Elsevier

Superlattices and Microstructures

Volume 75, November 2014, Pages 378-389
Superlattices and Microstructures

Structural and optical characterization of ZnO thin films for optoelectronic device applications by RF sputtering technique

https://doi.org/10.1016/j.spmi.2014.07.032Get rights and content

Highlights

Abstract

This work reports structural and optical study of ZnO thin films grown over p-type silicon (Si) and glass substrates by RF magnetron sputtering technique. Surface morphological and optical properties of thin film have been studied using X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray (EDX), ellipsometry and photoluminescence (PL) spectroscopy. Strong diffraction peak (0 0 2) obtained from XRD spectra of ZnO thin film indicates a preferential growth of single crystalline thin film along the c-axis oriented phase of hexagonal wurtzite structure. Surface morphological feature reveals uniform growth of undoped ZnO thin film over the substrate. Different important microstructural parameters for the film such as grain size, lattice parameters, defect density, stress and strain have been obtained. Optical properties such as transmittance, reflectance, absorption coefficient, refractive index and dielectric constant for a spectral range of 300–800 nm have been evaluated. A good optical transmittance of 83–92% has been observed for visible region, and the optical bandgap of ZnO films was found to be 3.23 eV. Energy Loss Function (ELF) and photoluminescence (PL) spectra for ZnO thin film has also been analyzed and reported.

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)

  • R. Romero et al.

    Mater. Sci. Eng.: B

    (2004)
  • S. Sharma et al.

    Superlattices Microstruct.

    (2014)
  • J.L. Deschanvres et al.

    Sens. Actuators A

    (1992)
  • L. Znaidi

    Mater. Sci. Eng. B

    (2010)
  • S.H. Bae et al.

    Appl. Surf. Sci.

    (2001)
  • R. Al Asmar et al.

    Microelectron. Eng.

    (2006)
  • V. Bilgin et al.

    Mater. Chem. Phys.

    (2005)
  • R.K. Gupta et al.

    Spectrochim. Acta Part A Mol. Biomol. Spectrosc.

    (2012)
  • A.M. Salem et al.

    Physica B: Phys. Condens. Mat.

    (2008)
  • B.J. Jin et al.

    Thin Solid Films

    (2000)
  • Q.P. Wang et al.

    Appl. Surf. Sci.

    (2002)
  • R. Ghosh et al.

    Appl. Phys. Lett.

    (2007)
  • S. Sharma et al.

    J. Electron Dev.

    (2014)
  • L. Beaur et al.

    Phys. Rev. B

    (2011)
  • Cited by (0)

    View full text