A Chemical Route to Monolithic Integration of Crystalline Oxides on Semiconductors

Abstract

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.

Atomic layer deposition of crystalline SrHfO3 directly on Ge (001) for high-k dielectric applications

“…This corresponds to a reduction of over seven orders of magnitude in leakage current when compared with a 15-nm-thick STO film on Ge. 23 The massive reduction in leakage current is attributed to the favorable conduction band offset (2.2 eV) for SHO on Ge; whereas, STO shows negligible conduction band offset (0.1 eV) under similar measurement conditions. 23 Using the conductance method, D it was estimated to be 1 Â 10 13 cm À2 eV À1 .…”

“…This is in contrast to STO films, where the initial deposition on Ge required a Sr-heavy subcycle ratio of 2:1 (Sr:Ti). 23 Upon deposition of the SHO film, the sample is transferred back into the annealing chamber for crystallization. Postdeposition annealing is carried in vacuum (<2 Â 10 À9 Torr) with a temperature ramp rate of 20 C/min.…”

“…To circumvent this leakage issue, we explore the Hf-based perovskite, SHO, with optimal band alignments to Ge and associated reduction in leakage current. As detailed in our previous report, 23 the controlled growth by ALD on a clean Ge (001) surface enables a chemical route to epitaxial oxide integration with semiconductors. Here, we confirm this growth and annealing protocol to realize crystalline SHO films, and evaluate the potential for epitaxial SHO films as a future gate dielectric for Ge-based transistors.…”

“…26 Recently, we reported the growth of single crystal STO (a ¼ 3.905 Å ) on Ge (001) by atomic layer deposition (ALD) followed by post-deposition annealing in vacuum. 23 Capacitor structures revealed a large dielectric constant (k $ 90) for the STO films, but a high leakage current of $10 A/cm 2 for an applied field of 0.7 MV/cm. To circumvent this leakage issue, we explore the Hf-based perovskite, SHO, with optimal band alignments to Ge and associated reduction in leakage current.…”

“…Materials related to TiO 2 , including STO and BTO, have high permittivities, but the conduction band offsets with Si and Ge are very small to negligible. [21][22][23] Alternatively, crystalline strontium hafnate, SrHfO 3 (SHO), meets many of the high-k requirements and has a reasonable lattice match to Ge ($1.9% mismatch), making it an ideal material candidate for Ge-based transistors.…”