WO1991006976A2

WO1991006976A2 - Process for producing an aluminum oxide layer on various substrates

Info

Publication number: WO1991006976A2
Application number: PCT/US1990/006391
Authority: WO
Inventors: Arold K. Green; Victor L. Rehn; Carey Scwartz
Original assignee: Department Of The Navy
Priority date: 1989-11-07
Filing date: 1990-11-06
Publication date: 1991-05-16
Also published as: AU6964591A; WO1991006976A3

Abstract

A process for producing an electrically insulating and protective layer of aluminum oxide on semiconductive, insulating, metallic and superconductive substrates is disclosed, which comprises forming an epitaxial or polycrystalline film of aluminum arsenide on the surface of the substrate, e.g. a gallium arsenide substrate, and exposing the aluminum arsenide coated substrate, e.g. gallium arsenide, to water vapor, e.g. the ambient atmosphere containing water vapor, at a sufficient water vapor pressure and for a period of time sufficient to substantially completely convert the aluminum arsenide to aluminum oxide.

Description

PROCESS FOR PRODUCING AN ALUMINUM OXIDE LAYER ON VARIOUS SUBSTRATES

TECHNICAL FIELD

This invention relates to applying aluminum oxide on various substrates, and is particularly directed to a process for producing an electrically insulating and protective layer or film of aluminum oxide on surfaces of se iconductive, insulating, metallic or superconductive materials. For the past quarter of a century the absence of a simple, commercially applicable process for producing high quality, electrically insulating layers or films on surfaces of gallium arsenide (GaAs) or other compound semiconductors has been recognized as a deficiency in the development of (GaAs) and related compound semiconductors for electron-device applications. In contrast the development of a process for producing such electrically insulating layers on surfaces of semiconducting silicon culminated in the late 1960's with the silicon dioxide -on-silicon process, and a few years later, the silicon nitride-on-silicon process. Both these processes are widely used in the silicon semiconductor industry presently, and constitute an essential component of the remarkably successful technology for production of electron devices based on silicon.

Indeed, if the situation had been reversed, i.e. a simple, reliable process for producing insulating layers had been discovered for the gallium arsenide technology, but not for the silicon technology, it would be reasonable to assume that silicon electron devices would never have come into wide usage, and that computers, consumer and military electronics, etc, would now be based on compound semiconductors such as

GaAs. Because of the superior intrinsic electronic properties of GaAs and related compound semiconductors

(compared to silicon) , the consequence would have been a more rapid achievement of high-performance electronics for the microwave and millimeter-wave applications based on GaAs and related compound semiconductors. However, over the past 25 years, the development of compound semiconductor devices has lagged, due in part to the absence of a simple, commercially applicable process for producing high- quality, electrically insulating layers or films on the surfaces of compound-semiconductors such as GaAs. In spite of rigorous effort by many researchers, no such process has been forthcoming to date.

Insulating layers have a wide variety of applications in electronic devices and device fabrication. A1₂0₃ has a large electron energy-band gap (variously reported between 7.0 and 9.9 electron volts, somewhat larger than that of silicon dioxide, and considerably larger than that of silicon nitride) , and a very high resistivity (reported as 1 x 10¹⁶ ohm-cm at room temperature, and 3 x 10¹³ ohm-cm at 300C) . It is physically hard (hardness of 9.0 on Mho's scale), refractory (melting point - 2015°C) , and chemically inert. Electrically, the large band gap produces a large intrinsic breakdown voltage, and a high 15 resistivity. These properties allow usage of A1₂0₃ films in any application requiring high voltages, or low leakage currents, or both. Use of A1₂0₃ films allows use of increased operating voltages. Examples of electron devices in which higher operating or breakdown voltages would provide a valuable improvement are mentioned below. Because they can support a large bias voltage, insulating A1₂0₃ layers can be used in a variety of tunneling applications such as impact-avalanche transit-time (IMPATT) diodes, barrier-injected transit-time (BARITT) diodes, and transferred-electron devices (TEDs or Gunn diodes) . IMPATT diodes are used to generate power at millimeter-wave frequencies. If the A1₂0₃ layer is doped, then it can be used in place of the intrinsic layer in an IMPATT diode. Because of the high voltage that can be applied across the A1₂0₃ insulator without breakdown, a larger avalanche-multiplication factor can be achieved than in the usual IMPATT diodes. Similarly, the films can be used in BARITTs and TEDS, which can be operated at higher voltage to yiel*-¹ nigher output power.

BACKGROUND ART

In the prior art, anodized aluminum has been applied to gallium arsenide. However, such anodized aluminum layer has not functioned satisfactorily as an electrically insulating and protective layer on semiconductors.

Other exemplary prior art includes U. S. Patent No. 2,754,456, which discloses an aluminum arsenide semiconductor provided with an alumina coating by heating the semiconductor body of aluminum compound in an oxygen-containing atmosphere such as substantially pure oxygen. The patent states that the coating need be given only the slight thickness sufficient for protection from humidity or the chemical effects of the ambient air. U. S. Patent 2,837,451 discloses the use of moisture in the oxidation of aluminum to aluminum oxide powder. U. S. Patent 4,615,747 discloses that when aluminum is allowed to stand in the presence of water or moisture a hydrated oxide coating is formed over the surface. Russian Patent 857,301 discloses depositing an oxide film on an aluminum mirror by exposure of the mirror to a gaseous mixture of oxygen and nitrogen over a period of several days.

Other prior art for production of alumina films includes plasma deposition, chemical vapor deposition and reactive evaporation. Alumina films produced by these processes have also not been fully satisfactory.

DISCLOSURE OF THE INVENTION One object of the invention is the provision of a process for applying an effective aluminum oxide insulating and protective layer on various substrates. Another object is to provide a process of the above type for application of such aluminum oxide layer to surfaces of semiconductor, insulating, metallic or superconductive materials. A still further object is the provision of an improved procedure for applying an effective electrically insulating and protective layer of aluminum oxide on gallium arsenide and related compound semiconductors, particularly for use in high performance electronics.

Yet another object is to provide a process of the above type for forming a layer of electrically insulating and protective aluminum oxide on substrates, particularly semiconductors such as gallium arsenide, by simple and complete conversion of aluminum arsenide to aluminum oxide. The above objects are achieved according to the invention by a simple process by which films or layers of aluminum oxide (A1₂0₃) can be grown or produced upon the surfaces of semiconductive, metallic, insulating or superconductive materials or thin films. Using gallium arsenide as a semiconductive substrate, a layer of aluminum arsenide (AlAs) , preferably in the form of an epitaxial film, is grown by, for example, molecular- beam epitaxy, upon the gallium arsenide substrate. The aluminum arsenide-coated gallium arsenide substrate is then exposed to water vapor at a water vapor pressure and for a period of time sufficient to substantially completely convert the aluminum arsenide to aluminum oxide. This chemical conversion process is complete in that the total thickness of the epitaxial aluminum arsenide film is converted to aluminum oxide. The aluminum oxide layer has excellent electrical insulating properties, as well as being physically protective and impervious to chemical attack or contamination.

In carrying out the conversion of aluminum arsenide to aluminum oxide at room temperature, the water vapor pressure employed can range from about 7 to about 24 torr. The time period of exposure of the aluminum arsenide film to the water vapor varies, depending on the temperature and the water vapor pressure, for a particular thickness of aluminum arsenide film. Thus, for a 500 angstrom (A) thick epitaxial aluminum arsenide layer, the product of water vapor pressure and time of exposure at room temperature ranges from about 10 to about 50 torr hours, usually about 15 to about 25 torr hours.

In carrying out the reaction the substrate containing the aluminum arsenide layer can be exposed to the ambient atmosphere containing a water vapor pressure within the range acted above and for a period of time e.g. 2 to 5 hours at room temperature, to effect complete conversion of a 500 A thick aluminum arsenide layer to aluminum oxide film. However, if desired, the conversion reaction can be carried out in an enclosed vessel containing an atmosphere composed only of water vapor at the desired pressure and oxygen, or water vapor at the desired pressure and nitrogen, or water vapor alone. The conversion of the aluminum arsenide to aluminum oxide according to the invention method is carried out at normal temperatures e.g. about 20°C.

In a known process for applying an aluminum oxide insulating layer to devices such as tunnel diodes, light-emitting diodes or infrared detectors, such devices are fabricated by a three-step process including the steps of depositing a layer of metallic aluminum on a suitable substrate, exposing the aluminum to gaseous oxygen at elevated temperatures and growing or depositing a second layer of metallic aluminum upon the oxide. The present procedure for depositing an aluminum oxide layer on a substrate is superior to the above process and that of above U. S. Patent No. 2,754,456, in that aluminum arsenide rather than aluminum is converted to aluminum oxide and neither oxygen nor elevated temperatures are necessary during the conversion process. In the process of the invention, accordingly, finer control of the oxide thickness and better oxide quality are obtained. BEST MODE FOR CARRYING OUT THE INVENTION According to the invention process, an electrically insulating and protective layer or film of aluminum oxide can be applied to the surfaces of semiconductor, insulating, metallic or superconductive materials as substrates.

One group of such materials include IIIB-VB semiconductor materials formed from elements of Groups IIIB and VB of the periodic table. These include for example gallium arsenide, gallium antimonide, indium arsenide, indium phosphide, gallium-indium arsenide, indium antimonide, gallium arsenide antimonide, gallium arsenide phosphide, indium-gallium arsenide phosphide, aluminum arsenide and gallium phosphide. Gallium arsenide is the preferred semiconductor material.

Other semiconductor materials include IIB-VIB semiconductors formed from elements of Groups IIB and VIB of the periodic table. Examples of these include zinc sulfide, zinc selenide, cadmium selenide, cadmium telluride, and zinc telluride. Other semiconductor materials include Group IVB semiconductors, formed of elements of Group IVB, for example, germanium, silicon, and diamond (carbon) , and including silicon carbide.

Examples of insulating materials to which the aluminum oxide layer can be applied according to the invention include, for example, quartz (SiO_z) , glass, magnesium oxide, beryllium oxide and alumina

(sapphire) .

Examples of metallic substrates to which an aluminum oxide coating can be applied according to the invention include the noble metals silver, gold, platinum, and palladium, and alloys thereof, as well as other metals such as copper, aluminum, iron, titanium, vanadium, chromium, and alloys thereof.

Examples of superconductive materials to which the aluminum oxide coating can be applied according to the invention include low temperature superconductors such as niobium and niobium nitride, and high temperature superconductors including materials such as yttrium barium copper oxide, bismuth calcium strontium copper oxide and thallium calcium strontium copper oxide. An epitaxial film can be grown on a single crystal substrate, such as a gallium arsenide substrate, by various methods including molecular-beam epitaxy. organometallic chemical-vapor deposition, organometallic vapor-phase epitaxy or atomic-layer epitaxy. These are all techniques known in the art. In one of the most widely used systems, the molecular-beam epitaxy (MBE) system is a commercial ultrahigh vacuum system, the internal surfaces of which are maintained at cryogenic temperatures to minimize contamination, and the substrate is held on a substrate holder which permits one to select the temperature of the substrate appropriate to growth of the aluminum arsenide film. Facing the growth surface of the substrate is a series of molecular beams so that for depositing aluminum arsenide, two beam sources are employed, an aluminum source and an arsenic source. Shutters are disposed in front of each source. When growing aluminum arsenide the temperature of the substrate appropriate for growing aluminum arsenide is established and then the aluminum and arsenic shutters are opened to allow the respective molecular beams of these materials to impinge on the substrate. The aluminum arsenide will grow epitaxially. The beams of aluminum and arsenic are impinged on the substrate for a period of time required to grow the desired thickness of epitaxial aluminum arsenide, e.g. a thickness of 500 angstroms. However, the thickness of such film can range from about 6 angstroms to about 1.5 microns, or greater. The film or layer of aluminum arsenide deposited on the selected substrate is generally in the form of an epitaxial film, but need not be in this form, and can be polycrystalline or amorphous. The term "epitaxial" layer is defined as an added layer of crystal that takes on the same crystalline orientation as the substrate.

The aluminum arsenide-coated substrate, e.g. gallium arsenide, is removed from the growth or deposition apparatus and is exposed to water vapor, e.g. as by exposure to the ambient atmosphere, according to the invention. As previously noted, water vapor pressure applied can range from about 7 to about 24 torr. For a given water vapor pressure, the time for complete conversion of the aluminum arsenide layer to aluminum oxide is equal to the exposure time, and will depend on the thickness of the aluminum arsenide layer, the temperature and the water vapor pressure. For a 500 angstrom layer of aluminum arsenide and for water vapor pressure within the above range, the time for exposure for complete conversion of aluminum arsenide to aluminum oxide can range from about 2 to about 5 hours, depending upon the magnitude of the temperature and the water vapor pressure. The product of water vapor pressure and time to accomplish complete conversion of the above layer of aluminum arsenide to aluminum oxide can range from about 10 to about 50 torr hours, e.g. from about 15 to about 25 torr hours, at ambient temperature. The number of torr hours to which the aluminum arsenide layer is subjected to the action of the water vapor can be greater than 50, but without any substantial advantage. While for convenience the preferred temperature of conversion is ambient, such temperature can vary and can be above or below ambient. In order to determine that water vapor is the active component for converting aluminum arsenide to aluminum oxide and that other components present in the air are ineffective for this purpose, a typical 500 angstrom thick aluminum arsenide epitaxial layer on gallium arsenide was placed in a closed container containing 24 torr water vapor and 800 torr oxygen, the increase to above atmospheric by adding oxygen being so that no unknown gas would leak into the reaction chamber. The sample was successfully converted to aluminum oxide. To show that the oxygen was not essential, a similar gallium arsenide substrate containing the 500 angstrom thick aluminum arsenide epitaxial layer was placed in a closed container containing 24 torr water vapor and 800 torr nitrogen to bring the reaction chamber up to above atmospheric. Again the aluminum arsenide layer was completely converted to aluminum oxide. Furthermore, conversion to aluminum oxide was shown not to occur at ambient temperature in an environment of pure 0₂ pure N₂ or a mixture of these, or such mixture also containing C0₂. Doping of the Al₂0₃ film can be achieved depending on what properties are required in such film. If the aluminum oxide film is to be highly insulating, then no doping is used. If doping of the aluminum oxide film is required, such doping may be made in the aluminum arsenide film by conventional doping procedures such as by standard MBE procedure. For gallium arsenide, the most common doping agent is silicon for producing an n-type electron conductor, or beryllium for producing a p-type hole conductor. Conversion of the aluminum arsenide film containing the doping agent to aluminum oxide according to the invention process does not alter the concentration, distribution or type of dopant included. Hence, aluminum oxide films can be produced according to the invention with controlled and characterized doping.

The following are examples of practice of the invention, it being understood that such examples are only intended as illustrative and not as limitative of the invention.

Example I A 500A epitaxial layer of aluminum arsenide is deposited on a surface of a gallium arsenide substrate by molecular-beam epitaxy as described above. The gallium arsenide substrate coated with aluminum arsenide is placed in an enclosed chamber containing 10 torr of water vapor and 860 torr of nitrogen, a total pressure above atmospheric (760 torr) so that no unknown gas can leak into the reaction chamber. Under these conditions of exposure to the water vapor the gallium arsenide is completely converted to aluminum oxide in a period of 5 hours.

The surface of the converted aluminum oxide layer remains as smooth and featureless as the original surface containing the epitaxial aluminum arsenide layer. The resulting aluminum oxide layer has excellent electrical insulating properties and is impervious to chemical attack.

Example 2

The procedure of Example 1 is repeated except that the 500-A⁰- thick aluminum arsenide layer on the gallium arsenide substrate is exposed to the atmosphere of water vapor and nitrogen for a period of 2.5 hours. The aluminum arsenide film is completely converted to aluminum oxide throughout the thickness of the aluminum arsenide layer, as in Example 1.

Example 3 The procedure of Example 1 is essentially repeated except that the period of exposure of the aluminum arsenide layer to the atmosphere of water vapor and nitrogen is reduced to 1.25 hours.

The conversion of aluminum arsenide to aluminum oxide in this example is about 90% complete.

Example 4

A gallium arsenide substrate coated with a 500A thick epitaxial aluminum arsenide film is placed in a container containing a mixture of approximately 24 torr water vapor and 800 torr oxygen. The aluminum arsenide epitaxial layer is successfully converted to aluminum oxide after an exposure period of 2 hours.

Example 5 A gallium arsenide substrate having a 500A thick aluminum arsenide epitaxial layer is subjected to the ambient atmosphere containing about 20 torr water vapor pressure.

After a period of about 2 hours the aluminum arsenide epitaxial layer is substantially completely converted to aluminum oxide.

Example 6 A silicon substrate having a 500A thick aluminum arsenide layer is subjected to the ambient atmosphere containing about 20 torr water vapor pressure. After a period of about 2 hours the aluminum arsenide layer is substantially completely converted to aluminum oxide.

The electrically insulating A1₂0₃ layer produced according to the invention can be used for a variety of purposes, such as an intrinsic part of an active electron device in which an insulating layer is required, such as in field-effect transistors (FETs) , metal-oxide-semiconductor field-effect transistors (MOSFETs) , semiconducting laser structures, avalanche photodiodes, tunnel diodes, as barriers in quantum- well devices, etc. Other uses include applications as protective or chemically passivating layers or coatings, or barriers against chemical reaction, interdiffusion or contamination. In addition the A1₂0₃ insulator can be grown or produced upon metallic surfaces, allowing for its application as the insulating layer in metal-insulator-metal (MIM) diodes, photodiodes, detectors of infrared, visible or ultraviolet radiation or ionizing particles, and superconductive tunnelling devices such as thin-film-type Josephson junctions and other superconductor-insulator-superconductor junction devices. In addition, the property that further layers of other semiconductive, insulating, metallic or superconductive materials may be grown, deposited, or produced upon the A1₂0₃ insulator layer or film allows it to be used as the insulating layer in complex metal-insulator or other structures useful in electronic, optical, or other applications.

With regard to use in semiconductor devices, the invention process can be used for production of passivating or protective A1₂0₃ coatings, or electrically, or chemically isolating A1₂0₃ layers. As a result of its chemical inertness, A1₂0₃ layers will protect underlying semiconductor materials from exposure to corrosive chemicals during device- fabrication and processing steps (etch-stop layers) . Inasmuch as A1₂0₃ has a large band gap, the A1₂0₃ film isolates the active semiconductor layers electrically as well as chemically. Hence, the A1₂0₃ film produced according to the invention process allows isolation of semiconductor materials from metals, making possible improved metal-semiconductor (MES) devices that utilize compound semiconductors, such as metal-semiconductor field-effect transistors (MESFETs) , or Schottky diodes. Such devices are of interest for their amplifying or rectifying capabilities. As noted above, the A1₂0₃ layer is frequently used to electrically isolate regions of a device. Inasmuch as a useful native oxide does not exist for GaAs, metal-oxide-semiconductor (MOS) diodes based on GaAs are difficult to produce. Because metals can be grown or deposited upon the A1₂0₃ films produced upon GaAs, MOS devices incorporating the high electron mobility of GaAs or indium-gallium arsenide (InGaAs) can be developed using the insulating A1₂0₃ layer in the role of the oxide layer. This application will permit development of MOS-type GaAs diodes by a simple, reliable and effective process.

IIIB-VB semiconductor materials such as GaAs possess qualities that allow their application in electron devices operable at much higher frequencies than are achievable with silicon devices. The development of IIIB-VB devices has been impeded for many years by the lack of an insulating native oxide. The simple process disclosed herein for growing or producing insulating A1₂0₃ films provides the high quality, electrically insulating, chemically inert film needed for the development of electron devices and circuits based on IIIB-VB compound semiconductor materials. In particular, the process provides highly resistive, impervious, chemically inert and stable, physically hard films of insulating A1₂0₃ with controllable thickness and excellent morphology and uniformity. This A1₂0₃ layer can play a role for IIIB-VB semiconductor devices and circuits similar to that played by Si0₂ or Si₃N₄ in silicon-based electron-device technology. Further, the process for production of the insulating A1₂0₃ film is simple and compatible with customary device-fabrication processes, and produces reliable and controllable A1₂0₃ films. INDUSTRIAL APPLICATION

Gallium Arsenide semiconductors can be used in any application where silicon based semiconductors are presently being used. By its very nature a gallium arsenide semiconductor is at least an order of magnitude faster than an identical silicon based semiconductor and is resistant to damage by electro magnetic pulses generated by a nuclear explosion. If the circuit is specifically designed for a gallium arsenide semiconductor then an improvement of several orders of magnitude can be achieved over a comparable silicon based design. Thus the process of the invention which can quickly, efficiently, and economically produce reliable gallium arsenide semiconductors has tremendous industrial applications. Since various changes and modifications can be made in the invention without departing from the spirit of the invention, the invention is not to be taken as limited except by the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

Claim 1 A process for producing an electrically insulating and protective layer of aluminum oxide on surfaces of semiconductive, insulating, metallic and superconductive substrates which comprises forming a layer of aluminum arsenide on a substrate selected from the group consisting of semiconductive, insulating, metallic and superconductive substances, and

exposing said aluminum arsenide coated substrate to water vapor at a water vapor pressure and for a period of time sufficient to substantially, completely convert said aluminum arsenide to aluminum oxide.

Claim 2 The process of claim 1, which includes carrying out said exposing step in a reaction chamber and including the step of adding a non-reactive gas to said reaction chamber to increase the pressure in said chamber to above atmospheric. Claim 3 The process of claim 1, wherein said aluminum arsenide is exposed to the ambient atmosphere containing said water vapor.

Claim 4 The process of claim 1, which includes carrying out said exposing step in a reaction chamber and including the step of adding oxygen to said reaction chamber to increase the pressure in said chamber to above atmospheric.

Claim 5 The process of claim 1, which includes carrying out said exposing step in a reaction chamber and including the step of adding nitrogen to said reaction chamber to increase the pressure in said chamber to above atmospheric.

Claim 6 The process of claim 1, wherein said water vapor pressure ranges from about 7 to about 24 torr.

Claim 7 The process of claim 1, wherein the product of water vapor pressure and time ranges from about 15 to about 50 torr hours, based on a 500 angstrom thick layer of aluminum arsenide and ambient temperature conversion.

Claim 8 The process of claim 1, wherein the product of water vapor pressure and time ranges for about 15 to about 25 torr hours, based on a 500 angstrom thick layer of aluminum arsenide and ambient temperature conversion.

Claim 9 The process of claim 1, wherein said substrate is a substance formed of elements of Group IIIB and of Group VB of the periodic table.

Claim 10 The process of claim 1, wherein said substrate is a substance formed of elements of Group II and of Group VIB of the periodic table.

Claim 11 The process of claim 1, wherein said substrate is a substance formed of elements of Group IVB of the periodic table.

Claim 12 The process of claim 1, wherein said substrate is a semiconductive substance selected from the group consisting of zinc sulfide, zinc selenide, zinc telluride, cadmium selenide and cadmium telluride, mercury selenide, mercury telluride, mercury sulfide, and alloys thereof.

Claim 13 The process of claim 1, wherein said substrate is a semiconductive substrate selected from the group consisting of, gallium arsenide, gallium antimonide, indium arsenide, indium phosphide, gallium indium arsenide, indium antimonide, gallium arsenide antimonide, gallium arsenide phosphide, indium- gallium arsenide phosphide, gallium phosphide and aluminum arsenide. Claim 14 The process of claim 1, wherein said substrate is gallium arsenide.

Claim 15 The process of claim 1, wherein said substrate is a metallic substance.

Claim 16 The process of claim 1, wherein said substrate is a metallic substance selected from the group consisting of silver, gold and palladium and platinum, and alloys thereof.

Claim 17 The process of claim 1, wherein said substrate is a metallic substrate selected from the group consisting of copper, aluminum, iron, titanium, vanadium, chromium and alloys thereof.

Claim 18 The process of claim 1, wherein said substrate is an insulating material Claim 19 The process of claim 1, wherein said substrate is an insulating material selected from the group consisting of quartz, glass, magnesium oxide, beryllium oxide and alumina.

Claim 20 The process of claim 1, wherein said substrate is a superconductive substance.

Claim 21 The process of claim 1, wherein said substrate is a superconductive substance selected from the group consisting of niobium, niobium nitride, yttrium barium copper oxide, bismuth calcium barium cooper oxide.

Claim 22 The process of claim 1, wherein said substrate is gallium arsenide, said layer of aluminum arsenide being an epitaxial film of aluminum arsenide, and which includes carrying out said exposing step in a reaction chamber and including the step of adding a substance selected from the group consisting of oxygen and nitrogen to said reaction chamber to increase the pressure to above atmospheric, and wherein the product of water vapor pressure and time ranges from about 10 to about 50 torr hours based on a 500 angstrom thick layer of aluminum arsenide and ambient temperature conversion.

Claim 23 The process of claim 22, wherein the product of water vapor pressure and time ranges from about 15 to about 24 torr hours, based on a 500 angstrom thick layer of aluminum arsenide and ambient temperature conversion.

Claim 24 The process of claim 1, wherein said substrate is gallium arsenide, said layer of aluminum arsenide being an epitaxial film of aluminum arsenide, and which includes carrying out said exposing step in the ambient atmosphere containing water vapor at a water vapor pressure of about 7 to about 24 torr, and wherein

the product of water vapor pressure and time ranges from about 10 to about 50 torr hours based on a 500 angstrom thick layer of aluminum arsenide and ambient temperature conversion. Claim 25 The process of claim 1, wherein said forming step is carried out by molecular-beam epitaxy to produce an epitaxial film of aluminum arsenide.

Claim 26 The process of claim 1, wherein said forming step is carried out by organometallic chemical-vapor deposition.

Claim 27 The process of claim 1, wherein said forming step is carried out by organometallic vapor-phase deposition.

Claim 28 The process of claim 1, wherein said forming step is carried out by metallorganic molecular-beam epitaxy.

Claim 29 The process of claim 1, wherein said forming step is carried out by chemical-beam epitaxy.

Claim 30 The process of claim 1, wherein said layer of aluminum arsenide is an epitaxial film having a thickness ranging from about 6 angstroms to about 1.5. microns, or greater. Claim 31 A substrate with an electrically insulting and protective layer of aluminum oxide on its surface, said layer being produced according to the process of claim 1.

Claim 32 A substrate with an electrically insulating and protective layer of aluminum oxide on its surface, said layer being produced according to the process of claim 2.

Claim 33 A substrate with an electrically insulating and protective layer of aluminum oxide on its surface, said layer being produced according to the process of claim 6.

Claim 34 A substrate with an electrically insulating and protective layer of aluminum oxide on its surface, said layer being produced according to the process of claim 22.