Tuning of defects induced visible photoluminescence by swift heavy ion irradiation and thermal annealing in zinc oxide films

Highlights

• Tuning of defects related photoluminescence by ion irradiation.

• Visible PL emission as a function of microstructure and irradiation parameters.

• Formation of vacancies cluster and ionized oxygen vacancies by ion irradiation.

Abstract

Present study reports the influence of electronic energy deposition on sol-gel grown zinc oxide thin films with different type of microstructures for tailoring the defects induced photoluminescence properties. The microstructure of the films was modified by the controlled thermal annealing and the nature of defects were tuned by swift heavy ions (SHI) irradiations at varying ion fluences of such thin films. It is observed that all the films show two emission bands in the spectral range of 350 nm–1000 nm; (i) a very weak UV-emission band and (ii) a broad visible emission band. The UV-emission band is attributed to the band to band transition while the visible emission to the deep-level defects. The maximum visible emission lies in the green emission region which enhanced with increasing annealing temperature for the samples annealed in controlled Argon gas environment in comparison of samples annealed in an oxygen environment. The intensity of the visible emission band was found to be increased with ion irradiation at low ion fluences while showing reduction at higher ion fluences along with peak broadening. The films annealed at higher annealing temperature exhibit shifting of peak maximum from green region to red region at higher fluences. The peak shifting and peak broadening at higher ion fluences could be attributed to the formation of the high density of complex defects such vacancies cluster and ionized oxygen vacancies in the lattice as tuned under the influence of electronic energy deposition through SHI irradiations. Thus, it is suggested that the intense luminescence induced by the defects could be tuned in controlled manner by selecting the proper irradiations parameters and thus it would be very interesting for the development of the optoelectronic applications.

Introduction

Zinc oxide is an n-type semiconductor exhibiting wurtzite hexagonal structure at ambient conditions. It has a wide bandgap of 3.37 eV and high excitonic binding energy of 60 meV (greater than room temperature thermal energy of around 25 meV) (Özgür et al., 2005a). ZnO unique properties make it a suitable candidate for many applications such as solar cell (Hüpkes et al., 2012; Bhosle et al., 2007), light-emitting diode (LED) (Xu et al., 2006; Willander et al., 2009; Ryu et al., 2006), gas sensors (Zhu and Zeng, 2017; Shishiyanu et al., 2005), UV detectors (Vettumperumal et al., 2014; Soni et al., 2020), etc. Investigation on the luminescence properties of ZnO is still going on to fully utilize its properties for solid-state light-emitting applications. Generally, ZnO shows a sharp emission band in the ultra-violet region (UV) and a broad emission band in the visible region of solar spectrum (Özgür et al., 2005b). The emission band in UV-region known as near band edge emission has been reported widely and attributed to exciton recombination or the band to band transition (conduction band to valence band or vice-versa). While the emission band in the visible region known as deep-level emission attributed to the extrinsic and intrinsic defects present in the ZnO films. The intrinsic defects (sometimes called as native point defects) in ZnO are zinc vacancy (V_Zn), zinc interstitial (Zn_i), oxygen vacancy (V_O), oxygen interstitial (O_i), zinc and oxygen antisites (O_Zn and Zn_O) and so on (Oba et al., 2011; Look et al., 2005). In which Zn_i acts as a shallow donor, V_Zn acts as a shallow acceptor, V_O act as a deep donor, and O_i act as a deep acceptor (Lyons et al., 2017; Janotti and Van de Walle, 2007). In particular, in the case of ZnO, it is difficult to avoid intrinsic defects completely due to its non-stoichiometric behavior. Nevertheless, the luminescence behavior of ZnO can be tuned in a controlled manner by controlling the number of defects according to the desire application. Luminescence properties of ZnO get affected by various parameters such as a method of deposition (Thongpool et al., 2019; Fan et al., 2005; Barbagiovanni et al., 2015), the structure of ZnO (Cao et al., 2019; Kabir et al., 2018; Vanheusden et al., 1996), type of dopant (Musavi et al., 2019; Naik et al., 2020; Khranovskyy et al., 2017), doping concentration (Gupta et al., 2019; Kim et al., 2012; Horng et al., 2016), annealing temperature (Ambrosio et al., 2019; Kang et al., 2004) and environment (Gruzintsev and Yakimov, 2005; Singh et al., 2011a) also plays a crucial role in luminescence properties of ZnO).

It has been reported previously by our group (Singh et al., 2011a), that annealing environment or annealing temperature has a significant effect on the luminescence properties of ZnO thin films. We showed that the efficiency of visible luminescence increases with increasing annealing temperature and also in the reduced environment (annealing in Ar-gas) the intensity of visible emission evolved which was attributed to increase in the concentration of deep-level defects. In another report by H. Kang et al. (2004), also supports our previous work that the intensity of visible emission increases with an increase in annealing temperature. But the origin behind visible luminescence is contradictory in many reports and the research is still going on to find out the exact origin of visible emission such as green emission. Further, it is well known that visible luminescence in ZnO can be tuned by controlling the type and concentration of defects present in the system. In this context, ion irradiation is the best tool to create defects into a material in a controlled manner by varying irradiation parameters such as ion species, ion beam current, ion fluences, and ion energy, etc. There are some reports available on the effect of ion irradiation on luminescence properties (Kumar et al., 2013a; Gautam et al., 2015; Singh et al., 2009; Sharma et al., 2011). Recently, H. Gupta et al. (2019), reported that with ion irradiation the visible emission can be tuned. Thus it would be interesting to study the luminescence behavior of ZnO thin films by combing annealing conditions and ion irradiation.

In view of this, the sol-gel spin coating grown films annealed at 500 °C in an oxygen environment, 500 °C in an argon environment, and 700 °C in argon environment are irradiated using 50 MeV Ni and 120 MeV Au ions with different fluences. Two annealing temperatures were chosen to have two different types of microstructure and surface morphology. The nanostructured films are modified by SHIs with varying electronic energy deposition into the ZnO films. These ions loose negligible energy in nuclear scattering as compared to the electronic energy deposition and the range of the ions is much larger than the thickness of the ZnO films. Therefore, ions are implanted deep inside the Si substrates and the modification in the ZnO thin films are mainly expected due the electronic energy deposition via these incident ions in the ZnO films.