Voltage Scaling of Graphene Device on SrTiO3 Epitaxial Thin Film

Multifunctional Magnetic Oxide-MoS2 Heterostructures on Silicon

Correlated oxides and related heterostructures are intriguing for developing future multifunctional devices by exploiting their exotic properties, but their integration with other materials, especially on Si-based platforms, is challenging. Here, van der Waals heterostructures of La0.7 Sr0.3 MnO3 (LSMO) , a correlated manganite perovskite, and MoS2 are demonstrated on Si substrates with multiple functions. To overcome the problems due to the incompatible growth process, technologies involving freestanding LSMO membranes and van der Waals force-mediated transfer are used to fabricate the LSMO-MoS2 heterostructures. The LSMO-MoS2 heterostructures exhibit a gate-tunable rectifying behavior, based on which metal-semiconductor field-effect transistors (MESFETs) with on-off ratios of over 104 can be achieved. The LSMO-MoS2 heterostructures can function as photodiodes displaying considerable open-circuit voltages and photocurrents. In addition, the colossal magnetoresistance of LSMO endows the LSMO-MoS2 heterostructures with an electrically tunable magnetoresponse at room temperature. This work not only proves the applicability of the LSMO-MoS2 heterostructure devices on Si-based platform but also demonstrates a paradigm to create multifunctional heterostructures from materials with disparate properties.

Two-Dimensional Layered Materials Meet Perovskite Oxides: A Combination for High-Performance Electronic Devices

As the Si-based transistors scale down to atomic dimensions, the basic principle of current electronics, which heavily relies on the tunable charge degree of freedom, faces increasing challenges to meet the future requirements of speed, switching energy, heat dissipation, and packing density as well as functionalities. Heterogeneous integration, where dissimilar layers of materials and functionalities are unrestrictedly stacked at an atomic scale, is appealing for next-generation electronics, such as multifunctional, neuromorphic, spintronic, and ultralow-power devices, because it unlocks technologically useful interfaces of distinct functionalities. Recently, the combination of functional perovskite oxides and two-dimensional layered materials (2DLMs) led to unexpected functionalities and enhanced device performance. In this paper, we review the recent progress of the heterogeneous integration of perovskite oxides and 2DLMs from the perspectives of fabrication and interfacial properties, electronic applications, and challenges as well as outlooks. In particular, we focus on three types of attractive applications, namely field-effect transistors, memory, and neuromorphic electronics. The van der Waals integration approach is extendible to other oxides and 2DLMs, leading to almost unlimited combinations of oxides and 2DLMs and contributing to future high-performance electronic and spintronic devices.

Room-Temperature Colossal Magnetoresistance in Terraced Single-Layer Graphene

Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in 2D materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, a room-temperature colossal MR of up to 5000% at 9 T is reported in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO₂ -terminated SrTiO₃ , a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices is demonstrated. Strikingly, a colossal MR of >1000% is also achieved in the terraced graphene even at a high carrier density of ≈10¹² cm^-2 . Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. The results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

Resistance hysteresis correlated with synchrotron radiation surface studies in atomic sp2 layers of carbon synthesized on ferroelectric (001) lead zirconate titanate in an ultrahigh vacuum

Graphene-like layers synthesized in ultrahigh vacuum, characterized by surface science techniques, exhibit resistance hysteresis depending on the carbon coverage.

Synergetic Behavior in 2D Layered Material/Complex Oxide Heterostructures

The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO₃ , graphene/LaAlO₃ /SrTiO₃ , graphene/ferroelectric oxide, MoS₂ /SrTiO₃ , and FeSe/SrTiO₃ heterostructures include voltage scaling in field-effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory functions based on charge traps, and greatly enhanced superconductivity. In this context, various properties and functionalities appearing in numerous different 2DLM/TMO heterostructure systems are reviewed. The results imply that the multidimensional heterostructure approach based on the disparate material systems leads to an entirely new platform for the study of condensed matter physics and materials science. The heterostructures are also highly relevant technologically as each constituent material is a promising candidate for next-generation optoelectronic devices.

Gigahertz Field-Effect Transistors with CMOS-Compatible Transfer-Free Graphene

High-quality graphene grown on metal-free substrates represents a vital milestone that provides an atomic clean interface and a complementary metal-oxide-semiconductor-compatible manufacturing process for electronic applications. We report a scalable approach to fabricate radio frequency field-effect transistors with a graphene channel grown directly on the sapphire substrate using the technique of remote-catalyzed chemical vapor deposition (CVD). A mushroom-shaped AlO _x top gate is used to allow the self-aligned drain/source contacts, yielding remarkable increase of device transconductance and reduction of the associated parasitic resistance. The quality of thus-grown graphene is reflected in the high extrinsic cutoff frequency and maximum oscillation frequency of 10.1 and 5.6 GHz for the graphene channel of length 200 nm and width 80 μm, respectively, potentially comparable with those of transferred CVD graphene at the same channel length and holding promise for applications in high-speed wireless communications.

Quantum Conductance Probing of Oxygen Vacancies in SrTiO3 Epitaxial Thin Film using Graphene

Quantum Hall conductance in monolayer graphene on an epitaxial SrTiO₃ (STO) thin film is studied to understand the role of oxygen vacancies in determining the dielectric properties of STO. As the gate-voltage sweep range is gradually increased in the device, systematic generation and annihilation of oxygen vacancies, evidenced from the hysteretic conductance behavior in the graphene, are observed. Furthermore, based on the experimentally observed linear scaling relation between the effective capacitance and the voltage sweep range, a simple model is constructed to manifest the relationship among the dielectric properties of STO with oxygen vacancies. The inherent quantum Hall conductance in graphene can be considered as a sensitive, robust, and noninvasive probe for understanding the electronic and ionic phenomena in complex transition-metal oxides without impairing the oxide layer underneath.