The current work explores the crystalline perovskite oxide, strontium hafnate, as a potential high-k gate dielectric for Ge-based transistors. SrHfO3 (SHO) is grown directly on Ge by atomic layer deposition and becomes crystalline with epitaxial registry after post-deposition vacuum annealing at ∼700 °C for 5 min. The 2 × 1 reconstructed, clean Ge (001) surface is a necessary template to achieve crystalline films upon annealing. The SHO films exhibit excellent crystallinity, as shown by x-ray diffraction and transmission electron microscopy. The SHO films have favorable electronic properties for consideration as a high-k gate dielectric on Ge, with satisfactory band offsets (>2 eV), low leakage current (<10−5 A/cm2 at an applied field of 1 MV/cm) at an equivalent oxide thickness of 1 nm, and a reasonable dielectric constant (k ∼ 18). The interface trap density (Dit) is estimated to be as low as ∼2 × 1012 cm−2 eV−1 under the current growth and anneal conditions. Some interfacial reaction is observed between SHO and Ge at temperatures above ∼650 °C, which may contribute to increased Dit value. This study confirms the potential for crystalline oxides grown directly on Ge by atomic layer deposition for advanced electronic applications.

Atomic layer deposition (ALD) is a proven technique for the conformal deposition of oxide thin films with nanoscale thickness control. Most successful industrial applications have been with binary oxides, such as Al2O3 and HfO2. However, there has been much effort to deposit ternary oxides, such as perovskites (ABO3), with desirable properties for advanced thin film applications. Distinct challenges are presented by the deposition of multi-component oxides using ALD. This review is intended to highlight the research of the many groups that have deposited perovskite oxides by ALD methods. Several commonalities between the studies are discussed. Special emphasis is put on precursor selection, deposition temperatures, and specific property performance (high-k, ferroelectric, ferromagnetic, etc.). Finally, the monolithic integration of perovskite oxides with semiconductors by ALD is reviewed. High-quality epitaxial growth of oxide thin films has traditionally been limited to physical vapor deposition techniques (e.g., molecular beam epitaxy). However, recent studies have demonstrated that epitaxial oxide thin films may be deposited on semiconductor substrates using ALD. This presents an exciting opportunity to integrate functional perovskite oxides for advanced semiconductor applications in a process that is economical and scalable.

Atomic layer deposition (ALD) of epitaxial c-axis oriented BaTiO3 (BTO) on Si(001) using a thin (1.6 nm) buffer layer of SrTiO3 (STO) grown by molecular beam epitaxy is reported. The ALD growth of crystalline BTO films at 225  °C used barium bis(triisopropylcyclopentadienyl), titanium tetraisopropoxide, and water as co-reactants. X-ray diffraction (XRD) reveals a high degree of crystallinity and c-axis orientation of as-deposited BTO films. Crystallinity is improved after vacuum annealing at 600  °C. Two-dimensional XRD confirms the tetragonal structure and orientation of 7–20-nm thick films. The effect of the annealing process on the BTO structure is discussed. A clean STO/Si interface is found using in-situ X-ray photoelectron spectroscopy and confirmed by cross-sectional scanning transmission electron microscopy. The capacitance-voltage characteristics of 7–20 nm-thick BTO films are examined and show an effective dielectric constant of ∼660 for the heterostructure.

This work demonstrates the growth of crystalline SrTiO3 (STO) directly on germanium via a chemical method. After thermal deoxidation, the Ge substrate is transferred in vacuo to the deposition chamber where a thin film of STO (2 nm) is deposited by atomic layer deposition (ALD) at 225 °C. Following post‐deposition annealing at 650 °C for 5 min, the STO film becomes crystalline with epitaxial registry to the underlying Ge (001) substrate. Thicker STO films (up to 15 nm) are then grown on the crystalline STO seed layer. The crystalline structure and orientation are confirmed via reflection high‐energy electron diffraction, X‐ray diffraction, and transmission electron microscopy. Electrical measurements of a 15‐nm thick epitaxial STO film on Ge show a large dielectric constant (k ≈ 90), but relatively high leakage current of ≈10 A/cm2 for an applied field of 0.7 MV/cm. To suppress the leakage current, an aluminum precursor is cycled during ALD growth to grow crystalline Al‐doped STO (SrTi1‐x­AlxO3‐δ) films. With sufficient Al doping (≈13%), the leakage current decreases by two orders of magnitude for an 8‐nm thick film. The current work demonstrates the potential of ALD‐grown crystalline oxides to be explored for advanced electronic applications, including high‐mobility Ge‐based transistors.

Epitaxial strontium titanate (STO) films have been grown by atomic layer deposition (ALD) on Si(001) substrates with a thin STO buffer layer grown by molecular beam epitaxy (MBE). Four unit cells of STO grown by MBE serve as the surface template for ALD growth. The STO films grown by ALD are crystalline as-deposited with minimal, if any, amorphous SiOx layer at the STO-Si interface. The growth of STO was achieved using bis(triisopropylcyclopentadienyl)-strontium, titanium tetraisopropoxide, and water as the coreactants at a substrate temperature of 250 °C. In situ x-ray photoelectron spectroscopy (XPS) analysis revealed that the ALD process did not induce additional Si–O bonding at the STO-Si interface. Postdeposition XPS analysis also revealed sporadic carbon incorporation in the as-deposited films. However, annealing at a temperature of 250 °C for 30 min in moderate to high vacuum (10−6–10−9 Torr) removed the carbon species. Higher annealing temperatures (>275 °C) gave rise to a small increase in Si–O bonding, as indicated by XPS, but no reduced Ti species were observed. X-ray diffraction revealed that the as-deposited STO films were c-axis oriented and fully crystalline. A rocking curve around the STO(002) reflection gave a full width at half maximum of 0.30° ± 0.06° for film thicknesses ranging from 5 to 25 nm. Cross-sectional transmission electron microscopy revealed that the STO films were continuous with conformal growth to the substrate and smooth interfaces between the ALD- and MBE-grown STO. Overall, the results indicate that thick, crystalline STO can be grown on Si(001) substrates by ALD with minimal formation of an amorphous SiOx layer using a four-unit-cell STO buffer layer grown by MBE to serve as the surface template.