Band gap widening and narrowing in Cu-doped ZnO thin films

doi:10.1016/j.jallcom.2016.04.093

Journal of Alloys and Compounds

Volume 680, 25 September 2016, Pages 252-258

https://doi.org/10.1016/j.jallcom.2016.04.093 Get rights and content

Highlights

•
Band gap widening and narrowing upon copper doping in ZnO.
•
Dissociation of CuO nanophase in ZnO host by high temperature annealing.
•
Mechanisms of band gap tuning in framework of band gap renormalization.
•
Effects of stress due to creation of intrinsic defects and secondary phase formation.

Abstract

Cu-doped Zinc Oxide (Cu single bond ZnO) thin films have been prepared on silicon and quartz substrates by sol-gel spin coating method. These films were further annealed at 700 °C in controlled oxygen environment for improving crystalline quality and stoichiometry. In order to understand the material properties of thin films, X-ray Diffraction, Scanning Electron Microscopy, Atomic Force Microscopy, Raman Spectroscopy and UV–Visible Spectroscopy methods have been used in the present study. Polycrystalline nature of the Cu single bond ZnO thin films is confirmed by structural analysis with increased crystallite size upon increasing the doping concentrations of the thin films. The phonon evolution at different doping concentrations is understood by Raman spectroscopy, thus confirming the wurtzite phase stability of the thin films with increase in crystallite size and presence of mixed phases have also been reported. In addition, widening and narrowing of band gap for the Cu single bond ZnO thin films have been discussed in optical studies. This anomalous behavior in band gap tuning such as widening (+0.5 eV) and narrowing (−0.6 eV) under different doping concentrations and annealing temperature is understood by the influence of induced stress effects due to creation of intrinsic defects with band gap renormalization and secondary phase formation.

Graphical abstract

Introduction

ZnO thin films are considered as one of the most promising optoelectronic materials due to its large band gap (E_g = 3.37 eV) at room temperature and also one of the versatile materials with vast range of applications in solar cells, gas sensors, optoelectronic devices etc, due to their unique optical and electrical properties [1], [2], [3], [4], [5], [6]. The role of doping in ZnO has to be explored further which may be greatly influenced by chemical doping or by inducing intrinsic lattice defects. Copper doping in ZnO modifies the photoluminescence transitions by creating localized impurity levels [7]. The diffusion of Cu into ZnO can lead to formation of various defect centers (Cu_Zn, Cu_i) since there is a possibility that Cu can replace either substitutional or interstitial Zn atoms in ZnO lattice leading to structural deformations [8], [9], [10], [11]. As Cu is a prominent luminescence activator in II–VI compounds and Cu atoms can further enhance the oxygen adsorption capacity of thin film surfaces. Hence, it is quite significant that ZnO thin films doped with Cu will assist possible modifications in physical and optical properties, since metal dopants play a decisive role for optoelectronic applications. Lattice dynamical properties are affected by partial cationic substitution in ZnO lattice due to change in the effective mass and force constant in the local lattice which results in vibration of optical modes at the center of the Brillouin zone. The presence of local vibrational modes related to the external impurities and the phase segregation can be observed as a result of doping. Raman spectroscopic technique is one of the non-destructive tools that has been exploited here to understand the phonon kinetics of Cu-doped ZnO thin films. This technique is also useful for studying the effects of increase in local strain, substitutional effects, lattice distortion, presence of structural defects and non stoichiometric incorporation either by doping or by any other method. Evolution of Raman modes can be ascribed to the effect of disorder and lattice defects induced either by doping or by annealing which helps in understanding the underlying interaction processes [12], [13], [14]. Impurity induced phonon modes for ZnO thin films doped with N, Fe, Al, Ga, Co and Mn which have been studied by Raman spectroscopy technique [15], [16]. Origin of modes has been attributed to the breakdown of translational symmetry of the lattice which is due to the defects or impurities induced by introducing the dopants into the undoped lattice [17]. Cu incorporation in ZnO thin films leads to intense evolution of longitudinal optical (LO) phonon modes even at low solubility limit for which a few reports are available. The aim of present investigation is to study and analyze the structural and optical modifications in undoped and Cu-doped ZnO thin films fabricated by sol-gel spin coating technique and thus investigating Raman spectra to correlate this analysis to lattice defects and then to predict the optical and structural properties of the system. The modifications in the physical and chemical properties of ZnO thin films are hence studied by introducing metal cationic dopant into the lattice structure at different concentrations in order to understand the observed modifications.

Section snippets

Experimental

Cu doped ZnO thin films were deposited on silicon and quartz substrate of about 300 nm thickness by sol-gel spin coating technique which is a chemical route and also cost effective procedure [18]. For designing Cu single bond ZnO thin film at different concentrations, CuZnO solution was prepared by mixing Copper chloride (CuCl) and Zinc acetate dihydrate Zn(CH₃COO)₂·2H₂O with 2-methoxyethanol at room temperature, followed by the addition of mono ethanolamine (MEA) which is a highly water soluble, non-ionic

Structural studies

Fig. 1 shows the characteristic X-ray diffraction (XRD) pattern of 700 °C annealed Cu-doped ZnO thin films. The polycrystalline nature with hexagonal wurtzite structure of ZnO has been confirmed from the obtained pattern. The diffraction peaks are oriented in (100), (002) and (101) planes in accordance with JCPDS# 36-1451. The peak positions of (002) orientations for all five samples are shown. The shift in the peak position with respect to undoped samples relates with uniform diffusion and

Conclusion

Cu-doped ZnO thin films have been synthesized by sol-gel spin coating technique. GAXRD results show the polycrystalline thin films with increased crystallite size and decreased stress and strain values along with phase segregation on varying doping concentration. Raman structural analysis also confirms the stress effect on suppressed E₂ (high) mode and CuO phase formation upon doping ZnO thin films with Copper. The increase in crystallite size is thereby confirmed by XRD, SEM and AFM analysis.

Acknowledgements

Authors are grateful to the Director (IUAC), New Delhi, for providing support throughout work. Authors are also thankful to Dr. S.A. Khan and Dr. (Mrs.) I. Sulania for experimental support in SEM and AFM investigations, respectively; while Mr. A. Das for the critical reading of the manuscript. One of the authors (KJ) is also thankful to UGC for providing the financial support under the scheme of Ph.D. fellowship.

References (39)

D. Ehrentraut et al.
Fabrication of homoepitaxial ZnO films by low-temperature liquid-phase epitaxy
J. Cryst. Growth
(2006)
L. Schmidt-Mende et al.
ZnO–nanostructures, defects, and devices
Mater. Today
(2007)
J. Chen et al.
Effect of sputtering oxygen partial pressures on structure and physical properties of high resistivity ZnO films
Appl. Surf. Sci.
(2004)
K. Yamamoto et al.
Preparation and characterization of ZnO nanowires
Physica E Low. Dimens. Syst. Nanostruct.
(2004)
V. Kumar et al.
Synthesis and characterization of aluminum–boron co-doped ZnO nanostructures
Mater. Res. Bull.
(2013)
D.C. Agarwal et al.
SHI induced modification of ZnO thin film: Optical and structural studies
Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms.
(2006)
R.G. Singh et al.
Growth kinetics of ZnO nanocrystallites: Structural, optical and photoluminescence properties tuned by thermal annealing
Curr. Appl. Phys.
(2011)
V. Kumar et al.
Highly transparent and conducting boron doped zinc oxide films for window of Dye Sensitized Solar Cell applications
J. Alloys Compd.
(2012)
M. Chen et al.
Surface characterization of transparent conductive oxide Al-doped ZnO films
J. Cryst. Growth
(2000)
H. Schulz et al.
Structure parameters and polarity of the wurtzite type compounds Sic-2H and ZnO
Solid State Commun.
(1979)

T.P. Rao et al.

Effect of stress on optical band gap of ZnO thin films with substrate temperature by spray pyrolysis

J. Alloys Compd.

(2009)

M.P.S. Rana et al.

Band gap engineering and low temperature transport phenomenon in highly conducting antimony doped tin oxide thin films

Ceram. Int.

(2016)

Z.F. Liu et al.

Photoluminescence of ZnO: Ga thin films fabricated by pulsed laser deposition technique

J. Electroceram.

(2004)

D.C. Look et al.

Characterization of homoepitaxial p-type ZnO grown by molecular beam epitaxy

Appl. Phys. Lett.

(2002)

S.J. Pearton et al.

Wide band gap ferromagnetic semiconductors and oxides

J. Appl. Phys.

(2003)

T.M. Hammad et al.

Optical and magnetic properties of Cu-doped ZnO nanoparticles

J. Mater. Sci. Mater. Electron.

(2013)

D.L. Hou et al.

Magnetic properties of n-type Cu-doped ZnO thin films

Appl. Phys. Lett.

(2007)

N.Y. Garces et al.

Role of copper in the green luminescence from ZnO crystals

Appl. Phys. Lett.

(2002)

S.Y. Li et al.

Field emission and photofluorescent characteristics of zinc oxide nanowires synthesized by a metal catalyzed vapor-liquid-solid process

J. Appl. Phys.

(2004)

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View full text

Band gap widening and narrowing in Cu-doped ZnO thin films

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Structural studies

Conclusion

Acknowledgements

J. Cryst. Growth

Mater. Today

Appl. Surf. Sci.

Physica E Low. Dimens. Syst. Nanostruct.

Mater. Res. Bull.

Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms.

Curr. Appl. Phys.

J. Alloys Compd.

J. Cryst. Growth

Solid State Commun.

J. Alloys Compd.

Ceram. Int.

Photoluminescence of ZnO: Ga thin films fabricated by pulsed laser deposition technique

J. Electroceram.

Characterization of homoepitaxial p-type ZnO grown by molecular beam epitaxy

Appl. Phys. Lett.

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J. Appl. Phys.

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J. Mater. Sci. Mater. Electron.

Magnetic properties of n-type Cu-doped ZnO thin films

Appl. Phys. Lett.

Role of copper in the green luminescence from ZnO crystals

Appl. Phys. Lett.

Field emission and photofluorescent characteristics of zinc oxide nanowires synthesized by a metal catalyzed vapor-liquid-solid process

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