Plasmon-phonon resonance at gate-electrode/gate-dielectric interface on carrier mobility of organic TFTs with high-k gate dielectrics

The resonance between the plasmons in gate electrode and the phonons in gate oxide of the omnipresent metal–oxidesemiconductor devices predicted in 2004 is experimentally demonstrated to degrade the mobility of the carriers in the semiconductor channel. By using bottom-gated pentacene organic thin-film transistors fabricated on n-type Si substrates with different resistivities, this work experimentally shows that even for thick HfLaO gate dielectric (~50 nm), the interaction between the gate plasmons and the dielectric phonons can significantly affect the carrier mobility. FTIR and Raman spectra reflect that the phonon frequency of the dielectric ranges from 13.2 to 24.9 THz, overlapping the gate-plasmon frequency (13.2 ~ 18.7 THz) of the 0.01 ~ 0.015 Ω·cm substrate. Therefore, the predicted mobility reduction can be explained by a simpler concept of “resonance”: when the oscillations of gate carrier and dielectric atom have about the same frequency, the remote phonon scattering of the gate dielectric on the channel carriers is greatly enhanced to reduce their mobility. In summary, this resonance-induced mobility reduction could pose a temperature-related reliability problem to electronic circuits when carrier freeze-out at low working temperature reduces the carrier density in the gate electrode to about 3 × 10¹⁸ cm⁻³ to result in a plasmon frequency close to the phonon frequency of the gate dielectric.