Patent Publication Number: US-9416464-B1

Title: Apparatus and methods for controlling gas flows in a HVPE reactor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119 to U.S. Provisional Application Ser. No. 60/829,093, filed on Oct. 11, 2006, the contents of which are incorporated herein by reference as though set forth in full. 
    
    
     FIELD OF THE INVENTION 
     The present inventions relate to hydride vapor phase epitaxy (HVPE) reactors and, more particular, to apparatus and methods for controlling gas flows within a HVPE reactor. 
     BACKGROUND 
     Group III nitride compounds and their alloys, including GaN, AlN, InN, AlGaN and InGaN, have been developed for various optoelectronic and electronic applications. Gallium nitride (GaN) has received increased attention for use in optoelectronic and electronic semiconductor devices. GaN materials including AlGaN, InGaN and InAlGaN may have desired material characteristics and open the door to new devices and applications including solar cells, high-speed electronics, and short-wavelength and long-wavelength light emitters such as ultraviolet (UV), violet, blue, and green, yellow, red and infrared light emitting devices. While GaN materials and related devices may hold promise for new devices and applications, growth of bulk GaN crystals for use as substrates in such devices and applications has presented a number of challenges. One major limitation is the inability to fabricate single crystal bulk GaN, AlN, and InN materials at high growth rates with desired material qualities. 
     For example, it is known to grow low defect single crystal GaN materials using Metal Organic Chemical Vapor Deposition (MOCVD). With MOCVD, group III nitride compounds are grown from the vapor phase using metal organic compounds as sources of the Group III metals. Trimethylindium (TMI) is typically used as an indium source material, trimethylaluminum (TMA) is used as an aluminum source material and trimethylgallium (TMG) is used as a gallium source material. Ammonia gas is typically used as a nitrogen source. 
     These materials are supplied to a MOCVD reactor from source tanks that are located outside of the reactor. Within the MOCVD reactor, a metal organic material source reacts with ammonia resulting in the deposition of an epitaxial layer of a group III nitride material on a substrate. Electrically active impurities are introduced into the MOCVD reactor during material growth to control the electrical conductivity of the grown materials. More specifically, undoped group III nitride compounds normally exhibit n-type conductivity. Donor impurities, such as silicon or germanium, can be introduced into the grown material to control n-type conductivity and form materials with low electrical resistivity. Magnesium impurities in the form of metal organic compounds are introduced into the MOCVD reactor to form nitride materials having p-type conductivity. For example, Publication No. WO 00/68470 to Solomon et al. describes a MOCVD epitaxial growth system that includes a bubbler containing a magnesium-containing compound cyclopentadienylmagnesium (Cp2Mg). Hydrogen gas is used to carry the magnesium containing compound from the Cp2Mg source into the reaction zone to form materials having p-type conductivity. 
     While MOCVD has been used with some effectiveness in the past for growth of certain materials, MOCVD has limited applicability for growth of bulk group III nitride materials such as GaN, including p-type GaN. A very low growth rate is a major limitation of MOCVD. For example, with MOCVD, GaN can be grown at a rate that does not exceed several microns per hour. Consequently, deposition of GaN crystals having thicknesses on the order of millimeters is not feasible using MOCVD. 
     In addition to growth rate limitations, MOCVD systems have other shortcomings. For example, the configuration described by Solomon et al. is not suitable for HCl etching of magnesium delivery tubes in order to remove any magnesium-containing deposits from the tube walls. Magnesium deposits may result in inconsistent doping, which is not desirable, particularly for use in applications that require consistent doping characteristics including various high speed communication electronics, light emitting diode and laser diode devices. 
     Hydride vapor phase epitaxy (HVPE) has also been investigated as an alternative to MOCVD for fabricating group III nitride materials including p-type GaN and other materials. HVPE offers a number of advantages over MOCVD and other fabrication techniques including materials having low defect densities, improved growth rates, controllable doping, less complicated equipment and reduced fabrication costs. Further, HVPE growth can be performed at atmospheric pressure, thereby eliminating the need for vacuum equipment. HVPE is also suitable for mass production of semiconductor materials, structures and devices due to its low cost, excellent material characteristics, flexibility of growth conditions, and reproducibility. Examples of known HVPE reactors and methods for epitaxial and bulk growth of group III nitride materials, including p-type group III materials, are described in U.S. Pat. No. 6,447,604 to Flynn et al.; U.S. Pat. No. 6,596,079 to Vaudo et al., U.S. Pat. No. 6,613,143 to Melnik et al., U.S. Pat. No. 6,616,757 to Melnik et al., U.S. Pat. No. 6,656,285 to Melnik et al. and U.S. Pat. No. 6,936,357 to Melnik et al., “Vertical-HVPE as a Production Method for Free-Standing GaN-Substrates” by B. Schineller et al. (2007), and “Growth of AlN films and their characterization” by R. Jain et al., the contents of all of which are incorporated herein by reference. 
     It is known that defect densities of group III nitride layers grown on foreign substrates such as sapphire rapidly decrease as the thickness of the grown layer increases. For this purpose, HVPE can be used to grow thick layers or boules of such materials, e.g., having thicknesses of about 10 to 100 microns and thicker, to provide higher quality materials and devices with reduced defect densities. This may be accomplished by growth of materials at high growth rates and/or with long growth cycles. Long growth cycles may be an option with the consequence that more time is required. Further, challenges exist in efficiently fabricating thicker material or boules at fast growth rates (e.g., greater than 1 mm per hour) with desired material quality and low defect densities. 
     For example, Vaudo et al. describe growth of group III-V nitride boules at growth rates in excess of 50 microns (0.05 mm) per hour, with growth rates of 200 microns (0.2 mm) per hour being preferred, and growth rates in excess of 500 microns (0.5 mm) per hour being most preferred. To the inventor&#39;s knowledge, while Vaudo et al. may generally state that is preferred to have growth rates in excess of 500 microns per hour, such capabilities have not been implemented or are difficult to implement while achieving desired material qualities. For example, as noted by Schineller et al., as GaN layers grown by HVPE become thicker, a transition in the surface morphology is observed and presents challenges in growing GaN material with desired thicknesses and material qualities. For example, Schineller et al. explain that growth of GaN in a vertical HVPE reactor at rates only as high as 400 microns (0.4 mm) per hour were achieved. Further, as noted by Jain et al., HVPE as been successfully utilized to grow free-standing GaN substrates with growth rates only as high as 200 microns (0.2 mm) per hour, citing a Journal of Crystal Growth article by Vaudo et al. (2002). 
     More particularly, single crystal GaN material degradation at high deposition rates is due, in part, to resulting parasitic deposition of GaN on internal components of a HVPE reactor during growth of single crystal GaN on a seed upon which the single crystal material is grown. Parasitic deposition degrades reactor components, shifts growth parameters and may even lead to a crash of the HVPE reactor. More particularly, parasitic deposition on reactor components may stress the reactor components leading to cracking and damage of the components. Additionally, parasitic deposition may result in growth of inclusions in the GaN crystal itself. Thus, although a HVPE reactor may be configured for single crystal growth, the resulting material may have inclusions. Material degradation may also result from non-stable growth rates that occur during extended periods of growth and non-uniform supplies of source materials over growing surfaces. 
     Thus, known HVPE reactors and methods have been utilized to achieve GaN growth at rates up to 0.2 mm to 0.4 mm per hour. However, to the inventors&#39; knowledge, known HVPE reactors and growth methods have not been successfully employed to grow high quality, uniform and low defect density GaN bulk single crystals at higher growth rates, e.g., greater than 1 millimeter per hour, due in part to parasitic deposition at these high growth rates. To increase the growth rate, it is necessary to increase amount of source materials in the growth zone which, in turn, increases the amount of source material in the growth apparatus, which leads to an increase in parasitic reactions and depositions. 
     A further limitation of known HVPE reactors and methods is growth of sufficiently thick p-type group III nitride materials having desired material qualities at high growth rates. Examples of known HVPE reactors and methods for fabricating p-type group III nitride materials including p-type GaN are described in U.S. Pat. No. 6,472,300 to Nikolaev et al., the contents of which are incorporated herein by reference, and WO 00/68470 to Solomon et al. While such system have been used with some effectiveness in the past, they may not provide stable p-type doping during long growth processes due to surface degradation and the changes in the size of the metallic acceptor sources (e.g., magnesium and/or zinc). 
     For example, a HVPE system described by Solomon et al. utilizes metallic magnesium a source of magnesium dopant. The metallic magnesium is housed within a dopant chamber that is positioned outside of a reactor or growth tube. The metallic magnesium is heated, and hydrogen carrier gas is used to deliver magnesium to the reactor and form p-type GaN. Solomon et al. also describe another HVPE system that involves passing HCl gas over a group III metal/Mg mixture to form a first reagent gas, which reacts with ammonia to form p-type GaN. 
     HVPE systems that utilize a metallic magnesium sources, however, are not suitable for bulk growth or long-duration growth cycles due to degradation of the metallic magnesium source which, in turn, results in inconsistent doping and doping characteristics that are not readily reproducible. Further, Solomon et al. describe using hydrogen as a carrier gas for the metallic magnesium. Hydrogen gas, however, is not desirable for this purpose due to the resulting high electrical resistivity of the grown gallium nitride material. Moreover, such systems are associated with accumulation of magnesium-containing compounds on the inner surfaces of gas delivery tubes of the reactor. Additionally, metallic magnesium have to be close to or above its melting point for purposes of high level doping of grown materials, but the resulting melted magnesium may react with boats or containers holding source materials, thereby contaminating internal components and source materials and resulting in inconsistent and unstable doping. Further, while it is known to use Cp2Mg in MOCVD reactors, such materials have not been utilized in HVPE reactors due to, for example, potential decomposition of Cp2Mg in HVPE reactor before magnesium-containing gases reach the growth zone, deposition of magnesium-containing compounds on internal reactor components and low and uncontrollable doping levels. 
     SUMMARY 
     According to one embodiment, an apparatus for controlling gas flows for growth a group III nitride material by HVPE includes an outer tube and first and second tubes positioned within the outer tube. The first and second tubes define a space there between. A reaction product and a reactive gas flow through the first tube, a gas source supplies gas that flows along the outer surfaces of the first and second tubes to contain flowing reaction product and reactive gas within the space as the reaction product and reactive gas flow through the space and into the second tube. 
     According to another embodiment, an apparatus for controlling gas flows for growth of a group III nitride material by HVPE includes an outer tube, a gas injection tube positioned within the outer tube, a gas collection tube positioned within the outer tube, a seed holder and a gas source. A reaction product resulting from a reaction of a metal source and a first reactive gas flows through the first tube. A second reactive gas also flows through the first tube. A space is defined between the first and second tubes, and the seed holder is positioned within the second tube and supports a seed on which group III nitride material is grown. Gas from the gas source flows along outer surfaces of the gas injection and gas collection tubes to contain flowing reaction product and second reactive gas within the space as the reaction product and the second reactive gas flow through the space, over the seed, and into the collection tube. 
     Another embodiment is directed to a method for controlling gas flows for growth a group III nitride material by HVPE. The method includes positioning a first tube and a second tube within an outer tube of a HVPE reactor such that a space is defined between the first and second tubes. The method also includes supplying a reaction product and a reactive gas that flow through the first tube, through the space, over a seed on which the group III nitride material is grown, and into the second tube. The method further includes directing a gas that flows along outer surfaces of the first and second tubes to contain flowing reaction product and reactive gas within the space as the reaction product and the reactive gas flow through the space, over the seed, and into the second tube. 
     A further embodiment is directed to a method for controlling gas flows for growth a group III nitride material by HVPE. The method includes positioning a gas injection tube and a gas collection tube within an outer tube of a HVPE reactor such that a space is defined between the gas injection and gas collection tubes. A reaction product resulting from a reaction of a metal source and a first reactive gas is supplied and flows through the first tube, through the space, over a seed on which group III nitride material is grown, and into the collection tube. Further, a second reactive gas is supplied and flows through the first tube, through the space, over the seed, and into the second tube. The method also includes directing a gas that flows along outer surfaces of the first and second tubes to contain flowing reaction product and second reactive gas within the space as the reaction product and the second reactive gas flow through the space, over the seed, and into the collection tube. 
     In one or more embodiments, the reaction product, e.g., gallium chloride is a result of a reaction of a halide reactive gas, e.g. HCl gas, and a gallium metal source and may flow through tubes and the space defined there between with a carrier gas such as Argon. Another reactive gas flowing through the tubes may be ammonia. In one embodiment, the gas that flows outer surfaces of the first and second tubes to serve as a “gas focusing” element or lens is a halide reactive gas, such as HCl gas. 
     A further embodiment is directed to a multi-tube apparatus for controlling gas flows in a HVPE reactor. The apparatus includes at least four tubes. A second tube is positioned within a first or outer tube to define first space there between for flow of a halide reactive gas. The third tube is positioned within the second tube to define a second space there between for flow of a reaction product. The fourth tube is positioned within the third tube to define a third space there between for flow of the halide reactive gas. The fourth tube defines an inner or fourth space for flow of ammonia. 
     Another embodiment is directed to a method of controlling gas flows in a HVPE reactor using a multi-tube structure. The method includes providing at least four tubes, positioning a second tube within a first or outer tube, positioning a third tube within the second tube, and positioning a fourth tube within the third tube. A first reactive gas comprising a halide reactive gas flows within a space defined between the first and second tubes, and within a space defined between the third and fourth tubes. A reaction product flows within a space defined between the second and third tubes. A second reactive gas flows within a space defined by the fourth tube. 
     A further embodiment is directed to a method of controlling gas flows in a HVPE reactor, comprising and includes providing at least four tubes and positioning a second tube within an outer or first tube, positioning a third tube within the second tube, and positioning a fourth tube within the third tube. HCl gas flows within a space defined between the first and second tubes, and within a space defined between the third and fourth tubes. A carrier gas that carries the reaction a reaction product flows within a space defined between the second and third tubes. Ammonia flows within a space defined by the fourth tube. 
     In one or more embodiments, the tubes may have different shapes or the same shapes, e.g., some tubes may have a rectangular cross-sectional shape whereas others may have a circular cross-sectional shape. All of the tubes may also have a circular cross-sectional shape and may be aligned to share a common central axis. 
     In one or more embodiments, the halide reactive gas is HCl gas, the reaction product is gallium chloride resulting from a reaction of a gallium metal source and HCl gas and the second reactive gas is ammonia. The reaction produce may flow with a carrier gas such as Argon to the growth zone. In one or more embodiments, the tubes are configured such that the reaction product, the halide reactive gas and ammonia are separated until they reach a growth zone in the HVPE reactor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout and in which: 
         FIG. 1  is a schematic illustration of a horizontal HVPE reactor constructed according to one embodiment that may include one or more or all of an external non-metallic magnesium source, a flexible member that covers a surface an internal component, and a gas focusing element; 
         FIG. 2  is a schematic illustration of a horizontal a HVPE reactor constructed according to one embodiment having a flexible member that covers one or more surfaces of one or more internal components; 
         FIG. 3  is a schematic illustration of a seed or substrate supported by a seed holder; 
         FIG. 4  is a schematic illustration of a flexible member covering a surface of a seed holder according to one embodiment; 
         FIG. 5  is a flow chart of a method of growing single crystal group III nitride materials with the reactor constructed as shown in  FIGS. 2-4 ; 
         FIG. 6  is a schematic illustration showing growth of single crystal gallium nitride material on the seed or substrate and parasitic deposition of gallium nitride material on the flexible cover member according to one embodiment; 
         FIG. 7  is a schematic illustration of an inner element or tube positioned within a main reactor tube of a vertical HVPE reactor; 
         FIG. 8  is a schematic illustration of a flexible member covering an inner surface of the inner tube shown in  FIG. 7  according to one embodiment 
         FIG. 9  is a flow chart of a method of growing single crystal group III nitride materials and moving or replacing a flexible member on which parasitic material has been deposited according to one embodiment; 
         FIG. 10  is a schematic illustration of a HVPE reactor having an inner tube and a seed positioned within the inner tube and in the path of active gases during growth of single crystal group III nitride material on the seed; 
         FIG. 11  is a schematic illustration of the HVPE reactor shown in  FIG. 10  and the seed and crystal material grown thereon being movable through an aperture defined by the inner tube to allow the inner tube to be replaced during a growth run according to one embodiment; 
         FIG. 12  is a schematic illustration of a HVPE reactor constructed according to another embodiment including an external non-metallic magnesium source and a magnesium-containing gas that is delivered to a growth zone by a carrier gas other than hydrogen; 
         FIG. 13  is a flow chart of a method of growing single crystal p-type group III nitride materials with the HVPE reactor constructed as shown in  FIG. 12 ; 
         FIG. 14  is a schematic illustration of a HVPE reactor constructed according to another embodiment including an external non-metallic magnesium source that is delivered through a tube having sections of different diameters; 
         FIG. 15  is a schematic illustration of a HVPE reactor and one manner in which the external, non-metallic magnesium source shown in  FIG. 14  can be constructed according to one embodiment; 
         FIG. 16  is a schematic illustration of a HVPE reactor and another manner in which the external, non-metallic magnesium source shown in  FIG. 14  can be constructed according to another embodiment; 
         FIG. 17  is a schematic illustration of a gas focusing element or gas lens constructed according to one embodiment; 
         FIG. 18  is a schematic illustration of a source concentrating boat for use with HVPE reactors according to a further embodiment; 
         FIG. 19  illustrates a schematic illustration of a multi-tube insert for use with the source concentrator shown in  FIG. 18  and configured to separate and focus flows of different gases through the source tube according to one embodiment; 
         FIG. 20A  is a schematic illustration of a cross-sectional view of a portion of a length of a multi-tube insert constructed according to another embodiment; and 
         FIG. 20B  is a schematic illustration of a cross-sectional view of  FIG. 20A  along line B-B. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
     Embodiments relate to HVPE growth apparatus or reactors, fabrication methods and resulting high quality, low defect density single crystal group III nitride materials. Single crystal group III nitride materials are group III nitride materials having a continuous, unbroken crystal lattice and no grain boundaries, as opposed to amorphous materials, polycrystalline materials that are made up of a smaller crystals referred to as crystallites, which are which are small single crystal structures having random or chaotic orientations, and isotropic materials. Examples of single crystal materials that may be grown with embodiments include single crystal gallium nitride (GaN), which may have n-type, insulating, or p-type conductivity, and related pn structures. Embodiments can be implemented for growth of various other group III nitride materials including AlN, AlGaN, InN and InGaN and related structures. For ease of explanation, reference is made to single crystal gallium nitride and related structures and devices. 
     With embodiments, single crystal gallium nitride materials were grown for long durations and at high growth rates, e.g., growth rates exceeding 1 millimeter per hour, or about 17 microns per minute while maintaining desired material qualities. According to one embodiment, gallium nitride single crystals may be grown for a growth duration exceeding 30 minutes, e.g., exceeding 1 hour and even exceeding 10 hours. According to one embodiment, the growth rate exceeds 2 millimeter per hour and may be 5 millimeter per hour. Single crystal gallium nitride materials grown by embodiments at high growth rates have improved material parameters and uniformity and may be used in or enable various new device structures, capabilities and applications. Embodiments achieve these advantages by utilizing novel HVPE reactor components, configurations and methods that eliminate or reduce parasitic deposition of amorphous, polycrystalline and crystallite materials or structures on internal reactor components and inclusions in grown single crystal materials while the single crystal materials are grown with consistent and reproducible p-type doping characteristics using stable dopant sources. Referring to  FIG. 1 , a HVPE reactor  100  constructed according to embodiments includes a main or outer reactor tube  110  (generally referred to as “outer tube  110 ”) and internal reactor components  120  including a seed or substrate  122  (generally referred to as “seed”  122 ) supported by a seed or substrate holder  124  (generally referred to as “seed holder”  124 ). For example, the seed  122  may be a relatively thin (e.g., 1-15 microns) gallium nitride layer having a low defect density less than 10 8  cm 2 , preferably less than 10 7  cm −2  and more preferably less than 10 6  cm −2 . The gallium nitride seed layer may be deposited directly on a sapphire substrate or on a buffer layer to form the seed  122  and may be grown in the same or in a separate HVPE process. For example, one suitable buffer layer is a AlN/AlGaN/GaN multi-layer buffer including layers having a thickness of about 1 to 50 nm. 
     The outer tube  110  may be made of quartz. Single crystal gallium nitride materials (not shown) are grown or deposited on the seed  122  in a growth zone  112  within the outer tube  110 . The internal components  120  may also include other internal tubes or structures  126 . For example, the tube  126  may be one of various internal reactor tubes  126  such as dopant delivery tubes, gallium source material delivery tube, aluminum source material delivery tube, etc., or may be a tube that may protect an inner surface of the main outer tube  110 . The reactor  110  also includes one or source tubes  130  that contain a metal source  132  such as gallium metal, e.g., for growth of gallium nitride. Gas supplies  140  provide reactive and carrier gases utilized in HVPE reactors to transport reaction products and active gases to the growth zone  112 . 
     According to one or more embodiments, an internal component  120  is partially or completely covered by a chemically stable, flexible or elastic material or member  150  (generally referred to as cover member  150 ) upon which a parasitic material, which may be substantially polycrystalline, is deposited during deposition of single crystal gallium nitride on the seed  122 . For ease of explanation, reference is made to parasitic materials, which are defined as amorphous, polycrystalline materials that are made up of a smaller crystallites and isotropic materials, in contrast to single crystal materials as such terms are understood by persons of ordinary skill in the art. Controllable deposition of parasitic material in this manner prevents or reduces deposition of parasitic materials on surfaces of internal reactor components  120  and also prevents or reduces formation of inclusions within group III nitride materials that are grown on the seed  122 . The flexible member  150  has the additional capability of protecting covered components from chemically active substances such as chemically active gases used during HVPE growth. 
     Embodiments also provide HVPE reactors and methods for growth of group III nitride material having desired and uniform p-type doping utilizing an external non-metallic magnesium source  160  as a p-type dopant source. During use, the source  160  evaporates, and the resulting gas is delivered directly to the growth zone  112  to react with other source and active gases  140  to grow p-type single crystal gallium nitride. 
     Embodiments further provide gas focusing elements or lenses located within the outer tube  110 . A gas focusing element  128  is generally illustrated in  FIG. 1 . A gas focusing element  128  directs or restricts the direction in which certain gases flow inside of the outer tube  110  to reduce or prevent certain internal reactor components from being exposed to chemically active gases which, in turn, reduces parasitic deposition on internal reactor components  120 . A gas focusing element  128  also increases the mass of source materials in the growth zone  112  and, in particular, on a growth surface to achieve higher single crystal material growth rates. 
     It will be understood that  FIG. 1  generally illustrates certain components within the outer tube  110 , e.g., an inner tube  126  and a gas focusing element  128 , but in practice, the inner tube  126  and gas focusing element  128  are configured and positioned as needed depending on, for example, the number and location of source tubes  130 , the number and position of active gas tubes and whether the reactor  100  is a vertical or horizontal reactor. Thus,  FIG. 1  is provided to generally illustrate reactor  100  components. Further, it will be understood that structural configurations and methods of embodiments may be utilized individually and in combination as described in further detail with reference to other figures. 
     Embodiments involving one or more cover members  150  applied over one or more surfaces of one or more internal reactor components  120  are described in further detail with reference to  FIGS. 2-8 . Embodiments involving moving or replacing a cover member  150  during a growth cycle are described with reference to  FIGS. 9-11 . Embodiments involving an external non-metallic magnesium source  160  are described in further detail with reference to  FIGS. 12-16 . Embodiments involving gas focusing elements are described in further detail with reference to  17 - 20 B. It will be appreciated that advantages of embodiments may be achieved with components of embodiments individually and in combination. 
     Referring to  FIG. 2 , a HVPE reactor  200  constructed according to one embodiment includes a horizontal furnace that defines different temperature zones using one or more heaters  202   a - c  (generally  202 ), each of which at least partially surrounds the outer tube  110 . Various internal components  120  are positioned within the outer tube  110 , e.g., one or more source tubes  130  (one source tube is illustrated), one or more inner tubes  126 , and a seed  122  supported by a seed holder  124 . Single crystal group III nitride material  210  is grown on the seed  122 . 
     With further reference to  FIG. 3 , in the illustrated embodiment, the seed holder  124  includes a top surface  302  that defines a cavity or pocket  304  in which the seed  122  is positioned. According to one embodiment, the seed holder  124  is made of quartz or sapphire. It should be understood that other seed holder  124  configurations may also be utilized. 
     Referring again to  FIG. 2 , a source boat or other suitable container  220  is located within each source tube  130  to hold the metallic gallium source material  132 . A control rod  222  is used to position the boat  220  within the outer tube  110 . A gas source  140  is coupled to each source tube  130 , and the flow of gas through the source tubes  130  is controlled by one or more valves  230  or other suitable devices or flow control systems. The number of source tubes  130  and source materials  132  are determined by the desired composition of the single crystal layers  210  to be grown on the seed  122 . 
     Embodiments may be utilized for growth of various single crystal group III nitride materials  210  including, for example, GaN, AlN, InN, AlGaN and InGaN. In the case of growth of gallium nitride, the source tube  130  includes a metallic gallium source material  132 . In the case of growth of aluminum nitride, the source tube  130  includes a metallic aluminum material  132 . For ease of explanation, this specification refers to growth of single crystal gallium nitride and a gallium source tube  130  and a metallic gallium source  132  held in a boat  220 . The source tube  130  is coupled to a supply of a halide active gas  141 , e.g., HCl gas and a supply of a carrier gas such as Argon (Ar)  143 . A source of ammonia gas  142  is also coupled to reactor  110 . 
     In the illustrated embodiment, internal components  120  of the reactor  200  include one or more source tubes  130 , one or more other internal tubes  126 , the seed holder  124  and the seed  122 . According to one embodiment, the inner tube  126  is comprised of quartz and configured to carry active gases (e.g. ammonia  143 ) to the growth zone  112  through the inner tube  110 . The tube may also protect an inner surface  111  of the outer tube  110  from active gases. 
     According to one embodiment, the flexible protective material  150  is in the form of a cover, lining, roll, or sheet (generally referred to as flexible cover  150 ) that is applied over one or more surfaces of one or more internal components  120  of the reactor  200 . In one embodiment, the cover member  150  is flexible, deformable, bendable and/or elastic. For ease of reference, the cover member  150  is described as being flexible. 
     According to one embodiment, the cover member  150  is a flexible pure graphite material in the form of a sheet that is removably applied over one or more surfaces of an internal reactor component  120 . The cover member  150  may be applied over the entire internal reactor component  120  or a portion thereof, and may be applied over or on one internal component  120  or multiple internal components  120 . In the embodiment shown in  FIG. 2 , the cover member  150  is applied to cover an inner surface of the inner tube  126  and one or more surfaces, e.g., one or both of the top surface  302 , of the seed holder  124 . The cover member  150  does not degrade when heated to high temperatures, e.g., 1200° C.-1400° C., or when exposed to aggressive chemicals. As such, a cover member  150  may be repeatedly utilized. 
     With further reference to  FIGS. 3 and 4 , the seed holder  124  defines a cavity  304 , and the flexible cover member  150  defines an aperture or window  152  so that the flexible member  150  can be applied over the top surface  302  of the seed holder  124 . According to one embodiment, the shape and size of the window  152  generally corresponds to the shape and size of the cavity  304  defined by the seed holder  124 . The thickness of the flexible member  150  may be on the orders or microns or millimeters and may vary depending on, for example, the type of material and desired strength and flexibility. Further, the flexible member  150  can be bent in various configurations, and the shape, size and number of windows  152  may vary. For example, the window  152  may define an area of about a square millimeter to several square centimeters, e.g., about 1 to about 1000 square centimeters, or larger. 
     Referring to  FIG. 5 , during use, the internal components  120  such as the seed holder  124 , the seed  122 , the inner tube  126  and one or more source tubes  130  are positioned inside of the outer tube  110  at stage  505 . At stage  510 , surfaces of internal components  120 , e.g., surfaces that would be exposed to active gases such as HCl gas  141 , ammonia gas  143 , a gas containing gallium chloride  134  (e.g., for growth of gallium nitride) and/or a gas containing aluminum chloride (e.g., for growth of aluminum nitride, and other active gases associated with growth of a particular group III nitride material, may be partially or completely covered by one or more flexible members  150 . For example referring again to  FIG. 4 , the flexible member  150  may be applied over the top surface  302  of the seed holder  124 . Further, the flexible member  150  may be applied to line an inner surface of the inner tube  126 . 
     Referring again to  FIG. 5  and with further reference to  FIG. 6 , at stage  515 , a single crystal group III nitride material  210  is grown on the seed  122  as a result of the reaction of gallium chloride  134  carried by Argon gas  142  and ammonia  143  while, at the same time, parasitic gallium nitride  600 , which may be polycrystalline, amorphous or isotropic, is deposited on the flexible sheet(s)  150  at stage  520 . 
     More particularly, for growth of the single crystal gallium nitride  210  (stage  515 ) and deposition of the parasitic gallium nitride  600  (stage  520 ), the reactor  200  is filled with Argon gas  142 , the flow of which is controlled by a valve  230 . The seed holder  224  and the seed  222  supported thereby are heated to the desired growth temperature, e.g., about 800-1200° C., more preferably to a temperature of between 1000-1100° C. Gallium source material  132  is heated to a suitable temperature (e.g., about 750-1050° C.), and gaseous HCl from a tank or source of HCl gas  141  (generally referred to as HCl gas  141  or a source of HCL gas  141 ) is introduced into the source tube  130 . HCl gas  141  reacts with the gallium metal source  132  to form a reaction product  134  in the form of gallium chloride, which is delivered to the growth zone  112  by Argon gas from a source or tank of Argon gas  142  (generally referred to as Argon gas  142  or a source of Argon gas  142 )  142  flowing through the source tube  130 . Simultaneously, ammonia gas from a source or tank of ammonia gas  143  (generally referred to as ammonia gas  143  or a source of ammonia gas  143 ) is delivered to the growth zone  112 . The gallium chloride  134  carried through the source tube  130  by Argon gas  142  and the ammonia  132  react, resulting in growth of gallium nitride  210  (e.g., n-type GaN  210 ) on the seed  122 . Further details of HVPE growth processes and parameters are described in U.S. Pat. No. 6,555,452 to Nikolaev et al., the contents of which were previously incorporated herein by reference. 
     Referring again to  FIG. 6 , with embodiments, parasitic gallium nitride  600  is deposited on the flexible member  150  while desired single crystal gallium nitride  210  is grown on the seed  122 . In the illustrated embodiment, parasitic gallium nitride  600  grows only on the flexible member  150 , and the single crystal gallium nitride  210  grows through the window  152  defined by the flexible member  150 . In this manner, embodiments advantageously manipulate parasitic deposition of gallium nitride  600  such that parasitic materials  600  are controllably deposited on the flexible member  150  rather than on other internal reactor components  120  or within the single crystal gallium nitride  210  itself as it is grown. Thus, embodiments are capable of protecting internal reactor components  120  from parasitic deposition  600 , thereby reducing stresses and damage to such components and providing for more consistent growth parameters, while also reducing or eliminating formation of inclusions within the single crystal gallium nitride  210 . Embodiments also reduce or prevent contact between the single crystal gallium nitride  210  and parasitic materials  600 , which may result in stress or cracking of single crystal gallium nitride  210  during growth. 
     Embodiments achieve these advantages and unexpected results by manipulating the material properties of the flexible member  150 . The flexibility or elasticity of the flexible member  150  allows the flexible member  150  to bend or deform, but not break, under the weight of the parasitic material  600 . Consequently, forces and weight resulting from deposition of parasitic material  600  are not directly translated to the seed holder  124  and do not cause stresses and fractures in the seed holder  124 . 
     For this purpose, according to one embodiment, the flexible sheet  150  is made of graphite and may be in the form of felt or paper. Parasitic materials  600  are deposited on the graphite member  150 , which does not contaminate the single crystal gallium nitride  210  grown on the seed  222  and through the window  152  defined by the flexible member  150 . If necessary, the flexible member  150  may be annealed before or during growth in order to remove impurities. For example, annealing may be performed in Argon gas  142  at a temperature of about 1100° C. 
       FIG. 2  illustrates a flexible member  150  that is part of a horizontal reactor  200 . However, flexible member  150  components may also be integrated within vertical reactors  700 . For example, referring to  FIGS. 7 and 8 , a vertical reactor  700  constructed according to one embodiment includes an inner tube  126  that is positioned within a main or outer vertical reactor tube  110 . The inner tube  126  includes an inner lining of flexible material  150 . The seed holder  124  is attached to a rod or shaft  710  that is controllably movable to position the seed holder  124  and the seed  122  supported thereby into and out of the growth zone  112 . In this manner, chemically active gases, e.g., HCl gas  141 , ammonia gas  142  and a gas containing gallium chloride  134 , flow through the inner tube  126  tube. The tube  126  may also carry a single chemically active gas. Parasitic material  600  is deposited on the flexible member  150  lining applied to the tube  126  rather than on the inner surface  127  of the inner tube  126  itself or on the inner surface  111  of the outer reactor tube  110 , while directing active gases to the growth zone  112 . The inner surfaces  127  of the inner tube  126  and the inner surfaces  111  of the outer tube  110 , both of which may be made of quartz, are not exposed to active gases or are exposed to smaller quantities of such gases. The flexible member  150 , therefore, advantageously protects the internal tube  126  and the outer tube  100  from parasitic deposition  600  and chemical etching, thereby reducing stress, cracking and damage to these components that may otherwise occur if due to active gas exposure. 
     Referring to  FIG. 9 , one embodiment is directed to a method  900  of moving or replacing the flexible member  150  or sections thereof having parasitic deposits  600  thereon. In the illustrated embodiment, stages  905 - 920  are similar to stages  505 - 520  sown in  FIG. 5 . At stage  905 , internal components  120  such as the seed holder  124 , the seed  122 , the inner tube  126  and one or more source tubes  130  are positioned inside of the reactor tube  110 . At stage  910 , one or more internal components  126  are covered with a cover member  150 . At stage  915 , single crystal gallium nitride  210  is grown on the seed  122  as a result of the reaction of gallium chloride  134  carried by Argon gas  142  and ammonia  143  while at stage  920 , parasitic gallium nitride  600  is deposited on the flexible member  150 . At stage  925 , the flexible member  150  having parasitic deposits  600  may be removed, e.g., slidably removed, from the top surface  302  of the seed holder  124  or slidably removed from the inner surface  127  of the inner tube  126 . 
     Whether the flexible member  150  is slidably removed from the seed holder  124  may depend on, for example, the depth of the cavity  304  defined by the seed substrate  120 , the height of the single crystal material  210  and the height of the single crystal material  210  relative to the flexible member  150 . For example, if the single crystal material  210  thickness is such that it extends above the flexible member  150 , then the flexible member  150  can be lifted off from the top surface  302  of the seed holder  224 . Otherwise, the flexible member  150  may be slidably removable from the seed holder  224 . 
     Further, with embodiments that utilize a flexible graphite member  150 , the single crystal gallium nitride  210  will not be contaminated if the single crystal gallium nitride  210  contacts the graphite flexible member  150  during growth or during movement or removal of the flexible member  150 . Additionally, the flexibility of the cover member  150  eliminates or reduces stresses on and cracking of the growing single crystal gallium nitride  210  in the event that the cover member  150  and the growing crystal  210  come into contact with each other during growth. 
     At stage  930 , a new portion of the flexible member  150  (e.g., a new portion having no parasitic deposits  600  or a previously used portion having less parasitic deposits  600 ) is positioned on the seed holder  224  or over the inner surface  111  of the inner tube  126  to replace the previously used flexible member  150  or portion thereof. In this manner, embodiments advantageously allow extended growth processes to be performed while replacing used flexible members  150  or components thereof to maintain controlled deposition of parasitic materials  600  on the flexible member  150  rather than on internal components  120  of the reactor. For this purpose, for example, the flexible member  150  may be stored on rolls such that new portions of the flexible member  150  may be unrolled from a first roll, and portions of the flexible member  150  having parasitic deposits  600  may be rolled up onto a take-up roll or other suitable storage device. 
     Referring to  FIGS. 10 and 11 , according to another embodiment, the inner tube  126  may be replaced during growth of single crystal gallium nitride  210 . In the illustrated embodiment, the inner tube  126  defines an aperture  1005 . The seed holder  124  is coupled to rod or shaft  710  that is controllably movable to move the seed  122  and single crystal gallium nitride  210  grown thereon into and out of the inner tube  126  and through the aperture  1005  and, therefore, into and out of a first gas environment  1010  and a second gas environment  1015  within the inner tube  126  that includes the growth zone  112 . 
     As shown in  FIG. 10 , during growth, the seed  122  and the seed holder  124  are positioned inside of the second gas environment  1015  in the growth zone  112 . The first gas environment  1010  includes flowing active gases, e.g., a gas including gallium chloride  134  and ammonia  143  so that single crystal gallium nitride  210  is grown on the seed  222 . The top surface  302  of the seed holder  224  may be covered by the flexible cover member  150  (as described above) so that single crystal gallium nitride  210  is grown on the seed  222  while parasitic material  600  is deposited on the flexible member  150 . The inner tube  126  or other suitable internal component may also be fabricated from flexible and elastic graphite similar to the flexible member  150 . Alternatively, the inner tube or other suitable component may be composed of quartz, graphite, sapphire and other suitable materials. 
     During growth, it may be necessary to replace the inner tube  126  due to deposition of parasitic material  600  on the flexible member  150 . For this purpose, active gas flows through the second gas environment  1015  can be stopped, an inert gas such as Argon gas  142  can flow through the second environment  1015 , and the seed holder  124  and seed  122  upon which single crystal gallium nitride  210  has grown can be moved from the second environment  1015  to a non-growth position in the first environment  1010  that includes inert gas. The inner tube  126  may then be moved, replaced or exchanged with another inner tube  126  or suitable internal component. The seed holder  124 , seed  122  and the single crystal material  210  grown thus far may then be re-inserted through the aperture  1005  into the growth zone  112 . Flow of active gases may be resumed to continue growth of the single crystal gallium nitride  210  on the seed  122  in the growth zone  112 , while parasitic material  600  is deposited on the new flexible member  150 . In this manner, embodiments advantageously move or exchange internal reactor components  120  during a growth cycle in order to prevent cracking and/or breakage of internal components  120  due to heavy parasitic deposition  600 . These capabilities are particularly advantageous during long growth runs during which larger quantities of parasitic material  600  are deposited on the flexible member  150 . The reduction or elimination of negative aspects of parasitic deposition  600  allows growth of large single crystal gallium nitride layers or structures at higher growth rates. 
     Referring to  FIGS. 12 and 13 , other embodiments are directed to a HVPE reactor  1200  having an external non-metallic material or source  160 , which serves as a dopant source for growth of p-type single crystal gallium nitride  210  and other p-type single crystal group III nitride materials, and a method  1300  for growing p-type gallium nitride  210  and other p-type group III nitride materials for long duration and at fast growth rates. For ease of explanation, reference is made to p-type single crystal gallium nitride  210 . 
     According to one embodiment, the external non-metallic material or source is an external non-metallic magnesium source  160  rather than metallic magnesium. According to one embodiment, the non-metallic magnesium source  160  is cyclopentadienylmagnesium (Cp2Mg)  1207 . 
     Embodiments utilize novel HVPE reactor configurations that can be adapted for use with Cp2Mg  1207  as a non-metallic source of magnesium for growth of p-type gallium nitride  210 . Embodiments address technical issues of known HVPE reactors by eliminating or reducing decomposition of Cp2Mg in HVPE reactor prior to gases reach the growth zone  112 , eliminating or reducing deposition of magnesium-containing compounds on internal reactor components  120  and providing for high and controllable an stable doping levels, which are consistent across the grown material  120 . These advantages are achieved while p-type gallium nitride is grown at fast growth rates, e.g., about 10 microns per minute and higher. 
     More specifically, a method  1300  of growing p-type gallium nitride  210  includes positioning the gallium metal source  132  inside of the reactor tube  110  at stage  1305 . A first reactive gas, such as HCl gas  141 , is supplied to the source tube  130  at stage  1310 , thereby forming a reaction product, such as gallium chloride  134 , at stage  1315 . At stage  1320 , a carrier gas  142 , such as Argon  142 , is supplied to the source tube  130  to deliver the gallium chloride  134  to the growth zone  112 . At stage  1325 , a second reactive gas, such as ammonia  143 , is simultaneously supplied to the growth zone  112 . 
     At stage  1330 , gaseous Cp2Mg  1209  is formed from the external Cp2Mg source  160 . The source  160  is solid Cp2Mg  1207  that may be maintained at ambient temperature and evaporates at ambient and elevated temperatures to form the magnesium-containing gas  1209 . At stage  1335 , a carrier gas  142  other than hydrogen flows through a dopant delivery tube  1210  to deliver the magnesium-containing gas  1209  to the growth zone  112 . For this purpose, Argon gas  142  can flow at low rates, e.g., about 1 sccm, to about 1000 sccm through a vessel that holds the solid Cp2Mg  1207 , and the magnesium-containing gas  1209  may flow through the delivery tube  1210  at a gas flow velocity of at least about 1 meter per second. Flow of Argon gas  142  through a vessel holding the solid Cp2Mg  1207  may be controlled independently of the Argon gas  142  flowing through the delivery tube  1210 . Additional Argon flow  1420  (generally referred to as Argon push flow) is used to controllably increase flow velocity of magnesium-containing gas  1209  inside of the HVPE reactor. The Argon push flow  1420  may be varied from about 0.1 to 5 slm. 
     The reaction of gallium chloride  134  and ammonia  143  in the presence of magnesium-containing gas  1209  in the growth zone  112  results in growth of low defect density p-type single crystal gallium nitride  210  on the seed  122 . With embodiments, low defect density, single crystal, uniformly doped, p-type GaN  210  may be grown at high growth rates, e.g., from about 1 micron per minute to more than 1 millimeter per hour, and even greater than 2 millimeters per hour. Growth durations greater than 10 hours were demonstrated while maintaining single crystal quality and stable p-type doping, as demonstrated by dopant variations of less than 5%. For bulk p-GaN  219  growth, process durations exceeding 100 hr may also be utilized. It should understood that various steps shown in  FIG. 13  may occur simultaneously, and reference is made to separate stages for ease of explanation. 
     HVPE reactor  1200  and method  1300  embodiments provide a number of advantages over known MOCVD and HVPE reactors. For example, in known MOCVD systems, hydrogen is used as a carrier gas to deliver Cp2Mg into the MOCVD reactor. Use of hydrogen gas, however, results inpassivated magnesium atoms within grown gallium nitride materials, which reduces the electrically active magnesium concentration and increases the electrical resistivity of the grown material. These results are not desirable. 
     With embodiments, on the other hand, Argon gas  143 , rather than Hydrogen gas, is used to deliver the magnesium-containing gas  1209  to the growth zone  112  to reduce passivated magnesium atoms within the grown material  210  and thereby prevent increased resistivities resulting from use of hydrogen gas. Further, with embodiments, magnesium delivery or source tubes  1210  can be etched with a mixture of HCL and Argon gases  141 ,  142  in order to remove any magnesium-containing deposits from the inner surfaces of the tubes  141 ,  142 . These capabilities advantageously improve the consistency and reproducibility of p-type doping of single crystal gallium nitride  210 . For example, HCl etching may be performed by flowing about 10 to 1000 sccm of HCl gas  141  through the magnesium delivery tube  1210  before and after a doping process. 
     Further, with embodiments, HVPE reactors can be configured such that a magnesium-containing gas  1209  can be carried through a hot zone of the HOVE reactor. In one embodiment, the magnesium-containing gas  1209  flows through the delivery tube  1210  and through several high temperature zones, including a source zone, which may be at a temperature of about 800-900° C., and through a gas mixing zone, which may be at a temperature of about 900°-1000° C. The magnesium-containing gas  1209  in the growth zone  112  is then used to grow low defect density p-type single crystal gallium nitride  210  at fast growth rates. This is in contrast to known MOCVD systems, which involve injection of relatively cold gases directly into the growth zone. 
     Referring to  FIG. 14 , according to an alternative embodiment, a HVPE reactor  1400  includes a source tube  1210  that delivers the magnesium-containing gas  1209  that is tapered or has one or more sections with a reduced diameter. In the illustrated embodiment, the deliver tube  1210  includes a first section  1401  associated with an end of the outer tube  110  and having a first diameter. A second section  1402  extends from the first section  1401  and has a second, smaller diameter. According to one embodiment, the first diameter of the first section  1410  is about 20 millimeters, and the second diameter of the second section  1402  is about 4 millimeters. 
     With this configuration, embodiments allow magnesium-containing gas  1209  to flow through the delivery tube  1210  and, in particular, through the reduced diameter section  1402  of the delivery tube  1210 , at a sufficiently high rate to reduce or prevent decomposition of the magnesium compound within the tube  1210  while the magnesium-containing gas  1209  is delivered through a hot portion of the reactor  1400 . With this configuration, embodiments advantageously increase the speed at which magnesium-containing gas  1209  flows through the delivery tube  1210 , thereby reducing the amount of time that the magnesium-containing gas  1209  is exposed to high temperatures in source and gas mixing zones which, in turn, reduces magnesium decomposition that would otherwise occur at these high temperatures. According to one embodiment, the flow rate of magnesium-containing gas  1209  through the first section  1401  is about 0.06 m/sec, and the flow rate of magnesium-containing gas  1209  through the second section  1402  is about 1.5 m/sec. 
       FIG. 15  illustrates one embodiment of a HVPE reactor apparatus  1500  that includes an external Cp2Mg source  160  and a delivery tube  1210  having a reduced diameter section  1402  (as shown in  FIG. 14 ). In the illustrated embodiment, HCl and Argon gases  141 ,  142  are provided for etching of the inner surface of the delivery tube  1210 . More particularly, etching is performed by HCl  141 , and Argon  142  is added to carry and dilute the HCl gas  141 . A vacuum pump  1510  is used for evacuating oxygen-containing gases to prevent contamination and oxidation of the source  132   
       FIG. 16  illustrates an external Cp2Mg source  160  and flow control system  1600  constructed according to one embodiment. The system  1600  includes a bubbler  1605  or other suitable vessel containing solid Cp2Mg  1207 . Solid Cp2Mg  1207  evaporates to form a magnesium-containing gas  1209 , which flows to the delivery tube  1210  using suitable valves and controllers, e.g., manual valves  1610  and pneumatic valves  1612 , pressure gauges  1614  and mass flow controllers  1616 . 
       FIGS. 17-20B  illustrate embodiments of systems and methods of controlling growth of and focusing source materials, such as a gas containing gallium chloride  134 , to control, direct or focus flows of source material gases onto the seed  122  to increase the source material efficiency and utilization, reduce parasitic deposition  600 , reduce clogging of tubes such as the source tube  130  and ammonia  134  delivery tube, and increase the growth rate of group III single crystal gallium nitride  210 . 
     Referring to  FIG. 17 , according to one embodiment, a HVPE reactor includes a gas focusing element or “gas lens”  1700  (generally referred to as “gas focusing element”  1700 ). According to one embodiment, an inner tube  126  in the form of an active gas injection tube  1701  is positioned adjacent to a gas collection tube  1702 . An intermediate space  1710  is defined between the gas injection and collection tubes  1701 ,  1702 . Chemically active gases, e.g., gallium chloride  134  carried by Argon  142  and ammonia  143 , flow through the injection tube  1701 , through the intermediate space  1710  and on and around the seed  222  within the growth zone  112 . Single crystal gallium nitride  210  is deposited on the seed  222 , and active gases flow past the seed  222  and into the collection tube  1702  for evacuation. 
     According to one embodiment, the gas focusing element  1700  is formed by a protective gas  1705  that flows along the outer surfaces of the injection and collection tubes  1701 ,  1702 . According to one embodiment, the protective gas is HCl gas  141 , which forms a gas wall or barrier around the open space  1710  and between adjacent ends of the injection and collection tubes  1701 ,  1702 . The protective gas  1705  blocks active gases (gallium chloride  134  and ammonia  143 ) from entering the intermediate space  1710  and contacting other internal components  120  inside of the reactor. The protective gas  1705  forms a gas focusing element or lens  1700  that focuses or restricts the direction that active gas flows between the tube  1701  and the collection tube  1702 , thereby increasing the mass of source materials provided to the growth surface, thereby increasing the growth rate. The protective gas  1705  also reduces deposition of parasitic gallium nitride  600  on various internal components, e.g., around the seed  122  and on or around other internal components  120  including nozzles of source delivery tubes. In this manner, embodiments reduce or eliminate stresses and breaking of quartz materials and reduce or prevent growth rate reductions and clogging of various tubes. Gas focusing elements  1700  also allow larger quantities of source materials to be mixed in the growth zone  112  in the vicinity of the seed  122 , thereby improving the efficiency at which source materials are supplied to the growth zone  112  and increasing the growth rate of single crystal gallium nitride  210 . Gas focusing elements  1700  also protect the growing single crystal gallium nitride crystal  210  from HCl gas  141  that may damage the crystal  210  during cleaning of internal components  120  by gas etching. 
     Referring to  FIG. 18 , another embodiment is directed to source concentrating boats or source concentrators  1800  (generally referred to as source concentrators  1800 ) that are used to control, direct or focus gases within source tubes  130  in order to increase the source material utilization and efficiency and increase growth rates of single crystal gallium nitride  210  on the seed  222  in the growth zone  112 . In the illustrated embodiment, a source concentrator  1800  includes main body  1801  and an inlet tube  1802  that serves as an inlet for HCl gas  141 . The length of the main body  1801  may be several centimeters to almost a meter, and the diameter of the main body  1801  may be about one to several centimeters. 
     In the illustrated embodiment, the HCl inlet tube  1802  extends along the length of the main body  1801 , which contains gallium metal  132 . During use, the inlet tube  1802  releases HCl gas  141  inside of the main body  1801  through an outlet  1804 , and the released HCl gas flows in a direction  1806  (indicated by arrow). As a result, the HCl gas  141  reacts with the gallium metal  132  to form gallium chloride  134 , which is released from the boat  130  through an outlet  1805  and is carried towards the growth zone  112  by Argon gas  142 . The reverse direction  1806  of HCl gas  141  over the gallium metal  132  advantageously improves the efficiency of the reaction between HCl gas  141  and the Ga melt  132  surface and also helps to increase the growth rate. 
     According to one embodiment, the size of outlet  1805  through which the gas containing gallium chloride  134  is released may be controllably altered during the growth process in order to controllably alter the supply of gas containing gallium chloride  134  to the growth zone  112  which, in turn, provides the ability to controllably alter the deposition rate of single crystal gallium nitride  210  on the seed  122 . In one embodiment, the size of the outlet  1805  may be controllably altered between a closed position and an open position. An open outlet  1805  may have a diameter up to several square centimeters). According to one embodiment, this is accomplished by a cover (not shown) that can be positioned and moved over the outlet  1805  to increase and reduce the size of the outlet  1805  opening through which gases can flow. 
       FIGS. 19 and 20A -C illustrate embodiments of gas flow control components  1900 ,  2000  that are inserted into or placed within the source concentrator  1800  shown in  FIG. 18 . The unique geometry and flow control provided by inserts  1900  and  2000  prevent premature reaction between gallium chloride  134  and ammonia  143  and parasitic deposition  600  on the seed  122 . 
     More particularly, in the embodiment illustrated in  FIG. 19 , a gas flow control component or insert  1900  is a multi-tube structure that is configured such that certain gases flow through different tubes or different spaces defined by the tubes, and are then released from the flow control  1900  to the growth zone  112  where they can react with other active gases for growth of single crystal gallium chloride  210 . In the illustrated embodiment, the flow control component  1900  includes four tubes: a first or outer tube  1910 , a second tube  1920  positioned within the inner space  1911  defined by the first tube  1910 , a third tube  1930  positioned within an inner space  1921  defined by the second tube  1920 , and fourth tube  1940  positioned within an inner space  1931  defined by the third tube  1930 . 
     In the illustrated embodiment, HCl gas  141  flows through the space  1911  or through a separate tube positioned within the space  1911 . A gas carrying gallium chloride  134  flows through the space  1921 . HCl gas  141  also flows through the space  1931 , and ammonia  143  flows through the space  1941 . With this configuration, flows of HCl gas  141 , Argon  142 /gallium chloride  134 , and ammonia  143  are separated from each other as these gases flow through the flow control component  1900  and are then combined in the growth zone to react and form single crystal gallium nitride  210  on the seed  222 . 
     With embodiments, HCl gas  141  flows are used to prevent deposition of parasitic material  600  on internal components  120 , e.g., the inner surfaces of the first or outer tube  1910  and the inner surface of the third tube  1930 , and to focus flow of Argon gas  142 /gallium chloride  134  towards the seed  122 . For example, HCl gas  141  flowing within the third tube  1930  cleans a nozzle of the fourth or ammonia delivery tube  1940  to prevent or reduce parasitic deposition  600 . HCl gas  141  flows as shown in  FIG. 19  also prevent premature reaction of ammonia  143  and gallium chloride  134  as a result of the arrangement of tubes  1920 ,  1930  and  1940 , which separate ammonia  143  and gallium chloride  134  until the gases exit the outlets of the respective tubes and flow towards the growth zone  112 . The cross-sectional area of a tube may, for example, be from about 1 mm 2  to several square centimeters, and the length of a tube may, for example, be from about 1 centimeter to more than 1 meter. 
     With the unique tubular geometry and resulting gas flows, embodiments effectively suppress parasitic deposition  600 , extend the lifetime of internal components  120  and achieve longer growth durations with higher growth rates. Further, embodiments shown in  FIG. 19  may be utilized with the flexible and elastic graphite cover member  150  to further suppress or control or manipulate parasitic deposition  600 . For this purpose, the thickness of the graphite cover member  150  may be from about 0.1 mm to more than about one millimeter, and the size of the cover member  150  may be less than about 1 cm 2  for covering a small component to more than 1000 cm 2  for covering larger components. 
       FIG. 19  illustrates an embodiment of a gas flow control component or insert  1900  that includes a rectangular outer tube  1910  and circular inner tubes  1920 ,  1930  and  1940 . Other embodiments of flow control components may include multiple tubes in different configurations. For example, referring to  FIGS. 20A and 20B , a multi-tube structure  2000  constructed according to another embodiment may include four circular tubes  2010 ,  2020 ,  2030 ,  2040  arranged around a central axis  2005 . The second tube  2020  is positioned within a space  2011  defined by the first tube  2010 , the third tube  2030  is positioned within a space  2021  defined by the second tube  2020 , and the fourth or innermost tube  2040  is positioned within a space  2031  defined by the third tube  2030 . 
     Similar to the structural configuration shown in  FIG. 19 , the embodiment illustrated in  FIGS. 20A and 20B  are structured so that HCl gas  141  flows through the space  2011  defined by the first or outer tube  2010 , Argon  142  and gallium chloride  134  generated by the reaction of gallium metal  132  and HCl gas  141  flows through the space  2021 . HCl gas  141  also flows through the space  2031  of the third tube  2030 , in which the fourth tube  1940  is positioned. Ammonia  143  flows through the space  2041  defined by the fourth tube  2040 . With this configuration, flows of HCl gas  141 , Argon  142 /gallium chloride  134 , and ammonia  143  are separated from each other as these gases flow through the flow control component  2000  and are then combined to react in the growth zone  112  to form gallium nitride  210  on the seed  222 . According to one embodiment, the fourth tube  2040  that may carry ammonia  143  is shorter than the third tube  2030  such that the nozzle of the fourth tube  2040  may be surrounded by HCl  141  gas within the space  2031  to prevent nozzle clogging by parasitic deposits. 
     Reactor embodiments were utilized to grow various low defect density single crystal gallium nitride layers  210  at fast growth rates. Following is a summary of tests utilizing reactor apparatus and method embodiments described above. For growth with embodiments, suitable seeds or substrates  222  include a gallium nitride layer on a sapphire substrate. The gallium nitride layer may be relatively thin (e.g., about 1 to 15 microns) and have a low defect density that is an average total density of treading dislocations that is less than 10 8  cm −2 , preferably less than 10 7  cm −2  and more preferably less than 10 6  cm −2 ). The uniformity of the thickness of the gallium nitride seed  122  may be better than 10%, and preferably less than 1%. The as-grown surface of the seed layer  122  may have a surface roughness (rms) of about 0.05 to 0.5 nm. The gallium nitride seed layer  122  may be undoped with a background impurity concentration less than 5×10 16  cm −3  and oxygen and carbon atomic concentrations less than 5×10 16  cm −3 , preferably below 1×10 16  cm −3 . The gallium nitride seed layer  122  may be deposited directly on the sapphire or on a buffer layer such as an AlN/AlGaN/GaN multi-layer buffer including layers having thickness from about 1 to 50 nm. Such a structure may be used to control and minimize strain and defects in the seed layer  222 . 
     For example, AlN/AlGaN super lattice structures may include AlN and AlGaN, e.g., from 10 to 90 mol. % of AlN, and the thickness of the layers may be about 1 to 100 nm and deposited as a buffer layer. The deposition temperature for this purpose may vary from 900 to 1100° C. Control of buffer composition/thickness and seed layer deposition parameters may allow control of bowing of the seed  222 . For example, bowing of 2-inch seeds  222  may be controlled from about 10 to 70 microns, and the resulting shape of grown gallium nitride  210  can be controlled such that gallium nitride  210  wafers have desired concave and convex shapes can be grown. 
     During certain growth test, the sapphire seed  122  that was utilized had a diameter of about two inches, but seeds  122  having other diameters may be utilized, e.g., three inch, four inch and six inch sapphire seed substrates  122 . The thickness of the sapphire seed  122  was about 440 microns, and may be about 0.2 mm to 3 mm and thicker as necessary. The surface orientation of sapphire of the seed  122  was 3 degrees off relative to the (0001) c-plane in m-direction. 
     During these growth tests, it was observed that certain misorientation angles produced the smoothest single crystal gallium nitride  210  surfaces at certain gallium nitride  210  deposition rates. For example, at a gallium nitride  210  deposition rate of about 1 micron per minute, the smoothest gallium nitride  210  layers were grown on sapphire seeds  122  with a misorientation angle of about 0.6 degree, whereas for a gallium nitride  210  deposition rate of about 20 microns per minute, smooth surfaces were observed on a sapphire seed  122  with a misorientation angle of about three degrees. Other crystallographic orientations, e.g., a-, r-, and m-plane seeds  122 , may also be suitable for growth at various growth rates. Further, single crystal gallium nitride  210  may be grown for sufficient durations to form a boule, and the boule may also have a corresponding crystallographic orientation 
     Gallium nitride boules  210  that were grown with embodiments had a length of about four millimeters and a diameter of about two inches. Grown boules were grown with growth durations of about 3.3 hours at a growth rate of about 1.2 mm/hr. The length of the gallium nitride  210  may vary to have lengths of several millimeters with different growth durations. For example, with reactor and method embodiments, a boule may be grown at a rate of about 20 microns per minute, i.e., greater than 1 millimeter per hour, and growth durations may exceed 100 hours. Further, embodiments were tested by growth of gallium nitride  210  on multiple seeds  122 . Embodiments may be used for growth of single crystal gallium nitride  210  at fast growth rates on multiple seeds  122 , e.g., more than six seeds  122 , e.g., more than 12 seeds. 
     Single crystal gallium nitride  210  grown using reactor and method embodiments was single crystal 2H—GaN, as determined by x-ray diffraction analysis. The grown gallium nitride  210  materials did not contain parasitic inclusions. The as-grown surface of the gallium nitride crystal  210  was smooth and free of pits and other macro-defects. Reflection high-energy electron diffraction (RHEED) analysis was performed to demonstrate that the surface of the gallium nitride  210  was single crystalline. The top surface of the gallium nitride  210  had Ga polarity, and the defect density was below 10 7  cm −2  and embodiments may be utilized for even lower defect densities, e.g., below 10 6  cm 2 . For grown gallium nitride materials,  210 , the full width at a half maximum (FWHM) of x-ray diffraction rocking curves measured in ω-scanning geometry was less than 300 arc seconds for both (00.2) and (10.2) GaN reflections. Background concentration Nd—Na for grown materials was in the range of 10 15 -10 17  cm −3 . 
     Although tests were conducted using gallium nitride to demonstrate HVPE reactor and method embodiments the same HVPE reactor configurations and methods may also be used to grow low defect density, single crystal layers and boules of other group III nitride materials including, for example, AlN and AlGaN. Further, grown materials may be doped to controllably provide GaN with n-type or p-type conductivity or semi insulating GaN. 
     Other tests also demonstrated effectiveness of embodiments. Desired material qualities were reflected in narrower widths of x-ray diffraction rocking curves. In this regard, x-ray rocking curve measurements can be performed for several x-ray diffraction reflections (peaks). Accepted reflections for group III nitride materials include the (00.2) symmetric and (10.2) asymmetric reflections based on w-scan geometry. Improved material qualities were also reflected in improved surface roughness and defect densities measured using etch pit method. 
     More specifically, single crystal gallium nitride materials  210  were growth with the reactor and method embodiments in which a HVPE reactor was equipped with an atmospheric-pressure horizontal hot-wall quartz reactor and two-zone resistively heated furnaces. A gas-handling system based on electronic mass flow controllers (MFCs) was used to for distribution and control of gas flows inside of the reactor. Ammonia  143  and HCl  141  served as input active gases, and metallic Ga (7N) and Al (5N) were used as sources  132  of group III materials. The volume of the Ga and Al metallic sources was up to 2.5 kg and may be increased to 5 kg and more. Growth was performed at temperatures ranging from about 900° C. to 1100° C. Argon  142  served as a carrier gas for delivering gallium chloride  134  to the growth zone  112 . 
     During a growth run, HCl gas  141  was passed over the Ga metal source  132 , and the reaction between metallic Ga  132  and HCl  141  resulted in gallium chloride reaction product  134  that was transported by Argon gas  142  to the growth zone  112 . In the growth zone  112 , gallium chloride  134  was mixed with ammonia  143  resulting in deposition of single crystal gallium nitride  210  on the seed  122 . 
     To increase the growth rate of gallium nitride  210 , active gases (Argon  142 /gallium chloride  134  and ammonia  143 ) were focused on the growing seed  122  surface utilizing the focusing element  1700  shown in  FIG. 17 . This was achieved by forming a “gas lens” of Argon gas  142  inside of the reactor tube. Faster growth rates were achieved by increasing the surface area of the Ga source  132  that reacts with the HCl gas  141  and increasing efficiency of the reaction. The total surface area of the Gallium metal source  132  exposed to HCl gas  141  was increased by increasing the number of Ga-containing boats  220  and increasing the open area of each boat  220 . During these tests, 5-10 boats  220  were utilized, and the flow of HCl gas  141  was about 10 to 400 cm 2  for different numbers of boats  220 . This test involved six channels or tubes  130  with Ga sources  132 . Separate tubes  130  with HCl gas  141  were directed to points of intense parasitic deposition in the gas mixing zone, prior to the growth zone  112 . 
     These HCl flows  141  served to reduce or eliminate the parasitic gallium nitride deposits  600 . For example, HCl gas flows  141  focused on the inlet of the supply of ammonia  143  and prevented parasitic deposition  600  that would otherwise clog the supply tube and allowed the supply tube remain clean and flowing during long-term growth. Elastic graphite materials  150  (e.g., as described with reference to  FIGS. 2-8 ) and graphite cartons were used to cover portions of internal quartz components  120  including seed holder  124  to prevent parasitic depositions on these internal components  120  and to prevent associated cracking (breakage) of these internal components  120 . During growth of single crystal gallium nitride  120 , the flow of HCl gas  141  was maintained at about 2.6 slm, the flow of ammonia gas  143  was maintained at about 6 slm and the flow of Argon gas  142  was maintained at about 18 slm. The linear gas flow rate in the growth zone  112  was about 20 cm/sec. After one hour of growth using HVPE reactor embodiments, and operating parameters, single crystal gallium nitride material  210  having a thickness of 1.4 mm was achieved representing a growth rate of 1.4 mm per hour, which is higher than known HVPE systems. 
     Further, for the gallium nitride material grown at this growth rate. The x-ray diffraction rocking curve width (FWHM) for ω-scanning measurements was about 100 to 400 arc seconds for the (00.2) GaN reflection and about 90 to 450 arc seconds for the (10.2) GaN reflections. The dislocation density of the grown gallium nitride  210  was from 10 4  to 10 6  cm −2 , and the grown gallium nitride  210  materials had mirror like surface. The surface roughness of the grown material  210 , measured by atomic force microscopy using 5×5 μm 2  scans, was about 0.5 to 5 nm. The background Nd—Na concentration measured on gallium nitride  210  surface using a mercury probe was about 1.5×10 17  to 9×10 17  cm −3 . Embodiments demonstrated stable growth on a large area substrate holder  124  that held four two-inch seeds  122 . No inclusions were detected in the grown gallium nitride  210  materials. 
     Other grown gallium nitride materials  210  had a full width at half maximum (FWHM) of an ω-scan x-ray rocking curve for grown gallium nitride was less than 500 arc seconds for the (10.2) peak and less than 300 arc seconds for a (00.2) peak and grown with a deposition rate exceeding 17 microns per minute. The resulting low defect density materials had dislocation densities less than 10 4  to 10 7 cm-2  and were grown at growth rates greater than 1 mm/hr, greater than 2 mm/hr and greater than 5 mm/hr. The thickness across the grown materials were uniform and varied by less than 1%, less than 5% and less than 10%, respectively. Additionally, material parameters including resistivity and optical transmission varied by less than 2-3%. Grown materials also exhibited uniform doping, which varied by less than 10%. The surface roughness of materials grown at rates greater than 1 mm/hour were confirmed to be better than 1 nm for a 5×5 atomic force microscopy scan. It was also confirmed that the materials that were grown were single crystal materials and did not include inclusions (e.g., due to parasitic deposition). 
     A similar method was applied to demonstrate growth of aluminum nitride using a metallic aluminum source. Further, growth of aluminum gallium nitride was demonstrated using both of gallium and aluminum meal sources. Commercial 2-inch, 3-inch, and 4-inch (100 mm) SiC and sapphire wafers were used as seed substrates  122 . For growth of AlGaN, HCl gas was passed separately over the Ga and Al metal sources, and the gallium and aluminum chlorides were carried by Argon gas  142  to the growth zone  112  where they were mixed with ammonia  143 , providing deposition of AlGaN solid solution layers on the seed  122 . In the grown materials, the [Al]/[Al]+[Ga] ratio in vapour phase determined AlGaN alloy composition. 
     Further tests demonstrated the effectiveness of HVPE reactor and method embodiments by growth of p-type bulk single crystal gallium nitride  210  as described with reference to  FIGS. 12-16 . Prior to doping, HCl  141  etching was used to eliminate magnesium memory effect on inner surfaces of internal components  120  including the tube  1210  that delivers a magnesium-containing gas to the growth zone  112 . 
     Magnesium impurities were supplied from and external Cp2Mg source  160 . Thick single crystal p-type gallium nitride materials  210  were grown and had Na—Nd concentrations exceeding 5×10 19  cm −3 , atomic Mg concentrations greater than 10 20  cm −3  and hole carrier concentrations at room temperature greater than 6×10 18  cm −3 . Hole mobility for grown materials ranged from 10 to 80 cm 2 /Vsec at 300 K as measured by the Hall method. Control of p-type doping within a range of doping parameters, e.g., from about 10 16  to 10 19  cm −3  was demonstrated by changing Argon gas  142  flows through the vessel holding the Cp2Mg  1207  and/or the magnesium-containing gas delivery tube  1210 . 
     These tests demonstrated growth of p-type single crystal gallium nitride  210  having a thickness or length ranging from several microns (e.g. about 0.1 to about 10 microns) to several millimeters (e.g., about 1 to about 4 mm) at high growth rates from about 1 micron per minute to more than 1 millimeter per hour, and even greater than 2 millimeters per hour, while maintaining low defect densities (e.g., about 10 6 - 10   8 cm-2 ).) Such capabilities were also demonstrated for long growth durations (e.g., more than 100 hours). P-type GaN wafers were fabricated after separation of grown GaN materials  210  from the seed  122 . 
     Dopant variations were less than 5%, and inverted domains, which are typical crystal defects within gallium nitride that is doped using Cp2Mg during MOCVD growth, were not observed. Grown gallium nitride  210  layers had smooth as-grown surface with rms roughness less than 1 nm measured by atomic force microscopy. RHEED characterization of the grown materials demonstrated that the as-grown surface is single crystalline. X-ray diffraction confirmed that the entire grown p-type gallium nitride was single crystal. It was observed that for grown p-type materials  210 , background impurity concentrations, including carbon background concentrations, were as low as background impurity concentrations in undoped GaN materials. Further, the hydrogen background atomic concentration was typically was several times (e.g., &gt;10×) less than magnesium atomic concentrations. 
     GaN materials  210  with sharp doping interfaces were grown, and the background concentration of an undoped GaN layer grown after highly Mg-doped layer in the same HVPE epitaxial process was in the 10 16  cm −3  range. 
     HVPE reactor and method embodiments may also be used to fabricate a GaN pn structures. For example, embodiments may also be applied to fabricate AlGaN and InGaN pn structures. 
     Grown structures utilized embodiments include thick single crystal gallium nitride  210  layers having controlled n-type doping and p-type doping. N-type doping may be performed using donor impurity, e.g., silicon. Similar processes may be done using other donor impurity such as oxygen, germanium and others. 
     To fabricate a structure, a first silicon doped n-type GaN layer having a thickness of about 12 microns may be grown on a seed  122 , e.g., a 4-inch sapphire with c-plane orientation and tilt angle of 0.6 degrees toward m-crystallographic direction. Other substrates such as Si, SiC, GaN, AlN and others may be also used. Two wafers were loaded at the same time. Low defect gallium nitride seeds  122  with a dislocation density less than 10 6  cm −2  are preferable and less than 10 4  cm −2  are more preferred. A gallium nitride layer  210  may be grown with at a deposition rate of 5 microns per minute. Doping concentration Nd—Na in the silicon doped layer may be about 5×10 18  cm −3 . This single crystal gallium nitride layer  210  may be formed to ensure low electrical resistivity of grown gallium nitride material. Other doping concentrations may be used still producing gallium nitride with low enough electrical resistivity. Doping reduction measurements may be acquired in situ to reduce dislocation density in grown layers. 
     When growth of this layer is completed, Si-containing gas flow may be reduced to grow a gallium nitride low doped region. Si doping concentrations can be controlled to ensure required reverse breakdown voltage of the pn structure. For example, for a reverse breakdown voltage of 10,000 V, the doping concentration can be Nd—Na of 5×10 15  cm −3  or lower values. The thickness of a low doped gallium nitride layer  210  may be controlled to withstand high voltage operation of the pn structure. For example, for 10,000 V pn structure, the thickness of the gallium nitride layer  210  may be about 100 microns, and the growth rate may be about 2 microns per minute. However, other growth rates may be used, e.g., about 1 to 20 microns per minute. 
     When a low doped GaN layer of sufficient thickness is reached, epitaxial wafers may be removed from the growth zone  112  to a growth interruption zone, and a p-type doping impurity may be introduced into the growth zone  112 . One suitable p-type doping impurity is an external Cp 2 Mg source  1207 , as described with reference to  FIG. 12 , or p-type doping. Gallium nitride materials  210  that are doped with magnesium with embodiments can have a Na—Nd concentration of about 10 19  cm −3 . P-type gallium nitride layers  210  may be grown with a growth rate that is controlled to be from about 0.01 to about 20 microns per minute. The thickness of grown gallium nitride  210  layers may be varied from 0.005 to 1000 microns. The surface roughness of grown structures, rms, is expected to be below 2 nm. The crystalline quality of grown gallium nitride  210  layers corresponds to the properties of gallium nitride layers described in previous embodiments. The thickness and doping uniformity are expected to be less than 10%. As a result, gallium nitride  210  epitaxial wafers with pn structures and a high breakdown voltage may be fabricated. 
     Magnesium doped gallium nitride materials  210  that were grown using HVPE reactor and method embodiments exhibit p-type conductivity. In most cases, no activation of acceptor impurity was used to form pn structure. Low resistivity Ohmic contacts were deposited top-type and n-type regions of the pn structures. The contacts were formed using e-beam metal evaporation in a high (better than 10 −6  Torr) vacuum. The contact resistivity was determined to be below 10 −3  Ohm cm 2  and 10 −6  Ohm cm 2  for respective p-type and n-type contacts to ensure low forward voltage drops for the diodes. Ni-based, Au-based, and Pt-based Ohmic contact metallizations were used. Mesa structures with a size from 10 −5  cm −2  to several square centimeters were fabricated using reactive ion etching. Ar-based, Fl-based and Cl-based plasmas were used for reactive ion etching. Reverse voltage of fabricated structures varied from 10 2  to 10 3  V depending on material doping and thickness. Pn structures having a reverse voltage exceeding 10 4  V may be grown by this approach. Higher operating voltages may be obtained using lower doped, thicker gallium nitride. The electric breakdown of fabricated structures had avalanche characteristics. The electric breakdown field was measured from 2.5 to 3.5 MV/cm. Leakage currents of fabricated structures were less than 10 −6  to 10 −9  A for reverse voltages less than 0.9×Vb, where Vb is a breakdown voltage. Surface protecting layers (field plates) were used to increase the breakdown voltage of the diodes. HVPE grown aluminum nitride, Si3N4 and other insulating layers may be used for mesa side wall protection. The turn-on voltage for gallium nitride pn junctions that were grown were about 2.8 V. The forward current at a forward voltage of 2 V was below 10 −6  A, and may preferably be below 10 −9  A. 
     Electro luminescence was observed when the diodes were operating at forward voltage exceeding 3 V. The color of the luminescence was determined by the radiative recombination processes in the light-emitting region of the pn structure ranged from ultraviolet to red. If the thickness of the light emitting region was sufficiently thin (typically thinner than 10 nm), quantum effect related recombination was observed in photo- and electro-luminescence spectra. 
     Although HVPE reactor and method embodiments are described with reference to GaN pn structures, embodiments may also be used to fabricate other pn structures including, but limited to, GaN/AlGaN, InGaN/GaN, and InGaN/AlGaN pn structures. Further, the same HVPE reactor and method embodiments may be applied to fabricate high voltage pn structures having several hetero junctions and pn-junctions, for example N-p-N, P-n-P, or N-p-n-P structures (for example p-AlGaN/n-GaN/-p-InGaN/-n-AlGaN). Low doped regions of pn structures fabricated with embodiments may have n-type or p-type conductivity. 
     Although particular embodiments have been shown and described, it should be understood that the above discussion is not intended to limit the scope of these embodiments. Various changes and modifications may be made without departing from the scope of the claims. 
     For example, it should be understood that the flexible cover member, such as a graphite sheet, may be capable of multiple uses depending on which internal components are covered. For example, there may be instances when the flexible cover member is use to receive parasitic deposition of group III nitride material. There may be other times when there is no parasitic deposition, but the flexible member protects internal components from chemically active substances such as chemically active gases. Further, there may be other times when the flexible member is used to both receive parasitic deposition and also protect internal components from chemically active substances. The use the flexible member may depend on, for example, the location within the reactor of the flexible member, the stage of HVPE processing, and the substances, chemicals or gases that are utilized. 
     As a further example, it should be understood that embodiments may be implemented within vertical and horizontal reactors, and the seed upon which single crystal group III nitride materials are grown may be positioned parallel to or perpendicular to gas flow. Further, although various figures illustrate a reactor and a single seed, it should be understood that embodiments can be implemented using multiple seeds and substrates, for simultaneously growth of multiple crystals. 
     Additionally, although the structure and functionality of embodiments are described individually and certain embodiments are described in combination with other embodiments, it should be understood that embodiments may be used individually or in combination with one or more other embodiments depending on the system configuration utilized and processing needs. For example, HVPE apparatus and method embodiments may involve a cover member, such as graphite felt or paper cover member, and an external non-metallic magnesium dopant source. 
     A cover member may also be used in combination with embodiments that involve exchanging or replacing one or more internal components or the cover member, e.g., after parasitic deposition of material. Embodiments may also involve use of a cover member in combination with a gas focusing element or lens for preventing flow of reactive gases to an inner surface of a main reactor tube. Embodiments may also involve an external non-metallic source and a gas focusing lens. Moreover, in another embodiment, a HVPE apparatus utilizes a cover member, an external non-metallic magnesium source and a focusing element or lens. The structural configurations described above show how embodiments may be implemented. It should be understood, however, that other combinations of components of embodiments may be utilized for different applications. 
     Embodiments may also be utilized for growth of various group III nitride materials including, but not limited to, AlN, AlGaN, InN and InGaN, and that various other impurities other than magnesium may be utilized for doping including, but not limited to, zinc and iron. Further, materials grown using embodiments may be n-type and p-type materials. 
     Although embodiments are described with reference to a GaN material, embodiments can also be utilized to fabricate a semiconductor structure including multiple layers. 
     Thus, embodiments are intended to cover alternatives, modifications, and equivalents that may fall within the scope of the claims.