GB1598051A

GB1598051A - Molecular-beam epitaxy

Info

Publication number: GB1598051A
Application number: GB2458/78A
Authority: GB
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1977-05-26
Filing date: 1978-01-20
Publication date: 1981-09-16
Also published as: JPS5528544B2; FR2391769A1; DE2806766A1; JPS53147462A; FR2391769B1; IT1109159B; CA1102013A; IT7820149A0

Description

(54) MOLECULAR-BEAM EPITAXY (71) We, INTERNATIONAL BUSINESS MACHINES CORPORATION, a Corporation organized and existing under the laws of the State of New York in the United States of America, of Armonk, New York 10504, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to molecular-beam epitaxy.

Molecular-beam epitaxy denotes the epitaxial growth of a semiconductor film by a process involving the reaction of one or more thermal molecular beams with a crystalline surface under ultra-high vacuum conditions, and is well known in the art.

A complete discussion of the molecularbeam epitaxy process and the systems for carrying it out is provided by the publication Progress in Solid State Chemistry, Vol. 10 part 3, 1975 in the article "Molecular Beam Epitaxy" by A. Y. Cho and J. R. Arthur at page 157.

Another extensive discussion of the prior art of molecular-beam epitaxy is found in the text Epitaxial Growth Part A of the Materials Science Series. The article "Molecular-Beam Epitaxy" by L. L. Chang and R. Ludeke, Section 2.2, pages 37-72 presents a treatise on the theory and techniques employed in the prior art.

According to the present invention there is provided a molecular-beam epitaxy system of the type including an enclosed vacuum chamber containing at least one source material and a substrate on which the source material is epitaxially grown, the system further comprising a source of hydrogen and means for introducing the hydrogen at a controlled rate into the vacuum chamber for interacting with the epitaxially grown mate rival.

An embodiment of the invention will now be described with reference to the accompanying drawing whose single figure is a schematic illustration of a molecular-beam epitaxy system according to the invention.

The molecular-beam epitaxy system shown includes a vacuum chamber or enclosure 10, the interior of which is maintained at an ultra-high vacuum by vacuum pumps. A single source 12, for example of Ga, Al, As or Sn, is shown inside chamber 10, however more than one source of the above or other materials may be present depending on the desired application. A substrate 20 is also included in chamber 10. The structure described and illustrated up to this point represents a conventional molecular beam epitaxy system well known in the prior art. It is to be appreciated that in actual practice there are several other components and devices employed in the system.A more complete arrangement is illustrated in Fig. 1 of the Chang and Ludeke article and includes such elements as source heaters, source shutters, substrate holders, substrate heaters, substrate shutters, shrouds, electron guns, screens and other known components of a working system. These elements have been omitted from the drawing for simplicity since their operation and purpose are well known.

A novel aspect of the molecular-beam epitaxy system shown in the drawing is the presence of the hydrogen source 14, which is used to introduce a beam of hydrogen into chamber 10 via a conduit controlled by valve 16. The hydrogen beam may optionally be atomized or ionized by the atomizer or ionizer structure 18.

Molecular-beam epitaxy is a term used to denote the epitaxial growth of a semiconductor film by a process involving the reaction of one or more thermal molecular beams with a crystalline surface under ultra-high vacuum conditions.

A molecular-beam is defined as a directed beam of neutral molecules in a vacuum system. The beam density is low and the vacuum high so that no appreciable collisions occur among the beam molecules and between the beam and the background vapor. The beam is usually produced by heating a solid substance contained in an effusion cell. The orifice dimension of the cell is small compared to the mean free path of the vapor in the cell so that flow of the molecules into the vacuum chamber is by effusion. Quasi-equilibrium exists in the cell so that both the vapor composition and the effusion rates of the beam are constant and are predictable from thermodynamics, in contrast to the case of free evaporation.

The beam is guided by the orifice and possibly by other slits and shutters onto the substrate where the situation is usually far from equilibrium. Under proper conditions, governed mainly by kinetics, the beam condenses resulting in nucleation and growth.

Referring to the accompanying figure it is again stated that the conventional elements employed in a typical molecular beam epitaxy system, such as ion pumps, sublimation pumps, liquid nitrogen shrouds, source ovens (i.e. resistively heated effusion cells composed for example of graphite or boron nitride), thermocouples, source shutters, substrate shutters, and substrate holders have been omitted from the accompany figure for simplicity since the operation of such system is well explained in the prior art literature.

The substrate 20 is usually a monocrystalline material that has been cleaned, polished and etched. It may or may not be the same material as that to be deposited, depending on whether homoepitaxy is desired. The substrate 20 during deposition is kept at an elevated temperature, which is usually necessary for epitaxial growth. It can also be heated before deposition primarily for cleaning and afterwards for various heat treatments.

Source 12, in the present application, is meant to represent either a single source material or a plurality of source materials for producing multilayered or compound films and the aforesaid supporting equipment, such as heaters, thermocouples and shroud.

Typically, sources of Ga and As may be provided for growth of GaAs, or sources of Ga, As and Al may be provided when GaAlAs is desired. By additionally providing a source of Sn, the grown film may be ndoped.

The present invention is directed to the improvement of the molecular-beam epitaxy system wherein a beam of hydrogen is introduced which can result in certain applications in improvements in surface smoothness, electron mobility, photoluminescence and doping incorporation. The hydrogen beam is provided by the hydrogen source 14 which selectively supplied hydrogen into chamber 10 through the valve 16 and the orifice of the structure 18.

The introduction of hydrogen into the molecular-beam epitaxy process produces superior results for the following reasons.

One of the most serious impediments to high quality molecular-beam epitaxy grown films is the presence of oxygen. When oxygen gets into the sample film it forms deep levels that act as traps for the charge carriers and greatly affect the electronic properties of the films grown. The oxygen problem is especially pronounced in the presence of Al, such as in the growth of Galas, a commonly desired film material. To overcome the oxygen problem, the present invention uses the introduction of hydrogen to remove the oxygen from the surface during growth.

In a given application in growing two Galas samples, one using hydrogen according to the present invention and one without hydrogen, it was found that the sample grown in the hydrogen environment exhibited a three times increase in measured carrier concentration and a five times increase in electron mobility. The simultaneous increase in carrier concentration normally leads to a decrease in mobility. A further result was a ten times increase in photoluminescence.

In a typical embodiment of the invention in the molecular-beam epitaxy process, the arrival rate of the hydrogen from source 14 of the accompanying figure into chamber 10 is controlled by valve 16 to be about 10X4 to 10'5 molecules per square centimeter per second. In comparison, the Ga arrival rate is 4x 1014 atoms per square centimeter per second for a growth rate of 2 Angstroms per second of GaAs. Thus, the hydrogen arrival rate is maintained about twice that of the Ga.

Of course, the hydrogen flow rate can be adjusted in accordance with the particular geometry of the molecular-beam epitaxy system being employed in order to obtain the desired hydrogen arrival rate.

WHAT WE CLAIM IS: 1. A molecular beam epitaxy system of the type including an enclosed vacuum chamber containing at least one source material and a substrate on which the source material is eptiaxially grown, the system further comprising a source of hydrogen and means for introducing the hydrogen at a controlled rate into the vacuum chamber for interacting with the epitaxially grown material.

2. A molecular-beam epitaxy system according to claim 1, wherein the at least one source material includes sources of Ga, Al, As and Sn.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. A molecular-beam is defined as a directed beam of neutral molecules in a vacuum system. The beam density is low and the vacuum high so that no appreciable collisions occur among the beam molecules and between the beam and the background vapor. The beam is usually produced by heating a solid substance contained in an effusion cell. The orifice dimension of the cell is small compared to the mean free path of the vapor in the cell so that flow of the molecules into the vacuum chamber is by effusion. Quasi-equilibrium exists in the cell so that both the vapor composition and the effusion rates of the beam are constant and are predictable from thermodynamics, in contrast to the case of free evaporation. The beam is guided by the orifice and possibly by other slits and shutters onto the substrate where the situation is usually far from equilibrium. Under proper conditions, governed mainly by kinetics, the beam condenses resulting in nucleation and growth. Referring to the accompanying figure it is again stated that the conventional elements employed in a typical molecular beam epitaxy system, such as ion pumps, sublimation pumps, liquid nitrogen shrouds, source ovens (i.e. resistively heated effusion cells composed for example of graphite or boron nitride), thermocouples, source shutters, substrate shutters, and substrate holders have been omitted from the accompany figure for simplicity since the operation of such system is well explained in the prior art literature. The substrate 20 is usually a monocrystalline material that has been cleaned, polished and etched. It may or may not be the same material as that to be deposited, depending on whether homoepitaxy is desired. The substrate 20 during deposition is kept at an elevated temperature, which is usually necessary for epitaxial growth. It can also be heated before deposition primarily for cleaning and afterwards for various heat treatments. Source 12, in the present application, is meant to represent either a single source material or a plurality of source materials for producing multilayered or compound films and the aforesaid supporting equipment, such as heaters, thermocouples and shroud. Typically, sources of Ga and As may be provided for growth of GaAs, or sources of Ga, As and Al may be provided when GaAlAs is desired. By additionally providing a source of Sn, the grown film may be ndoped. The present invention is directed to the improvement of the molecular-beam epitaxy system wherein a beam of hydrogen is introduced which can result in certain applications in improvements in surface smoothness, electron mobility, photoluminescence and doping incorporation. The hydrogen beam is provided by the hydrogen source 14 which selectively supplied hydrogen into chamber 10 through the valve 16 and the orifice of the structure 18. The introduction of hydrogen into the molecular-beam epitaxy process produces superior results for the following reasons. One of the most serious impediments to high quality molecular-beam epitaxy grown films is the presence of oxygen. When oxygen gets into the sample film it forms deep levels that act as traps for the charge carriers and greatly affect the electronic properties of the films grown. The oxygen problem is especially pronounced in the presence of Al, such as in the growth of Galas, a commonly desired film material. To overcome the oxygen problem, the present invention uses the introduction of hydrogen to remove the oxygen from the surface during growth. In a given application in growing two Galas samples, one using hydrogen according to the present invention and one without hydrogen, it was found that the sample grown in the hydrogen environment exhibited a three times increase in measured carrier concentration and a five times increase in electron mobility. The simultaneous increase in carrier concentration normally leads to a decrease in mobility. A further result was a ten times increase in photoluminescence. In a typical embodiment of the invention in the molecular-beam epitaxy process, the arrival rate of the hydrogen from source 14 of the accompanying figure into chamber 10 is controlled by valve 16 to be about 10X4 to 10'5 molecules per square centimeter per second. In comparison, the Ga arrival rate is 4x 1014 atoms per square centimeter per second for a growth rate of 2 Angstroms per second of GaAs. Thus, the hydrogen arrival rate is maintained about twice that of the Ga. Of course, the hydrogen flow rate can be adjusted in accordance with the particular geometry of the molecular-beam epitaxy system being employed in order to obtain the desired hydrogen arrival rate. WHAT WE CLAIM IS:

1. A molecular beam epitaxy system of the type including an enclosed vacuum chamber containing at least one source material and a substrate on which the source material is eptiaxially grown, the system further comprising a source of hydrogen and means for introducing the hydrogen at a controlled rate into the vacuum chamber for interacting with the epitaxially grown material.