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/T2 temperature dependence indicates dominant 4-phonon scattering. Nanosecond tl-TDTR

thus provides a rapid, nondestructive route to assess semiconductor thermal conductivity.