As new materials emerge, thermal conductivity measurements will need to be developed to handle materials with a small size and high thermal conductivity (k). A joint research group has developed two new optical techniques — photoluminescence mapping (PL-mapping) and time-domain thermo-photoluminescence (TDTP) — that provide rapid, nondestructive characterization of k in materials and that require minimal sample preparation. PL-mapping and TDTP could provide new tools for further development of semiconductor-based high-k materials, according to the team of scientists from Baylor University, the University of Electronic Science and Technology of China, the University of Houston, and Sichuan University. Schematic of time-domain thermo-photoluminescence. The temperature-dependent photoluminescence is used to directly measure the temperature of the sample. Courtesy of S. Yue et al., “Photoluminescence mapping and time-domain thermo-photoluminescence for rapid imaging and measurement of thermal conductivity of boron arsenide,” doi: 10.1016/j.mtphys.2020.100194. PL-mapping provides nearly real-time imaging of crystal quality and k over mm-size crystal surfaces. TDTP allows researchers to pick up any spot on the sample surface and measure its k using nanosecond laser pulses. The team’s nondestructive k measurement method and rapid k screening technique require no sample preparation and have no size limitation. Currently, the most common optical technique for measuring thermal conductivity is time-domain thermo-reflectance (TDTR), in which a femtosecond pump pulse is used to heat up the sample through a metal film, and a probe pulse with variable delay is used to probe the dynamics of the temperature. The metal film, which is used as a thermal transducer, is coated on the surface of the material to be measured. In addition to requiring expensive femtosecond lasers, the TDTR technique is invasive and requires careful sample preparation. In the TDTP approach, the researchers use a nanosecond pump pulse to heat a semiconductor as in TDTR, and obtain the temperature dynamics from the photoluminescence of the probe pulse at variable delay. No metal film is used. Temperature-dependent photoluminescence is used to directly measure the sample temperature. PL-mapping of a boron arsenide (BAs) sample. An optical image (a) and the PL-mapping (b). The selected spots P1-3 are marked by “+” along with their k measured with time-domain thermo-photoluminescence. The unit is W/m·K. Courtesy of S. Yue et al., “Photoluminescence mapping and time-domain thermo-photoluminescence for rapid imaging and measurement of thermal conductivity of boron arsenide,” doi: 10.1016/j.mtphys.2020.100194. The researchers demonstrated their techniques on a boron arsenide crystal, a material with k that is close to diamond and several times higher than silver. Using their new techniques, the researchers experimentally established 1.82 eV as the indirect bandgap of boron arsenide and observed room-temperature band-edge photoluminescence. PL-mapping provided a clear picture of thermal conductivity k over the whole crystal. TDTP allowed any spot on the mapped surface of the crystal to be arbitrarily picked up and its k obtained. According to the researchers, PL-mapping only takes seconds, and thus it provides a fast screening of a semiconductor’s thermal conductivity. The research was published in Materials Today Physics (www.doi.org/10.1016/j.mtphys.2020.100194).