Laser Energy and SnO2-Doping Effects on Structural, Morphological, and Optical Properties of GeO2 Thin Films Prepared by PLD

INTRODUCTION
Semiconductors are the cornerstone of modern electronic and optoelectronic technology, facilitating devices from integrated circuits to sensors [1]. Precise control over their electronic band structure is crucial. Among these, metal-oxide semiconductors (MOS) like ZnO, SnO2, and GeO2 are highly valued for their stability, wide bandgap (WBG) characteristics, and cost-effectiveness [2]. Their versatility makes them indispensable for next-generation transparent electronics, UV detectors, and chemical gas sensors. Recent advancements emphasize the crucial role of controlling material dimensions at the nanoscale, where structural evolution significantly impacts electronic performance, necessitating continuous research into advanced synthesis methods [3]. Germanium dioxide (GeO2) is a particularly interesting WBG metal oxide. Crystalline GeO2 exhibits a direct bandgap typically in the range of 4.5-5.5 eV, making it highly promising for deep UV applications and as a high-k dielectric material in microelectronics due to its high breakdown field strength and compatibility with Germanium substrates [4]. GeO2 is also a key material for integrated photonics due to its role as a host for luminescence and optical amplification [5]. However, tailoring GeO2 films often requires introducing defects or impurities to narrow the bandgap or enhance conductivity, especially for sensing applications where surface conductance changes are key. The structural crystallinity, surface morphology, and optical absorption edge of GeO2 thin films are highly sensitive to deposition parameters, making the choice of synthesis technique and subsequent post-processing steps critical. Achieving high-quality thin films requires precise control over stoichiometry and crystallinity. Pulsed Laser Deposition (PLD) is a superior technique for synthesizing complex oxide thin films due to its ability to maintain target stoichiometry, high deposition rate, and versatility [6]. Laser energy is a critical operational parameter in PLD, directly influencing the kinetic energy of ablated species, plasma temperature, and consequently, the film’s crystallinity, grain size, and defect concentration [7]. PLD enables non-equilibrium growth conditions, ideal for synthesizing metastable phases or inducing beneficial stress/strain, thereby enhancing functional properties [8]. Manipulating the laser energy offers a non-chemical pathway to control the structural and morphological features, which fundamentally dictate the optical and electronic properties of the resulting thin film. Therefore, a systematic study on the effect of varying laser energy is essential to optimize the growth parameters of GeO2 and GeO2: SnO2 thin films. To enhance GeO2 functional performance, particularly for gas sensing and optical properties, Tin Oxide (SnO2) is introduced as a dopant. SnO2 is a well-known n-type MOS with a wide bandgap (sim 3.6 eV) and excellent chemical sensitivity [9]. Integrating SnO2 into the GeO2 matrix is expected to create synergistic effects, leading to structural modifications and localized electronic states within the GeO2 bandgap. Introducing Sn ions can cause changes in lattice constant, crystal structure distortion, and the formation of oxygen vacancies, which are critical for surface chemical reactions and carrier mobility [10]. Substitutional doping mechanisms often lead to non-linear property changes due to competing effects of lattice strain and charge carrier concentration [11]. Furthermore, the interplay between the host (GeO2) and the dopant (SnO2) can be exploited for band gap engineering, allowing precise tuning of the film’s absorption edge for optoelectronic devices. Controlling the doping concentration is paramount, as an optimal amount can dramatically improve performance, while excessive doping often leads to phase segregation or crystallinity degradation. This study is therefore primarily focused on systematically investigating the dual influence of pulsed laser deposition energy (200-350 mJ) and SnO2 doping concentration (1, 2, 3, 5, 8 wt%) on the fundamental physical properties of GeO2 thin films deposited at room temperature. The primary objectives are: (1) To determine the effect of varying laser energy on the crystallinity, preferred orientation, and grain size of both pure and Sn-doped GeO2 films using XRD. (2) To analyze the evolution of surface morphology, grain distribution, and roughness via AFM and SEM. (3) To precisely calculate and correlate the optical constants, specifically (α) and the direct optical bandgap (Eg), with the induced structural and morphological changes by utilizing UV-Visible Spectroscopy. The insights gained will contribute significantly to the optimization of GeO2: SnO2 based materials for transparent electronic and sensor applications, providing valuable data for controlled synthesis via PLD.

MATERIALS AND METHODS
Target Materials
The base material used was high-purity Germanium Dioxide (GeO2) powder (99.999% purity, Sigma-Aldrich). The dopant material was high-purity Tin Oxide (SnO2) powder (99.99% purity, Sigma-Aldrich).

Target preparation
The GeO2:SnO2 targets were prepared for PLD by the standard solid-state reaction and cold pressing method. SnO2 powder was mixed with GeO2 powder at varying concentrations: 1, 2, 3, 5, and 8 weight percent (wt%) of SnO2. The pure GeO2 powder target was also prepared for comparison. The powders were thoroughly mixed and ground in an agate mortar for approximately 30 minutes to ensure homogeneity. The mixed powders were pressed into circular pellets (20 mm diameter) under a pressure of 5-10 tons for 10 minutes using a hydraulic press. The resulting pellets were subjected to a high-temperature sintering process in an ambient air atmosphere at a temperature of 800-900 oC for 6 hours to increase their density and mechanical stability, crucial for effective laser ablation.

PLD System Setup
The thin films were deposited using an in-house customized PLD system (e.g., using a high vacuum chamber).

Laser Source
Laser Type: Nd: YAG pulsed laser (e.g., Continuum Surelite III) operating at its fourth harmonic (λ= 266 nm) to ensure strong absorption by the oxide targets.
Pulse Duration: Typically, 5-10 ns.
Repetition Rate: Set to 5 Hz (pulses per second).
Pulse Number: A fixed number of 100 laser pulses was applied for the deposition of each film to control the film thickness.
Laser Energy Variation: The impact of laser energy was investigated by varying the laser energy density hitting the target at: 200, 250, 300, and 350 mJ.

Deposition Parameters
All films were deposited at room temperature on Si substrate to investigate low-temperature film growth. The deposition chamber was evacuated to a high vacuum pressure of 10-5 Torr. Maintained constant at 5 cm. The target was rotated during deposition to ensure uniform ablation and prevent pitting. The obtained samples thermal treatment a 750 oC for annealing. The thickness of the prepared thin films was measured using an ellipsometer technique (Gaertner L116C). Since the number of pulses (100) was fixed, Table 1 presents the thickness measurements of samples produced at room temperature with varying laser energy between 200 and 350 mJ, using a constant number of 100 pulses on the glass substrates (it is noteworthy that the thickness measurements exhibited an error margin of around 5.2 nm). While the laser energy rises, the overall thickness of these films also increases, indicating that higher laser energy results in the removal of more particles from the target area. Consequently, the thickness of these thin films escalates. 

Characterization Techniques
The structural, morphological, elemental, and optical properties of the GeO2 and GeO2:SnO2 thin films were investigated using the following high-precision instruments: X-ray Diffraction (XRD) was using type (Bruker D8 Advance system). Atomic Force Microscopy (AFM)was using type (AFM Veeco Dimension 3100). Scanning Electron Microscopy was using type (SEM) (SEMFEI Quanta FEG). UV-Visible Spectroscopy was using type (Shimadzu UV-1800spectrophotometer).

RESULTS AND DISCISSION
Fig. 1 and Table 2, derived from the X-ray Diffraction (XRD) patterns, confirms the structural evolution of the pure GeO2 thin films as a function of the Pulsed Laser Deposition (PLD) laser energy (200, 250, 300, and 350 mJ). The crystallite size (D) is the critical parameter quantified from the XRD data. It is calculated using the Scherrer equation of the diffraction peaks (Scherrer, 1918) [12]:

Where D is the crystallite size, K is the shape factor (typically 0.94), λ is the X-ray wavelength (CuKα), β is the FWHM (in radians), and θ is the Bragg angle.
The films are confirmed to be polycrystalline and possess the hexagonal crystal structure of GeO2 across all deposited energies, consistent with the standard reference card (Card No. 98−020−0731). Five main diffraction planes are observed: (010), (011), (110), (102), and (020). The presence of multiple peaks confirms the polycrystalline nature. The Experimental d-spacing (D-space. Exper.) values are in excellent agreement with the Standard d-spacing (d-space. Stand.) values for all peaks and laser energies. This highly consistent match validates the phase purity, confirming the material is indeed hexagonal GeO2 with minimal lattice distortion or secondary phases. Fig. 2 shows a clear dependence of the average crystallite size on laser energy. The data exhibits a monotonic increase in the average crystallite size from 24.938 nm at 200 mJ to 29.228 nm at 350 mJ. This positive correlation suggests that higher laser energy in this range provides greater kinetic energy to the ablated plasma plume species. This energy translates into increased surface adatom mobility upon condensation on the substrate, promoting the formation of larger, better-defined crystalline domains, thus enhancing the overall crystallinity of the film [13]. While some PLD studies on metal oxides (e.g., ZnO or TiO2) show that excessively high laser energies can cause plasma overheating and rapid nucleation, leading to smaller crystallites, the results here for GeO2 show the opposite trend (increasing size). This implies that the energy threshold for achieving maximum crystal growth in GeO2 films via PLD is at or above 350 mJ under these specific deposition conditions (room temperature, silicon substrate). Research by Kumar et al. (2023) [14], on oxide films supports the idea that optimizing PLD parameters is crucial. In systems where the film is under structural constraint (such as GeO2 on Si), increasing the kinetic energy (via higher laser energy) can sometimes provide the necessary activation energy to overcome potential barriers for grain boundary movement, thus promoting crystal growth. These results establish the baseline for the undoped films. The subsequent analysis of the SnO2-doped films must consider this established trend, as the introduction of SnO2 is likely to introduce strain and defects, competing with the growth-promoting effect of the laser energy [15].
Fig. 3 and Table 3 detail the effect of SnO2 doping concentration (from 1 wt% to 8 wt%) on the structural parameters of the GeO2 thin films prepared by PLD at laser energy 350 mJ. The GeO2 films primarily retain the hexagonal structure (Card No. 98−020−0731), but the appearance of new peaks, such as (121) and (031), indexed to the Tetragonal phase (Card No. 98−001−6635), is noted, particularly at higher concentrations (5 wt% and 8 wt%).This confirms that doping with SnO2 (which itself is typically tetragonal rutile structure) induces a mixed-phase regime or high lattice strain in the host GeO2 matrix [15].The hexagonal (011) peak remains a dominant orientation across all samples, but its intensity and FWHM fluctuate with SnO2 ratio. A slight shift in the 2θ values for the main peaks (2θ for (011) shifts from 26.714∘ at 1 wt% to 26.93∘ at 8 wt%) indicates a change in the d-spacing. This suggests that the Sn atoms (which have a different ionic radius than Ge) are being incorporated, leading to lattice distortion/strain within the GeO2 host structure, as predicted by the standard Vegard’s law effects [14]. The Average Crystallite Size shows a strong dependence on the SnO2 doping concentration, as shown in Fig. 4. The average crystallite size initially increases significantly with SnO2 doping, rising sharply from 35.39 nm (2 wt%) to 52.035 nm (3 wt%). The size continues to increase up to 58.05 nm (5 wt%) but then saturates at 58.19 nm (8 wt%). The addition of SnO2 acts as a crystal growth enhancer up to an optimum concentration (∼5 wt%). This enhancement is likely due to the Sn dopant providing energy during the deposition process that aids the GeO2 grain boundary movement, leading to larger grains. However, at very high concentrations (8 wt%), the beneficial effect saturates, possibly because the increasing lattice strain counteracts further growth. 
Atomic Force Microscopy (AFM) was used to investigate the surface topography, grain size, and roughness characteristics of undoped GeO2 thin films deposited via PLD. The AFM micrographs confirm high film uniformity and excellent adhesion. The morphological parameters, summarized in Table 4, show that the average grain size is consistently below 100 nm.A significant morphological transition was observed as the incident laser energy increased from 200 mJ to 350 mJ. This increase resulted in a systematic reduction in average grain size (decreasing from 90.01 nm to 65.01 nm). Conversely, both the average roughness (Ra) and the RMS roughness showed a marked increase (escalating from 13.1 nm and 15.6 nm to 20.2 nm and 25.1 nm, respectively). This dual trend (decreasing grain size and increasing roughness) is rooted in the dynamics of the PLD plume [16]. Increasing laser energy boosts the kinetic energy and flux of ablated species [17], resulting in the ejection of smaller particles that strike the substrate with higher energy. This high arrival rate and low surface mobility promote a significantly higher nucleation rate and hinder particle coalescence. Consequently, the film develops a denser structure of smaller crystallites, contributing to the measured increase in surface irregularities [18]. The particle distribution analysis (Fig. 6) further supports this conclusion, showing a clear shift towards smaller, more dominant size classes at higher deposition energy. This observed dependence is consistent with findings for other PLD oxide thin films, confirming that elevated kinetic energy of the plasma plume is a dominant factor in controlling grain refinement and surface morphology [18].
The surface morphology of GeO2 thin films doped with SnO2 (1 wt% to 8 wt%) was studied using AFM at an optimized laser energy of 350 mJ. Unlike the inverse relationship observed when increasing laser energy, increasing the SnO2 concentration leads to a simultaneous increase in both the average grain size and the surface roughness parameters (Table 5). Specifically, the average grain size systematically increases from 71.06 nm to105.34 nm. Concurrently, the RMS roughness dramatically escalates from 34.5 nm to a maximum of 60.4 nm for the 8 wt% film. The observed increase in grain size and roughness is attributed to the Sn dopant modifying film growth kinetics [18]. The incorporation of Sn atoms promotes two primary mechanisms: enhanced surface diffusion, which lowers the kinetic barrier enabling adatoms to coalesce into larger structures [19], and strain-induced nucleation, where doping-induced strain or the formation of secondary SnO2 phases favors the growth of fewer, larger grains [20]. This change confirms that SnO2 acts as a structural modifier, shifting the growth mode toward a more three-dimensional island-like growth as the concentration rises.
The surface morphology of the pure GeO2 thin films prepared via Pulsed Laser Deposition (PLD) was further investigated using Scanning Electron Microscopy (SEM). The SEM micrographs, presented in Fig. 9, confirm that the synthesized films consist of nanoparticles generally measuring less than 100 nm. The particle shapes are predominantly spherical, although some irregular and aggregated clusters are observable across all samples. This nanoscale morphology confirms the suitability of the PLD technique for producing high-performance GeO2 thin films. The images, captured at varying magnifications (micron and 100 nm scales), show that the films exhibit uniform homogeneity with minimal aggregation, attesting to the quality of the deposited coating. A critical observation derived from the precise size measurements (extracted using ImageJ software) is the dependency of the particle size on the incident laser energy, where the results showed that the particle size systematically decreased from 58.68 nm at 200 mJ to a minimum of 22.24 nm at 350 mJ. This trend nanoparticle size reduction with increased laser energy is attributed to the fundamental PLD mechanism. Higher laser energy enhances the momentum transfer to the target material, resulting in a plasma plume composed of species with higher kinetic energy and increased ablation speed [21]. This rapid ejection minimizes the time available for ablated particles to collide and aggregate within the plume before reaching the substrate, thereby reducing the size of the deposited nanoparticles [22]. These findings are strongly corroborated by recent literature concerning PLD growth of oxide nanomaterials. The observed inverse relationship between laser energy and particle size is consistent with the results reported by [23], who studied similar oxide nanostructures. Furthermore, this confirms the general understanding that increasing laser energy is an effective strategy for achieving finer nanoscale features in oxide thin films deposited by PLD [22].
The morphology of GeO2 thin films doped with SnO2 (1wt% to 8 wt%) was investigated using SEM at a fixed laser energy of 350 mJ. SEM micrographs (Fig. 10) revealed a consistent surface pattern of homogeneous porous structures composed of semi-spherical nanoparticles. The average particle diameter showed a complex, non-monotonic dependence on SnO2 concentration (Table 6). Ignoring anomalies at 2 wt% (171.96 nm) and 8 wt% (105.22 nm) attributed to technical system issues, the dominant trend showed nanoparticle size reduction with increasing doping, reaching a minimum of 18.73 nm at 5 wt%. This contrasts with the systematic increase in AFM grain size and roughness. The mechanism for this SEM particle size reduction (or intra-plume refinement) at specific doping levels is ascribed to the interaction between the pulsed laser and the larger ionic radius of Sn+4 relative to Ge+4 [24, 25]. This interaction potentially leads to Dopant Disruption of larger GeO2 clusters in the plume and Surface Strain upon deposition, favoring smaller, stable nanoparticles. While AFM showed inter-grain coalescence (macro-scale roughening), SEM indicates this concurrent nano-scale refinement, underscoring the precise morphological control achievable via PLD [26].
The optical absorption properties of the pure GeO2 thin films deposited on Si substrates using PLD at varying laser energies (200 to 350 mJ) were investigated by measuring the absorption spectrum in the wavelength range of 230 nm to 800 nm. As illustrated in Fig. 11, all prepared films exhibit high fundamental absorption within the ultraviolet (UV) region, specifically at wavelengths between 230 nm and 300 nm. This strong absorption peak is characteristic of semiconductor materials and is attributed to interbond electronic transitions, where valence band electrons are excited across the energy gap to the conduction band upon absorbing high-energy UV photons. Beyond 350 nm (into the visible spectrum), the absorption rapidly decreases due to the weak photon energy corresponding to longer wavelengths. A slight but systematic variation in absorbance values is observed based on the laser energy used for deposition, where, the film prepared at 350 mJ exhibits the highest absorbance (0.97), followed sequentially by films prepared at 300 mJ, 250 mJ, and 200 mJ. Also, the maximum difference in absorbance between the highest (350 mJ) and lowest energy films is minimal (0.02). This enhancement in absorbance with increasing laser energy is primarily correlated with the resulting film morphology and thickness. The higher absorbance is plausibly ascribed to the increased film thickness (typical at higher laser energy), coupled with the decrease in grain size (from 90.01 nm to 65.01 nm,), which generally increases the total available scattering area or density of states [8]. These optical findings are consistent with recent research on oxide films grown by PLD. The observed trend of slight absorbance enhancement with increasing laser energy, linked to morphological changes, is in agreement with the results reported by Deng et al. (2021) [27], who found that high PLD energy optimizes the absorption characteristics of metal oxide thin films due to improved structural density [28].
The optical band gap energy (Eg) for the pure GeO2 thin films deposited at varying laser energies was determined using the Tauc plot method. The Tauc plots, shown in Fig. 12, display the relationship between photon energy and the absorption coefficient term (α.hν)r, where r=2 for direct transitions. The calculated direct optical band gap energy (Eg) values were found to increase systematically with higher laser deposition energy as shown in Table 6. The Eg value shifts from 3.55 eV at 200 mJ to its maximum of 3.65 eV at 350 mJ. This increase in the optical band gap is primarily attributed to the improvement in the film’s structural quality and quantum confinement effects resulting from the high energy PLD process [29, 30]. Higher laser energy promotes a more efficient rearrangement of ablated atoms, leading to films with increased structural density and reduced defects. A decrease in structural defects generally enhances the periodicity of the crystal lattice, widening the band gap [31]. As confirmed by the AFM and SEM results, increasing the laser energy drastically reduces the average particle size (down to 22.24 nm). This size reduction pushes the system into the quantum confinement regime, where the confinement of charge carriers within the smaller nanocrystals leads to a blue shift (increase) in the optical band gap energy [30]. The observed correlation between the enhancement of Eg and the reduction in crystallite size is a well-established phenomenon in semiconductor nanomaterials. This finding is consistent with literature on GeO2 films [2], and aligns broadly with the general principles of the quantum size effect in nanocrystalline oxides prepared by various methods [31].
The UV-Visible absorption spectra of GeO2 thin films doped with SnO2 (1 wt% to 8 wt%) were recorded at a fixed laser energy of 350 mJ. Fig. 13 illustrates the spectra of UV-Vi’s absorption spectrum of all doped films showed a fundamental absorption edge, with the maximum absorption occurring consistently around 290nm, characteristic of GeO2 electronic transitions. A systematic trend was observed in the magnitude of absorption: the absorbance generally increases with increasing SnO2 doping concentration. The maximum absorbance (A = 0.95) was reached at the 5 wt% concentration, with a slight drop observed at 8 wt% (A = 0.918). This general enhancement in absorbance up to 5 wt% SnO2 is primarily attributed to the augmentation of the optical path length and changes in film structure induced by the dopant [32]. Sn incorporation can lead to a thicker film [33], increasing the number of absorbing particles. Additionally, structural disorder introduced by the dopant (as suggested by increased AFM roughness, Section 3.3) may introduce localized defect states or enhanced surface scattering, contributing to higher absorbance [34]. The slight drop at 8 wt% may be due to phase segregation or high surface roughness leading to increased light scattering. This correlation between increased dopant concentration and enhanced UV absorption aligns with prior findings in the literature [31, 32].
The direct optical band gap energy (Eg) for SnO2-doped GeO2 thin films was determined using the Tauc method, as shown in Fig. 14. The calculated Eg values (Table 8) show a non-monotonic dependence on SnO2 concentration, indicating a complex influence on the electronic structure. The band gap initially widens from 3.55 eV at 1 wt% to a maximum of 3.65 eV at 2 wt%. It then narrows progressively, reaching a minimum of 3.40 eV at 5 wt%, before slightly increasing to 3.45 eV at 8 wt%. This dual behavior stems from a competition between two dominant effects. At low doping (1wt% and 2 wt%), the initial Eg increase (blue shift) is linked to enhancement of the crystal structure (atomic rearrangement) [35] and, potentially, minor quantum confinement effects. Conversely, at higher doping (3 wt% and above), the band gap begins to narrow due to the formation of secondary energy levels (intermediate levels) within the forbidden gap [36]. These intermediate levels, arising from high dopant concentration and defects, facilitate lower-energy electron transitions, effectively reducing Eg [34]. This non-monotonic dependence is common in doped metal oxides, reflecting the complex interplay between quantum confinement and impurity-induced defect states [36].

CONCLUSION
This study demonstrates that the Pulsed Laser Deposition (PLD) technique successfully synthesized GeO2 and SnO2-doped GeO2 thin films with precisely tunable properties. Increasing the laser energy from 200 to 350 mJ significantly enhanced the crystallinity, increasing the crystallite size to 29.23 nm, and promoted grain refinement that reduced nanoparticle size to 22.24 nm, leading to a widened optical band gap (E.g) due to the quantum confinement effect. In contrast, SnO2 doping acted as a major structural modifier, inducing a mixed-phase regime (Hexagonal GeO2 and Tetragonal SnO2) and increasing the crystallite size up to 58.19 nm. Morphologically, while laser energy led to smaller particles with slight roughness, SnO2 doping caused significant grain agglomeration (up to 105.34 nm) and a shift toward a three-dimensional island-like growth pattern with increased surface roughness. Optically, SnO2 doping exhibited a dual effect, initially widening the band gap at low concentrations before causing it to narrow at higher levels (>2 wt%). Ultimately, while high laser energy is essential for maximizing structural quality and band gap width, SnO2 doping provides a versatile tool for tuning the absorption and electronic states of GeO2 films for optoelectronic applications.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.