Note: Descriptions are shown in the official language in which they were submitted.
13 Back~round of the Invention
14 Field of the Invention
The present invention relates to an improved molecular-beam epitaxy
16 system and method, and more particularly to a molecular-beam epitaxy system
17 wherein hydrogen is introduced and employed to improve the quality of the
18 epitaxy.
19 Description of the Prior Art
Molecular-beam epitaxy as a method for epitaxial growth of compound
21 semiconductor films by a process involving the reaction of one or more
22 thermal molecular beams with a crystalline surface under ultra-high vacuum
23 conditions is well known in the art.
24 A complete discussion of the molecular-beam epitaxy process and
the structures for carrying it out is provided by the publication Progress
26 in Solid,State Chemistry, Vol. 10 part 3, 1975 in the article '~olecular
27 Beam Epitaxy" by A. Y. Cho and J. R. Arthur at page 157.
,
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1 Another extensive discussion of the prior art of molecular-beam
2 epitaxy is found in the text Epitaxial Growth Part A of the Materials Sciènce
3 Series. The article "Molecular-Beam Epitaxy" by L. L. Chang and R. Ludeke,
4 Section 2.2, pages 37-72 presents a treatise on the theory and techniques
employed in the prior art.
6 There is no teaching in the prior art relative to the use of
7 hydrogen in the molecular-beam evaporation process for epitaxy growth as
8 provided by the present inVentiQn~ and a review of the prior art will
9 indicate that such use of hydrogen as in the present invention is an
unusual and unexpected technique.
11 Summary of the Invention
12 An ob~ect of the present invention is to provide an improved
13 process and system for the molecular-beam evaporatlon epitaxy growth including
14 the presence of hydrogen.
Another object of the present invention is to provide an improved
16 process and system for molecular-beam epitaxy including a hydrogen beam
17 directed onto the substrate.
18 A further object of the present invention is to provide an
19 improved molecular-beam epitaxy system and method for the formation of
GaAs or GaAlAs and Sn for n-dopant impurity wherein hydrogen is used in
21 the growth process.
22 The foregoing and other objects, features and advantages of the
23 invention will be apparent from the following re particular description
24 of a preferred embodiment of the invention, as illustrated in the accompany- ing drawings.
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1 Brief Descrlption of the Drawings
2 The single figure of the drawing is a schematic illustration
3 of a molecular-beam epitaxy system including mea~s for int-oduciag a
4 beam of hydrogen.
Detailed Description of the Preferred Embodiment
6 Referring to the drawing a schematic illustration of a molecular-
7 beam epitaxy system i8 shown including the basic vacuum chamber or enclosure8 10, the interior of which is maintained at an ultra-high vacuum condition
9 by vacuum pumps. A single source 12, such as Ga, Al As or Sn ls shown insldechamber lO, however more than one source of the above or other materials may
11 be generally represented by element 12 depending on the desired growth
12 application. A substrate 20 is also included in chamber 10. The structure
13 described and illustrated up to this point represents a conventional
14 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
16 devices employed in the system. A more complete arrangement is illustrated
17 in FIG. 1 of the Chang and Ludeke publication in the previously mentioned
18 text E~itaxial Growth and includes such elements as source heaters, source
19 shutters, substrate holders, substrate heaters, substrate shutters, shrouds,
electron guns, screens and other state-of-the-art components of a working
21 system. These elements have been omitted from the drawing for simplicity
22 since their operation and purpose are well known.
23 A novel aspect of the molecular-beam epitaxy system in the drawing
24 is 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
26 optionally be atomized or ionized by the atomizer or ionizer structure 18.
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1 ~lolecular-beam epitaxy is a term used to denote the epitaxial
2 growth of compound semiconductor films by a process involving the reaction
3 of one or more thermal molecular beams with a crystalline surface under
4 ultra-high vacuum conditions.
A molecular-beam is deflned as a directed ray of neutral molecules
6 or atoms in a vacuum system. The beam denslty is low and the vacuum
7 high so that no appreciable collisions occur among the beam molecules
8 and between the beam and the background vapor. The beam is usually
9 produced by heating a solid substance contained in an effusion cell.
The orifice dimension of the cell is small compared to the mean free
11 path of the vapor in the cell so that flow of the molecules into the
12 vacuum chamber is by effusion. Quasi-equilibrium exists in the cell so
13 that both the vapor composition and the effusion rates of the beam are
14 constant and are predictable from thermodynamics, in contrast to the case
of free evaporation.
16 The beam is guided by the orifice and possibly by other slits
17 and shutters onto a substrate where the situation is usually far from
18 equilibrium. Under proper conditions, governed mainly by kinetics, the
19 beam would condense resulting in nucleation and growth.
Referring to FIG. 1, it is again stated that the conventional
21 elements employed in a typical molecular beam epitaxy system, such as ion
22 pumps, sublimation pumps, liquid nitrogen shrouds, source overns (i.e.
23 resistively heated effusion cells composed for example of graphite or
24 boron nitride), thermocouples, source shutters, substrate shutters, and
substrate holders have been omitted from FIG. 1 for simplicity since the
26 operation of such system is well explained in the prior art literature.
27 The substrate 20 i6 usually a monocrystalline material that has
28 been cleaned, polished and etched. It may or may not be the same material
29 as that to be deposited, depending on whether homoepitaxy is desired.
The substrate 20 during deposition, is kept at elevated temperatures, which
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1 are usually ~ecessary for epitaxial growth. It can also be heated before
2 deposition primarily for clean$ng and afterwards for various heat treatments.
3 Source 12, ln the present application, is meant to represen;
4 either a single source material or a plurality of source materials for
S producing multilayered or compound films and the aforesaid supporting
6 equipment, such as heaters, thermocoupler and shroud.
7 The present invention is directed to the improvement of the
8 molecular-beam epitaxy method and system wherein a beam of hydrogen is
9 introduced which results in improvements in material surface smoothness,
electron mobility, photoluminescence and doping incorporation. The
11 hydrogen beam is provided by a hydrogen source 14 which selectlvely supplies
12 hydrogen into chamber 10 through valve 16 and orifice 18.
13 The introduction of hydrogen into the molecular-beam epitaxy
14 process produces superior results for the following reasons. One of the
most serious impediments to maximum quality molecular-beam epitaxy grown
16 samples is the presence of oxygen. When oxygen gets into the sample film
17 it forms deep levels that act as traps for the charge carriers and greatly
18 affect the electronic properties of the films grown. The oxygen problem
9 i9 especially pronounced in the presence of Al, such as in the growth of
GaAlAs, a commonly desired sample. To overcome the oxygen problem, the
21 present invention uses the introduction of hydrogen to remove the oxygen
22 from the surface during growth.
23 In a given application in growing two GaAlAs samples, one using
24 hydrogen according to the present invention and one without hydrogen, it
was found that the sample grown in the hydrogen environment exhibited a
26 three times increase in measured carrier concentration and a five times
27 increase in electron mobility. The simultaneous increase in carrier
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1 concentration normally leads to a decrease in mobllity. A further result
2 was a ten times increase in photoluminescence.
3 In a typical embodiment of the invention in the molecular-beam
4 epitaxy process, the arrival rate of the hydrogen from source 14 of FIG. 1
into chamber 10 is controlled by valve 16 to be about 1014 to 10
6 molecules per square centimeter per second. In comparison, the Ga
7 arrival rate is 4 X 1014 atoms per square centimeter per second for a growth
8 rate of 2 Angstroms per second of GaAs. Thus, the hydrogen arrival rate
9 is maintained about twice that of the Ga. Of course, the hydrogen flow
rate can be adjusted in accordance wtih the particular geometry of the
11 molecular-beam epitaxy system being employed in order to obtain the desired
12 hydrogen flow rate.
13 Whlle the invention has been particularly shown and described
14 with reference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in form and
16 details may be made therein without departing from the spirit and scope of
17 the invention.
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