<i>In situ</i> XRD study of strain evolution in AlGaN/GaN HEMT at high temperatures up to 1000 °C

The thermal stability and structural evolution of a GaN high-electron mobility transistor (HEMT) heterostructure grown on a Si (111) substrate were investigated using in situ high-temperature x-ray diffraction, reciprocal space mapping (RSM), Raman spectroscopy, and rocking-curve (RC) analysis at varying temperatures. The heterostructure, consisting of a p-GaN cap, an AlGaN barrier, and a GaN channel supported by two AlGaN/AlGaN superlattice buffer layers, maintained clear and periodic satellite peaks up to a temperature of 1000 °C, confirming structural integrity. Symmetric and asymmetric RSM results reveal that both the Si and GaN diffraction peaks shift with increasing temperature, consistent with thermal expansion, and show no significant broadening or relaxation throughout the heating process. The c-lattice constant follows the theoretical expansion predicted by the multi-frequency Einstein model, whereas the a-lattice expansion is slower due to in-plane strain constraints imposed by the underlying Si substrate and buffer layers. Irreversible strain relaxation and thermal mismatch-induced stress redistribution induce a residual compressive strain of roughly 0.3% in the GaN channel after cooling, which has been further confirmed in Raman spectra through a blue shift (∼1 cm−1) of the GaN E2 (high) phonon mode, corresponding to an in-plane strain of −0.15% ± 0.16%. RC analysis revealed an increase in both screw and edge dislocation densities of 28% and 12%, respectively. These results collectively demonstrate that the GaN HEMT heterostructure exhibits robust crystalline stability up to 1000 °C without cracking due to strain relaxation, with only minor strain redistribution and limited dislocation activity, providing experimental evidence for GaN devices' applications under high-temperature conditions.