The interaction of coherent acoustic phonons with electrons in semiconductor superlattices

• Main Author: Poyser, Caroline Louise
• Published: University of Nottingham 2015
• Subjects: 621.3815; QC501 Electricity and magnetism
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.682659

This thesis presents a study of the electron-phonon interaction in an n-doped weakly coupled semiconductor superlattice (SL). Two experiments were performed which studied different aspects of this interaction. Firstly, a coherent phonon optics chip was designed. This was used in an experiment where a phonon beam was passed through the SL while an electrical bias was applied to it. The experiment provided a sensitive measurement of the effects caused by bias in the SL on the phonon beam. Secondly, a train of strain pulses was passed through the SL and the charge transferred in the device due to the strain was investigated. A coherent phonon optics chip was formed using a semiconductor superlattice as a transducer structure and a p-i-n photodiode as a coherent phonon detector on the opposite side of the substrate. The doped weakly coupled superlattice structure, which is the main subject of investigation in this thesis was grown between the transducer and detector structures. Optical access mesas were processed on both sides of the substrate to allow the application of bias to both the doped superlattice and the p-i-n structures. A photocurrent pump-probe experiment was then performed using a femtosecond laser to excite the transducer structure and activate the detection mechanism. The application of bias to the weakly coupled SL was found to cause a small attenuation to the 378 GHz phonon beam passing through it. An investigation of the possible causes of this attenuation ruled out several trivial explanations, suggesting that it was caused by the interaction between electrons and phonons in the structure. The active control of phonon amplitude by electrical means has not previously been demonstrated and may offer exciting new prospectives for phonon devices and experiments. The coherent phonon optics technique was shown to be very sensitive and it will be a useful technique to increase our understanding of future acousto-electric devices. The electrical signal that acoustic excitation caused in the SL device was investigated using a pulse shaping technique in combination with an amplified femtosecond laser. A Fabry-Perot cavity was used in the laser path to create a train of equally spaced laser pulses with an adjustable pulse spacing. Focusing these pulses on an aluminium film transducer creates a train of equally spaced acoustic pulses simulating a monochromatic acoustic wave packet. The SL was processed and electrically contacted so that the charge transferred through it due to the acoustic pulse train could be monitored using a 12.5 GHz-bandwidth digital oscilloscope. The variation in charge transfer seen as a function of the DC bias applied to the device and as a function of the total energy of the acoustic pulse train was investigated. The behavior was compared to a theoretical model developed in the style of previous theories of electrical conversion in SLs excited by electromagnetic waves. The dependencies of the charge transfer on the bias and energy of the pulse train were well reproduced in the theory. The theory predicted that magnitude of the signal in the superlattice was independent of the frequency of the acoustic pulse train. This was verified by measuring the frequency dependence of the signal seen for a variety of transducer films. The frequency dependencies seen were well explained through simulations presuming the device response was independent of train frequency. This confirms the predictions of the theory. Both the experiments detailed in this thesis have helped increase our understanding of the nature of electron-phonon interactions in superlattices. It is hoped that a fuller understanding of these interactions may be instrumental in the creation of exciting new acousto-electrical devices.