Time-domain and Frequency-domain Thermoreflectance (TDTR, FDTR):
TDTR and FDTR are state-of-the-art photo-thermal metrology techniques for characterizing energy diffusion via various carriers both within materials and across material interfaces. TDTR utilizes femtosecond laser pulses to interrogate the temporal diffusion of heat from the surface, down through the layer(s) beneath over a timescale from single picoseconds (ps) out to 6 nanoseconds (ns). This range of timescales encompasses a variety of different carrier relaxation/scattering processes, including electron-phonon (1-5 ps) and phonon-dominated (100 ps to 6 ns), enabling thorough characterization of the energy transport landscape. FDTR utilizes continuous-wave (CW) lasers and high-frequency electro-optic modulators to interrogate the frequency-dependent thermal diffusion of heat through a nanostructure/material of interest. FDTR is a comparatively simpler technique than TDTR with regards to laser alignment, enabling the additional capability of scanning thermometry to map out the thermal properties of a material. Both techniques offer depth resolution down to 10’s of nm, while also being able to measure the thermal properties of bulk materials.
Nanosecond Transient Themoreflectance:
Nanosecond transient thermoreflectance is an important technique for measuring the thermal transport properties across deeply buried interfaces in semiconductor material system, such as those often encountered for epitaxially-grown films deposited on bulk substrates. The thicknesses of these epi-films can often reach the 2 – 10 μm range or even thicker, depending on the eventual application. In situations where the epi-layer itself presents a large thermal resistance due to factors such as dislocations or other crystal defects, degenerate-doping levels of impurity atoms or exotic nanostructuring in the form of superlatticies, the thermal interface resistance where the epi-layer meets the bulk crystal can be difficult, if not impossible to measure with TDTR. The nanosecond transient system uses nanosecond-width pulses and much longer time-delay windows to interrogate transport processes at longer diffusion times than the 6 ns window offered by TDTR. This system fills an important niche regarding the measurement of these buried interfaces which can often exhibit significant thermal resistances.
3-Omega (3ω) Thermometry:
The 3ω technique is an electrically-based thermometry technique used to characterize the thermal transport properties of micron-scale thin films, up to bulk materials. The technique utilizes a patterned metal structure on the top surface of a material as both the heater and thermometer, depositing energy at the sample surface through joule heating at a known frequency, ω, and employing phase-sensitive (lock-in) detection to monitor the change in resistance of the metal line due to the oscillating heating event at the third harmonic of the heating frequency, 3ω. By sweeping ω across a specific frequency range, the thermal properties of the material can be extracted. The 3ω technique is particularly useful for measuring materials that do not have the specularly/optically-smooth top surface required for the laser-based thermometry techniques.
Photo Gallery of Prior Experimental Setups:
Time-domain thermoreflectance (TDTR) at the University of Virginia:
Thermal switching over temperature measured via TDTR with applied electrical bias in a Janis cryogenic probe station:
Frequency-domain thermoreflectance (FDTR) system built at Penn State in October 2017:
FDTR (left half) and TDTR (right half) on a single table at Georgia Tech:
Electrical probing during TDTR at Georgia Tech:
Thermal measurements of transistor operating temperature via gate-resistance thermometry (GRT) at Georgia Tech: