Picosecond/THz acoustic measurements of semiconductor devices

This thesis investigates the electron-phonon interactions in fabricated Semiconductor Devices i.e. Schottky Diodes, Quantum Well (QW) embedded p-i-n photodiodes, two dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) structures all on (311) GaAs substrates. Fast electrical measurements were performed on Schottky diodes fabricated on the (311B) GaAs surface. Optical excitation generates a compressive strain pulse in an Al film which is at the opposite side of the GaAs substrate. Two coherent acoustic phonon modes were generated in the (311B) GaAs substrate: quasi-longitudinal phonons (QLA) and quasi-transverse phonons (QTA). These two modes were detected successfully using the Schottky diode, the dependence of QLA and QTA signals on different applied bias, pump powers and temperatures were investigated. The dependence of signals on bias reveals that the temporal position of the signals for QLA and QTA moves slightly earlier times with increasing reverse bias because the width of the depletion layer increases with increasing negative bias. The first peak arrival time for QLA phonons in the measured signal moves to earlier times with increasing pump power. This was caused by the nonlinear acoustic properties of the GaAs substrate. Conversely, QTA phonons arrival time was not dependent on pump power which indicates the elastic nonlinear effects are not crucial for QTA signals. Investigation of the temperature dependence reveals that the arrival time changes to later times with increasing temperature because of the decreasing sound velocity with increasing temperature. The polarity of the experimental signals was compared to the simulated signals of electron-phonon coupling interactions concludes the predominant role of piezoelectric phonon-electric transformation mechanism over deformation potential. Detailed theoretical calculations show the dominance of the piezoelectric mechanism up to frequencies of 70GHz and 250GHz for longitudinal and transverse phonons respectively. Low dimensional structures i.e. Quantum well embedded p-i-n photodiode detector (QW-p-i-n), two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG), fabricated on the (311) GaAs surface, were also investigated as detectors for longitudinal and transverse acoustic signals. The first experimental evidence of high frequency tail in transverse signal is observed using GaAs based QW-p-i-n detector. The longitudinal signal confirms the existence of the GHz oscillations, with the most significant occurring 170 GHz, while in transverse acoustic signal we observed a high frequency tail from 300 ps to 800 ps. Fast Fourier transforms (FFT) analysis of this tail shows the existence of high frequency phonons up to 200 GHz. Photocurrent pump-probe measurements was also performed with three different Al film thicknesses (20nm, 30nm and 100nm). Experiments performed with 20nm film thickness clearly shows a weak signal at 250 GHz in the amplitude spectrum which is not observed for 30 nm and 100 nm aluminium films. This observation indicates that the thinner aluminium films can generate higher frequency phonons. The longitudinal and transverse signals were also detected by the 2DHG; for QLA predominantly the peak is negative while for QTA it is positive in contrast to the 2DEG experiments due to the reason that in 2DEG experiments the origin of the signal is due to the decrease in carrier density but for 2DHG the signal is due to the increase in carrier density. This thesis studies the interaction of subterahertz acoustic phonons with charge carriers with the aim of developing high speed acoustoelectric detectors for subterahertz compressional and shear acoustic waves, and it has been shown that the Schottky diodes are successful in this application