Photoresponse of pulsed laser deposited ZnO:Cu thin films
Graphical abstract
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)
- et al.
Structural transformations of TiO2 films with deposition temperature and electrical properties of nanostructure n-TiO2/p-Si heterojunction diode
J. Alloys Compd.
(2014) - et al.
Thickness-controlled photoresponsivity of ZnO:Al/Si heterostructures: Role of junction barrier height
Mater. Lett.
(2014) - et al.
The heterojunction effects of TiO2 nanotubes fabricated by atomic layer deposition on photocarrier transportation direction
Nanoscale Res. Lett.
(2012) - et al.
An EXAFS and XANES study of MBE grown Cu-doped ZnO
Nucl. Instrum. Meth. Phys. Res. Sect B Beam Interact. Mater. Atoms
(2003) - et al.
Electrodeposition of Cu-doped p-type ZnO nanorods; effect of Cu doping on structural, optical and photoelectrocatalytic property of ZnO nanostructure
Superlattices Microstruct.
(2018) - et al.
ZnO based visible–blind UV photodetector by spray pyrolysis
Superlattices Microstruct.
(2014) - et al.
Metal oxide heterojunction (NiO/ZnO) prepared by low temperature solution growth for UV-photodetector and semi-transparent solar cell
Sol. Energy
(2018) - et al.
Advances in ZnO-based materials for light emitting diodes
Curr. Opin. Chem. Eng.
(2014) - et al.
White light-driven photo response of TiO2 thin films: influence of substrate texturing
Sol. Energy
(2018) - et al.
Growth of TiO2thin films on chemically textured Si for solar cell applications as a hole-blocking and antireflection layer
Appl. Surf. Sci.
(2017)
Effect of ultra-thin ZnO coating layer on the device performance of TiO2 dye sensitized solar cell
Sol. Energy
Light-trapping Al-doped ZnO thin films for organic solar cells
Sol. Energy
ZnO gap engineering by doping with III–V compounds
J. Phys. Condens. Matter
ZnO-based dye-sensitized solar cells
J. Phys. Chem. C
Electrical characterization of Au/ZnO/Si Schottky contact
J. Phys. Conf. Ser.
Defects induced luminescence and tuning of bandgap energy narrowing in ZnO nanoparticles doped with Li ions
J. Appl. Phys.
Thickness-dependent blue shift in the excitonic peak of conformally grown ZnO: Al on ion-beam fabricated self-organized Si ripples
J. Appl. Phys.
Quantum confined atoms of doped ZnO nanocrystals
Phys. Status Solidi
ZnO UV photodetector with controllable quality factor and photosensitivity
AIP Adv.
Direct probe of heterojunction effects upon photoconductive properties of TiO 2 nanotubes fabricated by atomic layer deposition
Nanotechnology
ZnO-based ultra-violet light emitting diodes and nanostructures fabricated by atomic layer deposition
Semicond. Sci. Technol.
A XANES study of Cu valency in Cu-doped epitaxial ZnO
Phys. Status Solidi
Role of copper in the green luminescence from ZnO crystals
Appl. Phys. Lett.
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.
Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review
Energy Environ. Sci.
Colloidal chemically fabricated ZnO: Cu-based photodetector with extended UV-visible detection waveband
Nanoscale
Green luminescence from Cu-doped ZnO nanorods: role of Zn vacancies and negative thermal quenching
Appl. Phys. Lett.
Zinc oxide—from synthesis to application: a review
Materials (Basel)
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