Electron-Phonon Coupling in the Barriers of GaAs Schottky Diodes

The first and the second derivatives ($\frac{\mathrm{dV}}{\mathrm{dI}}$ and $\frac{d^{2}V}{d{I}^2}$) of the tunnel current of GaAs-Pd Schottky diodes show structure at critical points of acoustical as well as optical phonons and multiphonon combinations, which are interpreted in terms of the excitation of collective modes of lattice vibrations in the barrier via point defects. The data are consistent with the theory of Duke, Silverstein, and Bennett and show that certain types of the zero-bias anomaly are caused by these excitations. The zero-bias anomaly in our data is found to terminate exactly at the transverse acoustical (TA) phonon energy of GaAs (8.7±0.4mV). A further drop in $\frac{d^{2}V}{d{I}^2}$ is interpreted as an opening of new tunneling channels by exciting two-TA-phonon modes. This drop terminates at 17.4 (±0.6) mV, in good agreement with twice the TA energy. Beyond the maximum caused by coupling of the electrons to the LO phonon, we observe another resistance increase, which is interpreted as a two-phonon process involving one LO and one TA phonon.