Elsevier

Applied Surface Science

Volume 316, 15 October 2014, Pages 456-462
Applied Surface Science

Effect of annealing treatment on electrical and optical properties of Nb doped TiO2 thin films as a TCO prepared by sol–gel spin coating method

https://doi.org/10.1016/j.apsusc.2014.08.029Get rights and content

Highlights

  • We synthesized Nb doped TiO2 thin films using a simple sol–gel spin coating method.

  • The effect of O2 in annealing process was studied on conductivity of TNO thin films.

  • A comparison between O2 and Burstein–Moss effect on optical band gap has been studied.

Abstract

Anatase niobium doped TiO2 thin films (Ti0.94Nb0.06O2) were prepared on quartz substrates by sol–gel spin coating technique and various initial concentrations ([Ti + Nb] in the solution) of 0.1 M and 0.4 M. All the films dried at 100 °C and then the annealing treatment performed by three different procedures. The influence of different annealing processes and total metal concentrations was studied on crystallinity, transparency, conductivity and morphology of thin films by XRD, EDX, FT-IR and UV–vis transmittance spectroscopy, four point probe, AFM and SEM images. It was found that two-step annealing procedure under vacuum is a more proper way to increase the conductivity of TNO thin films compared to one-step annealing process. Two-step method exhibited the minimum resistivity of 4.4 Ω cm and the transparency of about 50% in the visible region with 36 nm thickness.

Introduction

Transparent conducting oxides (TCOs) are a unique type of materials that combine electrical conductivity and optical transparency, simultaneously, with a wide range of applications e.g. displays, low emissive (low-e) windows, thin film photovoltaic (PV) and smart devices [1]. Tin-doped indium oxide (ITO) is a widely used material for TCO applications due to its high transmittance to visible light and high conductivity [2] but it is so important to find a substitute for it because of indium natural resources rarity. Although in recent years aluminum-doped zinc oxide (ZAO) [3] and fluorine-doped tin oxide (FTO) [4] are presented as new TCOs but the hardship in large area preparation of ZAO films with smooth surface morphology and easily etched ZAO films in both acid and alkaline environment [5] and also relatively low electrical conductivity of FTO and its difficulty to pattern via wet etching as compared to ITO [6] are some defects of ZAO and FTO so they are not adequate and appropriate candidates for the new and widespread requirements to the transparent conducting oxides.

In 2005 Furubayashi et al. [7] prepared the Nb-doped TiO2 (TNO) thin film by pulsed laser deposition (PLD) method and reported it as a novel TCO material with minimum resistivity of 2.3 × 10−4 Ω cm and the transparency of higher than 95% in the visible range. Yamada et al. [8] also fabricated highly conductive polycrystalline TNO film on glass substrate by sputtering method so that resistivity of the film was measured 3 × 10−4 Ω cm at room temperature and optical transmittance was achieved 75% in the visible range. Although highly conductive and transparent TNO thin films can be obtained by above methods, but industrial productions are limited by the expensive vacuum technique. Furthermore, the homogeneous and large-area films preparation is also an upfront challenge yet. A sol–gel method has several advantages including simplicity, low cost, easily controlled doping levels and feasible preparation of large area films [9]. Zhao et al. [5] produced a 6 at.% Nb-doped TiO2 thin film by sol–gel method and post annealing treatment where the resistivity and transparency of the derived film were reported 19.3 Ω cm and 75%. Elen et al. [10] also prepared TNO thin films by aqueous chemical solution deposition method with annealing in reduced atmosphere so that the minimum resistivity reached 0.28 Ω cm with a transparency in the visible range of higher than 80%. This was the minimum resistivity of TNO thin films as a TCO carried out by sol–gel method up to now. Vacuum annealing is usually introduced to improve the conductive properties of the TNO films. It is utilized to enhance oxygen vacancies in the films and these vacancies can help ion diffusion in TiO2 films which is beneficial to improve the special properties of the films [11].

In this study, anatase niobium doped TiO2 thin films were fabricated by a sol–gel method combined with spin coating technique with Nb content of 6 at.% on quartz substrate. The concentration amounts of [Ti + Nb] in the solution were kept at 0.1 M and 0.4 M. Three different annealing procedures applied to achieve the higher electrical conductivity of the TNO thin films. The effect of different annealing processes and total metal concentration on transparency and conductivity of the films are discussed.

Section snippets

Preparation of TNO thin films

Nb doped TiO2 (TNO) thin films are prepared using tetrabutylorthotitanate (Ti(C4H9O)4, TBT, 99.99%) and niobium (V) chloride (NbCl5, 99.9%) as precursors, glacial acetic acid (CH3COOH) as catalyst, acetylacetone (C5H8O2, acac, analytical grade) as chelating agent and absolute ethanol as solvent that all purchased from Merck Co. The Nb doped TiO2 sol solution is made in molar ratio of Ti:Nb = 94:6 for molarity amounts ([Ti + Nb] in the solution) of 0.1 M and 0.4 M. In a typical experiment for total

The FT-IR spectroscopy

The infrared spectrum of the sol solution and as-prepared TNO powder after annealing at 550 °C are shown in Fig. 1. The appeared peaks in Fig. 1(a) are assigned to the organic groups of the precursors. The broad absorption band in the range of 3200–3350 cm−1 is related to the Osingle bondH stretching band of ethanol molecules. The observed strong peaks located at 1640–1680 cm−1 are attributed to the stretching vibrations of the Cdouble bondO groups which belongs to acetic acid and acetylacetone. The CH3 symmetrical

Conclusions

In this study, anatase TNO thin films are prepared on quartz substrates by a sol–gel method combined with spin coating technique. The influences of concentration (two different amounts of 0.1 M and 0.4 M) and annealing treatment (three different processes) on electrical and optical properties are studied. The higher nanoparticle size of S2 compared to S1 observed by AFM images affirms the influence of higher total metal concentration on nanoparticle size increase but proximity of S2 and S1

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