Mechanisms of Field Emission from Cubic Boron Nitride Coated Molybdenum Emitters
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Mat. Res. Soc. Symp. Proc. Vol. 509 ©1998 Materials Research Society
Fig. 1:
Scanning electron micrographs of a tip-shaped Mo emitter (a) before and (b) after coating with c-BN powder (grain size typ. 100 nm).
EXPERIMENTAL RESULTS Experimental data from bare Mo tips were collected prior to the coating procedure and after an in vacuo thermal desorption at 500"C for several hours. Subsequently, the tips were coated and their field emission characteristics were recorded after an additional in vacuo thermal desorption similar to that applied to the bare tips. The desorption step was needed to produce stable emission in both cases. Fig. 2 shows I-V measurements taken before and after coating of a Mo tip; at a given voltage the field emission current was typically increased by 2 orders of magnitude after the coating had been applied. 10 -7 -
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I-V plots of a Mo emitter recorded before and after electrophoretic coating with c-BN powder.
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For the V-FEED measurements the emitter and the extraction gate were aligned with the entrance lens of a hemispherical electron analyzer (VG Instruments, CLAM II). The electron analyzer recorded the energy distribution of the field-emitted electrons which passed through the opening of the gate. The energy of field-emitted electrons was referenced to the Fermi level (EF) of Mo using the following relationship [5]: (1) (E - EF) = Ekin - eV+ P A, where V represents the potential applied between the gate and the tip, e the elementary char e, and Ekin the measured kinetic energy of field-emitted electrons. The analyzer work function, C'A was considered constant and was calibrated prior to the FEED experiments. It should be noted that the energy scale (E - EF) implicitly takes into account any changes in kinetic energy of the emitted electrons due to changes in applied voltage. This energy scale therefore displays the "energy loss" of the observed, field emitted electrons relative to the energy of electrons that would be field emitted from the Fermi level of Mo. Further details regarding the V-FEED technique can be found in Ref. [5]. The energy distribution of the electrons emitted from bare Mo tips was measured at different applied voltages. The energy (E - EF) of the electrons emitted from Mo was well defined and did not depend on the potential applied between gate and emitter. This behavior was expected since the applied field does not penetrate the metal. After a coating of c-BN was applied to the Mo tips, the energy (E - EF) of the electrons emitted from the c-BN-coated tips decreased monotonically with the applied voltage, as shown in Figure 3. 0-
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