Ferroelectricity in Polar Polymer-based FETs: A Hysteresis Analysis.

There is an increasing number of reports on polar polymer-based Ferroelectric Field Effect Transistors (FeFETs), where the hysteresis of the drain current - gate voltage (Id-Vg) curve is investigated as the result of the ferroelectric polarization effect. However, separating ferroelectric effect from many of the factors (such as charge injection/trapping and the presence of mobile ions in the polymer) that confound interpretation is still confusing and controversial. This work presents a methodology to reliably identify the confounding factors which obscure the polarization effect in FeFETs. Careful observation of the Id-Vg curves, as well as monitoring the Id-Vg hysteresis and flat band voltage shift as a function of temperature and sweep frequency identifies the dominant mechanism. This methodology is demonstrated using 15-nm thick high glass transition temperature polar polymer-based FeFETs. In these devices, room temperature hysteresis is largely a consequence of charge trapping and mobile ions, while ferroelectric polarization is observed at elevated temperatures. This methodology can be used to unambiguously prove the effect of ferroelectric polarization in FeFETs.

Two-dimensional hybrid layered materials: strain engineering on the band structure of MoS2/WSe2 hetero-multilayers.

In this paper, we report a comprehensive modeling and simulation study of constructing hybrid layered materials by alternately stacking MoS2 and WSe2 monolayers. Such hybrid MoS2/WSe2 hetero-multilayers exhibited direct bandgap semiconductor characteristics with bandgap energy (E g) in a range of 0.45-0.55 eV at room temperature, very attractive for optoelectronics (wavelength range 2.5-2.75 µm) based on thicker two-dimensional (2D) materials. It was also found that the interlayer distance has a significant impact on the electronic properties of the hetero-multilayers, for example a five orders of magnitude change in the conductance was observed. Three material phases, direct bandgap semiconductor, indirect bandgap semiconductor, and metal were observed in MoS2/WSe2 hetero-multilayers, as the interlayer distance decreased from its relaxed (i.e., equilibrium) value of about 6.73 Å down to 5.50 Å, representing a vertical pressure of about 0.8 GPa for the bilayer and 1.5 GPa for the trilayer. Such new hybrid layered materials are very interesting for future nanoelectronic pressure sensor and nanophotonic applications. This study describes a new approach to explore and engineer the construction and application of tunable 2D semiconductors.

Self-assembled nanowire array capacitors: capacitance and interface state profile.

Direct characterization of the capacitance and interface states is very important for understanding the electronic properties of a nanowire transistor. However, the capacitance of a single nanowire is too small to precisely measure. In this work we have fabricated metal-oxide-semiconductor capacitors based on a large array of self-assembled Si nanowires. The capacitance and conductance of the nanowire array capacitors are directly measured and the interface state profile is determined by using the conductance method. We demonstrate that the nanowire array capacitor is an effective platform for studying the electronic properties of nanoscale interfaces. This approach provides a useful and efficient metrology for the study of the physics and device properties of nanoscale metal-oxide-semiconductor structures.

Fabrication, characterization and simulation of high performance Si nanowire-based non-volatile memory cells.

We report the fabrication, characterization and simulation of Si nanowire SONOS-like non-volatile memory with HfO(2) charge trapping layers of varying thicknesses. The memory cells, which are fabricated by self-aligning in situ grown Si nanowires, exhibit high performance, i.e. fast program/erase operations, long retention time and good endurance. The effect of the trapping layer thickness of the nanowire memory cells has been experimentally measured and studied by simulation. As the thickness of HfO(2) increases from 5 to 30 nm, the charge trap density increases as expected, while the program/erase speed and retention remain the same. These data indicate that the electric field across the tunneling oxide is not affected by HfO(2) thickness, which is in good agreement with simulation results. Our work also shows that the Omega gate structure improves the program speed and retention time for memory applications.

The large-scale integration of high-performance silicon nanowire field effect transistors.

In this work we present a CMOS-compatible self-aligning process for the large-scale-integration of high-performance nanowire field effect transistors with well-saturated drain currents, steep subthreshold slopes at low drain voltage and a large on/off current ratio (>10(7)). The subthreshold swing is as small as 45 mV/dec, which is substantially beyond the thermodynamic limit (60 mV/dec) of conventional planar MOSFETs. These excellent device characteristics are achieved by using a clean integration process and a device structure that allows effective gate-channel-source coupling to tune the source/drain Schottky barriers at the nanoscale.

Influence of metal-MoS2 interface on MoS2 transistor performance: comparison of Ag and Ti contacts.

In this work, we compare the electrical characteristics of MoS2 field-effect transistors (FETs) with Ag source/drain contacts with those with Ti and demonstrate that the metal-MoS2 interface is crucial to the device performance. MoS2 FETs with Ag contacts show more than 60 times higher ON-state current than those with Ti contacts. In order to better understand the mechanism of the better performance with Ag contacts, 5 nm Au/5 nm Ag (contact layer) or 5 nm Au/5 nm Ti film was deposited onto MoS2 monolayers and few layers, and the topography of metal films was characterized using scanning electron microscopy and atomic force microscopy. The surface morphology shows that, while there exist pinholes in Au/Ti film on MoS2, Au/Ag forms a smoother and denser film. Raman spectroscopy was carried out to investigate the metal-MoS2 interface. The Raman spectra from MoS2 covered with Au/Ag or Au/Ti film reveal that Ag or Ti is in direct contact with MoS2. Our findings show that the smoother and denser Au/Ag contacts lead to higher carrier transport efficiency.