Diamond surface conductivity: Properties, devices, and sensors
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troduction Diamond, with a bandgap of 5.47 eV, is an insulating material when undoped. However, when terminated with hydrogen, the C–H bond at the terminated surface creates a dipole layer that induces a negative electron affinity reported to be as low as –1.3 eV and a concurrent lowering of the ionization energy to 4.2 eV.1 While this phenomenon has led to significant interest in electron emission devices due to the ease with which electrons excited into the conduction band can be emitted from the surface, it additionally drives the transfer of electrons from the diamond valence band into the accepting state of an appropriate adsorbate layer on the diamond surface.2 This gives rise to p-type surface conductivity carried by a resulting subsurface hole accumulation layer that extends a few nanometers into diamond and supports a carrier density of up to 4 × 1013 cm–2.2 In addition to lowering the ionization energy sufficiently to facilitate electron transfer from the valence band into the adsorbate layer, the hydrogen termination passivates the surface by removing interfering surface states from the gap that would otherwise accept the transferred charge preventing them from contributing to free carriers in the diamond. In its simplest form, the accumulation of holes and band bending at the surface of hydrogen-terminated diamond occurs as a consequence of charge transfer into an adsorbed water layer arising from exposure to air (Figure 1a).3 It was shown in this case that surface charge transfer arises from an
electrochemical redox reaction between the adsorbed water layer and the diamond surface, a mechanism that is unique to hydrogenated diamond among semiconductor materials under standard atmospheric conditions.3 Strobel et al. showed that p-type surface doping could also be achieved with fullerene (C60) and fluorinated fullerenes;4,5 in this case, charge is transferred from the diamond valence band into the lowest unoccupied molecular orbital (LUMO) of the fullerene surface acceptor. While providing supporting evidence for the surface transfer doping model proposed as being responsible for the charge accumulation at the surface of diamond, this work additionally presented the possibility of using other molecular species as surface acceptors on the hydrogen-terminated diamond surface. Surface transfer doping of diamond with other molecular species, such as 7,7,8,8-tetracyanoquinodimethane and its fluorinated derivative,6 and molecular heterolayers7 have subsequently been demonstrated. Recently, Russell et al. have extended this work further to include the use of the transition metal oxide MoO3 as a surface electron acceptor material, which may offer improved stability and performance in diamond electronic devices.8
Interfacial energy level alignment In contrast to doping with an adsorbed water layer, organic molecules provide a simpler acceptor system that can be probed using standard surface analysis techniques to aid the
Christopher I. Pakes, Department of Physics, La Trobe University, Victoria, Australia; c.pakes@la
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