ABSTRACT
Nanocrystalline lanthanum hexaboride (LaB(6)) films have been deposited on molybdenum foil by the pulsed laser deposition (PLD) technique. The as-deposited films were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The XRD pattern shows the cubic crystallinity of the LaB(6) film. The AFM studies reveal that the conical shaped LaB(6) nanostructures have height 60 nm, base 800 nm, and a typical radius of curvature â¼20 nm. A comparison of force and in situ current imaging AFM studies reveals that current contrast does not originate from the surface topography of the LaB(6) film. Field emission studies have been performed in the planar diode configuration. A current density of 4.4 × 10(-2) A cm(-2) is drawn from the actual emitting area. The Fowler-Nordheim plot is found to be linear, in accordance with the quantum mechanical tunneling phenomenon. The field enhancement factor is estimated to be 3585, indicating that the field emission is from LaB(6) nanocrystallites present on the emitter surface, as confirmed by the AFM. The emission current-time plots show current stability to the extent of 5% fluctuation about the average current over a period of 3 h.
ABSTRACT
Lanthanum hexaboride films were grown on tungsten and rhenium tips and foils by pulsed laser deposition. The X-ray diffraction spectra of the PLD LaB6 films on both the substrates show crystalline nature with average grain size approximately 125 nm. The field emission studies of pointed and foil specimens were performed in conventional and planar diode configurations, respectively, under ultra-high vacuum condition. An estimated current density of approximately 1.2 x 10(4) A/cm2 was drawn at the electric field of 3 x 10(3) and 6 x 10(3) V/microm from the LaB6 coated tips of tungsten and rhenium, respectively. The Fowler-Nordheim plots were found to be linear showing metallic behavior of the emitters. The field enhancement factors were calculated from the slopes of the Fowler-Nordheim plots, indicating that the field emission is from LaB6 nanoscale protrusions present on emitter surfaces. The emitters were operated for long time current stability (3 h) studies. The post-field emission surface morphology of the emitters showed no significant erosion of LaB6 films during 3 h continuous operation. The observed behavior indicates that it is linked with the growth of LaB6 films on W and Re. These results reveal that the LaB6 films exhibit high resistance to ion bombardment and excellent structural stability and are more promising emitters for practical applications in field emission based devices.
