Impact of oxygen vacancy on luminescence properties of dysprosium-doped zinc oxide thin films

Zinc oxide (ZnO) stands out as an inherently n-type, II–VI compound semiconductor possessing a wide, direct bandgap of 3.32 eV and an exciton binding energy of 60 meV at room temperature. Its unique combination of structural, electrical, optical, chemical, biocompatible, and radiation-resistant properties makes it a promising material for various applications.^1,2 Due to high exciton binding energy and tunable optoelectronic transport properties, it has received remarkable importance in delivering recent technology demands. To tune carrier concentration (∼10²⁰ cm⁻³) and the bandgap, which is the foremost requirement to fabricate optoelectronic devices, the material is usually doped using Sb, F, Al, Ga, etc.^3–6 Doping ZnO, especially using rare earth (RE) elements, can also enhance its optical and electronic properties significantly.^7–12 

The advantage of doping RE elements in the host matrix includes their trivalent states, which are a partially filled 4f shell exhibiting a rich intra-4f-shell energy level structure.^7,13–15 Although transitions between these levels are forbidden due to selection rules, optical emissions between these energy levels become possible because of perturbations such as crystal field effects, breakdown in translational symmetry caused by defects, and the introduction of impurities or dopants into the host matrix. In contrast, each rare earth ion's distinct emission spectrum, covering blue to near-infrared wavelengths, likely broadens the emission range of ZnO beyond its band edge, with visible emissions from diverse intra-4fⁿ transitions.^7–15 

Among various lanthanides, dysprosium (Dy) may be special if we consider all allowable transitions possible. The prominent observed transitions from Dy³⁺ ions in the blue (486 nm) region arises from the magnetic dipole transition (⁴F_9/2 to ⁶H_15/2), and in the yellow (577 nm) region, they are attributed to the electric dipole transition (⁴F_9/2 to ⁶H_13/2), which are necessary for the generation of white light emitting phosphors.^12,13,16 The hypersensitive (⁴F_9/2 to ⁶H_13/2)¹⁶ transition relies heavily on the host material, either incorporated or doped, while the less sensitive (⁴F_9/2 to ⁶H_15/2) transition is not as dependent on the host material. This makes the selection of a compatible host material crucial for its doping. In this exploration, a range of glass types, including oxide, fluoride, oxyfluoride, strontium lithium bismuth borate, fluorophosphate, lithium borate, and lithium fluoroborate glasses, have been employed as host materials.^16–18 Additionally, Dy activated Scheelite-type oxides such as NaLa_1−xDy_x(MO₄)₂ and Na₅La_1−xDy_x(MO₄)₄ (M = Mo, W)¹⁹ have been investigated as thermosensitive phosphors.

Given the numerous reports confirming successful doping of various rare earth elements (RE) in ZnO as a host material, it stands to reason that doping Dy could present new avenues for the production of white light emitters. In this connection, researchers have already explored Dy doping in ZnO in various forms, viz., thin film, nano form, and bulk form,^20–24 in different applications like UV detectors, white LED, sensors, and paramagnetic thin films.^21–24 

Typically, the surface morphology and defects in the thin films are contingent upon the specific synthesis method employed. Various synthesis techniques, e.g., RF magnetron sputtering, thermal evaporation, pulsed laser deposition (PLD), etc.,^20,24–26 are used to grow Dy-doped ZnO films. Among all these techniques, the use of PLD became a popular choice mainly for its advantages, which include freedom to use various ablation sources, control on the stoichiometry of the deposited films, and controlling the deposition parameters. Although PLD seemed to have these extraordinary advantages, to the best of our knowledge, there are very few reports in detail that investigate Dy-doped ZnO thin films grown under variable deposition parameter conditions.

Thus, this article reports the investigation of Dy-doped ZnO films grown by PLD by varying the oxygen partial pressure during deposition. This study aims to explore how varying oxygen ambient pressure influences the structural, chemical, and optical characteristics of Dy-doped ZnO thin films. Additionally, it seeks to enhance our understanding of the potential suitability of this material for various optoelectronic applications.

High-purity ZnO (99.99%) and high-purity Dy₂O₃ (99.99%) were procured from Sigma-Aldrich. The solid state ceramic route was used to synthesize pristine ZnO and 1 at. % Dy-doped ZnO (DZO) pellets. A Rietveld analysis of the recorded XRD data of the pellets ensured the formation of single phase, polycrystalline ZnO and DZO pellets. These pellets were used in the PLD as targets to deposit the respective thin films on the c-Al₂O₃ substrate under different ambient pressure conditions, respectively. Prior to depositing the thin films, the c-Al₂O₃ substrate was cleaned ultrasonically using trichloroethylene, acetone, and finally methanol for 3 min each. The cleaned substrate was mounted on the substrate heater and placed at a distance of 5 cm parallel to the target surface inside the deposition chamber. The chamber was evacuated to a base pressure of 1 × 10⁻⁴ Pa (base vacuum) using a turbo molecular pump backed up with a scroll pump. For ablating the target material, the fourth harmonic of the Nd:YAG laser (λ = 266 nm, pulse duration t_p = 6 ns) was used. The repetition rate of the laser beam was kept constant at 5 Hz, where the polar angle between the incident radiation and the normal to target surface was 45°. The energy density of the incident radiation on the surface of the target was kept constant at 2 J/cm² throughout the deposition process. The target was rotated at 10 RPM during deposition to prevent pitting or texturing of the surface of the target. Sets of ZnO and DZO thin films were deposited in vacuum and oxygen partial pressures of 0.01 and 0.1 Pa, respectively. The deposition was carried out for a period of 3600 s. The substrate temperature was maintained at 700 °C throughout the deposition period. After the deposition process, the temperature is maintained for an additional 600 s to facilitate annealing. The substrate with deposits were slowly cooled (at a rate of 2 °C/min) to room temperature under the same ambient pressure in which the film was grown. Henceforth, for convenience, the deposited thin films are abbreviated as ZnO, DZO, 10⁻⁴ Pa-ZnO/DZO, and so on. All the deposited ZnO and DZO thin films were then characterized for their thickness, micro/structural, optical, and chemical environment of Zn, O, and Dy in the films using different techniques. The thicknesses of the ZnO and DZO thin films were measured by using a Stylus profilometer (KLA Tencor P16+), and they were found to be 380 ± 10 and 410 ± 10 nm, respectively. X-ray diffraction (XRD) investigations were conducted to analyze the structural characteristics of the samples. The diffraction patterns in θ–2θ scan modes were recorded using Ni-filtered Cu-Kα radiation at 1.542 Å (HR-XRD Bruker D8 Advanced). Atomic force microscopy (Park systems NX20) was performed to study the surface morphology of the deposited thin films. Optical transmission was recorded using a UV–Vis (Jasco V750) spectrometer. For the investigation of room-temperature photoluminescence (RTPL) properties, two different systems were employed. Horiba iHR320 (Syncerity CCD) with a He–Cd laser at λ = 325 nm (3.82 eV) served as the excitation source, while Horiba Fluorolog FL3 (photomultiplier tube) utilized a monochromatized output of a xenon lamp at λ = 438 nm (2.83 eV) as the excitation source. To examine the chemical environment of Zn and O in the ZnO thin films and Zn, Dy, and O in the DZO thin films, x-ray photoelectron spectroscopy (XPS) (AXIS Supra, Kratos Analytical, UK) was employed. The XPS was calibrated using the Au 4f_7/2 line (83.8 eV), and monochromatic Al-Kα radiation at 1486.6 eV served as the x-ray source. The base pressure of the XPS chamber was 2.67 × 10⁻⁷ Pa. Prior to recording, the samples underwent sputter cleaning with 500 eV Argon ions for 10 min. Survey scans were conducted in the range of 0–1400 eV, while high-resolution (0.5 eV) scans utilized a 20 eV pass energy. The peak position of C1s in the survey scan was compared to its standard value of 284.5 eV to correct for any shift in the binding energy (spectral position) of Zn, Dy, and O due to the charging effect. High-resolution spectra of Zn, Dy, and O were subjected to a peak fit analysis with a Shirley-type background using a mixed Gaussian and Lorentzian line shape profile, and XPSPEAK41 software was employed for this purpose. The energy-dispersive x-ray (EDAX) (Zeiss Gemini Ultra 55) analysis was performed to determine the atomic percentage of the Dy present in the DZO thin films.

Before growing any thin films using PLD, the single-phase formation of each pellet (Dy-doped ZnO and ZnO) prepared by the solid state route was confirmed by recording x-ray diffractograms (provided as Fig. S1 in the supplementary material). Bragg's peaks positioned at different 2θ values, especially for Dy-doped ZnO pellets, were identified with the respective crystallographic orientations of wurtzite ZnO (JCPDS data card no 36–1451). No additional peaks corresponding to any other phase(s)/structure(s) of either ZnO or Dy-oxides were observed within the detection limit of XRD. Each XRD spectrum was also refined theoretically using Rietveld software (Fullprof). The lattice parameters extracted from the Rietveld refinement of each XRD spectrum are listed in Table S1 in the supplementary material. From the Rietveld analysis, a slight decrease in the unit cell volume of ZnO specimen as compared with standard values was observed, indicating the generation of oxygen vacancies during the synthesis of the ZnO pellet. An expected increase in the unit cell volume for Dy-doped ZnO specimen was also observed. The observed increase was due to the Dy³⁺ ions (ionic radius: 91.2 pm) replacing Zn²⁺ ions (ionic radius: 74 pm) in the ZnO matrix,^6,21 followed by the difference in the bond length of Dy-O (2.36 Å),²⁷ which was much longer than that of Zn-O (1.97 Å).⁶ These results clearly demonstrate successful doping of Dy in the ZnO specimen.

Table I lists the lattice strain (ɛ) extracted using the above relation. Negligible changes associated with the lattice strain produced in the film confirm the high-quality deposition of specimens. A prominent increase in the crystallite size as a function of oxygen pressure for both sets of specimens is clear from the values listed in the table.

Atomic force microscopy (AFM) was utilized to investigate the surface morphology of ZnO and DZO thin films deposited on the sapphire substrate. As illustrated in Figs. 2(a)–2(f), the surface morphology of ZnO and DZO thin films, deposited under different pressure conditions, is displayed. The dimensions of all captured AFM images are 1 × 1 μm². These images reveal a uniform surface morphology characterized by a well-covered, crack-free surface with distinct protrusions. Each film exhibits unique changes, distinguishing them from one another and dependent on the pressure condition during deposition. The average size of the protrusions increased as the oxygen partial pressure increased during deposition. This observation is consistent with the observation of XRD data. The root mean square (RMS) roughness is shown in Table S2 in the supplementary material, and it is observed that as the oxygen partial pressure increased, the RMS roughness also increased.

At reduced oxygen pressures, the films exhibit low surface roughness, which can be attributed to their uniform and dense surface structure. Conversely, at elevated oxygen pressures, the species ablated from the target during deposition are likely to collide more frequently with oxygen molecules. These collisions result in a reduction in the average kinetic energy of the species. Consequently, the ad-atoms that are deposited lack sufficient mobility to spread evenly across the substrate, which, in turn, leads to an increase in RMS roughness.³⁰ 

To investigate the effect of oxygen variation on the optical properties of ZnO and DZO films, room temperature UV-Vis transmission spectra and room temperature photoluminescence spectra from all specimens were recorded. In the inset, Figs. 3(a) and 3(b) show the UV-Vis transmission spectra recorded on ZnO and DZO thin films, respectively. All specimens were observed to have a transparency rate of more than 90% in the visible region, whereas a sharp absorption at ∼3.31 eV (375 nm) was due to the absorption of excitation photons, i.e., band-edge transistion.²⁸ Urbach energy was estimated for all the specimens and is shown in Table S3 in the supplementary material. From structural investigations reported in Table I and the estimated Urbach energy from Table S3 in the supplementary material, we are of the opinion that ZnO films deposited under vacuum has more defects, structural disorder, and relatively poor crystalline quality when compared to other specimens.^31,32 It can also be noted from Table S3 in the supplementary material that with the increase in oxygen partial pressure during the deposition process, there was an enhancement in the crystalline quality and a reduction in structural disorder, as evidenced by the reduced Urbach energy for both groups of samples. A Tauc plot was extracted using UV-Vis transmission spectra recorded for each specimen, as shown in Figs. 3(a) and 3(b). Slight variations with the band edge absorption were observed. The bandgap extracted from the Tauc plot is shown in Table S4 in the supplementary material.

To verify whether structural disorders and defects related to oxygen in these films can alter their optical emission characteristics, photoluminescence spectra for all samples were measured at room temperature. Figures 4(a) and 4(b) show the PL spectra for all specimens by exciting them with photons of ∼3.82 eV (325 nm wavelength). As expected for all specimens, two distinct emissions present in the UV and visible region are observed. The PL spectra recorded for ZnO specimens, i.e., films deposited under varying oxygen pressure [Fig. 4(a)], have a sharp peak positioned at ∼3.31 eV, confirming the presence of near band-edge emissions,^{12,20–22,24} whereas a broad hump observed in the visible region (i.e., 2.00–2.76 eV) almost in all specimen indicates the presence of various defect induced optical transitions already predicted for ZnO.¹ For example, the green luminescence centered at ∼2.4 eV is due to the optical transitions from deep donor levels such as oxygen vacancies (V_O) and Zn interstitials (Z_ni) to the valance band and/or optical transitions from shallow donor levels to zinc vacancies (V_Zn) present near the conduction band. The yellow emission centered at ∼2.2 eV is ascribed to the interstitial oxygen defects. From the PL investigation, it is clear that oxygen partial pressure kept during deposition plays a vital role in controlling the defect related emissions especially in the visible region. For, e.g., negligible visible emissions were observed when the ZnO film was deposited at vacuum (10⁻⁴ Pa). Prominent emission in the green region (∼2.4 eV) was observed when the film was deposited at 0.01 Pa, whereas yellow emission at ∼2.2 eV was observed when the film was deposited at 0.1 Pa oxygen partial pressure.^{1,2,26,33–36}

Similarly, Fig. 4(b) shows the PL spectra recorded for Dy-doped ZnO thin films deposited under varying pressure conditions. Compared with the ZnO PL spectra [Fig. 4(a)], a noticeable broadening in the linewidths of near band edge emission was clearly observed. Unexpectedly, in all Dy-doped ZnO samples, the emissions typically detected in the visible spectrum were found to be significantly reduced. However, expected characteristic 4fⁿ emissions^{12,20–25,37–44} due to Dy³⁺ ions were not observed. The reasons behind this may be related to the nonradiative energy transfer from Dy³⁺ ions to host ZnO and/or very fast excitonic decay in the ZnO matrix preventing direct energy transfer to Dy³⁺ ions.^{12,37,45–47}

To investigate the impact of Dy doping in the ZnO matrix and the oxygen partial pressure maintained during the deposition of these specimens on the near-band-edge optical transitions, the distinct UV peak observed in all samples was analyzed through deconvolution. Figures 5(a)–5(c) and 5(d)–5(f) show the Jacobian transformed wavelength to energy plot for both sets.⁴⁸ For ZnO thin films deposited at vacuum [Fig. 5(a)], the spectra can be deconvoluted into three peaks positioned at ∼3.29, ∼3.24, and ∼3.06 eV, whereas for the rest of the films, the spectra are deconvoluted into two peaks positioned at ∼3.30 and ∼3.24 eV, respectively. For all specimens, expected prominent emissions observed at ∼3.30 and ∼3.24 eV can be attributed to free exciton (FX_A) and near-band-edge emission (NBE), whereas the emission observed at ∼3.06 eV only for films deposited at vacuum is due to the LO phonon replica of bound exciton transitions (D⁰X), respectively.^1,2,33,49,50

Figures 5(d)–5(f) show the Jacobian transformed wavelength to energy plot for Dy-doped ZnO films deposited under varying pressure conditions. Due to the nature of the spectra observed herewith, each spectrum is deconvoluted into multiple peaks. For all specimens, the deconvoluted peaks are observed to be positioned at ∼3.31, ∼3.21, ∼3.01, and ∼2.82 eV (see Table S5 in the supplementary material). For DZO films deposited under vacuum, the emission peak present at ∼3.45 eV could be due to the interstitial doping of Dy and/or the crystal field effect caused by the presence of Dy ions in the ZnO matrix,^1,2,33,49,50 whereas the rest of the peaks positioned at ∼3.31, ∼3.21, and ∼3.01 eV are attributed to the optical transitions assigned to ZnO. In all DZO specimens, a pronounced long tail extending to approximately 2.7 eV on the lower energy side of the spectra is observed. This feature is likely due to radiative transitions between energy states created by defects from Dy doping,^23,24,37,47 confirming the presence of the ⁶H_15/2 energy level near the valence band of ZnO.^{16,21–23,25,37–43}

To ensure the existence of Dy doping in the ZnO matirx, i.e., the existence of intrinsic f–f transition of Dy³⁺ ions, we decided to record photoluminescence excitation (PLE) spectra at room temperature, particularly for Dy-doped ZnO films. Figure 6(a) shows the PLE spectra recorded in the range of 2.7–4.8 eV, monitoring the emission peak at ∼2.56 eV (reported).^{16,21–23,25,37–43} The excitation peak positioned at ∼2.83 eV in Fig. 6(a) can be assigned to the intrinsic f–f transition of Dy³⁺ ions mainly from the ⁶H_15/2 to ⁴I_15/2 excited state, which is also observed in the room temperature PL spectra recorded for Dy-doped ZnO specimens, i.e., Figs. 5(d)–5(f). The emission observed herewith, i.e., ∼2.83 eV, was then used to record room temperature PL for Dy-doped ZnO thin films. Figures 6(b)–6(d) show the RTPL for Dy-doped ZnO thin films deposited under varying partial pressure conditions.

From Figs. 6(b)–6(d), one can clearly see various intra-4f transitions of Dy³⁺ ions, viz., ⁴F_9/2 → ⁶H_15/2 (∼2.57 eV), a band related to ⁴F_9/2 → ⁶H_13/2 (∼2.07–2.16 eV) (two small peaks are observed at ∼2.13 and ∼2.08 eV), ⁴F_9/2 → ⁶H_11/2 (∼1.84 eV), ⁴F_9/2 → ⁶H_9/2 + ⁶F_11/2 (∼1.66–1.69 eV), and ⁴F_9/2 → ⁶H_7/2 + ⁶F_9/2 (∼1.48 eV).^{12,20–25,37–44} Notably, the intensity of these characteristic transitions from Dy³⁺ ions increases with higher oxygen partial pressure during deposition (see Fig. S3 in the supplementary material), indicating the integration of Dy into the ZnO matrix and how oxygen played a crucial role during deposition. Figure 6(e) illustrates the proposed energy level diagram of Dy³⁺ ions in the DZO thin films based on observed excitation and emission in PLE and PL spectra, respectively.^{12,20–25,37–44}

Finally, x-ray photoelectron spectra were recorded to investigate the charge states of Dy, which is bound to the oxygen. Figure 7(a) shows the survey scan of ZnO thin films deposited under different pressure conditions in the range of 0–1200 eV. All the characteristic emissions of Zn, viz., 2s, 2p, 3s, 3p, and 3d, along with LMM Auger transitions of Zn, match very well with Zn²⁺ in ZnO.⁵¹ Emissions from carbon (C1s) and oxygen, viz., O1s and KLL Auger transitions are also clearly observed. A similar survey scan was also recorded for DZO thin films in the range of 0–1400 eV and is shown in Fig. 7(b). In addition to the various transitions that emanated from Zn, O1s, and C1s, photoelectrons that emitted from 3d_5/2 and 3d_3/2 energy states of Dy⁵¹ from DZO thin films are also observed. The observation from the XPS analysis suggests successful doping of Dy into the ZnO matrix. The EDAX spectra (see Fig. S4 in the supplementary material) were recorded for DZO thin films deposited under various pressure conditions. The atomic% of the Dy element was found out to be ∼0.7 (see Table S6 in the supplementary material).

The spin orbit coupled energy states of Zn, viz., Zn 2p_3/2 and Zn 2p_1/2, are located at ∼1021 and ∼1044 eV, respectively, with a characteristic separation of ∼23 eV, which confirms the +2 oxidation state of Zn in both the ZnO and the DZO thin films⁵¹ [see Figs. S5(a)–S5(f) in the supplementary material].

The photoelectrons emitted from 3d_5/2 and 3d_3/2 energy states of Dy³⁺ ions in the DZO thin films are found to be located at ∼1296 eV and ∼1333 eV, respectively. The energy state position and their characteristic separation of ∼37 eV [see Figs. S5(g)–S5(i) in the supplementary material] match very well with the reported and standard values, which confirms the presence of Dy in the +3 oxidation state in the DZO thin films.⁵¹ 

Figures 8(a)–8(c) and 8(d)–(f) show the O1s spectra of ZnO and DZO thin films deposited under different pressure conditions, respectively. In all the films, it is observed that the recorded O1s spectra are highly asymmetric in nature and could be fitted with three Lorentzian–Gaussian curves centered at ∼530.5, ∼531.5, and ∼532.5 eV.^51,52 These peaks could be attributed to lattice oxygen present in perfect symmetry (O_ps), lattice oxygen in distorted symmetry (O_ds), and absorbed oxygen (O_ab) loosely bound to –H₂O, –CO₂, –CO, etc., confirming the presence of oxygen in three different chemical environments in these thin films.^51,52 The emission of O1s from oxygen bonded to Dy ions is overshadowed by the emission of O1s electrons from the oxygen ions within the ZnO matrix.^53,54

Figures 8(g) and 6(h) show the percentage contribution of O_ps, O_ds, and O_ab estimated from the area under the Lorentzian–Gaussian curves in the case of ZnO and DZO thin films, respectively. In both the thin films, it is observed that the contribution of O_ps is higher than that of O_ds, and as the oxygen partial pressure increased during deposition, the percentage contribution of O_ps also increased, while O_ds decreased. This indicates the improved crystalline quality of the ZnO and DZO thin films as the oxygen content in the distorted sites is reduced, which is also supported by XRD and Urbach energy analyses. However, in the case of the DZO thin films, O_ps is observed to be significantly greater than that of its ZnO counterpart. When comparing these observations from the O1s analysis with the XRD results mentioned in Table I and the Urbach energy analysis (Table S3 in the supplementary material), it is apparent that as the oxygen partial pressure during deposition is increased, the induced strain is released due to the filling up the oxygen vacancies, thus reducing crystal distortion/imperfection during growth. Hence, DZO thin films show improved crystalline quality than ZnO films in terms of increased crystalline sizes and decreased strain.

As laser radiation is incident on the solid target surface of ZnO or DZO, a plasma plume is generated due to laser–matter interaction. The plasma contains various species like Zn, ZnO, Zn⁺, Zn_xO_y, O⁺, O, and O²⁻ (in the case of the ZnO target) and O⁺, O, O²⁻, Zn, ZnO, Zn⁺, Zn_xO_y, Dy, Dy⁺, Dy_xO_y, and DyO (in the case of the DZO target). Hence, the oxygen carriers in both the cases are different and suggest that DZO thin films contain more oxygen (O_ps) than ZnO thin films. Comparatively, the O_ab content in the ZnO thin films is marginally greater than that in DZO thin films. The reduction in O_ab content in the DZO thin films implies a decrease in adsorption sites, possibly indicative of high crystalline quality in the thin films.

Thin films of ZnO, both pristine and doped with dysprosium, were grown on c-Al₂O₃ substrates via pulsed laser deposition. The growth procedure entailed adjustments in the oxygen partial pressure throughout the deposition process. Various advanced experimental techniques were employed to systematically analyze the structural, optical, and compositional characteristics of both highly c-axis-oriented, single phase pristine and dysprosium-doped (1 at. %) ZnO thin films. A confirmation of improvements in the structural properties, including crystalline quality, crystallite size, induced strain, and surface morphology, in the dysprosium-doped films was observed as a function of the oxygen partial pressure maintained during the deposition process, as evidenced by structural investigations. The optical investigations confirmed a transparency rate of more than 90% for all specimens in the visible region. Significant modifications associated with NBE, particularly for the Dy-doped ZnO films, were confirmed from PL investigations. These alterations validate the existence of diverse defect states within the bandgap of ZnO, positioned just below the minima of the conduction band edge. The emission peak positioned at ∼2.82 eV in RTPL spectra when excited by ∼3.82 eV confirmed the presence of ⁶H_15/2 energy state of Dy³⁺ ions close to the valance band. Different intra-4f transitions, viz., ⁴F_9/2 → ⁶H_J (J = 15/2, 13/2, 11/2, 9/2), from all the DZO thin films were observed when excited by a direct excitation of ∼2.83 eV in the present study. The XPS findings revealed that Zn consistently exhibited a +2 oxidation state in all samples. Additionally, the O1s spectra demonstrated the existence of oxygen in three distinct chemical settings. The oxygen contribution percentage indicated that thin films of superior crystalline quality could be achieved under optimized oxygen partial pressures, which led to fewer defects and reduced structural disorders, corroborated by structural and optical analyses. Furthermore, XPS validated the presence of Dy in a +3 oxidation state within the ZnO matrix. We anticipate that this study will stimulate additional research efforts focused on regulating the doping and growth techniques applied to oxide thin films produced through high-vacuum growth methodologies such as PLD. Such initiatives have the potential to expand the scope of applications for next-generation optoelectronic devices.

See the supplementary material, where Fig. S1 shows the XRD pattern of the (a) ZnO and (b) DZO pellets synthesized by the solid state route. The recorded XRD data for both ZnO and DZO pellets underwent Rietveld refinement. The data points are represented as black squares (Obs.), while the Rietveld fits are denoted by red circles (cal.). Beneath the XRD pattern, blue vertical lines (Bragg's positions) highlight the diffracted peaks corresponding to various crystallographic orientations of ZnO. Additionally, a plot (Obs.–Cal.) illustrating the difference between observed and calculated values is depicted in bluish-green (Difference) below the XRD pattern. Table S1 presents the lattice parameters of ZnO and DZO pellets derived from the Rietveld refinement of the recorded XRD data. These obtained parameters are compared with the standard values of ZnO (JCPDS card no. 36–1451). Figure S2 shows the recorded XRD diffractogram (raw data) of ZnO and DZO thin films deposited under a 0.1 Pa oxygen partial pressure without a log10 scale to compare with Figs. 1(a) and 1(b) (with a log10 scale). The RMS roughness was estimated from the AFM analysis and shown in Table S2. The Urbach energy of the samples was calculated using UV-Vis transmission spectra and presented in Table S3. From a Tauc plot, the estimated bandgaps of ZnO and DZO thin films deposited under different pressure conditions are shown in Table S4. Table S5 shows the peak positions and corresponding FWHM values from deconvoluted Jacobian-transformed wavelength-to-energy plots of RTPL spectra acquired from ZnO and DZO films deposited under various pressure conditions. Figure S3 shows the plot of intensity of f-f transitions vs oxygen partial pressure conditions for DZO thin films. Figure S4 and Table S6 show the EDAX spectra and atomic% of the elements in DZO samples, respectively. Figure S5(a)–S5(c) and Fig. S5(d)–S5(f) show the Zn 2p3/2 and Zn 2p1/2 energy state spectra of ZnO and DZO thin films grown under various pressure conditions, respectively. Figure S5(g)–S5(i) displays the high-resolution spectra depicting photoelectrons emitted from the 3d5/2 and 3d3/2 energy states of Dy3+ ions in the thin films of DZO deposited under pressure conditions of 10−4 Pa (vacuum) and oxygen partial pressures of 0.01 Pa, 0.1 Pa, respectively. Figure S6 shows the pictures of deposited samples and target pellets. The XPS survey scan and high-resolution scan of the DZO pellet are shown in Fig. S7.

A. Mandal acknowledges the Department of Instrumentation Science and Department of Physics, Savitribai Phule Pune University, Pune, India, for experimental support and financial assistance under DRDP. A. Mandal expresses gratitude for the support of the Department of Materials Engineering, Indian Institute of Science (IISc), Bangalore, India, for fellowship. A. Mandal extends gratitude to Dr. Sachin R. Rondiya from the Institute of Science (IISc), Bangalore, for his valuable time, support, and insightful scientific discussions pertaining to the current project. His productive suggestions greatly contributed to the results and discussions presented herein.

The authors have no conflicts to disclose.

Animesh Mandal: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Arun G. Banpurkar: Formal analysis (equal); Writing – review & editing (equal). Shashikant D. Shinde: Data curation (equal); Formal analysis (equal); Methodology (equal); Writing – review & editing (equal). Suhas M. Jejurikar: Conceptualization (equal); Formal analysis (equal); Writing – original draft (supporting); Writing – review & editing (equal).

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