Electrical Properties and Structural Defects in Epitaxial CaF2 on Si

Abstract

ABSTRACTIn this paper, a study is made of the electrical properties and of the types of structual defects which can occur in epitaxial CaF2 grown on Si(111) substrates by molecular beam epitaxy (MBE). High-resolution transmission electron microscopy (HRTEM) and high-energy (≥2 MeV) He+ ion channeling techniques are used to characterize defects in the epitaxial layer and at the CaF2 /Si interface while current-voltage (I–V) and capacitance-voltage (C–V)3 measurements are used to characterize the electrical properties. The HRTEM images show an atomically abrupt interface between the CaF2 and Si. The most common defect we have been able to identify is associated with an atomic step at the interface and is similar to a Shockley partial dislocation at a (111) twin interface in an fcc crystal structure. The ion channeling measurements also indicate the presence of defects at the CaF2 /Si interface. The apparent defect density measured by ion channeling decreases in the CaF layer as one moves away from the interface at a rate which depends on ihe final thickness of the epitaxial film. Ion channeling has also been used to measure strain and it is found that, while thin layers of epitaxial CaF2 on Si(111) have large strain, the strain becomes vanishingly small as the layers exceed 200 nm in thickness. This result can be adequately explained using an equilibrium model for the introduction of strain relieving misfit dislocations and indicates that epitaxial fluoride layers may be useful as thermal mismatch buffers in heteroepitaxial structures. In C–V measurements of epitaxial CaF2 layers on Si(111) which have been fabricated into metal-insulator-semiconductor structures, the capacitance is observed to remain constant over a large variation in applied voltage. This constant capacitance region can be explained as due to a high density of interface states in the band gap. In addition, I–V measurements indicate that, at low fields, the CaF2 resistivity exceeds 1014 Ω-cm. At high fields, the CaF2 starts to conduct in a manner which we speculate is due to defects at the CaF2/Si interface. The field at which this conduction takes place has been ovserved to exceed 1 MV/cm for a 42-nm-thick CaF2 film with the device geometry used for this work.