Field Effect and Strongly Localized Carriers in the Metal-Insulator Transition Material VO(2)

High-Mobility Carriers in Epitaxial IrO2 Films Grown using Hybrid Molecular Beam Epitaxy

Binary rutile oxides of 5d metals such as IrO2 stand out in comparison to their 3d and 4d counterparts due to limited experimental studies, despite rich predicted quantum phenomena. Here, we investigate the electrical transport properties of IrO2 by engineering epitaxial thin films grown using hybrid molecular beam epitaxy. Our findings reveal phonon-limited carrier transport and thickness-dependent anisotropic in-plane resistance in IrO2 (110) films, the latter suggesting a complex relationship between strain relaxation and orbital hybridization. Magnetotransport measurements reveal a previously unobserved nonlinear Hall effect. A two-carrier analysis of this effect shows the presence of minority carriers with mobility exceeding 3000 cm2/(V s) at 1.8 K. These results point toward emergent properties in 5d metal oxides that can be controlled using dimensionality and epitaxial strain.

Modulation-doping a correlated electron insulator

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2-especially without inducing structural changes-has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard X-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5 × 1021 cm-3 without inducing any measurable structural changes. We find that the MIT temperature (TMIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.

A three-terminal non-volatile ferroelectric switch with an insulator-metal transition channel

Ferroelectrics offer a promising material platform to realize energy-efficient non-volatile memory technology with the FeFET-based implementations being one of the most area-efficient ferroelectric memory architectures. However, the FeFET operation entails a fundamental trade-off between the read and the program operations. To overcome this trade-off, we propose in this work, a novel device concept, Mott-FeFET, that aims to replace the Silicon channel of the FeFET with VO₂- a material that exhibits an electrically driven insulator-metal phase transition. The Mott-FeFET design, which demonstrates a (ferroelectric) polarization-dependent threshold voltage, enables the read current distinguishability (i.e., the ratio of current sensed when the Mott-FeFET is in state 1 and 0, respectively) to be independent of the program voltage. This enables the device to be programmed at low voltages without affecting the ability to sense/read the state of the device. Our work provides a pathway to realize low-voltage and energy-efficient non-volatile memory solutions.

Ultrafast, Kinetically Limited, Ambient Synthesis of Vanadium Dioxides through Laser Direct Writing on Ultrathin Chalcogenide Matrix

Vanadium dioxide (VO₂) is a strongly correlated electronic material and has attracted significant attention due to its metal-to-insulator transition and diverse smart applications. Traditional synthesis of VO₂ usually requires minutes or hours of global heating and low oxygen partial pressure to achieve thermodynamic control of the valence state. Further patterning of VO₂ through a series of lithography and etching processes may inevitably change its surface valence, which poses a great challenge for the assembly of micro- and nanoscale VO₂-based heterojunction devices. Herein, we report an ultrafast method to simultaneously synthesize and pattern VO₂ on the time scale of seconds under ambient conditions through laser direct writing on a V₅S₈ "canvas". The successful ambient synthesis of VO₂ is attributed to the ultrafast local heating and cooling process, resulting in controlled freezing of the intermediate oxidation phase during the relatively long kinetic reaction. A Mott memristor based on a V₅S₈-VO₂-V₅S₈ lateral heterostructure can be fabricated and integrated with a MoS₂ channel, delivering a transistor with abrupt switching transfer characteristics. The other device with a VS_xO_y channel exhibits a large negative temperature coefficient of approximately 4.5%/K, which is highly desirable for microbolometers. The proposed approach enables fast and efficient integration of VO₂-based heterojunction devices and is applicable to other intriguing intermediate phases of oxides.

Barrier Formation at the Contacts of Vanadium Dioxide and Transition-Metal Dichalcogenides

Phase-transition field-effect transistors (FETs) are a class of steep-slope devices that show abrupt on/off switching owing to the metal-insulator transition (MIT) induced in the contacting materials. An important avenue to develop phase-transition FETs is to understand the charge injection mechanism at the junction of the contacting MIT materials and semiconductor channels. Here, toward the realization of high-performance phase-transition FETs, we investigate the contact properties of heterojunctions between semiconducting transition-metal dichalcogenides (TMDCs) and vanadium dioxide (VO₂) that undergoes a MIT at a critical temperature (T_c) of approximately 340 K. We fabricated transistors based on molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂) in contact with the VO₂ source/drain electrodes. The VO₂-contacted MoS₂ transistor exhibited n-type transport both below and above T_c. Across the MIT, the on-current was observed to increase only by a factor of 5, in contrast to the order-of-magnitude change in the resistance of the VO₂ electrodes, suggesting the existence of high contact resistance. The Arrhenius analyses of the gate-dependent drain current confirmed the formation of the interfacial barrier at the VO₂/MoS₂ contacts, irrespective of the phase state of VO₂. The VO₂-contacted WSe₂ transistor showed ambipolar transport, indicating that the Fermi level lies near the mid gap of WSe₂. These observations provide insights into the contact properties of phase-transition FETs based on VO₂ and TMDCs and suggest the need for contact engineering for high-performance operations.

Vacancy-Driven Robust Metallicity of Structurally Pinned Monoclinic Epitaxial VO2 Thin Films

Vanadium dioxide (VO₂) is a strongly correlated material with 3d electrons, which exhibits temperature-driven insulator-to-metal transition with a concurrent change in the crystal symmetry. Interestingly, even modest changes in stoichiometry-induced orbital occupancy dramatically affect the electrical conductivity of the system. Here, we report a successful transformation of epitaxial monoclinic VO₂ thin films from a conventionally insulating to permanently metallic behavior by manipulating the electron correlations. These ultrathin (∼10 nm) epitaxial VO₂ films were grown on NiO(111)/Al₂O₃(0001) pseudomorphically, where the large misfit between NiO and Al₂O₃ were fully relaxed by domain-matching epitaxy. Complete conversion from an insulator to permanent metallic phase is achieved through injecting oxygen vacancies ( x ∼ 0.20 ± 0.02) into the VO_{2- x} system via annealing under high vacuum (∼5 × 10^-7 Torr) and increased temperature (450 °C). Systematic introduction of oxygen vacancies partially converts V⁴⁺ to V³⁺ and generates unpaired electron charges which result in the emergence of donor states near the Fermi level. Through the detailed study of the vibrational modes by Raman spectroscopy, hardening of the V-V vibrational modes and stabilization of V-V dimers are observed in vacuum-annealed VO₂ films, providing conclusive evidence for stabilization of a monoclinic phase. This ultimately leads to convenient free-electron transport through the oxygen-deficient VO_{2- x} thin films, resulting in metallic characteristics at room temperature. With these results, we propose a defect engineering pathway through the control of oxygen vacancies to tune electrical and optical properties in epitaxial monoclinic VO₂.

Dynamic Electronic Doping for Correlated Oxides by a Triboelectric Nanogenerator

The metal-insulator transition of vanadium dioxide (VO₂ ) is exceptionally sensitive to charge density and electron orbital occupancy. Thus three-terminal field-effect transistors with VO₂ channels are widely adopted to control the phase transition by external gating voltage. However, current leakage, electrical breakdown, or interfacial electrochemical reactions may be inevitable if conventional solid dielectrics or ionic-liquid layers are used, which possibly induce Joule heating or doping in the VO₂ layer and make the voltage-controlled phase transition more complex. Here, a triboelectric nanogenerator (TENG) and a VO₂ film are combined for a novel TENG-VO₂ device, which can overcome the abovementioned challenges and achieve electron-doping-induced phase modulation. By taking advantage of the TENG structure, electrons can be induced in the VO₂ channel and thus adjust the electronic states of the VO₂ , simultaneously. The modulation degree of the VO₂ resistance depends on the temperature, and the major variation occurs when the temperature is in the phase-transition region. The accumulation of electrons in the VO₂ channel also is simulated by finite element analysis, and the electron doping mechanism is verified by theoretical calculations. The results provide a promising approach to develop a novel type of tribotronic transistor and new electronic doping technology.

Enhanced electronic-transport modulation in single-crystalline VO2 nanowire-based solid-state field-effect transistors

Field-effect transistors using correlated electron materials with an electronic phase transition pave a new avenue to realize steep slope switching, to overcome device size limitations and to investigate fundamental science. Here, we present a new finding in gate-bias-induced electronic transport switching in a correlated electron material, i.e., a VO₂ nanowire channel through a hybrid gate, which showed an enhancement in the resistive modulation efficiency accompanied by expansion of metallic nano-domains in an insulating matrix by applying gate biases near the metal-insulator transition temperature. Our results offer an understanding of the innate ability of coexistence state of metallic and insulating domains in correlated materials through carrier tuning and serve as a valuable reference for further research into the development of correlated materials and their devices.

A 1D Vanadium Dioxide Nanochannel Constructed via Electric-Field-Induced Ion Transport and its Superior Metal-Insulator Transition

Nanoscale manipulation of materials' physicochemical properties offers distinguished possibility to the development of novel electronic devices with ultrasmall dimension, fast operation speed, and low energy consumption characteristics. This is especially important as the present semiconductor manufacturing technique is approaching the end of miniaturization campaign in the near future. Here, a superior metal-insulator transition (MIT) of a 1D VO₂ nanochannel constructed through an electric-field-induced oxygen ion migration process in V₂ O₅ thin film is reported for the first time. A sharp and reliable MIT transition with a steep turn-on voltage slope of <0.5 mV dec^-1 , fast switching speed of 17 ns, low energy consumption of 8 pJ, and low variability of <4.3% is demonstrated in the VO₂ nanochannel device. High-resolution transmission electron microscopy observation and theoretical computation verify that the superior electrical properties of the present device can be ascribed to the electroformation of nanoscale VO₂ nanochannel in V₂ O₅ thin films. More importantly, the incorporation of the present device into a Pt/HfO₂ /Pt/VO₂ /Pt 1S1R unit can ensure the correct reading of the HfO₂ memory continuously for 10⁷ cycles, therefore demonstrating its great possibility as a reliable selector in high-density crossbar memory arrays.

Switching VO₂ Single Crystals and Related Phenomena: Sliding Domains and Crack Formation

VO₂ is the prototype material for insulator–metal transition (IMT). Its transition at T_IMT = 340 K is fast and consists of a large resistance jump (up to approximately five orders of magnitude), a large change in its optical properties in the visible range, and symmetry change from monoclinic to tetragonal (expansion by 1% along the tetragonal c-axis and 0.5% contraction in the perpendicular direction). It is a candidate for potential applications such as smart windows, fast optoelectronic switches, and field-effect transistors. The change in optical properties at the IMT allows distinguishing between the insulating and the metallic phases in the mixed state. Static or dynamic domain patterns in the mixed-state of self-heated single crystals during electric-field induced switching are in strong contrast with the percolative nature of the mixed state in switching VO₂ films. The most impressive effect—so far unique to VO₂—is the sliding of narrow semiconducting domains within a metallic background in the positive sense of the electric current. Here we show images from videos obtained using optical microscopy for sliding domains along VO₂ needles and confirm a relation suggested in the past for their velocity. We also show images for the disturbing damage induced by the structural changes in switching VO₂ crystals obtained for only a few current–voltage cycles.

A Steep-Slope Transistor Combining Phase-Change and Band-to-Band-Tunneling to Achieve a sub-Unity Body Factor

Steep-slope transistors allow to scale down the supply voltage and the energy per computed bit of information as compared to conventional field-effect transistors (FETs), due to their sub-60 mV/decade subthreshold swing at room temperature. Currently pursued approaches to achieve such a subthermionic subthreshold swing consist in alternative carrier injection mechanisms, like quantum mechanical band-to-band tunneling (BTBT) in Tunnel FETs or abrupt phase-change in metal-insulator transition (MIT) devices. The strengths of the BTBT and MIT have been combined in a hybrid device architecture called phase-change tunnel FET (PC-TFET), in which the abrupt MIT in vanadium dioxide (VO₂) lowers the subthreshold swing of strained-silicon nanowire TFETs. In this work, we demonstrate that the principle underlying the low swing in the PC-TFET relates to a sub-unity body factor achieved by an internal differential gate voltage amplification. We study the effect of temperature on the switching ratio and the swing of the PC-TFET, reporting values as low as 4.0 mV/decade at 25 °C, 7.8 mV/decade at 45 °C. We discuss how the unique characteristics of the PC-TFET open new perspectives, beyond FETs and other steep-slope transistors, for low power electronics, analog circuits and neuromorphic computing.

Current induced polycrystalline-to-crystalline transformation in vanadium dioxide nanowires

Vanadium dioxide (VO₂) exhibits a reversible insulator-metal phase transition that is of significant interest in energy-efficient nanoelectronic and nanophotonic devices. In these applications, crystalline materials are usually preferred for their superior electrical transport characteristics as well as spatial homogeneity and low surface roughness over the device area for reduced scattering. Here, we show applied electrical currents can induce a permanent reconfiguration of polycrystalline VO₂ nanowires into crystalline nanowires, resulting in a dramatically reduced hysteresis across the phase transition and reduced resistivity. Low currents below 3 mA were sufficient to cause the local temperature in the VO₂ to reach about 1780 K to activate the irreversible polycrystalline-to-crystalline transformation. The crystallinity was confirmed by electron microscopy and diffraction analyses. This simple yet localized post-processing of insulator-metal phase transition materials may enable new methods of studying and fabricating nanoscale structures and devices formed from these materials.

Controlling the Temperature and Speed of the Phase Transition of VO2 Microcrystals

We investigated the control of two important parameters of vanadium dioxide (VO2) microcrystals, the phase transition temperature and speed, by varying microcrystal width. By using the reflectivity change between insulating and metallic phases, phase transition temperature is measured by optical microscopy. As the width of square cylinder-shaped microcrystals decreases from ∼70 to ∼1 μm, the phase transition temperature (67 °C for bulk) varied as much as 26.1 °C (19.7 °C) during heating (cooling). In addition, the propagation speed of phase boundary in the microcrystal, i.e., phase transition speed, is monitored at the onset of phase transition by using the high-speed resistance measurement. The phase transition speed increases from 4.6 × 10(2) to 1.7 × 10(4) μm/s as the width decreases from ∼50 to ∼2 μm. While the statistical description for a heterogeneous nucleation process explains the size dependence on phase transition temperature of VO2, the increase of effective thermal exchange process is responsible for the enhancement of phase transition speed of small VO2 microcrystals. Our findings not only enhance the understanding of VO2 intrinsic properties but also contribute to the development of innovative electronic devices.