ZnO, TiO² and exotic materials for low temperature thin film electronic devices
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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.