Nanoimprinted high-frequency surface acoustic wave devices: Generation, characterization and acousto-electric transport
Başlık çevirisi mevcut değil.
- Tez No: 401254
- Danışmanlar: PROF. DR. WILFRED G. VAN DER WIEL
- Tez Türü: Doktora
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2014
- Dil: İngilizce
- Üniversite: University of Twente
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 117
Özet
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Özet (Çeviri)
This thesis deals with both the excitation and characterization of high-frequency SAWs, as well as with SAW-induced charge transport by using an electrical excitation method. SAWs are widely used and very important in both research and industry. In piezoelectric materials, they can be excited through the inverse piezoelectric effect by using interdigital transducers (IDTs). Because of the piezoelectricity of the material, a piezoelectric potential wave accompanies the mechanical wave. The unique SAW properties make them suitable not only for signal processing, but also for sensing applications, photonics, and charge transport. The SAW frequency is determined by the periodicity of the IDT electrodes and the acoustic velocity of the material. Almost for all applications, there is a strong demand for higher frequencies, for example to enhance the processing speed or to reach the single-charge manipulation regime. Chapter 1 gives a brief introduction to SAWs with a review of breakthrough results, specifically on SAW-assisted charge carrier transport from the literature. In the second part of this chapter, some background for the rest of the thesis is given by introducing a mathematical description of SAWs, their interaction with charge carriers, SAW-induced modulation mechanisms of the electronic structure of semiconductors, and finally their generation and detection methods. In Chapter 2, the experimental methods that have been used in the course of this research are introduced. The first section focuses on the fabrication methods and describes the nanoimprint lithography (NIL) process. A description of the etching and film formation techniques is also given. In the second half of the chapter, the characterization techniques are introduced, such as network analysis and the optical setup for the electron-hole pair transport measurements. In Chapter 3, a method to fabricate sub-100 nm structures by using a planarization technique and an appropriate material configuration is introduced. This chapter specifically addresses the lift-off issues and critical-dimension control at the nanometer scale. It has been shown that global contact planarization with HSQ can be achieved at room temperature with a flat and blank template. This technique has given an improvement of 45 % compared to standard spin coating planarization. Because of the inorganic nature of HSQ, HSQ shows excellent etch selectivity with respect to or- ganic underlying layers. This was demonstrated by pattern generation with UV-based NIL and reactive-ion etching (RIE) processes. Very precise control of the critical dimensions and well-defined undercut resist profiles were obtained, suitable for metal deposition followed by lift-off. HSQ contact planarization is a generic method and can therefore also be applied for other lithography techniques and multilayer applications because of its simplicity, fastness, low cost and efficiency. In Chapter 4, ultrahigh frequency IDTs on a ZnO/SiO2/Si multilayer system were fabricated, using NIL and electrical characterization was performed. Crucial to our novel approach is the application of the HSQ planarization layer mentioned above that provides excellent planarization combined with very high etching selectivity, critical dimension control and easy lift-off. The highest resonance frequency reported so far for ZnO-based transducers on silicon (16.1 GHz), was measured on an IDT with 65 nm finger width. Our experimental results are in very good agreement with numerical simulations of the specific multilayer system. The IDT performance is expected to improve after optimizing the ZnO growth parameters and lowering the Si substrate conductivity. Our technology has great potential to reach even higher frequencies in case of higher velocity substrates, such as SiC, sapphire or diamond. In Chapter 5, ultrahigh frequency SAW devices up to 23.5 GHz were fabricated on SiO2/ZnO/SiO2/Si by using a CMOS-compatible fabrication process. Excited modes are well verified by numerical calculations. Piezoelectric field distribution calculations showed that the SAW-induced field is strong enough for efficient charge (both electron and hole) transport at the Si/SiO2 interface. Our results show that the higher-order modes are advantageous to get higher acoustoelectric current owing to their higher frequency without losing the required mobility. Additionally, metal structures on the SiO2 surface modulating the piezoelectric field, might be used as an acoustic charge guide for certain vibration modes to enhance the transport. Finally, in Chapter 6, the oscillating piezoelectric fields accompanying SAWs were used to demonstrate carrier transport along micrometer distances in GaAs nanowires by using a high-frequency SAW (fSAW = 1.57 GHz). The short wavelength of the acoustic modulation, smaller than the length of the nanowire, allows the trapping of photogenerated electrons and holes at the spatially separated energy minima and maxima of conduction and valence bands respectively, and their transport along the nanowire with a well-defined acoustic velocity towards indium-doped recombination centers. The relative transport efficiency of this acoustic transport mechanism was found to be about 60%. To conclude, the developed NIL fabrication technique can be implemented to generate ultrahigh frequency SAWs on top of different substrates. This method enables very high critical-dimension control on the devices. Integration of SAWtechnology into silicon-based technology is of special interest for the realization of low-cost, integrated devices. Additionally, the importance of contactless manipulation of charge carriers has been pointed out with different proposals, such as the precise measurement of electron charge or single-photon generation. In this respect, the SAW frequency is of crucial importance for the device performance. We believe that this study provides an important contribution to future SAW applications.
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