Tetrahedrally bonded amorphous carbon for field-emission displays W. I. Milne J. Robertson A. Hart B. S. Satyanarayana Abstract — Field emission from a series of tetrahedrally bonded amorphous-carbon (ta-C) films, deposited in a filtered cathodic vacuum arc, has been measured. The threshold field for emission and current densities achievable have been investigated as a function of sp 3 /sp 2 bonding ratio and nitrogen content. Typical as-grown undoped ta-C films have threshold fields of the order 10–15 V/μm and optimally nitrogen doped films exhibit fields as low as 5 V/μm. In order to gain further understanding of the mechanism of field emission, the films were also subjected to H 2 , Ar, and O 2 plasma treatments and were also deposited onto substrates of different work function. The threshold field, emission current, and emission site densities were all significantly improved by the plasma treatment, but little dependence of these properties on work function of the substrate was observed. This suggests that the main barrier to emission in these films is at the front surface. Keywords — Field-emission cathodes, field-emission displays, flat cathodes. 1 Introduction Flat-panel displays with a picture quality comparable to a cathode-ray tube can, in principle, be obtained from field- emission displays (FEDs). A typical FED uses the emission of electrons from a matrix-addressed array of cathodes to excite phosphor pixels. Most current FEDs use Spindt-tip cathodes made from Mo, Si, or W. The tips are necessary to provide the high fields required to extract electrons from such high-electron-affinity materials and, although working displays based on this principle have been manufactured, they have exhibited low lifetimes due to the poisoning and erosion of these tips. Some of the problems have recently been overcome, but the complicated lithographic processes necessary to produce the tip arrays mean that yield is still a major problem. An alternative is to use flat cathodes pro- duced from low-electron-affinity materials. Recently, dia- mond and diamond-like carbon (DLC) have become attractive candidates for this application. DLC is especially interesting as, unlike diamond, it can be deposited over large areas at room temperature, which makes feasible the use of low-melting-temperature glass as the substrate material. DLC can be deposited using several different tech- niques leading to a variety of different DLC types (see Fig. 1). Techniques such as plasma-enhanced chemical vapor deposition (PECVD) 1 and the plasma beam source (PBS) 2 deposit films containing hydrogen while others such as fil- tered cathodic vacuum arc (FCVA), 3 laser ablation, and ion- beam deposition processes produce hydrogen-free films, with a high sp 3 bonding content, known as tetrahedrally bonded amorphous carbon (ta-C). A common feature of these processes is that the film grows from a beam of medium-energy ions. In this paper, we report on the field-emission behavior of ta-C produced using a FCVA system. These films contain a significant fraction of sp 3 bonds (up to 80%), which depends on the incident ion energy. We have therefore investigated emission behaviour as a function of sp 3 bonding content and also investigated the effects of nitrogen on field-emission behavior. Nitrogen has been found to modify the field-emission behavior in a-C:H films. To investigate whether the emission is limited by a barrier at the front or back interface we have also measured emission as a function of substrate work function and front-surface condition. 2 Emission measurements The ta-C films studied in this project were produced using a FCVA system described in detail elsewhere. 3 The sp 3 con- Received 4/8/99; accepted 10/22/99. The authors are with the Engineering Department, Cambridge University, Trumpington St., Cambridge CB2 1PZ U.K.; +44-1223-332-757, fax +44-1223-766-207, e-mail: wim@eng.cam.ac.uk.  Copyright 2000 Society for Information Display 1071-0922/00/0801-0057$1.00 FIGURE 1 — Ternary phase diagram of the various forms of DLC. Journal of the SID 8/1, 2000 57