Abstract:
A self aligned, index-guided, buried heterostructure AlGaInN laser diode provides improved mode stability and low threshold current when compared to conventional ridge waveguide structures. A short period superlattice is used to allow adequate cladding layer thickness for confinement without cracking. The intensity of the light lost due to leakage is reduced by about 2 orders of magnitude with an accompanying improvement in the far-field radiation pattern when compared to conventional structures. The comparatively large p-contact area allowed by the self-aligned architecture contributes to a lower diode voltage and less heat during continuous wave operation of the laser diode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to commonly assigned, concurrently filed Bour et al. U.S. patent application entitled “STRUCTURE AND METHOD FOR INDEX-GUIDED BURIED HETEROSTRUCTURE AlGaInN LASER DIODES”(application Ser. No. ______, Attorney Reference No. D/99241Q) which is included by reference in its entirety. 
     
    
     
       FIELD OF INVENTION  
         [0002]    This invention relates to nitride based blue laser diodes.  
         BACKGROUND OF INVENTION  
         [0003]    Nitride based blue laser diodes are being developed for printing and optical data storage applications. The first AlGaInN blue laser diodes were broad area lasers providing no control over the laser diode&#39;s various spatial modes. Most applications, however, require the laser diode to operate in a single spatial mode. One way of achieving single spatial mode operation for AlGaIN blue laser diodes is to use a ridge waveguide structure to define a lateral waveguide as described in “Ridge-geometry InGaN multi-quantum-well-structure laser diodes” by S. Nakamura et al., in Applied Physics Letters 69 (10), pp. 1477-1479 which is hereby incorporated by reference in its entirety. While a ridge waveguide provides for single spatial mode emission in blue lasers, the waveguiding provided is relatively weak. The lateral refractive index step is small and is influenced by heating and carrier injection. Additionally, there are fabrication difficulties because the ridge must be etched to extend sufficiently close to the laser active region without the ability to use an etch stop to prevent material damage to the laser active region since chemical etching is not applicable to GaN materials.  
           [0004]    To provide stronger mode stability and low threshold current operation, more strongly index-guided diode lasers are required such as those having buried heterostructures that are typically used for InGaAsP fiber optic-communication lasers, or the impurity-induced-layer-disordered waveguide structures used for high-power single-mode AlGaAs laser diodes. Additionally, the use of a buried heterostructure avoids certain fabrication difficulties.  
         BRIEF SUMMARY OF INVENTION  
         [0005]    Both index-guided buried heterostructure AlGaInN laser diodes and self-aligned index guided buried heterostructure AlGaInN laser diodes provide improved mode stability and low threshold current when compared to conventional ridge waveguide structures. A structure for the index-guided buried heterostructure AlGaInN laser diode in accordance with the invention typically uses insulating AlN, AlGaN or p-doped AlGaN:Mg for lateral confinement and has a narrow (typically about 1-5 μm in width) ridge which is the location of the narrow active stripe of the laser diode which is defined atop the ridge. The narrow ridge is surrounded by an epitaxially deposited film having a window on top of the ridge for the p-electrode contact. The ridge is etched completely through the active region of the laser diode structure to the short period superlattice n-cladding layer. The short period superlattice is used to allow adequate cladding layer thickness for confinement without cracking. Typically, use of a short period superlattice allows doubling of the cladding layer thickness without cracking. This reduces the intensity of the light lost due to leakage by about 2 orders of magnitude with an accompanying improvement in the far-field radiation pattern in comparison with conventional structures. Junction surfaces are exposed by the ridge etch and these junction surfaces contribute surface states which prevent injected carriers from filling conduction or valence band states needed for a population inversion. However, the epitaxial regrowth of a high bandgap material passivates the surface states because the interface between the overgrown material and the ridge structure is perfectly coherent.  
           [0006]    The structure for the self-aligned, index guided, buried heterostructure AlGaInN laser diode uses the p-cladding layer to also function as the burying layer to provide strong lateral optical confinement and strong lateral carrier confinement. The p-cladding layer/burying layer is typically AlGaN:Mg. The structure for the self-aligned, index guided, buried heterostructure laser diode is simpler than for the index-guided, buried heterostructure AlGaInN laser diode. The laser structure is grown through the active quantum well and waveguide region followed by etching a narrow laser ridge down to the n-bulk cladding layer. The p-type cladding/burying layer is then overgrown around the ridge along with the p-contact layer. Subsequent laser processing is simple since the two-step growth process results in a lateral waveguide and carrier confinement structure which does not require the creation of contact windows. Hence, the laser processing required is basically a broad area laser fabrication sequence. Additionally, the comparatively large p-contact area allowed by the self-aligned architecture contributes to a lower diode voltage and less heat during continuous wave operation of the laser diode.  
           [0007]    The advantages and objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention, its preferred embodiments, the accompanying drawings which are illustrative and not to scale, and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 shows an embodiment of an index guided, buried heterostructure laser diode structure in accordance with the invention.  
         [0009]    [0009]FIG. 2 shows an embodiment of an index guided, buried heterostructure laser diode structure in accordance with the invention.  
         [0010]    [0010]FIG. 3 shows an embodiment of a self-aligned, index guided, buried heterostructure laser diode in accordance with the invention.  
         [0011]    [0011]FIG. 4 shows the carrier paths for the embodiment shown in FIG. 3  
         [0012]    [0012]FIG. 5 shows an embodiment of a self-aligned, index guided, buried heterostructure laser diode in accordance with the invention.  
         [0013]    FIGS.  6 - 11  show process steps for making a self-aligned, index guided, buried heterostructure laser diode in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 1 shows index-guided, buried heterostructure AlGaInN laser diode structure  100  in accordance with the present invention. GaN:Si layer  115  is positioned on Al 2 O 3  growth substrate  110  and in one embodiment layer  115  may be made of AlGaN:Si to reduce optical leakage. Short period superlattice n-cladding structure  121 , typically made up of alternating layers of Al 0.15 Ga 0.85 N:Si and GaN:Si each with a typical thickness of about 20 Å, is positioned below the GaN n-waveguide layer (not shown in FIG. 1) at the bottom of InGaN multiple quantum well structure  145 . Introduction of short period superlattice n-cladding structure  121  allows increased cladding thickness to significantly reduce leakage of the transverse optical mode and results in an improved transverse far-field pattern for laser diode structure  100 . For example, a typical leakage of about 7% may be reduced to 0.5%. The far-field beam pattern approaches a Gaussian far-field beam.  
         [0015]    P-cladding layer  125 , typically Al 0.07 Ga 0.93 N:Mg, is positioned over the GaN p-waveguide layer (not shown in FIG. 1) which is adjacent to the tunnel barrier layer (not shown in FIG. 1), typically Al 0.2 Ga 0.8 N:Mg, present at the top of InGaN multiple quantum well structure  145 . Layer  185  serves as a capping layer to facilitate ohmic contact. Burying layer  155  is positioned over capping layer  185 , typically GaN:Mg, with windows through burying layer  155  to allow p-electrode  190  to contact GaN:Mg layer  185  and n-electrode  195  to contact GaN:Si layer  115 .  
         [0016]    Burying layer  155 , typically insulating AlN or AlGaN, has a low refractive index which results in strong lateral index guiding because the refractive index step is typically around 0.1. With such a large lateral index step, the lateral waveguiding in index-guided, buried heterostructure AlGaInN laser diode structure  100  overwhelms thermal or carrier injection influences to provide a more stable and less astigmatic beam pattern. Burying layer  155  also has a high bandgap energy which results in high lateral carrier confinement.  
         [0017]    Undoped AlN films are insulating and prevent formation of a shunt path around InGaN multiple quantum well structure  145 . In one embodiment of index-guided, buried heterostructure AlGaInN laser diode structure  100  in accordance with the invention, buried layer  155  is an AlGaN:Mg doped layer as undoped AlGaN may not be insulating depending on growth conditions and the aluminum content. The optoelectronic character of AlGaN depends on growth conditions. For example, it is possible to grow insulating GaN at low temperatures (approximately 900° C), while at higher growth temperatures GaN tends to have an n-type background conductivity. The precise mechanism for the n-conductivity is presumed to arise from either native defects and or impurities. Oxygen and silicon are both commonly encountered shallow, unintentional donors in GaN. Low aluminum-content AlGaN that is magnesium doped behaves similarly to GaN except that the magnesium acceptor&#39;s activation energy increases at the rate of about 3 meV for each percent aluminum added in the alloy up to a 20 percent aluminum content. For aluminum content above 20 percent, unintentional oxygen incorporation may result in uncontrollably high n-type background conductivity. Oxygen is readily incorporated into AlGaN because of the high affinity of aluminum for oxygen and oxygen impurities are typically available from various sources during MOCVD growth. While oxygen impurities may be compensated by magnesium acceptors this is difficult in practice and suggests it would be difficult to make high aluminum content AlGaN burying layers that are insulating. High aluminum content provides better optical and carrier confinement.  
         [0018]    Due to the nature of MOCVD growth where atomic hydrogen is available to form neutral complexes with magnesium acceptors, AlGaN:Mg films are insulating as grown and require thermal annealing to activate p-type conductivity. While an insulating burying layer is typically preferable, activated AlGaN:Mg (having p-type conductivity) is also suitable for burying layers if it is difficult or not possible to deposit an insulating burying layer. When buried layer  155  is p-type, a p-n junction is formed at the interface with short period superlattice n-type cladding layer  121 . However, the turn-on voltage of this p-n junction is greater than the p-n junction in InGaN multiple quantum well structure  145 . This favors the current path preferentially going through InGaN multiple quantum well structure  145 . Because no p-GaN cap is deposited over buried layer  155 , the contact of p-electrode  190  to buried layer  155  is significantly more resistive than the contact of p-electrode  190  to p-GaN:Mg  185 . This further favors current injection into multiple quantum well structure  145 .  
         [0019]    An n-type burying layer may also be used in order to further reduce optical losses because free-carrier loss is lower for n-type material or if it is only possible to grow n-type AlGaN material. FIG. 2 shows index-guided, buried heterostructure AlGaInN laser diode structure  200  with n-burying layer  255  in accordance with an embodiment of the invention. After regrowth of n-burying layer  255 , regrown burying layer is patterned by etching, typically CAIBE. In a second regrowth, n-burying layer  255  is buried with heavily p-doped GaN:Mg layer  250 , having a typical doping level of approximately 10 ° Mg atoms/cm 3 , which also functions as the contact layer. Alternatively, burying layer  255  may also be undoped. P-doped GaN:Mg layer  250  is needed to prevent p-electrode  290  from contacting n-burying layer  255 .  
         [0020]    [0020]FIG. 3 shows self-aligned index-guided, buried heterostructure AlGaInN laser diode structure  300  in accordance with the invention. GaN:Si layer  315  is positioned on Al 2 O 3  growth substrate  310  and in one embodiment layer  315  may be made of AlGaN:Si. Bulk n-cladding layer  320 , typically Al 0.07 Ga 0.93 N:Si, is positioned below the GaN n-waveguide layer (not shown in FIG. 3) at the bottom of InGaN multiple quantum well structure  345  and over n-cladding short period superlattice  321 . N-cladding short period superlattice  321  is typically made up of alternating layers of AlGaN:Si and GaN:Si each with a typical thickness of about 20 Å. Bulk n-cladding  320  prevents injection of carriers from overgrown layer  325 , typically Al 0.07 Ga 0.93 N:Mg to provide optimized transverse waveguiding, into the low bandgap portion of n-cladding short period superlattice  321 . Overgrown layer  325  functions both as the burying layer and as the upper p-cladding layer. Hence, the overall thickness of AlGaN in overgrown layer  325  positioned above GaN p-waveguide layer (not shown in FIG. 3) and the tunnel barrier layer, typically Al 0.2 Ga 0.8 N:Mg, (not shown in FIG. 3) that are located at the top of InGaN multiple quantum well structure  345  is on the order of the thickness used in conventional nitride lasers. Layer  385 , typically GaN:Mg, serves as a capping layer to facilitate ohmic contact to p-electrode  390 . Dashed line  303  shows the location of the p-n junction in laser diode structure  300 .  
         [0021]    Overgrown layer  325  functions as both the p-cladding layer and the burying layer to create both strong lateral current confinement and optical confinement. The strong lateral index guiding (typically an index step on the order of 0.1) provided by overgrown layer  325  allows low threshold current and beam stability. Strong index-guiding allows the laser stripe to be made very narrow which facilitates lateral heat dissipation and lowers the required threshold current. The lateral width of InGaN multiple quantum well structure  345  can be made very narrow because of the strong index guiding, typically less than 2 μm, to provide for a low threshold current and for lateral mode discrimination. Self-aligned index-guided, buried heterostructure AlGaInN laser diode structure  300  shown in FIG. 3 provides a greater p-contact area than index-guided, buried heterostructure AlGaInN laser diode structure  100  shown in FIG. 1. A greater p-contact area results in less contact resistance. Lowering contact resistance reduces laser diode heating particularly in continuous wave operation and a wider p-contact also serves to better dissipate heat. Current preferentially flows through InGaN multiple quantum well structure  345  because the p-n junction bandgap is lowest along that portion of dashed line  303 .  
         [0022]    [0022]FIG. 4 is an expanded view of InGaN multiple quantum well structure  345  in FIG. 3 and shows carrier injection paths  401  and  402  for self-aligned index-guided, buried heterostructure AlGaInN laser diode structure  300 . P-doped waveguide  407 , typically GaN, and n-doped waveguide  408 , typically GaN, are also shown. Dashed line  303  traces the location of the p-n junction. In an embodiment in accordance with the invention operating at a wavelength of about 400 nm, InGaN multiple quantum well structure  345  has a bandgap energy of about 3.1 eV while underlying n-waveguide  408  has a bandgap energy of about 3.4 eV. Hence, the turn-on voltage for the p-n junction associated with InGaN multiple quantum well region  414  is lower than that of the p-n junction associated with n-waveguide  408  and carriers are preferentially injected along injection path  401  into InGaN multiple quantum well region  414  when laser diode  300  is forward-biased.  
         [0023]    The 300 meV difference between the bandgap energy of InGaN multiple quantum well region  414  and n-waveguide  408  may in some cases be insufficient for confining carrier injection to injection path  401  and some carriers may be injected along injection path  402  across the p-n junction at the sidewalls of n-waveguide  408 . Because carriers injected across the p-n junction at the sidewalls of n-waveguide  408  do not populate the quantum wells, these carriers do not contribute to higher optical gain and cause a higher threshold current to be required. Operation at wavelengths higher than about 400 nm such as about 430 nm would increase the bandgap energy differential so that carrier injection across the p-n junction at the sidewalls of n-waveguide  408  is significantly reduced.  
         [0024]    Lateral injection of carriers across the p-n junction at the sidewalls of GaN n-waveguide  408  may be reduced by using an inverted asymmetric waveguide structure as shown in FIG. 5 which eliminates n-waveguide  408 . This eliminates carrier injection along injection path  402  shown in FIG. 4. Tunnel barrier layer  546  lies over InGaN multiple quantum well region  514  and is typically AlGaN with an aluminum content between 5 to 15 percent. P-waveguide  507 , typically GaN, is located over tunnel barrier layer  546 . P-cladding layer  525 , typically AlGaN:Mg, covers p-waveguide  507  and buries entire laser ridge structure  511 . Capping layer  585 , typically GaN:Mg, provides contact to p-contact  590 .  
         [0025]    InGaN multiple quantum well region  514  is positioned on bulk n-cladding layer  520 , typically AlGaN:Si. Bulk n-cladding layer  520  is placed over short period superlattice n-cladding structure  521 , typically made up of alternating layers of AlGaN:Si and GaN:Si with each with a typical thickness of about 20 Å. Bulk n-cladding layer  520  blocks charge carriers from being injected from p-cladding layer  525  into the typically lower bandgap GaN:Si layers of short period superlattice n-cladding structure  521 . Introduction of short period superlattice cladding structure  521  allows cladding layers with the same average aluminum content as bulk n-cladding layer  520 , typically about 8 percent, to be grown to a thickness of more than 1 micron whereas bulk n-cladding layer  520  is usually limited to a typical thickness of about 0.5 μm before cracking occurs. Increased thickness provided by short period superlattice cladding structure  521  significantly reduces leakage of the transverse optical mode and results in an improved transverse far-field pattern for laser diode structure  500 . For example, a typical leakage of about 7% may be reduced to 0.5% The far-field beam pattern approaches a Gaussian far-field beam. N-layer  515 , typically AlGaN:Si or GaN:Si, underlies short period superlattice cladding structure  521  and is placed over substrate  510 , typically Al 2 O 3 .  
         [0026]    Index-guided, buried heterostructure AlGaInN laser diode structure  100  in FIG. 1 may be fabricated by first CAIBE (chemically assisted ion beam etch) etching through layers  185 ,  125 ,  145  and  121  to expose n-type layer  115  for deposition of n-electrode  195 . Growth related to GaN is disclosed in U.S. patent application Ser. No. 09/288,879 entitled “STRUCTURE AND METHOD FOR ASYMMETRIC WAVEGUIDE NITRIDE LASER DIODE” by Van de Walle et al. hereby incorporated by reference in its entirety. A possible issue with p-type material growth is magnesium turn on delay due to Cp 2 Mg sticking to gas lines rather than entering the reactor. Magnesium turn on delay may be compensated for by pre-flowing Cp 2 Mg into the reactor prior to heating and growth. The magnesium is switched to vent during the heatup and then switched back into the reactor without turn on delay when magnesium doping is desired.  
         [0027]    In one embodiment in accordance with the present invention, photoresist is applied to GaN:Mg layer  185  to define the top of ridge structure  111 . However, before applying the photoresist it is advantageous to activate GaN:Mg layer  185 . The activation avoids possible hydrogen evolution during processing which causes bubbling under the photoresist. Activation is typically performed in one of two ways. Normal thermal activation may be used by heating to approximately 850° C. for 5 minutes in a nitrogen environment. Alternatively, GaN:Mg layer  185  may be exposed to intense UV light to release the hydrogen preventing possible thermal degradation of the surface. The photoresist stripe is lithographically patterned with the photoresist stripe aligned along the &lt;1100&gt; crystallographic direction of GaN layer  185 . Subsequently, the stripe is etched to produce a ridge structure  111 , typically having a width from 1 to 5 μm. Ridge structure  111  is formed by CAIBE etching through layers  185 ,  125 ,  145  to short period superlattice n-cladding structure  121 . Note that the length axis of ridge structure  111  is oriented perpendicular to the set of {1100} planes and aligned along the &lt;1100&gt;crystallographic direction due to the orientation of the photoresist stripe prior to etching. This orientation has been found to reduce surface pitting.  
         [0028]    Cleaning is performed prior to epitaxial regrowth and includes photoresist removal using a combination of dissolution in acetone and ashing in an oxygen plasma. Further cleaning is performed using aqua regia then H 2 SO 4 :H 2 O 2 :H 2 O mixed in the ratio 4:1:1, respectively and used as-mixed (hot). A final rinse is performed with de-ionized water followed by drying in pure nitrogen.  
         [0029]    The regrowth occurs at a stabilized temperature of 900° C. in an ammonia/hydrogen gas stream. When the growth temperature has stabilized the reactants trimethylaluminum, trimethylgallium and biscyclopentadienylmagnesium are introduced into the reactor. Insulating overgrowth of burying layer  155  is accomplished by growing an undoped film at low temperature (T growth &lt;900° C.). Epitaxial regrowth of burying layer  155 , typically made of insulating AIN or AlGaN, is performed to surround the ridge structure. Alternatively, a p-doped burying layer  155 , typically AlGaN:Mg may be grown. An opening is etched using CAIBE into burying layer  155  down to p-cap  185  to open up a narrow window for contacting p-cap layer  185  with p-electrode  190 .  
         [0030]    Further processing of index-guided, buried heterostructure AlGaInN laser diode structure  100  involves p-dopant activation by annealing at 850° C. for 5 minutes in a nitrogen ambient. Palladium p-contact metal deposition is evaporatively deposited and alloyed at 535° C. for 5 minutes. Mirror facets (not shown) are formed by cleaving or etching. If mirror facets are etched, the etch of the first mirror facet is performed along with the mesa etch. N-metal deposition is performed of Ti/Al. Finally, n-electrode  195  and p-electrode  190  are deposited and high reflection coatings TiO 2 /SiO 2  are applied to the first and second mirrors.  
         [0031]    Processing for laser structure  200  is similar to that of laser structure  100 . However, in FIG. 2 n-type burying layer  255 , typically AlGaN:Si, is regrown followed by regrowth of p-burying layer  250 , typically GaN:Mg. Additionally, after n-metal deposition takes place, high temperature dielectric deposition, typically of SiN or SiO 2 , is performed over the entire surface using PECVD (plasma enhanced chemical vapor deposition). The deposited dielectric is then patterned to create windows for n-electrode  195  and p-electrode  190 . Patterning is used instead of a photoresist mask because the deposition temperature for the dielectric is approximately 250° C. and photoresist is limited to temperatures below about 120° C. This makes a restricted contact window for p-electrode  290  contacting p-burying layer  250  to avoid current injection outside the laser stripe. Alternatively, ion implantation at energies typically from about 80-120 keV may be used to create the restricted window by masking the window regions and then performing the ion implantation.  
         [0032]    Processing for self-aligned, index guided, buried heterostructure AlGaInN laser diode structures  300  (see FIG. 3) and  500  (see FIG. 5) is similar to that for laser diode structures  100  and  200 . A key difference of the self-aligned, index guided, buried heterostructure AlGaInN laser structures  300  and  500  is that the deposited p-doped layers  325  and  525  serve both as p-cladding layers and as burying layers. Due to the self-aligned structure of laser diodes  300  and  500  there is also no etching through burying layers  325  and  525 , respectively. Note that processing is performed so that the length axis of both ridge structure  311  (see FIG. 3) and ridge structure  511  (see FIG. 5) is aligned along the &lt;1100&gt; crystallographic direction to reduce surface pitting.  
         [0033]    FIGS.  6 - 11  show processing steps for making a laser diode structure similar to self-aligned, index guided, buried heterostructure AlGaInN laser diode structure  300 . Tunnel barrier layer  646 , if desired, lies between multiple quantum well region  345  and p-doped waveguide  407 .  
         [0034]    [0034]FIG. 6 shows the deposited epitaxial structure up to and through p-doped waveguide region  407 . Note that no p-cladding or capping layers are present. FIG.  7  shows the CAIBE etching of trenches  710 , typically about 10 μm wide, surrounding ridge  720  which is typically about 1-2 μm wide. The etching must penetrate into, but not through bulk n-cladding layer  320 . This results in an etch of about 300 nm for a typical thickness of multiple quantum well region  345  and waveguides  407  and  408 . FIG. 8 shows MOCVD growth of p cladding layer  325  to a typical thickness of about 0.5-1.0 μm. P-capping layer  385  is also grown to a typical thickness of about 0.1 μm over the structured surface. The remaining process sequence is similar to that of conventional ridge-waveguide nitride lasers except that the ridge-etch step is not performed.  
         [0035]    [0035]FIG. 9 shows deposition of p-metal layer  390 , typically palladium alloy, at 535° C. for 5 minutes in a nitrogen ambient. FIG. 10 shows CAIBE etching of p-metal layer  390  and CAIBE etching to a depth of about 2 μm to penetrate through n-cladding short period superlattice  321  into GaN:Si layer  315 . This etch exposes the area for n-lateral contact  1101 . The first and second mirrors (not shown) are also CAIBE etched in this step. Liftoff metallization (typically Ti—Al) is performed for n-contact pad  395 . FIG. 10 shows metallization, typically Ti—Au, to build up metal thickness on n-contact  1101  and p-contact  1102 . Finally, SiO 2 /TiO 2  mirror coating evaporation is performed.  
         [0036]    While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.