Epitaxial growth by atomic layer deposition and properties of high-k barium strontium titanate on Zintl-templated Ge (001) substrates

Abstract

This work reports the atomic layer deposition (ALD) and epitaxial growth of 10–12 nm BaxSr1−xTiO3 (x ∼ 0.5) thin films on Zintl-templated Ge (001) substrates, formed with 0.5 monolayers of Ba. The best thin films were grown using two steps. An amorphous 2–3 nm layer was deposited at 225 °C and then was converted to a crystalline film by annealing for 15 min at 650 °C under vacuum to generate the seed layer that had an abrupt interface with the Ge (001) substrate. A 7–10 nm layer was subsequently grown and crystallized at the same conditions as the seed layer. Metal-insulator-semiconductor structures that employed Au electrodes were used to assess leakage currents and dielectric properties. Interfacial effects inherent to the ∼10 nm BaxSr1−xTiO3 films affect the capacitance measurements leading to k of 87 and 140 for 10.9 and 14.6 nm films, respectively. The epitaxial films have high k in the bulk. Using capacitance measurements for BaxSr1-xTiO3 films that are 13–18.4 nm thick, a bulk k of ∼2000 and low interfacial capacitance density (C/A) of ∼100 fF/μm2 were extracted from thickness dependent relationships. The 10 nm films display a high leakage current (∼102 A/cm2 at −1.0 V) that can be reduced (to ∼10−1 A/cm2 at −1.0 V) by capping the film with ∼1 nm Al2O3 using ALD before depositing the Au electrode or by annealing in O2 at 300 °C for 15 min. The thin low-k Al2O3 layer or the GeOx formed during the O2 annealing step, while improving leakage current, introduces a capacitor in series that further reduces the film dielectric constant such that a 9.9 nm Al-capped film and a 9.8 nm O2-annealed film yield dielectric constants of 80 and 41, respectively.