Electron Beam-Plasma Vacuum Deposition of Very Thin Nanostructured Carbon Films: Photocathode Application

Abstract

Very thin nanostructured carbon films were deposited on quartz substrates by electron beam-plasma (EBP) vacuum deposition. In EBP system, electrons emitted by the incandescent cathode are focused by the electrostatic lens and accelerated by the electric field between the cathode and the crucible filled with evaporated material. At an initial instant, the evaporated material is melted due to electron bombardment effect. On achieving a certain vapor density (specific for each material) a non-independent discharge in material's vapor is developed within the anode-cathode gap (-,6-10 mm). The directly heated cathode changes it's purely thermo emission mode of operation into a combined - i.e. thermo- plus ion-electronic emission - mode. In our experiment, the evaporated material was pyrolitic graphite. Substrate holder temperature during deposition was 500°C (samples Q1, Q2) and 350°C (samples Q3, Q4,). Film thickness were 20 nm (samples Q1, Q3) and 25 nm (samples Q2, Q4). The elements concentrations in the films were determined by RBS and ERD analytical method. Raman spectroscopy was used for chemical structural features determination of carbon films. Elements concentration results were practically the same for the all samples: carbon - 93 at.%, nitrogen - 2 at.%, hydrogen - 2 at.%, oxygen - 3 at.%. Raman spectrum of carbon film, which is typical for all samples, was deconvoluted. We used five peaks fitting for the range 1000–1800 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> and four peaks fitting for the range 2500–3300 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , Nanostructured carbon film contained several carbon phases. The photo-induced (pulsed laser −266 nm) electron emission properties of carbon very thin films were determined by the measurement of cathode bunch charge and calculate quantum efficiency (QE) of the prepared back-side illuminated transmission photocathode. Best result of QE showed photocathode Q4 and was <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{QE}(\%)\mathrm{x}10^{-2}=1.5$</tex> at extraction field 4 kV/mm. Results in this study call for optimize EBP technology for the fabrication of the robust and high QE transmission photocathode.