Elsevier

Solar Energy

Volume 207, 1 September 2020, Pages 228-234
Solar Energy

Photoresponse of pulsed laser deposited ZnO:Cu thin films

https://doi.org/10.1016/j.solener.2020.06.072Get rights and content

Highlights

  • 2 wt.% Cu-doped ZnO thin films are deposited on p-Si substrates using pulsed laser deposition without any gas insertion.

  • Current-voltage characteristics show a diode-like rectifying behaviour under both dark and white light illumination.

  • Upon white light illumination, junction barrier height and ideality factor reduce.

  • A superior and stable photo-response under reverse bias is exhibited by ZnO:Cu/p-Si heterojunction.

  • Device shows fast photoresponse (rise and fall times <150 ms) in reverse biased condition.

Abstract

Cu-doped ZnO (2 wt%) thin films are deposited on p-Si (1 0 0) substrates using pulsed laser deposition (PLD) technique at room temperature. X-ray diffraction study shows the amorphous nature of these films, whereas scanning electron microscopy and atomic force microscopy images show their granular nature. A diode-like rectifying behaviour is revealed by the current-voltage characteristics of ZnO:Cu/p-Si heterojunction under both dark and light illumination conditions. In addition, it is demonstrated that there is a large enhancement in the photocurrent along with high sensitivity and responsivity of the diode. This study will be useful for the fabrication of ZnO:Cu based photodetectors.

Introduction

Zinc oxide (ZnO) has drawn significant attention, in view of its plenty of potential applications in optoelectronics, which includes light-emitting diodes (Chen et al., 2012, Pearton and Ren, 2014, Wang et al., 2015, Willander et al., 2009), photodetectors (Campos et al., 2013, Inamdar et al., 2014, Law and Thong, 2006, Liu et al., 2010, Tian et al., 2014), solar cells (Anta et al., 2012, Gholizadeh et al., 2017, Gonzalez-Valls and Lira-Cantu, 2009, Klochko et al., 2018, Tseng et al., 2015, Wu et al., 2013, Yu et al., 2017), gas sensors (Kumar et al., 2015, Tiwale, 2015), field emitters (Marathe et al., 2006, Sui et al., 2013, Zhang et al., 2015), and piezoelectric devices (Özgür et al., 2010Özgür et al., 2010Özgür et al., 2010Özgür et al., 2010, Wu and Wang, 2016), etc. ZnO is a wide bandgap (3.37 eV) n-type semiconductor with a large exciton binding energy (60 meV) at 25 °C. Due to its wide bandgap, ZnO is active only under ultraviolet light and remains transparent for the visible light. Hence, to enable the visible light activity of ZnO its band gap tunability becomes exigent. The band gap can be engineered by creating vacancies, substitution, or interstitial-type doping of materials (Andriotis and Menon, 2016, Awan et al., 2014, Basu et al., 2015, Basu et al., 2014, Bhargava et al., 2002, Kumar et al., 2016, Saini et al., 2018, Wang et al., 2012). For instance, presence of Cu impurities yield acceptor states within the bandgap of ZnO and ion substitution is favoured due to the comparable atomic radii of Zn2+ (74 pm) with Cu+ (77 pm) and Cu2+ (73 pm) (Fons et al., 2002, Fons et al., 2003). Wang et al. have implanted Cu into ZnO nanorod arrays and shown the optical absorption in visible region (Wang et al., 2014). In another report, Garces et al. have shown green luminescence in Cu-doped ZnO crystals due to the presence of oxygen vacancies (Garces et al., 2002). Likewise, Kouklin has shown blue-green emission bands in photoluminescence spectrum of Cu-doped ZnO nanowires (Kouklin, 2008). Likewise, Huang et al. have shown the role of Zn vacancies towards green luminescence from ZnO:Cu nanorods (Huang et al., 2013). Further, the visible and UV light sensitivities of ZnO are modulated by varying the concentration of doped Cu atoms (in the range of 0.2 to 1.5 at.%) which reveal that visible light activity dominates over UV light for the highest amount of Cu-doping (Hu et al., 2013). It is also reported that oxygen vacancies help in narrowing the band-gap of ZnO and in turn improve its photo catalytic activity under visible light (Wang et al., 2012). However, the role of oxygen vacancies in photoresponse of ZnO:Cu thin films under visible light is still lacking. Thus, to push the photosensitivity of ZnO from UV to visible light and explore the photoresponse under white light illumination, it is proposed to study the synergetic effect of Cu-doping and oxygen vacancies in ZnO matrix.

In this study, we investigate the I-V characteristics of Cu-doped ZnO (ZnO:Cu) thin films deposited on p-type Si substrates using pulsed laser deposition technique at room temperature (RT). It is observed that ZnO:Cu/Si heterojunction shows a diode-like-rectifying behaviour under dark as well as white light illumination. In addition, it is demonstrated that there is a large enhancement in photocurrent with improved sensitivity and responsivity of the diode. We have also studied the visible light induced photo response under forward and reverse biased conditions. The present results are explained in light of electronic transport due to a change in the barrier height across the interface of differently biased CZO/Si heterojunction under white light illumination. The present study will be useful in fabricating ZnO:Cu thin film-based photodetectors operated under reverse biased condition with fast response times and an excellent stability.

Section snippets

Experimental detail

ZnO:Cu (2 wt%) films were deposited on ultrasonically cleaned p-Si(1 0 0) substrates (B-doped, Resistivity = 0.01–0.05 Ω-cm) at room temperature (RT) by pulsed laser deposition system (Excel Instruments, India). We have used KrF excimer laser (248 nm) with the repetition rate of 10 Hz for the ablation of ZnO:Cu (2 wt%) target. The measured laser energy density on the target was 2.5 J cm−2, whereas the working pressure during deposition was 2 × 10−6 mbar (without any reactive gas injection). The

Results and discussion

Fig. 1(a) shows the AFM topographic image of a CZO film which is granular in nature. Analysis reveals that the film has a root mean square (RMS) roughness of 9.5 nm and an average feature height of 26 nm. Thickness of CZO films is measured from various randomly chosen places at the interface of masked region on the sample (data not shown) and is observed to be 60 ± 2 nm. The deposited film is amorphous in nature which is confirmed by XRD measurements which does not show any crystalline peak [

Conclusions

In conclusion, we have deposited CZO thin films on p-type Si substrate using PLD to study the photoresponse of CZO/p-Si heterojunction. The as-deposited films are amorphous and granular in nature, as confirmed by XRD and AFM data, respectively. Optical studies show <20% reflectance in the visible region, revealing the visible light absorption at the CZO/p-Si heterojunction. The n-CZO/p-Si heterojunction shows a rectifying behaviour in I-V characteristics. Photoresponse of the heterojunction is

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge Dr. Sanjeev Kumar Srivastava from IIT Kharagpur, India for XPS measurements.

References (56)

  • K. Wu et al.

    Effect of ultra-thin ZnO coating layer on the device performance of TiO2 dye sensitized solar cell

    Sol. Energy

    (2013)
  • Xuan Yu et al.

    Light-trapping Al-doped ZnO thin films for organic solar cells

    Sol. Energy

    (2017)
  • A.N. Andriotis et al.

    ZnO gap engineering by doping with III–V compounds

    J. Phys. Condens. Matter

    (2016)
  • J.A. Anta et al.

    ZnO-based dye-sensitized solar cells

    J. Phys. Chem. C

    (2012)
  • M. Asghar et al.

    Electrical characterization of Au/ZnO/Si Schottky contact

    J. Phys. Conf. Ser.

    (2013)
  • S.U. Awan et al.

    Defects induced luminescence and tuning of bandgap energy narrowing in ZnO nanoparticles doped with Li ions

    J. Appl. Phys.

    (2014)
  • T. Basu et al.

    Thickness-dependent blue shift in the excitonic peak of conformally grown ZnO: Al on ion-beam fabricated self-organized Si ripples

    J. Appl. Phys.

    (2015)
  • R.N. Bhargava et al.

    Quantum confined atoms of doped ZnO nanocrystals

    Phys. Status Solidi

    (2002)
  • L.C. Campos et al.

    ZnO UV photodetector with controllable quality factor and photosensitivity

    AIP Adv.

    (2013)
  • Y.-H. Chang et al.

    Direct probe of heterojunction effects upon photoconductive properties of TiO 2 nanotubes fabricated by atomic layer deposition

    Nanotechnology

    (2010)
  • M.-J. Chen et al.

    ZnO-based ultra-violet light emitting diodes and nanostructures fabricated by atomic layer deposition

    Semicond. Sci. Technol.

    (2012)
  • P. Fons et al.

    A XANES study of Cu valency in Cu-doped epitaxial ZnO

    Phys. Status Solidi

    (2002)
  • N.Y. Garces et al.

    Role of copper in the green luminescence from ZnO crystals

    Appl. Phys. Lett.

    (2002)
  • A. Gholizadeh et al.

    Efficiency enhancement of ZnO nanostructure assisted Si solar cell based on fill factor enlargement and UV-blue spectral down-shifting

    J. Phys. D. Appl. Phys.

    (2017)
  • I. Gonzalez-Valls et al.

    Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review

    Energy Environ. Sci.

    (2009)
  • L. Hu et al.

    Colloidal chemically fabricated ZnO: Cu-based photodetector with extended UV-visible detection waveband

    Nanoscale

    (2013)
  • X.H. Huang et al.

    Green luminescence from Cu-doped ZnO nanorods: role of Zn vacancies and negative thermal quenching

    Appl. Phys. Lett.

    (2013)
  • A. Kołodziejczak-Radzimska et al.

    Zinc oxide—from synthesis to application: a review

    Materials (Basel)

    (2014)
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