稲富 裕光

Materials with enhanced electron and reduced phonon transport properties are preferred for thermoelectric applications. The defect engineering process can optimize the interrelated electron and phonon transport properties to enhance thermoelectric performance. As the influence of various crystalline defects on the functional properties of materials is diverse, it is crucial to scale, optimize, and understand them experimentally. With this perspective, crystalline defects in InGaSb ternary alloys were engineered and their influence on the thermoelectric properties was studied experimentally. Crystalline defects such as point defects, dislocations, and compositional segregations were induced in In0.95Ga0.05Sb crystals by the addition of excess constituent elements, In, Ga, or Sb. The addition of excess Ga increased point defects, whereas excess Sb reduced dislocation densities. The thermoelectric figure of merit value (ZT) of In0.95Ga0.05Sb+Ga0.02 was recorded to be 0.87 at 573 K, which is the highest among other reported values of III-V semiconductors. The collective interactions of compositional segregations, point defects, and dislocations with electrons and phonons enhanced the ZT in this study.

Thermoelectric materials with optimum carrier concentration of the order of 1019–1020/cm3 are required to obtain a high figure of merit (ZT) value. As undoped In0.8Ga0.2Sb has a lower carrier concentration (~1016/cm3), Te impurity was doped between low (1 × 1018/cm3) and high level (1 x 1021/cm3) to understand the effects of doping on its thermoelectric properties. The undoped and Te-doped In0.8Ga0.2Sb crystals retained cubic zinc blende crystal structure irrespective of heavy doping of Te element. In addition to the optical phonon vibrational modes, acoustic phonon modes were also present when the doping concentration exceeded 1 × 1018/cm3. The carrier concentration in Te-doped In0.8Ga0.2Sb crystals were varied in the range 1018–1020/cm3. Te-doped In0.8Ga0.2Sb with concentration 1 × 1018/cm3 was recorded a higher power factor because of its lower resistivity and higher mobility than other crystals. The ZT of Te-doped In0.8Ga0.2Sb (1 × 1018/cm3) was higher than other samples at 300–450 K. This study revealed that the optimum Te dopant concentration to enhance the ZT value of InxGa1−xSb is 1 x 1018/cm3 for optimizing its properties toward mid-temperature thermoelectric applications.

Thermoelectric devices require p-type and n-type semiconductors with similar chemical, mechanical and thermoelectric properties to achieve maximum efficiency. To match with n-type In0.95Ga0.05Sb crystals for the fabrication of thermoelectric device, zinc (Zn) element was doped with In0.95Ga0.05Sb crystal intentionally to change its conductivity from n-type to p-type and its thermoelectric properties were studied. The Zn-doped In0.95Ga0.05Sb crystals grown by directional solidification were free from micro-cracks and their composition was distributed homogeneously. The carrier concentration was increased upon doping with Zn element. The resistivity of Zn-doped In0.95Ga0.05Sb increased with increasing temperature that showed degenerate semiconducting characteristics resulted from heavy doping. The Peierls distortion resulting from Sb–Sb interaction was observed in Zn-doped In0.95Ga0.05Sb crystals. The higher electron contribution and lower phonon contribution to total thermal conductivity were obtained in Zn-doped In0.95Ga0.05Sb than undoped crystals. The maximum ZT of 0.24 at 573 K was achieved by Zn-doped In0.95Ga0.05Sb with dopant concentration 1 × 1020 atoms/cm3. The ZT achieved is the highest among other reported values of p-type III–V semiconductors.