Synthesis and Characterization of Hexagonal Boron Nitride for Integration with Graphene Electronics

Open Access
Bresnehan, Michael Stephen
Graduate Program:
Materials Science and Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
August 20, 2013
Committee Members:
  • David Robinson, Dissertation Advisor
  • Joshua Alexander Robinson, Dissertation Advisor
  • Joan Marie Redwing, Committee Member
  • Mauricio Terrones Maldonado, Committee Member
  • Graphene
  • Epitaxial Graphene
  • Boron Nitride
  • h-BN
  • Dielectric
  • Field Effect Transistor
  • CVD
  • High Mobility
Hexagonal boron nitride (h-BN) has attracted increased interest as a dielectric material to graphene electronics. Traditional dielectrics, such as SiO2 or various high-k materials, can introduce scattering from charged surface states, impurities, surface optical phonons, and substrate roughness; significantly degrading the transport properties of graphene. Hexagonal boron nitride boasts several key advantages over SiO2 and high-k dielectrics. Most notably, it exhibits an atomically smooth surface that is expected to be free of dangling bonds, leading to an interface that is relatively free of surface charge traps and adsorbed impurities. Additionally, h-BN’s high energy surface optical phonon modes lead to reduced phonon scattering from the dielectric. Using h-BN (grown via CVD on copper foil) as a gate dielectric to quasi-freestanding epitaxial graphene (QFEG) devices, a >2.5x increase in intrinsic current gain cut-off frequency and a >3x increase in mobility over HfO2 gated devices is obtained. In addition, this thesis presents the transfer-free deposition of boron nitride on sapphire and silicon for use as a supporting substrate to CVD-grown graphene. This is accomplished via a polymer-to-ceramic conversion process involving the deposition of polyborazylene at low temperature (≤400°C) and subsequent annealing at 1000°C to BN. Atomic force microscopy (AFM) confirms the deposition of an ultra smooth (RMS roughness <130pm) h-BN film without the need for a solution-based transfer process. However, x-ray photoelectron spectroscopy (XPS) shows that the stoichiometry is dependent on the initial polyborazylene deposition temperature. Despite a turbostratic structure and a boron-rich stoichiometry, CVD graphene transferred to boron nitride films deposited on Al2O3 at a polyborazylene deposition temperature of 400°C is nearly strain-free and results in an improvement in mobility of >1.5x and >2.5x compared to CVD graphene transferred to bare Al2O3 and SiO2, respectively, due to a low impurity density and reduced surface optical phonon scattering.