Fabrication and transport of large-scale molecular tunnel-junction arrays Giuseppe Maruccio * , Pasquale Marzo, Roman Krahne, Antonio Della Torre, Adriana Passaseo, Roberto Cingolani, Ross Rinaldi National Nanotechnology Laboratory of CNR-INFM, University of Lecce, Via per Arnesano, 73100 Lecce, Italy Available online 1 February 2007 Abstract We demonstrate a method for the simultaneous fabrication (without the need of expensive e-beam systems) of large arrays of nan- odevices working at room temperature. The electrode gap is defined by a selective wet-etching of a AlGaAs/GaAs quantum well structure and controlled with nanometer precision. A selective oxidation of the Al rich barrier reduces the bulk leakage current by six orders of magnitude and extends the applicability of the produced devices to room temperature functionality. As a demonstration, we employ here these nanojunctions to investigate transport in molecular tunnel-junctions based on individual Azurins, a blue copper protein, under ambient conditions. This approach opens the way to the fabrication of complex circuits consisting of different nanodevices. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Nanotechnology; Molecular electronics; Nanofabrication; Nanoelectronics; Lithography 1. Introduction Molecular electronics [1] is aimed at building complex and functional devices based on few/single molecules. As a proof of concept, a number of individual components [2–7] has been reported, revealing intriguing features such as rectification [8,9], negative differential resistance [10] and Kondo effect [3,11,12]. Moreover, logic circuits based on CNT or nanowires have been also demonstrated. How- ever, the economic fabrication of complex molecular elec- tronics circuits on a large scale is still a major challenge, due to the difficulties in interconnecting molecules and fab- ricating electrical contacts to the outside world. In the last years, different techniques (including mechan- ical break junctions, electron beam lithography, electromi- gration, electrodeposition, etc.) have been developed for patterning at the nanometer scale, beyond the intrinsic physical limitations of optical lithography. However, none of these methods equals the advantages of photolithogra- phy for low cost and high throughput and most of the pro- posed methods are appropriate for contacting single devices only. Thus, a major requirement in the establish- ment of molecular electronics consists in the demonstration of reproducible and economic methods for the fabrication of large-scale arrays of nanojunctions/nanodevices. Recently, the fabrication of a large array of nanojunc- tions by optical lithography and wet etching of an AlGaAs/GaAs quantum-well (QW) structure has been reported by Krahne et al. [13]. The main advantage in this method is that the thickness of the quantum well and of the deposited metal layer controls the nanogap size with sub- nanometer precision, without the need of expensive e-beam systems. However, this innovative approach to nanoscale electronics has some intrinsic drawbacks: because of the bulk leakage currents through the semiconductor substrate under ambient conditions, such nanojunctions can be employed at cryogenic temperatures and in the dark only. In this work, we significantly improve this method by means of selective oxidation of the two AlGaAs barriers above and below the QW in order to reduce the leakage current through the semiconductor layers and thus extend 0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.107 * Corresponding author. Tel.: +39 0832 298211; fax: +39 0832 298180. E-mail address: giuseppe.maruccio@unile.it (G. Maruccio). www.elsevier.com/locate/mee Microelectronic Engineering 84 (2007) 1585–1588