University of Twente Student Theses


Mobility analysis in silicon for high-frequency surface acoustic wave applications

Veen, J. van der (2013) Mobility analysis in silicon for high-frequency surface acoustic wave applications.

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Abstract:Preliminary work was done on the realization of acoustic charge transport devices in silicon. These devices utilize the inverse piezoelectric effect to induce surface acoustic waves (SAWs) using interlocking finger electrodes. SAWs are lattice deformations travelling across the surface of solids. In piezoelectric layers, these waves induce a piezoelectric field as well, modulating the band structure of the material. The moving band minima and maxima resulting from this can be used to transport charge carriers. For efficient charge transport, carrier velocity in the piezoelectric field should be higher than the SAW velocity. Thus, carrier mobility in the used substrate material should be sufficiently high to allow for this charge transport. Mobility characterizations of high resistive Si wafers were performed investigating the viability of efficient acoustic charge transport. These characterizations were done using Hall bars and field effect transistors (FETs). Measurements on the Hall bars proved troublesome due to difficulties in applying a gate voltage using the used measurement setup. The results that were obtained were mobility values that were higher than values found in literatures by at least a factor of magnitude. Measurements on FETs yielded more reliable results, measured FET-characteristics show expected behavior in general. Measurements were analyzed using an idealized FET model. Results lead us to believe that the high resistive Si wafers still have some background n-type doping. This is supported by multiple observations. First, n-type devices show relatively large leakage currents compared to p-type devices, especially for devices which have a high aspect ratio (long, narrow channels). Additionally, currents in n-type devices were found to be much higher in general, which can be attributed to the higher amount of charge carriers available. Effective mobility values were found in the range of 400-600 cm2/Vs at 4 K and 250-300 cm2/Vs at 300 K for electrons. For holes effective mobility values of around 200-300 cm2/Vs at 4 K and around 100-150 cm2/Vs at 300 K were found. Furthermore IDTs have been fabricated using nanoimprint lithography on Si using a novel fabrication method involving the application of an HSQ layer for etch selectivity and planarization. Utilizing this fabrication process, resonance frequencies up to 16 GHz were shown for devices made on a ZnO/SiO2/Si substrate. Even more important, resonance frequencies of up to 23.5 GHz were shown for devices made in a CMOS compatible process on a SiO2/ZnO/SiO2/Si substrate. Both these measurement results are supported by theoretical calculations. Further modeling has been done to determine electric field distribution in SAWs at the used frequencies and as a result, mobilities required for effective charge transport. The minimum required mobilities were found to be about the same order as the mobilities derived from FET measurements. As effective mobility values for acoustic charge transport devices are expected to be higher, they should allow for effective charge transport.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Nanotechnology MSc (60028)
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