Abstract:
The invention provides for growing semiconductor and other crystals by loading a vessel in its lower portion with a seed crystal, loading a charge thereon in the vessel, heating the charge to a molten state and electromagnetically stirring the melt using magnetic and electric fields to obtain a more uniform composition of melt and slowly reducing the temperature of the melt over the crystal to grow a more uniform crystal from such stirred melt.

Description:
RELATED APPLICATIONS 
   This application relates to provisional application 60/285,914, filed 24 Apr. 2001, from which domestic priority is claimed. This application is a Divisional of application Ser. No. 10/131,458, filed on 23 Apr. 2002, now U.S. Pat. No. 6,849,121 having the same title. 

   STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 

   FIELD OF THE INVENTION 
   This invention relates to controlled growth of crystals, particularly for growth of more uniform crystals. 
   BACKGROUND OF THE INVENTION 
   The favored research approach to achieve ultra high-quality compound semiconductor crystals is the bottom-seeded method—either the Vertical Gradient Freeze (VGF) or Bridgman technique. There are clear advantages of these methods over the more common top-seeded Czochralski (CZ) method, However, in some cases the crystal growth rate can be severely limited and in others the composition of the crystal can be non-uniform. 
   In the above bottom-seeded method, seed crystal  2  is positioned under growth crystal  4 , in turn under melt  6 , in vessel  8  as shown in  FIG. 1 . 
   In the VGF and Bridgman methods of crystal growth ( FIG. 1 ), solidification is initiated either from a seed or from spontaneous nuclei at the bottom of a molten charge as opposed to the CZ method in which a seed is dipped into the melt from the top. Convective stirring due to thermally driven buoyancy, that is found in CZ melts, is not present in bottom-seeded melts, where the thermal gradient increasing from the bottom to the top of the melt provides thermal stability. This absence of strong convection, in fact, provides some of the advantages of the bottom-seeded methods over the CZ method (fewer dislocations and lower twinning probability). However, the absence of melt convection also generates the melt condition that limits the growth rate and causes compositional non-uniformity. 
   Generally the chemical composition of a solid is not precisely the same as that of the melt from which it freezes. This is known as non-congruent melting, as opposed to congruent melting, in which the composition of the solid and liquid phases are identical. For alloy crystals such as Ga 1-x Al x Sb, for example, the solid composition can be very different from that of the melt. Therefore during solidification one, or more of the constituent elements is rejected into the melt to form a boundary layer of liquid with a chemical make-up that is different from that of the bulk of the melt. This boundary layer builds up just adjacent to the crystal-melt interface. Strong convective stirring due to thermally driven buoyancy, is not present in bottom-seeded melts because the top is hotter than the bottom. Therefore the most effective transport mechanism in the boundary layer of a VGF melt is diffusion, which tends to be quite slow. This slow rate of diffusion determines the crystal growth rate limit. Typical growth rates for bottom-seeded melts are nearly an order of magnitude less than those for top-seeded CZ growth and therefore the cost of producing VGF crystals is greater. In addition, if there is virtually no mixing in the melt, the composition of the grown crystal can exhibit radial non-uniformity if the melt composition is not initially uniform from the center to the periphery. 
   Accordingly there is need and market for an improved growth process for the above alloy crystals that overcomes the above prior art shortcomings. 
   There has now been discovered a process for more controlled crystal growth to obtain more uniform crystals; both alloy crystals and non-congruently melting binary crystals. 
   SUMMARY OF THE INVENTION 
   Broadly, the present invention provides a method for growing a more uniform crystal by bottom seeding which includes, 
   a) loading a vessel, in its lower portion with a seed crystal, 
   b) adding a charge thereon in said vessel, 
   c) heating the charge to a molten state to form a melt, 
   d) electromagnetically stirring the melt to promote uniformity over the crystal and 
   e) slowly reducing the temperature of the melt over such crystal to grow the latter. 
   The invention also provides more uniform crystals as grown by the above process. 
   Examples of crystal growth methods with electromagnetic stirring are found in the prior art, e.g., in U.S. Pat. No. 5,769,944 to Park et al (1998) and in a Paper entitled  Silicon Crystal Growth by the Electromagnetic Czochralski  ( EMCZ )  Method , Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L10-L13, by M. Watanabe et al. which, however, relate to the CZ or top seeding method of crystal growth, with the attendant problems of non-uniform composition and higher defect density due to convection and the steep temperature gradient. Thus the prior art above does not provide for forming a uniform alloy crystal. 
   DEFINITIONS 
   By “precursor charge”, as used herein, is meant a charge that can be heated to a melt that can grow, upon cooling, into a crystal of desired composition 
   By “crystal”, as used herein, is meant a sizeable (up to 2-inch diameter or more) solid body, with the same crystallographic structure and orientation. 
   By “slowly reducing the temperature of the melt”, as used herein, is meant to decrease linearly with time the temperature of the liquid above the growing crystal by reducing the heat emanating from various heaters. 
   By “vessel”, as used herein, is meant a container, a crucible of carbon, of quartz, of boron nitride or any other material that will not react with the melt. 
   By “alloy”, as used herein, is meant a mixture or combination of two or more constituents in a crystal whose melting temperature and crystallographic properties are determined by its composition. 
   By “dopant”, as used herein, is meant an additional minor element mixed with the major constituents, which alters some crystal properties, but does not significantly alter the melting temperature. 
   By “uniform”, as used herein, is meant having a constant composition (alloy or dopant) throughout the crystal body or nearly so. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more apparent from the following detailed specification and drawings in which; 
       FIG. 1  is a perspective, schematic view of a vessel for un-enhanced crystal growth per the prior art; 
       FIG. 2  is a perspective schematic view of an embodiment for enhanced crystal growth per the present invention; 
       FIG. 3  is an isometric vector diagram relative to  FIG. 2 ; 
       FIG. 4  is a perspective schematic view of another embodiment of the invention and 
       FIGS. 5&amp;6  are elevation schematic views, partly in section, of yet another embodiment of the invention for enhanced crystal growth. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring in more detail to the drawings, the present invention utilizes magnetic and electric fields to provide stirring during the growth of semiconductor crystals from bottom-seeded melts. One procedure for growing a crystal with such stirring, is to load a cylindrical crystal growth crucible  11  that is closed at the bottom and open at the top with a single seed crystal  12  at the bottom and a charge of one or more chunks or particles of a solid (at room temperature) on top of it (not shown). The so loaded crucible is then heated by a helical coil  22 , so that the temperature increases from bottom to top of the charge, until all of the charge and a small portion of the seed  12 , become molten to form melt  14 , as indicated in  FIG. 2 . 
   A small-diameter electrode  16  made of electrically conducting material that does not chemically react with the molten charge is inserted vertically into the center of the melt so that it extends to a point just above the seed crystal  12  but does not touch it. If the crucible is electrically conducting, it serves as the second electrode  17  and a voltage source  18  is connected between the two. In this case, if the melt  14  is a good conductor, an electric current will flow radially from (or to) the center electrode  16 . A magnetic field B (per  FIG. 3 ) is induced in the melt in the vertical direction by a magnetic coil or solenoid  21 , as indicated in  FIG. 2 . The magnetic field B, perpendicular to the current I, will induce a tangential force F (per  FIG. 3 ) and under many conditions, create a tangential flow  20  (per  FIG. 2 ) and cause mixing in the melt. 
   Alternatively the coil  22  can be the same one that serves as a heat source around the vessel. A magnetic field of about 50 gauss generated directly with a heating coil together with a current of several tens of amperes can be sufficient. 
   In one example, it has been demonstrated that for a two inch diameter melt of GaSb, about 5 amperes of current together with a magnetic field of 50 gauss produce melt rotation rates of about 15 RPM. 
   To grow the crystal, the temperature is slowly reduced so that the melt is progressively crystallized onto the seed. As the height of the crystal interface  23  increases, the electrode  16  is slowly raised, ideally at a rate equal to the crystal growth rate to, e.g., height of interface  24  per  FIG. 2  (or higher). If desired, the melt stirring can be somewhat confined to an area near the growth interface, e.g.,  23  to  24 , by utilizing an electrical insulator  15  over the upper length of the center electrode, per  FIG. 2 . 
   Also per  FIG. 4 , if the crucible  13  is not electrically conducting, a thin-walled cylinder  26  of conducting material or one, two or more small diameter electrodes (not shown, but similar in size to electrode  19  in  FIG. 4 ) near the periphery of the melt can be deployed within the vessel. If the melt is volatile, an inert encapsulant can be employed and the growth takes place in a high-pressure chamber (not shown). 
   Electromagnetic stirring can also be employed to improve the quality and increase the growth rate of crystals grown by a bottom-seeded technique in which a submerged heater  32 , in vessel  30 , per  FIGS. 5&amp;6 , is employed to promote good long-range chemical uniformity. A schematic cross-sectional elevation of the crystal growth apparatus  30  is shown in  FIGS. 5&amp;6 . The submerged heater  32 , in its housing  52 , effectively isolates two melt zones (upper and lower) during growth, to provide a constant alloy composition in the lower zone and thus also in the crystal to be formed. The system utilizes coil ( 34 ) and planar resistance heating elements ( 36 ) as shown. A fused silica crucible  38  contains a large-diameter hollow-core upper charge to supply melt  40  and a smaller diameter lower charge to supply melt  42  as well as a full-diameter seed crystal  44 . The graphite electrodes (including the small diameter electrode  46  in the center, and the outer electrode  48  surrounding the growing crystal) are used to pass the radial current which stirs the lower melt  42  when a vertical magnetic field is also applied in the manner of  FIGS. 2&amp;3  above. 
   As shown in  FIG. 5 , a copper wire coil  50 , wrapped on the outside of a lower insulating tube (not shown) provides the magnetic field. At the beginning of a growth run, the upper and lower charges are made molten or melts  40 &amp; 42 , by applying power to the heaters; i.e., the coil “side heaters”  34 , the “submerged heater  32 ”, and the “external lower heater  36 ”. A small fraction of the seed  44  is also melted just prior to growth. To grow the crystal, the temperature of the BN disc  36 , just below the seed  44 , is ramped down with a temperature controller (not shown) connected to the external lower heater. At the same time, the submerged heater housing  52  is slowly raised, and the temperatures of the side heaters are ramped down as well. As the crystal grows, per  FIG. 6 , molten material flows from the upper melt  40  to the lower one  42 , through small holes in the outer electrode tube and down through the annular space between the outer electrode  48  and the submerged heater housing  52 , as indicated in  FIGS. 5&amp;6 . The aim is to maintain a constant lower melt height of the order of, e.g., 1 cm while the crystal grows upwardly from the seed. A drawing of the system after some growth of crystal  56  has taken place, is shown in  FIG. 6 . Since the lower melt  42  is replenished with liquid from the upper melt  40  through the annular space  54 , if there were no mixing, the liquid composition of the annulus  54  could be quite different from that of the remainder of the lower melt  42 . In this case, stirring the molten melt  42  below the submerged heater  32  with electric and magnetic fields can provide a more uniform radial composition both in the melt  42  and in the crystal  56 , in addition to permitting a more rapid growth rate. 
   Thus the present invention employs magnetic and electric fields to provide stirring during crystal growth where uniform composition and homogeneous properties are required. The lack of stirring is a chronic problem for crystals that are grown from bottom-seeded melts. In the present invention, the melt is mixed by the Lorenz force arising when an axial magnetic field is applied together with a radial electric current. This novel technique is applied to solve a chronic problem associated with many semiconductor crystals and alloy crystals, which can be of great importance militarily or commercially. The problem has been that, in commercial practice, low defect density crystals can only be obtained by bottom-seeded methods, but these methods are very slow and lacking in uniformity of crystal properties. The bottom-seeded method of the present invention overcomes the above disadvantages and there is a need for crystals with low defect density and uniform properties such as GaSb for IR transparent windows for IR imaging arrays. Such crystals do not exist at present. The method of the present invention offers the following advantages: 
   1. Faster growth rates 
   2. Controlled crystal properties 
   3. Uniform alloy composition and doping concentration 
   The advantages of the electromagnetic stirring of bottom-seeded crystal growth melts are realized through the changes that stirring creates in the boundary layer above the growing crystal. Since the crystal is frozen from the liquid in this layer, its radial uniformity depends directly on the chemical uniformity of such layer, and stirring can improve it. 
   A value of this invention militarily is that new high-quality crystal substrates will become available for high-speed photonics and advanced micro-electronic circuits. Prior to this invention, the majority of semiconductor crystals have been produced commercially by top-seeded growth methods, which have typically high defect densities and are not suitable for advanced applications such as long wavelength IR lasers and detector arrays. 
   A benefit of this invention is to improve the quality and reduce the cost of compound melt-grown bulk semiconductor crystals by electromagnetic stirring. Such crystals are generally sliced into wafers and used as substrates for epitaxial growth or for ion implantation. These crystal wafers are the building blocks for structures that enable the fabrication of virtually every electronic and optical system being produced or in development. 
   The type of crystals grown herein include crystals of alloy crystals such as Ga 1-x Al x Sb, or In 1-x Ga x P, for example or any combination of mixed Group III and Group V elements of the periodic table. In general, any incongruently melting crystal material can be grown by this method.