Very low field electron emission from Hot Filament CVD grown microcrystalline diamond
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Very low field electron emission from Hot Filament CVD grown microcrystalline diamond. B.S.Satyanarayanaa, X.L.Peng*b, G.Adamopoulos, J.Robertson , W.I.Milne, & T.W.Clyne* Electronic Materials & Devices, Dept of Engineering, Cambridge University, Cambridge. CB2 1PZ. U.K. *Material Science dept, Cambridge University, Cambridge. U.K. ABSTRACT. Very low threshold field emission from undoped microcrystalline diamond films grown by the hot filament chemical vapour deposition process (HFCVD) is reported. The effect of crystal size, methane concentration and the temperature has been studied. The microcrystalline diamond films grown using 3% methane (CH4) / hydrogen (H2) gas mixture ratio under varying deposition temperatures exhibit very low emission threshold fields. The threshold fields varied from 0.4 V/ µm to 1 V/µm for an emission current density of 1 µA/cm2. A correlation between the emission characteristics and the material properties is presented. These films exhibit an emission site density of ~ 104-105/cm2 at an applied field of 3 V/µm. INTRODUCTION Low electron affinity was one of the prime factors for interest in diamond as a candidate for field emitters, besides its other physical properties like high thermal stability, mechanical hardness and chemical inertness. Recently low field electron emission has been reported from many carbon based materials including nanocrystalline diamond [1-8], nanotubes [9-11] and nanocluster or nanostructured carbons [12-15]. This clearly shows that electron affinity is not the only criteria but there are other factors which also influence the emission. The emission mechanism from these materials is still not clear. For efficient display applications the need is for high emission site densities ~ 106 and current preferably in the range of 10 mA / cm2 [8] at reasonably low fields. Undoped nano-crystalline / polycrystalline diamond films with threshold fields as low as 3-5 V/µm [4-6] and doped polycrystalline diamond with threshold fields as low as 0.5 V/µm [3] have been reported. In the case of doped diamond films, the doping efficiency is still poor and the films tend to be still resistive even after heavy doping.[3.16] It has been observed that the phosphorus doped films tend to show a decrease in threshold field with temperature while boron doped films seem to be relatively stable[16]. It has also been reported that nitrogen has a tendency towards phase transformation in carbon systems[17]. Therefore it is still not clear if the doped diamond cathode would be stable over a long period of time. Further the low field emitting graphitic materials [9-15] and the evidence that even in diamond the emission is possibly from the grain boundaries [1,2,18] suggest that there could be an optimum crystal dimension and sp2/sp3 bonding ratio for low field emission. This could possibly be done even without doping. Hence an effort has been made to systematically study the effect of crystal size, methane concentration and growth temperature on field emission from undoped diamond films grown usin
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