Structural, optical and electrical characteristics of silver ions irradiated ZnO film on flexible substrate

Highlights

• ZnO films were deposited on PET substrate by DC magnetron sputtering.

• 300 KeV Silver (Ag) ions were implanted in the films at different doses.

• The films ’crystallinity decreased due to damage produced by Ag implantation.

• The Ag ions irradiation of ZnO up to 10¹³ decreased its electrical resistivity.

Abstract

Zinc oxide (ZnO) films were coated on flexible Polyethylene Terephthalate (PET) substrate by direct current magnetron sputtering of zinc in the presence of reactive oxygen. Afterward, the films were irradiated with 300 keV silver ions (Ag⁺¹) through Pelletron Accelerator by changing irradiation dose from 1 × 10¹¹ to 1 × 10¹⁴ ions/cm² in four equal steps. The penetration depth of Ag⁺¹ inside the film was found to be about 70 nm by SRIM (stopping and range of ions in matter) software. Structural parameters of unirradiated and irradiated films were examined by X-ray diffraction. The comparison showed significant degradation of ZnO (002) peak intensity and increase in its widening after irradiation. The Ag⁺¹ irradiation of ZnO at 1 × 10¹¹ ions/cm² decreased its lattice constant (c). However, the lattice constant was increased with a further increase of the irradiation dose. The absorption edge of ZnO was shifted to higher wavelengths with the increase of Ag⁺¹ dose, as shown by the ultraviolet–visible absorption spectra. The Four Probe measurement revealed a consistent decrease in the electrical resistivity of ZnO after Ag⁺¹ irradiation up to 1 × 10¹³ ionscm⁻². Then it remained constant with a further increase of the ions dose.

Introduction

During the last two decades, semiconductor based electronic/optoelectronic devices have received significant attention from researchers worldwide owing to their wide range of applications [[1], [2], [3], [4], [5]]. The use of polymer substrates, for these devices, allows flexibility in electronic circuits [[6], [7], [8], [9], [10], [11], [12]]. In this regard, commonly used flexible substrates are polyethylene terephthalate (PET), polycarbonate (PC), Polyethylene naphthalate (PEN) [[12], [13], [14]]. These polymer based substrates exhibit unique properties of light weight, flexible, and stretchable which increase their demand in the electronic and optoelectronic industry.

Zinc oxide (ZnO) based devices have become popular these days due to the interesting properties of ZnO such as wide band gap, n-type conductivity, high exciton energy, non-toxicity, and high transparency to visible light [15,16]. Different techniques like direct current/radiofrequency magnetron sputtering [17,18], pulsed laser deposition [19], sol-gel coating [20], chemical spray pyrolysis [21,22], etc. were employed to synthesize ZnO thin films on various substrates. The fabrication of ZnO film on a polymer like PET is quite challenging due to its heat sensitivity and degradability at high temperature [16,23]. Usually, the chemical synthesis of ZnO is followed by its annealing at high temperature. In the case of the polymer substrate, the high temperature might deteriorate the substrate due to its low melting point. Therefore, few studies were conducted to synthesize ZnO film on polymer substrates by chemical methods [24] owing to problems linked with the post-synthesis annealing of the film. On the other side, the physical vapor deposition method like magnetron sputtering is more appropriate to synthesize ZnO film on a flexible polymer with better crystalline quality which may eliminate the use of post-deposition annealing process.

Doping of metallic and non-metallic elements is a commonly employed technique to enhance the electrical and optical properties of ZnO. For this purpose, various elements of groups III, IV, and V were doped in ZnO, both chemically and physically to alter its physical properties [[25], [26], [27]]. Silver (Ag) is one of the interesting metals whose doping in ZnO can provide attractive physical properties for use in different optoelectronic applications [28]. It has been reported that the silver doping could enhance the photo-catalytic mechanism of ZnO by tapping the charge carries induced by light and prevent the charge recombination process [29]. In most of the past studies, silver doping in ZnO was done using the chemical methods to alter ZnO properties [30,31]. The results showed that the silver ions are difficult to incorporate inside ZnO by conventional chemical doping techniques due to large ionic radius of Ag than that of Zn.

Ion implantation is another technique that can be used to implant ions of different elements into thin films with relatively more uniformity and controllability as compared to chemically doped ions [[32], [33], [34], [35]]. Previously, only a few studies have focused on the silver ion implantation in ZnO [[35], [36], [37], [38]]. For example, Singh et al. [36] investigated the optical and structural properties of ZnO film implanted with 120 keV Ag ions. The authors found an insignificant change in the band gap of ZnO film; however, the transmittance of the film was decreased. Francis and Lakshmi M [37] carried out the doping of silver (Ag) in ZnO by RF sputtering. They reported the reduction in resistance and an optical band of ZnO film by Ag doping. Lyadova et al. [38] implanted 30 keV Ag ions in ZnO film prepared using the ion beam assisted deposition technique. The authors reported the formation of Ag nano-particles inside the film that increased its optical transmittance. In this study, we have used a flexible PET substrate to deposit ZnO film by direct current (DC) magnetron sputtering of Zn in the oxygen environment. The effects of 300 keV singly charged Ag ions (Ag⁺¹) irradiation on structural, optical and electrical characteristics of ZnO are studied here.