Laser Ablation Ceramic Target for 8YSZ High-k Dielectric Electrolyte thin Films Precessed by PLD on Si(100) and Pt/Si(111)

The cubic structure 8YSZ (8%Yttria-Stabilized Zirconia) thin films deposited by PLD(Pulsed Laser Deposition) on substrates Si (100) and Pt/Si (111) by identical control parameters have potential applications as electrolytes for planar micro electrochemical devices like Lambda oxygen sensors and IT-μSOFC. It appearance differences in polycrystalline structural and optical characterization by XRD (X-ray Diffraction), SEM (Scanning Electron Microscope), AFM (Atomic Force Microscopy) and VVASE (Variable Angle Spectroscopic Ellipsometry. The differences are relating on crystalline dimensions, lattice parameters; surface roughness measured by VVASE and AFM are presented synthetic to evidence the differences generated by substrates.


1.Introduction
The 8 YSZ (8% Y2O3 doped ZrO2) thin films are high-k dielectrics advanced materials used as electrolytes in many fields like electrochemical devices -gas sensors for automotive application and intermediated temperatures (IT) solid oxide fuel cell (SOFC). 8YSZ electrolytes have high oxygen ionic conductivity and high ionic resistivity [1,2]. This material assures a good compatibility with electrodes, is gas tight and chemically stable at high temperatures (t < 1200 o C) [3]. The cubic phase of 8 YSZ is stabilized on a large field of temperatures including low temperature (t<450 o C) that involves applications in electrochemical devices with long life time of operation by avoiding thermal and mechanical degradation at high temperature [4]. Being operating according Nerst equation, the temperature is the main parameter for working with direct influence on the global efficiency of the cell. Other parameters like methods of deposition, new design configurations and new materials with optimal properties using thin films technologies it can be generate the reduction of thickness of electrolytes with low ohmic resistivity. In the range of 1-1000 nm, CVD (Chemical Vapor Deposition), plasma enhances CVD [5,6] and PLD [7] can be used to produce good quality thin films. PLD can grow cubic crystalline 8YSZ thin films with good adherence to the substrate and respecting the stoichiometric the target [8]. Planar 8YSZ thin films have lower degradation on different types of gas composition. The thickness and the structures of the film can be controlled by deposition parameters (laser fluency, laser wavelength, deposition time, substrate material, substrate temperature, distance target-substrate etc.).The task of this paper is to investigate the effects of Si (100) and Pt/Si (111) substrates on the optical and structural properties [10,11]. Such properties are studied by XRD, SEM, AFM and V-VASE.

2.Materials and methods
8YSZ target with 14 mm diameter x 5mm thick was manufactured by American Elements (SUA) by sintering powder of 99, 99%Zirconia and 8mol% Yttria -Stabilized. The deposition of 8 YSZ thin films with different substrates were fabricated on CompexPro 50 PLD equipment using ArF excimer Figure 1. A schematic diagram of PLD system laser ( = 193 nm) with energy of 230mJ pulse -1 and repetion rate = 10Hz, fluence = 5 J/cm 2 , in oxygen partial pressure of 8x10 -2 mbar, the laser beam was focused at an angle of incidence of 45 on YSZ target. Two sets of experiments were made by using Si (100) and Pt/Si (111) substrates (15 x 15mm) at 500 C and 50mm distance target-substrates with the same number of pulses and time of depositions sample (72.000/ Si (100)) and (72.000/ Pt/Si (111)) configurations. The vacuum chamber has a base pressure 2x10 -4 mbar for both samples by operating a turbo pump ( Figure 1).
The XRD measurements have been performed by using a Panalytical X'Pert PRO MRD diffractometer equipped with monochromatic X-ray radiation of CuKα 1 (0.154056nm) in Bragg-Bretano geometry with 2θ in the angular range 15-75, scanned in steps of 0.02 with an acquisition time of 0.2s/step. The Joint Committee on Powder Diffraction Standards (JCPDS) database from the International Center for Diffraction Data (ICDD) was utilized for structural characterization like phase identification, crystalline dimensions and lattice constant. The Scanning Electron Microscope (SEM) manufactured by FEI -SEM FEI Inspect S is designed to investigate the surface quality of 8YSZ on Si (100) and Pt/Si (111) thin films. Atomic Force Microscopy (AFM) XE-100 Park Systems is used to characterization the morphology and Root Means Square (RMS) roughness, in non-contact mode; allows a maximum horizontal scan area of about 50 x 50 µm 2 and a vertical movement up to 12µm. The investigated areas were 2 x 2 µm 2 , 5 x 5µm 2 ,20 x 20 µm 2 , for revealing both the general aspect of the samples as well as their topographic details. V-VASE with variable angels of incidence (60-70) is used for optical characterization, having high accuracy and precision with a wide spectral range 250-1700 nm. Optical models are generated by WVASE32 software; n, k parameters and roughness are measured [12].

Results and discussions
XRD characterization of 8YSZ ceramic target and thin films deposed on Si (100) and Pt/Si (111) are lay out in Figure 2. This is necessary to identify the crystalline structure that will be transfered on the substrates. Also, the structural characterization of the thin films of 8YSZ/Si (100) and Pt/Si (111) are presented in relation to the same diagrams for identification of crystalline phases. The (111) crystalline phases are cubic phase, and are identified using the standard base XRD JSCPDS file 089-9069.
AFM imagines shows nanoscale grains with homogeny distribution; variation in RMS is a function of dimensions of widows of exploration 2 x 2 μm 2 , 5 x 5 μm 2 , and 20 x 20 μm 2 [13].  The top view surface characterized by SEM are observed in Figure 4 a and 4b; the surfaces are free of cracks with small droplets and small particles. High -k ceramic electrolyte 8YSZ thin films are transparent over a large range of the spectrum and it was selected a Cauchy optical model for characterization. In the operating range (250-1700nm), optical characterizations were made using a VVASE (Variable Angle Spectroscopic Ellipsometry) (Woollam, SUA) assisted by the WVASE 32 software package for processing ellipsometric parameters, acquired at three angles of incidence (60, 65 and 70). The best fit of experimental data for deposition on Si (100) has been obtained by dispersion formula, called generalized Cauchy functions: and k= 0 all maseasured wavelength, where: An, Bn, Cn parameters [14]; Cauchy -Urbach was applied for deposition on Pt/Si (111) substrate. The first term An is a constant that has the largest contribution in shaping the curve and an initial estimate is required. The coefficient Bn has an influence on the curvature and Cn has a reduced contribution, also on the curvature, and in the last upgrade of VVASE 32 it is take Cn = 0. In Table 2 value of thickness of each layer and Cauchy -Urbach parameters are presented. Table 2. Values of estimated parameters for assumed models for both thin films These parameters give the best fitting between experimental and theortical curves. On Figure 5 a and b are measured ellipsometric parameters ( and ), shown in green lines that correspond to experimental and theoretical data. Almost perfectly arrange between yhese two curves is seen for that models are three different angles of incident. That mean our model describe real structure very well. ( Figure 5 a and b). 8YSZ Table 2 of Cauchy-Urbach for model B, optical constants can be found ( Figure 6). In Table 3 it is presented a comparison between roughness measured by SE and AFM; the differences are generated by the fundamental principal of measurement in SE and AFM.