Hysteretic electrical transport in BaTiO3/Ba1−xSrxTiO3/Ge heterostructures

Abstract

We present electrical transport measurements of heterostructures comprised of BaTiO 3 and Ba 1−x Sr x TiO 3 epitaxially grown on Ge. The Sr-alloying imparts compressive strain to the BaTiO 3 , which enables the thermal expansion mismatch between BaTiO 3 and Ge to be overcome to achieve c-axis oriented growth. The conduction bands of BaTiO 3 and Ba 1−x Sr x TiO 3 are nearly aligned with the conduction band of Ge, which facilitates electron transport. Electrical transport measurements through the dielectric stack exhibit rectifying behavior and hysteresis, where the latter is consistent with ferroelectric switching. PACS numbers: 74.50.+r, 74.72.Bk, 74.20.Rp, 74.25.Nf 1 Recent advancements in epitaxial growth have enabled single crystalline ferroelectrics to be integrated with conventional semiconductors, opening a pathway to exploit the polarization of the former in semiconducting based devices. Electrically coupling the re-orientable polarization of a ferroelectric to a semiconducting channel remains a long standing challenge in materials research. Single crystalline ferroelectrics on semiconductors are ideal for such applications, since they offer superior material characteristics in comparison to polycrystalline materials [1,2]. Ferroelectric -semiconductor heterojunctions were originally conceived to operate as capacitors, where the polarization could be utilized to maintain accumulation or depletion in the semiconducting electrode [3]. In such an application, DC charge transport through the ferroelectric gate stack is inhibited. However, ferroelectric thin films also exhibit semiconducting properties, which could provide additional functionality in ferroelectric -semiconductor heterostructures [4][5][6][7]. In conjunction with the re-orientable polarization, the semiconducting properties of thin film ferroelectrics could potentially be utilized to control charge transport through ferroelectric-semiconductor heterojunctions.In this letter, we introduce mobile carriers into a ferroelectric-semiconductor heterostructure and exploit the re-orientable polarization to modulate the Schottky barrier to a semiconductor. Our heterostructures are comprised of BaTiO 3 (BTO) and Ba 1−x Sr x TiO 3 (BST) grown epitaxially on a Ge wafer, with Pt serving as the counter electrode. X-ray photoemission spectroscopy measurements indicate that the conduction bands of Ge and BTO are nearly aligned, thus enabling charge transport through the heterojunction interface.Transport measurements of the current through the asymmetric capacitors reveals rectifying behavior and hysteresis. The former arises from the presence (absence) of a barrier at the metal-ferroelectric (ferroelectric-semiconductor) interface for charge transport, whereas the latter can be attributed to switching of the ferroelectric polarization.The BTO/BST heterostructures were grown using reactive molecular beam epitaxy (MBE) [8,9]. The (100)-oriented p-type (ρ ∼0.018 Ωcm) Ge wafers were cleaned by dipping in a 30:1 solution of H 2 O:HF for 60 s, followed by e...

A review of molecular beam epitaxy of ferroelectric BaTiO3films on Si, Ge and GaAs substrates and their applications

“…Ngai et al [115] have grown a 20 nm tri-layer Ba 1− x Sr x TiO 3 stack—with decreasing x values—as a buffer and have obtained c -axis oriented 40 nm thick BaTiO 3 films. The in-plane and out-of-plane parameters were 3.987 Å and 4.040 Å respectively.…”

“…The lattice parameters were a = 3.96 Å and c = 4.06 Å for 16 nm thick BaTiO 3 . Both in [115] and [116], the comparison of the x-ray diffraction θ /2 θ scans clearly showed the impact of the buffer insertion on the BaTiO 3 crystalline orientation. In the work of Ponath et al [116], STEM-HAADF images revealed that Ti atomic columns close to the top of the BaTiO 3 film are shifted downward from the cell center, meaning a ‘down’ polarization, which is in good agreement with their DFT calculations.…”

“…In the work by Merckling et al [112] where a mixture of both c -axis tetragonal phase and cubic phase was reported, ferroelectricity was not studied. Ferroelectricity was reported for BaTiO 3 c -axis oriented films on (Ba,Sr)TiO 3 -buffered Ge [115, 116]. Current versus voltage curves were measured on capacitive structures with 40 nm BaTiO 3 deposited on a trilayer Ba 1− x Sr x TiO 3 buffer (20 nm) stack and with top Pt electrodes, showing rectifying behavior [115].…”

“…Ferroelectricity was reported for BaTiO 3 c -axis oriented films on (Ba,Sr)TiO 3 -buffered Ge [115, 116]. Current versus voltage curves were measured on capacitive structures with 40 nm BaTiO 3 deposited on a trilayer Ba 1− x Sr x TiO 3 buffer (20 nm) stack and with top Pt electrodes, showing rectifying behavior [115]. A hysteresis consistent with ferroelectric switching was observed while no hysteresis was present for a heterojunction with a -axis 60 nm BaTiO 3 directly on Ge [115].…”

“…Current versus voltage curves were measured on capacitive structures with 40 nm BaTiO 3 deposited on a trilayer Ba 1− x Sr x TiO 3 buffer (20 nm) stack and with top Pt electrodes, showing rectifying behavior [115]. A hysteresis consistent with ferroelectric switching was observed while no hysteresis was present for a heterojunction with a -axis 60 nm BaTiO 3 directly on Ge [115]. In c -axis 16 nm BaTiO 3 film on 2 nm SrTiO 3 buffered-Ge [116], PFM measurements indicated ferroelectricity with a coercive voltage of −4 V and +5 V. The non-poled regions (as-deposited film) were found to be mono-domain with the polarization oriented towards the STO/Ge substrate, which was in agreement with their theoretical calculations [116].…”