Investigation of structural and morphological properties of high energy ion irradiated KNN films

The transfer of high localized energy density to target matrix via swift heavy ion provides a domain to engineer the properties of materials in a systematic and controlled mode. The present study describes the influence of ion irradiation on structural properties and surface morphology of (K,Na)NbO3 (KNN) films of thickness 650–750 nm irradiated with 100 MeV Ni ions at different fluences varying from 1 × 1012 to 1 × 1013 ions cm−2. Multiple ion impact induced reduction in crystalline behavior of KNN perovskite is observed as an effect of ion fluence. The films show partially amorphized nature with ion fluence, and are remained in crystalline perovskite phase after irradiation with decreased peak intensities. Quantitative surface roughness and surface scaling study via power spectral density (PSD) analysis were carried out using atomic force microscopy (AFM) micrographs. The root mean square roughness decreased at 1 × 1012 ions cm−2 and thereafter, increased monotonously with increasing ion fluence. The adatoms mobility and coalescence effect might have caused the variation in roughness. From the PSD results, modification of surface morphology of films irradiated at 1 × 1012 ions cm−2 is attributed to the competing mechanisms of viscous flow and evaporation-recondensation processes. At higher fluence, the evolution mechanism of morphology is turned out to be the combined effect of evaporation-recondensation and diffusion processes. The microstructures obtained using scanning electron microscopy are correlated with the AFM results. The dominating processes of irradiation induced modification in the morphology of KNN films are studied in detail, and this study will be useful from both fundamental and applied perspectives.


Introduction
The advancement of a new category of lead-free ceramics has become a broad research area for scientists and engineers. Potassium sodium niobate ((K,Na)NbO 3 ) (KNN) is one of the most potential candidates among various lead-free materials owing to its admirable piezoelectric properties with Curie temperature of above 400°C [1]. The lead-based piezoelectric ceramic material such as Pb(Zr, Ti)O 3 contains more than 60 wt.% lead, which is detrimental for the environment [1,2]. Such limitations have led to ever growing demands for lead-free ceramic-based products day by day. KNN-based thin films have found prominent use in diverse applications such as energy harvesters, microelectromechanical system, and optical devices [3][4][5]. Numerous techniques have been adopted for the deposition of KNN thin films that include chemical deposition [6], sol-gel [7], RF magnetron sputtering [3,4], pulsed laser deposition [8], etc. Among these techniques, RF magnetron sputtering is a potential tool for the deposition of high quality, dense, and uniform thin films with a large deposition area. Recently, tailoring growth modes by excess alkali addition in sputtered KNN thin films has been reported [9]. A few studies have already been implemented to improve the properties of KNN thin films such as doping, annealing, varying sputtering parameters, deposition methods, etc. In addition to these methods, ion-matter interaction is also a potential approach to tune the properties of materials in a controlled manner. The ion beam facility has offered a new possibility of synthesis, and modification of materials at nanoscale [10]. Therefore, a study dealing with the interaction of ion beam with KNN thin films for tuning its properties is of utmost importance. The study of swift heavy ion (SHI) irradiation has been established as a promising method for tuning the properties of nanostructured materials. SHI irradiation study has become a versatile technique which can engineer the properties of target materials in the controlled and selective way based on the energy, mass, and fluence of incident ion [11,12]. The formation mechanism of surface features of few nanometer sizes with impact of SHI and slow highly charged ions is discussed in a review report [13]. In the recent study, the improvement in piezoelectric properties of triglycine sulphate single crystal irradiated using 100 MeV Ni ion beam is reported [14]. It is explained that SHI irradiation can enhance the piezoelectric properties of materials probably due to induced defects density. It may create dangling bonds at the surface that lead to the local deformation of crystal structure. Further on applying the electric field in a particular direction on sample, it may increase the magnitude of deformation.
Furthermore, several reports can be found in literature that explain the irradiation induced modification in surface roughness and surface smoothening behavior for different materials [15,16]. The competing mechanism between roughening of the surface due to irradiation induced sputtering, and surface smoothening as a consequence of adatoms' motion by surface diffusion causes the evolution of surface features during ion beam irradiation. Saravanan et al [17] reported the effects of irradiation using a 45 MeV Li ion beam on structural and morphological properties of KNN single crystal synthesized by flux method. In the previous study, the modifications in structural and optical properties as an effect of ion beam irradiation on KNN films prepared by RF magnetron sputtering in pure Ar as well as the mixed environment of Ar and O 2 have been investigated [18,19]. However, there is no systematic study on the evolution of surface features of KNN thin films using surface scaling theory within the purview of the existing model. The properties of films deposited using sputtering are highly dependent on the morphology of thin films. It is essential to understand the factors affecting the surface morphology and its fundamental mechanism because roughness of thin films plays a crucial role in the performance of devices [20]. The smooth surface is required for most of the technological applications in electronic and optical devices because these technologies are hindered by inherent surface roughness by an escalation of separate islands during growth process. Furthermore, the dependency of decay lifetime on surface defects/grain boundaries is observed in our recent study on light-emitting defects in KNN thin films as an effect of ion irradiation [21]. This motivates us to investigate further SHI irradiation induced study on surface evolution and kinetics of KNN films prepared by RF magnetron sputtering.
The present work is mainly focused on the effect of SHI irradiation induced structural and surface modification of KNN films, which were deposited in pure Ar ambience using RF magnetron sputtering. Films were irradiated at different fluences varying from 1×10 12 to 1×10 13 ions cm −2 with 100 MeV Ni ions. After SHI irradiation, power spectral density (PSD) analysis from atomic force microscopy (AFM) micrographs was carried out to comprehend the smoothening mechanism of films. The structural and morphological studies of pristine and ion beam irradiated KNN films were studied using X-ray diffraction (XRD), AFM, and field emission scanning electron microscopy (FESEM).

Experiment
The conventional solid-state reaction process was adopted for synthesizing the KNN sputtering target by taking high purity powders of Na 2 CO 3 , K 2 CO 3 and Nb 2 O 5 (Sigma Aldrich, USA). The detailed procedure of target preparation has been published elsewhere [5]. KNN films were deposited at room temperature on Si substrates using RF magnetron sputtering (Advanced Process Technology, India). The deposition of KNN films was done at ∼2.6×10 −2 Torr pressure at 20 SCCM argon gas by maintaining a fixed sputtering power of 40 W. Substrates holder was mounted above at a gap of ∼5 cm from sputtering target, and during deposition, it was rotated consistently for the uniform deposition of films. The thickness of RF-sputtered KNN films is in the range of 650-750 nm. Afterward, to obtain crystalline KNN films (pristine films); as-deposited samples were annealed in air ambiance at a temperature of 700°C for an hour. Ultimately, the annealed films were subjected to SHI irradiation with 100 MeV Ni ions using the 15UD Pelletron Accelerator facility situated at Inter-University Accelerator Centre (IUAC), New Delhi, India. Ion irradiation of KNN samples was done at various fluences varying from 1×10 12 to 1×10 13 ions cm −2 . Figure 1(a) represents the schematic illustration of ion beam interaction with matter. The electronic energy loss (S e , transferred to target electrons) of 100 MeV Ni ions in the KNN matrix is 1.15 keV Å −1 , whereas nuclear energy loss (S n ) is 2.23 eV Å −1 , estimated using SRIM-2013 simulation program as shown in figure 1(b). Therefore, S e plays an imperative character in the modification of properties of KNN films in present study. The projected range of Ni ions, which go deep inside the substrate, into KNN is 12.41 μm and, therefore, neglects the possibility of Ni ions getting embedded in the KNN matrix.
The crystalline phase analysis of pristine and SHI irradiated KNN samples is carried out using Cu-K α (1.54 Å) monochromatic X-rays. The surface topography of pristine and irradiated films is analyzed using an atomic force microscope (Bruker, MultiMode 8, Germany). The 2D isotropic PSD data was taken out from AFM images in order to identify the evolution mechanism of surface features before and after ion beam irradiation. A field emission scanning electron microscope (FESEM; FEI Nova NanoSEM 450) was utilized to study surface morphology and grain size estimation. Figure 2 depicts the X-ray diffraction (XRD) pattern of pristine and irradiated films at different ion fluences. It is evident from the XRD spectra that pristine KNN film has a typical pseudocubic crystalline phase of perovskite structure with (001)-preferred orientation. Pristine KNN film exhibits polycrystalline nature with various crystal planes of (001), (110), (002), (012), and (112) situated at 22.63°, 32.17°, 46.17°, 51.81°and 57.26°, respectively. The induced effect of ion irradiation with different fluences on XRD spectra can be observed from figure 2. It is seen that the peak intensities of various crystal planes are decreased monotonously upon irradiation with increasing ion fluence. The decrease in the crystalline peak intensities of KNN perovskite with an increase in fluence is probably due to the overlapping of fluence, which creates defects in crystal lattice resulting in the partially amorphized behavior of films [18]. Since the electronic energy loss is very high compared to nuclear energy loss (shown in figure 1(b)), most of the incident Ni ion beam energy is transferred to electrons of the target atoms via electronic energy loss. According to thermal spike model, the incident energy is distributed  between electrons via electron-electron coupling, and subsequently, is transferred to the lattice by electronphonon interaction [22]. Therefore, an enormous amount of energy is transferred to the target atoms along the path of projectile and accounts for the formation of zones of high temperature in the surroundings of ion path. Due to temperature spike, the pressure waves might have generated that may lead to the structural order/ disordering and strain in the crystal structure and thus, affecting the peak intensity of KNN upon ion irradiation [23]. Moreover, the intensity ratio corresponding to more preferential (001) and (110) planes of pristine and irradiated films is calculated in order to estimate the degree of preferred orientation of KNN perovskite and is observed to be 2.7 for pristine films, as shown in table 1. The obtained value indicates that the grains are preferentially more orientated along [001]. This intensity ratio is found to increase after irradiation, and the values for irradiated films are observed in the range of 3.0-3.9, as given in table 1.

Results and discussion
Moreover, the average crystallite size (D) of pristine and irradiated samples is estimated using Scherrer's formula [24] given as: where β is full width at half maximum in radians, λ denotes the wavelength of Cu-K α X-ray (1.54 Å), and θ B represents Bragg diffraction angle. The average crystallite size of KNN was determined from the most intense (001) peak in XRD spectra. The average crystallite size of the pristine sample is ∼18.5 nm, while it is found to be decreased after ion beam irradiation, as tabulated in table 1. Reduction in the crystallite size from 18.5 nm (pristine sample) to 16.0 nm (irradiated films at fluence 1×10 12 ions cm −2 ) could be attributed to the structural disintegration of materials resulting from numerous ion impacts by SHI irradiation [25]. Further, a minor and non-monotonic improvement in the crystallite size is observed with ion fluences. The reduction in crystallite size after irradiation can be explained based on the amorphization of material with ion fluences, as discussed above. The considerable energy imparted to the target system leads to increase in the local temperature surrounding the path of projectile that may cause the breaking of crystallites. These crystallites may be rejoined upon multiple ion impact, which consequences the slight increase in crystallite size upon increasing fluence. Therefore, the increase in crystallite size at 5×10 12 ions cm −2 can be ascribed to the aggregation of these fragmented crystallites. The similar behavior of an increase in crystallite size at higher fluence has been reported in literature [16,26]. This aggregation after irradiation is further correlated with AFM results, as discussed in the next section. Moreover, it is identified that the crystalline peaks of KNN have shifted slightly towards lower angle, as shown in figure 3. This might be due to some strain relaxation as a result of SHI irradiation. Besides the shifting of peaks, it is also clearly seen from XRD spectra (represented in figure 4) that (012) crystalline peak of KNN is split into doublet more efficiently after irradiation. The relative intensity of these peaks is affected by irradiation at different fluences. The similar kind of outcomes has been observed previously for composite KNN ceramic [27]. The tendency of domain switching (increase in the intensity of shorter peak relative to other peak in doublet) indicates the co-existence of phases in KNN system caused by SHI irradiation. These results suggest that ion beam irradiation induced a structural order-disorder in KNN system that may lead to crystalline phase transition in KNN perovskite. Furthermore, films also show a significant amount of secondary phases (K 2 Nb 6 O 16 ) along with the KNN crystalline phase. The formation of this phase might be due to high volatilization rate of alkali species, and it forms at a lower temperature than that of KNN phase. Similar behavior for the formation of pyrochlore phases in various forms has been reported by different authors [28][29][30]. The weight percentage of K 2 Nb 6 O 16 in the films is quantified from the following relation [31]:  excitation, and ionization of target atoms is responsible for the partial amorphized zone along the path of ions, which might have caused the modification of structural properties of KNN perovskite. Before and after Ni ion irradiation, the surface topography of KNN samples is studied using AFM. Figure r x y , . The obtained R rms value for a pristine sample is ∼10.6 nm, whereas, for irradiated films at fluence 1×10 12 , 5×10 12 and 1×10 13 ions cm −2 , is found to be around ∼4.9, ∼8.3 and ∼8.8 nm, respectively (shown in  table 2). The variation in surface roughness shows the significant consequences of ion beam irradiation on the surface topography of KNN films. During the passage of energetic ions through the material, the transferred energy leads to enhancing the mobility and rearrangement of surface atoms of target material and can be accountable for the surface morphology modification [33] of KNN thin films. The R rms of KNN films is observed to decrease from 10.6 nm to 4.9 nm upon irradiation at 1×10 12 ions cm −2 . However, the roughness of films is slightly increased monotonically after irradiation beyond the initial fluence. The sharp decrease in surface roughness of films irradiated at fluence of 1×10 12 ions cm −2 shows the smoothening of surface occurring at an initial fluence. The smoothening can be due to viscous flow, evaporation and recondensation, volume, or surface diffusion, which takes place on the surface of films after the impact of ion irradiation. Furthermore, it is found that the average lateral size and height of surface features of AFM images also decreased upon irradiation at 1×10 12 ions cm −2 , as listed in table 2. Log-normal fitted size distribution of surface features of films before and after irradiation at different ion fluences is represented in figure 6. The obtained average features' size of pristine  KNN film is 231±10 nm, which is then decreased to 164±9 nm after irradiation at 1×10 12 ions cm −2 , and beyond this, it is further enhanced with an increase in ion fluence. The variation of surface roughness and size distribution of surface features with respect to ion fluence is depicted in figure 7. The surface roughness is found to vary in similar trend as that of average size with ion fluence. It can be perceived from AFM images that the areal density of surface features is increased at a fluence of 1×10 12 ions cm −2 due to decrease in average size and, thereafter, is decreased monotonically with ion fluence due to the evolution of bigger surface features. Initially, the reduction of average size may be due to the transfer of energy of incident ions by which adatoms might have moved on the surface, and thus, the height (shown in the inset of figure 6) and the size of surface features decreased at 1×10 12 ions cm −2 . The similar results have also been reported for 50 MeV Ni ion irradiated ZnO thin films [26], and CdZnO matrix irradiated with 100 MeV Au ion beam [34]. As the fluence  increased beyond 1×10 12 ions cm −2 , the size of surface features and roughness both are increased with ion fluence. This can be due to the augmentation of mobility of adatoms, which induced the coalescence effect on the surface and thus increases the growth along vertical height as well as the lateral size of surface features [35].
During the coalescence process, the grain growth increases, and therefore, areal density decreases with ion fluence beyond 1×10 12 ions cm −2 . The concept of surface roughness can also be related to the aspect ratio of surface height to size of surface features. It is generally considered that the largest aspect ratio of surface features indicates the roughest surface for a given value of surface roughness [36]. From table 2, it is clear that the aspect ratio of average vertical height and average size of surface features of films is decreased at 1×10 12 ions cm −2 and beyond that, it rises monotonously with ion fluence. The variation of aspect ratio of surface features is similar as that of change in R rms value with the ion fluence. As a result, the surface roughness of films is increased upon SHI irradiation beyond the fluence of 1×10 12 ions cm −2 in the present study. Therefore, the modification in size of surface features and surface roughness might be due to the modification in surface energy after deposition of energy from heavily energetic ion beams. The irradiation induced morphological evolution study has also been reported by different research groups [15,37,38].
To understand the mechanism of surface evolution, a surface scaling study through power spectral density (PSD) analysis is employed. PSD analysis can be quite useful as it provides the quantitative representation of surface growth in both lateral and vertical directions, and it remains independent from the scan area of sample. The 2D-PSD function is quantitatively obtained from the Fourier transform of the surface and is defined as [15,32]: where q is spatial frequency, L represents the length of scan area, h(r) is the surface height at a position ( ) = r x y , .The 2D-PSD curves of pristine and irradiated KNN films are shown in figure 8(a). AFM images of scan size 3×3 μm 2 are utilized for PSD analysis. The log-log plot of PSD spectrum shows two distinct spatial frequency regions; the horizontal low frequency region i.e., Region I, which corresponds to uncorrelated noise, while the tail of PSD curves in high frequency region i.e., Region II, resembles the correlated surface features. The significant parameters analogous to PSD curves are: (i) the slope of PSD curves in high spatial frequency region (at large q), which gives the predominant mechanism of surface evolution; (ii) correlation length ξ 0 (=1/q 0 ), which is related to the lateral surface roughness; (iii) the plateau height (w) in low frequency region, which defines the height of surface features. The horizontal part of PSD curve in a low value of q depicts the stochastic roughening (as shown in figure 8(a)). Stochastic roughening may be opposed by the various lateral mass transport mechanism and assists in making the surface smoother. The analysis of tail of the PSD curve provides information about smoothening mechanism depending upon the slope of curve in high spatial frequency region.
The surface corrugation is described as the slope of lines joining two points on the surface, which becomes minute for length longer than that of correlation length ξ 0 . The surface is considered to be flat for length greater than ξ 0 . Therefore, for real-space behavior, it can be expected that the PSD should remain independent for q<1/ξ 0 , while it should decrease with an increase in q for q>1/ξ 0 . The surface corrugation becomes significant for spatial frequency greater than q 0 , and PSD displays the power-law dependence as [39]: ( ) = -PSD q Aq ; n where A is a constant, and n denotes the power-law exponent and is related to the slope of tail of the PSD curve, which is a real number and predicts the mechanism of surface evolution. The surface transport mechanisms of surface features are relatively dependent on the numerical values of n, i.e., n=1, 2, 3, or 4. The numerical value of n is related to different smoothening mechanisms such as viscous flow caused by surface tension, evaporation-recondensation, volume diffusion, and surface diffusion, respectively [36,40]. The possible smoothening mechanisms are illustrated schematically in figure 8(b). The value of n is calculated from the linear fitting of PSD curve in region II (shown in figure 9) and is found to be 2.37±0.01 for pristine KNN films. Upon irradiation, the value of n is decreased to 1.85±0.02 at a certain fluence 1×10 12 ions cm −2 and, thereafter, increases with ion fluence, although the variation is relatively small. So, the smoothening of surface for a fluence of 1×10 12 ions cm −2 , evaporation-recondensation and viscous flow are the competing mechanisms of adatom' motion for surface smoothening, and thereafter, the evolution of surface morphology is due to evaporation-recondensation and volume diffusion mechanisms. It is observed that the surface smoothening mechanism for films irradiated at 1×10 12 ions cm −2 depends on spatial frequency. Thus, the high spatial frequency region (Region II) is differentiated into three parts for better understanding. Most of the part of PSD function exhibits the smoothening of surface through viscous flow driven by surface tension, and this mechanism is sharply turned to volume diffusion with frequency. This might be the reason for decrease in size and height of surface features at fluence 1×10 12 ions cm −2 . It is considered that the smoothening mechanism of viscous flow (n=1) plays an important role in smoothening of oxide films [36]. The smoothening of the surface at fluence 1×10 12 ions cm −2 can be explained from the cascade collision of ionsolid interaction. The energy acquired by surface atoms might be less as compared to that of surface binding energy; the adatoms may not leave the surface but can drift parallel to the surface and build the surface smoother. The significant increase in surface roughness of KNN films beyond 1×10 12 ions cm −2 ion fluence is assumed to be due to evaporation or sputtering of adatoms from the surface. From the PSD spectra, it is observed that the plateau height (w) of curves vary with the fluence of ion beam irradiation. The decrease in w is observed at initial fluence (1×10 12 ions cm −2 ) while it increases at higher fluences. The decrease in w at 1×10 12 ions cm −2 suggests that the surface roughness of films decreases. Thus, the nature of variation in w with ion fluences is similar to the R rms of films. Therefore, ion induced evaporation-recondensation and viscous flow processes seem to play a dominant role in smoothening of KNN films irradiated at a fluence of 1×10 12 ions cm −2 .
The induced effects of SHI irradiation on the surface morphology of KNN films were further analyzed by FESEM. The SEM micrographs and the grain size distribution of pristine and irradiated samples at different fluences are depicted in figure 10. The pristine sample exhibits that the grains are arranged in regular manner with rod-like and spherical structures along with some irregular shaped features. Upon irradiation at 1×10 12 ions cm −2 , the morphology of sample remained similar with more homogeneity, but the average grain size is found to reduce from 215±8 nm to 143±7 nm. A further increase in ion fluence, the spherical and rod-like structure disappears, and grains are found in bigger size with a well-defined structure due to agglomeration under induced modification by SHI irradiation. The average grain size is found to be 232±6 nm and 234±9 nm for samples irradiated at ion fluences of 5×10 12 ions cm −2 and 1×10 13 ions cm −2 , respectively. The increase in grain size with ion fluence is probably due to sudden enhancement of lattice temperature, which caused the coalescence with an increase in ion fluence [41]. The SEM results with variation in ion fluences are similar to AFM. Moreover, some pinholes and cracks on the surface of films also appeared in pristine sample as indicated by the arrow in figure 10(a), which might be due to the evolution of residual stress (compressive/or tensile) in films as an effect of post-annealing treatment [42]. The annihilation of these cracks and reduction in pinholes are found after subjected to ion beam irradiation. The strain relaxation behavior after irradiation with increasing ion fluence is also observed in XRD results where peaks shifted towards lower diffraction angle with ion fluence. In addition, small white regions can also be seen in the grain boundaries on the surface of samples, which indicates the evaporation of materials due to their volatilization behavior. This shows the presence of pyrochlore phase in the grain boundaries, which can be confirmed from the XRD results. Also, the large number of fragmented white regions in case of irradiation at 5×10 12 ions cm −2 substantiated the increased amount of secondary phase. The presence of pyrochlore phases after crystallization in grain boundary regions is also observed for similar piezoelectric perovskite material [43]. Therefore, SHI irradiation led to modify the surface morphology of KNN films as a function of ion fluence.

Conclusions
We have investigated the modification of surface topography and structural properties of KNN thin films irradiated using a 100 MeV Ni ion beam. The systematic decrease in peak intensities of XRD after irradiation of films is observed. The average crystallite size of KNN films varies non-monotonously with ion fluence. The amount of secondary phases other than crystalline KNN is decreased at a fluence of 1×10 12 ions cm −2 , and at this fluence, the more pronounced orientation-(001) is enhanced. SEM images revealed the improvement in surface morphology of KNN films as a result of ion beam irradiation with increasing ion fluence. AFM analysis shows that films irradiated at 1×10 12 ions cm −2 exhibits minimum surface roughness, and beyond this fluence, R rms value increases with fluence. Surface-scaling analysis of the AFM images gives a power-law exponent 'n' in range of 1.85−2.37 for pristine and irradiated KNN films. Irradiation at a particular fluence of 1×10 12 ions cm −2 suggests the smoothening mechanism by a combined effect of viscous flow and evaporation-recondensation dominated processes. Further, the smoothening mechanism is found to shift towards the competitive process between evaporation-recondensation and diffusion processes for higher fluences. A strong correlation is found between the R rms , size and height of surface features, their aspect ratio, and the power-law exponent value.