Student, IEEE Kepa Odriozola
email: kepa.odriozola@se.com

Fellow, IEEE Thierry A Meynard

Alain Lacarnoy
email: alain.lacarnoy@se.commeynard@laplace-univ.tlse.fr

Multi-Level Multiplexed

Keywords: Power Converter Topology for 1500V Applications Multilevel converter, Multiplexed topology, Uninterruptible Power Supply, Solar PV systems, ESS

come

I. INTRODUCTION

Over the last decade, there has been an increase in the installed capacity of utility scale solar PV and datacenter plants, and ESS is often strictly required to level the power delivered by high power PV plants. In these fields a power of several megawatts becomes quite standard and the power of some facilities even exceeds one hundred megawatts. This increase in power has also led to the increase of the DC bus voltage up to 1500V.

The choice of the 1500V standard voltage is mainly linked to economic gain. Although the length of PV panels and/or battery strings is longer than conventional installations, it allows saving on wiring, connections and combiner boxes, and reduces the number of inverter stations to be installed. This choice also increases the efficiency of the system, directly impacting the net profit of the operating company: omhic losses in DC conductors are reduced and allow using cables with a smaller cross-section and moreover the risk of system failure is lower which is translated into better overall reliability associated with reduced downtime because less DC components (wiring, connections, boxes, etc.). That said, 1500V is expected to be the next voltage class, which will reduce the total cost of PV and UPS systems on a large commercial and utility scale. These savings translate into a significant advantage that drives the growth of the 1500 V market; several GW/year and several MW/year are respectively expected in the PV and the UPS markets.

This new mainstream forces power converter manufacturers to redesign converter topologies with the aim of keeping the same levels of efficiency, cost and volume. From the introduction of the first well known multi-level topologies [START_REF] Nabae | A New Neutral-Point-Clamped PWM Inverter[END_REF][START_REF] Meynard | Multi-level conversion: high voltage choppers and voltage-source inverters[END_REF][START_REF] Marchesoni | A non conventional power converter for plasma stabilization[END_REF] to their industrial maturity, many applications have taken advantage of their improved performances [START_REF] Kieferndorf | A New Medium Voltage Drive System Based on ANPC 5L Technology[END_REF][START_REF] Rodriguez | Multilevel inverters: a survey of topologies, controls, and applications[END_REF][START_REF] Rodriguez | Multilevel Voltage-Source-Converter Topologies for Industrial Medium-Voltage Drives[END_REF].

Even if many industrial applications have been able to adopt these multi-level converter topologies, the transition to the 1500V standard is still a big challenge. Most manufacturers still choose to use traditional three-phase topologies such as classic 2L-VSI or multi-level topologies with 3 to 5 levels maximum. To meet the requirements of 1500V systems, topologies with a low number of output voltage levels require high blocking voltage semiconductor devices and topologies with a high number of output voltage levels, require a large number of low blocking voltage semiconductor devices, leading to high system cost. In addition, to achieve a high DC bus voltage, it may be necessary to serialize several switches and capacitors. The main challenge of these applications is to maintain a high efficiency at high voltage conversion ratio. This requirement is even stronger for UPS that need double conversion and for which losses are an extra cost for the user. In order to overcome the limitations and constraints of standard topologies at high voltage conversion ratio, some new proposals have been done. In [START_REF] Zhang | A multilevel converter topology with common flying capacitors[END_REF], it is proposed to share flying capacitors per each phase leg as a solution to high number of devices and high input voltage. However, each leg is generating almost the full amplitude of output voltage leading to high power switching losses. In [START_REF] Burguete | Multilevel voltage source conversion approach: Multiplexed Choppers[END_REF], with the introduction of Multiplexed choppers family concept, authors want to optimize the overall number of devices and efficiency by means of multiplexing the voltage generated by each stage. Nevertheless, extra balancing circuit is needed for regulating the dc bus points, leading to a bulky final solution. In [START_REF] Norambuena | Reduced Multilevel Converter: A Novel Multilevel Converter With a Reduced Number of Active Switches[END_REF] another topology with reduced number of switches is introduced but switches with a high voltage rating are required and the control is quite complex. Other recent developments in the field [START_REF] Concari | H8 Inverter for Common-Mode Voltage Reduction in Electric Drives[END_REF][START_REF] Concari | H8 architecture for reduced common-mode voltage three-phase PV converters with Silicon and SiC power devices[END_REF] show similar approach in the idea of reducing the common mode voltage but at the expense of higher cost and higher losses due to the need of bus balancer.

This paper proposes a new multilevel Multiplexed converter topology family [START_REF] Odriozola | Multi-Level Inverter Topologies for Medium and High Voltage Applications[END_REF] especially intended for high-power UPS, Solar PV systems and ESS applications, which achieves very high efficiency, up to 98,7% in single conversion architecture using only low-cost silicon (Si) low blocking voltage semiconductor devices.

The structure and conversion stage topologies of this MultiLevel Multiplexed topology will be described in Section. II. The control strategy for the proposed topology is the described in Section. III, starting with the basic principle and then describing more in depth chopper and inverter modulation. Simulation results are presented in Section. IV. Finally, the conclusions are given in Section. V.

II. NEW MULTIPLEXED CONVERTER TOPOLOGY

A. Power Converter Structure

The proposed power conversion architecture (introduced by Schneider Electric) is shown in Fig. 1 and it is composed of three parts: The input voltage source (PV strings, battery strings or conventional dc bus voltage made of capacitor banks as in typical back-to-back conversion systems), the two symmetric DC-DC Step-Down chopper stage and three phase DC-AC output inverter.

The main feature of the architecture is to supply the output three-phase DC-AC voltage inverter stage via two DCDC Step-Down chopper stage without any filtering elements between both stages. Thus, the inverter is directly cascaded by Top side (Top) and Bottom side (Bot) choppers and it plays the role of a multiplexer, hence the name of "Multiplexed Converter" (i.e. "xPlexed"). The proposed architecture can be considered as an entire single AC-DC (active rectifier) or DCAC (inverter) conversion stage. Depending on the application, several conversion configurations are possible. In this way, more than one conversion stage can be combined. For Solar PV system and ESS applications, a single conversion stage is needed (see Fig. 2), to connect the grid to a 1500V PV panel strings or battery pack system. For UPS application two conversion stages are required (see Fig. 3); a grid side rectifier or AFE (Active Front End) and inverter are connected in back-to-back configuration. A battery pack system can be connected via a DC-DC converter or even directly to the dc bus.

B. Converter Stage Topologies

In principle, the DC-DC chopper and DC-AC inverter can be any converter with two or more voltage levels. However, for a DC bus at 1500V we will consider different combinations of multi-level topology from 3 to n levels, where n is the number of voltage levels at the output of each converter. Fig. 4 shows the power circuit of the three-phase 5-level Multiplexed DC-AC inverter with 3-level Flying Capacitor chopper and 3L-TNPC inverter configuration. In this paper we will mainly focus on this configuration. The output voltage levels depend mainly on the number of levels of the chopper and inverter stage. Moreover, for a given topology for both the DC-DC chopper stage and the DC-AC inverter stage the output voltage levels will depend on the amplitude of the output voltage. In this way, for a Multiplexed topology consisting of 3-level chopper and inverter stages, under rated conditions, it is possible to obtain a five-level output voltage waveform at the output terminals of the converter.

C. Semiconductor selection

One of the main features being efficiency and low cost, the selection of semiconductors becomes essential. Only silicon components are used in order to reduce the total cost to the maximum but of course wide band gap components could be used to reduce losses and so improve efficiency. The first originality of this topology is that the choppers deliver square voltage waveforms with steps of one fourth of the DC bus; assuming that an appropriate control pattern is used, the voltage applied to the inverter switches may thus be limited to 3/4 or even 1/2 the DC bus voltage, which of course helps maintaining a high efficiency. In our case, the inverter is realized with with 1200V, 75A High Speed IGBTs IKY75N120CS6 from Infineon. The typical voltage across the semiconductors of the flying capacitor chopper is 1/4 of the DC bus voltage, and 650V, 75A Low-VCE,sat IGBTs IKZ75N65ES5 from Infineon have been selected here. In order to handle the high current of the load and to reduce the conduction losses, each switch consists of four devices in parallel.

III. CONTROL STRATEGY FOR XPLEXED ARCHITECTURE

A. Basic principle

Unlike conventional three-phase topologies, the multiplexed topology does not have independent phase legs. Therefore each conversion stage module must be controlled separately. As a basic principle of modulation, sinusoidal modulation can be used to make it simple. The main concept (see Fig. 5) is to generate the highest (in green) and the lowest (in red) part of the reference AC waveforms directly by the corresponding Top and Bot choppers so that at least two phases of the inverter can be saturated, the one connected to the phase with the highest voltage reference being saturated to a 100% duty cycle, the one connected to the phase with the lowest voltage reference saturated with a 0% duty cycle. Only the inverter leg connected to the phase with the intermediate voltage reference needs to switch to follow the sinusoidal middle reference (in blue). So, at any given moment there is only one inverter arm switching.

B. Chopper Modulation

Regarding the control of DC-DC choppers, the main objective is to generate the highest and lowest side of the reference voltage. The chopper section involving capacitors has a strong requirement to ensure that the average current is zero and that the voltage of this capacitor can be regulated. Another objective will be to minimize the static voltage seen by the inverter stage in order to respect the voltage rating of the inverter switches. As a first approach, self balancing mechanism can be tried by applying a carrier based PWM modulation scheme such as Phase Shifted (PS) [START_REF] Meynard | Multicell Converters: Basic Concepts and Industry Applications[END_REF]. Another possibility is to use Phase Disposition (PD) by means of state machines like in [START_REF] Cougo | PD modulation Scheme for Three-Phase Phase Parallel Multilevel Inverters[END_REF] and [START_REF] Mcgrath | Optimal Modulation of Flying Capacitor and Multicell Converters Using a State Machine Decoder[END_REF], but in this kind of schemes extra commutations are needed and the balancing of flying capacitor could become complex. We therefore use PS sawtooth carriers (cf. Fig. 6). Sawtooth type carriers at 8kHz switching frequency are used for having the absolute control of turn-on and turn off moments, or rising-up and fallingdown instants of the voltage created by each chopper. Rising sawtooth carriers for the Top chopper and falling sawtooth carriers for the BOT chopper have been selected. In order to minimize the static voltage seen by the inverter stage over the majority operating points, there will be a relative phase shift of some switching periods between the Top and Bot choppers carrier signals. In this way when high modulation index is required, consecutive steps of voltage levels on the differential voltage of the chopper outputs can be avoided. The feature of the Flying Capacitor converter to double the apparent frequency at the output leads to a chopper switching frequency equal to half that of the inverter. Consequently, the switching losses are reduced too. This switching frequency ratio between chopper and inverter must be respected if the calculation of the inverter duty cycle (explained in the next section) is to be accurate.

C. Inverter Modulation

The voltage generated at the output of the inverter must be synthetized by means of the voltage already switched by choppers. Hence, duty cycle for inverter is defined in function of choppers duty cycles and inverter voltage reference. For that, duty cycles of each output inverter arm are determined separately by means of two-dimensional look-up-tables using a precalculated PWM method. Contrary to the disadvantages of this type of techniques [START_REF] Leon | The Essential Role and the Continuous Evolution of Modulation Techniques for Voltage-Source Inverters in the Past, Present, and Future Power Electronics[END_REF][START_REF] Dahidah | A Review of Multilevel Selective Harmonic Elimination PWM: Formulations, Solving Algorithms, Implementation and Applications[END_REF], in this case, the computational cost of the algorithm is not high and duty cycles can be calculated with a high accuracy. The calculation of duty cycles is made offline throughout two piecewise interpolations. For that, first of all, in Eq. 1 a virtual inverter average voltage V m(i) is calculated using a default chopper duty cycle array and its associated chopper output mean voltage V c(i) as well as a default inverter duty cycle array αm(j) for ∀ i,j ∈ [1,n] with n being the length of the array. This virtual inverter average voltage is the result of what is obtained at the output of inverter for a given duty cycle. The final duty cycle α 0 m is calculated in Eq. 2 using the default stored array values and previously calculated inverter duty cycle values.

The two dimensional lookup tables are filled with αc 0 , V 0 m and α 0 m. The result is a non-linear function α 0 m = f(αc 0 ,V 0 m), as shown in Fig. It can be observed that for high values of chopper duty cycle and V 0 m, the inverter duty cycle is saturated to 100% duty cycle. And vice versa when is always high but V 0 m is low therefore is saturated to 0%. Hence, the applied modulation scheme has the intrinsic property of freezing the inverter phase legs when necessary. Once the duty cycle is computed, whatever scheme of modulation could be applied for the inverter output stage. Gate signals must be generated in function of selected topology. For our configuration, three-phase three-level TNPC has been chosen as inverter. For being compatible with the chopper modulation and mainly with the inverter duty cycle offline calculation we must choose carriers at 16kHz switching frequency such that they are in counterposition to the chopper carriers as shown in Fig. 6. In this case, the upper switch cell must depend on the carriers of the Top chopper and vice versa. Moreover, each switch cell will guarantee ZVS operation for inverter arms (at least one turn-on or turn-off per switching period in function of the selected rising or falling front carrier configuration) thanks to the extra slight phase shift introduced between Top and Bot choppers and inverter stage carriers. The generated PWM signals for each inverter leg are depicted in Fig. 8. Consequently, the desired switching pattern is obtained for the inverter, thus only one phase leg is swiching at a time.

IV. SIMULATION RESULTS

The Multiplexed converter has been simulated in the PLECS ® software using specifications described in Table I. Fig. 9 shows the multilevel performance of the three-phase inverter (illustrated in Fig. 4) in open loop during steady state over one period of fundamental output frequency. It can be seen that natural balance of the flying capacitors voltage can be achieved through the control algorithm, which allows to generate a five-level output voltage waveform in each phase of the converter.

To evaluate the efficiency of the proposed topology, it will be compared with an existing three-phase multi-level topology: the five-level flying capacitor converter (5-level FCC). This comparison has been carried out using the specifications that are given at Table I, for the same number of output voltage levels, the same rated power, unity power factor and under the assumption that the junction temperature within the semiconductor devices remains constant at 75°C at steady state (see Fig. 10). The proposed topology is able to achieve a maximum efficiency of 98,7% using only silicon components. V. CONCLUSIONS

In this paper, a new multilvel converter topology was proposed. This topology has the ability to achieve a high step down voltage conversion ratio which makes it suitable for new low-medium voltage applications with wide range input voltage such as PV solar inverters and UPS systems. The fact of sharing serialized stages and so cascading inverter by choppers without intermediate filtering elements leads to a reduction of the voltage applied to the switches of the inverter (in some cases ZVS operation is possible), thus reducing the switching losses and indirectly the conduction losses (since switches with a lower voltage rating can be used). The switching losses in the inverter are also reduced because the Top (Max) and Bot (Min) voltage waveforms are synthesized by the choppers, so at any time only one of the inverter legs is switching. These features enables switching at relatively high frequency and helps in the end to obtain a high power density. Moreover, compared to conventional three-phase multilevel topologies it reduces the number of switches thanks to a form of mutualization of chopper stages and besides it uses only low-cost silicon low blocking voltage semiconductors devices reducing the overall cost of converter. An appropriate control strategy for the novel topology is presented. Simulations have been conducted on the PLECS ® software environment with good results and it is shown that the proposed solutions should bring a high efficiency for the coming prototype.

Fig. 1 :

 1 Fig. 1: Proposed Multi-Level Semi-Multiplexed DC-AC Power Converter Architecture.

Fig. 2 :

 2 Fig. 2: Solar PV system or ESS application conversion architecture using xPlexed topology

Fig. 3 :

 3 Fig. 3: UPS application conversion architecture using xPlexed topology

Fig. 4 :

 4 Fig. 4: Proposed 5-level Multiplexed DC-AC Inverter with 3-level-Flying Capacitor Chopper topology and 3-level-T type NPC Inverter topology

Fig. 5 :

 5 Fig. 5: Voltage reference signals for chopper and inverter stage. green: Top chopper; blue: Inverter; red: Bot chopper

Fig. 6 :

 6 Fig. 6: Phase Shifted (PS) PWM carrier configuration for Top and Bot choppers (8kHz) and Inverter stage (16kHz)

7

 7

Fig. 7 :

 7 Fig. 7: Data points of inverter duty cycle for 2D-lookup tables.

Fig. 8 :

 8 Fig. 8: Inverter output phase duty cycle and gate control signals. Three phase voltage reference signals (a); Duty cycles for the top side of the three-phase inverter (b); Gate signals for the outer top switches of the inverter (c); Duty cycles for the bot side of the three-phase inverter (d); Gate signals for the outer bot switches of the inverter (e).

Fig. 9 :

 9 Fig. 9: Multiplexed inverter output voltage referenced to the DC-link midpoint and corresponding filtered phase load voltage, VU0(t), VU(t) (a). Filtered threephase load voltage, VU,V,W(t) (b).

Fig. 10 :

 10 Fig. 10: Effiency comparison and power loss breakdown for three-phase 5level xPlexed converter and three-phase 5-level FCC using specifications of Table I. Left (a): VLoad = 400V; Right (b): VLoad = 480V

TABLE I :

 I Specifications and characteristics of the Multiplexed Converter.

	Parameter	Symbol	Value
	Nominal Power DC	PN	50 kW
	bus voltage Load voltage Output frequency	VDC VLoad fLoad	900-1500 V 400V-480 V 50-60 Hz
	Chopper switching frequency	fChop fSw 8 kHz
	Inverter switching frequency		16 kHz
	Flying Capacitor	Cfly	150 µF
	Load Filter inductance	Lf	200 µH
	Load Filter capacitance	Cf	50 µF

  Table II and Table ??), making the converter an interesting solution in terms of efficiency, volume and price.

TABLE II :

 II Number of switch semiconductors in compared three-phase 5-level Converters.

	Topology	N° Switches	Max. Blocking
			Voltage
	5-level
	5-level FCC	24	VDC/4

TABLE III :

 III Number of Flying Capacitors in compared three-phase 5-level Converters.

	Topology	Flying Capacitors	Capacitance	Rated Voltage
	5-level xPlexed	2	150µF	VDC/4
		3	150µF	VDC/4
	5-level FCC	3	150µF	VDC/2
		3	150µF	3VDC/4


Martin Mickan
email: martin.mickan@univ-orleans.fr

Pierre Coddet

Julien Vulliet

Amael Caillard

Thierry Sauvage

Anne-Lise Thomann

Optimized magnetron sputtering process for the deposition of Gadolinia doped Ceria layers with controlled structural properties

Keywords: Reactive magnetron sputtering, Gadolinia doped ceria, Thin films

Gadolinia doped ceria (GDC) films have been deposited using reactive magnetron sputtering and the influence of the deposition parameters on the properties of the films has been investigated. The oxygen flow rate, the target-substrate distance and the radio frequency substrate bias have been varied and films were deposited with a relatively high power-normalized growth rate of 15-18 nm min -1 W -1 cm². Depending on the target-substrate distance and on the oxygen flow rate fully oxidized as-deposited films were obtained or substoichiometric films, which need to be oxidized by an ex-situ thermal annealing step. These two manufacturing routes (direct deposition of a fully oxidized compound or post-oxidation of a sub-stoichiometric oxide) lead to different preferential orientations ([200] or [111]) of the final oxide film. This is attributed to the oxygen/metal flux ratio, which is modified according to the experimental configuration and the process parameters. Furthermore, the substrate bias was found to further the [111] orientation and thus playing with this parameter allows a fine tuning of the preferential orientation.

Introduction

Solid oxide fuel cells, which can be reversibly used for high temperature steam electrolysis, are subject to chemical reactions during operation that can degrade their performance and therefore limit their lifetime. Especially in devices made of yttria-stabilized zirconia (YSZ) electrolytes and La1-xSrxCo1-yFeyO3-δ (LSCF) electrodes, the diffusion of Sr can lead to the formation of insulating SrZrO3 [START_REF] Tu | Ln0.4Sr0.6Co0.8Fe0.2O3-δ (Ln=La, Pr, Nd, Sm, Gd) for the electrode in solid oxide fuel cells[END_REF][START_REF] Sase | Interfacial reaction and electrochemical properties of dense (La,Sr) CoO3-δ cathode on YSZ (1 0 0)[END_REF]. Gadolinia doped ceria (GDC) thin films can be used in such devices as a Sr diffusion barrier in order to extend their lifetime [START_REF] Uhlenbruck | Thin film coating technologies of (Ce,Gd)O2-δ interlayers for application in ceramic hightemperature fuel cells[END_REF][START_REF] Sønderby | Magnetron sputtered gadolinia-doped ceria diffusion barriers for metal-supported solid oxide fuel cells[END_REF].

GDC layers are classically prepared by ceramic techniques such as screen printing [START_REF] Berkel | Enhanced ASC Performance at 600°C by Ceria Barrier Layer Optimisation[END_REF] or wetchemical techniques such as spin coating [START_REF] Plonczak | A high performance ceria based interdiffusion barrier layer prepared by spin-coating[END_REF]. However, the deposition of thin and dense layers is difficult with these techniques with typical thicknesses of a few micrometers [START_REF] Coddet | Characteristics and properties of a magnetron sputtered gadolinia-doped ceria barrier layer for solid oxide electrochemical cells[END_REF]. Physical vapor deposition techniques such as pulsed laser deposition [START_REF] Knibbe | Cathode-Electrolyte Interfaces with CGO Barrier Layers in SOFC[END_REF], electron beam evaporation [START_REF] Hong | Fabrication and characterization GDC electrolyte thin films by e-beam technique for IT-SOFC[END_REF] or magnetron sputtering can be used to produce high quality films with a controlled thickness (less than 1 µm). Among these techniques, magnetron sputtering offers the advantage of easy scalability to industrial processes. There are several reports of the deposition of GDC films using both radio frequency (RF) [START_REF] Kuo | Gadolinia-doped ceria films deposited by RF reactive magnetron sputtering[END_REF][START_REF] Lin | Characterization of electrolyte films deposited by using RF magnetron sputtering a 20mol% gadolinia-doped ceria target[END_REF][START_REF] Kuo | Growth of 20mol% Gd-doped ceria thin films by RF reactive sputtering: The O2/Ar flow ratio effect[END_REF][START_REF] Jiang | Fabrication of epitaxial zirconia and ceria thin films with arbitrary dopant and host atom composition[END_REF][START_REF] Wu | Influence of Deposition Temperature of GDC Interlayer Deposited by RF Magnetron Sputtering on Anode-Supported SOFC[END_REF][START_REF] Park | Effect of the thickness of sputtered gadolinia-doped ceria as a cathodic interlayer in solid oxide fuel cells[END_REF][START_REF] Bae | Three-dimensional hexagonal GDC interlayer for area enhancement of low-temperature solid oxide fuel cells[END_REF][START_REF] Kuo | Sputter-deposited 20mol% gadolinia-doped ceria films on 8mol% yttria-stabilized zirconia tapes for improved electrochemical performance[END_REF][START_REF] Kuo | Mechanism of oxygen ion diffusion in Gd-doped ceria electrolyte films deposited via reactive and direct sputtering[END_REF][START_REF] Coppola | Structural and Electrical Characterization of Sputter-Deposited Gd0.1Ce0.9O2-δ Thin Buffer Layers at the Y-Stabilized Zirconia Electrolyte Interface for IT-Solid Oxide Cells[END_REF] and direct current (DC) [START_REF] Sønderby | Magnetron sputtered gadolinia-doped ceria diffusion barriers for metal-supported solid oxide fuel cells[END_REF][START_REF] Coddet | Characteristics and properties of a magnetron sputtered gadolinia-doped ceria barrier layer for solid oxide electrochemical cells[END_REF][START_REF] Gourba | Electrical properties of gadoliniadoped ceria thin films deposited by sputtering in view of SOFC application[END_REF][START_REF] Briois | A comparison of electrical properties of sputter-deposited electrolyte coatings dedicated to intermediate temperature solid oxide fuel cells[END_REF][START_REF] Brahim | Electrical properties of thin bilayered YSZ/GDC SOFC electrolyte elaborated by sputtering[END_REF][START_REF] Fonseca | Properties of bias-assisted sputtered gadolinia-doped ceria interlayers for solid oxide fuel cells[END_REF][START_REF] Fonseca | Temperature and Bias Effects on Sputtered Ceria Diffusion Barriers for Solid Oxide Fuel Cells[END_REF][START_REF] Klemensø | High performance metal-supported solid oxide fuel cells with Gd-doped ceria barrier layers[END_REF][START_REF] Sakaliūnienė | Investigation of Structural and Optical Properties of GDC Thin Films Deposited by Reactive Magnetron Sputtering[END_REF][START_REF] Sønderby | Strontium Diffusion in Magnetron Sputtered Gadolinia-Doped Ceria Thin Film Barrier Coatings for Solid Oxide Fuel Cells[END_REF][START_REF] Solovyev | Deposition of a Thin-Film CGO Electrolyte for Solid Oxide Fuel Cells[END_REF][START_REF] Ionov | Reactive co-sputter deposition of nanostructured cermet anodes for solid oxide fuel cells[END_REF][START_REF] Solovyev | Magnetron-Sputtered YSZ and CGO Electrolytes for SOFC[END_REF][START_REF] Londono | Properties of Gadolinium-doped Ceria (GDC) Films Deposited by Reactive Magnetron Sputtering Processes[END_REF][START_REF] Coddet | YSZ/GDC bilayer and gradient barrier layers deposited by reactive magnetron sputtering for solid oxide cells[END_REF] magnetron sputtering, as well as high power impulse magnetron sputtering (HiPIMS) [START_REF] Rabotkin | Process stabilization during high power impulse magnetron sputtering of Ce/Gd target[END_REF][START_REF] Solovyev | Magnetron Sputtering of Gadolinium-doped Ceria Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells[END_REF]. The different methods have each their own advantages and drawbacks. One problem is the deposition rate. While high deposition rates can be generally achieved in DC or RF sputtering, HiPIMS (leading to dense layers) generally gives lower deposition rates. Another problem is the cost of the deposition equipment, which is the lowest for DC sputtering. For this work, we chose to use reactive pulsed DC sputtering, because it is a relatively low cost process, which can be tuned to obtain high deposition rates.

While many of the publications on sputtering of GDC focus on the properties of the deposited GDC films [START_REF] Kuo | Gadolinia-doped ceria films deposited by RF reactive magnetron sputtering[END_REF][START_REF] Lin | Characterization of electrolyte films deposited by using RF magnetron sputtering a 20mol% gadolinia-doped ceria target[END_REF][START_REF] Wu | Influence of Deposition Temperature of GDC Interlayer Deposited by RF Magnetron Sputtering on Anode-Supported SOFC[END_REF][START_REF] Park | Effect of the thickness of sputtered gadolinia-doped ceria as a cathodic interlayer in solid oxide fuel cells[END_REF][START_REF] Bae | Three-dimensional hexagonal GDC interlayer for area enhancement of low-temperature solid oxide fuel cells[END_REF][START_REF] Kuo | Sputter-deposited 20mol% gadolinia-doped ceria films on 8mol% yttria-stabilized zirconia tapes for improved electrochemical performance[END_REF][START_REF] Coppola | Structural and Electrical Characterization of Sputter-Deposited Gd0.1Ce0.9O2-δ Thin Buffer Layers at the Y-Stabilized Zirconia Electrolyte Interface for IT-Solid Oxide Cells[END_REF][START_REF] Gourba | Electrical properties of gadoliniadoped ceria thin films deposited by sputtering in view of SOFC application[END_REF][START_REF] Briois | A comparison of electrical properties of sputter-deposited electrolyte coatings dedicated to intermediate temperature solid oxide fuel cells[END_REF][START_REF] Brahim | Electrical properties of thin bilayered YSZ/GDC SOFC electrolyte elaborated by sputtering[END_REF][START_REF] Sakaliūnienė | Investigation of Structural and Optical Properties of GDC Thin Films Deposited by Reactive Magnetron Sputtering[END_REF][START_REF] Ionov | Reactive co-sputter deposition of nanostructured cermet anodes for solid oxide fuel cells[END_REF][START_REF] Londono | Properties of Gadolinium-doped Ceria (GDC) Films Deposited by Reactive Magnetron Sputtering Processes[END_REF][START_REF] Coddet | YSZ/GDC bilayer and gradient barrier layers deposited by reactive magnetron sputtering for solid oxide cells[END_REF], in this work, we want to focus on the process itself and on the optimization of the deposition conditions in order to achieve dense, stoichiometric GDC films at a high growth rate. In addition, we studied the influence of the process parameters such as the target-substrate distance, the oxygen flow rate and the substrate bias on the crystallographic orientation of the films. We investigated two routes, one furthering the deposition of fully oxidized films and another including the deposition of substoichiometric films followed by an ex-situ thermal oxidation treatment.

Materials and methods

GDC films have been deposited by reactive sputter deposition in a 35 L vacuum chamber using a 4-inch Ce0.8Gd0.2 target clamped on a balanced magnetron. A schematic of the chamber is shown in Figure 1. The target is powered in pulsed regime pDC (frequency of 50 kHz and an off-time of 5μs) using an Advanced Energy Pinnacle+ power supply regulated with a constant current (mean current of 1 A). Depending on the operating parameters (such as the oxygen flow rate), the mean voltage varies between 125 and 230 V, leading to a power range of 125-230 W.

The gas flow rate was fixed using Bronkhorst EL-FLOW mass flow controllers. The Ar flow rate was set to 20 sccm and the total pressure in the chamber was adjusted to 1 Pa using a gate valve before turning on the discharge. For one experiment, a higher Ar flow rate of 80 sccm was used in order to assess the influence of the pumping speed on the reactive sputter process. The pressure was measured using a capacitive gauge (Pfeiffer CCR375). A rotating substrate holder was used, which held pieces of [100] oriented P-doped Si wafers. The distance between the substrate and the target was fixed at 12.7 or 16.0 cm as shown in Figure 1. For some deposits, an RF bias of -40 to -80 V was applied on the substrate using an Advanced Energy Cesar 136 RF power supply corresponding to a delivered power of 4-9W and a reflected power of less than 1W. The deposition time for most of the samples was 15 min, leading to a film thickness of 600-700 nm. The influence of the gas inlet position was studied at two different locations (see gas inlet position 1 and 2 in Figure 1).

The deposition of the films took place in the metallic mode to achieve a high deposition rate of about 40 nm min -1 . In order to fully oxidize the films, in some cases a heat treatment after the deposition is necessary. This heat treatment was performed in air in a furnace (Nabertherm model LT5/13/B180) at 500°C with a heating rate 1°C/min and maintained at the maximum temperature for 1min as described in a previous work [START_REF] Coddet | Characteristics and properties of a magnetron sputtered gadolinia-doped ceria barrier layer for solid oxide electrochemical cells[END_REF].

The chemical composition of the deposited layers was analyzed using Rutherford backscattering spectrometry (RBS) with a 2 MeV He ion beam in a Pelletron accelerator at the CEMHTI laboratory, Orleans (scattering angle of 165°). The experimental data were plotted using the software RUMP [START_REF] Doolittle | Algorithms for the rapid simulation of Rutherford backscattering spectra[END_REF] and RBS spectra were fitted with simulated data in order to calculate the chemical composition. The morphology of the deposited films was investigated using a scanning electron microscope (Carl Zeiss SMT, Supra-40, FEG-SEM). Additionally, energy dispersive Xray spectroscopy (EDX, Bruker XFlash Detector 4010) was used to investigate the composition of the films. For the investigation of the crystalline properties, symmetrical θ/2θ scans were performed using a Bruker D8 Discover X-ray diffractometer in the parallel beam geometry with a Cu anode (Cu Kα line).

Results

In order to define the process parameters, the reactive sputtering of the CeGd target was investigated by following the target voltage evolution with respect to the oxygen flow rate between 0 and 6 sccm with a waiting time of 2 min between each point. Figure 2(a) shows the typical evolution of the discharge voltage for a pumping speed of 34 L s -1 (Ar flow rate of 20 sccm), while the corresponding evolution of the pressure is shown in Figure2(b). When the oxygen flow rate is increased, the discharge voltage stays at a high value of around 220 V at first. This is the metallic mode of the reactive sputtering process in which the target surface is in a metallic state and the oxygen is consumed by the growing film. Above a critical point at 2.5 sccm the discharge voltage decreases sharply (transition mode) to reach a lower level of 130-160V. As this low level is reached, the process is in the oxide mode, where the target is covered by an oxide layer. When decreasing the oxygen flow rate, the process encounters a transition back to the metallic mode, and a hysteresis behavior is observed. However, the discharge voltage does not directly return to its initial value. Instead, the target needs to be sputtered in a pure Ar atmosphere for more than 60 min as was observed in previous work [START_REF] Coddet | Characteristics and properties of a magnetron sputtered gadolinia-doped ceria barrier layer for solid oxide electrochemical cells[END_REF]. The same behavior can also be seen in the pressure curve, but less pronounced. This problem may be related to the high reactivity of Ce and Gd towards oxygen. Ershov et al. [START_REF] Ershov | Deposition of cerium oxide thin films by reactive magnetron sputtering for the development of corrosion protective coatings[END_REF] also observed that the voltage does not return to the initial value after a hysteresis experiment. In other publications, the voltage does return to its initial value as soon as 0 sccm oxygen is reached [START_REF] Solovyev | Magnetron-Sputtered YSZ and CGO Electrolytes for SOFC[END_REF][START_REF] Van Steenberge | Influence of oxygen flow and film thickness on the texture and microstructure of sputtered ceria thin films[END_REF]. This may be related to the relatively high pumping speed used in these works, i.e. 236 L s -1 [START_REF] Solovyev | Magnetron-Sputtered YSZ and CGO Electrolytes for SOFC[END_REF] or 109 L s -1 [START_REF] Van Steenberge | Influence of oxygen flow and film thickness on the texture and microstructure of sputtered ceria thin films[END_REF]. In order to test this hypothesis, we performed a hysteresis experiment at an Ar flow rate of 80 sccm and adjusted the pumping speed to 135 L s -1 , in order to keep the pressure (before adding oxygen) at 1 Pa. The resulting voltage and pressure curves are also shown in Figure 2. The lower slope in the pressure curve is due to the higher pumping speed. In this case, the critical point is shifted to a higher oxygen flow rate of 4.5 sccm.

When decreasing the oxygen flow rate, the voltage starts already to increase at an oxygen flow rate of 1.5 sccm. The initial value of the discharge voltage is reached after sputtering in pure Ar for 8 minutes. At high pumping speed the residence time of oxygen species close to the target is low, hindering the poisoning of the target and therefore limiting the growth of an oxide layer on the target. Thus, a high pumping speed allows the target to return to its initial state more easily.

However, in order to avoid overheating of the pump and excessive Ar consumption, all other experiments in the present work were performed with a pumping speed of 34 L s -1 . Thus, particular care had to be taken in order to avoid entering the oxide mode, in order to impede target poisoning.

In follow-up experiments, the oxygen flow rate was increased up to 4 sccm with the same increasing rate (0.5 sccm/2 min) and then kept at this fixed value during a time where the target was sputtered in the oxide mode. After this time (varying between 2 and 80 min), the oxygen flow rate is set to 0 sccm. The time that was needed for the voltage to return to its initial value (230 V) was measured from that moment. Figure 3(a) displays the evolution of the voltage during the ramp of increasing oxygen flow rate, during the time where the O2 is fixed to 4 sccm and during the time the voltage progressively increases without the presence of O2. Figure 3(b) shows that the time needed to return to the metallic mode increased with the time that the target was sputtered in the oxide mode. This suggests that the oxide layer continues to grow during the sputtering process in the oxide mode. This is an evidence of the high reactivity of Ce and Gd towards oxygen. Due to this property, the oxygen poisoning of the target surface is difficult to remove, especially when the metal surface is exposed to oxygen during a long period. It should also be noted that the target voltage in the oxide mode keeps increasing, even though the oxygen flow rate is kept constant, as can be seen in Figure 3(a). This is another hint that the target oxidation process does not reach a steady state in the oxide mode and could be related to the continued increase of the oxide layer thickness during sputtering.

In order to avoid excessive consumption of target material, further experiments were stopped (oxygen flow rate set to 0 sccm) as soon as the critical point was surpassed and the target could return to the metallic mode in less than 10 min. The evolution of discharge voltage and pressure was recorded by increasing the oxygen flow rate by 0.5 sccm steps with a waiting time of 2 min at each step, while operating the discharge with a constant current of 1 A. This is shown in Figure 4 for two different target-substrate distances (12.7 and 16.0 cm). For the gas inlet position 1 (Figure 4(a)), the critical point was shifted from 3 sccm at the shorter distance to 4 sccm at the longer distance.

In order to investigate the reason for the shift in the critical point of the transition, the gas inlet position 2 was tested (close to the bottom of the vacuum chamber below the magnetron as shown in Figure 1). The experiments were repeated for both target-substrate distances as plotted in Figure 4(b). In this case, the critical point was found at the same flowrate of 3 sccm for both targetsubstrate distances. This suggests that placing the gas inlet close to the bottom of the chamber (position 2) leads to a higher localized oxygen concentration at the target, whereas placing the gas inlet between the target and substrate induces a higher localized oxygen concentration close to the substrate and the chamber walls in front of the target. These areas are submitted to the sputtered atom flux and thus are coated by a thin metallic film sensitive to oxidation. As the target-substrate distance increases, a larger part of the sputtered atoms is deposited on the chamber walls meaning on a larger surface. This leads to an increased global consumption of oxygen, so a higher oxygen flow rate is necessary to reach the transition from metallic to oxide mode. This behavior is thus explained by the change in the relative consumption of oxygen at the substrate and the chamber walls, and at the target.

An oxygen flow rate close to the critical point was chosen for the deposition of GDC films, to further the incorporation of oxygen inside the film during the deposition, while keeping the deposition rate high and avoiding target poisoning [START_REF] Coddet | Characteristics and properties of a magnetron sputtered gadolinia-doped ceria barrier layer for solid oxide electrochemical cells[END_REF]. The gas was introduced at inlet position 1, in order to be able to use higher oxygen flow rates at a larger target-substrate distance. For targetsubstrate distances of 12.7 and 16.0 cm, the oxygen flow rate was set to 2 and 3 sccm, respectively.

In both cases, the target voltage is equal to 230 V which means an average power of 230 W applied on this 4-inch target (specific power density of 2.8 W cm -²).

First, the chemical composition of the films was investigated. EDX spectra were measured on a thick (>1µm) GDC film, to verify the Ce/Gd ratio. These results show, that the films contain a Ce/Gd ratio of 80%/20%, the same as the target. In order to evaluate the oxygen content of the films, RBS spectra were recorded on several deposited films, shown in Figure 5. The black line shows the fitted simulation that was obtained using the software RUMP. The fitted parameters include the concentration x of oxygen in Ce0.8Gd0.2Ox, the thickness of the films and a roughness factor (corresponding to the average thickness deviation expressed in 10 15 at/cm²) to take the surface roughness into account. The Ce/Gd ratio was fixed to 80%/20%, as confirmed by the EDX analysis. After deposition at the shortest target-substrate distance, the films are not fully oxidized as evidenced by the RBS spectra shown in Figure 5(a,b,c). The as-deposited films can be fitted by a model using a thin layer at the extreme surface that exhibits an oxygen concentration gradient and a thicker sublayer that has a constant oxygen concentration corresponding to Ce0.8Gd0.2O1.0 (floating and -40 V) or Ce0.8Gd0.2O0.9 (-80 V). Only in two cases (Figure 5(a) and (c) corresponding to thin films deposited at short distance and with a floating / -80 V biased substrate), the experimental data is not well fitted by the model at the interface between the layer and the substrate.

The presence of the thin top layer with an oxygen gradient at the extreme surface can be explained by the incorporation of oxygen inside the films when the samples are exposed to air. This confirms that the as deposited films obtained at low substrate/target distance are not saturated in oxygen and remain sensitive to oxidation. After the annealing, the spectra can be fitted by the RUMP model containing a single layer of Ce0.8Gd0.2O2, corresponding to a fully oxidized film. In case of the deposition at the longer target-substrate distance, the films are already fully oxidized in the asdeposited state, no thin top layer with oxygen gradient is visible and there is no difference before and after annealing in the RBS spectra shown in Figure 5(d,e,f). This confirms that a longer targetsubstrate distance (and therefore a change in the distance between the substrate and the gas inlet) can lead to an increased consumption of oxygen at the substrate as suggested by the transition shift shown in Figure 4.

The results of the fitting procedure are summarized in Table I. The thickness values correspond to the atom density (Ce, Gd and O) in the whole GDC layer and the thickness indicated in parentheses corresponds to the atom density of the oxygen-rich top thin film at the extreme surface. For films deposited at the same distance, the number of atoms is the same within an error margin of 10%.

This number is decreased in the case of the films deposited at short distance before the annealing, which was expected considering that these films contain less oxygen. The thickness of the top layer obtained at short distance containing the oxygen gradient decreases with the substrate bias, as can be also seen in Figure 5(a,b,c). This suggests that the substrate bias might lead to the formation of a denser film that would allow less oxygen diffusion inside the sub-stoichiometric film when exposed to air. This cannot be observed on the films deposited at long distance, since they are fully oxidized before air exposure (absence of gradient top layer). The roughness was similar for the samples deposited at short distance, whereas for the films deposited at longer distance, the roughness parameter is decreasing with the substrate bias. In most cases, the annealing leads to an increase of the film roughness. This effect is especially pronounced for the films deposited at short distance for which the oxygen content increased during this annealing, suggesting that roughness is related to volume expansion as the crystalline structure turns from a disordered metallic phase to oxide one. This is a well-known effect and is one of the causes of delamination when metal films are annealed in air [START_REF] Coddet | Characteristics and properties of a magnetron sputtered gadolinia-doped ceria barrier layer for solid oxide electrochemical cells[END_REF].

Figure 6 shows X-ray diffractograms of the as-deposited films for the two different target-substrate distances in the 2θ range from 20 to 60°. For the films deposited at the shortest target-substrate distance, several overlapping peaks are visible that can be attributed to a phase mixture of Ce, Ce7O12 and GDC. For the films deposited at the longest target-substrate distance, only peaks corresponding to GDC are visible. This confirms the RBS results: at the shortest distance, the films are substoichiometric in the as-deposited state, whereas for the longest distance, the films are fully oxidized. It should be noted, however, that for the films deposited at a longer target-substrate distance, the position of the peaks corresponding to GDC is slightly shifted to smaller values as compared to the reference positions. An elongation of the lattice parameter in the out-of-plane direction can indicate the presence of an in-plane compressive stress, which is often found in magnetron sputtered thin films due to bombardment of the growing film with energetic particles [START_REF] Windischmann | Intrinsic stress in sputter-deposited thin films[END_REF].

Figure 7 shows the X-ray diffractograms of the films after annealing at 500°C for the two different target-substrate distances in the 2θ range from 20 to 60°. For both distances and whatever the substrate bias, only peaks corresponding to GDC are visible. In this range, GDC has several peaks corresponding to the (111), ( 200), ( 220), ( 311) and (222) family of planes [START_REF] Brauer | Über heterotype Mischphasen bei Seltenerdoxyden. I[END_REF]. The positions of the peaks are close to the reference positions from the PDF card, indicating that the compressive in-plane stress could be relieved by the annealing procedure.

The XRD peaks in Figure 7 could be fitted with a Voigt function using the software Fityk [START_REF] Wojdyr | Fityk: a general-purpose peak fitting program[END_REF].

From the integral intensity 𝐼 of the peaks in the X-ray diffractograms the texture factor of the different crystallographic orientations can be calculated by taking into account the relative intensities 𝐼 ref of the peaks in case of a powder sample. The texture factor 𝑇 ℎ for the lattice plane ℎ can be calculated according to

𝑇 ℎ = 𝐼 ℎ m /𝐴 𝜃/2𝜃 (𝜃 ℎ ) 𝐼 ℎ ref / ∑ 𝐼 ℎ ′ m /𝐴 𝜃/2𝜃 (𝜃 ℎ ′ ) 𝐼 ℎ ′ ref ,
with 𝐼 m the measured intensity, 𝐼 ref the reference intensity and ∑ ℎ′ standing for the sum over all the diffraction peaks [START_REF] Barrett | Structure of metals: crystallographic methods, principles and data[END_REF]. The reference intensities of the ( 111), ( 200), ( 220) and (311) diffraction peaks are taken from the JCPDS card 01-075-0162. The absorption factor 𝐴 𝜃/2𝜃 can be calculated by

𝐴 𝜃/2𝜃 = (1 -exp (- 2𝜇𝑡 sin𝜃 )) ,
with 𝑡 the film thickness and 𝜇 the linear attenuation coefficient [START_REF] Birkholz | Thin film analysis by X-ray scattering[END_REF]. The linear attenuation coefficient can be calculated with the software xraylib using the chemical composition and the theoretical density of GDC [START_REF] Schoonjans | The xraylib library for X-ray-matter interactions. Recent developments[END_REF]. Considering that 4 different peaks are taken into account, using this definition of the texture factor means, that a completely random orientation would lead to a texture factor of 0.25 for each orientation, whereas a texture factor of 1 for a single orientation would describe a film fully oriented along that direction. Figure 8 shows the resulting texture factors. The films deposited at the shortest target-substrate distance show a random orientation without substrate bias and an increasingly preferential [111] orientation with increasing absolute value of the substrate bias. The films deposited at a longer target-substrate distance show a strong preferential [200] orientation without substrate bias that decreases with increasing the substrate bias (in absolute value).

Cross-sectional SEM micrographs of the films were observed before and after annealing and are shown in Figure 9. The images show that the as-deposited films have a columnar microstructure.

For the films deposited at a distance of 12.7 cm, the columnar character of the microstructure of the films is less visible after the annealing, leading to a denser microstructure. The increase of the density can be explained by the volume expansion of the sub-stoichiometric films due to the oxidation during the annealing [START_REF] Coddet | Planar Solid Oxide Fuel Cell: Electrolyte Deposited by Reactive Magnetron Sputtering and Cell Test[END_REF][START_REF] Briois | Synthesis and Characterization of Apatite Structure Sputter Deposited Coatings Dedicated to Intermediate Temperature Solid Oxide Fuel Cells[END_REF]. For the films deposited at a distance of 16.0 cm, the microstructure does not change during the annealing and remains columnar. This result is not surprising since these films do not exhibit a change of the composition after annealing, so there is no volume expansion.

The thickness of the films is about 750 nm (630 nm) for a target-substrate distance of 12.7 cm (16.0 cm), leading to a deposition rate of 50 nm min -1 (42 nm min -1 ). Taking into account the target power density of 2.8 W cm -², this leads to a power-normalized deposition rate of 18 nm min -1 W -1 cm² (15 nm min -1 W -1 cm²). Typical power-normalized deposition rates in the literature for the deposition of GDC films using magnetron sputtering vary from less than 1 nm min -1 W -1 cm² [START_REF] Lin | Characterization of electrolyte films deposited by using RF magnetron sputtering a 20mol% gadolinia-doped ceria target[END_REF][START_REF] Bae | Three-dimensional hexagonal GDC interlayer for area enhancement of low-temperature solid oxide fuel cells[END_REF][START_REF] Wu | Structure-designed gadolinia doped ceria interlayer for solid oxide fuel cell[END_REF] to around 2-8 nm min -1 W -1 cm² [START_REF] Uhlenbruck | Thin film coating technologies of (Ce,Gd)O2-δ interlayers for application in ceramic hightemperature fuel cells[END_REF][START_REF] Solovyev | Deposition of a Thin-Film CGO Electrolyte for Solid Oxide Fuel Cells[END_REF][START_REF] Solovyev | Magnetron-Sputtered YSZ and CGO Electrolytes for SOFC[END_REF][START_REF] Rabotkin | Process stabilization during high power impulse magnetron sputtering of Ce/Gd target[END_REF][START_REF] Solovyev | Magnetron Sputtering of Gadolinium-doped Ceria Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells[END_REF] as shown in Table II. Only a few works [START_REF] Kuo | Growth of 20mol% Gd-doped ceria thin films by RF reactive sputtering: The O2/Ar flow ratio effect[END_REF][START_REF] Coppola | Structural and Electrical Characterization of Sputter-Deposited Gd0.1Ce0.9O2-δ Thin Buffer Layers at the Y-Stabilized Zirconia Electrolyte Interface for IT-Solid Oxide Cells[END_REF] report power-normalized deposition rates of more than 10 nm min -1 W -1 cm² using RF sputtering, which requires more expensive deposition equipment.

The high power-normalized deposition rate in this work could be explained by the fact that the deposition takes place in the metallic mode.

Discussion

Both RBS as well as XRD results confirm that the films deposited at a longer target-substrate distance show an enhanced oxidation state as compared to films deposited at a shorter distance.

This result can be explained by the observed shift of the transition. The shift of the critical point towards higher oxygen flow rates at the longest target-substrate distance allowed to use a higher oxygen flow rate while staying in the metallic mode. The higher oxygen flow rate of 3 sccm instead of 2 sccm led to an increase in the flux of oxygen towards the substrate in this case. At the same time, the flux of sputtered metal atoms towards the substrate decreases with increasing targetsubstrate distance. So, at the longest target-substrate distance the oxygen/metal flux ratio is expected to be high which is favorable to the growth of fully oxidized films, whereas at the shortest target-substrate distance the ratio of oxygen over metal flux at the substrate is low, leading to the growth of substoichiometric films.

The difference in the growth mechanism of the films at the two different target-substrate distances can also explain the different preferential orientations. In the works reported on the sputter deposition of GDC, only few papers report information about the crystalline orientation of the deposited films. Most of them indicate a random orientation [START_REF] Uhlenbruck | Thin film coating technologies of (Ce,Gd)O2-δ interlayers for application in ceramic hightemperature fuel cells[END_REF][START_REF] Sønderby | Magnetron sputtered gadolinia-doped ceria diffusion barriers for metal-supported solid oxide fuel cells[END_REF][START_REF] Lin | Characterization of electrolyte films deposited by using RF magnetron sputtering a 20mol% gadolinia-doped ceria target[END_REF][START_REF] Kuo | Growth of 20mol% Gd-doped ceria thin films by RF reactive sputtering: The O2/Ar flow ratio effect[END_REF][START_REF] Wu | Influence of Deposition Temperature of GDC Interlayer Deposited by RF Magnetron Sputtering on Anode-Supported SOFC[END_REF][START_REF] Kuo | Sputter-deposited 20mol% gadolinia-doped ceria films on 8mol% yttria-stabilized zirconia tapes for improved electrochemical performance[END_REF][START_REF] Kuo | Mechanism of oxygen ion diffusion in Gd-doped ceria electrolyte films deposited via reactive and direct sputtering[END_REF][START_REF] Coppola | Structural and Electrical Characterization of Sputter-Deposited Gd0.1Ce0.9O2-δ Thin Buffer Layers at the Y-Stabilized Zirconia Electrolyte Interface for IT-Solid Oxide Cells[END_REF][START_REF] Gourba | Electrical properties of gadoliniadoped ceria thin films deposited by sputtering in view of SOFC application[END_REF][START_REF] Briois | A comparison of electrical properties of sputter-deposited electrolyte coatings dedicated to intermediate temperature solid oxide fuel cells[END_REF][START_REF] Ionov | Reactive co-sputter deposition of nanostructured cermet anodes for solid oxide fuel cells[END_REF][START_REF] Wu | Structure-designed gadolinia doped ceria interlayer for solid oxide fuel cell[END_REF] and only a few report a preferential [111] orientation [START_REF] Kuo | Gadolinia-doped ceria films deposited by RF reactive magnetron sputtering[END_REF][START_REF] Jiang | Fabrication of epitaxial zirconia and ceria thin films with arbitrary dopant and host atom composition[END_REF][START_REF] Sakaliūnienė | Investigation of Structural and Optical Properties of GDC Thin Films Deposited by Reactive Magnetron Sputtering[END_REF][START_REF] Solovyev | Magnetron-Sputtered YSZ and CGO Electrolytes for SOFC[END_REF][START_REF] Solovyev | Magnetron Sputtering of Gadolinium-doped Ceria Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells[END_REF]. The change from random to [111] orientation for the films deposited at the shortest distance in our work is in agreement with these works. In our case, a polycrystalline thin film consisting of a mixture of metallic and various substoichiometric phases with random orientation was oxidized to form a GDC film with mostly random orientation at the shortest target-substrate distance. With increasing substrate bias, positive ions can be accelerated towards the substrate, increasing their energy, which could suggest an increase in the energy per arriving atom. A high energy per arriving atom could favor the growth along the [111] orientation, as the (111) surfaces have the lowest surface energy in fcc metals such as Ce [START_REF] Zhang | Calculation of the surface energy of FCC metals with modified embedded-atom method[END_REF].

On the other hand, at the longest target-substrate distance, the oxygen flow rate was higher and an oxide layer grew directly onto the substrates with a preferential [200] orientation. A preferential

[200] orientation was not previously reported for sputter deposited GDC films except for epitaxial films deposited at high temperature on MgO substrates [START_REF] Jiang | Fabrication of epitaxial zirconia and ceria thin films with arbitrary dopant and host atom composition[END_REF]. However, it was found by Van Steenberge et al. [START_REF] Van Steenberge | Influence of oxygen flow and film thickness on the texture and microstructure of sputtered ceria thin films[END_REF] for the deposition of pure CeO2 films at oxygen flow rates close to the critical point. They explained the preferential [200] orientation by the suppression of the [111] orientation due to preferential sputtering of [111] oriented crystallites (planes with the highest surface atom density) by negative oxygen ion bombardment of the growing film [START_REF] Van Steenberge | transfer driven textural changes of CeO2 thin films[END_REF]. If such a negative oxygen ion bombardment is also causing the [200] orientation in our case, it could also explain why the preferential orientation is less pronounced with increasing substrate bias, as the negative substrate bias would slow down the negative oxygen ions coming from the target leading to a more random orientation.

It is difficult from the features evidenced in this work to predict which GDC films will be the most efficient as diffusion barrier in electrochemical cells. A fully oxidized and crystallized compound is required to ensure a good ionic conductivity and a dense structure to avoid atomic diffusion. Working at longer distance allows synthesizing fully oxidized films in a single step, but leads to a columnar structure. In the second processing route investigated in the present work an annealing step is required to complete the oxidation state of the compound that has the advantage to densify the films by volume expansion (disappearance of the columnar structure). Electrochemical tests are underway to validate one or the other route. They will also allow to check whether the preferential orientation has a key influence on the performance in the application of solid oxide fuel cells, as it is known that the preferential orientation of thin films can have a large influence on performance and reliability in the case of other applications such as in microelectronics [START_REF] Knorr | Applications of texture in thin films[END_REF].

Conclusion

The deposition of thin GDC films on Si substrates by reactive pulsed DC magnetron sputtering with a high growth rate has been shown (18 nm min -1 W -1 cm²). Mainly, two different growth conditions have been identified, that differ in their oxygen/metal flux ratio. A low oxygen/metal flux ratio leads to the deposition of substoichiometric films that need to be fully oxidized by an annealing post-treatment and show a stronger [111] orientation with increasing substrate bias. A high oxygen/metal flux ratio leads to the deposition of fully oxidized GDC films with a preferential

[200] orientation that decreases with increasing substrate bias.

In a wider perspective, our study shows the ability to control the preferential orientation as well as the stoichiometry of magnetron sputtered oxide films by simply varying the process parameters and choosing a proper experimental configuration, which is less conventional. Here the oxygen flow rate, the gas inlet location and the target-substrate distance were found to play a great role on the deposition process and the final film properties. A finer control of the preferential orientation is even possible with RF substrate bias, suggesting the possibility to control the energy of the different ions arriving at the substrate.

Figure 1 :

 1 Figure 1: Schematic representation of the deposition chamber, showing the different targetsubstrate distances as well as the positions of the gas inlet.

Figure 2 :

 2 Figure 2: Typical hysteresis curves for the CeGd target for 2 different pumping speeds. The waiting time between subsequent measurements is 2 minutes in the case of filled symbols and 6 minutes in the case of open symbols. The arrows indicate the direction of increasing or decreasing oxygen flow rate.

Figure 3 :

 3 Figure 3: (a) Hysteresis curve of a Ce/Gd target in an Ar/Oxygen atmosphere. The waiting time between two subsequent measurements is 2 minutes. (b) The time needed to return to the initial voltage after a hysteresis experiment as a function of the time that the target was sputtered in the oxide mode.

Figure 4 :

 4 Figure 4: Evolution of discharge voltage (full symbols) and pressure (open symbols) with the oxygen flow rate for two different target-substrate distances and two different gas inlet positions.

Figure 5 :

 5 Figure 5: RBS spectra of GDC films before and after annealing at 500°C for films deposited at 12.7 cm distance (a,b,c) and films deposited at a 16.0 cm distance (d,e,f) with varying substrate bias (floating, -40 V and -80 V). The black lines show the simulated spectra that were fitted to the experimental data.

Figure 6 :

 6 Figure 6: X-ray diffractograms of the as-deposited GDC films. The circular markers show the peak positions corresponding to GDC (PDF card 01-075-0162), the square markers correspond to metallic Ce (PDF card 01-071-7442) and the triangles correspond to Ce7O12 (PDF card 01-071-0567).

Figure 7 :

 7 Figure 7: X-ray diffractograms of GDC films after annealing at 500°C at short and long distance for various bias voltages in logarithmic scale. The diffractograms are offset by sequential multiplication by a factor of 10.

Figure 8 :

 8 Figure 8: Texture factors of the GDC films after annealing at 500°C at short and long distance for various bias voltages.

Figure 9 :

 9 Figure 9: SEM images of the films before and after the annealing at 500°C

Figure 9

 9 Figure 9

Table I :

 I Fitting parameters of the RBS spectra. The thickness values in parentheses correspond to the thickness of the top layer with an oxygen concentration gradient. The error in the thickness can be estimated to 10×1015 atoms/cm².

				As-deposited			Annealed
							Roughness
			Thickness	Roughness		Thickness	factor
	Distance		(10 15	factor	x in	(10 15	(10 15	x in
	(cm)	Bias	atoms/cm²)	(10 15 atoms/cm²)	Ce0.8Gd0.2Ox	atoms/cm²)	atoms/cm²)	Ce0.8Gd0.2Ox
		Floating 3997 (1400)	206	1.0	5711	322	2
	12.7	-40V	3887 (512)	200	1.0	5710	394	2
		-80V	3582 (364)	214	0.9	5802	355	2
		Floating	4335	356	2	4293	402	2
	16.0	-40V	4419	300	2	4420	330	2
		-80V	4366	248	2	4406	268	2
				18

Table II :

 II Comparison of power-normalized deposition rates reported in the literature. Transition mode in the sputter regime column means that the authors of the respective publication worked in the unstable transition between the metallic and the oxide mode.

				Power-normalized
	Reference	Type	Sputter regime	deposition rate
				(nm min -1 W -1 cm²)
	This work	pDC	Metallic mode	15-18
	[3]	not reported	not reported	8.0
	[46]	not reported	not reported	0.8
	[11]	RF	Oxide target	0.3
	[12]	RF	Metallic mode	22.1
	[16]	RF	Oxide target	0.5
	[19]	RF	Oxide target	13.3
	[28]	pDC	Transition mode	3.4
	[30]	pDC	Transition mode	3.0
	[33]	HiPIMS	Transition mode	2.1
	[34]	HiPIMS	Transition mode	5.5

Acknowledgements

The authors would like to thank Thomas Lecas and Hélène Lecocq for the technical support and the CERTeM for the funding for the SEM and XRD. This work was supported by the Region Centre Val de Loire in the Hytempe project in the framework of the Lavoisier program and the European Union through the European Regional Development Fund.