Technique For Growth Of Epitaxial Compound Semiconductor Films

Abstract

Epitaxial growth of Group III(a)-V(a) semiconductor compound films is effected in an ultrahigh vacuum by directing collimated molecular beams at the surface of a suitable substrate member preheated to a temperature ranging from 450.degree.-650.degree. C. The described process is a nonequilibrium growth technique which permits the growth of epitaxial films less than 1 micron in thickness at temperatures appreciably below those commonly employed in epitaxy.

Description

This invention relates to a technique for the growth of epitaxial compound semiconductor films. More particularly, the present invention relates to a technique for the growth of epitaxial semiconductor films of Group III(a)-V(a) compounds of the Periodic Table of the Elements by a novel physical vapor growth procedure.

The dynamic growth of the semiconductor industry and the sophistication of device technology over the past decade have created stringent demands upon materials from the standpoint of reliability and physical and electrical characteristics. In order to meet certain of these demands, workers in the art have focused their attention with increasing frequency upon epitaxial growth techniques.

Heretofore, epitaxial films suitable in such applications have been grown by several techniques, the most popular being solution epitaxy, chemical vapor growth and physical vapor growth. Although such techniques have generally been satisfactory from a device standpoint, the need for a procedure permitting greater flexibility with respect to doping profiles combined with film thicknesses of the order of one micron or less has not been met. Additionally, a need has long existed for a nonequilibrium epitaxial growth procedure which would permit growth at temperatures appreciably below those conventionally employed.

In accordance with the present invention, these needs have been realized by means of a novel physical vapor growth procedure wherein epitaxial growth is effected in an ultrahigh vacuum at temperatures ranging from 450.degree.- 650.degree. C. (which permits impurity profiles to be altered abruptly due to limited diffusion) the constituent components of the grown films being furnished to the substrate by collimated molecular beams.

The described technique is premised upon the discovery that Group III(a)-V(a) elements contained in compound semiconductors are adsorbed upon the surface of single crystal semiconductors at varying rates, the V(a) elements typically being almost entirely reflected therefrom in the absence of III(a) elements. However, it has been determined that growth of stoichiometric III(a)-V(a) semiconductor compounds, including mixed crystals, thereof, may be effected by providing vapors of Group III(a) and V(a) elements at the substrate surface, an excess of Group V(a) element being present with respect to the III(a) element, thereby assuring that the entirety of the III(a) element will be consumed while the nonreacted V(a) excess is reflected.

Briefly, the inventive technique involves forming an atomically clean substrate surface in a vacuum chamber, evacuating the chamber and directing at least one collimated molecular beam containing the constituent components of the desired crystalline material at the substrate for a time period sufficient to grow an epitaxial film of the required thickness. The collimated molecular beams employed herein furnish not only the constituent components of the film but also the desired impurities, so permitting the altering of the composition at will and the production of abrupt changes in composition or impurity levels, such end being of particular interest in certain device applications in which either an abrupt PN junction is required or a ternary composition.

The invention will be more readily understood by reference to the following detailed description taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a front elevational view, partly in section, of a typical apparatus employed in the practice of the present invention; and

FIG. 2 is a cross-sectional view of a cylindrical gun employed in the apparatus of FIG. 1.

With reference now more particularly to FIG. 1, there is shown a vacuum chamber 11 having disposed therein a gun port 12 containing cylindrical guns 13 and 14, a sputtering port 15 containing a sputtering gun 16 and a substrate holder 17 connected to a ceramic insulator 18 by means of shaft 19. Ceramic insulator 18 is connected by means of shaft 20 to a rotor 21 capable of effecting rotary motion of shafts 19 and 20. Also shown disposed within chamber 11 is a liquid nitrogen cooling shroud 22 and a collimating frame 23 having a collimating aperture 24. Substrate holder 17 is provided with an internal heater 25 and clips 26 and 27 for affixing a substrate member 28 thereto. Chamber 11 also includes an inlet 29 for the introduction of a sputtering gas from source 30 controlled by valve 31 and an outlet 32 for evacuating the chamber by means of a pump 33.

FIG. 2 is a cross-sectional view of a typical cylindrical gun, such as 14, shown in FIG. 1. Gun 14 typically comprises a refractory crucible 41 having a thermocouple well 42 and a thermocouple 43 inserted therein for the purpose of determining the temperature of the material contained therein.

For purposes of exposition, the present invention will be described in detail by reference to an illustrative example wherein the various operating parameters are given.

The first step in the inventive technique involves selecting a substrate member (relatively dislocation free), obtained from commercial sources. Suitable substrate members may be selected from among single crystal elemental and compound semiconductors as well as certain insulators manifesting lattice constants closely related to those of the desired epitaxial film. Prime examples of substrate materials meeting these requirements are silicon, germanium, gallium arsenide, gallium phosphide, gallium arsenic phosphide, indium arsenide, indium phosphide, sapphire and the like.

The substrate member selected is initially polished by any conventional polishing technique for the purpose of removing impurities from the surface thereof. An etchant such as a bromine-methanol or hydrogen peroxide-sulfuric acid solution may optionally be employed for the purpose of further purifying the substrate surface subsequent to polishing.

Next, the cleaned substrate is placed in an apparatus of the type shown in FIG. 1 and the system baked for a time period ranging from 5 to 10 hours at a pressure within the range of 10.sup..sup.-5 to 10.sup..sup.-8 torr for the purpose of removing water vapor from the system. Thereafter, a suitable inert sputtering gas such as argon is admitted to the vacuum chamber and sputtering initiated with the substrate member facing the sputtering gun. Sputtering is continued for a time period ranging from 1 to 3 hours employing a sputtering voltage ranging from 100 to 250 volts with a current density within the range of 100 to 500 microamps for the purpose of removing several monolayers of material from the substrate so as to form an atomically clean surface thereon. Then, the substrate member is rotated so as to face the gun port of the apparatus, inert gas pumped out of the system and the background pressure then lowered to at least 5.times.10.sup..sup.-8 torr and preferably to a value of the order of 1.times.10.sup..sup.-9 torr, thereby precluding the introduction of any deleterious components onto the substrate surface. The next step in the process involves introducing liquid nitrogen to the cooling shroud and heating the substrate member to the growth temperature which ranges from 450.degree.--650.degree. C. dependent upon the specific material to be grown, such range being dictated by considerations relating to surface diffusion.

Following, the gun or guns employed in the system, which have previously been filled with the requisite amounts of the constituent of the desired films to be grown, are heated to a temperature sufficient to vaporize the contents thereof to yield a molecular beam, that is, a stream of atoms manifesting velocity components in the same direction, in this case toward the substrate surface. The atoms of molecules reflected from the surface strike the cooled shroud and are condensed, thereby insuring that only atoms or molecules from the molecular beam impinge upon the surface.

As indicated, the present invention relates to the growth of Group III(a)-V(a) semiconductor compounds and mixed crystals thereof. Accordingly, the materials furnished to the gun or guns are either Group III(a)-V(a) compounds or Group III(a) elements. Additionally, a desired dopant may be added either to an independent gun or included with the III(a)-V(a) compound. For the purposes of the present invention, the amount of source materials furnished to the guns must be sufficient to provide an excess of the V(a) element with respect to the III(a) element. Similar considerations obtain with respect to the ternary compounds, such as GaAs.sub.x P.sub.1.sub.-x. However, it has been found that in regard to this material the phosphorous-to-arsenic ratio in the vapor must be about four times the desired phosphorous-to-arsenic ratio in the bulk.

Thereafter, growth of the desired epitaxial film is effected by directing the molecular beam or beams at the collimator which functions to remove velocity components therein in directions other than those desired, thereby permitting the desired beam to pass through the collimating aperture to effect reaction at the substrate surface. Growth is continued for a time period sufficient to yield an epitaxial film of the desired thickness, a feature of the subject technique residing in the growth of films appreciably less than one micron in thickness. Diffusion of a desired dopant into the grown layer may be effected simultaneously with the growth of that layer or following growth by rotation of the substrate in such manner that it faces a gun port containing a doping gun.

It will be understood by those skilled in the art that the composition of the grown layer can be altered at will. Thus, for example, ternary compounds of the type alluded to hereinabove can be grown by using three source beams and the value of x can be precisely controlled and altered at any time during growth by appropriate beam regulation.

Several examples of the present invention are given by way of illustration and are not to be construed as limitations, many variations being possible within the spirit and scope of the invention.

EXAMPLE I

This example describes a process for the growth of an epitaxial film of gallium arsenide upon a gallium arsenide substrate member.

A gallium arsenide substrate member evidencing few dislocations, obtained from commercial sources, and initially polished by conventional mechanical polishing techniques was inserted in an apparatus of the type shown in FIG. 1. In the apparatus actually employed, two guns were contained in the gun port, one gram of gallium arsenide and one-half gram of gallium being placed in the respective guns. Following, the vacuum chamber was evacuated to a pressure of the order of 10.sup..sup.-6 torr and the system baked at 250.degree. C. for 12 hours. Following the baking procedure, 10 microns of argon were admitted to the system, the substrate rotated in such manner as to face the sputtering port and sputtering effected at 200 volts with a current of approximately 500 microamps for a period of 2 hours, thereby effecting the removal of several monolayers of material from the substrate surface. Then, the argon was pumped out of the system, the substrate member rotated so as to face the gun port and heated to a temperature of approximately 600.degree. C., the background pressure of the system being 1.times.10.sup..sup.-9 torr. At this time, liquid nitrogen was introduced to the cooling shroud and the guns heated, the gallium arsenide gun to a temperature of 1,250.degree. K. and the gallium gun to 1,300.degree. K., thereby resulting in vaporization of the materials contained therein and the consequent flow of molecular beams toward the collimating frame which removed velocity components in the beams which were undesirable. The beams were focused upon the substrate surface for a period of 1 hour, so resulting in the growth of an epitaxial film of gallium arsenide upon the substrate 1 micron in thickness.

EXAMPLE II

The procedure of example I was repeated, with the exception that the gun port contained only one gun which was initially filled with one gram of gallium arsenide. At a temperature of 1,250.degree. K. (gun temperature) it was found that 50 times as much arsenic (in the form of diatomic species) was emitted from the gun as gallium so that for the growth of gallium arsenide, it was necessary to use but one source material. Growth was continued for a time period of 1 hour, so resulting in the growth of a gallium arsenide epitaxial film upon the gallium arsenide substrate one-half micron in thickness.

EXAMPLE III

The procedure of example II was repeated with the exception that the solitary gun contained 1 gram of gallium phosphide. Growth was continued for a period of 1 hour, so resulting in the growth of an epitaxial film of gallium phosphide upon the gallium arsenide substrate one-half micron in thickness.

EXAMPLE IV

The procedure of example III was repeated with the exception that a gallium phosphide substrate was employed. Growth was continued for a time period of approximately 1 hour, so resulting in the growth of an epitaxial film of gallium phosphide, one-half micron in thickness.

EXAMPLE V

This example describes the growth of an epitaxial film of GaAs.sub..25 P.sub..75. The procedure of example I was employed utilizing 1 gram of gallium phosphide and 1 gram of gallium arsenide in the respective guns. The gallium phosphide gun was heated to a temperature of 1,212.degree. K. and the gallium arsenide gun to a temperature of 1,140.degree. K., heating being continued for a time period of approximately 2 hours during which a film of GaAs.sub..25 P.sub..75 1 micron in thickness grew upon the substrate.

EXAMPLE VI

The procedure of example I was repeated with the exception that a third gun was employed containing one-half gram of tellurium which was heated to a temperature of 400.degree. C. during the operation of the procedure, so resulting in the formation of an N-type gallium arsenide epitaxial film, 1 micron in thickness.

Claims

What is claimed is:

1. A method for the growth of an epitaxial film of a Group III(a)-V(a) compound of the Periodic Table of the Elements upon a substrate surface at subatmospheric pressure, which comprises focusing collimated molecular beams at least one of which comprises a III(a)-V(a) compound of the desired epitaxial film upon a substrate surface, preheated to a temperature within the range of 450.degree.-650.degree. C., for a time period sufficient to effect growth of a film of the desired thickness.

2. A method in accordance with claim 1 wherein said substrate possesses an atomically clean surface.

3. A method in accordance with claim 1 wherein said pressure is less than 5.times.10.sup..sup.-8 torr.

4. A method in accordance with claim 1 wherein said molecular beam is formed by heating at least one gun member containing the constituent components of the desired epitaxial film to a temperature sufficient to vaporize said components and permitting the resultant vapor to impinge upon a collimating frame.

5. A method in accordance with claim 4 wherein said gun member contains gallium arsenide.

6. A method in accordance with claim 4 wherein a pair of gun members are employed and in which one contains gallium arsenide and the other contains gallium.

7. A method in accordance with claim 4 wherein said gun member contains gallium phosphide.

8. A method in accordance with claim 6 wherein one gun contains gallium phosphide and the other contains gallium arsenide.

9. A method in accordance with claim 8 wherein the phosphorus-to-arsenic ratio in the vapor is four times the phosphorus-to-arsenic ratio in the bulk material.

10. A method in accordance with claim 4 wherein three guns are employed and in which one gun contains a dopant.

11. A method in accordance with claim 10 wherein said dopant is an N-type material.