Light emitting device with an omnidirectional photonic crystal

Abstract

A light emitting device includes a light emitting diode and an omnidirectional photonic crystal formed on one of an upper outer surface and a lower outer surface of the light emitting diode and exhibiting a periodic variation in dielectric constant in such a manner so as to introduce an omnidirectional photonic band gap in a given frequency range such that the radiation generated by the light emitting diode in the frequency range for all incident angles and polarizations can be totally reflected by the omnidirectional photonic crystal, and that at least a portion of the radiation with frequencies outside the frequency range can pass through the omnidirectional photonic crystal.

1 . A light emitting device comprising: 
 a light emitting diode that defines an upper outer surface and a lower outer surface opposite to said upper outer surface; and    an omnidirectional photonic crystal formed on one of said upper outer surface and said lower outer surface of said light emitting diode and exhibiting a periodic variation in dielectric constant in such a manner so as to introduce an omnidirectional photonic band gap in a given frequency range such that the radiation generated by said light emitting diode in said frequency range for all incident angles and polarizations can be totally reflected by said omnidirectional photonic crystal, thereby enhancing radiation extraction from the other of said upper outer surface and said lower outer surface of said emitting diode, and that at least a portion of the radiation with frequencies outside said frequency range can pass through said omnidirectional photonic crystal.    
   
   
       2 . The light emitting device of  claim 1 , wherein said light emitting diode includes first and second semiconductor layers, an active layer sandwiched between said first and second semiconductor layers, and a substrate, said first and second semiconductor layers cooperatively defining a p-n junction therebetween, said first semiconductor layer being formed on said substrate, said substrate having an outer surface that is opposite to said first semiconductor layer and that defines said one of said upper outer surface and said lower outer surface of said light emitting diode.  
   
   
       3 . The light emitting device of  claim 2 , wherein said omnidirectional photonic crystal includes periodically stacked dielectric units, each of which includes first and second dielectric layers that have a refractive index difference greater than 0.58.  
   
   
       4 . The light emitting device of  claim 3 , wherein said omnidirectional photonic crystal defines a lattice constant a that is equal the total thickness of each of said dielectric units, said first dielectric layer having a thickness ranging from 0.24a to 0.69a and said refractive index difference ranging from 0.9 to 1.2 so as to obtain said omnidirectional photonic band gap between said frequency range ranging from 0.27c/a to 0.31c/a, wherein c is the speed of light.  
   
   
       5 . The light emitting device of  claim 4 , wherein said first dielectric layer is made from a compound selected from the group consisting of TiO 2 , Ta 2 O 5 , ZrO 2 , ZnO, Nd 2 O 3 , Nb 2 O 5 , In 2 O 3 , SnO 2 , Sb 2 O 3 , HfO 2 , CeO 2 , and ZnS, and said second dielectric layer is made from a compound selected from the group consisting of SiO 2 , Al 2 O 3 , MgO, La 2 O 3 , Yb 2 O 3 , Y 2 O 3 , Sc 2 O 3 , WO 3 , LiF, NaF, MgF 2 , CaF 2 , SrF 2 , BaF 2 , AlF 3 , LaF 3 , NdF 3 , YF 3 , and CeF 3 .  
   
   
       6 . The light emitting device of  claim 5 , wherein said first dielectric layer is made from TiO 2 , and said second dielectric layer is made from SiO 2 .  
   
   
       7 . The light emitting device of  claim 1 , wherein said light emitting diode includes first and second semiconductor layers, an active layer sandwiched between said first and second semiconductor layers, and a transparent substrate, said first and second semiconductor layers cooperatively defining a p-n junction therebetween, said first semiconductor layer being formed on said substrate, said second semiconductor layer having an outer surface that is opposite to said active layer and that defines said one of said upper outer surface and said lower outer surface of said light emitting diode.  
   
   
       8 . The light emitting device of  claim 7 , further comprising a thermal conductor that is connected to said first and second semiconductor layers for heat dissipation.