Evaluation of Thermal Transport of GaN Thin Films Using Raman Spectroscopy

Understanding the thermal transport properties of nanoscale gallium nitride (GaN) is essential to ensure reliable operation and prevent overheating in high-power applications. Although previous works have attempted to investigate the thermal properties of GaN films on various substrates, such as sapphire and Si, these substrates could influence the measured results. To address this issue, we aim to investigate the thermal properties of suspended GaN films with thicknesses down to 82nm using the Raman technique.

To achieve the suspended GaN structure, the samples composed of GaN/AlInN on GaN template/sapphire were processed through microfabrication and wet chemical etching of a lattice-matched Al_0.83In_0.17N layer. The Raman measurement was directly performed on the suspended bridge. The heat conduction model is defined as κ = χ_A (L/2Wh)(δω/δP)⁻¹, where κ is the thermal conductivity, L is the length from the center of the bridge to the anchor, W is the width, h is the thickness, χ_A is the temperature coefficient due to anharmonic phonon interaction, and δω/δP is the change in Raman shift at different power levels.

As a result, we obtained κ values of 70±7 and 87±5 Wm⁻¹K⁻¹ for the suspended GaN thin films with thicknesses of 82nm and 160nm, respectively. These values are consistent with theoretical predictions from first-principles lattice dynamics. In addition, it was previously reported that the κ value of non-suspended GaN on a Si substrate decreases from 136 to 127 Wm⁻¹K⁻¹ as the thickness increases from 1300 to 300nm, asmeasured by the time-domain thermoreflectance method. This suggests that the κ values of GaN films for both suspended and non-suspended structures decrease with decreasing thickness due to boundary and surface scattering.

The current study presents an accurate strategy to evaluate the thermal conductivity of the suspended GaN without the effect of the substrate.