Study and improvement of aluminium doped ZnO thin films: Limits and advantages
Introduction
Zinc oxide (ZnO) is a wide band gap (∼3.3 eV) n-type semiconductor [1], [2], [3], [4], with superior electron mobility [4], [5] and a high exciton binding energy of ∼60 mV [4], [6], high enough to be stable near room temperature, that makes it suitable for applications in short wavelength emitting devices such as light emitting diodes [4]. It also presents excellent thermal stability [7], high transparency [4], pyroelectric [8] and piezoelectric [2] properties, and chemical sensing capabilities that are appropriate for gas detector applications [5]. Aside from its interesting characteristics, it has a relatively low cost and low toxicity [7], [9].
In thin film based photovoltaic devices, ZnO has been used as buffer layer between the transparent conductive electrode and the CdS window layer in CdS/Cu(In,Ga)Se2 [10] and CdS/CdTe solar cells. On the other hand, aluminium-doped zinc oxide films (AZO) have been proposed as n-type layer instead of CdS, directly deposited on ITO substrates, because its higher band gap.
A wide variety of techniques have been reported in the literature for the preparation of ZnO films ranging from physical methods like chemical vapour deposition (CVD) [3], [11], sputtering [11], laser ablation [12], to wet chemistry methods [2] such as chemical bath deposition (CBD) [5], hydrothermal synthesis [1], [6], sol–gel [5], [13] and electrodeposition [2], [3], [14], [15], [16], [17], [18]. Electrodeposition offers unique features such as scalability, easiness of implementation, and very accurate control on the film thickness and morphology [16]. Exotic ZnO nanostructures [2] for instance pillars, stars, urchins, flowers [15] and of course nanowires [3], nanobelts [2] and nanorods [3] that are of interest, for example, in dye sensitized solar cells [3], have been successfully prepared by electrodeposition. Most of these electrochemical approaches reported for ZnO preparation use an aqueous solution of a Zn salt, namely ZnCl2 or Zn(NO3)2 [16], [17], as Zn source and an oxygen precursor like nitrates (NO3−), molecular oxygen (O2) dissolved in the solution or hydrogen peroxide (H2O2) [18]. Regardless of the oxygen precursor used, crystalline ZnO thin films with optical transparency can be obtained [19]. However, ZnO electrodeposition onto conductive substrates such as ITO is not straightforward since the lack of active sites of the ITO substrates often leads to non-uniform films that eventually cause device failure [14], [20], [21]. Strategies based on acid or alkaline activation as well as substrate seeding have been proposed to overcome this issue [22].
Although a lot of literature can be found on ZnO preparation, only very few contributions describe the electrochemical preparation of ZnO:Al [3], [14], [23] because it is usually deposited using evaporation techniques, mainly by sputtering [11]. A comparative study between layers formed using both techniques was reported by Welling et al. showing that despite the better optical properties of sputtered films, the resistivity values are in the same order of magnitude. So this drops a hint towards attempting a more accurate control of the morphology in electrodeposited films to achieve the same optical quality.
In the present contribution, we study ZnO:Al electrodeposition onto ITO substrates using Zn(NO3)2 as Zn precursor and Al(NO3)3·9H2O as Al3+ source. First, general conditions for ZnO electrodeposition such as the working temperature, the applied potential and substrate activation have been explored. In the optimized conditions for ZnO electrodeposition, films with different Al contents have been prepared. The structure, morphology and optical properties as well as the photoconductivity and apparent free carrier concentration of un-doped and Al-doped films have been assessed through X-ray diffraction (XRD), UV–vis optical transmittance, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS) and photocurrent spectroscopy (PCS).
Films were electrodeposited onto indium tin oxide (ITO) coated glass substrates (Solems, 10 mm × 25 mm, 23 Ω/sq). The substrates were first rinsed with ethanol and Milli-Q water, then N2 blown and directly mounted in the electrochemical cell. Electrodeposition, cyclic voltammetry (CV) and electrochemical impedance measurements (EIS) were performed using an Autolab PGSTAT-12 Galvanostat-Potentiostat (Metrohm Autolab) equipped with a Frequency Analyzer Module (FRA). ZnO electrodeposition was done in a home-made thermostatized electrochemical cell, working in the three electrode configuration. The ITO substrate was vertically placed at 2 cm from a Pt mesh acting as auxiliary electrode. A Ag/AgCl/Sat KCl (SSC) in contact with the solution through a Luggin capillary was used as true reference electrode. All potentials from herein will be quoted versus the SSC electrode. The temperature of the system was controlled with an ED5 water circulating bath (Julabo).
The electrodeposition bath was prepared by mixing solutions in the appropriate ratio of Zn(NO3)2 (Sigma–Aldrich, >99%) and Al(NO3)3·9H2O (Sigma–Aldrich, >98%) to achieve a final 0.01 M Zn2+ concentration and increasing Al3+ concentrations of 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1 and 2 mM for the different doping contents. Additionally, 1 M NaOH (Riedel de-Haën, >99%) solution was prepared for substrate pre-treatment. All solutions were prepared with MilliQ water (18.2 MΩ).
The film preparation involves three steps:
- I.
Substrate pre-treatment [24]: A 10 mA current is passed during 60 s through an ITO substrate immersed in a 1 M NaOH solution. The procedure enhances the film uniformity and decrease the air bubbles trapped during the subsequent film electrodeposition, reducing the amount of pinholes in the final films. Electrodeposited ZnO films with and without the pretreatment are compared in the Fig. S1 of the Supplementary Information. After this pre-treatment, the substrates are thoroughly rinsed with MilliQ water.
- II.
Nucleation: The substrate is introduced in the electrochemical bath, heated at 70 °C. And, a short cathodic pulse at −1.1 V is applied. The effect of pulse duration on the morphology of the films is presented in Fig. S2 of the Supplementary Information, and the effect on film transmittance is discussed in Fig. S3. After analysing those results, a pulse of 5 s is used from herein.
- III.
Film electrodeposition: After the system returns to the open circuit potential (OCP), the electrodeposition process is carried out by applying a fixed potential of −1.1 V during 600 s [4] in the same electrolyte and temperature conditions as the nucleation step.
X-ray photoelectron spectroscopy (XPS) measurements were carried out in a PHI 5500 Multitechnique System (Physical Electronics) with a base pressure between 5 × 10−9 and 2 × 10−8 Torr, using a monochromatic X-ray source (Al Kα line of 1486.6 eV energy and 350 W), placed perpendicular to the analyser axis and calibrated using the 3d5/2 line of Ag with a full width at half maximum (FWHM) of 0.8 eV. The analysed area was a circle of 0.8 mm diameter. The spectra resolution for the spectra was 23.5 eV for the general spectra and 0.1 eV/step for the spectra of the single elements. Before measurements, the sample surface was sputtered with an Ar+ ion source (4 keV) for 30 s. A low energy electron gun (less than 10 eV) was used in order to discharge the surface when necessary.
The X-ray diffraction (XRD) measurements were performed in PANalytical X’Pert Pro MPD Alpha1 diffractometer using a Ni filtered Cu Kα (λ = 1.5418 Å) radiation in the Bragg-Brentano geometry. Crystalline phases were identified using the JCPDS database [25]. Surface morphology was studied in Hitachi H-4100 Field Emission Scanning Electron Microscope (FE-SEM). The UV–vis transmittance spectra were acquired in a Shimadzu spectrometer (UV-1700 PharmaSpec) using a clean ITO substrate as reference.
Electrochemical impedance spectroscopy (EIS) and photocurrent spectroscopy (PCS) measurements were performed in an electrochemical cell with the ZnO film placed at the bottom and exposing to the electrolyte an area of 0.3 cm2. Both measurements were obtained in 0.1 M LiClO4 (Sigma–Aldrich, >98%) in carbonate propylene (Sigma–Aldrich, >99%) solution, to avoid ZnO decomposition [26]. EIS measurements were done using an excitation signal of 0.025 V amplitude (in the order of kT energy). The impedance versus frequency spectra were acquired at a fixed potential of 0.5 V to ensure surface carrier depletion at frequencies ranging from 10 kHz to 0.1 Hz. Impedance versus potential measurements were done and represented as Mott–Schottky plots (1/C2 versus potential) to calculate the flat band potential (EFB) and estimate the carrier density (ND). The photocurrent measurements were recorded at fixed potentials between 0.2 and 0.8 V by illuminating the sample from the solution side, using a 150 W illuminator (MI-150, Dolan Jenner), in an intermittent mode using 60 s on/off cycles.
Section snippets
ZnO electrodeposition: parameter optimization
The electrochemical mechanism of ZnO electrodeposition using NO3− as oxygen precursor has been extensively revised in the literature [2], [22], [24]. In general, it is accepted that the reactions occurring at the electrode surface are the following [22], [27]:2H+ + 2e− → H2NO3− + H2O + 2e− → NO2− + 2OH−Zn2+ + 2OH− → Zn(OH)2(s)Zn2+ + 2e− → Zn(s)
Briefly, the electrochemical reduction processes in reactions (1), (2), consume H+ and generate OH− species respectively, causing an increase in the
Conclusions
An effective method for the electrodeposition of uniform, adherent and crystalline n-type ZnO and ZnO:Al thin films on ITO substrates has been developed. The electrodeposition procedure consisted in 3 steps: (i) a substrate pre-treatment in NaOH to avoid the presence of pinholes during ZnO growth, (ii) a 5 s nucleation pulse at −1.1 V in 0.01 M Zn(NO3)2 at 70 °C that improves adhesion and uniforms nano-pillar morphology and (iii) film growth also at −1.1 V for 600 s.
ZnO:Al films with Al content up to
Acknowledgments
I. Díez-Pérez and P. Gorostiza are acknowledged for the valuable discussion on the EIS. ACA and APP thank MECD and MEC, respectively, for financial support through a FPU grant. FCB acknowledges financial support from CONACYT 151679 and SIP-2011-1267. The Scientific Technical Services of the University of Barcelona (CCiT-UB), particularly the SEM, XRD and XPS facilities, are also acknowledged. This project has been partially financed through the CTQ2011-25156 project from MEC.
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Both authors contributed equally to this work.