CVD Diamond for Ultraviolet and Particle Detectors

Abstract

Conventional solid-state photodetectors, such as those made using silicon, are typically diode structures operated with a reverse bias placed across them. Low dark currents result; carriers photo-generated in the depletion region form a drift current which is the basis for light detection. Light with an energy greater than the band gap of the material can be seen. Silicon has a 1.1 eV band gap and devices fabricated from this material therefore react to both ultra-violet and visible wavelengths. The high resistivity of diamond suggests that diode structures may not be needed to achieve low dark currents if this material were used for the fabrication of photodetectors. Metal/diamond/metal devices can be considered which simply rely upon photoconductivity for their operation; such a device could possess high gain, since many carriers may be able to flow around the detector circuit during the lifetime of a photo-generated electron-hole pair. The wide band gap (5.5 eV, 225 run) of diamond implies that this form of photodetector will be capable of detecting deep UV light while being essentially “blind” to visible wavelengths. This property is highly desirable; filtering conventional devices to make them visible blind significantly reduces their sensitivity to UV light. The physical and chemical robustness of diamond also suggests that such devices may be suitable for operation in hostile environments. Diamond can exhibit high carrier mobilities, saturated carrier velocities and electric field breakdown strength; these properties suggest that fast detectors may be realisable. Many industrial, military and environmental applications can thus be envisaged for diamond UV sensors. The emergence of commercially accessible thin-film diamond grown by chemical vapour deposition (CVD) has enabled reliable devices to be developed. In this chapter, the intrinsic and extrinsic photoconductivity of thin-film diamond is reviewed; the design, fabrication and performance of diamond UV photodetectors is then discussed, along with some of the uses for these devices. For some applications, it may be desirable to fabricate diamond photodiodes; the realisation of this type of device from p-type material is also addressed.