Generation of Coherent Acoustic Phonons in Nitride-Based Semiconductor Nanostructures

Abstract

In this chapter, we review experimental and theoretical aspects of coherent acoustic phonon generation in nitride-based semiconductor nanostructures, with particular application to InGaN/GaN multiquantum wells (MQWs). We first discuss the experimental generation and detection of coherent longitudinal-acoustic (LA) phonon oscillations in InGaN/GaN MQWs using the transmission-type pump--probe technique. With UV femtosecond pulse excitation, photogenerated carriers screen the piezoelectric field and initiate the displacive coherent phonon oscillations. The spatial wavevector of the periodic carrier distribution determines the phonon-oscillation frequency. The induced acoustic phonon oscillations result in a piezoelectric field modulation that then causes an absorption variation through the Franz--Keldysh effect. Injecting another control pulse can further control the resulting coherent phonon oscillations. Both magnitude and phase manipulation can be achieved by controlling the intensity and time delay of the control pulse. After reviewing the experimental results, we then present a microscopic theory of the generation and propagation of coherent LA phonons in wurtzite semiconductor MQWs. Under typical experimental conditions, the propagation of coherent LA phonons is described by a loaded-string equation for the lattice displacement, where the time- and position-dependent loading term is a function of the photoexcited carrier density. We note that this differs from the situation in which coherent LO-phonon scillations are generated in bulk systems where the coherent LO phonons obey a forced-oscillator equation as opposed to a loaded-string equation. Both deformation-potential and piezoelectric-coupling mechanisms contribute to the driving force in the loaded-string equation. We also discuss a macroscopic theory for the generation and dynamics of coherent acoustic phonons in wurtzite semiconductor nanostructures. This approach is based on macroscopic continuum constitution equations for piezoelectric wurtzite semiconductors. Starting from Poisson’s equation and the dynamic elastic equation, a vector-loaded wave equation is obtained. By projecting the corresponding equation to eigenvectors of the elastic Christoffel equation, the loaded-string equation can also be obtained. The macroscopic approach is then used to study the orientation effects on the generation of coherent acoustic phonons and it is found that large coherent transverse acoustic phonon oscillation can be generated when the growth direction of the nanostructure is along certain directions, e.g., [$10\bar {1}0$]. This coherent transverse wave may have special applications in THz ultrasonics. Finally, a phenomenological theory for the transient transmission measurement of coherent acoustic phonons is presented. Calculations based on the loaded-string equation with the optical-measurement theory agree well with the experiments.