SrTiO3 Schottky Junctions Research Articles

Controllable resistive switching behaviors in heteroepitaxial LaNiO3/Nb:SrTiO3 Schottky junctions through oxygen vacancies engineering

Perovskite oxide-based memristors have been extensively investigated for the application of non-volatile memories, and the oxygen vacancies associated with Schottky barrier changing are considered as the origin of the memristive behaviors. However, due to the difference of device fabrication progress, various resistive switching (RS) behaviors have been observed even in one device, deteriorating the stability and reproducibility of devices. Precisely controlling the oxygen vacancies distribution and shedding light on the behind physic mechanism of these RS behaviors, are highly desired to help improve the performance and stability of such Schottky junction-based memristors. In this work, the epitaxial LaNiO3 (LNO)/Nb:SrTiO3 (NSTO) is adopted to explore the influence of oxygen vacancy profiles on these abundant RS phenomena. It demonstrates that the migration of oxygen vacancy in LNO films plays a key role in memristive behaviors. When the effect of oxygen vacancies at the LNO/NSTO interface is negligible, improving the oxygen vacancies concentration in LNO film could facilitate resistance on/off ratio of HRS and LRS, and the corresponding conducting mechanisms attributes to the thermionic emission and tunneling-assisted thermionic emission, respectively. Moreover, it is found that reasonably increasing the oxygen vacancies at LNO/NSTO interface makes trap-assisted tunneling possible, also providing an effective way to improve the performance of the device. The results in this work have clearly elucidated the relationship between oxygen vacancy profile and RS behaviors, and give physical insights into the strategies for improving the device performance of Schottky junction-based memristors.

Temperature dependence of tunneling current in Pt/Nb:SrTiO3 Schottky junction

We investigate temperature-dependent electrical properties of Pt/Nb:SrTiO3 Schottky junctions by measuring current–voltage and capacitance–voltage curves at various temperatures (20 K–300 K). Interestingly, the Schottky junctions have shown different temperature dependences of resistance according to the polarity of bias voltage: insulating behaviors are displayed in the positive bias branch, while metallic behaviors are observed in the negative bias branch. These behaviors can be ascribed to Schottky barrier modulation that arises from the temperature and field dependences of dielectric permittivity of SrTiO3. The modulation of the barrier profile determines anomalous tunneling current in the negative bias branch as a function of temperature. These comprehensive analyses could not only reveal the rich physics of the Pt/Nb:SrTiO3 Schottky junction but also enhance our understanding for high dielectric materials.

Inhomogeneous barrier heights at dipole-controlled SrRuO3/Nb:SrTiO3 Schottky junctions

It has recently been shown that the Schottky barrier height (SBH) formed at metal-semiconductor perovskite oxide heterojunctions can be dramatically tuned by the insertion of atomic-scale dipole layers at the interface. However, in idealized form, this would only allow for specific values of the SBH, discretized by the dipole layer thickness. Here, we examine the effect of fractional unit cell LaAlO3 dipoles inserted between SrRuO3 and Nb:SrTiO3 in (001) Schottky junctions, as a function of their in-plane lateral distribution. When the LaAlO3 dipoles are finely dispersed, we observe uniformly rectifying junctions, with SBHs reflecting the fractional LaAlO3 coverage. For larger length-scale distributions, the junction characteristics reflect the inhomogeneous combination of regions with and without the interface dipole. The characteristic length scale dividing the two regimes corresponds to the semiconductor depletion width scaled by the dipole potential, determining the effective scale for which the SBH can be continuously tuned.

Asymmetric Resistive Switching Effect in Au/Nb:SrTiO3 Schottky Junctions

Asymmetric resistive switching processes are observed in Au/Nb:SrTiO3 (NSTO) Schottky junctions. The SET transition from high‐ to low‐resistance state occurs at 10 ns under +2 to +5 V bias, while the RESET transition from low‐ to high‐ resistance state occurs at 104 to 107 ns under −2 to −5 V bias. A model of electron trapping/detrapping coupled with the migration of oxygen vacancies toward/away from Au/NSTO Schottky junction interface is proposed to understand this fast SET and slow RESET process. This fast SET and slow RESET switch may have potential applications in some special regions.

Large room-temperature tunneling anisotropic magnetoresistance and electroresistance in single ferromagnet/Nb:SrTiO3 Schottky devices

There is a large effort in research and development to realize electronic devices capable of storing information in new ways - for instance devices which simultaneously exhibit electro and magnetoresistance. However it remains a challenge to create devices in which both effects coexist. In this work we show that the well-known electroresistance in noble metal-Nb:SrTiO3 Schottky junctions can be augmented by a magnetoresistance effect in the same junction. This is realized by replacing the noble metal electrode with ferromagnetic Co. This magnetoresistance manifests as a room temperature tunneling anisotropic magnetoresistance (TAMR). The maximum room temperature TAMR (1.6%) is significantly larger and robuster with bias than observed earlier, not using Nb:SrTiO3. In a different set of devices, a thin amorphous AlOx interlayer inserted between Co and Nb:SrTiO3, reduces the TAMR by more than 2 orders of magnitude. This points to the importance of intimate contact between the Co and Nb:SrTiO3 for the TAMR effect. This is explained by electric field enhanced spin-orbit coupling of the interfacial Co layer in contact with Nb:SrTiO3. We propose that the large TAMR likely has its origin in the 3d orbital derived conduction band and large relative permittivity of Nb:SrTiO3 and discuss ways to further enhance the TAMR.

Controllable resistive switching in Au/Nb:SrTiO3 microscopic Schottky junctions

The reversible resistive switching effect at oxide interface shows promising applications in information storage and artificial intelligence. However, the microscopic switching mechanism is still elusive due to the difficulty of direct observation of the electrical and chemical behavior at the buried interface, which becomes a major barrier to design reliable, scalable, and reproducible devices. Here we used a gold-coated AFM tip as a removable electrode to investigate the resistive switching effect in a microscopic Au/Nb:SrTiO3 Schottky junction. We found that unlike the inhomogeneous random resistive switching in the macroscopic Schottky junctions, the high and low resistance states can be reversibly switched in a controllable way on the Nb-doped SrTiO3 surface by the conductive tip. The switching between the high and low resistance states in vacuum is accompanied by the reversible shift of the surface Fermi level. We indicate that the transfer of the interface oxygen ion in a double-well potential is responsible for the resistive switching in both macroscopic and microscopic Schottky junctions. Our findings provide a guide to optimize the key performance parameters of a resistive switching device such as operation voltage, switching speed, on/off ratio, and state retention time by proper electrode selection and fabrication strategy.

Band alignment and photovoltaic effect of epitaxial α-PbO thin films

To explore a p-type semiconductor lattice-matched with perovskite transition-metal oxides, we have grown α-PbO(001) thin films on (Nb-doped) SrTiO3(001) and GdScO3(110) substrates by pulsed laser deposition. The photovoltaic effect in a Au/α-PbO/Nb:SrTiO3 heterojunction is enhanced compared with that in a Au/Nb:SrTiO3 Schottky junction. The band alignment is deduced from photocurrent action spectra. We conclude that α-PbO facilitates the separation of electron–hole pairs generated at the interface of the SrTiO3 side in the ultraviolet light region and of the α-PbO side in the visible light region. Our results indicate that α-PbO is a promising candidate for photovoltaic heterojunctions involving strongly correlated oxides.

Transient behavior in Pt/Nb-doped SrTiO3 Schottky junctions

We investigated the transient behavior of Pt/Nb-doped SrTiO3 Schottky junctions that exhibit rectifying and hysteric I-V characteristics. We found that regardless of the junction resistance state, the junction under the bias shows the relaxation behavior over 104 s while the behavior is strongly dependent on the junction resistance state. Detailed investigation of the time dependence of the current and the capacitance of the junctions indicates that the junctions have the inhomogeneous Schottky barrier heights and depletion layer widths while the mean junction profile remains unchanged regardless of the junction resistance state. Given the experimental results, we attribute the observed transient behavior to time-dependent changes in the population of electrons trapped in the local conduction regions under the bias, which play an important role in determining the resistance state of Pt/Nb-doped SrTiO3 junction.

Modulation of resistance switching in Au/Nb:SrTiO3 Schottky junctions by ambient oxygen

We investigated the room-temperature current-voltage characteristics of Au/Nb:SrTiO3 Schottky junctions under various atmospheres and working pressures. We observed that oxygen partial pressure reversibly modulates junction response, briefly individual specimens behave as high-quality rectifiers in oxygen-rich atmospheres and as bipolar resistive switches in vacuum and inert gases. A two orders of magnitude modulation of resistance switching characterizes samples with the highest content of interfacial oxygen vacancies. We attribute this behavior to oxygen ionosorption and chemical oxidation at the metal-oxide interface. Our results are relevant to oxide devices displaying resistive switching at ambient-exposed interfaces, and might be exploited for gas detection purposes.

Electric field penetration in Au/Nb:SrTiO3 Schottky junctions probed by bias-dependent internal photoemission

Electric field penetration into the metallic side of a Schottky junction is in principle a universal phenomenon, the magnitude of which increases with the semiconductor permittivity. Here, we quantitatively probe this effect using bias-dependent internal photoemission spectroscopy at the Schottky junction between a large dielectric permittivity semiconductor SrTiO3 and gold. A clear linear reduction in the barrier height with increasing interface electric field was observed, highlighting the importance of field penetration into the gold. The interfacial permittivity of SrTiO3 at the interface is reduced from the bulk value, reflecting intrinsic suppression at the interface.

Resistance switching properties in Pt/SrTiO3:Nb Schottky junctions studied by admittance spectroscopy

We have investigated the resistance switching behavior of Pt/Nb-doped SrTiO3 (STO:Nb) Schottky junctions, which were fabricated using STO:Nb single crystals with an as-polished surface, a thermally treated surface, and a freshly cleaved surface. The largest and smallest I–V curve hystereses were found in the samples with the cleaved and thermally treated surfaces, respectively. The I–V curves’ temperature dependence showed a steep increase of leakage current in reverse bias mode with decreasing temperature for all samples. Also cooling enhanced the magnitude of the hysteresis. The frequency dependence of the junction admittance indicated that the fundamental structure of the Pt/STO:Nb junction can be described by a parallel connection of a depletion layer (capacitance) and a leakage path. Furthermore, the leakage path seemed to be a simple conduction path without any obvious frequency dependency in conductance.

Resistance switching effect in Nb-doped SrTiO3 (100) bicrystal with (100) ∼45° twist boundary

We have fabricated Nb-doped SrTiO3 bicrystals with (100) ∼45° twist boundary by changing the fabricating conditions and demonstrated that they can exhibit a large resistance switching effect. Nb:SrTiO3 bicrystals cooled at the rate of −300 K/h show an asymmetric large hysteresis in current-voltage characteristics and the magnitude of resistance change attains two orders of magnitude. The resistance state can also be switched by the application of voltage pulses with different polarity and both states can be memorized for over 5 h at room temperature. From experimental results, it can be speculated that the twist boundary could be an origin of large resistance switching and memory effects and it behaves like an electrode interface in SrRuO3/Nb:SrTiO3 Schottky junction devices.

Electrical properties of a Ba1−xKxBiO3/Nb-doped SrTiO3 Schottky junction

Current-voltage properties for a Ba1−xKxBiO3/Nb-doped SrTiO3 (0.01 wt%) Schottky junction have been measured and investigated over a temperature range from 5.3 to 300 K. Excellent rectification properties of the junction were observed, with high reproducibility. The barrier height V60 decreased and the ideality factor n increased with decreasing temperature. The modified Richardson plot of saturation current density showed a straight-line dependence. These results may be explained by the existence of interfacial layer or the inhomogeneity of the Schottky barrier height.