Study on thermoreflectance linear relation of gallium nitride epitaxial structures at above-bandgap wavelength

Abstract

Increasing attention has been paid to the transducer-less thermoreflectance measurements on gallium-nitride (GaN) semiconductors to reveal their thermal properties as well as channel temperatures of GaN-based devices by employing above-bandgap light sources. However, quantitative studies of the thermoreflectance linearity for GaN-based materials are still challenging. In this work, the thermoreflectance linearity of different GaN-based epitaxial structures, including GaN on silicon carbide, GaN on silicon and GaN on sapphire substrate, under 320 nm light illumination has been studied by utilizing the transient thermoreflectance (TTR) technique. We found the thermoreflectance linear relationships are valid from room temperature to ∼110 °C, ∼180 °C and ∼125 °C, respectively. The corresponding Cth within the linear temperature range (Trange) is 8.7E-4 °C-1, 5.7E-4 °C-1 and 3.1E-4 °C-1, respectively. The different non-linear relationships are also observed for the three samples at the temperature above Trange. Additionally, both theory and experiments elucidate that the thin-film interference, due to the presence of an additional AlxGa1-xN layer on top of the GaN, can affect the reflectance characteristics and ultimately the thermoreflectance linear and non-linear behaviors. Finally, the role of thermoreflectance studies in transducer-less thermal characterization is illustrated, offering an effective solution to prevent excessive measurement error caused by thermoreflectance non-linearity. This study provides a deep insight into the fundamental thermoreflectance properties of GaN-based materials at above-bandgap illumination, opening avenues for more advanced transducer-less thermoreflectance-based techniques.