Single Crystal Diamond Schottky Photodiode

Thanks to its extreme optical and electronic properties, diamond appears to be a promising semiconducting material for photon detection. Its wide band-gap, 5.5 eV, results in a very low leakage current and its electronic properties as high carrier mobility allow fast time response (J. E. Field, 1979). Besides, it has a large breakdown electric field (∼10 V/┤m), a low dielectric constant (i.e. low capacitance), chemical inertness and low intrinsic carrier density, which makes cooling for noise reduction unnecessary (J.Prins, 1997). Its extreme radiation hardness is well known and another interesting feature, again related to the wide band-gap, is its selective sensitivity to radiation with wavelengths shorter than 225 nm (visible-blind detectors) (J.F. Hochedez et al., 2002). Several attempts have been made to build up UV detectors from natural or synthetic diamonds grown by Chemical Vapour Deposition (CVD). A detector often reported in literature is the photoresistor (A. Balducci e al., 2005; T. Teraji et al., 2004) having a planar structure and consisting of a photoconductive diamond film with metal electrodes placed on the top surface. It can operate only with external voltage applied and the signal is affected from secondary electron emission, which is known to strongly affect the detection properties in the UV and EUV spectral regions. A different geometry reported is a polycrystalline sandwiched photodiode structure (V.I. Polyakov et al., 1998, L. Thaiyotin et al., 2002) with a contact on the diamond growth surface and a backside contact on the silicon substrate. However, the CVD diamond performance is limited in this case by the polycrystalline structure due to defect states in the band gap introduced by the grain boundaries (R. D. McKeag&R. B. Jackman, 1998, L. Barberini, 2001), which affects the photoelectric properties and alters the detection characteristics. On the other hand, detector grade natural diamonds are extremely rare and expensive, while high pressure high temperature (HPHT) diamonds have their performance strongly worsened by defects and impurities (E. Pace et a., 2000). A great effort is therefore being devoted to produce device-grade Single Crystal Diamond films (SCD) by homoepitaxial CVD growth on low-cost diamond substrates (S. Almaviva et al., 2009, 2010a). A few years ago, at the University of Rome “Tor Vergata” laboratories, CVD single crystal diamond films were used to obtain a new class of detectors with a layered structure. Thanks to the combination of boron doped and intrinsic single crystal diamond films, together with the possibility to easily build Schottky junctions on intrinsic diamond by thermal evaporation of the metal contacts, it has been possible, by using simple multilayered a p-type/nominally intrinsic diamond/metal layered structures, to obtain high quality and highly reproducible devices which can be effectively used for detection (UV and X-rays) photons.