Electron Emission Properties of Liquid-Gallium on a Tungsten Field Emitter Tip

For d 1 0 > and 4112 oriented tungsten tips deposited with liquid-gallium, electroll emission patterns and emissiol~ current characteristics were investigated by field emission micrc~scopy. When the tip ~ o l t a g c was applied for the first time after deposition of gallium, stable liquid-gallium cones were forn~ed on the remolded facets for thicknesses d 1.5 5nm and DC electron emission occurred. The total emission current obeyed the Child-La~~gn~uir 's law. Tltis nlealls that t l~e stabilization mechanism of liquid-gallium cones is the space-cllarge effect. These cspcrimental results are very similar to those for the liquid-lithium deposited tungslcn tips already rel)orled. Wllcn the applied voltage was decreased to zero and again increased, weak distorted pattern appeared OII t l ~ c same facets and became bright gratluallj.. The emission current mo11otonous1y illcreased with t l~e appliecl voltage. In tllis case, no liquid-gallium cone seems to he formed and the electron emission probably occurs front tile liquid droplet, as mentioned by Rao a al. It is well known that a liquid metal droplet forms a Taylor cone [ I ] when an appropriate positive electric field is applied. Since the electrostatic force which acts on liquid rnetal hics no dcpendcncc 011 thc direction of applied field, a liquid metal coated field emitter has the possibility of providing a function of high-brightness ionlelectron source. Electron emissions from liquid 111-group metals which arc important for applications had already been tried by several researchers. Saranson and Schwind had carried out the cxperilnent of extracting field electrons from a liquid Ga-In alloy and observed the periodic pulsed emjssion [2J. Later, Rao ct al. found the DC-mode electron emission from liquid-gallium coatcd tungsten tips with radii of about 1OOnm [3], and they reported that no fieId-stabilized cone is fol-ined and thc general field emission occurs. We tried the in situ HV-TEM observation of tungsten tip coatcd with liquid Ga-In-Sn alloy and found that a field-stabilized small cone is formed provided the liquid-metal is thin [4]. Recently, Driesel et al. performed the HV-TEM in-situ investigations of the tip shapc of a galliu~n liquid metal electron emitter and obtained the rcsult supporting thc Rao's model 151. In thcse cxperimcnts, however: the liquid thickness was not controlleci. Wc have lately clarified that the cwrcful thickncss control of liquid-lithium makes the formation of stable lithium cone possil~le [h,7] . The liquid-mctai thickness is thc key factor for the stablc cone formation a i d the determination of emission modc (DC or pulsc). In this paper, therefore, the possibility of the stable cone formation as well as the clcctron e~nission characteristics were investigated for the tungsten tips deposited with liquid-gallium, by carefully controlling thc thickncss. A gallium-deposited tungsten tip is prepared as follows. A W<110> poly-crystalline or :I W<111> singlccrystal wire was spot-m7elded to the apex of a tungsten hair-pin filament. The one end of thc wire was electrolytically etched to be about a few hundreds nin of the apex raclius by 1 N NaOH. 111 an FEM Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1996512 JOURNAL DE PHYSIQUE IV column (base pressure of 1.3 X 10-'~a) [7], the tip was flashed for 30s at 2100K. Subsequently, the (100) plane of W<110> (or the (111) plane of W<111> tip) was remolded for 30s under the conditions of the-tip temperature of 1850K (or 2000Q and the field strength of 71.2MV/an (or 61.7MV/an). Gallium having meltlng polnt of 303K was deposited on the remolded tip from one direction perpendicular to the tip axis by using a chimney-type evaporator. The deposited thickness t was estimated by taking into account the directivity of evaporator to be t = t,( r + b t a n a ) / ( r ~ a t a n a ) , (1) where t, is the gallium thickness on the quartz thickness monitor, r the inner radius of chimney, and a and b are the distances from the end of chimney to the tip and to the quartz thickness monitor, respectively and a ( + 3" ) the spread half angle of gallium vapor. Setting the temperature of the gallium-deposited tip to about 600K so that the deposited gallium was in a liquid state and applying the negative volta e to the tip, Jield electrons were extracted. As the vapor pressure of gallium is very low at 600K, only a Tittle of liqurd-gallium evaporate. Fig.1a shows the scanning electron microscope (SEM) image of remolded W<110> tip just after deposition of gallium of 3nm thickness. It is found that thc gallium atoms are not deposited uniformly but form the droplets (shown by arrows) due to the high surface tension (718 mN/m). Fig.lb shows the SEM image of another gallium depositcd tip after extraction of electrons. This suggests that the liquid-gallium is uniformly diffused on the tip surface during operation. For li uid gallium deposited W<110> and W<111> tips, the variations of the electron emission pitterns withge applied voltage and the electron emission characteristics were studied. Figure 1: SEM images of the remolded W<110> tips deposited with gallium of 3nm thickness. (a) Just after deposition of gallium and (b) after extraction of electrons. 3. RESULTS AND DISCUSSIONS 3.1 Liquid-gallium thickness for stable cone formation Electron emission patterns from tungsten tips deposited with various thicknesses of liquid-gallium were observed to investigate whether stable Liquid-gallium cones are formed or not. For the thicknesses of 1.5 5nm, stable liquid-gallium cones were formed and DC mode electron emission occurred. For thicknesses more than 5nm, however, no stable cone was obtained and only explosive pulsed electron emission resulted. The deposition thickness for forming ~Pdble cones is one order of magnitude smaller than that for lithium[G]. This implies the necessity of precise control for deposition of gallium. 3.2 Cone formation sites Figs.2a and 2b show electron emission patterns from a W<110> tip just after flashing and after remolding of (100) facets, respectively. Figs.2~-2f show the emission patterns from the remolded tip deposited with liquid-gallium of 2.5nm thickness. When the applied voltage was negatively increased from zcro just after deposition, gallium droplets migrated from the shank to the remolded (100) facets by surface diffusion (2c and 2d). At the critical voltage of 3.19kV, a liquid-gallium cone was formed on the remolded facet (2e). When the voltage was further increased, a new cone was formed on another remolded facet (20. Figure 2: Electron emission patterns from the W<llOz tip (a,b) and the tip deposited with liquid-gallium of 2.5nm thich~ess (c-f). (a) after flashing, applied voltage V = 3.55kV, total emission current I = 1.05 u A ; (b) after (100)-remolding, V = S.IYkV, I = 5.51 uA; (c) V = 2.85kV, 1 = 0.35 /LA; (d) V = 3.04kV, I = 1.56 /LA; (e) V = 3.1YkV, I = 125 ,LA; (f) V = 3.36kV, 1 = 259 /LA. The electron emission patterns from a W<111> tip just after flashing and after remolding of (111) facets are shown in Figs3a and 3b, respectively. The emission patterns from the remolded tip deposited with liquid-gallium of 2.5nm thickness are shown in Figs.3~-3e. When the applied voltage was negatively increased from zero just after deposition, a liquid cone was suddenly formed on the remolded (111) facet C5-82 JOURNAL DE PHYSIQUE IV at the critical voltage of 3.36kV (3c). When the applied voltage was decreased to zero after cone formation and was again increased, a weak small pattern appeared on the same remolded facet (3d) and became bright gradually with the voltage (3e). It should be noted that these patterns are not circular, but distorterl. The similar phenomena were also observed for the Wtip. Figure 3: Electroil emission patterns from the W<111> tip (a&) and from the tip deposited with liquid-gallium of 2.Snm thickness (c e). (a) just after flashing, V = 4.35kV, I = 3.15 /LA; (b) after (111)-remolding, V= 3.48kV, I = 2.32 {LA; (c) V = 3.36kV, I = 134 IJA; (d) V = 2.08kV, I = 0.42 PA; (e) V = ?.56kV, I = 81.4 @A. From the experimental results described above, the formation sites of liquid-gallium conc werc found to correspond exactly to the remolded faccts. This is the same result as that for liquid-lithium cone[8]. When the applied voltage mas decreased to zero and was again increased, the results werc different from those of liquid-lithium. Because stable liquid-lithium cones arc rcprocluctively formcd. 3.3 Electron emission current characteristics Typical total emission currents for the (100)-remolded W<110> tip deposited with liquidallium of 2.5nm thickness are plotted against the applied voltage in Fig.4. The closed (a) and open (8) symbols indicate the currents when the tip voltage was applied for the first time after deposition, and the currents when the applied voltage was decreased to zero and then again increased, respectively. In the former case, the emission current jumped from 3.7 /LA to 1 9 0 u A at the applied .voltay o f 2.X4kV and simultaneously the emission pattern changed. As the work function o liquid-gal ]urn is constant, the current jump suggests the sharpening of liquid shape and therefore the cone formation. In the latter case, on the other hand, the emission current monotonously increased with the applied voltage and had no jump. This result is very similar to that of Rao et al. [3]. No liquid-gallium cone is formed in this casc. In the former case, the emission current after the cone formation suddenly deceased at the lower voltage than -2.84kV when the applied voltagc was decreased. I11 other words, the current showed the hystcsesis. For the fixed applied voltages, the emission current decreased linearly with time, as in the case of liquid2.5 3.0 Applied voltage [-kV] Figure 4: Typical total enlission characteristics for the liquid-gallium deposited W<llOz tip, 0: w11e11 the tip voltage \vas applied for the first time afier deposition and (.):when the applied voliage was decreased to zero anci thcn again increased. A: (100)-remolded tip without deposition and its vertical axis is shown on the right. Applied voltage [-kV] Figure 5 : Total emission current characteristics ut Fig.4, represented using the log scalc. JOURNAL DE PHYSIQUE IV lithium deposited tip [6]. This decrease may be causcd by the thermal evaporation of gallium due to the Joule's heating. In the fi ure, for reference, the currents for the (100)-remolded tip without deposition areshown by the symbol ? A). It is found that the deposition of liquid-gallium makes the emission current about two order of magnitude larger. Fig.5 shows the emission characteristics in Fig.4, represented using the log scale. When the tip voltage was applied for the first time after deposition(*), the increase rate of the emission cui~ent changed drastically at 2.84kV where a gallium-cone was formed. The increase rate after the cone formation was 1.64. Therefore it is reasonable to consider that the emission current after the cone formation obeys the Child-Langmuir's law, namely, I = ~ ~ 3 1 2 (2) with perveance P bf 3.40 x ~ O ~ ~ A / V ~ / ~ . In other words, the stabilization mechanism of liquid-gallium cones is the space charge effect as well as in liquid-lithium cones [9]. When the applied voltage was decreased to zero and then again increased(O), on the other hand, the increase rate of the emission current was about 11 and hence the current doesn't obey the Child-Langmuir's law.