Patent Publication Number: US-2003235225-A1

Title: Guided self-aligned laser structure with integral current blocking layer

Description:
[0001] This is a continuation-in-part of application Ser. No. . . . filed Jun. 21, 2002. (15399JD) 
    
    
     
       FIELD OF THE INVENTION  
       [0002] This invention relates to high power semiconductor diode laser structures but more particularly to a high power laser structure with a continuous active region and p-n-p blocking layers.  
       BACKGROUND OF THE INVENTION  
       [0003] Numerous semiconductor laser structure designs have been tried.  
       [0004] The two most common designs are the ridge laser and the buried heterostructure. Despite the simplicity of the ridge structure, the control of the lateral optical mode is complex. In the ridge structure design, both the air/semiconductor/air refractive index step in the lateral direction and the stress in the dielectric layers and the metal contacts contribute to the effective shape of the lateral optical mode. The advantages of this design are that only one growth step is required (two steps if a grating is included), the active region is continuous and the processing is simple which usually leads to a high yield process. Generally, the disadvantages are that the threshold current is high because lateral leakage current is important, and the requirement for large contact area for improved power handling capability leads to high beam ellipticity which degrades fiber coupling efficiency in some coupling schemes. The main cause for these drawbacks are that lateral current and optical confinement cannot be separated from one another. In wide ridge structures the mode control for single lateral mode operation is more difficult to obtain than in narrow ridge structures. The reason is that wide ridges require a relatively large distance of the air-semiconductor interface from the core of the waveguide for weak optical guiding making the device more sensitive to parasitic effects like current spreading and manufacturing induced strain.  
       [0005] In the p-n-blocking buried heterostructure, the lateral optical waveguide is fabricated by etching away part of the active layer. Subsequently the structure is overgrown. The advantages of this structure are that the threshold current is low because the lateral leakage current is low, the lateral mode control is good, and the contact area can be made large independently of the waveguide structure giving good heat dissipation. The disadvantages are that the structure requires three growth steps (four steps if a grating is included) that lower the yield and since the active region has been etched, the non-radiative recombination can increase significantly and can degrade the device performance and its reliability. Also, the high doping levels in the p-n current blocking layers are located in areas where the optical intensity is high resulting in increased optical losses, which prevent the use of long cavities usually necessary to achieve high power.  
       [0006] It would then be very attractive if the simplicity and high yield of the ridge structure could be combined with the good waveguiding characteristics and low leakage current of the buried heterostructure.  
       [0007] A need therefore exists for a self-aligned laser structure, which combines the advantages of the ridge structure and buried heterostructure while overcoming their shortcomings.  
       SUMMARY OF THE INVENTION  
       [0008] According to a first aspect of the present invention, there is provided a self-aligned laser structure with an integral active and guiding layer. A continuous active region and a current blocking region forms a lateral waveguide. The blocking region has an index of refraction n 1 . The continuous guiding layer has an index of refraction n 2 , wherein n 2  is greater than n 1 . The guiding layer and the blocking region are made of the same material. The blocking region has a real refractive index step forming a transverse optical mode, wherein the transverse laser mode is controlled by a change in doping level of the blocking region.  
       [0009] Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] The invention and its embodiments thereof will be described in conjunction with the accompanying drawings in which:  
     [0011]FIG. 1 is a diagram illustrating a ridge laser structure according to the prior art;  
     [0012]FIG. 2 is a diagram illustrating a buried heterostructure laser according to the prior art;  
     [0013]FIGS. 3 a  and  3   b  are schematic drawings of prior art Real Index Self-Aligned (RISA) structures which combine the good properties of the structures of FIGS. 1 and 2;  
     [0014]FIGS. 4 a  and  4   b  are structures of high power lasers according to a first and second embodiment of the present invention; and  
     [0015]FIGS. 5 a ,  5   b  and  5   c  are structures of high power lasers according to third and fourth embodiments of the present invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0016] Referring now to FIG. 1, we have shown a schematic diagram illustrating a cross-section of a typical ridge laser structure.  
     [0017] Despite the simplicity of the ridge structure, the control of the lateral optical mode is complex. The ridge laser comprises an n+InP substrate  10 , a layer of n−InP  11 , an active region  12  and a p−InP layer forms the ridge section  13 . A p+InGaAs layer  14  is then used as a contact layer to the ridge section  13 . In this design both the air/semiconductor/air refractive index step and the strain in the dielectrics  15  and metal layers  16  contribute to the effective shape of the lateral optical mode. The advantages of this design are that only one growth step is required (two steps if a grating is included), the active region is continuous and the processing is simple which leads to a high yield device. The disadvantages are that the threshold current is high because the lateral leakage current is high. The contact area is small so that the heat dissipation is poor, and the optical mode control is hard to control, which might result in poor coupling efficiency.  
     [0018] Referring now to FIG. 2, we have shown a cross-section of a Buried Heterostucture (BH) laser. In this design the lateral step of index of refraction that provides the lateral guiding of the optical mode is due to the etched active layer  20 . The advantages of this structure are that the threshold current is low, because the lateral leakage current is low, the mode control is good and the contact area is large giving good heat dissipation. The disadvantages are that the structure requires three growth steps (four steps if a grating is included), which lower the yield and since the active region  20  has been etched, the non-radiative recombination can increase significantly to degrade the device performance and its reliability. Also, the high doping levels in the pn current blocking layers at a location of high optical density tend to increase the optical losses, which prevent the use of long cavities usually necessary to achieve high power.  
     [0019] An attempt was made in combining the advantages from the ridge and the BH while limiting their disadvantages. This is shown in FIG. 3 a . This cross-section represents a high power A1GaAs laser diode. The structure combines a real index guide with a self-aligned p-n-p blocking layer for current confinement. In this design the lateral index of refraction step that provides the lateral optical mode confinement is the difference in refractive index between A1xbGa1xbAs  30  and A1xcGa1-xcAs  31  with xb being larger than xc. Namely the guiding in this structure is provided by the incorporation of the A1xcGa1-xbAs  30  discontinuous layer with lower index of refraction. This layer  30  plays also the role of the current blocking layer.  
     [0020] Despite the improvements of the RISA structure described in FIG. 3 a , this structure still has some drawbacks.. The RISA structure referred in FIG. 3 a  is made of a material system that does not operate well at wavelengths much beyond 980 nm.  
     [0021] Another alternative structure realized in the prior art is illustrated in Fig 3   b . Here material with lower band-gap is chosen for the blocking layers instead of being used for the cladding material. Lasers realized in this way experience lateral index anti-guiding and, depending on the wavelength of the mode, guiding through lateral absorption of the mode in the blocking layers is present. This results in unacceptably high optical losses, which degrade the device performance. Thus, a new structure with a material system capable of high output power operating at longer wavelengths, such as 1480 nm is required. As an example, a material system supporting operation at a longer wavelength like 1480 nm can be based on InP where InGaAsP compounds are used as a material with lower bandgap (i.e. higher index of refraction) and InP or InGaAsP compounds (with lower band-gap energy than InP) can be used where a lower index of refraction is required.  
     [0022] Referring now to FIGS. 4 a  and  4   b , we have shown a first and second embodiment of the real refractive index-guided self-aligned laser structure of the present invention. Here the lateral index of refraction step that provides the lateral optical mode confinement is the difference between InP and InGaAsP (The refractive index of InGaAsP is higher than InP). In FIG. 4 a , is shown a structure in InP where the lateral optical mode confinement is provided by the difference in refractive index between InP and InGaAsP weighted by the overlap of the optical mode with the named areas. In such a structure, InGaAsP is grown in the trench region while no material remains outside the trench. This can be achieved either by etching away the InGaAsP outside the trench or by selective overgrowth. In this case, the etch mask (preferrable a dielectric) could in principal be used for masking the area outside the trench. The trench would be partly or fully filled with the material of the waveguide. This requires no extra process step for masking but requires 3 growth steps instead of 2 for the structure of FIG. 4 b . This is the case since after the growth of the waveguide layer in the trench one need to remove the dielectric and continue with the homogenous growth of the p-cladding. Another difficulty is that quaternary materials are difficult to control in composition especially if the surface is not planar, i.e. if they are grown in trenches. The embodiment of FIG. 4 b  achieves the same purpose as the structure of FIG. 4 a  but is a structure which is easier to manufacture and provides additional benefits. This structure comprises a n or n+InP region  41 , a n−InP layer  43  and a n-doped blocking layer  44 . The blocking layer  44  can consist of InP. A continuous p−InGaAsP guiding layer is shown at  45 . It has an index of refraction greater than the index of refraction of the blocking layer  44 . A P−InP region  46  is used to cover the guiding layer  45 . A p contact  47  is then used to permit connection. In line with the teachings of this invention, a p−InP layer  48  can also be incorporated between the blocking layers  44  and the guiding layer  45 . Similarly a p−InP layer  49  can be placed below the n−InP blocking layer  44  to adjust the lateral guiding strength independently of the lateral current confinement. A special feature and distinct advantage of this type of structure compared to the RISA structure is that the material surrounding the blocking layers and located especially in the core of the waveguide can be made of a material with the highest band-gap possible. In material systems with relatively low energy barriers between the active region and the cladding layers (exemplary but not limited to the system InP/InGaAsP) cladding layers with lower energy barriers lead to an unfavorable degradation in the device performance due to vertical heterobarrier carrier leakage. In RISA structures this cannot be achieved due to their inherent property, which requires that the guiding be provided by the discontinuous blocking layers. In a RISA structure the core of the structure needs to be of a material with a lower band-gap to obtain a contrast in the index of refraction between the high index core of the structure and the lower index obtained by the blocking layers.  
     [0023] This determines the mechanism by which the mode is guided. For the lateral direction the structure in FIG. 4 b  is guiding the mode by use of the guiding layer  45  instead of guiding the mode by use of the blocking layer as realized in FIG. 3 a . This can most easily be seen if one looks at the lateral distribution of the refractive index at the position of the element providing the guiding. In FIG. 4 b , the index is higher in the core of the structure and drops to a background value towards the edge of the structure. In FIG. 3 a , for the RISA structure, the value in the core of the structure is the background value and drops to a lower value for the blocking layer providing the guiding. This means that in FIG. 4 b  the background value is the “low” refractive index while in FIG. 3 a  the background value is the “high” refractive index. The same holds for the realization in FIG. 4 a.    
     [0024] A second feature and advantage of the invention is that the discontinuous p-n blocking layer can be made of a material with highest band-gap embedded into a material of complementary conductivity but also highest band-gap energy. This is favorable since it maximizes the voltage for breakdown of the p-n-blocking region. It is therefore again preferable to build both the discontinuous blocking layer and the surrounding cladding layers from the same high band-gap material.  
     [0025] A third feature and advantage of the invention is that it is made of a material system that is suitable for operation at longer wavelength beyond 980 nm wavelength.  
     [0026] Referring now to FIG. 5 a , we have shown a further embodiment of the present invention illustrating a dopant-induced real refractive index-guided self-aligned laser structure with integral current blocking layer. This structure is similar to the structures of FIGS. 4 a  and  4   b , except that the blocking layer  50  in FIG. 5 a  is highly doped compared with the blocking layer  44  of FIGS. 4 a  and  4   b . Similarly, the InGaAsP “dielectric step layer”  45  of FIG. 4 b  is replaced with a p−InP layer  51 .  
     [0027] The blocking layer  50  can consist of n + -InP having a refractive index lower than the p−InP guide layer. The refractive index step between n + -InP  @˜1×10 19   cm −3  and p−InP @˜1×10 18  Cm −3  is ˜0.05, which is large enough to control a transverse optical mode. A p−InP region  52  is used to cover the active region  53 . The structure also comprises a n InP layer  54  over the n+InP substrate  55 . A p contact  56  is then used to permit connection.  
     [0028] A special feature and distinct advantage of this type of structure is that it requires two epitaxial steps (three steps if a grating is required), one less than the traditional BH. The first growth finishes with the n + -InP  50 , and the second growth starts with p−InP  51  over the slotted wafer surface. In addition, the MQW active region  53  is continuous and has not been etched through as in the BH. The structure is self-aligned thus making manufacture simpler and the transverse mode is easier to control as it only depends on the thickness and doping level in n−InP blocking layer  50 . Further, the regrowth of p−nP over an etched slot is more ready to planarise than InGaAsP. Similarly, it is much easier to control and reproduce the thickness and doping level of InP than it is to control the thickness, composition and doping level of InGaAsP, especially in an etched slot.  
     [0029] This is the first time that a transverse laser mode has been controlled by a change in doping level rather than a change in material composition. It is also the first time that an InP blocking layer has had a dual role where it acts as a blocking layer and as a mode control layer.  
     [0030] In the embodiment of FIG. 5 b , the blocking layer is comprised of a p InP layer  57  covered by a layer of n+InP  58 . The guiding layer  59  is made of n InP over an n substrate  60 .  
     [0031] In FIG. 5 c , the illustrated embodiment is shown on a p-substrate  61 , with a n++ blocking layer  62  and a pInP guiding layer  63 . A n−InP region  64  is used to cover the active region  65  and a n contact  66  is then used to permit connection.  
     [0032] It will be known to those knowledgeable in the art that various combinations of layer materials can be used to achieve the guiding and blocking layers wherein the index of refraction of the guiding layer is greater than the index of refraction of the blocking layer. Similarly, the type of conductivity of the materials used can be inverted and still offer the same characteristics. For example, the guiding and blocking layers can be made of GaN, InP, GaAs, etc.  
     [0033] In addition it is recognized that this invention can also be used for active regions consisting of a double-heterostructure active region as well as for single- (SQW) or multiple quantum well (MQW) active areas. In the preferred embodiment, the structure is realized preferably with a MQW active region.  
     [0034] It is also recognized that this invention can also incorporate one or more elements for wavelength stabilization of the device like DFB gratings.