The principal objective of HETERO is the development of chemical vapour deposition processes for the fabrication of heteroepitaxial diamond and silicon carbide semiconductor thin films. To achieve this goal the project has screened surface modifications, thin film buffer layers and novel substrates to promote diamond epitaxy ameliorating the effects of diamond's small lattice parameter and high surface energy compared with potential semiconductor substrate materials; developed silicon carbide deposition techniques using novel precursor chemicals in both plasma enhanced- and thermal-chemical vapour deposition processes. The final aim of HETERO is to establish fabrication routes for advanced wide bandgap semiconductor materials for applications requiring wide bandgap semiconductor electronic devices in hostile environments.
Surface engineering techniques to produce novel candidate substrates for thin film diamond heteroepitaxy have been investigated. The techniques have examined surface ion beam modifications, thin film buffer layers and novel substrates. Ion beam surface modification processes have been used to treat substrates with boron, carbon and nitrogen to promote the nucleation of diamond on single crystals. Thin film epitaxial buffer layers based on silicon carbide and titanium nitride have also been prepared as potential substrate materials for oriented diamond deposition. Silicon carbide films have been used in diamond growth experiments. Chemical vapour depostion have been investigated growth experiments have also examined the use of nickel alloys, boron nitride and oxide single crystal substrates. Low temperature diamond deposition processes have been developed in order to expand the range of potential epi-substrates for heteroepitaxial diamond thin films. Boron doping of CVD diamond films has been developed using novel precursor gases.
A lamp heated reactor to investigate thermal chemical vapour deposition of silicon carbide films has been constructed. Temperatures up to 1100 C have been achieved in the reactor. Reproducibility and uniformity of the thermal gradients within the deposition zone have been established. A 4-bubbler system has been constructed with one being used for novel precursors and impurity dopant precursors. Specimens have been prepared and characterized by infrared (IR) spectroscopy and photothermal deflection spectroscopy.
A plasma CVD SIC reactor has been constructed, comprizing a water cooled stage and resistance heater for the sample, to complement lamp heating work. The system allows transfer of samples between low energy electron diffraction (LEED) and Auger characterization facilities without exposure to atmosphere via a gate valve in the top of the chamber. A thermogravimetry and mass spectrometry system has also been constructed for in situ analysis.
APPROACH AND METHODS
The general methodology adopted in HETERO has been identification and preparation of epi-substrate material; development of advanced CVD precursors for the semiconductor and for dopant impurities; and tailoring of reactor geometries to the appropriate CVD processes. The dual development of diamond and silicon carbide semiconductor films is focused towards their eventual combination to enable the realisation of hybrid semiconductor material systems.
The use of surface and interface engineering techniques is required to obtain suitable substrates for diamond heteroepitaxy. Without modification most of the conventional substrate materials impose excessive lattice parameter mismatch which inhibits epitaxy. In the case of silicon carbide thin films the advanced nature of silicon wafer production is a strong influence on its use as substrate for heteroepitaxial deposition however the development of advanced organic precursor systems is playing a central role in the development of low defect density films. It is intended that heterojunctions consisting of diamond and silicon carbide semiconducting layers will be fabricated and characterised. This approach will target the use of both n- and p-type layers of each material if an n-type dopant precursor for diamond can be identified.
If successful, the industrial impact for Europe will be the provision of sources of device-quality diamond and silicon carbide semiconductor materials which will provide an important technological springboard into application areas such as switched-mode power convertors, medical electronics, aerospace electronics, and devices for motor control. European manufacturers will benefit strategically from freely available power semiconductor materials. Accomplishment of the projects aims will lead to use the arising material in advanced engineering systems and in the establishment of a European source for these materials. Overall, the programme provides a unique opportunity for the multi disciplinary skills in a SME, IME, university and a research institute to be combined to provide the basic research understanding of heteroepitaxial deposition that is required for a technological breakthrough in wide bandgap electronics.