1 Negative Photoconductance in Heavily Doped Si Nanowire Field- 2 Effect Transistors 3 Eunhye Baek, † Taiuk Rim, ‡ Julian Schü tt, † Larysa Baraban,* ,†,§ and Gianaurelio Cuniberti †,§ 4 † Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062 Dresden, Germany 5 ‡ Department of Creative IT Engineering, Pohang University of Science and Technology, 37673 Pohang, Korea 6 § Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany 7 * S Supporting Information 8 ABSTRACT: We report the first observation of negative photo- 9 conductance (NPC) in n- and p-doped Si nanowire field-effect transistors 10 (FETs) and demonstrate the strong influence of doping concentrations on 11 the nonconventional optical switching of the devices. Furthermore, we 12 show that the NPC of Si nanowire FETs is dependent on the wavelength of 13 visible light due to the phonon-assisted excitation to multiple conduction 14 bands with different band gap energies that would be a distinct 15 optoelectronic property of indirect band gap semiconductor. We attribute 16 the main driving force of NPC in Si nanowire FETs to the photogenerated 17 hot electrons trapping by dopants ions and interfacial states. Finally, 18 comparing back- and top-gate modulation, we derive the mechanisms of 19 the transition between negative and positive photoconductance regimes in 20 nanowire devices. The transition is decided by the competition between the 21 light-induced interfacial trapping and the recombination of mobile carriers, 22 which is dependent on the light intensity and the doping concentration. 23 KEYWORDS: Negative photoconductance, hot electron trapping, interfacial trapping, Si nanowire, indirect band gap semiconductor 24 N egative photoconductance (NPC) is a rare effect because 25 the photoexcitation of charge carriers normally enhances 26 the channel conductivity. 1 In order to reach the situation, when 27 the channel conductivity is decreased (NPC), additional 28 electronic states are required that can compensate a generation 29 of photoelectrons. Some of the low-dimensional materials (e.g., 30 nanoparticles, nanowires, and thin film) reveal a negative 31 photoconductance due to the surface effects originating from 32 the high surface-to-volume ratio. 2,3 Thus, the large surface area 33 of nanostructured materials can potentially generate high 34 density of localized energy states acting as traps for charge 35 carriers, sufficient to reverse the type of the channel 36 conductivity. For instance, arrays of metal nanoparticles, 37 which are capable of surface plasmon excitations upon light 38 illumination, can reveal the NPC due to the presence of 39 interfacial charges. 2 On the other hand, the NPC in 40 semiconductors is of different nature and is linked to the 41 energy band gap structure. In many cases, the NPC has been 42 observed in large band gap semiconductors such as AlN, 4 p- 43 ZnSe, 5 or Ga 2 O 3 6 with sub-band gap excitation where 44 photoexcited electrons can be captured by extrinsic (e.g., 45 surface oxygen) and intrinsic (e.g., defects) trap states in the 46 middle of the band gap. Moreover, because photoexcited 47 electrons are generated via the superband gap excitation, NPC 48 requires additional phenomena like scattering at recombination 49 centers in InN. 7 50 Photoconductivity studies of Si have a long history 8,9 as well 51 as numerous industrial realizations 10 because of the well-known 52 electronic properties and performance, e.g., high speed and 53 efficient signal processing and compatibility with various 54 electrical platforms by mature integration. The NPC of bulk 55 Si was observed for the first time in cobalt-doped Si under the 56 infrared light illumination. 8,11 The localized energy states of 57 dopants in the band gap of Si act as a powerful recombination 58 center, which is typical sub-band gap NPC phenomena. During 59 the past decade, Si and Si nanostructures, especially nanowires, 60 have been studied for various optical applications, such as 61 photodetectors, 12,13 photovoltaics, 14,15 and solar cells, 16,17 using 62 advantages from a one-dimensinal structure and relying mostly 63 on the phonon-assisted photoexcitation, due to its indirect 64 bandgap, and generating a conventional “positive” photo- 65 current. 66 However, despite the well-developed Si photodetectors 67 offered on the market and the enormous research and industrial 68 demands of Si nanowires for various optical applications, the 69 NPC in Si nanowire devices has not yet been reported. In 70 particular, modern Si nanowire field effect transistors (FETs) 71 need proper doping in the conduction channel for effective gate Received: July 1, 2017 Revised: August 30, 2017 Published: September 29, 2017 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.7b02788 Nano Lett. XXXX, XXX, XXX−XXX mac00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i12 HF02:4458 | 2.0 alpha 39) 2016/10/28 09:46:00 | PROD-JCAVA | rq_11261579 | 10/02/2017 11:32:49 | 8 | JCA-DEFAULT