Method for crystal growth of a metal-nonmetal compound using a metallophobic-metallophilic surfactant and a thin metal wetting layer

#The disclosed technique relates to crystal growth, in general, and to methods and systems for growing crystals of a metal-nonmetal compound using surfactants, in particular.

#Methods for growing crystals from a liquid melt are known in the art. For example, U.S. Pat. No. 7,097,707, issued to Xu, entitled “GaN boule grown from liquid melt using GaN seed wafers” is directed to methods for making single crystal GaN boules. A first method comprises the procedures of contacting a GaN seed wafer with a GaN source environment under process conditions. The process conditions include a thermal gradient in the GaN source environment for producing growth of gallium nitride on the GaN seed wafer, thus forming the GaN boule. The source environment can be selected from a gallium melt and a nitrogen source or a supercritical ammonia containing solubilized GaN.

#A second method comprises the procedures of providing a gallium melt and contacting a GaN seed wafer with the gallium melt in the presence of a nitrogen source and under a thermal gradient. This produces the growth of gallium nitride on the GaN seed wafer, thereby forming a GaN boule. The GaN seed wafer is attached to a rotatable rod. The rotatable rod is rotated, thus rotating the GaN seed wafer, while pulling the rod and the GaN seed wafer from the gallium melt during the growth of the GaN boule. The nitrogen source comprises a nitrogen plasma including atomic nitrogen, nitrogen ions and dinitrogen ions. Also an ambient environment is formed of the gallium melt and the GaN seed wafer. The nitrogen plasma is generated by a discharge technique selected from direct current discharge, radio frequency discharge and microwave discharge. The temperature of the gallium melt is from about 900° C. to about 1500° C. A GaN crust is formed on a surface of the gallium melt, from the reaction between the nitrogen source and the gallium melt. The thermal gradient comprises a temperature which is higher at the GaN crust than at the GaN seed layer, whereby gallium nitride is transported from the crust to the growth of gallium nitride on the GaN seed wafer via dissolved atomic nitrogen in the gallium melt. The GaN in the crust is decomposed into atomic nitrogen with an equilibrium concentration at the temperature at the crust. The atomic nitrogen equilibrium concentration is at supersaturation relative to the temperature at the GaN seed wafer, thus producing homoepitaxial growth of GaN at the seed wafer.

#The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

#FIG. 1 is a schematic illustration of a method for growing crystals of a metal-nonmetal compound from a surfactant, operative in accordance with an embodiment of the disclosed technique;

#FIG. 2A is a first schematic illustration of the atomic structure of a metal-nonmetal compound crystal grown using the method of FIG. 1, constructed and operative in accordance with another embodiment of the disclosed technique;

#The disclosed technique overcomes the disadvantages of the prior art by providing a novel crystal growth method for growing high quality metal-nonmetal compound crystals from a surfactant which rests above a thin liquid metal wetting layer. According to the disclosed technique, a thin liquid metal wetting layer is provided with nonmetal atoms or metal-nonmetal particles which do not dissolve in the thin liquid metal wetting layer. The nonmetal atoms and particles combine with metal atoms and particles of the thin liquid metal wetting layer and adsorb on the upper surface of the thin liquid metal wetting layer thus forming a surfactant monolayer of metal-nonmetal molecules which includes a specific orientation having a metallophobic side and a metallophilic side. When brought in close proximity to a seed crystal which is to be grown into a crystal, a physical mechanism enables the metal-nonmetal molecules of the surfactant monolayer to jump, tunnel or diffuse and couple with the seed crystal. A plurality of such jumps will form an epitaxial layer of the metal-nonmetal molecules on the seed crystal, thus increasing its volume and substantially growing a metal-nonmetal compound crystal on the seed crystal. Regulating the distance between the growing seed crystal and the upper level of the thin liquid metal wetting layer, with a continuous supply of metal-nonmetal molecules in the surfactant monolayer, will epitaxially grow the seed crystal into a metal-nonmetal compound crystal.

#The disclosed technique relates to a general method for crystal growth and is limited to crystals grown from compound materials. In general, crystals grown using the disclosed technique are grown from a precursor compound that includes at least one metal and at least one nonmetal, herein abbreviated and referred to as an MN compound. In the description of the disclosed technique, the term ‘metal’ is used to denote any element in the periodic table of elements classified as either an alkali metal, an alkaline earth metal, a transition metal, a lanthanide element, an actinide element or an other metal. In the description of the disclosed technique, the term ‘nonmetal’ is used to denote any element in the periodic table of elements classified as a nonmetal or a halogen, such as hydrogen (H), carbon (C), nitrogen (N), oxygen (O), phosphorus (P), sulfur (S), selenium (Se), fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). As is known to the worker skilled in the art, the following elements in the periodic table of elements are classified as metalloids: boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po). Metalloids exhibit certain properties which are similar to both metals and nonmetals. As such, in the description of the disclosed technique, any element classified as a metalloid can be considered a metal or a nonmetal depending on which other element or elements it is bonded with. For example, in the compound germanium silicide (GeSi), Ge acts as a metal and Si acts as a nonmetal. However, in the compound silicon carbide (SiC), Si acts as a metal, with C being the nonmetal. Examples of MN compounds that can be used as a precursor compound include gallium nitride (GaN), aluminum oxide (Al2O3), cadmium zinc selenide (CdZnSe), indium gallium nitride (InGaN), Spinel (MgAl2O4), yttrium barium copper oxide (YBa2Cu3O7) and the like.

#Reference is now made to FIG. 1 which is a schematic illustration of a method for growing crystals of a metal-nonmetal compound from a surfactant, operative in accordance with an embodiment of the disclosed technique. In a procedure 100, a seed crystal is provided. The seed crystal represents the substrate upon which crystals of the MN compound of the disclosed technique will be grown upon. The seed crystal can have any kind of crystallographic structure and orientation and is not limited in any manner. Examples of different crystallographic structures are shown below in FIG. 4A-4C. In addition, the seed crystal can be used to grow homogeneous crystals, where the elements comprising the seed crystal and the elements comprising the MN compound are substantially similar. The seed crystal can also be used to grow heterogeneous crystals, where the elements comprising the seed crystal and the elements comprising the MN compound are substantially different. The seed can further be a small crystallite which may be one of many polycrystalline seeds grown on an amorphous or crystalline substrate. According to the disclosed technique, a variety of growth environments is possible and depends upon the specific MN compound crystal to be grown. Also, as described below the precursor materials required to grow the MN compound crystals according to the disclosed technique can be introduced in a selected growth environment using known crystal growth methods such as chemical vapor deposition (herein abbreviated CVD), molecular beam epitaxy (herein abbreviated MBE), liquid phase epitaxy (herein abbreviated LPE), vapor phase epitaxy (herein abbreviated VPE) and the like. Therefore, in procedure 100, the seed crystal which is provided is substantially placed in a suitable growth environment, depending on the specific MN compound crystal to be grown and the selected method for introducing the required precursor materials. As a first example, in procedure 100, the seed crystal may be provided and placed in a crucible for melting a metal, such as used in solution growth or LPE. As a second example, in procedure 100, the seed crystal may be provided and placed as a wafer in a high vacuum chamber, such as used in MBE. As a third example, the seed crystal may be provided as a polyseed layer deposited on a crystalline structure or on an amorphous structure.