All-oxide Electronics Research Articles

Large increase in photo-induced conductivity of two-dimensional electron gas at SrTiO3 surface with BiFeO3 topping layer

In this work, we study and compare the photo-induced conductivity of a two-dimensional electron gas (2DEG) at the bare surface of SrTiO3 (STO) and in the heterostructure of BiFeO3 (BFO) and STO, where BFO was deposited by radio frequency magnetron sputtering. The photo-induced conductance of the BFO/STO interface shows a large increase which is 20.62 times more than the sum of photo-induced conductance from each individual BFO thin film and STO crystal. Since this photo-induced conductance of the BFO/STO heterostructure can be adjusted to become higher and lower by applying an electric field to the top surface, we attribute this large increase to the strong photo-induced electrical polarization of BFO. With the two-point setup of positive bias and negative bias, the conductivity also exhibits diode-like behavior where the forward and backward resistances are different. This work provides methods to interplay between light irradiation, electric field, and conductivity in all-oxide electronics.

Domain matching epitaxy stabilized metastable, tetragonal BiFeO3 on symmetry-mismatched c-plane ZnO

We report the stabilization of metastable tetragonal BiFeO3 epilayer on ZnO(0001) surface. X-ray reciprocal space map characterizations show that the BiFeO3 film is of true tetragonal symmetry, but not the commonly observed monoclinic structure. The critical thickness of the tetragonal BiFeO3 is higher than 140 nm, much larger than that reported previously. Despite the considerable lattice mismatch and symmetry mismatch, tetragonal BiFeO3 can be formed on ZnO(0001) though domain matching epitaxy which is featured by anisotropic growth. We show that by taking into account the elastic energy during the initial semi-coherent growth, the tetragonal phase is lower than the thermally stable rhombohedral phase in total energy by 70 meV per formula unit. Moreover, local piezoelectric characterizations reveal a coercive field of 360 kV cm−1 and a piezoelectric constant of 48 pm V−1. The integration of tetragonal BiFeO3 with robust ferroelectricity on the platform of ZnO has potentials for all-oxide electronics applications.

A comparison of LaAlO3/SrTiO3 heterointerfaces grown by spin coating and pulsed laser deposition methods

The two dimensional electron gas (2DEG) exhibits a considerable potential in all-oxide electronics due to the emergence of novel physical phenomena at the typical LaAlO3/SrTiO3 (LAO/STO) heterointerfaces. However, the quality and property of 2DEG are distinctly influenced by the preparation method. Herein, we compare the pulsed laser deposition (PLD) and spin coating method by obtaining high-quality 2DEGs. The comparative studies show that all the LAO/STO heterointerfaces exhibit a perfect metallic conductive behavior under the similar thickness. Moreover, the results by SC exhibit smaller sheet resistance and higher mobility, which is attributed to fewer defect states at heterointerfaces because the chemical method can effectively avoid the ablation of high-energy plume in PLD process. Therefore, our works provide a promising fabrication method for oxide heterointerfaces with a convenient, gentle, large scale and low cost, expanding the way for the application of all-oxide electronic devices.

Spectral weight reduction of two-dimensional electron gases at oxide surfaces across the ferroelectric transition

The discovery of a two-dimensional electron gas (2DEG) at the hbox {LaAlO}_3/hbox {SrTiO}_3 interface has set a new platform for all-oxide electronics which could potentially exhibit the interplay among charge, spin, orbital, superconductivity, ferromagnetism and ferroelectricity. In this work, by using angle-resolved photoemission spectroscopy and conductivity measurement, we found the reduction of 2DEGs and the changes of the conductivity nature of some ferroelectric oxides including insulating Nb-lightly-substituted hbox {KTaO}_3, hbox {BaTiO}_3 (BTO) and (Ca,Zr)-doped BTO across paraelectric-ferroelectric transition. We propose that these behaviours could be due to the increase of space-charge screening potential at the 2DEG/ferroelectric regions which is a result of the realignment of ferroelectric polarisation upon light irradiation. This finding suggests an opportunity for controlling the 2DEG at a bare oxide surface (instead of interfacial system) by using both light and ferroelectricity.

A foundation for complex oxide electronics -low temperature perovskite epitaxy

As traditional silicon technology is moving fast towards its fundamental limits, all-oxide electronics is emerging as a challenger offering principally different electronic behavior and switching mechanisms. This technology can be utilized to fabricate devices with enhanced and exotic functionality. One of the challenges for integration of complex oxides in electronics is the availability of appreciable low-temperature synthesis routes. Herein we provide a fundamental extension of the materials toolbox for oxide electronics by reporting a facile route for deposition of highly electrically conductive thin films of LaNiO3 by atomic layer deposition at low temperatures. The films grow epitaxial on SrTiO3 and LaAlO3 as deposited at 225 °C, with no annealing required to obtain the attractive electronic properties. The films exhibit resistivity below 100 µΩ cm with carrier densities as high as 3.6 · 1022 cm−3. This marks an important step in the realization of all-oxide electronics for emerging technological devices.

Probing Vacancy Behavior in Complex Oxide Heterostructured Films

Oxygen vacancies (Vo•• ) play a critical role in the transport mechanisms within complex oxides, analogous to electrons and holes within semiconductors. Systems including memristors, all-oxide electronics, and electrochemical cells comprise substrate-supported thin films either in metal-insulator-metal or complex oxide heterostructure configurations. As well-studied defect chemistry dictates mixed electronic/ionic functionality, improving oxide-oxide interfaces necessitates a direct, spatial understanding of vacancy distributions that define electrochemically active regions. Here we show that vacancies deplete over large, micron-level distances within single crystalline perovskite Nb-doped SrTiO3 substrate (Nb:SrTiO3) substrates after typical vacuum film deposition and post-annealing processes. We demonstrate the conversion of the surface potential across a four-layer strontium titanate / yttria-stabilized zirconia (STO/YSZ) heterostructured film to spatially defined (< 100 nm) [Vo•• ] profiles within STO through a unique combination of high temperature (500 °C), in situ scanning probes and classic semiconductor energy band diagram model analysis. Further comparison between room temperature and high temperature potential profiles clearly distinguishes between electronic-dominant and activated, ionic-dominant transport characteristics within the oxide layers. Consequently, we determined that oxygen scavenging by deposited films during pulsed laser deposition significantly reduce the Nb:STO, which is then partially reoxidized in the ambient environment during cooling. The results presented herein i) introduce the means to spatially resolve quantitative vacancy distributions across oxide films, and ii) pose the mechanism by which oxide thin film getters both enhance then deplete vacancies within the underlying substrate.

La interstitial defect-induced insulator-metal transition in the oxide heterostructures LaAlO3/SrTiO3

Perovskite oxide interfaces have attracted tremendous research interest for their fundamental physics and promising all-oxide electronics applications. Here, based on first-principles calculations, we propose a novel surface La interstitial promoted interface insulator-metal transition in LaAlO3/SrTiO3 (110). Compared with surface oxygen vacancies, which play a determining role on the insulator-metal transition of LaAlO3/SrTiO3 (001) interfaces, we find that surface La interstitials can be more experimentally realistic and accessible for manipulation and more stable in ambient atmospheric environment. Interestingly, these surface La interstitials also induce significant spin-splitting states with Ti dyz/dxz character at conducting LaAlO3/SrTiO3 (110) interface. On the other hand, for insulating LaAlO3/SrTiO3 (110) (<4 unit cells LaAlO3 thickness), a distortion between La (Al) and O atoms is found at the LaAlO3 side, partially compensating the polarization divergence. Our results reveal the origin of metal-insulator transition in LaAlO3/SrTiO3 (110) heterostructures, and also shed light on the manipulation of the superior properties of LaAlO3/SrTiO3 (110) for new possibilities of electronic and magnetic applications.

Avoiding polar catastrophe in the growth of polarly orientated nickel perovskite thin films by reactive oxide molecular beam epitaxy

By means of the state-of-the-art reactive oxide molecular beam epitaxy, we synthesized (001)- and (111)-orientated polar LaNiO3 thin films. In order to avoid the interfacial reconstructions induced by polar catastrophe, screening metallic Nb-doped SrTiO3 and iso-polarity LaAlO3 substrates were chosen to achieve high-quality (001)-orientated films in a layer-by-layer growth mode. For largely polar (111)-orientated films, we showed that iso-polarity LaAlO3 (111) substrate was more suitable than Nb-doped SrTiO3. In situ reflection high-energy electron diffraction, ex situ high-resolution X-ray diffraction, and atomic force microscopy were used to characterize these films. Our results show that special attentions need to be paid to grow high-quality oxide films with polar orientations, which can prompt the explorations of all-oxide electronics and artificial interfacial engineering to pursue intriguing emergent physics like proposed interfacial superconductivity and topological phases in LaNiO3 based superlattices.

Ink-Jet Printed CMOS Electronics from Oxide Semiconductors.

Complementary metal oxide semiconductor (CMOS) technology with high transconductance and signal gain is mandatory for practicable digital/analog logic electronics. However, high performance all-oxide CMOS logics are scarcely reported in the literature; specifically, not at all for solution-processed/printed transistors. As a major step toward solution-processed all-oxide electronics, here it is shown that using a highly efficient electrolyte-gating approach one can obtain printed and low-voltage operated oxide CMOS logics with high signal gain (≈21 at a supply voltage of only 1.5 V) and low static power dissipation.

Reversible insulator-metal transition of LaAlO3/SrTiO3 interface for nonvolatile memory

We report a new type of memory device based on insulating LaAlO3/SrTiO3 (LAO/STO) hetero-interface. The microstructures of the LAO/STO interface are characterized by Cs-corrected scanning transmission electron microscopy, which reveals the element intermixing at the interface. The inhomogeneous element distribution may result in carrier localization, which is responsible for the insulating state. The insulating state of such interface can be converted to metallic state by light illumination and the metallic state maintains after light off due to giant persistent photoconductivity (PPC) effect. The on/off ratio between the PPC and the initial dark conductance is as large as 105. The metallic state also can be converted back to insulating state by applying gate voltage. Reversible and reproducible resistive switching makes LAO/STO interface promising as a nonvolatile memory. Our results deepen the understanding of PPC phenomenon in LAO/STO, and pave the way for the development of all-oxide electronics integrating information storage devices.

ALD High-k for All-Oxide Electronics

not Available.

Aqueous Inorganic Inks for Low-Temperature Fabrication of ZnO TFTs

A simple, low-cost, and nontoxic aqueous ink chemistry is described for digital printing of ZnO films. Selective design through controlled precipitation, purification, and dissolution affords an aqueous Zn(OH)(x)(NH(3))(y)((2-x)+) solution that is stable in storage, yet promptly decomposes at temperatures below 150 degrees C to form wurtzite ZnO. Dense, high-quality, polycrystalline ZnO films are deposited by ink-jet printing and spin-coating, and film structure is elucidated via X-ray diffraction and electron microscopy. Semiconductor film functionality and quality are examined through integration in bottom-gate thin-film transistors. Enhancement-mode TFTs with ink-jet printed ZnO channels annealed at 300 degrees C are found to exhibit strong field effect and excellent current saturation in tandem with incremental mobilities from 4-6 cm(2) V(-1) s(-1). Spin-coated ZnO semiconductors processed at 150 degrees C are integrated with solution-deposited aluminum oxide phosphate dielectrics in functional transistors, demonstrating both high performance, i.e., mobilities up to 1.8 cm(2) V(-1) s(-1), and the potential for low-temperature solution processing of all-oxide electronics.

High Mobility inLaAlO3/SrTiO3Heterostructures: Origin, Dimensionality, and Perspectives

We have investigated the dimensionality and origin of the magnetotransport properties of LaAlO3 films epitaxially grown on TiO2-terminated SrTiO3(001) substrates. High-mobility conduction is observed at low deposition oxygen pressures (P(O2)<10(-5) mbar) and has a three-dimensional character. However, at higher P(O2) the conduction is dramatically suppressed and nonmetallic behavior appears. Experimental data strongly support an interpretation of these properties based on the creation of oxygen vacancies in the SrTiO3 substrates during the growth of the LaAlO3 layer. When grown on SrTiO3 substrates at low P(O2), other oxides generate the same high mobility as LaAlO3 films. This opens interesting prospects for all-oxide electronics.