The possibility of adding impurities to diamond (a process called doping) and so changing it from being an electrical insulator to a semiconductor, opens up a whole range of potential electronic applications. However, there are a number of major problems which still need to be overcome if diamond-based devices are to be achieved. First, CVD diamond films are polycrystalline containing many grain boundaries and other defects, which all reduce electrical conductivity. For effective device operation, single crystal diamond films are required, and this still hasn't been achieved. Another problem, which to some extent has recently been solved, is the requirement that the diamond films must be patterned to produce features of similar size to those used in microcircuitry, typically a few microns. Fortunately, diamond can be etched in oxygen-based plasmas, provided a suitable non-erodible mask is used. Diamond films can now be patterned to geometries suitable for all but the most demanding devices. The final, and probably the most difficult problem to solve in order to be able to create diamond devices, is that of doping - changing the conductivity of the diamond reliably and reproducibly by incorporation of suitable impurity atoms. Unfortunately, most electronic devices require two different types of impurities to be present, ones that lead to an excess of positive charge, and ones that lead to an excess of negative charge. Creating excess positive charge (p-type doping) is relatively straightforward, since addition of a small amount of a boron-containing gas such as diborane to the CVD process gas mixture is all that is required to incorporate boron into the structure. However, the close packing and rigidity of the diamond structure make incorporation of atoms larger than carbon very difficult. This means that impurities (such as phosphorus or arsenic) which are routinely used to create excess negative charge (n-type doping) in other semiconductor materials like silicon, cannot easily be used for diamond. The development of a successful n-type doping process has taken a considerable time, and only very recently have a few reports appeared from Japan claiming success in this area using sulphur as the necessary impurity. Despite these difficulties, diamond-based devices are gradually beginning to appear, and it may become the material of choice for electronic applications involving high power and/or high temperature.
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