Abstract:
A method of making an aluminum nitride (AlN) buffer layer over a silicon wafer for a light emitting diode (LED) includes preflowing a first amount of ammonia that is sufficient to deposit nitrogen atoms on the surface of a silicon wafer without forming SiNx, before flowing trimethylaluminum and then a subsequent amount of ammonia through the chamber.

Description:
RELATED APPLICATIONS 
     The present application is a divisional of co-pending, commonly assigned, patent application Ser. No. 13/190,420, entitled “NUCLEATION OF ALUMINUM NITRIDE ON A SILICON SUBSTRATE USING AN AMMONIA PREFLOW,” filed Jul. 25, 2011, which was abandoned on Sep. 17, 2014. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to methods of growing gallium nitride on silicon. 
     BACKGROUND INFORMATION 
     Thin films of Group III nitride, such as gallium nitride (GaN), are used in the production of efficient optoelectronic light emitters. Conventionally, GaN has been grown directly on sapphire substrates (Al 2 O 3 ). The GaN is grown in thin layers as opposed to in a single three-dimensional growth mode in order to achieve a high quality crystalline structure of the epitaxial growth. Growing the epitaxial layers of GaN on silicon as opposed to on sapphire offers considerable cost savings because of the economies of scale from the large production of silicon for the semiconductor industry. A large amount of equipment for the production of crystalline silicon has already been depreciated. That equipment can now be used in the production of light emitting diodes (LEDs). 
     Current attempts to grow high quality epitaxial layers of GaN on silicon substrates, however, have not been entirely successful. Because of the great difference between the lattice constants and thermal expansion coefficients, of GaN and silicon, GaN is not well suited for epitaxial growth directly on a silicon substrate. GaN epilayers often crack upon cooling to room temperature because even at growth temperatures above 1000° C. the lattice constant of GaN is much smaller than that of crystalline silicon. In addition, GaN has a much larger coefficient of thermal expansion than does silicon. So as the layers of GaN grown on silicon at high temperature cool to room temperature, the smaller lattice distance of the GaN crystals relative to the silicon crystals becomes even more pronounced. The GaN layers deposited directly on silicon are subjected to even more tensile stress as they cool and can even cause the underlying silicon substrate to bow. 
     Consequently, attempts have been made to grow buffer layers between the silicon substrate and the epitaxial GaN layers in order to compensate for the differing lattice constants and thermal expansion coefficients of GaN and silicon. For example, buffer layers of AlN, AlGaN and AlGaIN have been grown between the silicon substrate and the GaN layers. 
     The quality of the epitaxial GaN layers, however, that can be grown over existing buffer layers has been poor. Current methods of forming buffer layers of AlN and AlGaN have resulted in epitaxial growth of GaN layers that contain structural defects such as discontinuities, dislocations and faults. These defects degrade the morphology and optical properties of the GaN layers, rendering the GaN layers unsuitable for use in high qualify LEDs. 
     A method is sought for growing buffer layers on a silicon substrate that allows high quality epitaxial GaN layers with fewer structural defects to be grown over the buffer layers. 
     SUMMARY 
     A silicon wafer used in manufacturing crystalline gallium nitride (GaN) for light emitting diodes (LEDs) includes a silicon substrate, a buffer layer, of aluminum nitride (AlN), a second buffer layer of aluminum gallium nitride (Al x Ga 1-x N), and an upper layer of GaN. The silicon wafer has a diameter of at least 200 millimeters and an Si(111)1×1 surface (as opposed to a Si(111)7×7 reconstructed surface). The AlN buffer layer overlies the Si(111) surface of the substrate and is between 205 to 250 nanometers thick. The second buffer layer of aluminum gallium nitride is disposed between the buffer layer of aluminum nitride and the upper layer of gallium nitride. 
     Across the entire wafer substantially no aluminum atoms of the AlN are present in a bottom most plane of atoms of the AlN buffer layer, and across the entire wafer substantially only nitrogen atoms of the AlN buffer layer are present in the bottom most plane of atoms of the AlN. Thus, the AlN buffer layer has a single polarity. The silicon and AlN are oriented as AlN&lt;0001&gt;∥Si&lt;111&gt;. No amount of metallic aluminum is disposed between the silicon substrate and the AlN buffer layer. In addition, no layer of SiN x  is present between the silicon substrate and the AlN buffer layer. 
     A method of making an AlN buffer layer includes preflowing a first small amount of ammonia before flowing trimethylaluminum in order to form single polarity AlN. The crystallinity of the AlN buffer layer is influenced by the quality of the initial nucleation layer of AlN and the nature of the atomic bonding between the AlN and the silicon(111) surface. Because of the ammonia preflow step, the initial nucleation layer of AlN begins to grow with only nitrogen atoms bonded to the silicon(111) surface over the entire surface of the silicon wafer. 
     In a first cleaning step, a substrate of silicon (Si) is heated to a temperature above 950° C. in a reaction chamber of a metal-organic chemical vapor deposition (MOCVD) device. Then hydrogen (H 2 ) is flowed through the chamber in an amount between 106 and 118 cubic centimeters of hydrogen per minute over each square centimeter of the surface of the silicon substrate. In one aspect, the temperature in the chamber during the flowing of hydrogen is above 1100° C. 
     In the ammonia preflow step, a first amount of ammonia is flowed through the reaction chamber while the hydrogen is still flowing through the chamber. The first amount of ammonia is less than 0.01% by volume of the hydrogen flowing through the chamber. The first amount of ammonia does not exceed 0.006 cubic centimeters per minute over each square centimeter of the surface of the silicon substrate. The ammonia preflow step is performed for between thirty seconds to three minutes. The temperature in the chamber during the ammonia preflow step is between 1000° C. and 1050° C. 
     Then, trimethylaluminum (Al 2 (CH 3 ) 6 ) is flowed through the chamber while the hydrogen and first amount of ammonia are still flowing through the chamber. The trimethylaluminum is flowed through the chamber for between ten to twenty minutes in an amount of about ninety micromoles per minute. 
     Then a subsequent amount of ammonia is flowed through the chamber while the trimethylaluminum is still flowing through the chamber. The subsequent amount of ammonia is greater than 0.002% by volume of the hydrogen, flowing through the chamber. In one aspect, the subsequent amount of ammonia flowed through the chamber was just under 5% of the total amount of hydrogen, ammonia and trimethylaluminum. 
     Further details and embodiments and techniques are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
         FIG. 1  is a cross-sectional view showing the growth of a single crystal GaN film on buffer layers over a silicon substrate. 
         FIG. 2  is a flowchart of a method for growing an initial nucleation layer of AlN on a silicon substrate. 
         FIG. 3A  shows a model of the crystal structure of silicon. 
         FIG. 3B  is a diagram of the silicon atoms along an Si(111)1×1 surface of silicon. 
         FIG. 4  is a graph of gas flows of hydrogen, ammonia and trimethylaluminum through a reaction chamber during the method of  FIG. 2 . 
         FIG. 5  shows a model of the crystal structure of wurtzite aluminum nitride. 
         FIG. 6  is a diagram of the crystal structure of the aluminum hexagons in the C-plane of AlN superimposed over the crystal structure of silicon along the Si(111)1×1 surface. 
         FIG. 7  is a diagram of the crystal structures of a silicon substrate and an AlN nucleation layer viewed perpendicular to the Si(111) surface. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 1  is a schematic diagram showing the growth of a single crystal GaN film  10  on buffer layers over a silicon substrate  11 . A buffer layer of aluminum nitride (AlN)  12  is first grown on the silicon substrate  11 . Then higher buffer of layers of aluminum gallium nitride (Al x Ga 1-x N)  13  are grown over the AlN layer  12 . Finally, the GaN layer  10  is grown over the top layer of aluminum gallium nitride  13 . In some embodiments, the GaN layer includes several sublayers. The buffer layer of AlN  12  is made up of a lower initial nucleation layer  14  and thicker upper layers  15 . 
     There are several reasons for first growing buffer layers on a silicon substrate before the gallium nitride (GaN) layer  10  is grown. First, meltback etching of the silicon substrate  11  by gallium occurs if the gallium is allowed to react directly with the silicon substrate. The reaction between gallium and silicon results in poor crystal quality and morphology of the GaN layer  10 . Second, SiN x  can be formed as GaN is deposited on silicon, which leads to a 3-dimensional growth of GaN crystals instead of a 2-dimensional growth over the entire surface of the silicon substrate before the GaN layer thickens. Three-dimensional crystal growth leads to lower quality GaN layer than does 2-dimensional crystal growth. Third, the lattice mismatch between GaN and crystalline silicon causes a large tensile strain on the GaN layer at the interface with the silicon. The lattice mismatch at room temperature between GaN and the hexagonal surface orientation of silicon Si(111) is about 16.9%. Fourth, the in-plane thermal expansion coefficients of GaN on Si(111) vary widely (5.59×10 −6 K −1  for GaN and 2.6×10 −6 K −1  for Si). The difference in thermal expansion coefficients can result in cracking of the GaN layer upon cooling from the growth temperature to room temperature. 
     To solve the problems caused by growing GaN directly on silicon, buffer layers are typically deposited between the silicon and the GaN. For example, a nucleation layer of AlN  12  can first be grown on the silicon substrate  11 , followed by the additional buffer layers  13  of aluminum gallium nitride (Al x Ga 1-x N). The AlN nucleation layer and the other buffer layers solve the four problems described above. First, the AlN buffer layer  12  does not allow gallium to come into contact with the silicon substrate  11 . Second, GaN is not grown directly on the silicon substrate, so the formation of amorphous SiN x  can be prevented on the surface of the silicon, which would otherwise degrade the crystal formation of the GaN. Third, the lattice mismatch between GaN and Si(111) is compensated by the smaller lattice constant of AlN that applies compressive stress to the GaN to counteract the tensile stress of GaN from the underlying Si(111). Fourth, the smaller lattice constant of AlN compensates for the greater proportionate shrinking of the GaN crystal compared to the silicon crystal as both cool from the growth temperature to room temperature. 
     The quality of the GaN layer and other epitaxial layers, however, is also dependent on the quality of the AlN nucleation layer  12 . Consequently, simply growing a layer of AlN to solve the aforementioned four problems that result from growing directly on silicon will not necessarily result in high-quality GaN. The properties of the aluminum nitride nucleation layer  12 , such as its dislocation density and surface morphology, are critical in influencing the properties of the higher epitaxial layers. The AlN layer  12  acts as a crystallographic template for the higher buffer layers and ultimately for the GaN layer  10 . The properties of the AlN layer  12 , in turn, are determined in large part by the conditions under which the growth of the AlN is initiated and by how the silicon substrate  11  is treated prior to growth of the AlN. 
     It would seem that growing an AlN layer that has a low dislocation density over Si(111) would be hindered by the 23.4% lattice mismatch between AlN and Si(111). The distance between silicon atoms in the (111) plane of a silicon crystal is 3.840 angstroms, whereas the distance between aluminum atoms or between nitrogen atoms in the C-plane of wurtzite AlN is 3.112 angstroms. Smooth interface morphology between AlN and Si(111) can be achieved, however, due to the lattice coincidence between the (111) plane of silicon and the C-plane of AlN which permits the relaxation of the crystal stress at regular intervals of misfit dislocation at the AlN/Si interface. Achieving the same type of misfit dislocations at regular intervals is critical to obtaining a smooth interface morphology. 
     A method is disclosed for beginning the growth of AlN that results in a smooth interface between AlN and Si(111). The method grows a single polarity buffer layer of AlN having a low dislocation density. The subsequent buffer layers grown over the AlN buffer layer retain the high quality crystalline form and enable higher quality GaN and other epitaxial layers to be grown over the buffer layers. 
       FIG. 2  is a flowchart illustrating steps of a method  16  for growing a high-quality AlN nucleation layer on a silicon substrate. In a first step  17 , a substrate of Silicon (Si) is heated to a temperature above 950° C. in a chamber. In one aspect, the substrate was heated to a temperature of 1140° C. in the reaction chamber of a metal-organic chemical vapor deposition (MOCVD) system (also called a metal-organic vapor phase epitaxy system). The substrate was in the form of three 8-inch silicon wafers cut along the (111) plane. The three wafers were placed on a wafer receptacle having a diameter of 465 millimeters. 
       FIG. 3A  is a diagram illustrating the crystal structure  23  of silicon. The (111) plane along which the three silicon wafers are cleaved is the plane that intersects the silicon atoms A, C and F in  FIG. 3A . The silicon atoms A, F and  24  define the (010) plane, in which the silicon atoms form a square format. The distance between silicon atoms at adjacent corners of the squares of the identical (100), (010) and (001) planes is 5.431 angstroms. There is a shorter distance, however, between adjacent silicon atoms of the hexagonal format of the (111) plane. For example, the distance between atoms A and B in the (111) plane is 3.840 angstroms. This shorter distance between atoms in the hexagonal format of Si(111) better matches the distance between nitrogen atoms in the hexagonal format along the C-plane of AlN.  FIG. 3B  illustrates the atoms A-F of FIG. A in which the (111) plane coincides with the plane of the page. The surface of the three silicon wafers after first step  17  has a Si(111)1×1 structure  25  as shown in  FIG. 3B  as opposed to the dimer-adatom-stacking (DAS) fault structure of the Si(111)7×7 reconstruction. The more-stable, faceted Si(111)7×7 surface structure breaks down to the regular Si(111)1×1 hexagonal structure  25  as the silicon substrates are heated above about 850° C. 
     In step  18 , hydrogen is flowed through the chamber in order to remove SiO 2  from the wafers and generally to clean the surface of the silicon substrate. Between 106 and 118 cubic centimeters of hydrogen per minute flow over each square centimeter of the surface of the substrate. In one aspect, 180-200 liters of hydrogen per minute was flowed through the chamber. The silicon substrate was baked in the hydrogen flow at 1140° C. for about fifteen minutes to remove the native oxide. Then, the temperature in the chamber was lowered to about 1020° C. 
     In step  19 , a first amount of ammonia (NH3) is flowed through the chamber while the hydrogen is still flowing through the chamber. The first amount of ammonia is less than 0.01% of by volume of the hydrogen flowing through the chamber. The first amount of ammonia is flowed through the chamber for between thirty seconds to three minutes. In one aspect, less than ten cubic centimeters of ammonia per minute was flowed over the 465-mm wafer receptacle. Thus, less than 0.00588 cubic centimeters per minute of ammonia flowed over each square centimeter of the surface of the silicon substrate. At 1020° C., the first amount of ammonia is insufficient to form a layer of SiN x  over the surface of the silicon substrate. The first amount of ammonia is, however, sufficient to form a small number of Si—N bonds on the Si(111)1×1 surface. 
     In step  20 , trimethylaluminum (Al 2 (CH 3 ) 6 ) is flowed through the chamber while the hydrogen is still flowing through the chamber. The trimethylaluminum flows through the chamber in an amount of about ninety micromoles per minute. In one aspect, ninety micromoles per minute of trimethylaluminum flowed through the chamber for between ten to twenty minutes. 
     In step  21 , a subsequent amount of ammonia is flowed through the chamber while the trimethylaluminum is still flowing through the chamber. The subsequent amount of ammonia is greater than 0.002% by volume of the hydrogen flowing through the chamber. In one aspect, the subsequent amount of ammonia flowed through the chamber was just under 5% of the total amount of hydrogen, ammonia and trimethylaluminum. When the subsequent amount of ammonia was flowed through the chamber at just under 5% of the total flow for about fifteen minutes, an initial nucleation layer  14  of aluminum nitride (AlN) grew to a thickness of between 25-50 nanometers. The crystallinity of the AlN buffer layer  12  is related to the quality of the initial nucleation layer  14  and the nature of the atomic bonding between the silicon(111) surface and the AlN. Because of the ammonia preflow in step  19 , the initial nucleation layer  14  begins to grow with only nitrogen atoms bonded to the silicon(111) surface over the entire surface of the 8-inch wafers. 
     In a step  22 , the flow of trimethylaluminum is increased to about 180 micromoles per minute, and the temperature in the chamber is increased to about 1120° C. The buffer layer of AlN is grown an additional 180-200 nanometers to a total thickness of 205-250 nanometers under the increased flow of trimethylaluminum. 
       FIG. 4  is a graph that represents the flows of hydrogen, ammonia and trimethylaluminum through the reaction chamber during the cleaning, preflow, initial growth and thicker growth stages of the formation of the AlN buffer layer. In other embodiments, the additional 180-200 nanometers of AlN is grown in multiple stages of higher trimethylaluminum concentration as opposed to in one step. 
     The initial nucleation layer  14  of AlN first starts to form when the trimethylaluminum starts flowing through the chamber in step  20  and before the subsequent amount of ammonia is flowed through the chamber in step  21 . Consequently, a very small amount of nitrogen is present on the Si(111)1×1 surface of the silicon substrate  11  before the aluminum from the trimethylaluminum comes into contact with the substrate surface. As the first seed crystals of AlN are formed, the aluminum atoms form bonds to the nitrogen atoms that are present on the Si(111)1×1 surface as opposed to directly with silicon atoms on the substrate surface. The nitrogen from the ammonia preflow step ensures that the polarity of the alternating layers of aluminum and nitrogen in the AlN crystals that are formed across the entire silicon wafer will have a nitrogen layer facing the silicon substrate and an aluminum layer on top. 
       FIG. 5  is a diagram illustrating the crystal structure  26  of wurtzite aluminum nitride (AlN). The smaller spheres represent aluminum atoms  27 , and the larger spheres represent nitrogen atoms  28 . The C-plane of the AlN crystal intersects all six aluminum atoms that form a hexagon on the top surface of the crystal. The distance between adjacent aluminum atoms around the hexagon in the C-plane is 3.112 angstroms. The distance between adjacent nitrogen atoms around the middle hexagon is therefore also 3.112 angstroms. The hexagons of nitrogen and aluminum along the C-plane of AlN approximately match the hexagonal format of silicon atoms on the Si(111)1×1 surface of the silicon substrate. 
       FIG. 6  is a diagram of the crystal structure of the aluminum hexagons in the C-plane of AlN superimposed over the crystal structure of the silicon substrate on the Si(111)1×1 surface. Because the distance between atoms around the silicon hexagon is 3.840 angstroms and the distance between the atoms around the hexagons of AlN is 3.112 angstroms, the lattice distance of a silicon cell unit is 6.652 angstroms and the lattice distance of an AlN cell unit is 5.390 angstroms. Thus, there is a 23.4% lattice mismatch. However, the width of five AlN cell units (26.95 angstroms) approximately matches the width of four silicon cell units (26.61 angstroms), as illustrated in  FIG. 6 . Every fifth AlN cell unit can bond well to every fourth silicon cell unit. 
       FIG. 7  is a diagram of the crystal structures of the silicon substrate and AlN viewed perpendicular to the Si(111)1×1 surface and the C-plane of AlN.  FIG. 7  illustrates how each fifth cell unit of the AlN crystal approximately matches each fourth cell unit on the Si(111)1×1 surface. The regular mismatch between surface structures of AlN and silicon allows a crystal of AlN to be grown with a low dislocation density. 
     The dislocation density of the AlN buffet layer is considerably higher, however, if not all islands of AlN crystals that begin to form on the Si(111)1×1 surface have the same polarity. If some islands of AlN crystals form with nitrogen atoms bonded to the silicon while other islands of AlN crystals form with aluminum atoms bonded to the silicon, then discontinuities and stacking faults will form where the islands of crystals having opposite polarity grow together.  FIG. 7  shows that the initial nucleation layer of AlN formed using method  16  has only nitrogen atoms bonded to the Si(111)1×1 surface. Across the entire substrate wafer substantially only nitrogen atoms of the aluminum nitride are present in the bottom most plane of atoms of the aluminum nitride. Because the initial nucleation layer of AlN formed using method  16  has a single polarity, it was possible to grow a GaN layer above the nucleation layer that had a dislocation density of less that 2×10 9  cm −2 . 
     Some prior methods of growing an AlN buffer layer begin by depositing a metallic Al layer on the surface of the silicon substrate before the AlN is grown in order to prevent the formation of amorphous SiN x . The presence of aluminum atoms on surface of the silicon substrate probably causes at least some of the islands of AlN crystals to form with aluminum as the bottom most plane of atoms of the aluminum nitride. Because these prior art methods do not prevent at least some nitrogen atoms from bonding to the silicon substrate (not necessarily as amorphous SiN x ), some islands of AlN crystals form with nitrogen as the bottom most plane of atoms of the aluminum nitride, and the resulting AlN layer has mixed polarity. Method  16 , on the other hand, allows a single polarity material to be grown. 
     Thus, method  16  can be used to manufacture a wafer of silicon substrate over which an AlN buffer layer, AlGaN layers and finally an upper GaN layer are grown. The silicon substrate has an Si(111) surface, which converts from a 7×7 structure to a 1×1 structure when the silicon is heated above about 850° C. The AlN buffer layer is a means for compensating for the lattice mismatch between the GaN and the Si(111) surface of the silicon substrate so as to enable the upper GaN layer to grow under reduced stress. The AlGaN on top of the AlN buffer layer is a better lattice match for GaN than in silicon. The silicon substrate is a wafer with a diameter of at least 200 millimeters, such as an 8-inch wafer. The AlN buffer layer overlies the Si(111) surface of the substrate and is oriented as AlN&lt;0001&gt;∥Si&lt;111&gt;. The upper layer of GaN is grown on the AlGaN layers over the AlN buffer layer. Across the entire wafer substantially no aluminum atoms of the aluminum nitride are present in a bottom most plane of atoms of the aluminum nitride, and across the entire wafer substantially only nitrogen atoms of the aluminum nitride are present in the bottom most plane of atoms of the aluminum nitride. Thus, across the entire wafer substantially only nitrogen atoms of the AlN buffer layer form bonds to the Si(111) surface. There is neither metallic aluminum nor any layer of SiN x  present between the silicon substrate and the AlN buffer layer. 
     Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.