Phosphorus doped diamond electrode with tunable low work function for emitter and collector applications

Abstract

An apparatus includes an emitter electrode including a phosphorus doped diamond layer with low work function. The apparatus further includes a collector electrode and a vacuum gap disposed between the emitter and the collector. The collector has a work function of 0.84 eV or less.

What is claimed is: 
     
         1 . A method for forming an electrode, comprising:
 loading a substrate into a plasma enhanced chemical vapor deposition (PECVD) reactor and preparing a surface of the substrate for doped diamond growth;   exposing the substrate to a pure hydrogen plasma for a preset duration; and   depositing a phosphorus doped diamond layer on the substrate to form the electrode,   wherein the electrode comprises a collector that has a work function of 0.84 eV or less.   
     
     
         2 . The method of  claim 1 , wherein the wet-chemical cleaning procedure comprises:
 boiling the substrate in H2SO4/H2O2/H2O at 220° C. for at least fifteen minutes;   treating the substrate for at least five minutes using a solution comprising hydrofluoric acid; and   boiling the substrate in NH4OH/H2O2/H2O at 75° C. for at least fifteen minutes.   
     
     
         3 . The method of  claim 2 , wherein the wet-chemical cleaning procedure further comprises:
 rinsing the substrate with deionized water after each step.   
     
     
         4 . The method of  claim 1 , wherein exposing the substrate to the pure hydrogen plasma for the preset duration comprises:
 exposing the substrate to the pure hydrogen plasma at a preset temperature range for at least fifteen minutes.   
     
     
         5 . The method of  claim 4 , wherein the preset temperature range comprises a low temperature not greater than 800° C. and a high temperature of at least 900° C. 
     
     
         6 . The method of  claim 1 , wherein depositing the phosphorus doped diamond layer on the substrate comprises:
 depositing the phosphorus doped diamond layer using a methane flow rate of 2 sccm and a 200 ppm trimethylphosphine/hydrogen (the phosphorus source) flow rate between 10 sccm and 30 sccm with hydrogen as carrier gas.   
     
     
         7 . The method of  claim 6 , wherein the methane flow rate is 0.5% of a total gas flow rate of 400 sccm. 
     
     
         8 . The method of  claim 1 , wherein depositing the phosphorus doped diamond layer on the substrate comprises:
 depositing the phosphorus doped layer at around 900° C.-1200° C. using a microwave power of at least 2500 W and a preset chamber pressure range.   
     
     
         9 . The method of claim  17 , wherein the preset chamber pressure range comprises a range of 75 to 85 Torr. 
     
     
         10 . The method of  claim 1 , further comprising:
 controlling impurities during layer deposition by using a water-cooled sample stage in the PECVD reactor.   
     
     
         11 . The method of  claim 10 , further comprising:
 timing a growth period of the doped diamond growth for less than or equal to seven minutes;   terminating methane and TMP/H2 flow after the growth period; and   inducing a negative electron affinity to the surface of the substrate using hydrogen plasma exposure.