Co-reporter:Yuandong Hu, Fei Zeng, Chiating Chang, Wenshuai Dong, Xiaojun Li, Feng Pan, and Guoqi Li
ACS Omega February 2017? Volume 2(Issue 2) pp:746-746
Publication Date(Web):February 28, 2017
DOI:10.1021/acsomega.6b00392
Pt/Ca2+–polyethylene oxide/polymer poly[3-hexylthiophene-2,5-diyl]/Pt devices were fabricated, and their pulse responses were studied. The discharging peak, represented by the postsynaptic current (PSC), first increases and then decreases with increasing input number in a pulse train. The weight of the PSC decreased for low-frequency stimulations but increased for high-frequency stimulations. However, the peak of the negative differential resistance during the charging process varied following the opposite trend. These behaviors suggested the ability for transferring the signal bidirectionally, confirming the equivalence between the ionic kinetics of our device and the transmitter kinetics of one kind of synapse. A facilitation (F)–depression (D) interplay model corresponding to the ionic polarization and doping interplay at the electrolyte/semiconducting polymer interface was adopted to successfully mimic the weight modification of the PSC. The simulation results showed that the observed synaptic plasticity was caused by the great disparity between the recovery time constants of F and D (τF and τD). Moreover, such an interplay could inspire the features of responses to post-tetanic stimulations. Our study suggested a means to realize synaptic computation.Topics: Cell and Molecular biology; Electric transport processes and properties; Polarization; Polymers; Polyoxyalkylenes; Solid state electrochemistry;
Co-reporter:C. Song, J.H. Miao, F. Li, Y.N. Yan, B. Cui, F. Pan
Journal of Alloys and Compounds 2017 Volume 696(Volume 696) pp:
Publication Date(Web):5 March 2017
DOI:10.1016/j.jallcom.2016.11.313
•Coercivity of one monolayer cobalt is modified by ionic liquid gating.•Transition temperature of anomalous Hall effect is tuned by ionic liquid gating.•Unexpected inverted Hall loops above the transition temperature are observed.•It provides a way for exploring anomalous Hall effect by electrical control.We investigate electrical manipulation of magnetism in Ta/Pd/Co/HfO2 films probed by anomalous Hall Effect (AHE). The coercivity of the one monolayer cobalt and the transition temperature for the disappearance of usual AHE curves could be modified by a low voltage below ±2 V via the usage of ionic liquid. The unexpected emergence of inverted Hall loops above the transition temperature but far below the Curie temperature performed on the magnetometer could be explained by the competition between skew scattering and intrinsic/side-jump contributions of AHE. Our finding provides a different avenue for exploring the origin of AHE by electrical manipulation.
Co-reporter:Bin Cui;Cheng Song;Haijun Mao;Yinuo Yan;Fan Li;Shuang Gao;Jingjing Peng;Fei Zeng
Advanced Functional Materials 2016 Volume 26( Issue 5) pp:753-759
Publication Date(Web):
DOI:10.1002/adfm.201504036
The semiconductor industry has seen a remarkable miniaturization trend, where the size of microelectronic circuit components is expected to reach the scale of atom even subatom. Here, an orbital switch formed at the interface between BaTiO3 (BTO) and La0.5Sr0.5MnO3 (LSMO) is used to manipulate the electric field effect in the LSMO/BTO heterostructure. The orbital switch is based on the connection or breakdown of interfacial Ti–O–Mn bond due to the ferroelectric displacement under external electric field. This finding would pave the way for the tuning of the material performance or device operation at atomic level and introducing the orbital degree of freedom into the terrain of microelectronics.
Co-reporter:Shuang Gao, Guang Yang, Bin Cui, Shouguo Wang, Fei Zeng, Cheng Song and Feng Pan
Nanoscale 2016 vol. 8(Issue 25) pp:12819-12825
Publication Date(Web):02 Jun 2016
DOI:10.1039/C6NR03169B
Stateful logic circuits based on next-generation nonvolatile memories, such as magnetoresistance random access memory (MRAM), promise to break the long-standing von Neumann bottleneck in state-of-the-art data processing devices. For the successful commercialisation of stateful logic circuits, a critical step is realizing the best use of a single memory cell to perform logic functions. In this work, we propose a method for implementing all 16 Boolean logic functions in a single MRAM cell, namely a magnetoresistance (MR) unit. Based on our experimental results, we conclude that this method is applicable to any MR unit with a double-hump-like hysteresis loop, especially pseudo-spin-valve magnetic tunnel junctions with a high MR ratio. Moreover, after simply reversing the correspondence between voltage signals and output logic values, this method could also be applicable to any MR unit with a double-pit-like hysteresis loop. These results may provide a helpful solution for the final commercialisation of MRAM-based stateful logic circuits in the near future.
Co-reporter:M.J. Wang, F. Zeng, S. Gao, C. Song, F. Pan
Journal of Alloys and Compounds 2016 Volume 667() pp:219-224
Publication Date(Web):15 May 2016
DOI:10.1016/j.jallcom.2016.01.177
•Resistive switching behaviors are modulated by Ti interlayer and operation scheme.•Self-compliance effect appears in Pt/HfO_2/Ti(∼5 nm)/HfO_2/Pt devices.•Bipolar and complementary resistive switching obtained in the devices with 7 nm Ti.•The mechanism relies on the oxygen vacancy and Hf-rich filaments formation.To overcome the practical issues existed in RRAMs application, complementary resistive switching (CRS) and self-compliance effect are investigated to alleviate sneak current issue and avoid external compliance current (Icomp), respectively. It is remarkable that the two resistive switching (RS) behaviors are attempted to achieve in the same system of Pt/HfO2(7.5−x)/Ti(2x)/HfO2(7.5−x)/Pt (x = 0, 2.5, 3.5 nm) device fabricated by magnetron sputtering, i.e., multi-mode resistive switching behaviors are modulated by varying Ti interlayer thickness and operation scheme, such as initial Icomp and applied voltage during reset switching. As indicated by the results, typical bipolar resistive switching (BRS) and CRS behaviors exhibit in Pt/HfO2(∼4 nm)/Ti(∼7 nm)/HfO2(∼4 nm)/Pt devices, and self-compliance effect appears in Pt/HfO2(∼5 nm)/Ti(∼5 nm)/HfO2(∼5 nm)/Pt devices. After investigated the conductive mechanisms, it is proposed adopting conceivable switching mechanism associated with the formation of oxygen vacancy (VO) and metallic conduction filament (CF) to understand CRS and self-compliance effect. The exploration of multi-mode RS behaviors paves the way to improve the flexibility and convenience of RRAMs in actual application.
Co-reporter:Yuyan Wang;Xiang Zhou;Cheng Song;Yinuo Yan;Shiming Zhou;Guangyue Wang;Chao Chen;Fei Zeng
Advanced Materials 2015 Volume 27( Issue 20) pp:3196-3201
Publication Date(Web):
DOI:10.1002/adma.201405811
Co-reporter:Bin Cui;Cheng Song;Haijun Mao;Huaqiang Wu;Fan Li;Jingjing Peng;Guangyue Wang;Fei Zeng
Advanced Materials 2015 Volume 27( Issue 42) pp:6651-6656
Publication Date(Web):
DOI:10.1002/adma.201503115
Co-reporter:Bin Cui;Cheng Song;Gillian A. Gehring;Fan Li;Guangyue Wang;Chao Chen;Jingjing Peng;Haijun Mao;Fei Zeng
Advanced Functional Materials 2015 Volume 25( Issue 6) pp:864-870
Publication Date(Web):
DOI:10.1002/adfm.201403370
Electrical manipulation of lattice, charge, and spin is realized respectively by the piezoelectric effect, field-effect transistor, and electric field control of ferromagnetism, bringing about dramatic promotions both in fundamental research and industrial production. However, it is generally accepted that the orbital of materials are impossible to be altered once they have been made. Here, electric field is used to dynamically tune the electronic-phase transition in (La,Sr)MnO3 films with different Mn4+/(Mn3+ + Mn4+) ratios. The orbital occupancy and corresponding magnetic anisotropy of these thin films are manipulated by gate voltage in a reversible and quantitative manner. Positive gate voltage increases the proportion of occupancy of the orbital and magnetic anisotropy that were initially favored by strain (irrespective of tensile and compressive), while negative gate voltage reduces the concomitant preferential orbital occupancy and magnetic anisotropy. Besides its fundamental significance in orbital physics, these findings might advance the process towards practical oxide-electronics based on orbital.
Co-reporter:Shuang Gao, Fei Zeng, Fan Li, Minjuan Wang, Haijun Mao, Guangyue Wang, Cheng Song and Feng Pan
Nanoscale 2015 vol. 7(Issue 14) pp:6031-6038
Publication Date(Web):04 Mar 2015
DOI:10.1039/C4NR06406B
The search for self-rectifying resistive memories has aroused great attention due to their potential in high-density memory applications without additional access devices. Here we report the forming-free and self-rectifying bipolar resistive switching behavior of a simple Pt/TaOx/n-Si tri-layer structure. The forming-free phenomenon is attributed to the generation of a large amount of oxygen vacancies, in a TaOx region that is in close proximity to the TaOx/n-Si interface, via out-diffusion of oxygen ions from TaOx to n-Si. A maximum rectification ratio of ∼6 × 102 is obtained when the Pt/TaOx/n-Si devices stay in a low resistance state, which originates from the existence of a Schottky barrier between the formed oxygen vacancy filament and the n-Si electrode. More importantly, numerical simulation reveals that the self-rectifying behavior itself can guarantee a maximum crossbar size of 212 × 212 (∼44 kbit) on the premise of 10% read margin. Moreover, satisfactory switching uniformity and retention performance are observed based on this simple tri-layer structure. All of these results demonstrate the great potential of this simple Pt/TaOx/n-Si tri-layer structure for access device-free high-density memory applications.
Co-reporter:Jingjing Peng, Cheng Song, Fan Li, Bin Cui, Haijun Mao, Yuyan Wang, Guangyue Wang, and Feng Pan
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 32) pp:17700
Publication Date(Web):July 27, 2015
DOI:10.1021/acsami.5b04994
We investigate charge transfer, orbital reconstruction, and the emergence of exchange bias in (La,Sr)MnO3/LaNiO3 heterostructures. We demonstrate that charge transfer from Mn3+ ions to Ni3+ ions is accompanied by the formation of hybridized Mn/Ni 3z2 – r2 orbits at the interface, instead of strain-stabilized Mn and Ni x2 – y2 orbits in the bulk films. In the heterostructures with ultrathin LaNiO3, orbital reconstruction induced by charge transfer results in magnetization frustration of (La,Sr)MnO3 at the interface. But the strain effect exerted by the growth of the LaNiO3 top layer plays a dominant role on orbital reconstruction in the heterostructures with thick LaNiO3, stabilizing 3z2 – r2 orbits. In this case, robust spin glass, associated with larger magnetization frustration, accounts for the exchange bias effect. Our work builds a bridge between the microscopic electronic structure and the macroscopic magnetic property, providing the possibility of manipulating the exotic states with the aid of strain engineering in oxide-based electronics.Keywords: charge transfer; exchange bias; orbital reconstruction; oxide-based electronics; spin glass; strain effect;
Co-reporter:Shuang Gao, Fei Zeng, Minjuan Wang, Guangyue Wang, Cheng Song and Feng Pan
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 19) pp:12849-12856
Publication Date(Web):17 Apr 2015
DOI:10.1039/C5CP01235J
The common nonpolar switching behavior of binary oxide-based resistive random access memory devices (RRAMs) has several drawbacks in future application, such as the requirements for a high forming voltage, a large reset current, and an additional access device to settle the sneak-path issue. Herein, we propose the tuning of the switching behavior of binary oxide-based RRAMs by inserting an ultra-thin chemically active metal nanolayer, and a case study on Ta2O5–Ta systems is provided. The devices are designed to be Pt/Ta2O5(5 − x/2)/Ta(x)/Ta2O5(5 − x/2)/Pt with x = 0, 2, or 4 nm. The reference devices without the Ta nanolayer exhibit an expected nonpolar switching behavior with a high forming voltage of ∼−4.5 V and a large reset current of >10 mA. In contrast, a self-compliance bipolar switching behavior with a low forming voltage of ∼−2 V and a small reset current of <1 mA is observed after inserting a 2 nm Ta nanolayer. When the Ta nanolayer is increased to 4 nm, a complementary resistive switching (CRS) behavior is found, which can effectively settle the sneak-path issue. The appearance of CRS behavior suggests that a thin Ta nanolayer of 4 nm is robust enough to act as an inner electrode. Besides, the behind switching mechanisms are thoroughly discussed with the help of a transmission electron microscope and temperature-dependent electrical measurements. All these results demonstrate the feasibility of tuning switching behavior of binary oxide-based RRAMs by inserting an ultra-thin chemically active metal nanolayer and might help to advance the commercialization of binary oxide-based RRAMs.
Co-reporter:W. S. Dong, F. Zeng, S. H. Lu, X. J. Li, C. T. Chang, A. Liu, F. Pan and D. Guo
RSC Advances 2015 vol. 5(Issue 119) pp:98110-98117
Publication Date(Web):10 Nov 2015
DOI:10.1039/C5RA19938G
Heavy ion Nd3+ is introduced into the electrolyte layer to study frequency selectivity of a semiconducting polymer/electrolyte double-layer cell. This cell exhibits long-term depression under low-frequency stimulations and potentiation under high-frequency stimulations by positive triangular pulses, suggesting a conventional learning protocol, i.e., spike-rate-dependent plasticity. The frequency selectivity depends significantly on the input shape due to large ionic size and mass. The input threshold of the frequency selectivity is around the voltage inducing a negative differential resistance (VNDR) influenced by the loading rate. The typical value of VNDR is 0.3 V for a loading rate of 100 V s−1, but VNDR disappears when the loading rate exceeds 1000 V s−1. Besides, the frequency selectivity has not been observed under rectangular pulse input. Moreover, the possibility of bidirectional signal transfer has been tested simply by anti-connecting two individual cells. Our study suggests the possibility to realize signal pruning and synthetizing by changing ionic types.
Co-reporter:Yuyan Wang;Cheng Song;Guangyue Wang;Jinghui Miao;Fei Zeng
Advanced Functional Materials 2014 Volume 24( Issue 43) pp:6806-6810
Publication Date(Web):
DOI:10.1002/adfm.201401659
The requirement for high-density memory integration advances the development of newly structured spintronic devices, which have reduced stray fields and are insensitive to magnetic field perturbations. This could be visualized in magnetic tunnel junctions incorporating anti-ferromagnetic instead of ferromagnetic electrodes. Here, room-temperature anti-ferromangnet (AFM)-controlled tunneling anisotropic magnetoresistance in a novel perpendicular junction is reported, where the IrMn AFM stays immediately at both sides of AlOx tunnel barrier as the functional layers. Bi-stable resistance states governed by the relative arrangement of uncompensated anti-ferromagnetic IrMn moments are obtained here, rather than the traditional spin-valve signal observed in ferromagnet-based tunnel junctions. The experimental observation of room-temperature tunneling magnetoresistance controlled directly by AFM is practically significant and may pave the way for new-generation memories based on AFM spintronics.
Co-reporter:Bin Cui;Cheng Song;Guangyue Wang;Yinuo Yan;Jingjing Peng;Jinghui Miao;Haijun Mao;Fan Li;Chao Chen;Fei Zeng
Advanced Functional Materials 2014 Volume 24( Issue 46) pp:7233-7240
Publication Date(Web):
DOI:10.1002/adfm.201402007
The electronic phase transition has been considered as a dominant factor in the phenomena of colossal magnetoresistance, metal-insulator transition, and exchange bias in correlated electron systems. However, the effective manipulation of the electronic phase transition has remained a challenging issue. Here, the reversible control of ferromagnetic phase transition in manganite films through ionic liquid gating is reported. Under different gate voltages, the formation and annihilation of an insulating and magnetically hard phase in the magnetically soft matrix, which randomly nucleates and grows across the film instead of initiating at the surface and spreading to the bottom, is directly observed. This discovery provides a conceptually novel vision for the electric-field tuning of phase transition in correlated oxides. In addition to its fundamental significance, the realization of a reversible metal-insulator transition in colossal magnetoresistance materials will also further the development of four-state memories, which can be manipulated by a combination of electrode gating and the application of a magnetic field.
Co-reporter:Fei Zeng, Sizhao Li, Jing Yang, Feng Pan and D. Guo
RSC Advances 2014 vol. 4(Issue 29) pp:14822-14828
Publication Date(Web):11 Feb 2014
DOI:10.1039/C3RA46679E
A resistive switching (RS) device of Ti/PEDOT:PSS/Ti, which is favourable for simulating learning processes, was made in this study. The conventional synaptic potentiation, depression plasticity and spike-timing-dependent plasticity, widely studied in neuroscience, were realized in this RS system. Our RS cell can be potentiated under moderate stimulation, but intensive or strong stimulation will trigger the depression mechanism without changing the bias sign. The characterizations of the chemical state suggest that the Ti compound forms at the interface and that PEDOT:PSS contributes to resistance switching and synaptic plasticity. We constructed the energy band diagram for the pristine device to provide a RS cell prototype applied in neuromorphic computing.
Co-reporter:F. Pan, S. Gao, C. Chen, C. Song, F. Zeng
Materials Science and Engineering: R: Reports 2014 Volume 83() pp:1-59
Publication Date(Web):September 2014
DOI:10.1016/j.mser.2014.06.002
This review article attempts to provide a comprehensive review of the recent progress in the so-called resistive random access memories (RRAMs). First, a brief introduction is presented to describe the construction and development of RRAMs, their potential for broad applications in the fields of nonvolatile memory, unconventional computing and logic devices, and the focus of research concerning RRAMs over the past decade. Second, both inorganic and organic materials used in RRAMs are summarized, and their respective advantages and shortcomings are discussed. Third, the important switching mechanisms are discussed in depth and are classified into ion migration, charge trapping/de-trapping, thermochemical reaction, exclusive mechanisms in inorganics, and exclusive mechanisms in organics. Fourth, attention is given to the application of RRAMs for data storage, including their current performance, methods for performance enhancement, sneak-path issue and possible solutions, and demonstrations of 2-D and 3-D crossbar arrays. Fifth, prospective applications of RRAMs in unconventional computing, as well as logic devices and multi-functionalization of RRAMs, are comprehensively summarized and thoroughly discussed. The present review article ends with a short discussion concerning the challenges and future prospects of the RRAMs.
Co-reporter:Guangsheng Tang, Fei Zeng, Chao Chen, Hongyan Liu, Shuang Gao, Cheng Song, Yisong Lin, Guang Chen and Feng Pan
Nanoscale 2013 vol. 5(Issue 1) pp:422-428
Publication Date(Web):27 Nov 2012
DOI:10.1039/C2NR32743K
Through the one-step plasma oxidation of TiN thin films at room temperature (a simple semiconductor technology compatible method), a partly oxidised structure of titanium oxynitride (TiNxOy) with a TiO2−x nanolayer on top has been prepared for non-volatile resistive switching memory devices. The fabricated Pt/TiO2−x/TiNxOy/TiN memory devices demonstrate complementary resistive switching behaviours within an operation voltage of 1 V. The complementary resistive switching behaviours can be explained by redistribution of the oxygen vacancies between the Pt/TiO2−x top interface and the TiO2−x/TiNxOy bottom interface in the TiO2−x nanolayer. A model concerning the resistive switching mechanism as well as a recover program of a failed device is also proposed. Our work provides a possible cost-efficient solution to suppress the sneak-path problem in nanoscale crossbar memory arrays.
Co-reporter:Sizhao Li, Fei Zeng, Chao Chen, Hongyan Liu, Guangsheng Tang, Shuang Gao, Cheng Song, Yisong Lin, Feng Pan and Dong Guo
Journal of Materials Chemistry A 2013 vol. 1(Issue 34) pp:5292-5298
Publication Date(Web):24 Jun 2013
DOI:10.1039/C3TC30575A
In this study, a memristor with the simple structure Ag/poly(3,4-ethylenedioxythiophene):poly (styrenesulphonate) (PEDOT:PSS)/Ta was fabricated. Essential synaptic plasticity and learning behaviours were emulated using this memristor, including short-term plasticity, long-term plasticity, spike-timing-dependent plasticity and spike-rate-dependent plasticity. Important time constants were extracted from these synaptic modifications, which are associated with brain learning and memory functions. It was clearly demonstrated that the movement of the Ag interface upon the initiation of a redox reaction accounts for the resistive switching mechanism of our memristor. The conducting path in the polymer layer and the elastic effect of the polymer matrix were suggested to be considered in the memory and learning processes. Moreover, the energy band diagram of our memristor was drawn after the cross-sectional transmission electron microscopy images were analysed. It was found that a natural p–n junction in the PEDOT:PSS/Ta compound was formed. This resulted in rectifying, high resistance and low power consumption. Our device structure may be considered a feasible prototype for integrating memristors into a large-scale neuromorphic circuit.
Co-reporter:Chao Chen, Shuang Gao, Guangsheng Tang, Huadong Fu, Guangyue Wang, Cheng Song, Fei Zeng, and Feng Pan
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 5) pp:1793
Publication Date(Web):February 20, 2013
DOI:10.1021/am303128h
We report the complementary resistive switching (CRS) behaviors in aluminum nitride (AlN)-based memory devices as the promising new material system for large-scale integration of passive crossbar arrays. By utilizing different electrodes (Cu, Pt, and TiN), CRS characteristics are demonstrated in both TiN/AlN/Cu/AlN/TiN electrochemical metallization cells and Pt/AlN/TiN/AlN/Pt ionic resistive switching systems. The instability of Pt/AlN/Cu/AlN/Pt based CRS is explained by the relatively small reset voltage caused by the thermal effects enhanced reset process in the corresponding bipolar resistive switching element. It is concluded that the prerequisite for reliable and stable CRS is that the reset voltage of the bipolar resistive switching element must be much larger than half of the set voltage.Keywords: AlN; bipolar resistive switching; complementary resistive switching; filament; memristor; RRAM;
Co-reporter:Hongyan Liu, Fei Zeng, Guangsheng Tang, Guangyue Wang, Cheng Song, Feng Pan
Acta Materialia 2013 Volume 61(Issue 12) pp:4619-4624
Publication Date(Web):July 2013
DOI:10.1016/j.actamat.2013.04.032
Abstract
Obtaining electrodes with high electromigration resistance, which are used in high-frequency surface acoustic wave devices, is imperative for the rapidly developing mobile communications industry. The effect of a Ti underlayer on the electromigration resistance and texture of Al films was studied comprehensively. It was found that crystalline orientation, crystalline size and electromigration resistance varied significantly with the thickness of the Ti underlayer. A highly (1 1 1) oriented Al film with a full width at half maximum value of the rocking curve of 1.7° and a crystalline grain with a bamboo-like structure was successively deposited on an ultrathin titanium underlayer (∼1 nm). The median time to failure of Al interconnects with the ultrathin Ti underlayer was enhanced by one order of magnitude higher than those without the Ti underlayer. The influence of the Ti underlayer on the texture of Al films was investigated using a combination of X-ray diffraction and transmission electron microscopy techniques. The results revealed that the ultrathin discontinuous Ti underlayer facilitated the Al (1 1 1) texture growth. The mechanism of texturing was discussed in terms of surface-interface energy balance along with high resolution transmission electron microscopy analysis.
Co-reporter:Hongyan Liu, Fei Zeng, Shuang Gao, Guangyue Wang, Cheng Song and Feng Pan
Physical Chemistry Chemical Physics 2013 vol. 15(Issue 31) pp:13153-13161
Publication Date(Web):03 Jul 2013
DOI:10.1039/C3CP51894A
Diluted magnetic semiconductors have great potential in applications for biological detection and spintronics. However, the origin of magnetism is complex and it is of significant importance to clarify the contributions from various origins. We prepared epitaxial copper-doped ZnO films and investigated the origin of ferromagnetism by combining various characterization methods. The results show that, with nominal Cu concentrations of up to 7.3 at.%, the Cu atoms substitute for the Zn atoms and form strong covalence bonds (CuZn–O), which show a property commensurate with that of the Zn–O bonds in the ZnO host. With further increases in Cu concentrations, the substitutional CuZn effect is obscured, and the [CuZnO4] clusters, regulated by the wurtzite ZnO host, segregate into CuO phase after annealing in air. Magnetization in volume increases with increasing Cu content up to about 7.3 at.% and then decreases with further increase, while the magnetic moment per Cu atom decreases monotonically with the increase in Cu content. We have demonstrated that the substitution of Cu for Zn and the presence of strong CuZn–O bonds are necessary for ferromagnetism while the [CuZnO4] clusters are detrimental to the ferromagnetism. The enhancement of ferromagnetism in volume is strongly correlated with the moderate oxygen vacancy mediated Cu ions.
Co-reporter:Shuang Gao, Cheng Song, Chao Chen, Fei Zeng, and Feng Pan
The Journal of Physical Chemistry C 2013 Volume 117(Issue 22) pp:11881-11882
Publication Date(Web):May 14, 2013
DOI:10.1021/jp401828m
Co-reporter:Jingting Luo, Pingxiang Luo, Min Xie, Ke Du, Bixia Zhao, Feng Pan, Ping Fan, Fei Zeng, Dongping Zhang, Zhuanghao Zheng, Guangxing Liang
Biosensors and Bioelectronics 2013 49() pp: 512-518
Publication Date(Web):
DOI:10.1016/j.bios.2013.05.021
Co-reporter:G.S. Tang, H.Y. Liu, F. Zeng, F. Pan
Vacuum 2013 Volume 89() pp:157-162
Publication Date(Web):March 2013
DOI:10.1016/j.vacuum.2012.03.039
A simple method for fabricating self-organized Cu nano-dots on Si(100) substrate by low energy Ar+ ion beam bombardment of a Cu thin film at room temperature over a large area is demonstrated. The morphological evolution has been investigated using scanning electron microscopy and atomic force microscopy. It was found that nano-ripple patterns formed on a Cu grain surface on a 110 nm thick polycrystalline Cu thin film under normal ion incidence. Uniformly distributed Cu nano-dots were obtained by bombardment of 55 nm thick nano-crystalline Cu thin films. The formation mechanism of the Cu nanostructures was discussed with the aid of numerical simulations using a modified damped Kuramoto–Sivashinsky equation.
Co-reporter:Zhishun Wang, Fei Zeng, Jing Yang, Chao Chen, and Feng Pan
ACS Applied Materials & Interfaces 2012 Volume 4(Issue 1) pp:447
Publication Date(Web):December 27, 2011
DOI:10.1021/am201518v
We report the design and fabrication of Al/poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Cu resistive memory devices that utilize the Cu redox reaction and conformational features of PEDOT:PSS to achieve resistive switching. The top Cu electrode acts as the source of the redox ions that are injected through the PEDOT:PSS layer during the forming process. The Cu filament is confirmed directly using the cross-sectional images of transmission electron microscopy and energy-dispersive X-ray spectroscopy. The resultant resistive memory devices can operate over a small voltage range, i.e., the switching-on threshold voltage is less than 1.5 V and the absolute value of the switching-off threshold voltage is less than 1.0 V. The on/off current ratio is as large as 1 × 104 and the two different resistance states can be maintained over 106 s. Moreover, the devices present good thermal stability that the resistive switching can be observed even at temperature up to 160 °C, at which the oxidation of the Cu top electrode is the failure factor. Furthermore, the cause of failure for Al/PEDOT:PSS/Cu memory devices at higher temperature is confirmed to be the oxidation of Cu top electrode.Keywords: metal filament; nonvolatile; PEDOT:PSS; RRAM; switching voltage; thermal stability;
Co-reporter:Shuang Gao ; Cheng Song ; Chao Chen ; Fei Zeng
The Journal of Physical Chemistry C 2012 Volume 116(Issue 33) pp:17955-17959
Publication Date(Web):August 6, 2012
DOI:10.1021/jp305482c
Dynamic formation/rupture processes of metallic filament have been clarified in solid electrolyte- and oxide-based resistive memory devices, whereas they remain exclusive in organic ones. Here we report these dynamic processes in Cu/poly (3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester/indium–tin oxide (ITO) structure, which exhibits a typical bipolar resistive switching effect. Under illumination, an open circuit voltage of −0.15 V exists in high-resistance state, yet it vanishes in low-resistance state owing to the emergence of Cu filament. By combining the symmetry of current–voltage curves with corresponding energy band diagrams in different resistance states, it is demonstrated that the Cu filament grows from Cu/organics interface, ends at organics/ITO interface, and ruptures near organics/ITO interface. This work might advance the insight into resistive switching mechanisms in organic-based resistive memories.
Co-reporter:Feng Pan;JingTing Luo;YuChao Yang;XuBo Wang
Science China Technological Sciences 2012 Volume 55( Issue 2) pp:421-436
Publication Date(Web):2012 February
DOI:10.1007/s11431-011-4682-8
In recent years, with the growing concerns on environmental protection and human health, new materials, such as lead-free piezoelectric materials, have received increasing attention. So far, three types of lead-free piezoelectric systems have been widely researched, i.e., perovskites, bismuth layer-structured ferroelectrics, and tungsten-bronze type ferroelectrics. This article presents a new type of environmental friendly piezoelectric material with simple structure, the transition-metal(TM)-doped ZnO. Through substituting Zn2+ site with small size ion, we obtained a series of TM-doped ZnO with giant piezoresponse, such as Zn0.975V0.025O of 170 pC/N, Zn0.94Cr0.06O of 120 pC/N, Zn0.913Mn0.087O of 86 pC/N and Zn0.988Fe0.012O of 127 pC/N. The tremendous piezoresponses are ascribed to the introduction of switchable spontaneous polarization and high permittivity in TM-doped ZnO. The microscopic origin of giant piezoresponse is also discussed. Substitution of TM ion with small ionic size for Zn2+ results in the easier rotation of noncollinear TM-O1 bonds along the c axis under the applied field, which produces large piezoelectric displacement and corresponding piezoresponse enhancement. Furthermore, it proposes a general rule to guide the design of new wurtzite semiconductors with enhanced piezoresponses. That is, TM-dopant with ionic size smaller than Zn2+ substitutes for Zn2+ site will increase the piezoresponse of ZnO significantly. Finally, we discuss the improved performances of some TM-doped ZnO based piezoelectric devices.
Co-reporter:Yuchao Yang, Xiaoxian Zhang, Min Gao, Fei Zeng, Weiya Zhou, Sishen Xie and Feng Pan
Nanoscale 2011 vol. 3(Issue 4) pp:1917-1921
Publication Date(Web):11 Mar 2011
DOI:10.1039/C1NR10096C
We demonstrate nonvolatile resistive switching in single crystalline ZnO nanowires with high ON/OFF ratios and low threshold voltages. Unlike the mechanism of continuous metal filament formation along grain boundaries in polycrystalline films, the resistive switching in single crystalline ZnO nanowires is speculated to be induced by the formation of a metal island chain on the nanowire surface. Resistive memories based on bottom-up semiconductor nanowires hold potential for next generation ultra-dense nonvolatile memories.
Co-reporter:X.Y. Zhu, J.T. Luo, F. Zeng, F. Pan
Thin Solid Films 2011 Volume 520(Issue 2) pp:818-823
Publication Date(Web):1 November 2011
DOI:10.1016/j.tsf.2010.12.251
The microstructure and mechanical properties of Cu/Nb multilayers were investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and nanoindentation. Ultrahigh strength of 3.27 GPa is achieved at the smallest layer thickness of 2.5 nm, which agrees well with the theoretical prediction based on the deformation mechanism of crossing of dislocations across interfaces. After that, the strength decreases with the increasing layer thickness and the transition of the deformation mechanism to confined layer slip occurs at the layer thickness of 6.5 nm. Additionally, strength of the Cu/Nb multilayers increases with increasing loading strain rate because of enhanced strain hardening.
Co-reporter:Feng Pan, Olinda Conde
Thin Solid Films 2011 Volume 520(Issue 2) pp:675
Publication Date(Web):1 November 2011
DOI:10.1016/j.tsf.2011.08.098
Co-reporter:X.Y. Zhu, J.T. Luo, G. Chen, F. Zeng, F. Pan
Journal of Alloys and Compounds 2010 Volume 506(Issue 1) pp:434-440
Publication Date(Web):10 September 2010
DOI:10.1016/j.jallcom.2010.07.024
Cu/Co multilayers with periodicity of 4–40 nm were prepared by electron beam evaporation deposition. Microstructure and room temperature creep behavior were investigated by X-ray diffraction, transmission electron microscopy and nanoindentation test. The results show that superlattice structure forms with decreasing periodicity and coherent interfaces come into being at low periodicity of 4 nm. Size dependence of the creep behavior is observed and power-law creep parameters including stress exponent and size sensitivity index are calculated by dimension analysis. A dislocation model for predicting the steady-state deformation of multilayers with semi-coherent interfaces is presented. Nanoscale effects are explained by dislocation generation and annihilation mechanisms involving single dislocations slip in confined layers and dislocations climb at the interfaces, respectively. Model predictions agree well with experimental observation.Research highlights▶ Cu/Co superlattice structure forms with decreasing periodicity. ▶ It exhibits size dependence of the creep behavior and power-law creep parameters. ▶ A dislocation model for steady-state deformation of semi-coherent films is presented. ▶ Nanoscale effects are related to dislocation generation and annihilation at interfaces.
Co-reporter:J.T. Luo, F. Zeng, F. Pan, H.F. Li, J.B. Niu, R. Jia, M. Liu
Applied Surface Science 2010 Volume 256(Issue 10) pp:3081-3085
Publication Date(Web):1 March 2010
DOI:10.1016/j.apsusc.2009.11.077
Abstract
High-frequency surface acoustic wave (SAW) filters using undoped and V-doped ZnO films were fabricated on diamond. Compared with their counterparts, the SAW filters using V-doped ZnO films have higher electromechanical coupling coefficient of ∼2.9% and lower insertion loss. The filtering performance improvement is considered to be due to the ferroelectricity in V-doped ZnO films and the resultant high piezoresponse (∼110 pm/V), which is one order of magnitude larger than that of undoped ZnO films. In addition, more perfect (0 0 2) preferred orientation, better uniform grains and smoother surface of V-doped ZnO films also contribute to the filtering performance improvement.
Co-reporter:X.Y. Zhu, X.J. Liu, F. Zeng, F. Pan
Materials Letters 2010 Volume 64(Issue 1) pp:53-56
Publication Date(Web):15 January 2010
DOI:10.1016/j.matlet.2009.10.003
This paper deals with room temperature indentation creep behavior of nanoscale Ag/Fe multilayers. The constant-load nanoindentation test results reveal that all the multilayers exhibit steady-state creep after transient creep occurring at first 150 s and decreasing periodicity leads to a decrease in the stress exponent and an increase in creep rate. The dependence of the stress exponent and creep rate on the periodicity indicates that the creep process is dominated by dislocation glide–climb mechanism and the increasing fraction of grain boundaries and interfaces provide effective diffusion paths for the creep climb that determines the whole creep rate.
Co-reporter:X.Y. Zhu, X.J. Liu, R.L. Zong, F. Zeng, F. Pan
Materials Science and Engineering: A 2010 527(4–5) pp: 1243-1248
Publication Date(Web):
DOI:10.1016/j.msea.2009.09.058
Co-reporter:Yu Chao Yang, Feng Pan, Qi Liu, Ming Liu and Fei Zeng
Nano Letters 2009 Volume 9(Issue 4) pp:1636-1643
Publication Date(Web):March 9, 2009
DOI:10.1021/nl900006g
Through a simple industrialized technique which was completely fulfilled at room temperature, we have developed a kind of promising nonvolatile resistive switching memory consisting of Ag/ZnO:Mn/Pt with ultrafast programming speed of 5 ns, an ultrahigh ROFF/RON ratio of 107, long retention time of more than 107 s, good endurance, and high reliability at elevated temperatures. Furthermore, we have successfully captured clear visualization of nanoscale Ag bridges penetrating through the storage medium, which could account for the high conductivity in the ON-state device. A model concerning redox reaction mediated formation and rupture of Ag bridges is therefore suggested to explain the memory effect. The Ag/ZnO:Mn/Pt device represents an ultrafast and highly scalable (down to sub-100-nm range) memory element for developing next generation nonvolatile memories.
Co-reporter:Cheng Song, Changzheng Wang, Xuejing Liu, Fei Zeng and Feng Pan
Crystal Growth & Design 2009 Volume 9(Issue 2) pp:1235
Publication Date(Web):January 5, 2009
DOI:10.1021/cg800754b
Cobalt-doped (5 at. %) LiNbO3 (Co:LN) single crystalline films were prepared by combinatorial laser molecular-beam epitaxy on Al2O3 (001) substrates. The oxygen atmosphere should be severely controlled to be approximately 10 Pa to obtain stoichiometric Li/Nb concentration. Determined by asymmetric X-ray diffraction and high-resolution transmission electron microscopy, the epitaxial relationship of the sample follows (003)<100>F ∥ (003)<100>S, (110)<001>F ∥ (110)<001>S, and (113)<1̅10>F ∥ (113) <1̅10>S (F and S denote the film and the substrate, respectively). The Co:LN films have a single-phase, where Co is not metallic but in the 2+ state. Co K-edge X-ray-absorption near-edge structure spectrum determines that Co2+ ions substitute for Nb5+ lattice sites, producing oxygen vacancies to compensate for the charge nonequilibrium, and shows that the Co−O bond length is greater than that of Co:LN films grown on Si (100) substrates. The Co:LN/Al2O3 films exhibit a high Curie temperature of ∼550 K and room temperature ferromagnetism of 0.58 μB/Co arising from the bound magnetic polarons mechanism based on defects. This work opens a window to a class of single-phase multiferroics by introducing magnetic dopants to ferroelectric materials and to the mechanism of dopants induced ferromagnetism in insulators.
Co-reporter:S.P. Wen, R.L. Zong, F. Zeng, S. Guo, F. Pan
Applied Surface Science 2009 Volume 255(Issue 8) pp:4558-4562
Publication Date(Web):1 February 2009
DOI:10.1016/j.apsusc.2008.12.001
Abstract
The hardness, elastic modulus and scratch behaviors of Ag/Ni mulitlayers deposited by evaporation have been carried out by nanoindentation and nanoscratch. It has been found that the hardness (H) increases, while the modulus (E) decreases, that is to say an increase of H/E as the periodicity decreases. Many mechanisms are included in nanoscratch, including initial elastic contact, plowing and fracture stage, in each multilayer. Coefficient of friction during plowing decreases with the decrease of the periodicity, which can be ascribed to decreasing material pile-up due to the increase of H/E. Elastic recovery after scratching also increases as the periodicity decreases because of the increase of H/E, which leads to improved wear resistance. The fracture stage will be postponed with decreasing periodicity, which also leads to better wear behavior.
Co-reporter:X.J. Liu, X.Y. Zhu, J.T. Luo, F. Zeng, F. Pan
Journal of Alloys and Compounds 2009 Volume 482(1–2) pp:224-228
Publication Date(Web):12 August 2009
DOI:10.1016/j.jallcom.2009.03.162
Zn0.97Co0.03O films with and without ZnO buffer layer have been fabricated by magnetron sputtering to investigate the role of grain boundary defects in ferromagnetic ordering in this system. The deposited wurtzite films with (0 0 2) preferred orientation all show intrinsic room temperature ferromagnetism based on the substitutional behavior of Co2+. We found that the ZnO/Co:ZnO film grows in smaller grain size, compared with Co:ZnO film, which leads to the increase in grain boundary defects. Meanwhile the increase in oxygen vacancies is confirmed by Co K-edge X-ray-absorption near-edge spectra and the enhancement of saturated magnetization is observed in ZnO/Co:ZnO film. Hence the most important factor for mediating ferromagnetism is proposed to be grain boundary defects, i.e., oxygen vacancies. Bound magnetic polaron mechanism is adopted to explain the intrinsic origin and the mediative effects of grain boundary defects on ferromagnetism in Co-doped ZnO films.
Co-reporter:X.J. Liu, C. Song, F. Zeng, F. Pan
Thin Solid Films 2008 Volume 516(Issue 23) pp:8757-8761
Publication Date(Web):1 October 2008
DOI:10.1016/j.tsf.2008.07.002
We experimentally report the role of donor defects on the origin of room temperature ferromagnetism in Zn0.985Co0.015O films. Undoped ZnO, Co:ZnO and (Co, N):ZnO thin films are fabricated by radio-frequency magnetron sputtering. The local structural measurements indicate that Co2+ substitutes for Zn2+ in the ZnO wurtzite lattice with the upper limit of ~ 10% Co metal of the total cobalt dopants. The Raman spectra reveal vibrational modes at 273, 470, 639, 691 and 854 cm− 1 in addition to the host phonons of ZnO, indicating the substitutional behavior of N3− for O2− and the increase in donor defects. Acceptor doping with nitrogen compensates the electron carriers and hence decreases the electron carrier concentration. Combined with the enhancement of ferromagnetism, it is found that the donor defects other than electron carriers are responsible for the room temperature ferromagnetism in Co:ZnO films.
Co-reporter:S.P. Wen, R.L. Zong, F. Zeng, Y. Gao, F. Pan
Wear 2008 Volume 265(11–12) pp:1808-1813
Publication Date(Web):26 November 2008
DOI:10.1016/j.wear.2008.04.025
Wear behaviors of Ag/Cu multilayers were evaluated and compared using nanoscratch techniques under a ramping load. Scratch grooves were examined using scanning electron microscopy (SEM) and in situ cross-section scan. It was found that the friction coefficients for all samples have significant difference and the wear resistance is not linearly proportional to the hardness. These discrepancies were suggested to be a result of the transition of the wear mechanism from micro-cutting to micro-ploughing with decreasing periodicity. Multilayer with periodicity Λ = 100 nm has the highest friction coefficient and wear resistance due to formation of built-up edge. The elastic recovery increases consistently with increasing H/E.
Co-reporter:R.L. Zong, S.P. Wen, F. Zeng, Y. Gao, C. Song, B. He, F. Pan
Applied Surface Science 2007 Volume 253(Issue 6) pp:2993-2998
Publication Date(Web):15 January 2007
DOI:10.1016/j.apsusc.2006.06.025
Abstract
Fe–Ag alloy films were deposited by magnetron sputtering. Fe K edge X-ray absorption near-edge structure (XANES) was performed by synchrotron radiation to evidence the structure of the films. Annealing experiments were carried out to study their stability. The hardness and elastic modulus were measured by nanoindentation. The experimental and calculated XANES spectra both reveal that Fe atoms replace part of Ag atoms and supersaturated fcc Ag (Fe) solid solution alloy films are formed up to 38 at.% Fe. The solid solutions are stable and begin to precipitate at 400 °C The elastic modulus increases with the increase of Fe concentration and satisfies the rule of mixtures. The hardness of the as-deposited alloy film is larger than that calculated based on the rule of mixtures. The mechanism responsible for the enhancement of the hardness is discussed in terms of Labusch model of solid solution hardening.
Co-reporter:S.P. Wen, R.L. Zong, F. Zeng, Y. Gao, F. Pan
Materials Science and Engineering: A 2007 Volume 457(1–2) pp:38-43
Publication Date(Web):25 May 2007
DOI:10.1016/j.msea.2006.12.012
Ag/W multilayers with periodicity ranging from 15 to 200 nm were deposited by direct current magnetron sputtering. The microstructure, hardness and elastic modulus were investigated by X-ray diffraction, Rutherford backscattering, X-ray fluorescence, scanning electron microscopy and nanoindentation. The results show that multilayers with periodicity less than 50 nm have columnar porous structure, which leads to low modulus and brittle fracture. Multilayers with periodicity larger than 50 nm have continuous laminated structure, and they are relatively ductile. All the multilayers have abnormal low hardness far less than a rule of mixture value, which has been attributed to porous structure and the deformation localization due to the plasticity mismatch between Ag and W.
Co-reporter:D.M. Li, X.B. Wang, F. Pan
Materials Science and Engineering: A 2006 Volume 418(1–2) pp:218-222
Publication Date(Web):25 February 2006
DOI:10.1016/j.msea.2005.11.051
A highly textured Al/Ti films were deposited on 128° Y–X LiNbO3 substrates by ion beam assisted deposition. Influence of low energy Ar ion beam bombardment on the residual stress and resistivity of the Al/Ti films was investigated. It was found that the residual stress of the Al/Ti films varied from tensile to compressive with increases in ion energy or flux. The films fabricated at 0.5 keV and 3 μA/cm2 possessed zero residual stress. An increase in the compressive stress resulted in a decrease in resistivity. By controlling the ion incident energy and flux, we were able to control the film stress and resistivity that were related to the reliability of SAW devices.
Co-reporter:J.J. Chen, F. Zeng, D.M. Li, J.B. Niu, F. Pan
Thin Solid Films 2005 Volume 485(1–2) pp:257-261
Publication Date(Web):1 August 2005
DOI:10.1016/j.tsf.2005.04.028
Surface acoustic wave (SAW) device based on diamond is attracting much interest because diamond has the highest elastic constant of all substrates and the SAW velocity is more than 10,000 m/s. Although diamond is not piezoelectric, its high acoustic propagation makes it a desirable substrate for SAW device when coupled with piezoelectric thin films such as zinc oxide (ZnO) thin film. It is well known that, for these devices, performance is critically dependent on the quality of ZnO films. In this paper, highly-oriented, dense, and fine-grain polycrystalline ZnO thin films with excellent surface flatness and high resistivity were produced on diamond substrate by reactive magnetron sputtering. The properties of ZnO films were characterized by X-ray diffraction, high-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The influences of deposition condition are discussed. Interdigital-transducer/ZnO/diamond layered structure was fabricated, which are expected to be capable of wide bandwidth application in SAW devices at a high frequency of about 2.5 GHz.
Co-reporter:G.H. Yang, K.W. Geng, F. Zeng, F. Pan
Thin Solid Films 2005 Volume 484(1–2) pp:283-288
Publication Date(Web):22 July 2005
DOI:10.1016/j.tsf.2005.02.004
A series of Co–Cu multilayers with a constant Cu layer thickness of 60 Å and a Co layer thickness ranging from 12 to 90 Å, respectively, were prepared by electron beam evaporation at high vacuum. The magnetic properties and microstructure of the films and their dependences on the ferromagnetic layer thickness were investigated. It is found that Co layer grows into an fcc structure for thinner Co layer and stable hcp Co for thicker one. The fcc Co has more contributions to the magnetization than the hcp Co. Metastable Co–Cu solid solutions are formed at interfaces in all the multilayers, inducing alloying effect to reduce the magnetic moment. Lattice parameter of the Co–Cu multilayers is presented to determine the magnetic moment as a controlling parameter, and it explains above two influences successfully. The microstructure parameter, together with the microtopography characteristics, affects magnetoresistance in an inverse way.
Co-reporter:Y. Gu, F. Zeng, K.W. Geng, G.H. Yang, F. Pan
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 2005 Volume 227(Issue 3) pp:275-281
Publication Date(Web):January 2005
DOI:10.1016/j.nimb.2004.08.013
The effects of the incident angle on the microstructure and magnetic properties of Fe–Nb thin films grown by Ar ion beam assisted deposition have been investigated. A new metastable crystalline bcc phase and an amorphous phase were obtained at 45° ion incident angle, while a mixture of bcc Nb and bcc Fe was formed under the incident angles of the assisting ion beam of 30° and 60°. The difference caused by the variation of incident angle is discussed on the basis of energy transfer and competition between the effects of surface energy and ion beam channeling.
Co-reporter:G.H. Yang, B. Zhao, Y. Gao, F. Pan
Surface and Coatings Technology 2005 Volume 191(Issue 1) pp:127-133
Publication Date(Web):1 February 2005
DOI:10.1016/j.surfcoat.2004.02.006
The mechanical properties of evaporated Co/Mo multilayers with periodicity of 4–16 nm were investigated. The microstructures of the films were analyzed using X-ray diffraction and transmission electron microscopy. Continuous stiffness measurement technique was used in the nanoindentation tests to determine the hardness and elastic modulus of the films. A Hall–Petch-like behavior is observed in the Co/Mo multilayers, in which the fitting parameter n is found as large as 0.93. Other possible mechanisms are also proposed to explain this behavior. The peak hardness, approximately 2.1 times of the rule-of-mixtures value is obtained for the periodicity of 7 nm. When the periodicity is reduced to 4 nm, a hardness decrease observed is attributed to the interface alloying.
Co-reporter:F Zeng, B Zhao, F Pan
Thin Solid Films 2002 Volume 415(1–2) pp:88-93
Publication Date(Web):1 August 2002
DOI:10.1016/S0040-6090(02)00534-5
The ion beam assisted deposition (IBAD) technique is utilized to study amorphization in Fe–Nb binary metal systems. The amorphous range covers a Nb fraction of approximately 34–60 at.%. The Fe-rich and Nb-rich supersaturated solid solutions of the bcc phase were obtained in Fe66Nb34 and Fe31Nb69 films, respectively. In the Fe66Nb34 films, the ‘amorphous→amorphous+bcc→bcc’ evolution with increased ion beam energy from 2 to 8 keV is observed accompanying changes of film morphology. The phase formation in the IBAD process is determined by both thermodynamic and kinetic factors.
Co-reporter:T.S. Li, H. Li, F. Pan
Surface and Coatings Technology 2001 Volume 137(2–3) pp:225-229
Publication Date(Web):15 March 2001
DOI:10.1016/S0257-8972(00)01096-3
Ti/TiN multilayered films were deposited by a reactive magnetron sputtering method using a combination of a Ti target and an Ar–N2 mixture discharge gas. The microstructure and nanoindentation hardness was characterized by various methods. The experimental results indicated that the hardness of multilayered film depends on both the modulation period and the thickness ratio between Ti and TiN layers λTi/TiN. The hardness was considerably enhanced when the modulation period was in the range 35–60 nm for λTi/TiN=1:1 and it was up to 23 GPa. For λTi/TiN=1:3, the hardness enhancement appears in the films with the modulation period of 20–70 nm. The possible mechanism responsible for the modification of the hardness and periods of films is also discussed.
Co-reporter:F Zeng, B Zhao, F Pan
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 2001 Volume 183(3–4) pp:311-317
Publication Date(Web):October 2001
DOI:10.1016/S0168-583X(01)00747-9
The Cu–Nb alloy films with positive heat of mixing were prepared by ion-beam-assisted deposition (IBAD) technique. A new Nb-rich fcc phase and an amorphous phase, with composition range around 65–75% Nb fraction, were obtained in Cu–Nb films deposited under Ar+ ion bombardments with energy below 7 keV. The supersaturated bcc phase was also formed in Cu20Nb80 films by IBAD. The formation mechanism of metastable phases in Cu–Nb system is discussed in terms of the procedure far from equilibrium and compared with that in Fe–Zr system by IBAD.
Co-reporter:F. Pan, C. Song, X.J. Liu, Y.C. Yang, F. Zeng
Materials Science and Engineering: R: Reports (30 June 2008) Volume 62(Issue 1) pp:
Publication Date(Web):30 June 2008
DOI:10.1016/j.mser.2008.04.002
This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent ferromagnetic ordering of TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics.
Co-reporter:Sulei Fu, Qi Li, Shuang Gao, Guangyue Wang, Fei Zeng, Feng Pan
Applied Surface Science (30 April 2017) Volume 402() pp:
Publication Date(Web):30 April 2017
DOI:10.1016/j.apsusc.2017.01.025
•High-quality AlN epitaxial films were grown successfully on c-sapphire substrates using a thin ZnO buffer layer.•SAW devices with a center frequency of 1.4 GHz, a phase velocity of 5600 m/s were achieved on the obtained AlN films.•A thin ZnO buffer layer can significantly enhance the crystalline quality and release the strain in AlN films.•The mechanism accounting for enhanced performance of AlN/sapphire based SAW devices were proposed.AlN epitaxial films with a thin ZnO buffer layer were successfully deposited on c- sapphire by DC magnetron sputtering for surface acoustic wave (SAW) applications. The effect of ZnO buffer layer thickness on structural properties of AlN epitaxial films and the related SAW properties were investigated systematically. The results revealed that a thin ZnO buffer layer can significantly enhance the crystalline quality of AlN films and release the strain in AlN films. The AlN films were epitaxially grown on ZnO buffered-substrate with orientation relationship of (0001)[101¯0]AlN//(0001)[101¯0]ZnO//(0001)[2¯110]Al2O3. High frequency SAW devices with a center frequency of 1.4 GHz, a phase velocity of 5600 m/s were achieved on the obtained AlN films. The optimum ZnO buffer layer thickness was found to be 10 nm, resulting in high-quality epitaxial AlN films with a FWHM value of the rocking curve of 0.84°, nearly zero stress and low insertion loss of SAW devices. This work offers an effective approach to achieve high-quality AlN epitaxial films on sapphire substrates for the applications of AlN-based SAW devices.
Co-reporter:Hongyan Liu, Fei Zeng, Guangsheng Tang, Feng Pan
Applied Surface Science (1 April 2013) Volume 270() pp:
Publication Date(Web):1 April 2013
DOI:10.1016/j.apsusc.2013.01.005
The authors have investigated the variation of microstructure and piezoelectric response of TaxAl1−xN films with Ta concentrations. The results indicate that moderate Ta doping facilitates the c-axis orientation, crystallinity and enlargement of lattice constants. When the doping content is about 5.1 at.%, the c-axis orientation is optimized in the best with c constant of about 0.5086 nm. The Raman spectra of TaxAl1−xN films further determine the wurtzite structure of AlN after Ta doping. The high resolution transmission electric microscopy demonstrates that the column single crystal was obtained with presence of nano distortion domains. The Ta 4f7/2 binding energy of TaxAl1−xN shows a shift toward lower binding energy side, indicating that the coordination configuration of Ta with N changed as Ta contents increased. A significant enhancement in piezoelectric response was obtained from 4.2 pC/N to 8.2 pC/N, which is enhanced by ca. 100%, when the Ta content arrived to 5.1 at.%. The expansion of unit cell volume and the competition between Ta and Al atoms about the coordination of nitrogen atoms, which will enhance the internal nitrogen displacement under electric field, are proposed to be the main microscopic origin of the enhancement of piezoelectric response.Highlights► Highly c-axis oriented Ta:AlN films were deposited on Si(1 0 0) substrate using DC magnetron reactive sputtering at 400 °C. ► A large enhancement of piezoelectric response with piezoelectric coefficient of 8.2 pC/N was obtained. ► The mechanism of the large enhancement in piezoelectric response is supposed to be enhancement of internal nitrogen displacement under electric field.
Co-reporter:Hongyan Liu, Fei Zeng, Shuang Gao, Guangyue Wang, Cheng Song and Feng Pan
Physical Chemistry Chemical Physics 2013 - vol. 15(Issue 31) pp:NaN13161-13161
Publication Date(Web):2013/07/03
DOI:10.1039/C3CP51894A
Diluted magnetic semiconductors have great potential in applications for biological detection and spintronics. However, the origin of magnetism is complex and it is of significant importance to clarify the contributions from various origins. We prepared epitaxial copper-doped ZnO films and investigated the origin of ferromagnetism by combining various characterization methods. The results show that, with nominal Cu concentrations of up to 7.3 at.%, the Cu atoms substitute for the Zn atoms and form strong covalence bonds (CuZn–O), which show a property commensurate with that of the Zn–O bonds in the ZnO host. With further increases in Cu concentrations, the substitutional CuZn effect is obscured, and the [CuZnO4] clusters, regulated by the wurtzite ZnO host, segregate into CuO phase after annealing in air. Magnetization in volume increases with increasing Cu content up to about 7.3 at.% and then decreases with further increase, while the magnetic moment per Cu atom decreases monotonically with the increase in Cu content. We have demonstrated that the substitution of Cu for Zn and the presence of strong CuZn–O bonds are necessary for ferromagnetism while the [CuZnO4] clusters are detrimental to the ferromagnetism. The enhancement of ferromagnetism in volume is strongly correlated with the moderate oxygen vacancy mediated Cu ions.
Co-reporter:Shuang Gao, Fei Zeng, Minjuan Wang, Guangyue Wang, Cheng Song and Feng Pan
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 19) pp:NaN12856-12856
Publication Date(Web):2015/04/17
DOI:10.1039/C5CP01235J
The common nonpolar switching behavior of binary oxide-based resistive random access memory devices (RRAMs) has several drawbacks in future application, such as the requirements for a high forming voltage, a large reset current, and an additional access device to settle the sneak-path issue. Herein, we propose the tuning of the switching behavior of binary oxide-based RRAMs by inserting an ultra-thin chemically active metal nanolayer, and a case study on Ta2O5–Ta systems is provided. The devices are designed to be Pt/Ta2O5(5 − x/2)/Ta(x)/Ta2O5(5 − x/2)/Pt with x = 0, 2, or 4 nm. The reference devices without the Ta nanolayer exhibit an expected nonpolar switching behavior with a high forming voltage of ∼−4.5 V and a large reset current of >10 mA. In contrast, a self-compliance bipolar switching behavior with a low forming voltage of ∼−2 V and a small reset current of <1 mA is observed after inserting a 2 nm Ta nanolayer. When the Ta nanolayer is increased to 4 nm, a complementary resistive switching (CRS) behavior is found, which can effectively settle the sneak-path issue. The appearance of CRS behavior suggests that a thin Ta nanolayer of 4 nm is robust enough to act as an inner electrode. Besides, the behind switching mechanisms are thoroughly discussed with the help of a transmission electron microscope and temperature-dependent electrical measurements. All these results demonstrate the feasibility of tuning switching behavior of binary oxide-based RRAMs by inserting an ultra-thin chemically active metal nanolayer and might help to advance the commercialization of binary oxide-based RRAMs.
Co-reporter:Sizhao Li, Fei Zeng, Chao Chen, Hongyan Liu, Guangsheng Tang, Shuang Gao, Cheng Song, Yisong Lin, Feng Pan and Dong Guo
Journal of Materials Chemistry A 2013 - vol. 1(Issue 34) pp:NaN5298-5298
Publication Date(Web):2013/06/24
DOI:10.1039/C3TC30575A
In this study, a memristor with the simple structure Ag/poly(3,4-ethylenedioxythiophene):poly (styrenesulphonate) (PEDOT:PSS)/Ta was fabricated. Essential synaptic plasticity and learning behaviours were emulated using this memristor, including short-term plasticity, long-term plasticity, spike-timing-dependent plasticity and spike-rate-dependent plasticity. Important time constants were extracted from these synaptic modifications, which are associated with brain learning and memory functions. It was clearly demonstrated that the movement of the Ag interface upon the initiation of a redox reaction accounts for the resistive switching mechanism of our memristor. The conducting path in the polymer layer and the elastic effect of the polymer matrix were suggested to be considered in the memory and learning processes. Moreover, the energy band diagram of our memristor was drawn after the cross-sectional transmission electron microscopy images were analysed. It was found that a natural p–n junction in the PEDOT:PSS/Ta compound was formed. This resulted in rectifying, high resistance and low power consumption. Our device structure may be considered a feasible prototype for integrating memristors into a large-scale neuromorphic circuit.
![Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]](http://img.cochemist.com/ccimg/138200/138184-36-8.png)
![Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]](http://img.cochemist.com/ccimg/138200/138184-36-8_b.png)
