X-ray induced electrostatic graphene doping via defect charging in gate dielectric

www.nature.com/scientificreports OPEN Received: 15 December 2016 Accepted: 8 March 2017 Published: xx xx xxxx X-ray induced electrostatic graphene doping via defect charging in gate dielectric Pavel Procházka1,2, David Mareček2, Zuzana Lišková1,2, Jan Čechal1,2 & Tomáš Šikola1,2 Graphene field effect transistors are becoming an integral part of advanced devices. Hence, the advanced strategies for both characterization and tuning of graphene properties are required. Here we show that the X-ray irradiation at the zero applied gate voltage causes very strong negative doping of graphene, which is explained by X-ray radiation induced charging of defects in the gate dielectric. The induced charge can be neutralized and compensated if the graphene device is irradiated by X-rays at a negative gate voltage. Here the charge neutrality point shifts back to zero voltage. The observed phenomenon has strong implications for interpretation of X-ray based measurements of graphene devices as it renders them to significantly altered state. Our results also form a basis for remote X-ray tuning of graphene transport properties and X-ray sensors comprising the graphene/oxide interface as an active layer. Graphene has attracted an enormous attention for its unique mechanical, optical, and electronic properties with a wide technological perspective1–5. One of the most appealing graphene attributes is the possibility of controlling the type and concentration of charge carriers via application of an electrostatic potential between a grounded graphene layer and a gate electrode, so called gate voltage. The high intrinsic charge carrier mobility in graphene implies a high application promise for use of gated graphene devices – graphene field effect transistors (GFETs) – as high speed electronic devices6, 7. Although a direct application of GFETs in electronic circuits is largely hampered by the lack of a bandgap in a single-layer graphene8, there are advanced devices that do not require the bandgap for their functionality. In particular, graphene spintronic devices9, gas sensors with sensitivity down to the single molecule limit10, 11, and photodetectors12 show great potential for future applications. The research and development of these devices is intimately connected with analysis of their structural, chemical and optical properties. In this respect, the characterization tools based on X-ray radiation are invaluable to determine bond specific chemical composition13, graphene-adsorbate charge transfer, molecular orientation, and magnetic properties naming only the most prominent14. However, the possible effect of ionizing X-ray radiation on the GFET properties should be considered. In this paper we show that the X-ray radiation induces strong changes in graphene transport properties via charging of intrinsic defects in the gate dielectric. As the semiconductor field effect transistors (FETs) comprise the heart of a modern electronic industry the huge amount of work has been devoted to understanding their properties with respect to their further development. The quality of the gate dielectric has a profound impact on the long term stability of FETs15. More particularly, defects within the dielectric layer behave like charge traps, which can be ionized, e.g., by electron or hole injection or X-ray radiation, rendering FET sensitive to ionizing radiation16. The effect of charged impurities and adsorbates is even more pronounced in graphene devices17, 18. Recently, the photo-induced doping of graphene has been realized by visible or UV radiation exposure of GFETs19–26. Here, two distinct groups of GFET devices were introduced: in the first group the charges excited within the photoabsorbing medium (e.g., MoS2, Bi2Te3, nanoparticles, and plasmonic antennas) are transferred to graphene appearing as an increase of the graphene DC conductivity19–22. Within the second group the UV/Vis radiation ionize donor-like traps leaving the gate dielectric positively charged. This charge acts as a positive gate: it increases the electron concentration in graphene by capacitive coupling23–26. In contrast to direct graphene doping from adsorbed species causing also a decrease of carrier mobility27, the major advantage of “remote gating” is its minimal impact on the charge carrier mobility24, 26, 28 . 1 CEITEC - Central European Institute of Technology, Brno University of Technology, Purkyňova 123, 612 00, Brno, Czech Republic. 2Institute of Physical Engineering, Brno University of Technology, Technická 2896/2, 616 69, Brno, Czech Republic. Correspondence and requests for materials should be addressed to J.Č. (email: ) Scientific Reports | 7: 563 | DOI:10.1038/s41598-017-00673-z 1 www.nature.com/scientificreports/ Figure 1. Evolution of GFET resistivity as a function of the back gate voltage VBG (BG trace) or time (time trace), respectively. (a) Schematic of the device utilized in this study. All data presented in this figure are obtained for Al2O3 passivated devices except the case of the panel (e). The complementary data for open devices are presented in Supplementary Information, Figure S6. (b) BG trace measured for a pristine device (no X-ray irradiation). (c) Time trace recorded during the first exposure of the device at VBG = 0 V. The colored circles on the time trace mark the associated position on the BG trace portrayed in the inset. (d) BG traces acquired on the pristine (grey) and X-ray irradiated device while the X-ray is on (black) and off (red). (e) BG traces acquired for the open (non-passivated) device before (gray), during (black) and after (red) initial X-ray irradiation. (f) Time trace measured for VBG = −70 V during succeeding X-ray irradiation. The inset depicts the position of VBG relatively to X-On BG trace. (g) Schematic illustration of formation of a time trace via a CNP shift towards more positive VBG values upon succeeding X-ray irradiation. All presented sweeps in the figure are recorded in the direction from negative to positive VBG. Surprisingly, only little attention was paid to reveal the influence of X-ray radiation on graphene in the GFET configuration. In this respect, Copuroglu et al. studied the effect of the gate voltage on the shift of the core-level peaks associated with graphene and gate dielectric using X-ray photoelectron spectroscopy29. In a separate work, in pursuit for graphene application as an X-ray sensor, Cazalas et al. observed the change in graphene source-drain current of GFET upon hard X-ray irradiation (15 keV) of graphene on SiC30. While the latter work introduces the GFET as a device sensitive to X-ray irradiation, the basic description and understanding of the X-ray radiation effect on the GFET is still missing. Here we show that X-ray radiation induces the ionization of donor-like defects in the gate dielectric leading to a large increase of the electron concentration in graphene, i.e., to its strong negative electrostatic doping (n-doping). This r (...truncated)