Yi Shuang1,Yuji Sutou1
Tohoku University1
Thermoelectric properties have been widely studied in bulk materials compared to thin films. However, in some specific applications, such as flexible temperature sensors, small-scale power generation applications and on-chip energy harvesting, thermoelectric thin films are playing a significant role. Traditional thermoelectric thin films showing a high ZT value such as tellurides are limited for the applications at elevated temperatures due to their poor stability, low corrosion resistance and toxicity.<sup>[1,2]</sup> Therefore, in the recent years, the transition-metal nitrides and rare-earth nitrides have been developed as a new class of thermoelectric thin films due to their outstanding mechanical properties, relatively low thermal conductivity and a variable range of electrical properties. Among them, chromium nitride (CrN) exhibiting <i>n</i>-type conduction is one of the most promising candidates as they can show a low electrical resistivity together with a large absolute value of Seebeck coefficient. Besides, the high melting point of CrN makes it possible to be used in high-temperature thermoelectric applications. To practically use the <i>n</i>-type CrN into thermoelectric applications, <i>p</i>-type materials are also needed. The best choice of <i>p</i>-type materials is the material with similar crystal structure and thermal properties such as the thermal expansion. In addition, intermixing of atoms or chemical reactions should be avoided between <i>p</i> and <i>n</i> layers. Therefore, it is an ideal way to develop one thermoelectric material with both <i>n-</i> and <i>p</i>-type conducting properties. For example, <i>p</i>-type CrN can be made by alloying with a small amount of Al into <i>n</i>-type CrN.<sup>[3]</sup> The alloyed Al element can behave as an electron acceptor that reduces the carrier concentrations of pure CrN and leads to a <i>p</i>-type conducting eventually. Febvrier et al. also proposed a <i>p</i>-type CrN by controlling the stoichiometry without the need for dopants deposited by DC magnetron sputtering at 600 °C. They demonstrated that p-type conduction can be attributed from Cr vacancies, which pushes the Fermi level down in the valence band.<sup>[4]</sup> In this study, we proposed a much simpler method to convert the semiconducting type of CrN by introducing oxygen dopants into the CrN film using a radiofrequency (RF) magnetron sputtering at room temperature. By fixing the chamber base pressure to be 10<sup>-5</sup> Pa and varying the N<sub>2</sub> gas flow rate (<i>f</i><sub>N2</sub>) during reactive sputtering, the oxygen content in the CrN films can be controlled from 6.8 to 12.4 at. %. The <i>n</i>-type CrN can be successfully converted into <i>p</i>-type by tuning the oxygen content. The crystal structure, electrical property and thermoelectric property of the obtained CrN<sub>x</sub>O<sub>y</sub> thin films were investigated and discussed. The present method to tune the <i>p-n</i> type of CrN<sub>x</sub>O<sub>y</sub> films is simple and reproducible, which could contribute to a <i>p-n</i> homo-junctions enabling a better application in not only thermoelectric devices, but also wider fields such as photodetectors, selector devices etc. What is more, the room temperature growth of thin film would be beneficial for the flexible and wearable energy-harvesting, and compatible to the MEMS processing.<br/>[1] P. Mannu, M. Palanisamy, G. Bangaru, S. Ramakrishnan, R. Meena, C. L. Dong, A. Kandasami, <i>Appl. Surf. Sci.</i> <b>2020</b>, <i>505</i>, 144115.<br/>[2] R. Lan, S. L. Otoo, P. Yuan, P. Wang, Y. Yuan, X. Jiang, <i>Appl. Surf. Sci.</i> <b>2020</b>, <i>507</i>, 145025.<br/>[3] A. Le Febvrier, N. Van Nong, G. Abadias, P. Eklund, <i>Appl. Phys. Express</i> <b>2018</b>, <i>11</i>, 051003.<br/>[4] A. le Febvrier, D. Gambino, F. Giovannelli, B. Bakhit, S. Hurand, G. Abadias, B. Alling, P. Eklund, <i>Phys. Rev. B</i> <b>2022</b>, <i>105</i>, 104108.