Selective Thermal Mapping of Wide and Ultrawide Band Gap Semiconductor Devices Using Mechanically Exfoliated MoS₂ Flakes

This work demonstrates a method for 2D thermal mapping of the surface temperature rise of semiconductor devices using thermoreflectance imaging of mechanically exfoliated MoS2 flakes. Thickness-dependent thermoreflectance spectra were acquired, and the A, B, and C excitons of MoS2 were shown to have high thermoreflectance responses. The A exciton, corresponding to a probing wavelength of 675 nm, had the highest thermoreflectance response, but temperature rise (ΔT) measurements using this wavelength can be prone to nonlinear effects; nevertheless, these effects can be accounted for through calibration. Additionally, the high thermoreflectance response enables high measurement sensitivity for devices with low ΔT (e.g., <10 K), such as diamond-based electronics. The C exciton (probing wavelength of 470 nm) yielded a linear response over a larger ΔT range due to the broader energy band and inherently smaller wavelength shift per unit energy in this spectral region. Based on the findings in this work, the desired MoS2 flake thickness to optimize sensitivity for thermal measurements is <30 nm. The method was validated by comparing the surface temperature rise measured from a MoS2 flake, using 470 and 675 nm probing wavelengths, and directly from the semiconductor channel of a GaN high electron mobility transistor, using a 365 nm probing wavelength. To demonstrate the viability for thermometry of ultrawide band gap (UWBG) semiconductor devices, the method was applied to perform 2D mapping of the temperature rise of the semiconductor channel of a UWBG β-Ga2O3 heterostructure field effect transistor. Ultimately, this work provides a framework for employing the layered morphology and highly nonlinear exciton resonance of transition metal dichalcogenides to probe the local thermal environment and also provides an avenue for probing thickness-dependent and temperature-dependent excitonic characteristics of thin layered materials.