LOW VACUUM VAPOR PROCESS FOR PRODUCING EPITAXIAL LAYERS

Abstract: A method of fabricating superconductor articles with an epitaxial layer is described (105). The method can be performed under conditions of relatively high pressure and low substrate surface temperature (101). The vapors of a preselected gas in an inert or reactive carrier gas can be directed toward a target surface at a high velocity (102). The velocity of the vapors of the preselected gas can be reduced as they approach the target surface (103). The method includes conditioning the target surface to remove unwanted material from the substrate (104). The resulting epitaxial layers can demonstrate various advantageous features, including low pore density and/or inclusions with small average particle size diameter.

Description: LOW VACUUM VAPOR PROCESS FOR PRODUCING EPITAXIAL LAYERS Background of the Invention The invention relates to methods of growing epitaxial layers. An epitaxial layer is a layer of material that is grown on the surface of a substrate such that the crystallographic orientation of the layer of material is determined by the lattice structure of the substrate. Epitaxy is also meant to refer to materials with ordered surfaces whether formed by conventional epitaxy or by graphoepitaxy . Epitaxial layers have been grown using physical vapor deposition (PVD) , chemical vapor deposition (CVD) and sputtering techniques. Typically, PVD involves the evaporation of a solid material and transfer of the vapor to the substrate surface in a diffuse gas beam in which only a small portion of the total amount of evaporated solid may reach the substrate surface. Thus, the material usage efficiencies obtained with PVD can be low. In addition, PVD is usually performed at a chamber pressure of at most about 1 X 10 ~4 torr, so the flux of evaporated solid at the substrate surface can be small, resulting in low epitaxial layer growth rates . In CVD, one or more reactant gases within the chamber adsorb to the substrate surface and react to form the epitaxial layer with product gases desorbing from the substrate surface. Generally, the reactant gases reach the substrate surface by convection and diffusion, so the material usage efficiencies can be low. Furthermore, CVD is typically conducted at a chamber pressure of at least about 0.1 torr, and, to grow epitaxial layers at these elevated pressures, relatively high substrate temperatures are usually used. Thus, the selection of substrate materials used in CVD can be limited. Sputtering methods of growing epitaxial layers can also be limited by the aforementioned considerations. When using either PVD, CVD or sputtering the quality of the epitaxial layer can depend upon the chemical nature of the substrate surface during layer growth. For example, contaminants present at the substrate surface can interfere with epitaxial layer growth. In addition, native oxides present at the substrate surface can help or hinder epitaxial layer growth. Further, PVD, CVD and sputtering can be ineffective at providing control of the chemical nature of the substrate surface during layer growth, so the epitaxial layers formed by these techniques can be of poor quality. Summary of the Invention The present invention features a low vacuum vapor deposition process for producing epitaxial layers on substrates that can be of non-identical composition. In one aspect, the invention features a method that includes the steps of placing a textured or crystallographically oriented target surface of a substrate, typically including contaminant materials, in a low vacuum environment, and heating the target surface (substrate surface) to a temperature which is greater than the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in a high vacuum environment under otherwise comparable conditions. A layer-forming stream, including an inert carrier gas and a dispersion of a first species (layer forming gas) , which is a chemical component of the desired epitaxial layer, is directed at a positive velocity greater than about 1 m/sec toward the substrate surface through the low vacuum environment. A second species (conditioning gas) is provided in the low vacuum environment, directed toward the substrate surface at a velocity substantially similar to the velocity of the layer-forming stream, and reacted with one or more of the species present in the substrate surface, for example, a contaminant material . This reaction conditions the substrate surface and promotes nucleation of the epitaxial layer. A desired material chemically comprising the layer forming gas is deposited from the stream onto the substrate surface to form an epitaxial layer. As used herein, "layer forming gas" refers to a gas that can adsorb to a surface and become a component of an epitaxial layer. A layer forming gas can be formed of atoms, molecules, ions, molecular fragments, free radicals, atomic clusters and the like. As used herein, "conditioning gas" refers to a gas that can interact with a surface to remove surface contaminants, remove undesired native oxides present at the substrate surface or form desired native oxides or other compounds at the substrate surface. A conditioning gas can be formed of atoms, molecules, ions, molecular fragments, free radicals, atomic clusters and the like. In another aspect, the invention features a method of growing an epitaxial layer, at a chamber pressure of at least about 1 X 10 "3 torr, on a substrate surface having a temperature that is about the same as the PVD epitaxial growth threshold temperature for a chamber pressure of at most about 1 X 10 "4 torr. For a given substrate and epitaxial layer, the "PVD epitaxial growth threshold temperature for a chamber pressure of at most about 1 X 10 ~4 torr" refers to the minimum substrate temperature that can be used to grow the epitaxial layer at a chamber pressure of at most about 1 X 10 "4 torr, and typically at most about 1 X 10 "5 torr, using PVD with a diffuse gas beam. As used herein, a "diffuse gas beam" refers to a gas beam that expands from a beam source such that less than about 50% of the gas beam is incident on the substrate surface. In a further aspect, the invention features a method of growing an epitaxial layer on a substrate surface at a rate of at least about 50 Angstroms per second in a vacuum chamber having a pressure of at least about 1 X 10 "3 torr. In still a further aspect, the invention features a method of growing an epitaxial layer on a substrate surface. In this aspect, the method includes exposing the substrate surface to a gas beam having a layer forming gas. At least about 75% of the layer forming gas in the gas beam is incident on the substrate surface. In yet a further aspect, the invention features a method of growing an epitaxial layer on a substrate surface . The method includes exposing the substrate surface to a conditioning gas that interacts with the substrate surface to form a conditioned substrate surface. The method also includes exposing the conditioned substrate surface to a gas beam having a layer forming gas that becomes a component of an epitaxial layer on the conditioned substrate surface. The exposing steps are performed at a pressure of at least about 1 X 10 "3 torr. A "conditioned substrate surface" herein denotes a substrate surface from which a substantial portion of surface contaminants or undesired native oxides or other compounds have been removed by a conditioning gas, or on which a desired native oxide has been formed by a conditioning gas. In one aspect, the invention features an article that includes a substrate having an epitaxial layer disposed thereon. A surface of the epitaxial layer has a pore density of less than about 500 pores per square millimeter. As used herein, "pore density" refers to the number of pores at the surface of an epitaxial layer per unit area of the surface of the epitaxial layer. In another aspect, the invention features an article that includes a substrate having an epitaxial layer disposed thereon. A surface of the epitaxial layer has inclusions with an average particle size diameter of less than about 1.5 micrometers. As used herein, "inclusions" refer to surface defects, such as second phase particles, that can act as locations of initiation of non-epitaxial growth at a surface . The substrate material may be ceramaceous, such as an oxide, metallic, such as a metal or alloy, or intermetallic . It chemically comprises one or more substrate species, and at least one substrate species is different from the layer forming gas. In addition to the substrate species, the species present in the substrate surface may include native oxides of one or more of the substrate species, and adsorbed surface contaminants. Since the epitaxial layer typically includes a metal, the layer forming gas is usually a metal vapor. If the epitaxial layer includes more than one species, such as a metal oxide, the layer forming gas can comprise more than one gas. For example, if the epitaxial layer is formed of cerium oxide, the cerium vapor and oxygen can be used in the gas beam. The conditioning of a substrate surface can occur by exposing the substrate surface to a background pressure of a conditioning gas. Alternatively, the conditioning of a substrate surface can occur by exposing the surface to a gas beam that includes a conditioning gas. Such gas beams can be directed gas beams or diffuse gas beams, and the conditioning gas included therein may have a high velocity or a low velocity. In some embodiments, conditioning of the substrate can occur by exposing the substrate surface to a background pressure of a conditioning gas and a conditioning gas that is included in a gas beam. When using a background pressure of a conditioning gas to condition a substrate surface, the vacuum chamber typically has a partial pressure of the conditioning gas of at least about 0.5%, preferably from about 1% to about 10%, and more preferably from about 2% to about 6%. Thus, for example, if the pressure of the vacuum chamber were about 100 millitorr, the partial pressure of hydrogen in the chamber can be at least about 0.5 millitorr while conditioning a substrate surface. In embodiments in which a conditioning gas is included in a gas beam, the conditioning gas may be introduced in the same stream as the layer forming gas or in a separate stream. In some embodiments, other species may also be introduced which are reactive either with the layer forming gas or with one or more additional species present in the substrate surface, but the stream containing the layer forming gas preferably contains only one other reactive species. Additional reactive species are preferably introduced in separate streams of substantially similar velocity. Large dissimilarities in stream velocities can favor one set of species reactions at the expense of the others, and are generally to be avoided. The desired epitaxial layer material may be formed of the layer forming gas or the reaction product of the layer forming gas with one or more of the other species introduced into the low vacuum environment. The desired material may be ceramaceous, metallic, or intermetallic . The substrate surface can be exposed to the conditioning gas and the layer forming gas in series or in parallel. In parallel processes, one portion of the substrate surface is conditioned while the epitaxial layer is simultaneously grown on another portion of the substrate surface. The chemical nature of the conditioning gas depends upon the chemical species present at the substrate surface. For example, if the substrate has an undesired native oxide present at its surface, the conditioning gas can be a reducing gas, such as hydrogen. Alternatively, the conditioning gas can be an oxidizing gas, such as oxygen, if it is desirable to form a native oxide at the substrate surface or if sulfur or carbon are contaminants present at the substrate surface. The velocities of the layer forming gas, the conditioning gas and additional gaseous species dispersed in their streams should be positive, that is directed toward the substrate surface. In some embodiments, the velocities can be greater than about 1 m/sec. Velocities as high as supersonic may be used. Velocities in the range of from about 10 m/sec to about 400 m/sec are preferred at typical low vacuum conditions. In a preferred embodiment, the velocity of the layer forming gas is originally high and is reduced from a high velocity to a low velocity prior to contact with the substrate surface, which in essence reduces its kinetic energy. The reduction in velocity is accomplished by interference from the low vacuum environment and also from a gaseous boundary layer of carrier gas which forms at the substrate surface. The decrease in the kinetic energy assists in thermal equilibration of the desired material with the substrate. While the high initial velocity of the gaseous species aids in the efficient transport of the layer forming gas to the substrate surface and thereby facilitates higher deposition rates, low velocity gases may also be effectively used in the scope of this invention. The term high velocity refers to positive velocities which approach at least about sonic levels, i.e the speed of sound at operating vacuum levels, such as 100-400 m/sec. The term low velocity refers to positive velocities of less than 10 m/sec and more generally less than 100 m/sec, but greater than 1 m/sec. The term elevated temperature (s) refers to temperatures which are sufficient to allow diffusion of the desired material to low energy equilibrium sites after arrival at the substrate surface. The range of elevated temperatures will vary depending upon the materials involved and other specifics of the deposition process, such as vacuum level and deposition rate. Although this is a low vacuum process, the lower temperature limit is set as the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in a high vacuum environment at the predetermined deposition rate. As used herein, the term low vacuum (or high pressure) refers to a vacuum pressure which is achievable using mechanical pumping systems or a pressure greater than or equal to about 1 X 10 "3 torr, and the term high vacuum (or low pressure) refers to pressure less than about 1 X 10 "5 torr of base vacuum. Temperatures found suitable for the invention correspond to temperatures that are suitable for forming epitaxial layers with pressures that are from about 10 3 to 10 6 lower, and preferably from about 10 3 to 10 4 lower, than pressures used with methods that involve diffuse gas beams. Thus, for example if a pressure of about 1 X 10 "1 torr is used in the invention to form an epitaxial layer of the desired material on a given substrate material at a particular growth rate, the substrate temperature can be the same as would be used at a pressure of from 1 X 10 "4 to 1 X 10 "7 torr to grow the same epitaxial layer under the same conditions with a diffuse gas beam. An upper end of the elevated temperature range is set as 90% of the melting point of the selected substrate material. Typically the substrate surface is heated to a temperature which is less than the prior art threshold temperature for forming an epitaxial layer of the desired material on the substrate material in the low vacuum environment at the predetermined deposition rate . If a single conditioning gas is used, the conditioning gas can be reactive with the layer forming gas. Alternatively, if multiple conditioning gases are used, one or more of the conditioning gases can be reactive with the layer forming gas. In embodiments in which the substrate material is metallic and the epitaxial layer is an oxide of the layer forming gas, the conditioning gas may be chosen to be more oxidizing with respect to the layer forming gas than with respect to a native oxide of the substrate material. In embodiments in which the substrate and epitaxial layer are both metallic, the conditioning gas may be reducing with respect to both. In some embodiments, a layer forming gas can also serve as a conditioning gas. For example, oxygen can be used to remove impurities and/or form an oxide of the substrate surface as well as form an oxide epitaxial layer. When a substrate of preferred crystallographic orientation is used, such as a cubic textured substrate, deposition of an epitaxial layer onto the substrate according to the process of this invention provides a composite material which is suitable for preparing a composite article. The cubic texture of the substrate is transferred to the epitaxial layer which, in turn may transfer this orientation to additional layers deposited onto the epitaxial layer. The additional layers may be deposited onto the epitaxial layer using the process of this invention. The invention allows the application of high vacuum- like evaporation processes to the fabrication of large scale substrate areas without limitations which are typically imposed by a high vacuum system. Material usage efficiencies are typically increased from less than 50 percent to over 95 percent by the directed layer-forming deposition streams. Furthermore, the directed layer- forming stream in the low vacuum environment reduces the uncertainty associated with the formation of epitaxial films on metallic substrates. The deposition stream can produce a surface boundary layer which enhances the nucleation and growth of epitaxial films on the substrate and reduces the temperatures required for epitaxial film growth. The addition of at least one conditioning gas, as allowed by the low vacuum operation, can be used to disrupt a native surface oxide on substrate materials which allows the template of the crystallographically oriented substrate, preferably a cubic textured alloy, to be directly available for the depositing atoms of the preselected material. The layer forming gas and the conditioning gas can be selected to thermodynamically favor specific compounds and materials to improve epitaxial film growth. A consistent process is developed as well, which can eliminate the system to system variation in the deposition of epitaxial films in high vacuum systems. The high energy, low vacuum approach of the present invention also provides the opportunity to use a single manufacturing method for both the epitaxial layer and any additional layers deposited on the epitaxial layer without high vacuum processing. For epitaxial layers, it is generally disadvantageous to have a high pore density or inclusions with a large particle size. The invention can provide epitaxial layers having low pore densities and/or inclusions with small particle sizes. The epitaxial layers preferably have a pore density of less than about 500 pores per square millimeter, more preferably less than about 250 pores per square millimeter, and most preferably less than about 130 pores per square millimeter. The epitaxial layers preferably have inclusions with an average particle size of less than about 1.5 micrometers, more preferably less than about 1 micrometer and most preferably less than about 0.5 micrometers. In embodiments in which a conditioning gas is included in a gas beam, the gas beam which impinges upon - li the substrate surface should include a sufficient amount of conditioning gas to condition the substrate surface and allow the epitaxial layer to grow. In these embodiments, the gas beam that impinges upon the substrate surface preferably includes from about 0.5 molar percent to about 10 molar percent conditioning gas, more preferably from about 1 molar percent to about 8 molar percent conditioning gas, and most preferably from about 2 molar percent to about 6 molar percent conditioning gas. In some embodiments, it can be desirable to grow the epitaxial layer at a high rate. In these embodiments, the epitaxial layer is preferably grown at a rate of at least about 50 Angstroms per second, more preferably at least about 100 Angstroms per second, and most preferably at least about 150 Angstroms per second. These growth rates can be achieved at a vacuum chamber pressure of at least about 1 X 10 "3 torr. The epitaxial layers can be formed without using electron beams or ion beams. Brief Description of the Drawings Fig. 1 outlines the low vacuum vapor process to produce epitaxial layers; Fig. 2 illustrates an apparatus that can be used according to the method of the invention; Fig. 3 illustrates a boundary layer formed on a substrate; Fig. 4 is a close-up view of a portion of Figure 3 illustrating the motion of individual atoms in the boundary layer; Figs. 5A and 5B illustrate multistream vapor deposition processes in accordance with the invention; Fig. 6 illustrates a superconductor article fabricated by a low vacuum vapor deposition process; Fig. 7 illustrates another embodiment of an apparatus that can be used in the method of the invention; Fig. 8 is an x-ray diffraction pole figure of cerium oxide deposited on single crystal nickel under conditions where uniformly oriented epitaxy does not occur, as seen in Example 2; and Figs. 9A-9C are x-ray diffraction pole figures of a nickel substrate, the nickel substrate having a cerium oxide epitaxial layer disposed thereon, and the nickel substrate/cerium oxide layers having an epitaxial yttria- stabilized zirconia layer disposed thereon. Description of the Preferred Embodiments The invention provides a method of fabricating a composite article which has an epitaxial layer. A preferred embodiment of the method is outlined in Fig. 1 and includes placing a textured substrate surface in a low vacuum environment 101, and heating the substrate surface to an elevated temperature via a heat source. In addition to the substrate species, the species present in the substrate surface typically include certain contaminant materials such as undesired native oxides of one or more of the substrate species and adsorbed surface contaminants. In some embodiments, desired native oxides of one or more of the substrate species may be present. A stream including a layer forming gas, a conditioning gas and an inert carrier gas is directed at high velocity toward the substrate surface through the low vacuum environment 102. Alternatively, step 102 is performed by providing a background pressure of the conditioning gas and a stream including a layer forming gas and a carrier gas is directed at the substrate surface. In either embodiment, the conditioning gas reacts with one or more of the species present in the substrate surface to condition the substrate surface and promote nucleation of the epitaxial layer 104. In the preferred embodiment illustrated, the velocity of the layer forming gas is reduced from high velocity to low velocity 103 by interference with the substrate, a gaseous boundary layer adjacent the substrate and the low vacuum environment. The layer forming gas, or in some embodiments, its reaction product with the conditioning gas, is deposited onto the substrate at an elevated temperature to form an epitaxial layer 105. Additional layers can be deposited onto the epitaxial layer to form a composite article. The substrate surface is heated to an elevated temperature which is less than about 90% of the melting point of the substrate material but greater than the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in a high vacuum environment at the predetermined deposition rate. Typically the substrate surface is heated to a temperature which is less than the prior art threshold temperature for forming an epitaxial layer of the desired material on the substrate material in the low vacuum environment at the predetermined deposition rate. Temperatures suitable for forming epitaxial layers in vacuums from about 10 3 to 10 6 lower, and preferably from about 10 3 to 10 4 lower, than the actual vacuum conditions may be used. Epitaxial metal layers have successfully been produced at temperatures of 77K and below. Typical lower limit temperatures for the growth of oxide layers on metal are approximately 200 degrees Celsius. The growth of epitaxial oxide films on oxide substrates can occur at lower temperatures. In some embodiments, the substrate surface has a temperature of from 200°C to 800°C, preferably 500°C to 800°C and more preferably from 500°C to 650°C. Various well-known methods such as convection heating and conduction heating may be used to obtain the desired temperature elevation. Referring to Figs. 2-4, there is shown a directed vapor deposition machine for use, for example, with the low vacuum process of the present invention. Other apparatus for forming and directing a vapor stream may also be used. The machine 10 includes an evaporation housing 20 for a high pressure environment which includes a vapor source 80 for a layer forming gas and a source 82 for a conditioning gas and an inert carrier gas to form a dispersion which is ejected from the housing 20 by a nozzle 28 through a nozzle opening 30 at high velocity and energy. If the conditioning gas is provided in machine 10 at a background pressure, machine 10 can include a leak valve for entering the conditioning gas into machine 10. Such leak valves are well known for those skilled in the art . The dispersion forms a high speed stream 34 of the carrier gas, a layer forming gas and optionally a conditioning gas. In embodiments in which the conditioning gas is leaked into machine 10, stream 34 need not include a conditioning gas. This stream is usually a directed gas beam. The stream 34 is directed by the source 28 at near sonic velocity, such as 100 - 400 m/sec, through the low vacuum environment toward the substrate surface 42 of a work piece or substrate 40. The velocity of the layer forming gas is reduced from the high energy and high velocity of the stream to low energy and velocity as the individual atoms 36 of layer forming gas are deposited onto the surface 42 of the substrate 40. This is accomplished through the collision of the atoms or molecules of the layer forming gas with the gas molecules in a gaseous boundary layer which is created by the impingement of the carrier gas as the carrier gas strikes the substrate surface 42. The result is that the individual atoms or molecules 36 of the layer forming gas contact the substrate 40 at low kinetic energy and low velocity, thereby settling into a desirable and stable low energy configuration for epitaxial layer nucleation and growth. This forms the deposited layer 44 on substrate 40. The directed flow of gas from nozzle opening 30 can result in high material usage efficiencies. Preferably, at least about 75% of the layer forming gas that emanates from 32 nozzle opening 30 is incident on the substrate surface, more preferably at least about 90% and most preferably at least about 95%. With the present system, one or more substrates 40 can either be held stationary, or moved under computer control in an X-Y coordinate system as shown by arrows 41 and 43 in Fig. 2. Movement of the substrate facilitates the use of multiple directed streams, as further illustrated and discussed in connection with Figs. 5A and 5B. It can also be seen from Figs. 2, 5A and 5B that a wide variety of materials may be deposited to form different end products, such as metals, alloys, multilayers, composites, nitrides, oxides, or any other vapor depositable material . With the present system, a substrate of unlimited length and finite width may be transported across the field of deposition of the directed vapor stream 34 as shown in Fig. 5A. A substrate 40 in the form of a long tape may be mounted on reels external to the deposition chamber and passed in to and out of the low vacuum environment through conventional gas seals 75. As the substrate passes through the deposition chamber the substrate temperature is raised to the deposition temperature by the heaters 72. A directed vapor stream 34 impinges on the moving substrate both controlling the substrate surface condition and depositing the layer forming gas epitaxially upon the substrate surface. As the substrate traverses the deposition chamber, additional directed vapor streams 35, 36, 37 can be used to either condition the substrate surface or deposit one or more epitaxial species on the substrate surface. Multiple nozzles 20, 21, 22, 23 can be employed in-line on an axis coincident with the substrate transport direction to increase the amount of material which can be deposited in a given deposition chamber as shown in Fig. 5A. Alternatively, as shown in Fig. 5B, the nozzles 25, 26, 27, 28 may be offset from the transport axis to allow broader coverage of a wide strip as the substrate moves through the deposition chamber. Many alternative advantageous arrangements of the multiple deposition sources are readily apparent. With the addition of the baffles 78 each source may produce a different vapor composition with different reactive species and/or different functions. For example, a first stream 37 may be used to cleanse the substrate of contaminants and native oxide scale. A second stream 36 may be used to deposit a first epitaxial layer. A third stream 35 may be used to deposit a second epitaxial layer. A fourth stream 34 may be used to deposit an additional epitaxial layer. Further epitaxial layers can be added by additional gas beams. The low vacuum environment allows these multiple depositions to be accomplished with simple baffles, or gas seals between subchambers . If dissimilar directed vapor streams are used, it is preferable to maintain the subchamber pressures such that the initial vapors are not contaminated by later vapors, i.e. the pressure in each subsequent chamber must be slightly lower than in prior chambers and all chamber pressures must be less than the source pressures. This can be accomplished by simple valving to differentially pump the subchambers. The vapor deposition system of the present invention 10 employs an evaporation source 20 which can operate under low inert gas pressure of approximately 2-5 torr. Conventional deposition sources, such as sputtering, thermal, chemical or other sources such as those commercially available from Jet Process Corporation of New Haven, Connecticut, are used to produce the vapor. Such sources are disclosed in, for example, U.S. Patent Nos. 5,265,205; 5,356,672; 5,356,673 and 5,571,332, each of which is hereby incorporated by reference. The layer forming gas can be any material, with metallic and oxide sources preferred. Multiple evaporation sources and alloys may be used concurrently in a single stream for deposition of complex materials or sequentially in several streams for layered schemes. The layer forming gas mixes with an inert carrier gas which enters the evaporation source 20 through opening 24. The carrier gas can be chosen and tailored to the requirements of the system, thereby allowing greater freedom of choice for the vapor sources. A conditioning gas, such as a reducing gas, can be added to the carrier gas. Alternatively, a background pressure of a conditioning gas can be established in the vacuum chamber. Conditioning gases suitable for use with the present invention include, but are not limited to, oxygen, argon, helium, hydrogen, nitrogen, halogen-containing compounds and mixtures thereof . In embodiments in which the conditioning gas is included in a gas beam, a preferred combination of carrier gas and conditioning gas is argon with approximately 4 to 10 % hydrogen. In some embodiments, the hydrogen can be added downstream of the source 20 via a separate source. Alternatively, a conditioning gas such as the hydrogen can be supplied in a separate cleansing stream which contacts the substrate material before the substrate material is contacted by the layer-forming stream containing carrier gas and the layer forming gas. The concentration of the hydrogen is varied according to the specific materials selected for the epitaxial layer and according to the substrate material. In embodiments in which a background pressure of a conditioning gas is used, hydrogen should comprise at least 0.5 percent of the pressure of the chamber. Preferably, the hydrogen comprises from about 0.5 percent to about 10 percent of the pressure of the chamber, more preferably from about 1 percent to about 8 percent, and most preferably from about 2 percent to about 6 percent . The addition of a conditioning gas such as hydrogen or oxygen, either to the carrier gas or via a separate stream creates a reactive background for the layer forming gas which stimulates the nucleation of an epitaxial layer on the surface 42 of the substrate 40. As noted above, this reactive background can be established, at least in part, by establishing a background pressure of a conditioning gas. In one embodiment, a conditioning gas may be supplied in the layer-forming stream with the layer forming gas, or subsequently in a separate stream, to promote formation of an interlayer for the desired material. In a preferred embodiment, the conditioning gas is leaked into the chamber using a leak valve. For the case of a substrate where an epitaxial native oxide scale is preferred as an interlayer for the epitaxial layer, the conditioning gas may be oxygen, and an oxygen partial pressure can be selected to form a desired oxide from elements within the substrate alloy. For example, oxygen partial pressure on the order of 1 X 10 "5 torr in the low vacuum carrier gas may be selected to form a stable epitaxial alumina or chromia scale on nickel base alloys while avoiding the formation of nickel oxide. In another preferred embodiment, a background partial pressure of a conditioning gas reacts with a surface contamination of adsorbed elements, such as carbon or sulfur, forming a second gas phase which is carried away from the substrate surface in the flowing gas stream. This effect is emphasized in the presence of a native oxide scale on a metallic substrate surface. Native oxide scales can form in amorphous fashion, as random crystallographic scales or as epitaxial scales. The epitaxial scales often form in a different crystallographic orientation from the substrate due to changes in crystal lattice parameter as the oxide is formed. These oxide scales can interfere with the nucleation of epitaxially deposited oxide films from the desired metallic template. Adsorbed contaminants are cleaned from the surface of metallic substrates by forming a native oxide scale and then subsequently removing that scale by reduction in high temperature in an atmosphere of argon gas with a partial pressure of hydrogen. The present invention allows for this cleansing process to take place in situ with the desired epitaxial film deposition process. This invention also allows for precise control over the presence, controlled growth or removal of native oxide films by adjustment of background reactive gas composition in situ with the deposition process. In some embodiments, an additional conditioning gas which can react with the layer forming gas may be supplied in the layer-forming stream with the layer forming gas, or subsequently in a separate stream, to promote formation of the desired material. In a preferred embodiment, multiple separate directed vapor streams containing a series of conditioning gases and layer forming gases can be employed to sequentially cleanse contaminants and native oxide films from the substrate surface and to deposit the desired epitaxial oxide layer without removal from a high vacuum chamber or changes in source material. The carrier gas and the layer forming gas form a dispersion which is accelerated by the nozzle 28 to a positive velocity, which is greater than 1 m/sec and may be in the range of several hundred meters per second. The nozzle 28 imparts a given velocity and kinetic energy to the atoms or molecules 36 of the layer forming gas. This dispersion forms a directed vapor stream 34 which is directed through the low vacuum environment and toward the substrate 40. The kinetic energy of a given individual atom or molecule 36 is dependent upon the type of material and the initial velocity produced by the source 20. The low vacuum environment within the substrate deposition chamber is preferably maintained at a pressure of at least about 1 X 10 "3 torr, usually from about 1 X 10 "3 torr to about 2 torr, but typically at less than 50 percent of the source chamber pressure in order to ensure adequate vapor stream acceleration to the substrate. It has been discovered that a low vacuum process of the present invention provides improved results over conventional high vacuum processes. In conventional physical vapor deposition, increases in deposition chamber pressure result in more collisions of the layer forming gas with gas molecules, thereby reducing the mean free path of flight of the atoms or molecules of the layer forming gas, and dispersing the vapor. The present invention takes advantage of this otherwise detrimental effect to slow the individual atoms or molecules 36 of the layer forming gas to a lower velocity just above or at the substrate surface, and hence lower the kinetic energy level. This allows the interfacial mobility of the depositing atoms or molecules to be high, allowing them to settle into a stable low energy configuration upon deposition onto the substrate 40. This avoids random implantation of individual atoms 36 of the dispersion onto the substrate 40 at high velocity and high energy. The driving force for epitaxial nuclei to form is thus increased, while providing a uniform, high rate—deposited layer 44 on the substrate surface 42 at lower temperatures than could otherwise be utilized. Substrates for use in the process of the present invention are preferably cubic textured alloys or metals. However, single crystal oxide substrates and single crystal metal and alloy substrates have been used successfully. Those skilled in the art will recognize many such substrates which can be advantageously used under the scope of the present invention. A preferred substrate for use in a superconductor composite is a nonmagnetic nickel alloy with a cube texture. Additionally, other face centered cubic metals and alloys can be preferably employed as substrate materials under the present invention. The use of the invention allows the deposition of epitaxial oxide buffer layers on copper or copper-based alloys, such as copper-nickel without deleterious oxidation of the metal or alloy substrate. More generally, any crystallographically oriented or textured substrate may be used, provided conditions suitable for epitaxy such as lattice match and chemical compatibility may be arrived at. Examples of nickel-copper alloys are disclosed in commonly assigned U.S. Patent Application Serial No. 08/943,047, entitled "Substrates with Improved Oxidation Resistance", filed October 1, 1997, which is hereby incorporated by reference. As shown by Fig. 3, the deposition and growth of epitaxial layers are enhanced by the removal of surface species 85 which can be a native oxide and/or a surface contaminant on the substrate surface 42. The conditioning gas in the high energy deposition stream 34 disrupt species 85. This exposes the crystallographically oriented substrate template 87 while removing species 85 and allows direct deposit of the layer forming gas onto the substrate surface 42 to form an epitaxial layer 44. In this manner, the crystallographic orientation of the substrate is transferred to the epitaxial layer. This crystallographic orientation is then, likewise transferred to a superconducting oxide when a superconducting composite is formed. Referring to Fig. 4, the deposition of the layer forming gas onto the surface 42 of the substrate 40 can be described in greater detail. As the stream 34 is directed at the substrate surface 42, the carrier gas 26 forms a boundary layer 50 on the substrate 40. The boundary layer 50 is created by the carrier gas striking the surface of the substrate 40 at positive velocity and bouncing back upward toward the source 28. However, since the stream 34 continues to flow downward towards the substrate 40 at a positive velocity, the reflected carrier gas is forced downward again to the substrate, thereby forming a continuous oscillation and boundary layer above the substrate surface 42. The greater the velocity of the initial stream, the more significant the boundary layer effect. The boundary layer 50 assists in creating interference to change the velocity and hence, kinetic energy of individual atoms or molecules 36 of the layer forming gas from the higher energy and higher velocity to a significantly lower velocity and kinetic energy prior to deposition of the individual atom or molecule 36 onto the substrate surface 42. Thus, regardless of the initial speed of the stream, within the limits disclosed, the velocity and kinetic energy of the atoms or molecules 36 will be low at the substrate surface. To further illustrate this point, reference is made to individual atoms 54, 56, 58 and 60 of Fig. 4. Individual atom 54 is directed at the substrate 40 at near-sonic velocity and high kinetic energy by source 28 and strikes the boundary layer 50 above the substrate 40. The boundary layer 50 causes interference with the individual atoms, such as atom 56, which is slowed and reduced from relatively high energy and velocity to a much lower velocity relative to the substrate normal . An atom which has been slowed and interfered with by the boundary layer 50, such as atom 58, is then free to float or settle into a stable low energy configuration on the substrate surface 42. Once an atom is deposited on the substrate 40, such as atom 60, the elevated temperature of the substrate 40 allows interface mobility of the atom 60. This allows atom 60 to move on the surface 42 to a stable position, in contrast to other high energy deposition processes which embed atoms into a substrate surface. As several atoms are deposited on the surface and position themselves at desired low energy configurations, an epitaxial layer 44 is grown. Epitaxial layer 44 can reach thicknesses of 0.01 to 5 micrometers with the process of the present invention 10. Further, the temperature of the substrate 40 should be maintained at 25 °C to 800 °C for deposition and interface mobility. This is a significant improvement over other deposition processes, such as sputtering, which do not have the necessary low substrate arrival energy for individual atoms of the layer forming gas. A high arrival energy causes the atoms 36 to shoot off randomly in a static environment without the carrier gas creating a boundary layer. By the process of the present invention, material usage efficiencies can be increased from less than 50 percent to over 95 percent as materials are deposited without scattering. Materials are also not embedded into substrate surfaces with the present invention, thereby allowing a stable and uniform epitaxial layer to grow. Referring now to Fig. 6, there is shown a composite article formed according to the processes of the present invention. The composite 110 includes the substrate 112, an epitaxial layer 114 deposited on the substrate 112 and an additional layer 116 deposited on the epitaxial layer 114. In certain embodiments, epitaxial layer 114 is deposited on the substrate 112 primarily to provide a suitable template for growth of layer 116 and optionally to provide a barrier to diffusion of substrate elements into layer 116. To be effective, layer 114 should be uniform, continuous and smooth. These features can be achieved with the process of the present invention. A defined epitaxial relationship with the substrate 112 gives control of the crystallographic template for high temperature film nucleation and growth. Epitaxial layer 114 may include any of the materials listed previously and are preferably deposited in thicknesses of 0.01 micrometers to 0.5 micrometers. Orientation relationships between these epitaxial layer materials and a given substrate material can be determined individually. Pores present at the surface of an epitaxial layer represent one type of imperfection in the epitaxial layer. Thus, it is desirable to provide epitaxial layers having a low pore density. Preferably, the epitaxial layers have a pore density of less than about 500 pores per square millimeter, more preferably less than about 250 pores per square millimeter, and most preferably less than about 130 pores per square millimeter. Inclusions present at the surface of an epitaxial layer can initiate non-epitaxial growth, so it is advantageous to reduce the size of these inclusions. Preferably, the epitaxial layers have inclusions with an average particle size diameter of less than about 1.5 micrometers, more preferably less than about 1 micrometer and most preferably less than about 0.5 micrometers. Layer 116 can comprise any material that can be deposited onto epitaxial layer 114. Such materials include insulators, metals, metal oxides, intermetallics, oxides of intermetallics and semiconductors. While the above discussion has focused on embodiments in which multiple nozzles were used in series, in some embodiments, more than one nozzle can be used in parallel. Fig. 7 illustrates such an embodiment in which vacuum apparatus 200 includes beam sources 202 and 204 and a substrate surface 206 which can be similar to the corresponding components discussed above. The layer forming gas can emanate from one source, and the conditioning gas can emanate from another source. The substrate surface can be exposed to the conditioning gas and the layer forming gas in series or in parallel, as described above . Example I The effect of substrate temperature on epitaxial layer growth was studied for epitaxial layers of cerium oxide formed on a single crystal lanthanum aluminate substrate. Cerium was sputtered from a metallic cerium target to form cerium vapor. The cerium vapor was entrained in a mixture of argon/10% oxygen gas which was directed toward the substrate at high velocity through a nozzle. Thus, the carrier gas, layer forming and conditioning gas were argon, cerium vapor and oxygen, respectively. In this example, the conditioning gas reacted with the layer forming gas to provide the cerium oxide layer. The substrate was a polished LaAl0 3 single crystal chip with the {00l} sheet normal face exposed to the deposition stream. The linear deposition rate was approximately 12 nm/min. The substrate temperature during deposition was varied between 200°C to 450°C. The deposition chamber pressure was maintained at about 120 millitorr. A substrate temperature of less than about 350°C resulted in Ce0 2 layers of relatively poor quality; in one case a {ill} sheet normal texture with random in-plane texture, in a second case a {00l} sheet normal texture with an in-plane texture of approximately 15 degrees full width half maximum on a {ill} peak. A substrate temperature of at least about 400°C showed excellent epitaxial relationship with the LaA10 3 substrate, with full widths at half maximum ranging from 5 degrees to 7.7 degrees. Separate experiments on deposition of Ce0 2 on LaAl0 3 were conducted in an electron-beam chamber. Ce0 2 could not be deposited epitaxially from a metallic source but could be deposited epitaxially from an oxide source. The use of a metallic source was made possible in the low vacuum approach by the ability to add a conditioning gas, oxygen, to the carrier gas stream. Evaporation rates are generally higher from metallic sources, increasing the viability of the low vacuum process for an economical manufacturing capability. Example II The effect of removing a native oxide from the substrate surface was studied. Cerium vapor was formed as in Example I . The substrate was a polished single crystal nickel disk oriented with the {OOl} sheet normal. The vapor was entrained in a gas mixture of argon or argon plus 5-10% oxygen . The substrate temperature was raised in a series of experiments from the 400°C to 540°C. Epitaxial layers were grown in each case. All samples exhibited a mixed { 001} /{ill} sheet normal texture in the deposited film. The amount of {OOl} texture as measured by relative X-ray peak intensities increased with increasing temperature, however the {ill} texture was never eliminated. A representative pole figure is shown in Fig. 8. Both sheet texture variants appeared to be formed epitaxially at the highest temperature since very little random in-plane orientations were observed. The full width at half maximum for the {OOl} variant improved from over 45 degrees to less than 30 degrees with increasing temperature, indicating that higher temperatures may be required to obtain the best quality films. Further efforts to produce a single {OOl} oriented epitaxial film by temperature increases were curtailed since the results indicated that temperature alone could not eliminate the {ill} orientation. The naturally occurring nickel oxide on a {00l} oriented nickel single crystal is the {ill} orientation. These results indicate that the elimination of this oxide was advantageous in allowing reliable deposition of {00l} oriented epitaxial films on the nickel substrate. Example III The effect of controlling native oxides at the substrate surface was studied. A series of experiments were performed to evaluate the combined effects of temperature and the use of hydrogen to control the formation of native surface oxides and subsequent epitaxial growth of oxide films on metallic substrates . A cerium metal sputtering target was used as the source for the cerium vapor. The substrate was a polished, single crystal nickel with a {OOl} surface normal. The temperature was varied from 500°C to approximately 700°C. The hydrogen, which acted as a conditioning gas in the gas beam, content of the argon carrier gas was varied from 2% to nearly 10%. The hydrogen addition was eliminated and replaced with a small amount of oxygen to ensure formation of the stable cerium oxide once an initial oxide film layer was deposited (nucleated) on the metal surface. The amount of {OOl} Ce0 2 texture increased relative to the {ill} texture with the addition of 2% hydrogen as temperature increased from 500°C to 650°C, though the {ill} texture was not eliminated. A single, fully epitaxially oriented {OOl} Ce0 2 film was produced at 650°C with hydrogen contents of about 4% and about 8%. This established that lower thresholds for both temperature and reactive gas (hydrogen) content can exist for the successful high rate deposition of epitaxial oxide films on metallic substrates without requiring conditioning of the substrate surface and growth of the epitaxial layer in series. Full widths at half maximum for the Ce0 2 films ranged from 6.8 degrees to 11 degrees at temperatures and hydrogen pressures above the epitaxial threshold (in this case 650°C at 4% hydrogen, a temperature identical to its epitaxial temperature in a prior art high vacuum process) . Example IV An epitaxial layer of cerium oxide was grown on a nickel substrate according to the procedure of Example III using a nickel substrate temperature of about 650°C. An epitaxial layer of yttria-stabilized zirconia was grown on the cerium oxide layer at a surface temperature of about 700°C. Figs. 9A-9C show x-ray pole figures of the nickel substrate, the cerium oxide/nickel article and the yttria- stabilized zirconia/cerium oxide/nickel article, respectively. The full widths at half maximum for these figures were 10°, 12° and 12°, respectively. Micrographs of the epitaxial layer demonstrated a pore density of 130 pores per square millimeter. The micrographs also demonstrated an average second phase particle size diameter of 0.5 micrometers. The chamber pressure during growth of the cerium oxide layer and the yttria-stabilized zirconia layer was about 0.1 torr. The temperatures used during the growth of the cerium oxide and yttria-stabilized zirconia layers was comparable to those used for electron beam deposition at a pressure of 1 X 10 "5 torr. The growth rate achieved for the cerium oxide and yttria-stabilized zirconia layers was about five times higher than typically achieved at a pressure of about 1 X 10 "5 torr. Example V A cerium oxide/yttria-stabilized zirconia/cerium oxide buffer layer stack was formed on a textured (100) nickel substrate by passing argon through a deposition source and hydrogen through a second source such that the partial pressure of hydrogen in the chamber was from about 2 percent to about 4 percent of the total chamber pressure. After an initial treatment of the argon/hydrogen mixture to remove the nickel oxide scale on the substrate, the initial cerium oxide layer was deposited. Yttria-stabilized zirconia was then deposited with argon again passing through the deposition source and oxygen passing through another source such that the partial pressure of oxygen in the chamber was from about 2 percent to about 4 percent of the total chamber pressure. A cerium oxide cap layer was then formed in the same manner as the yttria-stabilized zirconia layer, with argon passing through the deposition source and oxygen passing through the other source. The substrate temperature during the formation of all three layers was from about 500°C to about 750°C. A Θ -2 Θ scan showed peak identifications as follows: Other embodiments are within the claims.

Claims: What is claimed is: 1. A method of forming an epitaxial layer of a desired material chemically comprising a first species on a deposition surface of a substrate chemically comprising one or more substrate species including at least one species different from the first species, at a predetermined deposition rate, comprising the steps of: placing a crystallographically oriented target surface of a substrate in a low vacuum environment, the species present in the target surface substantially consisting of the substrate species, and one or more materials selected from the group consisting of desired native oxides of one or more of the substrate species, and contaminant materials, which include undesired native oxides of one or more of the substrate species and adsorbed surface contaminants; heating the target surface to a temperature which is less than about 90% of the melting point of the second material but greater than the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in a high vacuum environment at the predetermined deposition rate; directing a layer-forming stream comprising a dispersion of the first species in an inert carrier gas at a first velocity greater than about 1 m/sec toward said target surface through said low vacuum environment; directing a dispersion of a beneficially reactive second species at a velocity greater than about 1 m/sec toward said target surface through said low vacuum environment at a velocity substantially similar to said first velocity; reacting said second species with one or more of the species present in the target surface to condition the target surface and promote the nucleation of the epitaxial layer; depositing a desired material chemically comprising said first species onto said target surface at the predetermined deposition rate to form an epitaxial layer on said substrate. 2. The method of claim 1 further comprising the step of reducing said first velocity to a second lower velocity by forming a boundary layer of said carrier gas at said target surface. 3. The method of claim 1 wherein the target surface is heated to a temperature which is less than the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in the low vacuum environment at the predetermined deposition rate. 4. The method of claim 1 wherein said substrate material is metallic. 5. The method of claim 4 wherein said substrate material is a cubic textured alloy. 6. The method of claim 4 wherein said second species is selected to be reducing with respect to said undesired native oxides and said reaction step is a reducing reaction which at least partially removes said undesired native oxides from said target surface. 7. The method of claim 4 wherein said desired material is an oxide of the first species and said second species is selected to be more oxidizing with respect to said first species than with respect to the substrate material thereby reducing undesired native oxides on the target surface while allowing formation of an epitaxial oxide layer from said first species. 8. The method of claim 4 wherein said desired material is metallic and said second species is selected to be reducing with respect to said undesired native oxides, said substrate material and said desired material thereby at least partially removing said undesired native oxides from said target surface while allowing formation of a metallic epitaxial layer from said first species. 9. The method of claim 1 wherein said substrate material is an oxide. 10. The method of claim 1 wherein said second species is selected to be reactive with respect to said adsorbed surface contaminants thereby at least partially removing said trace surface contaminants from said target surface . 11. The method of claim 1 wherein the desired material consists essentially of the first species. 12. The method of claim 1 wherein said desired material consists essentially of the reaction product of the first species and the second species. 13. The method of claim 1 wherein said second species is supplied in said layer-forming stream. 14. The method of claim 1 wherein said second species is supplied in a cleansing stream which contacts the substrate material before said substrate material is contacted by said layer- forming stream. 15. The method of claim 14 further comprising the step of supplying a third species reactive with said first species. 16. The method of claim 15 wherein said desired material consists essentially of the reaction product of the first species and the third species. 17. The method of claim 15 wherein said third species is supplied in said layer-forming stream. 18. The method of claim 15 wherein said third species is supplied in a separate stream after said substrate material is contacted by said layer-forming stream. 19. The method of claim 1 further comprising the step of supplying the first species to the stream from a metallic vapor source. 20. The method of claim 19 wherein said desired material is an oxide and said substrate material is metallic, and wherein said first species is selected to be more noble than said substrate material, thereby promoting both reduction of said undesired native oxides and formation of said desired material. 21. A composite article made according to the method of claim 1. 22. A method of growing an epitaxial layer on a surface of a substrate, the method comprising the steps of: growing the epitaxial layer on the surface of the substrate having a temperature that is about the same as a PVD epitaxial growth threshold temperature for a chamber pressure of about 1 X 10 "4 torr, wherein the growing step is performed in a chamber having a pressure of at least about 1 X 10 "3 torr. 23. The method according to claim 22, wherein the surface of the substrate has a temperature of from about 25°C to about 800°C. 24. The method according to claim 22, wherein the surface of the substrate has a temperature of from about 500°C to about 800°C. 25. The method according to claim 22, wherein the surface of the substrate has a temperature of from about 500°C to about 650°C. 26. The method according to claim 22, wherein the chamber has a pressure of at least about 0.1 torr. 27. The method according to claim 22, wherein the chamber has a pressure of at least about 1 torr. 28. A method of growing an epitaxial layer on a surface of a substrate, the method comprising the step of: growing the epitaxial layer at a rate of at least about 50 Angstroms per second, wherein the growing step occurs in a chamber having a pressure of at least about 1 X 10 "3 torr. 29. The method according to claim 28, wherein the chamber has a pressure of at least about 0.1 torr. 30. The method according to claim 28, wherein the chamber has a pressure of at least about 1 torr. 31. The method according to claim 28, wherein a temperature of the conditioned substrate is equal to about a PVD epitaxial growth threshold substrate temperature for a chamber pressure of about 1 X 10 "4 torr. 32. The method according to claim 28, wherein the surface of the substrate has a temperature of from about 25°C to about 800°C. 33. The method according to claim 28, wherein the surface of the substrate has a temperature of from about 500°C to about 800 °C. 34. The method according to claim 28, wherein the surface of the substrate has a temperature of from about 500°C to about 650°C. 35. A method of forming an epitaxial layer on a surface of a substrate, the method comprising the step of: exposing the surface of the substrate to a gas beam having a layer forming gas, wherein at least about 75% of the layer forming gas within the gas beam is incident on the surface of the substrate. 36. The method according to claim 35, wherein the exposing step is performed in a chamber that has a pressure of at least about 0.1 torr. 37. The method according to claim 35, wherein the exposing step is performed in a chamber having a pressure of at least about 1 X 10 "3 torr. 38. The method according to claim 37, wherein a temperature of the conditioned substrate is equal to about an epitaxial growth threshold temperature for a pressure of about 1 X 10 "4 torr. 39. The method according to claim 35, wherein the surface of the substrate has a temperature of from about 25°C to about 800°C. 40. The method according to claim 35, wherein the surface of the substrate has a temperature of from about 500 °C to about 800°C. 41. The method according to claim 35, wherein the surface of the substrate has a temperature of from about 500°C to about 650°C. 42. A method of forming an epitaxial layer on a surface of a substrate, the method comprising the steps of: exposing the surface of the substrate to a conditioning gas that interacts with surface of the substrate to form a conditioned surface of the substrate; and exposing the conditioned surface of the substrate to a gas beam having a layer forming gas that becomes a component of an epitaxial layer on the conditioned surface of the substrate, wherein the exposing steps are performed in a chamber having a pressure of at least about 1 X 10 "3 torr . 43. The method according to claim 42, wherein the pressure of the chamber is at least about 0.1 torr. 44. The method according to claim 48, wherein the pressure of the chamber is at least about 1 torr. 45. The method according to claim 42, wherein the surface of the substrate has a temperature of from about 25°C to about 800°C. 46. The method according to claim 42, wherein the surface of the substrate has a temperature of from about 500°C to about 800°C. 47. The method according to claim 42, wherein the surface of the substrate has a temperature of from about 500°C to about 650°C. 48. The method according to claim 39, wherein a temperature of the conditioned substrate is equal to about an epitaxial growth threshold substrate temperature for a chamber pressure of about 1 X 10 "4 torr. 49. An article, comprising: a substrate; and an epitaxial layer disposed on a surface of the substrate, wherein a surface of the epitaxial layer has a pore density of less than about 500 pores per square millimeter. 50. The article according to claim 49, wherein the surface of the epitaxial layer has a pore density of less than about 250 pores per square millimeter. 51. The article according to claim 49, wherein the surface of the epitaxial layer has a pore density of less than about 130 pores per square millimeter. 52. The article according to claim 49, wherein the surface of the epitaxial layer has inclusions with an average particle size diameter of less than about 1.5 micrometers . 53. An article, comprising: a substrate; and an epitaxial layer disposed on a surface of the substrate, wherein a surface of the epitaxial layer has inclusions with an average particle size diameter of less than about 1.5 micrometers. 54. The article according to claim 53, wherein the inclusions have an average particle size diameter of about less than about 1 micrometer. 55. The article according to claim 53, wherein the inclusions have an average particle size diameter of less than about 0.5 micrometers.

Inventors: BUCZEK DAVID M, FRITZEMEIER LESLIE G, BUCZEK, DAVID, M., FRITZEMEIER, LESLIE, G.
Applicants: AMERICAN SUPERCONDUCTOR CORP, AMERICAN SUPERCONDUCTOR CORPORATION
Application year: 1998
Main classification: C30B25
IPC codes: C04B35, C30B23, C30B25, H01L39