Taking the temperature of terahertz devices reveals heating causes

One promising pathway to practical terahertz (THz) technology involves intrinsic Josephson junction (IJJ) mesa devices, where a chip in which various areas are etched down to produce layers or “mesas,” made with Bi2212, a layered high-transition-temperature T_c superconductor. But their operation can be compromised by Joule heating effects. An article in the Journal of Applied Physics reports on a thermoreflectance microscopy (TRM) technique to measure surface temperature characteristics of IJJ-THz emitters with greater sensitivity, versatility and efficiency than previous methods.

CCD-based TRM is a noncontact technique to observe thermal performance and characteristics in microelectronics at high resolution. The experimenters used it to study several IJJ Bi2212 mesa structures. Under the application of voltage, they observed the formation of hot spots on the mesa surfaces, seen in IVC (current-voltage characteristics) images. In all cases, Joule heating appears to arise primarily from contact resistance.

This TRM technique measures Joule heating quickly and with great sensitivity and resolution, and unlike the generally used photoluminescence methods, requires no special premeasurement treatment of mesa surfaces. These qualities make it convenient for quality-checking electrical contacts and material defects in these devices. The authors believe this work may represent the first time TRM has been used in superconducting devices. They plan to expand their method to measure and study the direct correlation between temperature and THz emissions.

Source: “Thermoreflectance microscopy measurements of the Joule heating characteristics of high-T_c superconducting terahertz emitters,” by Takanari Kashiwagi, Taiga Tanaka, Chiharu Watanabe, Hiroyuki Kubo, Yuki Komori, Takumi Yuasa, Yuki Tanabe, Ryusei Ota, Genki Kuwano, Kento Nakamura, Manabu Tsujimoto, Hidetoshi Minami, Takashi Yamamoto, Richard A. Klemm, and Kazuo Kadowaki, Journal of Applied Physics (2017). The article can be accessed at https://doi.org/10.1063/1.5002743 .