Abstract:
A structure method for producing same provides suppressed lattice defects when epitaxially forming nitride layers over non-c-plane oriented layers, such as a semi-polar oriented template layer or substrate. A patterned mask with “window” openings, or trenches formed in the substrate with appropriate vertical dimensions, such as the product of the window width times the cotangent of the angle between the surface normal and the c-axis direction, provides significant blocking of all diagonally running defects during growth. In addition, inclined posts of appropriate height and spacing provide a blocking barrier to vertically running defects is created. When used in conjunction with the aforementioned aspects of mask windows or trenches, the post structure provides sign0ificant blocking of both vertically and diagonally running defects during growth.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     The U.S. Government has a fully paid-up license in this disclosure and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract number W911 NF-08-C-0003 awarded by DARPA-VIGIL. 
    
    
     BACKGROUND 
     The present disclosure is related to reducing defects which form during growth of semiconductor materials over a substrate, and more specifically to reducing certain dislocations which arise due to the lattice mismatch between a substrate and a nitride material grown thereover. 
     It is quite well known that when epitaxially growing a material such as a semiconductor over a substrate, a mismatch in the lattice constants of the substrate and the growth material can result in crystalline defects in the material as grown. This is illustrated in  FIG. 7  for a structure  100 . Substrate  102  is typically oriented such that its crystal boundaries  104  are oriented roughly perpendicular to the plane of the growth surface (i.e., vertically). As a growth material  106 , such as gallium nitride (GaN), forms over substrate  102 , lattice defects  108  form therein which are roughly parallel to the crystal boundaries  104  of substrate  102 . There are various techniques known in the art for suppressing these defects. 
     One material system of particular interest today is the nitride system (e.g. compounds formed between any group-III element and nitrogen). Nitride-based materials are able to produce light-emitting devices such as diode lasers and the like which emit light at shorter wavelengths corresponding to green, blue, and even ultra violet (UV) light as compared to other known material systems. Other applications for the nitrides are transistors and other electronic devices. However, nitride materials are typically grown by metalorganic chemical vapor (MOCVD) deposition techniques onto lattice-mismatched substrates like for example sapphire, silicon carbide, and silicon for which there are relatively fewer options to address and prevent lattice mismatch defects. 
     One known technique for suppressing lattice dislocations in MOCVD processes is known as lateral overgrowth, which is illustrated in  FIG. 8 . Again, a c-plane oriented substrate  102  such as Al 2 O 3  (sapphire), is the typical starting point. In order to suppress the vertical dislocation defects, a mask layer  112  is formed over the surface of substrate  102 , and one or more openings  114  are then formed in the mask. GaN layer  116  is then epitaxially grown over substrate  102 , beginning in openings  114  in order to initiate the epitaxy. The GaN grows both vertically and laterally. Due to the crystallographic orientation of substrate  102 , any vertical defects forming in layer  116  are limited to the opening areas, and are either suppressed or bent horizontally over the mask  112 . Thus the regions over the mask  112  are substantially free of vertical lattice dislocations. 
     While c-plane oriented substrates have been the most widely used substrates to date, other orientations such as semi-polar and m-plane orientations are becoming increasingly important. For example, bulk semi-polar GaN substrates are highly desired for (InGaAl)N-based light emitters such as light-emitting diodes (LEDs) and laser diodes (LDs), in order to reduce internal electric fields which impair the efficiency of the light emission process on conventional c-axis oriented nitride devices. However, such bulk substrates are not yet widely available and are limited to small sizes. As an alternative to bulk semi-polar GaN substrates, semi-polar GaN templates have been grown on large area sapphire substrates by conventional means such as Hydride Vapor Phase Epitaxy (HVPE). However, the defect density in such template layers is of the order of 10 10  cm −2 , unless defect reduction techniques are applied. 
     While lateral overgrowth is an effective technique for c-plane oriented substrates, it is not optimized for materials in which the c-axis is tilted with respect to the surface normal, such as any semi-polar oriented GaN, in which a significant portion of defects extend across the GaN layer at an angle corresponding to the tilt of the basal plane GaN(0001). With reference next to  FIG. 9 , one difficulty observed is that since the lattice defects  128  in semi-polar template layer  122  (or equivalently, a semi-polar substrate, not shown) run diagonally, e.g., at a given angle between 0 and 90 degrees relative to the plane of growth surface  132  of layer  122 , the effectiveness of the mask at limiting communication of the defects into the growth layer  116  is reduced.  FIGS. 10   a - 10   f  are TEM images of GaN(1122) layers grown by a lateral overgrowth technique on a semi-polar buffer layer, as known in the art. Cross-section images shown in  FIGS. 10   e  and  10   f  highlight the persistence of diagonally running defects despite the presence of lateral overgrowth mask  126 . To compound this problem, certain substrate orientations present defects in multiple different planes (e.g., perpendicular to the growth surface as well as angled relative to that plane). To date, there have been inadequate solutions for suppressing lattice defects in epitaxial growth layers formed over non-c-plane oriented layers. 
     SUMMARY 
     Accordingly, the present disclosure is directed to a method and structure for suppressing lattice defects in nitride-based epitaxial growth layers formed over non-c-plane oriented layers, such as a semi-polar oriented template layer or substrate. According to one aspect of the disclosure, a patterned mask with “window” openings formed therein with vertical dimensions according to the product of the window width times the cotangent of the angle between the surface normal and the c-axis direction for the semi-polar layer provides significant blocking of all diagonally running defects during growth. 
     According to another aspect of the disclosure, etched trenches in the template layer or substrate, with vertical dimensions according to the product of the trench opening width times the cotangent of the angle between the surface normal and the c-axis direction for the semi-polar layer provides significant blocking of all diagonally running defects during growth. 
     According to another aspect of the disclosure, etched trenches in the template layers or substrates are formed and selectively covered by a dielectric mask leaving only the facet in GaN[0001] direction uncovered. 
     According to a still further aspect of the disclosure, inclined posts of appropriate height and spacing provide a blocking barrier to vertically running defects is created. When used in conjunction with the aforementioned aspects of mask windows or trenches, significant blocking of both vertically and diagonally running defects during growth is provided. 
     The above is a summary of a number of the unique aspects, features, and advantages of the present disclosure. However, this summary is not exhaustive. Thus, these and other aspects, features, and advantages of the present disclosure will become more apparent from the following detailed description and the appended drawings, when considered in light of the claims provided herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings appended hereto like reference numerals denote like elements between the various drawings. While illustrative, the drawings are not drawn to scale. In the drawings: 
         FIG. 1  is a structure with lattice defect suppressing islands formed thereon according to one embodiment of the present disclosure. 
         FIG. 2  is a close-up illustration of a portion of the structure of  FIG. 1 , showing added details of the lattice defect suppressing islands according to one embodiment of the present disclosure. 
         FIG. 3  is a structure with lattice defect suppressing trenches formed therein according to one embodiment of the present disclosure. 
         FIG. 4  is a close-up illustration of a portion of the structure of  FIG. 3 , showing added details of the lattice defect suppressing trenches according to one embodiment of the present disclosure. 
         FIGS. 5A and 5B  are illustrations of a structure with selectively-masked lattice defect-suppressing trenches formed therein according to one embodiment of the present disclosure. 
         FIG. 6  is a structure with lattice defect suppressing islands and inclined posts formed thereon according to a another embodiment of the present disclosure. 
         FIG. 7  is an illustration of a prior art structure showing the formation of vertical lattice defects in a growth medium formed over a crystalline substrate. 
         FIG. 8  is an illustration of the lateral overgrowth process according to the prior art. 
         FIG. 9  is an illustration of a structure exhibiting both vertical and inclined lattice defects despite employing lateral overgrowth according to the prior art. 
         FIGS. 10   a  through  10   f  are TEM images of GaN layers grown by a lateral overgrowth technique on a semi-polar buffer layer, as known in the art. 
     
    
    
     DETAILED DESCRIPTION 
     We initially point out that descriptions of well known starting materials, processing techniques, components, equipment and other well known details are omitted so as not to unnecessarily obscure the details of the present invention. Thus, where details are otherwise well known, we leave it to the application of the present invention to suggest or dictate choices relating to those details. 
     As a solution to the problems of defect suppression when growing material over a non-c-plane oriented layer or substrate, the present disclosure describes a number of techniques for preparation of the growth surface according to the specific characteristics of the surface orientation. The present disclosure makes use of the fact that the tilt angle at which defects may form in the growth material is defined in part by the particular surface orientation of the layer or substrate on which the growth material is formed. 
     According to a first embodiment of the present disclosure, a growth material layer is formed over a template layer. A mask is formed over the template layer, and regular “windows” are formed therein. The thickness of the masking material is selected to be greater than or equal to the product of the window width times the tangent of the angle between the surface normal and the primary plane of the template material. As the growth proceeds in the window areas, all defects tilted according to the tilt of the basal plane will be blocked by the mask. 
     With reference to  FIG. 1 , a specific example of this first embodiment is next described. A structure  10  according to this embodiment comprises a template substrate  12 , which may be a 1 micron thick or thicker nitride layer (e.g., GaN, AlGaN, or InGaN or combinations thereof) of semi-polar orientation (e.g., other than (0001) or (1010) oriented surface) grown by MOCVD (or any suitable method) on sapphire or any other suitable substrate (not shown). While a gallium nitride (GaN) growth material layer formed over a GaN template layer over a Al 2 0 3  (sapphire) substrate is described in this example, other growth and substrate materials may be employed (as well as other layers of structure  10 ), and the particular materials of this example are selected merely for explanation purposes. In the case of wurtzitic nitride substrates, substrate  12  has a growth plane corresponding to a crystallographic plane forming an angle α other than 0 or 90 degrees to its primary crystallographic plane. For other crystallographic substrate materials, substrate  12  is oriented such that its primary crystallographic plane is at an angle α that is out of normal to said growth plane. 
     A dielectric layer  14 , for example SiO 2  2 microns thick, is deposited on template substrate  12 . Next, a photoresist layer  16  is deposited, and is shown in dashed outline to represent that it is subsequently removed from the final structure. Photoresist layer  16  is patterned by conventional photolithography to form a regular pattern of stripes spaced apart by a distance, for example, in the range of 1 to 20 microns or a geometrical pattern of openings, for example 1 micrometer by 1 micrometer squares, spaced apart by a distance in a range of 1 to 20 microns (or alternatively, hexagonal or other shaped openings), although other spacings are contemplated and depend on the application of the teachings of the present disclosure. 
     The SiO 2  layer  14  is then mask-patterned using the mask of photoresist  16 , for example by chemical assisted ion-beam etching (CAIBE) thereby defining islands  18  and windows  20 . Islands  18  define sidewalls  22  predominantly in the direction of the c-axis (i.e., perpendicular to a growth surface  24 ) of the GaN unit cell and parallel to the direction of the basal plane of the GaN unit cell. 
     Prior to transfer into the growth chamber, selective surface cleaning is performed to remove the photoresist layer  16  while not attacking the SiO 2  layer  14 . Following transfer to the MOCVD growth chamber, the substrate is heated to 1050° C. in an ammonia atmosphere and a layer growth (GaN, AlGaN) starts at a rate of 0.5-4 micron/hr. Layer growth proceeds both vertically and laterally, and proceeds until full coalescence is achieved (i.e., material grows uniformly both above islands  18  and windows  20 ), thereby forming layer  26 . Thereafter, device structures (not shown) consisting of a plurality of doped or undoped GaN, InGaN, AlGaN, etc. layers may be deposited. 
     As will be appreciated from  FIG. 1 , the crystallographic axis of growth substrate  12  is inclined relative to growth plane  24 . The thickness of layer  14 , and hence the height of islands  18 , is precisely chosen so that it is at least sufficiently tall that any dislocation defects  28  communicated from substrate  12  into growth layer  26  and continuing therein as defects  30  terminate at sidewall  22  (yet minimally thick for rapid surface smoothness upon coalescence of the growth island, and for ease of patterning). With reference to  FIG. 2 , this is calculated such that the height, h, of island  18  is greater than or equal to the product of the window width, w, times the cotangent of the angle α between the surface normal, N, and the primary plane, or
 
h=w cot α
 
     In a second embodiment, the growth surface is patterned by a mask and grooves are etched into the surface with a depth greater than or equal to the product of the width of the grooves times the tangent of the angle between the surface normal and the GaN[0001] direction. This takes advantage of a large difference of growth rates along the GaN[0001] and along the opposite GaN[000-1] direction. As the former is higher and leads to defect reduction, the grooves are filled with defect-reduced material and tilted defects do not propagate to the surface. 
     With reference to  FIG. 3 , there is shown therein a structure  40  according to this second embodiment of this disclosure. Structure  40  comprises a template substrate  42 , which may be a 2 micron thick or thicker nitride layer (e.g., GaN, AlGaN, or InGaN or combinations thereof) of semi-polar orientation (e.g., other than (0001) or (1010) oriented surface) grown by MOCVD (or any other suitable method) on sapphire or any other suitable substrate (not shown). A dielectric layer  44 , for example SiO 2 , of 0.2 micron thickness is deposited on template substrate  42 . 
     Next, a photoresist layer  46  is deposited, and is shown in dashed outline to represent that it is subsequently removed from the final structure. Photoresist layer  46  is patterned by conventional photolithography to form a regular geometrical pattern of openings, for example 1 micrometer by 1 micrometer square (or alternatively, hexagonal or other shaped openings, or stripes). The spacing of these regular patterns may, for example, be in a range of 1 to 20 micron, although other spacings are contemplated and depend on the application of the teachings of the present disclosure. 
     The SiO 2  layer  44  as well as substrate  42  are then mask-patterned, for example by chemical assisted ion-beam etching (CAIBE) using layer  46  as a mask, thereby defining channels  48 . Channels  48  extend into substrate  42  and have a height h measured from the surface  52  of SiO 2  layer  44 , as described further below. Channels  48  define sidewalls  50  predominantly in the direction of the c-axis (i.e., perpendicular to growth surface  51 ) of the GaN unit cell and parallel to the basal plane of the GaN unit cell. 
     Prior to transfer into the growth chamber, selective surface cleaning is performed to remove the photoresist layer  46  while not attacking the SiO 2  layer  44 . Following transfer to the MOCVD growth chamber, the substrate is heated to 1050° C. in an ammonia atmosphere and a layer growth (GaN, AlGaN) starts at a rate of 0.5-4 micron/hr. Layer growth proceeds both vertically and laterally, and proceeds until full coalescence is achieved and layer  54  is formed. Thereafter, device structures (not shown) consisting of a plurality of doped or undoped GaN, InGaN, AlGaN, etc. layers may be deposited. 
     As will be appreciated from  FIG. 3 , the crystallographic axis of growth substrate  42  is inclined relative to the plane of growth surface  51 . The depth of channels  48  are precisely chosen so that they are at least sufficiently deep that any dislocation defects  56  communicated from substrate  42  into growth layer  54  forming new defects  58  terminate at sidewall  50 . With reference to  FIG. 4 , this is calculated such that the height, h, of channel  48  is greater than or equal to the product of the channel width, w, times the cotangent of the angle α between the surface normal, N, and the primary plane, or
 
h=w cot α
 
     In a third embodiment, the growth surface is mask-patterned and grooves are etched into the surface with a depth greater than at least 0.1-0.5 μm. After etching, a masking layer is selectively deposited on all growth surfaces except for the specific facet of the grooves that points in GaN[0001] direction. This blocks any growth other than along the GaN[0001] from the highly defective seed template or substrate. Thereby, the grooves are filled with defect-reduced material and tilted defects do not propagate to the surface. 
     With reference to  FIG. 5A , there is shown therein a structure  200  according to this third embodiment of this disclosure. Structure  200  comprises a template substrate  202 , which may be a 2 micron thick or thicker nitride layer (e.g., GaN, AlGaN, or InGaN or combinations thereof) of semi-polar orientation (e.g., other than (0001) or (1010) oriented surface) grown by MOCVD (or any other suitable method) on sapphire or any other suitable substrate (not shown). A photoresist layer (not shown) is deposited and patterned by conventional photolithography to form a regular geometrical pattern of openings, for example 1 micrometer by 1 micrometer square (or alternatively, hexagonal or other shaped openings, or stripes). The spacing of these regular patterns may, for example, be in a range of 1 to 20 micron, although other spacings are contemplated and depend on the application of the teachings of the present disclosure. 
     The substrate  202  is then mask-patterned, for example by chemical assisted ion-beam etching (CAIBE), thereby defining channels  204 . Channels  204  extend into substrate  202  and have a height h measured from the surface  206  of substrate  202 , as described further below. Channels  204  define sidewalls  208  predominantly in the direction of the c-axis (i.e., perpendicular to growth surface  206 ) of the GaN unit cell and parallel to the basal plane of the GaN unit cell. 
     With reference to  FIG. 5B , surface cleaning is performed to remove the photoresist layer. Then, the sample is loaded into an evaporation chamber in such a way that the facet pointing in the GaN[0001] direction is the only facet shaded against the evaporation source. This can be obtained by tilting the surface normal in GaN[0001] direction by about 5-10 degree. A 100 nm SiO 2  layer  210  is then deposited using electron beam evaporation, for example. This results in the deposition of layer  210  over surface  20 , and the bottom  204   a  and a first side  204   b  of channel  204 , but does not result in the deposition of layer  210  on a second side  204   c  of channel  204 . 
     Returning to  FIG. 5A , the substrate is transferred to an MOCVD growth chamber, and heated to 1050° C. in an ammonia atmosphere and a layer growth (GaN, AlGaN) starts at a rate of 0.5-4 micron/hr. Layer growth proceeds both vertically and laterally, and proceeds until full coalescence is achieved and layer  212  is formed. Thereafter, device structures (not shown) consisting of a plurality of doped or undoped GaN, InGaN, AlGaN, etc. layers may be deposited. 
     As will be appreciated from  FIGS. 5A and 5B , the crystallographic axis of growth substrate  202  is inclined relative to the plane of growth surface  212 . All facets are SiO 2 -covered except for the facet pointing in the GaN[0001] direction. 
     According to a fourth embodiment  60  shown in  FIG. 6 , either one or both of the techniques described above are employed together with additional processes in order to produce a structure capable of suppressing dislocation defects both in the c-plane (i.e., perpendicular to the plane of growth) as well as inclined thereto. This embodiment accommodates substrates with complex crystallographies and a variety of conditions leading to the formation of defects in materials grown thereover. 
     Accordingly, embodiment  60  includes a plurality of posts  62  inclined at an angle β out of the surface normal, which are formed by reducing the growth rate along the GaN[0001]-direction while increasing the growth rate in the perpendicular GaN[10 1 0] direction (or vice versa). The relative growth rates depend on the composition of the nitride semiconductor layer and can be controlled by growth parameters such as temperature, pressure, and V/III ratio. Depending on the width of the mask opening, the GaN[0001] facet (or the GaN[10 1 0] of post  62  will intersect with all vertically running defects  64  upon reaching a certain critical post height, h c , and thereby terminate those defects. Coalescence of the posts can be forced above the critical height by changing the growth parameters in such a way that favors new growth along the GaN[0001] direction. 
     With reference to  FIG. 6 , structure  60  comprises a template substrate  66 , which may be a 2 micron thick or thicker nitride layer (e.g., GaN, AlGaN, or InGaN or combinations thereof) of semi-polar orientation (e.g., other than (0001) or (1010) oriented surface) grown by MOCVD (or any other suitable method) on sapphire or any other suitable substrate (not shown). A dielectric layer  68 , for example SiO 2  2 microns thick, is deposited on template substrate  66 . 
     Next, a photoresist layer  70  is deposited, and is shown in dashed outline to represent that it is subsequently removed from the final structure. Photoresist layer  70  is patterned by conventional photolithography to form a regular pattern of openings, for example 1-micrometer wide stripes. The pitch of these stripes may be in a range of 10 to 20 micron, although other spacings are contemplated and depend on the application of the teachings of the present disclosure. A larger pitch (i.e., stripe separation) is beneficial for delaying the coalescence process, as discussed further below. The mask defines sidewalls predominantly in direction of the c-axis of the GaN unit cell. 
     For purposes of illustration, this embodiment  60  is described employing the first “island” embodiment described above, although the second “trench” embodiment described above is equally useful herein. The SiO 2  layer  68  is then mask-patterned, for example by chemical assisted ion-beam etching (CAIBE) thereby defining islands  72  and windows  74 . Islands  72  define sidewalls  76  predominantly in the direction of the c-axis (i.e., perpendicular to a growth surface  78 ) of the GaN unit cell and parallel to the basal plane of the GaN unit cell. 
     Prior to transfer into the growth chamber, selective surface cleaning is performed to remove the photoresist layer  70  while not attacking the SiO 2 . The growth chamber is a vertical quartz tube with a rotating two-inch SiC-coated graphite susceptor. The susceptor body is heated by inductive heating which is controlled via pyrometric temperature reading from the backside of the susceptor. Following transfer to the MOCVD growth chamber, the substrate is heated to 1050° C. in an ammonia atmosphere and a layer growth (GaN, AlGaN) starts at a rate of 2 micron/h (for growth on a planar substrate) and a reactor pressure of 200 Torr using input flow rates of 6.7 μmol/min trimethylgallium, 4 slpm ammonia, and 6 slpm hydrogen. Growth proceeds in the GaN[0001] direction at a rate of about 1 μm/h, and greater than 5 μm/h in the GaN[1010] direction. Thereby, pre-dominant growth occurs in the GaN[10 1 0] direction, yielding posts with side facets inclined by 58 degrees with respect to the surface. The GaN layer growth may proceed until full coalescence of layer  80  is achieved, much as described above. 
     Alternatively, after reaching a thickness where the sidewalls of the GaN posts are completely intercepting the surface normal over the mask openings the growth conditions may be changed to favor faster coalescence. For example, an AlGaN layer may be grown on top of the GaN post ends such that layer  80  is comprised of AlGaN. Afterwards, device structures consisting of a plurality of doped or undoped GaN, InGaN, AlGaN layers may be deposited. 
     The physics of modern electrical devices and the methods of their production are not absolutes, but rather statistical efforts to produce a desired device and/or result. Even with the utmost of attention being paid to repeatability of processes, the cleanliness of manufacturing facilities, the purity of starting and processing materials, and so forth, variations and imperfections result. Accordingly, no limitation in the description of the present disclosure or its claims can or should be read as absolute. The limitations of the claims are intended to define the boundaries of the present disclosure, up to and including those limitations. To further highlight this, the term “substantially” may occasionally be used herein in association with a claim limitation (although consideration for variations and imperfections is not restricted to only those limitations used with that term). While as difficult to precisely define as the limitations of the present disclosure themselves, we intend that this term be interpreted as “to a large extent”, “as nearly as practicable”, “within technical limitations”, and the like. 
     Furthermore, while a plurality of preferred exemplary embodiments have been presented in the foregoing detailed description, it should be understood that a vast number of variations exist, and these preferred exemplary embodiments are merely representative examples, and are not intended to limit the scope, applicability or configuration of the disclosure in any way. Various of the above-disclosed and other features and functions, or alternative thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications variations, or improvements therein or thereon may be subsequently made by those skilled in the art, which are also intended to be encompassed by the claims, below. 
     Therefore, the foregoing description provides those of ordinary skill in the art with a convenient guide for implementation of the disclosure, and contemplates that various changes in the functions and arrangements of the described embodiments may be made without departing from the spirit and scope of the disclosure defined by the claims thereto.