Guodong Zhu

Tunable Charge-Trap Memory Based on Few-Layer MoS2

Charge-trap memory with high-κ dielectric materials is considered to be a promising candidate for next-generation memory devices. Ultrathin layered two-dimensional (2D) materials like graphene and MoS2 have been receiving much attention because of their fantastic physical properties and potential applications in electronic devices. Here, we report on a dual-gate charge-trap memory device composed of a few-layer MoS2 channel and a three-dimensional (3D) Al2O3/HfO2/Al2O3 charge-trap gate stack. Because of the extraordinary trapping ability of both electrons and holes in HfO2, the MoS2 memory device exhibits an unprecedented memory window exceeding 20 V. Importantly, with a back gate the window size can be effectively tuned from 15.6 to 21 V; the program/erase current ratio can reach up to 10(4), allowing for multibit information storage. Moreover, the device shows a high endurance of hundreds of cycles and a stable retention of ∼ 28% charge loss after 10 years, which is drastically lower than ever reported MoS2 flash memory. The combination of 2D materials with traditional high-κ charge-trap gate stacks opens up an exciting field of nonvolatile memory devices.

Magneto-electric tuning of the phase of propagating spin waves

The utilization of a magnetoelectric film composite to control, by an electric field, the phase of magnetostatic surface spin waves propagating along thin films is reported. Laminates of ferromagnetic films of Ni and NiFe are deposited on a ferroelectric substrate, lead magnesium niobate-lead titanate. The phase of propagating spinwaves is shown to be modulated by an electric field while traveling a finite distance along the surface. The observed phase change in the spinwaves is in agreement with the anisotropy field changes measured with magneto optical Kerr effect hysteresis loops. A quantitative agreement is demonstrated.

Polarization fatigue in ferroelectric vinylidene fluoride and trifluoroethylene copolymer thin films

In recent years polarization fatigue in ferroelectric polymers has attracted much attention due to their potential use in nonvolatile memories, though the mechanism is still an open question. Here we mainly focus on the experimental observations on two physical phenomena occurring during fatigue process: The frequency dependence of the fatigue rate and the change in the coercive field with increased fatigue cycles. The frequency dependence of fatigue rate shows that at lower fatigue frequencies the rate obeys a universal scaling behavior with respect to N/f, where N is the fatigue cycles and f is the fatigue frequency, while at much higher frequencies this scaling behavior is broken and the fatigue rate is N/f dependent. In addition, both the increase and the decrease of the coercive field are observed, which indicates that the change in the coercive field should be dominated by different mechanisms. Based on the understanding of the crucial influence of space charges on electrical fatigue, we give a deta...

Fabrication of electrically bistable organic semiconducting/ferroelectric blend films by temperature controlled spin coating.

Organic semiconducting/ferroelectric blend films attracted much attention due to their electrical bistability and rectification properties and thereof the potential in resistive memory devices. During film deposition from the blend solution, spinodal decomposition induced phase separation, resulting in discrete semiconducting phase whose electrical property could be modulated by the continuous ferroelectric phase. However, blend films processed by common spin coating method showed extremely rough surfaces, even comparable to the film thickness, which caused large electrical leakage and thus compromised the resistive switching performance. To improve film roughness and thus increase the productivity of these resistive devices, we developed temperature controlled spin coating technique to carefully adjust the phase separation process. Here we reported our experimental results from the blend films of ferroelectric poly(vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) and semiconducting poly(3-hexylthiophene) (P3HT). We conducted a series of experiments at various deposition temperatures ranging from 20 to 90 °C. The resulting films were characterized by AFM, SEM, and VPFM to determine their structure and roughness. Film roughness first decreased and then increased with the increase of deposition temperature. Electrical performance was also characterized and obviously improved insulating property was obtained from the films deposited between 50 and 70 °C. By temperature control during film deposition, it is convenient to efficiently fabricate ferroelectric/semiconducting blend films with good electrical bistability.

Imprint effect in ferroelectric poly(vinylidene fluoride-trifluoroethylene) thin films

The imprint effect in ferroelectrics often hinders their performance in practical applications. Large numbers of observations have been reported on the imprint effect in inorganic ferroelectrics, while the imprint effect in ferroelectric polymers has attracted attention just in recent years. Here we reported our recent studies on the imprint effect in ferroelectric poly(vinylidene fluoride-trifluoroethylene) thin films. We measured the shift of ferroelectric switching peaks and the increase in switching time as a function of the waiting time. The influences of imprint pulse profile and short-circuit condition on the imprint effect were also determined. According to the interface screening model, we discussed the origin of the imprint effect in ferroelectric polymers and explained our experimental observations.

Observation of asymmetric ferroelectric switching in fatigued ferroelectric vinylidene fluoride and trifluoroethylene copolymer films

In this paper, we illuminated the observation of asymmetric ferroelectric switching phenomena in fatigued P(VDF-TrFE) copolymer films. Two types of asymmetric switching were observed: one was the occurrence of switching dual peaks obtained from films with asymmetric electrode structure and the other was the jump of remanent polarization during fatigue process, as was obtained mostly from films with asymmetric electrodes and occasionally from films with symmetric electrodes. No evidence indicated the dependence between the switching dual peaks and the jump of remanent polarization. The possible mechanism of such asymmetric phenomena was also discussed.

Electrical fatigue in ferroelectric P(VDF-TrFE) copolymer films

In recent years, ferroelectric polymer-based nonvolatile memory devices have attracted much attention due to their advantages over inorganic ferroelectrics. Polarization fatigue is a major problem in ferroelectric memories and seldom studies have been reported in this field. Here we will summarize our recent work on electric fatigue in polymers as well as some of work from the other researchers. Our experimental observations indicated that the injection and the trap of space charges acted a crucial role in the origination of polarization fatigue in ferroelectric polymers.

Epitaxy of Ferroelectric P(VDF-TrFE) Films via Removable PTFE Templates and Its Application in Semiconducting/Ferroelectric Blend Resistive Memory

Ferroelectric polymer based devices exhibit great potentials in low-cost and flexible electronics. To meet the requirements of both low voltage operation and low energy consumption, thickness of ferroelectric polymer films is usually required to be less than, for example, 100 nm. However, decrease of film thickness is also accompanied by the degradation of both crystallinity and ferroelectricity and also the increase of current leakage, which surely degrades device performance. Here we report one epitaxy method based on removable poly(tetrafluoroethylene) (PTFE) templates for high-quality fabrication of ordered ferroelectric polymer thin films. Experimental results indicate that such epitaxially grown ferroelectric polymer films exhibit well improved crystallinity, reduced current leakage and good resistance to electrical breakdown, implying their applications in high-performance and low voltage operated ferroelectric devices. On the basis of this removable PTFE template method, we fabricated organic semiconducting/ferroelectric blend resistive films which presented record electrical performance with operation voltage as low as 5 V and ON/OFF ratio up to 105.

Solvent vapor annealing of ferroelectric P(VDF-TrFE) thin films.

Ferroelectric polymers are a kind of promising materials for low-cost flexible memories. However, the relatively high thermal annealing temperature restricts the selection of some flexible polymer substrates. Here we report an alternative method to obtain ferroelectric poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films under low process temperatures. Spin-coated P(VDF-TrFE) thin films were solvent vapor processed at 30 °C for varied times. Structural analyses indicated that solvent vapor annealing induced crystallization to form a ferroelectric β phase, and electrical measurements from both macroscopic ferroelectric switching and microscopic vertical piezoresponse force microscopy further proved the films enduring solvent vapor annealing for suitable short times possessed good ferroelectric and piezoelectric properties. To illuminate the application of solvent vapor annealing on ferroelectric devices, we further fabricated ferroelectric capacitor memory devices with a structure of Al/P(VDF-TrFE)/Al2O3/p-Si/Al where the ferroelectric layer was solvent vapor annealed. Ferroelectric capacitors showed obvious bistable operation and comparable ON/OFF ratio and retention performance. Our work makes it possible to structure ferroelectric devices on flexible substrates that require low process temperatures.

The influence of in-plane ferroelectric crystal orientation on electrical modulation of magnetic properties in Co60Fe20B20/SiO2/(011) xPb(Mg1/3Nb2/3)O3-(1 − x)PbTiO3 heterostructures

(011) cut xPb(Mg1/3Nb2/3)O3-(1 − x)PbTiO3 (PMN-PT) ferroelectric crystal is usually used in ferroelectric/ferromagnetic (FE/FM) heterostructures due to its strong voltage-induced anisotropic in-plane strain. (011) PMN-PT crystal includes two in-plane crystal orientations, 〈100〉 and 〈01 − 1〉, with different piezoelectric strength resulting in anisotropic in-plane strain. Few systematic studies have been conducted to determine the influence of in-plane orientation on magnetoelectric (ME) coupling characteristic in ferroelectric/ferromagnetic composites. In this paper, we report our work to distinguish the contributions of in-plane orientations on electric modulation of magnetic properties. Magneto-optical Kerr effect and propagating spin wave spectroscopy are measured to define the influence of in-plane orientations on electric control of magnetic parameters and spin wave propagation. Magnetoelectric coupling coefficients and frequency modulation coefficients are also calculated.