Trapped Charge Effect on Composite Lithium Niobate-Silicon Acoustoelectric Delay Lines

Acoustoelectric delay lines (AEDL) fabricated on a composite lithium niobate-silicon (LN-Si) platform could enable acoustoelectric (AE) nonreciprocity and gain provided that the material properties of the LN-Si heterostructure are properly selected. Among such properties are the carrier density and mobility in the Si substrate. However, the bulk Si properties are subject to substantial perturbation at the LN-Si interface as a result of interfacial trapped charges at dislocations and dangling bond sites. The metal-insulator-semiconductor (MIS) capacitor inherently formed in such heterostructures, however, could allow for some level of control over the Si carrier distribution at the LN film interface. In this work, we demonstrate that the AE gain achieved by the momentum transfer from the carriers drifting in Si and the subsequent nonreciprocity could be fine-tuned and the efficiency of the device could be improved by utilizing the MIS capacitor. The device efficiency is found to be enhanced once the majority electron carriers in n-type Si are slightly depleted at the LN-Si interface resulting in ∼1.5 times improvement in the AE gain at a lower bias current, increasing the efficiency by ∼60%.