Optical properties of ZnO and ZnO:Ce layers grown by spray pyrolysis

Abstract

In this paper, zinc oxide (ZnO) and cerium-doped zinc oxide (ZnO:Ce) films were deposited by reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors. The effects of Ce concentration on the structural and optical properties of ZnO thin films were investigated in detail. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements. All deposited ZnO layers at the temperature 450 °C are polycrystalline and indicate highly c-axis oriented structure. The dimension of crystallites depends on incorporation of Ce atoms into the ZnO films. The photoluminescence spectra of the films have been studied as a function of the deposition parameters such as doping concentrations and post grows annealing. Photoluminescence spectra were measured at the temperature range from 13 K to 320 K.

Introduction

The electronic and optoelectronic properties of transparent, high conductive, oxide semiconductors have been intensively studied over the last decade. Zinc oxide (ZnO) presents interesting electrical, optical, acoustical and chemical properties, which finds wide applications in optical and electrical devices [1].

Stoichiometric zinc oxide is an insulator that crystallizes with the wurtzite structure to form transparent needle-shaped crystals. ZnO is a direct band-gap semiconductor having an energy gap of 3.37 eV at room temperature with high exciton binding energy (60 meV) [2].

Recently, ZnO has been extensively studied because of its potential applications in various fields, such as: gas sensor, solar cells [3], photodetectors [4], light emitting diodes (LEDs) [5] and laser systems etc. Especially since optical pumped UV laser of ZnO films was reported [6], ZnO has received more and more attention of researchers. ZnO films can be used for applications in surface acoustic wave devices, low-loss optical waveguide, and transparent conductive electrodes.

Until now semiconducting ZnO thin films have been prepared by the number of techniques such as spray pyrolysis, RF magnetron sputtering, pulsed laser deposition (PLD), laser molecular beam epitaxy (MBE), sol–gel (SG), liquid phase epitaxy (LPE), metal-organic chemical vapour deposition (MOCVD) and vacuum evaporation (VE) [7], [8], [9], [10]. The spray deposition method presents several advantages (compared with other methods) such as: low cost, possibility of large-scale deposition, low temperature processing and direct control of film thickness. Generally the electrical and optical properties of polycrystalline films deposited by spin coating, dip coating or spray pyrolysis techniques can be significantly different from the homoepitaxial layers grown on single-crystal with the same lattice constant (without mismatch) [11].

Therefore, in order to improve efficiency of the devices contained ZnO, better knowledge of the structure, optical and electrical properties of ZnO layers is needed.

The polycrystalline structure of ZnO contains large voids, which can easily accommodate interstitial atoms. Consequently, it is really impossible to prepare entirely pure crystalline layers, even when these layers are heated, because they tend to lose oxygen [12]. For these reasons, the ZnO layers show degenerate n-type semiconducting properties with many defects, such as the lack of O atoms and the excess of Zn one [13]. A study of the photoluminescence kinetics (PL) of ZnO is interesting because it can provide valuable information about the quality and purity of the materials [14]. The photoluminescence (PL) spectra of ZnO thin films show near band edge (NBE) transitions and deep level (DL) emission related to the defects and impurities [15], [16]. Stoichiometric thin films of ZnO usually show strong UV luminescence (NBE). The origin of the observed near band edge lines was identified in terms of bound exciton complexes and the phonon replicas caused by emission of a single optical phonon or two phonons. It is known that emission in the visible range of spectrum is mainly due to the defects, which are related to the deep levels emission, such as Zn interstitials (Zn_i) and oxygen vacancies [17], [18]. Vanheusden et al. [15] found that oxygen vacancies are responsible for the green luminescence in ZnO. Oxygen vacancies occur in the three different charge states: the neutral oxygen vacancy ($V_{\text{O}}^0$), the singly ionized oxygen vacancy ($V_{\text{O}}^{\ast }$) and the doubly ionized oxygen vacancy ($V_{\text{O}}^{\ast \ast }$). However, only the singly ionized oxygen vacancy ($V_{\text{O}}^{\ast }$) can act as luminescent center [17], [18].

While the impurities can be partially eliminated by the rigorous control over the purity of the materials and the film-depositing process, the minimization of structural defects such as vacancy of oxygen and interstitial zinc remains difficult. It should be pointed out that this is particularly important to obtain a strong photoluminescence in near energy gap region of ZnO. In view of the presence of inherent oxygen vacancy in ZnO films deposited in vacuum and the extreme difficulties connected with the expitaxial growth of ZnO films on silicon wafers, the thermal oxidation method of ZnO films provides simple and convenient technique to produce ZnO films with high optical properties. Using spray pyrolysis method, makes it easy to minimize the oxygen vacant resulting from oxidized nature of ZnO compound as well as to control the morphology and crystallite size through modulating the deposition process of ZnO films.

In this paper, undoped and cerium-doped zinc oxide films (ZnO, ZnO:Ce) were deposited by reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors at temperatures up to 450 °C. The effects of Ce concentration, on the structural and optical properties of ZnO thin films, were investigated.