Integration of Oxide Semiconductor Thin Films with Relaxor-Based Ferroelectric Single Crystals with Large Reversible and Nonvolatile Modulation of Electronic Properties

Engineering Co Vacancies for Tuning Electrical Properties of p-Type Semiconducting Co3O4 Films

Spinel oxide Co₃O₄ has attracted more and more attention for energy- and environment-related applications. In order to tune the electrical properties of Co₃O₄, p-type semiconducting Co₃O₄ films were fabricated on the Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (PMN-PT), MgAl₂O₄ (MAO), and SrTiO₃ substrates by reactive magnetron sputtering. The Co₃O₄ film on the MAO substrate exhibits perfect epitaxial growth. However, the Co₃O₄ film on the PMN-PT substrate presents dislocation defects between the [011] and [112] orientations. The special ferroelectric domain shape surface and phase transition of the PMN-PT substrate induce the higher concentration of Co vacancies in the Co₃O₄ film, which further reduce the resistivity by several orders of magnitude. The calculated results indicate that introducing Co vacancies can enhance the electrical properties of Co₃O₄ by building impurity levels near the Fermi level, which is beneficial to form free-moving holes in the valence band. The free-moving holes can also be accumulated/dissipated by the ferroelectric field effect of PMN-PT substrates, leading to upward/downward bending of conduction, valence bands, and low/high-resistance states. This work helps us to tune and improve the electrical properties of Co₃O₄.

Electro-opto-mechano driven reversible multi-state memory devices based on photocurrent in Bi0.9Eu0.1FeO3/La0.67Sr0.33MnO3/PMN-PT heterostructures

A single device with extensive new functionality is highly attractive for the increasing demands for complex and multifunctional optoelectronics. Multi-field coupling has been drawing considerable attention because it leads to materials that can be simultaneously operated under several external stimuli (e.g. magnetic field, electric field, electric current, light, strain, etc.), which allows each unit to store multiple bits of information and thus enhance the memory density. In this work, we report an electro–opto–mechano-driven reversible multi-state memory device based on photocurrent in Bi_0.9Eu_0.1FeO₃ (BEFO)/La_0.67Sr_0.33MnO₃ (LSMO)/0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) heterostructures. It is found that the short-circuit current density (J_sc) can be switched by the variation of the potential barrier height and depletion region width at the Pt/BEFO interface modulated by light illumination, external strain, and ferroelectric polarization reversal. This work opens up pathways toward the emergence of novel device design features with dynamic control for developing high-performance electric–optical–mechanism integrated devices based on the BiFeO₃-based heterostructures.

The mutual interaction between polarization switching, light and piezoelectric strain.

Nonvolatile Control of the Electronic Properties of In2-xCrxO3 Semiconductor Films by Ferroelectric Polarization Charge

A series of Cr-doped In_2-xCr_xO₃ (ICO) semiconductor thin films were epitaxially grown on (111)-oriented 0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃ (PMN-0.29PT) single-crystal substrates by the pulsed laser deposition. Upon the application of an electric field to the PMN-0.29PT substrate along the thickness direction, we realized in situ, reversible, and nonvolatile control of the electronic properties and Fermi level of the films, which are manifested by abundant physical phenomena such as the n-type to p-type transformation, metal-semiconductor transition, metal-insulator transition, crossover of the magnetoresistance (MR) from negative to positive, and a large nonvolatile on-and-off ratio of 5.5 × 10⁴% at room temperature. We also strictly disclose that both the sign and the magnitude of MR are determined by the electron carrier density of ICO films, which could modify the s-d exchange interaction and weak localization effect. Our results demonstrate that the ferroelectric gating approach using PMN-PT can be utilized to gain deeper insight into the carrier-density-related electronic properties of In₂O₃-based semiconductors and provide a simple and energy efficient way to construct multifunctional devices which can utilize the unique properties of composite materials.

Ferroelectric field manipulated nonvolatile resistance switching in Al:ZnO/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures at room temperature

Resistance switching was obtained in Al:ZnO/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ heterostructures at room temperature by applying an external electric field. The modulation of the resistance is more pronounced in the thinner samples, indicating that it is an interfacial effect. In addition, the resistance of Al:ZnO films is significantly reduced by the photoexcited carriers when illumination is applied. The results indicate that the carrier density in the Al:ZnO films is modulated under external electric fields, due to the accumulation and depletion of charge at the interface between Al:ZnO and Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃. Hence, reversible and nonvolatile resistance states can be achieved by the ferroelectric field effect, and it is expected that multilevel storage will be realized.

Nonvolatile and Reversible Ferroelectric Control of Electronic Properties of Bi2Te3 Topological Insulator Thin Films Grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 Single Crystals

Single-phase (00 l)-oriented Bi₂Te₃ topological insulator thin films have been deposited on (111)-oriented ferroelectric 0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃ (PMN-PT) single-crystal substrates. Taking advantage of the nonvolatile polarization charges induced by the polarization direction switching of PMN-PT substrates at room temperature, the carrier density, Fermi level, magnetoconductance, conductance channel, phase coherence length, and quantum corrections to the conductance can be in situ modulated in a reversible and nonvolatile manner. Specifically, upon the polarization switching from the positively poled P_r⁺ state (i.e., polarization direction points to the film) to the negatively poled P_r^- (i.e., polarization direction points to the bottom electrode) state, both the electron carrier density and the Fermi wave vector decrease significantly, reflecting a shift of the Fermi level toward the Dirac point. The polarization switching from P_r⁺ to P_r^- also results in significant increase of the conductance channel α from -0.15 to -0.3 and a decrease of the phase coherence length from 200 to 80 nm at T = 2 K as well as a reduction of the electron-electron interaction. All these results demonstrate that electric-voltage control of physical properties using PMN-PT as both substrates and gating materials provides a simple and a straightforward approach to realize reversible and nonvolatile tuning of electronic properties of topological thin films and may be further extended to study carrier density-related quantum transport properties of other quantum matter.