Thermal Conductivity above 2,000 W/m·K in Boron Arsenide by Nanosecond Transducer-Less Time-Domain Thermoreflectance

Time-domain thermoreflectance (TDTR) has been a standard technique for measuring thermal conductivity (κ) for more than 3 decades, yet its reliance on femtosecond lasers and metal transducers has limited its broader adoption in the materials community. Recent attempts to eliminate the metal layer have achieved partial success but have been hampered by dominant reflectance from photoexcited carriers, arising from the continued use of femtosecond pump and 800-nm probe pulses. Here, we introduce a nanosecond transducer-less TDTR (tl-TDTR) method that overcomes this challenge. Using ~80-ns pump pulses and a 450-nm continuous-wave probe, we suppress carrier-induced negative transients, yielding positive signals characteristic of pure thermoreflectance. Thermal conductivity is extracted via heat transport simulations and direct time-domain curve fitting. The method is validated on benchmark semiconductors (Si, Ge, InP) and cross-checked on Si and diamond using an Al-film transducer. Applied to cubic boron arsenide crystals, the technique reveals room-temperature κ exceeding 2,000 W/m·K-comparable to single-crystal diamond-and confirmed by traditional TDTR on the same samples. Raman, photoluminescence (PL), and PL lifetime measurements indicate high crystal quality. Sub-10-ns lifetimes remain shorter than expected for an indirect bandgap semiconductor, suggesting headroom for further κ improvement. The observed ~1/T ² temperature dependence indicates dominant 4-phonon scattering. Nanosecond tl-TDTR thus provides a rapid, nondestructive route to assess semiconductor thermal conductivity.