Electroluminescent device comprising porous silicon

Abstract

An electroluminescent device ( 10 ) comprises a porous silicon region ( 22 ) adjacent a bulk silicon region ( 20 ), together with a top electrical contact ( 24 ) of transparent indium tin oxide and a bottom electrical contact ( 26 ) of aluminium. The device includes a heavily doped region ( 28 ) to provide an ohmic contact. The porous silicon region (22) is fabricated by anodizing through an ion-implanted surface layer of the bulk silicon. The silicon remains unannealed between the ion-implantation and anodization stages. The device ( 10 ) has a rectifying p-n junction within the porous silicon region ( 22 ).

1 . An electroluminescent device ( 10 ) biasable to produce electroluminescence and comprising an electroluminescent porous silicon region ( 22 ) and electrical connections to the porous silicon region ( 24 ,  26 ,  28 ,  20 ), characterized in that electroluminescence from the porous silicon region is detectable when the device is biased such that a current having a current density of less than 1.0 Am −2  flows through the device ( 10 ).  
   
   
       2 . An electroluminescent device according to  claim 1 , characterized in that electroluminescence is detectable when the device is biased such that a current having a current density of less than 0. 1 Am −2  flows through the device.  
   
   
       3 . An electroluminescent device according to  claim 2 , characterized in that electroluminescence is detectable when the device is biased such that a current having a current density of less than 0.01 Am −2  flows through the device.  
   
   
       4 . An electroluminescent device according to  claim 3 , characterized in that electroluminescence is detectable when the device is biased such that a current having a current density of less than 0.0001 Am −2  flows through the device.  
   
   
       5 . An electroluminescent device according to claims  1 , characterized in that the device is biasable to produce electroluminescence with an external quantum efficiency greater than 0.1%.  
   
   
       6 . An electroluminescent device according to  claim 1 , characterized in that the device is biasable to produce electroluminescence with an external quantum efficiency of at least 0.4%.  
   
   
       7 . A solid state electroluminescent device ( 10 ) comprising an electroluminescent porous silicon region ( 22 ) and electrical connections to the porous silicon region ( 24 ,  26 ,  28 ,  20 ), characterized in that the device is biasable to produce electroluminescence from the porous silicon region with an external quantum efficiency greater than 0.01%.  
   
   
       8 . An electroluminescent device according to  claim 7 , characterized in that the device is biasable to produce electroluminescence with an external quantum efficiency greater than 0.1%.  
   
   
       9 . An electroluminescent device according to  claim 7 , characterized in that the device is biasable to produce electroluminescence with an external quantum efficiency in the range 0.01% to 0.18%.  
   
   
       10 . An electroluminescent device according to  claim 7 , characterized in that the device is biasable to produce electroluminescence with an external quantum efficiency of at least 0.4%.  
   
   
       11 . An electroluminescent device according to  claim 7  comprising a luminescent porous silicon region and electrical connections to the porous silicon region, characterized in that the porous silicon region incorporates a p-type porous silicon region ( 930 ,  950 ) and an n-type porous silicon region ( 960 ) having a p-n junction ( 970 ) therebetween.  
   
   
       12 . An electroluminescent device according to  claim 11 , characterized in that the device comprises: 
 i) an n-type bulk silicon region ( 910 ),    ii) an n-type porous silicon region adjacent the n-type bulk silicon region ( 960 ),    iii) a p-type porous silicon region ( 930 ,  950 ) adjacent the n-type porous silicon region, and    iv) electrical contacts ( 920 ,  26 ,  28 ) to the bulk silicon region and the p-type porous silicon region.    
   
   
       13 . An electroluminescent device according to  claim 1  or  claim 7  characterized in that the device is operable to produce a modulated light output, modulatable at a frequency of 1 MHz.  
   
   
       14 . An electroluminescent device according to  claim 1  or  claim 7  characterized in that the device is encapsulated.  
   
   
       15 . A method of fabricating an electroluminescent device including the steps of: 
 a) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity;    b) anodizing the wafer under illumination to produce a luminescent porous silicon region extending through the surface region; and    c) depositing an electrode on the porous silicon region;    characterized in that the surface region has a sheet resistivity greater than 100 Ω□ −1  immediately prior to the anodizing step.    
   
   
       16 . A method of fabricating an electroluminescent device including the steps of: 
 a) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity;    b) anodizing the wafer under illumination to produce a luminescent porous silicon region extending through the surface region; and    c) depositing an electrode on the porous silicon region;    characterized in that less than 1% of the acceptor impurity is electrically active prior to the anodizing step.    
   
   
       17 . A method of fabricating an electroluminescent device including the steps of: 
 a) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity and at least a part of the region has an acceptor impurity volume concentration comparable with the solid solubility limit of the acceptor impurity in silicon;    b) anodizing the wafer under illumination to produce a porous silicon region extending through the surface region; and    c) depositing an electrode on the porous silicon region.    
   
   
       18 . A method of fabricating an electroluminescent device including the steps of: 
 a) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity;    b) anodizing the wafer under illumination to produce a luminescent porous silicon region extending through the surface region; and    c) depositing an electrode on the porous silicon region;    characterized in that the silicon wafer does not receive an anneal between steps (a) and (b).    
   
   
       19 . A method of fabricating an electroluminescent device including the steps of: 
 a) implanting a surface region of a silicon wafer, doped with a donor impurity to render the wafer n-type, with an acceptor impurity such that the surface region has a volume concentration of the acceptor impurity which is greater than a volume concentration of the donor impurity;    b) anodizing the wafer under illumination to produce a luminescent porous silicon region extending through the surface region; and    c) depositing an electrode on the porous silicon region;    characterized in that the anodization step causes surface doping of silicon quantum wires within the porous silicon region, rendering the surface doped quantum wires p-type.    
   
   
       20 . P-type porous silicon, characterised in that the porous silicon has a porosity greater than 30%.  
   
   
       21 . Substantially wholly microporous visibly luminescent porous silicon, characterized in that the porous silicon is derived from n-type bulk silicon.  
   
   
       22 . An electroluminescent device comprising a porous silicon region and electrical connections to the porous silicon region, characterized in that the porous silicon region contains a p-n junction therein.  
   
   
       23 . An electroluminescent device comprising a porous silicon region and electrical connections to the porous silicon region, characterized in that the porous silicon region is a wholly microporous visibly luminescent region fabricated from n-type bulk silicon.