In Situ Monitoring of Pulsed Laser Annealing of Eu-Doped Oxide Thin Films

Eu3+-doped oxide thin films possess a great potential for several emerging applications in optics, optoelectronics, and sensors. The applications demand maximizing Eu3+ photoluminescence response. Eu-doped ZnO, TiO2, and Lu2O3 thin films were deposited by Pulsed Laser Deposition (PLD). Pulsed UV Laser Annealing (PLA) was utilized to modify the properties of the films. In situ monitoring of the evolution of optical properties (photoluminescence and transmittance) at PLA was realized to optimize efficiently PLA conditions. The changes in optical properties were related to structural, microstructural, and surface properties characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The substantial increase of Eu3+ emission was observed for all annealed materials. PLA induces crystallization of TiO2 and Lu2O3 amorphous matrix, while in the case of already nanocrystalline ZnO, rather surface smoothening0related grains’ coalescence was observed.

Post-deposition thermal treatment is usually required to activate the Eu ions and optimize the photoluminescence (PL) response [4,8,9,13,14,16,17,[19][20][21]26,[28][29][30]32,33]. However, the methods used for conventional thermal annealing require heating the samples to high temperature (>700 • C) for sufficiently long times in order for thermal diffusion of the defects to occur. The high-temperature processing limits the flexibility and practical applicability of these methods toward development of various optical and electro-optical devices. To address just the main disadvantages, only heat-resistant substrates can be used, undesirable crystallization of amorphous host might also occur, and, finally, the treatment of the entire sample may be undesirable in the device production.
The utilization of Pulsed Laser Annealing (PLA) to obtain and tune optically active Eu 3+ -doped films may profit from numerous advantages [34,35], i.e., (if necessary) horizontally structured local-rapid laser rating, ability to achieve ultra-high temperature in local scale, better controlled vertical thermal profile, controlled thermal profile, rapid processing, possibility of remote sample processing, and easy process automation. PLA plays, therefore, an important and challenging role in the development of high-tech electronic, optoelectronic, photonics, and sensing devices.
PLA technique offers a great flexibility in the number of control parameters available, i.e., laser wavelength, fluence (F L ), repetition rate (f rep ), number of shots, and ambient atmospheres [36][37][38]. Finding the optimal PLA conditions strongly depends on sample properties, i.e., film thickness, optical properties, and microstructure. Therefore, in order to find the optimal PLA conditions for maximizing Eu 3+ PL response, a number of testing samples will be necessary. In situ monitoring of PLA can be an effective way to reach the optimal conditions accurately and fast.
In this paper, we report on in situ monitoring optical properties of Eu-doped ZnO, TiO 2 , and Lu 2 O 3 thin films prepared by PLD within their processing by PLA using ArF excimer laser. Photoluminescence and optical transmittance measurement techniques were implemented. The effect of PLA on morphology, microstructure, and composition was evaluated.

Materials and Methods
ZnO:Eu, Lu 2 O 3 :Eu, and TiO 2 :Eu thin films were fabricated by PLD using Nd:YAG laser (λ = 266 nm, τ = 6 ns) and KrF laser (λ = 248 nm, τ = 20 ns). The targets were Eu 2 O 3 :ZnO (1% at. Eu), Eu 2 O 3 :Lu 2 O 3 (3% at. Eu), and Eu 2 O 3 :TiO 2 (1% at. Eu). Depositions were performed in an oxygen ambient. The vacuum chamber was pumped down to an ultimate pressure of 5.10 −4 Pa. The films were grown on fused silica substrates maintained at room temperature. The distance between substrate and target was 50 mm. The deposition conditions are listed in Table 1. PLA was performed using an ArF laser (λ = 193 nm, τ = 20 ns) Compex PRO 205 F (Coherent, Inc., Santa Clara, CA, USA). To perform PLA the laser beam was focused using converging lens on the sample at the angle of 70 • . The optical properties were analyzed sequentially after each PLA pulse or burst of pulses that were fired at f rep = 1 Hz. The signal for in situ PL was taken from the sample edge using an optical fiber connected to a spectrometer Horiba JY Triax iHR550 (Kyoto, Japan) equipped with LN cooled CCD Symphony. The same ArF laser was used for spectra excitation at lowered F L = 10 mJ·cm −2 with f rep = 5 Hz. Optical transmittance was in situ probed perpendicularly using light generated by a DH 2000 source (Ocean Insight, Orlando, FL, USA) and analyzed by a spectrometer. The PLA conditions are listed in Table 1 and the experimental setup is shown in Figure 1. The structure of the films was characterized by X-ray diffraction (XRD) using an X-Ray diffractometer (EMPYREAN, Malvern Panalytical, Malvern, UK) with Cu anode (λ = 1.5406 Å) by grazing incidence (omega = 0.85 • ) and Bragg-Brentano measurements at room temperature. Morphology was examined by atomic force microscopy (AFM) (Dimension Icon, Bruker, Billerica, MA, USA). Chemical composition was determined by Scanning electron microscopy-Energy-dispersive X-ray spectroscopy (SEM-EDX) (Fera 3, TESCAN, Brno, Czech Republic) and the Orbis ® PC Micro-XRF spectrometer (EDAX, Ametek, Berwyn, PA, USA), Rh X-ray tube, 40 kV, polycapillary optics, a diameter of 30 µm of incident beam, and a silicon-drift detector with resolution FWHM~130 eV/Mn. The thickness measurement took place on an Alphastep IQ device (KLA-Tencor Corporation, Milpitas, CA, USA) with a scan length of 5 mm, stylus force of 5.5 mg, and scan speed of 50 µm·s −1 . Each line was measured two times. The stylus diamond tip had a radius of 5 µm and an angle of 60 • .

Results and Discussion
The deposited films of all investigated materials exhibited characteristic Eu 3+ PL 5 D 0 → 7 F j (j = 0, 1, 2, 3, and 4) emission related to f-f transition. The observation of the PL emission confirmed that the energy transfer processes in Eu 3+ excitation must take place considering excitation wavelength λ = 193 nm (6.42 eV). It can be mainly related to host absorption and marginally in case of Lu 2 O 3 :Eu also to the O 2− -Eu 3+ charge transfer band (CTB) [39,40]. Following hyper-sensitive electric dipole transition 5 D 0 → 7 F 2 , its Stark splitting and 5 D 0 → 7 F 2 / 5 D 0 → 7 F 1 (magnetic dipole transition) emission ratio (asymmetry ratio), we could correlate the effect of PLA on the structural properties' modification. It provided information related to the surrounding defects and disorder around the Eu 3+ ions. The 5 D 0 → 7 F 1 transition was not considerably influenced by the surrounding of Eu 3+ in the particular host. The transition 5 D 0 → 7 F 2 dominated all emission spectra from Eu 3+, , suggesting the local symmetry around Eu 3+ was low and deviated from an inversion center. The 5 D 0 → 7 F 0 transition was only allowed, taking into account the electric dipole selection rule, in the following 10 site symmetries: C s , C 1 , C n , and C nv (n = 2, 3, 4, 6) [12].
The values of E g listed in Table 2 were derived using Tauc plot-plotting (αhν) 1/m against photon energy (hν), where m is a parameter related to the type of transition, m = 1 /2, and 2 corresponds to direct allowed transition and indirect allowed transition, respectively. TiO 2 in anatase phase is considered as an indirect semiconductor, where m = 2 [41] while ZnO is a direct semiconductor (m = 1 /2) [42] and Lu 2 O 3 is considered as a direct ultrawide band gap semiconductor (m = 1 /2) [43]. The E g values were obtained with error of ±0.02 eV. As for the chemical composition of the films, Eu dopant concentrations obtained by EDX and XRF analyses corresponded to those of the targets and any substantial variation after PLA was not detected.
In the following, let us discuss the properties' modifications of each material, Eudoped ZnO, TiO 2, and Lu 2 O 3 by PLA, in more details.

ZnO:Eu
The evolution of optical properties (PL emission and transmittance spectra) of ZnO:Eu films with increasing number of PLA shots is shown in Figures 2 and 3, respectively. Characteristic Eu 3+ red emission was detected at 587.3 nm and 593.8 nm ( 5 D 0 → 7 F 1 ), 609.9 nm and 618.8 nm ( 5 D 0 → 7 F 2 ), 654.8 nm ( 5 D 0 → 7 F 3 ), and 691.5 nm, 692.8 nm and 704.4 nm ( 5 D 0 → 7 F 4 ). The spectra in Figure 2 revealed that different emitting centers related to Eu may coexist in ZnO:Eu films, which is commonplace for wide band gap materials of wurtzite structure [44]. Eu 3+ ions in ZnO:Eu films generally occupy the substitutional sites of Zn 2+ ions. This fact turns 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 transitions to be allowed. The observation of 5 D 0 → 7 F 2 transition splitting suggests that Eu 3+ can be at a substitutional Zn site (C 3v symmetry). We could not observe clear evidence of 5 D 0 → 7 F 0 transition whose appearance in PL spectra of ZnO:Eu was related with Eu 3+ presence in other sites (e.g., the interstitial sites or surface/grain boundary of ZnO nanocrystals) [33,45,46]. A potential drawback of our interpretation can be the relatively low intensity of the PL signal. However, if we expect only a small fraction of Eu 3+ in the sample located at ZnO nanocrystals' surface, Eu 3+ emission lines' intensities and asymmetric ratio would increase with the increasing number of PLA shots, while the asymmetric ratio saturated at N # = 400, as it is depicted in Figure 2. The enhancement of Eu 3+ PL intensity at 609.9 nm by a factor of 4.1× was obtained. The PL signal intensity and asymmetric ratio variations might be related to microstructure modification, as displayed in Figure 4, where grains coalescing took place. It was reported that the energy relaxation process of excited Eu 3+ ions should play a more important role toward PL efficiency enhancement than the energy transfer process from ZnO matrix to Eu 3+ ions [47].
The transmittance spectra shown in Figure 3 confirm good quality of the film because transmittance of~80% was reached in the visible light spectrum (VIS) band. There was not noticeable any variation in transmittance spectra, which confirmed that defects like oxygen vacancies were not introduced during the PLA processing. Band gap value decreased after PLA from E g~3 .28 eV to 3.25 eV. Recently, we observed similar E g values and trends at ZnO:Eu film PLD deposited at a substrate temperature of 300 • C [24].
AFM images of the film surface presented in Figure 4 revealed that PLA caused interconnecting of the grains accompanied by substantial lowering of surface roughness, as depicted in Table 2. We could attribute this effect to reaching the melting threshold of ZnO:Eu thin film. Similar results of UV PLA on the surface morphology of ZnO thin film of comparable thickness deposited by PLD on quartz substrate were reported at higher F L ≥ 140 mJ/cm 2, where coalesced nanoclusters kept the wurtzite crystalline structure [36]. The PLA-induced coalescence of nanoparticles was also reported at ZnO [48] and ZnO:Eu [37] films of lower thickness < 60 nm on metal and Si substrates, respectively.   XRD patterns of as-deposited and PLA-treated ZnO:Eu film, shown in Figure 5, did not reveal any substantial changes in structural properties. This fact is in good correlation with our previous results dedicated to PLA of ZnO:Eu film by KrF laser [37]. The film exhibited a hexagonal wurtzite structure of fibrous texture along the c-axes with random lateral orientations of crystallites. The FWHM of the strongest (002) peak was used to estimate the average crystallite size by using the Scherrer's formula and were ≈19 nm. This correlated with the AFM measurement data. The presence of additional phases, such as Eu 2 O 3, was not detected. Considering the larger ionic radius of Eu 2+ (1.13 Å) than Eu 3+ (0.95 Å) in comparison with Zn 2+ (0.74 Å), substitution of Eu 3+ ions at Zn 2+ sites seemed to be more favorable than that of Eu 2+ ions but charge compensation must take place [24].

TiO 2 :Eu
The dependence of optical properties of TiO 2 :Eu films, i.e., PL emission and transmittance spectra, on the number of PLA shots is presented in Figures 6 and 7, respectively. The transmittance spectra underline the high quality of the film because any evident absorption in VIS that can be related to defects such as oxygen vacancies and a presence of Ti suboxides did not appear [28]. We could observe a weak broadened emission line of Eu 3+ for the as-grown, which is characteristic for the amorphous phase [49]. The increase of Eu 3+ PL intensity accompanied by sharpening lines of split 5 D 0 → 7 F 2 transition, as depicted in Figure 6, as well as a slight increase of transmittance (1.3%) are corelated to surface modification and gradual crystallization of TiO 2 matrix. The significant increase of Eu 3+ PL intensity (~5×) was detected at N # = 50, where transmittance decreased and further remained constant. TiO 2 nanocrystallites formed by PLA may act as sensitizers absorbing the excitation energy more efficiently [31,49]. We can recognize splitting 5 D 0 → 7 F 2 transition to peaks located at 611.1 nm, 614 nm, 616.9 nm, 624 nm, and 626.1 nm. The splitting may indicate Eu 3+ located in D 2 and S 4 symmetry sites of TiO 2 [49]. The slight decrease of Eu 3+ PL intensity of 5 D 0 → 7 F 2 transitions at 616.9 nm and 624 nm after N # = 200 can be attributed to the creation of well-crystalized anatase TiO 2 matrix [28,50]. Because the intensity of 5 D 0 → 7 F 1 transition was weak, it made it impossible to conclude on its splitting. The 5 D 0 → 7 F 0 transition could not be followed at all, which may exclude Eu 3+ occupying C 2v symmetry sites. Concerning transmittance, we must notice that its variation within PLA processing was generally rather weak. Its relative variation was less than 2%, which is slightly above the measurement error.
We did not obtain any shift in E g value after PLA, as listed in Table 2. Similar values of E g~3 .22 eV for TiO 2 material [51] and anatase-like thin TiO 2 films [52] were reported. There are reports of higher Eg~3.3 eV for TiO 2 :Eu films deposited by the spray pyrolysis technique. Our E g values suggested we did not introduce defects in the films since lowering E g from 3.03 eV to 2.52 eV was observed after aerodynamic levitated laser annealing of defect-rich (oxygen vacancies) TiO 2 nanoparticles [53]. Lower E g~2 .7 eV was also reported for TiO 2 :Eu films prepared by MAPLE, where E g value shift was related to interstitial Eu doping [28].  An AFM image of the film surface, shown in Figure 8, revealed clear evidence of crystallization of the film after PLA, as observed by XRD. The growth of grains is reflected by the increasing roughness, as depicted in Table 2. XRD analyses, shown in Figure 9, revealed the amorphous character of as-grown film, while the PLA-treated film exhibited polycrystalline tetragonal anatase phase TiO 2 , as referred in the JCPDS Card NO: 96-500-0224 file. Peak positions and reflections' planes derived from XRD patterns were 25.38 • (011), 36.78 • (013), 37.68 • (004), 48.03 • (020), 53.68 • (015), and 55.08 • (121). Crystallites' size was derived using Sherrer's formula and was ≈ 46.1 nm. Anatase TiO 2 phase was also obtained by KrF PLA of nanoparticles at F L = 34 mJ/cm 2, while at a higher F L a rutile phase appeared [54]. The crystallization of divalent-or trivalent europium-containing oxide compounds or europium titanate [55] was not detected by XRD. The large mismatch in the ion radii of Eu 3+ and Ti 4+ , 0.95 A and 0.68 A, respectively, may cause a crystal structural distortion and also make difficult the substitution of Ti 4+ by Eu 3+ within the anatase lattice [31].
We noticed the most substantial change in the film optical properties, shown in Figures 10 and 11, after the first PLA shot. The most substantial increase of Eu 3+ intensity emission and optical transmittance was obtained. The E g value shifted from 5.47 eV to 5.51 eV, where the latter values corresponded to those reported for evaporated Lu 2 O 3 films [43,59]. This may be attributed to both surface and structure modifications (as observed by AFM and XRD in Figures 12 and 13, respectively). The asymmetric ratio increased from 7.9 to 9.5 reached at N # = 2, which is reflecting the site symmetry decreasing related with film structure modification. The asymmetric ratio value was then decreasing to 9 and not changing for N # = 4-7. PLA led to the maximum enhancement of 2.8 times for Eu 3+ intensity emission at 611.0 nm. The energy transfer from Eu 3+ residing in S 6 to Eu 3+ in C 2 may take place because the levels of Eu 3+ in the site S 6 lie above those for site C 2 . It was reported, based on following the 5 D 0 → 7 F 1 / 5 D 0 → 7 F 0 ratio, that the rate of the energy transfer may change with the crystallites' sizes (it is lower for larger crystallites) or because the location of Eu 3+ in C 2 site is more common for small crystallites [57,60]. We did not observe any considerable variation of this ratio within the PLA process, although the crystallites' size varied, as confirmed by XRD.    AFM analyses, shown in Figure 12, revealed similar morphology for as-deposited and PLA-treated film with typical S q = 3.7 nm. Slight surface smoothing was observed after PLA and S a decreased from 2.04 nm to 1.48 nm.
A substantial improvement of the crystalline structure after PLA treatment was evidenced by XRD diffraction patterns, depicted in Figure 13. While as-deposited film was rather amorphous, the film revealed a polycrystalline structure after seven PLA shots. The estimated crystallites' size was 10.9 nm. The similar XRD pattern was observed for Lu 2 O 3 :Eu films deposited by PLD at a substrate temperature higher than 400 • C [25] and by magnetron sputtering with thermal postdeposition treatment at 900 • C for 2 h [20]. The reflections in the XRD pattern can be assigned to a body-centered cubic (I 21 3) structure of Lu 2 O 3 , according to JCPDS Card NO: 96-101-0596 file. The Eu 2 O 3 phase was not detected in the patterns, confirming the Eu dopant was substituting Lu in Lu 2 O 3 host, which was supported by the closed values of the ionic radii of 0.95 Å and 0.85 Å for Eu 3+ and Lu 3+ , respectively [25]. The broad hump around 2θ = 21 • is characteristic for amorphous fused silica substrate.

Conclusions
The in situ monitoring system of optical properties (photoluminescence and transmission) at pulsed laser annealing processing of Eu-doped oxide thin films was demonstrated. Eu-doped ZnO, TiO 2 , and Lu 2 O 3 films were fabricated by PLD at room temperature. The system allowed us to find quickly the optimal PLA conditions (ArF laser at λ = 193 nm, fluence, and number of PLA shots) for maximizing Eu 3+ photoluminescence. The Eu 3+ emission enhancement of 4.1×, 5× and 2.8×, after 800, 400, and 7 PLA shots, for ZnO, TiO 2 , and Lu 2 O 3 matrix was obtained, respectively. Transmission was markedly modified at Lu 2 O 3 :Eu film, where also E g increased to 5.51 eV. On the other hand, ZnO:Eu and TiO 2 :Eu films exhibited lowering E g to 3.25 eV and 3.23 eV, respectively. TiO 2 :Eu and Lu 2 O 3 :Eu as-deposited films exhibited an amorphous character that was modified by PLA to anatase and cubic nanocrystalline phase, respectively. In the case of ZnO:Eu, the application of PLA induced rather surface smoothening-related grains' coalescence.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.