Improved optical properties of ion beam irradiated (K,Na)NbO3 thin films

Journal Pre-proof Improved optical properties of ion beam irradiated (K,Na)NbO3 thin films Radhe Shyam, Apurba Das, Pamu Dobbidi, Fouran Singh, Pargam Vashishtha, Govind Gupta, Srinivasa Rao Nelamarri PII:

S0925-8388(20)30157-2

DOI:

https://doi.org/10.1016/j.jallcom.2020.153794

Reference:

JALCOM 153794

To appear in:

Journal of Alloys and Compounds

Received Date: 16 October 2019 Revised Date:

7 January 2020

Accepted Date: 9 January 2020

Please cite this article as: R. Shyam, A. Das, P. Dobbidi, F. Singh, P. Vashishtha, G. Gupta, S.R. Nelamarri, Improved optical properties of ion beam irradiated (K,Na)NbO3 thin films, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153794. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

CRediT authorship contribution statement Radhe Shyam: Investigation, Methodology, Writing - original draft, Writing - review & editing. Apurba Das: Formal analysis, Methodology. Pamu Dobbidi: Formal analysis, Methodology. Fouran Singh: Formal analysis, Methodology. Pargam Vashishtha: Formal analysis, Methodology. Govind Gupta: Formal analysis, Methodology. Srinivasa Rao Nelamarri: Conceptualization, Supervision, Writing - review & editing.

Improved optical properties of ion beam irradiated (K,Na)NbO3 thin films Radhe Shyama, Apurba Dasb, Pamu Dobbidib, Fouran Singhc, Pargam Vashishthad, Govind Guptad, Srinivasa Rao Nelamarria,* a

Department of Physics, Malaviya National Institute of Technology Jaipur, J.L.N. Marg, Jaipur 302017, India b

Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India c

Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India

d

CSIR-National Physical Laboratory, K.S. Krishnan Marg, New Delhi 110012, India *Corresponding author email: [email protected]

Abstract In the present study, we have demonstrated the effect of swift heavy ion (SHI) irradiation on photoluminescence (PL) and time-resolved photoluminescence (TRPL) properties of potassium sodium niobate (KNN) thin films deposited on Si and quartz substrates using RF magnetron sputtering. Ion beam irradiation of crystalline KNN films was carried out at room temperature using 100 MeV Ni ions with different fluences such as 1×1012, 5×1012 and 1×1013 ions/cm2. Various modes obtained in Raman spectra of films are related to NbO6 octahedron which confirms the crystalline phase of KNN. Moreover, the decrease in peak intensities with ion fluence is attributed to defects produced after SHI irradiation. X-ray photoelectron spectroscopy results show the increase in oxygen vacancies after irradiation. The optical properties of pristine and irradiated films were evaluated using UV-Vis-NIR spectroscopy and a significant improvement in optical transmittance upon irradiation was observed. The optical band gap of films is decreased to 3.14 eV upon irradiation at 1×1012 ions/cm2. PL spectra of films were obtained with excitation wavelength of 274 nm and, the results depict the emission wavelengths (band-to-band or near band edge) of KNN thin films in regime of 313−367 nm (3.96−3.38 eV). The origin of blue and green emission is due to defects created as a result of ion irradiation. The green luminescence evolved at higher fluence (i.e. 1×1013 ions/cm2) may be attributed to intrinsic defects such as oxygen vacancies. The obtained Commission Internationale de I'Elcairage (CIE) color coordinates are shifted towards white point in chromaticity diagram as a function of ion fluence. TRPL result reveals the decay lifetime of KNN films in nanosecond regime and, is varied with ion fluence. At fluence of 1×1013 ions/cm2, film exhibited the minimum average decay lifetime (1.75 ns) which suggests that KNN can be a potential candidate for optical switching, optical display and sensors, and opto-electronic device applications. Keywords: KNN thin films, SHI irradiation, photoluminescence, time-resolved photoluminescence, defects

1

1. Introduction From the start of an era in 1950, the electronics industry has witnessed an uprising of lead-based piezoelectrics such as lead zirconate titanate [Pb(Zr,Ti)O3], with its potential applications in actuators, piezoelectric transformers, micro-electromechanical systems, ultrasonic motors, sensors [1–4] and many more. However, in the last few decades, the environmental hazards associated with the lead content has oriented the research towards developing environment friendly lead-free materials [5]. Among various lead-free piezoelectrics, potassium sodium niobate (KNN) has gained major attraction due to its high Curie temperature and superior piezoelectric properties [6,7]. In recent years, a lot of research has been carried out to exploit its piezoelectric, ferroelectric, and dielectric properties for different applications [6–11]. In addition, the research for integrated optical applications on KNN is still sparse. However, certain reports on optical properties of KNN thin films have provided substantial evidence on the application of KNN in optical coatings, integrated optical devices, electro-optic modulators, LEDs, and optical storage technology [12–17]. Recent research on luminescent properties of various crystalline oxides has opened up a new dimension in thin film domain due to their multifunctional properties and vast area of applications such as optoelectronic devices, display technology, sensor technology [18–23]. Luminescent properties of many semiconductors have been studied for optical sensor and other applications [20,24]. Over past few years, various crystalline materials of large band gap are being continually used due to their novel properties for above mentioned applications in commercial devices. Motivated by the innovation behind such applications, the photoluminescence (PL) study of KNN thin films has been explored in this article in detail. A careful literature survey of the research work done by various research groups revealed a significant volume of literature on PL study of KNN for optical applications but no systematic study is dedicated to calculate the decay lifetime. Therefore, it is very important to systematically explore the decay lifetime of KNN using time-resolved photoluminescence (TRPL) from a fundamental as well as technical point of view. TRPL is a sophisticated stateof-the art technique that can be used to reveal a wealth of information. It is a non-destructive and one of the most powerful optical characterization tools that is generally used for the estimation of decay lifetime of excitons, which is further related to material quality and device performance. The decay lifetime corresponding to a particular transition is directly relative to the radiative recombination efficiency and is predominantly useful in designing opto-electronic device. 2

Tuning various properties of KNN thin films using different methods such as doping, pre- and post annealing, varying deposition parameters etc. has been extensively studied by researchers for device based applications [16,25,26]. However, modification of its properties via SHI irradiation can also open a wide scope from the basic as well as application point of view. SHI irradiation can be a very effective technique to alter various properties of materials in a controlled way depending upon the incident ions, their energies (from few keV to GeV), ion fluences, incident angle etc and is being used for engineering the properties of materials including metals, semiconductors, insulators, oxides and polymers [27–29]. When high energy ion passes through a material, it loses its energy via two ways, namely, electronic energy loss and nuclear energy loss. Therefore, SHI can be a potential method to engineer various properties of KNN in a selective way for specific application. Many reports are available on the tuning of structural and optical properties of various materials using SHI irradiation, but there is a serious dearth of literature on irradiation induced luminescent study of KNN material. However, a temporal report exists on optical study of KNN solid solution co-doped with La-Mn and Eu-Fe in which average lifetime is calculated for transitions between dopants [30]. Based on a careful and in depth literature survey, the effect of SHI irradiation on decay lifetime of KNN has been presented in this article for first time. Among the multitude of techniques available for deposition of films, spin coating [12], sol-gel [13,14], RF magnetron sputtering [16,17] have been widely used to deposit KNN thin films. This study deals with thin films deposited using RF magnetron sputtering, which is one of the most powerful techniques used globally for the deposition of high quality thin films owning to its high sputtering yield, large area deposition and good adhesion of films to substrates. In the present study, crystalline KNN films were irradiated using 100 MeV Ni ions at different ion fluences, such as 1×1012, 5×1012 and 1×1013 ions/cm2, to investigate high energy ion irradiation effects on structural and optical properties of KNN thin films. The irradiation induced effects on optical behavior of KNN films are reported in detail using UV-Vis-NIR spectroscopy, photoluminescence spectroscopy, and time-resolved photoluminescence spectroscopy.

2. Experimental Details 2.1. Deposition of KNN thin films KNN sputtering target was synthesized by solid state reaction method using high purity (99.99%) powders of K2CO3 (Sigma Aldrich, USA), Na2CO3 (Sigma Aldrich, USA) 3

and Nb2O5 (Sigma Aldrich, USA) as starting materials. The details of the preparation method are reported elsewhere [16,17]. KNN films were deposited on Si and quartz substrates at room temperature using RF magnetron sputtering in an Ar ambience with a flow rate of 20 SCCM. In brief, the deposition chamber was initially evacuated to a base pressure of ∼4.5×10−6 mbar and then Ar was introduced into the chamber to maintain a constant deposition pressure of ∼3.5×10−2 mbar during the entire process. The substrates were kept at a separation of ∼5 cm from the sputtering target and a power density of ∼1.97 W/cm2 was used to carry out the deposition of KNN thin films. The as-deposited KNN films were annealed in a furnace (Nabertherm, Germany) at 700°C in air ambience for 1 hr to achieve crystalline films. 2.2. SHI irradiation of KNN films The crystalline KNN films (pristine films) were eventually irradiated with 100 MeV Ni ions at different fluences such as 1×1012, 5×1012 and 1×1013 ions/cm2 at room temperature using the Pelletron Accelerator facility available at Inter-University Accelerator Centre (IUAC), New Delhi, India. The energetic ion beam was rastered at normal incidence to the sample surface and the surfaces were scanned over a cross sectional area of 1×1 cm2. The beam current was kept constant at 2 pnA (particle nanoampere) during irradiation of samples. Since, incident ions interact with target atoms, it losses energy via electronic energy loss (Se) and nuclear energy loss (Sn) during the penetration through the films. The energy losses (Se and Sn) are estimated from SRIM-2013 simulations and, the Se and Sn values for 100 MeV Ni ions into KNN are 1.15×103 eV/Å and 2.23 eV/Å, respectively. The projected range of incident ions is ∼12.41 µm which is very large enough to avoid any possibility of Ni ions implantation into KNN matrix. 2.3. Characterization of KNN films Films were characterized using confocal micro-Raman spectrometer (Airix Corp, STR 500, Japan) to confirm the crystalline phase of KNN. Raman spectra were obtained using a solid state laser source with excitation wavelength of 532 nm and 12.5 mW power. The chemical analysis of pristine and irradiated films was examined using X-ray photoelectron spectroscopy (XPS; Omicron NanoTechnology, ESCA+, Germany). The absorbance and transmittance spectra of films before and after irradiation were carried out using UV-Vis-NIR spectroscopy (Lambda 750, Perkin Elmer). The photoluminescence spectroscopy (Edinburgh FLS-980 d2d2) was carried out with a Xenon (Xe) flash lamp of excitation wavelength of 4

274 nm, and time-resolved photoluminescence (TRPL) spectra were taken with excitation wavelength of 266 nm by picosecond diode laser.

3. Results and discussion 3.1 Raman analysis: Raman analysis of thin films was carried out in order to extract information regarding structural modifications and extent of crystallization from phonon modes. Raman spectroscopy is one of the most sensitive techniques, utilized for the examination of structural deformation in a crystal lattice. In ABO3 class of perovskites, the major vibrations are associated with the internal modes of

octahedron and therefore, deformation in

octahedron and displacement of cations causes the change in structural properties. According to earlier reports, KNN exhibits 12 Raman active modes of 4A1 + 4B1 + 3B2 + A2. These vibrations are associated with translational modes of isolated cations and internal modes of NbO6 octahedra. Raman modes of NbO6 octahedra consist of 1A1g(ν1) + 1Eg(ν2) + 2F1u(ν3,ν4) + F2g(ν5) + F2u(ν6), where vibrations corresponding to 1A1g(ν1) + 1Eg(ν2) + 1F1u(ν3) are stretching modes and rest of them are bending modes [31,32] as shown schematically in Fig. 1 [31]. In the present study, the various vibrational modes are identified based on the interpretation of Raman spectra of KNN reported by K. Kakimoto [31] and Y. Nakashima et al. [33]. The obtained Raman spectra of pristine as well as irradiated films are shown in Fig. 2. The vibrational modes of NbO6 octahedra appear in a wide range from 150-900 cm-1. Moreover, a weak peak around 196 cm-1 is also observed left to the shoulder of ν5 mode and that is related to the translational mode of K+ cations versus rotation of NbO6 octahedra [31,34]. In particular, wide band of ν5 (∼237 cm-1) and ν1 (∼627 cm-1) are found as relatively strong scattering modes in pristine samples because of near-perfect equilateral octahedral symmetry. An intense peak observed around 520 cm-1 is related to the typical signal from Si substrate. The observed modes of NbO6 octahedra and a weak translational mode of K+ cations reveal the crystalline nature of the KNN films. Upon irradiation, intensities of different Raman modes are found to decrease monotonously with ion fluence and at a particular fluence of 1×1013 ions/cm2, the Raman bands almost disappeared. Furthermore, evolution of ν1+ν5 (827 cm-1) mode is also observed upon irradiation at ion fluence of 1×1012 ions/cm2 and this mode is disappeared at higher fluence. Irradiation induced defects are generated upon irradiation that can promote the local structural distortion of KNN. This can be understood from the fundamental concept of ion5

solid interaction. In the present work, 100 MeV Ni ions were used as projectile for irradiation of KNN thin films and the electronic energy loss is much higher than that of nuclear loss (values have been mentioned in experimental details). According to thermal spike model, large amount of energy is deposited to the electronic subsystem of target by the projectile ions. This energy is then distributed between the electrons of target atoms through electronelectron coupling and later, shared to lattice atoms by electron-lattice interaction resulting in local heating in the vicinity and along the path of projectile ions. Due to high thermal spikes, pressure waves are generated and it leads to strain in crystal lattice. Thus, it is expected the O−Nb−O bonds in NbO6 octahedra would be broken by energetic Ni ions leading to a disordered lattice which will contribute to internal strain in ordered lattice and therefore, the intensities of various Raman modes are decreased with ion fluence. This indicates the induced partial amorphization of crystalline KNN thin films upon SHI irradiation. At a certain fluence of 1×1013 ions/cm2, films seem to be almost amorphized as the intensity of Raman modes is almost diminished. Surface amorphization of films might be the possible reason for the reduction of intensity of the Raman modes. Moreover, softening of ν1 mode is observed upon irradiation which may be attributed to confinement of phonons by irradiation induced structural defects. The similar results of decrement in peak intensities, softening of Raman modes along with broadening upon irradiation are reported earlier by F. Singh [35] and S.K. Gautam [36] for oxide films. The reduction in intensity and disappearance of Raman modes upon irradiation may also be related to change in co-ordination number of central Nb5+ in NbO6 octahedron which is due to oxygen vacancies by displacement of oxygen atom from its lattice site to interstitial site. Therefore, reduction in co-ordination symmetry is responsible for the broadening of modes and reduction in mode intensities at higher fluence [37]. Moreover, Raman coupled mode evolved around 827 cm-1 after irradiation at fluence of 1×1012 ions/cm2 might be due to niobium interstitial defects and/or irradiation induced oxygen vacancies. Irradiation induced evolution of silent Raman modes due to intrinsic lattice defects (interstitials/vacancies) is reported for other oxide films [38,39]. This mode (around 827 cm-1) disappears at higher fluence which could be due to impact of multiple ions. 3.2. XPS analysis: Fig. 3 depicts the core level XPS spectra of Nb3d for pristine and irradiated KNN films. The XPS spectrum for Nb3d peak could be de-convoluted into spin-orbit components of Nb3d5/2 and Nb3d3/2 with binding energies of ∼207.3 and ∼210 eV, respectively. These are related to Nb2O5 (Nb5+) phase present in KNN. Moreover, the shoulder of Nb3d at ∼205.5 eV 6

and ∼208.5 eV are related to Nb4+ (NbO2) chemical state of niobium [40–45]. Upon irradiation, it is observed that the intensity of Nb4+ state of niobium is slightly increased with increase in ion fluence. To understand the formation of oxygen vacancies after irradiation, O1s spectra were de-convoluted into two components as O2- (lattice oxygen) contributing in KNN structure with binding energy ∼530.5 eV and another peak with high binding energy associated to Nb4+ oxygen species (NbO2). The XPS results are in good agreement with the reported literature [43–45]. The oxygen peak intensity (related to NbO2) is increased remarkably with ion fluence, and the peak becomes intense at fluence of 1×1013 ions/cm2 as evident from O1s core level spectra in Fig. 4. The increase in intensity of O1s peak at higher binding energy indicates the presence of large number of oxygen vacancies in KNN after irradiation at higher fluence [46–48]. 3.3. UV-Vis analysis: Fig. 5 shows the optical transmittance spectra of pristine and irradiated KNN thin films in the wavelength range of 200−1000 nm. It is found that the transmittance of films increases upon irradiation at various fluences. The average transmittance of irradiated films at higher fluence is around 78%, in wavelength region of above 400 nm, which is much higher than that of pristine samples. The increase in transmittance with ion fluence can be correlated with decrease in particle size as evident from the broadening of modes in Raman analysis and therefore, the scattering of incident light might have decreased. The similar result of slight increase in average transmittance of oxide films after irradiation has already been reported [49–51]. Moreover, the sharp decrease in transmittance of films is observed in wavelength region of 300-400 nm. This could be due to the fundamental absorption of films in lower wavelength region. Fig. 6 represents the absorption coefficient of pristine as well as irradiated films at different fluences in the wavelength ranging from 280−560 nm. The sharp absorbance in the low wavelength region is mainly due to equal or higher energy of incident photons, than the band gap of KNN, that offers sufficient energy to carriers so that they jump from valance band to conduction band. Thus, films exhibit higher transmittance in wavelength region of above 320 nm. As clearly seen from the absorption coefficient spectra, the absorption edge is slightly changed after irradiation that may cause slight modification of band gap of KNN films. For calculation of optical band gap of pristine and irradiated samples, the Tauc’s formula [52,53] is used which is expressed by: ℎν =

ℎν − 7

(1)

where, α is absorption coefficient, ‘hν’ represents incident photon energy, A is a constant, and m represents the allowed or forbidden electronic transitions. In the present study, the band gap energy of KNN films was estimated by considering an allowed indirect (

= 2)

electronic transition from the state of highest occupied valance band to lowest unoccupied state of conduction band [14,17]. Fig. 7 shows the band gap energy of pristine and irradiated films, at various fluences ranging from 1×1012 to 1×1013 ions/cm2 and is calculated by extrapolating the linear portion of corresponding Tauc plot. The band gap energy of KNN films is varied from 3.14−3.69 eV. The band gap energy of pristine film is found to be 3.25 eV which decreased to 3.14 eV for films irradiated at initial fluence (1×1012 ions/cm2) indicating the red shift of absorption edge. The decrease in band gap at initial fluence may be due to irradiation induced lattice defects that create extra shallow defect levels between conduction band and valence band [50]. The shallow energy levels effectively modified the band gap and promote the transition from valence band edge to these energy levels rather than band to band transition. Since, the optical band gap is related to degree of structural order-disorder of crystal lattice [54], the intermediate energy levels might have increased upon irradiation at initial fluence. The decrease in band gap energy can be attributed to structural defects and distortion in NbO6 octahedra. Further increase in ion fluence, an increase in band gap is observed. This can be correlated with shifting of absorption edge towards lower wavelength side (as shown in Fig. 6) as ion fluence is increased from 1×1012 to 1×1013 ions/cm2 and this shift indicates the pressure effect on lattices as discussed in the previous section. The widening of band gap at higher fluence may be explained in terms of Burstein-Moss (B-M) shift [55]. The narrowing and widening of band gap with ion fluence is also reported earlier for oxide films [49]. 3.4. Photoluminescence study: Photoluminescence is the process of light emission after recombination of excited carriers [56]. It is an important tool that provides the information of electronic energy band structure, optical band gap, quality of films, interfaces and defects. In the present study, PL spectra were recorded in wavelength range of 295−560 nm (4.20−2.21 eV) at room temperature in order to examine the effect of ion beam irradiation on luminescent properties of KNN thin films. An excitation wavelength of 274 nm (4.52 eV) was used for the present study. In general, the intrinsic behavior (band edge structure) and defects are the possible reason for any material to exhibit the luminescence phenomena. Fig. 8 depicts multiple peak fitted PL spectra of pristine as well as irradiated KNN thin films at various ion fluences. 8

Pristine and irradiated films show two PL bands in ultraviolet and blue wavelength region. The short wavelength emission is ascribed to band-to-band /or near band edge emission due to radiative recombination of free carriers or shallow donor/acceptor bound excitons while the long wavelength emission is related to deep structural defects [57,58]. The defects may be generated due to interstitial, point, and antisite defects (niobium present at A-site) and other complexes. However, the exact origin of PL emission is still unclear. Generally, the visible PL emission arises due to either the recombination of delocalized electrons in conduction band to deep traps containing holes or recombination of deep trap electrons to holes in valence band. In present study, it is found that the pristine films exhibit the characteristic peak in the wavelength range of 313−367 nm (3.96−3.38 eV) which is related to nearultraviolet PL emission and indicates band-to-band transition because this emission is near to the energy of absorption edge. Therefore, the emission band arisen in ultraviolet region is not attributed to defects level. The broad emission corresponding to violet emission is also observed in region of 406−427 nm (3.05−2.90), and an intense blue emission at 435−457 nm (2.85−2.71 eV) along with the some weak shoulders in the regime of 473−492 nm (2.62−2.52 eV). This broad emission is related to defects present due to distortion in NbO6 octahedron in KNN perovskite and, the present results are supported by literature [59]. The luminescent property is highly dependent on the O-Nb-O bonds where the conduction band is related to Nb5+ 4d-orbitals and valence band is composed of O2- 2p-orbitals. Since 4d-orbitals of Nb split in to Eg (dx2-y2, dz2) at higher energy level and T2g (dxz ,dyz, dxy) at lower energy level [60]. The disorder present in NbO6 octahedra may leads to further splitting of these 4d-orbitals. Upon irradiation at 1×1012 ions/cm2, the evolution of sharp peak of near-ultraviolet emission is found around 343 nm (3.61 eV) along with the diminishing of 318 nm (3.90 eV) peak. Although, growth of the new emission around 3.61 eV after irradiation may not be associated with defects of films but can be assumed that it might have evolved probably due to atomic emission. Also, the violet emission corresponds to ∼3.00 eV became sharper and intense upon irradiation at initial fluence. Moreover, an evolution of a broad peak corresponds to cyan is observed between the wavelength region of 512−525 nm (2.42−2.36 eV). At higher fluence of 1×1013 ions/cm2, it is noticed that the near ultraviolet band emission became sharp and the number of defect peaks is increased. Moreover, three more significant peaks evolved around 380 nm (3.26 eV) and 524, 551 nm (2.36, 2.25 eV) correspond to violet and cyan-green emission in visible region, respectively. This might be due to increase in number of the defects upon irradiation. With ion fluence, more defect emissions are 9

produced towards higher wavelength region and it can be confirmed from the Commission Internationale de I'Elcairage (CIE) color coordinates as shown in Fig. 9. These color coordinates are also termed as projective coordinates which help in the determination of wavelength, purity of light emission, and mixing ratio of two colors. The outer boundary of gamut represents the spectral locus of primary colors (RGB) of the chromaticity which is mainly described by hue (angular / radial representation of colors) and colorfulness. The CIE coordinates as dark spots of KNN films are obtained from chromaticity diagram which was formed when light intensity of various photoluminescence emission mapped. Colors of KNN can be modified from violet-blue to bluish-green by disturbing the local environment of NbO6 octahedra by projecting fast moving energetic heavy ion. It is clearly observed that the (x, y) coordinates are shifting towards

= 1⁄3, = 1⁄3 (white centre) at higher fluence as

the defects increased upon SHI ion irradiation. So, CIE result reveals that KNN can be a suitable candidate for the production of violet-blue light emitting applications, and can be further developed for white light emitting materials after ion irradiation. Upon irradiation, the intensity ratio of defect to characteristic emission is relatively increased. It is seen from PL results that the number of defect centers increased with ion fluence. Moreover, it is reported that the disorder structure causes PL in ABO3 perovskites and the energy gap can be reduced about 20-30% depending upon the bond length of oxygen anion with central transition metals [61]. It can be correlated with present case where the symmetry of NbO6 octahedron might be reduced by weakening / breaking of O-Nb-O bond. The relative increase in defect intensity of irradiated samples as compared to pristine can also be

explained

on

the

basis

of

generation

of

shallow

donor/acceptor

(antisite/interstitial/vacancy) and deep-level defects (probably oxygen vacancies) after irradiation. However, the origin of PL is not exactly identified. It is assumed that deep-traps are formed due to oxygen vacancies while the shallow donors may be arisen due to niobiuminterstitial (Nbi) or antisite (niobium present at cationic sites). When highly energetic ions pass through KNN films, they interact with target electrons inelastically resulting in the electronic excitation and ionization of target atoms. The energy transferred to target atoms tends to increase the local heating/amorphization which may result in increase of defect centers in KNN. Therefore, electronic excitation caused by high electronic stopping may weaken /or break the oxygen bond and results in creation of oxygen vacancies /or distortion in NbO6 octahedra. The green emission exhibited by films irradiated at higher fluence is generally attributed to distortion in NbO6 octahedron by oxygen vacancies [12] and it can be 10

well seen from spectra that the intensity of broad emission, probably due to defects produced after irradiation, is enhanced. Moreover, it is noticed that at fluence of 1×1012 ions/cm2, the optical band gap energy is near to defects energy level in violet region. So, the decrease in band gap energy is probably due to the transition from defects level rather than band to band transition. The formation of sharp emission band in the regime of near ultra-violet and the evolved blue as well as green emission band in visible region reveals that KNN can be used as a potential candidate for the applications in optical storage, LED’s, and electro-optic modulators. 3.5. TR-PL analysis: Time-resolved photoluminescence (TRPL) spectroscopy offers the aspects regarding the dynamical processes such as recombination, relaxation, and excitation processes [58]. In order to probe the effect of SHI irradiation on carrier dynamics, TRPL spectroscopy was performed at room temperature (shown in Fig. 10). The decay time of material is generally determined from the concentration of defects present and these defects are responsible for the trapping of electrons and/or holes. In the present study, an excitation wavelength of 266 nm using a picosecond diode laser was used to record the TRPL spectra of KNN thin films. Fig. 11 depicts the fitted TRPL curves of pristine and irradiated (1×1012, 5×1012 and 1×1013 ions/cm2) KNN thin films. The curves are fitted using bi-exponential function which was used to estimate the decay time of pristine as well as irradiated KNN films. The fitted curves give the decay time constants and the exponential function is given by: =

+

⁄!"

+

#

⁄!$

(2)

where, I(t) represents luminescence intensity, A is a constant, Bi is the PL intensity of ith component ( = 0), τ1 and τ2 represent the decay time constants. Short decay time (τ1) is related to fast decay process whereas long decay time (τ2) represents the slow decay process and these decay processes correspond to trap-assisted recombination (free exciton lifetime) and radiative recombination (might characterize the lifetime of free carriers together with trapping and emission processes), respectively [62,63]. The slow and fast decay processes can also be strongly correlated with the data given in Table 1 in which τ2 is greater than τ1. In case of bi-exponential decay, the average decay time (&'( ) is estimated using the relation as follows: ) !$ *) !$

&'( = )" !" *)$ !$ " "

$ $

11

(3)

Table 1 Decay parameters of pristine and irradiated KNN thin films +,-

Decay parameters Sample Pristine 1×1012 ions/cm2

τ1 (ns)

τ2 (ns)

B1

B2

χ2

(ns)

2.15

7.00

926.27

188.27

0.988

4.08

1.36

3.34

878.04

223.06

0.911

2.12

12

2

1.59

11.09

848.38

141.86

0.977

6.70

13

2

0.89

2.63

820.41

275.45

0.964

1.75

5×10 ions/cm 1×10 ions/cm

The average decay time, decay constants, and value of χ# for pristine as well as irradiated KNN samples are listed in Table 1. It is clear from table that the decay time is decreased upon SHI irradiation except for films irradiated at 5×1012 ions/cm2. The reduction in the decay time of SHI irradiated KNN thin films suggests the increase in defects density, after ion irradiation, which can trap the energy of excitons and might have resulted in decrease in decay time [64]. The increase in defects may be due to freezing out of the carriers and the weakening/breaking of O−Nb−O bonds that might be due to multiple ion impact at higher ion fluence, which has already been discussed in Raman analysis section. The average decay time of KNN films may also be directly correlated with the photoluminescence results. At initial fluence, the contribution of intensity of defects to the characteristic peak is relatively increased as clearly seen from the PL spectra. At a fluence of 5×1012 ions/cm2, the relative defect intensity is observed to minimum as compared to other samples. Due to this reason, the average decay time of films irradiated at this fluence is increased. The increase in decay lifetime at this particular fluence also indicates the reduction of non-radiative recombination centers [22]. The variation in density of non-radiative recombination centers with ion fluence can be seen from the TRPL spectra as shown in Fig. 10. It is considered that defect species present in material govern the non-radiative processes. However, the dominant mechanism of formation of non-radiative recombination centers is still ambiguous. But, further at higher fluence (1×1013 ions/cm2), the intensity of defects is relatively increased as well as more defect peaks are evolved (as shown in Fig. 8) that might be the possible reason for the minimum value of average decay time. SHI irradiation effects on KNN thin films lead to decrease in the decay lifetime (1.75 ns for films irradiated at a fluence of 1×1013 ions/cm2) and suggest the possibility of KNN being a suitable candidate for designing devices meant for fast optical switching/sensing applications as well as in field of opto-electronic devices.

12

4. Conclusions In summary, we have investigated the SHI irradiation induced modification on structural, optical and luminescent properties of RF sputtered KNN thin films. Films were subjected to ion irradiation of 100 MeV Ni ions with fluences ranging from 1×1012 to 1×1013 ions/cm2. The various modes of NbO6 octahedron in Raman spectrum of pristine films confirmed the crystalline phase of KNN. The decrease in peak intensities upon ion irradiation is observed which is probably due to various defects produced in films after irradiation. Irradiation induced increase in oxygen vacancies with ion fluence is explained using XPS results. The optical band gap of films observed to be decreased at initial fluence and further increased with ion fluence. Photoluminescence spectra revealed the evolution of defects (deep-traps or shallow donor/acceptor) after irradiation of KNN thin films. The intensity of blue emission band increases upon irradiation and, the evolution of green emission at higher fluence indicated the intrinsic defects (i.e. oxygen vacancies) produced in KNN films. The average decay lifetime in nanosecond regime suggest the possible use of KNN films in optical switching, optical display and sensors based applications as well as in opto-electronic devices.

Acknowledgements RS is highly grateful to University Grants Commission (UGC), New Delhi for providing UGC-JRF fellowship. Authors are thankful to Materials Research Centre, Jaipur for various characterization facilities. Inter-University Accelerator Centre (IUAC), New Delhi is highly ackowledged for providing the ion beam facilities.

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19

Figures

Fig. 1. Schematic illustration of various internal vibrational stretching modes (ν1, ν2, ν3) and bending modes (ν4, ν5, ν6) of NbO6 octahedron.

Fig. 2. Raman spectra of pristine and irradiated KNN thin films.

20

Fig. 3. XPS spectra of Nb3d core level of (a) pristine, and irradiated KNN films at (b) 1×1012, (c) 5×1012, (d) 1×1013 ions/cm2.

21

Fig. 4. XPS spectra of O1s core level of (a) pristine, and irradiated KNN films at (b) 1×1012, (c) 5×1012, (d) 1×1013 ions/cm2.

Fig. 5. Transmittance spectra of pristine and irradiated KNN thin films at different fluences

22

Fig. 6. Absorption coefficient spectra of pristine and irradiated KNN thin films at different fluences.

Fig. 7. Tauc plots of pristine and irradiated KNN thin films.

23

Fig. 8. PL spectra of (a) pristine, and irradiated KNN films at (b) 1×1012, (c) 5×1012, (d) 1×1013 ions/cm2.

Fig. 9. CIE color co-ordinates and spectrum of pristine and irradiated KNN films.

24

Fig. 10. Room temperature time-resolved photoluminescence spectra of pristine and irradiated KNN films

Fig. 11. Bi-exponential fitted time-resolved photoluminescence spectra of (a) pristine, and irradiated KNN films at (b) 1×1012, (c) 5×1012, (d) 1×1013 ions/cm2.

25

Highlights: •

Structural and luminescent properties of ion beam irradiated KNN films are investigated

•

Possible mechanism of variation of energy band gap of KNN films with ion fluence is reported

•

PL and TRPL results suggest the use of irradiated KNN films in opto-electronic application

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: