Structural, Electrical and Optical Properties of Zn-Doped SrVO3 Thin Films Grown By Co-Sputtering
Authors: Rouviller Axel , Jolivet Aline , Misiak Alex , Mezhoud Moussa , Labbé Christophe , Cardin Julien , Portier Xavier , Dufour Christian , Marie Philippe , David Adrian , Lüders Ulrike , Gourbilleau Fabrice 
Published: February 2024

Due to its optical and electrical characteristics, SrVO3 is a strongly correlated metal that has received extensive research in recent years. This makes it a promising transparent conducting oxide (TCO) for a variety of optoelectronic applications. The most widely used TCO at the moment, indium tin oxide, suffers from resource depletion. By analyzing and improving these interesting properties, SrVO3 might be able to take its place [1]. Unfortunately, in order to obtain SrVO3 as a crystallized phase, non-compatible with microelectronic industry thin film growth techniques must be used. Moreover, they require specific substrates for achieving the crystalline state, such as SrTiO3, LaAlO3, and (LaAlO3)0,3(Sr2TaAlO6)0,7 (LSAT) [2]. Nevertheless, recent studies made by our groups have demonstrated that crystalline SrVO3 layers may be produced on less expensive substrates such as glass or silicon substrates, with the simple use of a TiO2 buffer layer. This recent finding is covered by a global patent [3].

In the present work, we report experimental investigations on the reactively co-sputtering of SrVO3 and ZnO targets in a H-rich plasma, on Si substrates, with and without a TiO2 buffer layer, to grow transparent and conductive films. TiO2 buffer layer has been deposited on Si Substrate by Atomic Layer Deposition as reported elsewhere [4]. We looked at the effects of growth temperatures (TG) and the hydrogen rate rH (the ratio of H2 to Ar) on the thin films’ structural, electrical, and optical characteristics. XRD, high-resolution TEM, and AFM techniques were used to examine the films’ structural characteristics. The 4 probes approach, Van der Pauw measurements using a PPMS, and Hall effect measurements were used to examine the electrical properties. Finally, spectroscopic ellipsometry was used to conduct optical characterizations.

The structural analysis showed that it is possible to favor the growth of crystalline SrVO3 layers on top of the TiO2 buffer layer by optimizing TG and rH (Figure 1a). In some specific conditions, a partially crystallized layer of SrVO3 was also directly deposited on a Si substrate without the use of such a buffer layer, which has never been reported in the literature and thus is encouraging for the future growth of such material on-low-cost substrate (Figure 1b). The Zn concentration in the film is only 0.15 at% because the radio-frequency (RF) power density applied to the ZnO target is much lower than the one applied to the SrVO3 target. The presence of dopants during the development process may favor the crystallization of SrVO3, which may help to explain this partial crystallization. This finding might pave the way for buffer-free complete crystallization of SrVO3 on Si substrates.

When the films are grown on a TiO2 buffer layer, measurements of the physical properties have evidenced that it is possible to form thin films with optical transparency ranging from 70 to 75%, between 475 and 800 nm (Figure 1c). The thin films’ electrical resistivities at ambient temperature reach values of 1.2 × 10−3 Ohm.cm, according to electrical characterizations performed on them using the 4 probes method. Moreover, the PPMS data reveal a decrease in resistivity as a function of temperature (Figure 1d) which is the signature of semiconductor behavior. Such a feature has never been reported elsewhere and is probably due to an excess of oxygen in the layer induced 2 by the reactive growth approach. The vanadate films detailed in this paper present a sufficiently low resistivity to be used for microelectronics applications since the films produced with optimal values of rH and TG are more conductive than the undoped semiconductor films typically used as TCOs, such as ZnO and SnO2. In addition, these films feature an optical band gap, and therefore offer the opportunity to create materials with photoluminescence properties by doping them with rare earth ions for example. This is extremely promising for the design of light-emitting diodes or sensors.

[1] L. Zhang et al, “Correlated metals as transparent conductors,” Nature materials, vol. 15,12 2015.

[2] A. Boileau et al, “Tuning of the optical properties of the transparent conducting oxide SrVO3 by electronic correlations,” Advanced Optical Materials, vol. 7, p. 1801516, 01 2019.

[3] Patent FR3113185

[4] A. Jolivet et al, “Structural, optical, and electrical properties of TiO2 thin films deposited by ALD: Impact of the substrate, the deposited thickness and the deposition temperature,” Applied Surface Science, vol. 608, p. 155214, 2023.

Figure 1