Guijun Lian

Comprehensive study of the resistance switching in SrTiO3 and Nb-doped SrTiO3

We have demonstrated that the resistance switching (RS) effect can be controlled by the modification of the electrode configurations and the carrier densities in the Ag/SrTiO3 and Ag/Nb-doped SrTiO3(Nb:STO) structures. The elimination of the Schottky junction in the metal/Nb:STO completely destroys the RS effect, which suggests that the RS effect originates from the modification of Schottky-like barrier formed at the interface of metal/Nb:STO. The rectifying I-V curves revealed that the change in resistance was attributed to the trapping or detrapping carriers at the interface. The carrier density plays an important role in the determination of RS effect. The presence of the RS in SrTiO3 requires an appropriate doping level to provide conditions for trapping carriers at the interface.

Resistance and superconductivity switching caused by carrier injection: Evidences of self-trapping carriers in oxide electronics

Carrier injection performed in doped-manganite junction exhibits stable resistance switching effect and variety of interface barrier adjusted by electric fields, which demonstrates roles of the carrier movement across interface and suggests the importance of injected carriers’ staying. All observations suggest that the staying of injected carriers in the functional oxides leads to a carrier self-trapping mechanism to understand resistance switching phenomenon. More results in other oxide junctions and epitaxial doped manganite films support carrier self-trapping model and exhibit the importance of heterointerface on stabilizing the self-trapping carriers. Superconductivity achieved and adjusted by carrier injection can be distinct evidence that the self-trapping carriers as minority played important roles in operating of oxides’ intrinsic property. This phenomenon could relate to a novel area of researches and applications in oxide electronics.

Crystal-orientation controlled epitaxial CeO2 dielectric thin films on Si(100) substrates using pulsed laser deposition

Abstract Cerium oxide (CeO 2 ) is the potential dielectric candidate for the Si-based devices due to its stable chemical properties, high dielectric constant and good lattice match with silicon. In this paper, the impacts of process conditions on the structural and electrical characteristics of CeO 2 thin films deposited on Si(100) substrates have been studied. Epitaxial CeO 2 thin films with different crystal orientations on Si(100) substrates have been grown by pulsed laser deposition (PLD). The structural and electrical characteristics of CeO 2 thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), and high frequency capacitance–voltage measurements (C–V).

The growth mechanism and topography of superconducting YBa2Cu3O7-δ and BiSrCaCuO-2201 films studied by scanning tunneling microscopy

The growth mechanism and surface topography of YBa2Cu3O7 − δ (YBCO) and BiSrCaCuO-2201 (BSCCO) epitaxially grown superconducting films have been studdied by scanning tunneling microscopy. We report here two growth mechanisms: screw dislocation growth and layered growth, and also the surface characterization and surface modification of thin films under a controlled manner. High-quality epitaxial thin films were grown in situ by excimer laser ablation. YBCO thin films were epitaxially grown with the c-axis perpendicular to the SrTiO3 (100) substrate surface. On the flat substrate, the YBCO films were nucleated and grown in the screw dislocation manner. On the titled substrate (α<1 deg.), the films show layered growth. The etching caused by field-induced evaporation has been utilized to observe the initial stage of growth. Epitaxially grown BSCCO-2201 films were firstly synthesized in our laboratory with the c-axis perpendicular to the ZrO2 substrates. Layered growth has been observed by STM. No screw dislocations were identified in our samples. Larger atomic flat areas were present, which are more stable than YBCO films. As a parent phase for the BSCCO 2212, 2223 series, the study of the growth mechanism of 2201 films is very important to the synthesis of superconducting 2212, 2223 or other artificial layered thin films.

Trap-assisted tunneling resistance switching effect in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 heterostructure

We reported the resistance switching (RS) behavior in the epitaxially grown CeO2/ La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate. The CeO2/LSCMO device displayed improved switching characteristics as compared to that of metal/manganite device. The switching threshold voltage showed a strong dependence on the thickness of the CeO2 layer, where a minimum/maximum thickness was required for the appearance of the resistance switching. Both set and reset threshold voltages increase with the increase of the CeO2 layer thickness due to the trap-assisted electron tunneling effect. In the meantime, the defects or vacancies in the CeO2 films, in particular, the concentration of the defects or vacancies in the interface between CeO2 and LSCMO, have a significant impact on the switching effect. These results suggest that the electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the RS effect in the insulator/manganite system.

Anticrosstalk characteristics correlated with the set process for α-Fe2O3/Nb–SrTiO3 stack-based resistive switching device

A resistive switching device based on the stacked α-Fe2O3/Nb–SrTiO3 is proposed and fabricated that demonstrates excellent bipolar resistive switching behaviors including the uniformity, endurance, and retention performance. The Schottky-like current-voltage characteristics correlated with set process were observed in both high resistive states (HRSs) and low resistive states (LRSs) of the device. Importantly, the anticrosstalk characteristic, possessing higher reversed-biased LRS resistance than the forwarded-biased HRS resistance, indicates the potential applications of the stacked α-Fe2O3/Nb–SrTiO3 for multilevel storage in the cross-bar memory arrays. The carrier injection and trapping mechanism is suggested to explain the observed phenomena.

Superconducting HgBa2CaCu2Oy thin films growth on NdGaO3, SrTiO3, LaAlO3 and Y–ZrO2 substrates

Abstract Superconducting HgBa 2 CaCu 2 O y (Hg-1212) films have been fabricated on (110) NdGaO 3 , (100) SrTiO 3 , (100) LaAlO 3 and (100) Y–ZrO 2 substrates by a two-step process. Good reproducible superconducting properties, zero resistance temperatures ( T c )>115 K and critical current density ( J c )>4000 MA/m 2 at 77 K in zero field, are obtained. X-ray diffraction patterns indicate that the films have an epitaxial structure with the c -axis perpendicular to the surface. Scanning electron microscopy measurements show a layered growth structure with square and octagonal grains on the Hg-1212 film surfaces. (110) NdGaO 3 is demonstrated to be a good substrate for growing the Hg-cuprate thin films. A lower superconducting transition temperature is observed in films deposited on Y–ZrO 2 substrates, which can be attributed to variations of the microstructure in the films.

Switching of saturation magnetization by carrier injection in YBa2Cu3O7−δ/α-Fe2O3/Nb–SrTiO3 junctions

Ferromagnetic (FM) α-Fe2O3 layers were deposited on n-type oxide semiconductor substrates of Nb-doped SrTiO3 (NSTO) to form YBa2Cu3O7−δ/α-Fe2O3/NSTO junctions. Multilevel resistance switching behavior was observed in YBa2Cu3O7−δ/α-Fe2O3/NSTO junctions with stable resistive states. The saturation magnetization MS of the YBa2Cu3O7−δ/α-Fe2O3/NSTO junctions was modulated by carrier injection and correlated with the conductivity of the switched state. This is the first report of simultaneous modulation of intrinsic magnetic and transport properties for FM oxide devices by using carrier injection.

Understanding the intermediate initial state in TiO2−δ/La2/3Sr1/3MnO3 stack-based bipolar resistive switching devices

Bipolar resistive switching (RS) behavior was observed in epitaxially deposited TiO2−δ/La2/3Sr1/3MnO3 stacks. An intermediate initial state of this device demonstrated that the original resistive state would switch to either the higher or the lower state, depending only on the electrical polarity of the initial measurement. As the analogue intermediate states were obtained by device fabrication, the direct observation of these switching states was possible, knowing the microscopic physical origin of the RS effect. Based on x-ray photoelectron spectroscopy analysis, an understanding of this interesting phenomenon was proposed, well supporting the oxygen vacancy formation and recombination mechanism.

Preparation of superconducting HgBa2CaCu2Ox films with a zero-resistance transition temperature of 121 K

Abstract Hg-1212 films were prepared by means of annealing of mercury-free precursor films with bulk Hg-1212 pellets in an evacuated quartz tube. The mercury free precursor films were deposited by laser ablation on (100) SrTiO 3 substrates. The annealed films exhibited the (00 n ) oriented X-ray diffraction pattern of Hg-1212. The superconducting transition temperature, T c ( ϱ = 0), was 121 K.