Abstract:
Purely gain-coupled diffraction gratings may be realized for use in QCLs and other edge emitting lasers that lack a typical p-n junction. The periodic, typically heavily n-doped regions of doped diffraction gratings are replaced with p-type regions having significantly lower doping.

Description:
BACKGROUND  
       [0001]     Quantum cascade lasers (QCLS) use electronic intersubband transitions for lasing action in semiconductor superlattices. For light to be either strongly emitted or absorbed by intersubband transitions, the electric field of the light is typically perpendicular to the epitaxial layers and transverse magnetic (TM) polarized light is predominantly absorbed or emitted by intersubband transitions in quantum wells.  
         [0002]     Plasmon-waveguide structures have been introduced for transverse-mode confinement in QCLs because of the impracticality of growing cladding layers sufficiently thick to contain the long evanescent tail of the transverse mode present at the longer emission wavelengths of intersubband semiconductor lasers such as QCLs. Plasmon-waveguide structures provide optical confinement by significantly lowering the refractive index of the cladding layers by use of high doping to increase the refractive index contrast. When the doping level is sufficiently high, the plasma frequency of the semiconductor approaches the QCL emission frequency so that the optical character of the semiconductor becomes more metal-like with a complex refractive index, n+ik, a small real component, n, and a large imaginary component, k. Adjusting the doping and thickness of the plasmon-waveguide structures allows the modal loss and the overlap with the quantum cascade to be optimized.  
         [0003]     The requirements for doping in the visible and near-infrared wavelengths for plasmon confinement are typically too high to be practicable. However, at the longer, mid and far infrared (IR) wavelengths typically associated with QCLs, doping levels on the order of about 10 18 /cm 3  are sufficient to reduce the refractive index of the cladding layers at the operational wavelength of the QCL to provide transverse-mode confinement. This approach has been explored in U.S. patent application Ser. No. 11/076599 entitled “Quantum Cascade Laser with Grating Formed by a Periodic Variation in Doping” incorporated herein by reference.  
       SUMMARY OF THE INVENTION  
       [0004]     In accordance with the invention, purely gain-coupled diffraction gratings may be realized for use in QCLs and other edge emitting lasers that lack a typical p-n junction. The periodic, typically heavily n-doped regions of doped diffraction gratings are replaced with p-type regions having significantly lower doping. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  shows an embodiment in accordance with the invention.  
         [0006]      FIG. 2   a  shows an embodiment in accordance with the invention.  
         [0007]      FIG. 2   b  shows an embodiment in accordance with the invention.  
         [0008]      FIG. 3   a  shows a side view of an embodiment in accordance with the invention.  
         [0009]      FIG. 3   b  shows a side view of an embodiment in accordance with the invention.  
         [0010]      FIG. 3   c  shows a top view of an embodiment in accordance with the invention.  
         [0011]      FIGS. 4   a - e  show steps for making an embodiment in accordance with the invention.  
         [0012]      FIGS. 5   a - d  show steps for making an embodiment in accordance with the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 1  shows the partial structure of an embodiment in accordance with the invention. QCL structure  100  has purely gain-coupled grating  125 . Periodic p-type regions  115  with acceptor doping replace periodic, heavily n-type doped regions. In periodic p-type regions  115  of gain-coupled grating  125 , the acceptor doping should be large enough to block current flow. The acceptor doping depends on the thickness of the dimensions of p-type regions  115  and the donor concentration of surrounding n-type upper cladding layer  116 , typically n-InP. For a thickness of about 0.5 μm and a typical n-doping concentration in the range of about 1×10 16 /cm 3  to about 5×10 7 /cm 3 , an acceptor doping typically in the range from about 10 16 /cm 3  to about 10 17 /cm 3  is typically sufficient. Hence, p-doping levels need not be so large as to create a large change in the real part of the refractive index and contribute significant loss.  
         [0014]     Because p-type regions  115  block current flow, the current is no longer uniformly distributed along the length of QCL active region  118 . The local current density varies periodically, with high current density  114  in the regions between p-type regions  115  and lower current density beneath p-type regions  115 . As long as the periodic variation in current density is preserved to some extent in active region  118 , a corresponding periodic variation is imposed on the gain of QCL structure  100 . Under p-type regions  115 , the striped current density pattern will be subject to spreading because of lateral electron diffusion. Lower cladding layer  117  lies below QCL active region  118 .  
         [0015]     The characteristic length that determines the range over which the current distribution remains inhomogeneous is typically the electron diffusion length. Charge distribution inhomogeneities occurring on a spatial dimension significantly less than the electron diffusion length typically do not persist and dissipate instead. Therefore, to maintain the periodically-varying current density in active region, gain-coupled grating  125  should typically be placed as close as possible to active region  118  and limiting the p-doping concentration in p-type regions  115  to minimize loss. Because the electron diffusion length is typically on the order of about 1 μm and the 1 st  order grating period for QCL structure  100  is also on the order of about 1 μm, it is possible to preserve an inhomogeneous current distribution in a significant portion of active region  118 . This results in a periodic axial variation in the gain of QCL structure  100  to create gain-coupled grating  125 . This provides stable single mode operation for QCL structure  100 .  
         [0016]     Because gain-coupled grating  125  has the opposite polarity of surrounding upper cladding layer  116 , depletion regions  230  (see  FIG. 2   a ) surround each p-type region  115 . Conduction electrons are depleted from depletion regions  230 . Carrier depletion further limits current flow between periodic p-type regions  115 . However, the size of depletion regions  230  can typically be controlled by applying an electrical bias to p-type regions  115 . For example, by reverse biasing periodic p-type regions  115  with respect n-type upper cladding layer  116 , depletion regions  230  expand into depletion regions  230 ′ as shown in  FIG. 2   b . Hence, a bias applied to gain-coupled grating  125  is analogous to a gate contact of a field effect transistor. The control the bias provides over the conductivity of n-type upper cladding layer and the associated current distribution provides a way to modulate the output of QCL structure  100 .  FIG. 2   b  shows the case where an applied reverse bias is sufficient to pinch off the current flow between p-type regions  115  which quenches the output from QCL structure  100 .  
         [0017]      FIGS. 3   a - c  show three views of QCL structure  300  including the electrical contact scheme in accordance with the invention. Electrical contact  301 , typically gold, is the top laser contact while electrical contact  302 , typically gold, is the bottom laser contact applied to substrate  320 , typically n-InP. By biasing electrical contact  301  with respect to electrical contact  302 , QCL structure  300  may be operated conventionally. However as shown  FIG. 3   b , electrical contact  303 , typically gold, may be formed by etching down to gain-coupled grating  325  and applying a p-ohmic metal.  FIG. 3   c  shows the top view of QCL structure  300 . If electrical contact  303  is reverse-biased with respect to electrical contact  301 , depletion regions  230  increase in size as shown in  FIGS. 2   a - b . This allows QCL structure  300  to be modulated in two ways.  
         [0018]     First, expanded depletion regions  230 ′ reduce the area of the conduction path thereby increasing the resistance of upper cladding layer  316  so the QCL current is reduced for a given voltage bias applied between electrical contacts  301  and  302 . Second, the current distribution is altered by the narrowing of the conduction path. Consequently, because the periodically varying current distribution generates gain-coupled grating  325 , a corresponding change in the grating strength occurs as the conduction path is narrowed. This results in a change of the distributed feedback QCL output.  
         [0019]     For example, if p-type regions  315  are placed within about a few hundred nanometers to active region  318 , depletion regions  230  and  230 ′ penetrate into active region  318 . This allows a relatively large change in gain coupled grating strength to be achieved by modulation of electrical contact  303 . In accordance with the invention, p-type regions  315  may extend into active region  318  to enhance gain-coupling and modulation response.  
         [0020]      FIGS. 4   a - e  show a fabrication sequence for QCL structure  400  in accordance with the invention.  FIG. 4   a  shows QCL structure  400  grown by metalorganic chemical vapor deposition (MOCVD) or by molecular beam epitaxy (MBE) on n-type InP substrate  320  including n-type InP lower cladding layer  317 , AlInAs—GaInAs quantum cascade active region  318  and n-type InP upper cladding layer  316 . Then, dielectric mask  355 , typically SiO 2  or Si 3 N 4 , is patterned into dielectric mask stripes  350  on the surface of upper cladding layer  316 .  
         [0021]      FIG. 4   b  shows dry or wet etching of periodically spaced grooves  370  into upper cladding layer  316 , reproducing the pattern defined by dielectric mask stripes  350 . The etch of periodically space grooves  370  is typically stopped near or into AlInAs—GaInAs quantum cascade active region  318 .  
         [0022]      FIG. 4   c  shows the selective growth of p-type regions  315  typically p-type InP, in periodically spaced grooves  370 . If QCL structure  400  is to have three electrical contacts with an electrically addressable grating as shown in  FIGS. 3   a - c , provisions for electrical contact  303  are incorporated in the masking step.  
         [0023]      FIG. 4   d  shows removal of dielectric mask stripes  350  and  FIG. 4   e  shows overgrowth of remaining n-type upper cladding layer  316 , typically n-InP.  
         [0024]      FIGS. 5   a - d  show a fabrication sequence for QCL structure  500  in accordance with the invention. Here p-type grating layer  515  is deposited over AlInAs—GaInAs quantum cascade active region  318  in the first MOCVD or MBE epitaxy. No selective overgrowth is required because p-type grating layer  515  is subsequently patterned by dry or wet etching.  FIG. 5   a  shows QCL structure  500  grown on n-type InP substrate  320  including n-type InP lower cladding layer  317 , AlInAs—GaInAs quantum cascade active region  318  and p-type grating layer  515  over AlInAs—GaInAs quantum cascade active region  318 , alternatively, a portion of n-type InP upper cladding layer  516  may be grown over AlInAs—GaInAs quantum cascade active region  318  before deposition of p-type grating layer  515 . Then dielectric mask  555 , typically SiO 2  or Si 3 N 4 , is patterned into dielectric mask stripes  550  on the surface of p-type grating layer  515 .  
         [0025]     In  FIG. 5   b , unmasked portions of p-type grating layer  515  are then etched down to active layer  318  or into n-type upper cladding creating grooves  570 . In FIG.  5   c , dielectric stripes  550  are removed. In  FIG. 5   d , overgrowth of the rest of n-type upper cladding layer  516  is completed.