Patent Publication Number: US-2023135911-A1

Title: MOLECULAR BEAM EPITAXY (MBE) REACTORS AND METHODS FOR n+GaN REGROWTH

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority benefit of the filing date of U.S. Provisional Patent Application No. 63/275,580, filed on Nov. 4, 2021, the disclosure of which application is herein incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to molecular beam epitaxy (MBE) reactor structures for the unit process of n+GaN contact regrowth using ammonia as a nitrogen source, and more particularly to structures and methods for enhancing evacuation of ammonia in a GaN regrowth process. 
     BACKGROUND OF THE INVENTION 
     Molecular beam epitaxy (MBE) is a technique for thin-film deposition of single crystals in a high-vacuum system. The present disclosure relates to the design of a highly specialized molecular beam epitaxy (MBE) reactor for the unit process of n+GaN contact regrowth, which is needed for reducing the contact resistance of highly scaled, high-frequency gallium nitride (GaN) high electron mobility transistor (HEMT) transistors. The disclosed processes are based on deposition of highly doped GaN material in etched Ohmic contact regions to reduce the contact resistance of the device. The disclosed processes are much more reliable and affordable for manufacturing compared with current general purpose MBE reactors. 
     Molecular beam epitaxy (MBE) tools have been around for many years, but they are very large and are designed for growing complex epitaxial structures and typically include many types of elemental or gas sources. For a single process, a much simpler tool can be designed. In addition to the size and complexity of common MBE reactors, the use of ammonia (NH 3 ) as a process gas leads to many issues regarding the reliability of the system and the maintenance requirements that make this process expensive to implement for a high-volume production line. The need for lower contact resistance for high-frequency, highly scaled gallium nitride (GaN) high electron mobility transistor (HEMT) devices that contain wide bandgap barrier layers for high charge density requires that these processes be made practical for manufacturing. 
     Most of the production-scale MBE reactors in industry were designed for arsenide-based and phosphide-based material systems and were designed for use with evaporated or sublimated source materials. The unreacted arsenide and phosphide materials are very effectively pumped on the liquid nitrogen-filled panels (cryopanels) in the vacuum system, where they condense as solids and remain so when these panels are warmed to room temperature. These MBE reactors were not designed for handling high gas loads, which are characteristic of the plasma nitrogen source required for nitride-based materials. As a result, most MBE reactors that have been used for GaN growth have been modified with higher throughput vacuum pumping systems, but even with these modifications the pumping speed is limited due to the original reactor design. The use of NH 3  for nitride-based material growth is much less common than the plasma-based processes. When NH 3  is introduced into the MBE reactor, the liquid nitrogen-filled cryopanels pump (condense) unreacted NH 3 , which freezes as NH 3  (ice) on the surface of the panels, as shown in  FIG.  1   . Not only is unreacted NH 3  condensed, but the other unreacted source materials condense and are trapped in this ice. Unfortunately, when the cryopanels are warmed up, this ice sublimes as NH 3  gas and the other trapped constituents are released and fall as particles onto the shutters and into the source effusion cells, contaminating them. The liberation of condensed NH 3  also becomes a problem as the thickness of the accumulated ice increases and is radiatively heated by the high temperature substrate heater and gallium silicon, germanium, and other sources in the reactor during normal operation. 
     Most conventional GaN regrowth reactors and methods use nitrogen-plasma as the source of nitrogen. However, growth of GaN on wafers for use in highly scaled, high-frequency gallium nitride (GaN) high electron mobility transistor (HEMT) transistors require that the GaN growth be selective. Nitrogen-plasma is not as selective as alternative nitrogen sources. The reactors and methods of the present invention use ammonia (NH 3 ) as a nitrogen source because it is useful for selectivity. However, using ammonia as a nitrogen source has its various challenges. When ammonia is introduced into an MBE reactor, unreacted portions of the ammonia gas build up as ammonia ice on the surface of the cryoshroud within the MBE reactor. Although some of the unreacted ammonia gas is pumped out of the reactor using a pumping system, conventional MBE reactors are not equipped to handle the high gas loads characteristic of nitride-based growth. As a result, conventional MBE reactors are unable to efficiently and effectively evacuate ammonia gas from the reactor during GaN regrowth using these pumping systems, and more ammonia ice is accumulated on the cryoshroud to maintain the pressure during growth. 
     In order to remove the ammonia ice accumulated on the cryoshroud, a regeneration process is performed to heat the cryoshroud, sublime the ammonia ice, and remove the sublimed ammonia gas from the reactor. Because a substantial amount of ammonia ice accumulates on the cryoshroud rather than being evacuated from the system during conventional methods, there is a significant amount of downtime required to remove the ammonia ice. As such, removal is performed less frequently and the ammonia ice continues to accumulate on the cryoshroud, leading to significant wear and damage to the reactor over time. Further, more contaminants and particles are trapped in the ammonia ice during GaN regrowth. These contaminants and particles may be released and fall during regrowth, regeneration, or both, risking damage to gas injectors positioned at the bottom of the reactor, or other components of the reactor. 
     Accordingly, there is a need for improved reactors that can enhance evacuation of ammonia gas and avoid accumulation of ice on the cryoshroud during GaN growth. The reactors and methods of the present invention address these and other needs. 
     Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the detailed description included herein in association with the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention include molecular beam epitaxy (MBE) reactors for GaN regrowth using ammonia as a nitrogen source, the reactor comprising: a chamber; a wafer port through which a wafer is introduced into the chamber; one or more pump ports; a cryoshroud positioned within the chamber and configured to enhance evacuation of ammonia; and a plurality of gas injectors configured to introduce reactants into the chamber. 
     In some embodiments, the cryoshroud comprises one or more openings configured to enhance evacuation of ammonia. In some embodiments, the one or more openings comprise a helical geometry. In some embodiments, the cryoshroud comprises a plurality of separate components, and the separate components are configured to enhance evacuation of ammonia. In some embodiments, the plurality of separate components comprise an upper component and a lower component, and the upper and lower components are positioned to form a cylindrical gap between them. In some embodiments, the plurality of separate components are positioned to form one or more vertical gaps between the separate components. In some embodiments, the plurality of separate components are arranged in an interdigitated manner. In some embodiments, the cryoshroud comprises one or more liquid nitrogen-filled cryopanels. 
     In some embodiments, the wafer port, the one or more pump ports, or any combination thereof are centered on one or more openings in the cryoshroud. In some embodiments, the wafer port is positioned above or below the cryoshroud. In some embodiments, the cryoshroud at least partially overlaps the wafer port, the one or more pump ports, or any combination thereof. In some embodiments, the wafer port, the one or more pump ports, or any combination thereof are not centered on one or more openings in the cryoshroud. In some embodiments, the width of one or more openings in the cryoshroud ranges from 2 inches to 8 inches. In some embodiments, a height of the cryoshroud is less than a height of the chamber. In some embodiments, a height of the cryoshroud is equal to a height of the chamber. In some embodiments, at least one of the one or more openings in the cryoshroud is positioned in a central region of the cryoshroud. In some embodiments, at least one of the one or more openings in the cryoshroud is positioned in a peripheral region of the cryoshroud. 
     In some embodiments, the plurality of gas injectors enter through a bottom surface of the chamber. In some embodiments, the gas injectors are angled towards the wafer. In some embodiments, at least one of the plurality of gas injectors comprises a distal end, and the distal end is positioned above a bottom level of the cryoshroud. In some embodiments, at least one of the plurality of gas injectors comprises a hydride source and at least one of the plurality of gas injectors comprises a gallium source. In some embodiments, the hydride source is configured to introduce at least one reactant selected from the group consisting of: NH 3 , SiH 4 , Si 2 H 6 , GeH 4 , and any combination thereof. In some embodiments, the gallium source is configured to introduce at least one reactant selected from the group consisting of: TEGa, TMGa, GaCl 3 , and any combination thereof. In some embodiments, at least one of the plurality of gas injectors comprises a distal end, and the reactor further comprises a mechanical shutter configured to cover the distal end of one or more of the gas injectors. 
     Aspects of the invention include molecular beam epitaxy (MBE) reactors for GaN regrowth using ammonia as a nitrogen source, the reactor comprising: a chamber; a wafer port through which a wafer is introduced into the chamber; one or more pump ports; a cryoshroud positioned within the chamber, the cryoshroud comprising an upper component and a lower component, wherein the lower component is spaced from the upper component by a fixed distance and wherein the spacing of the upper and lower components enhances evacuation of ammonia from the reactor; and a plurality of gas injectors configured to introduce reactants into the chamber. 
     In some embodiments, the cryoshroud comprises one or more liquid nitrogen-filled cryopanels. In some embodiments, the height of the upper component of the cryoshroud is greater than the height of the lower component of the cryoshroud. In some embodiments, the height of the upper component of the cryoshroud is less than the height of the lower component of the cryoshroud. In some embodiments, the height of the upper component of the cryoshroud is the same as the height of the lower component of the cryoshroud. In some embodiments, the wafer port, the one or more pump ports, or any combination thereof are centered between the upper and lower components of the cryoshroud. In some embodiments, the wafer port, the one or more pump ports, or any combination thereof are not centered between the upper and lower components of the cryoshroud. In some embodiments, the wafer port is positioned above or below the cryoshroud. In some embodiments, the upper component, the lower component, or both at least partially overlap the wafer port, the one or more pump ports, or any combination thereof. In some embodiments, the distance between the upper component and the lower component ranges from 2 inches to 8 inches. In some embodiments, the distance from the bottom edge of the lower component to the top edge of the upper component is less than the height of the chamber. In some embodiments, the distance from the bottom edge of the lower component to the top edge of the upper component is equal to the height of the chamber. 
     In some embodiments, the plurality of gas injectors enter through a bottom surface of the chamber. In some embodiments, the plurality of gas injectors are angled towards the wafer. In some embodiments, at least one of the plurality of gas injectors comprises a distal end, and the distal end is positioned above a bottom level of the lower component of the cryoshroud. In some embodiments, at least one of the plurality of gas injectors comprises a hydride source and at least one of the plurality of gas injectors comprises a gallium source. In some embodiments, the hydride source is configured to introduce at least one reactant selected from the group consisting of: NH 3 , SiH 4 , Si 2 H 6 , GeH 4 , and any combination thereof. In some embodiments, the gallium source is configured to introduce at least one reactant selected from the group consisting of: TEGa, TMGa, GaCl 3 , and any combination thereof. In some embodiments, at least one of the plurality of gas injectors comprises a distal end, and the reactor further comprises a mechanical shutter configured to cover the distal end of one or more of the gas injectors. 
     Aspects of the invention include systems for GaN regrowth using ammonia as a nitrogen source, the system comprising: a molecular beam epitaxy (MBE) reactor comprising a chamber, a wafer port through which a wafer is introduced into the chamber, one or more pump ports, a cryoshroud positioned within the chamber and configured to enhance evacuation of ammonia, and a plurality of gas injectors configured to introduce reactants into the chamber; one or more pumps connected to the chamber via the one or more pump ports; and a wafer introducing means configured to introduce the wafer into the chamber through the wafer port. 
     In some embodiments, the one or more pumps comprise a turbomolecular vacuum pump. In some embodiments, the system further comprises a wafer platform coupled to a shaft positioned through a top surface of the chamber, wherein the wafer platform is configured to accept the wafer from the wafer introducing means. In some embodiments, the wafer platform is positioned above an upper edge of the cryoshroud. In some embodiments, the wafer platform is positioned within one or more openings in the cryoshroud. In some embodiments, at least one of the plurality of gas injectors comprises a distal end, and the reactor further comprises a mechanical shutter configured to cover the distal end of one or more of the gas injectors. In some embodiments, the one or more pumps further comprise a low vacuum, high throughput pump. 
     In some embodiments, the cryoshroud comprises one or more openings configured to enhance evacuation of ammonia. In some embodiments, the one or more openings comprises a helical geometry. In some embodiments, the cryoshroud comprises a plurality of separate components, and the separate components are configured to enhance evacuation of ammonia. In some embodiments, the plurality of separate components comprises an upper component and a lower component, and the upper and lower components are positioned to form a cylindrical gap between them. In some embodiments, the plurality of separate components are positioned to form one or more vertical gaps between the separate components. In some embodiments, the plurality of separate components are arranged in an interdigitated manner. In some embodiments, the cryoshroud comprises one or more liquid nitrogen-filled cryopanels. 
     In some embodiments, the wafer port, the one or more pump ports, or any combination thereof are centered on one or more openings in the cryoshroud. In some embodiments, the wafer port is positioned above or below the cryoshroud. In some embodiments, the cryoshroud at least partially overlaps the wafer port, the one or more pump ports, or any combination thereof. In some embodiments, the wafer port, the one or more pump ports, or any combination thereof are not centered on one or more openings in the cryoshroud. In some embodiments, the width of one or more openings in the cryoshroud ranges from 2 inches to 8 inches. In some embodiments, a height of the cryoshroud is less than a height of the chamber. In some embodiments, a height of the cryoshroud is equal to a height of the chamber. In some embodiments, at least one of the one or more openings in the cryoshroud is positioned in a central region of the cryoshroud. In some embodiments, at least one of the one or more openings in the cryoshroud is positioned in a peripheral region of the cryoshroud. 
     In some embodiments, the plurality of gas injectors enter through a bottom surface of the chamber. In some embodiments, the gas injectors are angled towards the wafer. In some embodiments, at least one of the plurality of gas injectors comprises a distal end, and the distal end is positioned above a bottom level of the cryoshroud. In some embodiments, at least one of the plurality of gas injectors comprises a hydride source and at least one of the plurality of gas injectors comprises a gallium source. In some embodiments, the hydride source is configured to introduce at least one reactant selected from the group consisting of: NH 3 , SiH 4 , Si 2 H 6 , GeH 4 , and any combination thereof. In some embodiments, the gallium source is configured to introduce at least one reactant selected from the group consisting of: TEGa, TMGa, GaCl 3 , and any combination thereof. 
     Aspects of the invention include methods for GaN regrowth using ammonia gas as a nitrogen source with reduced formation of ammonia ice, the method comprising: cooling a cryoshroud of a molecular beam epitaxy (MBE) reactor; introducing a wafer into the reactor; introducing ammonia gas into the reactor; introducing one or more additional reactants into the reactor configured to react with the ammonia gas on the wafer; reacting at least a portion of the ammonia gas with the one or more additional reactants to facilitate GaN regrowth on the wafer; accumulating a first portion of the unreacted ammonia gas on the cryoshroud as ammonia ice; and evacuating a second portion of the unreacted ammonia gas through one or more openings in the cryoshroud to reduce the accumulation of ammonia ice on the cryoshroud. 
     In some embodiments, the one or more additional reactants are selected from the group consisting of: SiH 4 , Si 2 H 6 , GeH 4 , TEGa, TMGa, and GaCl 3 . In some embodiments, evacuating the second portion of the unreacted ammonia gas comprises high throughput vacuum pumping. In some embodiments, accumulating the first portion of the unreacted ammonia gas as ammonia ice further comprises accumulating unreacted portions of the one or more additional reactants within the ammonia ice. 
     Aspects of the invention include methods for GaN regrowth using ammonia as a nitrogen source with enhanced evacuation of ammonia, the method comprising: cooling a cryoshroud of a molecular beam epitaxy (MBE) reactor; introducing a wafer into the reactor; introducing ammonia gas into the reactor; introducing one or more additional reactants into the reactor configured to react with the ammonia gas on the wafer; reacting at least a portion of the ammonia gas with the one or more additional reactants to facilitate GaN regrowth on the wafer; accumulating a first portion of the unreacted ammonia gas on the cryoshroud as ammonia ice; and evacuating a second portion of the unreacted ammonia gas through one or more openings in the cryoshroud, wherein the one or more openings enhances the evacuation of ammonia from the reactor. 
     In some embodiments, the one or more additional reactants are selected from the group consisting of: SiH 4 , Si 2 H 6 , GeH 4 , TEGa, TMGa, and GaCl 3 . In some embodiments, evacuating the second portion of the unreacted ammonia gas comprises high throughput vacuum pumping. In some embodiments, accumulating the first portion of the unreacted ammonia gas as ammonia ice further comprises accumulating unreacted portions of the one or more additional reactants within the ammonia ice. 
     Aspects of the invention include methods for improving the turnover time of molecular beam epitaxy (MBE) reactors after GaN regrowth processes using ammonia as a nitrogen source, the method comprising: performing a plurality of GaN regrowth unit operations utilizing methods as described herein and forming a reduced thickness of ammonia ice on the cryoshroud; heating the cryoshroud to sublime the ammonia ice accumulated on the cryoshroud thereby forming ammonia gas; and evacuating the sublimed ammonia gas through one or more openings in the cryoshroud; wherein the time required to heat the cryoshroud, sublime the accumulated ammonia ice, and evacuate the sublimed ammonia gas is reduced due to the reduced thickness of ammonia ice on the cryoshroud. 
     In some embodiments, the improved turnover time for the MBE reactor ranges from 4 to 24 hours. In some embodiments, the improved turnover time for the MBE reactor is less than 4 hours. In some embodiments, the plurality of GaN regrowth unit operations comprises 10 to 20 unit operations. In some embodiments, the plurality of GaN regrowth unit operations comprises 12 unit operations. 
     These and further aspects will be further explained in the rest of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. In addition, the present disclosure may repeat reference numerals, letters, or both in the various embodiments. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
         FIG.  1    is a diagram showing a conventional molecular beam epitaxy (MBE) reactor design. 
         FIG.  2    is a schematic illustration of a MBE regrowth reactor for GaN regrowth using ammonia as a nitrogen source according to embodiments of the present disclosure. 
         FIG.  3    is a cross-sectional view of a MBE regrowth reactor in accordance with one or more embodiments. 
         FIG.  4 A  is a cross-sectional view of a cryoshroud comprising a helical geometry for use in a MBE reactor in accordance with one or more embodiments. 
         FIG.  4 B  is a perspective view of a cryoshroud comprising a helical geometry for use in a MBE reactor in accordance with one or more embodiments. 
         FIG.  5 A  is a cross-sectional view of a cryoshroud comprising vertical gaps or openings for use in an MBE reactor in accordance with one or more embodiments. 
         FIG.  5 B  is a perspective view of a cryoshroud comprising vertical gaps or openings for use in a MBE reactor in accordance with one or more embodiments. 
         FIG.  6 A  is a cross-sectional view of a cryoshroud comprising a plurality of separate components arranged in a horizontal interdigitated manner for use in an MBE reactor in accordance with one or more embodiments. 
         FIG.  6 B  is a perspective view of a cryoshroud comprising a plurality of separate components arranged in a horizontal interdigitated manner for use in an MBE reactor in accordance with one or more embodiments. 
         FIG.  7 A  is a cross-sectional view of a cryoshroud comprising a plurality of separate components arranged in a vertical interdigitated manner for use in an MBE reactor in accordance with one or more embodiments. 
         FIG.  7 B  is a perspective view of a cryoshroud comprising a plurality of separate components arranged in a vertical interdigitated manner for use in an MBE reactor in accordance with one or more embodiments. 
         FIG.  8    is a system for GaN regrowth using ammonia as a nitrogen source in accordance with one or more embodiments. 
         FIG.  9    is a flowchart of a process for GaN regrowth using ammonia gas as a nitrogen source with reduced formation of ammonia ice in accordance with one or more embodiments. 
         FIG.  10    is a flowchart of a process for GaN regrowth using ammonia as a nitrogen source with enhanced evacuation of ammonia in accordance with one or more embodiments. 
         FIG.  11    is a flowchart of a process for improving the turnover time of an MBE reactor after a GaN regrowth process using ammonia as a nitrogen source in accordance with one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention. 
     In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features and procedures well known to those skilled in the art have not been described in order to avoid obscuring the invention. 
     All references cited throughout the disclosure, including patent applications and publications, are incorporated by reference herein in their entirety. 
     I. Definitions 
     By “comprising” it is meant that the recited elements are required in the composition/method/kit, but other elements may be included to form the composition/method/kit etc. within the scope of the claim. 
     By “consisting essentially of”, it is meant a limitation of the scope of composition or method described to the specified materials or steps that do not materially affect the basic and novel characteristic(s) of the subject invention. 
     By “consisting of”, it is meant the exclusion from the composition, method, or kit of any element, step, or ingredient not specified in the claim. 
     By “nitrogen source,” it is meant the reactant or constituent which provides Nitrogen in the reaction for GaN regrowth. In the embodiments described herein, ammonia is used as the nitrogen source. 
     By “hydride source,” it is meant the reactant or constituent that provides a negative hydrogen ion. 
     By “gallium source,” it is meant the reactant or constituent that provides Gallium in the reaction for GaN regrowth. 
     By “cryoshroud,” it is meant a shroud that is cryogenically cooled using, for example, liquid nitrogen. A cryoshroud may be formed from one or more cryopanels. 
     By arranged in an “interdigitated” manner, it is meant that the separate components of the cryoshroud are arranged in an interlocking manner, such that the finger-like projections on one portion of the cryoshroud interlock with the finger-like projections on a second portion of the cyroshroud. 
     By “centered on,” it is meant that the center of one element aligns with the center of another element. 
     By “central region of the cryoshroud,” it is meant plus or minus 20% from the center of the cryoshroud. 
     By “peripheral region of the cryoshroud,” it is meant more than plus or minus 20% from the center of the cryoshroud. The peripheral region is any region of the cryoshroud outside the central region of the cryoshroud. 
     By “mechanical shutter,” it is meant a device or mechanism comprising one or more shutter curtains that are capable of covering a component. 
     By “evacuation,” it is meant removal using, for example, a pumping system. 
     By “sublime,” it is meant to transform a solid directly into a gas or vapor upon heating without going through the liquid phase. 
     By “sublimed ammonia gas,” it is meant the ammonia gas resulting from ammonia ice transforming directly into a gas (i.e., subliming) upon heating of the cryoshroud. 
     By “turnover time,” it is meant the time to regenerate a reactor to remove the ammonia ice formed in the reactor. Specifically, this is the time to heat a cryoshroud in the reactor, sublime the ammonia ice accumulated on the cryoshroud, and evacuate the sublimed ammonia ice. 
     By “unit operation,” it is meant the operation of a single, GaN regrowth process. For example, a unit operation may be performance of method  900  or method  1000  a single time. 
     All scientific and technical terms used herein have meanings commonly used in the art unless otherwise specified. The definitions provided herein are to facilitate understanding of certain terms used frequently herein and are not meant to limit the scope of the patent disclosure. 
     As used here, the singular forms “a,” “an,” and “the” encompass examples having plural referents, unless the content clearly dictates otherwise. 
     As used here, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. The term “and/or” means one or all of the listed elements or a combination of any two or more of the listed elements. 
     As used here, “have,” “having,” “include,” “including,” “comprise,” “comprising,” or the like are used in their open-ended sense, and generally mean “including, but not limited to.” It will be understood that “consisting essentially of,” “consisting of,” and the like are subsumed in “comprising” and the like. As used herein, “consisting essentially of,” as it relates to a composition, product, method, or the like, means that the components of the composition, product, method, or the like are limited to the enumerated components and any other components that do not materially affect the basic and novel characteristic(s) of the composition, product, method, or the like. 
     The words “preferred” and “preferably” refer to examples of the invention that may afford certain benefits, under certain circumstances. However, other examples may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred examples does not imply that other examples are not useful and is not intended to exclude other examples from the scope of the disclosure, including the claims. 
     The recitations of numerical ranges by endpoints include all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.8, 4, 5, etc. or 10 or less includes 10, 9.4, 7.6, 4.3, 2.9, 1.62, 0.3, etc.). Where a range of values is “up to” a particular value, that value is included within the range. 
     Any direction referred to here, such as “top,” “bottom,” “left,” “right,” “upper,” “lower,” and other directions and orientations are described herein for clarity in reference to the figures and are not to be limiting of an actual device or system or use of the device or system. Devices or systems as described herein may be used in a number of directions and orientations. 
     II. DETAILED DESCRIPTION 
     The present disclosure provides structures and methods for enhancing evacuation of ammonia in a GaN regrowth process that uses ammonia gas as a nitrogen source. As compared to conventional MBE reactor structures, systems for GaN regrowth, and processes for GaN regrowth, embodiments of the present disclosure enhance evacuation of ammonia gas during GaN regrowth processes, improve the vacuum pumping efficiency of a MBE reactor, reduce the formation of ammonia ice during GaN regrowth processes, reduce turnover time during regeneration of a MBE reactor, reduce contamination and damage of effusion or injector cells, improve the reliability of MBE-related equipment, reduce the overall footprint of systems using MBE reactors for GaN regrowth, and improve manufacturing efficiency of highly scaled, high-frequency gallium nitride (GaN) high electron mobility transistor (HEMT) devices. Due to the formation of ammonia ice in MBE unit operations that employ ammonia gas as a nitrogen source, enhancing the evacuation of ammonia gas from an MBE reactor provides various benefits as described herein. 
     Examples and embodiments described herein may be used, for example with the methods and devices described in, for example, U.S. Pat. No. 9,865,721 (filed Nov. 17, 2016) to Beam, III et al., entitled “High electron mobility transistor (HEMT) device and method of making the same, which is incorporated by reference herein in its entirety. 
     Molecular Beam Epitaxy (MBE) Regrowth Reactor 
       FIG.  1    shows a conventional MBE reactor  100  with a cryoshroud  104  positioned inside a chamber  102  of the MBE reactor  100 . The cryoshroud  104  in a conventional MBE reactor  100  contains openings for the wafer holder  106  and the various gas injectors  108 . However, the cryoshroud  104  does not include additional gaps beyond what is necessary for exposing various accessories (e.g., gas injectors, effusion cells) to the wafer. The lack of additional gaps in the cryoshroud  104  is to maximize the surface area of the cryoshroud  104  and to facilitate cooling of the cryoshroud  104  and condensation of unreacted source materials on the surface of the cryoshroud  104 , such as ammonia ice as depicted in  FIG.  1   . By condensing unreacted source materials on the cryoshroud  104 , the cryoshroud  104  assists pumping systems in maintaining the pressure of the MBE reactor  100  during a GaN regrowth process. However, conventional MBE reactors, like MBE reactor  100  in  FIG.  1   , were not designed for handling the high gas loads that are characteristic of nitride-based material growth. As a result, most MBE reactors that have been used for GaN growth have been modified with higher throughput vacuum pumping systems, but even with these modifications the pumping speed is limited due to the original reactor design. For example, the cryoshroud in a conventional MBE reactor must bear more of the pumping load considering the inefficiencies of the pumping systems in such conventional MBE reactors. As a result, the cryoshroud  104  of the conventional MBE reactor  100 , lacking any additional gaps, facilitates condensation of a substantial amount of unreacted source material, such as ammonia ice, on the surface of the cryoshroud. 
     To address the issues presented by conventional MBE reactors as discussed herein, a highly specialized single-function reactor is disclosed that is more suitable for long-term manufacturing. In some embodiments, the MBE reactor of the present invention accommodates 6-inch wafers, may be based on an ammonia nitrogen source for growth selectivity, and may be designed to protect any effusion or injector cells and shutters from particle contamination and damage.  FIG.  2    shows a schematic illustration of an MBE regrowth reactor  200  for GaN regrowth using ammonia as a nitrogen source according to embodiments of the present disclosure. The reactor  200  includes a chamber  202 . The chamber volume is minimized both to improve the vacuum pumping efficiency and to reduce the overall footprint of the system. The reactor  200  also includes a wafer port  204  through which a wafer may be introduced, and a pump port  206 . The pump port  206  is configured to connect one or more pumps to the chamber  202 . In some embodiments, high-conductance throughput pumping with turbomolecular vacuum pumps combined with dry roots pumps are used to handle both the gas loads during epitaxial growth and the very high gas loads that occur when the liquid nitrogen (LN 2 ) cryopanels  208  are warmed up. The reactor also includes a cryoshroud  208  that may include one or more cryopanels as depicted in  FIG.  2   . The reactor also includes a plurality of gas injectors  210 . Gas injectors, which are well within the inner circumference of the cryoshroud as depicted in one embodiment in  FIG.  2   , are located such that particles falling from the cryopanels do not fall into them. In the vacuum system, particles fall straight down since the pressure is low enough that gas turbulence is avoided. Gas injectors potentially can be used for all growth constituents including Ga, Si, Ge and NH 3 . Example sources include silane (SiH 4 ) or disilane (Si 2 H 6 ) and germanium hydride (GeH 4 ) diluted in nitrogen or hydrogen for the dopant sources and triethylgallium for the gallium source. Such a total gas source configuration eliminates all high-temperature effusion sources, significantly reducing the heat load within the reactor. As a risk reduction, a single gallium effusion cell may be added with an integrated shutter. This cell can be used if the organometallic gallium source results in too much carbon incorporation in the n+GaN material. Fortunately, gallium evaporates at relatively low temperatures and is one of the easier sources to deal with in an MBE reactor. Finally, a mechanical shutter (not shown) can be designed to move into place above the injector/effusion cell nozzles to further protect the sources during cryoshroud warmups. 
     Referring to  FIG.  3   , depicted is an embodiment of an MBE reactor  300  for GaN regrowth, like the MBE reactor  200  depicted in  FIG.  2   . In one or more embodiments, the MBE reactor  300  is used for the unit process of n+GaN contact regrowth using ammonia as a nitrogen source. The MBE reactor  300  may be implemented using, for example, system  800  described herein with respect to  FIG.  8    or a similar system. The reactor  300  includes a chamber  302 . The reactor also includes a wafer port  304 . In one or more embodiments, the wafer port is used to introduce a wafer  316  into the chamber  302 . The reactor  300  also includes a pump port  306 . The wafer port  304  and the pump port  306  are operably connected to the chamber  302 . The reactor also includes a cryoshroud  308  positioned within the chamber  302 . As depicted in  FIG.  3   , cryoshroud  308  comprises an upper component and a lower component which are positioned apart from one another to form a gap or opening  318  between them. In the depicted embodiment, the wafer port  304  and the pump port  306  are centered on the gap or opening  318  formed in the cryoshroud  308 . The reactor also includes a plurality of gas injectors  310  configured to introduce reactants into the chamber  302  for use in a GaN regrowth process. As depicted in  FIG.  3   , gas injectors  310  enter through a bottom surface of the chamber  302 . In a GaN regrowth process using the reactor  300  according to one or more embodiments, the cryoshroud  308  is cooled using, for example, liquid nitrogen. A wafer  316  is introduced into the chamber  302  of the reactor  300  through the wafer port  304  and further through the gap  318  in the cryoshroud  308 . Ammonia gas and one or more additional reactants are introduced into the chamber  302  via the gas injectors  310 . To maintain the pressure in the chamber  302  during operation, unreacted portions of the ammonia gas are pumped or evacuated out of the chamber  302  through the gap  318  in the cryoshroud  308  and further through the pump port  306 . While some of the unreacted ammonia gas is condensed on the surface of the cryoshroud  308  as ammonia ice, the depicted design of the cryoshroud  308  enhances evacuation of ammonia gas through the gap  318 , thereby reducing the formation of ammonia ice on the cryoshroud  308 . As a result, the pumping efficiency of the reactor  300  is improved due to the gap  318  in the cryoshroud  308 . Methods of using the reactor depicted in  FIG.  3    are described further herein. 
     Wafer port  304  is operably connected to chamber  302  to facilitate introduction of wafer  316  into the reactor  300 , and removal of wafer  316  after completion of a GaN regrowth unit operation. The wafer port  304  may be connected to a device or mechanism configured to introduce wafer  316  into the chamber. For example, in some embodiments the wafer port is connected to a load chamber. Further aspects of MBE regrowth systems including means for wafer introduction are discussed further herein. In some embodiments, the wafer port  304  is positioned on one or more gaps  318  in the cryoshroud  308 . More specifically, in some embodiments, as depicted in the embodiment of  FIG.  3   , wafer port  304  is centered on one or more gaps  318  in the cryoshroud  308 . In other embodiments, the wafer port  304  is positioned above the cryoshroud  308 ; in other embodiments, the wafer port  304  is positioned below the cryoshroud  308 . In one or more embodiments, the cryoshroud  308  may partially overlap the wafer port  304 . For example, as shown in  FIG.  3   , the upper component and the lower component of the cryoshroud  308  both partially overlap the wafer port  304 . In one or more embodiments where the cryoshroud  308  comprises separate components, such as the upper and lower components of cryoshroud  308  depicted in  FIG.  3   , one or more of the separate components may partially overlap the wafer port  304 . As depicted in  FIG.  2   , in one or more embodiments, the cryoshroud  208  does not overlap the wafer port  204 . 
     Pump port  306  is operably connected to chamber  302  to facilitate evacuation of unreacted constituents, such as unreacted ammonia gas, out of the reactor  300  during a GaN regrowth process. In some embodiments, the reactor  300  includes a plurality of pump ports  306 . The pump port  306  may be connected to one or more pumps configured to pump unreacted ammonia gas out of the reactor  300 . Further aspects of MBE regrowth systems including one or more pumps are discussed further herein. In one or more embodiments, the pump port  306  is positioned on one or more gaps  318  in the cryoshroud  308 . In some embodiments, as depicted in the embodiment in  FIG.  3   , the pump port  306  is centered on one or more gaps  318  in the cryoshroud  308 . Preferably, the pump port  306  is centered on the gap  318  in the cryoshroud  308  to maximize the pump throughput during a GaN regrowth process. In one or more embodiments, the cryoshroud  308  may partially overlap the pump port  306 . For example, as shown in  FIG.  3   , the upper component and the lower component of the cryoshroud  308  both partially overlap the pump port  306 . In one or more embodiments where the cryoshroud  308  comprises separate components, such as the upper and lower components of cryoshroud  308  depicted in  FIG.  3   , one or more of the separate components may partially overlap the pump port  306 . As depicted in  FIG.  2   , in one or more embodiments, the cryoshroud  208  does not overlap the pump port  206 . 
     The positions of the wafer port  304  and the pump port  306  on either side of the reactor  300  as depicted in  FIGS.  2  and  3    are not intended to be limiting. One of ordinary skill in the art will readily appreciate that various changes and modifications in the wafer port  304  and the pump port  306  can be made without departing from the spirit or scope of the invention. 
     CRYOSHROUDS. Cryoshrouds in accordance with embodiments of the invention can include one or more cryopanels which function to cool the cryoshroud  308  and provide a surface on which unreacted materials may condense in order to maintain the pressure in the reactor  300 . In some embodiments, the cryoshroud may include liquid-nitrogen filled cryopanels, such that the cryoshroud  308  includes tubes connected to a liquid nitrogen source for cooling the cryoshroud  308 . Cooling of the cryoshroud  308  facilitates condensation of a portion of unreacted ammonia gas on the cryoshroud  308  during a GaN regrowth process using the reactor  300 . The cryoshroud  308  also facilitates evacuation of ammonia during the GaN regrowth process using ammonia as a nitrogen source by allowing unreacted ammonia gas to escape the reactor  300 . This is accomplished by one or more gaps or openings  318  formed in the cryoshroud  308  in accordance with one or more embodiments of the invention. Compared to cryoshroud  104  of conventional MBE reactors  100 , as depicted in  FIG.  1   , which do not include additional gaps or openings beyond what is necessary for exposing various accessories (e.g., gas injectors, effusion cells) to the wafer in an effort to maximize the surface area of the cryoshroud, the cryoshroud  308  of reactor  300  enhances the evacuation of ammonia as a result of the gaps  318  through which unreacted ammonia gas is pumped out of the reactor chamber  302  during operation. As a result, less unreacted ammonia accumulates on the cryoshroud  308  as ammonia ice, which provides additional benefits as described herein.  FIGS.  4 A- 7 B  show alternative designs of the cryoshroud  308  in accordance with one or more embodiments. 
     In one or more embodiments, the cryoshroud  308  may be a single structure with one or more openings  318  to facilitate evacuation of ammonia. In some embodiments, as depicted in  FIGS.  4 A and  4 B , the cryoshroud  400  includes one or more openings  402  with a helical geometry. For example, the cryoshroud  400  may be formed from a helix, coil, or similar structure with a plurality of turns spaced apart by a distance, also referred to as the pitch of the helix. The separation between turns in the cryoshroud  400  having a helical geometry defines the one or more openings  402  and facilitates evacuation of ammonia through the one or more openings  402 . In some embodiments, a larger gap  402  may be formed between turns of the cryoshroud having a helical geometry to facilitate introduction and retrieval of wafer  316  in the chamber  302  via the wafer port  304 . 
     In one or more embodiments, the cryoshroud  308  may be formed from a plurality of separate components. In some embodiments, as depicted in  FIG.  3   , cryoshroud  308  comprises an upper component and a lower component which are positioned to form a gap or opening  318  between them. The gap or opening  318  may be, for example, a cylindrical gap. The lower component may be spaced from the upper component by a fixed distance which ranges from 2 inches to 8 inches in one or more embodiments. For example, in some embodiments, the distance between the upper and lower components may be 2, 3, 4, 5, 6, 7, or 8 inches. Preferably, in some embodiments the distance between the upper and lower components of the cryoshroud  308  is in the range of 3 to 6 inches, such as 4 or 5 inches. The spacing between separate components of the cryoshroud  308  forms one or more gaps or openings  318  in the cryoshroud  308  that enhance evacuation of ammonia from the reactor  300 . In some embodiments the height of the upper component is greater than the height of the lower component; in other embodiments, the height of the upper component is less the height of the lower component; in other embodiments, as depicted in  FIG.  3   , the height of the upper component is equal to the height of the lower component. 
     In one or more embodiments, as depicted in  FIGS.  5 A and  5 B , the cryoshroud  500  includes one or more vertical gaps  502  in the cryoshroud  500 . The cryoshroud may be a single structure with one or more vertical gaps  502  in some embodiments, or in other embodiments the cryoshroud  500  may be formed from a plurality of separate components positioned to form one or more vertical gaps  502  in between the separate components. The cryoshroud  500  depicted in  FIGS.  5 A and  5 B  includes three separate components with three vertical gaps  502  between the separate components. In some embodiments, the cryoshroud  500  includes two separate components with two vertical gaps  502  between the separate components. In other embodiments, the cryoshroud  500  includes four or more separate components positioned to form a plurality of vertical gaps  502  in between the separate components. The vertical gaps  502  in the cryoshroud  500  facilitate evacuation of unreacted ammonia gas through the vertical gaps  502 . In some embodiments, one or more of the vertical gaps  502  is positioned to coincide with the wafer port  304  to facilitate introduction and retrieval of wafer  316  in the chamber  302  via the wafer port  304 . 
     In one or more embodiments, as depicted in  FIGS.  6 A,  6 B,  7 A, and  7 B , the cryoshroud  600 ,  700  may include a plurality of separate components arranged in an interdigitated, or interlocking, manner to form horizontal, vertical, or a combination of horizontal and vertical gaps  602 ,  702  in the cryoshroud  600 ,  700 . In some embodiments, as shown by the cryoshroud  600  in  FIGS.  6 A and  6 B , the separate components are arranged in a horizontal interdigitated manner to form one or more gaps  602  in the cryoshroud. In other embodiments, as shown by the cryoshroud  700  in  FIGS.  7 A and  7 B , the separate components are arranged in a vertical interdigitated manner to form one or more gaps  702  in the cryoshroud. To facilitate introduction and retrieval of wafer  316  in the chamber  302  via wafer port  304 , a larger gap  602 ,  702  may be formed in between one or more of the digits  604 ,  704  in the interdigitated cryoshroud  600 ,  700 . One of ordinary skill in the art will appreciate that various modifications could be made to the measurements of digits  604 ,  704 , cryoshroud  600 ,  700 , or one or more of gaps  602 ,  702  to form a larger gap suitable for introduction of wafer  316 . For example, the height of two of the digits  704  in  FIGS.  7 A and  7 B  are shorter than the heights of the remaining digits  704  to form a larger gap  702  through which wafer  316  may be introduced and retrieved. 
     With continuing reference to  FIG.  3   , the height of the cryoshroud  308  extends from a bottom edge of the cryoshroud  308  to the top edge of the cryoshroud  308 , including any intervening gaps or openings  318 . In some embodiments the height of the cryoshroud  308  is less than the height of the chamber  302 , as depicted in  FIG.  3   . In other embodiments, the height of the cryoshroud  308  is equal to the height of the chamber  302 . 
     In one or more embodiments, the width of the gaps or openings  318  formed in the cryoshroud  308  ranges from 2 inches to 8 inches, such as 2, 3, 4, 5, 6, 7, or 8 inches. Preferably, in some embodiments, the width of the openings  318  ranges from 3 inches to 6 inches, such as 4 or 5 inches. 
     The gaps or openings  318  may be positioned in various regions of the cryoshroud  308  to enhance evacuation of ammonia. In some embodiments, the gaps or openings  318  are positioned in a central region of the cryoshroud  308 , where the central region is defined as plus or minus 20% from the center of the cryoshroud  308 . In other embodiments, the gaps or openings  318  are positioned in a peripheral region of the cryoshroud  308 , where the peripheral region is defined as more than plus or minus 20% from the center of the cryoshroud  308 . Further, in other embodiments of the cryoshroud  308  comprising one or more gaps or openings  318 , some of the gaps or openings  318  may be positioned in the central region of the cryoshroud  308 , and some of the gaps or openings  318  may be positioned in the peripheral region of the cryoshroud  308 . In some embodiments, one or more of the gaps or openings  318  can be positioned such that it occupies both a central region of the cryoshroud and a peripheral region of the cryoshroud (i.e., the opening extends from a central region of the cryoshroud into a peripheral region of the cryoshroud). 
     GAS INJECTORS. Gas injectors in accordance with the embodiments of the invention can include a plurality of gas injectors which are configured to introduce reactants into the chamber  302  to be used for GaN regrowth. In one or more embodiments, at least one of the gas injectors  310  comprises a hydride source and at least one of the gas injectors comprises a gallium source. In some embodiments, the hydride source introduces ammonia (NH 3 ) used for GaN regrowth. In some embodiments, the hydride source introduces one or more additional reactants to be used as a dopant in the regrowth process, as described, for example, in U.S. Pat. No. 9,865,721 (filed Nov. 17, 2016) to Beam, III et al., entitled “High electron mobility transistor (HEMT) device and method of making the same”, the disclosure of which is incorporated by reference herein in its entirety. 
     Examples of additional hydride source reactants include, but are not limited to, NH 3 , SiH 4 , Si 2 H 6 , and GeH 4 . In some embodiments, the gallium source introduces one or more reactants used for GaN regrowth. Examples of gallium source reactants include, but are not limited to, TEGa, TMGa, and GaCl 3 . 
     In one or more embodiments, the gas injectors  310  are angled towards the wafer  316 . In this way, the reactants are introduced into the chamber  302  in a direction towards the wafer  316  to prevent the reactants from interacting with other components in the reactor  300 , such as the cryoshroud  308 , and to prevent the reactants from reacting with each other prior to reaching the surface of the wafer  316 . Once the reactants reach the surface of the wafer  316 , at least a portion of the reactants react to facilitate GaN regrowth. In some embodiments, the gas injectors  310  enter through a bottom surface of the chamber. Further, in some embodiments, as depicted in  FIG.  2   , the gas injectors  210  include a distal end positioned above a bottom level of the cryoshroud  208 , where the distal end is the end through which reactants are released and introduced into the reactor  200 . When the injectors  210  are positioned in this way, they are protected from particles or contaminants falling from the cryoshroud  208 , such as unreacted materials trapped in the ammonia ice that accumulates on the cryoshroud  308  during the regrowth process. In other embodiments, the reactor  300  also includes a mechanical shutter (not shown) that is used to cover the gas injectors  310  from falling particle or contaminants. In use, the mechanical shutter moves above one or more of the gas injectors  310  when they are not operating, such as during a regeneration process as described herein, and shields the injectors  310  from particle contamination or damage. 
     Systems for GaN Regrowth 
     Referring to  FIG.  8   , depicted is a system  800  for GaN regrowth using ammonia as a nitrogen source in accordance with one or more embodiments. System  800  includes an MBE reactor  802 . The MBE reactor  802  may be any MBE reactor according to embodiments of the present invention described herein. For example, MBE reactor  300  as described herein with respect to  FIG.  3    may be implemented in system  800  as reactor  802 . System  800  also includes one or more pumps  804 . With reference to  FIG.  8   , and continuing reference to  FIG.  3   , pumps  804  are connected to reactor  802  via one or more pump ports  306 . System  800  also includes a wafer introducing means  806 , such as a wafer load chamber  806  as depicted in  FIG.  8   , which is connected to reactor  802  via wafer port  304 . In some embodiments, system  800  also includes a wafer platform  214 ,  314  as shown in  FIGS.  2  and  3   , coupled to a shaft  212 ,  312 , which is configured to accept the wafer from the wafer introducing means  806 . In some embodiments, wafer platform  214 ,  314  includes a heater (not shown) to heat the wafer  316  in preparation for GaN regrowth on the surface of the wafer  316 . 
     PUMPS. Pumps  804  in accordance with embodiments of the invention function to pump unreacted ammonia gas, and in some embodiments, one or more other unreacted materials out of the reactor  802  during performance of the methods as described herein. High gas loads are characteristic of nitride-based material growth processes such as the regrowth of GaN using ammonia according to the embodiments of the present invention. By pumping ammonia gas out of the reactor  802 , pumps  804  help maintain the pressure in the reactor  802  during the regrowth process. Similarly, pumps  804  pump unreacted ammonia gas that sublimes as a result of heating the cryoshroud during a regeneration process to melt the ammonia ice as described further herein. In some embodiments, a single pump  804  is used; in other embodiments, more than one pump  804  is used. One of ordinary skill will readily appreciate that any of a variety of suitable pumps can be used in connection with the systems and methods described herein. In some embodiments, pump  804  is a turbomolecular vacuum pump. In some embodiments, pumps  804  may include an additional pump, such as a low vacuum, high throughput pump. In use, these MBE reactor systems typically involve two-stage pumping systems in which a primary pump, e.g., a turbomolecular vacuum pump, is backed by a low vacuum, high throughput backing pump. One non-limiting example of a backing pump is a dry roots pump. 
     WAFER INTRODUCTION. Wafer introducing means  806  in accordance with embodiments of the invention include a device or mechanism configured to introduce a wafer ready for GaN regrowth into the reactor  802 . In some embodiments, the wafer introducing means  806  is capable of heating the wafer  316  to prepare it for GaN regrowth on the surface of the wafer  316 . Once the wafer  316  is ready for GaN regrowth, the wafer introducing means  806  introduces the wafer  316  through the wafer port  304  and further through one or more openings  318  in the cryoshroud  308  in some embodiments. In other embodiments, the wafer introducing means  806  introduces the wafer  316  above the cryoshroud  308 ; in other embodiments, the wafer introducing means  806  introduces the wafer  316  below the cryoshroud  308 . In some embodiments where system  800  includes wafer platform  214 ,  314  coupled to shaft  212 ,  312 , the wafer introducing means  806  introduces the wafer  316  to wafer platform  214 ,  314  where the wafer  316  is positioned for GaN regrowth. One of ordinary skill will readily appreciate that any of a variety of suitable wafer introducing means can be used in connection with the systems and methods described herein. In some embodiments, the wafer introducing means may be a wafer load chamber  806 , as depicted in  FIG.  8   . 
     It will be appreciated that MBE reactors as described herein may be much smaller and simpler than conventional MBE reactors used for growing complex epitaxial structures. Rather, MBE reactor  802  (and similarly reactors  200  and  300  depicted in  FIGS.  2  and  3   ), are designed for a single, unit process of n+GaN contact regrowth. Thus, in contrast to conventional MBE reactors, reactor  802  may be designed with a smaller chamber volume and less reactant sources (i.e., less gas injectors). Minimizing the volume of the chamber, such as those depicted in  FIGS.  2  and  3    with reference numerals  202  and  302 , also improves the pumping efficiency of the system  800 . In some embodiments, the footprint of the system  800  ranges from about 28 to about 40 square feet, such as about 30, 32, 34, 36, or 38 square feet. For example, in some embodiments, the footprint of system  800  is estimated at about 32 square feet. Table 1 compares the current state-of-the-art MBE reactor system with the expected performance of this custom single-function system  800  according to the present disclosure. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 N+ Regrowth Reactor Systems 
               
            
           
           
               
               
               
            
               
                   
                   
                 Disclosed 
               
               
                   
                   
                 single-function 
               
               
                 Parameter 
                 Riber 49* 
                 reactor 
               
               
                   
               
               
                 Footprint 
                 140 sq. ft. 
                 32 sq. ft. 
               
               
                 Reactor cost 
                 &gt;$2M 
                 ~$500K 
               
               
                 Capacity 
                 16-inch wafer/run 
                 16-inch wafer/run 
               
               
                 Wafer handling 
                 Semi-automatic or 
                 Semi-automatic or 
               
               
                   
                 fully automatic 
                 fully automatic 
               
               
                   
                 wafer transfer 
                 wafer transfer 
               
               
                 Sources 
                 10 source ports 
                 1 to 3 source ports 
               
               
                 Maintenance time (a.u.) 
                 10 
                 2 
               
               
                 Maintenance cost (a.u.) 
                 10 
                 1 
               
               
                 Throughput (6-in. 
                 ~24 
                 ~24 
               
               
                 wafers/day) 
               
               
                   
               
               
                 *Representative of the current state-of-the-art tools in use for GaN epitaxy. 
               
            
           
         
       
     
     Methods 
     Various method and systems embodiments described herein enable enhanced evacuation of ammonia gas during GaN regrowth processes. Due to the formation of ammonia ice in MBE unit operations that employ ammonia gas as a nitrogen source, enhancing the evacuation of ammonia gas from an MBE reactor provides various benefits, including, reduced formation of ammonia ice during GaN regrowth processes, reduced turnover time during regeneration of MBE reactors, among others as described herein. 
     GaN Regrowth Using Ammonia Gas. 
     In use, a method  900  for GaN regrowth using ammonia gas as a nitrogen source with reduced formation of ammonia ice is illustrated in  FIG.  9   . In one or more embodiments, method  900  is used for the unit process of n+GaN contact regrowth using ammonia as a nitrogen source. Method  900  is illustrated as a set of operations or blocks  902  through  914  and is described with continuing reference to  FIGS.  3  and  8   . One or more blocks that are not expressly illustrated in  FIG.  9    may be included before, after, in between, or as part of the blocks  902  through  914 . In one or more embodiments, the blocks  902  through  914  are performed by an MBE reactor system, such as system  800  in  FIG.  8   , using a reactor in accordance with the embodiments described herein with respect to  FIGS.  2 - 7 B . In some embodiments, the method  900  may take between about 30 to 40 minutes to complete. For instance, in some embodiments, the method  900  may take 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, or 40 minutes to complete. 
     At step  902 , a cryoshroud of an MBE reactor is cooled. In one or more embodiments, the cryoshroud may be cooled using liquid nitrogen. In some embodiments, the cryoshroud may include one or more cryopanels having tubes connected to a cooling source, such as a liquid nitrogen source, and the cryoshroud may be cooled by pumping liquid nitrogen through tubes in the cryoshroud. 
     At step  904 , a wafer is introduced into the reactor. In one or more embodiments, the wafer may be introduced into the reactor by a wafer introducing means, such as wafer introducing means  806  in  FIG.  8   , through the wafer port. In some embodiments, the wafer is further introduced through one or more openings in the cryoshroud, such as gap  318  in  FIG.  3   ; in other embodiments, the wafer is introduced above the cryoshroud; in other embodiments, the wafer is introduced below the cryoshroud. In some embodiments, the wafer is accepted by a wafer platform, such as wafer platform  314  coupled to shaft  312  in  FIG.  3   . In some embodiments, the wafer is positioned on the wafer platform for GaN regrowth. 
     At step  906 , ammonia gas is introduced into the reactor. The ammonia gas is used as a nitrogen source for regrowth of GaN in accordance with the embodiments of the invention. In contrast, most conventional nitride-based material growth processes use plasma as a nitrogen source. Gas injectors, such as injectors  310  in  FIG.  3   , may be used to introduce ammonia gas in step  906 . Specifically, a hydride source gas injector may be used to introduce the ammonia gas. The introduced ammonia gas flows from the gas injectors towards the surface of the wafer as a result of the gas injectors being angled towards the wafer in some embodiments. 
     At step  908 , one or more additional reactants are introduced into the reactor. These reactants are introduced to react with the ammonia gas on the surface of the wafer. Gas injectors, such as injectors  310  in  FIG.  3   , may be used to introduce the reactants in step  908 . In one or more embodiments, at least one of the gas injectors comprises a hydride source and at least one of the gas injectors comprises a gallium source. In some embodiments, one or more additional reactants introduced into the reactor is a hydride introduced using a hydride source gas injector. A hydride may be used as a dopant for the GaN regrowth. Non-limiting examples of reactants that may be introduced into the reactor as hydrides include NH 3 , SiH 4 , Si 2 H 6 , GeH 4 , and any combination thereof. In some embodiments, one or more additional reactants introduced into the reactor is a source of gallium in the GaN regrowth introduced using a gallium source injector. Non-limiting examples of gallium sources include TEGa, TMGa, GaCl 3 , and any combination thereof. In some embodiments, the introduced reactants flow from the gas injectors towards the surface of the wafer as a result of the gas injectors being angled towards the wafer. 
     At step  910 , at least a portion of the ammonia gas reacts with the one or more additional reactants to facilitate GaN regrowth on the wafer. The ammonia gas and the one or more additional reactants introduced into the reactor at steps  904  and  906  flow directly to the surface of the wafer and react to facilitate GaN regrowth. The use of ammonia gas as the nitrogen source in GaN regrowth allows for more selective growth on the wafer. That is, ammonia gas as a nitrogen source allows for more control over where growth occurs on the wafer. At least a portion of the ammonia gas and at least a portion of the additional reactants do not react, making up excess material that must be pumped from the reactor in order to maintain the pressure. Continuous injection of ammonia gas, one or more additional reactants, or both will cause pressure to build up in the reactor. To prevent pressure build-up (i.e., to maintain an appropriate pressure) the excess unreacted material is pumped out of the reactor. 
     At step  912 , a first portion of the unreacted ammonia gas accumulates on the cryoshroud as ammonia ice. After step  910 , some of the unreacted ammonia gas accumulates on the cryoshroud as ammonia ice as a result of the cryoshroud being cooled in step  902 . In some embodiments, at least a portion of the unreacted one or more additional reactants may accumulate within the ammonia ice on the cryoshroud. 
     At step  914 , a second portion of the unreacted ammonia gas is evacuated through one or more openings in the cryoshroud. The evacuation of unreacted ammonia gas through the openings in the cryoshroud facilitates reduced accumulation of ammonia ice on the cryoshroud. Whereas a traditional MBE reactor does not include additional gaps or openings for enhanced evacuation of ammonia, MBE reactors and systems in accordance with embodiments of the present invention include one or more openings through which a portion of the ammonia gas escapes the reactor. As a result, a reduced amount of ammonia ice is formed on the cryoshroud because the unreacted portions of ammonia that would otherwise condense on the cryoshroud in conventional MBE reactors may evacuate the reactor through the gaps or openings. Further, a reduced amount of unreacted additional reactants (i.e., contaminants) are trapped within the ammonia ice on the cryoshroud, which provides additional benefits as described herein. 
     One or more pumps as described herein are used to pump unreacted portions of ammonia gas out of the reactor through the gaps or openings in the cryoshroud. In one or more embodiments, the evacuation in step  914  includes high throughput vacuum pumping. The pumping speed and efficiency in a conventional MBE reactor remains limited due to its design. In contrast, MBE reactors in accordance with embodiments of the present invention improve the pumping efficiency of an MBE reactor. This is accomplished, in part, by the gaps or openings formed in the cryoshroud through which unreacted ammonia gas may escape during GaN regrowth. 
     Another non-limiting example of a method for GaN regrowth using ammonia gas as a nitrogen source with enhanced evacuation of ammonia is illustrated in  FIG.  10   . As shown in  FIG.  10   , method  1000  is similar to the method  900  in  FIG.  9   , but step  1014  in method  1000  provides an improvement of enhanced evacuation of ammonia from the reactor as described herein. 
     Regeneration of MBE Reactor. 
     One non-limiting example of a method for improving the turnover time of an MBE reactor after a GaN regrowth process using ammonia as a nitrogen source is illustrated in  FIG.  11   . In use, method  1100  involves heating a cryoshroud in a reactor after one or more GaN regrowth unit operations to facilitate melting and removal of condensed ammonia ice from the cryoshroud. This process may be referred to as regeneration. Method  1100  is illustrated as a set of operations or blocks  1102  through  1106 , and is described with continuing reference to  FIGS.  3 ,  8 , and  9   . One or more blocks that are not expressly illustrated in  FIG.  11    may be included before, after, in between, or as part of the blocks  1102  through  1106 . In one or more embodiments, the blocks  1102  through  1106  are performed by an MBE reactor system, such as system  800  in  FIG.  8   , using a reactor as described by the embodiments herein with respect to  FIGS.  2 - 7 B . 
     At step  1102 , a plurality of GaN regrowth unit operations are performed according to the method  900  as described herein with respect to  FIG.  9   . Compared to the ammonia ice accumulated during conventional methods using conventional MBE reactors, method  900  forms a reduced thickness of ammonia ice on the cryoshroud. In some embodiments, the number of unit operations performed before proceeding to step  1104  to begin regeneration of the cryoshroud ranges from 10 to 20 unit operations. For example, in some embodiments, the number of unit operations performed may be 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 unit operations. Typically, in use according to the embodiments described herein, 12 unit operations are performed prior to regeneration of the reactor. 
     At step  1104 , the cryoshroud is heated to sublime the ammonia ice accumulated on the cryoshroud thereby forming ammonia gas. Step  1104  is the first step in the regeneration process to melt and remove the ammonia ice and prepare the reactor for maintenance or further GaN regrowth unit operations. In one or more embodiments, prior to step  1104 , the wafer with GaN regrowth may be removed from the reactor using wafer introducing means. After the wafer is removed from the reactor, one or more gases may be injected into the cryoshroud to warm the cryoshroud. During step  1104 , the ammonia ice accumulated on the cryoshroud sublimes, transforming directly into ammonia gas. In some embodiments, the one or more additional reactants (i.e., contaminants) trapped within the ammonia ice are released from the ammonia ice. Some of these reactants may be evacuated along with the sublimed ammonia gas in step  1106 . Some of these reactants may fall towards the bottom of the reactor. As discussed herein, the reduced formation of ammonia ice on the cryoshroud also reduces the amount of unreacted additional reactants trapped within the ammonia ice. As a result, fewer reactants may fall towards the bottom of the reactor as compared to conventional MBE reactors and methods, thereby reducing the risk of contamination or damage to the gas injectors or other components of the MBE reactor during regeneration. 
     At step  1106 , the sublimed ammonia gas is evacuated through one or more openings in the cryoshroud. One or more pumps as described herein are used to pump sublimed ammonia gas out of the reactor through the gaps or openings in the cryoshroud. In one or more embodiments, the evacuation in step  1106  includes high throughput vacuum pumping. The pumping speed and efficiency in a conventional MBE reactor remains limited due to its design. In contrast, MBE reactors in accordance with embodiments of the present invention improve the pumping efficiency of an MBE reactor. This is accomplished, in part, by the gaps or openings formed in the cryoshroud through which sublimed ammonia gas may escape during a regeneration process. Increased pumping efficiency of the MBE reactor during regeneration improves the time required to evacuate the sublimed ammonia gas, thereby improving turnover time of the MBE reactor. 
     The time required to heat the cryoshroud, sublime the accumulated ammonia ice, and evacuate the sublimed ammonia gas may be referred to as the turnover time, or the time for regeneration of the reactor. The turnover time of method  1100  is reduced, as compared to that of a conventional MBE reactor, as a result of the reduced thickness of ammonia ice formed on the cryoshroud at step  1102 . The turnover time of method  1100  is an improvement over conventional methods because less ammonia ice is condensed on the cryoshroud as a result of embodiments of the improved reactor design described herein. Specifically, less ice is accumulated on the cryoshroud because gaps or openings formed in the cryoshroud in accordance with the embodiments described herein provide enhanced evacuation of unreacted ammonia gas that would otherwise condense on the cryoshroud. In some embodiments, the improved turnover time for the MBE reactor ranges from 4 to 24 hours. For example, the turnover time may be 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 hours. In other embodiments, the improved turnover time for the MBE reactor is less than 4 hours. For example, the turnover time may be 1, 2, or 3 hours. In use, the typical turnover time for the structures and systems described herein is about 4 hours. In contrast, the turnover time for a conventional MBE reactor is greater than 24 hours. 
     Whereas a traditional MBE reactor is only taken offline for maintenance infrequently due to the long downtime to fully melt the condensed ammonia ice, MBE reactors in accordance with embodiments of the present invention may be taken offline more frequently due to the reduced thickness of the ammonia ice condensed on the cryoshroud. In some embodiments, the number of unit operations performed before performing steps  1104  and  1106  ranges from 10 to 20 unit operations. For example, in some embodiments, the number of unit operations performed may be 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 unit operations. Typically, in use according to the embodiments described herein, 12 unit operations are performed prior to regeneration of the reactor. 
     A further benefit of the improvements in the turnover time of an MBE reactor after a GaN regrowth process according to embodiments of the invention is an improvement in the manufacturing efficiency of highly scaled, high-frequency gallium nitride (GaN) high electron mobility transistor (HEMT) devices. In some embodiments, the methods described herein are used in the manufacturing of HEMT devices. As such, improvements in the efficiencies of the methods described herein provide further improvement in the efficiencies of the manufacturing processes utilizing these methods. 
     While preferred embodiments of the present invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.