Abstract:
The present invention provides a distributed feedback laser diode (DFB-LD), in which the active region may be easily flattened. The active region of the invention is optically coupled with the feedback grating made of the n-type InP layer and the n-type AlGaInAs layer. Since both layers show the n-type conduction, the n-type impurities, which are typically silicon (Si), introduced from the ambient or tools may not increase the resistivity of the layers. Moreover, the difference in the refractive index between AlGaInAs and InP is greater than that between InGaAsP and InP. Accordingly, even when the magnitude of the undulation formed in the interface between AlGaInAs and InP is small, the coupling coefficient between the grating the active layer, which is equal to the product of the magnitude of the undulation H and the difference in the refractive index An, may be prevented from the extreme decrease.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention related to a light-emitting device made of III-V compound semiconductor materials.  
         [0003]     2. Related Prior Arts  
         [0004]     A rapid increase of the mass to be transmitted by the optical communication requests a light-emitting device capable of modulating in high frequencies with a low price. A distributed feedback laser diode (DFB-LD) in a 1.3 μm wavelength band is one of solution to meet such requests. The DFB-LD is able to modulate in direct and to operate without any temperature-control means. Without temperature-controlling means, such as a Peltier device, the DFB-LD is required to show a good performance especially at high temperatures. The DFB-LD with an active layer made of AlGaInAs may show good characteristics at high temperatures.  
         [0005]     The DFB-LD has a grating typically formed by burying the periodic undulation formed on the surface of the ground layer with another semiconductor layer having a composition different to that of the ground layer. This feedback grading is disposed above or below the active layer that emit light.  
         [0006]     Nakahara et al. has disclosed, in Journal of lightwave technology 22(1), (2,004) pp. 159 to 165, a DFB-LD with a ridge waveguide structure, a feedback grating formed by the p-type InP and the p-type InGaAsP and an active region including a AlGaInAs multiple quantum well (MQW) coupled with the feedback grating. Kobayashi et. al has disclosed, in the proceeding of 15th Indium Phosphide and related materials, (2003) pp. 239 to 242, a DFB-LD with a buried hetero-structure that provides a feedback grating made of n-type InGaAsP and InP, and an active region including AlGaInAs with the MQW structure..  
         [0007]     When the feedback grating is formed, the surface thereof is exposed to the ambient, which may cause a contamination of the surface by impurities such as silicon (Si) derived from the process tools. Since Si behaves as an n-type dopant in the III-V compound semiconductor such as InP, when p-type layers constitute the feedback grating, the Si atoms accumulated on the exposed surface reduce the hole concentration thereby increasing the resistivity of the layers. To increase the intrinsic resistance of the DFB-LD results in not only the increase of the driving voltage of the LD but also the increase of the operating temperature due to the large heat generated at the region Si impurities are accumulated, which degrades the performance of the DFB-LD. Nakahara has reported that the resistance at the hetero interface of the grating may be reduced by adjusting the composition of InGaAsP layer and the p-type doping concentration of InGaAsP and InP layers, both comprising the feedback grating. However, Nakahara has not mentioned nor suggested to reduce the influence of the impurities accumulated in the interface.  
         [0008]     The band discontinuity may be formed at the interface between layers each constituting the feedback grating and having the composition different to each other. This band discontinuity causes the increase of the resistivity in a direction intersecting the interface. Generally in the III-V compound semiconductor material, the electron mobility is greater than the hole mobility, accordingly, when the electron is the majority carrier, the increase of the resistivity due to not only the hetero-interface between two materials may be suppressed but also the accumulation of the n-type impurities therein.  
         [0009]     Kobayashi&#39;s DFB-LD includes feedback grating made of the n-type InGaAsP and the n-type InP, and the active layer disposed above this feedback grating with a large undulation in spite of the active layer, in particular the well layers thereof, is required to be flat enough in an atomic layer level. The Kobayashi has reported to obtain such flat semiconductor layer on the undulated feedback grating by adjusting the growth condition of the n-type InGaAsP and the n-type InP each constituting the grating.  
         [0010]     Therefore, the present invention is to provide a laser diode that includes a feedback grating capable of obtaining an active layer formed thereon flat enough compared as a combination of the n-type InGaAsP and then n-type InP.  
       SUMMARY OF THE INVENTION  
       [0011]     According to one aspect of the present invention, a light-emitting device is to be provided. The light-emitting device of the invention comprises an n-type InP region, an n-type AlGaInAs layer disposed on this n-type InP region, and an active region disposed on the n-type AlGaInAs layer. A feedback grating is formed in the interface between the n-type InP region and the n-type AlGaInAs layer, and is optically coupled with the active region.  
         [0012]     Since the preset light-emitting device provides the feedback grating made of n-type semiconductor material, the accumulation of n-type impurities in the interface may not decrease the majority carrier, electrons in this case. Moreover, since the hetero-interface between the n-type AlGaInAs and the n-type InP shows a smaller discontinuity in the conduction band and a larger difference in the refractive index compared with the conventional combination of InGaAsP and InP, the former effect may not disturb the transportation of the majority carrier, which suppresses, accompanied with the combination of the n-type materials, the increase of the resistivity along a direction intersecting the interface, and the latter effect makes the depth of the undulation at the interface smaller, which results in the easy process to bury the undulation to obtain a flattened surface for the active region disposed thereon.  
         [0013]     The n-type AlGaInAs in the present light-emitting device may have a band gap wavelength greater than 1.07 μm, and smaller than 1.2 μm. By setting the band gap wavelength of the n-type AlGaInAs, the discontinuity of the conduction band may be smaller than 0.05 eV and the difference in the refractive index to the n-type InP may be greater than 0.15.  
         [0014]     The present light-emitting device may further provide, in the active region thereof, first and second graded layers, first and second separated confinement hetero-structure (SCH) layers, and a quantum well region. Two SCH layers put the quantum well region therebetween, and two graded layers put the two SCH layers with the quantum well region therebetween. Moreover, the quantum well region may include a plurality of well layers and a plurality of barrier layers. These layers, the well layers, the barrier layers, two SCH layers, and two graded layers, may be made of AlGaInAs with compositions different to other layers. The well layers have the band gap wavelength about 1.4 μm. Thus, a distributed feedback (DFB) laser capable of emitting the single-mode light at comparably high temperatures can be obtained, in which the primary semiconductor material is AlGaInAs and the emission wavelength is in the 1.3 μm band. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  is a perspective view with partially exposing a section;  
         [0016]      FIG. 2  shows a configuration of the active region;  
         [0017]      FIG. 3  shows a band discontinuity of the conduction band between InGaAsP and InP, and that between AlGaInAs and InP against the band gap wavelength of InGaAsP and AlGaInAs, respectively;  
         [0018]      FIG. 4  is shows a difference in the refractive index between InGaAsP and InP, and between AlGaInAs and InP at the wavelength of 1.3 μm;  
         [0019]      FIGS. 5A  to  5 C show the process for manufacturing the DFB-LD of the present invention; and  
         [0020]      FIGS. 6A and 6B  shows the process, subsequent to the step shown in  FIG. 5C , for manufacturing the DFB-LD of the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]     Next, preferred embodiments of the present invention will be described as referring to accompanying drawings. In the specification and drawings, same symbols or numerals will refer the same elements without overlapping explanations.  
         [0022]      FIG. 1  is a perspective view with partially exposing a cross section of a semiconductor laser according to the present invention. The semiconductor laser (hereinafter denoted as LD)  1  includes a p-type layer  3 , an n-type InP region  5 , an n-type AlGaInAs layer  7 , and an active region  9  put between the n-type InP region  5  and the p-type layer  3 . The active region optically couples with a distributed feedback grating  11  comprised of the n-type InP region  5  and n-type AlGaInAs layer  7 .  
         [0023]     The feedback grating  11  may be formed by burying an undulation formed on the surface  5   a  of the n-type InP region  5  with the n-type AlGaInAs layer  7 , and causes the optical diffraction by a difference in the refractive index between two regions,  5  and  7 , and the magnitude of the undulation. The strength of the optical diffraction due to the coupling of the light generated in the active region  9  with the feedback grating  11  is expressed by the coupling coefficient, which considerably affects various characteristics of the LD with the feedback grating, which is called as the distributed feedback laser diode, DFB-LD, such as the threshold current, the emission efficiency, and the stability of the single mode operation thereof. The coupling coefficient may be nearly determined by the difference in the refractive index of two materials comprising the feedback grating and the magnitude of the undulation.  
         [0024]     When the band gap wavelength of the n-type AlGaInAs layer  7  is 1.1 μm, a difference in the refractive index An between the n-type InP region  5 , n  (InP) , and that of the n-type AlGaIAs layer  7 , n (AlGaInAs) , becomes Δn =0.19, which is greater than that of the conventional combination of the InGaAsP and InP materials. The band gap wavelength denotes a wavelength corresponding to the fundamental absorption edge of a semiconductor material, that is, when the band gap energy of 1 eV corresponds to the band gap wavelength of 1.24 μm.  
         [0025]     A large difference in the refractive index between the n-type AlGaInAs layer  7  and the n-type InP region  5  may result in the large coupling coefficient, which shows the lower threshold current and the higher stability in the single mode operation of the DFB-LD. Moreover, since the difference in the refractive index becomes greater, the undulation formed on the n-type InP region  5  and buried by the n-type AlGaInAs layer  7  may be made smaller, to obtain the same coupling coefficient, compared as the conventional grating buried by the InGaAsP layer. The smaller undulation makes it possible to obtain a flat surface of the n-type AlGaInAs layer  7 . To make the surface of the n-type AlGaInAs layer  7 , which is beneath the active region  9 , flat enough by an atomic level, is a key point t obtain the active layer with high quality.  
         [0026]     On the other hand, the electrical resistance in a direction intersecting the feedback grating  11  does not increase because two layers,  5  and  7 , are made of n-type materials. That is, n-type impurities introduced at the process for the feedback grating does not reduce the majority carrier, electrons in this case, of the n-type AlGaInAs layer  7  and the n-type InP region  5 .  
         [0027]     The n-type AlGaInAs region  7  disposes the active region thereon, and the active region disposes the p-type layer  3  thereon. The LD  1  has the so-called ridge waveguide structure. The p-type layer  3  includes a planar portion  3   a  that covers the whole surface of the active region  9  and a stripe portion  3   b  disposed on the planar portion  3   a . On the stripe portion  3   b  is formed with a contact layer  13  with an anode electrode  15  thereof. While, the n-type InP region  5  forms a cathode electrode  17  on a back surface  5   b  thereof.  
         [0028]      FIG. 2  is a cross section of the active region  9 . The active region  9  of the present embodiment includes a plurality of well layers  21  and a plurality of barrier layers  23 , each barrier layer  23  being put between well layers  21 . The well and barrier layers,  21  and  23 , are made of AlGaInAs, but compositions of each element are different in both layers. In the present embodiment, the band gap wavelength of the well layer  21  is 1.4 μm, while that of the barrier layer  23  is 1.05 μm. Here, the band gap wavelength is one index denoting the composition of the semiconductor material. The well layer  21  suffers a compressive strain of 1 %. These well and barrier layer,  21  and  23 , are put between two SCH (Separated Confinement Hetero-structure) layers,  25  and  27 . These two SCH layers,  25  and  27 , are also made of AlGaInAs with the band gap wavelength of 1.05 μm.  
         [0029]     The p-type layer  3  is, similar to the n-type InP layer  5 , made of InP. The first SCH layer  25  and the n-AlGaInAs layer  7  put a graded layer  29 , whose composition continuously varies from the first SCH layer  25  to the n-type AlGaInAs layer  7 . The second SCH layer  27  accompanies a second graded layer  31  in the outer side thereof. The composition of the second graded layer  31  continuously varies from the second SCH layer  27  to a semiconductor layer  33  explained below. Two graded layers,  29  and  31 , each removes the band discontinuity between the layers sandwiching the graded layers,  29  and  31 , to reduce the resistivity along the direction intersecting the active region  9 . The p-type InP layer  3  and the second graded layer  31  put the layer  33  made of AlInAs, which prevents electrons from overflowing into the p-type InP layer  3 .  
         [0030]     An exemplary arrangement of each layer is as following table:  
                                     TABLE                       Typical arrangement of the DFB laser according to the       present invention                                    Anode 15   Ti/Pt/Au                   contact layer 13   InGaAs   0.4   μm           p-type layer 3   p-InP   2   μm           active region 9           AlInAs layer   AlInAs   20   nm           second grader layer 31   AlGaInAs   80   nm           second SCH layer 27   AlGaInAs   80   nm           well layer 21   10 layers of AlGaInAs   5   nm           barrier layer 23    9 layers of AlGaInAs   8   nm           first SCH layer 25   AlGaInAs   40   nm           first grade layer 29   AlGaInAs   40   nm           n-type AlGaInAs layer 7   n-AlGaInAs   30   nm           n-type region 5   n-InP substrate   100   pm           Cathode 17   AuGe/Ni/Au                      
 
         [0031]      FIG. 3  shows the discontinuity in the conduction band between InGaAsP and InP (line C 1 ), and that between AlGaInAs and InP (line C 2 ), where the horizontal axis corresponds to the band gap wavelength (μm) of InGaAsP and AlGaInAs, respectively. For the n-type layer, which buries the feedback grating  11 , it is preferable to have the band gap wavelength from 1.0 to 1.2 μm in the emission wavelength of 1.3 μm band. As shown in  FIG. 3 , the combination of InGaAsP and InP appears the band discontinuity in the conduction band below-0.05 eV, which means that the bottom of the conduction band of InGaAsP is lower than that of InP, in a whole range of the band gap wavelength from 1.0 to 1.2 μm. That is, the band discontinuity is greater than 0.05 eV in absolute in the combination of InGaAsP and InP.  
         [0032]     While, for the combination of AlGaInAs and InP and in the band gap wavelength from 1.07 μm to 1.2 μm, the band discontinuity in the conduction band is less than 0. 05 eV in absolute, which means that the discontinuity in the conduction band may become smaller in the combination of AlGaInAs and InP compared to the combination of InGaAsP and InP. The small band discontinuity results in the low resistivity between layers, thereby reducing the threshold voltage and the heat generation of the LD due to the reduction of the resistance. The band gap wavelength of AlGaInAs is further preferable in a range from 1.1 μm to 1.2 μm to gain the band discontinuity less than 0.03 eV in absolute.  
         [0033]      FIG. 4  shows the difference in the refractive index between InGaAsP and InP (C InGaASP ) and that between AlGaInAs and InP (C AlGaInAs ) in the wavelength band of 1.3 μm. The horizontal axis corresponds to the band gap wavelength of InGaAsP or AlGaInAs, while the vertical axis denotes the difference of the refractive index. Thus, the difference in the refractive index is always greater for the combination of AlGaInAs and InP than that for the combination of InGaAsP and InP in the range of the band gap wavelength from 1.0 to 1.2 μm.  
         [0034]     Next, the process for manufacturing the DFB-LD of the present invention will be described.  FIG. 5A  shows a process to form the n-type InP region. First, the n-type INP layer  43  is grown on the substrate  41  by the Organo-Metallic Vapor Phase Epitaxy (OMVPE) technique.  FIG. 5B  illustrates a process for forming the feedback grating. A mask  45 , which may be apatterned silicon dioxide (SiO 2 ), placed on the InP layer  43  to form the undulation for the feedback grating. To etch the InP layer  43  by using the mask  45  lefts the undulation with a depth of about 15 nm. The mask is removed after this etching.  
         [0035]     The n-type AlGaInAs layer  47  is grown on the patterned InP layer  43   a  as shown in  FIG. 5C . It is preferable for a thickness of the n-type AlGaInAs layer to be around 30 nm or more to bury the undulation with a 15 nm depth in the InP layer 43. The surface of the InP layer  43  or the patterned InP layer  43   a  is exposed until the n-type AlGaInAs is grown thereon, during which an n-type impurities such as silicon (Si) may be transported onto the exposed surface of the InP layer  43  from the ambient or the tools made of silica glass. The Si behaves, within the InP layer, as an n-type dopant. However, the InP layer  43  is the n-type layer. Therefore, even the n-type dopant is transported into the n-type semiconductor layer, the conductivity of the n-type layer does not reduce.  
         [0036]     Subsequently, the active region  49  is grown in the n-type AlGaInAs layer  47 . The active region may be configured as shown in  FIG. 2 . On the active region  49  is sequentially grown with the p-type layer  51  and the p-type contact layer  53 . The semiconductor regions put the active region  49  therebetween may operate as cladding layers. Thus, the epitaxial growth has completed to obtain the substrate  55  with a series of semiconductor layers grown thereon.  
         [0037]     Next,  FIG. 6A  shows the process for forming the ridge waveguide  59 . First, a mask  57 , may be made of patterned silicon dioxide (SiO 2 ), is formed on the p-type contact layer  53 . The p-type contact layer  53  and the p-type layer  51  are etched by using this mask  57  to form the p-type layer  51   a  and the contact layer  53   a . The p-type layer  51  includes the planar portion  51   b  and the ridge portion  51   c . The planar portion  51   b  is left at the etching so as to protect the whole surface of the active region  49  from the etching damage or the contamination due to the etchant. For example, the planar portion  51   b  is left with 0.15 μm thickness thereof for the original thickness of 2.0 μm in the layer. The ridge portion  51   c  and the contact layer  53   a  constitute a ridge stripe  59 . After the formation of this ridge stripe  59 , the etching mask  57  is removed.  
         [0038]      FIG. 6B  illustrates a process for forming electrodes. On the contact layer  53   a  is formed with the p-type electrode  61  made of, for example, titanium (Ti), platinum (Pt), and gold (Au) While, the n-type electrode  63  is formed on the back surface of the substrate  41  after grinding the substrate  41  to the thickness thereof about 100 μm. The n-type electrode is made of, for example, multi-layered metals of AuGe eutectic metal, nickel (Ni), and gold (Au). By cleaving the LD thus completed and forming an anti-reflecting film with the reflectivity below 1 % on one cleaved edge surface, while a high-reflecting film with the reflectivity greater than 80 %, the DFB-LD is completed. This LD, including AlGaInAs in the well and barrier layer in the active region, and the combination of AlGaInAs and InP in the feedback grating, shows a superior performance especially in high temperatures. The DFB-LD of the present invention can emit light in a single mode even at 100° C.  
         [0039]     The embodiment thus described concentrates on the ridge waveguide structure. However, the feedback grating of the present invention may be applied to an LD with the buried hetero-structure. Moreover, the feedback grating may be the λ/4 shift type. In this case, anti-reflecting films may be formed on both cleaved edge surfaces.  
         [0040]     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from this invention and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of this invention. Furthermore, it is to be understood that the invention is solely defined by the appended claims.