We study the effect of carbon to oxygen ratio (C/O) on the electrical resistance of functionalized graphene sheets prepared by thermal exfoliation and reduction of graphite oxide at various temperatures. Using a 2-probe technique in conjunction with Kelvin probe force microscopy, we observe a transition from high-resistance (>400 kΩ/sq) nonlinear current/voltage characteristics at low C/O to low-resistance (<10 kΩ/sq) linear behavior at high C/O, indicating a transition from hopping to diffusive electron transport. Simultaneously, the metal-graphene contacts change from high-resistance Schottky-type behavior to nearly non-invasive metal-metal contact characteristics.
The exploitation of the excellent intrinsic electronic properties of graphene for device applications is hampered by a large contact resistance between the metal and graphene. The formation of edge contacts rather than top contacts is one of the most promising solutions for realizing low ohmic contacts. In this paper the fabrication and characterization of edge contacts to large area CVD-grown monolayer graphene by means of optical lithography using CMOS compatible metals, i.e. Nickel and Aluminum is reported. Extraction of the contact resistance by Transfer Line Method (TLM) as well as the direct measurement using Kelvin Probe Force Microscopy demonstrates a very low width specific contact resistance down to 130 Ωμm. The contact resistance is found to be stable for annealing temperatures up to 150°C enabling further device processing. Using this contact scheme for edge contacts, a field effect transistor based on CVD graphene with a high transconductance of 0.63 mS/μm at 1 V bias voltage is fabricated.
Hexagonal boron nitride
(hBN) is widely used as a protective layer
for few-atom-thick crystals and heterostructures (HSs), and it hosts
quantum emitters working up to room temperature. In both instances,
strain is expected to play an important role, either as an unavoidable
presence in the HS fabrication or as a tool to tune the quantum emitter
electronic properties. Addressing the role of strain and exploiting
its tuning potentiality require the development of efficient methods
to control it and of reliable tools to quantify it. Here we present
a technique based on hydrogen irradiation to induce the formation
of wrinkles and bubbles in hBN, resulting in remarkably high strains
of ∼2%. By combining infrared (IR) near-field scanning optical
microscopy and micro-Raman measurements with numerical calculations,
we characterize the response to strain for both IR-active and Raman-active
modes, revealing the potential of the vibrational properties of hBN
as highly sensitive strain probes.
We fabricate graphene/p-Si heterojunctions and characterize their current-voltage properties in a wide temperature range. The devices exhibit Schottky diode behaviour with a modest rectification factor up to 10 2 . The Schottky parameters are estimated in the framework of the thermionic emission theory using Cheung's and Norde's methods. At room temperature, we obtain an ideality factor of about 2.5 and a Schottky barrier height of ∼ 0.18 eV, which reduces at lower temperatures. We shed light on the physical mechanisms responsible for the low barrier, discussing the p-doping of graphene caused by the transfer process, the exposure to air and the out-diffusion of boron from the Si substrate. We finally propose a band model that fully explains the experimental current-voltage features, included a plateau observed in reverse current at low temperatures.
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