Browsing by Subject "Zinc oxide."

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Open Access
Electrical properties and device applications of atomic layer deposited ZnO and GaN thin films
(2014) Bolat, Sami
Zinc oxide (ZnO), a semiconducting material with a wide band gap of 3.37 eV, has become a promising material for wide range of electronic and optoelectronic applications. One of the most important properties of this material is its large exciton binding energy of 60 meV, which makes ZnO a strong candidate for ultraviolet light emitting diodes and lasers. In addition, potentially high electron mobility and the transparency in the visible region strengthen the future of the ZnO based transparent electronics. Although several applications of ZnO have taken their places in the literature, use of ZnO in the thermal imaging applications is yet to be explored. In the parts of this thesis related to ZnO, the temperature coefficient of resistance and electrical noise together with resistivity and contact resistance properties of atomic layer deposition based ZnO are investigated. Due to its remarkably high temperature coefficient of resistance value and suitable 1/f noise corner frequency, this material is proposed as an alternative material to be used in the active layers of uncooled microbolometers. GaN is another wide gap semiconductor which has been intensely investigated throughout the last decades for its potential usage in both optical and electrical applications. Especially, high saturation velocity of the electric carriers of this material has made it a strong candidate to be used in high power applications. Furthermore the high electron mobility transistors based on the 2-dimensional electron gas region formed between the AlGaN and GaN, have found wide range of applications in radio frequency (RF) electronics area. Currently, most commonly used techniques for growing GaN, are metal organic chemical vapor deposition and molecular beam epitaxy. Both of these techniques offer single crystalline layers; however, the process temperatures used in the growth of the GaN disable the use of this material in low temperature flexible electronic/optoelectronic applications. In order to solve this problem, hollow cathode plasma assisted atomic layer deposition technique is utilized and GaN thin films with polycrystalline structures are successfully grown at 200°C. In the parts of this thesis related to GaN, the electrical properties, the effect of contact annealing on the resistivity of the GaN thin films and the contact resistance between this material and Ti/Au metallization scheme are investigated. Afterwards, we present the world’s first thin film transistor with atomic layer deposition based GaN channel and discuss its electrical characteristics in detail. Finally, the GaN thin film transistors are fabricated by performing all fabrication steps at temperatures below 250°C. This is the lowest process thermal budget for the GaN based thin film transistors reported so far. Electrical characteristics as well as the stability of the proposed device are investigated and the results obtained are discussed. Proposed devices are believed to pave the way for the GaN-based stable flexible/transparent electronics after further materials and process optimization.
Open Access
Fabrication and characterization of zinc oxide based surface acoustic wave devices
(2013) Noyan, Mehmet Alican
Surface acoustic wave (SAW) devices, as applied to today’s technology, were first described in 1965. Since then, these devices were applied to a wide variety of fields. Bandpass filter is their most common application, which is an important component in consumer products such as televisions and mobile phones. SAW devices can also be utilized as chemical and biological sensors. Driving force behind the development of SAW sensors is their small size, high sensitivity, reliability, and durability. This thesis presents the development and characterization of ZnO/Si based SAW devices. ZnO thin films with c-axis orientation were deposited using rfmagnetron sputtering. Effect of post deposition annealing on the structure of ZnO and on the SAW device performance was studied. It was found that annealing ZnO above 600o C is detrimental for SAW device performance. Surface roughness of ZnO increases as the annealing temperature increases. In literature, roughness increase is presented as one of the reasons behind device breakdown. This work shows that roughness is not the primary cause for the breakdown. In addition, effect of SiO2 interlayer insertion between ZnO/Si structure on the device performance was examined together with the effect of ZnO thickness.
Open Access
Resistive switching mechanism and device applications of ZnO and Ain thin films
(2014) Özcan, Ayşe
Resistive switching memories are potential candidates for next generation nonvolatile memory device applications due to natural simplicity in structure, fast switching speed, long retention time, low power consumption, suitability for 3D integration, excellent scalability and CMOS compatibility. However, the atomic scale mechanisms behind resistive switching are still being debated. In this work we investigate resistive switching mechanisms in ZnO and AlN thin films. The structural and physical changes in ZnO thin films during resistive switching are investigated via TEM, EDX, EFTEM techniques. We also investigate application of resisitive switching to reconfigurable optical surfaces. Recently, resistive switching in nitride films such as AlN is attracting increasing attention. The wide band gap, high electrical resistivity, and high thermal conductivity of AlN make it a good candidate for a resistive switching memory device. We report self-compliant resistive switching behavior in AlN films which is deposited by atomic layer deposition.
Open Access
ZnO based photo-thin-film-transistors with actively tunable photoresponse in the visible spectrum
(2013) Aygün, Levent Erdal
Zinc oxide, ZnO, is an important material for wide range of optoelectronic device applications. Especially, ZnO is famous with its large exciton binding energy of 60 meV which makes it a good candidate for ultraviolet light emitting diodes and lasers. Moreover, its high carrier mobility and wide band gap of 3.37 eV (368 nm) makes it a promising material for transparent electronics and UV photodetectors. However, ZnO has crystallographic defect states (e.g. oxygen vacancies, zinc interstitials) which degrade the performance of ZnO based LEDs, lasers and UV photodiodes. In this thesis, ZnO based photo-thin-film-transistors (photo-TFTs) with visible light response by using their defect states to absorb subbandgap photons are investigated. The design, fabrication and characterization of ZnO based photoTFTs are presented. A photo-TFT is a three-terminal optoelectronic device that is a photoconductor structure with an additional gate terminal which actively tunes electrical and optical properties of photoconductive material. In a clean room environment, ZnO based photo-TFTs with various device sizes are fabricated at different ZnO channel layer deposition temperatures (ranging from 80 to 250 °C). Initially, TFT characteristics of fabricated devices are characterized to show that the gate terminal dynamically modulates ZnO’s channel conductivity. Moreover, the effects of the device size and the deposition temperature on device performance are investigated. Then, the optical characterization of ZnO film deposited at 250° C is conducted via absorption and photoluminescence measurements in order to investigate its visible light absorption characteristics and the energy levels of its defect states in the forbidden band gap of ZnO. After that, the responsivity measurements are reported from ZnO based photo-TFTs fabricated at 250 °C and the active tuning mechanism of visible light photoresponse is discussed. Finally, the effects of the deposition temperature and the device size on the visible light responsivity are presented.