Growth of high-quality InGaN thin films over whole In content range and evaluation of thermoelectric properties

Abstract

III-nitride semiconductors are emerging as promising thermoelectric (TE) materials when integrated with III-nitride optical and electronic devices for exhaust heat management. Among these, InGaN has been predicted to exhibit high thermoelectric performance. However, comprehensively evaluating the TE properties of InGaN across the full In content range is challenging owing to difficulties in crystal growth, especially at higher In contents. In this study, InGaN was grown over the entire In content range using radio frequency plasma-assisted molecular beam epitaxy. Optimizing the growth temperature and maintaining the flux relationship of In + Ga > N* > Ga enabled the growth of InGaN films with smooth surfaces, single In content, and c-axis orientation. A high-quality InN film was also achieved through droplet elimination by radical beam irradiation method, effectively eliminating In droplets on the growing surface. The surface flatness and crystal coherency of InGaN degraded in the mid-range of In content, reflecting the thermodynamic instability characteristic of immiscible alloy systems. With increasing In content, the absolute value of the Seebeck coefficient decreased, while electrical conductivity, Hall mobility, and carrier concentration increased. These trends were consistent with the previously reported calculated values. The discrepancies between experimental results and calculation are discussed in terms of carrier and phonon scattering, linked to crystal quality such as surface flatness, dislocation density, and impurity concentration. Although the maximum power factor was observed in InN, the highest dimensionless thermoelectric figure of merit (ZT) was obtained for In0.8Ga0.2N, attributable to the reduction in thermal conductivity (κ) through alloy scattering.