Patent Publication Number: US-8993996-B2

Title: Superlattice structure

Description:
REFERENCE TO RELATED APPLICATIONS 
     The current application claims the benefit of U.S. Provisional Application No. 61/610,636, which was filed on 14 Mar. 2012 and U.S. Provisional Application No. 61/768,799, which was filed on 25 Feb. 2013, both of which are hereby incorporated by reference. Additionally, the current application is a continuation-in-part of co-pending U.S. patent application Ser. No. 13/162,895, titled “Superlattice Structure,” which was filed on 17 Jun. 2011, which is a continuation-in-part of co-pending U.S. patent application Ser. No. 12/987,102, titled “Superlattice Structure,” which was filed on 8 Jan. 2011, and which claims the benefit of co-pending U.S. Provisional Application No. 61/293,614, titled “Superlattice Structures and Devices,” which was filed on 8 Jan. 2010, all of which are hereby incorporated by reference. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of the Grant No. IIP-0839492 awarded by the National Science Foundation. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to semiconductor devices, and more particularly, to a superlattice structure configured to reduce polarization effects of the semiconductor materials forming the devices. 
     BACKGROUND ART 
     In nitride based semiconductor materials and devices, including visible and ultraviolet (UV) light emitting diodes (LEDs), polarization effects play a dominant role causing strong built-in fields and spatial separation of electrons and holes. These polarization effects can negatively impact the performance of nitride-based visible and ultraviolet light emitting diodes. For example,  FIGS. 1A-1C  show illustrative band diagrams of a positive-intrinsic-negative (p-i-n) quantum well structure according to the prior art. In particular,  FIG. 1A  shows a band diagram of the structure without external bias and illumination;  FIG. 1B  shows a band diagram of the structure with the p-i-n field compensated by external bias; and  FIG. 1C  shows a band diagram of the structure with the total electric field compensated by external bias and intense optical excitation. 
     Polarization effects were evaluated for illustrative aluminum indium gallium nitride-based (Al x In y Ga 1-x-y N-based) multiple quantum well (MQW) structures. The MQW structures comprise an Al molar fraction in the quantum wells and barrier layers close to 20% and 40%, respectively, and In content in both the quantum wells and barriers of approximately 2% and 1%, respectively. The MQW structures comprise a total of three wells, each of which is two to four nanometers thick, separated by four five nanometer thick barriers. 
     Calculations indicated that the barriers and wells undergo tensions of 0.815% and 0.314%, respectively. These tensions correspond to piezoelectric charges at interfaces induced by this mismatch of −0.0484 coulombs per meter squared (C/m 2 ) for the well and −0.0134 C/m 2  for the barrier. The polarization charge was calculated as −0.041 C/m 2  and −0.049 C/m 2  for the wells and barriers, respectively. The total electric field in the well for an alternating sequence of barriers and wells was found to be 1.2 Megavolts per centimeter (MV/cm). About fifty percent of the field was due to piezoelectric effect and the remaining fifty percent was caused by spontaneous polarization, both having the same direction. This corresponds to a 0.12 eV band bending in a one nanometer wide quantum well. Such band bending precludes using wide quantum wells in deep UV LEDs, which decreases the overall LED efficiency by limiting the MQW design optimization to very narrow (i.e., one to two nanometer thick) quantum wells. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention provide a superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. The superlattice layer can be incorporated in various types of devices, and can allow for, for example, utilization of much wider quantum wells by avoiding the detrimental confined Stark effect, which prevents efficient radiative recombination. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation. 
     A first aspect of the invention provides a structure comprising: a first layer; and a superlattice layer having a first side adjacent to the first layer, the superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first composition and a first polarization; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second composition distinct from the first composition and a second polarization opposite the first polarization. 
     A second aspect of the invention provides a method comprising: creating a structure design for a device, the structure design including a first layer and a superlattice layer having a first side adjacent to the first layer, the superlattice layer comprising a plurality of periods, the creating the structure design including: selecting a first composition having a first polarization for a first sub-layer of each of the plurality of periods; and selecting a second composition having a second polarization for a second sub-layer of each of the plurality of periods, wherein the second sub-layer is adjacent to the first sub-layer, and wherein the second composition is distinct from the first composition and the second polarization is opposite the first polarization. 
     A third aspect of the invention provides a group III nitride-based device comprising: a p-type contact comprising: a first p-type layer; and a p-type superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride-based composition and a first polarization; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride-based composition distinct from the first composition and a second polarization opposite the first polarization, wherein the first polarization and the second polarization comprise at least one of: a strain-induced polarization or a spontaneous polarization. 
     A fourth aspect of the invention provides a structure comprising: a first layer; and a superlattice layer having a first side adjacent to the first layer, the superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride composition and a first polarization, wherein the first group III nitride composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride composition distinct from the first group III nitride composition and a second polarization opposite the first polarization. 
     A fifth aspect of the invention provides a method comprising: creating a structure design for a device, the structure design including a first layer and a superlattice layer having a first side adjacent to the first layer, the superlattice layer comprising a plurality of periods, the creating the structure design including: selecting a first group III nitride composition having a first polarization for a first sub-layer of each of the plurality of periods, wherein the first group III nitride composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and selecting a second group III nitride composition having a second polarization for a second sub-layer of each of the plurality of periods, wherein the second sub-layer is adjacent to the first sub-layer, and wherein the second group III nitride composition is distinct from the first group III nitride composition and the second polarization is opposite the first polarization. 
     A sixth aspect of the invention provides a group III nitride-based device comprising: a p-type contact comprising: a first p-type layer; and a p-type superlattice layer including a plurality of periods, each of the plurality of periods including: a first sub-layer having a first group III nitride-based composition and a first polarization, wherein the first group III nitride-based composition is selected such that the first sub-layer has a transparency of at least a target transparency to ultraviolet radiation of a target wavelength; and a second sub-layer adjacent to the first sub-layer, the second sub-layer having a second group III nitride-based composition distinct from the first composition and a second polarization opposite the first polarization, wherein the first polarization and the second polarization comprise at least one of: a strain-induced polarization or a spontaneous polarization. 
     The illustrative aspects of the invention are designed to solve one or more of the problems herein described and/or one or more other problems not discussed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the disclosure will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various aspects of the invention. 
         FIGS. 1A-1C  show illustrative band diagrams of a p-i-n quantum well structure according to the prior art. 
         FIGS. 2A and 2B  show illustrative structures according to the prior art and an embodiment, respectively. 
         FIG. 3  shows a conduction band diagram comparing a conduction band profile for a conventional quantum well and a conduction band profile for a quantum well according to an embodiment. 
         FIG. 4  shows another illustrative structure according to an embodiment. 
         FIG. 5  shows a chart of a calculated electric field at a heterointerface between gallium nitride (GaN) and aluminum indium nitride (AlInN) as a function of the indium molar fraction in the AlInN according to an embodiment. 
         FIG. 6  shows an illustrative light emitting device structure according to an embodiment. 
         FIG. 7  shows a dependence of the absorption coefficient on the wavelength for various aluminum molar fractions (x) of an Al x Ga 1-x N alloy according to an embodiment. 
         FIG. 8  shows an illustrative chart for selecting an aluminum content of an AlGaN alloy to maintain a target transparency for a corresponding emitted wavelength according to an embodiment. 
         FIG. 9  shows an illustrative lattice configuration of a gallium nitride layer including domain inversion according to an embodiment. 
         FIG. 10  shows an illustrative flow diagram for fabricating a circuit according to an embodiment. 
     
    
    
     It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As indicated above, aspects of the invention provide a superlattice layer including a plurality of periods, each of which is formed from a plurality of sub-layers. Each sub-layer comprises a different composition than the adjacent sub-layer(s) and comprises a polarization that is opposite a polarization of the adjacent sub-layer(s). In this manner, the polarizations of the respective adjacent sub-layers compensate for one another. The superlattice layer can be incorporated in various types of devices, and can allow for, for example, utilization of much wider quantum wells by avoiding the detrimental confined Stark effect, which prevents efficient radiative recombination. Furthermore, the superlattice layer can be configured to be at least partially transparent to radiation, such as ultraviolet radiation. As used herein, unless otherwise noted, the term “set” means one or more (i.e., at least one) and the phrase “any solution” means any now known or later developed solution. 
     Turning to the drawings,  FIGS. 2A and 2B  show illustrative structures  2 ,  10  according to the prior art and an embodiment, respectively. As illustrated in  FIG. 2A , structure  2  includes a superlattice layer  4 , which includes a plurality of repeating sub-layers  6 A- 6 C. Each sub-layer  6 A- 6 C can be separated from another sub-layer by a second set of sub-layers  8 A- 8 B in the superlattice layer  4 . Superlattice structure  4  can be configured to perform any type of function as part of a device incorporating structure  2 . For example, sub-layers  6 A- 6 C can comprise a set of quantum wells and sub-layers  8 A- 8 B can comprise a set of barriers. In this case, superlattice layer  4  can comprise a multiple quantum well structure. 
     As shown in  FIG. 2B , an embodiment of the invention provides a structure  10  including a superlattice layer  12  that is configured, for example, to reduce polarization effects. In particular, the superlattice layer  12  includes multiple periods  14 A- 14 C, each of which includes two or more sub-layers  16 ,  18  having different compositions. Adjacent sub-layers  16 ,  18  in each period  14 A- 14 C are configured to have polarizations (e.g., built-in electric fields) that at least partially cancel one another. For example, sub-layer  16  can comprise a spontaneous polarization having an opposite sign as a spontaneous polarization of sub-layer  18 . Similarly, sub-layer  16  can comprise a strain-induced polarization having an opposite sign of a strain-induced polarization of sub-layer  18 . Still further, one type of polarization in sub-layer  16  can have an opposite sign of another type of polarization in sub-layer  18 , thereby reducing the net polarization present due to a combination of multiple types of polarizations (e.g., spontaneous and strain-induced). 
     In an embodiment, sub-layer  16  can comprise a positive or negative spontaneous polarization, while sub-layer  18  comprises the other of the positive or negative spontaneous polarization. In a more particular embodiment, the absolute values of the spontaneous polarizations of sub-layers  16 ,  18  are substantially equal, so that the net spontaneous polarization for the period  14 A- 14 C is close to zero. In another embodiment, sub-layer  16  can comprise a strain-induced (e.g., piezoelectric) polarization due to stretching or compression, while sub-layer  18  comprises a strain-induced polarization due to the other of stretching or compression. In a more particular embodiment, the absolute values of the strain-induced polarizations of sub-layers  16 ,  18  are substantially equal, so that the net strain-induced polarization for the period  14 A- 14 C is close to zero. It is understood that the respective spontaneous and/or strain-induced polarizations of sub-layers  16 ,  18  can be configured to only partially reduce the net spontaneous and/or strain-induced polarization for the period  14 A- 14 C. 
     In still another embodiment, the spontaneous and/or strain-induced polarization of one sub-layer  16 ,  18  is configured to at least partially compensate the other of the spontaneous and/or strain-induced polarization of the other sub-layer  16 ,  18 . For example, sub-layer  16  can comprise a spontaneous polarization of a first sign, and sub-layer  18  can comprise a strain-induced polarization of the opposite sign. In this case, the net polarization for the period  14 A- 14 C will be reduced due to the two types of polarizations of the sub-layers  16 ,  18  compensating one another. 
     The various periods  14 A- 14 C in superlattice layer  12  can be separated from one another by a set of additional sub-layers  20 A- 20 B. In an embodiment sub-layers  20 A- 20 B comprise inactive layers having no polarization. In another embodiment, each period  14 A- 14 C comprises a quantum well, while each sub-layer  20 A- 20 B comprises a barrier. In this case, superlattice layer  12  comprises a multiple quantum well structure. The periods (e.g., quantum wells)  14 A- 14 C in superlattice layer  12  can be wider than the conventional sub-layers (e.g., quantum wells)  6 A- 6 C. For example, in an embodiment, the width of superlattice layer  12  can be greater than two nanometers. In a more particular embodiment, the width of superlattice layer  12  is between approximately three nanometers and eight nanometers. In particular, periods  14 A- 14 C will comprise a much smaller polarization field than that of a conventional sub-layer  6 A- 6 C of a similar width. As a result, the detrimental confined Stark effect is avoided, which separates electrons and holes within a quantum well and prevents efficient radiative recombination. 
       FIG. 3  shows a conduction band diagram comparing a conduction band profile  22  for a conventional quantum well  6 A ( FIG. 2A ) and a conduction band profile  24  for a quantum well  14 A ( FIG. 2B ) according to an embodiment. As illustrated, the conduction band profile  24  comprises a more shallow profile than that of the conduction band profile  22 . As a result, electrons in quantum well  14 A can spread out within the quantum well  14 A more than the electrons in quantum well  6 A, providing for a more efficient radiative recombination. 
     Returning to  FIG. 2B , superlattice  12  can perform any function as part of a device formed using structure  10 . To this extent, superlattice  12  is located between a first layer  26  and a second layer  28  of the structure  10 . In an illustrative embodiment, first layer  26  and second layer  28  can be formed from two dissimilar materials (e.g., two dissimilar nitride based semiconductor materials), and superlattice  12  can be graded in such a manner that it compensates (e.g., reduces) strain exerted by the dissimilar materials of layers  26 ,  28 . For example, the lattice structure of each sub-layer  16 ,  18  of superlattice  12  can gradually change from a lattice structure similar to first layer  26  to a lattice structure similar to second layer  28 . 
     While periods  14 A- 14 C are each shown including two sub-layers  16 ,  18 . It is understood that each period  14 A- 14 C can include any number of sub-layers  16 ,  18 . Similarly, while superlattice layer  12  is shown including three periods  14 A- 14 C, it is understood that superlattice layer  12  can include any number of two or more periods  14 A- 14 C. For example,  FIG. 4  shows another illustrative structure  30  according to an embodiment. Structure  30  includes a superlattice layer  32 , which comprises four periods  34 A- 34 D that are separated by three sub-layers  36 A- 36 C. Each period  34 A- 34 D is formed by a set of six sub-layers of alternating compositions and polarizations. To this extent, each sub-layer of each period  34 A- 34 D is immediately adjacent to one or two sub-layers having a different composition and an opposite polarization (e.g., spontaneous and/or strain-induced as described herein). In this manner, the periods  34 A- 34 D can be made even wider than the conventional sub-layers  6 A- 6 C of the prior art with smaller polarization fields than a conventional sub-layer  6 A- 6 C of a similar thickness. 
     In an embodiment, structures  10  ( FIG. 2B) and 30  ( FIG. 4 ) can comprise nitride-based heterostructures. In a more specific embodiment, the structures  10 ,  30  comprise group III nitride-based heterostructures. In this case, the periods  14 A- 14 C,  34 A- 34 D of each structure  10 ,  30 , respectively, each can be formed of group III nitride materials. Group III nitride materials comprise one or more group III elements (e.g., boron (B), aluminum (Al), gallium (Ga), and indium (In)) and nitrogen (N), such that B W Al X Ga Y In Z N, where 0≦W, X, Y, Z≦1, and W+X+Y+Z=1. Illustrative group III nitride materials include AlN, GaN, InN, BN, AlGaN, AlInN, AlBN, AlGaInN, AlGaBN, AlInBN, and AlGaInBN with any molar fraction of group III elements. In an even more specific embodiment, the sub-layers described herein are quaternary or ternary group III nitride sub-layers, such as such as AlInN, AlGaN, InGaN, or AlInGaN. For further strain and/or polarization reduction, one or more sub-layers  16 ,  18  forming each period  14 A- 14 C,  34 A- 34 D can be doped. The sub-layers  16 ,  18  can be doped p-type or n-type. Furthermore, a sub-layer  16 ,  18  can comprise a monolayer. 
       FIG. 5  shows a chart of a calculated electric field at a heterointerface between gallium nitride (GaN) and aluminum indium nitride (AlInN) as a function of the indium molar fraction in the AlInN according to an embodiment. As illustrated, the calculated electric field drops to zero and goes negative as the indium molar fraction exceeds 0.7. In an illustrative embodiment, each sub-layer  16 ,  18  comprises AlInN with differing molar fractions of In. For example, sub-layer  16  can comprise an In molar fraction of approximately 0.65, which results in a calculated electric field of approximately 0.5 MV/cm, and sub-layer  18  can comprise an In molar fraction of approximately 0.77, which results in a calculated electric field of approximately −0.5 MV/cm. In this manner, the electric fields of both sub-layers  16 ,  18  can substantially cancel one another. 
     The superlattices  12 ,  32  described herein can be implemented as part of structures  10 ,  30  utilized for various types of devices, e.g., which are fabricated using semiconductor materials where polarization effects play a role. A superlattice  12 ,  32  described herein can be utilized as, for example, a multiple quantum well, an integral part of an ohmic and/or Schottky contact, a cladding layer, a buffer layer, a barrier layer, and/or the like, for the device. In an illustrative embodiment, structure  10  comprises a p-type contact including superlattice  12  and a metal layer  26  located thereon. 
     A structure  10 ,  30  described herein can be implemented as part of, for example, a light emitting device, such as a light emitting diode (LED), a superluminescent diode, or a laser. The light emitting device can comprise a visible light emitting device, an ultraviolet light emitting device, and/or the like. In this case, the light emitting device can include one or more superlattices as cladding layer(s), ohmic contact(s), and/or the like. In a more particular embodiment, the superlattice is formed as part of an ohmic contact for an ultraviolet light emitting device where a top p-type contact layer (e.g., layer  26  of  FIG. 2B ) of the ohmic contact, which is transparent to ultraviolet radiation, is located directly on the superlattice  12 ,  32 . In a still more particular embodiment, the top p-type contact layer comprises AlInN. 
     It is understood that any combination of one or more layers (or sub-layers) in a structure  10 ,  30  can be configured to be at least partially transparent (e.g., semi-transparent or transparent) to radiation, such as ultraviolet radiation. As used herein, a layer is at least partially transparent to ultraviolet radiation if it allows more than approximately 0.001 percent of the ultraviolet radiation to pass there through. In a more particular embodiment, an at least partially transparent layer is configured to allow more than approximately five percent of the ultraviolet radiation to pass there through. In an embodiment, the at least partially transparent layer(s) are configured to be at least partially transparent to ultraviolet radiation emitted by the structure  10 ,  30 . For example, the at least partially transparent layer(s) can be configured to be at least partially transparent to ultraviolet radiation in a range including the peak emission wavelength of the structure  10 ,  30  and at least five nanometers above and/or below the peak emission wavelength. 
     The layer(s) at least partially transparent to the ultraviolet radiation can be formed using any solution. For example, a transparent layer can comprise a p-type layer formed of a group III nitride material described herein. Illustrative at least partially transparent group-III nitride materials include AlGaN, AlInGaN, boron-containing alloys (GaBN, AlBN, AlGaBN, AlInGaBN, InGaBN, and/or the like), and/or the like. Furthermore, the at least partial transparency of a layer can be achieved using any solution. For example, at least partial transparency can be achieved in materials with bandgaps smaller than a photon energy of the ultraviolet radiation due to tunneling, thermionic transport via impurity states, and/or the like. 
     Similarly, it is understood that any combination of one or more layers in a structure  10 ,  30  can be configured to reflect ultraviolet radiation. As used herein, a layer is reflective of ultraviolet radiation when it reflects more than approximately five percent of the ultraviolet radiation. In an embodiment, the reflective layer(s) are configured to reflect ultraviolet radiation emitted by the structure  10 ,  30 . For example, the reflective layer(s) can be configured to reflect ultraviolet radiation in a range including the peak emission wavelength of the structure  10 ,  30  and at least five nanometers above and/or below the peak emission wavelength. 
     The ultraviolet reflective layer(s) can be formed using any solution. For example, a reflective layer can comprise a metal coating formed of Al, Rhodium (Rh), enhanced Al, enhanced Rh, Gold (Au), Aluminum Silicon Monoxide (AlSiO), Aluminum Magnesium Fluoride (AlMgF 2 ), and/or the like. Furthermore, the reflectivity of a layer can be achieved using any solution. For example, reflectivity can be achieved by the formation of a reflecting photonic crystal, a distributed Bragg reflector (DBR) structure, and/or the like. 
     The at least partially ultraviolet transparent and/or reflective layer(s) can comprise any of various layers of a structure  10 ,  30  based on a desired operating configuration for the structure  10 ,  30 . For example, a structure  10 ,  30  can include an at least partially ultraviolet transparent contact. Such a contact can comprise, for example, a p-type at least partially ultraviolet transparent layer  26  ( FIG. 2B ) and an interlayer, such as an at least partially ultraviolet transparent superlattice  12  ( FIG. 2B ), for making a p-type ohmic contact, Schottky contact, non-ohmic contact, and/or the like. Similarly, a structure  10 ,  30  can include an ultraviolet reflecting contact, which is configured to reflect a desired amount of the ultraviolet radiation generated by the structure  10 ,  30 . Such a reflecting contact also can include, for example, a p-type ultraviolet reflective layer  26  and a superlattice  12  for making a p-type ohmic contact, Schottky contact, non-ohmic contact, and/or the like. 
     A structure  10 ,  30  can include various other layers, which are at least partially ultraviolet transparent and/or ultraviolet reflective, such as a p-type superlattice  12 ,  32 , an electron blocking layer located between a superlattice  12 ,  32  and a multiple quantum well structure, and/or the like. In each case, the at least partially ultraviolet transparent and/or ultraviolet reflective layer can be formed using any type of material. In an embodiment the at least partially ultraviolet transparent and/or ultraviolet reflective layer is formed using a group-III nitride material, such as boron-containing layers. 
       FIG. 6  shows an illustrative light emitting device structure  40  according to an embodiment. As illustrated, the device structure  40  comprises an n-type contact layer  50  adjacent to a radiation generating structure  52 . Radiation generating structure  52  can comprise any type of structure, such as a multiple quantum well structure, for generating any type of radiation, such as ultraviolet light. Furthermore, device structure  40  includes a p-type contact layer  54  on an opposing side of the radiation generating structure  52  as the n-type contact layer  50 . 
     The device structure  40  further includes a superlattice layer  12 , which can be formed as described herein. Superlattice layer  12  is shown located on the same side of the radiation generating structure  52  as the p-type contact layer  54 . In an embodiment, the superlattice layer  12  is at least partially transparent to radiation generated by radiation generating structure  52 . It is understood that superlattice layer  12  is only illustrative of the types of superlattices that can be included in the device structure  40 . For example, the device structure  40  could include superlattice  32  and/or a variant of the superlattices shown herein. 
     The device structure  40  also can include an electron blocking layer  56 , which can be located between the superlattice layer  12  and the radiation generating structure  52 . In an embodiment, the electron blocking layer  56  has a thickness in a range between approximately two and approximately one hundred nanometers. The electron blocking layer  56  can comprise a p-type composition having a larger band gap than the barrier(s) located within the superlattice layer  12 , which can result in an improved transparency of the electron blocking layer to radiation generated by the radiation generating structure  52 . Furthermore, the electron blocking layer  56  can comprise a graded composition, which can be configured to decrease a resistance of the electron blocking layer  56 . For example, the electron blocking layer  56  can have a graded doping that increases or decreases by approximately 10 4  cm −3 , e.g., between approximately 10 16  and approximately 10 20  cm −3 . Alternatively, the electron blocking layer  56  can have a homogeneous doping within the range of approximately 10 16  and approximately 10 20  cm −3 . 
     The device structure  40  can include a contact  60 . Contact  60  can comprise any type of contact. In an embodiment, the contact  60  comprises a p-type metal contact, such as a Schottky contact, a leaky Schottky contact, a rectifying contact, and/or the like. In a more specific embodiment, the contact  60  at least partially reflects the radiation generated by the radiation generating structure  52  and can be formed from, among other things, aluminum, enhanced aluminum, aluminum silicon monoxide, aluminum magnesium fluoride, rhodium, enhanced rhodium, gold, and/or the like. In another more specific embodiment, the contact  60  is at least partially transparent to the radiation generated by the radiation generating structure  52  and can be formed from, among other things, a metallic superlattice, in which each layer is at least partially transparent to the radiation. In either case, the contact  60  can be directly adjacent to a transparent adhesion layer  58 . The transparent adhesion layer  58  can be configured to improve ohmic properties of the contact  60  and promote adhesion of the contact  60  to a surface of the semiconductor (e.g., layer  54 ). In an embodiment, the transparent adhesion layer  58  is formed of nickel. However, it is understood that transparent adhesion layer  58  can be formed of any suitable material, including Nickel oxyhydroxide (NiOx), Palladium (Pd), Molybdenum (Mo), Cobalt (Co), and/or the like. 
     The various layers in the device structure  40  can be formed using any type of materials. In an embodiment, the device structure  40  comprises a group III nitride-based heterostructure, in which one or more of the layers  50 ,  56 ,  12 , and  54  and radiation generating structure  52  are formed of various group III nitride materials using any solution. Additionally, contact  60  can be implemented without a transparent adhesion layer  58 , and be formed of one or more layers of metal, such as for example, one or more layers of titanium, aluminum, gold, chromium, nickel, platinum, lead, rhodium, and/or the like. 
     In an embodiment, one or more of the contacts  50 ,  54 ,  60  comprises graphene, which can be configured to be transparent to radiation generated by the radiation generating structure  52  and very conductive. For example, the p-type contact layer  54  to the superlattice layer  12  and/or contact  60  can be at least partially formed of p-type graphene. Similarly, the n-type contact layer  50  can be at least partially formed of n-type graphene. In an embodiment, a contact  50 ,  54 ,  60  comprises a graphene composite contact, which includes a graphene sub-layer adjacent to a thin sub-layer of metal, which can improve current spreading in the contact  50 ,  54 ,  60 . In a further embodiment, the graphene composite contact is at least partially transparent to the radiation generated by the radiation generating structure  52 . It is understood that the device structure  40  can include one or more layers, such as transparent adhesion layer  58  and/or contact  60 , adjacent to a contact formed of graphene, such as contact  54 , which are configured to improve light extraction from the device structure  40 , e.g., via a textured surface. 
     In an embodiment, a structure described herein can include one or more layers having a composition selected such that the layer has a transparency of at least a target transparency to radiation, such as ultraviolet radiation, of a target set of wavelengths. The layer can comprise, for example, a p-type contact layer  54  ( FIG. 6 ), an electron blocking layer  56  ( FIG. 6 ), a superlattice layer  12  ( FIG. 6 ), and/or the like. For example, a layer can be a group III nitride-based layer, which is composed of Al x Ga 1-x N where the aluminum molar fraction (x) is sufficiently high in some domains of the layer to result in the layer being at least partially transparent to ultraviolet radiation. In an embodiment, the layer can comprise a superlattice layer located in an emitting device configured to emit radiation having a dominant wavelength in the ultraviolet spectrum, and the composition of at least one sub-layer in each period of the superlattice layer is configured to be at least partially transparent to ultraviolet radiation having a target wavelength corresponding to the ultraviolet radiation emitted by the emitting device. 
     In an embodiment, the sub-layer has a thickness in a range between approximately one and approximately one thousand nanometers. Furthermore, the sub-layer can have a graded doping that increases or decreases by approximately 10 4  cm −3 , e.g., between approximately 10 16  and approximately 10 20  cm −3 . Alternatively, the sub-layer can have a homogeneous doping within the range of approximately 10 16  and approximately 10 20  cm −3 . The doping can be any type of doping. For example, the doping can be: modulation doping; unintentional doping by impurities from one or more of oxygen, hydrogen, and magnesium; a dopant, such as magnesium and/or carbon, diffused from another doped layer or present in the growth chamber as residual elements; and/or the like. In an embodiment, one or more sub-layers can be co-doped with magnesium and carbon, where both the carbon and magnesium doping levels are within the range of approximately 10 16  and approximately 10 20  cm −3 , but the combined concentration of the dopants does not exceed approximately 10 20  cm −3 . In another embodiment, the doping can alternate between two or more dopants. For example, a sub-layer can include carbon doping, while the adjacent sub-layer(s) can include magnesium doping. 
     An amount of transparency of a short period superlattice (SPSL) can be approximated by computing the averaged band gap of the SPSL, and deducing average absorption coefficient of the SPSL. The absorption coefficients depend on an absorption edge of the semiconductor material, which for materials formed of an AlGaN alloy, is a function of the molar fractions of the Al x Ga 1-x N semiconductor alloy. 
     In an embodiment, the target transparency for the material is at least ten times more transparent than the least transparent layer of material in the structure (e.g., GaN for a group III nitride-based device). In this case, an absorption coefficient of the semiconductor layer can be on the order of 10 4  inverse centimeters or lower. In this case, a one micron thick semiconductor layer will allow approximately thirty-six percent of the ultraviolet radiation to pass there through. 
       FIG. 7  shows a dependence of the absorption coefficient on the wavelength for various aluminum molar fractions (x) of an Al x Ga 1-x N alloy according to an embodiment. In order to maintain an absorption coefficient of the semiconductor layer at orders of 10 4  inverse centimeters or lower, the content of aluminum in an SPSL barrier layer can be chosen based on the corresponding target wavelength or range of wavelengths. For example, for a target wavelength of approximately 250 nanometers, the aluminum molar fraction can be approximately 0.7 or higher, whereas for a target wavelength of approximately 300 nanometers, the aluminum molar fraction can be as low as approximately 0.4.  FIG. 8  shows an illustrative chart for selecting an aluminum content of an Al x Ga 1-x N alloy to maintain a target transparency for a corresponding emitted wavelength, λ, according to an embodiment. In this case, the target transparency corresponds to an absorption coefficient of the semiconductor layer on the order of 10 4  inverse centimeters. Note that in  FIG. 8 , the dependence of x=x(λ) is linear, with x=C·λ+B, where C=−0.0048 nm −1 , and B=1.83. 
     In an embodiment, one or more sub-layers of the SPSL can have a graded composition. For example, a sub-layer of the SPSL can be formed of an Al x Ga 1-x N alloy, where the aluminum molar fraction, x, is continually varied in the vertical direction of the sub-layer. 
     In an embodiment, a device can include one or more layers with lateral regions configured to facilitate the transmission of radiation through the layer and lateral regions configured to facilitate current flow through the layer. For example, the layer can be a short period superlattice, which includes barriers alternating with wells. In this case, the barriers can include both transparent regions, which are configured to reduce an amount of radiation that is absorbed in the layer, and higher conductive regions, which are configured to keep the voltage drop across the layer within a desired range. As used herein, the term lateral means the plane of the layer that is substantially parallel with the surface of the layer adjacent to another layer of the device. As described herein, the lateral cross section of the layer can include a set of transparent regions, which correspond to those regions having a relatively high aluminum content, and a set of higher conductive regions, which correspond to those regions having a relatively low aluminum content. 
     The set of transparent regions can be configured to allow a significant amount of the radiation to pass through the layer, while the set of higher conductive regions can be configured to keep the voltage drop across the layer within a desired range (e.g., less than ten percent of a total voltage drop across the structure). In an embodiment, the set of transparent regions occupy at least ten percent of the lateral area of the layer, while the set of higher conductive regions occupy at least approximately two percent (five percent in a more specific embodiment) of the lateral area of the layer. Furthermore, in an embodiment, a band gap of the higher conductive regions is at least five percent smaller than the band gap of the transparent regions. In a more particular embodiment, the transparent regions comprise a transmission coefficient for radiation of a target wavelength higher than approximately sixty percent (eighty percent in a still more particular embodiment), while the higher conductive regions have a resistance per unit area to vertical current flow that is smaller than approximately 10 −2  ohm·cm 2 . As used herein, the term transmission coefficient means the ratio of an amount of radiation exiting the region to an amount of radiation entering the region. 
     The transparent and conductive regions can be formed using any solution. For example, a layer can be grown using migration-enhanced metalorganic chemical vapor deposition (MEMOCVD). During the growth, inhomogeneities in the lateral direction of a molar fraction of one or more elements, such as aluminum, gallium, indium, boron, and/or the like, can be allowed in the layer. In an embodiment, such compositional inhomogeneities can vary by at least one percent. 
     In an embodiment, a light emitting device structure can include one or more structures configured to reduce an overall polarity of the structure. In embodiment, the structure can form a cladding layer, a p-type contact layer, and/or the like, of a light emitting device. In order to confine a polarization charge within a sub-layer, the sub-layer thicknesses can be larger than a Bohr radius of the carriers. Using a p-type contact layer as an illustrative example, the Bohr radius, R B , can be calculated for hole carriers. In this case, the Bohr radius is given by R B =4π∈ℏ 2 /m h e 2 , where ∈ is the permittivity of the material, ℏ is the reduced Planck&#39;s constant, m h  is the hole rest mass, and e is the elementary charge. For the composition Al 0.5 Ga 0.5 N, the mass of an “average” hole is about four times the electron rest mass (m h ˜4m e ), the permittivity is approximately nine times the permittivity of free space (∈˜9∈ 0 ), and the resulting Bohr radius, R B , is approximately 9/4 of the Bohr radius of hydrogen, R H , that is R B ˜1.2 nm. A group III semiconductor layer having a higher concentration of gallium will have a smaller hole mass (e.g., for GaN, m h ˜1.4). As a result, such a group III semiconductor layer can have a Bohr radius, R B ˜6×R H =3.2 nm. 
     An AlGaN film deposited by MOCVD on a substrate formed of sapphire, SiC, Si, and/or the like, typically grows with its gallium face up. This growth corresponds to the growth direction of the film being [0001], the positive c-axis direction. However, growth of a heavily Mg-doped AlGaN layer by MOCVD can produce a negative c-axis direction (N-face growth) of AlGaN. The inversion of polarity can reduce an overall “average” polarity within a given sub-layer. To this extent,  FIG. 9  shows an illustrative lattice configuration of a gallium nitride layer including domain inversion according to an embodiment. As illustrated the layer includes a plurality of lateral domains, at least one of which is a nitride facing domain (N-face) and at least one of which is a gallium facing domain (Ga-face). As illustrated, the polarization (P S ) and electric field (E) vectors are inversed on either side of the boundary between the domains. 
     Structures  10 ,  30  described herein can be incorporated as part of, for example, a transistor (e.g., a field effect transistor), a photodetector, a monolithic and/or optoelectronic integrated circuit, a metal-semiconductor diode, a p-n junction diode, a switch, and/or the like. In this case, the device can include one or more superlattices as buffer layer(s), barrier layer(s), contact layer(s), and/or the like. In a more particular embodiment, the periods of the superlattice layer are formed from AlInN. 
     While shown and described herein with respect to the fabrication of a superlattice layer, it is understood that an embodiment of the invention can be applied to the fabrication of a heterostructure comprising a set of quantum wells and a set of barriers. The various sub-layers shown and described herein can be formed using any solution. For example, the superlattice layers  12 ,  32  can be grown using a combination of metallo organic chemical vapor deposition (MOCVD) and/or migration enhanced MOCVD (MEMOCVD), in which each period in the superlattice layer  12 ,  32  requires at least two growth steps. 
     While shown and described herein as a method of designing and/or fabricating a structure and/or a corresponding semiconductor device including the structure, it is understood that aspects of the invention further provide various alternative embodiments. For example, in one embodiment, the invention provides a method of designing and/or fabricating a circuit that includes one or more of the semiconductor devices designed and fabricated as described herein (e.g., including one or more superlattice layers  12 ,  32 ). 
     To this extent,  FIG. 10  shows an illustrative flow diagram for fabricating a circuit  126  according to an embodiment. Initially, a user can utilize a device design system  110  to generate a device design  112  using a method described herein. The device design  112  can comprise program code, which can be used by a device fabrication system  114  to generate a set of physical devices  116  according to the features defined by the device design  112 . Similarly, the device design  112  can be provided to a circuit design system  120  (e.g., as an available component for use in circuits), which a user can utilize to generate a circuit design  122  (e.g., by connecting one or more inputs and outputs to various devices included in a circuit). The circuit design  122  can comprise program code that includes a device designed using a method described herein. In any event, the circuit design  122  and/or one or more physical devices  116  can be provided to a circuit fabrication system  124 , which can generate a physical circuit  126  according to the circuit design  122 . The physical circuit  126  can include one or more devices  116  designed using a method described herein. 
     In another embodiment, the invention provides a device design system  110  for designing and/or a device fabrication system  114  for fabricating a semiconductor device  116  by using a method described herein. In this case, the system  110 ,  114  can comprise a general purpose computing device, which is programmed to implement a method of designing and/or fabricating the semiconductor device  116  as described herein. Similarly, an embodiment of the invention provides a circuit design system  120  for designing and/or a circuit fabrication system  124  for fabricating a circuit  126  that includes at least one device  116  designed and/or fabricated using a method described herein. In this case, the system  120 ,  124  can comprise a general purpose computing device, which is programmed to implement a method of designing and/or fabricating the circuit  126  including at least one semiconductor device  116  as described herein. 
     In still another embodiment, the invention provides a computer program fixed in at least one computer-readable medium, which when executed, enables a computer system to implement a method of designing and/or fabricating a semiconductor device as described herein. For example, the computer program can enable the device design system  110  to generate the device design  112  as described herein. To this extent, the computer-readable medium includes program code, which implements some or all of a process described herein when executed by the computer system. It is understood that the term “computer-readable medium” comprises one or more of any type of tangible medium of expression, now known or later developed, from which a copy of the program code can be perceived, reproduced, or otherwise communicated by a computing device. For example, the computer-readable medium can comprise: one or more portable storage articles of manufacture; one or more memory/storage components of a computing device; paper; and/or the like. 
     In another embodiment, the invention provides a method of providing a copy of program code, which implements some or all of a process described herein when executed by a computer system. In this case, a computer system can process a copy of the program code to generate and transmit, for reception at a second, distinct location, a set of data signals that has one or more of its characteristics set and/or changed in such a manner as to encode a copy of the program code in the set of data signals. Similarly, an embodiment of the invention provides a method of acquiring a copy of program code that implements some or all of a process described herein, which includes a computer system receiving the set of data signals described herein, and translating the set of data signals into a copy of the computer program fixed in at least one computer-readable medium. In either case, the set of data signals can be transmitted/received using any type of communications link. 
     In still another embodiment, the invention provides a method of generating a device design system  110  for designing and/or a device fabrication system  114  for fabricating a semiconductor device as described herein. In this case, a computer system can be obtained (e.g., created, maintained, made available, etc.) and one or more components for performing a process described herein can be obtained (e.g., created, purchased, used, modified, etc.) and deployed to the computer system. To this extent, the deployment can comprise one or more of: (1) installing program code on a computing device; (2) adding one or more computing and/or I/O devices to the computer system; (3) incorporating and/or modifying the computer system to enable it to perform a process described herein; and/or the like. 
     The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to an individual in the art are included within the scope of the invention as defined by the accompanying claims.