Abstract:
The present invention provides a vertical-cavity surface emitting laser (VCSEL) diode and a method for producing the same. In this method, an n-type and a p-type ohmic contact electrodes are previously disposed, and then two pairs of distributed Bragger reflectors (DBRs) are formed. At last, a permanent metal substrate is plated. According to the present invention, reflectivity of the DBRs can be preserved without damage during rapid thermal annealing, and thus brightness of the laser diode is improved.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a laser diode and a method for producing the same, in particular, to a vertical-cavity surface emitting laser diode and a method for producing the same.  
         [0003]     2. Description of the Related Art  
         [0004]     For conventional vertical-cavity surface emitting laser (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.  
         [0005]     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.  
         [0006]     Therefore, it&#39;s desirable to find a laser diode and a method for producing the same to overcome the above disadvantages.  
       SUMMARY OF THE INVENTION  
       [0007]     The object of the present invention is to provide a vertical-cavity surface emitting laser diode, which includes Bragg mirrors with superior reflectivity.  
         [0008]     Another object of the present invention is to provide a method for producing the above laser diode, which is easily achieved and needs low cost.  
         [0009]     To produce the laser diode of the present invention, a substrate is first provided for sequentially epitaxying an n-type cladding layer, an active layer with quantum well structure and a p-type cladding layer thereon. The p-type cladding layer, the active layer and an upper portion of the n-type cladding layer are partially etched so as to expose the n-type cladding layer. Lateral surface of the p-type cladding layer is then oxidized to obtain a surrounding insulating area by wet oxidation. Next, an n-type ohmic contact electrode is deposited on the exposed n-type cladding layer, and an annular p-type ohmic contact electrode is deposited on the p-type cladding layer close to the insulating area. An upper DBR pair of dielectric material is then deposited on the p-type cladding layer at least within the annular p-type ohmic contact electrode. A bottom DBR pair of dielectric material is deposited beneath the n-type cladding layer. A glass substrate is then bonded to the upper DBR pair and the substrate aforementioned is removed. A permanent substrate is then plated beneath a metal conductive layer which is previously deposited beneath the bottom DBR pair. At last, the glass substrate is removed.  
         [0010]     In accordance with the above method, a vertical-cavity surface emitting laser (VCSEL) diode is obtained. The VCSEL diode primarily includes an n-type cladding layer, an active layer, a p-type cladding layer, an insulating area, an n-type ohmic contact electrode, an annular p-type ohmic contact electrode, an upper DBR pair, a bottom DBR pair, a metal conductive layer and a permanent substrate. The n-type cladding layer has a top surface being partially etched. The active layer has a quantum well structure and is formed on the un-etched surface of the n-type cladding layer. The p-type cladding layer is formed on the active layer. The insulating area is formed surrounding the p-type cladding layer. The n-type ohmic contact electrode is deposited on the etched surface of the n-type cladding layer. The annular p-type ohmic contact electrode is deposited on the p-type cladding layer close to the insulating area. The upper DBR pair is made of dielectric material and formed on the p-type cladding layer at least within the annular p-type ohmic contact electrode. The bottom DBR pair is also made of dielectric material and formed beneath the n-type cladding layer. The metal conductive layer is formed beneath the bottom DBR pair. The permanent substrate is formed beneath the metal conductive layer.  
         [0011]     Moreover, a spacer without shading the emitted light can be formed between the bottom DBR pair and the n-type cladding layer by appropriately applying photolithographic techniques. Similarly, the metal substrate is plated only where corresponding to the spacer, so that light beams can be transmitted from the bottom DBR pair. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIGS. 1-7  illustrate cross sections of the first embodiment during manufacturing.  
         [0013]      FIG. 8  illustrates the structure of the first embodiment with an additional transparent conductive film.  
         [0014]      FIGS. 9-11  illustrate cross sections of the second embodiment different from the first embodiment during manufacturing.  
         [0015]      FIG. 12  illustrates the second embodiment having a plated permanent substrate without shading scrub lines. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]      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 .  
         [0017]     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 area  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 area  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.  
         [0018]      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.  
         [0019]      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.  
         [0020]      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 .  
         [0021]     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.  
         [0022]      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.  
         [0023]     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.  
         [0024]      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 .  
         [0025]     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 shaded by the metal conductive layer  41  and the copper substrate  42 .  
         [0026]     In like manner, the wafer is diced after removing the glass substrate  92 , and a laser diode as shown in  FIG. 11  is obtained.  
         [0027]     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.  
         [0028]     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.  
         [0029]     According to description of the preferred embodiments, advantages of the present invention can be roughly summarized as follows: 
        a) production cost is low and the laser diode retains good light-emitting efficiency;     b) processes are easily completed by providing the DBR pairs of dielectric material (or companied with metal mirrors); and     c) heat dissipation of the diode is promoted by plating the metal permanent substrate, which also facilitates preserving the DBRs without damage.