Direct observation of a gate tunable band-gap in electrical transport in ABC-trilayer graphene

Few layer graphene systems such as Bernal stacked bilayer and rhombohedral (ABC-) stacked trilayer offer the unique possibility to open an electric field tunable energy gap. To date, this energy gap has been experimentally confirmed in optical spectroscopy. Here we report the first direct observation of the electric field tunable energy gap in electronic transport experiments on doubly gated suspended ABC-trilayer graphene. From a systematic study of the non-linearities in current \textit{versus} voltage characteristics and the temperature dependence of the conductivity we demonstrate that thermally activated transport over the energy-gap dominates the electrical response of these transistors. The estimated values for energy gap from the temperature dependence and from the current voltage characteristics follow the theoretically expected electric field dependence with critical exponent $3/2$. These experiments indicate that high quality few-layer graphene are suitable candidates for exploring novel tunable THz light sources and detectors.Comment: Nano Letters, 2015 just accepted, DOI: 10.1021/acs.nanolett.5b0077

Electronic transport properties of few-layer graphene materials

Since the discovery of graphene -a single layer of carbon atoms arranged in a honeycomb lattice - it was clear that this truly is a unique material system with an unprecedented combination of physical properties. Graphene is the thinnest membrane present in nature -just one atom thick- it is the strongest material, it is transparent and it is a very good conductor with room temperature charge mobilities larger than the typical mobilities found in silicon. The significance played by this new material system is even more apparent when considering that graphene is the thinnest member of a larger family: the few-layer graphene materials. Even though several physical properties are shared between graphene and its few-layers, recent theoretical and experimental advances demonstrate that each specific thickness of few-layer graphene is a material with unique physical properties.Comment: 26 pages, 8 figure

Probing the electrical properties of multilayer graphene

Graphene is a new two-dimensional (2D) material with unique electrical transport, optical and mechanical properties. However, monolayer graphene (MLG) is a gapless semiconductor, which limits its relevance for transistor applications where a large on/off ratio of the current is required. In this work the investigation of transport properties of few-layer graphene (FLG) is presented. These 2D electronic systems offer a novel solution to the problem concerned the absence of an energy gap in single layer graphene, since they exhibit an electric field and stacking-dependent band gap in the energy dispersion. Thus far, a clear observation of a band-gap in multilayer graphene (e.g. Bernal-stacked bilayers) in transport measurements was hindered by the presence of disorder. Here we develop a reliable and effective method of fabrication of high-quality suspended double-gated graphene devices, which are of crucial importance for probing the low energy dispersion of few-layer graphene. The current annealing technique, described in details, improves transport characteristics like carrier mobility, which is typically higher than ∼ 104 cm2/Vs for our multilayer devices. Electrical transport experiments on suspended dual-gated ABC-stacked trilayer are performed. We report the direct evidence of the opening of a tunable band-gap with an external perpendicular electric field, ranging from 0 meV up to 5.2 meV for an electric field of 117 mV/nm. Thermally activated transport is observed in these samples over the temperature range 0.5 - 80 K. The values of energy gap extracted from both temperature dependence of minimum conductivity measurements and non-linear I –V characteristics correlate well. Our experimental results are in a good agreement with theoretical approximation, based on self-consistent tight-binding calculations. The high quality of our ABC trilayer samples is also demonstrated by a particularly high on/off ratio of the current (250 at applied electrical displacement as low as 80 mV/nm), which makes these devices promising for future semiconductor electronics. FLG samples with reduced disorder allow us to observe quantum Hall effect (QHE) at magnetic field as low as 500 mT. We present the first study of electric field- induced new QH states in ABC trilayer graphene (TLG). The transitions between spin-polarized and valley polarized phases of the sample at the charge neutrality point are investigated. Resolved novel broken symmetry states along with observed Lifshitz transition in rhombohedral TLG display exciting phenomena attributed to rich physics in these interactive electronic systems

Limitations to Carrier Mobility and Phase-Coherent Transport in Bilayer Graphene

We present transport measurements on high-mobility bilayer graphene fully encapsulated in hexagonal boron nitride. We show two terminal quantum Hall effect measurements which exhibit full symmetry broken Landau levels at low magnetic fields. From weak localization measurements, we extract gate-tunable phase coherence times $\tau_{\phi}$ as well as the inter- and intra-valley scattering times $\tau_i$ and $\tau_*$. While $\tau_{\phi}$ is in qualitative agreement with an electron-electron interaction mediated dephasing mechanism, electron spin-flip scattering processes are limiting $\tau_{\phi}$ at low temperatures. The analysis of $\tau_i$ and $\tau_*$ points to local strain fluctuation as the most probable mechanism for limiting the mobility in high-quality bilayer graphene

Tunable mechanical coupling between driven microelectromechanical resonators

We present a microelectromechanical system, in which a silicon beam is attached to a comb-drive actuator, that is used to tune the tension in the silicon beam, and thus its resonance frequency. By measuring the resonance frequencies of the system, we show that the comb-drive actuator and the silicon beam behave as two strongly coupled resonators. Interestingly, the effective coupling rate (~ 1.5 MHz) is tunable with the comb-drive actuator (+10%) as well as with a side-gate (-10%) placed close to the silicon beam. In contrast, the effective spring constant of the system is insensitive to either of them and changes only by $\pm$ 0.5%. Finally, we show that the comb-drive actuator can be used to switch between different coupling rates with a frequency of at least 10 kHz.Comment: 5 pages, 4 figures, 1 tabl

Fabrication of comb-drive actuators for straining nanostructured suspended graphene

We report on the fabrication and characterization of an optimized comb-drive actuator design for strain-dependent transport measurements on suspended graphene. We fabricate devices from highly p-doped silicon using deep reactive ion etching with a chromium mask. Crucially, we implement a gold layer to reduce the device resistance from $\approx51.6$ k$\mathrm{\Omega}$ to $\approx236$ $\mathrm{\Omega}$ at room temperature in order to allow for strain-dependent transport measurements. The graphene is integrated by mechanically transferring it directly onto the actuator using a polymethylmethacrylate membrane. Importantly, the integrated graphene can be nanostructured afterwards to optimize device functionality. The minimum feature size of the structured suspended graphene is 30 nm, which allows for interesting device concepts such as mechanically-tunable nanoconstrictions. Finally, we characterize the fabricated devices by measuring the Raman spectrum as well as the a mechanical resonance frequency of an integrated graphene sheet for different strain values.Comment: 10 pages, 9 figure

Electrical transport in suspended and double gated trilayer graphene

We present a fabrication process for high quality suspended and double gated trilayer graphene devices. The electrical transport measurements in these transistors reveal a high charge carrier mobility (higher than 20000 cm^2/Vs) and ballistic electric transport on a scale larger than 200nm. We report a particularly large on/off ratio of the current in ABC-stacked trilayers, up to 250 for an average electric displacement of -0.08 V/nm, compatible with an electric field induced energy gap. The high quality of these devices is also demonstrated by the appearance of quantum Hall plateaus at magnetic fields as low as 500mT.Comment: to appear in Applied Physics Letters. Typos corrected and references update

Integrated impedance bridge for absolute capacitance measurements at cryogenic temperatures and finite magnetic fields

We developed an impedance bridge that operates at cryogenic temperatures (down to 60 mK) and in perpendicular magnetic fields up to at least 12 T. This is achieved by mounting a GaAs HEMT amplifier perpendicular to a printed circuit board containing the device under test and thereby parallel to the magnetic field. The measured amplitude and phase of the output signal allows for the separation of the total impedance into an absolute capacitance and a resistance. Through a detailed noise characterization, we find that the best resolution is obtained when operating the HEMT amplifier at the highest gain. We obtained a resolution in the absolute capacitance of 6.4~aF$/\sqrt{\textrm{Hz}}$ at 77 K on a comb-drive actuator, while maintaining a small excitation amplitude of 15~$k_\text{B} T/e$. We show the magnetic field functionality of our impedance bridge by measuring the quantum Hall plateaus of a top-gated hBN/graphene/hBN heterostructure at 60~mK with a probe signal of 12.8~$k_\text{B} T/e$.Comment: 7 pages, 5 figure

Vacancy induced zero energy modes in graphene stacks: The case of ABC trilayer

The zero energy modes induced by vacancies in ABC stacked trilayer graphene are investigated. Depending on the position of the vacancy, a new zero energy solution is realised, different from those obtained in multilayer compounds with Bernal stacking. The electronic modification induced in the sample by the new vacancy states is characterised by computing the local density of states and their localisation properties are studied by the inverse participation ratio. We also analyse the situation in the presence of a gap in the spectrum due to a perpendicular electric field.Comment: 6 pages, 4 figures Published in special issue: Exploring Graphene, Recent Research Advance