Dirch Hjorth Petersen

Anisotropic heat–conducting materials play crucial roles in designing electronic, optoelectronic, and thermoelectric devices, where temperature and thermal stress are important. Despite substantial research efforts, a major obstacle to determining the anisotropic thermal diffusivity tensor in polycrystalline systems is the need for a robust, direct, and nondestructive technique to distinguish between distinct thermal diffusivities. Here, we demonstrate a conceptually unique thermal diffusivity microscope capable of performing high-resolution local measurements of anisotropic thermal diffusivity. The microscope features a unique micro four-point probe for fast, nondestructive scanning without calibration or extra sample preparation. It measures anisotropic thermal diffusivity based on thermal delay from a single heater. Through a series of experiments, we demonstrate that the anisotropy of the measured thermal diffusivity correlates excellently with the crystallographic direction of prototypical Bi ₂ Te ₃ . The anisotropic heat transport shows that the lattice contribution dominates the heat transport for both in- and out-of-plane directions.

The micro four-point probe (M4PP) technique has become a well-established method for characterizing the electrical properties of materials. However, extra attention must be paid when measuring the resistivity of thermoelectric materials due to the possibility of an additional Seebeck voltage. This issue vanishes when measuring at a sufficiently high frequency, but the threshold frequency is substantial due to the small separation between the pins of the probes. Typical M4PP measurements are far from reaching this frequency, and their accuracy on thermoelectric materials is severely compromised. In this work, we explain the experimental conditions needed to measure reliably the electrical conductivity of thermoelectric materials and present a new method for measuring this property that reduces the frequency requirements by two orders of magnitude. The method is proven using two skutterudites and bismuth telluride material. It is also found that the resistance overestimation in the bismuth telluride sample is larger than that in the skutterudites due to its superior thermoelectric properties; the overestimate reaching 35%. The advances reported here enable the M4PP technique to be used for the measurement of the electrical conductivity of thermoelectric materials.

Micro four-point probing is a branch of electrical metrology where electrical (and electromagnetic) properties of charge carriers such as conductance, mobility, and tunneling magnetoresistance can be accurately and precisely determined at the μm scale and below. Here, we propose and demonstrate a novel application of micro four-point probe (M4PP) aimed at quantifying the thermoelectric properties of a sample. Specifically, we show that for an AC current passing through a bulk material at a low angular frequency ω, the voltage drop across the sensing electrodes at 2ω is to first order proportional to the ratio (α/κ) of its Seebeck coefficient (α) to its thermal conductivity (κ). Verified by numerical simulations, our analytic theory is then put into practice on a suite of p- and n-type bulk semiconductors (Si, Ge, and BiTe). The M4PP estimates of the Seebeck coefficient in these materials are characterized both by high accuracy and precision, suggesting a novel in-situ metrology of thermoelectric properties at the µm scale.

Electrical four-terminal sensing at (sub-)micrometer scales enables the characterization of key electromagnetic properties within the semiconductor industry, including materials’ resistivity, Hall mobility/carrier density, and magnetoresistance. However, as devices’ critical dimensions continue to shrink, significant over/underestimation of properties due to a by-product Joule heating of the probed volume becomes increasingly common. Here, we demonstrate how self-heating effects can be quantified and compensated for via 3ω signals to yield zero-current transfer resistance. Under further assumptions, these signals can be used to characterize selected thermal properties of the probed volume, such as the temperature coefficient of resistance and/or the Seebeck coefficient.

Pyrolytic carbon microelectrodes (PCMEs) are a promising alternative to their conventional metallic counterparts for various applications. Thus, methods for the simple and inexpensive patterning of PCMEs are highly sought after. Here, we demonstrate the fabrication of PCMEs through the selective pyrolysis of SU-8 photoresist by irradiation with a low-power, 806 nm, continuous wave, semiconductor-diode laser. The SU-8 was modified by adding Pro-Jet 800NP (FujiFilm) in order to ensure absorbance in the 800 nm range. The SU-8 precursor with absorber was successfully converted into pyrolytic carbon upon laser irradiation, which was not possible without an absorber. We demonstrated that the local laser pyrolysis (LLP) process in an inert nitrogen atmosphere with higher laser power and lower scan speed resulted in higher electrical conductance. The maximum conductivity achieved for a laser-pyrolyzed line was 14.2 ± 3.3 S/cm, with a line width and thickness of 28.3 ± 2.9 µm and 6.0 ± 1.0 µm, respectively, while the narrowest conductive line was just 13.5 ± 0.4 µm wide and 4.9 ± 0.5 µm thick. The LLP process seemed to be self-limiting, as multiple repetitive laser scans did not alter the properties of the carbonized lines. The direct laser writing of adjacent lines with an insulating gap down to ≤5 µm was achieved. Finally, multiple lines were seamlessly joined and intersected, enabling the writing of more complex designs with branching electrodes and the porosity of the carbon lines could be controlled by the scan speed.