Preparation of Bi_xSb_2-xTe₃ thermoelectric thin films by electrodeposition from basic solutions

Bismuth antimony telluride Bi_xSb_2-xTe₃ thermoelectric thin films were prepared by electrodeposition from alkaline solution adjusted to pH=9.0 in order to inhibit hydrogen from generating. NTA (nitrile-tri-acetic acid: C₆H₉NO₅) was used so as to increase solubility of bismuth in the alkaline solution. Electrochemical behaviors of Bi (III), Te(W), Sb( III) and their mixtures were firstly investigated by LSV (linear sweep voltammetry) for elucidation of electrochemical condition of diffusion controlled deposition. Then, Bi_xSb_2-xTe₃ electrochemical deposition experiments were conducted from the mixed solution under potential of diffusion controlled condition, i.e. -0.7 V, -0.8 V, and -0.9 V, where [Bi]/[Sb] ratio in solution was varied from 0 to 13 while keeping [Bi] + [Sb] =2 mmol/L, [Tel = 1.6 mmol/L, ENTA] = 0.1 mol/L and 333 K using rubber heater. Thermoelectric power properties such as semiconductor type, electric conductivity, Seebeck factor and power factor were measured and evaluated for the Bi_xSb_2-xTe₃. The experiment results revealed are as follows: (1) Bi_xSb_2-xTe₃ thin films can be obtained by electrochemical deposition at electric potential (-0.7-1.0 V) under limiting current density from alkaline solution including NTA, (2) composition of films can be controlled by the mixed solution composition as well as imposed electric potential, (3) the (015) crystalline face orientation becomes stronger when absolute value of imposed potential becomes smaller, and (4) n-type semiconducting thermoelectric thin films can be prepared when x is smaller than 0.6, and p-type thin films can be prepared when x is larger than 0.6.