The effect of annealing temperature on optical and structural properties of ZnO thin films

ZnO ince filmlerin optik ve yapısal özelliklerine ısıl işlem sıcaklığının etkisi

Abstract

Zinc oxide (ZnO) is an n-type semiconducting material with a ~3.3 eV band gap and high transparency over a wide range of the electromagnetic spectrum. Zinc oxide has been studied widely due to its inexpensiveness, non-toxicity and since it is a potential alternative to other transparent conducting materials. ZnO thin films are important due to their potential for applications such as gas sensors, solar cells, optoelectronic devices, transparent conducting electrodes and optical waveguides. ZnO thin films have been prepared by various methods such as chemical vapor deposition, pulsed laser deposition, sputtering, r.f. magnetron sputtering, spray pyrolysis and the sol-gel process. Among them, the sol-gel method is preferred since it has the advantages of easy control of the film composition and easy fabrication of large-area films with low cost. There are many factors that affect the optical and microstructural properties of sol-gel-made films. Among them are the chemical composition and concentration of the solution, the coating parameters, the thickness of the film, the preheating temperature and time between each layer deposition, and the final annealing temperature. In this study, transparent ZnO thin films were prepared on Corning 2947 substrates by the sol-gel spin coating method. ZnO films were annealed at 100, 250, 350 and $550^0C$ for 1 hour. The effects of annealing temperature on microstructure and optical properties were investigated. The coating solution was prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)2 .2H2O, 98%, Aldrich) in 2-propanol (99.5%, Aldrich) and stirring by a magnetic stirrer at 60oC for 10 min. Diethanolamine (DEA, 99%, Aldrich) was added drop by drop to the solution during stirring. Then, distilled water was slowly added to the solution and stirred 10 min more. Finally, a clear and homogeneous solution was obtained. (DEA:ZnAc=1:1 [mol ratio] and H2O:ZnAc=2:1 [mol ratio]). An amount of ZnO solution was dropped onto Corning 2947 glass substrates on the spin coater, which were rotated at 3000 rpm for 30 s. The films were dried at 100oC for 2 min to evaporate the solvent and remove organic residuals. The coating procedure was repeated six times to reach the desired thickness. Finally, the films were heat treated at 100, 250, 350 and $550^0C$ for 1 hour. All coatings were made at room temperature (~21oC) and ~55% relative humidity. Optical transmittance and reflectance of the ZnO films were measured in the spectral range of 300– 1000 nm using an NKD 7000 (Aquila, UK) spectrophotometer. Measurements were made with a beam incidence angle of 30o using s-polarized light. AFM images of the films were obtained using a SPM- 9500J3 (Shimadzu, Japan) scanning probe microscope in contact mode. The crystalline structures of the films were analyzed by an X-ray diffractometer (Philips PW3710, CuKα radiation). All the ZnO thin films were highly transparent with a transmittance of ~85% in the visible range. The ZnO thin films annealed at 100, 250, 350 and $550^0C$ had transmittance values of 88.4%, 86%, 87.1%, 86.7% and reflectance values of 10.5%, 13.8%, 12.7%, 13.2% at the wavelength of 550 nm, respectively. The effect of the annealing temperature was significant between the wavelength range of 300–400 nm. The peaks in this range for films annealed at 350 and $550^0C$ are a property of semiconducting materials due to excitonic absorption. ZnO has a high excitonic binding energy (60 meV) which causes exciton- related emission in the ultraviolet region. The XRD diffraction peaks belonging to the (100), (002), (101), (102) (110), (103) and (112) planes were seen only in ZnO films annealed at $550^0C$ for 1 hour. The film annealed at $550^0C$ was crystalline with hexagonal wurtzite structure. However, there were no peaks for the other films annealed at 100, 250 and $350^0C$ for 1 hour. ZnO thin films had smooth surfaces with RMS values of 7.5, 1.2, 0.5 and 3.3 nm for annealing temperatures of 100, 250, 350 and $550^0C$, respectively. This study showed that annealing temperature has a considerable effect on the optical and structural properties of ZnO thin films.

Özet

Çinko oksit (ZnO) yasak band aralığı yaklaşık 3.3 eV olan ve elektromanyetik spektrumun geniş bir aralığında yüksek geçirgenliğe sahip yarıiletken bir malzemedir. Çinko oksit ucuzluğu, sağlığa zararlı olmaması ve diğer şeffaf iletken malzemelere alternatif olma potansiyelinden dolayı son yıllarda yaygın olarak çalışılmaktadır. Bu çalışmada sol-jel döndürerek kaplama metodu ile Corning 2947 taşıyıcılar üzerine çinko oksit ince filmler hazırlanmıştır. Hazırlanan filmlere 100, 250, 350 ve 550oC’de ısıl işlem uygulamıştır. Farklı ısıl işlem sıcaklıklarının çinko oksit ince filmlerin optik ve yapısal özellikleri üzerine etkisi araştırılmıştır. Optik parametreler NKD spektrofotometresinden elde edilmiştir. Filmlerin kristal yapısı ve yüzey morfolojisi sırasıyla X-Işını Kırınımı (X-Ray Diffraction, XRD) ve Atomik Kuvvet Mikroskobu (Atomic Force Microscopy, AFM) ile analiz edilmiştir. Farklı ısıl işlem uygulanan filmlerin tamamının görünür bölgede oldukça yüksek geçirgenliğe sahip oldukları görülmüştür. 300–400 nm dalgaboyu aralığında ise ısıl işlem sıcaklığının etkisi daha belirgin ortaya çıkmıştır. XRD sonuçlarına göre 100, 250 ve $350^0C$ ısıl işlem sıcaklığına tabi tutulan filmlerin amorf yapıda oldukları; $550^0C$’de ısıl işlem uygulanan filmin ise kristal yapıda olduğu görülmüştür. AFM resimlerinden elde edilen sonuçlara göre ısıl işlem sıcaklığının artmasıyla filmlerin yüzey morfolojilerinin de önemli ölçüde değiştiği görülmüştür. Hazırlanan ZnO filmlerin tamamı uygulanan bütün ısıl işlem sıcaklıklarında 7.5 nm ve altındaki Rms değerleriyle oldukça pürüzsüz yüzeylere sahiptir. En düşük pürüzlülüğe sahip film $350^0C$’de elde edilmiştir.