news digest ♦ Novel Devices
spectra (red and blue) recorded at the off-chain tip positions indicated in a, revealing quantized electron states with quantum numbers n = 1-7. The reference spectrum of pristine InAs(111) A (green) reveals that the Fermi level is pinned in the conduction band due to intrinsic electron accumulation at the surface. d, Spatial DOS maps D(x,y) obtained by constant-height dI/dV scanning at the bias voltages corresponding to the resonances in c. Quantized states for n = 1-6, each with n lobes and n- 1 nodes, are clearly revealed.
This work is described in Nature Nanotechnology 9, 505-508 (2014) ‘Quantum dots with single-atom precision’ by Stefan Folsch et al.
doi:10.1038/nnano.2014.129
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
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www.compoundsemiconductor.net Issue VI 2014
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
highly frequency-selective acousto-electric resonant amplifier.
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