Voltage Dependent Field Emission Energy Distribution Analysis of Wide Bandgap Materials
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c-BN features negative electron affinity [7]. Field emission studies on flat, n-type c-BN have shown that considerable emission currents are attainable [8]. Recent studies on diamond coated Mo emitters have yielded promising results as well, and have demonstrated that diamond also features a negative electron affinity [4,5,9]. Characterization of diamond and c-BN coatings for tip-shaped Mo emitters is the subject of this study. The following describes how the use of voltage-dependent field emission energy distribution (V-FEED) measurements can yield crucial information about the origin of and mechanism responsible for field emission which cannot be obtained through classical current versus voltage (I-V) measurements. MEASUREMENT TECHNIQUE V-FEED analysis consists of measuring the energy spectra of field emitted electrons at various applied voltages by means of an electron spectrometer. The field emission current and energy distribution of the electrons emitted from bare Mo and c-BN coated or diamond coated tips were measured at different applied voltages. The Fermi level (EF) of Mo serves as an energy reference through the following relationship for the energy of field-emitted electrons [6]: (E - EF) = Ekin - eV + O~A,
(1)
where V represents the potential applied between the tip and gate, e the elementary charge, and Ekin the measured kinetic energy of the field-emitted electrons. The analyzer work function, 131 Mat. Res. Soc. Symp. Proc. Vol. 509 0 1998 Materials Research Society
(PA, was measured to be 4.3 ± 0.1 eV by x-ray photoelectron spectroscopy (XPS) of a clean Cu sample. The positions of the Cu 2 pi/2 and 2 P3/2 XPS peaks served as energy references for spectrometer calibration purposes. EXPERIMENT The Mo emitters used in this study were sharpened by electrochemically etching a 125
gtm initial diameter Mo wire. The Mo wire was biased at +10 V DC and submerged in a
concentrated KOH solution with a Pt counterelectrode. The tip morphology and radii of curvature were determined by Scanning Electron Microscopy (SEM). Typical radii of curvature were less than 100 nm. The c-BN and diamond coatings were applied by an electrophoretic procedure where the Mo tips were again biased at +10 V DC (also with a Pt counter electrode) and submerged in an ultrasonically prepared suspension of c-BN or diamnond particles in ethanol. Typical particle size was approximately 100 nm. Coating thickness and uniformity were monitored by SEM. Coatings were usually a few hundred nanometers thick. For analysis, the coated and bare emitters were placed in a sample holder so that each tip was positioned 500 jtm beneath a gate with a circular opening of 500 jtm. The holder and sample were then placed in a UHV analysis chamber with a base pressure of 10-9 torr and were aligned with the entrance lens of a hemispherical electron analyzer (VG Instruments, CLAM II). The sample holder could be transferred in vacuo to a heating stage where the surface of the emitters was cleaned by thermal desorption and the coatings were annealed. The energ
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