# Interface control layer in BaySr1-yTiO3 based varactors for 5G technology

Santiago Alberto Agudelo Estrada, Nick Barrett, Christophe Lubin, Jérôme

Wolfman, Béatrice Negulescu, Mudit Upadhyay

► To cite this version:

Santiago Alberto Agudelo Estrada, Nick Barrett, Christophe Lubin, Jérôme Wolfman, Béatrice Negulescu, et al.. Interface control layer in BaySr1-yTiO3 based varactors for 5G technology. JNSPE 2023 - Journees Nationales Des Spectroscopies De Photoemission, May 2023, Sophia -Antipolis, France. hal-05400366

HAL Id: hal-05400366

https://hal.science/hal-05400366v1

Submitted on 5 Dec 2025

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# Interface Control Layer in Ba<sub>y</sub>Sr<sub>1-y</sub>TiO<sub>3</sub> Based Varactors for 5G technology

**Santiago Agudelo**<sup>a*</sup>, **Nick Barrett**<sup>a</sup>, **Christophe Lubin**<sup>a</sup>, **Jérôme Wolfman**<sup>b</sup>, **Beatrice Negulescu**<sup>b</sup>, **Mudit Upadhyay**<sup>b</sup>

*Contact: santiago.agudeloestrada@cea.fr

## Résumé

The deployment of 5G technology has raised issues of energy consumption, reception quality, and call failure rate. All three can be minimized by continuously adjusting the antenna impedance. A voltage controllable impedance matching circuit with a highly tunable, ferroelectric (FE) capacitance is the preferred solution. Voltage-tunable capacitors (varactors) based on perovskite ferroelectric solid solution BaSrTiO<sub>3</sub> (BST) have excellent tunability/losses compromise leading to a far better quality factor (i.e. low dielectric losses at GHz frequencies) than other competing technologies<sup>1</sup>. Working in 5G range or lower voltage requires the reduction of the film thickness. In thin films (~50 nm), the formation of a chemically distinct interface layer of up to 10 u.c. with altered dielectric properties and the increased leakage current can be detrimental to device performance<sup>2</sup>. Within the framework of an ANR collaborative project "Be-Polar", the idea is to mitigate these limitations by the introduction of an interface control layer or ICL, typically La<sub>1-x</sub>Sr<sub>x</sub>MnO<sub>3</sub> that allows engineering the chemistry, promotes atomic rearrangement and hence preserves the best dielectric response. For a proper integration of an ICL in BST based varactors, it is mandatory to perform a systematic study of the ICL chemistry (LSMOx with 0<x<1), and effect of BST overlayer thickness (0-10uc).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** XPS study of variable thickness BST on LSMOx Interface Control Layer. a) Sr 3d core-level, b) Ti 2p core-level, c) Mn 2p core-level, d) relative binding energy as a function of Sr</figcaption></figure>

Samples were prepared by combinatorial pulsed laser deposition (CPLD)<sup>3</sup> at GREMAN (Univ. Tours). CPLD allows fabrication of capacitors with continuously varying ICL stoichiometries and BST overlayer thickness in orthogonal directions on a single 10x10 mm<sup>2</sup> sample substrate (Fig.1a). The LSMOx ICL varies from LaMnO<sub>3</sub> to SrMnO<sub>3</sub> and the BST thickness from 0 to 10 u.c.. High-resolution core-level photoemission spectroscopy with lateral resolution on the scale of the stoichiometry gradient is used to study the BST/ICL LSMOx interface chemistry in BST/LSMOx/LSMO<sub>0.7</sub>.

The Ti 2p XPS spectra on BST films indicate the presence of two components: Ti<sup>4+</sup> and Ti<sup>3+</sup> (Fig.1b), consistent with the presence of V<sub>O</sub> at the surface of BST. The Mn 2p<sub>3/2</sub> XPS spectra (Fig.1c) present Mn<sup>3+</sup> and Mn<sup>4+</sup> species. The

results suggest that the Mn<sup>3+</sup> ions of LSMO are oxidized at the interface with BST. Crucially the change of the Mn<sup>3+</sup>/Mn<sup>4+</sup> ratio (for a specific BST overlayer thickness) leads to an insulating interface layer with respect to the stoichiometric LSMO<sub>0.7</sub> electrode<sup>4</sup>. In addition, the free surface showed evidence of Ba rich surfactant<sup>5</sup>. We found that overall the features of the valence band as well as the core levels are shifted as x was increased (Fig.1d), indicating the systematic chemical-potential shift of LSMO thin films with hole doping.

## Références

1. Tiggelman M. P. J., Reimann K., Van Rijs F., Schmitz J., Hueting R. J. E., IEEE Trans. Electron Devices, **2009**, *56*, 2128-2136

2. Stengel M., Vanderbilt D., Spaldin N., Nat. Mater., **2009**, *8*, 392-397

3. Wolfman J., Negulescu B., Ruyter A., Niang N., Jaber N., Practical Applications of Laser Ablation, **2020**

4. Jedrecy N., Jagtap V., Hebert C., Becerra L., Hrabovsky D., Barbier A., Portier X., Adv. Electron. Mater., **2021**, *7*, 2000723

5. Agudelo-Estrada S., Barrett N., Lubin C, Wolfman J., Negulescu B., Andreazza P., Ruyter A., EPJ Web Conf., **2022**, 273, 01008

## Remerciements

This project has received funding from the Agence Nationale de la Recherche under Grant No. ANR-20-CE24-0008-BePolar.

<footnote><sup>a</sup>SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, F-91191 Gif-sur-Yvette, France</footnote>

<footnote><sup>b</sup>GREMAN, UMR7347 CNRS, Univ. de Tours, Parc de Grandmont, F-37200 Tours, France</footnote>