Diamond is a material with semiconductor properties that are superior to Silicon (Si), Germanium (Ge) or Gallium Arsenide (GaAs), which are now commonly used in semiconductor devices. In particular, diamond provides a higher band gap, a higher breakdown voltage and a greater saturation velocity which produces a substantial increase in its projected cutoff frequency and maximum operating voltage compared to devices fabricated from Si, Ge, or GaAs. Furthermore, diamond has the highest thermal conductivity of any solid at room temperature and excellent conductivity over a temperature range up to and beyond 500.degree. k. Diamond therefore holds the potential for efficient semiconductor operation at high frequency and power. Finally, diamond, by virtue of its small molecular size compared to other materials, provides a smaller neutron cross section which reduces its degradation rate in radioactive environments.
To date, these advantages of diamond as a semiconductor have not been exploited, in part because of the difficulty in forming electrical contacts on diamond surfaces which allow access to and control of diamond semiconductor devices. It is desirable to form both ohmic contacts (i.e., those whose resistance to current flow is symmetrical with respect to direction of flow) and diode-type Schottky contacts (i.e., those whose resistance to current flow is asymmetrical). It is also desirable to form insulating films on diamond which might be used, for example, in metal-oxide-semiconductor (MOS) devices.
One method for forming ohmic contacts on diamond may be found in U.S. Pat. No. 5,002,899 by Geis et al., entitled: "Electrical Contacts on Diamond." Known methods for forming Schottky contacts on diamond involve direct application of metals to p-doped diamond substrates as discussed in "Electronic and Optical Processes in Diamond", Nauka Publishing House, Moscow Office of Physio-Mathematical Literature, Moscow, 1985 by Vavilov et al. Presently, the inventors are not aware of a prior method for the information of high quality insulating films on diamond substrates, or formation of Schottky contacts upon such films.
Notwithstanding the foregoing, the formation of insulating silicon dioxide films on silicon semiconductor substrates is well known. Silicon dioxide (SiO.sub.2) is a particularly desirable insulator because of its inherent high insulating properties, and its compatibility with silicon wafer processing.
Several methods of forming these films are available. For example, silicon dioxide may be deposited on a silicon substrate by either thermal or plasma chemical vapor deposition (CVD). Using this technique, decomposition of a mixture such as of an oxygen-containing and silicon-containing gas or gasses by plasma or heat produces silicon and oxygen atoms which react to form silicon dioxide. The latter is then deposited onto a substrate surface held in the reaction region. In another method, known as the "Spin on Glass" technique, an organic compound containing a silicon oxide moiety is applied to the substrate surface and is then heated. The organic component is cleaved from the silicon oxide moiety and then removed from the surface by evaporation or sublimation while the silicon radical reacts with ambient oxygen to produce a film of silicon dioxide. Finally, in the "thermal growth" method, which is only applicable to a silicon substrate, such a substrate is heated to temperatures in a range of 900.degree. C.-1200.degree. C. in an oxygen atmosphere to form a silicon dioxide film directly on its exposed surface.
Critical to the formation of a device quality insulating film for use in semiconductors are the particular properties of the interface between the insulator and the semiconductor. In non-critical applications, the interface is known to contain interface surface states which are thought to arise from interruptions in the lattice structures at the surface of the insulator and are capable of changing charge by acceptance or donation of charge carriers (holes or electrons) from the semiconductor. These states are affected by voltages applied through the insulator and thereby degrade control of the carriers in the semiconductors. The presence or absence of these interface states is known to be highly dependent both on the nature of the semiconductor as well as on the method used to obtain the insulating film.