JPH0393608A - Production of thin film of oxide superconductor - Google Patents

Abstract

PURPOSE:To form a thin film of oxide superconductor showing excellent superconducting characteristics on a substrate without carrying out heat treatment after film formation by feeding active oxygen seed made into plasma from an active oxygen seed generator arranged in a vacuum container to a substrate. CONSTITUTION:An active oxygen seed generator 9 opening a nozzle 9a with given diameter is arranged in a vacuum container 1 in a reduced pressure state of <=1X10<-3>Torr. Then pressure in the container 9 is made into a reduced pressure state of >=1X10<-3>Torr higher than the pressure in the container 1 and gas molecules containing O atom in the container 9 are made into plasma by high-frequency power 11. Simultaneously, as active oxygen seed formed by the plasma is fed to the vicinity of a substrate 3, a thin film of oxide superconductor is formed on the substrate 3. By the above-mentioned method, as constituent elements of the oxide superconductor are sufficiently evaporated (evaporation sources 4, 5 and 6), the active oxygen seed such as oxygen radical and oxygen ion having high density and low kinetic energy can be fed to the vicinity of the substrate 3 and a deposited material is sufficiently oxidized in the film formation.

Description

[Detailed description of the invention]

[Object of the Invention] (Industrial Application Field) The present invention relates to a method for producing an oxide superconductor thin film. (Prior art) Since it was announced in 1988 that Ha-La-Cu-0-based layered perovskite oxides have a high critical temperature of 40K or more, oxide-based superconductors have attracted attention. Research is being actively conducted to collect and search for new materials. Among them, defective perovskite-type oxide superconductors represented by the Y-Ba-Cu-0 system, which have a high critical temperature higher than the liquid nitrogen temperature, the Bl-Sr-Ca-Cu-0 system and the TI-Ba -Ca-Cu-0-based oxide superconductors have important industrial value because inexpensive liquid nitrogen can be used as a coolant instead of expensive liquid helium. Research into these applications is also actively being conducted, including ultra-high-speed superconducting devices, SQLIID sensors,
Many applications are being considered, including superconducting wiring, superconducting coils, and power transmission lines. For example, if superconductors are used as wiring for various electronic devices, signal delays caused by the wiring will be reduced, making it possible to speed up signal processing or calculations. When considering the application of oxide superconductors to such superconducting wiring, it is advantageous to use a thin film whose critical current density is one order of magnitude higher than that of a bulk material. Oxide superconductor thin films with such excellent superconducting properties are
In many cases, it is manufactured using a vacuum process, such as sputtering, laser sputtering, vacuum evaporation,
Thin film forming methods such as the MBE method, MOCVD method, and JCB deposition method are applied. By the way, among the various thin film forming methods mentioned above, vacuum evaporation method, M
BE method, ICB vapor deposition method, etc. are usually IXIO-'Tar
Since the process is carried out under a high vacuum of less than t, the amount of oxygen introduced into the thin film cannot be sufficiently increased, and a thin film exhibiting good superconducting properties cannot be directly obtained. In order to solve these problems, attempts have been made to increase the amount of oxygen introduced and improve the superconducting properties by supplying activated oxygen by converting it into plasma. Specifically, this is a method of accelerating electrons to a high energy state using electron cyclotron resonance (ECR) and colliding them with oxygen to turn them into plasma. According to this method, IXIO'
Plasma can be generated even under a vacuum of less than 1 Torr or under a high vacuum of less than I x 10-' Torr. However, ECR requires a magnetic field to be created within the reaction chamber, which requires surrounding the reaction chamber with a coil or permanent magnet.
There was a problem in that there were significant restrictions on the equipment, such as the equipment being large-scale. In addition, the reaction chamber that generates plasma must be installed at a certain distance from the substrate, and the active species that reach the substrate are drawn out and accelerated by the leakage magnetic field from the ECR reaction chamber. Although the kinetic energy was large, the density was low. In other words, since this is a method in which oxygen active species with low density are introduced into the thin film using high kinetic energy, the resulting thin film is damaged by high-energy particles. On the other hand, a method has been proposed in which a coil for introducing high frequency waves is installed in a vacuum container and a glow discharge is caused there to turn oxygen into plasma, but in reality, glow discharge does not occur under a high vacuum of less than 104 Torr. Therefore, although it is not a practical method to carry out vapor deposition while generating oxygen plasma under a high vacuum of less than l x 10-' Torr, which is required for vacuum evaporation, MBE, IcB evaporation, etc. do not have. (Question 11 to be solved by the invention!) In order to improve the properties of an oxide superconductor thin film without damaging it, it is necessary to supply oxygen active species with low kinetic energy and high density to the film forming surface. However, the currently proposed method maintains the pressure inside the vacuum container required for vacuum evaporation, MBE, ICB evaporation, etc., i.e., the high vacuum below txto-'minorr, while achieving the above-mentioned It is extremely difficult to supply such active species of oxygen to the film-forming surface. The present invention was made to deal with such yAIfi, and satisfies the internal pressure of a vacuum container of at least IX 10" Torr or less required for vacuum evaporation, MBE, ICB evaporation, etc. To form an oxide superconductor thin film with excellent superconducting properties with a relatively simple equipment configuration by making it possible to supply active oxygen species with high density and low kinetic energy near the adherend substrate. The purpose of this invention is to provide a method for manufacturing an oxide superconductor thin film that makes it possible to achieve the following.

[Structure of the invention]

(Means for solving the problem) That is, the method for producing an oxide superconductor thin film of the present invention
When producing a thin film of the oxide superconductor by evaporating each metal element constituting the oxide superconductor and depositing it on the surface of the substrate in a vacuum container with a reduced pressure of X 10' Torr or less, there are no penalties. An oxygen active species generation container having a nozzle of a predetermined diameter opened in the vacuum container is disposed in the vacuum container, and the oxygen is brought into a reduced pressure state at a pressure higher than the internal pressure of the vacuum container of 1×10'' Torr or more. Gas molecules containing oxygen atoms are turned into plasma by glow discharge using high-frequency power in an active species generation container, and the oxide superconductor thin film is grown while supplying the oxygen active species formed by the plasma to the vicinity of the substrate. It is characterized by forming a film. (Function) By arranging an oxygen active species generation container in which a nozzle with a predetermined diameter is opened inside the vacuum container, the inside of the vacuum container can be evacuated to a high vacuum state of 1 x 10' Torr or less. Since the conductance inside the oxygen active species generating container is set to be low by the nozzle portion, it is possible to create a reduced pressure state higher than the internal pressure of the vacuum container. As a result, a high vacuum state of less than IX 10' Torr, which is necessary to sufficiently evaporate the constituent elements of the oxide superconductor, is achieved inside the vacuum vessel, while glow discharge using high-frequency power is generated within the oxygen active species generation vessel. This makes it possible to generate plasma. Therefore, while sufficiently vaporizing the constituent elements of the oxide superconductor, active species of oxygen such as oxygen radicals and oxygen ions with high density and low kinetic energy can be sufficiently supplied to the vicinity of the substrate, and deposits can be removed during film formation. It becomes possible to oxidize sufficiently. (Example) Next, an example of the present invention will be described with reference to the drawings. FIG. 1 shows a film apparatus to which a method for producing an oxide superconductor thin film according to an embodiment of the present invention is applied. In the figure, 1 is a vacuum vessel connected to an exhaust system 2, and inside this vacuum vessel 1, a substrate 3 and a single substance or compound of each metal element constituting the target oxide superconductor thin film are contained. Evaporation sources 4, 5, and 6 consisting of are arranged facing each other. The above-described substrate 3 is made of, for example, MgO SSrT1 (h,
Al203, Y stabilized Zr02 (YSZ), Sl,
It is made of Ag, Nozosteroy, Inconel, etc., and its shape is not limited to a plate-like shape, but can be any shape such as a rod-like shape, a wire-like shape, or a tape-like shape. Furthermore, a thin film of SrT103 or the like provided as a buffer layer on a metal substrate may be used, and when such a deposited substrate is used, the resulting film has higher C-axis orientation. Note that a heater 8 for heating the substrate is provided in the holder 7 that holds the adherend substrate 3.
is installed, and the deposited substrate 3 is heated to 400° C. or higher, preferably 850° C. or higher during film formation. As a result, a thin film with good crystallinity can be obtained. In addition, the evaporation sources 4, 5, and 6 are selected depending on the oxide superconductor thin film to be formed, and in this example, the metal structure or elements of the Y-Ba-Cu-0 based oxide superconductor are selected. Metal Y4, metal Ba5, and metal Cu6 are arranged, respectively. There are several heating methods for these evaporation sources 4, 5, and 6. For example, high melting point substances such as metal Y4 and metal Cu6 are evaporated by electron beam heating, and metals with relatively low melting points such as Ba5 are evaporated by resistance heating. let The evaporated substances from the evaporation sources 4, 5, and 6 heated by these methods simultaneously fly toward the deposition substrate 3. In addition, as the oxide superconductor applied to the present invention, Y
-Not limited to Ba-Cu-0-based oxide superconductors, but also belobite-type oxide superconductors containing various rare earth elements, B
1-Sr-Ca-Cu-0 based oxide superconductor, TI-B
An a-Ca-Cu-0 based oxide superconductor or the like is applied. The oxide superconductor containing a rare earth element and having a belobusite structure may be one that can realize a superconducting state, for example, REMCuO system 287-δ (RE is Y. La1ScSNds Ss, Eu
, Gds Dys 110%Ers Tm, Wb,
At least one element selected from rare earth elements such as Lu, H is at least one element selected from Has Sr, Ca, δ represents an oxygen defect, and is usually a number equal to or less than 1,
Part of Cu is TiSV 1Crs MnsFes CO
Can be replaced with SN1% Zn, etc. ) are exemplified. In addition, rare earth elements are defined in a broad sense, and are defined as Sc,
It shall include Y and La systems. In addition, the Bi-Sr-Ca-Cu-0 based oxide superconductor has the chemical formula: B12 Sr2Caz Cux OX
------- (1): B1z<Sr. C
a>3cu20x - (II) (in the formula, a part of Bl can be replaced with pb etc.), and is a TI-Ba-Ca-Cu-0 system oxidation. The physical superconductor has the chemical formula: TI2 Baz Caz Cu30!1
- (I[[): TI2 (Ba.Ca) 3
It is represented by Cuz Ox -...-- (IV) and the like. When forming a thin film of an oxide superconductor other than Y-Ba-Cu-0, an appropriate evaporation source is selected and used. Further, in the vicinity of the adherend substrate 3 in the vacuum container 1, an oxygen active species generating container 9 made of a quartz tube with a nozzle 9a formed by narrowing the aperture at one end is arranged.
is directed toward the 3 directions of the adherend substrate. A coil 12 connected to a high frequency power source 11 via a matching box 10 is wound around the outer periphery of the oxygen active species generation container 9 . Further, at the other end of the oxygen active electrode generation container 9, a gas containing oxygen atoms (not shown), such as o2, o3, co
An oxygen atom-containing gas supply pipe 13 is connected to a supply source of , CO2, N20, etc. via a valve (not shown). The nozzle 98 at the tip of the oxygen active species generation container 9 has a narrow diameter and is therefore set to have a low conductance.
When a gas of about 20 SCCM is supplied from the oxygen atom-containing gas supply pipe 13 while exhausting the gas at a rate of about 0,000 g/sec,
The pressure inside the vacuum container 1 is IXIO゜4～1×1G' To
At the time of rr, the inside of the oxygen active species generation container 9 is IX 10-2
A reduced pressure state of approximately Torr can be achieved, and in this state, glow discharge can be caused by applying high frequency power of, for example, Nllz order to the coil 12, and plasma is generated. The diameter of the nozzle 9a is made small when it is desired to set a large pressure difference with the inside of the vacuum container 1, and made relatively large when a large pressure difference is not required. For example, pumping speed 3000g
/sec, flow rate 208 CCH, when the pressure inside the oxygen active species generation container 9 reaches I x 1 G' Torr or more and plasma is generated, if the diameter of the nozzle 9a is lam, then
The pressure within the vacuum vessel 1 is below I x 10-s Torr, and if the diameter of the nozzle 9a is 211, the pressure within the vacuum vessel 1 is below l x 10-' Torr. Therefore, when forming a film under high vacuum, the nozzle 9a
On the other hand, in order to generate plasma as widely as possible, the diameter of the nozzle 9a may be increased. In this way, the diameter of the nozzle 9a can be arbitrarily selected in consideration of the vacuum conditions, the size of the adherend substrate 3, and the like. Further, the plasma generation state can also be changed by changing the diameter of the quartz tube constituting the oxygen active species generation container 9. In other words, when the diameter of the quartz tube is increased, the flow velocity inside the tube decreases, and the diameter of the coil 12 also increases.
Even if the output from 1 is the same, the power applied to the discharge region increases and the rate of oxygen decomposition increases. Normal nozzle 9a
When the diameter of the quartz tube is set to approximately 1vi to 211, it is appropriate that the diameter of the quartz tube is approximately 10sm to 50g+n. However, these diameters vary depending on the pumping speed, flow rate, etc., and are not absolute values. Normally, the diameter of the nozzle 9a is preferably lO11 or less, more preferably 511 or less, and the tube diameter is preferably lO1 or less. More generally speaking, the nozzle 9a
The diameter of the tube is preferably 1/4 or less of the pipe diameter, more preferably 1/10 or less. On the other hand, the nozzle 9a of the oxygen active species generation container 9 and the S target substrate 3
The distance from the substrate is also an important factor that greatly affects the oxidizing power during film formation. A feature of the present invention is that plasma can be generated close to the adherend substrate 3, and the distance between the adherend substrate 3 and each of the evaporation sources 4, 5, and 6 is 1/2 or less. is preferable, and more preferably 1/10 or less. The pressure inside the vacuum container 1 during film formation varies somewhat depending on the film formation method, but is preferably set to IX 10' Torr or less so as not to hinder the evaporation from each evaporation i[4, 5, 6]. is less than 5×1G' Torr. In addition, the pressure inside the oxygen active species generation container 9 is higher than the pressure inside the vacuum container 1, and glow discharge can occur. IX
Set to more than Torr. Further, oxygen in the oxide superconductor is particularly markedly desorbed when heated to a high temperature in a vacuum, and therefore desorption of oxygen may also occur during cooling. Therefore, it is preferable to slowly cool the film while exposing it to oxygen plasma even after the film formation is completed. The oxygen plasma generation conditions during this cooling are preferably such that the pressure inside the oxygen active species generation container 9 is higher than that during film formation.
It is approximately 1G' Torr to 1 Torr. In addition, the cooling rate is 1-10℃/min up to about 400℃, and then 2
400℃ at a rate of 1 to 20℃/min until about 00℃.
It is preferable to set the cooling rate up to 200°C higher than the cooling rate up to 200°C. By the way, the method of causing glow discharge is not limited to the method of wrapping the coil 12 around the oxygen active species generation container 9 as described above, but also the methods shown in FIGS. 2 to 5, for example, can be adopted. You may. FIG. 2 shows a quartz tube 21 connected to an electrode 2 made of a stainless steel tube.
2, and a ground electrode 23 is provided inside the quartz tube 21. Note that the ground electrode 23 may also serve as the oxygen atom-containing gas supply pipe 13. In addition, as shown in FIG. 3, the electrode 2 is divided into two parts.
4a, 24b may be used, in which case these electrodes 2
4a and 24b may be at the same potential, or as shown in the figure, one electrode 24a may be at a high frequency potential and the other electrode 24a may be at the same potential.
b may be set to the ground potential. Furthermore, as shown in FIG. 4, a nozzle 25a can be created by squeezing one side of a tubular body 25 made of a conductive material such as a stainless steel tube, so that the nozzle 25a can serve both as the oxygen active species generation container 9 and a high-frequency electrode. In this case, a ground electrode 27 insulated from the tubular body 25 by an insulator 26 is inserted inside. By configuring the oxygen active species generation capacity rA9 in this manner, there is no power loss due to the low dielectric constant of the quartz tube, resulting in good efficiency. Further, in FIG. 5, the inner electrode 28 is set to a high frequency potential, and the two outer electrodes that also serve as the container are set to ground electrodes. By the way, in each of the oxygen active species generation containers 9 shown in FIGS. 1 to 4, the pressure inside the vacuum container 1 increases.
' When the Torr stage is reached, since the vacuum vessel 1 is grounded, discharge occurs between the outer high-frequency electrodes 12, 22, 24, 25 and the inner wall of the vacuum vessel 1, and plasma is generated within the vacuum vessel 1. At the same time, the load impedance fluctuates greatly, making it necessary to adjust the matching, making it difficult to obtain stable plasma. For this reason, it is preferable that the pressure within the vacuum chamber be 1x10' Torr or less. On the other hand, in the oxygen active species generation container shown in FIG. 5, the vacuum container 1 and the outer electrode 29 are always at the same potential;
Regardless of the pressure inside the vacuum vessel 1, no discharge occurs outside. Therefore, the load impedance depends only on the pressure within the two tubular bodies, matching can be easily adjusted, and stable plasma can be generated at all times. Therefore,
The pressure inside the vacuum vessel can be raised to the order of 1 Torr. Here, the glow discharge generation mechanism of each oxygen active species generating container shown in FIGS. 1 to 5 will be explained using an equivalent circuit. In the mechanism using the coil 12 shown in FIG.
In addition to the L component due to 2, there is a C component (capacitor capacitance) between the coil 12 and the vacuum container internal jig at ground potential. Therefore, an equivalent circuit as shown in FIG. 6 is formed, in which the coil 12 and the capacitor 31 are connected in series. The impedance caused by this capacitor 31 also differs depending on the pressure inside the vacuum container 1 and whether or not plasma is generated. Note that 32 in the figure is a matching circuit. Furthermore, the mechanisms shown in Figures 2 to 5 are called capacitively coupled types, and the only load is the capacitor, but in the case of the mechanisms shown in Figures 2 to 4, the electrodes and the vacuum vessel are connected. There is a C component, and the equivalent circuit is capacitor 3 as shown in Figure 7.
3 and 34 are connected in parallel. In this case as well, the total impedance changes depending on the pressure inside the vacuum vessel 1. The mechanism shown in FIG. 5 has only one capacitor as a load, resulting in an equivalent circuit shown in FIG. 8. In this case, the capacity of the condenser 35 is determined only by the size of the container and the pressure inside the tube, and is not dependent on the pressure inside the vacuum container 1. Therefore, it is a method that is easy to match and can generate plasma most stably. Note that in any of the above methods, sufficient oxidizing power can be obtained during film formation. Next, an example in which a Y-based oxide superconductor thin film was actually produced using the film forming apparatus having the above configuration will be described. First, as the substrate 3 to be inspected, SrTI (
Set the substrate in the holder 7, and as the oxygen active species generation container 9, use
The mechanism shown in the figure was adopted and placed in the vacuum container 1 so that the nozzle 9a was located at a position 3011 from the adherend substrate 3. In addition, metal Y4 and metal B are used as evaporation sources.
A5 and metal Cu6 were placed at a distance of 300 cm from the adherend substrate 3, respectively. Note that the adherend substrate 3 was heated to 800°C. Then, the inside of the vacuum container 1 was evacuated at 3000 e/sec, and the inside of the vacuum container 1 was set to 2X 10' Torr. In addition, this allows the inside of the oxygen active species generation container 9 to be
'Torr and supply oxygen at 208CCM,
While applying a high frequency power of 13.58 Mllz to generate plasma and supply active species of oxygen to the vicinity of the deposition substrate 3, the evaporation sources 4, 5, and 6 were evaporated to perform ternary simultaneous evaporation. After that, it was cooled to 400°C at 2°C/min.
00°C for 1 hour, then 200°C at 5°C/min.
After cooling to 50° C. in oxygen at 1 atm, an oxide superconductor thin film was produced. The pressure inside the vacuum vessel 1 during cooling was set to 2X 1G-'' Torr. The obtained oxide superconductor thin film had a film thickness of 5000 mm,
After slow cooling, Yl ”1.950u3.050B.8
It had a critical temperature of 85K or higher and a critical current density of IX 1G' A/cj, which were good values. Also, X
It was confirmed from the results of line diffraction that the film was oriented along the C axis. When oxide superconductor thin films were similarly prepared using the various substrates described above as the adherend substrate 3, thin films exhibiting superconducting properties were obtained for all substrates, although the characteristics were dependent on the substrate. . Next, when an oxide superconductor thin film was produced under the same conditions as the above-described vapor deposition method except that each evaporated substance was used as an ionized cluster beam, even better results were obtained. One reason for this is that in ICB, charged particles such as electrons or ionized clusters fly by due to emission, making the oxygen plasma more stable, and the ICB vapor deposition method is most preferred in the present invention. The results of X-ray diffraction showed that although the film was formed under the same conditions, the peak intensity of the diffraction pattern was one order of magnitude larger than that of the film formed by the above-mentioned vapor deposition method, indicating that the crystallinity was improved. Therefore, the critical current density is 1 × 10' To
Improved to rr level. Furthermore, with this method, the orientation on the metal substrate is improved.
On the Ag substrate, a polycrystalline film with strong C-axis orientation was obtained. This is thought to be due to the fact that the migration effect that occurs when ionized clusters are accelerated by an electric field and collide with the substrate is greater than in normal vapor deposition methods. Next, we took advantage of the fact that an oriented film can be easily obtained on a metal substrate by using this ICB deposition method, and used Ag,・^U tape or wire, Ag-plated SUS, N1,
Using nichrome, inconel tape or wire, unplated tape or wire of each of the above metals,
An oxide superconductor thin film was continuously formed while moving at a speed of l c -/min, and after a certain amount of film was formed, the film was wound up to produce a coil. In the film forming apparatus, as shown in FIG.
Except for arranging the wire supply roller 45 and the winding roller 46 so as to pass through a position facing the B ion sources 42, 43, and 44, and arranging the heating heater 47 behind the wire passage section, the first The configuration is the same as in the figure. However, the film position was limited to the region where a uniform film could be obtained by the aperture 48, and a shutter 49 was provided in front of the film forming region. The thus obtained superconducting coil had a critical current density of 77K and a good value of IX 10' kid. As described above, in this example, since the tube with the tip opening narrowed and the inductance set small is installed in the vacuum container as the oxygen active species generation container, the inside of the vacuum container can be evacuated to a predetermined degree of vacuum. In this case, it becomes possible to set the pressure inside the oxygen active species generating container to be high. Therefore, plasma can be generated by causing a glow discharge in this oxygen active species generation container, and the oxide superconductor is It becomes possible to form a film. Therefore, an oxide superconductor thin film with excellent superconducting properties can be stably obtained. In the above example, the film formation of a V-based oxide superconductor was described, but B1-based oxide superconductors, TI-based oxide superconductors, Bl-based oxide superconductors added with PB, etc.
It can be applied to various oxide superconductors and similarly good results can be obtained. In addition to 02 gas, 03
, N2 0 , NO2, CO, co2, etc., can also produce similar effects, and in particular, 03 and N20 are preferable because they have a higher rate of becoming oxygen radicals and oxygen ions than 02 gas. [Effects of the Invention] As explained above, according to the method for producing an oxide superconductor thin film of the present invention, the kinetic energy is low but the activity is high without reducing the degree of vacuum in the vacuum vessel in which the film is formed. It becomes possible to supply oxygen active species such as oxygen radicals and oxygen ions to the vicinity of the adherend substrate at high density. Therefore, it becomes possible to form an oxide superconductor thin film exhibiting good superconducting properties without performing heat treatment after film formation.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of a film forming apparatus to which the method for producing an oxide superconductor thin film according to an embodiment of the present invention is applied, and FIGS. 2 to 5 show modified examples of the oxygen plasma generation mechanism. Figures 6 to 8 are equivalent circuit diagrams showing the oxygen plasma generation mechanism shown in Figures 1 to 5, and Figure 9 shows the configuration of a film forming apparatus to which another embodiment of the present invention is applied. FIG. 1... Vacuum container, 2... Exhaust system, 3.
...Adherent substrate, 4, 5, 6... Evaporation source,
9...Oxygen active species generation container, 9a...
Nozzle, 10...Matching box, 11.
...High frequency power supply, 12... Coil.

Claims (1)

[Claims] (1) Each metal element constituting the oxide superconductor is evaporated and deposited on the surface of the substrate in a vacuum container with a reduced pressure of 1×10^-^3 Torr or less to form a thin film of the oxide superconductor. When manufacturing, an oxygen active species generation container having a nozzle of a predetermined diameter opened in the vacuum container is placed in the vacuum container, and a 1×1
Gas molecules containing oxygen atoms are turned into plasma by glow discharge using high-frequency power in the oxygen active species generation container which is in a reduced pressure state higher than the internal pressure of the vacuum container of 0^-^3 Torr, and formed by this plasma. A method for producing an oxide superconductor thin film, characterized in that the oxide superconductor thin film is formed while supplying active species of oxygen to the vicinity of the substrate.