Influence of deposition parameters on constitution, microstructure, and ionic conductivity of magnetron sputtered thin film solid state electrolytes in the material system Li-V-Si-O

C. Ziebert¹ , A. Knorr¹, N. Thiel¹, M. Rinke¹, K. Seemann¹, S. Ulrich¹,
D. Gerthsen², T. Schimmel³

¹ Karlsruhe Institute of Technology (KIT), Institute for Applied Materials, IAM-AWP, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany;

² KIT, Laboratory for Electron Microscopy, Kaiserstrasse 12, 76131 Karlsruhe, Germany;

³ KIT, Institute of Applied Physics, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe, Germany

In general the ionic conductivity in solid state electrolytes is smaller than in liquid systems. Thus there is a strong need for the development of thin film solid state electrolytes, which can compensate this handicap by reducing the diffusion path of lithium ions due to their small thickness. The material system Li-V-Si-O (LVSO) is a promising candidate for further optimisation as a thin film solid state electrolyte because a value of the ionic conductivity at room temperature of 1´10^-6 S/cm was already reached by using a ceramic target of the composition Li_3.4V_0.6Si_0.4O₄.

Due to the large number of parameters to be varied and optimized in such a multinary material system, the classic one-by-one trial and error approach was replaced in this work by a combinatorial materials science approach based on non-reactive magnetron sputtering from segmented targets in a Leybold Z550 coating device. In each experiment, several coatings of different composition and/or microstructure were simultaneously obtained by placing different substrates in individual positions relative to the segmented target. The thin films have been deposited onto Si and stainless steel substrates using two different segmented targets. Target 1 consisted of two half parts of circular LiVO₃ and SiO₂ ceramics, target 2 was composed of Li₄SiO₄ and V₂O₅.

The influence of a systematic variation of the deposition parameters sputter pressure (0.075 –25 Pa) and bias voltage (up to -150 V) on composition, crystal structure, morphology and topography was studied by using inductive coupled plasma optical emission spectroscopy, inert gas fusion analysis, X-ray diffraction, Raman spectroscopy, atomic force microscopy and scanning electron microscopy. Thin film properties such as the intrinsic stress and the density were determined by wafer bending and X-Ray reflectivity, respectively. The ionic conductivity was analyzed by impedance spectroscopy.

Films deposited from target 1 at a pressure of 0.15 Pa and a substrate bias of -40 V remained X-ray-amorphous and Raman-inactive even after a heat treatment in a furnace for 3 h at 600 °C in an argon/oxygen atmosphere (Ar:O₂ = 4.5:5) of 10 Pa. The ionic conductivity of these Li_1.33V_0.77Si_0.35O₄ films at room temperature was determined to be 2.8´10^-5 S/cm.

At a pressure of 0.5 Pa without bias voltage amorphous Li_1.2V_1.3Si_0.7O₄ thin films could be deposited from target 2 that showed an even higher ionic conductivity of up to
6.5´10^-5 S/cm, which is significantly higher than all values for the Li-V-Si-O system or any other thin film electrolyte system reported in literature up to now.

This clearly confirms the potential of the combinatorial materials science approach with a segmented target arrangement.