Yemei Han

Synthesis of Large-Area Highly Crystalline Monolayer Molybdenum Disulfide with Tunable Grain Size in a H2 Atmosphere

Large-area and highly crystalline monolayer molybdenum disulfide (MoS2) with a tunable grain size was synthesized in a H2 atmosphere. The influence of introduced H2 on MoS2 growth and grain size, as well as the corresponding mechanism, was tentatively explored by controlling the H2 flow rate. The as-grown monolayer MoS2 displays excellent uniformity and high crystallinity evidenced by Raman and high-resolution transmission electron microscopy. The Raman results also give an indication that the quality of the monolayer MoS2 synthesized in a H2 atmosphere is comparable to that synthesized by using seed or mechanical exfoliation. In addition, the electronic properties and dielectric inhomogeneity of MoS2 monolayers were also detected in situ via scanning microwave microscopy, with measurements on impedance and differential capacitance (dC/dV). Back-gated field-effect transistors based on highly crystalline monolayer MoS2 shows a field-effect mobility of ∼13.07 cm2 V(-1) s(-1) and an Ion/Ioff ratio of ∼1.1×10(7), indicating that the synthesis of large-area and high-quality monolayer MoS2 with H2 is a viable method for electronic and optoelectronic applications.

VO 2 -Based Selection Device for Passive Resistive Random Access Memory Application

Low operation voltage, large resistance ratio, and good uniformity were achieved in W/VO2/Pt selection device. The selector is applied to Ti/HfO2/Pt resistive random access memory (RRAM) device, forming 1S1R configuration, to reduce the sneak path current. Ti/HfO2/Pt crosspoint array size can be improved from 8×8 to 128×128 by introducing the W/VO2/Pt selection device from readout margin simulation. In addition, the selector can be used in sub-10-nm scale RRAM to suppress sneak leakage current.

Ultra-Low Power Ni/HfO 2 /TiO x /TiN Resistive Random Access Memory With Sub-30-nA Reset Current

In this letter, we report ultra-low power (sub-30-nA reset current, Ireset) Ni/HfO2/TiOx/TiN RRAM devices that were fabricated with the rapid thermal oxidation of evaporated titanium. RRAM devices show forming-free, bipolar resistive switching behavior, low-resistive state (LRS) nonlinearity, good data retention, and stability. The resistive switching mechanism is mainly attributed to Schottky barrier modulation induced by O2- migration at the Ni/HfO2 interface. LRS/high-resistive state current conduction is controlled by Schottky emission/trap-controlled space-charge-limited current. The TiOx film is believed to provide a local high-density current for the device, confirmed by conductive atomic force microscope results.

Scalable Synthesis of Highly Crystalline MoSe2 and Its Ambipolar Behavior

Atomically thin, two-dimensional material molybdenum diselenide (MoSe2) has been shown to exhibit significant potential for diverse applications. The intrinsic band gap of MoSe2 allows it to overcome the shortcomings of the zero-band-gap graphene, while its higher electron mobilities when compared to molybdenum disulfide (MoS2) make it more appropriate for practical devices in electronics and optoelectronics. However, its controlled growth has been an ongoing challenge for investigations and practical applications of the material. Here, we present an atmospheric pressure chemical vapor deposition (CVD) method to achieve highly crystalline, single- and few-layered MoSe2 using a SiO2/Si substrate. Our findings suggested that careful optimization of the flow rate can result in the controlled growth of large-area MoSe2 with desired layer numbers due to the adjustment of gaseous MoSe2 partial pressure and nucleation density. The FETs fabricated on such as-synthesized MoSe2 displayed different transport behaviors depending on the layer numbers, which can be attributed to the formation of Se vacancies generated during low flow rates. Monolayer MoSe2 showed n-type characteristics with an Ion/Ioff ratio of ∼106 and a carrier mobility of ∼19 cm2 V-1 s-1, whereas bilayer MoSe2 showed n-type-dominant ambipolar behavior with an Ion/Ioff ratio of ∼105 and a higher mobility of ∼65 cm2 V-1 s-1 for electrons as well as ∼9 cm2 V-1 s-1 for holes. Our results provide a foundation for property-controlled synthesis of MoSe2 and offer insight on the potential applications of our synthesized MoSe2 in electronics and optoelectronics.

A novel magnetoelectric memory cell based on bilayer ferroelectric films of (1 − x)[Ba(Zr0.2Ti0.8)O3]–x(Ba0.7Ca0.3TiO3)

We demonstrate that a thin film magnetoelectric memory cell, comprised of Fe70Ga30 sputtered on top of bilayer ferroelectric films which consists of a tetragonal (T) 0.7Ba(Zr0.2Ti0.8)O3–0.3(Ba0.7Ca0.3TiO3) (70BZT–30BCT) film deposited on a rhombohedral (R) 0.3Ba(Zr0.2Ti0.8)O3–0.7(Ba0.7Ca0.3TiO3) (30BZT–70BCT) film. The insertion of rhombohedral layer improves the quality of the films and enhances the piezoelectric properties, which could be attributed to that the ferroelectric domains are tethered only by a soft R under layer, and not by the hard substrate. The working mechanism is that the 70BZT–30BCT layer acquires different strain states when an electric field is applied to it. Mechanical transduction couples this strain to the mechanically coupled Fe70Ga30 layer, which then changes its magnetic anisotropy and thus the magnetoresistance. The high (low) resistance states were realized when different voltages were applied due to the anisotropy magnetoresistance of Fe70Ga30 films. The demonstrated magnetoelectric memory device using resistance as the media and electric field as the writing field shows great promises towards exploring magnetoelectric devices for low-power and high density magnetic data storage applications.

Electric field induced modulation of transport characteristics in multiferroic BZT–BCT/FeCo thin films

We report the fabrication of 60Ba(Zr0.2Ti0.8)O3–40(Ba0.7Ca0.3TiO3)/Fe65Co35 (60BZT–40BCT/Fe65Co35) films on Pt/Ti/SiO2/Si substrates. The 60BZT–40BCT films exhibit well defined piezoelectric properties and the magnetic properties of Fe65Co35 films were characterized, which suggests the multiferroic nature of the films. The transport characteristics of Fe65Co35 films could be modified when the samples are subjected to a bias voltage along the thickness direction for 60BZT–40BCT/Fe65Co35 films, which manifests itself in the variation of current–voltage (I–V) curves of Fe65Co35 films under the influence of applied bias voltage. This produces more than an order of magnitude change in resistance. The manipulation could be attributed to the strain-mediated coupling and the charge accumulation and depletion at the interface.

Optimization of slurry and process parameter on chemical mechanical polishing of CR-doped Sb 2 Te 3 thin film

In this paper, we studied the composition of slurry including pH and the oxidizing agent Hydrogen Peroxide (H2O2) for Cr-doped Sb2Te3 (CST) thin film chemical mechanical polishing (CMP). Also the effects of the process parameters including down force and platen rotation rate were studied in detail. The results demonstrate that Material Removal Rate (MRR) has a relatively large dependence on pH values as well as the concentration of the oxidizing agent. Moreover, the MRR still exists when there is no down force and rotation, indicating that it is a mechanical abrasion assisted by chemical corrosion. Eventually, the root mean square (RMS) roughness was reduced from 4.02nm to 0.425nm and the MRR can be achieved at 100.45nm/min.

Resistive switching characteristic and uniformity of low-power HfO x -based resistive random access memory with the BN insertion layer*

In this letter, the Ta/HfO x /BN/TiN resistive switching devices are fabricated and they exhibit low power consumption and high uniformity each. The reset current is reduced for the HfO x /BN bilayer device compared with that for the Ta/HfO x /TiN structure. Furthermore, the reset current decreases with increasing BN thickness. The HfO x layer is a dominating switching layer, while the low-permittivity and high-resistivity BN layer acts as a barrier of electrons injection into TiN electrode. The current conduction mechanism of low resistance state in the HfO x /BN bilayer device is space-charge-limited current (SCLC), while it is Ohmic conduction in the HfO x device.