Structural and optical characterization of ZnO thin films for optoelectronic device applications by RF sputtering technique
Introduction
In recent years, ZnO has emerged as a promising material for a variety of optoelectronic and piezoelectric device applications. Its large bandgap (3.37 eV) and high exciton binding energy (60 meV) ensures its suitability for a variety of applications such as photodetectors (UV), room temperature LEDs, solar cells and gas sensors [1], [2], [3], [4]. High exciton binding energy of ZnO attributes to stability of electron–hole pairs at room temperature for electroluminescence. Therefore, ZnO is a potential candidate for highly efficient lasers and LEDs, provided a good quality of p-doped ZnO thin film can be grown. Good and stable photoluminance of ZnO at higher temperature ranges makes it an extremely promising material for variety of optoelectronic device applications [5]. ZnO’s blue region electromagnetic spectrum emission capability, high light–matter coupling strength and stability of its excitons at room temperature is very useful for the realization of new generation optoelectronic devices such as polariton lasers at room temperature [6]. High piezoelectric coefficient and large electromechanical coupling coefficient of ZnO nanostructures ensure its usability for piezoelectric and micro-electromechanical (MEMS) device applications [7], [8], [9].
ZnO nanostructures with single crystalline orientation along the c-axis are important for improved device performance in a variety of nanoelectronic applications. In the past, different techniques such as sol–gel, thermal vapor deposition, pulsed laser deposition, e-beam deposition and RF sputtering have been reported for growing ZnO thin films [10], [11], [12], [13]. Among these methods, RF sputtering has drawn wide attention. In RF sputtering, different parameters such as pressure, temperature, deposition time, gas flow rate and RF power can control the properties of grown thin films. Easy control for desired crystalline orientation, good interfacial adhesion with substrate, epitaxial growth at relatively low temperature and high packing density of grown thin film are some properties which make RF sputtering a suitable choice for growing ZnO thin films. This work reports a systematic study of structural and optical properties of ZnO thin film deposited on p-type Si and glass substrates using RF sputtering technique. Different microstructural (crystallographic orientation, roughness, grain size, lattice parameters, defect density, stress, strain) and optical parameters (transmittance, reflectance, optical bandgap, refractive index, dielectric constant, surface and volume energy loss function, photoluminescence) of the deposited ZnO thin film have been studied and reported.
Section snippets
ZnO thin film preparation
Nanocrystalline ZnO thin film was deposited on p-Si 〈1 0 0〉 and glass substrate using RF magnetron sputtering technique. Thickness and resistivity of p-Si substrate was 380 μm and 8–10 Ω cm respectively. Prior to deposition, both Si wafer and glass substrate were cleaned properly. Standard RCA-1 and RCA-2 cleaning process were used for wafer cleaning. RCA-1 (Solution of NH4OH, H2O2 and Deionized Water (DI) in the ratio of 1:1:5) was used for removal of organic contamination and RCA-2 (Solution of
Structural and surface morphology study
Fig. 1 shows XRD spectra of ZnO thin film. A unique diffraction peak corresponding to 0 0 2 orientation at 2θ = 34.48° was obtained. This confirms single crystalline nature of the deposited ZnO thin film. The unique peak obtained from XRD results also attributes to good crystallinity of ZnO thin film along the c-axis. Various important micro-structural parameters such as grain size, lattice parameters, defect density, residual stress and lattice strain for ZnO thin film have also been derived.
Conclusion
Preparation of nanocrystalline ZnO thin film grown by RF sputtering technique has been reported. It has been observed that RF sputtered ZnO thin film has shown a good crystalline nature with minimal surface roughness. Different microstructural parameters of thin film have been estimated using XRD analysis. A very good transmittance of 83–92% has been observed in the visible region for ZnO thin film. Optical band gap was found to be 3.23 eV. Various other optical parameters such as reflectance,
Acknowledgement
The authors are thankful to Material Research Centre, Malaviya National Institute of Technology, Jaipur, Rajasthan, India and Centre of Interdisciplinary Research, Motilal Nehru National Institute of Technology, Allahabad, U.P., India for extending fabrication and characterization related facilities to complete this work.
References (28)
- et al.
Mater. Sci. Eng.: B
(2004) - et al.
Superlattices Microstruct.
(2014) - et al.
Sens. Actuators A
(1992) Mater. Sci. Eng. B
(2010)- et al.
Appl. Surf. Sci.
(2001) - et al.
Microelectron. Eng.
(2006) - et al.
Mater. Chem. Phys.
(2005) - et al.
Spectrochim. Acta Part A Mol. Biomol. Spectrosc.
(2012) - et al.
Physica B: Phys. Condens. Mat.
(2008) - et al.
Thin Solid Films
(2000)