Junwoo Son

Reversible phase modulation and hydrogen storage in multivalent VO2 epitaxial thin films

Hydrogen, the smallest and the lightest atomic element, is reversibly incorporated into interstitial sites in vanadium dioxide (VO2), a correlated oxide with a 3d(1) electronic configuration, and induces electronic phase modulation. It is widely reported that low hydrogen concentrations stabilize the metallic phase, but the understanding of hydrogen in the high doping regime is limited. Here, we demonstrate that as many as two hydrogen atoms can be incorporated into each VO2 unit cell, and that hydrogen is reversibly absorbed into, and released from, VO2 without destroying its lattice framework. This hydrogenation process allows us to elucidate electronic phase modulation of vanadium oxyhydride, demonstrating two-step insulator (VO2)-metal (HxVO2)-insulator (HVO2) phase modulation during inter-integer d-band filling. Our finding suggests the possibility of reversible and dynamic control of topotactic phase modulation in VO2 and opens up the potential application in proton-based Mottronics and novel hydrogen storage.

Enhancing electron mobility in La-doped BaSnO3 thin films by thermal strain to annihilate extended defects

We report the enhancement of room-temperature electron mobility in La-doped BaSnO3 (LBSO) thin films with thermal strain induced by high temperature nitrogen (N2) annealing. Simple annealing under an N2 environment consistently doubled the electron mobility of the LBSO films on the SrTiO3 (STO) substrates to as high as 78 cm2 V−1 s−1 at a carrier concentration of 4.0 × 1020 cm−3. This enhancement is mainly attributed to annihilation of extended defects as a consequence of compressive strain induced by the difference in the thermal expansion coefficients of LBSO and STO. Our study suggests that thermal strain can be exploited to reduce extended defects and to facilitate electron transport in transparent oxide semiconductors.

Voltage-induced insulator-to-metal transition of hydrogen-treated NbO2 thin films

We report on the voltage-induced insulator-to-metal transition (IMT) of the NbO2 thin films that are deposited under forming gas in the growth chamber. It is shown that the hydrogen in the forming gas gives rise to the abrupt voltage-induced IMT characteristics in NbO2 thin films that are sandwiched between top and bottom Pt electrodes. By a catalytic reaction at the triple boundary between NbO2 and Pt, hydrogen appears to be easily incorporated into the NbO2 lattice and doping significantly lowers the IMT temperature of NbO2 thin films, along with the reduction of NbO2 films.

Bistable resistive states of amorphous SrRuO3 thin films

We fabricated amorphous SrRuO3 thin films which exhibited the electronic transport behavior of an insulator that showed a three-dimensional hopping transport. Depending on the polarity of a sweep bias, bistable resistive states were observed in the capacitor consisted of an amorphous SrRuO3 thin film and Pt electrodes, which gives the opportunity for nonvolatile memory applications. From electric transport and optical conductivity data, we indirectly confirmed a probability of the mixed phase of SrO and RuO2 in the amorphous SrRuO3 thin film. This supports the applicability of a filament model as a mechanism for the bistable resistive states.

Off-state current reduction in NbO2-based selector device by using TiO2 tunneling barrier as an oxygen scavenger

We report a significant off-state current reduction by an order of magnitude in the NbO2-based selector devices by inserting an ultrathin TiO2 (∼2 nm) tunneling barrier. Moreover, the ultrathin TiO2 layer improves the reliability and uniformity of voltage-induced insulator-to-metal transition (IMT) in the NbO2 selector devices by thermodynamically suppressing the formation of a surface Nb2O5 layer. Our study suggests that the suitable combination of tunneling barrier and IMT materials can minimize the “off” current of IMT selector devices and improve their applicability in high-density three dimensional cross point array memory devices.

Correlated memory resistor in epitaxial NdNiO3 heterostructures with asymmetrical proton concentration

The electronic devices using correlated transition metal oxides are the promising candidates to overcome the limitation of the current electronics due to the rich electronic phases and the extreme sensitivities. Here, we report proton-based resistive switching memory that uses correlated oxides, i.e., epitaxial NdNiO3 heterostructure with asymmetrical concentration of protons (H+) to obtain multilevel states. By designing such metal-NdNiO3-metal device structures with asymmetrical proton concentration, we demonstrate that the correlated oxides exhibit resistive switching by ionic transport of protons at the metal-hydrogenated NdNiO3 (H-NNO) interface. This finding will guide the development of energy-efficient switching devices for non-volatile memory and neuromorphic applications.

Modulation of metal-insulator transitions by field-controlled strain in NdNiO3/SrTiO3/PMN-PT (001) heterostructures

The band width control through external stress has been demonstrated as a useful knob to modulate metal-insulator transition (MIT) in RNiO3 as a prototype correlated materials. In particular, lattice mismatch strain using different substrates have been widely utilized to investigate the effect of strain on transition temperature so far but the results were inconsistent in the previous literatures. Here, we demonstrate dynamic modulation of MIT based on electric field-controlled pure strain in high-quality NdNiO3 (NNO) thin films utilizing converse-piezoelectric effect of (001)-cut - (PMN-PT) single crystal substrates. Despite the difficulty in the NNO growth on rough PMN-PT substrates, the structural quality of NNO thin films has been significantly improved by inserting SrTiO3 (STO) buffer layers. Interestingly, the MIT temperature in NNO is downward shifted by ~3.3 K in response of 0.25% in-plane compressive strain, which indicates less effective TMI modulation of field-induced strain than substrate-induced strain. This study provides not only scientific insights on band-width control of correlated materials using pure strain but also potentials for energy-efficient electronic devices.

Prepulse effect on laser-induced water-window radiation from a liquid nitrogen jet

The authors show the prepulse effect on the conversion efficiency of a visible laser into water-window (λ=2.3–4.4nm) x ray from a liquid nitrogen jet. It is observed that a prepulse of only 2mJ enhances the conversion efficiency by 10–15 times for the main pulse of 15–60mJ at a delay of 3–6ns. The photon flux is ∼1.2×1012photons∕pulsesr at a delay of 4ns for a main pulse of 60mJ with a prepulse of 4–8mJ. It is noticed that the conversion efficiency increases with the delay up to 3ns and is then saturated.

Facile Phase Control of Multivalent Vanadium Oxide Thin Films (V2O5 and VO2) by Atomic Layer Deposition and Postdeposition Annealing

Atomic layer deposition was adopted to deposit VOx thin films using vanadyl tri-isopropoxide {VO[O(C3H7)]3, VTIP} and water (H2O) at 135 °C. The self-limiting and purge-time-dependent growth behaviors were studied by ex situ ellipsometry to determine the saturated growth conditions for atomic-layer-deposited VOx. The as-deposited films were found to be amorphous. The structural, chemical, and optical properties of the crystalline thin films with controlled phase formation were investigated after postdeposition annealing at various atmospheres and temperatures. Reducing and oxidizing atmospheres enabled the formation of pure VO2 and V2O5 phases, respectively. The possible band structures of the crystalline VO2 and V2O5 thin films were established. Furthermore, an electrochemical response and a voltage-induced insulator-to-metal transition in the vertical metal-vanadium oxide-metal device structure were observed for V2O5 and VO2 films, respectively.

Influence of tensile-strain-induced oxygen deficiency on metal-insulator transitions in NdNiO 3−δ epitaxial thin films

We report direct evidence that oxygen vacancies affect the structural and electrical parameters in tensile-strained NdNiO3−δ epitaxial thin films by elaborately adjusting the amount of oxygen deficiency (δ) with changing growth temperature TD. The modulation in tensile strain and TD tended to increase oxygen deficiency (δ) in NdNiO3−δ thin films; this process relieves tensile strain of the thin film by oxygen vacancy incorporation. The oxygen deficiency is directly correlated with unit-cell volume and the metal-insulator transition temperature (TMI), i.e., resulting in the increase of both unit-cell volume and metal-insulator transition temperature as oxygen vacancies are incorporated. Our study suggests that the intrinsic defect sensitively influences both structural and electronic properties, and provides useful knobs for tailoring correlation-induced properties in complex oxides.