Phonon and Polaron Interaction in Germanium-Gallium Arsenide Tunnel Heterojunctions

The differential conductance vs voltage at 4.2^∘{}K of germanium-gallium arsenide tunnel heterojunctions exhibits structure similar to that found in tunnel diodes made in one material. In some units, a dip $V=0$, which has been attributed to polaron interaction in polar semiconductors, is observed. In addition, in these same units, bumps at voltages characteristic of phonon-assisted tunneling in germanium are found. This behavior is characteristic of neither germanium nor gallium-arsenide homojunctions alone. It shows that the units are not merely a germanium contact to a gallium-arsenide homojunction or vice versa. It is strong indication that the interface between the materials is within the space-charge region of the junction and that the interface is sharp and well ordered. The conditions for these observations are discussed.