Applied Materials Today
Volume 20, September 2020, 100705
Journal home page for Applied Materials Today

Fabrication of flexible UV-B photodetectors made of MgxZn1-xO films on PI substrate for enhanced sensitivity by piezophototronic effect

https://doi.org/10.1016/j.apmt.2020.100705Get rights and content

Highlights

  • Fabrication of flexible UV-B photodetectors made of MgxZn1-xO films and Selective detection of the UV-B region (280 nm–320 nm) of the electromagnetic spectrum.

  • MgZnO photodetector on the flexible substrate has achieved high light to dark current ratio (~1630) and the UV-B-visible rejection ratio (~2.34×105).

  • Large boost in the sensitivity (~20 %) to light under the application of strain.

  • Performance of the photodetector strongly depends on the Mg alloy concentration.

  • Investigation of the synergistic effect of the flexible substrate, piezopotential, large band gap over the piezophototronic effect.

Abstract

In spite of intense research in MgZnO films for photodetector application, growth of MgZnO over a flexible substrate especially utilizing piezophototronic effect could hold a more promising approach. We demonstrate high performance UV-B photodetectors fabricated by using high quality, single phase, wurtzite MgxZn1-xO thin films deposited on a polyimide substrate by magnetron sputtering. The Mg content in the MgxZn1-xO films varies from 38.5 at.% to 44 at.% by changing the substrate temperature from room temperature to 250 °C. The MgZnO films have columnar nanorod like morphology and are highly oriented along the c-axis. The metal-semiconductor-metal photodetectors of the MgZnO films have achieved significantly high light to dark current ratio (~1630). The peak responsivity is 0.3 mA/W at 292 nm with a cutoff wavelength of 305 nm at 9 V bias voltage, selectively detecting the UV-B region (280 nm-320 nm) of electromagnetic spectrum and the UV-B-visible rejection ratio (~2.34×105) is much higher than MgZnO PDs on rigid substrates. The performance of the MgxZn1-xO photodetector is further enhanced under applied strain leading to significantly higher photocurrent (~4 µA) and light to dark contrast ratio (~11000), resulting in large boost in the sensitivity to light by 20 % depending on Mg concentration. The sensitivity to light (~20%) at a fixed strain of +0.29 % or -0.29% is also four times higher as that with 0 strain (~5%) demonstrating the synergistic effect of the piezophototronic effect along with the flexible substrate over the performance of the PDs.

Graphical abstract

UV-B photodetectors based on MgxZn1-xO films enhanced by piezophototronic effect over a flexible substrate.

We demonstrate high performance UV-B photodetectors fabricated by using high quality, single phase, wurtzite MgxZn1-xO thin films deposited on a flexible substrate (polyimide) by magnetron sputtering. The photodetector selectively detects the UV-B region (280 nm-320 nm) of electromagnetic spectrum and the performance is further enhanced under applied strain demonstrating the synergistic effect of the piezophototronic effect along with the flexible substrate over the performance of the PDs.

Image, graphical abstract
  1. Download : Download high-res image (119KB)
  2. Download : Download full-size image

Introduction

Ultraviolet (UV) photodetectors (PDs) have been the subject of magnificent research over the years because of their wide range of applications, including basic scientific research (for example: use in lithography), commercial and military requirements, optical/satellite communications, chemical/environmental/biological analysis, water sterilization, flame sensing and early missile detection [1], [2], [3], [4], [5]. The UV region (10 nm to 400 nm wavelength) of the electromagnetic spectrum is broadly classified as UVA (400 nm-320 nm), UVB (320 nm-280 nm) and UVC (wavelength less than 280 nm) [1]. Given that the UVC of solar irradiance is completely absorbed on the way to earth, only UVB has a direct impact on human health [6], since a moderate (excess) dose boosts the formation of vitamin D to possibly cause skin cancer by suppressing the immune system and, therefore, the detection/quantification of UVB is highly essential. The UV PDs that detect all UV radiations with wavelengths below 400 nm are visible blind PDs. An ideal PD should have low dark current, better signal-to-noise ratio, great sensitivity, superior responsivity, high speed, and improved stability at high temperature [3]. So far, a large variety of PDs are developed such as pyroelectric devices, vacuum tubes, charge coupled devices (CCD), photomultipliers tubes (PMT) and Si photodiodes [4,5]. However, all these devices have limitations. For example, the response in the CCD detectors is slow and not dependent on the wavelength; high voltage operation at ultrahigh vacuum conditions restricts the use of PMTs; and Si photodiodes essentially need external filters to block the low energy photons for UV detection which degrade the efficiency of the PDs. Wide band gap semiconductors such as diamond, SiC, III-nitrides (GaN, AlxGa1-xN) and II-VI materials (ZnO and its alloys, MgxZn1-xO) are envisioned as the potential alternatives because of their great performance at low cost and high stability against harsh chemical/thermal environments [1,2,4]. Of which, although nitrides are promising candidates for UV detection, the formation of a large concentration of defects (dislocation density = 107−9 cm−2 in AlxGa1-xN) significantly weaken the performance. In addition, the grain boundaries as well as non-diamond impurities in polycrystalline diamond greatly affect the response of the PDs.

ZnO [7], [8], [9], [10] has been studied extensively for the potential applications in many different fields, specifically in optoelectronics (UV PDs [11], light emitting diodes [12,13], laser diodes [14]), sensors [15] (ozone sensor [16], UV sensors [17], piezoresistive E-Skin Sensors [18]), thin film transistors and transparent conducting oxides [19]. ZnO is attributed with high chemical stability, strong radiation hardness, low cost, ease of fabrication, environmentally friendly, high exciton binding energy (60 meV) and a large bandgap (3.37 eV) at room temperature [2,4]. Further practical implementation of ZnO in optoelectronics is limited by the difficulties in the fabrication of stable p-type ZnO and corresponding p-n junctions. Nevertheless, to engineer the band gap of ZnO-based family from 3.37 eV to 7.7 eV by alloying with MgO (commonly known as MgxZn1-xO) at a low growth temperature opens the possibility for wider applications of MgxZn1-xO in optoelectronics. The research in MgxZn1-xO semiconductor can be traced back to the pioneering discovery of Ohtomo et al., [20] where they have obtained highly c-axis oriented MgxZn1-xO (x=0.33) films over ZnO/sapphire (0001) substrates using pulsed laser deposition (PLD) and the band gap can be tuned from 3.36 eV (x=0) to 3.87 eV (x=0.33). Since then, investigations have been carried out to study the structural, optical and electrical properties of MgZnO films deposited on a variety of substrates (sapphire [20], [21], [22], [23], [24], c-Al2O3, ZnO, MgO, SrTiO3 [25], Si or glass [26,27], quartz [28], LaAlO3 [29], and Pt/Ti/SiO2/Si [30]) by different methods, including PLD [[20], [21], [22], [23],25,29,30], sputtering [28], molecular beam epitaxy (MBE) [24], sol-gel [26] and spray pyrolysis [27]. Structural studies indicate that the lattice constants of these alloys do not change significantly due to the close ionic radius of Zn2+ (0.60 Å) to Mg2+ (0.57 Å) [20]. Although, the bulk solid solubility of MgO in ZnO is very low of 2 wt. %, they are completely miscible in thin films. However, the crystal structure of MgxZn1-xO varies with Mg alloying concentration, being hexagonal for x<0.37, mixed phases for 0.37<x<0.62 and cubic for 0.62<x<1 [25].

UV PDs (solar blind [31], [32], [33], [34], [35], [36], [37], [38] and visible blind [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55]) based on MgZnO films have been reported previously, and the first report by Yang et al., [38] illustrates a metal-semiconductor-metal (MSM) structure Mg0.34Zn0.66O visible blind PD fabricated by PLD over c-plane sapphire substrates, where they have achieved relatively high responsivity (1200 A/W at 308 nm and 5 V bias). Table 1 summarizes the performance (dark current, photoresponsivity, UV to visible rejection ratio) of previously reported MgZnO PDs (MSM, p-n, schottky) obtained by a variety of deposition techniques on different substrates. However, all these PDs are fabricated over brittle substrates and there is hardly any investigation about the PD performance of MgZnO films over a flexible substrate. Most importantly, despite of the superior piezoelectric properties, piezo-phototronic effect of MgZnO films is scarcely reported and demands for further investigation into the implementation of this effect into PDs. Here, we demonstrate the deposition of MgxZn1-xO nanorod-like thin films on a flexible substrate (polyimide) by radio frequency magnetron sputtering without using any buffer layer and investigation of photo-sensing performance of the MSM PDs that have achieved distinct light to dark current ratio (~1630). The peak responsivity is 0.3 mA/W at 292 nm with a cutoff wavelength of 305 nm at 9 V bias voltage signifying its suitability as UV-B (280 nm–320 nm) PD and UV-B-visible rejection ratio (~2.34×105) is much higher than previously reported MgZnO films grown on other substrates (Si, glass, quartz). The efficiency of the PD is further enhanced by the application of piezophototronics leading to substantially higher photocurrent (~4 µA), light to dark contrast ratio (~11000) and sensitivity to light (20%). Application of strain at (-0.29% to +0.29%) also contribute significantly in enhancing the sensitivity to 20 % which is also four times as that with 0 strain illustrating the combined effect of the piezophototronic effect along with the flexible substrate and the Mg concentration over the performance of the PDs on rigid substrates.

Section snippets

Experimental

The MgxZn1-xO thin films were grown on polyimide (PI) substrates by r.f. magnetron sputtering at different substrate temperatures (25, 100, 150, 200, 250 °C) to modify the Mg content and crystallinity of the thin films. In the sputtering system, the target was Mg0.3Zn0.7O (99.99% purity, 2 in. in diameter) in the RF mode. The pure ZnO (99.99% purity, 2 in. in diameter) was deposited by using similar conditions. The substrates (3×1 cm2) were cleaned consecutively in acetone, ethanol and

Results and discussion

The XRD spectra of the as-grown ZnO, MgZnO thin films (Fig. 1a) on polyimide (PI) at different substrate temperature reveals the prominent (0002) peak, evidenced for highly oriented films along the c direction. The two XRD weak peaks at low Bragg angles of 22° and 26° are assigned to the PI substrate. Table 2 summarizes the peak position, d-spacing, full width at half maximum (FWHM) of the (0002) peak for all substrate temperatures. The intensity of the (0002) peak of the MgZnO films initially

Conclusions

In summary, IDT-Schottky photodetectors are fabricated over high quality MgxZn1-xO thin films grown on PI substrate by RF-magnetron sputtering. The Mg content (x =38.54–44 at.%) can be controlled by changing substrate temperature (25-250 °C). All the MgZnO films show high crystallinity and hexagonal wurtzite structure. Surface morphology analysis indicates that the MgxZn1-xO films have a dense surface with high crystalline quality. The PD fabricated by using MgZnO films with 38.6 at.% and

CRediT authorship contribution statement

Jr-Shiang Shiau: Investigation, Data curation. Sanjaya Brahma: Formal analysis, Visualization, Data curation, Writing - original draft, Writing - review & editing. Jow-Lay Huang: Supervision, Resources, Methodology, Project administration, Funding acquisition. Chuan-Pu Liu: Supervision, Conceptualization, Methodology, Writing - review & editing, Project administration, Funding acquisition.

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.

Acknowledgements

This work was financially supported by the Hierarchical Green-Energy Materials (Hi-GEM) Research Center, from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) and the Ministry of Science and Technology (MOST 108-3017-F-006-003) in Taiwan.

References (63)

  • Y.-Y. Chen et al.

    Self-powered n-MgxZn1-xO/p-Si photodetector improved by alloying-enhanced piezopotential through piezo-phototronic effect

    Nano Energy

    (2015)
  • Y.J. Chen et al.

    Enhancement of the piezoelectric coefficient in hexagonal MgxZn1-xO films at lower Mg compositions

    J. Alloy Compd.

    (2017)
  • E. Monroy et al.

    Wide-bandgap semiconductor ultraviolet photodetectors

    Semicond. Sci. Tech.

    (2003)
  • K. Liu et al.

    ZnO-based ultraviolet photodetectors

    Sensors

    (2010)
  • L. Sang et al.

    A comprehensive review of semiconductor ultraviolet photodetectors: from thin film to one-dimensional nanostructures

    Sensors

    (2013)
  • Y. Hou et al.

    Semiconductor ultraviolet photodetectors based on ZnO and MgxZn1− xO

    J. Phys. D Appl. Phys.

    (2014)
  • Z.-F. Shi et al.

    Semi-transparent all-oxide ultraviolet light-emitting diodes based on ZnO/NiO-core/shell nanowires

    Nanoscale

    (2016)
  • Y.-C. Yao et al.

    Enhancing UV-emissions through optical and electronic dual-function tuning of Ag nanoparticles hybridized with n-ZnO nanorods/p-GaN heterojunction light-emitting diodes

    Nanoscale

    (2016)
  • D. Vanmaekelbergh et al.

    ZnO nanowire lasers

    Nanoscale

    (2011)
  • D. Nunes et al.

    Metal oxide nanostructures for sensor applications

    Semicond. Sci. Technol.

    (2019)
  • R. Martins et al.

    Zinc oxide as an ozone sensor

    J. Appl. Phys.

    (2004)
  • J. Figueira et al.

    Sustainable fully printed UV sensors on cork using zinc oxide/ethylcellulose inks

    Micromachines

    (2019)
  • A. dos Santos et al.

    Piezoresistive E‐skin sensors produced with laser engraved molds

    Adv. Electron. Mater.

    (2018)
  • A. Ohtomo et al.

    Mg x Zn 1− x O as a II–VI widegap semiconductor alloy

    Appl. Phys. Lett.

    (1998)
  • A. Sharma et al.

    Optical and structural properties of epitaxial Mg x Zn 1− x O alloys

    Appl. Phys. Lett.

    (1999)
  • C. Teng et al.

    Refractive indices and absorption coefficients of Mg x Zn 1− x O alloys

    Appl. Phys. Lett.

    (2000)
  • J.W. Kim et al.

    Variation of structural, electrical, and optical properties of Zn 1− x Mg x O thin films

    J. Appl. Phys.

    (2006)
  • L. Liu et al.

    Self-compensation induced high-resistivity in MgZnO

    J. Phys. D Appl. Phys.

    (2017)
  • S. Hullavarad et al.

    Homo-and hetero-epitaxial growth of hexagonal and cubic MgxZn1− x O alloy thin films by pulsed laser deposition technique

    J. Phys. D Appl. Phys.

    (2007)
  • Y.-J. Lin et al.

    Mechanisms of enhancing band-edge luminescence of Zn1− x MgxO prepared by the sol–gel method

    J. Phys. D Appl. Phys.

    (2008)
  • P. Madahi et al.

    Deposition and characterization of ZnO: Mg thin films: the study of antibacterial properties

    Phys. Scr.

    (2011)
  • Cited by (12)

    • 2D Ti<inf>3</inf>C<inf>2</inf> nanoflakes anchored ZnO photodetector with substantially improved deep-ultraviolet photoresponse and on/off ratio

      2022, Journal of Alloys and Compounds
      Citation Excerpt :

      Moreover, by alloying, such as with MgO, the bandgaps of ZnO can be extended to 7.8 eV in principle, which is even wider than AlGaN material system [21–23]. Unfortunately, the preparation of high Mg-component ZnMgO alloy film is still difficult owing to the limited solubility of Mg atoms in ZnO, and the degradation of the crystal quality further hinders the performances of deep-UV PDs [24–26]. Consequently, ZnO-based alloys are still hardly to satisfy requirements for high stability, low energy consumption and high responsivity deep-UV PDs in practical applications.

    • Large linear electrostrain of acceptor-donor Co-doped ZnO films

      2022, Journal of Materials Science and Technology
      Citation Excerpt :

      Zinc oxide piezoelectric films with high sound velocity, large electromechanical coupling factor and silicon IC compatibility, have continued to arouse considerable attention because of applications in surface acoustic wave devices and microelectromechanical systems [1–4].

    • All solution processed flexible p-NiO/n-CdS rectifying junction: Applications towards broadband photodetector and human breath monitoring

      2021, Applied Surface Science
      Citation Excerpt :

      High stability and crystalline inorganic semiconductors are not easy to obtain at the same time. For this reason, the creation and production of thin films with excellent photovoltaic characteristics on flexible substrates can support future photoelectric systems, such as flat panel displays, curvilinear photovoltaic cells, and other wearable devices [18–22]. The chemical-bonding, intrinsic characteristics and crystal defects can have an important influence on the flexibility of the material.

    • Recent advances in development of nanostructured photodetectors from ultraviolet to infrared region: A review

      2021, Chemosphere
      Citation Excerpt :

      In such a way, all of the above features demonstrate that metal oxide semiconductors (ZnO, SnO2, Ga2O3, WO3, and TiO2) can be widely used in highly sensitive UV photodetectors. Among various metal oxide semiconductors, ZnO has been extensively studied and used in various applications (Khan et al., 2020; Saric et al., 2019; Shiau et al., 2020; Chauhan et al., 2020; Rabani et al., 2021). In particular, ZnO exhibit a wide bandgap (∼3.2 eV) and n-type semiconductor behavior with exciton binding energy of ∼60 meV (Pearton et al., 2003).

    • Energy harvesting from g-C<inf>3</inf>N<inf>4</inf> piezoelectric nanogenerators

      2021, Nano Energy
      Citation Excerpt :

      In the past decade, PENGs have been developed by means of the piezoelectric effects of noncentrosymmetric materials, such as semiconductors (ZnO [19,20], GaN [21]), ceramics (lead zirconate titanate (PZT) [22], BaTiO3 [23,24]), polymers (polyvinylidene fluoride (PVDF) [25,26], poly(vinylidene fluorideco-trifluoroethylene (PVDF-TrFE)) [27–29], and 2D materials [2–4,30]. To improve the performances of PENGs, much effort has been devoted including varying morphologies of nanomaterials [31] (nanofibers, nanowires, nanorods, etc.) [19–22,32,33], using flexible and stretchable substrates [34] (mica [19], Kapton (polyimide, PI) [28,35], polydimethylsiloxane (PDMS) [36], silicon rubber [37]), modifying piezoelectric materials by poling [26], doping [37], compositing [38] or creating nanopores inside [39,40]. However, the exploration of new materials for PENGs is relatively limited.

    View all citing articles on Scopus
    1

    These authors (Jr-Shiang Shiau, Sanjaya Brahma) have contributed equally.

    View full text