nanomaterials Article Bias Tunable Photocurrent in Metal-Insulator-Semiconductor Heterostructures with Photoresponse Enhanced by Carbon Nanotubes Antonio Di Bartolomeo 1,2, * , Filippo Giubileo 2 , Alessandro Grillo 1,2 , Giuseppe Luongo 1,2 , Laura Iemmo 1,2 , Francesca Urban 1,2 , Luca Lozzi 3 , Daniele Capista 3 , Michele Nardone 3 and Maurizio Passacantando 3 1 Physics Department “E.R. Caianiello”, University of Salerno, via Giovanni Paolo II 132, 84084 Fisciano, Salerno, Italy; agrillo@unisa.it (A.G.); giluongo@unisa.it (G.L.); liemmo@unisa.it (L.I.); furban@unisa.it (F.U.) 2 CNR-SPIN Salerno, via Giovanni Paolo II 132, 84084 Fisciano, Italy; filippo.giubileo@spin.cnr.it 3 Department of Physical and Chemical Science, University of L’Aquila, via Vetoio, 67100 Coppito, L’Aquila, Italy; luca.lozzi@aquila.infn.it (L.L.); daniele.capista@student.univaq.it (D.C.); michele.nardone@aquila.infn.it (M.N.); maurizio.passacantando@aquila.infn.it (M.P.) * Correspondence: adibartolomeo@unisa.it Received: 8 October 2019; Accepted: 6 November 2019; Published: 11 November 2019   Abstract: Metal-insulator-semiconductor-insulator-metal (MISIM) heterostructures, with rectifying current-voltage characteristics and photosensitivity in the visible and near-infrared spectra, are fabricated and studied. It is shown that the photocurrent can be enhanced by adding a multi-walled carbon nanotube film in the contact region to achieve a responsivity higher than 100 mA W −1 under incandescent light of 0.1 mW cm −2 . The optoelectrical characteristics of the MISIM heterostructures are investigated at lower and higher biases and are explained by a band model based on two asymmetric back-to-back Schottky barriers. The forward current of the heterojunctions is due to majority-carrier injection over the lower barrier, while the reverse current exhibits two different conduction regimes corresponding to the diffusion of thermal/photo generated carriers and majority-carrier tunneling through the higher Schottky barrier. The two conduction regimes in reverse bias generate two plateaus, over which the photocurrent increases linearly with the light intensity that endows the detector with bias-controlled photocurrent. Keywords: carbon nanotubes; heterostructures; photoconductivity; Schottky junctions; MISIM 1. Introduction Carbon nanotubes (CNTs) have been attracting a lot of attention in the past three decades due to their remarkable chemical, mechanical, and electrical properties [1–4]. In both single-walled (SWCNT) and multi-walled (MWCNT) forms, carbon nanotubes have been considered in several electronic applications, such as transistors [5–8], diodes [9,10], memory devices [11,12], photovoltaic cells [13,14], photodetectors [15–17], strain [18] and chemical sensors [19–22], and field emitters [23–26], etc. Although standalone CNT photodetectors have shown limited performance, low-noise and enhanced light detection can be achieved by combining nanotubes with traditional semiconductors in hybrid devices [27,28]. The CNT type, whether single- or multi-walled, their quality and configuration in bundles or films of different density and thickness as well as the role of the semiconductor substrate in such hybrid structures have been widely investigated [14,29–31]. In this context, photodetectors based on MWCNT [32–34] or SWCNT [35,36] films over Si have been the preferred devices for their fast response and high detection capability combined with easy fabrication, low cost, high reliability, and compatibility with existing technologies. Optimized devices realized with SWCNT films have Nanomaterials 2019, 9, 1598; doi:10.3390/nano9111598 www.mdpi.com/journal/nanomaterials