UV sensitivity enhancement in Fe-doped ZnO films grown by ultrafast spray pyrolysis

Highlights

• ZnO thin films are fabricated using ultrafast spray pyrolysis for 30 s.

• Fe doping decreases the crystallinity and crystallite size of ZnO.

• Fe concentration of 3% has the highest UV sensitivity.

• Lower dark current and higher photocurrent are found in Fe concentration of 3%.

Abstract

The effect of Fe dopant on structural and photosensitivity properties of ZnO thin films have been studied. The Fe-doped ZnO films were fabricated by ultrafast spray pyrolysis for 30 s. As a result, X-ray diffraction analysis demonstrates that the deposited films are polycrystalline with the hexagonal wurtzite structure. Fe doping in the ZnO system provokes not only the decrease of crystallinity and crystallite size (~50% for Fe concentration of 0.5%) but also the increase of interplanar spacing and strain. Surface morphology images show that the grain size of 3% Fe-doped ZnO (67 nm) is smaller than that of the pure ZnO (114 nm). Based on I–V characterizations, Fe doping increases photocurrent as well as UV sensitivity up to 21 times (1.5% Fe) and 70 times (3% Fe) than that of the undoped ZnO. This study is important to improve the functionality of ZnO as a UV photodetector.

Introduction

Many semiconductors such as SiC, GaN, CuO, TiO_2, and ZnO have been employed for photodetector devices. ZnO has attracted attention for this application due to its nontoxicity, biocompatibility, and low cost. ZnO is an n-type semiconductor with a wide bandgap of 3.37 eV and a large exciton binding energy of 60 meV [1]. The n-type formation is commonly occurred in ZnO due to intrinsic defects such as oxygen vacancy and zinc interstitial, which act as donors [2]. ZnO has also high transparency (commonly >80% for wavelength >400 nm) and tunable electrical resistivity by inserting various dopants [1]. Also, ZnO can be grown as low dimensional materials like nanowires and nanorods, which is a potential candidate for high-performance UV detectors due to the high surface-to-volume ratio [3,4]. Besides, the better performance of UV photodetectors can be achieved by inserting impurities [5]. Various 3d transition metals are considered effective ZnO dopants because they have ionic radii close to ZnO [6,7]. Iron (Fe), which is one of 3d metal transition elements, is considered an effective ZnO dopant for optical communication and optoelectronic devices [6,8,9], especially UV photodetector applications [10,11].

By considering the sensitivity and response times of the detectors, M. Salem et al. [11] have examined Fe-doped ZnO (FZO) as a sensitive photodetector material under visible light. C. O. Chey et al. [10] examined the sensitivity of ZnO doped by 5% Fe under UV irradiation of 368 nm. C. S. Prajapati et al. [12] reported high UV sensitivity (388 nm) of ZnO film using a Fe concentration of 1%. These studies show that the Fe concentration and deposition method determine the sensitivity. Fe doping can increase sensitivity up to 471 as compared to pure ZnO (43), and improve rise time (46 s) as well as decay time (37 s) compared to the other dopant [13]. On the other hand, FZO was used to enhance the photocatalytic activity for methylene blue [14] and methyl orange [15,16] degradations, and ammonia detection [17]. FZO coating on silicon solar cells has also shown an increase in solar power conversion [18]. FZO-based diluted magnetic semiconductors without metal clustering formation emerge room-temperature ferromagnetism, which is crucial for spintronic applications [2,19,20].

FZO thin films have been successfully grown with various methods both physical and chemical methods including sputtering methods [21,22], molecular beam epitaxy [23], sol-gel [24], spin coating [25], dip-coating [26], chemical bath deposition [13], dan spray pyrolysis [6,27]. The spray pyrolysis is a good chemical method to grow thin films by spraying a solution on a hot substrate [[28], [29], [30]]. The advantages are simple, fast, low cost, and vacuum-free growth process [6,31]. To produce a thin film within nanoscale, a reduction of initial size can be produced by an ultrasonic nebulizer that is capable to produce μm size droplets. Even though spray pyrolysis is a simple method, the effect of Fe doping on the structural and electrical properties of the ZnO-based UV detector is still not explored. Also, decreasing the deposition time without the decrease of crystalline quality is still an issue to be solved in mass-scale production.

In this paper, FZO thin films were grown by ultrafast spray pyrolysis for 30 s, which results in a good crystalline as compared to other physical and chemical deposition methods. The doping concentration was varied (from pristine ZnO to 3 at.% Fe) to obtain an optimum concentration with good UV sensing performance. The Planar configuration of the metal-semiconductor-metal junction was employed to investigate the UV sensitivity.