Abstract:
A semiconductor light-emitting device with a new layer structure is disclosed, where the current leaking path is not caused to enhance the current injection efficiency within the active layer. The device provides a mesa structure containing active layer and a p-type lower cladding layer on a p-type substrate and a burying layer doped with iron (Fe) to bury the mesa structure, where the burying layer shows a semi-insulating characteristic. The device also provides an n-type blocking layer arranged so as to cover at least a portion of the p-type buffer lower within the mesa structure. The n-type blocking layer prevents the current leaking from the burying layer to the p-type buffer layer, and the semi-insulating burying layer that covers the rest portion of the mesa structure not covered by the n-type blocking layer prevents the current leaking from the n-type blocking layer to the n-type cladding layer within the mesa structure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for forming a semiconductor light-emitting device, in particular, the method relates to a method to form a semiconductor laser diode with a mesa structure buried with a semi-insulating semiconductor layer doped with iron (Fe). 
     2. Related Prior Art 
     One type of a semiconductor laser diode (hereafter denoted as LD) has been well known where a mesa structure including an active layer formed on an InP substrate is buried by a burying layer. A Japanese Patent Application published as JP-H06-085390A has disclosed an LD with such an arrangement where the LD includes the mesa structure containing an n-type lower cladding layer, an active layer, a p-type upper cladding layer and a p-type contact layer, each sequentially grown on the n-type InP substrate, and a semi-insulating burying layer made of InP doped with iron (Fe) formed so as to bury the mesa structure. 
     Because irons in the burying layer behaves as an electron trap to show the semi-insulating characteristic, when it is in contact with a p-type layer, electrons trapped by iron atoms in the burying layer may recombine with holes injected from the p-type contact layer, which causes a current leaking path from the contact layer to the burying layer; accordingly, the efficiency to inject carriers within the active layer is reduced. 
     One solution to reduce such a leak current has been proposed, where an additional current blocking layer made of n-type InP is formed on the semi-insulating burying layer to electrically isolate the p-type contact layer from the semi-insulating burying layer. However, the path from the p-type cladding layer to the semi-insulating burying layer still exists and the current injection efficiency has a scope to be further enhanced. 
     SUMMARY OF THE INVENTION 
     A semiconductor light-emitting device according to the present invention has a feature that provides a p-type InP substrate; a mesa structure including a p-type buffer layer, an active layer, and an n-type cladding layer; an n-type blocking layer; and a semi-insulating burying layer. The n-type blocking layer covers the p-type InP substrate and at least the p-type buffer layer within the mesa structure to isolate the semi-insulating burying layer from the substrate and the p-type buffer layer. 
     The invention further provides a feature in a method to form the semiconductor light-emitting device, comprising steps of: (a) growing semiconductor layers including the p-type buffer layer, the active layer and the n-type cladding layer on the p-type semiconductor substrate in this order; (b) forming the mesa structure; (c) growing the n-type blocking layer on the p-type substrate, this blocking layer including a plane portion deposited on the p-type substrate and a wall portion deposited on both side surfaces of the mesa structure; (d) selectively etching the wall portion of the n-type blocking layer; and (e) growing the semi-insulating burying layer doped with iron so as to bury the mesa structure. 
     The light-emitting device of the invention may further provide an un-doped layer between the active layer and the p-type buffer layer. This un-doped layer may relax the condition required in the n-type blocking layer, in particular, a thickness of the layer. The n-type blocking layer may cover at least a portion of the un-doped layer within the mesa structure to isolate the p-type buffer layer from the semi-insulating burying layer. 
     According to the method of the invention, the n-type blocking layer may isolate the burying layer from the p-type substrate so as to prevent the inter-diffusion of dopants in the p-type substrate and in the burying layer, which may reduce the leak current. Because the selective etching forms the n-type blocking layer, the dimensions of the n-type blocking layer becomes optional; accordingly, the n-type cladding layer in the mesa structure may be reliably isolated from the n-type blocking layer. 
     Moreover, the process may further include the etching of the p-type substrate after the formation of the mesa structure. This additional etching may expose the surface of the p-type substrate with the (111) and its equivalent orientations. Because of the crystallographic characteristic of the semiconductor material, the surface with the (100) or its equivalent orientations is hard to be etched compared to surfaces with the (011) or (111) and their equivalent orientations, which enhances the selectiveness of the etching of the n-type blocking layer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A to 1C  illustrate processes to grow semiconductor layers on the p-type semiconductor substrate and to form the mesa structure including grown layers; 
         FIG. 2A  illustrates a process to etch the p-type substrate to appear the surface with the (111) orientation, and  FIG. 2B  illustrates a process to grow the n-type blocking layer on the p-type substrate and the side of the mesa structure; 
         FIG. 3A  illustrates a process to etch the wall portion of the n-type blocking layer deposited on the side of the mesa structure, and  FIG. 3B  illustrates a process to bury the mesa structure by growing the burying layer; 
         FIG. 4A  illustrates a process to remove the cap layer, and  FIG. 4B  illustrates a process to form the passivation layer on the mesa structure and on the burying layer; 
         FIG. 5A  illustrates a process to form the opening in the passivation layer, and  FIG. 5B  illustrates a process to form the n-type electrode in the opening and the p-type electrode in the back surface of the p-type substrate; and 
         FIG. 6  illustrates a cross section of a modified light-emitting device that includes an un-doped layer between the p-type buffer layer and the active layer. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, preferred embodiments of the present invention will be described as referring to accompanying drawings. 
       FIGS. 1A to 1C  show processes to form the mesa structure M. First, a series of semiconductor layers,  12   a ,  14   a ,  16   a  and  18   a , is grown on the primary surface  10  with the (001) surface orientation of a p-type InP substrate  10  by the conventional Organo-Metallic Vapor Phase Epitaxy (OMVPE) method. 
     The substrate is a p-type InP doped with zinc (Zn) by a concentration of about 1×10 18  cm −3  and with a thickness of about 350 μm. The layer  12   a  becomes the p-type buffer layer  12 , which is a p-type InP doped with Zn by a concentration of about 1×10 18  cm −3  and with a thickness of about 550 nm. 
     The layer  14   a  becomes the active layer  14 . The layer  14   a  may include a plurality of well layers and a plurality of barrier layers alternately stacked with each other, which constitutes the multiple quantum well (MQW) structure. The well layer may be InGaAsP with a band gap wavelength of 1.6 μm and a thickness of one layer of about 5 nm, while the barrier layer may be also InGaAsP but with a composition different from the well layer. The band gap wavelength of the barrier layer is about 1.25 μm and a thickness of one layer is about 10 nm, then a total thickness of the active layer becomes 224 nm. This active layer with the MQW structure may emit light with a wavelength of about 1.55 μm. 
     The layer  16   a  becomes the n-type cladding layer  16 . The layer  16   a  may be an n-type InP doped with silicon (Si) by a concentration of about 1×10 18  cm −3  and with a thickness of about 2000 nm. The layer  18   a  becomes a cap layer  18  in the mesa structure M. The layer  18   a  may be an n-type InGaAs doped with Si by a concentration of about 1×10 19  cm −3  and with a thickness of about 200 nm. 
     Next, as shown in  FIG. 1B , the process forms a mask layer  20  with a striped pattern on the layer  18   a , a position of the mask layer  20  is aligned with the mesa structure M. The mask layer  20  may be made of silicon oxide (SiO 2 ) and with a thickness of about 2 μm. The striped pattern of this mask layer  20  may be formed by the ordinary photolithography with a subsequent etching process. 
     The process etches a portion of the semiconductor layers,  12   a  to  18   a  not covered by the mask layer  20  to form the mesa structure M. The conventional reactive ion etching (RIE) may carry out this etching process by a mixed reactive gas of methane (CH 4 ) and hydrogen (H 2 ). One exemplary composition of the reactive gas may be obtained by the flow rate of respective gasses of 25 sccm, which realizes an etching rate of about 1.8 μm an hour. However, the etching rate of the semiconductor layer strongly depends on various process conditions, such as the electrical power of the RF source, the pressure within the etching apparatus, and so on. The present rate described above enables the deep etching of about 3.6 to 3.7 μm to form, what is called as the high-mesa structure M shown in  FIG. 1C . 
     After carrying out the dry etching using the RIE technique, the process etches a portion of the substrate  10 . Specifically, the process first removes the residual carbons accumulated around the mesa structure M due to methane (CH 4 ) in the etching gas by an etchant containing sulfuric acid for 2 minutes. Subsequently, the process etches a portion of the p-type InP substrate  10  with a mixed solution of hydrochloric acid (HCl) 150 cc, acetic acid (CH 3 COOH) 150 cc, hydrogen peroxide (H 2 O 2 ) 60 cc, and water 150 cc for 30 seconds to remove the surface layer damaged by the foregoing RIE process. This second wet-etching exposes the InP surface  10   b  with the (111) orientation adjacent to the mesa structure M by removing the surface of the substrate  10  by about 100 nm as shown in  FIG. 2A . 
     Next, the n-type blocking layer  26  is grown on the substrate  10 .  FIG. 2B  illustrates the process to grow the n-type blocking layer  26  on the surface  10   b  of the substrate  10  and the side surfaces of the mesa structure M. This n-type blocking layer  26  is an n-type InP doped with silicon (Si) by a concentration of about 1×10 19  cm −3  and having a thickness of 1.2 to 1.3 μm grown by the OMVPE technique. Exemplary growth conditions are that a mixture of tri-methyle-indium (TMIn) and phosphine (PH 3 ) with mono-silane (SiH 4 ) for a dopant as the sources, a growth temperature of 620° C., and a reaction period of 35 to 40 minutes. The growth rate of about 2 μm an hour may be obtained under the conditions above. 
     The n-type blocking layer  26  includes a plane portion  26   a  deposited on the surface  10   b  of the substrate  10  and a wall portion  26   b  deposited on the side surfaces of the mesa structure M. The plane portion  26   a  may isolate the substrate  10  from the burying layer  32  and is preferable to have a thickness thereof smaller than a distance from the bottom  14   b  of the active layer  14  to the top  10   b  of the substrate  10  in the mesa structure, which is equivalent to a thickness of the p-type buffer layer  12 . Specifically, the thickness of the plane portion  26   a  of the n-type blocking layer  26  is preferably 0.2 to 0.3 μm. This is due to the reason that the subsequent etching described below does not cause the current leaking path from the n-type cladding layer  16  to the n-type blocking layer  26 . When the n-type blocking layer  26  has such a thickness, the burying layer  32 , which has a semi-insulating characteristic primarily for electrons, may come in contact with the p-type buffer layer  12 . However, because of its high resistivity of the semi-insulating burying layer  32 , the contact between the p-type buffer layer  12  and the burying layer  32  is not significant. 
     More preferably, the thickness of the plane portion  26   a  of the blocking layer  26  is substantially equal to a thickness of the p-type buffer layer  12  added with the depth of the substrate etched in the foregoing process to level the bottom  14   b  of the active layer  14  with the top  26   c  of the blocking layer  26 . The thickness of the blocking layer  26  may be adjustable by changing the growth time by the OMVPE technique. 
     Next, the process selectively etches the blocking layer  26  as shown in  FIG. 3A . The wet-etching carried out in this step using a solution containing hydrochloric acid (HCl) 60 cc, acetic acid (CH 3 COOH) 300 cc and water 60 cc for 45 seconds selectively removes the wall portion  26   b  of the blocking layer  26 . The etchant above mentioned selectively etches the surface of the n-type InP layer  26  with the (011) and (111) orientations but hardly etches the surface with the (100) and its equivalent orientations. Accordingly, this etchant may selectively remove the wall portion  26   b  deposited on the side surfaces of the mesa structure M because the plane portion  26   a  of the layer  26  reflects the (100) orientation of the substrate  10 , while, the wall portion  26   b  shows the (011) and (111) surface orientations. 
     Next, the mesa structure M is buried with the semi-insulating burying layer  32  as illustrated in  FIG. 3B . This burying layer  32  is an InP doped with iron (Fe) by a concentration of about 1.5×10 18  cm −3  with the OMVPE technique using tri-methyle-indium (TMIn) and phosphine (PH 3 ) as the source materials and ferocene (C 10 H 10 Fe) as the dopant material. Exemplary growth conditions are that the growth temperature of 620° C. and the growth rate of 2 μm an hour, where it is necessary to take one hour and fifteen to twenty minutes to obtain a thickness of 2.5 μm enough to bury the mesa structure M. 
     After the growth of the burying layer  32 , the mask  20  to form and to bury the mesa structure M is removed as shown in  FIG. 4A . This mask  20  may be removed by, for instance, fluoric acid. 
     Next, the process forms the passivation layer  38  made of silicon oxide SiO 2  to cover the top of the mesa structure M and the burying layer  32 , as illustrated in  FIG. 4B . Subsequently, this passivation layer  38  is processed so as to form an opening  38   a  to expose a portion above the mesa structure M ( FIG. 5A ). The upper electrode  42  fills the opening  38   a  of the passivation layer  38  and is put on the layer  38 . This upper electrode  42  corresponds to the n-type electrode made of AuGe eutectic metal. On the other hand, another electrode  44  is processed on the back surface of the substrate  10  ( FIG. 5B ). This electrode  44  corresponds to the p-type electrode and is made of AuZn eutectic metal. Thus, the light-emitting device of the present invention is completed. 
     According to the method of the present invention thus explained, the n-type blocking layer  26  may prevent the inter-diffusion between dopants in the p-type substrate  10  and those in the semi-insulating burying layer  32 . Moreover, because the n-type blocking layer  26  is formed by the OMVPE technique and the subsequent selective etching, the physical shape of the blocking layer  26 , especially the thickness of the plane portion  26   b  may be optionally controlled and the blocking layer  26  may be escaped from being in contact with the n-type cladding layer  16  in the mesa structure M, which effectively prevents the current leaking path from causing. 
     The present invention has various modifications not restricted to those embodiments described above. It would be possible for an ordinal artisan in the fields to vary semiconductor materials of respective layers, their physical dimensions and conditions to process the semiconductor layers depending on requests. 
     For instance, it is possible to put separate confinement hetero-structure (SCH) layers between the MQW active layer  14  and the p-type buffer layer  12  and between the MQW active layer  14  and the n-type cladding layer  16 . These SCH layers may separately confine the carries within the MQW active layer  14  and the light within the MQW active layer  14  and these SCH layers. These SCH layers may have a thickness of about 50 nm and may be made of un-doped GaInAsP when the MQW active layer  14  is made of GaInAsP. These SCH layers, in particular, the layer between the MQW active layer  14  and the p-type buffer layer  12  may relax the condition of the thickness of the n-type blocking layer  26 , that is, the top level of the plane portion  26   b  of the blocking layer  26  may be within the range of the thickness of this SCH layer. 
     Moreover, the mesa structure M may further include an un-doped semiconductor layer  50  between the lower SCH layer above mentioned and the p-type buffer layer  12  as illustrated in  FIG. 6 . This additional layer  50  may be made of un-doped InP and have a thickness of about 100 nm, and may further relax the thickness condition of the n-type blocking layer  26 . The present invention, therefore, is limited only as claimed below and the equivalents thereof.