Abstract:
A method for improving the efficiency for an optoelectronic device, such as semiconductor lasers, Superluminescence Light Emitting Diodes (SLDs), Gain Chips, optical amplifiers is disclosed, see FIG.  4 B. In accordance with the principles of the invention, at least one blocking layer ( 70 ) is interposed at the interface between materials composing the device. The at least one blocking layers creates a barrier that prevents the leakage of electrons from a device active region contained in the waveguide region, to a device clad region ( 66 ). In one aspect of the invention, a blocking layer ( 70 ) is formed at the junction of the semiconductor materials having different types of conductivity. The blocking layer prevents electrons from entering the material of a different polarity. In another aspect of the invention, a low-doped or undoped region ( 64 ) is positioned adjacent to the blocking layer ( 70 ) to decrease optical losses.

Description:
FIELD OF THE INVENTION 
     This invention relates to semiconductor opto-electronic technology, and more particularly, high power opto-electronic devices, such as high power semiconductor lasers, amplifiers, gain chips and superluminescence Light Emitting Diodes (SLEDs) having improved efficiency and low beam divergence for telecom applications. 
     BACKGROUND OF THE INVENTION 
     Opto-electronic devices, such as semiconductor lasers and SLEDs provide the light source for different aspects of fiber-optic telecommunication systems. Opto-electronic radiation source devices, such as lasers, having wavelength outputs ranging from 1.31 to 1.55 microns (μm) are the most commonly used lasers for telecommunication operation. Radiation source devices with wavelengths of 1.31 μm are typically used for short distance transmission and cable TV signals, while wavelengths of 1.42–1.6 μm are typically used for fiber optic communications. Lasers having wavelengths in the range of 1.42 to 1.48 μm are typically used as pump lasers for Raman and Erbium Doped Fiber Amplifiers (EDFA). In long distance transmission one desires to use, for example, lasers having wavelengths between 1.54 to 1.56 μm. For all telecom applications and especially for EDFA pump sources, operating currents for semiconductor sources exceed by more than an order of magnitude the threshold current. 
     Essentially, as one can ascertain from the prior art, a weak temperature dependence of the main laser characteristics is extremely important for telecom lasers. As one knows in the prior art, it is not easy to keep high performance at elevated temperatures for lasers operating in the wavelength range of 1.3–1.5 μm. A widely used material for telecom lasers is InGaAsP compounds grown on InP substrates. Numerous investigations of InGaAsP/InP lasers have demonstrated that the strong temperature degradation of their parameters is partially associated with insufficient high energy level barriers for confinement of the electrons in the laser active region. As a result of the low barrier in the conduction band, some of the electrons injected from n-doped cladding layer passes through the active region and are lost in the p-doped cladding layer. There are many investigators that have looked at this problem, and people have provided various solutions to decrease electron leakage at elevated temperatures. For example, see an article entitled, “Lasing Characteristics Under High Temperature Operation of 1.55 μm Strained InGaAsP/InGaAlAs MQW Laser With InAIAs Electron Stopper Layer” by H. Murai et al., published in Electronics Letters on Nov. 23, 1995, Volume 31, Number 24. In that paper there is shown a 1.55 μm strained InGaAsP/InGaAlAs MQW laser in which InGaAIAs cladding layers and InAIAs electron stopper layer have been incorporated to reduce electron leakage current. The device showed low threshold current with high maximum temperature of CW operation. As indicated in the paper, superior lasing characteristics were demonstrated through comparison with conventional strained MQW lasers without electron stop layer. A similar design was used for the fabrication of 1.3 μm lasers that could operate without thermoelectric coolers. Such lasers are required for subscriber networks and optical interconnection systems. See, for example, an article entitled, “High Temperature Operation of AlGaInAs Ridge Waveguide Lasers With a p-AlInAs Electron Stopper Layer” published in Japanese Journal of Applied Physics on pages 1230 through 1233 by Takemasa et al. 
     As can be seen in these papers, there are band diagrams that show the active region of the laser including Quantum Wells (QW) sandwiched between layers of materials with a band gap larger than that of the active region. Thus energy barriers confining the injected electrons and holes in the active region are created. The prior art utilized InGaAsP and InP to create an active region and barriers for lasers operating in the 1.2 to 2.0 μm range. The disadvantage of these material is that the energy barriers for electrons is lower than that for holes and when one wants to inject electrons to generate photons in the QWs, there is a leakage of electron current from active region to the p-doped InP cladding layer that limits laser performance. This leakage current component is superlinearly related to total current and temperature sensitive, in that as the leakage current increases at a rate greater than an increase in the total current. The problem is more severe for lasers operating at 1.3 μm than for lasers that operate at 1.55 μm. In fact, the problem is so severe that one employs AlInGaAs as a cladding layer material or AlInAs as a stop-electron (blocking) layers. 
     However, aluminum compounds are not desirable for telecommunication devices because aluminum easily oxidizes and thus creates problems for laser fabrication as well as for long term reliability. The prior art has attempted to solve this problem by using a InGaP large band gap material as a blocking layer. The additional problem for InGaAsP/InP telecom lasers arises at operation at high current densities. Measurements of output power versus current (P-I characteristics) at both short pulsed regime and continuous wave (CW) regime demonstrate the transition from a linear performance to a sublinear performance at high current density. This phenomenon is called the P-I characteristic rollover or saturation effect. In the short pulse regime, where device heating is negligible, the saturation effect is purely current induced and is associated with an increase of the electron concentration and electrical field at the active region/p-doped cladding layer interface. The result is that the electron current component that is directed from the active region to the p-doped clad layer increases. In the CW regime this current component increases additionally due to heating of the active region caused by the high current. Several publications (See, for example, an article entitled, “Effect of Heterobarrier Leakage on the Performance of High-Power 1.5 μm InGaAsP Multiple-Quantum Well Lasers” published in Journal of Applied Physics on pages 2211–2215 by Shterengas et al.) indicated that the increase of p-doping of the active region/p-doped clad interface allows the decreasing of the electron leakage due to the reduction of the interface electrical field. However, the increase of the hole concentration can lead to the decrease of laser efficiency caused by the additional optical losses associated with the strong photon absorption by free holes. In order to solve this problem, a broad waveguide (BW) design was suggested in U.S. Pat. No. 5,818,860, entitled “High Power Semiconductor Laser Diode,” issued Oct. 6, 1998, to Garbuzov, the inventor herein. In the case of BW lasers, 99% of the optical mode is confined in the broadened waveguide with total thickness of about 1 μm. The waveguide material is undoped thus providing minimum optical losses for lasing mode. The broad waveguide design was very useful for the fabrication of high power, high efficient diode lasers for numerous but non-telecom applications. The drawback of broad waveguide lasers for telecom applications is the large beam divergence in the direction perpendicular to the laser plane (fast direction). For 1.5 μm lasers with a 1 μm thick waveguide this divergence exceeds 40 degrees at half maximum intensity (FWHM&gt;40°). This value of divergence is not compatible with effective laser coupling in single mode optical fiber. Methods of improving fast axis divergence are well known. For example, one method is to decrease the waveguide thickness down to 10–30 nm. However in the case of such narrow (NW) waveguide structures, a considerable portion (about 40%) of lasing mode propagates in the p-doped clad layer and optical losses caused by free hole absorption is high. As discussed above, the decrease of p-clad doping is not desirable because of electron leakage enhancement. 
     Thus, there is a need for a new design of telecom MQW low divergence lasers (especially pump lasers for fiber amplifiers) that provide high efficiency device operation at high current densities and elevated temperatures. 
     SUMMARY OF THE INVENTION 
     A new design of telecom narrow waveguide low divergence diode light emitting sources, particularly for fiber amplifier pump laser, is disclosed. The design provides high efficiency operation at high current densities and elevated temperatures. To achieve this design, a large band gap blocking layer is incorporated coincident with the p/n junction within the device structure. The large band gap blocking layer material creates an electron energy barrier having a minimum 200 meV greater than adjacent material levels in the conduction band to prevent electron leakage from the active region to a p-doped clad region. In addition, a low doped, region of a wide band gap material is used in combination with the at least one blocking layer to decrease optical losses. In one aspect of the invention, a blocking layer is imposed between the low-doped region and the active region. In a second aspect of the invention the low-doped region is imposed between the blocking layer and the active region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1   a  illustrates a perspective view of a conventional semiconductor laser; 
         FIG. 1   b  illustrates a conventional semiconductor laser conduction band diagram; 
         FIG. 2   a  illustrates an exemplary semiconductor laser conduction band diagram in accordance with one aspect of one embodiment of the invention; 
         FIG. 2   b  illustrates a second exemplary semiconductor laser conduction band diagram in accordance with a second aspect of the inventive concept illustrated in  FIG. 2   a;    
         FIG. 2   c  illustrates an exemplary semiconductor laser conduction band diagram in accordance with another aspect of the inventive concept illustrated in  FIGS. 2   a;    
         FIG. 2   d  illustrates an exemplary semiconductor laser conduction band diagram in accordance with an aspect of the inventive concept illustrated in  FIGS. 2   a ,  2   b  and  2   c;    
         FIG. 3   a  illustrates an exemplary semiconductor laser conduction band diagram in accordance with one aspect of a second embodiment of the invention; 
         FIG. 3   b  illustrates a second exemplary semiconductor laser conduction band diagram in accordance with a second aspect of the inventive concept illustrated in  FIG. 3   a;    
         FIG. 3   c  illustrates an exemplary semiconductor laser conduction band diagram in accordance with a third aspect of the inventive concept illustrated in  FIGS. 3   a  and  3   b;    
         FIG. 4   a  illustrates an exemplary cross-sectional epistructure of a conventional Quantum Well (QW) semiconductor radiation source device; 
         FIG. 4   b  illustrates an exemplary cross-sectional epistructure of a QW semiconductor radiation source device in accordance with the principles of the invention; 
         FIG. 5  illustrates a comparison of performance results of a conventional 1.48 micron pump laser and 1.48 micron pump laser having a conduction band diagram characteristic as illustrated in  FIG. 2   a ; and 
         FIG. 6  illustrates P-I characteristics and power output spectra of a DFB laser fabricated in accordance with the principles of the invention. 
     
    
    
     It is to be understood that these drawings are solely for purposes of illustrating the concepts of the invention and are not intended as a level of the limits of the invention. It will be appreciated that the same reference numerals, possibly supplemented with reference characters where appropriate, have been used throughout to identify corresponding parts. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1   a , which corresponds to  FIG. 1  of the referenced U.S. Pat. No., 5,818,860, a semiconductor laser diode is designated as  10 . Laser diode  10  comprises a body  12  of a semiconductor material or materials having a bottom surface  14 , top surface  16 , end surfaces  18  and side surfaces  20 . The body  12  includes a waveguide region  22  extending thereacross. Within the waveguide region  22  is an active region  24  in which photons are generated when an appropriate electrical bias is placed across the diode  10 . The active region  24  may be of any structure well known in the laser diode art that is capable of generating photons. The active region  24  comprises one or more quantum wells. The waveguide region  22  includes layers  26  on each side of the active region  24 , which are of undoped semiconductor material. 
     On each side of the waveguide region  22  is a separate clad region  28  and  30 . The clad regions  28  and  30  are layers of a semiconductor material of a composition, which has a lower refractive index than the materials of the layers  26  of the waveguide region  22 . Also, the clad regions  28  and  30  are at least partially doped to be of opposite conductivity type. The doping level in the clad regions  28  and  30  are typically between about 5*10 17 /cm 3  and 2*10 19 /cm 3 . For example, the clad region  28  between the waveguide region  22  and the top surface  16  of the body  12  may be of p-doped conductivity and the clad region  30  between the waveguide region  22  and the bottom surface  14  of the body  12  may be of n-doped conductivity. 
     A contact layer  32  of a conductive material, such as a metal, is on and in ohmic contact with the p-type conductivity clad region  28 . The contact layer  32  is in the form of a strip that extends between the end surfaces  18  of the body  12  and is narrower than the width of the body  12 , i.e., the distance between the side surfaces  20  of the body  12 . A contact layer  34  of a conductive material, such as a metal, is on and in ohmic contact with the n-type conductivity clad region  30 . The contact layer  34  extends across the entire area of the bottom surface  14  of the body  12 . 
     The various regions of the body  12  may be made of any of the well known semiconductor materials used for making laser diode, such as but not limited to gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), indium phosphide (InP), indium gallium arsenide (InGaAs) and such quaternary materials as indium gallium arsenide phosphide (InGaAsP). However, the materials used for the various regions must have refractive indices, which provide the desired confinement of the optical mode. The clad regions  28  and  30  may be doped uniformly throughout their thickness or may be graded with little or no doping at their junction with the waveguide region  22  and the heaviest doping at the respective surface of the body  12 . 
     Referring to  FIG. 1   b , which corresponds to FIG. 2 of the referenced U.S. Pat. No., 5,818,860, there is schematically shown the conduction band diagram  36  of a laser diode corresponding to the structure of laser diode  10  shown in  FIG. 1   a . In this schematic representation there is included a waveguide region  38  having therein a single quantum well region  40  of undoped material, e.g., In 20 Ga 80 As, and a separate confinement layer  42  of undoped material, e.g., Al 30 Ga 70 As, on each side of the quantum well region  40 . A p-doped conductivity clad region  44  is on one side of the waveguide region  38  and an n-doped conductivity clad region  46  is on the other side of the waveguide region  38 . Each of the clad regions  44  and  46  are of Al 60 Ga 30 As. Although the laser diode  36  is shown as having only a single quantum well region  40 , it may have a plurality of quantum well regions which are spaced apart by barrier regions as is well known in the laser diode art. Photons are generated as electrons drop into quantum well region  40  and confinement layer  42  inhibits the flow of electrons toward p-doped conductivity clad layer  44  and directs the generated photons travel along a known path. 
     Referring now to  FIG. 2   a , there is schematically shown a conduction band diagram  200  of one aspect of an opto-electronic device, e.g., a semiconductor laser diode, in accordance with one embodiment of the present invention. In this representation of one embodiment of the invention, the semiconductor waveguide region  38  comprises a plurality of quantum well regions  52  composed of a semiconductor material, such as InGaAsP, which are spaced apart by barrier regions  54 . Barrier regions  54  are composed of a semiconductor material, such as InGaAsP having a bandgap energy level represented herein as Eg 54 . At each side of quantum well region  52  is illustrated inner confinement layer  56 . Inner confinement layer  56  is composed of a material, such as InGaAsP having a bandgap energy level represented herein as Eg 56 . As would be appreciated, the material of barrier level  54  may be the same or different than that of inner confinement layer  56 . Adjacent to each of the inner confinement layers  56  is outer confinement layer  58 . Outer confinement layer  58  is composed of a material such as InGaAsP having a bandgap energy level represented herein as Eg 58 . As illustrated, the bandgap energy levels of the confinement regions are formulated such that:
 
Eg 52 &lt;Eg 54 &lt;=Eg 56 &lt;Eg 58 
 
     Adjacent to the outer confinement layers  58  are clad regions  60  and  62  composed of a material, such as InP, that is doped with impurities. Known impurities are added into clad region  60  to achieve an electron concentration, i.e., n-doped, and known impurities are added into clad region  62  to achieve a high hole concentration, i.e., p-doped. Clad regions  60  and  62  are generally doped, as previously discussed, to a level of greater that 5*10 17 /cm 3 . Furthermore, n-dope type clad region  60  generally is uniformly doped throughout its thickness, but the p-dope type clad region  62  can have a doping level that is graded from a lowest level at the interface with the outer confinement layer  58  to a highest level at its surface. The energy band gap of clad regions  60  and  62  are significantly greater than the energy bandgap of outer confinement region  58 . 
     In accordance with the principle of the invention, a tensile strained blocking layer  70  composed of a large band gap material, such as InGaP, which may also be p-doped, is interposed between outer confinement layer  58  and clad layer  62 , i.e., coincident with p/n junction formed between p-type materials and n-type materials. Blocking layer  70  creates an energy barrier that prevents electrons from escaping, i.e., leaking, from the outer confinement region  58  to clad region  62 . In this case, bandgap energy level of blocking layer  70 , represented as Eg 70  is such that:
 
Eg 58 &lt;Eg 62 &lt;Eg 70 
 
     Blocking layer  70  should create a barrier of not less than 200 meV. The barrier created by blocking layer  70  decreases electron leakage down to one percent of the total drive current. Furthermore, blocking layer  70  is a relatively thin layer, 20–50 nanometers (nm). In a preferred embodiment, blocking layer  70  should not be less than 20 nm to avoid electron tunneling and not greater than 50 nm to decrease series resistance of the device and eliminate the formation of defects when the blocking layer is lattice mismatched with the substrate material. 
     In another aspect of the invention, clad region  62  is composed of a spacer region  64  and cladding region  66 . In this aspect, cladding region  66  is a p-doped layer, as previously described. However, spacer region may be a p-doped InP material or InGaAsP material such that the electron bandgap is represented as:
 
Eg 66 &gt;Eg 64 &gt;Eg 58 
 
     In one aspect of the invention, spacer region  64  is a low p-doped in the order of p=1*10 17  to 2*10 17 /cm 3 . Furtherstill, the hole concentration in cladding region  66  can be sharply increased up to 10 18 /cm 3 . 
     Referring now to  FIG. 2   b , there is schematically shown a conduction band diagram  210  of a second aspect of a semiconductor laser diode in accordance with the embodiment of the invention illustrated in  FIG. 2   a . In this aspect of the invention, a second blocking layer  80  composed of a p-doped material, such as InGaP, is introduced at the interface of high-doped cladding region  66  and low-doped spacer layer  64 . Second blocking layer  80  preferably is composed of a p-doped material having a doping level providing hole concentration greater than 2*10 17 /cm 3 . This second blocking layer provides further suppression of the electron leakage current if some portion of the electrons penetrate blocking layer  70 . 
     Referring now to  FIG. 2   c , there is schematically shown a conduction band diagram  220  of another aspect of a semiconductor laser diode in accordance with the embodiment of the invention illustrated in  FIG. 2   a . In this aspect of the invention, second blocking layer  90  is introduced at the interface of between outer confinement layer  58  and inner confinement layer  56 . Preferably, blocking layer  80  is composed of n-doped material of a composition similar to that of blocking layer  70 . In this aspect of the invention, the inclusion of second blocking layer  90  prevents electron accumulation in waveguide  38 . This accumulation can cause additional optical losses due to free hole absorption in the waveguide layers. 
       FIG. 2   d  schematically shows a conduction band diagram  230  of a fourth aspect of the present invention. In this aspect of the present invention, each of the previously described second blocking layers  80  and  90 , respectively, are included within a fabricated semiconductor laser. 
     It should be understood that blocking layers  80  and  90  can be included individually or in combination to achieve a desired limitation of the leakage current outside waveguide region  38 . Thus, although the inclusion of illustrated blocking layers to control the leakage current has been progressively shown in  FIGS. 2   a – 2   d , it is understood that the incorporation of one blocking layer is independent of other blocking layers. Hence, other combinations of the illustrated blocking layers, although not illustrated, are contemplated as being within the scope of the invention. 
     Referring now to  FIGS. 3   a – 3   c , there are shown conduction band diagrams corresponding to a second embodiment of the invention. In this illustrative example, blocking layers are progressively included within the semiconductor material in accordance with the principles of the invention. Referring to  FIG. 3   a , there is illustrates a conduction band diagram  300 , depicting a blocking layer  100  of p-doped semiconductor material layer, such as InGaP, formed at the junction of p-doped portion  66  and n-doped portion  64  of clad layer  62 . In one aspect of the invention, blocking layer may be tensile strained. The p-doped blocking layer  100 , as previously discussed, creates an electron barrier that inhibits the flow of electrons from waveguide region  38  to p-doped portion of cladding region  66 . The adjacent low n-doped portion  64  of clad layer  62  has an electron concentration less than 5*10 17 /cm 3 . At the interface between the low n-doped layer  64  and the p-doped portion  66  of clad layer  62  the hole concentration increases sharply up to 10 18 /cm 3 . This increase in the p-doping provides a low resistance for the device. 
       FIG. 3   b  illustrates a second aspect of the invention, wherein an n-doped, strained, blocking layer  110  of a material, such as InGaP, is formed at the interface of outer confinement layers  58  and low doped spacer region  64 . In this aspect of the invention, spacer region  64  is an undoped material, such as InP. This additional blocking layer provides further suppression of a electron leakage current. 
       FIG. 3   c  illustrates another aspect of the invention, wherein a second n-doped blocking layer  120  of a material, such as InGaP, is incorporated at the interface between inner confinement layer  56  and outer confinement layer  58 . Blocking layer  120  prevents electron accumulation in waveguide  38 . Accumulation of electrons in waveguide  38  can create additional optical losses due to free hole absorption in the waveguide layers. 
       FIG. 4   a  depicts a cross-sectional view of an exemplary conventional diode radiation source. In this view, the material layers that compose the device are shown. Hence, n-doped clad layer  60  is composed of a InP material. Active region  38  composed of outer confinement layer  58 , inner confinement layer  56  and quantum well  52  is composed on a InGaAsP material. And p-doped clad layer  62  is composed of an InP material. 
       FIG. 4   b  depicts a cross-section view of an exemplary semiconductor light emitting device constructed in accordance with one aspect of the invention. More specifically,  FIG. 4   b  illustrates an opto-electronic device having a conduction band diagram similar to that illustrated  FIG. 2   a . In this illustrative example, blocking layer  70  in interposed at the interface between waveguide layer  38  and p-doped clad layer  62 . As is further illustrated, p-doped clad layer  62  is composed of a high-doped  66  portion and a low-doped portion  64 . An etch-stop layer is included in the fabrication process to isolate the high-doped layer  66  from low-doped layer  64 . In one aspect of the invention, the thickness of low-doped layer  64  may be selected to as: 
     
       
         
           
             d 
             ≥ 
             
               
                 λ 
                 2 
               
               ⁢ 
               ϕ 
             
           
         
       
         
         
           
             where d is said low doped material thickness;
           λ is an operational wavelength; and   φ is a desired beam divergence expressed in radians.   
         
           
         
       
    
       FIG. 5  depicts a graph  500  illustrating the performance of output power with respect to input current for both a conventional semiconductor laser and a semiconductor laser incorporating the features of the present invention. In this example, the semiconductor laser of the present invention is constructed having a conduction band diagram similar to that illustrated in  FIG. 2   a . As is represented by graph  510 , the output power of the conventional laser increases with increased input current up to a known level  512 . Thereafter, the output power remains substantially constant for any increase in current. On the other hand, the output power, as represented by graph  520 , of the laser device incorporating blocking layer technology of the present invention, increases linearly with increasing input current achieving a power of 750 mW. Hence, the radiation source device in accordance with principles of the invention experiences a substantially linear increase in output power with input current and does not experience a saturation output power with increased input current. 
       FIG. 6  graphically illustrates the P-I characteristics, in the insert box, and the output power spectra of a single-mode, single-frequency DFB laser fabricated in accordance with the principles of the present invention. As illustrated, as input current increases, the output power increases, up to 370 mW, while the spectra remains substantially single frequency. Furthermore, the intensity of the side modes remains significantly lower than the main output. This performance is advantageous, particularly in telecommunication systems, as the narrow frequency operation limits interference from one carrier wavelength to another. 
     Although the invention has been described and discussed with regard to semiconductor lasers as a preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form, has been made only by way of example, and that numerous changes in the details may be made without departing from the spirit and scope of the invention as hereinafter claimed. For example, the method of incorporating blocking layers to prevent the leakage of electron current may also be used in other opto-electronic devices, such as gain chips, SLEDs, optical amplifiers, DFB lasers, etc. It is intended that the patent shall cover by suitable expression in the appended claims, those features of patentable novelty that exists in the invention disclosed.