Akademik Veri Yönetim Sistemi
Tezin Türü: Yüksek Lisans
Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Fen Bilimleri Enstitüsü, Türkiye
Tezin Onay Tarihi: 2023
Tezin Dili: İngilizce
Öğrenci: MERYEM TUNÇKANAT
Asıl Danışman (Eş Danışmanlı Tezler İçin): Muhsine Bilge Imer
Eş Danışman: ALPAN BEK
Özet:
Transparent conductive oxides (TCOs) that have high transparency in the visible spectra with outstanding electrical conductivity have drawn great interest with their wide-range use in optoelectronic applications. While indium tin oxide (ITO) is the most commercially used TCO, the replacement of it with in indium-free TCOs is essential due to the toxicity, scarcity, and high cost of indium. Zinc oxide (ZnO), an n-type semiconductor with a direct band gap of 3.37 eV, and its doped forms are suitable candidates for indium-free TCOs along with several additional advantages over ITO, such as their abundancy in nature, non-toxicity, low cost, and easy fabrication. If fluorine anion is selected as a dopant, the formed material is called fluorine doped ZnO (FZO). In literature, lowest resistivity and highest visible transmittance of FZO was reported as 4x10-4 Ωcm and 87%, respectively and the resistivity and the visible transmittance of ITO reported as 5.1x10-4 Ωcm and 88.5%, respectively. Thus, the electrical and optical properties of FZO are compatible with those of ITO.
ALD is a unique deposition technique that has several benefits, such as providing conformal coverage and producing highly uniform thin films at low temperatures. Due to lack of fluorine precursors suitable for ALD process, there was only one study that reported FZO growth utilizing home-made HF/H2O as a source. But HF is an extremely strong acid that damages the equipment when used and it should be used with teflon coating in equipment and with extremely diluted (approximately 0.5%) solution to obtain 1.2 at.% fluorine doping. . In this study, fluorination of zinc oxide films in ALD was demonstrated for the first time in the literature by using a new homemade precursor, a mixture of ammonium fluoride (NH4F/H2O) solution, as a fluorine doping source. Compared with HF, NH4F is much weaker acid (0.125% HF has 1.1 nm/s etch rate while 10% NH4F has 1.1 nm/s etch rate for ZnO) and therefore, it is also much easier to fine tune the amount of fluorine amount incorporated in zinc oxide films. To study fluorine incorporation with this new precursor in ALD, experiments were set with changing variables. All FZO films were grown over silicon and quartz substrates. Diethyl zinc (DEZ) was used as the zinc source, and a mixture of NH4F/H2O solutions with different concentrations (1%-40% NH4F) was used as the oxygen and fluorine source. FZO thin films were grown at temperatures between 160°C-200°C. The effect of canister temperature on fluorine doping was observed at 180 °C with a 30% NH4F/DI mixture, and the canister temperature was varied at room temperature, 40 °C, and 50°C and kept at those temperatures during the deposition. The lowest fluorine concentration was founded in deposition temperature at 180 °C with a 30% NH4F/DI mixture, 40 °C canister temperature as 2.62 at.% and the highest fluorine doping was achieved at 180 °C with a 40% NH4F/DI mixture, 50 °C canister temperature as 29.08 at.%. In literature the resistivity and optical transmittance at 400-800 nm for CVD-grown FZO film with 3 at. % F doping was reported as 18.57 Ω.cm and 84% respectively. In this study, the best electrical properties were achieved at 2.62 at. % fluorine doping with 1.24 x1018 1/cm3 carrier concentration, 2.14 Ω.cm resistivity, and 2.36 Hall mobility cm2/Vs. The average transmittance values 400 nm to 800 nm of all FZO films are approximately 80% and above.