Effect of hot-press treatment on electrochemically deposited antimony telluride film

Abstract

Antimony telluride thin film electrochemically deposited in a triethanol based alkaline electrolyte features amorphous structure, high electrical resistance, as well as fine morphology, minor impurity incorporation and anti-corrosivity. To further improve film thermoelectric performance, this film was subjected to hot-uniaxial-press (HUP) treatment at 170–250 °C. HUP treated films revealed crystallized structures, and exhibited 2–3 orders of magnitude improvement of electrical conductance. The [TeO₃²⁻]/[SbO₂⁻] of the deposition electrolyte was utilized to fine tune film composition and thermal electrical performance. Ni diffusion from the substrate into the film was also studied, and it can be reduced by using lower temperature and shorter time of HUP treatment. Film Seebeck coefficient and power factor reached 138 μV/K and 337 μW/K²∙m, respectively, at elaborated deposition and HUP conditions.

Introduction

Thermoelectricity is used for the direct energy conversion between heat and electricity, and thermoelectric semiconductors are efficient materials for thermoelectricity applications. Thermoelectric devices made of thermoelectric semiconductors can be used over a wide temperature range as solid-state coolers, power generators, and sensors. Compared to traditional cooling techniques, thermoelectric refrigeration has received substantial attention due to its high cooling power density, fast response, long lasting working without maintenance. Since the invention of thermoelectric semiconductors in the 1950s, there is a persistent pursuit for higher efficiency materials. Beginning in the 1990s, nanotechnology has brought new approaches for improving thermoelectric properties [1], such as super-lattice [2], [3], powder metallurgy [4], [5], hydrothermal synthesis [6], [7], and electrochemical atomic layer epitaxy (ECALE) [8], [9]. Electrochemical deposition (ECD) features cost-effective, high-throughput, ease for process control, and nanostructure feasibility. As bismuth telluride based materials remain the most efficient among various optional materials in the temperature range from − 70 to130 °C, a variety of ECD researches of this family have been published [10], [11], [12], [13], [14], [15], [16]. However, low electrical conductance common in ECD films caused by poor crystal structure and defective morphology have hold back all the efforts for commercial application, even if annealing has been performed [17], [18] on these films. ECD in alkaline electrolyte is another option to improve film morphology [15]. We have developed a triethanolamine (TEA)-based alkaline electrolyte to deposit antimony telluride film, with delicate morphology and high Seebeck coefficient. The as-deposited film had a near-amorphous structure and high electrical resistance between 5000 and 20,000 μΩ∙m. Normal annealing in vacuum of these films showed no improvement on electrical conductance, due to the fact that film cracks cannot be re-patched during annealing. We have noticed that hot-uniaxial-press (HUP) is commonly applied to make bulk material from ball-milled powders. Besides, the as-deposited film had minor impurity incorporation, and was free of the corrosive residue that is generally unavoidable in films deposited in acidic electrolytes. Thus a proper combination of temperature and pressure in HUP would promote the crystallization while “patching” up morphological defects of ECD films. In this paper, HUP of electrochemically deposited antimony telluride film was presented. The aim of this work is to explore another way different from conventional annealing, which can be more effective in improving crystal structure as well as morphology, and bring a significant enhancement of film thermoelectric performance.