Abstract:
A semiconductor laser structure having confinement layers to confine electrons to an active region (quantum wells) and having separate antimonide-based cladding layers to provide additional electron confinement and photon confinement is suited to high temperature operation. The structure is suitable for lasing across telecommunications wavelengths from 980 nm to 1.55 μm (microns). The cladding layer uses AlAsSb which can be lattice-matched to InP and can be used to achieve large conduction band offsets. It is very useful for coolerless (without thermo-electric cooler) operation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Patent application Ser. No. 60/517,400 filed Nov. 6, 2003. 
     
    
     MICROFICHE APPENDIX  
       [0002]     Not Applicable.  
       TECHNICAL FIELD  
       [0003]     The present invention relates to semiconductor laser diodes and in particular, to a semiconductor laser diode which has excellent temperature characteristics.  
       BACKGROUND OF THE INVENTION  
       [0004]     Semiconductor laser diodes can be divided into two groups, those for use in short wavelength applications (λ=0.78-0.89 μm) (1 μm=1 micron) and those for use in long wavelength applications (λ=0.98-1.6 μm). Gallium arsenide (GaAs) based material systems are well suited to short wavelength applications and present excellent high temperature performance but they are generally not suited to applications beyond about 1.2 μm. However, modern optical telecommunications systems operate at long wavelengths, typically 980 nm to 1.55 μm and thus indium-phosphide (InP) based materials are typically used because they are better suited to long wavelength applications, especially at 1.3 to 1.6 μm, which is the typical signal transmission wavelength range. InP material systems usually exhibit poor high temperature performance, thus in order to operate InP based devices reliably, external cooling is usually required. It is well known in the art to package semiconductor laser diodes with integral thermoelectric coolers which increase cost, complexity and power dissipation.  
         [0005]     Generally, the high temperature operation capability of laser diodes is assessed through the use of a characteristic temperature T 0 , linking threshold currents and temperature operation as summarized in Equation (1) below: 
 
 I=I   0  exp( T/T   0 )   (1) 
 
         [0006]     Where I is the threshold current, I 0  is a scaling factor and T is a temperature in degrees Kelvin (° K). Therefore, higher T 0  permits higher temperature operation because for higher T 0 , the threshold current varies less with temperature. Higher T 0  has been linked to larger conduction band offsets. Conversely, the poor temperature performance in typical InP material systems is usually attributed to the small conduction band offset, which is also often due to the lack of a suitable available material with a higher energy bandgap and a lower index of refraction than InP.  
         [0007]     A first example of a known InP based laser structure is shown in  FIG. 1 . This laser uses the InGaAsP/InP material system which usually has a poor characteristic temperature of T 0 ≈60 K. Referring to  FIG. 1 , the laser structure  100  comprises cladding layer  101  of p-InP, confinement layers  102 ,  108  and barrier layers  104 ,  106  of InGaAsP (indium-gallium-arsenide-phosphide), and quantum wells  103 ,  105 ,  107  also of InGaAsP but of a different composition than the barrier layers and confinement layers, and another cladding layer  109  of n-InP. (Note that in the figures, an asterisk is used to represent a different composition of the same material.)  
         [0008]     Referring to the band diagram of  FIG. 1 , the conduction band offset  110  between the cladding layers ( 101 ,  109 ) and the confinement layers ( 102 ,  108 ) is 109 meV. (Note that in the figures, these values are displayed in parentheses in units of eV (electron Volts) to avoid ambiguity with the reference numbers). The conduction band offset  111  between the barrier and confinement layers ( 102 ,  104 ,  106 ,  108 ) and the quantum wells ( 103 ,  105 ,  107 ) is 111 meV. The valence band offset  112  between the cladding layers ( 101 ,  109 ) and the confinement layers ( 102 , 108 ) is 164 meV. The valence band offset  113  between the barrier and confinement layers ( 102 ,  104 ,  106 ,  108 ) and the quantum wells ( 103 ,  105 ,  107 ) is 166 meV. The energy bandgap  114  of InP ( 101 ,  109 ) is 1.35 eV. Referring now to the index of refraction diagram of  FIG. 1 , for an optical wavelength of 1.55 μm, the index of refraction of the cladding layers ( 101 ,  109 ) is 3.17, the index of refraction of the barrier and confinement layers ( 102 ,  104 ,  106 ,  108 ) is 3.31 and the index of refraction of the quantum wells ( 103 ,  105 ,  107 ) is 3.6. (Note that in the figures, these values are displayed in parentheses to avoid ambiguity with the reference numbers).  
         [0009]     A second example of a known InP based laser structure is shown in  FIG. 2 . This laser uses the InGaAlAs/InP material system and has a better characteristic temperature of T 0 ≈90 K than the structure of the first example. Referring to  FIG. 2 , the laser structure  200  comprises connection layer  201  of p-InP, cladding layers  202 ,  210  of InAlAs (indium-aluminide-arsenide), confinement layers  203 ,  209  and barrier layers  205 ,  207  of InGaAlAs, and quantum wells  204 ,  206 ,  208  of InGaAlAs (indium-gallium-aluminide-arsenide) but of a different composition than the confinement and barrier layers  203 ,  205 ,  207 ,  209 , and a substrate layer  211  of n-InP. Note that in general, semiconductor laser diodes are constructed on a substrate ( 211  in  FIG. 2 ) and have a connection layer ( 210  in  FIG. 2 ) for connecting to the external world. The connection and substrate layers are illustrated in  FIG. 2  for completeness but these layers do not play a significant role in the structure in terms of optical and electrical confinement and are therefore not illustrated in the other figures for brevity. Referring to the band diagram of  FIG. 2 , the conduction band offset  212  between the connection and substrate layers ( 201 ,  211 ) and the cladding layers ( 202 ,  210 ) is −185 meV. The conduction band offset  213  between cladding layers ( 202 ,  210 ) and the confinement and barrier layers ( 203 ,  205 ,  207 ,  209 ) is 297 meV. The conduction band offset  214  between the confinement and barrier layers ( 203 ,  205 ,  207 ,  209 ) and the quantum wells ( 204 ,  206 ,  208 ) is 165 meV. The valence band offset  215  between the connection and substrate layers ( 201 ,  211 ) and the cladding layers ( 202 ,  210 ) is 75 meV. The conduction band offset  216  between cladding layers ( 202 ,  210 ) and the confinement and barrier layers ( 203 ,  205 ,  207 ,  209 ) is 127 meV. The conduction band offset  217  between the confinement and barrier layers ( 203 ,  205 ,  207 ,  209 ) and the quantum wells ( 204 ,  206 ,  208 ) is 71 meV. The energy bandgap  218  of InP ( 201 ,  211 ) is 1.35 eV and the energy bandgap  219  of InAlAs ( 202 ,  210 ) is 1.46 eV. Referring now to the index of refraction diagram of  FIG. 2 , for an optical wavelength of 1.55 μm, the index of refraction of the connection and substrate layers ( 201 ,  211 ) is 3.17, the index of refraction of the cladding layers ( 202 ,  210 ) is 3.2, the index of refraction of the confinement and barrier layers ( 203 ,  205 ,  207 ,  209 ) is 3.35 and the index of refraction of the quantum wells ( 204 ,  206 ,  208 ) is 3.6.  
         [0010]     Another example of a known laser structure is shown in  FIG. 3 . This laser differs from the first two examples in that it is based on GaAs. It uses the InGaNAs/GaAs material system (indium-gallium-nitride-arsenide/gallium-arsenide) and has an improved characteristic temperature of T 0 ≈120 K than the structures of the first two examples. However, it is not necessarily suitable or desirable to use the GaAs system, especially for optical telecommunications wavelengths. Referring to  FIG. 3 , the laser structure  300  comprises cladding layer  301  of p-AlGaAs, confinement layers  302 ,  308  and barrier layers  304 ,  306  of GaAs, and quantum wells  303 ,  305 ,  307  of GaInNAs and another cladding layer  309  of n-AlGaAs. Referring to the band diagram of  FIG. 3 , the conduction band offset  310  between the cladding layers ( 301 ,  309 ) and the confinement layers ( 302 ,  308 ) is 224 meV. The conduction band offset  311  between the confinement and barrier layers ( 302 ,  304 ,  306 ,  308 ) and the quantum wells ( 303 ,  305 ,  307 ) is 434 meV. The valence band offset  312  between the cladding layers ( 301 ,  309 ) and the confinement layers ( 302 ,  308 ) is 150 meV. The valence band offset  313  between the confinement and barrier layers ( 302 ,  304 ,  306 ,  308 ) and the quantum wells ( 303 ,  305 ,  307 ) is 186 meV. The energy bandgap  314  of AlGaAs ( 301 ,  309 ) is about 1.90 eV and the energy bandgap  315  of GaAs ( 302 ,  304 ,  306 ,  308 ) is 1.52 eV. Referring now to the index of refraction diagram of  FIG. 3 , for an optical wavelength of 1.55 μm, the index of refraction of the cladding layers ( 301 ,  309 ) is 3.26, the index of refraction of the confinement and barrier layers ( 302 ,  304 ,  306 ,  308 ) is 3.40 and the index of refraction of the quantum wells ( 303 ,  305 ,  307 ) is 3.6. Note that the structure of  FIG. 3  would in practice be sandwiched between a n-GaAs substrate and a pGaAs connection layer for mechanical and electrical connection to the external world.  
         [0011]     The above described prior art laser structures have poor temperature performance or other disadvantages. Accordingly, a semiconductor laser structure for optical telecommunications wavelengths, capable of improved high temperature operation remains highly desirable.  
       SUMMARY OF THE INVENTION  
       [0012]     It is therefore an object of the present invention to provide an improved semiconductor laser structure, capable of high temperature operation.  
         [0013]     Accordingly, an aspect of the present invention provides a semiconductor laser structure having an active region, a confinement layer adjacent to the active region and a cladding layer adjacent to the confinement layer. The active region is capable of emitting radiation, and is constructed of antimony-free material. The confinement layer is adapted to confine electrons in the active region, and is constructed of antimony-free material. The cladding layer comprises an antimony-based (Sb) alloy.  
         [0014]     In some embodiments, the cladding layer has a lower index of refraction than the confinement layer.  
         [0015]     In some embodiments, the cladding layer has a larger bandgap than the confinement layer.  
         [0016]     In other embodiments, the cladding layer is lattice-matched to InP.  
         [0017]     In some embodiments, the cladding layer comprises AlAsSb.  
         [0018]     In other embodiments, the cladding layer comprises a compound comprising predominantly Al, As and Sb.  
         [0019]     In still other embodiments, the cladding layer comprises AlGaAsSb.  
         [0020]     In some embodiments, the active region comprises at least one quantum well and in other embodiments, the active region comprises a plurality of quantum wells separated by barrier layers.  
         [0021]     In some embodiments, the barrier layers comprise the same material as the confinement layer.  
         [0022]     In some embodiments, the quantum well(s) comprises InGaAsP.  
         [0023]     In some embodiments, the confinement layer comprises InP.  
         [0024]     In other embodiments, the quantum well(s) comprises InGaAlAs.  
         [0025]     In some embodiments, the confinement layer comprises InAlAs.  
         [0026]     In some embodiments, the active region is adapted to emit radiation at a wavelength of about 980 nm.  
         [0027]     In other embodiments, the active region is adapted to emit radiation at a wavelength of about 1.3 μm  
         [0028]     In still other embodiments, the active region is adapted to emit radiation at a wavelength of about 1.55 μm  
         [0029]     In some embodiments, the laser structure comprises a Fabry-Perot laser.  
         [0030]     In other embodiments, the laser structure comprises a distributed feedback (DFB) laser.  
         [0031]     In still other embodiments, the laser structure comprises a semiconductor optical amplifier (SOA).  
         [0032]     According to another aspect of the present invention, there is provided a semiconductor laser structure having an active region having a first side and a second side, the active region being capable of emitting radiation, a first confinement layer adjacent the first side of said active region, the first confinement layer adapted to confine electrons in the active region, a second confinement layer adjacent the second side of the active region, the second confinement layer adapted to confine electrons in the active region, a first cladding layer adjacent the first confinement layer, the first cladding layer comprising an antimony-based (Sb) alloy; and a second cladding layer adjacent the second confinement layer, the second cladding layer comprising an antimony-based (Sb) alloy.  
         [0033]     In some embodiments, the first confinement layer and the second confinement layer cooperate to confine electrons in the active region.  
         [0034]     In some embodiments, the first cladding layer and the second cladding layer are adapted to confine electrons in the active region.  
         [0035]     In some embodiments, the first cladding layer and the second cladding layer are adapted to cooperate with the first confinement layer and the second confinement layer to confine electrons in the active region.  
         [0036]     In some embodiments, the first cladding layer and the second cladding layer are lattice-matched to InP.  
         [0037]     In some embodiments, the first cladding layer and the second cladding layer comprise AlAsSb.  
         [0038]     In other embodiments, the first cladding layer and the second cladding layer comprise a compound comprising predominantly Al, As and Sb.  
         [0039]     In some embodiments, the first cladding layer and said second cladding layer comprise AlGaAsSb.  
         [0040]     In some embodiments, the active region comprises at least one quantum well.  
         [0041]     In some embodiments, the first confinement layer and the second confinement layer comprise InP.  
         [0042]     In other embodiments, the quantum well(s) comprise InGaAsP.  
         [0043]     In some embodiments, the first confinement layer and said second confinement layer comprise InAlAs.  
         [0044]     In other embodiments, the quantum well(s) comprise InGaAlAs.  
         [0045]     In still other embodiments, the active region comprises at least one quantum well, the quantum well(s) comprise InGaAlAs, the first confinement layer and the second confinement layer comprise InAlAs, and the active region is adapted to emit radiation at a wavelength of about 980 nm.  
         [0046]     According to another aspect of the present invention, there is provided a semiconductor laser structure based on an InP material system and having an active region capable of emitting radiation; a confinement layer adjacent the active region, the confinement layer adapted to confine electrons in the active region; and a cladding layer adjacent the confinement layer, the cladding layer comprising an antimony-based (Sb) alloy.  
         [0047]     The laser structure can operate at high temperatures and is very useful for coolerless operation required for low power dissipation in optical systems. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]     Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:  
         [0049]      FIG. 1  is a diagram showing the band structure and index of refraction characteristics of a first prior art InP-based laser structure;  
         [0050]      FIG. 2  is a diagram showing the band structure and index of refraction characteristics of a second prior art InP-based laser structure;  
         [0051]      FIG. 3  is a diagram showing the band structure and index of refraction characteristics of a prior art GaAs-based laser structure;  
         [0052]      FIG. 4  is a diagram showing the band structure and index of refraction characteristics of a first embodiment of the semiconductor laser structure of the present invention  
         [0053]      FIG. 5  is a diagram showing the band structure and index of refraction characteristics of a second embodiment of the semiconductor laser structure of the present invention; and  
         [0054]      FIG. 6  is a diagram showing the band structure and index of refraction characteristics of a third embodiment of the semiconductor laser structure of the present invention. 
     
    
       [0055]     It will be noted that, throughout the appended drawings, like features are identified by like reference numerals  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0056]     The present invention provides a semiconductor laser structure that can be grown lattice-matched to InP and which opens up the possibility of achieving conduction band energy offsets similar to the InGaNAs/GaAs material system.  
         [0057]     One way to improve temperature performance of laser structures using InP based materials, is to use a waveguide cladding material having an index of refraction less than that of InP at the optical wavelengths of interest, and having a bandgap energy greater than that of InP. The present invention uses antimony-based materials such as AlAsSb (aluminum-arsenide-antimonide) as a waveguide cladding. When used in conjunction with active regions and confinement layers containing no antimony, such antimony-based cladding layers present excellent electron confinement and waveguide characteristics. One advantage of these materials is that they can be lattice-matched to InP.  
         [0058]      FIG. 4  illustrates a first embodiment of the semiconductor laser structure of the present invention. This laser uses a InP material system traditionally used for telecommunications systems but with novel AlAsSb waveguide cladding layers. Referring to  FIG. 4 , the laser structure  400  comprises an active region comprising quantum wells  403 ,  405 ,  407  of InGaAsP and separated by barrier layers  404 ,  406  of InP. The active region is bounded by confinement layers  402 ,  408 . The confinement layers  402 ,  408  are bounded respectively by cladding layer  401  of p-AlAsSb and cladding layer  409  of n-AlAsSb. These layers are deposited on a InP substrate (not shown). Referring to the band diagram of  FIG. 4 , the conduction band offset  410  between the cladding layers ( 401 ,  409 ) and the confinement layers ( 402 ,  408 ) is  594  meV. The conduction band offset  411  between the confinement and barrier layers ( 402 ,  404 ,  406 ,  408 ) and the quantum wells ( 403 ,  405 ,  407 ) is 220 meV. The valence band offset  412  between the cladding layers ( 401 ,  409 ) and the confinement layers ( 402 ,  408 ) is −25 meV. The valence band offset  413  between the confinement and barrier layers ( 402 ,  404 ,  406 ,  408 ) and the quantum wells ( 403 ,  405 ,  407 ) is 330 meV. Note that in spite of the cladding layers ( 401 ,  409 ) having a band energy higher than the confinement and barrier layers ( 402 ,  404 ,  406 ,  408 ), holes are still well confined in the quantum wells ( 403 ,  405 ,  407 ) because of their high density of states, large effective mass and low mobility, compared to electrons.  
         [0059]     The energy bandgap  414  of AlAsSb ( 401 ,  409 ) is 1.91 eV and the energy bandgap  415  of InP ( 402 ,  404 ,  406 ,  408 ) is 1.35 eV. Referring now to the index of refraction diagram of  FIG. 4 , for an optical wavelength of 1.55 μm, the index of refraction of the cladding layers ( 401 ,  409 ) is 3.02, the index of refraction of the barrier layers ( 402 ,  404 ,  406 ,  408 ) is 3.17 and the index of refraction of the quantum wells ( 403 ,  405 ,  407 ) is 3.6. The cladding layer can be considered as an optical cladding layer or a waveguide cladding layer.  
         [0060]     The laser structure thus has an active region capable of emitting radiation, the active region is bounded by confinement layers on each side to confine electrons, and the confinement layers are bounded by waveguide cladding layers to further confine electrons and to confine radiation (photons).  
         [0061]      FIG. 5  illustrates a second embodiment of the semiconductor laser structure of the present invention using a newer material system than that of the embodiment of  FIG. 4 , and exhibits better high temperature performance. This laser uses AlAsSb waveguide cladding layers with InAlAs barriers and InGaAlAs quantum wells. Referring to  FIG. 5 , the laser structure  500  comprises an active region comprising quantum wells  503 ,  505 ,  507  of InGaAlAs, separated by barrier layers  504 ,  506  of InAlAs. The active region is bounded by confinement layers  502 ,  508 . The confinement layers  502 ,  508  are bounded respectively by cladding layer  501  of p-AlAsSb and cladding layer  509  of n-AlAsSb. Referring to the band diagram of  FIG. 5 , the conduction band offset  510  between the cladding layers ( 501 ,  509 ) and the confinement layers ( 502 ,  508 ) is about  334  meV. The conduction band offset  511  between the confinement and barrier layers ( 502 ,  504 ,  506 ,  508 ) and the quantum wells ( 503 ,  505 ,  507 ) is 462 meV. The valence band offset  512  between the cladding layers ( 501 ,  509 ) and the confinement layers ( 502 ,  508 ) is 125 meV. The valence band offset  513  between the confinement and barrier layers ( 502 ,  504 ,  506 ,  508 ) and the quantum wells ( 503 ,  505 ,  507 ) is 198 meV. The energy bandgap  514  of AlAsSb ( 501 ,  509 ) is 1.91 eV and the energy bandgap  515  of InAlAs ( 502 ,  504 ,  506 ,  508 ) is 1.46 eV. Referring now to the index of refraction diagram of  FIG. 5 , for an optical wavelength of 1.55 μm, the index of refraction of the cladding layers ( 501 ,  509 ) is 3.02, the index of refraction of the confinement and barrier layers ( 502 ,  504 ,  506 ,  508 ) is 3.20 and the index of refraction of the quantum wells ( 503 ,  505 ,  507 ) is 3.6.  
         [0062]     The confinement layers ( 502 ,  508 ) provide electron confinement. The cladding layers ( 501 ,  509 ) provide additional electron confinement and also help control the electron flow into the quantum wells ( 503 ,  505 ,  507 ), providing better performance than can be expected from the increase in barrier height alone. The cladding layers ( 501 ,  509 ) also provide optical confinement due to the low index of refraction.  
         [0063]     The use of ternary AlAsSb composition as a cladding layer provides excellent high temperature performance. Other embodiments of the present invention use quaternary compositions having small quantities of other elements such as Gallium (Ga) for example, thereby using AlGaAsSb as the cladding layer.  
         [0064]      FIG. 6  illustrates a third embodiment of the semiconductor laser structure of the present invention. This embodiment is similar to the second embodiment of  FIG. 5  but adapted to operate at a wavelength of 980 nm. Referring now to  FIG. 6 , the laser structure  600  comprises an active region comprising quantum wells  603 ,  605 ,  607  of InGaAlAs separated by barrier layers  604 ,  606  of InAlAs. The active region is bounded by confinement layers  602 ,  608 . The confinement layers  602 ,  608  are bounded respectively by cladding layer  601  of p-AlAsSb and cladding layer  609  of n-AlAsSb. Referring to the band diagram of  FIG. 6 , the conduction band offset  610  between the cladding layers ( 601 ,  609 ) and the confinement layers ( 602 ,  608 ) is about 334 meV. The conduction band offset  611  between the confinement and barrier layers ( 602 ,  604 ,  606 ,  608 ) and the quantum wells ( 603 ,  605 ,  607 ) is 137 meV. The valence band offset  612  between the cladding layers ( 601 ,  609 ) and the confinement layers ( 602 ,  608 ) is 125 meV. The valence band offset  613  between the confinement and barrier layers ( 602 ,  604 ,  606 ,  608 ) and the quantum wells ( 603 ,  605 ,  607 ) is 59 meV. The energy bandgap  614  of AlAsSb ( 601 ,  609 ) is 1.91 eV and the energy bandgap  615  of InAlAs ( 602 ,  604 ,  606 ,  608 ) is 1.46 eV. Referring now to the index of refraction diagram of  FIG. 6 , for an optical wavelength of 980 nm, the index of refraction of the cladding layers ( 601 ,  609 ) is 3.10, the index of refraction of the confinement and barrier layers ( 602 ,  604 ,  606 ,  608 ) is 3.38 and the index of refraction of the quantum wells ( 603 ,  605 ,  607 ) is 3.6.  
         [0065]     The embodiment of  FIG. 6  illustrates that the present invention is useful at 980 nm in addition to the longer wavelengths (980 nm to 1.55 μm) of typical optical telecommunications systems.  
         [0066]     The present invention is applicable to many types of semiconductor laser configurations such as, but not limited to Fabry-Perot pump lasers, distributed feedback (DFB) lasers using gratings and semiconductor optical amplifiers (SOA).  
         [0067]     The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.