Abstract:
A method has been developed to overcome deficiencies in the prior art in the properties and fabrication of semi-polar group III-nitride templates, films, and materials. A novel variant of hydride vapor phase epitaxy has been developed that provides for controlled growth of nanometer-scale periodic structures. The growth method has been utilized to grow multi-period stacks of alternating AlGaN layers of distinct compositions. The application of such periodic structures to semi-polar III-nitrides yielded superior structural and morphological properties of the material, including reduced threading dislocation density and surface roughness at the free surface of the as-grown material. Such enhancements enable to fabrication of superior quality semi-polar III-nitride electronic and optoelectronic devices, including but not limited to transistors, light emitting diodes, and laser diodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/US2014/035042 filed Apr. 22, 2014 which claims the benefit of U.S. Provisional Patent Application No. 61/814,653 filed Apr. 22, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of semi-polar III-nitride films and materials and method for making the same. 
     2. Prior Art 
     Group III-nitrides, which include but are not limited to Al x In y Ga 1-x-y N compositions in which 0≦x, y≦1, are of considerable interest in many fields, such as the fabrication of high brightness light emitting diodes (LEDs), laser diodes, and power electronics. Virtually all group III-nitrides grown today are produced such that the maximum surface area available for device fabrication lies on the (00.1) c-plane, wherein the notation (hk.l) is a shorthand form of Miller-Bravais crystallographic notation to identify crystal planes in hexagonal crystals. The “.” Represents the i index in (hkil) four-index notation, which is redundant as h+k+i=0. One skilled in the art further understands that (hkil) notation using parentheses refers to a specific crystal plane while notation using curly brackets such as {hkil} refers to a family of related crystallographic planes. For the purposes of this invention, { } and ( ) notation will be understood to be interchangeable as the invention typically applies to all specific planes that belong to any family of planes. 
     Conventional c-plane-oriented nitrides can be referred to as “polar” nitrides because of the substantial piezoelectric and spontaneous polarization fields that exist parallel to the c-axis and therefore perpendicular to the c-plane. Such polarization fields restrict performance of polar group III-nitride devices by causing color shifting, limiting radiative recombination efficiency, and reducing high-current density efficiency. 
     An alternate set of group III-nitride crystal orientations are referred to as “semi-polar.” Semi-polar nitrides are nitride crystal planes having at least two non-zero h, k, or i indices and a non-zero l index in Miller-Bravais notation. Some common semi-polar planes include, but are not limited to, the {10.1}, {10.2}, {10.3}, {20.1}, {30.1}, and {11.2} planes. 
     Group III-nitrides are commonly fabricated by several techniques, including but not limited to metalorganic chemical vapor deposition (MOCVD or OMVPE), molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE). The overwhelming majority of group III-nitride development has been focused on polar c-plane oriented material. Semi-polar group-III nitride planes, however, have historically proven difficult to grow by any technique using comparable parameters to polar nitrides. Indeed, one skilled in the art will recognize that growth of a semi-polar group-III nitride film, template, or free-standing layer using production parameters optimized for polar nitrides generally yields low-quality, rough, and defective material that is virtually unusable for fabrication of optoelectronic and electronic devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a simple embodiment of the present invention having two pairs of Periodic Epi-Structure Layers grown upon a graded or stepped AlGaN layer. 
         FIG. 2  illustrates an embodiment of the present invention wherein the Graded Al x Ga 1-x N Layer has been eliminated and defect reduction is achieved by way of the use of Periodic Epi-Structure inclusion. 
         FIG. 3  illustrates an embodiment of the present invention wherein the Periodic Epi-Structure has been eliminated and defect reduction is achieved by way of the use of a graded AlGaN layer. 
         FIG. 4  is a Nomarski optical contrast micrograph showing an approximately 15 of (11.2) GaN grown upon a Al 0.35 Ga 0.65 N intermediate layer grown upon an AlN nucleation layer on a m-plane sapphire substrate without the benefit of the present invention. 
         FIG. 5  is a Nomarski optical contrast micrograph showing a GaN film grown upon a 10-period Periodic Epi-Structure consisting of (a) layers consisting Al 0.35 Ga 0.65 N and (b) layers consisting of GaN. The Periodic Epi-Structure was grown on a Graded Al x Ga 1-x N layer that transitioned from AlN to GaN in five steps in accordance with an embodiment of the invention. 
         FIG. 6  is a Nomarski optical contrast micrograph showing a GaN film grown upon a 10-period Periodic Epi-Structure consisting of (a) layers consisting Al 0.85 Ga 0.15 N and (b) layers consisting of GaN. The Periodic Epi-Structure was grown on a Graded Al x Ga 1-x N layer that transitioned from AlN to GaN in five steps in accordance with an embodiment of the invention. 
         FIG. 7  is a Nomarski optical contrast micrograph showing a GaN film grown upon a 10-period Periodic Epi-Structure consisting of Al 0.85 Ga 0.15 N and (b) layers consisting of GaN in accordance with an embodiment of the invention. The Periodic Epi-Structure was grown on a Graded Al x Ga 1-x N layer that transitioned from AlN to GaN in five steps. The terminal GaN layer grown at a reduced growth rate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a means of significantly reducing defect densities, reducing surface roughness, and improving functionality of semi-polar group III-nitrides. The invention utilizes a novel variant of HVPE to grow nanometer-scale periodic epi-structures on semi-polar III-nitride planes. The invention further includes the use of stepped and/or graded AlGaN layers to improve phase purity and reduce macroscopic defect densities. 
     Key novel elements of the invention include one or more of the following: 
     1. Nanometer-scale control of semi-polar AlGaN and GaN growth rates with HVPE, a technique that is known for much higher growth rates 
     2. Incorporation of stepped or graded AlGaN layers as a transition from optional AlN nucleation layers on the m-plane Al 2 O 3  substrates to GaN at the free surface. In one embodiment, the film layer is transitioned from AlN to GaN in five composition steps 
     3. Growth of nanometer-scale periodic structures that feature alternating thin layers of AlGaN and GaN of different compositions 
     4. Application of the invention specifically to the growth of high-quality semi-polar group III-nitride films, templates, free-standing substrates, and bulk materials 
     5. Ability to grow the nanometer-scale graded AlGaN layers and periodic epi-structures in the same growth run as thin and thick AlGaN and GaN films, enabling low-cost template production compared to methods that rely on MBE or MOCVD for group III-nitride growth 
     6. Achievement of reduced surface roughness, reduced macroscopic defect density, and/or reduced micro-structural defect densities compared to semi-polar group III-nitrides as described in the prior art 
     The invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements. References in the following detailed description of the present invention to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristics described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in this detailed description are not necessarily all referring to the same embodiment. The following figures illustrate several embodiments of the invention. 
       FIG. 1  illustrates a template structure  100  that includes the invention. An optional nucleation layer  120  is deposited on a suitable substrate  110 . A Graded AlGaN region  130  is deposited on the nucleation layer, upon which a nanometer-scale periodic epi-structure  140  is grown. A terminal group III-nitride layer such as GaN is grown upon the periodic epi-structure, represented by block  150 . 
     Referring to  FIG. 1 , the substrate  110  may be any substrate that is capable of supporting group III-nitride growth, either heteroepitaxially or homoepitaxially. Examples of suitable substrate materials include, but are not limited to, sapphire, silicon, lithium aluminate, spinel, silicon carbide, gallium nitride, aluminum nitride, and silica glass. The substrate orientation can be any orientation that supports group III-nitride epitaxial growth, including but not limited to the sapphire c-plane, m-plane, r-plane, or n-plane; the silicon {100}, {110} or {111} planes; the lithium aluminate {100} plane; the silicon carbide {00.1}, {10.0}, {11.0}, {11.1}, {11.2}, {10.1}, {10.2}, {10.3}, and {20.1} planes. One skilled in the art will recognize that other substrate materials and orientations not listed here may be utilized in the practice of the invention. 
     The nucleation layer  120  may be any group III-nitride composition, may be deposited at any temperature ranging from 400 to 1300 degrees Celsius, and may be of any thickness from 0.1 nm to 1000 μm. The nucleation layer may further be the result of a deposition process, such as the deposition of AlN on a sapphire substrate; or may be the result of a modification of the substrate top surface, such as may be achieved by nitridizing a sapphire top surface by flowing ammonia over sapphire during annealing at high temperature, converting a one or more mono-layers of Al2O3 into AlN. The nucleation layer may further be omitted completely from the structure if so desired. 
     The Graded AlGaN region  130  is deposited on the nucleation layer. The Graded Al x Ga 1-x N region involves a transition from a initial group III-nitride composition, such as Al 1.0 Ga 0.0 N to a terminal group III-nitride composition, such as Al 0.0 Ga 1.0 N, over a total thickness ranging from approximate 5 nm to approximately 10 μm. The transition may be executed continuously, varying the composition as a function of growth time with no distinct layer structure. For example, in one embodiment the Graded AlGaN region involves a transition from AlN to GaN with a linear composition change as a function of thickness over a region thickness of 200 nm. 
     In an alternate embodiment, the Graded Al x Ga 1-x N region may be executed in a series of steps from an initial composition to a terminal composition. For example, in this alternate embodiment the Graded Al x Ga 1-x N region consists of a transition from AlN to GaN including six distinct layers having compositions of Al 1.00 Ga 0.00 N, Al 0.80 Ga 0.20 N, Al 0.60 Ga 0.40 N, Al 0.40 Ga 0.60 N, Al 0.20 Ga 0.80 N, and Al 0.00 Ga 1.00 N, respectively. 
     In another embodiment, a portion of the Graded Al x Ga 1-x N region is compositionally varied continuously while another portion is varied stepwise. The thickness of each gradation need not be constant throughout the Graded Al x Ga 1-x N layer. One skilled in the art will recognize that the specific number of gradations in and the total thickness of the Graded Al x Ga 1-x N region may be varied without deviating from the scope of the present invention. 
     The Periodic Epi-Structure  140  consists of pairs of group III-nitride layers having dissimilar composition that are grown upon one another. Referring to  FIG. 1 , one of the layers has been denoted Layer (a) and the other denoted as Layer (b) in block  140 . It is essential that the (a) and (b) layers consist of dissimilar group III-nitride compositions from one another. For example, in one embodiment Layer (a) is represents Al 0.80 Ga 0.20 N, while Layer (b) is Al 0.00 Ga 1.00 N. In the simplest implementation of the invention, all (a) layers in the Periodic Epi-Structure would consist of similar compositions to all other (a) layers, while all (b) layers in the Periodic Epi-Structure would consist of similar compositions to all other (b) layers. However, in some embodiments it is desirable to vary the composition of either the (a) layers or (b) layers (or both) through the thickness of the Periodic Epi-Structure. Such variation is compatible with the invention provided that each layer consists of a compositionally distinct group III-nitride from the immediately adjacent layers. 
     In the simple embodiment illustrated in  FIG. 1 , two pairs of Periodic Epi-Structure Layers  140  are illustrated. The number of pairs of Periodic Epi-Structure Layers used in practice will vary from approximately two pairs to approximately 200 pairs. 
     The thickness of the (a) and (b) layers in the Periodic Epi-Structure will each typically range from approximately 1 nm to approximately 100 nm. There is no requirement that identical thicknesses be used for all (a) layers and all (b) layers, respectively. In one embodiment, the thickness of the (a) layers is approximately 5 nm and the thickness of the (b) layers is approximately 20 nm. The thickness of the layers can be varied throughout the thickness of the Periodic Epi-Structure as well. For example, it may be desirable to utilize a structure in which the layer thicknesses are approximately 5 nm each for five pairs, followed by thicknesses of 10 nm each for five pairs. One skilled in the art will recognize that many variations of layer thicknesses can be utilized successfully in the practice of the invention. 
     The top layer represented by block  150  represents the terminal composition of the thin film or template that is grown utilizing the invention. Typically, this top layer will consist of GaN, but in practice it can consist of any group III-nitride alloy composition. This layer can be grown at different growth rates and can also be doped with modifying atoms or ions, including but not limited to Si, C, O, Mg, and Zn. 
     The thickness of the top layer may range from 1 nm in the case of thin templates to 50 mm in the case of bulk nitride materials grown for use as free-standing substrates. Typically, the top layer thickness will be approximately five to ten micrometers for group III-nitride template fabrication. Similarly, typically thicknesses for free-standing film production are on the order of 250-1000 μm. One skilled in the art will recognize that many ranges of thicknesses are compatible with the practice of the invention. 
     The invention can also be practiced with the exclusion of the Graded Al x Ga 1-x N Layer, as illustrated in  FIG. 2 . In the embodiment illustrated by block  101 , the Periodic Epi-Structure  140  is grown upon the Nucleation Layer  120 . 
     The invention can also be practiced with the exclusion of the Periodic Epi-Structure  140 , as illustrated in  FIG. 3 . In the embodiment illustrated in block  102 , the terminal GaN Layer  150  is grown upon the Graded Al x Ga 1-x N layer  130 . 
     One skilled in the art will further recognize that the order of the blocks as illustrated in  FIGS. 1-3  can be rearranged without fundamentally deviating from the scope of the invention. For example, the Graded Al x Ga 1-x N Layer could be grown upon the Periodic Epi-Structure instead of being grown in the order described in  FIG. 1 . It should also be emphasized that additional layers not described herein could be inserted into the structure. For example, in one embodiment a GaN layer is grown upon the nucleation layer, upon which the Graded Al x Ga 1-x N layer is grown. Such additions of supplemental layers are consistent with the scope and practice of the invention. 
       FIGS. 4-7  provide Nomarski optical contrast micrographs illustrating improved surface morphology of {11.2} GaN and Al x Ga 1-x N films incorporating the present invention. In  FIG. 4 , an Al x Ga 1-x N surface grown without the invention is shown. This film consisted of approximately 15 μm of (11.2) GaN grown upon a Al 0.35 Ga 0.65 N intermediate layer grown upon an AlN nucleation layer on a m-plane sapphire substrate. 
       FIG. 5  shows a Nomarski optical contrast micrograph of a GaN film grown upon a 10-period Periodic Epi-Structure consisting of (a) layers consisting of Al 0.35 Ga 0.65 N and (b) layers consisting of GaN. The Periodic Epi-Structure was grown on a Graded Al x Ga 1-x N layer that transitioned from AlN to GaN in five steps. A comparison of surface roughness between  FIG. 5  and  FIG. 4  clearly shows the superior quality of the surface in  FIG. 5 . 
       FIG. 6  shows a Nomarski optical contrast micrograph of a GaN film grown upon a 10-period Periodic Epi-Structure consisting of (a) layers consisting Al 0.85 Ga 0.15 N and (b) layers consisting of GaN. The Periodic Epi-Structure was grown on a Graded Al x Ga 1-x N layer that transitioned from AlN to GaN in five steps. A comparison of surface roughness between  FIG. 6  and  FIG. 4  clearly shows the superior quality of the surface in  FIG. 6 . 
       FIG. 7  shows a Nomarski optical contrast micrograph of a GaN film grown upon a 10-period Periodic Epi-Structure consisting of (a) layers consisting Al 0.85 Ga 0.15 N and (b) layers consisting of GaN. The Periodic Epi-Structure was grown on a Graded Al x Ga 1-x N layer that transitioned from AlN to GaN in five steps. In this epi-growth example the terminal GaN layer was grown at a reduced growth rate, yielding further reduction in surface roughness. A comparison of surface roughness between  FIG. 7  and  FIG. 4  clearly shows the superior quality of the surface in  FIG. 7 . 
     The incorporation of the present invention into the growth of semi-polar group III-nitrides can reduce terminal layer surface roughness by 75% or more compared to semi-polar group III-nitride films grown without the invention. The invention further improves micro-structural quality of the terminal group III-nitride layers by blocking propagation of micro-structural defects and relieving strain related to lattice mismatch and thermal expansion mismatch.