In the present PhD thesis, we study the electronic transport properties of graphene nanoribbons under a large magnetic field (up to 55 T) to unveil the electronic confinement effects on the band structure of graphene.
Graphene nanoribbons (GNRs) are promising materials to promote graphene for all carbon based nanoelectronics, including electronic wave guides and digital logic or radiofrequency devices. When graphene is structured at a nano-scale, with an accurate control of the edge symmetry, it is possible to open an energy gap in the band structure and to tune the number of conducting channels by a local electrostatic gate. Unfortunately, such a gap engineering goes along with a poor control of the edges' quality and, as a consequence, a drastic drop of the electronic mobility. Most of the confining potential effects on the band structure are masked by the presence of disorder and experimental evidence of the intrinsic electronic band structure of GNR remain challenging.
Here, we demonstrate that, by playing with a large magnetic confinement, we bring experimental evidence of anomalous Landau spectra, signature of the confining potential in GNRs.
The magneto-transport experiments are mainly obtained on monolayer GNRs on Si/SiO2 substrates patterned by e-beam lithography and reactive ion etching, with nominal widths between 55 and 100 nm. The carrier mobility varies from 600 to 3500 cm2/Vs. For the larger GNR (100 nm), presenting a weakly diffusive regime, we give evidence, at high doping levels, of anomalous Shubnikov-de Haas oscillations, fingerprint of the presence of the electronic confinement. At lower doping levels, we observe Landau quantization of the two probe conductance, for filling factor values expected for graphene. In narrower GNRs, new quantum oscillations develop in the Landau regime. This singular Landau spectrum, directly compared to magneto-electric subbands calculations, is assigned to a possible valley degeneracy lifting driven by the confining potential in presence of armchair symmetry at the edges.
Additionally, we present high magnetic field transport results obtained in bilayer graphene nanoribbons and we compare them to magneto-transport obtained on un-structured graphene bilayer. The comparison unveils new magneto-signatures in bilayer GNRs, originating from the electronic confining effects.
As a complementary part, we finally study the phase coherence in our GNR devices that manifests itself by magnetic field and back-gate voltage induced conductance fluctuations. We mainly focus on conductance fluctuations in the out-of-equilibrium regime. The analysis of the amplitude of the conductance fluctuations and of the correlation voltage unveils the phase coherence length and its bias voltage dependence, signature of electron-electron interaction as the main mechanism responsible for the decoherence. Additionally, we investigate the response of graphene to a THz radiation. The experiments show a high cross correlation between the photo-response and the second harmonic conductance fluctuations of electronic transport. This opens a new possibility to study non-linear mesoscopic effects using THz radiation. |
In the present PhD thesis, we study the electronic transport properties of graphene nanoribbons under a large magnetic field (up to 55 T) to unveil the electronic confinement effects on the band structure of graphene.
Graphene nanoribbons (GNRs) are promising materials to promote graphene for all carbon based nanoelectronics, including electronic wave guides and digital logic or radiofrequency devices. When graphene is structured at a nano-scale, with an accurate control of the edge symmetry, it is possible to open an energy gap in the band structure and to tune the number of conducting channels by a local electrostatic gate. Unfortunately, such a gap engineering goes along with a poor control of the edges' quality and, as a consequence, a drastic drop of the electronic mobility. Most of the confining potential effects on the band structure are masked by the presence of disorder and experimental evidence of the intrinsic electronic band structure of GNR remain challenging.
Here, we demonstrate that, by playing with a large magnetic confinement, we bring experimental evidence of anomalous Landau spectra, signature of the confining potential in GNRs.
The magneto-transport experiments are mainly obtained on monolayer GNRs on Si/SiO2 substrates patterned by e-beam lithography and reactive ion etching, with nominal widths between 55 and 100 nm. The carrier mobility varies from 600 to 3500 cm2/Vs. For the larger GNR (100 nm), presenting a weakly diffusive regime, we give evidence, at high doping levels, of anomalous Shubnikov-de Haas oscillations, fingerprint of the presence of the electronic confinement. At lower doping levels, we observe Landau quantization of the two probe conductance, for filling factor values expected for graphene. In narrower GNRs, new quantum oscillations develop in the Landau regime. This singular Landau spectrum, directly compared to magneto-electric subbands calculations, is assigned to a possible valley degeneracy lifting driven by the confining potential in presence of armchair symmetry at the edges.
Additionally, we present high magnetic field transport results obtained in bilayer graphene nanoribbons and we compare them to magneto-transport obtained on un-structured graphene bilayer. The comparison unveils new magneto-signatures in bilayer GNRs, originating from the electronic confining effects.
As a complementary part, we finally study the phase coherence in our GNR devices that manifests itself by magnetic field and back-gate voltage induced conductance fluctuations. We mainly focus on conductance fluctuations in the out-of-equilibrium regime. The analysis of the amplitude of the conductance fluctuations and of the correlation voltage unveils the phase coherence length and its bias voltage dependence, signature of electron-electron interaction as the main mechanism responsible for the decoherence. Additionally, we investigate the response of graphene to a THz radiation. The experiments show a high cross correlation between the photo-response and the second harmonic conductance fluctuations of electronic transport. This opens a new possibility to study non-linear mesoscopic effects using THz radiation. |