Production of Doped and Undoped DLC Films

Deposition of Diamondlike carbon (DLC) Films

This work was performed in the lab for the previous few years, but since we have now moved to a new laboratory space we have stopped working on RF plasma DLC deposition to concentrate on CVD diamond. The following details relate to the previous work.

We worked on a project to produce diamondlike carbon (DLC) films using a 13.56 MHz radio frequency capacitively-coupled parallel-plate plasma reactor (photo, right). We used methane and other hydrocarbons for the process gas at pressures of a few 10's of mTorr. The films were smooth on the nm scale, very hard (micro-indentation showed them to have Knoop hardness values of around 30 GPa), and about 0.1-0.5 μm thick (depending on deposition time and conditions). The films also contained up to 30% hydrogen, which gave them different properties to the H-free ta-C made by laser ablation.

We looked at doping these films by adding process gases such as NH₃, PH₃and H₂S in varying proportions, in an effort to make films with semiconducting and electron emission properties suitable for use in cold cathode devices such as field emission displays. These films produced very promising results, with threshold voltages of less than 15 V/μm and good longevity and maximum currents.

Production of carbon phosphide thin films

By adding larger quantities of PH₃ to the gas mixture, we were able to make thin films with P:C ratios of up to 3:1. Such films could no longer be considered as P-doped DLC, so we have termed them 'amorphous carbon phosphide' films. These films were relatively hard and had a band gap ~2 eV, and may have applications in opto-electronics. We annealled these films in vacuum to remove any impurities (such as hydrogen) and to attempt to crystallise them. The aim was to make a solid phase of crystalline carbon phosphide (or phosphorus carbide). Theoretical calculations in Prof Neil Allan's group suggested that crystalline material of stoichiometry C₃P₄ might be stable and have a pseudo-cubic structure. Such a material would be new to science, and may possess many useful electronic properties.

Production of Carbon-Sulfur thin films

Addition of H₂S to the CH₄ gas mixture allowed amorphous carbon films containing significant quantities of S to be deposited. Initial results indicated that S incorporation made the films more stable and more uniform, and inhibited delamination. We also determined some of the electronic properties of these highly S-doped DLC films.