Electrochemical Deposition Of Bi₂Te₃-based Thermoelectric Thin Films

Thermoelectric materials belong to the family of functional material used for direct transformation of heat and electric energy. The majorities of these materials are semiconductors, and feature long-lasting performance, intense energy density and low maintenance in thermoelectric application. The prominent species used for 200-400 K range come from bismuth telluride based semiconductors. There is a variety of method in preparing these semiconductors, such as the convential Bridgeman method, thermal coevaporation, metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), pulsed laser deposition, solvo-thermol and powder metallurge, etc. Among these methods, electrochemical deposition (ECD) is a relatively new method developed in last two decades. It has economic advantage, ease for process control and feasibility for material nano structures.In this paper, the ECD of bismuth telluride based material was explored in three parts, or in three stages.The first was ECD in commonly used acidic electrolyte. In this part (Chapter III) pulsed electrodeposition was the focus of study. It was proved that high frequency rectangle pulse can render the as-deposited film a smooth morphology, as well as increase bismuth and/or antimony content. Pulse patterns, amplitude, and average deposition potential were also compared with regards to theirs effect on film composition and morphology. And their mutual relation and reactions was also disclosed via experiment results. A new method, frequency shifting mode (FSK), was firstly applied in ECD and a periodic change versus shifting time of film composition was found. This can has some latent application in preparing nano structures, such as multilayers or super lattices.The acid radicals present in acidic electrolyte were studied as per their effects on film composition and morphology. Cl-was found to be easily incorpated in film in acidic electrolyte, which has undefirable impact on film thermoelectrical performance. Other additives, such as levelers like formaldehyde, acetone, and Arabic gum as surfactant were also studied in similar methods.In general, bismuth telluride based films deposited in acidic electrolyte have two inevitable defects, i. e. impurity and poor film morphology. Though these defects can be lessened via pulsed deposition and addition of suitable additives, they stay as major obstacles in improving material performance. ECD in alkaline electrolyte was discussed in second part (Chapter IV). The alkaline electrolyte has only a few reports before and is a favorable substitution of acidic electrolyte. The low to medium deposition speed, less-corrosive electrolyte helps to obtain better film morphology and purity. A triethanolamine (TEA) based electrolyte was promoted for the first time. In order to increase film tellurium content, cationic polymer, such as diaminourea polymer (PUB) and polyaminosulfone (PAS) was introduced in the electrolyte. Fine morphorlogy was obtained via proper combination of these additives. PUB also brought an unexpected high Seebeck coefficient (532μ,V/K), whereas very low Seebeck coefficient (<50μV/K) was observed in films deposited without cationic polymer. Furthermore, some cationic polymers were synthesized and proved to have similar effects. Bi-Se-Te and Bi-Sb-Te alloy deposition was also tried in this electrolyte.Pulsed electrodeposition was also studied in this alkaline electrolyte. It was found to have reverse effect on film tellurium content compared to acidic electrolyte, which can be owe to the deposition dynamic mechanism difference of elecment ions in acidic and alkaline electrolyte. All the films deposited in this alkaline electrolyte revealed a near-amorphous crystals structure, which is common in ECD studies. Together with some invisible cracks to eyesight caused by inner tension, the as-deposited films has very low electric conductance. Besides, these films also have different thermoelectric behaviours with the bulk materials of the same family, which can be caused by the unknown thermaldynamically unstable nano structures formed at the relatively low electrolyte temperature (near room temperature).In order to further improve material thermoelectric performance, hot-uniaxial-press (HUP) was chosen to anneal the amorphous films while improving morphology via high pressure. This new method was decribed in detail in the third part (Chapter V). DSC analysis was used to identify film crystallization procedure, and HUP temperature's effect on film crystal structure and thermoelectric performance was presented in detail. HUP was proved to be effective in improving thermoelectric performance as well as film morphology. The power factor of as-pressed antimony tellurium film surpassed that of Bi-Sb-Te alloy film elaborated with pulsed electrodeposition. Provided a desirable ECD film morphology and composition, this method can be applied to various amorphous thermoelectric films.