Photoassisted Fowler-Nordheim-like tunneling from passivated GaAs microcantilevers

Abstract

We have investigated the mechanisms of photoelectron tunneling into gold from tipless, nitridized, gallium arsenide microcantilevers through thin polymers. A study of the tunnel bias, tunnel distance, and light excitation power dependence of the tunneling photo-current reveals the tunneling process to be Fowler-Nordheim like. Both the value of the dielectric constant and the sign of the tunneling photocurrent at low excitation power suggest the important role of polymer defects.While the relevant processes for electron transport in reverse-biased metal-insulator-structures have been identified as standard tunneling, Fowler-Nordheim (FN) tunneling 1,2 or hopping between defects in the insulator, 3 there are at least three types of systems for which a better understanding of this transport is the subject of active research: (i) tunneling between metals through polymer or organic layers, 4,5 both for fundamental investigations of molecular energy levels and for applications exploiting light emission from organic compounds (calculations and experiments reveal FN-like behavior governed by the localized energy levels of the polymer), 6 (ii) charge and spin injection in hybrid semiconductormagnetic metal devices in the dark 7 or under light excitation, 8 and (iii) spin-polarized electron tunneling from optically pumped GaAs using a tunnel gap of adjustable width. 9,10 In this work the tunneling mechanism for photoelectron transport from a GaAs microcantilever through a polymer and into a gold surface is investigated. Gold is chosen since it has an energy-independent density of states. 11 As shown in the inset of Fig. 1, the GaAs microcantilever, fabricated using an assembly technique developed by the authors, 12 is held at a distance d from the gold surface and is excited from the rear by a laser whose energy (hν = 1.59 eV) is close to the bandgap, E g , of GaAs. By monitoring the position of the laser beam reflected from the cantilever, we ensure that all tunneling current measurements are made prior to mechanical contact. The tunnel gap between the GaAs and the gold is filled with low-viscosity (20 cSt), hydrophobic, polydimethylsiloxane (PDMS) by spin coating onto the heated metal surface (130 • C) in order to desorb all traces of water. The native oxide covering the GaAs cantilever is replaced by a monolayer of nitrogen using a recently developed hydrazine-sulfide treatment followed by annealing in an inert gas atmosphere at 500 • C. 13,14 The experimental setup and procedure have been described before. 15 After stabilization of the dark current (I dark ) to a set current at a bias of V c = −1.5 V, the feedback loop is opened and two bias scans are performed, one with the laser on and the second one in the dark. The difference between the measured currents is the tunneling photocurrent (I ph ). Since relatively small set currents are used, both I dark and I ph are stable over a period of several hours.The band structure of the metal-insulator-semiconductor structure is shown in Fig. 1. Optical excitation of photoele...