Patent Publication Number: US-2016240729-A1

Title: Plasmonic light emitting diode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of recently allowed U.S. patent application Ser. No. 13/145,995, filed on Jul. 22, 2011, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Light emitting diodes (LEDs) can convert electrical energy into optical energy for lighting and optical signaling. In general, LEDs are semiconductor diodes, typically containing a p-i-n junction. When an LED is forward biased, a current of electrons from the n-type material of the diode and holes from the p-type material of the diode combine. LEDs generally employ materials that create a suitable energy difference between the conduction band of electrons and the valence band of holes, so that the combination of an electron and a hole can spontaneously emit a photon. The energy difference is generally limited by the available materials but can otherwise be tuned or chosen to produce a desired frequency of light. Additionally, an LED can employ multiple layers of materials with conduction bands of different energies to create a quantum well that tends to confine electrons or holes and enhance the rate of spontaneous emissions, thereby improving energy efficiency of light production. 
     The spontaneous emission rate of a quantum well in an LED is not an intrinsic property of the quantum well, but instead depends on the electromagnetic environment of the quantum well. A plasmonic LED can exploit this phenomenon by positioning a quantum well close to a metal that supports the formation of surface plasmon polariton with electron-plasma oscillations extending into the quantum well. These electron-plasma oscillations or plasmons increase the electron-hole pair recombination rate within the quantum well via the Purcell effect and decrease the delay between a change in the current driving the LED and the corresponding change in the light emitted from the LED. Plasmonic LEDs can emit light with a modulation speed of about 10 GHz or faster while maintaining a radiative efficiency above about 20%, which compares well with the modulation speeds and efficiencies of VCSELs and other semiconductor lasers. International App. No. US/2008/001319, entitled “PLASMON ENHANCED LIGHT-EMITTING DIODES” describes some prior plasmonic LEDs that are fast enough for use in high data rate signaling. 
     One concern in manufacture of plasmonic LEDs is the materials available that are able to support surface plasmons of the proper frequencies for a plasmonic LED. Considering the limitations on the frequency of the emitted light placed by the available materials suitable for LEDs, silver and gold have been found to have surface plasmons with a desirable coupling for improving the response of an LED. Unfortunately, silver and gold, which must be close to a quantum well to provide the desired enhancement, have a tendency to migrate or diffuse in the semiconductor materials used in LEDs, and this diffusion can cause rapid degradation and shorting of the LED. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  schematically illustrate cross-sectional views of plasmonic LEDs in accordance with embodiments of the invention using alternative barrier structures to prevent unwanted diffusion but permit plasmon interactions with quantum wells. 
         FIG. 2  shows a more detailed cross-sectional view of a plasmonic LED in accordance with another embodiment of the invention. 
         FIG. 3  shows a cross-sectional view of a plasmonic LED in accordance with another embodiment of the invention. 
     
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION 
     In accordance with an aspect of the invention, a plasmonic LED can include a barrier between the semiconductor structures and a metal layer (e.g., a silver or gold layer) that supports plasmon oscillations at a frequency that enhances LED performance. In one embodiment, the barrier can be thin (e.g., about 10 nm or less) and include an insulating material such as an oxide and a contact structure of a conductive material such as a non-diffusive metal (e.g., platinum). The barrier being relatively thin and mostly made of a dielectric material allows the surface plasmon oscillations of the metal layer to interact with the quantum well in the LED, but the barrier can still block diffusion or spiking of metals such as silver or gold from the enhancement layer into the semiconductor layers. The patterned contact is an ohmic contact for injection of current into the LED and can be made of a non-diffusive metal such as platinum. Further, the contact can be patterned to improve light extraction, and the contact area can be minimized to ensure light-plasmon interaction between the quantum well and the enhancement layer while still guaranteeing good current injection. In an alternative embodiment, the barrier can be even thinner (e.g., about 2 nm) and made of non-diffusive conductive material such as platinum that blocks diffusion or spiking a metal such as silver or gold from the metal layer. While the barrier metal may have poor plasmon characteristics for enhancement of spontaneous emissions in the quantum well, the barrier being sufficiently thin still allows interactions of the desired surface plasmons in the metal layer with the quantum well. 
       FIG. 1A  shows a schematic representation of a cross-section of a plasmonic LED  100  in accordance with an embodiment of the invention. LED  100  has a p-i-n structure, which broadly includes a p-type structure  110 , an intrinsic structure  120 , and an n-type structure  130 . Intrinsic structure  120  is generally a multi-layer structure that includes a quantum well, which is a source of the light (i.e.., photons) produced by spontaneous emissions when electrons injected from n-type structure  130  combine with holes injected from p-type structure  110 . An enhancement structure  140  contains a layer  142  of material that supports surface plasmon oscillations having a frequency that enhances the rate of spontaneous emissions from the quantum well. Layer  142  may be a blanket layer or may be patterned or roughened if desired to alter properties of the plasmons in layer  142 . In general, greater enhancements can be achieved by placing enhancement structure  140  (particularly layer  142  since contacts  146  may have poor plasmonic properties) nearer to the quantum well so that the effects of plasmon oscillations extend into the quantum well. Enhancement layer  142  typically needs to be less than about 50 nm from the quantum well for significant enhancement of spontaneous emissions at photon wavelengths around 800 nm. The separation may be greater in LEDs producing longer wavelength light. In  FIG. 1A , enhancement structure  140  is adjacent to n-type structure but may be better placed adjacent to p-type structure  110  in embodiments where p-type structure  110  is thinner than n-type structure  130 . 
     Layer  142  in enhancement structure  140  may be made of a metal such as pure or alloyed silver or gold but other metals might be suitable. Diffusion or spiking of metal atoms from layer  142  into the semiconductor structure is an issue, particularly because layer  142  needs to be close to the quantum well to enhance spontaneous emissions. For example, it has been observed that GaAs dissolves readily into gold and gold based alloys. This dissolution results in equal amounts of galliumn (Ga) and arsenic (As) entering into the gold lattice. Arsenic has been shown to be able to pass easily through the gold lattice and evaporate from the free surface of the gold. It is likely that atoms of such materials enter the metallization along grain boundaries or other such imperfections, although it is possible that the diffusion may enter as a very low concentration of highly mobile interstitial atoms. This phenomenon is also observed for the InGaP contact layer used in other LEDs. 
     To prevent diffusion from layer  142  into adjacent semiconductor layers, LED  100  includes an insulating barrier layer  144  containing a patterned conductive contact  146  that electrically connects layer  142  and n-type structure  130 . Barrier layer  144  and patterned contact  146  can be less than about 10 nm thick and are preferably about 5 nm. In general, barrier layer  144  and contact  146  can be as thin as possible provided that barrier layer  144  and contact  146  sufficiently block diffusion from layer  142 . 
     LED  100  can be operated by applying an appropriate voltage in a forward bias direction across LED  100 . For example, for the p-i-n architecture of  FIG. 1A , an electrical signal having positive polarity can be applied to layer  142  of LED  100  while layer  110  is connected to a reference voltage or ground. Electrical signals would generally be applied to LED  100  through a contact structure (not shown in  FIG. 1A ). The relatively negative voltage on n-type structure  110  can be thought of as driving electrons toward the quantum well in intrinsic structure  120 , and the relatively positive voltage on layer  142  can be thought of as driving holes toward the quantum well. The quantum well may be made of a direct bandgap semiconductor material having an electronic bandgap energy that is smaller than the electronic bandgaps of the remaining layers of the LED  100 . When the applied voltage difference is large enough to inject electrons from n-type structure  130  and holes from p-type structure  110  into the quantum well, spontaneous emissions resulting from combination of electrons and holes in the quantum well generate light that can be output from the LED  100  through p-type structure  110 , that is opposite from enhancement structure  140 . 
     The enhancement that structure  140  achieves can be understood by treating the combining of electrons and holes as decays of electron-hole dipoles. In general, the spontaneous emission rate of a decaying dipole depends not only on the strength of the dipole, but also on the electromagnetic environment of the dipole. By changing the electromagnetic environment near a dipole, the spontaneous decay rate of the dipole can be tuned (i.e., suppressed or enhanced), which is called the “Purcell effect.” In the present case, introducing enhancement structure  140 , which supports plasmon oscillations that couple to desired frequencies of light, enhances the rate at which the electron-hole dipoles decay into the desired electromagnetic mode or frequency. The Purcell factor F P  quantifies the enhancement and is given by: 
     
       
         
           
             
               F 
               p 
             
             = 
             
               
                 Spontaneous 
                  
                 
                     
                 
                  
                 Emission 
                  
                 
                     
                 
                  
                 rate 
                  
                 
                     
                 
                  
                 in 
                  
                 
                     
                 
                  
                 complex 
                  
                 
                     
                 
                  
                 environment 
               
               
                 Spontaneous 
                  
                 
                     
                 
                  
                 Emission 
                  
                 
                     
                 
                  
                 rate 
                  
                 
                     
                 
                  
                 in 
                  
                 
                     
                 
                  
                 bulk 
                  
                 
                     
                 
                  
                 material 
               
             
           
         
       
     
     where the complex environment refers to the quantum well with adjacent enhancement structure  140 , and the bulk material refers to the surrounding material, such as n-type and p-type structures  130  and  110 , without enhancement structure  140 . The larger the Purcell factor, the faster the spontaneous emission rate. 
       FIG. 1B  illustrates an LED  150  using an enhancement structure  145  with a barrier layer  148  in accordance with an alternative embodiment of the invention. LED  150  includes a p-type structure  110 , an intrinsic structure  120 , an n-type structure  130 , and a metal layer  142 , which can be identical to the corresponding structures in LED  100  of  FIG. 1A . LED  150  differs from LED  100  in that barrier  148  is a very thin (less than 5 nm) layer of a non-diffusive metal such as platinum between the thicker metal (e.g., Ag or Au) layer  142  and the underlying semiconductor structure. Barrier layer  148  may have poor plasmon properties for enhancement of spontaneous emission, but layer  148  is thin enough that the surface plasmon enhancement of the combination of layers  148  and  142  can still be as efficient as that of a single thick layer  142 . In particular, a combination Pt/Au layer when the Pt portion is thin enough (e.g., less than 5 nm) can still be as efficient as a single Au layer at enhancing spontaneous emissions. Further, if barrier layer  148  is a platinum layer as thin as 2 to 3 nm, barrier layer  148  can still prevent unwanted diffusion between layer metal layer  142  and underlying semiconductor structures. Barrier layer  148  being conductive also has the advantage of providing a low resistance connection between layer  142  and the underlying semiconductor structure. 
       FIG. 2  shows an LED  200  in accordance with a specific embodiment of the invention that produces light having a wavelength of about 800 nm. LED  200  includes a gallium arsenide (GaAs) substrate  250 , a multi-layer n-type structure  130  on substrate  250 , a multi-layer intrinsic structure  120  on n-type structure  130 , a multi-layer p-type structure  110  on intrinsic structure  120 , and an enhancement structure  140  on p-type structure  110 . The description of LED  200  below provides details of one specific embodiment of the invention. However, as will be understood by those in the art, the details regarding specific structural parameters such as materials, dopants, doping concentrations, the number of layers, the order of layers, and layer thicknesses are subject to variations in different embodiments of LEDs. 
     The n-type structure  130 , which can be deposited or grown on substrate  250 , includes five layers  232 ,  234 ,  235 ,  236 , and  238  in the illustrated embodiment of  FIG. 2 . The bottom layer  232  is an n-type layer of indium-gallium-phosphorus (InGaP) about 20 nm thick and is doped with silicon (Si) to a concentration of about 2×10 18  cm −3 . The next three layers  234 ,  235 , and  236  are mixtures of aluminum (Al), gallium (Ga), and arsenic (As). Layer  234 , which is on layer  232 , is Al .35 Ga .65 As about 300 nm thick and doped with silicon to a concentration of about 2×10 18  cm −3 . Layer  236 , which is on layer  235 , is Al .65 Ga .35 As about 500 nm thick and doped with silicon to a concentration of about 5×10 17  cm −3 . Layer  235 , which is between layers  234  and  236 , is a graded layer that is an Al x Ga 1-x As mixture in which x ranges from 0.35 to 0.65 so that the composition of layer  235  transitions smoothly from the composition of layer  234  to the composition of layer  236 . Graded layer  235  is about 15 nm thick and doped with silicon to a concentration of about 2×10 18  cm −3 . The top n-type layer  238  is another graded layer of Al x Ga 1-x As about 15 nm thick, where x ranges from 0.65 to 0.35 so that layer  238  transitions smoothly from the composition of layer  236  to the composition of an overlying layer  222 . The compositionally graded semiconductor layers  235  and  238  have the electronic bandgaps that vary with position and can be produced by changing the composition or ratios of the constituents used during a deposition process. The graded layers are used to improve the current flow by minimizing junction discontinuities and thereby reducing the series resistance between the semiconductor layers. 
     Intrinsic structure  120  includes three layers  222 ,  225 , and  228  to create a quantum well with a bandgap structure that produces photons with the desired wavelength of about 800 nm. In the illustrated embodiment, bottom layer  222  is an undoped or intrinsic mixture of Al .35 Ga .65 As and about 80 nm thick. Layer  225  is a mixture GaAs .885 P .115  that is about 10 nm thick, and layer  228  is another layer of undoped Al .35 Ga .65 As but is about 10 nm thick. The bandgaps of layer  222 ,  225 , and  228  are such that layer  225  corresponds to a quantum well. Further, quantum well layer  225  has tensile strain of about +0.42% which results because of the thickness of layer  225  and the difference in the lattice constant of quantum well layer  225  and layers  222  and  228 . 
     The p-type structure  110  includes three layers  212 ,  214 , and  216  in the embodiment of  FIG. 2 . Layer  212  is the same mixture Al .35 Ga .8 As as intrinsic layer  228  but is about 40 nm thick and is p-type with a doping of carbon at a concentration of about 1×10 18  cm −3 . Layer  214  is Al .2 Ga .8 As that is about 7 nm thick and doped with a dopant such as carbon to a concentration of about 1×10 18  cm −3 . Layer  216  is p-type InGaP that is about 3 nm thick and doped with zinc to a concentration of about 1×10 18  cm −3 . In general, to maximize the Purcell factor, p-type structure  110  is as thin as possible to minimize the separation between overlying enhancement structure  140  and the quantum well in intrinsic structure  120 . 
     Enhancement structure  140  can have substantially the same structure as described above in regard to  FIG. 1A . In particular, enhancement structure  140  includes a barrier layer  144  made of an insulating material such as silicon dioxide or more preferably a high refractive index insulator such as titanium dioxide, which more closely matches the refractive indices of adjacent semiconductor structures. Barrier layer  144  is preferably less than about 10 nm thick. Contact  146  is made of a conductive material such as a non-diffusive metal and has a pattern with openings that permit interaction of optical modes of the quantum well with surface plasmons at the interface between layer  142  and barrier  144 . Contact  146  can be made of a material having poor plasmon properties for enhancement of spontaneous emissions from the quantum well and accordingly may block the desired plasmon interactions in the areas of contacts  146 . Ideally, the area occupied by contact  146  is kept to a minimum since contact  146  contributes little to the surface plasmon enhancement. Making contacts smaller may therefore improve enhancement of spontaneous emissions but may also increase the resistance to currents driven through LED  200 . The area of contact  146  can be chosen to balance concerns for enhancement of spontaneous emissions and diode resistance. Alternatively, enhancement structure  140  can be replaced with the enhancement structure  145  of  FIG. 1B , which provides a low resistance contact between metal layer  142  and underlying semiconductor structures. 
       FIG. 3  illustrates a plasmonic LED  300  that includes external electrodes  310  and  360 . LED  300  includes a p-type structure  110  and an intrinsic structure  120 , which can be of the type described above. An n-type structure  330  of LED  300  can include layers  234 ,  235 ,  236 , and  238  of  FIG. 2 . Layer  232  acts as an etch stop layer for a process that etches through substrate  250  ( FIG. 2 ) to leave a region  350  ( FIG. 3 ) surrounding the light emitting area of LED  300 . Electrode  360  is on the remaining region  350  of the substrate and can be made of any suitable composition and may, for example, include a titanium adhesion layer and a gold contact layer. A transparent conductor such as indium tin oxide could alternatively or additionally be employed over the light emitting area of LED  300 . 
     An enhancement structure of LED  300  includes a layer  142  of material such as AgZn, or Pt/AgZn with a very thin (&lt;5 nm) Pt diffusion barrier, which can support surface plasmons with a strong coupling to photons produced by spontaneous emissions in the quantum well. This layer  142  can be deposited by standard techniques such as e-beam deposition or sputtering. Layer  142  is electrically connected to contact  310 . Barrier layer  144  and contact  146  are between layer  142  and p-type structure  110  in the embodiment of  FIG. 3 . The active area of LED  300  can be defined by an oxygen implant into an outer portion of the semiconductor structure to create insulating oxide regions  340  that surround the active region through which drive current is channeled. Alternatively, a mesa structure can be formed by etching past the quantum well into the bottom n-type AlGaAs layer. The active region of LED  300  for high data rate signaling would typically have a width or diameter of about 10 to 50 μm because larger areas tend to increase capacitance and cause signal delays. An insulating layer  320  of a material such as polyimide can also be deposited to better confine the drive current through electrode  310  to the active area of LED  300 . 
     LED  300  can be operated by applying a positive polarity electrical signal, which may have a high frequency modulation for data transmissions, to electrode  310 . Electrical current then flows from electrode  310 , through layer  142  and contacts  146  into p-type structure  110 , and p-type structure  110  injects holes into (i.e., empties electron valence states in) intrinsic structure  120 . The drive current also corresponds to electrons flowing from electrode  360 , through region  350 , layer  232 , and n-type structure  330  into intrinsic structure  120 , where conduction electrons fall into emptied valence states, causing spontaneous emission of photons. The availability of plasmon oscillations in layer  142  enhances spontaneous emissions into the desired electromagnetic mode. 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.