Thermal vacuum evaporation peculiarities of perovskite-type barium-strontium hafnate

Experimental results are presented on the thermal vacuum evaporation of barium hafnate BaHfO3 and double barium-strontium hafnate Ba0.5Sr0.5HfO3, which are included together with finely dispersed tungsten as components in sintered composites. It is found that at T = 1900 K only barium and barium oxide are the products of BaHfO3 evaporation, i.e., congruent evaporation of BaHfO3 does not occur. Investigations of Ba0.5Sr0.5HfO3 evaporation at T = 1600 K showed that it also does not evaporate congruently. At the same time, the nucleation of a new phase of BaO·SrO in the form of structures with D6 symmetry was detected. Using a high-current vacuum arc discharge, congruent evaporation of Ba0.5Sr0.5HfO3 takes place, forming various crystalline formations of double barium-strontium hafnate on the substrate.


Introduction
Considerable progress has been made recently in the study of perovskite-based materials [1,2] and in the development of various new devices using these materials.Perovskites include a large number of compounds that have the same crystal structure as calcium titanate CaTiO 3 .Perovskites are used as semiconductor materials, for example, in the fabrication of high-efficiency optoelectronic devices (LEDs, solar cells) and for gate oxide replacement in future metal-oxide semiconductor field-effect transistors.
In the vast majority of devices, perovskite materials are used in the form of thin films.Techniques used directly to grow perovskite films include inkjet printing, spray coating, chemical vapour deposition and spin coating [3].The latter method is currently the most widely used.However, the spin coating method has a number of disadvantages: It is difficult to obtain largearea films with a homogeneous surface and there are problems in determining the film thickness.In addition, this method can lead to defects in the synthesis of precursor [3].
It has been proposed to solve these problems by using the thermal evaporation method.In this method, a perovskite powder is evaporated by heating, for example in a tungsten crucible, and deposited on a selected substrate.In this way, the rate of film growth (formation) can be controlled, and there is the possibility of shortening the deposition time and significantly improving reproducibility.
It should be mentioned that there is currently no information on the preparation of barium or strontium hafnate films by thermal evaporation.This is most likely due to the high melting points of these compounds.The melting points of BaHfO 3 and SrHfO 3 are 2893 K and 3200 K, respectively [10].
Both individual barium hafnate BaHfO 3 and double barium and strontium hafnates Ba x Sr 1−x HfO 3 in sintered and pressed composites with finely dispersed tungsten exhibit extremely high thermionic properties [11,12].The mixture of 63% BaHfO 3 and 37% W (by mass) provides a thermionic current density of 230 A/cm 2 at 2000 K [11] and the composite of 80% Ba 0.75 Sr 0.25 HfO 3 -20% W (by mass) is characterized by a thermionic current density of 1, 014 A/cm 2 at T = 1620 K [12].The latter thermionic cathode material is currently the most emissive.In the studies of the thermionic properties of BaHfO 3 and Ba 0.75 Sr 0.25 HfO 3 carried out in high vacuum, contamination of the anode and structural parts of the cathode heating unit by evaporation products of the composite cathode components was always observed.The evaporation rate of barium hafnate previously measured in thermionic tests at T = 1900 K was χ ev = (5 ± 2) • 10 −6 kg/(m 2 •s) [13].
Thus, there are no data yet on the thermal vacuum evaporation of BaHfO 3 or SrHfO 3 and double barium-strontium hafnates (Ba,Sr)HfO 3 and on the properties of films deposited on different substrates.The present work fills this gap.
The aim of this work is to study the thermal vacuum evaporation properties of BaHfO 3 and Ba x Sr 1−x HfO 3 hafnates (or their components) and to determine the composition and structure of the resulting coatings at different deposition modes.

Methods
Sintered composite cathode materials based on hafnate with finely dispersed tungsten: BaHfO 3 with 37% W (by mass) and double hafnate Ba 0.5 Sr 0.5 HfO 3 with 24% W (by mass) were used to prepare films by thermal vacuum evaporation.The test specimens were prepared by powder metallurgy standard methods.The specimens were cylindrical in shape and 3 mm in diameter.The specimens were heated in the cathode heating unit (figure 1) using a tantalum heater 4, which allows the heating of specimen 1 to a temperature of 2100 K.During thermal heating, the evaporating components of the samples deposit on the centering tantalum disk 2, the forming electrode 3 made of a W -47Re alloy and on the tantalum anode 5, which is 1.5 mm from the end of the sample.The experiments were performed in a vacuum chamber evacuated by a magnetic discharge pump.The operating pressure in the area of the cathode heater was not more than 5 • 10 −6 − 2 • 10 −5 Pa.The composition of the gases in the vacuum system was monitored using an MX-7304 radio-frequency monopole mass spectrometer with electron impact ionization.No molecular oxygen was found in the residual gas spectrum obtained during the experiments.
Heating at the selected temperature could be performed simultaneously with a pulse take-off thermionic current from the heated samples to the anode.The take-off current mode was as follows: Pulse duration τ = 5 µs, pulse frequency 0.5 − 4 Hz, maximum extraction voltage 6.5 kV.The experimental setup offers the possibility to increase τ to 2.7 ms.
Two thermal evaporation modes were used for the evaporation of barium hafnate and double barium-strontium hafnates.The first mode is ordinary indirect thermal heating of the samples at constant temperature with such a heating rate to the desired temperature that the pressure in the vacuum chamber does not exceed the specified limits during degassing.The second mode is film deposition with an additional high-current vacuum arc discharge between the sample and the anode.The arc discharge can be initiated, for example, by increasing the duration of the high-voltage pulse or by increasing the temperature, during which there is a sharp increase in thermionic current density and the development of electrical breakdown in the gap between the sample and the anode.The true (thermodynamic) surface temperature was measured with a LOP-72 pyrometer using a blackbody model.The structure and composition of the films obtained were studied using a scanning electron microscope SEM-106 with an energy dispersive X-ray microanalyser.The evaporation rates of the perovskite components were calculated from the values of mass decrease during heating using the gravimetric method, the accuracy of which is ±0.1 mg.

Results and discussion
First, the results for the deposition of BaHfO 3 evaporation products on a polycrystalline tantalum substrate (anode) are presented.The first thermal evaporation mode was used to deposit the films.After raising the temperature to T = 1900 K, the barium hafnate sample was heated isothermally for 20 hours.The choice of such a long heating time is due to the fact that the previously measured evaporation rate of BaHfO 3 components at T = 1900 K, which corresponds to χ ev = 5 • 10 −6 kg/(m 2 •s) [13], is considerably lower than the evaporation rate of barium oxide even at lower temperatures than 1900 K.According to the data of [14], where the pressure p s of saturated BaO vapor was measured in the range from 1200 K to 1800 K, p s = 9.49 • 10 −1 Pa at T = 1800 K can be calculated.If the value of the saturated vapor pressure is known, the evaporation rate of a substance χ ev can be determined using the following relationship: where µ is the molar mass of the evaporating substance and R is the universal gas constant.The evaporation rate of BaO at T = 1800 K calculated according to equation (1) corresponds to χ ev = 4.30•10 −3 kg/(m 2 •s) and is almost 10 3 times higher than the evaporation rate of BaHfO 3 components at T = 1900 K.
It is well known that both tensile and compressive stresses can be generated during the deposition of films, leading to cracks in the films.The cracking at the film surface is essentially related to the different values of the thermal expansion coefficients of the film material itself and the substrate.It was expected that no cracks would occur when barium hafnate was deposited on a tantalum substrate because the values of the thermal expansion coefficients of BaHfO 3 (α = 6.93 • 10 −6 K −1 [15]) and Ta (α Ta = 6.4 • 10 −6 K −1 [16]) are close to each other.
Figure 2 shows electron microscopic images of the tantalum surface with products of thermal vacuum evaporation of barium hafnate.
X-ray dispersion microanalysis results show the presence of barium and oxigen, as well as W, Al, Ni, and Ca impurities in the evaporation products.A line of Ta as substrate is also present in the spectrum.No hafnium is found in the evaporation products on the substrate.Mass-spectrometric studies of the thermal evaporation products of barium oxide carried out in [17] have shown that mainly Ba 2+ ions (65% of total amount of ions), as well as Ba + ions (23%) and BaO + (12%) are evaporated in the temperature range from 1033 K to 1203 K.
In view of the above, it can be concluded that thermal vacuum evaporation of BaHfO 3 leads to the formation of a film on a tantalum substrate and the film consists of a mixture of Ba and BaO.This film exhibits cracks (see figure 2a) associated with significant differences in the linear thermal expansion coefficients of Ba and BaO compared to tantalum (α BaO = 12.8•10 −6 K −1 [16] and α Ba = 20.6 • 10 −6 K −1 [18]).At higher magnification (see figure 2b), many barium and barium oxide fragments of different shapes and sizes (from tenths to 10 µm) can be seen on the tantalum surface.Thus, the thermal vacuum evaporation experiments of barium hafnate show that congruent BaHfO 3 evaporation does not occur.
Further experiments on vacuum thermal evaporation of barium-strontium hafnate Ba 0.5 Sr 0.5 HfO 3 at T = 1600 K were performed.The evaporation rate of Ba 0.5 Sr 0.5 HfO 3 components measured at this temperature was χ ev = 2.22 • 10 −5 kg/(m 2 •s).In the study of the deposited evaporation products, special attention was paid to the structures formed on the surface of the tungsten-rhenium alloy substrate (forming electrode) after 30 minutes of heating.Electron microscopic studies revealed peculiar "snowflake" island structures on the substrate surface (see figure 3), consisting of a complex barium-strontium oxide BaO•SrO or a mixture of BaO and SrO.It is clear that the film system minimizes the surface area in this way to reduce the effective surface energy.According to the results of microanalysis, there are no hafnium lines in the X-ray spectra of the deposited evaporation products, but only barium, strontium, and oxygen lines.In this case, the mutual percentage of Ba and Sr is the same as in Ba 0.5 Sr 0.5 HfO 3 .Despite the fact that BaO and SrO form a face-centered cubic lattice [16], the nuclei of the new phase are characterized by a six-fold dihedral symmetry (belong to the D6 symmetry group [19]).When analyzing the images in figure 3, note that the structure in figure 3a has smaller (by a factor of 1.6) linear dimensions compared to the structures in figures 3b and 3c and is characterized by the sharp tips of the "snowflake" branches.Subsequently, these tips split, become broader (see figures 3b, 3c), and branch to form fern-like dendritic structures.Similar dendritic growth occurs in true snowflakes [20].It should be noted that these branched dendritic structures are mainly formed by strontium oxide.
Thus, the products of thermal vacuum evaporation of hafnate Ba 0.5 Sr 0.5 HfO 3 are only barium and strontium oxides.There is no congruent evaporation of barium-strontium hafnate Ba 0.5 Sr 0.5 HfO 3 .
Since congruent evaporation of hafnates is not observed in general thermal vacuum evaporation, experiments on the sputtering of hafnates by a vacuum arc discharge (the second mode of evaporation) were carried out.This discharge was induced either by increasing the duration of the high-voltage pulse between the sample and the anode from 5 µs to 0.8 − 1.2 ms or by increasing the sample temperature from 1600 K in most cases to values where the thermionic current density exceeds 80 A/cm 2 .In [20], the characteristics of the initiation of pre-breakdown states and the breakdown of the interelectrode gap between the cathode and anode (arc discharge) were studied for the case where sintered composite Ba 0.75 Sr 0.25 HfO 3 with 20% W (by mass) was used as the cathode.Breakdowns caused considerable erosion of this material with the formation of numerous craters on its surface.
In the present work, identical results (erosion and craters) were obtained when sputtering hafnate Ba 0.5 Sr 0.5 HfO 3 .figure 4 shows electron microscope images of structures deposited on a tantalum substrate during a vacuum arc discharge (centering disk 2 in figure 1).X-ray microanalysis showed that the composition of the formed structures was in complete agreement with the stoichiometric composition of Ba 0.5 Sr 0.5 HfO 3 hafnate.In a vacuum arc discharge, barium-strontium hafnate sputters at the cluster level forming dynamically stable particles of the complex oxide 0.5BaO•0.5SrO•HfO 2 in the plasma.These particles represent the vapor phase for the formation of a coating of crystals of various shapes and sizes, but not in the form of a thin film.
Thus, the use of an arc discharge allows congruent evaporation of Ba 0.5 Sr 0.5 HfO 3 and the production of coatings with the original sputtered composition.

Conclusions
For the first time, experiments on thermal vacuum evaporation of barium hafnate BaHfO 3 and barium-strontium hafnate Ba 0.5 Sr 0.5 HfO 3 were carried out to obtain film coatings on various metal substrates.
A feature of the evaporation of these materials is their incongruent evaporation with the formation of a Ba and BaO mixture (for BaHfO) or BaO•SrO crystals (for Ba 0.5 Sr 0.5 HfO 3 ).
Nucleation features of BaO•SrO film structure with six-fold dihedral symmetry and dendritic growth are revealed.
It is shown that congruent evaporation of Ba 0.5 Sr 0.5 HfO 3 can be performed by sputtering the material in vacuum arc discharges between the hafnate and the anode.
As a result of the evaporated barium-strontium hafnate deposition, crystal structures of different shapes and sizes are formed.

Figure 2 .
Figure 2. SEM images (in secondary electrons) of BaHfO 3 evaporation products on tantalum: a -crack in the barium and barium oxide coating, b -Ba and BaO fragments.

Figure 3 .
Figure 3. SEM images of initial growth patterns of BaO•SrO crystals on a W-Re substrate: a and c -in primary electrons, b -in secondary electrons.Magnification: a -100x, b and c -60x.

Figure 4 .
Figure 4. SEM images of crystal structures (Ta substrate) after vacuum arc sputtering of Ba 0.5 Sr 0.5 HfO 3 at various magnifications.