Effects of Mn and Cu doping on the microstructures and optical properties of sol–gel derived ZnO thin films

Abstract

Both doped (Cu or Mn) and undoped zinc oxide thin films were deposited on glass substrates by a sol–gel technique. Zinc acetate, copper acetate and manganese nitrate were used as metal sources. A homogeneous and stable solution was prepared by dissolving ZnO acetate (ZnAc) in the solution of 2-propanol and ethanolamine (EA) followed by mixing with the doping solutions. ZnO:(Cu or Mn) thin films were obtained after preheating the spin coated films at 250 °C for 1 min after each coating. A post-annealing at 550 °C was applied to all films for 1 h after the deposition of the last layer. XRD analysis revealed that all films consist of single phase ZnO with zincite structure (Card no: 36-1451). While undoped films showed the strongest orientation, c-axis grain orientation was apparent in all films. TGA analysis of the undoped dried gel showed that weight loss continued until ∼400 °C. Compared to the undoped film, grain size of the films decreased by Mn doping and increased by Cu doping. All films had a very smooth surface with RMS surface roughness values between 0.23 and 1.15 nm and surface roughness increased by doping. Both Mn and Cu doping resulted in a slight decrease in the optical band gap of the films. The largest width of band tail was measured in Mn-doped film.

Introduction

ZnO is an inexpensive, n-type, wide band gap semiconductor with optical transparency in the visible range. It crystallizes in a hexagonal wurtzite structure (zincite) with the lattice parameters of (c = 5.205 Å, a = 3.249 Å) [1]. The n-type semiconductor behavior is originated by the ionization of excess zinc atoms at interstitial positions and the oxygen vacancies [2]. The resistivity values of ZnO films may be adjusted between 10⁻⁴ and 10¹² Ω cm by doping and changing the annealing conditions [3]. Both doped and undoped ZnO thin films are promising materials for the development of gas sensors [4], [5], solar cell windows, [6] and transparent electrodes [7]. Furthermore, since the grain growth of ZnO films shows preferential orientation along the c-axis, they are useful in optical wave-guides, and surface acoustic wave (SAW) and acoustic-optic devices [8].

ZnO thin films have been prepared by various techniques such as rf sputtering [9], spray pyrolysis [10], chemical vapor deposition (CVD) [11], [12], pulsed laser deposition [13], and sol–gel processing [14], [15]. One of the most important advantages of sol–gel processing, over conventional thin film deposition techniques, is the ease of chemical composition control. This advantage makes sol–gel processing a very attractive method especially for doped ZnO thin film fabrication.

Doping of ZnO with Ib and IIb transition elements is relatively less common [16] compared to IIIb elements such as Al. Mn-doped ZnO films have been studied to evaluate electro-optic [16] and (anti)ferromagnetic properties [17], and photoluminance [14], [15]. There have been different approaches to the defect chemistry of Mn-doped ZnO. Cao et al. [18] assumed that doping ZnO with manganese dominantly forms Mn²⁺ with a small amount of Mn⁴⁺. According to their model, when Mn⁴⁺ ions substitute Zn²⁺ ions, they act as donor atoms generating two free electrons while Mn²⁺ ions only generate oxygen vacancies. Han et al. [19] observed that Mn doping makes ZnO more resistive at room temperature and highly conductive at high temperatures and proposed a different model where manganese acts as a deep donor in ZnO:

$${\mathrm{Mn}}_{\mathrm{Zn}}^x\to {\mathrm{Mn}}_{\mathrm{Zn}}^{}+{\mathrm{e}}^{\prime }$$

where superscript “x” represents no effective charge. In this model, the energy level of Mn donor was estimated to be ∼2.0 eV below the conduction band.

ZnO:Cu films have been usually fabricated for their electrical and ferromagnetic properties [17]. In addition, they have potential in surface acoustic wave device applications [20]. Due to its similar electronic shell structure, Cu has many physical and chemical properties similar to those of Zn [21]. The solubility of Cu in ZnO lattice is estimated to be around 1.0 mol% Cu [22]. It is well known when ZnO is doped by Cu atoms, Zn²⁺ ions are substituted by Cu¹⁺ ions in the ZnO lattice. Hall coefficient measurements shown that number of carriers is reduced by Cu doping at room temperature since some of the n-type ZnO electrons occupies empty lower energy 3d Cu states leading to Cu¹⁺ ions [17]. An interesting associate donor–acceptor model for Cu_zn was proposed [23] where

$${\mathrm{Cu}}_{\mathrm{Zn}}^{}+{\mathrm{Zn}}_i^{}\to [{\mathrm{Cu}}_{\mathrm{Zn}}+{\mathrm{Zn}}_i{]}^x$$

The ionization of these deep neutral defects requires high energy (∼3.0 eV) and the presence of this type of complex defects compensates for the n-type of conductivity of ZnO.

The aim of this work is to evaluate the effect of doping (Mn or Cu) on the microstructure, optical properties, and the grain orientation of ZnO thin films prepared by sol–gel spin-coating.