Electronic and Sensing Properties of Diamond

Abstract

Diamond has attractive properties as an advanced electronic material. Its combination of high mobility, breakdown and thermal conductivity results in the largest Johnson’s and Keyes’ figures of merit by far. For example, the cutoff frequency of diamond transistors, as governed by the semiconductor material, suggests that if diamond devices could be realized, the enhancement in electronic performance would be extensive. Trew’s calculations of the frequency performance of realistically scaled diamond MESFETs noted “performance significantly better than possible with GaAs MESFETs.”

The high pressure diamond fabrication processes developed in the mid 1950’s synthesize particulate diamond. The direct deposition of diamonds layers from chemical vapor processes, pioneered by B. V. Derjaguin, B. V. Spitsyn and J. Angus, provide new vistas for diamond applications. These efforts have clearly demonstrated that diamond film with electronic potential can be routinely formed. The realization of that potential is another matter. There are material problems that curtail exploitation for circuits, principal among these is that the deposited films are often polycrystalline and hence contain grain boundaries, twins, stacking faults and other lineage and area defects which reduce the mobility and minority carrier lifetime. Although the potential of diamond has been demonstrated and active devices has been made, they were typically achieved by using homoepitax on natural or synthetic high pressure diamond substrates. To date there have been no corroborated observations of a means of achieving homoepitax (single crystal diamond grown on a non diamond substrate) and, therefore, no practical means of achieving diamond devices. Also, the excellent mobility values for carriers measured in natural single crystal diamond have not been routinely achieved in synthetic diamond film.

Nevertheless, polycrystalline diamond films (PDF) possess the same basic semiconductor materials properties which are desired for use in electronics and sensors, such as a wide band gap (5.45 eV), low thermal coefficient of expansion (~1 x 10^-6 / °C) and high thermal conductivity (20 W(cmK)^-1). Because of these properties, diamond devices could potentially be used in high-power, high-temperature environments. The development of a PDF passive device, such as a resistor, discussed in this presentation, facilities the refinement of techniques required for the development of diamond active devices. These techniques are, in part, adapted from standard microelectronic technologies. Furtheremore, there is a growing interest in the utilization of polycrystalline diamond film as a basic materials for sensor application, and with good cause. The feasibility of using the new diamond technology as a piezoresistor structure for strain sensing applications has been established and demonstrated by the author. The use of PDF resistor structures for strain sensing applications brings together many desirable characteristics, such as, high temperature operation, relatively high factor, small size, good mechanical, thermal, and chemical properties, electrical stability, and compatibility with hostile environments.

In this paper we review some of the reported achievements related to diamond as an electronic and sensor material. The state of heteroepitaxial films and polycrystalline diamond active electronics will be covered; negative electron affinity is briefly covered, and the pierzoresistive effect in diamond and its relevence to sensing is described.