Direct Determination of Energy Level Alignment and Charge Transport at $\mathrm{Metal}\mathrm{\text{−}}{\mathrm{Alq}}_3$ Interfaces via Ballistic-Electron-Emission Spectroscopy

Using ballistic-electron-emission spectroscopy (BEES), we directly determined the energy barrier for electron injection at clean interfaces of ${\mathrm{Alq}}_3$ with Al and Fe to be 2.1 and 2.2 eV, respectively. We quantitatively modeled the sub-barrier BEES spectra with an accumulated space charge layer, and found that the transport of nonballistic electrons is consistent with random hopping over the injection barrier.

Figure 1

(Color figure) (a) Schematic energy-level diagram of BEES. (b) A photograph of an $\mathrm{Al}\mathrm{\text{−}}{\mathrm{Alq}}_3\mathrm{\text{−}}\mathrm{Al}$ device. (c) $I_E$ and (d) $\Delta I_C$ at several values of $V_{\mathrm{CB}}$ are plotted against $V_{\mathrm{BE}}$. The solid curves are fits described in the text. (e) $I_C(0)$ vs $V_{\mathrm{CB}}$. The solid curve is a fit to the Arkhipov model [22].

Figure 2

(a) Calculated $n$ as a function of $V_{\mathrm{BE}}$ for the $\mathrm{Al}\mathrm{\text{−}}{\mathrm{Alq}}_3/\mathrm{Al}$ device in Fig. 1. Inset: The position dependence of $V_{\mathrm{SC}}$. (b) The fitting coefficient $C$ as a function of $V_{\mathrm{CB}}$.

Figure 3

(a) $I_C$ and $d{I}_C/d{V}_{\mathrm{BE}}$ vs $V_{\mathrm{BE}}$ for an $\mathrm{Fe}\mathrm{\text{−}}{\mathrm{Alq}}_3\mathrm{\text{−}}\mathrm{Au}$ device. (b) Schematic diagrams of energy-level alignment at $\mathrm{Al}\mathrm{\text{−}}{\mathrm{Alq}}_3$ and $\mathrm{Fe}\mathrm{\text{−}}{\mathrm{Alq}}_3$ interfaces.