Reactive pulsed laser deposition of WOx layers for SiC-based hydrogen sensor fabrication

V. Yu. Fominskia*, A. G. Gnedovetsb, R. I. Romanova, M. V. Demina

aMEPhI National Research Nuclear University, Kashirskoe sh. 31, Moscow, Russia 115409; bA. A. Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences,

Leninsky Prospect 49, Moscow, Russia 119991

ABSTRACT Peculiarities of WOx films fabrication by reactive pulsed laser deposition for high temperature Pt-oxide-SiC devices formation were investigated. Deposition of the oxide film was also carried out in such a way as to prevent deposition of droplet fraction (deposition with anti-droplet screen). Direct Simulation Monte Carlo and Kinetic Monte Carlo methods were performed for the deposition processes modeling. The response of the SiC-based devices to hydrogen-containing gases depends on the conditions of deposition of the oxide layer. The best properties were found in the sensor obtained by depositing the scattered flux of W atoms in a shady area on SiC substrate at an oxygen pressure of 10 Pa. Keywords: Reactive pulsed laser deposition, MOSiC device, high temperature SiC sensor, Monte Carlo simulations.

1. INTRODUCTION Metal-oxide-semiconductor devices based on the wide bandgap semiconductor SiC (MOSiC) are attractive for gas sensing device applications in harsh environments. Emissions control for both real time monitoring and feedback control of exhaust products in the automotive and energy sectors require sensors that can operate in chemically reactive high temperature environments. The performance of SiC-based devices in these harsh, high temperature ambients will depend on the chemical properties/structure/morphology/adhesion of the metal and oxide thin films in MOSiC device. The operation of field-effect chemical sensors based on refractory gate SiC devices is dominated by electronic and chemical interaction at interfaces (the environment/metal interface, the metal/oxide interface and the oxide/SiC interface)1. The selectivity, sensitivity and stability of such sensors can be increased with the addition of “reactive oxide” layer between the SiC and catalytic metal. It is envisaged that coupling WO3 with SiC, would be promising for high temperature gas sensing applications2. WO3 semiconductor is promising for hydrogen-containing gas sensing2 –4. This is because of its chemical stability and high diffusion coefficient of oxygen vacancies. WO3 has been used in various gas sensing applications, and also is well known for being an excellent sensing material for NOx–containing/exhaust gases4, 5. Reactive pulsed laser deposition (reactive PLD) is widely used for fabrication of various oxide layers, including WOx films6,7. Varying the deposition conditions can significantly change the structure, chemical composition, morphology, and properties of oxide-SiC and metal-oxide interfaces. These properties of the films depend greatly on the deposition rate of laser-initiated flow of atoms and energy characteristics of atomic flux. One effective way to change such deposition parameters is to change the laser fluence at the irradiated target. However, during irradiation of refractory metals, there are certain restrictions when choosing fluence due to the effective formation of droplet fraction. Another way to change (reduce) energy flux is to increase the gas pressure. In collisions with background gas, atoms of laser plume lose their kinetic energy, as well as changing the trajectory of motion. The effect of scattering by molecules of gas is used in the method of deposition of atomic flux in the shadow region behind a protective screen mounted on the expansion path of the laser plume8. Application of the screen prevents the deposition of droplets of micron and submicron sizes. Presumably, this would enable to create higher-quality thin-film layers for MOSiC sensors. The purpose of this work was in the experimental study of peculiarities of reactive pulsed laser deposition of WOx thin film layers with various methods of deposition: deposition without the use and with application of the screen. * E-mail address: VYFominskij@mephi.ru

Investigations on the reaction of hydrogen with WOx/SiC and Pt/WO3/SiC structures obtained by pulsed laser deposition were performed and the conditions for the structures with the best properties in terms of efficiency of detection of hydrogen at elevated temperatures were identified. Also, mathematical models of expansion of laser-induced atomic flux in the background gas and film growth during deposition of this flux were developed which allowed explaining the experimental results.

2. EXPERIMENTAL STUDY OF REACTIVE PULSED LASER DEPOSITION OF WOx FILMS Figure 1 shows a schematic diagram of the technique used to obtain thin films on silicon carbide substrates. The plasma–vapor flow from the targets (W, Pt) was initiated by radiation of a neodymium-doped yttrium aluminum garnet laser. The pulse duration was 20 ns. The radiation pulse energy was ~50 mJ, and the energy density on the target at the focus spot was ~10 J/cm2. The pulse repetition rate was 25 Hz. Thin Pt films were deposited in vacuum at a residual gas pressure no higher than 10-4 Pa. Thin WOx films were deposited in an oxygen atmosphere at a pressure varied from 2 to 10 Pa. The number of pulses was as large as 3×104 . The laser plume from W target propagated freely from the target to the substrate and, in some case, a disk screen 5 mm in diameter was placed on the plume expansion path at the distance z = 2 cm from the target. The substrate was placed at the distance z = 5 cm from the target. Drops and particles produced during laser exposure of the target expanded mostly along straight trajectories and were deposited on the screen. A flow of atoms that were scattered at fairly large angles in collisions with vapor atoms of the ablation plume and with O2 molecules propagated beyond the screen. Silicon and nitrogen-doped 6H-SiC wafers with resistivity ρ =0.03–0.2 Ω cm were used. Preliminarily (before thin films deposition), a nickel film was applied onto the rear ground wafer surface of SiC substrate by pulsed laser deposition, which was annealed in vacuum to obtain ohmic contact. Laser deposition of thin films on the polished substrates surface was performed at room temperature through a diaphragm with a circular aperture 2 mm in diameter close to the wafer surface. WOx films deposited on the SiC substrate were annealed in air for 2 hours at a temperature of 600 °C. Then, platinum films were deposited on the surface of oxide films. The grown film was characterized by a certain structure and thickness dependent on the distance r from the deposition area center. The deposited film thickness (in units of atom/cm2) and the relation x=O/W as a function of r was studied by Rutherford backscattering spectroscopy (RBS) of helium ions. The film surface topography was studied by atomic force microscopy (AFM). Measurements of current flow through the structures of Pt/WOx/SiC were carried out in a mixture of air with hydrogen at temperatures from 100 to 500 °C. The quality of sensor structures formed was determined by the results of these measurements.

Fig. 1. Schematic diagram of pulsed laser deposition of WOx films using a disk screen protecting the substrate from large particles and

droplets deposition. The coordinate system used in numerical simulation is shown.

a b

c d

Fig. 2. Tungsten deposition rate (a, c) and compositional distribution (b, d) of WOx films formed by PLD in the oxygen gas at the different pressure for free laser plume transport conditions (a, b) and for deposition using a screen (c, d). r – radial distance from the

center of the film deposition zone on a SiC substrate. The results of the compositional and structural studies are shown in Figs. 2 and 3. The use of the screen indeed made it possible to almost completely exclude deposition of large particles; however, an appreciable decrease in the W atom deposition rate was observed in this case. Even at pressure of 10 Pa at the shadow area center, the film deposition rate was lower by a factor of ~4 than in the area open for free plume deposition. The shadow area diameter was ~2 cm. At relatively low oxygen pressure (2 Pa) the effective oxidation of the tungsten film proceeded only in the center of the shadow area at very low rates of deposition. Increasing the pressure of oxygen enhanced the efficiency of oxidation of tungsten films at higher deposition rates. However, it should be noted that the screen have some negative impact on the oxidation process at an oxygen pressure of 10 Pa. The reason could be that the screen protected the substrate from UV and X-ray radiation of the plasma and the impact of high energy atoms/ions. These factors could significantly activate the chemical interaction between the film surface and the surrounding gas.

a b

Fig. 3. AFMs show the difference in the surface topography of the WOx films, formed on a Si substrate by reactive PLD (10 Pa oxygen pressure) for free laser plume transport conditions (a) and for deposition using a screen (b).

According to AFM studies thin films formed by reactive PLD in an oxygen atmosphere at room temperature of a Si substrate had a dense amorphous-like structure and a rather smooth surface. The characteristic size of large particles, which were rarely detected in the open area, was about 0.5 μm. Particles of nanometer size (Fig. 3 a) also presented on the surface in significant amount. The structure of the films deposited in the shadow zone behind the screen differed substantially from the structure of the films formed in the open area. The film surfaces almost had no spherical particles several tens of nanometers in size. However at relatively high oxygen pressure these films had a low-density structure and a developed surface (Fig. 3 b). The characteristic size of roughness was 1-2 nm, which is practically an order of magnitude higher than the surface roughness of the film obtained in the case of deposition without a screen. To analyze the W atom velocity spectrum in the laser plume, experiments with vacuum deposition of the laser-induced flow on a rotating disk were performed. A plate with a slit the width of which did not exceed 0.3 mm was placed on the laser plume propagation path. A disk 20 mm in diameter is placed behind the slit. The disk rotation frequency was 140 Hz. To measure the velocity spectrum with sufficient accuracy, the target should be multiply irradiated. The reproducibility of conditions of plume deposition on the disk was achieved using a special electronic–mechanical synchronization system. The film formed on the rotating disk after a preset number of laser pulses was studied by RBS. Figure 4 shows the measured velocity spectrum of W atoms in the laser plume used for film deposition. It was assumed that the velocity v distribution of atoms can be described by the “shifted” Maxwellian distribution

⎥⎦

⎤⎢⎣

⎡ −−⎟

⎠⎞

⎜⎝⎛=

kTm

kTmNf

2)(exp

2)(

22/3 uvvπ

, (1)

where k is the Boltzmann constant; m is the atomic mass; and N, T, and u are the density, temperature, and mass velocity of the laser plume, respectively. Experimentally measured velocity distribution of tungsten atoms is well described by the distribution (1) with the following parameters kT ~ 16 eV, u ~ 8 km/s. The model distribution does not include high speed components (atoms, moving at velocities over 15 km/s). It was assumed that this component of the velocity distribution is formed by deposition of high-speed ion flow. A measurement of time-of-flight (TOF) ion pulses and their mathematical treatment according to the Kelly-Dreyfus model9 were performed. Measurement of ion pulses during the expansion of the laser plume in a vacuum showed that the high-speed distribution of the ions dominates the flow with the following parameters of the Maxwellian distribution: kT ~ 50 eV, u ~ 20 km/s. The analysis of the process of passing through the laser plasma of oxygen at different pressures was performed (Fig. 4 b). Analysis of the ionic form of pulses showed that in the selected pressure range of oxygen, a mechanism of ion penetration through the plasma gas was realized. At pressures of oxygen for more than 20 Pa ion signal was split, suggesting the formation of the shock wave. These studies also showed that during deposition in oxygen at pressures up to 10 Pa WOx films were bombarded by high-velocity ions, which could have a significant impact on their structure and chemical composition.

a b

Fig. 4. Velocity spectrum of tungsten atoms (a) and time-of-flight signals of tungsten ions (b) in the pulsed laser plume. Ion pulses were measured at the different oxygen gas pressure.

3. ELECTRICAL CHARACTERISTICS OF Pt/WOx/SiC STRUCTURES IN HYDROGEN-CONTAINING ATMOSPHERE

Measurements of the dependence of current on voltage in the range from -3 V to 3V showed that in laboratory air current value was low for all samples at all temperatures studied (up to 500 °C). Hydrogen puffing caused a significant increase in current in the samples obtained by deposition WOx at the highest oxygen pressures. The maximum response was detected in a sample obtained with the use of the screen (Fig. 5 a). The temperature dependence showed that the increase in temperature causes a general enhancement of the reaction sensor. The highest response was found at 400 °C (Fig. 5 b).

a

b

Fig. 5. Dependence of current in the detector Pt/WOx/SiC on (a) hydrogen concentration in air (temperature 300 °C , 1 – deposition

WOx with the screen, 2 – deposition WOx without a screen) and (b) temperature, with temporal monitoring of the current. Measurements were performed at a voltage of 1 V.

4. SIMULATION OF PULSED LASER DEPOSITION OF TUNGSTEN FILMS IN OXYGEN GAS ATMOSPHERE

Numerical simulation of laser plume expansion was based on one of the variants of the Direct Simulation Monte Carlo (DSMC) method, in which only collisions of laser-evaporated target material atoms with gas molecules are considered, while mutual collisions of evaporated atoms with each other during their expansion from the target to the substrate were ignored. Gas was considered as remaining unperturbed. Due to axial symmetry of laser plume expansion, the problem was considered in the 2D3V statement10. Laser plume expansion was numerically simulated by the standard Monte Carlo algorithm which, in the problem at hand, was reduced as follows. After initial loading of numerical particles, i.e., atoms injected from the target surface for a short time t0 ≈ 10-6 s with velocity distribution (1), the operations of their motion in space according to the equations of motion at each time step Δt, simulation of all possible collisions, and determination of new particle velocities were cyclically repeated. Motion of ~107 numerical particles was simulated with time step Δt ≈ 10-7 s. The macroscopic flow parameters (density, temperature, and so on) were output to a spatial grid of 100×100 cells at a geometric size of the simulation region between the target and substrate equal to 5 cm. The simulation region was considered uniformly filled with gas molecules with given temperature Tg and density Ng (or pressure Pg = NgkTg). In numerical experiments, the gas temperature was taken as Tg = 300 K, and a pressure range from ideal vacuum to 10 Pa was studied. The probability of collisions of plume atoms and gas molecules was determined from the dependence

)exp(1 sNW pggpg Δ−−= σ , where σpg is the elastic collision cross section, Δs = vΔt is the distance passed by a vapor atom for the time Δt, and subscripts p and g signify the laser-induced flow of atoms (laser plume) and gas, respectively.

Collisions were described using the semi-empirical modified variable hard sphere model of molecule elastic scattering10, which describes isotropic scattering taking into account the kinetic energy E of colliding particles. According to this model, the collision cross section varies as

⎪⎪⎪

⎩

⎪⎪⎪

⎨

⎧

≤

>⎟⎟⎠

⎞⎜⎜⎝

⎛

=

−

refref

ref

w

refref

EE

EEEE

E,

,

)(σ

σ

σ , (2)

where σref = σpg(Eref) is the collision cross section at a given energy Eref = 1 eV. The exponent value w = 0.24 was used in the calculations. The reference cross section σref was specified by the results of the calculation of the hard sphere collision cross section for tabulated values of atomic radii of chosen elements. In the case under consideration, i.e., W atom scattering by oxygen molecules, the cross section was taken as σref = 0.32 nm2. The model describes the deposition of the vapor flux in the chamber without or with an disc screen. Location of the antidroplet screen and its dimensions are reproduced the experimental conditions of deposition. Figure 6 a shows the model radial distributions of deposited W atoms of the low-velocity flow. These distributions are in quite good agreement with the experimentally measured ones (Fig. 2 с). Simulation of the high-velocity component expansion showed that it is less efficiently scattered to the shadow region than the low-velocity one. In a gas at a pressure of 10 Pa, the velocity distribution of the deposited tungsten atoms and their angular diagram changes significantly (Fig. 6 b, c). It should be noted that the model predicted a more intense scattering of atoms of tungsten into the shadow region than that observed in the experiment. According to the model at oxygen pressure of 10 Pa the deposition rate of tungsten atoms in the center of the shadow area is only 1.3 times lower than at the edge of this area. In the experiment, the ratio of the deposition rates in these regions of the substrate was 1.6. The main reason for some discrepancy between the model and the experiment could be a simplification of the scattering mechanism of tungsten atoms on the molecules of oxygen. These simplifications consist in the choice of spherical form of diatomic molecules, as well as ignoring the inelastic and reactive collision processes of tungsten atoms with oxygen molecules. One can not also exclude the mechanism of dissociation of diatomic molecules into atoms under the influence of plasma radiation and as a result of collisions with atoms of W. The scattering of W atoms on the atomic oxygen will not proceed as efficiently as for molecular oxygen.

a b c

Fig. 6. Model deposition rate distribution on the substrate (a), energy (b) and angular distributions (c) of incident W atoms at the center of the substrate during the pulsed laser deposition of W films in vacuum and oxygen gas atmosphere

at the different pressures.

Calculations showed that at oxygen pressures of 10 Pa WOx film is formed mainly by the mechanism of deposition of the scattered flux of tungsten atoms. Energy and angular distributions of incident on the substrate tungsten atoms slightly differ for different methods of deposition (deposition without a screen and deposition with the screen). Therefore, the main difference in the film deposition conditions is that during the deposition with the screen high-energy ions and atoms (the energy higher than 200 eV) do not reach the surface without suffering a collision with gas molecules. Bombardment of the deposited layer by high-energy ions/atoms can significantly change the atomic structure of the formed film.

5. SIMULATION OF THE THIN FILM GROWTH

The substantial effect of bombardment by high-energy atoms and ions on the atomic structure of thin films formed by PLD is supported by computer simulation results. The thin film growth during bombardment by high-energy particles was simulated using the Kinetic Monte Carlo method10. This method, which is based on the lattice kinetic Monte Carlo algorithm in a 3D setting, was supplemented with a stochastic model of sputtering and densification of the near-surface layer in the film. During the simulation, an atom of the deposited material with a low (under threshold) energy was placed in a site neighboring to the film atom it collided with. If the incident atom had an energy sufficient to sputter the surface, a surrounding region was analyzed, the size of which was controlled by the collision cascade length Ld = a Nd, where Nd is the number of displaced atoms and a is the lattice parameter. Inside this region on the film surface, an atom with the smallest coordination number was randomly chosen, removed from the film, and the incident atom under the surface on a free lattice site at a distance up to Ld from the collision point was placed. In the calculations, it was assumed that the high-energy atoms were incident along the normal to the film surface. To estimate the displacement cascade size, the Khinchin–Pease formula was used, according to which the number of displaced atoms Nd depends on incident ion energy Eimpl and effective threshold displacement energy Ed and is calculated by the expression Nd = Eimpl/2Ed . The threshold displacement energy in the near-surface region depends substantially on a number of factors (structural defects, chemical state, and local environment) and may be 10–20 eV. If one assumes Ed ~ 10 eV to estimate the cascade, then ions with an energy Eimpl ~ 200 eV can displace Nd ~ 10 atoms in the film. It was assumed that the oxygen atoms adsorbed on the surface of the film are associated with tungsten atoms, and dissolve in the film. Structure/density of the film was determined mainly by deposition and packing of tungsten atoms. Chemical bonding with oxygen atoms prevented surface migration of tungsten atoms in the case of deposition WOx at room temperature of the substrate.

a b

Fig. 7. Top and side views of the simulated film structures (256×256×128 interatomic distances) that form during pulsed laser deposition using a screen (a) and without it (b) in oxygen gas atmosphere at a pressure of 10 Pa.

Figure 7 shows the simulated atomic structures that form during pulsed laser deposition using a screen and without it in an oxygen gas atmosphere at a pressure of 10 Pa. The substrate region size is 256 interatomic distances, and the film thickness is 128 interatomic distances. The angular and energy distributions of the incident W atomic flux were determined from the results of simulating the laser plume expansion. Formation of a sparse structure, containing a large number of structural defects, is predicted behind the screen as well as notable nanorelief of the film surface. Relatively dense structure and a smooth relief are achieved in the case where the fraction of high-energy (> 200 eV) atoms is above 10%. Such conditions of bombardment with high-energy ions and atoms are realized during the deposition of films in the gas without a screen. To initiate the bombardment of the films deposited using the screen, a bias of -200 V can be attached to the substrate11.

6. CONCLUSIONS Sensory properties of the structure of Pt/ WOx/SiC depend strongly on the conditions of pulsed laser deposition of oxide layer. The relatively prolonged annealing does not provide leveling of the effects and structural/chemical features that form in these films under deposition. The effect of oxygen pressure and deposition conditions (without a screen or with the screen) on the properties of these structures was discovered. The best properties are possessed by sensors, obtained by deposition with the screen at a maximum pressure of oxygen (~10 Pa). The mathematical simulation of laser plume expansion and deposition of WOx films was performed. Theoretical analysis revealed a factor that could have a significant influence on the properties of the oxide layer. During the deposition in oxygen using the screen a highly defective atomic structure of oxide with developed nanorelief is formed. During deposition by the traditional way (without a screen) conditions for the formation of a more perfect structure exist, which probably affects adversely on the sensory properties of Pt/WOx/SiC sensor in the hydrogen-containing medium. This work was carried out within the framework of the Federal special-purpose program “Scientific and scientific-educational personnel of innovative Russia” in 2009-2013.

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