Sb grew amorphously at room temperature, but that Bi and Te were crystalline. Further to this, the co-electrodeposition of antimony telluride and bismuth telluride onto TiN was attempted. Deposits were collected at several potentials and it was possible to electrodeposit stoichiometric Sb₂Te₃ from electrolytes containing 1.5 mM [NⁿBu₄ ][SbCl₄ ] and 3 mM [NⁿBu₄ ]2 [TeCl₆ ] in oDCB, and 1.75 mM [NⁿBu₄ ][SbCl₄ ] and 3 mM [NⁿBu₄ ]2 [TeCl₆ ] in DCE. Deposition of Bi₂Te₃ from both solvents was also successful, with baths composed of 2 mM [NⁿBu₄ ][BiCl₄ ] and 3 mM [NⁿBu₄ ]2 [TeCl₆ ] for oDCB, and 2.5 mM [NⁿBu₄ ][BiCl₄ ] and 3 mM [NⁿBu₄ ]2 [TeCl₆ ] for DCE. Characterisation of the deposits with scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction showed that the deposits were nanocrystalline. Finally, the effect of temperature on electrochemistry in oDCB and DCE was studied. Its influence was initially explored with measurements of the redox potential and diffusion coefficient of decamethylferrocene and cobaltocenium hexafluorophosphate as a function of temperature. Sb was then electrodeposited at various temperatures between 25 ^◦C and 140 ^◦C. Scanning electron microscopy indicated that the deposits became smoother and more uniform with increasing temperature. Additionally, X-ray diffraction and Raman spectroscopy showed that Sb deposited in a crystalline form at temperatures above 120 ^◦C, rather than amorphously at lower temperatures. Attempts at the electrodeposition of antimony telluride showed similar improvements in the properties of the deposit, but the composition was no longer stoichiometric.

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ORCID for Philip Bartlett: orcid.org/0000-0002-7300-6900

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