In this work, epitaxial GaN thin films were mechanically characterized. Residual stresses of the films were systematically investigated with micro-Raman spectroscopy and their elastic-plastic properties were studied by conducting nanoindentation tests at temperatures of 23 °C, 100 °C, and 180 °C....[ Read more ]

In this work, epitaxial GaN thin films were mechanically characterized. Residual stresses of the films were systematically investigated with micro-Raman spectroscopy and their elastic-plastic properties were studied by conducting nanoindentation tests at temperatures of 23 °C, 100 °C, and 180 °C.

A series of works were carried out to investigate the mechanical stress- or strain-induced change in Raman spectrum of the GaN films. A home-made four-point bending device allows us to in-situ measure the uniaxial stress- or strain-induced change in Raman spectrum. The phonon frequency shifts were observed in-situ under four-pint bending and the experimental data were analyzed successfully. The present work developed a state-of-the-art biaxial stress modulation method, which was aided with the microelectromechanical system (MEMS) patterning technique and finite element analysis, to examine more thoroughly the change in Raman spectrum of GaN films under biaxial stress or strain. Based on the fact that residual stress is relaxed in a coin-shaped island, and the smaller the island is, the much more severe the residual stress will be relieved, we fabricated coin-shaped islands on an originally stressed GaN film and had the island radius ranging from xx μm to xx μm to modulate the biaxial stress. With this approach, the phonon deformation potentials (PDPs) of GaN were accurately and reliably determined to be 3.43 cm^-1/GPa and 2.34 cm^-1/GPa for E^H₂ and A₁ (LO) phonons, respectively. With the determined PDPs, the residual stresses in various GaN films were convincingly characterized with the micro-Raman mapping technique.

The GaN thin films were also characterized mechanically by using the nanoindentation technique. The elastic modulus of the GaN films were evaluated at temperatures of 23 °C, 100 °C, and 180 °C with the “O-P method” on unloading curve and the Hertzian contact analysis on the initial stage of loading curve. The experimental results show that the reduced modulus of GaN decreased with temperature from around 240 GPa (23 °C) to around 140 GPa (180 ºC) according to the “O-P method”, or from 270 GPa (23 °C) to around 230 GPa (180 ºC) according to the Hertzian analysis. The results might suggest that the Hertzian analysis could produce more reliable results in spite of the thermal drift accumulated at a high temperature. This is because the Hertzian analysis uses the initial experimental data which suffer less influence from the thermal drift, while the unloading curve bears more influence from the thermal drift.

This research investigated the first pop-in event systematically. The first pop-in event in force-displacement curves reflects the elastic-plastic deformation transition of GaN films. A large number of data on the first pop-in event allowed statistical analysis. Based on Schuh et al.’s model, the statistical analysis yielded the activation energy of 849.76±35.85 meV, the activation volume of 10.76±1.61 ų and the frequency factor of 4.23×10²¹ m^-3s^-1. The shear strength, defined as the critical maximum shear stress at the first pop-in event, was examined by the nanoindentation tests with indentation tips of different tip radii, showing a tip-radius-dependent behavior, which could be also explained by Schuh et al.’s model.