Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24318−24330
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Reduced Graphene Oxide/Amorphous Carbon P−N Junctions: Nanosecond Laser Patterning Siddharth Gupta and Jagdish Narayan*
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Department of Materials Science and Engineering, North Carolina State University, Centennial Campus, Raleigh, North Carolina 27695-7907, United States ABSTRACT: The device integration of graphene and reduced graphene oxide (rGO) is impeded by scalability and high temperature (>2000 K) treatment required for effective reduction into high-quality rGO. In this article, we present a novel approach for direct laser writing of heavily reduced graphene oxide films by nanosecond laser melting of amorphous carbon on silicon (001) substrates under ambient conditions. Ultrafast quenching from the undercooled melt state above the melting threshold energy density (Ed) of 0.4 J/cm2 leads to the formation of large-area rGO films. The first-order phase transformation of liquid carbon into graphene is triggered by low undercooling at the C melt/silicon interface. The laserirradiated rGO films exhibit electron mobility of 12.56 cm2/V s and charge carrier concentration of −1.2 × 1021/cm3 at 300 K. Temperature-dependent electrical measurements and Raman spectroscopic investigations suggest low disorder and charge transport via 2D Mott variable range hopping between the graphene islands for rGO films. The localization length corresponding to the size of these graphitic domains is 3 nm. The ultrafast regrowth of rGO creates an atomically sharp interface between n-type rGO and p-type amorphous carbon, resulting in p−n junction heterojunction diodes with a turn-on voltage of 0.3 V, rectification ratio of 110@±1.5 V, and activation energy of 0.13 eV under reverse bias. This unique laser processing method solves the problems of traps and defects associated with equilibrium-based rGO fabrication methods, enabling high conductivity and mobility, providing insights into the fundamental mechanism driving laser writing of graphenebased materials on silicon. KEYWORDS: graphene, p−n junction, diode, laser patterning, transmission electron microscopy, Raman spectroscopy, molecular dynamics the added advantage of tunability13,14 in its bandgap as a function of reduction. The GO films exhibit low electrical conductivity due to the presence of functional groups, such as C−OOH, −OH, and −O− which scatter the electrons conducting along the −C− basal plane. The tunability of sp2/sp3 ratio by thermal and chemical reduction of GO is a powerful way to modulate its bandgap, thereby transforming it from an insulator to a semiconductor or a graphene-like semimetal.15 The GO films undergo controlled reduction using hydrazine monohydrate with a change in bandgap from 2.00 to 0.02 eV.16 The reduction of GO creates sp2 nanoclusters with O removal which provide percolation pathways for electronic hopping.17,18 However, the equilibrium-based processing techniques pertaining to thermal reduction of GO have demerits of increased thermal budget, while the chemical reduction route injects unwanted chemical impurities and traps which drastically deteriorate the mobility and conductivity. Because of the requirement of high-temperature treatment for sufficiently long timescales, the reduction processes such as rapid thermal annealing (RTA) damage the pre-existing device architecture, limiting use of rGO in devices. To integrate rGO
1. INTRODUCTION Since its discovery, graphene has attracted significant attention due to its excellent electro-optical, mechanical, and thermal properties.1 Notably, the ballistic transport of its carriers2 due to their massless Dirac fermionic behavior gives rise to record carrier mobility.3 The transport properties coupled with the high Young’s modulus of 0.5 TPa4,5 and the optical transparency of 97.7%6 have resulted in combinatorial usage of graphene in multifaceted device architectures enabling its usage in field-effect transistors ,7 memory devices,8 and energy storage applications.9 However, to date, micromechanical cleavage remains the most effective process to fabricate pristine graphene.1 The low yield by mechanical processing hampers the industrial implementation of integrated graphenebased circuits. Having said that, progress has been made toward scalable, large area graphene production via chemical route (PMMA) exfoliation.10 This route leaves residual impurities, thickness deviations, and wrinkles in the graphene basal plane, making it unfit for scalable device manufacturing. The intrinsic zero-gap semimetal nature of graphene results in extremely short carrier lifetime (∼picoseconds),11,12 which limits its applications in the semiconductor industry. This has created interest in graphene derivatives, such as graphene oxide (GO) and reduced GO (rGO)a two-dimensional semiconductor with distinct bandgap and high carrier density, with © 2019 American Chemical Society
Received: March 26, 2019 Accepted: June 11, 2019 Published: June 11, 2019 24318
DOI: 10.1021/acsami.9b05374 ACS Appl. Mater. Interfaces 2019, 11, 24318−24330
Research Article
ACS Applied Materials & Interfaces
2. EXPERIMENTAL SECTION
into multifunctional device architectures, it is essential to locally fabricate rGO, while the rest of the wafer remains under ambient conditions. Such an operational opportunity is not presented by equilibrium-based techniques. Hence, there is a need to develop processing techniques to fabricate efficient highly reduced GO films. The formation of graphene-related materials such as GO and rGO by the current equilibrium processing faces fundamental challenges as the free energy of materials remains unchanged with the number of layers. Hence, it is not possible to fabricate identical structures using equilibrium techniques. Therefore, we propose a highly nonequilibrium technique of nanosecond pulsed laser annealing (PLA) to transform amorphous carbon into heavily reduced GO films in air at room temperature. Post-PLA, the heat flow is spatially and temporally confined, making it an ideal technique to melt materials like carbon, which are susceptible to sublimation. As the PLA processing ends in 1000 K, a new state of carbon: Q-carbon20 is formed, which is a new phase of solid carbon with a higher mass density than amorphous carbon and a mixture of mostly fourfold sp3 (75− 85%) bonded carbon (with distinct entropy). The Q-carbon is harder than diamond,19,21 exhibits room-temperature ferromagnetism,22 field-emission,23 and high-temperature superconductivity24 upon B doping. The controlled melting of amorphous carbon leads to the formation of rGO films on Si which exhibit epitaxial nature locally, with ID/IG ratio of 0.33, highlighting the heavily reduced nature of these films. The reduction of rGO causes increased n-type conduction (with 1.2 × 1021 electrons/cm3), with high conductivity of 2600 S cm−1 and mobility of 12.56 cm2/V s. The reduction of GO creates disorder making the low-temperature electronic transport in rGO films inherently similar to bulk disordered semiconductors, where electron localization and hopping conduction drive the carrier transport.25 For highly disordered rGO films, the density of states vanishes, creating a wide coulomb gap (ECG), which makes ECG dominant at all measurable temperatures. This results in variable range hopping (VRH) governed by the Coulomb gap, known as Efros−Shklovskii-VRH (ES-VRH).26 For electronic systems with low disorder, ECG is low, and carriers have enough energy to overcome the barrier, resulting in Mott-VRH which is dependent on the dimensionality of transport.27 The PLAprocessed rGO films are shown to follow the Mott-VRH model of charge carrier conduction. The improved carrier transport for rGO films is shown to arise from the denser structure with higher graphitic nanodomains (localization length of 3 nm). With the intrinsic p-type character of diamond-like carbon (DLC) films, nanosecond PLA leads to the formation of atomically sharp p-DLC/n-rGO heterojunction diodes with a low turn-on voltage of 0.3 eV and low leakage current. Finite element simulations are performed to attain depth-dependent solid regrowth profiles for the C system. The conversion of amorphous carbon into graphitic carbon was simulated by molecular dynamics (MD) simulations employing the “liquid quench” method. Nanosecond PLA processing has enabled precise patterning of pure carbon-based diodes, which may further propel the field of biocompatible nanoelectronics.
2.1. Synthesis of rGO/DLC Diodes. DLC films are deposited on Si by laser ablation of a glassy carbon target, using KrF excimer laser having 25 ns pulse duration, 248 nm wavelength, and energy density 3.5−4.0 J cm−2, to a thickness of 100−400 nm under the high vacuum of 1 × 10−6 Torr inside the pulsed laser deposition (PLD) chamber at room temperature. Subsequently, PLA was performed on DLC films, using a single pulse of ArF excimer laser (193 nm; 20 ns) with 0.4− 0.6 J cm−2 energy density at room temperature and pressure in air. The energy density was controlled using a converging lens, generating a spot size of 1.0 ± 0.01 cm2. 2.2. Characterization. The rGO/DLC heterostructures are characterized by Raman spectroscopy, X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), high-resolution scanning electron microscopy (HR-SEM), transmission electron microscopy (TEM) imaging, and selected-area electron diffraction pattern (SAED), in addition to the temperature-dependent IV and resistance measurements. A Horiba Jobin Yvon LabRAM HR Evolution confocal micro-spectrometer is utilized to perform Raman spectroscopy measurement, in a backscattering configuration with a HeCd source at the 442 nm wavelength for laser excitation. The arrangement allows for a spatial resolution better than 1 μm and a spectral resolution of 3 cm−1 to characterize the Raman-active vibrational modes in the asdeposited and laser-annealed samples. The Raman spectra were calibrated using single-crystal silicon with a characteristic Raman peak at 520.6 cm−1. High-resolution scanning electron backscattering images were acquired for structural analysis and phase identification was performed by FEI Verios 460L field-emission scanning electron microscope (FESEM). The out-of-plane orientation of these films was determined using PANalytical X’pert PRO X-ray diffractometer with Cu Kα radiation at 40 kV and 40 mA current. To analyze the Hall mobility and charge carrier concentration for the DLC and rGO films the Ecopia HMS-3000 Hall measurement system was used. The calibration for Hall measurements was performed using standard indium tin oxide thin films. Resistivity measurements down to 5 K were performed using an AC transport measurement option in a Quantum Design Physical Property Measurement System (PPMS). Electrical contacts were made to the sample in a four-probe configuration with silver wires and conductive silver paint for the resistivity measurements. The devices were then bonded to a chip carrier in four-probe configuration and loaded into a variable temperature cryostat for temperature-dependent electronic transport measurements. The measurements were performed using a Keithley 2400 source meter, and a current preamplifier (DL1211) capable of measuring picoampere signal interfaced with the custom-built LABVIEW program. The electrical characteristics of a diode and a resistor were measured by a vacuum chamber current−voltage (I−V) probe station connected to a Keithley 4200 semiconductor analyzer. XPS is performed with X-ray energy 10−14 kV for Al/Mg and Al/Ag sources, employing superior analyzer (PHOIBOS 150) having
