Young, Eric Sze Kit
(2014)
Ultrafast acoustic strain generation and effects in semiconductor nanostructures.
PhD thesis, University of Nottingham.
Abstract
The nature of ultrafast acoustic strain generation and effects in III-V semiconductor-based nanostructures is explored in this thesis via experimental observations that are supported by theoretical analysis.
Specifically, coherent phonon generation processes in bulk gallium arsenide (GaAs) are investigated through remote hypersonic detection using a double quantum well-embedded p-i-n diode, after which strain-induced effects in a double barrier quantum well resonant tunnelling diode are examined. Finally, preliminary studies on acoustic modulation of a double barrier quantum dot resonant tunnelling diode are also considered, with recommendations for future experimentation.
It was experimentally observed that the transduction of strain in bulk GaAs produces an initial acoustic wavepacket that is strongly asymmetric with a heavily damped leading edge. This was determined to be due to photogeneration of a supersonically expanding electron-hole plasma near the irradiated GaAs surface. Coupled with its propagation from the free surface, the plasma generates stress and therefore strain in the system that is caused by a combination of the deformation potential and thermoelasticity; the former and latter are shown to be dominant for low and high optical excitation densities, respectively. These acoustic waves cannot escape the plasma until it has decelerated to subsonic velocities, which is achieved in a finite time, thus resulting in the observed asymmetry and damped leading edge. This finite acoustic escape time was reduced at high optical excitation densities due to plasma expansion limitation by increased non-radiative Auger recombination of electron-hole pairs.
This conclusion is substantiated by analytical expressions derived from the inhomogeneous wave equation, and analysis of the spatially- and temporally-expanding plasma density based on the deformation potential mechanism only. Numerical simulations based on these expressions are fitted to the experimental data, and the thermoelasticity contribution at high excitation densities is deduced from a non-linear deviation of the electron-hole recombination rate and a change in the duration of the leading edge. This contribution expressed a square-law behaviour in the former parameter, which is attributed to non-radiative Auger processes.
Strain-induced effects on a double barrier quantum well resonant tunnelling diode resulted in the detection of current modulation on a picosecond timescale only when the device was biased within its resonance region, with the largest modulations at the resonance threshold and peak biases. Through analysis of the device structure and stationary current-voltage characteristics, it is demonstrated that the observed current changes are due to variations of the resonant tunnelling rate caused by acoustic modulation of the confined ground state energies in the diode itself.
Numerical analysis of the tunnelling rates provided excellent agreement with the experimental data, particularly when comparing charge transfer rates, where the limited temporal response of the experimental device could be ignored. Furthermore, the charge transferred at the resonance threshold and peak has a set polarity regardless of optical excitation density, and therefore the device possesses “rectifying” behaviour. As such, it has been demonstrated that, by exploiting this acoustoelectronic pumping effect, control of picosecond charge transfer in a resonant tunnelling diode or its application as a hypersonic detector are possible.
In closing, the mechanisms for strain generation in bulk GaAs and the utilisation of the acoustoelectronic pumping effect in a double barrier quantum well resonant tunnelling diode are both exhibited in this work, and provide promising evidence and novel hypersonic detection methods for future research into ultrafast acoustic effects in semiconductor nanostructures.
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