Low temperature processed highly conducting, transparent, and wide bandgap Gd doped CdO thin films for transparent electronics

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Abstract

Gadolinium (Gd) doped cadmium oxide (CdO) thin films are grown at low temperature (100 °C) using pulsed laser deposition technique. The effect of oxygen partial pressures on structural, optical, and electrical properties is studied. X-ray diffraction studies reveal that these films are polycrystalline in nature with preferred orientation along (1 1 1) direction. Atomic force microscopy studies show that these films are very smooth with maximum root mean square roughness of 0.77 nm. These films are highly transparent and transparency of the films increases with increase in oxygen partial pressure. We observe an increase in optical bandgap of CdO films by Gd doping. The maximum optical band gap of 3.4 eV is observed for films grown at 1 × 10−5 mbar. The electrical resistivity of the films first decreases and then increases with increase in oxygen partial pressure. The lowest electrical resistivity of 2.71 × 10−5 Ω cm and highest mobility of 258 cm2/Vs is observed. These low temperature processed highly conducting, transparent, and wide bandgap semiconducting films could be used for flexible optoelectronic applications.

Research highlights

▶ The present manuscript entitled ‘Low temperature processed highly conducting, transparent, and wide bandgap Gd doped CdO thin films for transparent electronics’ is believed to be the first study on the structural, optical, and electrical properties of gadolinium doped CdO thin films. ▶ The effect of oxygen partial pressure on structural, optical, and electrical properties are studied. ▶ These (1 1 1) preferred oriented films are highly transparent. ▶ The optical bandgap of the films depends on oxygen partial pressure and varies from 3.0 eV to 3.4 eV. ▶ The lowest electrical resistivity and highest mobility of 2.71 × 10−5 Ω cm and 258 cm2/Vs, respectively, is observed. ▶ These low temperature processed high mobility and wide bandgap semiconducting films could be used for flexible optoelectronic and photovoltaic applications.

Introduction

Cadmium oxide (CdO) is widely used as transparent conductors for its high electrical conductivity and optical transparency [1]. CdO is an n-type semiconductor having bandgap of 2.27 eV [2]. The high conductivity of undoped CdO is due to defects of oxygen vacancies and cadmium interstitials [1]. CdO was the very first reported transparent conducting film made by oxidation of sputtered metallic cadmium [3]. After that different techniques such as sol–gel [4], dc magnetron sputtering [5], radio-frequency sputtering [6], spray pyrolysis [7], chemical vapor deposition [2], chemical bath deposition [8], and pulsed laser deposition [9] is used for deposition of doped and undoped CdO films. Although CdO is highly conducting, but due to its relatively small bandgap, it is not extensively studied compared to other transparent conducting oxides such as zinc oxide, indium oxide, titanium oxide, etc.

There are few reports on improvement of optical bandgap of CdO by doping [10], [11]. Saha et al. have reported an improvement in optical bandgap of CdO by titanium incorporation [12]. Yan et al. have observed that the optical band gap of tin doped CdO films first increases with increase in tin doping level and then decreases with further increase in tin doping [9]. The maximum bandgap of 2.87 eV is observed for 6.2 at% tin doping. The effect of growth temperature on optical bandgap of tin doped CdO film is studied [1]. Fluorine doped CdO films show an improvement in optical bandgap from 2.2 eV to 2.42 eV by 4 at% fluorine doping [13]. Recently, it is reported that the rare earth oxides are potential doping candidates to improve the optical and electrical properties of conducting metal oxides because of their high optical band gap [14], [15].

The literature survey indicates that there is no report on optoelectrical properties of rare earth oxide doped CdO films using pulsed laser deposition technique. The aim of the present work is to study the effect of oxygen partial pressure on structural, optical, and electrical properties of Gd doped CdO (CdO:Gd) thin films prepared by pulsed laser deposition technique.

Section snippets

Experimental details

Standard solid-state reaction was used for preparation of Gd (2 at%) doped CdO target. High purity Gd2O3 (Alfa Aesar, USA) and CdO (Alfa Aesar, USA) were used. The well-ground mixture was heated at 900 °C for 10 h. The powder mixture was cold pressed at 6 × 106 N/m2 load and sintered at 950 °C for 12 h. The thin films were deposited on quartz substrate under different oxygen partial pressures at 100 °C. KrF excimer laser (Lambda Physik COMPex, λ = 248 nm and pulsed duration of 20 ns) was used for

Structural characterization

Fig. 1 shows the X-ray diffraction patterns of CdO:Gd films grown under different oxygen partial pressures. The observed diffraction patterns indicate the polycrystalline nature of the CdO with cubic structure on the basis of PDF Card No: 005-0640 [17]. No extra peaks due to the addition of gadolinium in CdO oxide films were observed and this indicates absence of an impurity phase in the films. It is seen that all the films have preferred orientation along (1 1 1) direction. The average particle

Conclusions

Pulsed laser deposition technique is used for growth of Gd-doped CdO films at low temperature. The effect of oxygen partial pressure on structural, optical, and electrical properties are studied. These (1 1 1) preferred oriented films are highly transparent. The optical bandgap of the films depends on oxygen partial pressure and varies from 3.0 eV to 3.4 eV. The lowest electrical resistivity and highest mobility of 2.71 × 10−5 Ω cm and 258 cm2/Vs, respectively, are observed. These low temperature

Acknowledgement

This work is supported by National Science Foundation (Award Number DMR-0907037). Authors are thankful to Prof. R. Mayanovic, Missouri State University, for providing XRD facility.

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