Transport studies of mono-, bi- and tri- layer graphene

Open Access
- Author:
- Zou, Ke
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 29, 2012
- Committee Members:
- Jun Zhu, Dissertation Advisor/Co-Advisor
Jun Zhu, Committee Chair/Co-Chair
Vincent Henry Crespi, Committee Member
Nitin Samarth, Committee Member
Theresa Stellwag Mayer, Committee Member - Keywords:
- graphene
bilayer graphene
trilayer graphene
transport
dielectrics
band structure - Abstract:
- This dissertation focuses on the transport studies of mono-, bi- and tri- layer graphene. Although these two-dimensional forms of carbon differ by only one atomic layer, their transport properties are dramatically different. Monolayer graphene is a gapless material with a linear band dispersion. Bilayer graphene has four hyperbolic bands and a band gap tunable by a perpendicular electric filed. ABC stacked trilayer graphene shows a cubic band dispersion near the charge neutrality point and a tunable band gap, while ABA trilayer graphene is a semi-metal with monolayer-like and bilayer-like subbands. Studies in this dissertation reveal some of the unique transport properties of mono-, bi- and trilayer graphene. Chapter 1 introduces the band structures of mono-, bi- and trilayer graphene. We show the hallmark of mono- and bilayer graphene in a perpendicular magnetic field, i.e., their unique quantum Hall effect. The main scattering mechanisms and the effect of electron-electron interaction are also introduced in this chapter. In Chapter 2, we give a brief description of fabrication procedures for devices shown in this dissertation. The equipments and methods used for measurements are also introduced. The effect of surface oxide (SO) phonon scattering in graphene transistors is studied in Chapter 3. The effect of the dielectric materials is critical for any gated devices. We demonstrate the atomic layer deposition of high-quality dielectric HfO_2 films on graphene, which results in one of the highest reported carrier mobility 20,000 cm^2/Vs among graphene field effect transistors (GFETs). We study the effect of SO phonon scattering in two dielectric materials, namely, SiO_2 and HfO_2, at both low and high source-drain (s-d) bias. At low s-d bias, the contribution to resistivity and the mobility limit set by SO phonons from SiO_2 and HfO_2 are experimentally determined. When both HfO_2 and SiO_2 gate oxides are used in the same GFET, SO phonon scattering from HfO_2 dominates at high temperature and sets the mobility limit to ~20,000 cm^2/Vs at room temperature. At high s-d bias V_sd, the current I_sd in a graphene channel tends towards saturation in both single- and dual-gated GFETs. We have examined GFETs using a SiO_2 single-gated or SiO_2/HfO_2 dual-gated structure. Comparing theory and experiment, we show that the inclusion of SO phonon scattering is crucial to explain the experimental I_sd(V_sd) and account for the heat dissipation in graphene transistors. These studies illuminate the critical role played by gate oxide in graphene transistors. In Chapter 4, we demonstrate the control of the tunable band gap in bilayer graphene and study the conduction mechanisms when the Fermi level lies in the band gap. The presence of a band gap is crucial to many modern electronics and photonics applications. In bilayer graphene, the band gap is tunable by an electric field perpendicular to the sheet. We fabricate high quality SiO_2/HfO_2 dual-gated GFETs to control the band gap \Delta in bilayer graphene. The resistance at the charge neutrality point R_CNP increases with increasing perpendicular electric field, which demonstrates the opening of the band gap. To study the conduction mechanism in the band gap, the temperature T-dependence of R_CNP(T) is measured. R_CNP(T) exhibits two thermally activated processes and exp[(T)^{-1/3}] temperature dependence in different temperature regimes. Based on the existence of the potential energy fluctuations in GFETs, we develop a simple model which uses localized states related conduction mechanisms to explain the data. The high-temperature conduction is attributed to thermal activation to the mobility edge with an activation energy approaching \Delta/2 at large band gap. At intermediate and low temperatures, the dominant conduction mechanisms are nearest-neighbor hopping and variable-range hopping through localized states. Our study clarifies conduction mechanisms in gapped bilayer GFETs, which is critical for its actual applications. The band asymmetry between electrons and holes and the effect of electron-eletron interaction in bilayer graphene are studied in Chapter 5. We accurately determine the effective mass m* by the temperature dependence of the Shubnikov-de Haas oscillations. Within common experimental density range, both the hole mass m*_h and the electron mass m*_e increase with increasing carrier density, which proves the bands of bilayer graphene are not parabolic but hyperbolic. The hole mass m*_h is approximately 20-30% larger than the electron mass m*_e. The tight-binding parameter controlling the electron-hole band asymmetry v_4 = 0.063, is extracted accurately from the difference of m*_h and m*_e. Both m*_h and m*_e are significantly suppressed compared to single-particle values, providing clear evidence for the strong renormalization from electron-electron interaction. In Chapter 6, we study transport properties of both ABA and ABC stacked trilayer graphene with band structure tuning by a HfO_2/HfO_2 dual-gated structure. The high efficiency and large breakdown voltage of the HfO_2 gates enable independent tuning of the perpendicular electric field and the Fermi level over an unprecedentedly large range. We observe that resistance at the charge neutrality point increases by six orders of magnitude in the ABC trilayer, which demonstrates the opening of a band gap. The gap size appears to saturate at a large displacement field of D ~ 3 V/nm, in agreement with density functional theory calculations. In contrast, the ABA trilayer remains gapless but the band structure of the ABA trilayer continues to evolve with increasing D. We observe signatures of two-band conduction at large D fields. Our self-consistent Hartree calculation reproduces many aspects of the experimental data, but also points to the need for more sophisticated theory for the exact screening in ABA trilayer graphene. In Chapter 7, we give a summary of the studies discussed in this dissertation and provide some future research interests in mono-, bi- and tri- layer graphene.