Improved structural, optical, and ultraviolet sensing properties of ZnO thin films and nanorods by using lithium and aluminum doping

다국어 초록 (Multilingual Abstract)

We have conducted the three researches on the influence of doping on structural, optical, and ultraviolet sensing properties of zinc oxide (ZnO) thin films and nanorods.
Firstly, we prepared Li-doped ZnO (LZO) thin films spin-coated by the sol-gel method on Si substrates. The analysis of X-ray diffraction (XRD) patterns showed that the c-axis orientation was improved with increased Li doping concentration and the highest crystal quality was obtained at 2.5 at.% Li content. Analysis of photoluminescence (PL) spectra indicates an increase in the formation of substitutional Li at the Zn site. For investigating ultraviolet (UV) photoresponse properties, we fabricated metal-semiconductor-metal UV sensors through thermal evaporation of indium. LZO thin film-based UV sensors showed higher UV sensitivity and photoresponse rate compared with ZnO thin film-based UV sensor.
Secondly, we prepared Al-doped ZnO (AZO) thin films using sol-gel spin-coating method. The XRD patterns revealed that the c-axis lattice constant increased and the average grain size decreased with the Al content. In field-emission scanning electron microscopy (FE-SEM) images, the grain size decreased and the film density increased with increased Al concentration. The absorbance spectra showed that ultraviolet absorbance of the AZO thin films increased and absorption onset shifted towards longer wavelength with Al content. The PL spectra indicated that the intensity of the visible emission significantly decreased and the visible emission peak was red-shifted with Al content. The AZO thin film-based UV sensor exhibited a higher responsivity than ZnO thin film-based UV sensor.
We finally fabricated ZnO nanorod array (NA) on undoped ZnO and AZO seed layer using hydrothermal method. In FE-SEM, the average grain size and density of AZO seed layer decreased and increased, respectively, compared with that of undoped ZnO seed layer, resulting in increase in the length and density of ZnO NA on AZO seed layer. The XRD patterns and PL spectra showed that the intensity of (002) peak and UV emission for ZnO NA on AZO seed layer were higher than that of ZnO NA on undoped ZnO seed layer. On the other hand, the intensity of visible emission for ZnO NA on AZO seed layer was lower than that for ZnO NA on undoped ZnO seed layer. The UV-vis spectra exhibited the absorbance of ZnO NA on AZO seed layer in UV region increased compared to ZnO NA on undoped ZnO seed layer. The responsivity and sensitivity of UV sensor based on ZnO NA on AZO seed layer were higher than that based on ZnO NA on undoped ZnO seed layer.

목차 (Table of Contents)

• Contents
• Abstract ------------------------------------------------------------------------------------- i
• Contents ---------------------------------------------------------------------------------- iv
• List of Figures ---------------------------------------------------------------------------- vi
• 1. Introduction ---------------------------------------------------------------------- 1
• References ------------------------------------------------------------------------------- 3
• 2. Background Review ------------------------------------------------------------------- 6
• 2.1 Characteristic of ZnO -------------------------------------------------------------- 6
• References ------------------------------------------------------------------------------- 7
• 2.2 Applications of ZnO --------------------------------------------------------------- 8
• 2.2.1 UV sensors ---------------------------------------------------------------------- 8
• 2.2.2 Gas sensors--------------------------------------------------------------------- 10
• 2.2.3 Dye-sensitized solar cells --------------------------------------------------- 11
• 2.2.4 Quantum dot light emitting diodes----------------------------------------- 12
• References ------------------------------------------------------------------------------- 13
• 3. Experimental Details ---------------------------------------------------------------- 16
• 3.1 Growth and deposition method ------------------------------------------------- 16
• 3.1.1 Sol-gel process -------------------------------------------------------- 16
• 3.1.2 Hydrothermal synthesis --------------------------------------------- 18
• 3.2 Characterization techniques ----------------------------------------------------- 19
• 3.2.1 X-ray diffraction ----------------------------------------------------------- 19
• 3.2.2 Scanning electron microscopy -------------------------------------------- 20
• 3.2.3 Absorbance -------------------------------------------------------- 22
• 3.2.4 Photoluminescence--------------------------------------------------------- 23
• 3.2.5 UV sensing---------------------------------------------------------------- 25
• 4. Li-doped ZnO thin films ------------------------------------------------------------ 27
• 4.1 Experimental process and measurement --------------------------------------- 27
• 4.2 Structural properties -------------------------------------------------------------- 28
• 4.3 Optical properties ----------------------------------------------------------------- 29
• 4.4 UV photoresponse properties --------------------------------------------------- 29
• 4.5 Conclusions ------------------------------------------------------------------------ 33
• References ----------------------------------------------------------------------------- 34
• 5. Al-doped ZnO thin films ----------------------------------------------------------- 35
• 5.1 Experimental process and measurement ---------------------------------- 35
• 5.2 Structural and morphological properties --------------------------------------- 36
• 5.3 Optical properties ----------------------------------------------------------------- 38
• 5.4 UV photoresponse properties --------------------------------------------------- 40
• 5.5 Conclusions ------------------------------------------------------------------------ 44
• References ------------------------------------------------------------------------------ 45
• 6. ZnO nanorods on Al-doped ZnO seed layer ------------------------------------ 47
• 6.1 Experimental process and measurement ---------------------------------- 47
• 6.2 Morphological and structural properties --------------------------------------- 48
• 6.3 Optical properties ------------------------------------------------------------------ 51
• 6.4 UV photoresponse properties ---------------------------------------------------- 52
• 6.5 Conclusion -------------------------------------------------------------------------- 55
• References ------------------------------------------------------------------------------ 56
• Acknowledgements --------------------------------------------------------------------- 58