  Scanning tunneling spectroscopy (STS) has yielded significant insight on the
electronic structure of graphene and other two-dimensional (2D) materials. STS
directly measures a fundamental and directly calculable quantity: the single
particle density of states (SPDOS). Due to experimental setup limitations,
however, STS has been unable to explore 2D materials in ultra-high magnetic
fields where electron-electron interactions can drastically change the SPDOS.
Recent developments in the assembly of heterostructures composed of graphene
and hexagonal boron nitride have enabled a device-based alternative to
potentially overcome these roadblocks. Thus far, however, these nascent efforts
are incomplete in analyzing and understanding tunneling spectra and have yet to
explore graphene at high magnetic fields. Here we report an experiment at
magnetic fields up to 18 T that uses graphene tunneling field effect
transistors (TFETs) to establish a clear benchmark for measurement and analysis
of graphene planar tunneling spectroscopy. We acquire gate tunable tunneling
spectra of graphene and then use these data and electrostatic arguments to
develop a systematic analysis scheme. This analysis reveals that TFET devices
directly probe electronic structure features near and far from the Fermi level.
In particular, our study yields identification of the Dirac point and numerous
Landau levels as they fill and empty with charge via application of a gate
voltage. Our work demonstrates that TFET devices are a viable platform for
directly probing the electronic structure of graphene and other 2D materials in
high magnetic fields, where novel electronic states emerge.
