Field Emission Energy Distribution and Current-Voltage Characteristics Using Single Tip Gated Diodes
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b(D
I = aV 2 exp
J3(21
In Eq. (1). D is the work function, the parameter (a) depends on emission area, and the parameter (b) depends on tip shape. The slope of a plot of ln(I/V 2) verses I/V is (bI 3/2). If D is determined from FEED measurements, then (b) can be determined from I-V measurements. The FEED was calculated for a range of thermal conditions by Young [2]. At 0°K with infinite analyzer resolution, the FEED consists of a vertical step at the work function energy followed by an exponential fall-off at higher energies. Young and Kuyatt calculated the FEED broadening from finite analyzer resolution, which adds a low energy Gaussian tail and thus makes the onset less steep [3]. However, the inflection point of the Gaussian tail does not move as the energy distribution gets wider. When photons of energy hv are incident on a surface with work function (D, 85
Mat. Res. Soc. Symp. Proc. Vol. 558 ©2000 Materials Research Society
photoelectrons leave the surface with kinetic energy KE where (2) KE =hv- 0D When the photoelectric effect is examined closely, the electron band structure may contain both direct and indirect transitions [4,5,6]. To separate these contributions photoelectric thresholds must be determined using several different incident photon energies. EXPERIMENT In this work, field emission tips are mounted on a precision stage manipulator located above a gate aperture with a set of decelerating electrostatic lens and a simulated hemispherical energy analyzer/detector shown schematically in Fig. 1. The analytical chamber contains a simulated hemispherical energy analyzer made up of a small number of easily machined cylindrical and disk shaped electrodes [7]. The energy analyzer was designed to generate spherical equipotentials in the center space between two cylindrical electrodes with two parallel auxiliary electrodes across the ends of the cylindrical electrodes. This spectrometer is a low cost and more compact alternative to a hemispherical analyzer of the same inner and outer diameters [8]. The system is contained in an ultrahigh vacuum 8-inch dia cross. An emission tip is mounted pointing down on the manipulator from the top flange with the energy analyzer
800 V -90V -70V -87 V -60V OV -80V -82 V
Figure 1: Scale drawing of aperture, lenses, and energy analyzer with Simion plotted electron trajectories (electronic hardware and connections included).
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assembly below. Turbo and ion pump, leak valve, ion gauge, and residual gas analyzer are mounted on side flanges. The chamber achieves a base pressure of 10-9 Torr. The manipulator has stepper motors for XYZ and rotational motion with a positioning precision of 2 [tm. A tip is positioned using a He-Ne laser to illuminate it from below and then the position is optimized by maximizing the electron count rate to a channel plate detector located behind the energy analyzer. The analyzer has a base plate that contains the analyzer entrance and exit apertures, a cylindrical inner electrode, a cylindrical outer electrode that is made from wire mesh
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