A FEW WORDS ABOUT
surface acoustic waves
SAWs are elastic vibrations traveling along a surface. They are analogous to the waves generated by earthquakes that travel around the surface of the earth (Rayleigh waves), but with much smaller wavelengths (typically in the micro- and sub-micrometre range, in contrast with the wavelength of several metres for seismic waves). SAWs can be generated electrically in a piezoelectric medium using interdigital transducers (IDTs). As illustrated in Figure 2, the IDTs consist of two interlocking arrays of metal electrodes deposited on the surface of the material, which is normally fabricated using planar semiconductor technology.
The application of a radio-frequency (RF) voltage to the IDT launches a strain field, which propagates in the form of a strain wave along the surface to the region outside the transducer. In a piezoelectric material, the strain wave is accompanied by a piezoelectric field, i.E. a wave of electrostatic potential. SAWs on piezoelectric insulators have been used for numerous applications, most notably in signal processing, sensors, and acousto-optics, where they have a well-established place. The hundreds of millions of SAW devices incorporated in mobile phones worldwide attest to the economical and societal impact of SAW technology.
Present-day electronic devices normally rely on electric fields to control the properties of semiconductor crystals. Semiconductors are also very sensitive to other parameters, e.g. strain, temperature and magnetic fields: their exploitation towards novel functionalities forms the basis of the “More than Moore” pathway in the development of integrated devices. SAWtrain aims at exploring novel functionalities provided by SAWs in semiconductor nanostructures and related materials, where the dynamic strain and piezo-electric fields produced by the SAW modulate the material properties and create moving potentials for the confinement and transport of carriers. SAWtrain will mainly focus on the exploitation of these fields for the control of excitations (carriers, spins, photons, and phonons) in semiconductor and related structures.
Figure 2: Surface acoustic wave (SAW) generated by an interdigital transducer (IDT) on a semiconductor crystal containing quantum well structure.
Semiconductor-based SAW research
SAWtrain will focus on the exploitation of the moving SAW fields as tool for the control of carriers, spins, phonons, and photons in semiconductor and related structures. In addition, the research activities will develop materials and technological tools for the generation and control of high frequency SAWs, which will also support the conventional application areas of SAWs.
The SAWtrain members have contributed to many scientific and technological breakthroughs in the field of SAWs since the late 1990’s. The original work on acoustic transport of single electrons was carried out at UCAM, thus demonstrating the possibility of SAW-modulated quantum control. Further developments by this group and CNRS led to the demonstration of SAW-induced transfer of single carriers between quantum dots. The seminal works on SAW-induced light storage as well as on long-range transport of carriers and spins were carried out at UAU and PDI. Examples of advanced applications in electro-optical control are provided by single-photon sources, tuneable photonic crystals and integrated photonic devices developed recently by PDI, TREL, UCAM, UAU, and UVEG.
Recent experiments by CHALMERS demonstrated detection of pulses containing single SAW quanta. SAWtrain members have also been very active in the development of technologies to generate SAWs with very high frequencies, some of the highest values being reported by TWENTE, NTD and PDI.
Novel approaches for the application of SAWs to new materials (e.g. graphene, UPM and PDI), advanced sensors (CNR), as well as for the control of chemical reactions (UAU) have been introduced by the consortium beneficiaries. Finally, SAW-based sensors are key elements in the road map for semiconductor technologies, due to their simplicity, high sensitivity, and good time response. These developments have opened the way for novel SAW-based concepts and functionalities for signal processors, sensors, optical modulators and switches, nano-mechanical structures, and quantum control of single electrons, photons, and phonons, which are key enabling technologies to be investigated within SAWtrain.