Abstract:
A method for manufacturing a semiconductor laser element includes forming a semiconductor laminated structure, having an active layer, on a substrate; etching the semiconductor laminated structure to form a mesa; exposing the mesa to an oxygen-containing ambient forming an oxide layer on the mesa; removing a first part of the oxide layer from the mesa at a temperature lower than a critical temperature at which bonds between atoms of the oxide layer become stronger, by etching with a gas; removing the remainder of the oxide layer from the mesa at a temperature higher than the critical temperature by etching with a gas; and forming a burying layer coating the mesa.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing a semiconductor laser element wherein a semiconductor laminated structure having an active layer is etched to form a mesa, and the circumference of the mesa is buried with a burying layer. 
     2. Background Art 
     In a semiconductor laser element, the current pathway must be narrowed to efficiently supply a current to the active layer. Therefore, in many semiconductor lasers, after fabricating a semiconductor laminated structure having an active layer, a mesa is formed using a technique to transfer micro-patterns to an inductive film and an etching technique to limit the region where a current flows, and to narrow the current pathway. At this time, from the standpoint of the protection of the active layer exposed on the side of the mesa, heat dissipation, or the parasitic capacity of the element, a buried structure wherein the circumference of the mesa is coated with semiconductors is formed. 
       FIG. 11  is a sectional view showing a semiconductor laser element wherein the circumference of the mesa in a semiconductor laminated structure having an active layer is buried with n/p/n/p-type semiconductor layers. A mesa of a semiconductor laminated structure wherein a p-type InP clad layer  12 , an AlGaInAs lower optical confinement layer  13 , an AlGaInAs-MQW active layer  14 , an n-type AlGaInAs upper optical confinement layer  15 , and an n-type InP clad layer  16  are sequentially grown on a p-type InP substrate  11 , is formed. The circumference of the mesa is buried with p-type InP burying layers  17   a  and  17   b , an n-type InP current blocking layer  18 , a p-type InP burying layer  19 , and an n-type InP burying layer  20 . Thereon, an n-type InP contact layer  21 , an n-type InGaAs contact layer  22 , and an n-type InP cap layer  23  are formed. 
     Here, the side of the mesa must be coated with the p-type InP burying layer  17   a . This is because if the n-type InP current blocking layer  18  contacts the mesa, a current flows from the mesa to the burying layers, and the current to the active layer  14  cannot be narrowed. 
     The growth of the p-type InP burying layer  17   a  on the side of the mesa is much influenced by the surface state and the shape of the side of the mesa, which becomes the burying boundary. The portion exposed on the surface may be oxidized depending on the methods or conditions of etching for forming the mesa, or the semiconductor materials composing the mesa. On such an oxide layer, since the p-type InP burying layer  17   a  is difficult to form epitaxially, growing so as to coat such a portion begins after the semiconductor layers below the surface-oxidized semiconductor layer has been completely buried. 
     Therefore, there has been a problem wherein the n-type InP current blocking layer  18  contacts the mesa, and an invalid current pathway  24  wherein a current flows from the mesa to the burying layers is formed as shown in  FIG. 12 . Also in the mesa composed of Al-containing semiconductor materials, Al on the side of the mesa is oxidized when exposed to the atmosphere disturbing burying growth, and defective burying such as pitted growth occurs easily. In addition, since oxygen or impurities on the boundary might cause crystalline defects, there has been a problem wherein they become the cause of element deterioration. For these reasons, it is important for the characteristics and reliability of semiconductor lasers to reduce the amount of oxygen on the side of the mesa composed of Al-containing semiconductor materials. 
     Conventionally, therefore, the oxidation of Al-containing semiconductor layers has been prevented by placing a wafer in the state having a protective layer on the Al-containing semiconductor layer in a burying growth furnace, and, after surface cleaning is performed using HCl gas at a low temperature of not higher than 450° C. to remove oxygen on the surface of the protective layer, elevating the temperature to 450° C. or higher to perform mesa formation and burying growth using HCl gas etching (e.g., refer to Japanese Patent Laid-Open No. 7-263355). However, if the Al-containing semiconductor layer is exposed to the atmosphere and oxidized, it is difficult to remove oxygen only by HCl cleaning at a temperature of 450° C. or lower. 
     SUMMARY OF THE INVENTION 
     To solve the above-described problems, it is an object of the present invention to provide a method for manufacturing a semiconductor laser element that can prevent the formation of an invalid current pathway wherein a current flows from the mesa to burying layers. 
     According to one aspect of the present invention, a method for manufacturing a semiconductor laser element according to the present invention has a step for forming a semiconductor laminated structure having an active layer on a substrate; a step for etching the semiconductor laminated structure to form a mesa; a first cleaning step for cleaning the side of the mesa using a gas having an etching effect at a temperature lower than the critical temperature at which the oxide layer on the side of the mesa is firmed; a second cleaning step for cleaning the side of the mesa using a gas having an etching effect at a temperature higher than the critical temperature after the first cleaning step; and a step for forming a burying layer so as to coat the side of the mesa after cleaning the side of the mesa. 
     According to the present invention, the formation of an invalid current pathway wherein current flows from the mesa to the burying layer can be prevented. Also, since the content of impurities in the boundary between the mesa and the burying layer can be lowered, the improvement of characteristics and reliability can be expected. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-5  are sectional views for explaining a method for manufacturing a semiconductor laser element according to First Embodiment of the present invention. 
         FIG. 6  is a diagram schematically showing the growing temperature and the transition of time for the growth of each layer in burying and growing in the first embodiment. 
         FIG. 7  is a graph showing the results of an XPS analysis that shows change in the bonding state of O and Al when AlInAs was heated to each temperature. 
         FIGS. 8-10  are sectional views for explaining a method for manufacturing a semiconductor laser element according to Second Embodiment of the present invention. 
         FIG. 11  is a sectional view showing a semiconductor laser element wherein the circumference of the mesa in a semiconductor laminated structure having an active layer is buried with n/p/n/p-type semiconductor layers. 
         FIG. 12  is a sectional view showing a semiconductor laser element wherein an invalid current pathway wherein a current flows from the mesa to the burying layers is formed. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     The method for manufacturing a semiconductor laser element according to the first embodiment of the present invention will be described below referring to the drawings. 
     First, as  FIG. 1  shows, a p-type InP clad layer  12 , an AlGaInAs lower optical confinement layer  13 , an AlGaInAs-MQW active layer  14 , an n-type AlGaInAs upper optical confinement layer  15 , and an n-type InP clad layer  16  are sequentially grown on a p-type InP substrate  11  using metal organic vapor phase epitaxy (MOVPE) to form a semiconductor laminated structure having an active layer composed of an Al-containing semiconductor material, Al x Ga y In 1-x-y As (0&lt;x&lt;1, 0&lt;y&lt;1). 
     Next, the wafer is taken out from the MOVPE apparatus, an SiO 2  film is formed on the wafer to fabricate an SiO 2  mask  25  using photo engraving and transfer processes. Then, wet etching is performed using the SiO 2  mask  25  as a mask to form a mesa of the semiconductor laminated structure as shown in  FIG. 3 . 
     Next, the wafer on which the mesa has been formed is placed in the MOVPE apparatus again, and the mesa is buried. However, the side of the mesa is oxidized during wet etching and wafer transporting. Therefore HCl gas is introduced into the apparatus before burying growth to clean the surface. Specifically, after cleaning the side of the mesa using HCl gas at a first surface cleaning temperature of Tcl_ 1  (first cleaning step), the side of the mesa is cleaned using HCl gas at a second surface cleaning temperature of Tcl_ 2  (second cleaning step). 
     Thereafter, as  FIG. 4  shows, a p-type InP first burying layer  17   a  (burying layer) is formed at a growing temperature Tg_p 1  so as to coat the side of the mesa, and a p-type InP second burying layer  17   b , an n-type InP current blocking layer  18 , a p-type InP burying layer  19 , and an n-type InP burying layer  20  (burying layers) are formed at a growing temperature Tg_p 2  on the p-type InP first burying layer  17   a  to bury the circumference of the mesa. 
     Next, as  FIG. 5  shows, the wafer is taken out from the MOVPE apparatus, and the mask  25  is etched off. Thereafter the wafer is placed again in the MOVPE apparatus, and an n-type InP contact layer  21 , an n-type InGaAs contact layer  22 , and an n-type InP cap layer  23  are formed. By the above-described manufacturing process, a semiconductor laser element having an n/p/n/p buried structure is manufactured. 
     Here,  FIG. 6  is a diagram schematically showing the growing temperature and the transition of time for the growth of each layer in burying and growing in the first embodiment. As  FIG. 6  shows, the first surface cleaning temperature Tcl_p 1  is lower than Tc, and the second surface cleaning temperature Tcl_p 2  is higher than Tc. Tc is the critical temperature at which the oxide layer on the side of the mesa is firmed. It is considered that the linkage with O is broken by cleaning at a temperature lower than Tc, and the vapor pressures of remaining Cl compounds or O compounds by thereafter cleaning at a temperature higher than Tc are elevated to enhance the effect for removing the oxide layer. 
       FIG. 7  is a graph showing the results of an XPS analysis that shows change in the bonding state of O and Al when AlInAs was heated to each temperature. O is bonded to Al to create firmer bonding as a hydroxyl group at a temperature lower than 450° C., and as O alone at a temperature higher than 560° C. From these results, it is known that the above-described Tc is about 450° C. Therefore, the present invention is characterized in that the cleaning of the surface of an oxidized Al-containing semiconductor is performed at both a temperature lower than 450° C. and a temperature higher than 450° C. 
     Since the orientation of the crystal face exposed on the side of the mesa is not the ( 100 ) face, and the atomic arrangement on the outermost surface and the strength of bonding with oxygen is different from those of the ( 100 ) face, it is considered that the oxygen removing effect is different compared with the surface cleaning of the ( 100 ) face. Whereas, the oxide on the side of the mesa can be effectively decreased by the above-described cleaning, and the side of the mesa can be coated with the p-type InP first burying layer  17   a . Thereby, since the n-type InP current blocking layer  18  can be grown without contacting the mesa, the formation of the invalid current pathway wherein a current flows from the mesa to the burying layer can be prevented. Also, since the content of impurities in the boundary between the mesa and the burying layer can be lowered, the improvement of characteristics and reliability can be expected. 
     In the first embodiment, although surface cleaning at Tc or higher is once performed after surface cleaning at Tc or lower, surface cleaning can be performed at three or more different temperatures. When the temperature is changed, although the supply of HCl gas is interrupted, the temperature can be changed without interrupting the supply of the gas. Also in the first embodiment, although the p-type InP burying layer is divided into a p-type InP burying layer  17   a  and a p-type InP burying layer  17   b  grown at a higher temperature, the growing temperature can be constant. 
     The present invention is not limited to the n/p/n/p-type burying growth, but can be applied to any burying growth. The present invention can also be applied to the burying growth of a mesa in a semiconductor laminated structure composed of any semiconductor materials, such as InP, AlGaInAs, InGaAs, InGaAsP, AlInAs, AlGaAs, GaAs, AlGaInP, InGaP, AlGaN, GaN, and InGaN. However, if the material differs, since the temperature Tc at which bonding with O is firmed is different and the Cl compounds or O compounds remaining on the surface are different, optimum cleaning temperature must be selected depending on the semiconductor material composing the element. 
     Although HCl gas is used in the first embodiment, other gases having the etching effect, such as TBCl and CCl 4 , can be used in the cleaning before burying growth. The present invention can be applied not only to the fabrication of a semiconductor laser, but also to the fabrication of any semiconductor elements, such as a modulator and a light receiving element. 
     Second Embodiment 
     A method for manufacturing a semiconductor laser element according to the second embodiment of the present invention will be described referring to the drawings. 
     First, as in the first embodiment, and as  FIG. 1  shows, a semiconductor laminated structure composed of InP, AlGaInAs, and AlInAs is formed on a p-type InP substrate  11 ; and as  FIG. 2  shows, an SiO 2  mask  25  is fabricated. Dry etching is performed using the SiO 2  mask  25  as a mask to form a mesa of a semiconductor laminated structure as  FIG. 8  shows. Specifically, since the mesa is formed by dry etching, the orientation of the crystal face of the exposed mesa is different from that of the first embodiment. The shape of the mesa in  FIG. 8  has a vertically sheer drop against the substrate; however, the following is the same when the cross-section of the mesa is trapezoidal. 
     Next, the wafer on which the mesa has been formed is placed in the MOVPE apparatus again, and the mesa is buried. However, the side of the mesa is oxidized during wet etching and wafer transporting. Therefore, as in the first embodiment, HCl gas is introduced into the apparatus before burying growth to clean the surface. Specifically, after cleaning the side of the mesa using HCl gas at a first surface cleaning temperature of Tcl_ 1  (first cleaning step), the side of the mesa is cleaned using HCl gas at a second surface cleaning temperature of Tcl_ 2  (second cleaning step). 
     Thereafter, as  FIG. 9  shows, a p-type InP first burying layer  17   a  (burying layer) is formed at a growing temperature Tg_p 1  so as to coat the side of the mesa, and a p-type InP second burying layer  17   b , an n-type InP current blocking layer  18 , a p-type InP burying layer  19 , and an n-type InP burying layer  20  (burying layers) are formed at a growing temperature Tg_p 2  on the p-type InP first burying layer  17   a  to bury the circumference of the mesa. 
     Next, as  FIG. 10  shows, the wafer is taken out from the MOVPE apparatus, and the mask  25  is etched off. Thereafter, the wafer is placed again in the MOVPE apparatus, and an n-type InP contact layer  21 , an n-type InGaAs contact layer  22 , and an n-type InP cap layer  23  are formed. By the above-described manufacturing process, a semiconductor laser element having an n/p/n/p buried structure is manufactured. 
     Even for the mesa having a different orientation of the crystal face exposed to the side by forming using dry etching, by cleaning in the same manner as in the first embodiment, the same effect as in the first embodiment can be obtained. The scope of the application of the second embodiment is the same as in the first embodiment. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention maybe practiced otherwise than as specifically described. 
     The entire disclosure of a Japanese Patent Application No. 2006-230856, filed on Aug. 28, 2006 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.