Patent Publication Number: US-6990134-B2

Title: GaN series surface-emitting laser diode having spacer for effective diffusion of holes between P-type electrode and active layer, and method for manufacturing the same

Description:
This application is a Divisional application of application Ser. No. 10/055,999, filed Jan. 28, 2002 U.S. Pat. No. 6,754,245, which is hereby incorporated by reference and claims priority to Patent Application Number 2001-5065 filed in Rep. of Korea on Feb. 2, 2001, herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a surface-emitting laser diode formed of a GaN series III-V nitride compound and a method for manufacturing the same, and more particularly, toga GaN series surface-emitting laser diode having a spacer for effective diffusion of holes between a p-type electrode and an active layer, and a method for manufacturing the same. 
     2. Description of the Related Art 
     As shown in  FIG. 1 , a general GaN series surface-emitting laser diode includes an active layer  11  of an InGaN multi-quantum well (MQW) structure, a cavity  10  having an n-AlGaN carrier barrier layer  12  under the active layer  11  and a p-AlGaN carrier barrier layer  13  on the active layer  11 , each of which confines carriers to the MQW structure, and distributed Bragg reflectors (DBRs)  20  and  30  which are formed on and underneath the cavity  10 , respectively, with a reflectivity of about 99%. 
     DBRs are classified according to materials used for the DBRs: those formed of semiconductor materials having a similar lattice constant by epitaxial growth, and those formed of dielectric materials. The former has advantages in that current can be injected through semiconductor layers and the resultant material layers have good quality. In this case, suitable semiconductor materials should have bandgap energies greater than a desired oscillation wavelength so as not to cause absorption. A greater difference in refractive index between semiconductor materials for the two DBRs is preferable. For a GaN surface-emitting laser diode, as shown in  FIG. 1 , suitable semiconductor materials for the DBRs  20  and  30  include GaN (for layers indicated by reference numerals  22  and  32 ), AlN (for layers indicated by reference numerals  21  and  31 ), and AlGaN. Here, AlN and AlGaN including 30% or greater Al have too large bandgap energies. For this reason, when current is injected through DBRs formed of the materials, drive voltage becomes high, causing a heat related problem. In particular, AlGaN series materials have a small difference in refractive index, and thus multiple layers, e.g., tens of layer pairs, should be deposited for DBRs to satisfy a high-reflectivity requirement for laser oscillation. Due to narrow width of a high-reflectivity region, there is a difficulty in designing surface-emitting semiconductor laser diodes. In addition, laser oscillation requirements cannot be satisfied by slight deviations in thickness of the cavity  10  or slight changes in composition of the active layer  11 . 
     For these reasons, dielectric materials, instead of semiconductor compounds, have been widely used. In this case, current cannot be directly injected through DBRs, so a separate electrode (not shown) is required around the DBRs. The mobility of electrons is high and a doping concentration in an n-type compound semiconductor layer between an n-type electrode and an active layer can be increased. Meanwhile, the mobility of holes is smaller than that of electrons and it is impossible to increase a doping concentration in a p-type compound semiconductor layer between a p-type electrode and the active layer. Thus, there is a problem in injecting current. In addition, due to an electrode being formed around a laser output window, it is not easy to effectively diffuse holes toward the center of the laser output window and thus it is difficult to provide effective laser oscillation characteristics. 
     SUMMARY OF THE INVENTION 
     To solve the above-described problems, it is a first object of the present invention to provide a GaN series surface-emitting laser diode in which a stable optical mode is ensured by effectively diffusing holes toward the center of a laser output window. 
     It is a second object of the present invention to provide a method for manufacturing a GaN series surface-emitting laser diode. 
     To achieve the first object of the present invention, there is provided a surface-emitting laser diode comprising: an active layer; p-type and n-type material layers on opposite sides of the active layer; a first distributed Bragg reflector (DBR) layer formed on the n-type material layer; an n-type electrode connected to the active layer through the n-type material layer such that voltage is applied to the active layer for lasing; a spacer formed on the p-type material layer with a laser output window in a portion aligned with the first DBR layer, the spacer being thick enough to enable holes to effectively migrate to a center portion of the active layer; a second BDR layer formed on the laser output window; and a p-type electrode connected to the active layer through the p-type material layer such that voltage is applied to the active layer for lasing. It is preferable that the laser output window is formed in a lens-like shape having a predetermined curvature to compensate for a drop in characteristics of a laser beam caused by the spacer. It is preferable that the spacer has a protrusion portion, and the laser output window is formed on the top of the protrusion portion. It is preferable that the p-type electrode is formed to surround the protrusion portion of the spacer. It is preferable that the spacer comprises: a first spacer formed on the p-type material layer; and a second spacer formed on the first spacer on which the laser output window is formed and around which the p-type electrode is formed. It is preferable that the second spacer has a protruded shape on which the laser output window is formed. It is preferable that one of the first and second spacers is a p-type doped substrate or an undoped substrate. 
     To achieve the second object of the present invention, there is provided a method for manufacturing a surface-emitting laser diode, the method comprising the steps of: (a) sequentially forming a p-type material layer for lasing, an active layer, and an n-type material layer for lasing on a substrate; (b) forming a first distributed Bragg reflector (DBR) on the n-type material layer, around which an n-type electrode is formed; (c) forming a laser output window on a bottom surface of the substrate, the laser output window having a shape suitable for compensating for a drop in characteristics of a laser beam caused by the presence of the substrate; (d) forming a p-type electrode on the bottom surface of the substrate to surround the laser output window; and (e) forming a second DBR layer on the laser output window: 
     Preferably, step (b) comprises: forming a conductive layer on the n-type material layer; forming a mask pattern on the conductive layer to expose a portion of the conductive layer in which the first DBR layer is to be formed; removing the portion of the conductive layer which is exposed through the mask pattern, using the mask pattern as an etch mask; forming the first DBR layer on a portion of the n-type material layer from which the conductive layer is removed; and removing the mask pattern. 
     Preferably, in step (c), the laser output window is formed in a convex lens-like shape having a predetermined curvature suitable for compensating for diffraction of the laser beam. Preferably, in processing the mask pattern, the mask pattern is processed into a convex lens-like shape by reflowing, the convex lens-like shape having a predetermined curvature suitable for compensating for diffraction of the laser beam. 
     It is preferable that the substrate is formed of multiple layers including a first substrate and a second substrate on the first substrate. In this case, etching the bottom surface of the substrate on which the processed mask pattern is formed is continued until the second substrate is exposed. 
     It is preferable that the substrate is a p-type doped substrate or an undoped substrate. It is preferable that one of the first and second substrates is a p-type doped substrate or an undoped substrate. It is preferable that the first substrate is formed as a substrate on which a gallium nitride based material is grown and the second substrate is formed as a p-type spacer. 
     The present invention also provides a method for manufacturing a surface-emitting laser diode, the method comprising the steps of: (a) sequentially forming on a substrate an n-type material layer for lasing, an active layer, a p-type material layer for lasing, and a p-type spacer; (b) forming a laser output window in a predetermined area of the p-type spacer; (c) forming a p-type electrode on the p-type spacer to surround the laser output window; (d) forming a first distributed Bragg reflector (DBR) layer on the laser output window; (e) removing the substrate; and (f) forming a second DBR layer on a predetermined portion of a bottom surface of the n-type material layer and forming an n-type electrode around the second DBR layer. It is preferable that the substrate is formed of an n-type substrate or a sapphire substrate. It is preferable that the laser output window is formed in a convex lens-like shape having a predetermined curvature suitable for compensating for diffraction of the laser beam. 
     The surface-emitting layer diode according to the present invention comprises a spacer between a p-type electrode and an active layer to effectively cause holes to migrate towards the active layer. In addition, a DBR layer is formed on a portion of the spacer in a shape suitable for compensating for diffraction caused by the spacer and for minimizing the radius of laser mode in the active layer. Thus, with the surface-emitting laser diode according to the present invention, holes as well as electrons can effectively be provided to the center of the active layer, and thus the current threshold for laser emission is reduced. Energy conversion efficiency becomes high, and the laser beam emitted by the surface-emitting laser diode has stable transverse mode characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a sectional view of a conventional GaN series surface-emitting laser diode; 
         FIG. 2  is a sectional view of a first embodiment of a GaN series surface-emitting laser diode according to the present invention; 
         FIG. 3  is a sectional view of a second embodiment of the GaN base surface-emitting laser according to the present invention; 
         FIGS. 4 and 5  are diagrams illustrating calculation of the radius of curvature of a laser output window suitable for compensating for diffraction caused by a spacer of the GaN series surface-emitting laser diode according to the present invention, in which  FIG. 4  shows the geometric relation between the spacer thickness, the radius of curvature of the laser output window, the radius (W) of laser mode at the laser output window, and the radius (W 0 ) of laser mode at an active layer, and  FIG. 5  is a graph of the radius (W) of laser mode at the surface of the laser output window versus the thickness of the spacer; 
         FIGS. 6 through 13  are sectional views illustrating each step of a method for manufacturing a GaN series surface-emitting laser diode according to a first embodiment of the present invention; 
         FIGS. 14 and 15  are sectional views illustrating steps of a method for manufacturing a GaN series surface-emitting laser diode according to a second embodiment of the present invention; 
         FIGS. 16 through 18  are sectional views illustrating steps of a method for manufacturing a GaN series surface-emitting laser diode according to a third embodiment of the present invention; 
         FIGS. 19 and 20  are sectional views illustrating steps of a method for manufacturing a GaN series surface-emitting laser diode according to a fourth embodiment of the present invention; and 
         FIGS. 21 through 23  are sectional views illustrating steps of a method for manufacturing a GaN series surface-emitting laser diode according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A GaN series surface-emitting laser diode having a spacer between a p-type electrode and an active layer for effective hole diffusion and a method for manufacturing the same will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. In the drawings, the thickness of layers and regions are exaggerated for clarity. For convenience of explanation, the surfaces of the active layers are referred to as “first surface” and “second surface”, the first surface contacting a first material layer for lasing and the second surface contacting a second material layer for lasing. 
     Preferred embodiments of the GaN series surface-emitting laser diode according to the present invention now will be described. 
     &lt;Embodiment 1&gt; 
     Referring to  FIG. 2 , a surface-emitting laser diode according to the first embodiment of the present invention includes an active layer  40  in which lasing occurs with application of a voltage, and p-type and n-type material layers m 1  and m 2  formed around the active layer  40  facing each other. The n-type material layer m 2  includes a n-type barrier layer  41  formed underneath the active layer  40  and an n-type compound semiconductor layer  43  formed underneath the n-type barrier layer  41 . The n-type barrier layer  41  is preferably formed of a material layer having a bandgap smaller than that of the n-type compound material layer  43  and larger than that of the active layer  40 . For example, the n-type barrier layer  41  may be an n-type doped conductive compound semiconductor layer, and preferably, an n-Al x Ga 1-x N layer containing a predetermined ratio of Al. The n-type compound material layer  43  may be an n-type doped conductive compound semiconductor layer, and preferably, an n-Al x Ga 1-x N layer. Alternatively, the n-type barrier layer  41  and the n-type compound semiconductor layer  43  may be formed of undoped material layers. 
     The active layer  40  is a material layer in which lasing occurs and thus it is formed of, preferably, a lasing-capable material layer. More preferably, the active layer  40  is formed of a GaN series III-V nitride compound semiconductor layer having a multi-quantum well (MQW) structure. 
     The p-type material layer m 1  for lasing includes a plurality of compound semiconductor layers, for example, a p-type barrier layer  42  formed on the active layer  40  and a p-type compound semiconductor layer  44  formed on the p-type barrier layer  42 . The p-type barrier layer  42  may be formed of the same material layer as that of the n-type barrier layer  41 , but with conductive dopant providing for opposite electrical characteristics to those of the n-type barrier layer  42 . Likewise, the p-type compound semiconductor layer  44  may be formed of the same material layer as that of the n-type compound semiconductor layer  43 , but with conductive dopant providing for opposite electrical characteristics to those of the n-type compound semiconductor layer  43 . Alternatively, the p-type barrier layer  42  and the p-type compound semiconductor layer  43  may be formed of undoped material layers. 
     An n-type electrode  47  is formed underneath a portion of the n-type compound semiconductor layer  43  of the n-type material layer m 2 , and a first distributed Bragg reflector (DBR) layer  49  is formed underneath the remaining portion of the n-type compound semiconductor layer  43 . The n-type electrode  47  may have a variety of shapes, and preferably, has a symmetrical shape around the first DBR layer  49  by considering uniform carrier injection in all directions. Although not apparent in  FIG. 2 , more preferably, the n-type electrode  47  has an annular shape. The first DBR layer  49  as a material layer having a high reflectivity of about 99% is formed of multiple dielectric material layers having a predetermined dielectric constant, for example, including SiO 2 , AlO 3 , TiO 2 , and ZnO 2 . 
     A spacer  48 , and preferably, a p-type spacer, is formed on the p-type compound semiconductor layer  44  of the p-type material layer m 1 . The spacer  48  is a material layer for effectively supplying carriers, i.e., holes, into the center of the active layer  40 , and is formed of a material layer updoped or doped with p-type conductive impurity. The spacer  48  is formed to be thick enough for an excess of holes to reach the center of the laser, i.e., the active layer  40 , with the application of lasing voltage. 
     The spacer  48  has a protrusion portion  48   a  aligned with the first DBR layer  49 . A laser output window  48   b  is formed on the surface of the protrusion portion  48   a . A p-type electrode  50  is formed around the protrusion portion  48   a . The p-type electrode  50 , which induces migration of holes toward the active layer  40  when forward voltage is applied, is formed around the protrusion portion  48   a . The p-type electrode  50  preferably has the same shape as the n-type electrode  47 . 
     The spacer  48  is effective in diffusing an excess of holes into the center of the active layer  40 , but may degrade laser oscillation characteristics of the active layer  40 . For this reason, it is preferable that the laser output window  48   b  has a shape suitable for compensating for the laser characteristic degradation. For example, the laser output window  48   b  may have a lens shape, e.g., a convex lens shape having curvature such that laser light diffraction caused by the spacer  48  is offset or the radius of the laser mode is minimized at the center of the active layer  40 . 
     The laser output window  48   b  is capped by a second DBR layer  52 . Like the first DBR layer  49 , the second DBR layer  52  is formed of a high-reflectivity material layer including multiple dielectric layers having a predetermined dielectric constant. 
     Preferred shapes of the laser output window  48   b  are shown in  FIGS. 4 and 5 .  FIG. 4  shows the relation between the radius (R) of curvature of the laser output window  48   b , the radius (W) of the laser mode on the surface of the laser output window  48   b , the radius (W 0 ) of the laser mode in the active layer  40 , and the thickness (Z) of the spacer  48 . In  FIG. 5 , graphs G 1  and G 2  show variation in the radius (W) of the laser mode on the surface of the laser output window  48   b  and the radius (R) of curvature of the laser output window  48   b  with respect to thickness (Z) of the spacer  48 . As shown in  FIG. 5 , the radius (W) of the laser mode on the surface of the laser output window  48   b  becomes smaller with reduced thickness (Z) of the spacer  48 , and the radius (R) of curvature of the laser output window  48   b  is the smallest at a spacer thickness (Z) in the range of 200–250 μm. The spacer thickness (Z) and the radius (R) of curvature of the laser output window  48   b , at which the radius (W 0 ) of the laser mode in the active layer  40  becomes least, can be calculated with reference  FIGS. 4 and 5 . 
     &lt;Embodiment 2&gt; 
     A second embodiment of the GaN series surface-emitting laser according to the present invention is shown in  FIG. 3 . Referring to  FIG. 3 , a p-type spacer  54  is formed on the p-type material layer m 1  and a separate protrusion portion  48   a ′, which has the same shape as the protrusion portion  48   a  of the first embodiment, is formed on the p-type spacer  54  aligned with the first DBR layer  49 . The p-type electrode  50  is formed on the p-type spacer  54  to surround the protrusion portion  48   a′.    
     Preferred embodiments of a method for manufacturing a surface-emitting laser diode having a structure as described above now will be described. 
     &lt;Embodiment 1&gt; 
       FIGS. 6 through 13  are sectional views illustrating each step of a method for manufacturing a GaN series surface-emitting laser diode according to a first embodiment of the present invention. First, referring to  FIG. 6 , a p-type compound semiconductor layer  102  and a p-type barrier layer  106  for carrier confinement are sequentially formed on a p-type substrate  100 , resulting in a p-type material layer M 1  for lasing. An active layer  108  is formed on the p-type barrier layer  106 . An n-type barrier layer  110  and an n-type compound semiconductor layer  112  are sequentially formed on the active layer  108 , resulting in an n-type material layer M 2  for lasing. The p-type and n-type material layers M 1  and M 2  are the same as the p-type and n-type material layers m 1  and m 2  described with reference to  FIG. 2 . The active layer  108  is also the same material layer as the active layer  40  of  FIG. 2 . Thus, descriptions of these layers will not be provided hear. 
     Next, a conductive layer  115  is formed on the n-type material layer M 2  and a mask pattern  118  is formed to expose a region of the conductive layer  115  to be a first DBR layer. The mask pattern  118  may be formed of a soft mask pattern such as a photoresistive pattern or a hard mask pattern such as a silicon nitride or nickel pattern. 
     Referring to  FIG. 7 , the exposed region of the conductive layer  115  is etched using the mask pattern  118  as an etch mask, thereby forming a conductive pattern  116  (hereinafter, referred to as “n-type electrode”) on the n-type material layer M 2 . The n-type electrode  116  may have various forms. The n-type electrode  116  is preferably formed to be symmetrical at the center of the exposed region by considering migration of carriers (electrons). For example, the n-type electrode  116  may be formed in an annular shape. A first DBR layer  120  is formed in an exposed region of the n-type material layer M 2  excluding the region of the n-type electrode  116 . Here, the first DBR layer  120  is also formed on the mask pattern  118 . The first DBR layer  120  is the same as the first DBR layer  29  of  FIG. 2  and thus a description thereof is not provided here. Next, the mask pattern  118  is removed along with the first DBR layer  29  formed thereon. A chemical used for removing the mask pattern  118 , which is different from a chemical used for removing the first DBR layer  120 , does not affect the first DBR layer  120  formed on the n-type material layer M 2  during the removal process. As a result, the n-type electrodes  116  and the first DBR layer  120  are formed on the n-type material layer M 2 . 
     Referring to  FIG. 8 , after inverting the resultant structure having the first DBR layer  120  and the n-type electrode  116 , a mask pattern  122  is formed on a bottom surface of the p-type substrate  100  aligned with the first DBR layer  120 . The mask pattern  122  is formed of the same material layer used for the mask pattern  118  of  FIG. 6 . The mask pattern  122  will be reflowed into a lens shape, as shown in  FIG. 9 . For this reason, a soft mask pattern is preferred to a hard mask pattern as the mask pattern  122 . The mask pattern  122  is etched along with the p-type substrate  100  in a following process such that the shape of the mask pattern  122  is transferred to the p-type substrate  100 . Thus, it is preferable that the mask pattern  122  has an etching selectivity not smaller than that of the p-type substrate  100 , i.e., similar or equal to that of the p-type substrate  100 . 
     Next, the mask pattern  122  is reflowed at a predetermined temperature into a convex lens-shaped mask pattern  122   a  having a predetermined curvature, as shown in  FIG. 9 . An exposed region of the p-type substrate  100  is etched using the lens-shaped mask pattern  122   a  as an etch mask, resulting in a lens surface having the same curvature as that of the lens-shaped mask pattern  122   a  in the p-type substrate  100 . As a result, a laser output window  100   b  through which a laser beam generated from the active layer  108  is emitted to the outside, is formed on the bottom surface of the p-type substrate  100 , as shown in  FIG. 10 . Here, the surface of the laser output window  100   b  has the same curvature as that of the lens-shaped mask pattern  122   a . Although an electrode can be formed around the laser output window  100   b , it is preferable that the etching with the lens-shaped mask pattern  122   a  is performed until the exposed region of the p-type substrate  100  is removed by a predetermined thickness to form a protrusion portion  100   a , and then an electrode is formed around the protrusion portion  100   a.    
     In particular, as shown in  FIG. 11 , a mask pattern  124  is formed on the entire surface of the protrusion portion  100   a . A conductive layer  126  is formed on the p-type substrate  100  around the protrusion portion  100   a  covered by the mask pattern  124  and also on the mask pattern  124 . The conductive layer  126  which is formed on the mask pattern  124  is removed during removal of the mask pattern  124 . The conductive layer  126  which is formed on the p-type substrate  100  around the protrusion portion  100   a  is unaffected during the removal of the mask pattern  124  for the reason described above. As a result, only the conductive pattern  126  formed on the p-type substrate  100  remains around the sidewall of the protrusion portion  100   a . The remaining conductive layer  126  serves as a p-type electrode. Like the n-type electrode  116 , the p-type electrode  126  may have various forms as long as voltage can be applied. Preferably, the p-type electrode  126  is formed to have a symmetrical shape like the n-type electrode  116 . More preferably, the p-type electrode  126  is formed in the same annular shape as the n-type electrode  116 . 
     As shown in  FIG. 12 , a mask layer (not shown) is deposited on the resultant structure in which the p-type electrode  126  is formed, and then patterned into a mask pattern  128  covering the p-type electrode  126  and exposing the protrusion portion  100   a  around which the p-type electrode  126  is formed. A second DBR layer  130  is formed on the top of the protrusion portion  100   a  and on the mask pattern  128 . The second DBR layer  130  is the same as the second DBR layer  52  of  FIG. 2 , and thus a description thereof is not provided here. The second DBR layer  130  formed on the mask pattern  128  is removed during removal of the mask pattern  128 . The result, as shown in  FIG. 13 , is a surface-emitting laser diode in which the p-type substrate  100  interposed between the p-type electrode  126  and the active layer  108  serves as a spacer with the protrusion portion  100   a  having the lens-shaped top surface on which the second DBR layer  130  is formed, and the p-type electrode is formed around the protrusion portion  100   a.    
     As described above, due to the p-type substrate  100  formed as a spacer between the p-type electrode  126  and the active layer  108 , to be relatively thicker than other material layers of the laser diode, diffusion of holes to the center of the active layer  108  from the p-type electrode  126  is easy, and thus an excess of holes for laser emission can be provided to the center of the active layer  108 . However, the characteristics of a laser emitted from the active layer  108  may degrade due to diffraction of the laser beam: For this reason, the laser output window  100   b  on the protrusion portion  100   a  is formed in an appropriate shape such that the drop in characteristics of the laser beam, which may occur due to the presence of the p-type substrate  100  serving as a p-type spacer, can be compensated for. In view of this, the laser output window  100   b  is formed on the top of the protrusion portion  100   a  in a convex lens-like shape having a predetermined curvature suitable for compensation of the degradation of laser beam quality. In other words, it is preferable that the laser output window  100   b  is designed to have a curvature suitable for guiding a laser beam being diffracted toward the center of the active layer  108 . 
     &lt;Embodiment 2&gt; 
     Multiple substrates rather than a single substrate are used: a first substrate corresponding to a substrate  101  to be described below and a second substrate corresponding to a p-type spacer  140  formed on the substrate  101 . 
     Referring to  FIG. 14 , the p-type spacer  140  is formed to be thick on the substrate  101 . Here, the substrate  101  may be formed of a p-type substrate, but gallium nitride (GaN) or other GaN series materials are preferred as the substrate  101 . For example, a sapphire substrate or a silicon carbide (SiC) substrate is preferably used. As a p-type material layer M 1  for lasing, a p-type compound material layer  102  and a p-type barrier layer  106  for carrier confinement are formed on the p-type spacer  140 . Here, the p-type spacer  140  may be included in the p-type material layer M 1  for lasing. An active layer  108  is formed on the p-type barrier layer  106 , and an n-type barrier layer  110  and an n-type compound semiconductor layer  112  are sequentially formed on the active layer  108 , resulting in an n-type material layer M 2 . Following this, a conductive layer  115  is formed on the n-type compound semiconductor layer  112  in the same manner as in Embodiment 1 and patterned into an n-type electrode  116 , as shown in  FIG. 15 , using an mask pattern  118 . Next, a first DBR layer  120  is formed on an exposed portion of the n-type compound semiconductor layer  112 . 
     After removing the mask pattern  118  and inverting the resultant structure, a protrusion portion  101   a  is formed on the bottom surface of the substrate  101  in the same manner as in Embodiment 1. In etching the substrate  101  to form the protrusion portion  101   a , the etching is continued until the bottom of the p-type spacer  140  is partially exposed, resulting in the protrusion portion  101   a  as a substrate pattern with a laser output window  101   b  thereon. Next, a p-type electrode  126  and a second DBR layer  130  are formed in the same manner as in Embodiment 1. 
     &lt;Embodiment 3&gt; 
     In the present embodiment, a first DBR layer  120  is formed prior to formation of an n-type electrode  116 . 
     Referring to  FIG. 16 , the steps up to forming the n-type compound semiconductor material layer  122  are performed in the same manner as in Embodiment 1 or 2. Next, a first DBR layer  120  is formed on the n-type compound semiconductor layer  112 . A mask pattern  150  is formed to cover a predetermined region of the first DBR  120 . The mask pattern  150  is formed of a soft mask pattern or a hard mask pattern described above. A portion of the first DBR layer  120  exposed around the mask pattern  150  is removed using the mask pattern  150  as an etch mask. 
     Referring to  FIG. 17 , a conductive layer  116  serving as an n-type electrode is formed on the n-type compound semiconductor layer  112  exposed around the mask pattern  150 . The conductive layer  150  is removed along with the mask pattern  150  formed thereon. As a result, as shown in  FIG. 18 , the first DBR layer  120  and the n-type electrode  116  around the first DBR layer  120  are formed on the n-type compound semiconductor layer  112 . The following steps are performed in the same manner as in Embodiment 1. 
     &lt;Embodiment 4&gt; 
     A second DBR layer  130  is formed prior to formation of a p-type electrode  126 . Referring to  FIG. 19 , the steps up to forming a protrusion portion  100   a  with a laser output window  100   b  on the bottom surface of a p-type substrate  100  are performed in the same manner as in Embodiments 1 through 3. Next, a second DBR layer  130  is formed on the p-type substrate  100  having the laser output window  100   b . A mask pattern  160  is formed to cover the second DBR layer  130  formed on the laser output window  100   b . The mask pattern  160  is formed of a soft mask pattern or hard mask pattern. The second DBR layer  130  formed on a portion of the p-type substrate  100  surrounding the protrusion portion  100   a  is removed, exposing the portion of the p-type substrate  100 . 
     Following this, as shown in  FIG. 20 , a conductive layer  126  serving as a p-type electrode is formed on the exposed portion of the p-type substrate  100  around the protrusion portion  100   a . At this time, the conductive layer  126  is also formed on the mask pattern  160 , but it is removed along with the mask pattern  160 . As a result, a surface-emitting laser diode which allows smooth migration of holes existing between the second DBR layer  130  and an active layer  108  toward the active layer  108  and can compensate for diffraction of a laser beam emitted from the active layer  108 , is obtained. 
     &lt;Embodiment 5&gt; 
     Referring to  FIG. 21 , an n-type material layer M 2 , an active layer  108 , and a p-type material layer M 1  are sequentially formed on a substrate  200 . The substrate  200  is a high-resistance substrate, such as an n-type substrate or a sapphire substrate. Alternatively, the substrate  100  may be an undoped substrate. Next, a p-type spacer  210  is formed on the p-type material layer M 1 . 
     Referring to  FIG. 22 , the p-type spacer  210  is patterned into a protrusion portion  210   a  in the same manner as in Embodiment 1 or 2. A laser output window  210   b  is formed on the protrusion portion  210   a . A second DBR layer  130  is formed on the laser output window  210   b , and a p-type electrode  126  is formed on the p-type spacer  210  to surround the protrusion portion  210   a.    
     The resultant structure, on which the p-type electrode  126  is formed, is inverted, and the substrate  200  is removed from the structure. As shown in  FIG. 23 , a first DBR layer  120  is formed on the n-type compound semiconductor layer  112 , and a n-type electrode  116  is formed to surround the first DBR layer  120  formed on the n-type compound semiconductor layer  112 . 
     The present invention may be embodied in many different forms, and the embodiments described herein are merely illustrative and not intended to limit the scope of the invention. For example, it will be appreciated to those skilled in that art that the spacer can be applied to a surface-emitting laser diode whose laser output window is not a convex lens-like shape. Alternatively, a surface-emitting laser diode may be constructed such that a spacer is formed in any positions between the active layer and the p-type electrode, and a laser output window, which has a predetermined curvature and around which the p-type electrode is formed, is formed without a protrusion portion. The structures of the first and second material layers, or materials used for the same may be varied from the embodiments described above. In another surface-emitting laser diode, the DBR layers may be formed by air gap. 
     As described above, the present invention provides a surface-emitting laser diode including a spacer formed between a p-type electrode and an active layer to enable the effective migration of holes to the center of the active layer, a laser output window designed in a particular shape on a portion of the spacer such that diffraction of a laser beam caused by the formation of the spacer can be compensated for or the radius of the laser mode in the active layer can be minimized, and a DBR layer on the surface of the laser output window. With the surface-emitting laser diode according to the present invention, holes as well as electrons can effectively be provided to the center of the active layer, and thus emission of a laser beam is achieved with reduced current. Energy conversion efficiency becomes high, and the laser beam emitted by the surface-emitting laser diode has stable transverse mode characteristics. 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the sprit and scope of the invention as defined by the appended claims.