Research Group Develops Thermo-Photoluminescence for Rapid Imaging and Measurement

Today, the most common optical technique for imaging and measurement of thermal conductivity of semiconductors is time-domain thermoreflectance (TDTR), in which a femtosecond pump pulse is used to heat up the sample through a metal film. While widely adopted, the difficulties are well known. Besides the expensive femtosecond lasers, the samples have to be polished to provide a clean, flat surface for a controlled thermal contact with the metal film, so TDTR is invasive and requires careful sample preparation.

To overcome these shortcomings, a joint research group from University of Electronic Science and Technology of China, University of Houston, Sichuan University, and Baylor University developed a nondestructive k measurement method and a rapid k screening technique that require no sample preparation and have no size limitation. Figure 1 shows the schematic of optical setup. A nanosecond pump pulse is employed to heat a semiconductor as in TDTR, while the temperature dynamic is obtained from the photoluminescence of the probe pulse at variable delay. Here no metal film is used, and the temperature-dependent photoluminescence is used to directly measure the sample temperature, so the technique is called time-domain thermophotoluminescence (TDTP).

Figure 1. Schematic of time-domain thermophotoluminescence.

Materials with high thermal conductivity, especially semiconducting materials, will find a wide range of applications in electronics and optoelectronic devices because they not only act as an efficient heat sink, but also provide more integrated functionalities. As new materials emerge, new thermal conductivity measurements should be developed to handle materials with a small size and a high thermal conductivity k. Unlike a simple temperature sensor, any k measurement techniques must combine the function of a heater and a thermometer because k is determined from the amount of heat dissipated through space and time in the material. Conventional techniques use an electrical heater to generate a specific amount of heat and contact thermometers such as thermal couples to measure temperatures, and as such, they are only good for bulk materials. Optical methods have the potential for noncontact and more accurate measurement.

Photoluminescence (PL) can also reveal the quality of a semiconductor: Higher crystal quality and lower defect will result in stronger PL. Coincidentally, the exact same condition will lead to a higher thermal conductivity because of longer mean free path of phonons. Thus, PL can directly reflect a semiconductor’s thermal conductivity. Figure 2 shows an optical image and PL-mapping of a boron arsenide crystal, which has k only next to diamond and several times higher than that of silver. The nonuniform PL-mapping reveals nonuniform k even for an apparent single crystal. The nonuniform k is further confirmed by TDTP from three representative spots in the same figure. PL-mapping takes only seconds, and thus it provides a fast screening of a semiconductor’s thermal conductivity. PL-mapping and TDTP provide new tools for further development of semiconductor-based high-k materials.

Figure 2. PL-mapping of a BAs sample. An optical image (a) and the PL-mapping (b). The selected spots P1-3 are marked by “+” along with their κ measured with time-domain thermophotoluminescence.