Growth and characterization of a-axis textured ZnO thin films

doi:10.1016/j.jcrysgro.2004.12.162

Journal of Crystal Growth

Volume 277, Issues 1–4, 15 April 2005, Pages 26-31

https://doi.org/10.1016/j.jcrysgro.2004.12.162 Get rights and content

Abstract

Smooth a-axis oriented ZnO thin films are grown on (0 0 1) MgO substrates by pulsed laser deposition assisted by a very directional radio frequency oxygen plasma. The structure of the film is examined by X-ray diffraction, electron diffraction and high resolution electron microscopy. The film grows with vertical columns and is highly textured. Two preferential orientation relations between the ZnO film and the MgO substrate are observed: ( $1 0 \bar{1} 0$ )_ZnO||(0 0 2)_MgO; [0 0 0 2]_ZnO||[2 0 0]_MgO and ( $1 0 \bar{1} 0$ )_ZnO || (0 0 2)_MgO; [ $\bar{1} 2 \bar{1} 0$ ]_ZnO || [2 0 0]_MgO. The film/substrate interface is flat and strained because of the rather large lattice mismatch between the ( $1 0 \bar{1} 0$ ) plane of ZnO and the (0 0 2) plane of MgO.

Introduction

Zinc oxide (ZnO) is a wide band gap semiconductor (3.3 eV) with hexagonal wurtzite structure showing several outstanding physical properties for optical and electronic applications. ZnO has good photoconductivity, is transparent in visible and infrared and has a large exciton binding energy which enables the use of ZnO thin films for light emitting diodes and efficient UV lasers. With a large electro-mechanical coupling factor and a high piezoelectric coefficient, ZnO thin films are good candidates for high frequency and low loss surface acoustic wave (SAW) devices [1], [2]. However, the electrical and optical properties of the films are critically dependent on their microstructure, texture and surface morphology, on the presence of defects and the structure of the film–substrate interface. Highly oriented ZnO thin films have been deposited by various methods such as molecular beam epitaxy (MBE) [3], [4], [5], chemical vapor deposition (CVD) [1], [6], pulsed laser deposition (PLD) [7], [8], [9] and magnetron sputtering [2], [10] on different substrates. In all cases the films have a c-axis texture, because in materials with wurtzite structure, the growth rate along the c-axis is higher [1].

Good quality, highly oriented ZnO thin films are needed for the SAW applications to reduce the propagation loss and to increase the electro-mechanical coupling coefficient. Moreover, for operating surface wave devices in liquids it is necessary to generate surface shear displacements, where the wave displacement is perpendicular to the direction of the wave propagation and in the plane of the crystal surface. In common piezoelectric materials used to produce shear horizontal SAWs (SH-SAWs), a part of the energy is lost to a bulk acoustic wave which propagates normal to the surface, instead of along it. Propagation along the surface is best accomplished by using an acoustic wave guide made from a material for which the waves are trapped near the surface. ZnO can be such a material, but for the generation of detectable SH-SAWs, it is mandatory to grow ZnO thin films with the c-axis tilted away from the normal to the film surface [11].

In this paper we present a radiofrequency (RF) plasma assisted PLD method to produce a-axis oriented ZnO thin films, i.e. with the c-axis lying in the film plane at 90° from the normal to the film. The morphology and structural qualities of the films are investigated by X-ray diffraction (XRD), electron diffraction (ED), transmission electron microscopy in diffraction contrast (TEM) and high resolution transmission electron microscopy (HRTEM).

Section snippets

Experimental procedure

The experimental set-up [12], [13], [14] consists in a conventional PLD system used in conjunction with a RF discharge plasma beam. The deposition chamber contains the target and the substrate placed parallel to each other at a distance of 5.5 cm. The (0 0 1) MgO substrate was heated at 500 °C with a ramp of 20 °C/min and cooled with 10 °C/min after deposition. The vacuum system allows to obtain a base pressure better than 10⁻³ Pa. A Nd:YAG laser (10 Hz and incident fluence of 20 J/cm²) was directed to

Results and discussion

An optical emission spectrum obtained at the nozzle exit is presented in Fig. 1. It shows that the PLD deposition is assisted by a plasma beam of highly reactive species, such as oxygen radicals (the most important emission at 777 nm corresponds to the transition O(3p⁵P-3s⁵S⁰)) and molecular oxygen ions (bands of the first negative series, transitions O₂⁺(b⁴Σ_g⁻–a⁴Π_u)). Compared to the discharge-off conditions, i.e. a PLD deposition from a Zn target in non-excited flowing oxygen gas, in this

Conclusions

We have succeeded in growing a good quality a-axis textured ZnO thin film on a (0 0 1) MgO substrate by a directional RF plasma assisted PLD method. Since by conventional PLD only c-axis oriented ZnO films were obtained, a directional oxygen RF plasma beam, placed at a high angle towards the normal to the substrate surface plays a determinant role for designing the structure of the film. The ZnO thin film is highly oriented. In different crystallites, the a-axis is always normal to the film

Acknowledgements

This work is part of the bilateral Flemish-Romanian research project BIL 01/73, with financial support from the European Community Information Society Technologies program, Project PISARRO (IST-2001-33326) and the Romanian Ministry of Education and Research (projects CERES 104/2001 and 105/2001).

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Growth and characterization of a-axis textured ZnO thin films

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgements

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

J. Crystal Growth

Vacuum