Coaxial p-Si/n-ZnO nanowire heterostructures for energy and sensing applications

https://doi.org/10.1016/j.matchemphys.2012.05.034Get rights and content

Abstract

Radial p–n nanowire heterojunction devices represent a favorable geometry to maximize the interfacial area and charge carrier separation due to the built-in field established across the junction. This report presents the functional characterization of a heterojunction device based on a single coaxial p-Si/n-ZnO nanowire that was integrated in a circuit by FIB nanolithography to study the electrical properties. Specifically, their photovoltaic and gas sensing performances were preliminary assessed. The gas sensing response of the p–n heterojunction could be usefully modulated by controlling the bias currents through the device, showing a complementary functionality of these nanoarchitectured devices.

Highlights

► Coaxial p-Si/n-ZnO core/shell nanowires (NWs) were produced. ► Heterojunction devices based on one single NW were produced by FIB nanolithography. ► These devices displayed photovoltaic and gas sensing properties. ► Non-linear IV characteristic allowed us to tune their sensibility to gases by modulating the bias.

Introduction

Metal oxide nanowires are a broad class of materials which are gaining a growing interest due to their potential in gas sensing, optoelectronics and energy applications [1], [2], [3]. Although preliminary studies revealed promising outcomes, further research is necessary in order to reach complete control on their properties [4]. Working into the direction of nanowire-based systems, the integration of active characteristics in a single nanostructure by combinatorial materials architectures is a promising approach to develop diodes, transistors or any other active element equivalent to those employed in standard microelectronics [5]. Among all n-type metal oxide nanomaterials, ZnO is considered an excellent candidate to obtain functional devices [6], [7], [8]. To date, the development of ZnO nanosystems has been mainly focused on the use of bare nanowires as, for instance, mere passive resistors, gas sensors or building-blocks of battery electrodes [9], despite some interesting attempts to integrate them as active elements in new technologies and systems [10]. In this context, ZnO-based heterojunctions have recently attracted much focus for highly sophisticated electronic devices [11], [12], [13], [14], [15], such as p-Si/n-ZnO nanowire heterojunctions that are seen as an interesting alternative to overcome the pending issue of growing high quality and stable p-type ZnO [16].

In fact, radial p-Si/n-ZnO nanowires are expected to exhibit superior performances compared to their conventional planar p–n junction counterparts due to their favorable geometry that maximizes the junction interface area and the built-in field established across it [17]b), [17], [17]a), [18]. This is particularly interesting for any application involving massive charge carrier separation processes, as for instance, photovoltaics [19], [20]. Nevertheless, the electrical access to the inner core of these nanostructures without shorting the final device remains as a challenging avenue from a fabrication point of view.

In this work, we report on the prototyping and characterization of single coaxial p-Si/n-ZnO nanowire heterojunction devices, the study of their electrical response, and the preliminary assessment of their photovoltaic and gas sensing properties, which up to date has not been reported, to the best of our knowledge, on ZnO-based heterojunctions at single nanowire scale. Actually, in most of the previous devices the n–p junctions were formed at the interface of a nanowire or nanowall with the underlying substrate [15], [21], or at the cross interface of two nanowires [22]. Here, these stand-alone p-Si/n-ZnO nanowire prototypes are considered to be useful tools to achieve a good comprehension of the intrinsic phenomena in p–n radial nanowires, since the statistical dispersion typical of bulk-type systems is eliminated and contact effects are minimized [2], [4]; paving thus, the way to further and necessary developments in this incipient research area, and showing the potential to integrate heterostructures with modulated compositions, interfaces, and complementary functionalities in a single nanoarchitectured device.

Section snippets

Experimental

Silicon (Si) nanowires were synthesized by a metal-assisted wet chemical etching of p-doped (100) silicon wafers, following a published procedure (see Supporting information) [23]. In a following step, a zinc (Zn) layer was deposited on the Si nanowires via direct current (DC) sputtering after removing the native silicon oxide by etching. Annealing the ZnO–Si structure in ambient air at 500 °C for 1 h resulted in ZnO films. The phase composition of as-prepared sample was characterized on an

Results and discussion

XRD patterns of the as-prepared p-Si/n-ZnO nanowire arrays indicated a predominant growth along the (100) direction flanked by distinctive minor peaks in the 2θ ∼ 30°–40° range ascribable to the wurzite phase of ZnO (Fig. S1). SEM images revealed Si nanowires aligned perpendiculary to the substrate, with ca. 80 nm polycrystalline ZnO shell (Fig. S2). After the device fabrication step, final heterostructures were typically 20-30 microns long with a diameter of 300 nm (Fig. 2). It is worth to

Conclusions

In summary, single coaxial p-Si/n-ZnO nanowire devices have been fabricated using FIB-assisted nanolithography technique. Their p–n characteristics evaluated and modeled in dark and under illumination showed photovoltaic characteristics. Moreover, their use as gas sensors was demonstrated, showing that the sensing response could be modulated by changing the bias current through the device; with the maximum gas sensing response obtained at low reverse values. This working principle could be

Acknowledgments

The University of Cologne is gratefully acknowledged for the financial support. The research was also supported by the Framework 7 program under the project S3 (FP7-NMP-2009-247768). M. Hoffmann and H. Shen thank BMBF for the support of Nanofuture program (FKZ 03X5512). Authors from IREC acknowledge the financial support given by the XaRMAE Network of Excellence on Materials for Energy of the “Generalitat de Catalunya”. J.D. Prades and F. Hernandez-Ramirez thank DAAD for the support. A. E. Gad

References (32)

  • E. Comini et al.

    Prog. Mater. Sci.

    (2009)
  • S. Barth et al.

    Prog. Mater. Sci.

    (2010)
  • J.H. Choi et al.

    Solid State Electron.

    (2010)
  • J. Melngailis

    Ion Beam Lithography

    J. Melngailis et al.

    Focused Ion Beams in Semiconductor Manufacturing

  • M. Law et al.

    Annu. Rev. Mater. Res.

    (2004)
  • F. Hernandez-Ramirez et al.

    Phys. Chem. Chem. Phys.

    (2009)
  • J.M. Martínez-Duart et al.

    Nanotechnology for Microelectronics and Optoelectronics

    (2006)
  • M. Huang et al.

    Science

    (2001)
  • S. Xu et al.

    Nat. Nanotechnol.

    (2010)
  • J.I. Sohn et al.

    Nano Lett.

    (2010)
  • M. Riaz et al.

    Adv. Funct. Mater.

    (2010)
  • Q. Yang et al.

    ACS Nano.

    (2010)
  • M. Devika et al.

    Chem. Phys. Chem.

    (2010)
  • H.S. Song et al.

    Cryst. Growth Des.

    (2011)
  • K. Sun et al.

    J. Am. Chem. Soc.

    (2010)
  • C. Cheng et al.

    ACS Appl. Mater. Interfaces

    (2010)
  • Cited by (0)

    View full text