Abstract:
An optical semiconductor device such as a light emitting diode is formed on a transparent substrate having formed thereon a template layer, such as AlN, which is transparent to the wavelength of emission of the optical device. A mixed alloy defect redirection region is provided over the template layer such that the composition of the defect redirection region approaches or matches the composition of the regions contiguous thereto. For example, the Al content of the defect redirection region may be tailored to provide a stepped or gradual Aluminum content from template to active layer. Strain-induced cracking and defect density are reduced or eliminated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of copending U.S. Application for Letters Patent titled “Variable Period Variable Composition Superlattice And Devices Including Same”, Ser. No. 11/470,569, filed on Sep. 6, 2006, which is incorporated by reference herein in its entirety, and to which priority is hereby claimed. 
    
    
     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(s) to license others on reasonable terms as provided for by the terms of contract number N66001-02-C-8017 awarded by the Defense Advanced Research Projects Agency, and contract number DAAH01-03-9-R003 sponsored by the U.S. Army Aviation and Missile Command. 
    
    
     BACKGROUND 
     The present disclosure is related generally to the field of semiconductor light emitting devices, and more specifically to an architecture for an improved high-Al content, low defect quantum well light emitting device formed directly on a final substrate. 
     In the III-V compound semiconductor family, the nitrides have been used to fabricate visible wavelength light emitting device active regions. They also exhibit a sufficiently high bandgap to produce devices capable of emitting light in the ultraviolet, for example at wavelengths between 290 and 400 nanometers. In particular, InAlGaN systems have been developed and implemented in visible and UV spectrum light emitting diodes (LEDs), such as disclosed in U.S. Pat. No. 6,875,627 to Bour et al., which is incorporated herein by reference. These devices are typically formed on an Al 2 O 3  (sapphire) substrate, and comprise thereover a GaN:Si or AlGaN template layer, an AlGaN:Si/GaN superlattice structure for reducing optical leakage, an n-type electrode contact layer, a GaN n-type waveguide, an InGaN quantum well heterostructure active region, and a GaN p-type waveguide region. In addition, the complete device may also have deposited thereover a p-type AlGaN:Mg cladding layer and a capping layer below a p-type electrode. 
     While significant improvements have been made in device reliability, optical power output, and mode stability, the performance of the nitride-based lasers and light emitting diodes emitting in the ultraviolet (UV) is still inferior to that of their blue or violet counterparts. It is particularly true that for deep UV lasers and light emitting diodes operating at wavelengths below 340 nm, the nature of the substrate and template layer have a critical impact on the overall device performance. For example, differences in lattice constant between the substrate and the structural layers of the device significantly affects optical output and device lifetime. While Al 2 0 3  (sapphire) as a substrate has numerous advantages, it is highly lattice mismatched to the structural layers of typical deep UV epi-layers. The prior art AlGaN template layer formed over the typical Al 2 0 3  substrate mitigates the problem somewhat, but the resulting crystal quality of the high aluminum-containing structural layers in typical deep UV light-emitting devices utilizing these templates are still very poor. 
     The dislocation densities in AlGaN or AlN template layers on sapphire are typically in the mid 10 9  to high 10 10  cm −2  range. As a consequence, the external quantum efficiencies of deep UV light emitting diodes in the 290 nm to 340 nm range are still well below the external quantum efficiencies for blue GaN-based LED structures. The high dislocation densities also reduce the operating lifetime of devices utilizing such template layers. 
     Efforts to improve the quality of the LED structure in the ultraviolet range on Al x Ga 1-x N/sapphire templates have presented significant challenges due to the high defect density of epitaxial layers formed over the AlGaN crystallographic template. These defects tend to propagate upward, perpendicular to the layer planes, in the direction of crystal growth, forming features known as threading dislocation defects (or simply threading dislocations). If not mitigated, threading dislocations can permeate throughout the structure and reach the active layer, where the transmitted defects compromise light emission efficiency through non-radiative recombination. 
     In many cases, mechanical stresses lead to cracks in the heterostructure formed thereon. These issues are exacerbated when the Al content of layers formed above the AlGaN/sapphire system increases. Yet, as previously mentioned, an increased Al content (e.g., up to ˜50% in the MQWH active region of a 280 nm light emitting diode, and 60% to 70% in the surrounding AlGaN current and optical confinement layers) is required to obtain devices which emit in the mid- to deep-UV. 
     Various groups have published approaches to dealing with these shortcomings. All references referred to herein, and specifically each of the following references, are incorporated herein by reference. For example, Han et al., Appl. Phys. Lett, Vol 78, 67 (2001), discuss the use of a single AlN interlayer formed at low temperatures to avoid strain development. This low-temperature AlN interlayer approach has proven unsuccessful in the case of heterostructure growth with high Al mole fractions. Nakamura et al., J. J. Appl. Phys., vol. 36, 1568 (1997) has suggested short period GaN/AlGaN superlattice layers as a way of extending the critical layer thickness of AlGaN films grown pseudomorphically on GaN/sapphire. But the average Al mole fraction in these AlGaN/GaN systems is at such a low level (˜10% or less) that it is not compatible with deep UV light emitting diodes. Chen et al., Appl. Phys. Lett., vol. 81, 4961 (2002) suggests an AlGaN/AlN layer as a dislocation filter for an AlGaN film on a AlGaN/sapphire template. But again, the AlGaN/sapphire template presents the aforementioned series resistance problem. And Wong et al. in U.S. patent application Ser. No. 11/356,769, filed on Feb. 17, 2006, proposes a GaN/AlN superlattice formed between the GaN template layer and the MQWH active region. But again, the GaN template layer must be removed prior to light output for such a device. 
     There is a need for a UV light emitting device with improved operation characteristics. Accordingly, there is a need for a method and structure facilitating a high Al content MQWH active region with reduced threading dislocations, cracking, and related damage. 
     SUMMARY 
     The present disclosure is directed to facilitating the growth of high aluminum content heterostructure active regions on an initial AlGaN surface for UV light emitting devices such as light emitting diodes (LED) and laser diodes (LD). A defect redirection layer is described, which is grown between the active layer and the substrate to redirect or “bend” threading dislocations so that the dislocations propagate at an angle other than perpendicular to the direction of crystal growth. The net effect is a reduced threading dislocation density at the active region where light is emitted. 
     The initial AlGaN surface can, for example be an AlN or a GaN template on sapphire, an AlGaN template on silicon carbide, or a bulk AlN or GaN substrate. More specifically, the present disclosure is directed to systems and methods for providing an improved transition from an initial Al x Ga 1-x N surface (where 0≦x≦1) to a high-Al content MQWH active region. According to one embodiment of the present disclosure, a structure is formed beginning with a sapphire substrate on which is deposited an AlN template layer. A defect redirection region is next formed over the template layer such that the average Al content of the strain region varies over its thickness. For example, the average Al content may go from a relatively high value, such as 80% or higher, adjacent the template layer to a relatively lower value, such as 60% or lower, adjacent the MQWH region. In this way, the average Al content of the defect redirection region more closely matches the Al content of the regions contiguous thereto. 
     According to one aspect of the disclosure, the defect redirection region is comprised of a mixed alloy region. The mixed alloy region may be comprised of two or more subsections of alternating layers comprising a great percentage of AlN (an “AlN layer”) of a first thickness and a greater percentage of GaN (a “GaN layer”) of a second thickness. The thickness of the AlN layer decreases from subsection to subsection along the height of the defect redirection region. The effect of this varying thickness of AlN is to vary the average Al content of that subsection. In this way, the average Al content may be decreased from one subsection to the next until an uppermost layer has the desired Al content. In one embodiment, the defect redirection region comprises two such subsections. In another embodiment of the present disclosure the defect redirection region comprises more than two subsections. 
     According to another aspect of the disclosure, the mixed alloy region may be comprised of a continuum of alternating layers of a higher percentage of AlN (again, an “AlN layer”) and a high percentage of GaN (and again, a “GaN layer”). The thicknesses of the AlN layers gradually decrease from one AlN/GaN pair to the next. In this way, the average Al content of the defect redirection region decreases from bottom to top, such that the bottom portion thereof matches (or approaches) the Al content of a layer contiguous thereto (e.g., the template layer), and the average Al content of the top portion matches (or approaches) the Al content of a layer contiguous thereto (e.g., the MQWH) so that an improved lattice match is provided at the region interfaces. 
     According to still another aspect of the disclosure, a pure AlN layer is deposited over the AlN template layer prior to deposition of the defect redirection region. This AlN interface layer is generally thicker than the AlN layers of the defect redirection region, and provides a transition from the template layer to the defect redirection region. 
     Thus, in one embodiment, the present disclosure provides a defect redirection region for a light emitting semiconductor device, said defect redirection region formed above a substrate and below a multiple quantum well heterostructure active region, the multiple quantum well heterostructure active region composed in part of a first element so as to have an average composition of the first element, said defect redirection region comprising a plurality of groups of at least two layers, at least one layer of each said group comprised at least in part of the first element such that each group has an average concentration of the first element, the average concentration of the first element varying from group to group from a first concentration to a second concentration along the height of the defect redirection region such that the average concentration of the first element in the group nearest the multiple quantum well heterostructure active region approaches the concentration of the first element in said multiple quantum well heterostructure active region. A number of variation of this embodiment are also provided. 
     In another embodiment, the present disclosure provides a defect redirection region for a light emitting semiconductor device, said defect redirection region formed above a first semiconductor layer and below a second semiconductor layer, the bandgap of the first semiconductor layer being different from the bandgap of the second semiconductor layer, said defect redirection region comprising a plurality of groups of layers, each group comprising a periodic ordering of layers, the average bandgap of the group closest to the first semiconductor layer being closer to the bandgap of the first semiconductor layer than to the bandgap of the second semiconductor layer. A number of variation of this embodiment are also provided. 
     Thus, the defect redirection region according to the present disclosure provides a transition between a starting surface (such as a substrate, possibly with a template layer formed thereon) and the MQWH. Strain-induced cracking and defect density are reduced or eliminated. 
     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 cross-sectional illustration of the general architecture of a heterostructure AlGaInN light emitting device structure in accordance with the present disclosure. 
         FIG. 2  is an illustration of the general architecture of a mixed alloy defect redirection region, and surrounding layers, according to one aspect of the present disclosure. 
         FIG. 3  is a cross-sectional illustration of an exemplary light emitting diode structure in accordance with the present disclosure. 
         FIG. 4  is a graphical depiction of a variable period variable composition defect redirection region comprising two short-period groups of AlN/GaN layer pairs, illustrating the two respective periods of said groups. 
         FIG. 5  is an x-ray spectrum of a mixed alloy region grown on a reference GaN sample. 
         FIG. 6  is cross sectional view of a complete LED structure fabricated according to the present disclosure. 
         FIG. 7  is a comparison of the performance of an LED utilizing the mixed alloy defect redirection region according to the present disclosure to the performance of a prior art LED of identical structure with the exception of a GaN/AlN single-period superlattice strain relief region. 
         FIG. 8  is an optical micrograph of the top-most surface of an as-grown LED heterostructure manufactured according to the present disclosure. 
         FIGS. 9A and 9B  are cross-section representations and microphotographs, respectively, of an LED heterostructure manufactured according to the present disclosure. 
         FIG. 10  is a cross-section microphotograph of a defect redirection region of an LED heterostructure manufactured according to the present disclosure. 
         FIGS. 11A and 11B  are illustrations of gas flow for compositional precursors, and  FIG. 11C  is an illustration of the resulting alloy composition, respectively, for a method of forming a defect redirection layer comprising a periodic mixed alloy according to the present disclosure. 
         FIGS. 12A and 12B  are illustrations of gas flow for compositional precursors, and  FIG. 12C  is an illustration of the resulting alloy composition, respectively, for a method of forming a defect redirection layer comprising a variable-period periodic mixed alloy according to the present disclosure. 
         FIG. 13  is an x-ray diffraction graph for a dual-period periodic mixed alloy LED according to the present disclosure. 
         FIGS. 14A and 14B  are illustrations of gas flow for compositional precursors, and  FIG. 14C  is an illustration of the resulting alloy composition, respectively, for a method of forming a defect redirection layer comprising a pulse Ga method of forming a mixed alloy according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to  FIG. 1 , there is shown therein the general architecture of a heterostructure AlGaInN light emitting device structure  10  in accordance with the present disclosure. Diode structure  10  comprises a substrate  12 . According to one embodiment of the present disclosure, substrate  12  may be Al 2 O 3  (sapphire) on which is formed a template layer  14 . As described further below, other substrates such as Silicon Carbide, bulk AlN, or bulk GaN may be employed. Template layer  14  may be AlN, but may also be Al x Ga 1-x N where x is not equal to 1. In some cases, template layer  14  is not necessary and is absent. Formed thereon is an optional interface layer  16 . In the embodiment in which template layer  14  is AlN, interface layer  16 , if present, is preferably also AlN. 
     Formed above interface layer  16  is mixed alloy defect redirection region  18  comprising a number of layer pairs, such as AlN/GaN, described further below. Additional layers, such as AlGaN:Si buffer layer  20 , n-contact layer  21 , AlGaN/AlGaN:Si mixed alloy n-strain layer  22  (which allows for increased cladding thickness and hence reduced optical leakage of subsequent layers), AlGaN:Si n-cladding (index guiding) layer  24 , and active MQWH layer  26  (such as InAlGaN) may then be formed thereover. 
     Subsequent layer such as the following may also be formed on MQWH layer  26 : an AlGaN:Mg p-cladding (index guiding) layer  28 , an AlGaN:Mg buffer layer  30 , an AlGaN/AlGaN:Mg p-strain layer  32 , and a GaN:Mg capping layer  34 . The aforementioned layers may be formed by any method know in the art, including but not limited to methods described in U.S. Pat. No. 6,875,627 to Bour et al., which is incorporated by reference herein. It will be appreciated that a complete device will also include electrodes, not shown, as well as other similar or alternative devices formed in the manner of an array in appropriate embodiments. 
     Prior art devices comprising a template layer may include a material such as GaN for the template which must be removed prior to device operation, or which result in significant layer cracking and/or high defect density. Other prior art devices that comprise a high Al-content layer grown directly on an AlN template layer will exhibit high strain due to lattice mismatch between the two adjacent materials. One aspect of the present disclosure addresses these problems through the introduction of a transition layer between an initial growth surface and a high Al containing active layer, the transition layer comprising of a novel mixed alloy defect redirection region. 
       FIG. 2  is an illustration of the general architecture of mixed alloy defect redirection region, and surrounding layers, according to one aspect of the present disclosure. In one embodiment, a layer  42 , typically Al x Ga 1-x N (0≦x≦1), is formed on substrate  40 . While layer  42  is often referred to as a template layer, the combination of substrate  40  and layer  42  together form the template for the growth of additional layers. Over this template a mixed alloy defect redirection region  46  is formed which acts as a transition from the template to the MQWH active region, gradually or in step-wise fashion transitioning from the aluminum content of the template to the aluminum content of the active region. 
     As shown in  FIG. 2 , defect redirection region  46  consists of a plurality of pairs of layers of the form Al xi Ga 1-xi N, with a thickness t xi , and Al yi Ga 1-yi N, with a thickness t yi , where 0&lt;x≦1 and 0&lt;y≦1. The plurality of layers are arranged in i groups where 2≦i≦n. Thus, xi represents the aluminum content in a first layer of a layer pair of the i th  group, and yi represents the aluminum content in a second layer of that layer pair in the i th  group. The average aluminum content of each group, i, of layer  46  can be determined as follows: 
                   t   xi     ⁢     x   i       +       t   yi     ⁢     y   i             t   xi     +     t   yi             
Accordingly, by varying xi, yi, t xi  and t yi , the average aluminum content of each group of layer pairs can be controlled. Variable periodicity is achieved by varying the thickness t xi  and t yi  for different periods i, while variable composition is achieved by varying the compositions xi and yi for different periods i.
 
     With reference now to  FIG. 3 , in order to demonstrate the concept forming the present disclosure, we grew a light emitting diode (LED) structure  60  utilizing a two-group mixed alloy defect redirection region. We chose an AlN/GaN mixed alloy design with fixed composition (xi≅1 and yi≅0) for all periods. The LED is designed to operate at a wavelength λ of about 325 nm, requiring an active region heterostructure Al composition of about 35%. The template layer  64  was a 1 μm thick epitaxial layer of primarily AlN grown on a sapphire substrate  62  (in other words, with reference to  FIG. 2 , x≅1 in layer  42 ). A 25-30 nm thick AlN interface layer  66  was formed over template layer  64 . 
     A first group  68  of 40 layer pairs of AlN/GaN were then formed over layer  64 . We chose a first region average Al composition of 80%, and a second region average Al content of 60%, and tailored the layer thicknesses for xi≅1 and yi≅0 to produce these compositions as follows. In the first group  68  the thicknesses were t xi =1 nm, t yi =0.25 nm for 40 pairs (i=1 to 40). In the second group  70  the thicknesses were t xi =0.38 nm, t yi =0.25 nm for the remaining 40 pairs (i=41 to 80). This produced an AlN/GaN mixed alloy region with an average Al composition of 80% for the first 40 pairs at the template end of the mixed alloy region and 60% for the second 40 pairs at the heterostructure end. The thicknesses of the individual layers of the mixed alloy region, t xi  and t yi , are made very thin to minimize strain due to lattice mismatch.  FIG. 4  is a graphical depiction of groups  68 ,  70  of the aforementioned LED  60 , illustrating the two respective periods of said groups. As will be appreciated from  FIG. 4 , while the number of layer pairs in each group is the same, the difference in layer thickness results in group  68  being thicker than group  70 . 
     Referring next to  FIG. 5 , the graph shows an x-ray spectrum taken from a sample comprising the mixed alloy region of  FIG. 3  grown on a GaN on sapphire template. The GaN template produces a large main peak that is used as reference. The two side peaks come from regions  68  and  70  of  FIG. 3 . Region  70  of  FIG. 3  has a lower average Al content than region  68 , so it corresponds to the peak near the GaN reference peak. The X-ray peak occurring at the higher omega-2 theta angle comes from region  68 . The two peaks correspond to the two different average Al contents within the two sections of the mixed alloy region. Mixed alloy regions with more than two different periods will have more than two X-ray peaks. Similar test samples comprising single period redirection regions such as those employed in the aforementioned U.S. patent application Ser. No. 11/356,769 will produce one peak. 
     With reference to  FIG. 6 , a complete LED structure  60  according to the present disclosure is illustrated in cross section. In addition to the previously described elements, the structure incorporates n contact layer  71 , n-cladding layer  74 , n waveguide  76 , barrier layers  78  (10.49 nm),  80  (89.19 nm), with quantum well  82  (5.25 nm) therebetween, tunnel barrier layer  84 , and p waveguide and contact  86 ,  88 . Some of these layers such as waveguide layers  76  and  84  allow the LED design to be easily extendable to laser diodes but do not perform actual waveguiding functions when the device is operated in LED mode. 
     Referring now to  FIG. 7 , the performance of an LED utilizing the mixed alloy defect redirection region according to the present disclosure is compared to a prior art LED of identical structure with the exception of a GaN/AlN single-period superlattice strain relief region. As can be seen, the light output of the LED incorporating mixed alloy defect redirection region according to the present disclosure demonstrated significantly brighter optical output than the LED grown on prior art single period binary superlattice strain relief regions. From this we conclude that devices incorporating the mixed alloy defect redirection region taught herein benefit from enhanced optical output, due to the more gradual transition in Al content provided by the defect redirection structure. 
       FIG. 8  shows an optical micrograph of the top-most surface of an as-grown LED heterostructure manufactured with the variable period variable composition defect redirection region of the present disclosure. As can be seen, a substantially crack-free surface is produced. 
       FIGS. 9A and 9B  are cross-section representations and microphotographs, respectively, of an exemplary LED heterostructure  100  incorporating an AlN template layer  104  grown on a sapphire substrate  102 , a defect redirection layer  106 , a transition region  108 , and a MQW active region  110  (among other layers). The defect redirection layer  106  bends threading dislocation out of the growth direction, and reduces defects at the active region  110 . The transition region  108  can contain an electron transport layer  112 , an n-cladding layer  114 , and an n-side separate confinement heterostructure (SCH)  116 . The n-cladding layer  114  frequently utilizes a superlattice to improve carrier transport and to reduce strain. The n-side SCH  116  is usually doped n-type in the case of LEDs, but is usually left undoped to lower optical losses in the case of lasers. In lateral injection devices, a mesa structure  120  is usually etched down to the electron transport layer to form n-contacts. 
       FIG. 10  shows a magnified transmission electron micrograph (TEM) of a defect redirection layer  130  situated between a template layer  132  and a transition region  134 . Template layer  132  has high threading dislocation density, illustrated by the many vertical shadow lines. Threading dislocations originating from the substrate and propagating through template layer  132  encounter a “discontinuity” in crystal structure upon reaching defect redirection layer  130 . As is evident in the micrograph, this discontinuity bends the propagation direction of the threading dislocations away from the growth direction, as evidenced by the non-vertical shadow lines, so fewer threading dislocations reach transition region  134 . Moreover, for those dislocations that enter transition region  134 , a large fraction have bent (non-vertical) propagation directions. Consequently, even fewer threading dislocations reach the MQW active region (not shown). Transition region  130  is designed to be sufficiently thick such that a substantial portion of threading dislocations are bent out of the growth direction so that relatively few such defects reach the active region. Defect redirection layer  130  thus functions as a threading dislocation filter by creating a discontinuity in the crystal structure that bends the propagation direction of the defects. In one example, the defect density was reduced by a factor of 4.3, from 1.2×10 10  cm −2  at the substrate to 2.8×10 9  cm −2  at the active region. 
     In one family of embodiments of the present disclosure, the defect reduction layer comprises of a periodic mixed alloy. With reference to  FIGS. 11A through 11C , according to a first embodiment, the mixed alloy is formed by modulating TMA and TMG precursor gasses during growth.  FIGS. 11A and 11B  illustrate modulating the precursor gasses TMG and TMA, respectively, at 180° out of phase during epitaxial growth, but other modulation phase relationships are also possible. The resulting periodic mixed alloy AlGaN composition is shown in  FIG. 11C . It is important that the width, w, of the gallium-rich region is designed to be sufficiently narrow in order to prevent crystal dislocation and/or film cracking. Typically, w&lt;0.5 nm. 
     With reference to  FIGS. 12A through 12C , according to a second embodiment of a defect reduction layer comprising a dual period periodic mixed alloy, the average aluminum composition within the defect reduction layer varies as a function of position. The change in average aluminum composition is accomplished by reducing the thickness of the high aluminum component of the periodic structure. The thickness, w, of the gallium-rich region is not changed.  FIGS. 12A and 12B  illustrate modulating the precursor gasses TMG and TMA, respectively, to obtain the structure illustrated in  FIG. 12C . It is important that the thickness, w, is designed to be sufficiently narrow such that crystal dislocation and/or film cracking is prevented. Typically, w&lt;0.5 nm. The concept of a dual period mixed alloy can be expanded to a general variable period structure. 
       FIG. 13  shows the x-ray spectrum of a UV LED incorporating a dual period periodic mixed alloy defect reduction layer. The x-ray peaks indicate the average aluminum composition of the mixed alloy at each of the two periods. Unlike a superlattice, the mixed alloys do not produce satellite peaks in the x-ray spectrum. 
     Finally,  FIGS. 14A through 14C  illustrate a third embodiment of a defect reduction layer formed by pulsing the TMG metal organic precursor.  FIG. 14A  illustrates modulating the precursor gasses TMG. In this embodiment, the precursor gas TMA is provided at a substantially steady state. The structure illustrated in  FIG. 12C  is thereby obtained. 
     It will be appreciated that while the foregoing describes an embodiment of the present disclosure utilizing a two-group, mixed alloy design, the concept extends to include many different region profiles, such as three or more groupings (e.g., with an average composition of approximately 80%, 70% and 60%, respectively), or continuously varying composition profiles, varying linearly, parabolically, exponentially or otherwise, each providing a different transition profile for the Al content in the region. For example, a three step mixed alloy region would have three layers per period, each layer with aluminum contents of, say, x i , y i , z i  and thicknesses tx i , ty i , and tz i  for period i. A three-group mixed alloy region would transition step-wise, with for example two steps per period, from the Al content matching or approaching that of the transition layer to the Al content matching or approaching that of the active region. The abrupt transition between layers within each period can also be replaced with a transition layer whose Al content varies continuously from a starting composition near that of the starting layer to an ending composition near that of the adjacent layer. The general case would be a defect redirection region comprising a continuously varying composition profile starting with a composition close to that of the initial surface and ending with an Al composition close to that of the heterostructure active layer. The continuous composition profile can be linear, parabolic, or can consist of curves with multiple points of inflection. 
     It is also common to add a small amount of Indium in the aluminum containing alloys to improve structural quality. An example of a structure utilizing Indium quaternary alloys in the structural layers has already been described in FIG.  6 . Indium, typically at a concentration of about 1% to 2%, can also be added to some or all of the layers within the defect redirection region. 
     It should be appreciated that according to embodiments of the present disclosure, a modulated section of each period in the periodic mixed alloy may have a thickness of the order of one monolayer (such as is illustrated in  FIGS. 11A-C ,  12 A-C, and  14 A-C). Therefore, unlike traditional superlattices, there is no well-defined thickness in that section over which the composition is constant. Instead, the alloy composition varies to form a “mixed alloy” as indicated in the profiles in  FIG. 11A-C ,  12 A-C, and  14 A-C, and as evidenced in the x-ray spectrum of  FIG. 13 . 
     Furthermore, while the discussion above has been focused towards multiple quantum well active regions, it will be appreciated by one skilled in the art that other types of light-emitting active regions such as double heterojunction (DH), homojunction, quantum wire, active regions incorporating nanometer scale compositional inhomogeneities (NCI), and single quantum well active regions could also be employed. Moreover, while the discussion has been focused on light emitting diodes (LEDs), it will be appreciated by one skilled in the art that the structures and methods described also applies to other types of light emitting devices such as laser diodes and pump lasers. 
     Thus, 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. Rather, the foregoing detailed 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.