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Method for improving the transparency and quality of group-iii nitride crystals ammonothermally grown in a high purity growth environment

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Publication number
WO2013010121A1
Authority
WO
Grant status
Application
Patent type
Prior art keywords
group
growth
nitride
crystal
ill
Prior art date
Application number
PCT/US2012/046761
Other languages
French (fr)
Inventor
Siddha Pimputkar
Shuji Nakamura
James S. Speck
Original Assignee
The Regents Of The University Of California
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date

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Classifications

    • CCHEMISTRY; METALLURGY
    • C30CRYSTAL GROWTH
    • C30BSINGLE-CRYSTAL-GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B7/00Single-crystal growth from solutions using solvents which are liquid at normal temperature, e.g. aqueous solutions
    • C30B7/10Single-crystal growth from solutions using solvents which are liquid at normal temperature, e.g. aqueous solutions by application of pressure, e.g. hydrothermal processes
    • CCHEMISTRY; METALLURGY
    • C30CRYSTAL GROWTH
    • C30BSINGLE-CRYSTAL-GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10Inorganic compounds or compositions
    • C30B29/40AIIIBV compounds wherein A is B, Al, Ga, In or Tl and B is N, P, As, Sb or Bi
    • C30B29/403AIII-nitrides

Abstract

A method for improving the transparency and quality of Group-Ill nitride crystals ammonothermally grown in a high purity growth environment. Boron is used to improve the transparency of the Group-Ill nitride crystals, while a semipolar growth surface of the seed crystal is used to improve the quality of the Group-Ill nitride crystals. The high purity growth environment is achieved using a containment element within a pressure vessel, wherein the containment element is comprised of at least two segments that together form an inner volume that allows for matter to be transferred into and out of the inner volume.

Description

METHOD FOR IMPROVING THE TRANSPARENCY AND QUALITY OF GROUP-III NITRIDE CRYSTALS AMMONOTHERMALLY GROWN

IN A HIGH PURITY GROWTH ENVIRONMENT CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) of copending and commonly-assigned applications:

U.S. Provisional Application Serial No. 61/507,212, filed on July 13, 2011, by Siddha Pimputkar and Shuji Nakamura, entitled "HIGHER PURITY GROWTH ENVIRONMENT FOR THE AMMONOTHERMAL GROWTH OF GROUP-III NITRIDES," attorneys' docket number 30794.422-US-P1 (2012-023-1);

U.S. Provisional Application Serial No. 61/551,835, filed on October 26, 2011, by Siddha Pimputkar, Shuji Nakamura, and James S. Speck, entitled "USE OF BORON TO IMPROVE THE TRANSPARENCY OF AMMONOTHERMALLY GROWN GROUP-III NITRIDE CRYSTALS," attorneys' docket number 30794.438- US-P1 (2012-248-1); and

U.S. Provisional Application Serial No. 61/552,276, filed on October 27, 2011, by Siddha Pimputkar, Shuji Nakamura, and James S. Speck, entitled "USE OF SEMIPOLAR SEED CRYSTAL GROWTH SURFACE TO IMPROVE THE QUALITY OF AN AMMONOTHERMALLY GROWN GROUP-III NITRIDE CRYSTAL," attorneys' docket number 30794.439-US-P1 (2012-249-1);

all of which applications are incorporated by reference herein.

This application is related to the following co-pending and commonly- assigned applications:

U.S. Patent Application Serial No. 13/128,092, filed on May 6, 2011, by

Siddha Pimputkar, Derrick S. Kamber, James S. Speck and Shuji Nakamura, entitled "USING BORON-CONTAINING COMPOUNDS, GASSES AND FLUIDS

DURING AMMONOTHERMAL GROWTH OF GROUP-III NITRIDE

CRYSTALS," attorneys' docket number 30794.300-US-WO (2009-288-2), which application claims the benefit under 35 U.S.C. Section 365(c) of P.C.T. International Patent Application Serial No. PCT/US09/63233, filed on November 4, 2009, by Siddha Pimputkar, Derrick S. Kamber, James S. Speck and Shuji Nakamura, entitled "USING BORON-CONTAINING COMPOUNDS, GASSES AND FLUIDS

DURING AMMONOTHERMAL GROWTH OF GROUP-III NITRIDE

CRYSTALS," attorneys' docket number 30794.300-WO-U1 (2009-288-2), which application claims the benefit under 35 U.S.C. Section 119(e) of U.S. Provisional Application Serial No. 61/112,550, filed on November 7, 2008, by Siddha Pimputkar, Derrick S. Kamber, James S. Speck and Shuji Nakamura, entitled "USING BORON - CONTAINING COMPOUNDS, GASSES AND FLUIDS DURING

AMMONOTHERMAL GROWTH OF GROUP-III NITRIDE CRYSTALS," attorney's docket number 30794.300-US-P1 (2009-288-1);

U.S. Utility Patent Application Serial No. 13/048,179, filed on March 15, 2011, by Siddha Pimputkar, James S. Speck, and Shuji Nakamura, and entitled "GROUP-III NITRIDE CRYSTAL AMMONOTHERMALLY GROWN USING AN INITIALLY OFF-ORIENTED NONPOLAR OR SEMIPOLAR GROWTH SURFACE OF A GROUP-III NITRIDE SEED CRYSTAL," attorney's docket number 30794.376- US-U1 (2010-585-1), which application claims priority under 35 U.S.C. § 119(e) to co-pending and commonly-assigned U.S. Provisional Patent Application Serial No. 61/314,095, filed on March 15, 2010, by Siddha Pimputkar, James S. Speck, and

Shuji Nakamura, and entitled "GROUP-III NITRIDE CRYSTAL GROWN USING AN INITIALLY OFF-ORIENTED NONPOLAR AND/OR SEMIPOLAR GROUP- III NITRIDE AS A SEED CRYSTAL USING THE AMMONOTHERMAL METHOD AND METHOD OF PRODUCING THE SAME," attorney's docket number 30794.376-US-P1 (2010-585-1);

P.C.T. International Patent Application Serial No. PCT/US12/xxxxx, filed on July 13, 2012, by Siddha Pimputkar and James S. Speck, entitled "GROWING A GROUP-III NITRIDE CRYSTAL USING A FLUX GROWTH AND THEN USING THE GROUP-III NITRIDE CRYSTAL AS A SEED FOR AN AMMONOTHERMAL RE-GROWTH," attorneys' docket number 30794.419-WO- Ul (2012-020-2), which application claims the benefit under 35 U.S.C. Section 119(e) of co-pending and commonly-assigned U.S. Provisional Application Serial No. 61/507,170, filed on July 13, 2011, by Siddha Pimputkar and James S. Speck, entitled "USE OF GROUP-III NITRIDE CRYSTALS GROWN USING A FLUX METHOD AS SEEDS FOR AMMONOTHERMAL GROWTH OF A GROUP-III NITRIDE CRYSTAL" attorneys' docket number 30794.419-US-P1 (2012-020-1), and U.S. Provisional Application Serial No. 61/507,187, filed on July 13, 2011, by Siddha Pimputkar and James S. Speck, entitled "METHOD OF GROWING A BULK

GROUP-III NITRIDE CRYSTAL USING A FLUX BASED METHOD THROUGH PREPARING THE FLUX PRIOR TO BRINGING IT IN CONTACT WITH THE GROWING CRYSTAL" attorneys' docket number 30794.421 -US-PI (2012-022); and

P.C.T. International Patent Application Serial No. PCT/US12/xxxxx, filed on July 13, 2012, by Siddha Pimputkar and James S. Speck, entitled "GROWTH OF BULK GROUP-III NITRIDE CRYSTALS AFTER COATING THEM WITH A GROUP-III METAL AND AN ALKALI METAL," attorneys' docket number 30794.420-WO-U1 (2012-021-2), which application claims the benefit of U.S.

Provisional Application Serial No. 61/507,182, filed on July 13, 2011, by Siddha Pimputkar and James S. Speck, entitled "GROWTH OF BULK GROUP-III NITRIDE CRYSTALS AFTER COATING THEM WITH A GROUP-III METAL AND AN ALKALI METAL," attorneys' docket number 30794.420-US-P1 (2012-021-1);

all of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

The invention is related generally to the field of Group-Ill nitride

semiconductors, and more particularly, to a method for improving the transparency and quality of Group-Ill nitride crystals ammonothermally grown in a high purity growth environment.

2. Description of the Related Art.

Ammonothermal growth of Group-Ill nitrides, for example Gallium Nitride

(GaN), involves placing within a reactor vessel Group-Ill containing source material, Group-Ill nitride seed crystals, and a nitrogen-containing fluid or gas, such as ammonia, sealing it and heating it to conditions such that the reactor is at elevated temperatures (for example, between approximately 0°C and 1000°C) and high pressures (for example, between approximately 1 and 30,000 atm). Under these temperatures and pressures, the nitrogen-containing fluid becomes a supercritical fluid and normally exhibits enhanced solubility of Group-Ill nitride material. The solubility of Group-Ill nitride into the nitrogen-containing fluid is dependent on the

temperature, pressure and density of the fluid, among other things.

By creating two different zones within the vessel, it is possible to establish a solubility gradient where in one zone the solubility will be higher than in a second zone. The source material is then preferentially placed in the higher solubility zone and the seed crystals in the lower solubility zone. By establishing fluid motion between these two zones, for example, by making use of natural convection, it is possible to transport Group-Ill nitride material from the higher solubility zone to the lower solubility zone where it then deposits itself onto the seed crystals.

Nonetheless, there remains a need in the art for improvements to the ammonothermal growth of Group-Ill nitrides. The present invention satisfies this need.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method for improving the transparency and quality of Group-Ill nitride crystals ammonothermally grown in a high purity growth environment. The high purity growth environment is achieved using a containment element within a pressure vessel, wherein the containment element is comprised of at least two segments that together form an inner volume that allows for matter to be transferred into and out of the inner volume. Boron is used to improve the transparency of the Group-Ill nitride crystals, while a semipolar growth surface of the seed crystal is used to improve the quality of the Group-Ill nitride crystals.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

FIG. 1 is a schematic of a high-pressure vessel according to an embodiment of the present invention.

FIG. 2 is a flowchart illustrating the method according to an embodiment of the present invention.

FIG. 3 illustrates the basic outline of a capsule-like design of an apparatus which is used for the growth of Group-Ill nitride containing materials or a Group-Ill nitride crystal.

FIG. 4 is a schematic of ammonothermal growth on a Group-Ill nitride seed crystal.

FIG. 5 is a schematic of ammonothermal growth on a Group-Ill nitride seed crystal along a growth surface comprising a (11-22) plane.

FIG. 6 is a schematic of five generations of ammonothermal growth on an initial Group-Ill nitride seed crystal and then subsequent Group-Ill nitride seed crystals cut from the ammonothermally-grown crystal of the prior generation.

FIG. 7 is a schematic illustrating the reduction in dislocation density resulting from five generations of ammonothermal growth on an initial Group-Ill nitride seed crystal and then subsequent Group-Ill nitride seed crystals cut from the

ammonothermally-grown crystal of the prior generation.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

Apparatus Description

FIG. 1 is a schematic of an ammonothermal growth system according to one embodiment of the present invention. The system comprises a high-pressure reactor vessel 10, which may be an autoclave, that includes a lid 12, gasket 14, inlet and outlet port 16, and external heaters/coolers 18a and 18b. A baffle plate 20 divides the interior of the vessel 10 into two zones 22a and 22b, wherein the zones 22a and 22b are separately heated and/or cooled by the external heaters/coolers 18a and 18b, respectively. An upper zone 22a may contain one or more Group-Ill nitride seed crystals 24 and a lower zone 22b may contain one or more Group-Ill containing source materials 26, although these positions may be reversed in other embodiments. The vessel 10 and lid 12, as well as other components, may be made of a Nickel- Chromium (Ni-Cr) alloy. Both the seed crystals 24 and source materials 26 may be contained within baskets or other containment devices, which are typically comprised of a Ni-Cr alloy as well. Finally, the interior of the vessel 10 is filled with a nitrogen- containing solvent 28 to accomplish the ammonothermal growth. Additional elements 30, alloys 32, and/or compounds 34 may also be added to the vessel 10, as described in more detail below. Process Description

FIG. 2 is a flow chart illustrating a method for obtaining or growing a Group- Ill nitride-containing crystal using the apparatus of FIG. 1 according to one embodiment of the present invention.

Block 36 represents placing one or more Group-Ill nitride seed crystals 24, one or more Group-Ill containing source materials 26, and a nitrogen-containing solvent 28 in the vessel 10, wherein the seed crystals 24 are placed in a seed crystals zone (i.e., either 22a or 22b, namely opposite the zone 22b or 22a containing the Group-Ill containing source materials 26), the source materials 26 are placed in a source materials zone (i.e., either 22b or 22a, namely opposite the zone 22a or 22b containing the seed crystals 24). The seed crystals 24 comprise a Group-Ill containing crystal; the source materials 26 comprise a Group-Ill containing compound, a Group- Ill element in its pure elemental form, or a mixture thereof, i.e., a Group-Ill nitride monocrystal, a Group-Ill nitride polycrystal, a Group-Ill nitride powder, Group-Ill nitride granules, or other Group-Ill containing compound; and the solvent 28 comprises supercritical ammonia or one or more of its derivatives, which may be entirely or partially in a supercritical state. An optional mineralizer may be placed in the vessel 10 as well, wherein the mineralizer increases the solubility of the source materials 26 in the solvent 28 as compared to the solvent 28 without the mineralizer.

Block 38 represents growing Group-Ill nitride crystal on one or more surfaces of the seed crystals 24, wherein the environments and/or conditions for growth include forming a temperature gradient between the seed crystals 24 and the source materials 26 that causes a higher solubility of the source materials 26 in the solvent 28 in the source materials zone and a lower solubility, as compared to the higher solubility, of the source materials 26 in the solvent 28 in the seed crystals zone.

Specifically, growing the Group-Ill nitride crystals on one or more surfaces of the seed crystals 24 occurs by changing the source materials zone temperatures and the seed crystals zone temperatures to create a temperature gradient between the source materials zone and the seed crystals zone that produces a higher solubility of the source materials 26 in the solvent 28 in the source materials zone as compared to the seed crystals zone. For example, the source materials zone and seed crystals zone temperatures may range between approximately 0 °C and 1000 °C, and the temperature gradients may range between approximately 0 °C and 1000 °C.

Moreover, the reactor 10 may maintain high pressures (for example, between approximately 1 and 30,000 atm).

Block 40 comprises the resulting product created by the process, namely, a Group-Ill nitride crystal grown by the method described above. A Group-Ill nitride substrate may be created from the Group-Ill nitride crystal, and a device may be created using the Group-Ill nitride substrate.

Containment Element

The present invention includes the addition of a containment element within the reactor vessel 10 to improves the purity of the ammonothermal growth

environment.

As noted above, typical reactor vessels 10 that are used for this process of ammonothermal growth are typically made of Ni-Cr super alloys, for example, Inconel 718, Inconel 625, or Rene 41. These materials are alloys and contain a wide range of elements to improve on the structural and chemical properties.

Under the corrosive environment used in the ammonothermal growth of

Group-Ill nitride materials, it has been observed that some material dissolves from the reactor walls and ultimately incorporates into the growing Group-Ill nitride crystal. This is undesirable as it modifies the growing crystal, for example, by reducing its transparency, reducing its the growth rate, changing the formation rate of point defects (vacancies, vacancy complexes, etc.), modifying the surfaces, etc. It is therefore desirable to create a high purity growth environment for the Group-Ill nitride with the ability to control the chemical composition of elements within the growth environment. A high purity growth environment for the growth of Group-Ill nitride materials is accomplished by the present invention through the use of a capsule-like containment element 42, as shown in FIG. 3. The containment element 42 is an inner containment element placed within the reactor vessel 10. A lid or upper part 44 of the capsule-like containment element 42 may or may not be welded or permanently attached to a base or bottom part 46 of the containment element 42. Moreover, the containment element 42 lid 44 and/or base 46 may or may not be malleable at growth temperatures or other temperatures within the vessel 10.

The containment element 42 may contain all of the elements 22-28 placed in the interior of the vessel 10 in FIG. 1, or it may only contain a subset of those elements, for example, only the seed crystals 24 and the solvent 28 transporting the source materials 26.

In one embodiment, the containment element 42 may form an inner volume comprised of at least two segments, parts or pieces 44, 46 that do not seal completely, allowing some mass transfer in and out of the containment element 42. This configuration significantly simplifies handling and loading the containment element 42, while potentially reducing the cost of a growth run.

For example, one of the two or more segments 44, 46 of the inner containment element 42 that together form the inner volume may comprise a tube, which may be permeable to the materials contained within the pressure vessel 10. In another example, one of the two or more segments 44, 46 of the inner containment element 42 that together form the inner volume may comprise a plate, a plug type device, or a cap. In yet another example, any of the segments 44, 46 may contain threads or other means of bringing the segments 44, 46 into contact with each other without permanently binding or sealing them.

Additionally, a containment element 42 that is permeable to the fluid contained within it allows for gas and fluid flow in and out of the containment element 42 to balance pressure. Preferably, the design minimizes matter transfer between inside and outside of the containment element 42. Furthermore, the containment element 42 may be designed to be more permeable at lower pressure than higher pressure.

Note that, once fully assembled, the capsule-like containment element 42 creates an enclosed space within the vessel 10, wherein the fluid is primarily, if not exclusively, in contact with the materials used to create the capsule-like containment element 42.

Note also that this material can be made of any substance, though it is preferable to be made of an ultra high purity material that is resistant to significant corrosion by the fluid and materials present within the vessel 10. Such materials may include metals, such as, but not limited to, nickel, copper, rhodium, palladium, silver, iridium, platinum, gold, vanadium, niobium, tantalum, and tungsten; oxides; ceramics, such as, but not limited to, boron nitride, alumina, zirconia, yttria stabilized zirconia, yttrium aluminum garnet (YAG), and terbium aluminum garnet (TAG); and glasses.

The ultra high purity materials of the containment element 42 preferably are of a higher purity than the materials comprising the vessel 10. Moreover, the

containment element 42 may be comprised of one or more continuous or non- continuous layers of such ultra high purity materials. For example, the containment element 42 may be comprised one or more layers of the ultra high purity materials and one or more layers of the lower purity materials, wherein an outer most layer in contact with gas and/or fluid is comprised of the ultra high purity materials.

When filling the vessel 10, a nitrogen-containing gas used to grow the crystals ammonothermally may be placed within the vessel 10. During growth, however, a large transfer of gas into or out of the containment element 42 may be undesirable, e.g., when equal pressure is obtained during growth, no transfer of gas into or out of the containment element 42 may be achieved using the present invention, for example.

However, the containment element 42 may also provide enough gas transfer to balance pressure between the outside and inside of the containment element 42. For example, when filling, a large pressure may exist on the outside of the containment element 42, and a smaller pressure may exist on the inside of the containment element 42. On the other hand, during growth, the pressure may be larger on the inside of the containment element 42 as compared to the outside of the containment element 42. When there is a pressure imbalance, a transfer of mass is typically desired. The containment element 42 may allow transfer of mass to reduce or eliminate the pressure imbalance, e.g., during growth and filling. Only the smallest amount of mass transfer may be needed in some examples.

Possible Modifications of the Containment Element

The containment element 42 creates a growth environment within which the levels of impurities are primarily controlled by the materials that are placed within the containment element 42 and the materials from which the containment element 42 is made. In a variation of this invention, the containment element 42 has the chemical ability to partake and/or modify the ongoing chemical reactions.

This may include one or more of the following reactions:

(a) the surface of the containment element 42 provides nucleation sites for compounds to form that may or may not remain on the containment element 42 wall after forming;

(b) the surface of the containment element 42 provides catalytic enhancements for ongoing chemical reactions, such as the decomposition or formation of compounds;

(c) the surface of the containment element 42 is slightly dissolved into the growth environment, thereby providing materials that in turn may:

(1) modify the kinetics of the growth, through, for example, acting as a surfactant on the growing crystal surfaces;

(2) interact with the existing chemical compounds, favoring the formation of certain compounds by modifying the equilibrium

constants for the prevailing chemical reactions; (3) create new alloy s/compounds with the existing alloys/

chemical compounds, thereby forming materials that:

(i) partake in the growth of the Group-Ill nitride

crystal;

(ii) neutralize inactive species, or undesirable

compounds;

(iii) function as getters of undesirable chemical

species, such as oxygen. Advantages and Improvements of the Containment Element

The containment element 42 may enable higher purity and higher transparency Group-Ill nitrides crystals grown at a faster growth rate.

One of the benefits of this design over other existing state of the art equipment includes the relaxation of requiring the capsule to be impermeable to the fluid it contains. This has many benefits, some of them include:

(a) loading and unloading of the vessel 10 is significantly simplified as the containment element 42, and objects contained therein, can be easily removed and replaced;

(b) pressure balancing between the outside and inside of the capsule-like containment element 42 is simplified as it will happen naturally without any external inputs, thereby reducing probability of a failure of the capsule-like containment element 42;

(c) it is now possible to load the environment outside of the containment element 42 with materials that are different from that contained inside the capsule-like containment element 42, which materials can then gradually diffuse into the containment element 42, and which materials can be used to modify the chemistry during growth, and/or control the incorporation level of said materials into the crystal during growth, thereby achieving greater control achieved by modifying the permeability and position of the opening relative to the vessel 10 and materials contained within it;

(d) the containment element 42 allows for easy purging and refilling of the inner volume of the vessel 10 before, during, after growth, allowing one to completely remove the contained fluids, without having to expose the vessel 10 or the

containment element 42 to oxygen containing matter;

(e) the lid or opening 44 of the containment element 42 may be a porous medium that has the ability to selectively remove matter from the stream flowing through it, which includes the removal of contaminants, such as oxygen, carbon, transition metals, etc., thereby effectively acting as a filter.

Additional benefits to this capsule design include the fact that the containment element 42 does not necessarily have to be in contact with the walls of the vessel 10, although it may be desirable to do so.

This can be used advantageously, to generate temperature gradients within the containment element 42 that are different, or similar to those of the vessel 10. The space between the containment element 42 and the walls of the vessel 10 could be packed with a layer of thermally conductive material to selectively enhance heat transfer from the vessel 10 to the containment element 42.

Another benefit to having the containment element 42 floating with respect to the vessel 10 is that solid state diffusion of elements from the vessel 10 through the walls or opening 44 of the containment element 42 is minimized, therefore essentially eliminating any impurity incorporation into the growth environment. This allows for designs of a thin capsule-like containment element 42 reducing the amount of material needed to make up the capsule-like containment element 42.

Use Of Boron-Containing Solvent During Ammonothermal Growth

The present invention includes the addition of Boron or Boron-containing compounds or alloys to the ammonothermal growth environment during the growth of a Group-Ill nitride crystal to improve the transparency of the crystal. As noted above, typical vessels 10 that are used for ammonothermal growth are typically made of Ni-Cr alloys, for example, Inconel 718, Inconel 625, or Rene 41, which may contain a wide range of elements to improve on its structural and chemical properties. However, under the corrosive environment used in the ammonothermal growth of Group-Ill nitride materials, it has been observed that some material dissolves from the walls of the vessel 10 and ultimately incorporates into the growing Group-Ill nitride crystal. This is undesirable as it modifies the growing crystal, for example, by reducing its transparency, reducing its the growth rate, changing the formation rate of point defects (vacancies, vacancy complexes, etc.), modifying the surfaces, etc. It is therefore desirable to create a high purity growth environment for the Group-Ill nitride with the ability to control the chemical composition of elements within the growth environment. Further, it is also desirable to control the incorporation of impurities and formation of optically active defects within the growing crystal.

The present invention describes the use of Boron (B) to decrease the absorption coefficient for one or more wavelengths in ammonothermally-grown Group-Ill nitride crystals. The result is Group-Ill nitride crystals of higher transparency for use as substrates in optoelectronic devices with lower absorption losses.

In one embodiment, this is achieved by adding a finite amount of boron- containing material 30, 32, 34 to the growth environment. The Boron of the boron- containing material 30, 32, 34 may incorporate into the crystal, thereby modifying, among other things, the optical properties of the crystal. However, the Boron of the boron-containing material 30, 32, 34 does not necessarily need to be incorporated into the crystal, but instead may modify the growth environment and/or the surface of the crystal, thereby modifying the growth of the crystal.

The boron-containing material 30, 32, 34 may be added to the growth environment in any form (liquid, solid, gaseous, plasma). Additionally, the boron- containing material 30, 32, 34 may be added in any chemical form, such as an elemental form 30 (pure Boron), and/or in the form of alloys 32 (BGaN, etc.), and/or in the form of compounds 34 (borane (BH3), diborane (B2H6), borazane (BNH6), borazine (B3N3H6), sodium borohydride (NaBH4), etc.). Although the boron- containing materials 30, 32, 34 are represented by ellipses in FIG. 1, the boron- containing material 30, 32, 34 may be added to and may exist in the growth environment in any form (liquid, solid, gaseous, plasma).

Using the boron-containing materials 30, 32, 34, a Group-Ill nitride crystal may be grown that has a better optical transparency as compared to a crystal grown under comparable conditions without the boron-containing materials 30, 32, 34. For example, the crystal may have an absorption coefficient less than 10 cm"1 at wavelengths between 450 nm and 800 nm.

Use Of Semipolar Growth Surfaces On The Seed Crystals

The present invention also includes the use of semipolar planes as growth surfaces of Group-Ill nitride seed crystals, in particular, planes close to the {11-22} plane.

Currently, when growing Group-Ill nitride crystals using the ammonothermal method, the growth along one crystallographic direction may be slower than along another crystallographic direction. For example, the growth rate of GaN along the polar c-direction {0001 } is approximately four to ten times faster than the growth rate along a perpendicular, stable nonpolar direction, such as the m-direction <10-10>. Additionally, the absolute growth rate along the nonpolar direction may be relatively small, on the order of about 10-50 μm/day. In order to fabricate substrates from bulk Group-Ill nitride crystals, it is desirable to obtain the highest possible growth rates, while still maintaining crystal quality.

In addition to the problems with slow growth rates in the nonpolar direction, there is a fundamental issue of the initial seed crystal quality as any defects and non- idealities in the seed crystal can, and most likely will, propagate into the growing crystal, thereby reducing its quality. One of the non-idealities of a seed crystal, particularly those grown using the HVPE (Hydride Vapor Phase Epitaxy) process, is strain, which can, upon regrowth of thick enough layers, lead to cracking within the crystals, which may result in a loss of quality and a possible loss of the entire crystal. Cracks can easily occur in bulk layers grown on tensile strained layers or substrates when a thick enough layer is grown such that the force within the layer is larger than the bonding strength between a set of planes. Typically, the crack will occur between planes that possess the lowest bond density or surface energy. Cracking is typically observed only after growing a certain thickness. Below this critical thickness, the growing layer can elastically

accommodate the strain without relaxing by means of breaking bonds, as the bonds merely stretch. The forces in the layer are large, but not necessarily larger than the bond strength between any set of planes.

In addition to the problems of strain within the seed crystals, a larger than desired density of dislocations within the seed crystal may lead to inferior growth of bulk crystals. The lowest possible density of dislocation is typically desired in the growing crystal. If the seed crystal already possesses a certain density of dislocation, it is desired to reduce this density. Crystals grown using HVPE on sapphire substrates typically have dislocation densities around 1E6 dislocations per cm2 in the c-direction. While still lower than the number of dislocations seen when growing thin layers on sapphire using MOCVD techniques (~ 1E8 dislocations per cm2), it is still higher than desired. Dislocations on the order of 1 dislocation per cm2 or less are desired when growing bulk crystals. A technique needs to be found to produce a low defect density seed crystal, or alternatively, reduce the amount of dislocations in the growth seed crystal by eliminating or redirecting the dislocations.

The present invention uses semipolar planes as growth surfaces of Group-Ill nitride seed crystals, in particular, planes close to the {11-22} plane, as a means of reducing strain within the ammonothermally-grown crystal layer or of reducing dislocation densities by systematic regrowths and reuse of the newly formed material. This newly grown material can then be used as a seed crystal for another growth using any crystal growth technique. Specifically, the present invention allows for low strain, low dislocation, seed crystal generation with subsequent use for the generation of bulk Group-Ill nitride crystals, which can be used to fabricate substrates for use in optoelectronic and electronic devices.

The present invention produces a viable Group-Ill nitride crystal with reduced strain or dislocations by starting the ammonothermal growth on a seed crystal surface oriented in a particular direction. In addition to reducing strain and dislocation density, the present invention allows for rapid seed generation by growing on a non- steady-state surface, leading to potentially significantly higher than steady-state surface growth rates.

The method used to produce an improvement in crystal quality is to grow on a semipolar plane of a Group-Ill nitride seed crystal that is either a (11-22) plane or a (1 l-22)-approaching plane, wherein the (1 l-22)-approaching plane includes all planes whose vector normal to the plane surface form an angle of no more than 15 degrees in any direction with respect to the (11-22) plane normal.

The benefit of growing on a (11-22) plane or a (1 l-22)-approaching plane includes the ability for the thick ammonothermally grown layer to crack while still maintaining crystal quality in the growing layer as described by the FWHM (full width at half maximum) of the Omega rocking curve using XRD (X-ray diffraction).

The resulting grown Group-Ill nitride crystal has a more relaxed crystal lattice or improved crystal quality along at least one direction as compared to the initial seed crystal. Moreover, the resulting grown Group-Ill nitride crystal may have one or more cracks embedded within the crystal.

In addition to healing formed cracks in the growing layer, the (11-22) or (11- 22)-approaching planes have a significantly higher growth rate than steady-state growth planes of the Group-Ill nitride seed crystal, including the c-plane and m-plane. It has been observed that the (11-22) plane can grow up to 2-3 times faster than c- plane. This significant increase in growth rate allows for the rapid growth along the (11-22) direction, leading to significant increases in crystal mass over a shorter time period than when only growing along steady-state growth planes.

This can be used advantageously to further reduce dislocations through a method described in FIG. 4. When growing along the (11-22) plane, it is assumed that the steady-state growth planes will eventually emerge and overtake the growing surface leading to a habit of c-plane and m-plane facets. The starting (11-22) plane 48 is no longer visible as a (11-22) plane, but rather it could have faceted into two c- plane facets 50 and at least two m-plane facets 54, as shown in the schematic of FIG. 4 (note that only the growth of an m-plane facet 54 on one side of the seed crystal is depicted in FIG. 4, and growth also occurs in the opposite direction, but is omitted for clarity's sake).

FIG. 5 illustrates another view looking along the (11-22) plane 56 so that it appears as a line. Multiple growths can now be performed by cutting and reusing crystals appropriately.

In doing so, it is possible to reduce the overall concentration of dislocations with each cut, if done, for example, in the manner shown in FIG. 6. In FIG. 6, the first line 58 represents the first, initial seed crystal. The second generation seed (the line 60) represents the seed crystal that can be cut from the growth on the first generation (initial) seed crystal. Growth performed on the second generation seed can then result in the generation of a third generation seed (the line 62), leading to the fourth generation seed (the line 64), leading to the fifth generation seed (the line 66), and so on.

With each growth the quality of the seed crystal can be improved, and can be correlated to the initial seed in the following schematic drawing. By superimposing the various seed crystals and neglecting the removed material on the right, FIG. 7 portrays the situation with dislocations 68 that propagate primarily along the c- direction and how they are sequentially removed from the seed crystal of newer generations 70, 72, 74, 76, 78. As can be seen with each new generation 70, 72, 74, 76, 78, the heavily dislocated region 68 of the crystal moves upwards towards the +c-plane and eventually, in the current schematic, is eliminated in the fifth generation seed 78 altogether. The fifth generation 78 seed therefore, from this simplified few, should be completely dislocation free and can then used for the production of high quality crystals.

In addition to the reduction of dislocation with each new generation, each generation 70, 72, 74, 76, 78 can also reduce any strain through the formation of cracks or relaxation through other methods.

While in the current depiction of the invention, only one seed was fabricated from the growth, it is possible to generate more than one (11-22) seed crystal for regrowth. In particular, a seed can be generated from the growth on the +c facing side and one seed from the -c facing side. Therefore, this invention can easily be used for multiplying seed crystals and/or crystals for sequential use for substrates.

Nomenclature

The terms "Group-Ill nitride" or "Ill-nitride" or "nitride" as used herein refer to any composition or material related to (B, Al, Ga, In)N semiconductors having the formula BwAlxGayInzN where 0 < w < l, 0 < x < l, 0 < y < l, 0 < z < l, and w + x + y + z = 1. These terms as used herein are intended to be broadly construed to include respective nitrides of the single species, B, Al, Ga, and In, as well as binary, ternary and quaternary compositions of such Group III metal species. Accordingly, these terms include, but are not limited to, the compounds of AIN, GaN, InN, AlGaN, AlInN, InGaN, and AlGalnN. When two or more of the (B, Al, Ga, In)N component species are present, all possible compositions, including

stoichiometric proportions as well as off-stoichiometric proportions (with respect to the relative mole fractions present of each of the (B, Al, Ga, In)N component species that are present in the composition), can be employed within the broad scope of this invention. Further, compositions and materials within the scope of the invention may further include quantities of dopants and/or other impurity materials and/or other inclusional materials.

This invention also covers the selection of particular crystal orientations, directions, terminations and polarities of Group-Ill nitrides. When identifying crystal orientations, directions, terminations and polarities using Miller indices, the use of braces, { }, denotes a set of symmetry-equivalent planes, which are represented by the use of parentheses, ( ). The use of brackets, [ ], denotes a direction, while the use of brackets, < >, denotes a set of symmetry-equivalent directions.

Many Group-Ill nitride devices are grown along a polar orientation, namely a c-plane {0001 } of the crystal, although this results in an undesirable quantum- confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. One approach to decreasing polarization effects in Group- Ill nitride devices is to grow the devices along nonpolar or semipolar orientations of the crystal.

The term "nonpolar" includes the {11-20} planes, known collectively as a- planes, and the {10-10} planes, known collectively as m-planes. Such planes contain equal numbers of Group-Ill and Nitrogen atoms per plane and are charge-neutral. Subsequent nonpolar layers are equivalent to one another, so the bulk crystal will not be polarized along the growth direction.

The term "semipolar" can be used to refer to any plane that cannot be classified as c-plane, a-plane, or m-plane. In crystallographic terms, a semipolar plane would be any plane that has at least two nonzero h, i, or k Miller indices and a nonzero 1 Miller index. Subsequent semipolar layers are equivalent to one another, so the crystal will have reduced polarization along the growth direction.

References

The following references are incorporated by reference herein.

[1] U.S. Patent No. 7625446, issued December 1, 2009, to M.P. D'Evelyn, K.J. Narang, R.A. Giddings, S.A. Tysoe, J.W. Lucek, S.S. Vagarali, R.V. Leonelli Jr, J.R. Dysart, and entitled "High Temperature High Pressure Capsule for Processing Materials in Supercritical Fluids."

[2] U.S. Patent 7125453, issued October 24, 2006, to M.P. D'Evelyn, K.J. Narang, R.A. Giddings, S.A. Tysoe, J.W. Lucek, S.S. Vagarali, R.V. Leonelli Jr, J.R. Dysart, and entitled "High Temperature High Pressure Capsule for Processing Materials in Supercritical Fluids."

Conclusion

This concludes the description of the preferred embodiments of the present invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:
1. An apparatus for growing Group-Ill nitride containing materials, comprising:
a pressure-containing vessel for performing an ammonothermal growth of the Group-Ill nitride containing materials; and
a containment element within the pressure-containing vessel, wherein the containment element is comprised of at least two segments that together form an inner volume that allows for matter to be transferred into and out of the inner volume during the ammonothermal growth of the Group-Ill nitride containing materials in the pressure-containing vessel.
2. The apparatus of claim 1, wherein the containment element is comprised of materials having a higher purity than the vessel, such as metal, metal alloy, or ceramic materials.
3. The apparatus of claim 1, wherein the containment element is comprised of one or more continuous or non-continuous layers of materials.
4. The apparatus of claim 3, wherein the layers are comprised of higher purity materials and lower purity materials.
5. The apparatus of claim 4, wherein at least an outer most one of the layers is comprised of the higher purity materials.
6. The apparatus of claim 1, wherein one of the segments comprises a tube, which is permeable to materials contained within the pressure vessel.
7. The apparatus of claim 6, wherein one of the segments comprises a plate, plug, or cap for the tube.
8. The apparatus of claim 1, wherein the segments are in contact with each other without permanently binding or sealing the segments.
9. A method for growing Group-Ill nitride crystals, comprising:
(a) placing source materials and seed crystals into a vessel;
(b) filling the vessel with a solvent for dissolving the source materials and transporting the dissolved source materials to the seed crystals for growth of the
Group-Ill nitride crystals; and
(c) adding boron-containing materials to the solvent, thereby improving the Group-Ill nitride crystals' transparency.
10. The method of claim 9, wherein the boron-containing materials include one or more of elemental Boron, a Boron alloy, or a Boron compound comprised of borane (BH3), diborane (B2H6), borazane (BNH6), borazine (B3N3H6), or sodium borohydride (NaBH4).
11. The method of claim 9, wherein the Group-Ill nitride crystals have a better optical transparency as compared to Group-Ill nitride crystals grown under comparable conditions without the boron-containing materials.
12. The method of claim 9, wherein the Group-Ill nitride crystals have an absorption coefficient less than 10 cm"1 at wavelengths between approximately 450 nm and 800 nm.
13. A Group-Ill nitride crystal grown by the method of claim 9.
14. A method for growing Group-Ill nitride crystals, comprising:
(a) placing Group-III-containing source materials and Group-Ill nitride seed crystals into a vessel, wherein the seed crystals have one or more or semipolar growth surfaces; and
(b) growing the Group-Ill nitride crystals by filling the vessel with a nitrogen- containing solvent for dissolving the source materials and transporting a fluid comprised of the solvent with the dissolved source materials to the seed crystals for growth of the Group-Ill nitride crystals on the seed crystals.
15. The method of claim 14, wherein the semipolar growth surfaces includes at least one {11-22} surface.
16. The method of claim 14, wherein the semipolar growth surfaces includes at least one surface whose surface normal is within 15 degrees of a nearest surface normal from a {11-22} plane.
17. The method of claim 14, wherein the grown Group-Ill nitride crystals have a relaxed crystal lattice or improved crystal quality along at least one direction as compared to the seed crystal.
18. The method of claim 14, wherein the grown Group-Ill nitride crystal has one or more embedded cracks.
19. A crystal grown by the method of claim 14.
PCT/US2012/046761 2011-07-13 2012-07-13 Method for improving the transparency and quality of group-iii nitride crystals ammonothermally grown in a high purity growth environment WO2013010121A1 (en)

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Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3525594A (en) * 1967-02-14 1970-08-25 Westinghouse Electric Corp Nucleation chamber for crystal growth
US4923561A (en) * 1988-09-23 1990-05-08 American Telephone And Telegraph Company Crystal growth method
US7288152B2 (en) * 2003-08-29 2007-10-30 Matsushita Electric Industrial Co., Ltd. Method of manufacturing GaN crystals and GaN crystal substrate, GaN crystals and GaN crystal substrate obtained by the method, and semiconductor device including the same
US7368015B2 (en) * 2002-03-27 2008-05-06 Momentive Performance Materials Inc. Apparatus for producing single crystal and quasi-single crystal, and associated method
US20090309105A1 (en) * 2008-06-04 2009-12-17 Edward Letts Methods for producing improved crystallinity group III-nitride crystals from initial group III-Nitride seed by ammonothermal Growth
US20100075175A1 (en) * 2008-09-11 2010-03-25 Soraa, Inc. Large-area seed for ammonothermal growth of bulk gallium nitride and method of manufacture
WO2010053960A1 (en) * 2008-11-07 2010-05-14 The Regents Of The University Of California Using boron-containing compounds, gasses and fluids during ammonothermal growth of group-iii nitride crystals

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3525594A (en) * 1967-02-14 1970-08-25 Westinghouse Electric Corp Nucleation chamber for crystal growth
US4923561A (en) * 1988-09-23 1990-05-08 American Telephone And Telegraph Company Crystal growth method
US7368015B2 (en) * 2002-03-27 2008-05-06 Momentive Performance Materials Inc. Apparatus for producing single crystal and quasi-single crystal, and associated method
US7288152B2 (en) * 2003-08-29 2007-10-30 Matsushita Electric Industrial Co., Ltd. Method of manufacturing GaN crystals and GaN crystal substrate, GaN crystals and GaN crystal substrate obtained by the method, and semiconductor device including the same
US20090309105A1 (en) * 2008-06-04 2009-12-17 Edward Letts Methods for producing improved crystallinity group III-nitride crystals from initial group III-Nitride seed by ammonothermal Growth
US20100075175A1 (en) * 2008-09-11 2010-03-25 Soraa, Inc. Large-area seed for ammonothermal growth of bulk gallium nitride and method of manufacture
WO2010053960A1 (en) * 2008-11-07 2010-05-14 The Regents Of The University Of California Using boron-containing compounds, gasses and fluids during ammonothermal growth of group-iii nitride crystals

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