Elsevier

Optical Materials

Volume 72, October 2017, Pages 618-625
Optical Materials

Thermally evaporated InZnO transparent thin films: Optical, electrical and photoconductivity behavior

https://doi.org/10.1016/j.optmat.2017.07.009Get rights and content

Highlights

  • InZnO thin films (100 nm and 200 nm) were prepared by thermal evaporation technique.

  • Higher transmittance (80%) and wide band gap energy (3.46 and 3.49 eV).

  • Low resistivity, high conductivity and low activation energy.

  • Better photo sensitivity observed for 100 nm film.

Abstract

Indium Zinc Oxide (InZnO) thin films were deposited on pre-cleaned glass substrate by thermal evaporation technique. X-ray diffraction pattern revealed mixed phase with polycrystalline structure. The uniform distribution of spherical shape grains over entire film surface was observed through scanning electron microscopy. Optical study revealed the higher transmittance (80%) and wide band gap energy (3.46 and 3.49 eV). Low resistivity, high conductivity and low activation energy were obtained from Four Point Probe method. Dark and illuminated light on 100 nm film showed better photo sensitivity than 200 nm film. The observed uniform surface morphology, higher transmittance, wide band gap energy, low resistivity, high conductivity and good photoconductivity properties indicated that these thermally evaporated InZnO thin films could be used as TCOs instead of ITO in electronic and opto-electronic devices in future.

Introduction

The need of an efficient electric and opto-electronic devices has motivated the interests for transparent conductive oxides (TCOs). The first TCO thin film was prepared by Badeke in 1907 [1], [2]. The work on thin film based TCOs (In2O3, SnO2, ZnO, ITO, FTO, AZO, IGZO and IZO have been significantly increased over the last three decades [3], [4], [5], [6], [7], [8], [9], [10]. TCOs have been touted as the natural successor material for microelectronic devices because of their good optical transparency, wide optical band gap (Eg > 3 eV), high absorption coefficient (<105 cm−1) extinction coefficient k as low as 1 × 10−4, high electrical conductivity (102–1.2−6(S)) and resistivity as low as 1 × 10−4 [11], [12], [13]. TCOs with a high electron mobility and low carrier concentration possess enhancement in optical transmission for long wavelength regions without sacrificing electrical conductivity [14], [15]. Among many kinds of TCOs, indium tin oxide (ITO) is the most effective industrial material presently being used in device fabrications. For technological applications, the TCO should have electrical resistivity (ρ) ∼1.0 × 10−4Ωcm or less, with an absorption coefficient (α) smaller than 1.0 × 104 cm−1 in the near-UV and VIS range, and with an optical band gap > 3.0 eV [11]. The mobility of TCO thin films have been enhanced by improving the crystalline structure by heat treatment, choice of deposition technique, choice of substrate, etc [16], [17]. Usually thin films require annealing greater than 400 °C to provide good surface quality, transparency and conductivity. Without heating the substrate or low process temperature thin film exhibit low electron mobility as a result of scattering by grain boundaries and/or point defects [16], [17]. The grain boundaries or point defects are reduced by post deposition heat treatment, which leads to increase in grain size, improvement of crystal structure and enhancement of the electron mobility. It is important to understand the range of the electrical properties of the TCO thin films during high-temperature annealing, especially ITO thin films that exhibit both lowest resistivity and highest durability [18], [19]. The majority of known TCOs are n-type semiconductors where defects such as oxygen vacancies, impurity substitution and interstitials donate electrons to the conduction band providing charge carriers for the flow of electric current [20], [21].

Some of the important inorganic TCO materials such as ITO, In2O3, ZnO, SnO2, CdO, ternary compounds Cd2SnO4, CdIn2O4, CdSnO3, ZnSnO4, ZnSnO3, Zn2In2O5, Zn3In2O3, In2SnO4 and quaternary compound (ZnO-SnO2-In2O3-Ga2O3) have been extensively investigated due to their characteristic features and potential applications in wide areas of science and technology [22], [23], [24], [25]. Among the existing TCOs, indium zinc oxide (IZO or In2O3-ZnO) thin films are expected as superior transparent conductors which could be used as transparent electrodes and may provide an alternative to traditional ITO thin films used in optoelectronic applications [26], [27]. The oxygen vacancies are the most common defects observed for In2O3 and ZnO composites due to Zn2+ ions mixed with In3+ ions, which clearly indicated that the optoelectronic features of the IZO films are very sensitive to the cationic coordination [28], [29]. The significance of optical and photoluminescence study on high quality IZO thin films for opto-electronic devices have been reported by Ramamoorthy et al., [28]. In addition to that, IZO have been researched for semiconducting oxide/transparent electrode layer for thin film transistors (TFTs) and solar cells [13], [26], [30], [31], [32], [33], [34], [35], [36]. Also, combination of indium oxide (In2O3) with zinc oxide (ZnO), gallium oxide (Ga2O3) and tin oxide (SnO2) resulted in indium zinc oxide (IZO), indium gallium oxide (IGO), indium tin oxide (ITO) and indium gallium zinc oxide (IGZO) that have been used as channel layer for thin film transistors (TFTs) [37], [38]. Several research groups have studied the structural, electrical and optical properties of In2O3-ZnO (IZO) thin films prepared by DC magnetron sputtering [39], RF magnetron sputtering [27], metal-organic chemical vapor deposition (MOCVD) [40], pulsed-laser deposition [41], sol-gel processing [42], [43], electron beam evaporation [44], dip coating [45] and facing targets sputtering method [46]. The first report on the structure, morphology, optical and photoluminescence properties and gas sensing behavior of 100 nm InZnO thin films prepared by thermal evaporation from InZnO nanoparticles was published recently by our research group [47], [48], [29]. But so far, there is no report on the electrical and photoconduction properties of InZnO thin films prepared by thermal evaporation technique.

The present work is the first report on the electrical and photoconduction behavior of 100 nm and 200 nm thickness InZnO thin films prepared by thermal evaporation technique. The structure, morphology, optical, electrical and photoconduction behavior were investigated to find out the feasibility of utilizing InZnO thin films as an alternative to ITO in electronic and opto-electronic devices.

Section snippets

Preparation of InZnO thin films

Thermal evaporation technique (Hind Hivac-124A) was used to prepare InZnO thin films (100 nm and 200 nm) on pre-cleaned glass substrate from InZnO NPs. The pressure inside the chamber was maintained at 2 × 10−5 torr to avoid impurities if any present in the chamber. Pirani and Penning gauges were used to measure the pressure. Digital quartz crystal thickness monitor was used to measure the thickness of the prepared films. Then the as-deposited 100 nm and 200 nm films were annealed at 400 °C for

Structure

Fig. 2(a and b) shows the XRD patterns of InZnO thin films. The observed diffraction peaks at 2θ = 23, 29, 35 and 60° are respectively corresponds to the reflection of (211), (222), (400) and (622) orientation planes of cubic In2O3(JCPDS 06-0416) and peaks at 2θ = 31, 34, 35, 47, 56, 64, 66, 68 and 69° are respectively corresponds to (100), (002), (101), (102), (110), (103), (200), (112) and (201) orientation planes of hexagonal (wurtzite) ZnO structure (JCPDS 36-1451) [47], [49], [50], [51],

Conclusions

InZnO transparent thin films (100 nm and 200 nm) were prepared from thermal evaporation technique. The presence of elements (In, Zn and O) and mixed phase of cubic-In2O3 with hexagonal wurzite ZnO structure were confirmed from SEM-EDS analysis and XRD patterns. The optical, electrical and photoconduction studies revealed the prime properties such as uniform surface morphology, higher transmittance (80%) with wide band gap energy (3.46–3.49 eV), low resistivity, high conductivity and good photo

Acknowledgements

The authors wish to express their gratitude to the Kongunadu Arts and Science College, Coimbatore, India for their technical and instrumental supports throughout this research work.

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    Present address: Institute for Solid State Physics (ISSP), Division of Nanoscale Science, University of Tokyo, 5-1-5 Kashiwanoha, Chiba 277-8581, Japan.

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