Atomistic investigation into the interface engineering and heteroepitaxy of functional oxides on hexagonal silicon carbide through the use of a magnesium oxide template layer for the development of a multifunctional heterostructure

Abstract

Advancements in integrated circuit technology are quickly approaching the threshold of silicon semiconductor electronics. In order to break away from the confinements of standard device architecture and silicon's intrinsic material limitations, it is necessary to make an innovational change toward a new generation of novel materials with diverse functionality and superior mechanical, electrical, and magnetic properties that can perform under high-power, high-frequency, high-temperature application requirements. In order for the realization of a next-generation device, it will be necessary to diverge from traditional semiconductor processing into a wide bandgap semiconductor platform. Further, the realization of a next-generation device necessitates the development of novel functional materials that can accommodate the increased performance requirements of both the wide bandgap semiconductor platform and enable multifunctionality, one device interacting with the environment in multiple ways. The novel materials proposed are functional oxides, which can be tuned statically or dynamically to interact with their environment in different ways and can couple with each other to make multifunctional heterostructure devices. Through molecular beam epitaxy, this research explores the use of a magnesium oxide (MgO) template layer and the interface formation mechanism of an oxygen bridge for effective heteroepitaxy of high-quality, ferroelectric barium titanate (BTO) on 6HSiC. High quality, single crystalline MgO(111) is obtained with a smooth surface (RMS < 0.5 nm) and a stepped morphology conformal to the underlying 6H-SiC morphology, but is inherently twinned due to the ionic nature of a (111) oriented rocksalt structure. The smooth, conformal 2-D growth mechanism of MgO prefers to grow in tension with a 3.3 percent lattice mismatch, requires the presence of atomic oxygen, and transitions to a more 3-D growth mode when the thickness reaches ~10 nm. The engineered MgO surface is both effective and necessary to promote the pseudo-hexagonal, heteroepitaxy of BTO(111). Similar to MgO, BTO(111) prefers to grow in tension with a 5.3 percent lattice mismatch and is inherently twinned with a 6-fold symmetry, due to 60 degree in-plane rotations. The resulting epitaxy alignments for the BTO/MgO/6H-SiC results in a BTO{111}||MgO{111}||6H-SiC{0001} out-of-plane relationship and a BTO{110}||MgO{110}||6H-SiC{11-20} in-plane relationship. A multilayered heterostructure was fabricated consisting of ferrimagnetic barium hexaferrite (BaM), ferroelectric BTO, the MgO template layer, and the wide bandgap semiconductor 6H-SiC (BaM/BTO/MgO/6H-SiC). The BTO layer of the heterostructure has ferroelectric properties with a saturated polarization around 4.7 μC/cm2 and a striped domain structure. The BaM layer shows little uniaxial magnetic anisotropy due to mixed orientations. Although the individual functional properties were not optimized, the integration and demonstration of multiple functionality within a single heterostructure is an important contribution toward the realization of a next-generation, multifunctional device.