US3617381A

US3617381A - Method of epitaxially growing single crystal films of metal oxides

Info

Publication number: US3617381A
Application number: US748757A
Authority: US
Inventors: Joseph John Hanak
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1968-07-30
Filing date: 1968-07-30
Publication date: 1971-11-02
Anticipated expiration: 1988-11-02

Abstract

A method of epitaxially growing single crystal ferrite films by vapor transport comprises forming gaseous metal halides from metal oxides and decomposing these metal halides to form a single crystal film of a desired ferrite composition upon a single crystal substrate.

Description

United States Patent Joseph John Hanak Trenton, NJ. 748,757

July 30, 1968 Nov. 2, 197 1 RCA Corporation Inventor Appl. No. Filed Patented Assignee METHOD OF EPITAXIALLY GROWING SINGLE CRYSTAL FILMS OF METAL OXIDES References Cited UNITED STATES PATENTS Moest Chu et a1. Mee et al....

Mee Pulliam Burd et a1.

Primary Examiner-Herbert T. Carter Attorneys-Glenn H. Bruestle and Joel F. Spivak 23/50X 117/106 ll7/169X 117/169X 1l7/106X 117/106X ABSTRACT: A method of epitaxially growing single crystal ferrite films by vapor transport comprises forming gaseous metal halides from metal oxides and decomposing these metal halides to form a single crystal film of a desired ferrite composition upon a single crystal substrate.

PATENTEUuuv 2 I97! 3.617. 381

1; a 4n! fig lllli/ INVENTOI ATTOIIIY METHOD OF EPITAXIALLY GROWING SINGLE CRYSTAL FILMS OF METAL'OXIDES BACKGROUND OF THE INVENTION This invention relates to the synthesis of single crystal thin films of metal oxides and particularly to an improved process for growing single crystal films of ferrimagnetic oxides.

Ferrimagnetic films are useful, for example, as memory elements, as microwave devices such aslimitors and delay lines, as magnetic recording head surfaces, and in magneto-optic devices. The magnetic properties of ferrimagnetic materials, such as ferrites, depend largely upon the chemical composition, the valence of the constituentelements and;the micro structure of the material. In order to obtain high quality ferrite films, many variables must be carefully controlled these variables include temperature, time and-atmosphere.

Recently, single crystal films of magnetic oxides have been prepared by the chemical vapor transport of the constituent metal halides in a carrier gas. In accordance with these techniques, each metal halideisseparately heated toatemperature above its melting points and a carrier. gas is passed thereover so as to carry vapors of the metalhalide. The. gas flow over each halide mustbe separately adjusted inorder to obtain the desired proportionsof the various metals which react to form the magnetic film. The gaseous reactants in these prior art techniques are brought together to form a reaction mixture, which mixture also includes water vapor. The introduction of the water vapor into the reaction mixture'must also be separately controlled. .In addition tothe aforementioned controls over the various constituents of the reaction mixture, the temperature of the reaction chamber must also be separately controlled in order to cause the reactants to form the desired product. These many variables which must be controlled according to the prior art techniques make these techniques difficult to perform, difficult to control, and difficult to duplicate.

Other prior art methods for producing ferrite films involve vacuum evaporation, sputtering, and the like. These methods either result in single crystal films having poor magnetic characteristics or in polycrystalline films.

Thepresent invention discloses a method of producing single crystal thin films, which method is significantly easier to control than the prior art procedures, and resultsi'n magnetic sin glecrystal films having good magnetic properties.

SUMMARY or THEINVENTION A vapor transport method for the deposition of single crystal metal oxide films comprises the steps of converting constituent metal oxides to gaseous products and then converting said gaseous products to single crystal metal oxide layers by epitaxially depositing said metal oxides on a single crystal substrate.

BRIEF DESCRIPTION OF THE DRAWINGS H6. 1 is an elevational partially sectional view of an apparatus useful in carrying out one embodiment ,of the novel method.

FIG. 2 is an elevational, partially sectional view of an apparatus used to carry out another embodiment of the novel method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel method is applicable to the production of single crystal films of metal oxides generally, and is particularly useful for the production of single crystal magnetic thin films.

A. Close-Space Vapor Transport C lose-space vapor transport is similar to closed vapor transport systems in that the chemical reactions involved are essentially the same. However, the close-space system differs from the prior art closed systems in that 1. it is done in an open tube under a very low flow rate of reactant hydrogen halide gases as compared to the completely sealed tube equilibrium conditions of the prior art systems,

2. single crystal'films are grown by epitaxially-depositing the reaction product on a single crystalsubstrate as compared to the growth of crystallites'at one end of the sealed reaction tube as occurs in the prior art systems.

The, novel close-space transport method employs a starting material of a metal oxide, for example, a sintered ferrite of essentially the same composition as the desired film to be produced. This starting material is preferably held in acontainer such as platinum, which is inert to the initial reactants as well as to the intermediate metal halide vapors produced during the reaction. According to the novel method, theflow rate of the reactant hydrogen halide gas is very low, preferably in the orderof l to 5 milliliters per minute, and typically less than about 10 milliliters per minute, providing near equilibrium conditions. The substrate upon whichthe ferrite film is formed is placed in close proximity to the ferrite starting material. Typically, the distance between the starting material and the substrate is inthe order of l to 5 millimeters; although distances of up to about 10 millimeters are practical and distances of from about 1 to 3 millimeters are preferred. This distance is important since at the operating gas pressure the gasis moved essentially by diffusion andthe process is limited by the diffusion of the gases, thereby making the rate of deposition approximately inversely proportionalto the square of thedistance between the reactant metaloxideand the substrate. There is a temperature gradient between the reactant and the substrate in the close-space technique. The temperature. gradient is such that the temperature at the surface of the substrate is less than that of the reactant. Typically, the temperature of the substrate is about 20to 30 degrees centigrade cooler than that of'the reactant metal oxide.

An apparatus 10 useful for the operation of this novel closespace vaportransport method for producing thin metaLoxide films is shown in FIG. 1. The apparatus 10 comprisesia 're- .sistance furnace 11 which has a flat or planar inner bottom surface 12. Under the surface 12 is a heating element 13 extending below and along'the length of the inner bottom surface 12 of;the furnace 11. A reaction tube '14 of a material such as quartz which is relatively inert to the gases present therein during the reaction and which can be heated to the temperatures required, is provided. The reaction tube 14 which is placed in the furnace 11 as shown, is closed at one end 15 and has a flat bottom surface 16 which rests on the fiat bottom surface 12 of the furnace '11. The open end of the tube 14-is provided with a gasinlet 17 and a gas exhaust 18 which may be connected to the tube 14 by means of a ball joint assembly 19.

In operation, the reaction tube 14 is positioned in the furnace 11 and the furnace is loaded with two inert containers 20 and 21, which containers 20 and 21 contain powdered ferrite 22 of essentially the same composition as the final film to be deposited. The containers 20 and 21, which can be made of platinum, are preferably rectangular in cross section so as to fit flatly on the bottom of the tube 14. The containers 20 and 21 are placed in the tube 14 one behind the other. The container 20 farthest from the gas inlet 17 has a single crystal substrate 23, of for example magnesium oxide, placed directly over an opening 24 in its top lid or surface. The distance between the substrate 23 and the level of the ferrite 22 within the container 20 is preferably between about I to 3 millimeters. The container 21 nearer the gas inlet is shown as having a plurality of openings 25 in its top portion so as to provide for the flow of gas in and out of the container 21.

After placing the containers 20 and 21 in the furnace 11 as described, it is preferable to place a quartz wool plug 26 in the tube 14 as shown. The plug 26 acts both as a filter for incoming gas and a trap and condensation area for outgoing gases.

The ball joint assembly 19 which is provided with the gas inlet 17 and the gas outlet 18 is then connected onto the tube 14 and the furnace 11 is brought to operating temperature under an inert gas atmosphere. The operating temperatures typically range from about 850 to 1 150 C. and are preferably between 950 to 1100 C. The inert atmosphere is provided, for example, by allowing helium gas to flow into the tube 14. Since the sintered ferrite sources 22 are nearer the heating element 13, a temperature gradient exists whereby the substrate 23 is maintained at a temperature of from about 20 to 30 C. below that of the ferrite 22 within the container 20. When the furnace 11 reaches the desired operating temperature, as determined by the particular ferrite and the desired deposition rate, the gas admitted to the tube is altered to include either hydrogen chloride or hydrogen bromide of possibly a mixture thereof along with an inert carrier gas. Typically, the flow rate of the gas mixture is about 3 to 6 milliliters per minute with about onehalf the gas being the hydrogen halide. As the gases reach the ferrite 22, the following reaction occurs: M 20 (s)+bh8HX(g)=MX,(g)+2Fe X -,(g)+4H 0.(g), wherein M represents the metal ion in the ferrite 22 other than iron and X represents the particular halide used in the reaction. The gaseous products produced, which are indicated on the right hand side of the above equilibrium equation, diffuse from the surface of the ferrite 22 in container 20 to the cooler surface of the single crystal substrate 23 where they then recombine to form a single crystal ferrite film 28 on the surface of the substrate 23. The thickness of the film 28 depends upon the rated deposition which is generally in the range of about 2 to 8 microns per hour, and the total deposition time.

Generally, the rate of formation and the rate of diffusion of the ferric halide and the water vapor are greater than that of the other metal halide vapors formed during the reaction. This results in a small portion of these materials escaping from the region of the container 20 and the substrate 23 thereon, and consequently causes a deviation from equilibrium conditions. This in turn can cause a variation in the composition of the ferrite film. which variation would be greater with time. Such a variation is effectively and substantially eliminated by the presence of the ferrite 22 in the container 21, which acts as a makeup device for maintaining relatively constant equilibrium conditions around the region of the substrate 23 by being a source for the replenishment of the lost gases. In addition, the very slow flow ratesemployed and the quartz wool plug 26 also minimizes this effect.

lt should be noted that the novel method may be used to produce single crystal films of ferrites doped with or containing several, metal ions by, for example, either adding such metals to the ferrite powder starting material in the form of their oxides or other forms which will ultimately react with the hydrogen halide to produce a volatile metal halide at the reac tion temperature or by starting with a ferrite powder containing these metals therein or by starting with a mixture of oxides rather than ferrite and including oxides of these dopant metals.

Example 1 This example describes the preparation of single crystal films of magnesium manganese ferrite. The platinum containers 20 and 21 are 4 inches long, 1 inch wide and onefourth inch high and they are filled to within about 2 millimeters from the top with powdered magnesium manganese ferrite comprised of 20, 30 and 60 mole percent of MgO, MnO and Fe o respectively. The temperature of the surface 12 is brought to and held at about 1050 C. A flow ofa mixture of hydrogen bromide gas at about 1 milliliter per minute and of helium at about milliliters per minute is passed into the tube 14 through the gas inlet 17. The substrates 23 are four single crystal M g0 wafers %X-'%X1/32 inches each. The temperature of the substrates is approximately 1020 C. The desired ferrite film deposits on the surface of these substrates.

The total deposition time of 72 hours yields a film which is about 0.015 inch thick on eachof the substrates. The composition of the deposit produced was analyzed to be about 2 percent lower in magnesium and about 2 percent higher in iron than the starting material.

Example 2 Synthesis of manganese ferrite. The conditions and the manner of carrying out the deposition are similar to example 1, except that the temperature at the surface 12 is brought to 900 C., and the flow of HBr is 2 milliliters per minute and that of He 3 milliliters per minute. Correspondingly, the source material is powdered manganese ferrite. ln a 50-hour period a 0.012 inch thick deposit is obtained on single crystal substrate of manganese zinc ferrite.

Example 3 Synthesis of lithium ferrite. The same procedure is followed as set forth in example 1 except that the surface 12 is held at 1,000 O, and the flow of HBr and helium are both 2 milliliters per minute. The substrate of this example is single crystal sapphire and the source is powdered lithium ferrite. Epitaxial films 0.003 inch thick are obtained in 24 hours.

Example 4 Synthesis of nickel ferrite and cobalt ferrite. The same procedure as set forth in example 1 is employed except as follows. The starting material is powdered nickel ferrite or cobalt ferrite respectively. The gas mixture is a 50 percent mixture of HBr in He with a total rate of flow of 5 milliliters per minute and the temperature at surface 12 is set at 875 C. Single crystal films, 0.008 inch thick, of nickel ferrite and cobalt ferrite respectively are obtained in 30 hours. With a mixture of 50 percent HCl in He similar results are obtained when a ferrite temperature of 925 C. is employed.

B. Isothermal Transport In the isothermal transport method, the ferrite is again decomposed and converted to gaseous products according to the reaction described above with relation to the close-space transport method. However, according to this method, the temperature of the substrate and the ferrite sources are preferably essentially the same (hence, isothermal) such that epitaxial film growth of the ferrite does not occur due to a difference in temperature between the substrate and the ferrite starting material. According to the isothermal transport method, the deposition of the single crystal ferrite thin film upon the single crystal substrate is caused to occur by injecting water vapor into the equilibrium mixture of the gases so as to shift the equilibrium and to drive the aforementioned reaction toward the formation of the ferrite.

It has been found that although it is possible to obtain single crystal ferrite in this manner, there is a gradual change in the deposited ferrite composition as the reaction proceeds. This change is believed to be caused by an unequal depletion of the difierent metals and/or of oxygen in the ferrite. For this reason, as well as for greater flexibility in control of the film composition, instead of using a ferrite as a source material. separate multiple sources of individual metal oxides are preferred. For example, in the deposition of manganese ferrite, MnFe O the two source oxides are MnO and Fe O It has also been found that during the halogenation of the ferric oxide, especially when hydrogen bromide is used as the reaction gas, a small quantity of the ferric ion is reduced to the ferrous state. This reaction is undesirable and can be prevented by including a small amount of oxygen gas to be mixed with the hydrogen halide gas which passes over the iron oxide. Typically, the volume ratio of oxygen to hydrogen halide is in the range of about 1:10 to 1:100 and, preferably. is about 1:20.

An apparatus useful for carrying out the isothermal transport method for the deposition of epitaxial single crystal thin ferrite films is shown in FIG. 2. The apparatus 30 is comprised of a circular tube furnace 31 through which extends a quartz reaction tube 32. The furnace is provided with heating elements 33 which uniformly heats the reaction tube so as to create an essentially isothermal condition within the central portion of the reaction tube. The reaction tube is open at both ends to allow for the insertion of the reactants and the'substrate 34. The reaction tube 32 as shown, is provided with an inert gas inlet 35 and two gas outlets 36 and 37 through which unused gases may be exhausted from the reaction tube. The substrate 34 (or substrates) is contained on a substrate holder 38 which enters the reaction tube 32 through one end thereof and is held therein by means of a first ball joint assembly 39. The substrate extends into a portion of the reaction tube hereinafter called the deposition zone. This zone is preferably of uniform temperature. The metal oxide reactants 41 and 42 which act as sources for the metals which are part of the final ferrite film, are provided by means of oxide containers 43 and 44 which extend into the deposition zone of the reaction tube 32. The oxide container tubes 43 and 44 are secured by means of a second ball joint assembly 45 which connects to the open end of the reaction tube 32 opposite that of the substrate holder 38. The apparatus, as shown, indicates the container tubes 43 and 44 as open glasses tubes each of which are widened in the region in which the oxide is held and each of which is provided with an opening 46 and 47 for the entry and exhaustion of gases therethrough. Only two such container tubes are shown, however, one may provide a separate container for each metal ion to be included in the ferrite composition. A second gas inlet 48 is provided through which the water vapor is admitted to the reaction tube 32. The water vapor is preferably admitted directly into the deposition region of the reaction tube 32.

In operation, the metal oxide containers 43 and 44 are loaded with the respective metal oxides 41 and 42 and inserted into the reaction tube 32, as shown. The substrate 34 or substrates are placed upon the substrate holder 38 and similarly inserted into the reaction tube 32. An inert carrier gas is then admitted into the furnace, preferably simultaneously through all of the gas inlets so as to purge the furnace 31 of normal atmosphere. While being purged, the furnace 31 is brought to operating temperature which is, typically, between about 900 and Il50 C. When the desired temperature is achieved, a mixture of HCl or HBr with an inert carrier gas such as helium, is passed into the container tubes 43 and 44 through inlets 46 and 47 and over the metal oxides therein. In addition to the above gases, oxygen is included in the gas mixture which passes into the container having iron oxide therein. The unused gases and the gaseous reaction products of the reaction between the hydrogen halide and the metal oxide in each tube, passes out of the container tubes 43 and 44 and over the substrate 34 in the deposition zone of the reaction tube 32. It is preferable to allow this passage of gas to take place for a period of at least about minutes to insure that the gas mixture in the region of the substrate is an equilibrium gas mixture. After such time has elapsed, water vapor mixed with an inert carrier gas is caused to enter the deposition zone of the reaction tube 32. The water vapor causes a shift in the equilibrium toward the formation of ferrite. This shift in equilibrium results in the epitaxial deposition of thin ferrite single crystal films upon the substrate.

This technique is extremely flexible in that one can easily achieve a control of the composition of the ferrite film. That is, one can easily add a number of metal ions to the film by simply providing additional metal oxide containers containing such ions or by mixing metal oxides containing such ions with other metal oxides the ions of which are included in the film. Control of the composition can also be achieved in that the relative ratio of the metals present in the ferrite can easily be controlled. For example, the composition of manganese ferrite M nFe O expressed as the ratio of iron to manganese in the ferrite can be controlled between 1.0 and 5.5 by adjusting the relative ratios of the hydrogen halide flow over the respective metal oxide sources.

Example 5 Manganese ferrite (MnFe O is prepared in accordance with the general procedure given above and under the following specific conditions:

Isothermal reaction temperature equals 1,000 C.

Gas flow over MnO consists of 58.5 ml./min. HBr and245 ml./min. He.

The gas mixture over Fe O consists of 95.5 ml./min. HBr, 245 ml./min. He and 5 ml./min. 0

After the above gas mixtures were passed over the respective oxides for 10 minutes, a gas mixture consisting of 4,220 ml./min. of He and 6.2 grams/hr. of water vapor was passed into the deposition zone of the reaction tube 32 through the water vapor inlet 47. In addition to the above gases, He gas, at a rate of 4,220 ml./min. entered the reaction tube through the inlet 35 and 50 mL/min. of He gas was passed into the reaction tube through inlet 37. Under these conditions, the ferrite composition, as given by the ratio of FezMn, was typically in the range of 2.0i0.05:l and the rate of deposition was about 0.0008 inches per hour.

Example 6 In this example singly crystal films of nickel ferrite are synthesized in accordance with the general procedure set forth above. The starting material in this example is MO and Fe O The furnace temperature is approximately 950 C. Hydrogen chloride is used in the following amounts to decompose the starting oxides:

Flow ofHCl over NiO 60 ml./min. Flow of HCI over F6303 mL/min.

Example 7 In this example single crystal films of magnesium ferrite are synthesized in accordance with the general procedure set forth above. The starting oxides used are magnesium oxide and ferric oxide. The furnace temperature of 1050' C. is higher than for most other ferrites because of the low volatility of magnesium bromide formed during the reaction. Gas flow conditions were similar to those given in example 5, except that the flow of hydrogen bromide over the magnesium oxide was two to four times higher than that given for the manganous oxide in example 5. The reason for this is that magnesium oxide is attacked to a lesser degree by hydrogen bromide than manganous oxide.

As indicated, the novel techniques described above have been used to yield single crystal ferrite films of many ferrites including magnesium-manganese ferrite, magnesium ferrite, lithium ferrite, nickel ferrite, and cobalt ferrite. A solid solution of the above ferrites can also be deposited by these methods. In addition, films of metal oxides, for example, magnesium oxide, can also be deposited in single crystalline form by these methods.

The substrates useful for epitaxial growth of single crystal films preferably have lattice constants which approximate that of the ferrite to be deposited. Examples of generally useful substrates include magnesium oxide, ferrites with spinel structures, such as manganese zinc ferrite, magnetite and magnesium ferrite, and sapphire.

Iclaim:

1. A vapor transport method for the deposition of single crystal ferrite films comprises the steps of reacting at a temperature of from 850 C.-l l50 C. metal oxides selected from the group consisting of oxides of the constituent metals of said ferrite and said ferrite with a reactive halogen-containing gas in an open gas flow system to form gaseous reaction products consisting of metal halides and water vapor and the converting said gaseous reaction products to said single crystal ferrite film on a single crystal substrate positioned in said open gas flow system by causing a shift in equilibrium conditions of said gaseous reaction products in the vicinity of said substrate, the temperature of said metal oxides being maintained during deposition at a difference of from C. to 30 C. greater than the temperature of said substrate such that when the temperature difference is 0 C., said shift in equilibrium is caused by the addition of water vapor to the gaseous products and when said temperature difference is greater than 0 C. the said difference shall be between 20-30 C., said temperature difference creating said shift in equilibrium and the rate of said 7 gas flow being less that 10 ml. per minute with said substrate positioned from l-l 0 mm. from said metal oxides.

2. A close-space vapor transport method for the formation of single crystal ferrite films comprising the steps, as recited in claim 1, wherein said reactive halogen-containing gas comprises a hydrogen halide gas selected from hydrogen chloride and hydrogen bromide, said reactive gas flowing at a rate of about 1-10 ml. per minute, and wherein said shift in equilibrium occurs at the surface of said substrate by causing said substrate to be at a temperature of from -30 C. lower than that of said heated metal oxides and said substrate being separated from said metal oxides by a distance of from l to 10 mm.

3. An isothermal transport method for the formation of single crystal ferrite films comprising the steps as recited in claim '1 wherein said reactive halogen-containing gas comprises a gas selected from hydrogen chloride and hydrogen bromide and wherein said shift in equilibrium is caused by the addition of water vapor into said gaseous reaction products in the vicinity of said substrate.

4. The isothermal transport method recited in claim 3 wherein said metal oxides are comprised of powdered individual oxides of the constituent metals in said ferrite film to be prepared and wherein said oxides are heated to a temperature between about 850 C. and ll50 C., said hydrogen halide gas is mixed with an inert gas to form a gas mixture having a flow rate of at least several hundred mL/min.

5. The isothermal transport method recited in claim 3 wherein one of said metal oxides is ferric oxide and wherein the gas mixture flowing over said ferric oxide includes oxygen.

6. The isothermal transport method recited in claim 3 wherein one of said metal oxides is ferric oxide and the gas mixture flowing over said ferric oxide contains oxygen in a ratio of oxygen to hydrogen halide of from about one-tenth to one-hundredth.

7. A close-space vapor transport method for the epitaxial deposition of single crystal ferrite films on a single crystal substrate comprises the steps of reacting a powdered ferrite having substantially the same composition as the ferrite film to be deposited with a hydrogen halide gas selected from hydrogen chloride and hydrogen bromide in an open gas flow system at a temperature between about 950 C. and 1 C., said hydrogen halide gas being mixed with an inert gas to form a gas mixture, the flow rate of which is less than about 10 ml./min., less than 5 ml./min. of which is said hydrogen halide gas, and wherein said substrate is heated to a temperature of from about 20 to 30 lower than that of said powdered ferrite and said substrate being separated from said powdered ferrite by a distance offrom about 1 to 5 mm.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pa 3,617,381 Dated November 2 1971 Inventor(s) Joseph John Hanak It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the specification, column 3, line 20, the formula that reads M 20 (s)+bh8HX(g)=MX (g)+2Fe X (g)+4H 0(g) should read:

M Pe O (s) +8HX(g)=MX [g)+2FeX (g)+4H O (g) Column 5, line 24, "glasses" should read -glass--, column 6, line 25, "singly" should read single-.

Signed and sealed this 18th day of April 1972.

(SEAL) Attest:

EDWARD M.FLETCHER ,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents ORM PO-IOSO (10-69) USCOMM-DC eoavm 0 U 5 GOVERNMENT PRINYING OFFILF IQBDO- 165434

Claims

2. A close-space vapor transport method for the formation of single crystal ferrite films comprising the steps, as recited in claim 1, wherein said reactive halogen-containing gas comprises a hydrogen halide gas selected from hydrogen chloride and hydrogen bromide, said reactive gas flowing at a rate of about 1-10 ml. per minute, and wherein said shift in equilibrium occurs at the surface of said substrate by causing said substrate to be at a temperature of from 20*-30* C. lower than that of said heated metal oxides and said substrate being separated from said metal oxides by a distance of from 1 to 10 mm.
3. An isothermal transport method for the formation of single crystal ferrite films comprising the steps as recited in claim 1 wherein said reactive halogen-containing gas comprises a gas selected from hydrogen chloride and hydrogen bromide and wherein said shift in equilibrium is caused by the addition of water vapor into said gaseous reaction products in the vicinity of said substrate.
4. The isothermal transport method recited in claim 3 wherein said metal oxides are comprised of powdered individual oxides of the constituent metals in said ferrite filM to be prepared and wherein said oxides are heated to a temperature between about 850* C. and 1150* C., said hydrogen halide gas is mixed with an inert gas to form a gas mixture having a flow rate of at least several hundred ml./min.
5. The isothermal transport method recited in claim 3 wherein one of said metal oxides is ferric oxide and wherein the gas mixture flowing over said ferric oxide includes oxygen.
6. The isothermal transport method recited in claim 3 wherein one of said metal oxides is ferric oxide and the gas mixture flowing over said ferric oxide contains oxygen in a ratio of oxygen to hydrogen halide of from about one-tenth to one-hundredth.
7. A close-space vapor transport method for the epitaxial deposition of single crystal ferrite films on a single crystal substrate comprises the steps of reacting a powdered ferrite having substantially the same composition as the ferrite film to be deposited with a hydrogen halide gas selected from hydrogen chloride and hydrogen bromide in an open gas flow system at a temperature between about 950* C. and 1100* C., said hydrogen halide gas being mixed with an inert gas to form a gas mixture, the flow rate of which is less than about 10 ml./min., less than 5 ml./min. of which is said hydrogen halide gas, and wherein said substrate is heated to a temperature of from about 20* to 30* lower than that of said powdered ferrite and said substrate being separated from said powdered ferrite by a distance of from about 1 to 5 mm.