Patent Publication Number: US-8121165-B2

Title: MQW laser structure comprising plural MQW regions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of U.S. patent application Ser. No. 12/336,050 filed on Dec. 16, 2008, now U.S. Pat. No. 7,983,317, the content of which is relied upon and incorporated herein by reference in its entirety, and the benefit of priority under 35 U.S.C. §120 is hereby claimed. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates to semiconductor lasers and, more particularly, to enhanced optical confinement in laser structures. 
     2. Technical Background 
     The present inventors have recognized that, to improve the optical confinement of light propagating in the waveguide of a semiconductor laser, confined mode out-coupling to the laser substrate should be reduced or eliminated. In addition, the optical field profile around the active region of the laser should be narrowed to ensure efficient overlap between the optical propagating mode and the gain region and to prevent optical loss due to optical mode penetration into metal contacts in the vicinity of the active region. These challenges are particularly acute for semiconductor lasers operating at wavelengths between approximately 450 nm and approximately 600 nm because such lasers are often prone to optical leakage. 
     BRIEF SUMMARY 
     Semiconductor lasers may comprise optical heterostructures including, for example, a waveguide layer with higher refractive index sandwiched between two cladding layers with an index of refraction lower than that of the waveguide. The cladding layers serve to narrow the optical mode width and the mode decays exponentially in the cladding layers because the cladding layer refractive index is lower than the waveguide effective refractive index. The greater the difference between the effective refractive index n eff  of the waveguide and the refractive index of the cladding layer, the less the mode penetration in the cladding and the narrower the mode. Thus, a narrow mode can be achieved either by increasing the refractive index in the waveguide or by decreasing the refractive index of cladding. 
     If the effective refractive index n eff  of the waveguide is lower than the refractive index of the substrate, then tunneling of light through the bottom cladding into the substrate is probable. To reduce this probability, the difference between n eff  and the cladding layer refractive index should be as large as possible. Ideally, one needs to have the cladding layer as thick as possible and to have n eff  close to or higher than the substrate index. Unfortunately, in the context of Group III nitride semiconductor lasers, the lattice-mismatch induced strain in the heterostructure and the thermal instability of InGaN place significant design constraints on the laser. For example, it is challenging to grow an AlGaN cladding layer that is thick enough and has a high enough Al content to reduce cladding layer refractive index because AlGaN tensile strain generates cracking issues in the structure. It is also difficult to grow an InGaN heterostructure that has a sufficient In content because of factors like high compressive strain, poor thermal stability, and difficulties in doping the material. 
     The present inventors have also recognized that reductions in the cladding layer refractive index will not yield a high waveguide effective refractive index n eff , relative to the refractive index of the laser substrate, because, according to optical confinement physics, reductions in the cladding layer refractive index lead to reductions of the waveguide effective refractive index n eff . According to the subject matter of the present disclosure, the effective refractive index n eff  of the waveguide region of a semiconductor laser operating, for example, at lasing wavelengths greater than 450 nm, can be increased to enhance optical confinement in the laser structure by introducing a plurality of MQW regions in the laser structure. This enhanced optical confinement reduces mode leakage to the laser substrate and helps prevent optical loss due to optical mode penetration into metal contacts in the vicinity of the active region of the laser structure. For example, where a contact metal is deposited on the top of the upper cladding layer of the laser structure, even slight mode tail penetration into the metal layer through the upper cladding layer can be a source of significant optical losses because absorption in the metal can be extremely high. The aforementioned increases in the effective refractive index n eff  of the waveguide region can reduce this mode tail penetration. 
     In accordance with one embodiment of the present disclosure, a multi-quantum well laser diode is provided comprising a laser substrate, a semiconductor active region, a waveguide region, and a cladding region. The active region comprises at least one active MQW region and at least one passive MQW region. The active MQW region is configured for electrically-pumped stimulated emission of photons. The passive quantum well region is optically transparent at the lasing photon energy of the active MQW region. Each of the MQW regions comprises a plurality of quantum wells and intervening barrier layers of barrier layer thickness a. Adjacent MQW regions are separated by a spacer layer of spacer thickness b. The spacer thickness b is larger than the barrier layer thickness a. The bandgap of the quantum wells is lower than the bandgap of the intervening barrier layers and the spacer layer. The respective active, waveguide, and cladding regions are formed as a multi-layered diode over the laser substrate such that the waveguide region guides the stimulated emission of photons from the active region, and the cladding region promotes propagation of the emitted photos in the waveguide region. 
     In accordance with another embodiment of the present disclosure, a multi-quantum well laser structure is provided where the active region comprises one or more active MQW regions configured for optically-pumped stimulated emission of photons. Each of the MQW regions comprises a plurality of quantum wells, which comprise a bandgap-reducing Group III nitride component, and intervening nitride barrier layers of barrier layer thickness a. Adjacent MQW regions are separated by a nitride spacer layer of spacer thickness b. The spacer thickness b is larger than the barrier layer thickness a. The bandgap of the quantum wells is lower than the bandgap of the intervening nitride barrier layers and the nitride spacer layer. The respective active, waveguide, and cladding regions form a multi-layered structure over the laser substrate such that the waveguide region guides the stimulated emission of photons from the active region, and the cladding region promotes propagation of the emitted photons in the waveguide region. 
     In accordance with yet another embodiment of the present disclosure, the spacer thickness b is larger than the barrier layer thickness a and is between approximately 10 nm and approximately 150 nm. The barrier layer thickness a is between approximately 2 nm and approximately 30 nm. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
         FIG. 1  is an illustration of a multi-quantum well laser structure suitable for electrical pumping according to one embodiment of the present disclosure; and 
         FIG. 2  is an illustration of a multi-quantum well laser structure suitable for optical pumping according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The multi-quantum well (MQW) laser structures  100  illustrated in  FIGS. 1 and 2 , comprise a semiconductor active region  10 , a waveguide region comprising a pair of waveguide layers  20  disposed on opposite sides of the active region  10 , and a cladding region comprising a pair of cladding layers  30  disposed on opposite sides of the waveguide region. The respective active, waveguide, and cladding regions are formed as a multi-layered structure over a laser substrate  35  such that the waveguide layers  20  of the waveguide region guide the stimulated emission of photons from the active region  10 , and the cladding layers  30  of the cladding region promote propagation of the emitted photons in the waveguide region. 
     The active region  10  comprises a plurality of MQW regions  40 ,  50 ,  60 , at least one of which is configured for stimulated emission of photons. The MQW regions  40 ,  50 ,  60  enhance optical confinement without introducing optical loss by providing a refractive index that is as high as possible at the lasing wavelength. As a result, the effective refractive index n eff  of the waveguide, as describe above, increases and optical confinement in the laser structure is enhanced. The refractive index provided by the MQW regions  40 ,  50 ,  60  typically increases superlinearly with reductions of the bandgap of the semiconductor material used in the MQW regions  40 ,  50 ,  60 . Thus, in practicing the present invention, materials with relatively low bandgaps should be used in constructing the MQW regions  40 ,  50 ,  60  to enhance optical confinement. 
     In electrically and optically pumped laser structures  100 , including those illustrated in  FIGS. 1 and 2 , the active region  10  is constructed using a plurality of relatively thin quantum wells  70 . Although not required, the quantum wells  70  can be fabricated using a Group III nitride selected for the aforementioned bandgap reduction. For the purposes of defining and describing the present invention, a component that reduces the bandgap when its concentration is increased will be referred to as a “bandgap-reducing” component of the quantum well. The bandgap-reducing Group III nitride component is selected such that the refractive index of the quantum well  70  at the stimulated emission wavelength can be increased super-linearly as the concentration of the bandgap-reducing Group III nitride component of the quantum well  70  is increased. For example, according to one embodiment, the quantum wells  70  comprise InGaN quantum wells, which are typically grown under compressive strain on GaN substrates or buffer layers, and the concentration of the InN component of the InGaN is increased to create a superlinear increase in the refractive index. In contrast, if the concentration of the GaN component of the InGaN is increased, it does not reduce the bandgap of the quantum well and, as such, this component is not generally considered to be a bandgap-reducing component of the quantum well. In another embodiment, the quantum wells  70  comprise AlGaN quantum wells grown under compressive strain on AlGaN or AlN substrates. For InGaN quantum wells  70 , the bandgap-reducing Group III nitride component would be InN with the reduced bandgap discussed above. For AlGaN quantum wells  70 , the bandgap-reducing Group III nitride component would be GaN with the reduced bandgap discussed above. AlGaAs quantum wells, AlGaAsP quantum wells, GaAs quantum wells, InGaAs quantum wells and combinations thereof are also contemplated. 
     As is noted above, the refractive index of the MQW regions  40 ,  50 ,  60  increases superlinearly with increases in the concentration of the bandgap-reducing component of the quantum well. However, compressive strain in the semiconductor layers utilizing the bandgap-reducing component merely accumulates in a relatively linear fashion as the concentration of the component and the thickness of the layers utilizing the component increases. As a result, in order to benefit from the higher refractive index, and the better optical confinement provided thereby, without increasing total strain, it is preferable to use thinner layers with higher bandgap-reducing component concentrations, as opposed to single, relatively thick layers with relatively low bandgap-reducing component concentrations. Although compressive strain in the waveguide region  10  typically accumulates almost linearly as the content of the bandgap-reducing component increases, optical confinement can be enhanced without sacrificing structural integrity by decreasing the thickness of the relatively thin quantum wells  70  while increasing the content of the bandgap-reducing component in the quantum wells  70 . As a result, the MQW regions  40 ,  50 ,  60 , which also comprise intervening barrier layers  80 , represent a relatively compact waveguide region  10  that concentrates the propagating optical mode in a relatively narrow region. 
     In practicing the various embodiments disclosed herein, it should be recognized that, although quantum wells  70  with a lower bandgap generally increase optical confinement in the quantum wells  70 , the composition of the MQW regions  40 ,  50 ,  60  should be held such that it would not yield excessive compressive strain accumulation or deterioration of the growth morphology, which, for example, often results in the formation of v-pits in the structure. To help address these issues while permitting a low bandgap in the MQW regions  40 ,  50 ,  60 , adjacent MQW regions  40 ,  50 ,  60  are separated by spacer layers  90  fabricated with a larger bandgap material. The bandgap of the quantum well  70  is lower than the respective bandgaps of the intervening nitride barrier layers  80  and the spacer layer  90 . Although it is contemplated that laser structures within the scope of the present disclosure may employ a variety of conventional and yet-to-be developed nitrides or other materials that yield the aforementioned characteristics, in one embodiment, the MQW regions  40 ,  50 ,  60  comprise InGaN and the spacer layers  90  comprise GaN or InGaN. 
     As is illustrated schematically in  FIGS. 1 and 2 , the spacer layers  90  define a spacer thickness b that is larger than the barrier layer thickness a. The spacer layers  90  may be of nanometer-scale but should be sufficiently thick to at least partially mitigate strain accumulation across the MQW regions  40 ,  50 ,  60  and to recover any morphology deterioration introduced during MQW growth. For example, in some embodiments, the spacer thickness b can be larger than the barrier layer thickness a. More specifically, where the spacer thickness b is between approximately 20 nm and approximately 100 nm, the barrier layer thickness a can be between approximately 2 nm and approximately 30 nm. In other embodiments, the spacer thickness b is at least twice as large as the barrier layer thickness a. In still other embodiments, the spacer thickness b is greater than approximately 20 nm and the barrier layer thickness a is less than approximately 20 nm. 
     In the electrically pumped MQW laser structure  100  illustrated in  FIG. 1 , the respective active, waveguide, and cladding regions are formed as a multi-layered laser diode and the active region  10  comprises an active MQW region  50  sandwiched between a pair of passive MQW regions  40 ,  60 . The active MQW region  50  is configured for electrically-pumped stimulated emission of photons. To ensure that the optical transition energy of the passive quantum well regions  40 ,  60  is higher than the lasing photon energy of the laser structure  100  and that the passive quantum well regions  40 ,  60  are optically transparent at the lasing photon energy of the active MQW region  50 , the bandgap of the passive MQW regions  40 ,  60  needs to be as close as possible but higher than the lasing emission photon energy of the active MQW region  50 . As will be appreciated by those familiar with semiconductor structures, the respective quantum well optical transition energies of the active and passive MQW regions  40 ,  50 ,  60  can be tailored in a variety of ways. For example, in the context of an InGaN/GaN MQW region, the quantum well optical transition energy can be tailored by adjusting the InN mole fraction in InGaN. 
     Accordingly, as is illustrated in Table 1 below, to ensure transparency of the passive MQW regions  40 ,  60  in cases where the MQW regions comprise InGaN, the In content of the active quantum well region  50  can be tailored to be greater than the In content of the passive quantum well regions  40 ,  60  if the quantum well thickness is the same. In this manner, the optical transition energy of the passive quantum well regions  40 ,  60  can be made higher than the lasing photon energy of the multi-quantum well laser structure  100  and the passive quantum well regions  40 ,  60  will be transparent at the lasing photon energy of the multi-quantum well laser structure. The material refractive index of the passive MQW regions  40 ,  60  of the electrically pumped laser diode structure  100  of  FIG. 1  increases superlinearly with bandgap decrease. Generally, the bandgap cannot be less than the point at which the wavelength of the absorption edge of the MQW regions  40 ,  60  approaches the wavelength of laser emission, i.e., the absorption photon energy of the MQW regions  40 ,  60  has to be higher than lasing photon energy. In some embodiments, for example, the lasing photon energy of the active quantum well region will be approximately 50 meV to approximately 400 meV lower than the photon energy of the passive quantum well regions. Table 1, below, presents some specific design parameters suitable for practicing particular embodiments of the present invention in the context of an electrically pumped laser structure. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 (components listed from top to bottom, as illustrated in FIG. 1). 
               
            
           
           
               
               
               
               
               
            
               
                 Layer 
                 Thickness 
                 Composition 
                 Doping 
                 Notes 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 contact layer 
                 100  
                 nm 
                 GaN 
                 p ++   
                   
               
            
           
           
               
               
               
               
               
            
               
                 45 
                   
                   
                 doped 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 cladding  
                 &gt;500  
                 nm 
                 AlGaN or 
                 p- 
                   
               
            
           
           
               
               
               
               
               
            
               
                 layer 
                   
                 AlGaN/ 
                 doped 
                   
               
               
                 30 
                   
                 GaN SL, 
                   
                   
               
               
                   
                   
                 AlN average  
                   
                   
               
               
                   
                   
                 mole 
                   
                   
               
               
                   
                   
                 fraction  
                   
                   
               
               
                   
                   
                 0-20% 
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Waveguide  
                 0-150  
                 nm 
                 (In)GaN,  
                 p- 
                   
               
            
           
           
               
               
               
               
               
            
               
                 20 
                   
                 InN mole 
                 doped 
                   
               
               
                   
                   
                 fraction  
                   
                   
               
               
                   
                   
                 0-10% 
                   
                   
               
               
                 passive  
                 InGaN 
                 InGaN/  
                 p-type 
                 passive  
               
               
                 MQW 
                 thickness  
                 GaN, 
                   
                 InGaN 
               
               
                 60 
                 1-10 nm,  
                 InN 
                   
                 transition 
               
               
                   
                 GaN 
                 mole 
                   
                 energy 
               
               
                   
                 thickness 
                 fraction in 
                   
                 50-400  
               
               
                   
                 2-30 nm 
                 InGaN is 
                   
                 meV 
               
               
                   
                   
                 5-30% 
                   
                 higher 
               
               
                   
                   
                   
                   
                 than 
               
               
                   
                   
                   
                   
                 lasing 
               
               
                   
                   
                   
                   
                 photon  
               
               
                   
                   
                   
                   
                 energy 
               
            
           
           
               
               
               
               
               
               
            
               
                 spacer  
                 10-100  
                 nm 
                 (In)GaN,  
                 Partially  
                 AlGaN  
               
            
           
           
               
               
               
               
               
            
               
                 90 
                   
                 InN  
                 p- 
                 electron 
               
               
                   
                   
                 mole 
                 doped, 
                 stop  
               
               
                   
                   
                 fraction 
                 or 
                 layer in, 
               
               
                   
                   
                 0-10% 
                 undoped 
                 above, or 
               
               
                   
                   
                   
                   
                 below 
               
               
                   
                   
                   
                   
                 this 
               
               
                   
                   
                   
                   
                 layer 
               
               
                 Active  
                 InGaN 
                 InGaN/  
                 GaN 
                   
               
               
                 MQW  
                 thickness  
                 GaN, 
                 barriers 
                   
               
               
                 50 
                 1-10 nm,  
                 InN 
                 can be  
                   
               
               
                   
                 GaN 
                 mole 
                 n- 
                   
               
               
                   
                 thickness 
                 fraction 
                 doped 
                   
               
               
                   
                 2-30 nm 
                 in 
                   
                   
               
               
                   
                   
                 InGaN is 
                   
                   
               
               
                   
                   
                 10-50% 
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 spacer 
                 10-100  
                 nm 
                 (In)GaN, 
                 n- 
                   
               
            
           
           
               
               
               
               
               
            
               
                 90 
                   
                 InN mole 
                 doped 
                   
               
               
                   
                   
                 fraction 
                   
                   
               
               
                   
                   
                 0-10% 
                   
                   
               
               
                 passive  
                 InGaN 
                 InGaN/  
                 n- 
                 passive 
               
               
                 MQW 
                 thickness  
                 GaN, 
                 type 
                 InGaN 
               
               
                 40 
                 1-10 nm, 
                 InN 
                   
                 transition 
               
               
                   
                 GaN 
                 mole 
                   
                 energy 
               
               
                   
                 thickness 
                 fraction 
                   
                 50-400 
               
               
                   
                 2-30 nm 
                 in 
                   
                 meV 
               
               
                   
                   
                 InGaN is 
                   
                 higher 
               
               
                   
                   
                 5-30% 
                   
                 than 
               
               
                   
                   
                   
                   
                 lasing 
               
               
                   
                   
                   
                   
                 photon 
               
               
                   
                   
                   
                   
                 energy 
               
            
           
           
               
               
               
               
               
               
            
               
                 waveguide  
                 0-150  
                 nm 
                 (In)GaN,  
                 n- 
                   
               
            
           
           
               
               
               
               
               
            
               
                 20 
                   
                 InN mole 
                 doped 
                   
               
               
                   
                   
                 fraction  
                   
                   
               
               
                   
                   
                 0-10% 
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 cladding  
                 &gt;500  
                 nm 
                 AlGaN or 
                 n- 
                   
               
            
           
           
               
               
               
               
               
            
               
                 layer 
                   
                 AlGaN/ 
                 doped 
                   
               
               
                 30 
                   
                 GaN super 
                   
                   
               
               
                   
                   
                 lattice, 
                   
                   
               
               
                   
                   
                 AlN mole  
                   
                   
               
               
                   
                   
                 fraction 
                   
                   
               
               
                   
                   
                 0-20% 
                   
                   
               
               
                 substrate 35 
                 varies 
                 GaN 
                 n- 
                   
               
               
                   
                   
                   
                 doped 
                   
               
               
                   
               
            
           
         
       
     
     It is noted that the laser diode structure illustrated in  FIG. 1  may further comprise an electron stop layer  85  interposed between the active and passive MQW regions  50 ,  60 . The electron stop layer  85  could, for example, be positioned in the spacer layer  90 , as is illustrated in  FIG. 1 , between the spacer layer  90  and the active MQW region  50 , or between the spacer layer  90  and the passive MQW region  60 . For laser diode structures employing InGaN MQW regions  40 ,  50 ,  60 , the electron stop layer  85  can be made of p-doped AlGaN. If the spacer layer  90  is fully or partially above the electron stop layer  85 , the spacer material above the electron stop layer  85  should be p-doped. 
     As is illustrated in  FIG. 1  and Table 1, in one embodiment, the waveguide region comprises P-doped and N-doped layers  20  disposed on opposite sides of the active region  10 . The cladding region also comprises P-doped and N-doped cladding layers  30  disposed on opposite sides of the active region  10 . As such, the active MQW  50  is disposed between a p-doped side of the laser diode structure  100  and an n-doped side of the laser diode structure  100 . The spacer layers  90  on the n-doped side of the laser diode structure  100  are fully or partially n-doped, while the spacer layers  90  on the p-doped side of the laser diode structure  100  are fully or partially p-doped. The intervening barrier layers  80  between the quantum wells  70  on the n-doped side of the laser diode structure  100  can be n-doped, while the intervening barrier layers  80  between quantum wells  70  on the p-doped side of the laser diode structure  100  can be p-doped to ensure good carrier transport through the passive MQW regions  40 ,  60 . It is contemplated that the quantum wells  70  may also be n or p-doped in a manner consistent to that which is described for the barrier layers  80 . 
     In the optically pumped MQW laser structure  100  illustrated in  FIG. 2 , the active region  10  comprises one or more active MQW regions  40 ,  50 ,  60  configured for optically-pumped stimulated emission of photons. The MQW regions  40 ,  50 ,  60  of the laser structure  100  illustrated in  FIG. 2  can be substantially identical and, as such, the laser structure  100  of  FIG. 2  is suitable for use as an optically pumped laser structure because each of the MQW regions  40 ,  50 ,  60  can function as an active MQW region, although it is contemplated that one or more of the MQW regions  40 ,  50 ,  60  may be passive. 
     Each of the MQW regions  40 ,  50 ,  60  in the optically pumped MQW laser structure  100  illustrated in  FIG. 2  comprises a plurality of quantum wells  70  formed using a Group III nitride semiconductor material and intervening nitride barrier layers  80  of barrier layer thickness a. Adjacent MQW regions are separated by a nitride spacer layer  90  of spacer thickness b. The spacer thickness b is larger than the barrier layer thickness a and the bandgap of the quantum wells  70  is lower than the respective bandgaps of the intervening nitride barrier layers  80  and the nitride spacer layer  90 . As is the case with the electrically pumped laser diode structure  100  illustrated in  FIG. 1 , the respective active, waveguide, and cladding regions of the laser structure  100  illustrated in  FIG. 2  form a multi-layered structure where the waveguide layers  20  of the waveguide region guide the stimulated emission of photons from the active region  10 , and the cladding layers  30  of the cladding region promote propagation of the emitted photons in the waveguide region. 
     Typically, the respective active, waveguide, and cladding regions are formed from undoped semiconductor material layers because the laser structure is optically pumped, as opposed to being electrically pumped. Of course, it is contemplated that some level of doping in the semiconductor material layer could be tolerated as long as the doping does not lead to excessive optical loss. The respective compositions of the active MQW regions can be functionally equivalent and any or each of the active MQW regions  40 ,  50 ,  60  can be configured to be capable of lasing under optical pumping at a common wavelength. 
     As is illustrated in detail in Table 2, below, for some embodiments of the optically pumped laser structure  100 , the spacer thickness b is between approximately 20 nm and approximately 150 nm, while the barrier layer thickness a is between approximately 2 nm and approximately 30 nm. By utilizing these thickness relationships, those practicing the presently disclosed embodiments, will find it easier to maintain a low quantum well bandgap to optimize optical confinement while avoiding the morphology degradation commonly associated with excessive compressive strain or low growth temperatures in the active region  10 . Additional thicknesses are contemplated within and outside of the aforementioned ranges. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 (components listed from top to bottom, as illustrated in FIG. 2). 
               
            
           
           
               
               
               
               
            
               
                 Layer 
                 Thickness 
                 Composition 
                 other 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 cladding layer 
                 &gt;500 
                 nm 
                 AlGaN or AlGaN/GaN 
                   
               
            
           
           
               
               
               
               
            
               
                 30 
                   
                 super lattice, AIN mole 
                   
               
               
                   
                   
                 fraction 0-20% 
                   
               
            
           
           
               
               
               
               
               
            
               
                 spacer 90 
                 10-150  
                 nm 
                 (In)GaN, InN mole 
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 fraction 0-10% 
                   
               
               
                 Active MQW 
                 InGaN QW  
                 InGaN/GaN, or 
                 adjacent 
               
               
                 40, 50, 60 
                 thickness 
                 InGaN/InGaN InN 
                 MQWs are 
               
               
                   
                 1-10 nm, 
                 mole fraction in InGaN 
                 separated 
               
               
                   
                 InGaN 
                 QW is 10-50%. In 
                 by spacer  
               
               
                   
                 barrier 
                 mole fraction in 
                 90 
               
               
                   
                 thickness 
                 barriers and spacers is 
                   
               
               
                   
                 2-30 nm 
                 0-10% 
                   
               
            
           
           
               
               
               
               
               
            
               
                 spacer 90 
                 10-150  
                 nm 
                 (In)GaN, InN mole 
                   
               
            
           
           
               
               
               
               
            
               
                   
                   
                 fraction 0-10% 
                   
               
            
           
           
               
               
               
               
               
            
               
                 cladding layer 
                 &gt;500  
                 nm 
                 AlGaN or AlGaN/GaN 
                   
               
            
           
           
               
               
               
               
            
               
                 30 
                   
                 SL, 
                   
               
               
                   
                   
                 AlN average mole 
                   
               
               
                   
                   
                 fraction 0-20% 
                   
               
               
                 substrate 35 
                 varies 
                 GaN 
                   
               
               
                   
               
            
           
         
       
     
     Although the embodiments illustrated in  FIGS. 1 and 2  show the use of three MQW regions  40 ,  50 ,  60  in a MQW laser structure, it is contemplated that enhanced optical confinement can be achieved with two or more MQW regions separated by the aforementioned spacer layer  90 . In addition, it is contemplated that each MQW region may comprise any number of quantum wells  70 , provided the quantum wells are separated by the intervening barrier layer  80 . It is also noted that the embodiments described and contemplated herein can be used in both Al-free laser structures and structures with AlGaN cladding layers. 
     For the purposes of describing and defining the present invention, it is noted that reference herein to a variable being a “function” of a parameter or another variable is not intended to denote that the variable is exclusively a function of the listed parameter or variable. Rather, reference herein to a variable that is a “function” of a listed parameter is intended to be open ended such that the variable may be a function of a single parameter or a plurality of parameters. In addition, reference herein to “Group III” elements is intended to refer to boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (TI), and ununtrium (a synthetic element in the periodic table that has the temporary symbol Uut and has the atomic number 113). 
     It is noted that recitations herein of a component of the present disclosure being “configured” to embody a particular property, or function in a particular manner, are structural recitations, as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “configured” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component. 
     It is noted that terms like “preferably,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure. 
     For the purposes of describing and defining the present invention, it is noted that the term “approximately” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
     It is specified that the p-side of the structure is referred to as “top” and n-side as “the bottom” of the structure. According to this the term “above” means toward the structure top and “below” means toward the structure bottom. 
     Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present disclosure are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects. For example, although the laser structures described herein are, in some cases, identified as comprising a Group III nitride where the bandgap-reducing Group III nitride component is InN, it is contemplated that the laser structure may include any number of group III components, regardless of whether they are capable of reducing the bandgap of the quantum well. 
     It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.”