Characterization and fabrication of Mg2Ge thin film for thermoelectric conversion at moderate-temperature

From the point of view of environmental concerns as well as the decline of energy resource reserves, finding alternative energy resources has been considered as one of the most critical tasks in the modern days. The conventional heat engines and energy-intensive process industries produce numerous amount of waste heat to the atmosphere. To make use of this wasted heat, thermoelectric (TE) technology plays an important role as it enables the conversion of thermal energy directly into electrical energy or vice versa. Intermetallic compound of Mg2Ge has attracted attention as p-type moderate-temperature thermoelements for power-generation applications due to its superior TE properties and its environmentally friendly constituents in nature. Efficiency, however, is still the major limiting factor for its widespread use. The essential step towards achieving good TE properties of this material is to explore a reliable process for sample preparation. In this dissertation, two techniques are demonstrated: 1) d.c. magnetron sputtering, and 2) pulsed laser deposition (PLD), which successfully fabricate high quality single-phase polycrystalline Mg2Ge thin films. Furthermore, p-type Ag- and n-type Sb-doping are incorporated into Mg2Ge matrix via thermal diffusion and ion-implantation processes, respectively. The main focus of this dissertation is to investigate the correlation of TE performance (i.e. Seebeck coefficient, electrical transport properties, etc.) and optical bandgap properties with microstructures (i.e. grain size and strain mismatch, etc.) and materials processing techniques (i.e. deposition temperature and film thickness). In the sputtering process, it was found that stoichiometirc Mg2Ge thin films were formed at a depositing temperature window of Ts = 400 – 600 ˚C which exhibited a superior TE properties, whilst stoichiometric Mg2Ge thin films with different crystallinity were also obtained by PLD within a substrate temperatures of 300 – 600 ˚C. Furthermore, a threefold increase in power factor was observed in the highly Ag-doped sample by modifying its carrier concentration as compared to those of un-doped samples. Another focus of this dissertation is to study the correlation of bandgap energy with intrinsic and extrinsic defect in the Mg2Ge system. It is found that the bandgap structure of this material can be tailored through various approaches, including: 1) precisely controlling of its nanostructure to induce quantum confinement effect, 2) defect engineering through varying lattice strain and intrinsic carrier concentration, and 3) introducing density of impurity state, thus shifting Fermi energy level. As a result, it can become potential candidates for optical applications and be developed into a new class of prospective TE materials