Hole Injection from Silicon to Oxide Using Graphene as Transparent Electrode

We demonstrate a novel application of graphene as a transparent electrode in internal photoemission (IPE) spectroscopy. Owing to its low absorption in the IR/Visible/UV range, graphene enables the direct observation of hole injection, and thus the measurement of both conduction and valence band offsets at the semiconductor-oxide hetero-interface. The photocurrents, consisting of electron or hole transitions between Si substrate and graphene as a function of incident photon energy under various applied gate voltage are measured. The barrier height is further determined from the photoemission quantum yield, which is defined as the ratio of photocurrent and light intensity. As a result, the barrier heights, $\varphi _{e}^{0}$, from the valence band top in Si to the bottom of the conduction band in Al$_{2}$O$_{3}$, and $\varphi _{h}^{0}$, from the bottom of the conduction band in Si to the top of the valence band in Al$_{2}$O$_{3}$ are extracted to be 3.5 eV and 4.1~eV, respectively. Furthermore, the bandgap of Al$_{2}$O$_{3}$ can be simply obtained by$\mathop E\nolimits_{g}^{\mathop {Al}\nolimits_{2} \mathop O\nolimits_{3} } =\mathop \phi\nolimits_{e} +\mathop \phi\nolimits_{h} -\mathop E\nolimits_{g}^{Si} =3.5+4.1-1.1=6.5$eV, close to previously reported values. Similar phenomenon is also observed and confirmed by replacing Al$_{2}$O$_{3}$ with 10 nm SiO$_{2}$.