Interfacial varactor characteristics of ferroelectric thin films on high-resistivity Si substrate

Ferroelectric $Ba(Zr0.25Ti0.75)O3$ (BZT) thin films were deposited on high-resistivity Si substrate without or with inserting a high-$k$ buffer layer of $Ta2O5$⁠. The varactor characteristics of the BZT capacitors in metal-oxide-semiconductor structure were studied. At low frequency $(1MHz)$⁠, the capacitors exhibit a negatively tunable characteristic, i.e., $[C(V)−C(0)]∕C(0)<0$⁠, against dc bias $V$⁠, but opposite tunable characteristics were found at microwave frequencies $(>1GHz)$⁠. The change of voltage-dependent characteristic is attributed to the effect of low-resistivity interface induced by charged defects formed from interfacial oxidation of Si in screening the microwave from penetrating into the bulk of Si.