Theses supervised by Yrd. Doç. Dr. Ali Kemal Okyay

Plazmonik ve soğutmasız kızılötesi görüntüleme uygulamaları için alternatif malzeme olarak atomik katman kaplama yöntemi ile büyütülmüş çinko oksit

Plasmonics is touted as a milestone in optoelectronics as this technology can form a bridge between electronics and photonics, enabling the integration of electronics and photonic circuits at the nanoscale. Noble metals such as gold and silver have been extensively used for plasmonic applications due to their ability to support plasmons, yet they suffer from high intrinsic losses. Recently, there is an increased effort in the search for alternative plasmonic materials including Si, Ge, III-Nitrides and transparent conductive oxides. The main appeal of these materials, most of them semiconductors, is their lower optical losses, especially in the infrared (IR) regime, compared to noble metals owing to their lower number of free electrons. Other advantages can be listed as low-cost and control on plasma frequency thanks to the tunable electron concentration i.e. effective doping level. This work focuses on atomic layer deposition (ALD) grown ZnO as a candidate material for plasmonic applications. Optical constants of ZnO are investigated along with figures of merit pertaining to plasmonic waveguides. It is shown that ZnO can alleviate the trade-off between propagation length and mode confinement width owing to tunable dielectric properties. In order to demonstrate plasmonic resonances, a grating structure is simulated using finite-difference-time-domain (FDTD) method and an ultra-wide-band (4-15 µm) infrared absorber is computationally demonstrated. Finally, an all ZnO microbolometer is proposed, where ALD grown ZnO is employed as both the thermistor and the absorber of a microbolometer which is an uncooled infrared imaging units that relies on the resistance change of the active material (thermistor) as it heats up due to the absorption of incident electromagnetic radiation by the absorber material. The material complexity and process steps of microbolometers could be reduced if the thermistor layer and the absorber layer were consolidated in a single layer. Computational analysis of an all-ZnO microbolometer structure using FDTD method is conducted in order to calculate the absorptivity in the long-wave infrared (LWIR) region (8-12 μm). In addition, thermal simulations of the microbolometer structure are conducted using finite element method, and time constant and noise-equivalent-temperature-difference (NETD) values are extracted.

Atomik katman kaplama metoduyla büyütülen ZnO ve GaN ince filmlerin elektriksel özellikleri ve aygıt uygulamaları

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.

Uçucu olmayan belleklerde büyütme koşullarının yük-tuzaklamaya etkisi

Until recently, memories have been a critical bottleneck for the computer industry. Many research based (revolutionary) devices are in the research arena, however, technical concerns such as integration with the current semiconductor manufacturing processes and the feasibility prevent them from being mainstream. In contrast, charge trapping based memories are the evolutionary improvement to the currently ubiquitous floating gate (FG) based nonvolatile memory. This is predominantly due to their superior scalability and reduced leakage compared to FG based memories. This work attempts to investigate the influence of the deposition condition on charge trapping based non-volatile memories for two different gate stacks. The first one involves ZnO as the charge trap layer. This single-step grown ALD has the advantage of having low contamination and manufacturing simplicity. The second type of device uses Ge2Sb2Te5 as a charge trap memory. The nature of the defect states is different in the two materials, and hence the variation of the trap density with temperature.

Terahertz uygulamaları için optimize edilmiş bir yüzeysel schottky diyot: Elektromanyetik modelleme tabanlı bir yaklaşım

Terahertz (THz) spectrum, also called submillimeter spectrum, is a portion of electromagnetic (EM) band within the ITU-designated band of frequencies from 0.3 THz to 3 THz sandwiched between microwave and infrared/optical in EM spectrum. However compared to its microwave and infrared/optical neighbors, 'THz gap' is not heavily explored and it became a hot topic for researchers in recent years. As a result, progress in THz electromagnetics is currently undergoing a renaissance, with burgeoning wide range of applications such as biomedical imaging, sensing and communication applications. Schottky diode, the fastest conventional detection technology, is a promising component for future THz communication receiver systems. However, the overall performance of planar Schottky diode is limited by parasitic elements at THz frequencies. Recently, a lot of efforts are devoted to decreasing the impact of parasitic components. In this work, we introduce a systematic approach for analyzing the Schottky diode structure based on electromagnetic modeling using high frequency structural simulator (HFSS). S-parameters obtained from this 3-dimensional EM simulation were imported to Microwave Office environment to extract all parasitic elements (resistances and capacitances) from lumped-equivalent circuit model. Using this methodology, the effect of geometrical design parameters on the performance of the diode is investigated by which an optimized diode can be obtained. Using this methodology, we propose two concepts to minimize the amount of shunt capacitance and series resistance. The proposed design shows a cut-off frequency that is about 4 times greater than that of reference conventional diode. This methodology is not just limited to the diode but also it can be extended to all integrated planar devices where high frequency cross talk noise is detrimental.

Çinko oksit tabanlı yüzey akustik dalga aygıtlarının deneysel ve hesaplamalı olarak geliştirilmesi

Piezoelectric materials are used in different types of transducers such as microphones, accelerometers, speakers, hydrophones, pressure sensors etc. Compared to traditional bulk piezoelectric crystals, thin film piezoelectric materials are promising to realize integrated devices with CMOS technology. Among thin film materials, zinc oxide (ZnO) is attractive due to the giant piezoelectric effect when doped with vanadium. In this study, we investigate the deposition of thin film ZnO and V-doped ZnO films. Materials characterization of ZnO thin films is performed. We also investigate surface acoustic wave (SAW) devices based on ZnO thin films. SAW devices are formed by a pair of interdigitated transducers (IDTs), input and output IDTs. IDTs are fabricated onto the piezoelectric thin film. Applied oscillating electric field from input IDT creates surface acoustic waves in the piezoelectric thin film and these acoustic waves are converted back into an electrical signal at the output IDT. SAW devices based on ZnO and V-doped ZnO films were designed and fabricated. Frequency response of SAW devices is measured. In addition, finite element simulations of SAW devices are shown to be in agreement with measurement results. We discuss resonance frequency and insertion loss of SAW devices.

Sıcak elektron temelli düşük maliyetli geniş bant yakın kızılötesi silisyum fotodedektörler

Silicon is at the heart of all of the end-user digital devices such as smart phones, laptops, and wearable technologies. It is the holy grail for the largescale production of semiconductor devices since start of the semiconductor era due to its relatively good electrical, mechanical and chemical properties. Silicon's mediocre optical properties also make it an acceptable material for energy harvesting and ultraviolet photodetection applications. But its relatively large bandgap (1.12 eV) makes it infrared blind. So Silicon photodetectors fail to detect infrared light using traditional techniques. Hence, an all-Silicon solution is of interest for low-cost civil applications like telecommunication and imaging. Silicon based Schottky junction is a promising candidate for infrared photodetection. Internal photoemission is the main mechanism of photodetection in the Schottky junctions. Incident photons elevate the kinetic energy of the electrons in the metal so that the energetic electrons can jump over the Schottky barrier or tunnel through it. Carefully designed metal contact of the Schottky junction can, at the same time, give rise to hot electron generation through plasmon resonances. Here we introduce ultra-low-cost broad-band near-infrared Silicon photodetectors with a study over types of metal and nanostructures and fabrication techniques. The devices exhibit photoresponsivity as high as 2 mA=W and 600 A=W at 1300 nm and 1550 nm wavelengths, and can see beyond 2000 nm wavelengths. Their dark current density is as low as 50 pA= m2. Simplicity and scalability of fabrication in this type of structures make them the most cost e ective infrared detectors due to lack of expensive fabrication steps such as sub-micron lithography and high temperature epitaxial growth techniques.

Atomik katman kaplama tekniği ile III-nitrür ve metal-oksit bileşik temelli fotodedektörlerin araştırılması

Gallium Nitride (GaN), one of the most attractive optoelectronic materials today with a direct wide band gap of 3.4eV and high electron saturation velocity, has found many applications from blue/UV LEDs to UV photodetectors, from high electron mobility transistors (HEMT) to solar cells. Traditional techniques to grow GaN films require high temperature (over 600ºC) processes. Such techniques cannot be used to synthesize GaN films on temperature sensitive substrates such as plastics or even paper for large area optoelectronic applications. To circumvent this setback, atomic layer deposition (ALD) stands out with its unique features such as low temperature process, precise thickness control and step coverage. Our work marks the demonstration of the first optical device on hollow cathode plasma assisted atomic layer deposition (HCPA-ALD) grown GaN films. The fabricated devices showed promising electrical and optical performance. A UV/VIS contrast ratio of 15 is obtained with very low dark current of 14pA at 20V applied bias. Annealing the films improved the device performance. Dark current was reduced more than two orders of magnitude while the responsivity was increased by two times. In the second part of the thesis, optoelectronic device applications on ALD grown ZnO layers will be presented. ZnO is also an attractive wide direct band gap semiconductor. It is utilized in many optical devices such as photodetectors and solar cells as well as thin film transistors and biomedical applications. In this work, device applications of ZnO on Silicon heterojunctions are investigated. A high rectification ratio of 103 is achieved with 80ºC grown ZnO-Si heterojunction photodiodes. High responsivity values are also recorded for these devices. At 350nm incident wavelength maximum responsivity of 35mA/W and at 585nm incident wavelength maximum responsivity of 90mA/W are obtained.

İşlevsel nanoplazmonik aygıtlar ve yeni fotonik malzemeler

Plasmonics is one of the pillars of nanophotonics involving light matter interactions. Its applications found very wide range covering photovoltaics, photo-detection, optical communication, surface enhanced infrared absorption and Raman spectroscopy, infrared and THz imaging. Although the number of applications is very high, the underlying plasmonic structures are limited. In this thesis, we utilize a common plasmonic resonator structure namely metal-insulator-metal (MIM resonators) to realize active beam steering in the infrared spectrum. We investigate radiation characteristics of a phased array antenna formed by MIM resonators. Materials-wise, low intrinsic loss, CMOS compatibility and bio-compatibility are among the crucial requirements for various applications of plasmonics. Noble metals are the dominant materials used in plasmonics to get high localization of the incident field among which gold and silver face serious challenges due to high intrinsic loss and lack of CMOS compatibility. We introduce InN as a novel plasmonic material thanks to its high concentration of free carriers and investigate its optical characteristics in the IR spectrum. We form a proof-of-concept absorber and investigate its plasmon excitation characteristics. On the other hand, we introduce another material ZnO, non-plasmonic, suitable for infrared imaging purposes with strong absorption characteristics. Optical modulators are at the very heart of active light manipulation technologies such as integrated optics, bio-sensing, telecommunications, radio frequency and terahertz applications. Although various modulation schemes have been realized, the underlying mechanisms providing modulation did not change significantly. The common modulation methods can be listed as free carrier dispersion, thermo-optic method, use of liquid crystals, magneto-optical, optically nonlinear materials and recently introduced solid-state phase-change materials. Here we introduce another mechanism called resistive switching for optical modulation in the infrared spectrum. We investigate electrical resistive switching characteristics of an Al/ZnO/Si stack and optical modulation characteristics under electrical bias. We obtain hysteretic modulation in the reflection spectrum. We also investigate the thermo-optic modulation characteristics of atomic layer deposited ZnO through spectroscopic ellipsometry and realization of actively reconfigurable reflector surface.

Yüksek verimli boya uyarımlı güneş pilleri için fotoanot geliştirilmesi

Tümü silisyum fotonik ve plazmonik mükemmel ışık soğurucularının tasarımı ve uygulamaları

Majority of the optoelectronic devices works either in infrared regime or in visible spectrum. Among these, perfect absorbers attracted great attention due to their high applicability in solar cells and high performance photodetectors as well as special applications such as surface enhanced sensing. However, high material costs and elaborate nano-fabrication procedures to build perfect absorbers are prohibitive issues that researchers or processors have to deal with. In this work, all-Silicon (Si) practical low-cost photonic and plasmonic perfect absorbers are investigated by theoretical modeling and the designed devices are fabricated by utilizing standard CMOS technology. In order to model the optical response of Si, the effect of charge carrier mobility on the dielectric is analyzed. We showed that high performance devices that can perform better than the state of the art are possible without requiring high cost materials and elaborate fabrication techniques. Photonic perfect absorbers that have promising band properties in infrared are designed and fabricated. Experimental results support theoretical predictions. We used computational approach to investigate the effect of temperature.

Mikrobolometre uygulamaları için çinko oksidin atomik katmanlama yöntemiyle titanyum ile alaşımlanması

Microbolometers are attractive candidates for both military applications and consumer electronics among the uncooled thermal detectors due to their compactness, low cost, comparable performance with photon detectors and CMOS compatibility. The temperature sensitive active material is the most important part of resistive type microbolometers where change in electrical response occurs upon IR radiation. Typical active materials used for this purpose are YBCO and VOx. It was reported in several studies that ZnO has higher TCR value than commercially available active materials. However a temperature insensitive TCR property has not been achieved yet. To improve the TCR property of ZnO, doping with Titanium is proposed in this work. According to Ellingham diagram of oxides which is used generally in extractive metallurgy, it is obvious that titanium oxide is more stable than zinc oxide. Therefore doping with Titanium may reduce oxygen related defects and improve TCR property. Atomic layer deposition (ALD) is used for digital alloying of ZnO with Titanium. Titanium doped ZnO (TZO) films with Ti concentrations 2.5\%, 5.9\% and 12.2\% were deposited using precursors diethylzinc, mili-Q water and tetrekis(dimethylamido)titanium. Then intrinsic defect related elemental characterization were made. Effect of Titanium doping on structure of TZO thin films was discussed. After material characterization, planar microresistors were fabricated in UNAM cleanroom facility. Piranha-HF and solvent cleaning of silicon substrate were performed before microfabrication. Vaksis Handy Plasma Enhanced Chemical Vapor Deposition (PECVD) was used to deposit insulation layer on silicon wafer. Photolithograpy steps were performed using Laurell Spinner system and EVG 620 mask aligner to pattern TZO thin film. Then contact material metallization was performed using Vaksis MIDAS Thermal Evaporator system. Current-voltage characterization of microfabricated resistors was performed before Temperature Coefficient of Resistance (TCR) measurements to see the contact type resistors. Then TCR measurements were done between 15C and 25C by applying constant current to the contact pads of resistor. Approximate resistivity values of 5 different samples were calculated. It is shown that proposed TZO active material for resistive type microbolometers has a temperature insensitive and high TCR value.

Silisyum germanyum çoklu kuantum kuyuları içeren yüksek verimli optoelektronik aygıtlar

Silicon (Si) based complementary metal-oxide-semiconductor (CMOS) technology provides a low cost platform and reproducible processing for electronic signal processing and electronic industry has significantly flourished in terms of increased device densities and speeds. All the same, advanced CMOS processes associated with the very large scale integrated (VLSI) circuits are required for the fabrication of today?s advanced microprocessors comprising billions of transistors per single chip with higher operational data rates. Herein, increasing trend for the speed of current circuits is hampered due to the delays of electrical interconnects that basically arise from RC factor. Signal attenuation due to power dissipation and electromagnetic interference issues are further physical bottlenecks that affect the performance of current technology. At this point, an idea proposes the development of optical interconnect network layers on Si CMOS circuitry operating at telecommunications wavelength range (~1.3-1.5 ?m). This promising conception enables optical information processing and offers a platform that overcomes the physical restrictions of electrical interconnects. Si based optoelectronic devices are near-infrared-blind due to large indirect (~1.12 eV) and direct band gap (~3.4 eV) energies. To date, III-V compounds and especially InGaAs based detectors are ?state-of-the-art? optical devices that are used for near-infrared (NIR) applications. However, due to high material cost and difficult integration with Si CMOS technology they have limited market growth. At this point, germanium (Ge) is a promising candidate for NIR applications with its suitable indirect (~0.66 eV) and direct (~0.8 eV) band gap, which overcomes spectral limitations of Si. Additionally, lattice mismatch (~4.2%) problem among Si and Ge is worked out by novel heteroepitaxial growth technique (MHAH), which reduces the threading dislocations and enables the monolithic integration with conventional CMOS technology. For the scope of the thesis, fabrication, material characterization and device characterizations of p-i-n photodetector and electro-absorption modulator architectures, which are the key elements of a basic optical interconnect structure, are performed. Ge/SixGe1-x multi quantum well (MQW) structures are utilized in the intrinsic layer to exploit the electro-absorption mechanism of quantum-confined Stark effect (QCSE). Represented photodetectors exhibit low dark current (~5 mA/cm2 at -1 V) and high responsivity (~0.33 A/W at 1310 nm and 0 V). Demonstrated electro-absorption modulators exhibit very high absorption coefficient contrast (3.41 at 1550 nm and 3 V) especially for conventional C-band (1530-1565 nm) telecommunication wavelength range and low insertion loss (0.2 dB at 1540 nm). Modulation performance is highly comparable with the electro-absorption modulators based on direct gap III-V compounds at the wavelength range of interest. Demonstrated photodetector and electro-absorption modulators are the high performance building blocks of an optical interconnect system.

Plazmonik yapılarla geliştirilmiş silikon kızılötesi schottky algılayıcı

The holy grail of near infrared (NIR) photonics is a purely-Silicon-based solution. Today, infrared detection is dominated by III-V compound semiconductors, mainly InGaAs devices in the NIR. The high cost of integration with established Si VLSI technology, prevent the market growth of compound semiconductor device technology despite high efficiencies attained. On the other hand, Silicon is the second most abundant element on the earth and is the predominant material of the complementary metal-oxide-semiconductor (CMOS) technology. Since Silicon is near infrared blind due to its large energybandgap (~1.12 eV) it cannot absorb infrared light efficiently. However, Silicon based Schottky photodetectors can operate at infrared wavelengths due to their absorption mechanism at the interface of metal and semiconductor. The metal-semiconductor (M-S) diode is among the most investigated junction devices. Schottky diodes have been very attractive for their high speeds. The potential of high frequency operation and the ease of integration render Schottky devices a strong candidate in CMOS compatible NIR photonics. Silicon based Schottky photodetectors can be a promising candidate if NIR efficiencies could be increased. We design, fabricate and characterize plasmonic gratings to increase the efficiency of Silicon based Schottky photodetectors at the near infrared region. Our design is optimized for the conventional telecommunication C-band (1530 nm-1650 nm). We also study the effects of an interlayer dielectric and metal layer on the plasmonic fields. We use atomic layer deposited Alumina (Al2O3) between the metal and the semiconductor at the M-S junction. Our micro/nano-fabricated photodetector devices exhibit low dark current densities (~21 mA/cm2 at -3 V) and high responsivity values (~83 mA/W at 1530 nm and -3V). Plasmonic gratings provide an increase of responsivity by more than a factor of 2 at desired wavelengths.

Nanoaralık temelli etiketsiz çeliölçer biyoalgılayıcılar

Despite lots of research going on to find a hope, cancer is still a major cause of death in today?s world. It has been reported that cancer has some biomarkers in human body and detecting these biomarkers timely can pave the way for early detection and successful treatments.Point-of-care biosensors are highly promising for this mission. If these biosensors can achieve sensitivity and reliability with a low-cost and simple platform, they can address a large mass of people who are at the early stages of cancer without any clear symptoms yet.For this purpose, various biosensing mechanisms can be used to convert the signal coming from the recognition elements on the biosensor surface to the digital domain for signal processing. One of these mechanisms, impedimetric (impedance based) sensing is a very appealing electrical biosensing method since this method can offer label-free, low-cost, low-power requirement, miniaturizable, and chip-integrable detection platforms. However, impedimetric sensing in liquid medium is problematic, since during the electrical measurements, ion-based undesired layers (electrical double layers) are formed over the electrodes in the target liquid. Unfortunately, these layers act like a shield against the applied electric field to the liquid and can prevent the detection of the target biomarkers.In this thesis, a nanogap based label-free biosensor structure is designed and using this design impedimetric sensing in liquid medium is demonstrated at low frequencies (1 kHz ? 100 kHz). Low frequency platforms are quite amenable to low-cost applications like point-of-care biosensing.The designed structure utilizes nanometer scale electrode separation (nanogap). Theoretical calculations show that nanogap reduces the undesired effect of electrical double layer. Moreover, nanogap also helps in minimizing the volume of the required liquid for the measurement.Design, fabrication, surface functionalization and biotinylation stages of the biosensor are realized in a cleanroom environment and biomimetic materials laboratory. The fabricated biosensor is tested by introducing the target molecules (streptavidin) in a phosphate-buffered saline solution. A parameter analyzer with a capacitance-voltage unit and a probe station are used for the impedance measurements.With these biosensors, label-free detection of streptavidin is observed for100 µg/mL, 10 µg/mL, 1 µg/mL, 100 ng/mL and 10 ng/mL concentrations. This is, to the best of our knowledge, the first demonstration of streptavidin detection in nanogap based label-free impedimetric biosensors. The above-mentioned concentrations show that these biosensors are promising for commercial applications. Sensitivity to the dielectric constant of the target medium is measured to be 132 pF per unit change in the dielectric constant at 10 kHz measurement frequency. Reliability tests are performed: stable and repeatable operation of the sensors are checked and verified.In conclusion, this proof-of-concept study shows that nanogap based biosensors would be a suitable and appealing choice for sensitive, reliable, simple, low-power and low-cost point-of care biosensing applications. Next step would be utilizing the platform presented in this work in detecting specific cancer biomarkers like PSA or CA125. Thereby, developed further and commercialized, nanogap based label-free impedimetric biosensors can act in the battle of human being against cancer in the future.

Zno, tio2 ve bazı egzotik malzemeler kullanılarak düşük sıcaklıkta oluşturulmuş ince film elektronik aygıtlar

The metal-oxide-semiconductor field-effect transistor (MOSFET) technology is the core of integrated circuit industry. Nearly all electronic devices around us contain transistors for various purposes like electronic switches, amplifiers or sensors. As the need for more complex and miniature circuits has arisen, scaling down transistor sizes become the top priority. As Moore?s law indicates, number of transistors on integrated circuits doubles every two years but in future fabrication challenges and limitations like quantum effects seen in small devices will block further miniaturization. New growth techniques are required for depositing conformal, high quality films -like high-k dielectrics instead of SiO2- with atomic thickness control to reduce possible problems. Atomic layer deposition techniques are developed to meet these requirements.The field of thin film transistors (TFT), which is a subset of MOSFET?s have first started to be used in flat panel displays but now they are used in various fields, since their functional properties make them powerful candidates for sensor applications.ALD technology is important also for TFT applications since its low temperature growth mechanism allows fabricating TFT?s on various substrates like flexible and/or transparent ones. With ALD technique, transistors can be built even on cloths which makes the dream of e-suits real.In this thesis, thin film transistors are designed and fabricated using atomic layer deposition technique both for channel and dielectric layer growth.Design and fabrication steps of the TFT devices are realized in a cleanroom environment. The fabricated TFT?s are mainly characterized by measuring their current-voltage relations. A parameter analyzer with a probe station is used for such measurements.ALD grown ZnO TFT?s and the effect of growth temperature on performance characteristics are examined. High performance devices having very high Ion/Ioff ratios are fabricated at a temperature low as 80°C. ALD grown TiO2 TFT?s are also fabricated and effects of annealing temperature on device performance are analyzed. This study is, to the best of our knowledge, the first demonstration of TiO2 TFT?s grown by a thermal-ALD system. GaN and pentacene TFT?s are also fabricated and showed promising results. Pentacene TFTs have a special importance since it is a p-type organic semiconductor which gives us the opportunity to work on hybrid organic-inorganic structures.In conclusion, TFT devices based on ALD grown channel and/or dielectric layers show very encouraging results in terms of low cost, low temperature fabrication opportunities and freedom of using any substrate that can handle ALD processing temperature.Keywords: TFT, ALD, ZnO, TiO2, Al2O3, pentacene, GaN

Görünür ışığa tepkisi aktif olarak ayarlanabilen ZnO bazlı foto-ince-film- transistörler

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 photo-TFTs 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.