Abstract:
A crucible for growing III-nitride (e.g., aluminum nitride) single crystals is provided. The crucible includes an elongated wall structure defining an interior crystal growth cavity. The crucible includes a plurality of tungsten grains and a wall thickness of at least about 1.5 times the average tungsten grain size. In particular embodiments, the crucible includes first and second layers of tungsten grains the first layer including grains forming an inside surface thereof and the second layer being superposed with the first layer. The crucible may be machined from a bar, plate, or billet of powder metallurgy tungsten.

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/323,947, entitled Powder Metallurgy Tungsten Crucible for AlN Crystal Growth, filed Sep. 21, 2001. 
    
    
     Part of the work leading to this invention was made under a United States Government STTR Contract (N00014-97-C-0362). The U.S. Government may have certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to III-nitride semiconductors and more particularly to crucibles for growing III-nitride semiconductor substrates such as aluminum nitride (AlN). Use of the crucible of this invention may enable the growth of relatively large and high quality III-nitride single crystals. 
     (2) Background Information 
     Wide bandgap semiconductor devices, based on III-nitride semiconductors, are expected to find application in several opto-electronic technologies in the areas of short wavelength emission and detection. Aluminum nitride and high aluminum concentration alloys of aluminum nitride with gallium nitride and/or with indium nitride are potentially important III-nitride semiconductors for producing deep-UV light emitting diodes with potential applications including solid-state white lighting, sterilization and disinfectant devices, compact analytical devices for the biotechnology and pharmaceutical markets, bioagent detection systems, compact uv light sources for covert communication by the Department of Defense, and for short wavelength lasers for high density data storage. In addition, single-crystal substrates of aluminum nitride are attractive for the fabrication of III-nitride semiconductor, high power radio frequency, millimeter wave, and microwave devices needed for future wireless communication base stations and for Department of Defense applications. However, one of the factors limiting the maturation of aluminum nitride, and other III-nitride, technology has been the absence of high-quality bulk nitride substrates. 
     One promising method for the growth of aluminum nitride single crystals for such substrates is the sublimation-recondensation technique first developed by Slack and McNelly (“Growth of High Purity AlN Crystals”,  J. Cryst. Growth  34, 263 (1976) and “AlN Single Crystals”,  J. Cryst. Growth  42, 560 (1977)), both of which are fully incorporated by reference herein. However, one of the drawbacks that limited the maximum size of the crystals was the development of leaks in the tungsten crucibles, which ultimately lead to the failure thereof, through which aluminum gas may escape. The development of crucibles that substantially eliminates this problem may provide for the growth of relatively large aluminum nitride single crystals. 
     Therefore there exists a need for an improved crucible for growth of III-nitride semiconductor single crystals and in particular AlN single crystals. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention includes a sealable tungsten crucible for growing a III-nitride semiconductor crystal. The crucible includes an elongated wall structure extending in a longitudinal direction. The wall structure defines an interior crystal growth cavity and includes a plurality of tungsten grains. The wall structure has a thickness dimension extending in a direction substantially perpendicular to the longitudinal direction, the thickness dimension being at least about 1.5 times the average tungsten grain size. 
     In another aspect, this invention includes a sealable tungsten crucible for growing a III-nitride semiconductor crystal. The crucible includes an elongated wall structure extending in a longitudinal direction. The wall structure defines an interior crystal growth cavity and includes a plurality of tungsten grains. The grains effectively form at least first and second layers, the first layer including grains disposed on an inside surface of the crucible and the second layer being adjacent to the first layer. 
     In still another aspect, this invention includes a method for fabricating a tungsten crucible for use in growing aluminum nitride single crystals. The method includes providing a bar of powder metallurgy tungsten and machining an elongated wall structure extending in a longitudinal direction. The wall structure defines an interior crystal growth cavity and includes a plurality of tungsten grains. The grains effectively form at least first and second layers, the first layer including grains disposed on an inside surface of the crucible and the second layer being adjacent to the first layer. 
     In yet another aspect, this invention includes a method for fabricating an aluminum nitride single crystal. The method includes providing a tungsten crucible having an elongated wall structure extending in a longitudinal direction; the wall structure defining an interior crystal growth cavity and including a plurality of tungsten grains, which effectively form at least first and second layers, the first layer including grains disposed on an inside surface of the crucible and the second layer being adjacent the first layer. The method further includes charging the crucible with aluminum nitride, sealing the crucible, and heating at least a portion of the crucible to a temperature in excess of about 2000 degrees C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional schematic representation of a prior-art thin wall tungsten crucible; 
     FIG. 2A is a cross-sectional schematic representation of a portion of the crucible of FIG. 1 prior to grain growth; 
     FIG. 2B is a cross-sectional schematic representation of the portion of the crucible shown in FIG. 2A after exposure to Al vapor at growth temperatures; 
     FIG. 3A is a view similar to that of FIG. 2A, of a portion of a tungsten crucible of this invention prior to grain growth; and 
     FIG. 3B is a cross sectional schematic representation of the portion of the crucible shown in FIG. 3A after exposure to Al vapor at growth temperatures. 
    
    
     DETAILED DESCRIPTION 
     Referring briefly to FIGS. 3A and 3B, the present invention includes a crucible that may be useful in growing relatively large aluminum nitride and/or other III-nitride single crystals for use in semiconductor applications. The tungsten crucible  100  of this invention includes a wall structure having a thickness  126  that is at least about 1.5 times, and preferably greater than about 3 times, that of the average tungsten grain  122 . In one embodiment, the tungsten crucible  100  is formed using powder metallurgy techniques. 
     The crucible of the present invention may advantageously provide for the growth of relatively large III-nitride single crystals, and in particular may provide for the growth of AlN single crystals having a diameter in the range from about 20 to about 50 mm or greater. AlN crystals of this size may be advantageously used to fabricate relatively high quality nitride semiconductor devices with relatively better yield than prior-art approaches. These and other advantages of this invention will become evident in light of the following discussion of various embodiments thereof. 
     Referring now to FIGS. 1-3B, prior art and the apparatus and method of the present invention are described. Referring to FIG. 1, prior-art crucible  10  typically includes a cylindrical body portion  12  and a tapered conical end portion  14  and is used in a tungsten tube furnace such as that shown in the above-referenced Slack and McNelly (“AlN Single Crystals”). To grow AlN crystals using the sublimation-recondensation technique of Slack and McNelly, the sharp tip  16  of the crucible is placed in the nominal center of the furnace at the start of run with a charge of AlN (e.g., about 5 grams) placed at the opposite end  18  of the crucible  10 . The crucible  10  is rotated (e.g., at about 2 revolutions per hour) about its longitudinal axis  15  and simultaneously pushed through the hot zone (i.e., the center) of the tube furnace (with end  18  moving towards tip  16 ) at a rate of about 0.3 mm per hour. The center of the furnace is held at a temperature of about 2250 degrees C. during the duration of the run. The total running time may be on the order of 100 hours or more. 
     Referring now to FIG. 2A, these conventional crucibles  10  are formed by a chemical vapor deposition process (CVD). Crucibles  10  formed by a CVD process (e.g., using either the conventional tungsten fluoride or tungsten chloride processes) may be characterized as having a columnar grain structure in which the W grains  22  are disposed substantially perpendicular to the inner wall  24  of the crucible  10  and extend substantially the entire thickness  26  of the crucible wall  20  (i.e., from the inner wall  24  to the outer wall  25 ). Referring now to FIG. 2B, and as briefly described hereinabove, crucible  10  tends to develop pinhole leaks  30  at the grain boundaries  32  (FIG. 2A) thereof, which disadvantageously permit aluminum vapor to escape from the interior of the crucible  10 . The pinhole leaks  30  also generally lead to crucible failure which thus limits the maximum achievable crystal size. 
     This problem may be circumvented, in theory, by employing single-crystal tungsten crucibles. However, such a solution is relatively difficult to implement and prohibitively expensive for commercialization. 
     One aspect of this invention is the realization that the above-described pinhole leak problem may be related to swelling of the tungsten grains  22  caused by the absorption of aluminum during use. Measurements conducted by the inventors have shown that after growth at 2330 degrees C. the tungsten crucible contained 5.3 atom percent aluminum (which is approximately equivalent to the saturation value of aluminum in tungsten at that temperature). At this aluminum concentration, the lattice constant was measured to decrease slightly to 3.162 angstroms from 3.165 angstroms for pure tungsten (with aluminum atoms substitutionally replacing tungsten atoms in the lattice). Thus the absorption of aluminum should theoretically result in swelling of slightly less than 5.3 volume percent, which was corroborated by an observed swelling of about 5 volume percent. 
     Not wishing to be bound by a particular theory, it is believed that the swelling results in pinhole  30  formation, particularly at the triple points where three tungsten grains  22  meet. Since the grains  22  of the CVD tungsten crucibles  10  are columnar and extend the thickness  26  of the crucible wall  20 , the pinholes  30  likewise extend the thickness  26  of the crucible wall  20 . 
     Referring now to FIGS. 3A and 3B, a cross sectional schematic illustration of the tungsten grain  122  structure of the crucible  100  of the present invention is illustrated. Crucible  100  is substantially similar to crucible  10  (FIG. 1) in that it may include a cylindrical body portion  12  (FIG. 1) and a tapered conical end portion  14  (FIG.  1 ). Crucible  100  differs from that of crucible  10  in that it utilizes a granular, multigrain structure in which the thickness  126  of the crucible wall is at least 1.5 times that of the average grain diameter. The grain structure of this invention may also be thought of as including at least two layers, a first layer  144  adjacent to (or forning) the inner wall  124  of the crucible  100  and a second layer  145  adjacent to the first layer  144 . While not wishing to be bound by a particular theory it is believed that the present invention uses the normally deleterious effects of the grain swelling to advantage, to nominally block grain boundary diffusion. It is believed that once swelling has occurred along grain boundaries  132  of the first grain layer  144 , further diffusion is severely hampered by the second grain layer  145 , grains of which have swollen to effectively close off any diffusion pathways or pinholes along the grain boundaries of the first layer  144 . Thus, any further diffusion of aluminum into the walls of the crucible generally occurs only by relatively slow bulk diffusion. Further, the three-dimensional, substantially random arrangement of the grains  122 , as illustrated in FIGS. 3A and 3B, is believed to slow the formation of pinholes in any of the layers. Nevertheless, aluminum transport through any pinholes that may be formed in the first layer  144  of tungsten grains  122  is nominally sealed off by the second layer  145  (or subsequent layers) thereof. 
     One method of implementing this invention is to machine crucibles from a tungsten bar formed using powder metallurgy techniques. These powder metallurgy bars include tungsten grains having substantially no columnar grain structure. As stated above, this structure is believed to reduce the penetration rate of aluminum along the grain boundaries and its subsequent escape from the crucible, particularly after the tungsten grains have swelled due to the uptake of Al by bulk in-diffusion. Crucibles formed by the above described powder metallurgy technique have been demonstrated to exhibit a significantly longer life than would be expected simply by the increased wall thickness, indicating that the granular structure slows the permeation of aluminum through the crucible walls. 
     The artisan of ordinary skill in the art will readily recognize that there may be alternate approaches to forming a multilayered tungsten crucible wall. One such possible approach may be to fabricate crucibles using multiple CVD steps while using an agent between steps that disrupts further columnar growth of the individual tungsten grains. 
     The crucible of the present invention may be useful in fabricating III-nitride single crystals such as aluminum nitride for the electronics and opto-electronics applications. For example, the inventive crucible may be used in a tube furnace, such as that disclosed by Slack and McNelly as referenced hereinbove. The crucible is typically first charged with relatively pure aluminum nitride (e.g., greater than about 99%) and then sealed before inserting into the tube furnace. At least a portion of the crucible is heated to a temperature in excess of about 2000 degrees C. at which aluminum nitride crystal formation is favored. 
     The crucibles of the present invention have been demonstrated to successfully grow AlN crystals having a diameter of up to 20 mm and greater. Moreover, the powder metallurgy tungsten crucibles of this invention may be used to grow AlN crystals having a diameter of up to 50 mm or greater. As described hereinabove, AlN crystals of this size may be advantageously used in the fabrication of relatively high quality nitride semiconductor devices with relatively better yield than prior approaches. 
     Specific exemplary embodiment(s) have been disclosed herein in which a multilayered and/or three-dimensional nominally random tungsten grain structure is produced resulting in the boundaries between tungsten grains being nominally completely blocked owing to the swelling generated by aluminum absorption. In light thereof, the skilled artisan will readily recognize that any approach to tungsten crucible construction that creates multiple layers of relatively dense tungsten grains, may be considered within the spirit and scope of this invention. 
     Other embodiments are also envisaged in which tungsten crucibles are used with metals other than aluminum. For example, Be, Cr, Pd, Ti, Pt, Rh, Ru, and V each have high temperature solubilities in tungsten (S. V. Nagender Naidu and P. Rama Rao, Editors,  Phase Diagrams of Binary Tungsten Alloys , Indian Institute of Metals, Calcutta, (1991)). Thus they would be expected to swell the tungsten grains and nominally stop further grain boundary diffusion in the crucible of this invention. Metals with somewhat lower solid solubility in tungsten (such as Fe) may also be effectively contained by grain swelling although the solid solubility of Fe in tungsten is less than three percent. It may also be possible to fabricate crucibles out of metals other than tungsten in which the solid solubility of the metal contained by the crucible causes the grains of the crucible to swell and thus impede further grain boundary diffusion of the metal contained therein. 
     The modifications to the various aspects of the present invention described hereinabove are merely exemplary. It is understood that other modifications to the illustrative embodiments will readily occur to persons with ordinary skill in the art. All such modifications and variations are deemed to be within the scope and spirit of the present invention as defined by the accompanying claims.