Abstract:
The present invention provides a VCSEL (vertical-cavity surface emitting laser) diode, in which a p-type cladding layer is formed on an active layer and surrounded with an insulation edge. An annular p-type electrode is formed on the ptype cladding layer close to the insulation edge and an upper DBR mirror is formed therewithin. According to the present invention, light beams emitting from the active layer will not be shielded by a central electrode and brightness of the laser diode is improved.

Description:
CROSS REFERENCE  
       [0001]     The present application is a Division of co-pending U.S. application Ser. No. 10/668553 by the same inventors filed on Sep. 22,2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a laser diode, in particular, to a VCSEL (vertical-cavity surface emitting laser) diode comprising a DBR mirror within an annular p-type electrode.  
         [0004]     2. Description of the Related Art  
         [0005]     For conventional VCSEL diode, the cavities of distributed Bragg reflectors (DBR) can be formed by epitaxial growth. In general, reflectivity of the DBR higher than 99% is required. To obtain such reflectivity, appropriate pair numbers of DBRs with appropriate refractive index deviation (Δn) are provided. For VCSEL devices of wavelength at 1,310 or 1,550 nm, only the InGaAsP/InP Bragg mirror grown on an active layer of InP series is considered. However, heat dissipation of the InGaAsP/InP mirror is poor and Δn thereof is too small when compared with GaAs/AlAs or dielectric Bragg mirrors. Therefore, lots of Bragg reflector pairs are associated to achieve desired reflectivity. As a result, complicated epitaxial processes including thousands of MBE or MOCVD during at least 4-8 hours is necessary. In addition, to maintain growth deviation of production less than 1% is very hard for manufacturing.  
         [0006]     The above problems may be solved by applying direct wafer-bonding technology once or twice during manufacturing. For example, a laser diode of wavelength at 1,310 nm can be obtained by bonding an epitaxial structure to a GaAs substrate on which another epitaxial AlGaAs/GaAs DBR structure is grown. Such processes need an epitaxial system complying requirement of lattice matching which is not necessary for VCSEL epitaxial system. However, direct wafer-bonding needs to be performed at high temperature and through lattice alignment, which significantly limit production yields and increase manufacturing cost.  
         [0007]     Therefore, it&#39;s desirable to find a VCSEL diode to overcome the above disadvantages.  
       SUMMARY OF THE INVENTION  
       [0008]     The object of the present invention is to provide a VCSEL (vertical-cavity surface emitting laser) diode, in which light beams can emit from a central area without shielding and exhibit superior brightness.  
         [0009]     The VCSEL diode of the present invention comprises an n-type cladding layer with a top surface partially etched; an active layer with quantum well structure formed on the un-etched surface of said n-type cladding layer; a p-type cladding layer surrounded with an insulating edge and formed on said active layer; an n-type ohmic contact electrode deposited on said etched surface of said n-type cladding layer; an annular p-type ohmic contact electrode deposited on said p-type cladding layer close to said insulating edge; an upper DBR pair of dielectric material formed on said p-type cladding layer at least within said annular p-type ohmic contact electrode; a bottom DBR pair of dielectric material formed beneath said n-type cladding layer; a metal conductive layer formed beneath said bottom DBR pair; and a permanent substrate formed beneath said metal conductive layer.  
         [0010]     The upper and/or bottom DBR pair can be a metal reflective layer or made from a composite material selected from the group consisting of ZnSe/MgF 2 , SiO 2 /Si, Si 3 N 4 /Si, TiO 2 /Si, Ta 2 O 5 /Si, HfO 2 /SiO 2 , Ta 2 O 5 /SiO 2 , ZrO 2 /SiO 2 , TiO 2 /SiO 2 .  
         [0011]     The VCSEL diode can further comprises a transparent conductive film formed on said ptype ohmic contact electrode.  
         [0012]     The VCSEL diode can also further comprises an insulating spacer between said n-type cladding layer and said bottom DBR pair, and said insulating spacer is formed beneath said n-type cladding layer but not overlaps main area of said active layer, and said metal conductive layer and said permanent substrate are formed beneath said bottom DBR pair only corresponding to said insulating spacer. Therefore, the emitted light beams will not shielded by the spacer and can be transmitted from the bottom DBR pair. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIGS. 1-7  illustrate cross sections of the first embodiment during manufacturing.  
         [0014]      FIG. 8  illustrates the structure of the first embodiment with an additional transparent conductive film.  
         [0015]      FIGS. 9-11  illustrate cross sections of the second embodiment different from the first embodiment during manufacturing.  
         [0016]      FIG. 12  illustrates the second embodiment having a plated permanent substrate without overlapping scrub lines. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]      FIGS. 1-7  illustrate cross sections of the first embodiment during manufacturing. In  FIG. 1 , an InP substrate  91  is provided for sequentially epitaxy an n-type cladding layer  11 , an active layer with quantum well structure  12  and a p-type cladding layer  13  thereon. In this embodiment, both electrodes are formed at the top side. Therefore, according to size of each laser diode die, the p-type cladding layer  13 , the active layer  12  and an upper portion of the n-type cladding layer  11  are partially etched. As a result, trenches deep to the n-type cladding layer  11  are formed as shown in  FIG. 1 .  
         [0018]     In  FIG. 2 , lateral surface of the p-type cladding layer  13  and the active layer  12  along the trenches are oxidized by wet oxidation to form an surrounding insulating edge  14  for each laser diode die. For each laser diode die, an annular p-type ohmic contact electrode  31  is then disposed on the top edges of the p-type cladding layer  13  close to the insulating edge  14 , and an n-type ohmic contact electrode  32  is disposed on the exposed n-type cladding layer  11 , i.e., bottom of the trenches aforementioned. The electrodes  31 ,  32  can be formed on predetermined positions by a lift-off process, and then generate ohmic contact interfaces with the semiconductor layer by rapid thermal annealing above 350° C.  
         [0019]      FIG. 3  illustrates an upper DBR pair  21  coated on the p-type cladding layer  14  and within the annular p-type ohmic contact electrode  31 . Sputtering is preferably applied for completing the DBR pair due to suitable coating rate and adhesion effect. Particularly, the DBR pair is deposited after annealing, and therefore reflectivity thereof can be preserved without damage.  
         [0020]      FIG. 4  shows that a glass substrate  92  coated with wax  93  is bonded to the top surface of the wafer, and associated with the upper DBR pair  21  and the ohmic contact electrodes  31 ,  32 . By supporting the epitaxial structure with the glass substrate  92 , the InP substrate  91  is no longer necessary and can be removed by chemical mechanical polishing or etching. The n-type cladding layer  11  is thus exposed.  
         [0021]      FIG. 5  shows a bottom DBR pair  22  is coated beneath the n-type cladding layer  11  by sputtering. In the present invention, both the DBR pairs  21 ,  22  are made from dielectric material, for example, ZnSe/MgF 2 , SiO 2 /Si, Si 3 N 4 /Si, TiO 2 /Si, Ta 2 O 5 /Si, HfO 2 /SiO 2 , Ta 2 O 5 /SiO 2 , ZrO 2 /SiO 2 , TiO 2 /SiO 2 .  
         [0022]     To enhance heat dissipation of the laser diode, a metal permanent substrate  42  is plated beneath a metal conductive layer  41  which is previously deposited beneath the DBR pair  22  as shown in  FIG. 6 . The plating process can be completed in an electrolyte containing Cu +2 , to obtain a stable copper substrate  42 . The glass substrate  92  used for temporarily supporting the structure can be then removed by melting the wax  93  below 100° C. At last, a laser diode die as shown in  FIG. 7  is obtained after dicing.  
         [0023]      FIG. 8  shows that an additional transparent conductive film  33  of ITO material is deposited between the p-type cladding layer  13  and the p-type ohmic contact electrode  31  to enhance current spreading.  
         [0024]     For the laser diode of  FIGS. 7 and 8 , light is emitted out through the upper DBR pair  21 . The present invention also provides another embodiment in which light is emitted out through the bottom DBR pair  22 .  FIGS. 9-11  illustrate cross sections of such laser diode different from the first embodiment during manufacturing.  
         [0025]      FIG. 9  shows a photoresist layer  60  and an insulating layer  50  are coated beneath the n-type cladding layer  11  after the InP substrate  91  is removed. The photoresist layer  60  is coated where mainly corresponding to the active layer  12 . The insulating layer  50  is deposited on other bottom surface of the cladding layer  11 , i.e., opposite bottom edges of the n-type cladding layer  11  as shown in  FIG. 9 .  
         [0026]     The bottom DBR pair  22  is then deposited beneath the photoresist layer  60  and the insulating layer  50 . After the photoresist layer  60  is removed, a spacer formed by the insulating layer  50  is obtained, as shown in  FIG. 10 . Next, the metal conductive layer  41  is deposited beneath the bottom DBR pair  22  corresponding to the insulating layer  50 ; and the copper substrate  42  is plated beneath the metal conductive layer  41 . Accordingly, light passing through the bottom DBR pair  22  will not be shielded by the metal conductive layer  41  and the copper substrate  42 .  
         [0027]     In like manner, the wafer is diced after removing the glass substrate  92 , and a laser diode as shown in  FIG. 11  is obtained.  
         [0028]     Furthermore, by applying a voltage to the substrate  42  and the electrode  32  of the second embodiment, wavelength of the laser diode can be modulated by an electrostatics means.  
         [0029]     In the present invention, the substrate  42  is not necessarily plated through the bottom surface of the diode. Scrub lines  43  of the wafer can be optionally exposed as shown in  FIG. 12 , so that wafer dicing can be performed conveniently.  
         [0030]     According to description of the preferred embodiments, advantages of the present invention can be roughly summarized as follows:  
         [0031]     a) production cost is low and the laser diode retains good light-emitting efficiency;  
         [0032]     b) processes are easily completed by providing the DBR pairs of dielectric material (or companied with metal mirrors); and  
         [0033]     c) heat dissipation of the diode is promoted by plating the metal permanent substrate, which also facilitates preserving the DBRs without damage.