Heteroepitaxy of semiconductor-insulator layers and their interface properties

Abstract

The epitaxial growth of Ba₂SiO₄ thin films on Si(001) by co-deposition of Ba and Si in an oxygen background pressure is systematically investigated with a focus on the epitaxial interface. A structural investigation is performed by employing x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and aberration-corrected scanning transmission electron microscopy (STEM). In addition, an electrical characterization is done using MOS test capacitors.

The stoichiometry at the interface turns out to be critically dependent on the oxygen background pressure during deposition. Films grown with an oxygen pressure just above the saturation point for a complete oxidation of the film still feature 1/4 ML of O atoms in Si-O-Si bonding states. In comparison, the Ba₂SiO₄ bulk structure has only O atoms in Si-O-Ba bonding states. STEM shows that these films form an atomically sharp interface to Si(001) and that the Ba₂SiO₄ bulk structure is maintained up to the penultimate layer at the interface. Only one silicate layer is changed to a (2x3) structure, which is also observed in LEED, to match the (2x1.5) bulk structure to Si(001), neglecting relaxations. An interface model is proposed for these films, which features a pseudo-(2x1) reconstruction of the Si surface and helps to understand the formation process of the epitaxial interface in greater detail.

The growth in a high oxygen pressure leads to the formation of Si-rich silicate at the interface, which does not prevent the epitaxial growth but modifies the interface into a (2x6) structure. Moreover, a Ba surplus results in the formation of interfacial silicide, which is characterized by a (4x2) structure.

A dielectric constant of k=22.5 ± 1.1 is found for Ba₂SiO₄, as well as band offsets to Si(001) larger than 1.8 eV for crystalline layers. Moreover, leakage current densities as low as 2 ⋅ 10⁻⁶ A/cm² at -1 V are measured for a 10 nm thick film. Interface trap densities at midgap of (1.14 ± 0.78) ⋅ 10¹² eV⁻¹cm⁻¹² are measured for crystalline films with an abrupt interface. Amorphous films show slightly higher interface trap densities of (2.72 ± 0.82) ⋅ 10¹² eV⁻¹cm⁻¹² at midgap. A further reduction of the interface trap density is possible by incorporating a Si-rich silicate layer at the interface, which results in interface trap densities of (3.32 ± 0.45) ⋅ 10¹¹ eV⁻¹cm⁻¹² at midgap for crystalline layers. However, even though no SiO₂ forms at the interface, the epitaxial interface still contributes an offset of (0.56 ± 0.08) nm to the overall CET, which greatly limits the achievable minimum CET of the gate stack.