Xinjun Liu

Forming-free colossal resistive switching effect in rare-earth-oxide Gd2O3 films for memristor applications

The reproducible forming-free resistive switching (RS) behavior in rare-earth-oxide Gd2O3 polycrystalline thin film was demonstrated. The characteristic of this forming-free RS was similar to that of other forming-necessary binary RS materials except that its initial resistance starts from not the high resistance state (HRS) but the low resistance state (LRS). An ultrahigh resistance switching ratio from HRS to LRS of about six to seven orders of magnitude was achieved at a bias voltage of 0.6 V. Mechanism analysis indicated that the existence of metallic Gd in the Gd2O3 films plays an important role in the forming-free RS performance. Our work provides a novel material with interesting RS behavior, which is beneficial to deepen our understanding of the origin of RS phenomenon.

All-ZnO-based transparent resistance random access memory device fully fabricated at room temperature

A transparent resistance random access memory (RRAM) structure consisted of all-ZnO-based film is fabricated by the pulsed laser deposition method at room temperature. The device is based on transparent Mg-doped ZnO films, sandwiched by Al-doped ZnO as electrodes. Reliable and reproducible bipolar resistance memory switching performances are achieved. Fast and stable switching behaviour in the voltage pulse mode is demonstrated with set and reset durations of 50 ns and 100 ns, respectively. The transmittance of the device is from 64% to 82% in the visible region. All-ZnO-based transparent RRAM will open a route towards see-through memory devices.

The polarity origin of the bipolar resistance switching behaviors in metal/La0.7Ca0.3MnO3/Pt junctions

Bipolar resistance switching behaviors in the M/La0.7Ca0.3MnO3/Pt (M=Pt, Ag, Cu, Al, Ti, and W) junctions were investigated. We found that the switching polarities of the junctions for M=Pt, Ag, and Cu were opposite to those for M=Al, Ti, and W. This phenomenon was attributed to the different Gibbs free energy of the metal oxide formation. Based on Auger electron spectroscopy measurement of the M/La0.7Ca0.3MnO3 interfaces, the switching mechanisms were further discussed in terms of metal electrode redox reaction for M=Al, Ti, and W and oxygen vacancy generation/annihilation in the La0.7Ca0.3MnO3 film for M=Pt, Ag, and Cu, respectively.

Stable bipolar resistance switching behaviour induced by a soft breakdown process at the Al/La0.7Ca0.3MnO3 interface

The current–voltage characteristics, carrier transport processes and electric pulse induced resistance switching behaviour have been investigated in Al/La0.7Ca0.3MnO3 (LCMO)/Pt sandwich structures. A stable resistance switching effect was obtained when a soft breakdown process occurred during negative voltage sweeping. Moreover, the trigger condition of the high resistance state is completely different from that of the low resistance state. Based on the investigation on the carrier transport processes of different resistance states, it is suggested that the resistance switching effect is closely related to the resistance change of the AlOx layer formed at the Al/LCMO interface. The local oxidation/reduction of the AlOx layer associated with the rupture/formation of conductive filaments is proposed to explain the resistance switching effect. A simple method to control the oxidation/reduction of the AlOx layer has been devised to improve the resistance switching properties by adjusting the thickness of the Al electrode.

Multiform Resistance Switching Effects in the Al / La0.7Ca0.3MnO3 / Pt Structure

Multiform resistance switching (RS) effects, including two modes of bipolar RS effects and unipolar RS effect, have been observed in the same Al/La 0.7 Ca 0.3 MnO 3 (LCMO)/Pt structures by adjusting the thickness of Al electrode and implementing a forming process. When Al electrode is ultrathin (10 nm), a polarity reversal in bipolar RS can be obtained. The conversion from bipolar RS to unipolar RS can also be realized while increasing the thickness of Al (such as 50 nm). The transitions between these multiform RS effects derive from the changes in the Al/LCMO interface, which highlights the importance of the interface structure on the RS properties.

Structural properties and resistive switching behaviour in MgxZn1−xO alloy films grown by pulsed laser deposition

MgxZn1−xO alloy films with Mg concentration ranging from 0 to 0.5 were fabricated by a pulsed laser deposition method. The effect of Mg content on the microstructure and resistive switching behaviour was investigated. It was found that the film structure changed from pure hexagonal to a coexistence of hexagonal and cubic with increasing Mg content from 0 to 0.5. In addition, the ratio of the high-resistance state to the low-resistance state improved from ~14 to ~2 × 108. Furthermore, rapid thermal annealing of the samples reduced the forming voltage from ~18 V to ~10 V. The resistive switching behaviour in the MgxZn1−xO films was explained by the filament model based on the variation of band gap and crystalline grains induced by the Mg content modulation and the thermal treatment.

Reversible change in magnetic moment and specific heat of La0.9Ca0.1MnO3 at different resistance states

The magnetic moment and the specific heat of La0.9Ca0.1MnO3 (LCMO) at different resistance states induced by electric pulses were studied. It was found that the magnetic moment and the specific heat increase with decreasing resistance, and these changes are reversible. The reversible change in specific heat was shown to be attributed mainly to the ferromagnetic-spin-waves component by fitting specific heat. These results indicated that the electric pulses modulated the magnetic configuration of LCMO at low temperature, which would result in a large variation in magnetization and specific heat associated with ferromagnetic spin waves.

The properties of magnetic moment and specific heat of La 0.7Ca0.3MnO3 polycrystalline at different resistance state

The electric-pulse-induced resistance switching behavior was studied in bulk La0.7Ca0.3MnO3 polycrystalline at room temperature. The relationship between the thickness and the threshold value of electric pulse on La0.7Ca0.3MnO3 bulk was studied. It was found that the threshold value of electric pulse depends on the thickness of samples. The decrease of the magnetic moment and specific heat in the low resistance state on La0.7Ca0.3MnO3 are opposite to that observed on La0.9Ca0.1MnO3. These phenomena may be associated with the different ground states of La0.9Ca0.1MnO3 and La0.7Ca0.3MnO3. The change of magnetic configuration would change the specific heat arising from ferromagnetic spin-waves component and charge carriers.

Change of the magnetic moment and specific heat of La0.9Ca0.1MnO3 after heat treatment in oxygen and argon

对具有电脉冲诱发电阻可逆变化（EPIR）特性的多晶La0.9Ca0.1MnO3在相同条件下经氩气和氧气退火后样品的磁矩和比热进行了研究.实验结果表明,经氧气退火后的磁矩和比热分别比氩气退火后的磁矩和比热增大.对氧气退火后的比热和氩气退火后的比热做差处理后发现,两种气氛退火后的比热差与T3/2成线性关系.经氧气和氩气退火后的区别在于氧离子浓度的变化.经氧气退火后,氧离子浓度增加,导致样品中铁磁区域的增大,引起磁矩和比热增大.退火后磁矩和比热的变化与EPIR效应中磁矩和比热随电阻状态的变化趋势一致,表明EPIR效应与氧含量的变化有关.

Fabrication and resistance-switching behaviors of NiO thin films by thermal oxidation of evaporated Ni films

Polycrystalline NiO thin films were fabricated on Pt (111)/Ti/SiO2/Si substrates by thermal oxidation of the evaporated Ni films. Pt/NiO/Pt structures were prepared, and they showed reversible resistance switching behaviors. When the compliance set current was varied from 5 mA to 40 mA, the on-state currents increased, while the on-state resistances decreased. It is probably attributed to higher current compliance resulted in the formation of stronger and less resistive filaments, which in turn need more energy and power for their rupture. The resistive switching in NiO thin films is closely related to the formation and rupture of conducting filaments.