Elsevier

Journal of Alloys and Compounds

Volume 587, 25 February 2014, Pages 724-728
Journal of Alloys and Compounds

Electro-optical characterization of ZnO/ZnMgO structure grown on p-type Si (1 1 1) by PA-MBE method

https://doi.org/10.1016/j.jallcom.2013.10.256Get rights and content

Highlights

  • The ZnO:N/ZnMgO:N/i-ZnO/ZnMgO/ZnO heterostructure was grown by MBE technique on Si.

  • The eterostructure shows a near band edge emission in the PL and CL.

  • The IV characteristics of the studied structures exhibit diode-like behavior.

  • PPCap was detected up to 280 K confirming the presence of metastable centers.

Abstract

We report on the electro-optical properties of ZnO-based heterojunction with i-ZnO layer. The structure was grown on (1 1 1) p-Si by plasma-assisted molecular beam epitaxy at 570 °C. Quality of the obtained epilayers was verified by X-ray diffraction. Optical properties of the heterostructures were studied by photo- and cathodoluminescence. Photoluminescence spectra were measured within temperature range of 5.4–300 K and cathodoluminescence measurements were performed at room temperature. A defect-related radiative recombination was observed. In order to recognize the origin of the defects, responsible for the radiative recombination process, electrical measurements were performed on a diode of ZnMgO:N/i-ZnO/n-ZnMgO. Rectifying properties of the junction were confirmed by the current–voltage (IV) and capacitance–voltage (CV) measurements. Metastability of the IV characteristics as well as persistent photocapacitance observed within a temperature range of 40–280 K were explained by the presence of metastable defects, presumably related oxygen vacancies, and slow interface states.

Introduction

Zinc oxide is becoming the second most widely studied semiconductor material after Si. It is a II–VI direct wide band gap semiconductor with good optical, electrical, chemical, piezoelectric and mechanical properties [1], [2]. ZnO is an efficient luminescent material. It is very attractive for applications in electronics, photonics, sensing and acoustics for example in blue/UV LEDs and solar cells. ZnO contains a large number of deep-level impurities and defect complexes that emit light of different colors, covering the whole visible spectrum [3]. The dominant emitted colors can be controlled by the growth methods and growth conditions [4]. ZnO is quite often considered as an alternative to GaN for device applications due to many reasons, such as lower production cost and excellent optical properties. The high exciton binding energy of 60 meV is one of the properties that make ZnO superior to other semiconductors. Its direct band gap is 3.437 eV (compared to 3.507 eV in GaN) at low temperature range, and it also shows strong spontaneous piezoelectric polarization being the result of hexagonal wurtzite structure in which Zn and O planes are alternately stacked along the c-axis direction of ZnO. However, group III-nitrides (AlGaN, InGaN) hold a large advantage of their own, namely the existence of a viable p-type doping technology.

ZnMgO has been proposed and widely investigated because of a band gap of MgO that is of 7.83 eV – thus much wider than in ZnO, therefore MgO makes a good solid solution with ZnO [5]. However, because of the structural mismatch of ZnO (hexagonal wurtzite) and MgO (cubic, rock salt) the band gap energy of ZnMgO alloys varies between approx. 3.437 eV and 4.2 eV for hexagonal phase and from 5.3 eV to 7.8 eV for cubic phase [6], [7]. The band gap energy and the crystalline structure of ZnMgO films can be controlled by the growth conditions.

The achievement of stable and reproducible p-type ZnO is still a challenge because of problems with doping of ZNO to p-type. This makes the main obstacle for the fabrication of pn homojunctions in ZnO-based devices. There have been reported studies on the heterojunction LEDs using n-type ZnO deposited on different p-type semiconductors such as GaN [8] or Si [9]. The progress in ZnO-based LEDs, bandgap engineering, and growth of n- and p-type ZnO thin films was summarized by Choi et al. [10] and Przezdziecka et al. [11] developed heterojunctions prepared with arsenic doped ZnO films grown on n-type GaN templates. Successful deposition of ZnO/ZnMgO structures on silicon substrates is very important for possible ZnO-based optoelectronic devices on silicon platform. With reference to this goal, it is crucial to understand the electrical properties of ZnO-based heterostructures.

In this paper the results of electrical and optical studies of ZnO:N/ZnMgO:N/i-ZnO/ZnMgO/ZnO heterostructures fabricated on p-type Si (1 1 1) substrate using plasma-assisted molecular beam epitaxy (PA-MBE) are presented.

The direct growth of ZnO on Si usually results in oxidation of the Si surface and formation of amorphous silica layer. The growth of amorphous SiO2 layer on the Si substrate significantly degrades the quality of ZnO films grown on it. We have solved this problem by evaporating a very thin layer (about 2 nm) of Zn in low temperature before growing ZnO layer.

Optical properties of the heterostructures were studied by photoluminescence (PL) and cathodoluminescence (CL) measurements. Electrical properties of diodes based on the fabricated heterostructures were characterized by IV and CV measurements. The diodes exhibited rectifying properties. The presence of metastable defects was confirmed by IV and thermally stimulated photocapacitance (TSCAP) studies.

Section snippets

Experimental

The heterostructures of ZnO:N/ZnMgO:N/i-ZnO/ZnMgO/ZnO were grown on p-type Si (1 1 1) substrate using plasma-assisted molecular beam epitaxy (PA-MBE) technique. Conventional Knudsen-cells were used for the evaporation of Zn (6N) and Mg (6N) elements. A radio-frequency (rf) cell was used for generation of oxygen plasma. During the growth, the rf power of oxygen plasma was fixed at 400 W. Before the growth, the Si substrates were chemically and thermally cleaned at 700 °C. The growth temperature was

Structural properties

Fig. 2a shows θ XRD patterns measured with Soller slits and Pixel detector. The 0 0 0 2 and 0 0 0 4 ZnO and ZnMgO diffracted peaks can be indentified confirming the c-orientation of the ZnO/ZnMgO structures. The average composition of Mg determined from XRD and PL measurements is about 15%. Owing to this, the diffracted peaks from ZnO and ZnMgO layers are separated quite well, as depicted in Fig. 2b and c. The FWHM of the 0 0 0 2 reflection for ZnO and ZnMgO are 0.23 and 0.29, respectively. At room

Summary and conclusions

The pin ZnO:N/ZnMgO:N/i-ZnO/ZnMgO/ZnO heterostructures were grown by PA-MBE technique on Si substrate with low-temperature Zn thin films. The results indicate that the use of a thin Zn film deposited directly on Si results in the high structural quality of nanostructures grown on silicon. The near band edge excitonic emission in the PL and CL spectra also confirmed good optical quality of the obtained films. The room temperature CL analysis revealed that visible emission consists of a few

Acknowledgment

Work supported in part within European Regional Development Fund, through Grant Innovative Economy (POIG.01.01.02-00-008/08).

References (23)

  • J. Wu et al.

    Sci. Adv. Mater.

    (2011)
  • J.H. Lee et al.

    J. Kor. Phys. Soc.

    (2010)
  • E. Przeździecka et al.

    Sensor Actuat. A: Phys.

    (2013)
  • F. Yang et al.

    Superlattice. Microst.

    (2012)
  • K. Iwata et al.

    J. Cryst. Growth

    (2000)
  • K. Li et al.

    Mater. Focus

    (2012)
  • M. Willander et al.

    Nanotechnology

    (2009)
  • H. Ahn et al.

    J. Appl. Phys.

    (2009)
  • K. Li et al.

    Int. J. Nanotechnol.

    (2013)
  • S.B. Ogale

    Thin Films and Heterostructures for Oxide Electronics

    (2005)
  • K. Li et al.

    Funct. Mater. Lett.

    (2010)
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