Abstract:
Refractory metal ELOG mask are used for GaN based VCSELs and edge emitter structures to serve as intracavity contacts. In these structures the refractory metal ELOG masks serve both as ohmic contact metals as well as masks for ELOG.

Description:
BACKGROUND  
       [0001]     An issue in fabricating GaN and other nitride based lasers concerns the high-resistance intra-cavity contacts that are typically formed with nitride semiconductor material. This typically arises due to the poor p-type conductivity of GaN and typically requires that the metal contacts be placed close to the active region to reduce heating and voltage drops. Typically involved etching process are required to place the metal contacts in the required locations of the GaN lasers.  
       SUMMARY  
       [0002]     Refractory metal masks are used in accordance with the invention with an epitaxial layer overgrowth process (ELOG) and positioned relative to the laser active region to provide intracavity contacts and such that the refractory metal masks introduce minimal optical absorption loss. Refractory metal masks are used in place of SiO 2  or Si 3 N 4  masks for selective ELOG and also function as ohmic contact metals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]      FIG. 1  shows an embodiment in accordance with the invention.  
         [0004]      FIGS. 2   a - f  show steps for making the embodiment in accordance with the invention shown in  FIG. 1 .  
         [0005]      FIG. 3  shows an embodiment in accordance with the invention.  
         [0006]      FIGS. 4   a - f  show steps for making the embodiment in accordance with the invention shown in  FIG. 3 .  
         [0007]      FIG. 5  shows an embodiment in accordance with the invention.  
         [0008]      FIGS. 6   a - g  show steps for making the embodiment in accordance with the invention shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0009]      FIG. 1  shows an embodiment in accordance with the invention of GaN edge emitting laser structure  100 . Substrate  105  is typically Al 2 O 3  or SiC with GaN buffer layer  110  separating substrate  105  from n-GaN layer  120 . Refractory metal ELOG masks  130  is a layer that overlies n-GaN layer  120 . Refractory metal ELOG masks  130  may be made from Ti, Pt, W, Re Mo, Cr, Ni, Pd or other suitable refractory metal. ELOG layers, n-GaN layer  138 , n-AlGaN lower cladding layer  140 , InGaN separate confinement heterostructure layer  150 , InGaN multiple quantum wells  160 , AlGaN electron blocking layer  170  which prevents electron leakage, InGaN separate confinement heterostructure layer  180 , p-type AlGaN upper cladding layer  190  and p-type GaN layer  195  overlie refractory metal ELOG mask  130 . N-metal contact  134  contacts refractory metal layer  130  to provide efficient current injection into active region  199  of GaN edge emitting ridge waveguide laser structure  100 . P-metal contact  136  is positioned over p-type GaN layer  195 .  
         [0010]     Typically GaN edge emitting laser structure  100  is made by taking substrate  105 , typically Al 2 O 3  or SiC and depositing GaN buffer layer  110  over it to a typical thickness of about 30 nm. Then planar n-type GaN layer  120  is deposited over GaN buffer layer  110  as shown in  FIG. 2   a.  Refractory metal ELOG mask  130  is deposited by sputtering or evaporation and patterned by chemically assisted ion beam etching (CAIBE) or reactive ion-etching (RIE) over n-GaN layer  120 . Then ELOG growth is started using refractory metal ELOG mask  130  for growing n-GaN layer  138  to a typical thickness of about 1 to about 2 μm; n-AlGaN lower cladding layer  140  has a typical thickness of about 1 μm; active region  199  which comprises InGaN separate confinement heterostructure layer  150  has a typical thickness of about 0.1 μm, InGaN multiple quantum wells  160 , AlGaN electron blocking layer  170  has a typical thickness of about 20 nm, and InGaN separate confinement heterostructure layer  180  has a typical thickness of about 0.1 μm (see  FIG. 1 ); p-type AlGaN upper cladding layer  190  has a typical thickness of about 0.5 μm and p-type GaN layer  195  has a typical thickness of about 0.1 μm.  
         [0011]      FIG. 2   d  shows etching typically by CAIBE or RIE to make the typical wave guide structure by etching through p-type GaN layer  195  and into p-type AlGaN upper cladding layer  190 . A second etch by CAIBE or RIE down to refractory metal layer  130  is performed as shown in  FIG. 2   e  to provide a contact area for n-metal contact  134 . Finally, n-metal contact  134  and p-metal contact  136  are deposited and annealed.  
         [0012]      FIG. 3  shows an embodiment in accordance with the invention of GaN VCSEL laser structure  300  having ELOG n- and p-refractory metal masks with a lower DBR ELOG mask. Substrate  305  is typically Al 2 O 3  or SiC with GaN buffer layer  310  having a typical thickness of about 30 nm and separating substrate  305  from n-GaN layer  315  with a typical thickness of about 1 μm to about 2 μm. Lower dielectric distributed Bragg reflector (DBR)  318  overlies GaN buffer layer  310 . ELOG n-GaN layer  320  with a typical thickness of about 3 μm overlies lower dielectric DBR  318  and n-refractory metal ELOG mask  330  is a layer that overlies ELOG n-GaN layer  320 . N-refractory metal ELOG mask  330  may be made from Ti, Pt, W, Re Mo, Cr, Ni, Pd or other suitable refractory metal. Care must be taken to place n-refractory metal ELOG mask  330  at a null of the standing wave set up between lower dielectric DBR  318  and upper dielectric DBR  319 . N-refractory metal ELOG mask  330  typically has a thickness of about 50 nm or less. ELOG layer n-GaN  340  with a typical thickness of about 1 μm to about 2 μm, InGaN multiple quantum well active region  345 , p-AlGaN layer  346  having a typical thickness of about 20 nm and p-GaN layer  350  with a typical thickness of about 1 μm to about 2 μm overlie refractory metal ELOG mask  330 . P-refractory metal ELOG mask  321  is a layer that overlies p-GaN layer  350 . Care must be taken to place p-refractory metal ELOG mask  321  at a null of the standing wave set up between lower dielectric DBR  318  and upper dielectric DBR  319 . P-refractory metal ELOG mask  321  typically has a thickness of about 50 nm or less. P-doped refractory metal ELOG mask  321  may be made from Ti, Pt, W, Re Mo, Cr, Ni, Pd or other suitable refractory metal. P-GaN layer  360  with a typical thickness of about 1 μm to about 5 μm overlies p-doped refractory metal ELOG mask  321  and upper dielectric DBR  319 . N-metal contact  334  contacts n-refractory metal layer  330  and p-metal contact  336  contacts p-refractory metal layer  321  to provide efficient current injection into VCSEL structure  300 .  
         [0013]     Typically, GaN VCSEL structure  300  is made by taking substrate  305 , typically Al 2 O 3  or SiC and depositing GaN buffer layer  310  to a typical thickness of about 30 nm over it. Then planar n-type GaN layer  315  is deposited to a thickness of about 1 μm to about 2 μm over GaN buffer layer  310  as shown in  FIG. 4   a.  Lower dielectric DBR  318  is then deposited and patterned as shown in  FIG. 4   b.  Lower dielectric DBR  318  serves as an ELOG mask for ELOG of n-GaN layer  320  having a typical thickness of about 3 μm and is shown in  FIG. 4   c.  Then n-refractory metal ELOG mask  330  is deposited and patterned as shown in  FIG. 4   d.  With reference to  FIG. 4   e,  n-refractory metal ELOG mask  330  is then used to ELOG grow n-type GaN layer  340  having a typical thickness of about 1 μm to about 2 μm, InGaN multiple quantum well active region  345 , p-type AlGaN layer  346  with a typical thickness of about 20 nm and p-type GaN layer  350  with a typical thickness of about 1 μm to about 2 μm. After growing p-doped GaN layer  350 , p-refractory metal ELOG mask  321  is deposited on p-doped GaN layer  350  and patterned as shown in  FIG. 4   f.  ELOG of p-doped GaN layer  360  is then performed to a typical thickness of about 1 μm to about 5 μm using p-refractory metal ELOG mask  321  as shown in  FIG. 4   g.  Upper DBR  319  is then deposited on p-doped GaN layer  360  and etched as shown in  FIG. 4   h.  Finally, as shown in  FIG. 4   i,  etches are performed down to refractory metal layers  321  and  330  where n-electrode  334  and p-electrode  335  are deposited, respectively.  
         [0014]      FIG. 5  shows an embodiment in accordance with the invention of GaN VCSEL laser structure  500  having an ELOG p-refractory metal mask and using a lower DBR deposited on an n-GaN layer after removal of the substrate by laser liftoff or other suitable technique. N-type GaN layer  520  with a typical thickness of about 4 μm has n-contacts  534  attached on the bottom surface along with lower DBR  518 . InGaN multiple quantum well active region  545  overlies n-type GaN layer and is topped by AlGaN layer  546  having a typical thickness of about 20 nm. P-type GaN layer  547  having a typical thickness of about 0.2 μm to about 2 μm overlies AlGaN layer  546 . P-refractory metal ELOG mask  535  is a layer that overlies p-type GaN layer  547  and ELOG p-type GaN layer  560  having a typical thickness of about 1 μm to about 4 μm overlies p-type GaN layer  547 . Care must be taken to place p-refractory metal ELOG mask  535  at a null of the standing wave set up between lower dielectric DBR  518  and upper dielectric DBR  519 . P-refractory metal ELOG mask  321  typically has a thickness of about 50 nm or less. Upper DBR mirror  519  sits on ELOG p-type GaN layer  560  and p-type electrodes  536  are attached to p-type refractory metal layer  535 .  
         [0015]     Typically, GaN VCSEL structure  500  may be made by taking substrate  505 , typically Al 2 O 3  or SiC and depositing GaN buffer layer  510  to a typical thickness of about 30 nm over it. Planar growth is performed for n-type GaN layer  520  having a typical thickness of about 4 μm, InGaN multiple quantum well active region  545 , AlGaN layer  546  having a typical thickness of about 20 nm and p-type GaN layer  547  with a typical thickness of about 0.2 μm to 2 μm as shown in  FIG. 6   a.  P-refractory metal ELOG mask  535  is deposited on p-type GaN layer  547  and patterned as shown in  FIG. 6   b.  Then ELOG growth of p-GaN layer  560  to a typical thickness of about 1 μm to about 4 μm is performed as shown in  FIG. 6   c.  Upper DBR  519  is deposited on p-GaN layer  560  and patterned as shown in  FIG. 6   d.  Substrate  505  is subsequentally removed by laser liftoff leaving the VCSEL structure shown in  FIG. 6   e.  Lower DBR  518  is deposited on the bottom of GaN buffer layer  510  and patterned as shown in  FIG. 6   f.  Finally, an RIE or CAIBE etch is performed through p-GaN layer  560  down to p-refractory ELOG metal mask  535  to deposit p-type electrodes  536  and n-type electrodes  534  on the bottom of GaN buffer layer  510 .