Ultrafast probing of heterostructure dependent coherent acoustic phonon decoherence in AlGaN/GaN HEMT heterostructures

We investigate the influence of heterostructure engineering on the generation, propagation, and decoherence of coherent acoustic phonons (CAPs) in GaN-based materials using ultrafast pump–probe transient differential transmission spectroscopy. Measurements are performed on an AlGaN/AlN/GaN high electron mobility transistor (HEMT) heterostructure, the same structure after selective removal of the AlGaN/AlN layers, and a bare Mg-doped GaN epilayer with embedded InGaN/GaN superlattice layers. Femtosecond excitation launches longitudinal acoustic phonons whose coherent oscillations are tracked in the time and frequency domains using wavelength-resolved probing. The intact AlGaN/AlN/GaN heterostructure exhibits long-lived, narrowband CAP oscillations with a dominant frequency near 40 GHz, whereas etched GaN shows pronounced decoherence, spectral broadening, and mode mixing due to enhanced phonon–phonon and interfacial scattering. The quality factor of the coherent phonons in the heterostructure is approximately three times higher than that of the etched sample, highlighting the role of top layers in preserving phonon coherence and facilitating heat transport. In contrast, Mg-doped GaN epilayer displays strong waveform distortion and rapid dephasing, consistent with phonon scattering from superlattice interfaces. Time–frequency analysis reveals probe wavelength-dependent detection of phonon modes via Brillouin scattering, with shorter probe wavelengths accessing higher-frequency components. These results demonstrate that phonon coherence in GaN can be engineered through heterostructure design and doping control, with direct implications for thermal management and reliability of high-power GaN HEMTs.