Propagation of THz acoustic wave packets in GaN at room temperature

Abstract

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.

Experimental Evidence of Suppression of Subterahertz Phonons and Thermal Conductivity in GaAs/AlAs Superlattices Due to Extrinsic Scattering Processes

“…While the generation and detection of THz coherent acoustic phonons has been a subject of considerable interest in prior literature (readers are referred to ref for a comprehensive overview of this topic), other works specifically focusing on direct lifetime measurements of subterahertz phonon modes have been limited to semiconductors based on InGaN/GaN SLs. In this regard, a recent work by Maznev et al has shown that 1–1.4 THz coherent acoustic phonons in GaN can propagate more than 2 μm with a lifetime of ∼136 ps measured for a 1.06 THz phonon mode at RT, which they ascribe as a lower bound due to both intrinsic and extrinsic scattering processes. Similarly, Liu et al measure an intrinsic phonon lifetime of ∼150 ps at 380 GHz, which the authors ascribe as dictated by three-phonon scattering processes.…”

Long mean free paths of room-temperature THz acoustic phonons in a high thermal conductivity material

“…This calculation method is the same as that previously published in Ref. [26]. four-phonon scattering is expected to also be negligible.…”

“…where is the phonon propagation time in the GaN spacer layer ( =288 ps for sample 1, while =141.3 ps for sample 2), is the intrinsic phonon lifetime in GaN, and the factor accounts for the temperature-independent attenuation such as GaN thickness non-uniformity [26], as well as scattering by sample imperfections. Since these extrinsic factors are strongly related to quality of the sample and may vary with the laser spot position, we used a diamond pen and scratched a pattern on the sample to define a reference position and used an optical microscope to in situ ensure that the laser spot was at the same position (relative to the reference position) during temperature-dependent measurements.…”

“…However, inelastic x-ray and neutron scattering techniques are incapable of dealing with long-lived phonons in high k materials due to their limited instrumental resolution. Picosecond ultrasonics, in which coherent phonons are excited and probed by femtosecond laser pulses, is capable of measuring phonon lifetimes and MFPs directly [20][21][22][23][24][25][26]. However, despite the demonstrated ability to generate coherent longitudinal acoustic (LA) phonons at frequencies above 1 THz [27,28], there are no experimental data on RT intrinsic (associated with phonon-phonon interactions) phonon lifetimes in high thermal conductivity materials above 0.47 THz [23].…”

“…Meanwhile, InGaN/GaN multiple quantum well (MQW) structures yield excellent performance in the laser generation and detection of coherent phonons [27,31]. Measurements of phonon lifetimes in GaN both below and above 1 THz have been attempted [23][24][25][26], but only yielded lower bounds for the intrinsic phonon MFP due to extrinsic factors such as scattering by interface roughness and non-uniformity of GaN layer thickness within the measurement spot [26].…”