Stability of Field Emission Current for Hydrogen-Free Diamond-Like Carbon Films Prepared by Layer-by-Layer Technique

Abstract

We have studied the field emission characteristics and the stabilities of the hydrogen free diamond-like carbon (DLCj Jilms deposited by a layer-by-layer technique using plasma enhanced chemical vapor deposition(PECVDj, in which the deposition of a thin layer of DLC and a CF4 plasma exposure on its surface were curried out alternatively. The emission current of the hydrogen-free DLC increases ut first and then stabilizes with increasing bias-stress time. The surface of the DLC was not changed after 32h bias-stress, which was confirmed from scanning electron microscopy and Raman scattering analysis. The change in the emission current appears to be the change in the density of states in the gap and/or the formingprocess of the DLC. I. INTRODUCTION Diamond-like carbon (DLC) is an interesting material for field-emission displays due to its unique properties such as high hardness, high thermal conductivity and chemical inertness[ 1-21. PECVD (plasma enhanced chemical vapor deposition) is a conventional method for large area uniform deposition of DLC films[3]. Hydrogen-free DLC is n-type [4] and thus is considered as a promising material for efficient electrm emission. it is noted that other types of DLC are p-type. The hydrogen-free DLC films are smooth and can be deposited at low temperatures, making them potentially useful for electronic device applications. When considering the possible use of hydrogen-free DLC as a cold cathode material of field emission displays, the control of the Fermi energy through the band gap by doping as well as the stability of the material under bias-stress are very important[5-61. in this work we studied the stability of the electron emission currents under bias-stress for a conventional DLC and the hydrogen-free DLC deposited by layer-by-layer technique. 11. EXPERIMENT We used a conventional PECVD system, in which rf power was applied to the substrate holder, for the deposition of DLC and hydrogen-free DLC. The flow rates of He, H2 and CH4 were fixed at 20 sccm, 6 sccm and 3 sccm, respectively, for DLC deposition. The selfbias voltage of the substrate holder was found to be -80 V at a fixed rf power of 100 W. The self-bias voltage depends strongly on the deposition conditions. The hydrogen-free DLC films were grown in the same deposition system, by repeated deposition of 5 nm thin DLC layers and subsequent 140s CF4 plasma treatment on its surface. The detailed deposition conditions are described in the literature[7,8]. The electron emission currents were measured in a vacuum of 3~10-~ Torr, between metal and DLC plates with an area of 0.5 cm2 and 70 pm spacing. The DLC and hydrogenfree DLC were bias-stressed with an electric field giving an emission current of 2-4 PA. The current-electric field (I-E) measurements were performed with a scanning speed of 10