Propagation of THz acoustic wave packets in GaN at room temperature

• Published in: Applied physics letters 2018-02, Vol.112 (6)
• Main Authors: Maznev, A. A., Hung, T.-C., Yao, Y.-T., Chou, T.-H., Gandhi, J. S., Lindsay, L., Shin, H. D., Stokes, D. W., Forrest, R. L., Bensaoula, A., Sun, C.-K., Nelson, K. A.
• Format: Article
• Language: English
• Subjects: Acoustic propagation; Applied physics; Electronic devices; Femtosecond pulses; First principles; Free surfaces; Gallium nitrides; Laser applications; Lower bounds; MATERIALS SCIENCE; Phonons; Quantum wells; Room temperature; Specular reflection; Wave packets; Wave propagation
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/1.5008852

We use femtosecond laser pulses to generate coherent longitudinal acoustic phonons at frequencies of 1–1.4 THz and study their propagation in GaN-based structures at room temperature. Two InGaN-GaN multiple-quantum-well (MQW) structures separated by a 2.3 μm-thick GaN spacer are used to simultaneously generate phonon wave packets with a central frequency determined by the period of the MQW and detect them after passing through the spacer. The measurements provide lower bounds for phonon lifetimes in GaN, which are still significantly lower than those from first principles predictions. The material Q-factor at 1 THz is found to be at least as high as 900. The measurements also demonstrate a partial specular reflection from the free surface of GaN at 1.4 THz. This work shows the potential of laser-based methods for THz range phonon spectroscopy and the promise for extending the viable frequency range of GaN-based acousto-electronic devices.