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Research On GaN-based Resonators Gives New Insights Into Phonon-Electron Interactions

Studies suggest possibility of new class of acousto-electrically amplified resonant devices

Piezoelectric semiconductors (PS), such as ZnO, GaN and CdS, rely on interactions between electronic and mechanical domains. But when used to make real world devices such as resonators, these interactions are lossy and have limited conversion efficiency. 

Now recent work on GaN-based bulk acoustic standing wave (BAW) resonators by Vikrant Gokhale and Mina Rais-Zadeh at the University of Michigan, has shown that with the right design and material properties, it is possible to achieve low-loss in such devices with unprecedented ability to dynamically tune resonator Q.

The study, published in Nature, is claimed to be the first comprehensive investigation of phonon-electron interactions in piezoelectric semiconductor  BAW resonators. The results indicate that it is possible to design such resonators with: a) minimum phonon-electron scattering loss under normal operation; b) reduced total energy loss via acousto-electric interaction; and c) acousto-electric gain that can overcome all other losses, effectively creating a highly frequency-selective acousto-electric resonant amplifier.

As well as showing that phonon-electron interactions can lead to acoustic gain of standing waves in PS-BAW resonators, the researchers say that they have presented, for the first time, a comprehensive model that explains the resulting enhanced mechanical Q of PS-BAW resonators under the acousto-electric effect.

The dynamics of acoustic waves (phonons) trapped in resonant cavities made of solid elastic materials have been studied extensively over the years. At resonant frequency, mechanical energy is confined in the form of standing waves in the cavity, which is  the basis of BAW resonators. Ideal standing wave BAW resonators are lossless but the energy confinement in practical materials is not ideal due to a number of phonon-scattering processes. This attenuation limits the quality factor (Q).

Expressions for maximum Q-limits for scattering processes such as anharmonic phonon-phonon loss and thermoelastic damping (TED) are well known. A neglected scattering process is the phonon-electron interaction, which is significant in piezoelectric semiconductor materials such as ZnO, GaN and CdS that have both moderate-to-high doping concentrations and a mechanism facilitating strong electromechanical interactions. Similar to well-known lattice loss mechanisms such as the phonon-phonon loss, the phonon-electron scattering is dependent on the bulk material properties and is not design dependent.

Gokhale and Rais-Zade used thin-film GaN-based BAW resonators as test platforms for dynamic performance enhancement via acousto-electric amplification. The films were unintentionally doped (UID) bulk GaN. The researchers compared theoretical estimates with measured results obtained using the GaN BAW resonators.

The results showed that, in effect, pumping electrical energy into the system can be used to offset the mechanical losses. This dynamic and reversible improvement in the Q of PS-BAW resonators can be distinctly observed on applying a DC electric field. The experimental work focused on GaN as it is a high-quality acoustic PS material predicted to be optimal for demonstrating acousto-electric amplification. Other PS materials (CdS, ZnO H-SiC, GaAs, InP, InGaAs, and AlAs41) could also be potentially used to achieve Q-amplified BAW resonators, say the researchers.

An important question arising from this work, say the researchers, is whether one can overcome all other sources of loss in a practical PS-BAW resonator and achieve frequency-selective resonant BAW amplifiers. To demonstrate this experimentally, they say, further investigation is necessary into PS-BAW resonators made with high-quality thin-film materials, such as GaN, CdS, or ZnO. Ideally, the resonators should be optimized for low reflection loss, operated under controlled pressure/temperature, and stimulated with continuous wave or pulsed DC excitation. This would enable exhaustive investigation into acousto-electric interactions in PS materials and encourage the design of a new class of high-performance acousto-electrically amplified resonant devices.

This is a very brief summary of the paper Phonon-Electron Interactions in Piezoelectric Semiconductor Bulk Acoustic Wave Resonators by Vikrant J. Gokhale and Mina Rais-Zadeh. from Nature Scientific Reports 4, Article number: 5617 doi:10.1038/srep05617

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