Electron Transport and Charge Control in Epitaxial Graphene

Doctoral thesis, 2016

Graphene monolayers and bilayers have attracted research interest in both the physics and electronic materials communities owing to their unique band structures. In a pristine monolayer, carriers travel at the Fermi velocity v_f = 1e8 cm/s and exhibit linear dispersion. However, the lack of an energy gap in monolayer material makes applications in active devices challenging. In bilayers, carriers behave classically with an effective mass of m^* = 0.035m_e and exhibit parabolic dispersion along with adequate transport properties. Graphene bilayers are attractive as a narrow energy gap may be opened in the presence of a symmetry breaking potential. Native and intercalated graphene produced by epitaxy on semi-insulating 4H(6H)-SiC substrates are particularly relevant as highly crystalline material may be prepared on large area substrates. In this work, the low field and high field transport properties of native and intercalated epitaxial layers are investigated in electron devices. The results are then considered within the contextual framework provided by fundamental principles. The influence of charged impurity, neutral impurity, and acoustic phonon scattering on conductivity and carrier mobility is considered alongside low field transport data. High field measurements are performed in order to estimate a saturated electron velocity of 2e7 cm/s in epitaxial graphene on SiC. Mobility and saturation velocity influence transconductance and frequency performance in active devices. In both native and intercalated devices, velocity saturation occurs as a consequence of scattering with surface optical modes in the SiC substrate. In this respect, SiC is an ideal substrate for graphene as it demonstrates a high energy surface optical mode (115meV) relative to SiO2 (55meV). Charge control is also investigated via low temperature capacitance voltage measurements in large area metal oxide graphene (GMOS) capacitors with a low temperature Al2O3 dielectric. Results are then correlated with current voltage characteristics in active devices abricated in graphene monolayers and bilayers in which bias dependent instabilities and hysteresis are frequently observed. The CV results highlight the influence of surface potential fluctuations (90meV) and interface states (2e12 eV^{-1}cm^{-2}) on the transfer characteristics of active devices. The density of interface states is found to be relatively high in graphene MOS relative to the 5e10 eV^{-1}cm^{-2} achieved in thermally oxidized silicon MOS. Collecting results, it is possible to gain insight into the physical properties of the material while simultaneously outlining perspectives and challenges for technological applications. The electron transport results motivate further investigations into substrate engineering in graphene MOS in order to improve the saturation velocity and transport characteristics. Meanwhile, the charge control experiments demonstrate a need for improved dielectric films in graphene MOS structures. Together, the electron transport and charge control results provide a thorough description of the electronic properties of MOS devices in epitaxial graphene.