# Er3+-doped amorphous tellurite thin films

Pradeep Ramanan, Jan Gutwirth, Stanislav Slang, Anupama Viswanathan, Taghrid Ghanawi, Florent Starecki, Virginie Nazabal, Matthieu Roussey, Petr Nemec, Laeticia Petit

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Pradeep Ramanan, Jan Gutwirth, Stanislav Slang, Anupama Viswanathan, Taghrid Ghanawi, et al. Er3+-doped amorphous tellurite thin films. Journal of the American Ceramic Society, 2025, pp.e20589. 10.1111/jace.20589. hal-05050672

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Submitted on 29 Apr 2025

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RESEARCH ARTICLE

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Er<fnref n="3" />⁺-doped amorphous tellurite thin films

Pradeep Ramanan<fnref n="1" />,<fnref n="2" /> ID

Jan Gutwirth<fnref n="2" />

Stanislav Slang<fnref n="2" />

Anupama Viswanathan<fnref n="2" />

Taghrid Ghanawi<fnref n="3" />

Florent Starecki<fnref n="3" />,<fnref n="4" />

Virginie Nazabal<fnref n="3" />,<fnref n="2" />

Matthieu Roussey<fnref n="5" />

Petr Němec<fnref n="2" /> ID

Laeticia Petit<fnref n="1" /> ID

**Correspondence**

Pradeep Ramanan and Petr Němec, Faculty of Chemical Technology, University of Pardubice, Pardubice 53210, Czech Republic.

Email: pradeep.ramanan@tuni.fi, petr.nemec@upce.cz

**Funding information**

Horizon 2020 Framework Programme, Grant/Award Number: 101092723; Ministry of Education, Youth and Sports, Czech Republic; Research Council of Finland, Flagship Programme, Photonics Research, and Innovation (PREIN)

**Abstract**

Radio-frequency (RF) magnetron sputtering was used to deposit amorphous tellurite thin films from glassy targets from TeO₂-ZnO-Na₂O/BaO/Bi₂O₃ system doped with 2.5 mol% of Er₂O₃. The impact of the sputtering power on the optical, luminescence, and structural properties of the films is discussed. The deposition process leads to amorphous and transparent films which emit light in the visible and near-infrared confirming the presence of Er<fnref n="3" />⁺ in the films' network. Owing to their large refractive index and stability over time, the Bi-containing films are potential materials for integrated photonics and optical switches.

**KEYWORDS**

erbium/erbium compounds, glass, magnetron sputtering, tellurium/tellurium compounds, thin films

1 | INTRODUCTION

Rare-earth (RE) doped materials are of great interest as they are a solution for lasers and optical amplifiers, widely used in integrated photonics and optical telecommunications. When doped with erbium ions (Er<fnref n="3" />⁺), these

materials can emit light in the visible, Near-infrared (NIR) and mid-infrared (MIR).<fnref n="1" /> Glasses are good hosts for RE, especially the tellurite glasses since they possess interesting properties such as low phonon energy (~700 cm⁻<fnref n="1" />), a transmission window from ~0.4 to ~5 µm and high linear refractive index (RI, from ~1.8 to ~2.3 at 532 nm).<fnref n="2" />⁻<fnref n="5" />

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). Journal of the *American Ceramic Society* published by Wiley Periodicals LLC on behalf of American Ceramic Society.

<footnote n="1">¹Faculty of Engineering and Natural Sciences (Photonics Laboratory), Tampere University, Tampere, Finland</footnote>

<footnote n="2">²Faculty of Chemical Technology, University of Pardubice, Pardubice, Czech Republic</footnote>

<footnote n="3">³Institut des Sciences Chimiques de Rennes (ISCR), University of Rennes, Rennes, France</footnote>

<footnote n="4">⁴École nationale supérieure d'ingénieurs de Caen (ENSICAEN), Research Center on Ions, Materials and Photonics (CIMAP), Caen, France</footnote>

<footnote n="5">⁵University of Eastern Finland, Center for Photonics Sciences, Joensuu, Finland</footnote>
Compared to silica glass, they usually have a broad emission band and high-emission cross-section.⁶ Furthermore, they can incorporate large amount of RE without forming clusters which is important when developing Er³⁺-doped amplifiers for integrated photonics.⁷

Recently, Lemiere et al.⁸ reported a study on the impact of tellurite glass composition on the Er³⁺ spectroscopic properties and demonstrated that Er³⁺-doped tellurite glasses in the TeO₂-ZnO-Na₂O/BaO/Bi₂O₃ system exhibit promising Er³⁺ spectroscopic properties. These glasses were prepared with a high concentration of Er³⁺ (2.5 mol% of Er₂O₃) and the variation in the glass composition was found to change the glass structure impacting the Er³⁺ spectroscopic properties. The tellurite glass containing Bi₂O₃ was reported with the highest intensity of upconversion and emission in NIR and MIR under 980 nm pumping and lowest hydroxyl group (-OH) content owing to its least polymerized network.⁹ Consequently, this glass is expected to be promising for integrated photonics.

For integrated photonics, the glass needs to be deposited as a thin film onto a substrate.¹⁰ Various techniques based on physical and chemical methods have been developed to deposit glasses as films.¹¹⁻¹⁴ Within the scope of physical deposition methods, the pulsed laser deposition (PLD) technique has been successfully used for the deposition of thin films with good quality and in various glassy systems.¹⁵,¹⁶ For example, amorphous tellurite films were deposited using argon fluoride (ArF) excimer laser (193 nm) from glasses in the TeO₂-P₂O₅-Na₂O-ZnF₂¹⁷ and TeO₂-ZnO-ZnF₂¹⁸ systems. Radio-frequency (RF) magnetron sputtering is another approach to fabricate thin films. Compared to evaporation based deposition techniques, RF magnetron sputtering can be performed at lower temperatures with less deterioration to the target material since it is momentum based while resulting in better adhesion of the film onto the substrate.¹⁹,²⁰ It is clear that both techniques are suitable to deposit thin films from glass. PLD does offer advantages over RF magnetron sputtering when it comes to depositing polymeric materials.²¹,²² Interestingly, in the study performed by Schröder et al., both PLD and RF magnetron sputtering have been used in conjunction to deposit smoother and denser TiO₂ and Bi₂FeCrO₆ films.²³ Nevertheless, the good reproducibility of deposited films while having uniformity in thickness is inarguably a well-known benefit of the sputtering process while also aiding to keep the substrate at a stable desired temperature during deposition.²⁴,²⁵ Subsequently, RF sputtering has been widely used to deposit films for various applications related to integrated circuits, sensing, solar cells, etc.²⁶,²⁷ Demonstration of film deposition from glass within more complex system such as the MgO-CaO-P₂O₅-SiO₂ system was reported.²⁸ Surpris-

ingly, very few studies on the deposition of tellurite glasses with compositions such as TeO₂-ZnO-Bi₂O₃ and TeO₂-WO₃-Bi₂O₃ using RF magnetron sputtering have been reported.²⁹⁻³¹

This paper presents a study focused on the deposition of amorphous thin tellurite films using RF magnetron sputtering. The chemical composition as well as the structural, optical, and spectroscopic properties of the films are assessed and discussed as a function of the glass composition and deposition parameters used.

## 2 | METHODS

The melt quenching technique was used to prepare tellurite glasses with the composition 68.25 TeO₂-19.5 ZnO-9.75 X-2.5 Er₂O₃ (mol%) with X = BaO, Bi₂O₃, Na₂O.⁸ The glasses are hereby referred to as TeBa, TeBi, and TeNa, respectively. Commercially available raw materials were used: TeO₂ (Sigma-Aldrich, 99.99%), ZnO (Sigma-Aldrich, 99.99%), BaO (Sigma-Aldrich, 99.99%), Na₂CO₃ (Sigma-Aldrich, 99.5%), Bi₂O₃ (Sigma-Aldrich, 99.99%), and Er₂O₃ (Sigma-Aldrich, 99.99%). The 65 g glass batches were placed in a platinum crucible and melted at 775°C for 40 min. After melting, the glasses were quenched in a cylindrical 2" mold to obtain glass targets with the desired shape required for sputtering. After quenching, the glasses were annealed for 6 h at 40°C below their respective glass transition temperature. Finally, the annealed glass disks with a 2" diameter were surface grinded to achieve a thickness within 3-3.5 mm and flat parallel surfaces using a bench-top polisher (PM5 Precision Lapping & Polishing system, Logitech).

The films deposition was carried out using RF magnetron sputtering (MPE600, Plassys-Bestek) using argon plasma at room temperature (RT). The argon-oxygen (Ar:O₂) gas flow ratio was fixed at 40:10 sccm for all depositions following a study performed on the tellurite-bismuth glass system.²⁹ Sputtering power was varied in 10 W increment from 30 to 50 W for TeBa depositions. TeBi and TeNa depositions were performed at lower power levels, 20 and 30 W to ensure no damage to the sputtering targets. The substrates were rotated at 5 rpm and the chamber was maintained at an overall pressure of 0.5 mbar. Substrates used for the depositions were BK7 glass (Schott), < 1 0 0 > single-crystalline p-type silicon (Si) wafers and oxidized silicon (SiO₂/Si) wafers. After deposition, the films were stored in nitrogen atmosphere.

The X-ray diffraction (XRD) patterns of the films were measured using an X-ray diffractometer (PANalytical Empyrean) under a Cu Kα line with 99 s step time, 0.026° step size.
The elemental composition of the films was analyzed using a scanning electron microscope (SEM) LYRA 3 (Tes- can) equipped with energy dispersive spectroscopy (EDX) analyzer AZtec X-Max 20 (Oxford Instruments) coupled with AZtec software (Oxford Instruments). The bulk and film samples were coated with carbon (Leica ACE200) to minimize the charging.

Variable Angle Spectroscopic Ellipsometry (VASE) was used to determine the thickness, the RI, and the optical bandgap. The measurements were performed with a rotating analyzer ellipsometer (VASE, J. A. Woollam) at an angle of incidence from 50° to 70° in 10° increments over the wavelength range 280–2300 nm. The Cody–Lorentz model³² and Cauchy model³³ were used to fit the VASE data of thin films and targets, respectively. The RI dispersion of the thin films and the targets were also measured in the 2300–20000 nm range using a second ellipsometer (IR-VASE, J. A. Woollam) with a 16 cm⁻¹ resolution. The Gaussian oscillator model³⁴,³⁵ was used for data fitting using WVASE software. Modeling required the use of two Gaussians. The phonon absorption (PA) edge of the glasses was determined from the absorption spectrum of the glasses³⁶ and was found to be at ~855 cm⁻¹ for the TeBa target and films, at ~810 and at ~865 cm⁻¹ for the TeBi target and films respectively, and for the TeNa target and films at ~836 and ~865 cm⁻¹ respectively. These PA edge values are similar to those reported for tellurite glasses.³⁶ The accuracy for the RI determination is ± 0.01.

The transmittance spectra of the films were measured using an UV–VIS–NIR spectrophotometer (LAMBDA 1050+, Perkin Elmer) at step size of 1 nm over the wavelength range 180–2500 nm. The optical bandgap (E_g) of the sputtered films was estimated by generating a Tauc plot.³⁷,³⁸

Surface topography and roughness of the film were measured using atomic force microscopy (AFM) (NTEGRA ACADEMIA, NT-MDT) with a single-crystal silicon tip (HA_NC, ScanSens GmbH) having a force constant and resonant frequency of 12 N/m and 235 kHz respectively under the semi-contact mode. Scan resolution of 512 lines was chosen for an area of 5 × 5 sq.µm. Macroscale surface was mapped using an optical profiler (Wyko NT1100, Veeco Instruments) at 1.2× magnification (2.5× for objective lens and 0.5× for field-of-view lens). The entire surface was mapped using the stick option. The surface roughness in terms of root mean square (RMS) was measured for all the investigated films immediately after deposition and 8 months after deposition.

The emission spectra of the bulk glasses and deposited films were measured in the visible range using a micro-Raman system (alpha300R, Witec; Oxford Instruments) using a silicon drift detector. The excitation source was a 532 nm continuous wave diode laser with the measure-

<table><caption>TABLE 1 Thickness of deposited films determined from VASE for different sputtering power values along with deposition rate.</caption><thead><tr><th>Sputtering power (W)</th><th>Film thickness (nm)<br>± 5 nm</th><th>Deposition rate (nm/min)<br>± 0.1 nm/min</th></tr></thead><tbody><tr><td colspan="3"><strong>TeBa</strong></td></tr><tr><td>30</td><td>800</td><td>4.4</td></tr><tr><td>40</td><td>885</td><td>6.6</td></tr><tr><td>50</td><td>700</td><td>9.3</td></tr><tr><td colspan="3"><strong>TeBi</strong></td></tr><tr><td>20</td><td>940</td><td>3.9</td></tr><tr><td>30</td><td>1205</td><td>8.1</td></tr><tr><td colspan="3"><strong>TeNa</strong></td></tr><tr><td>20</td><td>975</td><td>4.1</td></tr><tr><td>30</td><td>745</td><td>7.1</td></tr></tbody></table>

Abbreviation: VASE, Variable Angle Spectroscopic Ellipsometry.

ment performed at 17.5 mW power and 1 s integration time. The NIR emission spectra of the films were collected using an excitation at 808 nm from a pulsed laser with a 5 MHz repetition rate. Measurements were performed using a 300-line grating, 1 µm blazed. A lock-in amplifier and a liquid nitrogen cooled indium arsenide antimonide detector were used.

The Raman spectra of the films were measured using a 405 nm excitation source (alpha300R, Witec; Oxford Instruments) and the experimental setup consisted of a 600-line grating, 100× microscope objective and were averaged on a 20 × 20 sq.µm zone.

## 3 | RESULTS AND ANALYSIS

Er³⁺-doped tellurite thin films within the TeO₂–ZnO–Na₂O/BaO/Bi₂O₃ system were deposited using different RF magnetron sputtering power as summarized in Table 1. For a comparable sputtering power (30 W), the largest deposition rate was observed for the TeBi thin films.

The XRD patterns of all the films shown in Figure 1 did not depict any peaks, suggesting that no significant crystallization had occurred during the film deposition for all glass compositions. The crystals might have also been too small and/or too few to be detected.

The composition of the films was verified using SEM coupled with EDX (Table 2). The deposition process led to an increase in the amount of barium (Ba) and bismuth (Bi) at the expense of tellurium (Te). However, the sputtering power had no significant impact on the film composition (within the accuracy of the measurement). The at% of erbium (Er) in all the investigated films was as targeted within the accuracy of the measurement (± 1 at%).
<table><caption>TABLE 2 Chemical composition of the investigated films compared to that of the bulk target glasses.</caption><thead><tr><th rowspan="3">Element</th><th colspan="5">Composition (at%)<br>(± 1 at%)</th><th colspan="5">TeBi</th><th colspan="5">TeNa</th></tr><tr><th colspan="5">TeBa</th><th colspan="5">TeBi</th><th colspan="5">TeNa</th></tr><tr><th>Te</th><th>Zn</th><th>Ba</th><th>Er</th><th>O</th><th>Te</th><th>Zn</th><th>Bi</th><th>Er</th><th>O</th><th>Te</th><th>Zn</th><th>Na</th><th>Er</th><th>O</th></tr></thead><tbody><tr><td>Theoretical</td><td>25</td><td>7</td><td>4</td><td>2</td><td>63</td><td>22</td><td>6</td><td>6</td><td>2</td><td>63</td><td>24</td><td>7</td><td>7</td><td>2</td><td>61</td></tr><tr><td>Bulk target</td><td>25</td><td>8</td><td>3</td><td>2</td><td>62</td><td>22</td><td>7</td><td>6</td><td>2</td><td>63</td><td>24</td><td>6</td><td>9</td><td>2</td><td>59</td></tr><tr><td>Film - 20 W</td><td></td><td></td><td></td><td></td><td></td><td>17</td><td>7</td><td>8</td><td>1</td><td>67</td><td>20</td><td>6</td><td>9</td><td>1</td><td>63</td></tr><tr><td>Film - 30 W</td><td>15</td><td>10</td><td>6</td><td>3</td><td>67</td><td>15</td><td>7</td><td>10</td><td>1</td><td>67</td><td>21</td><td>6</td><td>9</td><td>2</td><td>62</td></tr><tr><td>Film - 40 W</td><td>17</td><td>9</td><td>5</td><td>2</td><td>66</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Film - 50 W</td><td>18</td><td>9</td><td>6</td><td>2</td><td>65</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Note: The theoretical compositions of bulk glasses are also shown.

<figure><img src="image_4.png" /><figcaption>FIGURE 1 XRD pattern of the investigated thin films.</figcaption></figure>

The structure of the films was investigated using Raman spectroscopy. The Raman spectra of the films, normalized to the band at 780 cm⁻¹, are depicted in Figure 2.

The Raman spectra showed various bands which are similar to those reported for the bulk glasses.⁸ As compared to the Raman spectra of the bulk glasses, a band (1) with large intensity could be seen at ~520 cm⁻¹ which could be related to the signal from the Si substrate but it overlapped with the well-known band in the 400–500 cm⁻¹ range assigned to symmetrical bending of Te–O–Te/O–Te–O bonds³⁹ and to Zn–O–Zn/Zn–O–Te bridges.⁴⁰ The band at 660 cm⁻¹ ((2) in Figure 2A) was assigned to the asymmetric Te–O stretching and symmetric stretching of TeO₄ units. The band at 770 cm⁻¹ ((3) in Figure 2A) was assigned to the symmetric stretching in TeO₃ and TeO₃+1 units.⁴¹,⁴² Finally, the band at ~980 cm⁻¹ (420.7 nm) ((4) in Figure 2A) was assigned to the ⁴F₃/₂ → ⁴I₁₅/₂ emission

from Er³⁺. An increase in the sputtering power led to a slight decrease in intensity of the band at 660 cm⁻¹ compared to that of 770 cm⁻¹ suggesting an increase in number of TeO₃ and TeO₃+1 units at the expense of TeO₄ units. The TeBi films are suspected to have the most depolymerized network as evidenced by the lowest intensity of the band at 660 cm⁻¹ compared to the band at 770 cm⁻¹.

Irrespective of the deposition power, highly transparent thin films were obtained as verified from their transmittance spectra (Figure 3).

Over the entire scan range of the UV–VIS–NIR spectrum, the interference maxima reached 92%, equal to that of the BK7 substrate. The interference fringes were well pronounced and displayed a good optical quality of the films.

TeO₂ being a conditional glass former requires the presence of modifiers such as ZnO to alter the trigonal bipyramidal TeO₄ units to TeO₃+1 or trigonal pyramidal TeO₃ units. The incorporation of BaO, Bi₂O₃, and Na₂O into the glass matrix further influences the depolymerization which in turn leads to different ratios of TeO₃:TeO₃+1 units. In their study, Santos et al. observed that an increase in concentration of different modifiers into the TeO₂ glass resulted in a lowering of bandgap values.⁴³ This change in bandgap was expected since moving from crystalline to the amorphous state, the *k*-space was bound to change, thereby modifying the electron density and position of valence and conduction band. As a result, the optical bandgap was influenced by factors, with one such as the non-bridging oxygens.⁴⁴⁻⁴⁶ Owing to these, it was possible for the material to display a bandgap that was either purely direct or indirect or display both simultaneously. Furthermore, even for the bulk and film form of amorphous TeO₂, the optical bandgap was found to vary between direct and indirect.⁴⁷,⁴⁸ In this study, all the investigated films displayed purely an indirect optical bandgap as shown in Figure 4, depicted by a Tauc plot for indirect allowed transition (n = 2 in (αhν)¹/n).
<figure><img src="image_5.png" /><figcaption>FIGURE 2 Normalized Raman spectra of the TeBa (A), TeBi (B), and TeNa (C) films deposited using different sputtering powers on silicon substrate.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>FIGURE 3 Transmittance spectra of the TeBa (A), TeBi (B), and TeNa (C) films deposited using different sputtering powers on BK7 substrate.</figcaption></figure>

<table><caption>TABLE 3 Optical bandgap of deposited films determined from Tauc plot and from VASE with mean squared error (MSE) fit values for different sputtering power values.</caption><thead><tr><th>Sputtering power (W)</th><th>Optical bandgap from Tauc plot (eV)</th><th>Optical bandgap from VASE (eV)</th></tr></thead><tbody><tr><td colspan="3"><strong>TeBa films</strong></td></tr><tr><td>30</td><td>3.37 ± 0.01</td><td>3.68 ± 0.06</td></tr><tr><td>40</td><td>3.37 ± 0.01</td><td>3.65 ± 0.11</td></tr><tr><td>50</td><td>3.42 ± 0.01</td><td>3.65 ± 0.11</td></tr><tr><td colspan="3"><strong>TeBi films</strong></td></tr><tr><td>20</td><td>3.29 ± 0.01</td><td>3.43 ± 0.01</td></tr><tr><td>30</td><td>3.27 ± 0.01</td><td>3.37 ± 0.02</td></tr><tr><td colspan="3"><strong>TeNa films</strong></td></tr><tr><td>20</td><td>3.25 ± 0.01</td><td>3.40 ± 0.07</td></tr><tr><td>30</td><td>3.24 ± 0.01</td><td>3.51 ± 0.09</td></tr></tbody></table>

Abbreviation: VASE, Variable Angle Spectroscopic Ellipsometry.

Table 3 lists the optical bandgap which was obtained by extrapolating the linear fit of the linear portion to the intercept on the X-axis. The sputtering power had no significant

impact on the position of the bandgap. TeBi and TeNa films displayed similar bandgap values whereas the TeBa films had the highest band gap. Indeed, a bandgap located at the highest wavelengths was also reported for the TeBi glass bulk in.⁸

Also listed in Table 3 is the optical bandgap of the films estimated from VASE, confirming that TeBa films had larger bandgap than the TeBi and TeNa films. However, these bandgaps tended to be higher than the ones obtained from the Tauc plots.

Due to the lower amount of Te in the films in comparison with the targets, the films had a lower RI than the corresponding targets as shown in Figure 5. From all the investigated films, the TeBi films had the highest RI due to the high polarizability of Bi.⁴⁹

The presence of Er³⁺ in the films was confirmed by recording the emission spectra of the films using excitation at 532 nm (Figure 6) and at 808 nm (Figure 7). The emission spectra under 532 nm excitation, shown in Figure 6, were normalized to the band at 660 nm. They depicted two emission bands centered at 550 and 660 nm which could be
<figure><img src="image_7.png" /><figcaption>FIGURE 4 Tauc plots for an indirect allowed transition shown for TeBa, TeBi, and TeNa films (A–C) deposited using various sputtering power, respectively.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>FIGURE 5 RI dispersion curves from 300 nm to 20 µm for TeBa (A), TeBi (B), and TeNa (C) films deposited using different sputtering powers on BK7 glass substrate. RI, refractive index.</figcaption></figure>

related to the $ ^{4}S_{3/2} \rightarrow ^{4}I_{15/2} $ and $ ^{4}F_{9/2} \rightarrow ^{4}I_{15/2} $ transitions of Er$^{3+}$, respectively. Signal from the Si substrate could also be seen at ~550 nm as well as the signal from the pump at 532 nm. As observed from the parent bulks (Figure 6D), the green emission compared to the red one was the highest from the TeNa films and the lowest from the TeBi films when deposited using 30 W. A variation could be noted in the shape of the green emission band from the films compared to the corresponding targets suggesting that the Er$^{3+}$ sites were probably impacted during the film deposition. No sharp peaks except those related to the Si substrate can be seen in the emission spectra, suggesting that the Er$^{3+}$ are located in amorphous sites in the films. One should point out the absence of correlation between the ratio of the green to red emissions and the sputtering power.

The emission spectra normalized at 1612 nm which helped to take into account the reabsorption of Er$^{3+}$ are presented in Figure 7. They showed an emission centered

at ~1530 nm which could be related to the $ ^{4}I_{13/2} \rightarrow ^{4}I_{15/2} $ transitions of Er$^{3+}$. The emission band was also broad without the appearance of sharp peaks confirming the location of Er$^{3+}$ in amorphous sites. No significant changes in the shape of the emission could be seen when changing the film composition nor the sputtering power, indicating that the sites of the Er$^{3+}$ are similar in all the films. This was in agreement with Lemiere et al. who also reported similar Er$^{3+}$ sites in the TeBa, TeBi, and TeNa bulk glasses.⁸ However, the shape of the emission band of the films is different from that of the corresponding bulks (Figure 7D). This discrepancy should be related to the sample thickness. As the bulk samples are ~1 mm thick, reabsorption processes are expected to occur, modifying the shape of the emission band. Moreover, compared to bulk, the films are closer to the nanoscale regime and with the ions being uniformly spaced in the glass film matrix, a single broad emission was noticed. A similar type of emission band
<figure><img src="image_9.png" /><figcaption>FIGURE 6 Normalized emission spectra of the TeBa (A), TeBi (B), and TeNa (C) films deposited using various sputtering power on Si substrate ($\lambda_{exc}$ = 532 nm). Insets show the red emission. Shown in (D) is the emission spectra of the parent bulk glasses ($\lambda_{exc}$ = 532 nm). The spectra are normalized to the band at 660 nm.</figcaption></figure>

was observed by Güsken et al. for a plasmonic waveguide containing Er$^{3+}$ implanted on a 25 nm sputtered SiO$_{2}$ film.⁵⁰

Several factors contribute to the surface roughness of the films. For example, temperature and pressure tend to influence the growth kinetics based on the type of deposition process being used. A study performed by Amirzada et al. demonstrated the difference in film surface roughness depending on the deposition process, highlighting the importance of growth kinetics and the environmental factors present during deposition while also providing insight on the influence of the type of substrate being used.⁵¹ The type and surface roughness of the substrate are major factors in the case of epitaxial film growth, where the film critical thickness is in the order of atomic monolayers and the overall film thickness is less than tens of nanometers.⁵²,⁵³ However, in this work the substrate should not have influence since the material of interest is

not crystalline nor an alloy and the film thickness is in the range of hundreds of nanometers. Here, the surfaces of the as-deposited films were scanned with AFM and with an optical profiler so the film quality could be estimated from smaller and larger scale surfaces. The surface topography of the 30 W films measured with profilometer and AFM is presented in Figure 8A–C and in Figure 8D–F, respectively.

As shown in Figure 8A–C, the surface of the as-deposited films is uniform without voids adding evidence about the absence of significant crystallization occurring during film deposition. Using AFM, 5 × 5 sq.µm areas were scanned and low RMS was obtained for the as-deposited TeBi and TeNa films whereas higher RMS was detected for the as-deposited TeBa films (Table 4). On using an optical profiler, larger RMS was measured on scanning a larger area. Nonetheless, the newly deposited films had similar RMS than films deposited for the photovoltaic technology⁵⁴ confirming the feasibility of depositing the
<figure><img src="image_10.png" /><figcaption>FIGURE 7 Normalized emission spectra of the TeBa (A), TeBi, (B) and TeNa (C) films deposited using various sputtering power on Si substrate (λ<sub>exc</sub> = 808 nm). Shown in (D) is the emission spectra of the parent bulk glasses (λ<sub>exc</sub> = 808 nm).</figcaption></figure>

<table><caption>TABLE 4 RMS of the investigated films measured using AFM (scanning size of 5 × 5 sq.µm) and optical profiler (scanning size depending on the sample).</caption><thead><tr><th rowspan="2">Sputtering power (W)</th><th colspan="2">RMS from AFM (nm) (± 0.1 nm)</th><th colspan="3">RMS from optical profiler (nm) (± 10%)</th></tr><tr><th>as-deposited</th><th>after 8 months</th><th>scanned area</th><th>as-deposited</th><th>after 8 months</th></tr></thead><tbody><tr><td colspan="6"><strong>TeBa films</strong></td></tr><tr><td>30</td><td>1.25</td><td>2.5</td><td>27 mm × 25 mm</td><td>190</td><td>220</td></tr><tr><td>40</td><td>0.3</td><td>1.5</td><td>30 mm × 28 mm</td><td>250</td><td>335</td></tr><tr><td>50</td><td>0.4</td><td>1.2</td><td>27 mm × 26 mm</td><td>180</td><td>225</td></tr><tr><td colspan="6"><strong>TeBi films</strong></td></tr><tr><td>20</td><td>0.1</td><td>0.1</td><td>29 mm × 25 mm</td><td>240</td><td>345</td></tr><tr><td>30</td><td>0.1</td><td>0.1</td><td>41 mm × 31 mm</td><td>300</td><td>510</td></tr><tr><td colspan="6"><strong>TeNa films</strong></td></tr><tr><td>20</td><td>0.1</td><td>1.5</td><td>34 mm × 27 mm</td><td>360</td><td>550</td></tr><tr><td>30</td><td>0.4</td><td>1.6</td><td>36 mm × 23 mm</td><td>525</td><td>610</td></tr></tbody></table>

Abbreviations: AFM, atomic force microscopy; RMS, root mean square.
<figure><img src="image_11.png" /><figcaption>FIGURE 8 Surface profiles of the as-deposited TeBa, TeBi, and TeNa films deposited using 30W, measured using the optical profiler (A–C) and AFM (D–F), respectively.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>FIGURE 9 Surface profiles of the TeBa, TeBi, and TeNa deposited using 30W, measured using the optical profiler (A–C) and AFM (D–F), respectively. They were measured 8 months after the deposition. AFM, atomic force microscopy.</figcaption></figure>

new developed tellurite glasses into films with high surface quality using RF magnetron sputtering.

Also shown in Figure 9 are the surfaces of the same films measured 8 months after deposition.

As shown in Table 4, the roughness of the TeBa and TeNa films (from both AFM and optical profiler) increased sig-

nificantly over time, whereas no significant changes in surface roughness were observed for the TeBi films suggesting that these TeBi films were less sensitive to physical aging. A complete study of the (bulks and films) aging process is in progress to understand how to control the aging phenomena and so to improve the stability of the films over time.
4 | CONCLUSION

This work demonstrates that the RF magnetron sputtering
technique is a promising approach to deposit Er³⁺-doped
tellurite films of complex composition. For the first time
to the best of our knowledge, films with the composition
68.25 TeO₂–19.5 ZnO–9.75 X–2.5 Er₂O₃ (in mol%), where
X = BaO, Bi₂O₃, Na₂O were successfully deposited with
low surface roughness. In comparison with bulk glass tar-
gets, a small decrease in Te content was observed in the
films leading to films with lower RI than the targets. The
films prepared from the Bi₂O₃ containing glass target have
the highest RI. Nonetheless, amorphous and transparent
films were obtained with Er³⁺ emissions in the visible and
NIR under excitation at 532 and 808 nm, respectively. The
glass structure of the films was found to depend on the
deposition parameters. An increase in sputtering power
leads to films with more depolymerized network. The thin
films containing Bi₂O₃ are found to be the most stable ones
over time demonstrating that they are the most promising
for integrated photonics and optical switches.

ACKNOWLEDGMENTS

The work leading to this publication has received fund-
ing from the European Union's Horizon Europe research
and innovation programme under grant agreement num-
ber 101092723/IBAIA. UK participants in Horizon Europe
Project IBAIA are supported by UKRI Grant number:
10062902 (MODUS). The authors would also like to
acknowledge the Academy of Finland (Flagship Pro-
gramme, Photonics Research, and Innovation PREIN No.
320165 and 346518). Authors also appreciate the finan-
cial support from grant LM2023037 from the Ministry of
Education, Youth and Sports of the Czech Republic. Dr.
Rakesh Dhama, from Tampere University (Finland) is also
acknowledged for fruitful discussion.

Open access publishing facilitated by Univerzita Pardu-
bice, as part of the Wiley - CzechELib agreement.

ORCID

Pradeep Ramanan https://orcid.org/0000-0002-4111-2389

Petr Němec https://orcid.org/0000-0003-2426-3078

Laeticia Petit https://orcid.org/0000-0002-1673-8996

REFERENCES

1. Miniscalco WJ. Erbium-doped glasses for fiber amplifiers at 1500 nm. J Lightwave Technol. 1991;9(2):234-50.

2. Wang JS, Vogel EM, Snitzer E. Tellurite glass: a new candidate for fiber devices. Opt Mater. 1994;3(3):187-203.

3. Sidebottom D. Structure and optical properties of rare earth-
doped zinc oxyhalide tellurite glasses. J Non-Cryst Solids.
1997;222:282-89.

4. Layne CB, Lowdermilk WH, Weber MJ. Multiphonon relaxation of rare-earth ions in oxide glasses. Phys Rev B. 1977;16(1):10–20.

5. El-Mallawany R. The optical properties of tellurite glasses. J Appl Phys. 1992;72(5):1774–77.

6. Ohishi Y, Mori A, Yamada M, Ono H, Nishida Y, Oikawa K. Gain characteristics of tellurite-based erbium-doped fiber amplifiers for 1.5-µm broadband amplification. Opt Lett. 1998;23(4):274.

7. Jha A, Shen S, Naftaly M. Structural origin of spectral broadening of 1.5-µm emission in Er³⁺-doped tellurite glasses. Phys Rev B. 2000;62(10):6215–27.

8. Lemiere A, Bondzior B, Aromäki I, Petit L. Study of visible, NIR, and MIR spectroscopic properties of Er³⁺-doped tellurite glasses and glass-ceramics. J Am Ceram Soc. 2022;105(12):7186–95.

9. Kang S, Xiao X, Pan Q, Chen D, Qiu J, Dong G. Spectroscopic properties in Er³⁺-doped germanotellurite glasses and glass ceramics for mid-infrared laser materials. Sci Rep. 2017;7(1):43186.

10. Richardson K, Krol D, Hirao K. Glasses for photonic applica-
tions. Int J Appl Glass Sci. 2010;1(1):74–86.

11. Tosello C, Rossi F, Ronchin S, Rolli R, Righini GC, Pozzi F, et al. Erbium-activated silica–titania planar waveguides on silica-on-silicon substrates prepared by rf sputtering. J Non-Cryst Solids. 2001;284(1):230–36.

12. Grand G, Jadot JP, Denis H, Valette S, Fournier A, Grouillet AM. Low-loss PECVD silica channel waveguides for optical communications. Electron Lett. 1990;26(25):2135-37.

13. Kawachi M, Yasu M, Edahiro T. Fabrication of SiO₂–TiO₂ glass planar optical waveguides by flame hydrolysis deposition. Electron Lett. 1983;19(15):583–84.

14. Oke JA, Jen TC. Atomic layer deposition and other thin film deposition techniques: from principles to film properties. J Mater Res Technol. 2022;21:2481–514.

15. Caricato AP, Fazzi A, Jha A, Kar A, Leggieri G, Luches A, et al. Er-doped oxyfluoride silicate thin films prepared by pulsed laser deposition. Opt Mater. 2007;29(9):1166–70.

16. Munoz-Martin D, Fernandez-Navarro JM, Gonzalo J, Jose G, Jha A, Fierro JLG, et al. Structural and optical properties of tellurite thin film glasses deposited by pulsed laser deposition. Thin Solid Films. 2011;520(1):131–37.

17. Irannejad M, Jose G, Jha A, Steenson DP. A parametric study of Er³⁺-ions doped phospho-tellurite glass thin films by pulsed laser deposition. Opt Mater. 2010;33(2):215–19.

18. Morea R, Miguel A, Fernandez TT, Maté B, Ferrer FJ, Maffiotte C, et al. Er³⁺-doped fluorotellurite thin film glasses with improved photoluminescence emission at 1.53 µm. J Lumin. 2016;170:778-84.

19. Mattox DM. Physical vapor deposition (PVD) processes. Met Finish. 2002;100:394–408.

20. Chen P, Wang X. Cerium oxide film growth using radio-frequency sputtering process. Mater Sci Appl. 2020;11(5):305-18.

21. Norton DP. Pulsed laser deposition of complex materials: progress toward applications. In: Eason R, editor. Pulsed laser deposition of thin films. Wiley; 2006. p. 1–31.

22. Huber N, Heitz J, Bäuerle D. Polytetrafluoroethylene (PTFE) films prepared by F₂-laser deposition. EPJ Appl Phys. 2005;29(3):231–38.

23. Benetti D, Nouar R, Nechache R, Pepin H, Sarkissian A, Rosei F, et al. Combined magnetron sputtering and pulsed laser deposition of TiO₂ and BFCO thin films. Sci Rep. 2017;7(1):2503.
24. Kurdesau F, Khripunov G, da Cunha AF, Kaelin M, Tiwari AN. Comparative study of ITO layers deposited by DC and RF magnetron sputtering at room temperature. J Non-Cryst Solids. 2006;352(9):1466-70.

25. Garg R, Gonuguntla S, Sk S, Iqbal MS, Dada AO, Pal U, et al. Sputtering thin films: materials, applications, challenges and future directions. Adv Colloid Interface Sci. 2024;330:103203.

26. Ta MT, Briand D, Guhel Y, Bernard J, Pesant JC, Boudart B. Growth and structural characterization of cerium oxide thin films realized on Si(111) substrates by on-axis r.f. magnetron sputtering. Thin Solid Films. 2008;517(1):450-52.

27. Marina O. A solid oxide fuel cell with a gadolinia-doped ceria anode: preparation and performance. Solid State Ionics. 1999;123(1):199-208.

28. Mardare CC, Mardare AI, Fernandes JRF, Joanni E, Pina SCA, Fernandes MHV, et al. Deposition of bioactive glass-ceramic thin-films by RF magnetron sputtering. J Eur Ceram Soc. 2003;23(7):1027-30.

29. Ogbuu O, Du Q, Lin H, Li L, Zou Y, Koontz E, et al. Impact of stoichiometry on structural and optical properties of sputter deposited multicomponent tellurite glass films. J Am Ceram Soc. 2015;98(6):1731-38.

30. Del Cacho VD, da Silva DM, de Assumpção TAA, Kassab LRP, Alayo MI, Melo EG. Fabrication of Yb³⁺/Er³⁺ codoped Bi₂O₃–WO₃–TeO₂ pedestal type waveguide for optical amplifiers. Opt Mater. 2014;38:198-203.

31. Intyushin EB, Novikov VA. Tungsten-tellurite glasses and thin films doped with rare-earth elements produced by radio frequency magnetron deposition. Thin Solid Films. 2008;516(12):4194-200.

32. Ferlauto AS, Ferreira GM, Pearce JM, Wronski CR, Collins RW, Deng X, et al. Analytical model for the optical functions of amorphous semiconductors from the near-infrared to ultra-violet: applications in thin film photovoltaics. J Appl Phys. 2002;92(5):2424-36.

33. Pietralunga SM, Lanata M, Feré M, Piccinin D, Cusmai G, Torregiani M, et al. High-contrast waveguides in sputtered pure TeO₂ glass thin films. Opt Express. 2008;16(26):21662.

34. Peiponen KE, Vartiainen EM. Kramers-Kronig relations in optical data inversion. Phys Rev B. 1991;44(15):8301-3.

35. De Sousa Meneses D, Malki M, Echegut P. Structure and lattice dynamics of binary lead silicate glasses investigated by infrared spectroscopy. J Non-Cryst Solids. 2006;352(8):769-76.

36. Hrabovsky J, Strizik L, Desevedavy F, Tazlaru S, Kucera M, Nowak L, et al. Optical, magneto-optical properties and fiber-drawing ability of tellurite glasses in the TeO₂-ZnO-BaO ternary system. J Non-Cryst Solids. 2024;624:122712.

37. Tauc J. Absorption edge and internal electric fields in amorphous semiconductors. Mater Res Bull. 1970;5(8):721-29.

38. Tauc J. Optical properties and electronic structure of amorphous Ge and Si. Mater Res Bull. 1968;3(1):37-46.

39. Sekiya T, Mochida N, Ohtsuka A, Tonokawa M. Raman spectra of Mo1/2–TeO2 (M = Li, Na, K, Rb, Cs and Tl) glasses. J Non-Cryst Solids. 1992;144:128-44.

40. Manikandan N, Ryasnyanskiy A, Toulouse J. Thermal and optical properties of TeO₂-ZnO-BaO glasses. J Non-Cryst Solids. 2012;358(5):947-51.

41. Shen SX, Jha A. Raman spectroscopic and DTA studies of TeO₂–ZnO–Na₂O tellurite glasses. Adv Mater Res. 2008;39–40:159–64.

42. Yadav AK, Singh P. A review of the structures of oxide glasses by Raman spectroscopy. RSC Adv. 2015;5(83):67583–609.

43. Santos FA, Figueiredo MS, Barbano EC, Misoguti L, Lima SM, Andrade LHC, et al. Influence of lattice modifier on the nonlinear refractive index of tellurite glass. Ceram Int. 2017;43(17):15201-4.

44. Chacalaza-Ricaldi J, Lozano C G, Clabel H JL, Ferri FA, Rodrigues AD, Messaddeq Y, et al. Influence of Pr³⁺ ions on the structural properties of Er³⁺-doped tellurite-tungsten glasses. J Non-Cryst Solids. 2023;616:122471.

45. Jlassi I, Elhouichet H, Ferid M. Thermal and optical properties of tellurite glasses doped erbium. J Mater Sci. 2011;46(3):806-12.

46. Rivera VAG, Ledemi Y, Pereira-da-Silva MA, Messaddeq Y, Marega Jr E. Plasmon-photon conversion to near-infrared emission from Yb³⁺: (Au/Ag-nanoparticles) in tungsten-tellurite glasses. Sci Rep. 2016;6(1):18464.

47. Kim S-H, Yoko T, Sakka S. Linear and nonlinear optical properties of TeO₂ glass. J Am Ceram Soc. 1993;76(10):2486–90.

48. Łapiński M, Walas M, Gapska A, Kulik D, Szmytke A, Twardowski P, et al. Structure and optical parameters of Eu doped tellurium oxide thin films prepared by reactive magnetron sputtering method. Thin Solid Films. 2019;691:137592.

49. El-Mallawany R, Abou-Taleb WM, Naeem MA, Krar ME, Talaat S. Synthesis, physical, optical properties and gamma-ray shielding parameters of some tellurite glasses. Optik. 2021;242:167171.

50. Güsken NA, Fu M, Zapf M, Nielsen MP, Dichtl P, Röder R, et al. Emission enhancement of erbium in a reverse nanofocusing waveguide. Nat Commun. 2023;14(1):2719.

51. Amirzada MR, Tatzel A, Viereck V, Hillmer H. Surface roughness analysis of SiO₂ for PECVD, PVD and IBD on different substrates. Appl Nanosci. 2016;6(2):215-22.

52. Aihara W, Shiota T, Sakurai O, Shinozaki K. Improvement of crystallinity and surface roughness in epitaxial CeO₂/Ce₁⁻ₓZrₓO₂/Y₀.₁₅Zr₀.₈₅O₁.₉₃ buffer layers deposited on a Si(100) substrate by pulsed laser deposition. J Ceram Soc Jpn. 2016;124(6):721-24.

53. Lai C-C, Boyd R, Svensson P-O, Höglund C, Robinson L, Birch J, et al. Effect of substrate roughness and material selection on the microstructure of sputtering deposited boron carbide thin films. Surf Coat Technol. 2022;433:128160.

54. Al-Mansoori M, Al-Shaibani S, Al-Jaeedi A, Lee J, Choi D, Hasoon FS. Effects of gas flow rate on the structure and elemental composition of tin oxide thin films deposited by RF sputtering. AIP Adv. 2017;7(12):125105.

**How to cite this article:** Ramanan P, Gutwirth J, Slang S, Viswanathan A, Ghanawi T, Starecki F, et al. Er³⁺-doped amorphous tellurite thin films. J Am Ceram Soc. 2025;e20589.
https://doi.org/10.1111/jace.20589