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Transparentni prevodni oksidi (TCO) so električno prevodni materiali, ki v vidnem območju elektromagnetnega spektra absorbirajo majhno količino svetlobe in so posledično prosojni. Uporabljajo se v različnih optoelektronskih napravah, kot so solarne celice, termokromna stekla, prikazovalniki na dotik in senzorji. Večinoma se kot material za električno prevodne komponente v teh napravah uporabljajo tanke plasti na osnovi indijevega oksida z dodatkom kositra (ITO). Za pripravo tankih ITO plasti se uporabljajo fizikalne metode, kot sta lasersko nanašanje in magnetronsko naprševanje, ki zahtevajo nanos v vakuumu in uporabo drage opreme. Poleg tega je In2O3 drag, zato se raziskujejo alternativni materiali, ki so cenejši in dostopnejši od In2O3. Iščejo se tudi enostavne metode priprave tankih plasti, ki ne zahtevajo drage opreme in potekajo pri normalnih pogojih. Eden od obetavnih materialov je cinkov oksid (ZnO), ki ima širok prepovedan pas, dobro mehansko/kemijsko/toplotno stabilnost, v obliki tanke plasti na steklu pa ima tudi visoko prosojnost. Poleg tega lahko na strukturne, električne in optične lastnosti tankih plasti ZnO vplivamo s spreminjanjem kemijske sestave (dopirajem) ter s spreminjanjem pogojev pri procesu njihove priprave. V sklopu magistrskega dela sem pripravil tanke plasti IZO z atomskim razmerjem Zn/(In+Zn) = 0,36 na steklu in siliciju iz raztopin s tehniko vrtenja podlage. Raztopine IZO, koncetracij 0,3 mol/l in 0,147 mol/l, so bile pripravljene z raztapljanjem izhodnih surovin v ustreznih topilih. Cinkov acetat hidrat sem raztopil v etilen glikolu, indijev nitrat hidrat pa v etanolu z dodatkom ocetne kisline. Z mešanjem izhodnih raztopin sem pridobil homogeno in bistro raztopino IZO. S pomočjo termične analize (TG/DTA/EGA) in infrardeče spektroskopije s Fourierjevo transformacijo (FTIR) raztopin in prahov, dobljenih s sušenjem raztopin, sem določil pogoje sušenja posameznih nanosov in žganje plasti. Plasti IZO sem okarakteriziral z vrstično elektronsko mikroskopijo s poljsko emisijo (FE-SEM), mikroskopom na atomsko silo (AFM) in rentgensko praškovno difrakcijo (XRD). Električna upornost pripravljenih tankih plasti IZO je bila izmerjena s štiri-točkovno metodo. Pri sušenju plasti na 350 0C sem na siliciju pridobil tanko plast IZO s plastno mikrostrukturo, z znižanjem temperature sušenja na 200 0C pa sem uspel pridobiti plast s homogeno mikrostrukturo. Primerjal sem tudi vpliv temperature žganja na mikrostrukturo plasti IZO na steklu. Ugotovil sem, da je najbolj primerna temperatura žganja pri 600 0C. Debelina plasti IZO s plastno in homogeno mikrostrukturo je okoli 80 nm z velikostjo zrn do 20 nm. Površina vseh analiziranih plasti IZO je ravna in struktura le-teh nima urejenosti dolgega reda. Najnižjo upornost je imela plast IZO žgana na zraku (pri 6000C) s plastno mikrostrukturo in sicer 0,12 Ω*cm. Z dodatnim popuščanjem plasti IZO v Ar/H2 atmosferi pa se je upornost znižala za približno dva velikostna reda na 4*10-3 Ω*cm. Padec upornosti je posledica tvorbe vrzeli na kisikovih mestih v mikrostrukturi plasti IZO. Ugotovil sem tudi, da plasti IZO v vidnem delu elektromagnetnega spektra prepuščajo med 80 in 92% svetlobe, boljšo prosojnost pa izkazuje plast IZO s plastno mikrostrukturo. Transparent conductive oxides (TCO) are electrically conductive materials that absorb a small amount of light in the visible range of electromagnetic spectrum and are consequently transparent. They are used in various optoelectronic devices such as solar cells, thermochromic glasses, touch displays and sensors. Thin films based on indium oxide with the addition of tin (ITO) are mostly used as material for electrically conductive components in these devices. Physical methods, such as laser deposition and magnetron sputtering, are used for the preparation of ITO thin films, which require deposition in vacuum and use of expensive equipment. In addition, In2O3 is expensive, therefore alternative materials, that are cheaper and more accessible than In2O3, are being researched as well as methods for preparing thin films, that do not require expensive equipment and take place under normal conditions. One of the promising materials is zinc oxide (ZnO) with a wide band gap, good mechanical/chemical/thermal stability, and in the form of a thin film, it is also highly transparent. In addition, the structural, electrical and optical properties of ZnO thin films can be influenced by changing the chemical composition (doping) and changing the conditions during thin film preparation process. As part of my master`s thesis, I prepared thin films IZO with the atomic ratio Zn/(In+Zn) = 0,36 on glass and silicon, using the spin-coating method. IZO solutions, with concentrations 0,3 mol/l and 0,147 mol/l, were prepared by dissolving the starting raw materials in appropriate solvents. Zinc acetate hydrate was dissolved in ethylene glycol, and indium nitrate hydrate in ethanol with the addition of acetic acid. Mixing the starting solutions, I obtained a homogeneous and clear IZO solution. With thermal analysis and infrared spectroscopy with Fourier transformation of solutions and powders, obtained with drying of solutions, I determined the drying conditions of individual coatings and annealing of thin films. IZO thin films were characterized by field-emission scanning electron microscopy, atomic force microscopy and X-ray powder diffraction. The electrical resistivity of the prepared IZO thin films was measured by the four-point probe method. When drying the film at 350 0C, I obtained IZO thin film with a layered microstructure, and by lowering the drying temperature to 200 0C, I managed to obtain a film with a homogeneous microstructure. I also compared the influence of annealing temperature on the microstructure of the IZO thin films on glass. I found that the most suitable annealing temperature is 600 0C. The thickness of the IZO thin films with the layered and homogeneous microstructure is around 80 nm with grain size of up to 20 nm. The surface of all analysed IZO thin films is flat and their structure does not have long-range order. IZO film with a layered microstructure had the lowest resistivity of 0,12 Ω*cm. By additionally annealing IZO thin films in an Ar/H2 atmosphere, the resistivity was reduced by approximately two orders of magnitude to 4*10-3 Ω*cm. This is the result of the formation of oxygen vacancies in the microstructure of IZO thin films. I also found that these thin films transmit between 80% and 92% of light in the visible part of electromagnetic spectrum and the IZO thin film with a layered microstructure shows better transparency. |
