An apparatus for high resolution field emission spectroscopy

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ELSEVIER

Advances in Colloid and Interface Science 71-72 (1997) 353-369

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ADVANCESIN COLLOIDAND INTERFACE SCIENCE

An apparatus for high resolution field emission spectroscopy Chuhei Oshima Department of Applied Physics, Waseda University, 3-4-10kubo, Shinjuku, Tokyo 169,Japan

Abstract

We have constructed new apparatus for high resolution field emission spectroscopy. This apparatus, which we call HRFES system, consist of a new spectrometer, a computer controlled low-noise power supply, an extremely-high-vacuum chamber, and a new pressure gauge. The high resolution of better than 0.5 meV was realized by the new spectrometer. By using the HRFES system, we measured the energy distributions of field electrons emitted from tungsten (W) (310) and niobium (Nb) (111) tips at -193°C (80 K) and room temperature in the emission current region from 10 -8 to 10 -6 A. The high-intensity part of observed curves at emission of ~ 10 -8 A agreed with the calculated ones based on the Fowler-Nordheim theory, while there are discrepancies in two energy regions. © 1997 Elsevier Science B.V.

Keywords: High-resolution electron microscopy; Coherent electron beam; Field emission spectroscopy; Extremely high vacuum; Ionization pressure gauge

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Experimental apparatus and materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Vacuum system and evacuation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Development of a new pressure gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Development of a new high resolution electron (HRE) spectrometer . . . . . . . . . 2.4 Development of a new low noise power supply . . . . . . . . . . . . . . . . . . . . . . . Report of PRESTO, structure and functional properties, JRDC. 0001-8686/97/$32.00 © 1997 Elsever Science B.V. All rights reserved. PII S 0 0 0 1 - 8 6 8 6 ( 9 7 ) 0 0 0 2 6 - 2

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2.5 Performance of the HRE spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Tip preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Tungsten (310) tip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Niobium (111) tip at room temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

362 362 365 365 368 369

1. Introduction

Field electron emission is extensively used as an electron source of the high brightness beam for scanning electron microscopes and analytical electron microscopes, because of its characteristic features such as a point source, stable emission and a long life [1]. The performance of these instruments largely depends on the properties of an electron source. The conventional electron sources utilize either the thermionic emission at high temperature above ~ 1600°C or the field emission at room temperature. The energy width of these conventional electron guns are 0.2-0.3 eV. On the other hand, it is possible to obtain the electron beam with the narrow energy width of ~ 1 meV with a monochromator, but the width is limited now by the emission current. In 1969, Gadzuk predicted the electron emission, of which the energy width ( << 0.05 meV) is extremely narrow, from superconducting states [2]. If the monocromatized beam is realized, it will decrease the color aberration of the electron optics and energy resolution in energy loss spectra. Although some trials were carried out for the observation of emission phenomena from the superconducting states [3-5], no one succeeded to confirm it because of a poor energy resolution of the spectrometer and an insufficient vacuum condition. In this work, we have tried to improve the experimental techniques to realize these experiments. We prepared the XHV chamber, of which the base pressure was ~ 10-11 Pa under the cooled condition of an electron gun with a liquid helium. We confirmed the possibility of the electron emission in 10-10 Pa without any outgassing. Furthermore, to confirm the existence of the monochromatized beam, we needed a resolution of 0.5 meV, which was realized by using a three cylindrical energy analyzers combined with a computer-controlled low-noise power supply developed in this study. At present, the system was not completely constructed for the measurement of the emission from superconducting states, and hence, I review the present situation of development of the new instrument and preliminary results. In the present report, we first describe our HRFES system with a new H R E spectrometer and a new pressure gauge, and its performance characteristics. We then describe the preparation of Nb single crystal wire, fabrication of the Nb (111) tip. Finally we present preliminary experimental results for the energy distribution of the field electrons emitted from the Nb and W tips using the H R E spectrometer. We found that the high-intensity portion of typical spectra with low emission agreed with theoretical ones calculated based on the Fowler-Nordheim

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355

(FN) theory [1]. We found the changes in energy distribution as functions of the emission current. The large discrepancies in two energy regions, namely, below and above the main peak have been reported.

2. Experimental apparatus and materials 2.1. Vacuum system and evaluation process A vacuum system was constructed for easy evacuation to extremely high-vacuum (XHV) pressures. The fundamental design of the system was the same as that of the other XI-IV chambers, of which the base pressure is 10-11 Pa [6-8]. Our XHV chamber used in this experiment was made of special stainless steel (NKK sus316L 'clean Z'), and all the vacuum components, except the feedthrough, viewport and the vacuum gauge, were pre-baked at ~ 600°C under the vacuum of 10-4 Pa. The XHV chamber was evacuated by a diffusion pump (Edwards EO4) with a liquidnitrogen cooled trap (VG-CCT100) and a titanium sublimation pump with a liquid-nitrogen-cooled shroud. After the first baking at ~ 200°C for 12 hours, the pressure decreased to ~ 10-10 Pa. Fig. l(a) shows an evacuation process measured with an extractor gauge (Leybold IE514)just after the end of baking. At the lowest pressure, we measured species of the residual gas by the quadrupole mass spectrometer (Balzers QMG420). As shown in Fig. l(a), the pressure decreased rapidly with cooling the system. Ten hours later after the end of baking, we introduced liquid nitrogen into the trap. The pressure decreased to the order of 10 -9 Pa after 30-40 hours. Eighty hours later after the end of baking, we stopped the evacuation with the diffusion pump by closing an isolation valve, and cooled the shroud of the sublimation pump by introducing liquid nitrogen into the shroud. The extractor gauge indicated rapid pressure decreased to 3 × 10-10 Pa. Fig. l(b) shows a typical mass spectrum obtained at 3 × 10 -1° Pa. A large peak of mass number 2 was dominant, and faint peaks appeared at the mass numbers of 4, 16, 17, 28 and 44 [8]. The base pressure of 10-,0 Pa increased slightly after the installation of the field emission (FE) gun and the HRE spectrometer. When we introduced liquid-helium into the cryostat of the FE gun, the pressure decreased to the order of 10-1 ~ Pa. 2.2. Development of a new pressure gauge A conventional pressure gauge could not measure the pressure in our XHV vacuum system because of large noises due to soft X-rays and electron stimulated desorption (ESD) ions. Therefore, we constructed a new pressure gauge in this experiment, which we call XHV gauge [9,10]. Fig. 2 shows the schematic diagram of the gauge constructed for pressure measurement in XHV, which consists of an electron source, an ionization cage, ion deflector, and channeltron as an ion detector. The improvements of the XHV gauge over a conventional gauge are (a) reduction of outgassing from the gauge, and (b) reduction of the soft X-rays and ESD ions. To achieve the first improvement, we carefully chose low-outgassing

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materials to construct the gauge. All the electrodes, and the components except the channeltron and conflat flanges, were pre-baked at ~ 1000°C for 24 hours in a vacuum of ~ 10 -4 Pa. Because the noise due to the soft X-rays and ESD ions decreased with the use of the ion deflectors, it was possible to use the gauge itself to evaluate the outgassing from the gauge from the emission dependence of the ion current [9]. Most of the electrodes are made of stainless steel with a low hydrogen

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357

eltron Electron Source I Me

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Fig. 2. Schematic diagram of the XHV gauge.

impurity (NKK clean Z), of which the outgassing is extremely low. To further reduce the outgassing, a cold electron source fabricated by the techniques developed in vacuum micro-electronics was installed [10]. To achieve the second improvement, we used the ion deflector with the deflection angle, 256.4 ° which have the function of the energy analysis of ions by the electrostatic fields. Thus, the signal ions are efficiently discriminated from the X-ray noise and the electron stimulated desorption (ESD) ions, which are main noises in the conventional ionization gauge for ultra high vacuum. We estimated these noises for XHV gauge to be an order of 10 -13 Pa [9]. The channeltron increases the sensitivity of the ion detection, and hence, the emission current of 0.1-0.01 mA is sufficient to precisely measure the total pressure of 1 x 10- ~ Pa. Fig. 3 shows a typical energy spectrum measured by changing the deflection voltage. The pressure was 2 x 10 -l° Pa. The gas phase and ESD peaks are observed at 43.8 and 47.3 V in the spectrum, respectively. To determine the species of the residual gas and the ESD ions, we measured the flight-of-flight (TOF) time of ions passing through the deflector. We measured the TOF spectra for different voltages corresponding to different ion energies; the number of ions for each flight time was accumulated for 2 rain (4 x 10 6 ionization events). Fig. 4 is the TOF spectrum of the gas-phase ions at a deflection voltage of V0 = 43.8 V. A sharp single peak appears around 3.5 /zs corresponding to H~, which is much different from the species of ESD ions; H + and O ÷ ions were detected as the ESD ions [11]. The performance of the XHV gauge is summarized in Table 1. Although the XHV gauge have an ability to measure the pressure of 10 -13 Pa as shown in Table 1, the lowest pressure observed in this chamber was 3 x 10-11 Pa.

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2.3. Development of a new high-resolutionenergy (HRE) spectrometer Fig. 5 shows the schematic diagram of the HRE spectrometer developed in this experiment. We used a system of three cylindrical analyzers for compactness, a low background and high energy resolution. The fundamental design and the surface treatments of the HRE spectrometer are identical to the techniques for high resolution electron energy loss spectroscopy (HREELS) [12]. The use of many electrodes for control of the electric fields in the HRE spectrometer allows precise control of the electron trajectory under the low pass energy condition. The resolving power (AE/E) is approximated using the slit width (s) and the center radius of the i-th energy analyzer (R;) if we neglect the angle aberration [13]:

AE/E = s/( ~i R~)

(1)

Here, AE is the energy resolution and E is the pass energy o f the electron. In our design, w e used the values o f s = 0.15 ram, R 1 ffi 33.6 ram, R 2 - - 4 2 ram, and

C. Oshima /Adv. ColloidInterface ScL 71-72 (1997)353-369

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Table 1 Performance of the XHV gauge Lowest measurable limit due to soft X-rays (1 rain measurement) Lowest measurable limit of ESD ions Sensitivity of the gauge for hydrogen Operating emission current

4 x 10-13 Pa 2 x 10-13 Pa 1.8 x 10-2 Pa-1 10-100/~A

R 3 = 45.8 ram; and hence, A E / E is 0.0012. If we use E = 0.6 eV, then the energy resolution is ~ 0.7 meV. In our H R E spectrometer (Fig. 5), a portion of the emitted electrons pass through two apertures mounted in front of the lens system, which then focuses the electrons onto slit 1. For low pass energy conditions, it is essential that the surface potential in the system be uniform. To achieve this, (a) all the surface of the electrodes facing the electron trajectories were double-coated with evaporated Au film and aquadag (fine carbon powder) and (b) the H R E spectrometer were pre-baked at ~ 200°C in ultra-high vacuum. F o r the H R E spectrometer, stray magnetic fields must be either eliminated or shielded. First, we determined the magnetization of all the components of the H R E spectrometer

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C. Oshima /Adv. Colloid Interface $cL 71-72 (1997) 353-369

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Fig. 5. Schematic diagram of the HRE spectrometer (Topview).

using a highly sensitive magnetometer (FGM-3D1 Walker), and we confirmed that all the components in the H R E spectrometer were nonmagnetic. Therefore, we decided to triple-shield the electron trajectories inside the spectrometer with 0.8-mm thick 'permalloy' tubes and covers. The residual magnetic fields at the points of the electron trajectory were below 1 mG. 2.4. Development of a new low-noise power supply

We have developed a new low-noise power supply for our H R E spectrometer [14,15]. First, we designed electronics that supply voltages to 23 electrodes of the H R E spectrometer. Fig. 6 shows a schematic diagram of a computer-controlled system with a combination of a commercial digital-analog (AD) converter and custom-built amplifiers. The system is used not only for data processing, but also for automatic optimization of the tuning by adjusting all the potentials of the spectrometer. All the output voltages are automatically measured within 30 s. This makes it easy to retrieve a particular tuning condition, such as highest-resolution or highest-current, from the store data. Fig. 7 shows a schematic of the electric circuit of the custom-built amplifier. We made 24 amplifiers each of which was optimized to meet the potential requirements of each electrode. For example, we equipped fine adjustments for the outputs that allow very sensitive control of the potentials (e.g., slits, inner and outer electrode of analyzer). Furthermore, scan inputs can change the output voltages of the lens in proportion to the scanning energy. Also, current-voltage converters for the slits are made in the amplifiers to keep the output impedances low. Since the current-voltage converters have a sensitivity of ~ 10-~° A, one can measure the current of 10-~° A entering the electrodes. The sensitivity of 10 -]° A is sufficient to optimize the better potential automatically under the given conditions.

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C. Oshima /Adv. Colloid Interface Sci. 71-72 (1997)353-369

To achieve high resolution (0.5 meV) and high stability in our H R E spectrometer, we designed the electronics to meet the following requirements; low drift of ~ 0.1 m V / h , low ripple of ~ 0.1 mV, low output impedance of 1 /2 and a quick sweep faster than 1 msec. The most difficult requirements to meet were the low drift and the low ripple. To achieve the required stability, we used temperaturecompensated low-noise active devices and passive elements with the same temperature characteristics. In addition, to reduce thermal noise we kept the resistances as low as possible. We carefully removed external disturbances in a number of ways: (1) the loop circuits were removed from all the ground lines; (2) the ground lines in the analog part were separated from the digital component; (3) analog circuits are carefully wired in the manner of single point earth geometry; (4) special twist-paired lines sealed with a ground shield were used as the signal lines; (5) a noise-cuttransformer was used for reducing the noise coming through the power lines; (6) to eliminate noise from the computer, during the spectra measurements, the computer bus lines were cut by three-state buffers during the spectra measurements (Fig. 6) ; and (7) RC low-pass filters were mounted to the power pin of operational amplifiers. The performance of the system was as follows. The drift of the voltages per 12 hours were from 0.3 mV to 1 mV differing from slot to slot. The ripples of all outputs were less than 30/xV in the low-frequency band, and the ripple of 1 meV was measured in the wide-frequency band (up to 20 MHz). The largest noise was not a 50 Hz component of the power lines, but a thermal noise of the electronic devices. A low output impedance of less than 1 was realized. By using this power supply together with the H R E spectrometer a high resolution of ~ 0.5 meV was achieved as discussed below.

2.5. Performance of the HRE spectrometer Fig. 8 shows a typical energy spectrum obtained with our H R E spectrometer. To examine the energy resolution of the spectrometer, we analyzed the energy distribution of the electron beam entering the third analyzer using the third analyzer. We did this by scanning only the voltage difference between the outer and inner deflection electrodes of the third analyzer. The measured full width at half-maximum (FWHM) was 1.5 meV. Since the pass energy in the experiment was 0.4 eV, we expected the FWHM obtained by the first two analyzers to be 0.8 meV, and theoretical resolution of the third analyzer alone is 1.3 meV. In our theoretical analysis, we assumed a Gaussian distribution to deconvolute the two distributions (from the first two analyzers and from the third analyzer). The results was an FWHM of 1.5 meV, which agrees with the experimental results. Hence, Fig. 8 shows that the spectrometer operated well in accordance with the theoretical prediction under the low pass energy condition of 0.4 eV. We expected the overall resolution of the three analyzers to be 0.5 meV at the pass energy.

26. Tip preparation Since the crystal structures of Nb and W are a body centered cubic structure, the

C. Oshima /Adv. Colloid Interface Sci. 71-72 (1997) 353-369

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(310) and (111) surfaces have low work function. We have chosen the (111) and (310) orientations as the tip direction, because of high emission. After the W <310> wire (FE CORP.) was spot-welded on a Ta loop filament, the tip was fabricated electro-chemically form the wire. The tip was cleaned by flash heating in XHV. Concerning Nb tips [16], the tips were constructed in three stages. First, after the <111> orientation of the large Nb single crystal was aligned by the back-reflection X-ray Lane method, and long rectangular crystals with 1 x 1 x 8 mm along the <111> orientation was cut by a spark erosion method. Next, we polished each crystal mechanically to make a fine single crystal wire (0.05¢). Lastly, the Nb <111> tip was fabricated by electrochemical etching with a HNO3, HF, and H2SO 4. Figs. 9(a), 9(b), and 9(c) are photographs of a single cxystal, a wire and a fabricated tip, respectively. The emission patterns from the tips were evaluated in the separate vacuum chamber, and the tips with the best symmetry pattern were mounted in the FE gun connected with the HRE spectrometer in XHV chamber (Fig. 6). The emitted electrons are deflected by the four deflector electrodes in front of the tips. The electrons emitted in the forward direction within the angle + 4 ° were analyzed by the HRE spectrometer. The final cleaning of the tip was done by the field

364

C. Oshima /Adu. Colloid Interface ScL 71-72 (1997) 353-369

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evaporation, which is ascertained from the FE patterns on the anode screen. We operated the HRE spectrometer at the energy resolution of ~ 4 meV, which is high enough to measure the energy distribution from the metals at two tempera-

C. Oshima /Ado. ColloidInterfaceScL 71-72 (1997)353-369

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tures of - 193°C (80 K) and 27°C (300 K). Prior to the field emission, electrodes of deflectors and an anode were completely degassed by electron bombardment emitted from the hot filament. The tip was cooled to - 1 9 3 ° C (80 K) by a liquid nitrogen cryostat. The tip temperature was estimated by using a thermocouple attached to the Ta filament close to the tip.

3. Experimental results 3.1. Tungsten (310} tip Fig. 10 shows typical energy spectra at the emission current of 3 x 1 0 - 7 A at two different temperatures. Open circles are data points at room temperature, and d o s e d circles are those at - 2 7 3 ° C (80 K). The dotted lines in Fig. 10 are theoretical ones based on the Fowler-Nordheim (FN) theory. According to the FN theory [1], the energy distribution of FE electrons from a metal is

] ( • ) at f ( ~ )exp( • / d ) .

(2)

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C. Oshima /Adv. Colloid Interface Sci. 71-72 (1997)353-369

Where, e = E - E F eV is the total energy relative to the Fermi level EF, f ( e ) is the Fermi-Dirac distribution, and d -~ = 1 . 0 2 5 c h ~ / 2 t ( y ) / F eV, where 4~ eV is the work function, F V / A is the electric field, and t ( y ) is tabulated correction term [1]. Hereafter, we call Eq. (2) FN distribution. In this calculation, we have used three fitting parameters, ~b, T, and d. For spectrum (a) in Fig. 10, these parameters are ~b = 4.38 eV, T = 300 K, d = 0.130 eV; and for spectrum (b), ~b = 4.38 eV, T = 90 K, d = 0.120 eV. The energy spreads (FWHM) of the spectra (a) and (b) were 0.19 eV and 0.12 eV, respectively and agree with theoretical FWHM. Furthermore, for both temperatures, the observed spectra agree with the FN distribution. No influence from the surface effect was found [15]; this contrasts with the results by Swanson and Cruoser [16] for a W (100) plane. According to Eq. (2), therefore, the high-energy side of the spectrum was determined mainly by the Fermi distribution, and the tip temperature is presumably determined from the observed distribution. However, the discrepancies (i.e., tails) appear on the both sides of the peak. Fig. 11 shows the energy spectra at room temperature (300 K) for various emission currents; Intensity in the high-energy tail increased with increasing the emission current (I¢). Fig. 12 shows this dependence of the high-energy tail (i*) on

10°i

-1

-0.5 0 0.5 Energy from Peak Level (eV)

1

Fig. 11. Energy spectra at room temperature (300 K) of various emission currents.

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< v

10"7 Total Emission Current

10-6 I e (A)

Fig. 12. Dependenceof the high energytail current (i*) on the total emissioncurrent (le). I,, where I* is the current equivalent to this tail. The solid line represent the phenomenological relationship, i* = 4.4 x 104 le2"2, indicating an approximately parabolic relation of I e. This agrees with the reported curves for tips of other orientations, e.g., <111> [17-20]. At present, there are three theories to explain the tail. They are a Coulomb interaction model [17], a hot-hole electron cascade model [18] and a tunneling life model [19]. The magnitude of the tail based on the coulomb interaction model is at least four order of magnitude lower than our observed values [17]. However, the hot-hole model explains well the I~ dependence and the observed energy distribution of the high-energy tail by assuming that a thin surface region of 5 A is the cause of the tail [18]. But this model explains only the high-energy tail, and not the low-energy tail. Therefore, we must consider the third model to interpret lowenergy tails. The tunneling life time model reproduce both the parabolic function of I c and the order of magnitude both high-energy and low-energy tail (within an order of magnitude). At present, however, no energy distribution was calculated using the third model. In our experiment, moreover, we sometimes observed the spectra without the low-energy tail, although the high-energy tail appeared there, which indicates that the origin of each tail is different. Further theoretical and experimental studies are required to clarify these origins.

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1

1

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---=1 30min 10-6 -1 0 Energy from Peak Level (eV) Fig. 13. Energyspectra of a Nb(111) tip. With increasingthe lapse time, new peaks appeared.

3.Z N i o b i u m ( 1 1 1 ) tip at room temperature

Fig. 13 shows a typical energy distribution of the electrons emitted from the (111) tip at room temperature [21]. The total emission current was ~ 10 -8 A. These spectra were obtained sequentially, after the tip was cleaned. The solid curve is the theoretical spectra calculated using the FN theory and T = 300 K, d = 0.15 ( F = 35.4 MV/cm), and ~b = 4.36 eV. The theoretical spectra quantitatively explain the high-intensity part of the observed curve, but the difference appears on both high and low sides similarly to the W (310) tip. Below - 0 . 5 eV, the observed spectra have a clear shoulder decreasing with time. This shoulder is quite similar to the Swanson hump observed for the W (100) tip [16], which is attributed to the effects of the surface electronic structure. Another large discrepancy between the theoretical and observed spectra occurred at high energy above Fermi level. The same tail in high-energy region was observed for the W tip and their intensities increase with increasing the emission as mentioned above. As time lapsed, the emission current from the N-b tip drastically decreased, when compared with the W tip. Accordingly, the energy spread (FWHM) of the electron emission increases. This is also much different from the W (310) tip, in which an increase in work function due to adsorption

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leads to both the emission decrease and slight FWHM decrease, according to the simple FN theory. These changes strongly suggest that the adsorbates on the Nb (111) tip not only increase the work function, but also change the surface electronic state determining the field emission.

4. Conclusion In this work, we have constructed a new HRFES system consisting the XHV gauge, a HRE spectrometer and a computer-controlled low-noise power supply. Using our HRFES system, the energy spectra of the electrons emitted from tips were measured with a high resolution of 0.5 meV in XHV. In these preliminary experiments, we have measured the energy distribution of the field emission electrons from W (310) and Nb (111) tips at room temperature and 80 K. The high-intensity part of the observed distributions were in good agreement with the theoretical ones based on the FN theory. However, there were discrepancies in low- and high-energy regions. At present, we are now designing the low temperature FE gun according to the system of Bergeret, who observed successfully the emission from the superconducting state [5]. After constructing low-temperature electron gun, we plan to observe the fine structure of the emission from the superconducting states in order to detect an electron beam with the extremely narrow energy spread.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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