Investigation of electrical contact resistances in graphene-based devices by Kelvin Probe Force Microscopy

Graphene is a carbon atom-thick layer with outstanding mechanical and electrical properties. It has been envisioned as a promising candidate for its use as a conductive medium in polymers, a flexible, transparent and highly conductive electrode in optoelectronic devices and as the conductive channel of high frequency transistors. In the pathway to use graphene and its exceptional properties in such applications there are still open questions to be answered. One key issue always present for the fabrication of devices is that of the electrical contacting of the conductive channel. Such is the case for graphene where it is still not clarified how the oxygen content in Functionalized Graphene Sheets (FGS), the lithography process or the contact design in graphene produced by Chemical Vapor Deposition (CVD) impact the contact resistance between the metal electrode and the graphene channel. This understanding is however of utmost importance since contact resistances significantly affect the conductivity of electronic devices. Hence, the aim of this work is the characterization of electrical contacts in graphene-based devices on ambient conditions. For a better understanding of the electrical contacts, information on the submicrometer scale is advantageous, therefore Kelvin Probe Force Microscopy (KPFM) was used as the main characterization technique. Since an improvement is expected in the resolution of KPFM by detecting the force gradient instead of the electrostatic force itself, phase modulation is implemented in the existing system and its performance is examined. A feature of less than 20 nm and 80 mV in surface potential variation could be clearly resolved in ambient conditions. In the following section, it is shown that with decreasing oxygen content in FGS, the transport mechanism of the charge carriers has a transition from predominantly hopping to predominantly diffusive transport, along with a reduction of the sheet resistance from > 400 kΩ/□ to 100 kΩµm) to linear, ohmic behavior with low contact resistance (~ 1 kΩµm). In the last part of this work, the influence of the fabrication process and contact design in the contact resistance of CVD graphene is investigated. It is determined that optical lithography systematically produced devices with contact resistances up to an order magnitude larger (>> 1 kΩµm) than e-beam lithography (< 1 kΩµm). It is determined that this is caused by a 3 - 4 nm thick residual layer from the optical lithography process which is present between graphene and the metal electrode. An elegant solution to prevent the effect of this residual layer in contact resistance is the use of novel one dimensional contacts instead of the conventional two dimensional contacts. In the former type of contact the charge carriers transit the metal/graphene interface not vertically but horizontally. It can be shown that such novel type of contact design, even with the use of optical lithography, can reach contact resistances lower than 200 Ωµm

Quantitative protein sensing with germanium THz-antennas manufactured using CMOS processes

The development of a CMOS manufactured THz sensing platform could enable the integration of state-of-the-art sensing principles with the mixed signal electronics ecosystem in small footprint, low-cost devices. To this aim, in this work we demonstrate a label-free protein sensing platform using highly doped germanium plasmonic antennas realized on Si and SOI substrates and operating in the THz range of the electromagnetic spectrum. The antenna response to different concentrations of BSA shows in both cases a linear response with saturation above 20 mg/mL. Ge antennas on SOI substrates feature a two-fold sensitivity as compared to conventional Si substrates, reaching a value of 6 GHz/(mg/mL), which is four-fold what reported using metal-based metamaterials. We believe that this result could pave the way to a low-cost lab-on-a-chip biosensing platform

Program FFlexCom — High frequency flexible bendable electronics for wireless communication systems

Today, electronics are implemented on rigid substrates. However, many objects in daily-life are not rigid — they are bendable, stretchable and even foldable. Examples are paper, tapes, our body, our skin and textiles. Until today there is a big gap between electronics and bendable daily-life items. Concerning this matter, the DFG Priority Program FFlexCom aims at paving the way for a novel research area: Wireless communication systems fully integrated on an ultra-thin, bendable and flexible piece of plastic or paper. The Program encompasses 13 projects led by 25 professors. By flexibility we refer to mechanical flexibility, which can come in flavors of bendability, foldability and, stretchability. In the last years the speed of flexible devices has massively been improved. However, to enable functional flexible systems and operation frequencies up to the sub-GHz range, the speed of flexible devices must still be increased by several orders of magnitude requiring novel system and circuit architectures, component concepts, technologies and materials

Electronic properties of graphene/p-silicon Schottky junction

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 102 . 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

Graphene Schottky Junction on Pillar Patterned Silicon Substrate

A graphene/silicon junction with rectifying behaviour and remarkable photo-response was fabricated by transferring a graphene monolayer on a pillar-patterned Si substrate. The device forms a 0.11 eV Schottky barrier with 2.6 ideality factor at room temperature and exhibits strongly biasand temperature-dependent reverse current. Below room temperature, the reverse current grows exponentially with the applied voltage because the pillar-enhanced electric field lowers the Schottky barrier. Conversely, at higher temperatures, the charge carrier thermal generation is dominant and the reverse current becomes weakly bias-dependent. A quasi-saturated reverse current is similarly observed at room temperature when the charge carriers are photogenerated under light exposure. The device shows photovoltaic effect with 0.7% power conversion efficiency and achieves 88 A/W photoresponsivity when used as photodetector. © 2019 by the authors

Graphene Schottky Junction on Pillar Patterned Silicon Substrate

A graphene/silicon junction with rectifying behaviour and remarkable photo-response was fabricated by transferring a graphene monolayer on a pillar-patterned Si substrate. The device forms a 0.11 eV Schottky barrier with 2.6 ideality factor at room temperature and exhibits strongly bias- and temperature-dependent reverse current. Below room temperature, the reverse current grows exponentially with the applied voltage because the pillar-enhanced electric field lowers the Schottky barrier. Conversely, at higher temperatures, the charge carrier thermal generation is dominant and the reverse current becomes weakly bias-dependent. A quasi-saturated reverse current is similarly observed at room temperature when the charge carriers are photogenerated under light exposure. The device shows photovoltaic effect with 0.7% power conversion efficiency and achieves 88 A/W photoresponsivity when used as photodetector

Current Modulation of a Heterojunction Structure by an Ultra-Thin Graphene Base Electrode

Graphene has been proposed as the current controlling element of vertical transport in heterojunction transistors, as it could potentially achieve high operation frequencies due to its metallic character and 2D nature. Simulations of graphene acting as a thermionic barrier between the transport of two semiconductor layers have shown cut-off frequencies larger than 1 THz. Furthermore, the use of n-doped amorphous silicon, (n)-a-Si:H, as the semiconductor for this approach could enable flexible electronics with high cutoff frequencies. In this work, we fabricated a vertical structure on a rigid substrate where graphene is embedded between two differently doped (n)-a-Si:H layers deposited by very high frequency (140 MHz) plasma-enhanced chemical vapor deposition. The operation of this heterojunction structure is investigated by the two diode-like interfaces by means of temperature dependent current-voltage characterization, followed by the electrical characterization in a three-terminal configuration. We demonstrate that the vertical current between the (n)-a-Si:H layers is successfully controlled by the ultra-thin graphene base voltage. While current saturation is yet to be achieved, a transconductance of ~230 μ S was obtained, demonstrating a moderate modulation of the collector-emitter current by the ultra-thin graphene base voltage. These results show promising progress towards the application of graphene base heterojunction transistors

Terahertz subwavelength sensing with bio-functionalized germanium fano-resonators

Localized Surface Plasmon Resonances (LSPR) based on highly doped semiconductors microstructures, such as antennas, can be engineered to exhibit resonant features at THz frequencies. In this work, we demonstrate plasmonic antennas with increased quality factor LSPRs from Fano coupling to dark modes. We also discuss the advances in the biofunctionalization of n-doped Ge antennas for specific protein immobilization and cell interfacing. Finally, albumin biolayers with a thickness of a few hundred nanometers are used to demonstrate the performance of the fano-coupled n-Ge antennas as sensors. A resonant change of over 10% in transmission, due to the presence of the biolayer, can be detected within a bandwidth of only 20 GHz

Terahertz subwavelength sensing with bio-functionalized germanium fano-resonators

Localized Surface Plasmon Resonances (LSPR) based on highly doped semiconductors microstructures, such as antennas, can be engineered to exhibit resonant features at THz frequencies. In this work, we demonstrate plasmonic antennas with increased quality factor LSPRs from Fano coupling to dark modes. We also discuss the advances in the biofunctionalization of n-doped Ge antennas for specific protein immobilization and cell interfacing. Finally, albumin biolayers with a thickness of a few hundred nanometers are used to demonstrate the performance of the fano-coupled n-Ge antennas as sensors. A resonant change of over 10% in transmission, due to the presence of the biolayer, can be detected within a bandwidth of only 20 GHz

Quantitative protein sensing with germanium THz-antennas manufactured using CMOS processes

The development of a CMOS manufactured THz sensing platform could enable the integration of state-of-the-art sensing principles with the mixed signal electronics ecosystem in small footprint, low-cost devices. To this aim, in this work we demonstrate a label-free protein sensing platform using highly doped germanium plasmonic antennas realized on Si and SOI substrates and operating in the THz range of the electromagnetic spectrum. The antenna response to different concentrations of BSA shows in both cases a linear response with saturation above 20 mg/mL. Ge antennas on SOI substrates feature a two-fold sensitivity as compared to conventional Si substrates, reaching a value of 6 GHz/(mg/mL), which is four-fold what reported using metal-based metamaterials. We believe that this result could pave the way to a low-cost lab-on-a-chip biosensing platform