Thermal Conductivity above 2,000 W/m·K in Boron Arsenide by Nanosecond Transducer-Less Time-Domain Thermoreflectance
Oral-In-person
Abstract
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.
(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.
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Presenters
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Jiming Bao
- University of Houston