Structural and Phase Engineering in Two-Dimensional Polymorphic In2Se3

Nonvolatile ferroelectric-gate field-effect transistors (Fe-FETs) memories with solution-processed ferroelectric polymers are of great interest because of their potential for use in low-cost flexible devices. In particular, the development of a process for patterning high-performance semiconducting channel layers with mechanical flexibility is essential not only for proper cell-to-cell isolation but also for arrays of flexible nonvolatile memories. We demonstrate a robust route for printing large-scale micropatterns of solution-processed semiconducting small molecules/insulating polymer blends for high performance arrays of nonvolatile ferroelectric polymer memory. The nonvolatile memory devices are based on top-gate/bottom-contact Fe-FET with ferroelectric polymer insulator and micropatterned semiconducting blend channels. Printed micropatterns of a thin blended semiconducting film were achieved by our selective contact evaporation printing, with which semiconducting small molecules in contact with a micropatterned elastomeric poly(dimethylsiloxane) (PDMS) mold were preferentially evaporated and absorbed into the PDMS mold while insulating polymer remained intact. Well-defined micrometer-scale patterns with various shapes and dimensions were readily developed over a very large area on a 4 in. wafer, allowing for fabrication of large-scale printed arrays of Fe-FETs with highly uniform device performance. We statistically analyzed the memory properties of Fe-FETs, including ON/OFF ratio, operation voltage, retention, and endurance, as a function of the micropattern dimensions of the semiconducting films. Furthermore, roll-up memory arrays were produced by successfully detaching large-area Fe-FETs printed on a flexible substrate with a transient adhesive layer from a hard substrate and subsequently transferring them to a nonplanar surface.

The integration of oxides with semiconductors is important for the technological advancement of the next generation electronics. Concomitant ferroelectric and antiferromagnetic (AF) behavior is demonstrated in single crystal BiFeO3 (BFO) films grown on 20 nm SrTiO3 (STO) virtual substrates on Si(100) using molecular beam epitaxy (MBE). STO thin films are grown in an oxide MBE chamber by co-deposition of Sr, Ti, and molecular O2. Careful control of the O2 during nucleation produced commensurate growth of STO on Si. The sequence of the steps allows for the suppression of an amorphous SiO2 layer. This STO(20 nm)/Si structure was used as a virtual substrate for MBE deposition of BFO on Si without breaking vacuum. BFO was deposited using Fe and O2 plasma with an overpressure of Bi flux, the growth rate was controlled by the incoming Fe flux. The reflection high energy electron diffraction image shows a 2-D growth front with a 6-fold surface reconstruction under optimized O2 pressure of 5 × 10−8 mbar. Cross-sectional transmission electron microscopy (TEM) confirms the high crystallinity of the films and shows sharp, atomically flat interfaces. The selected area diffraction pattern (SADP) reveals that BFO grows in a distorted rhombohedral crystal structure. X-ray diffraction does not show formation of second phases and is consistent with the TEM and SADP results. The BFO films show AF behavior with a Neel temperature that exceeds 350 K, as expected (TN = 673 K) and with a residual ferromagnetic behavior that decreases with film thickness and is consistent with the G-type AF due to the canted spins. The saturation magnetization per unit volume for a 40 nm thick film was 180 emu/cm3 at an in-plane magnetic field of 8 kOe. The ferroelectric behavior of the films was verified using piezoresponse force microscopy.

Colloidal synthesized metal nanoparticles (NPs) have great potential to be utilized as economically viable and efficient charge storage floating gate for the non-volatile memory (NVM). Present research work focuses on the NVM device application of colloidal synthesized cobalt (Co) NPs using simple spin coating process. The metal-oxide-semiconductor (MOS) NVM capacitor devices with and without Co-NPs of average size of 5 nm have been fabricated. The MOS NVM and control devices have tunnel oxide (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) thickness of 3.9 nm grown using rapid thermal processing and control dielectric (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) thickness of 10 nm deposited using atomic layer deposition. Colloidal synthesized Cobalt (Co) NPs deposition over silicon dioxide wafer has been done using optimized spin coating process. Electrical characterization of fabricated devices has been done using proxima B1500 semiconductor device parameter analyzer. Capacitance-voltage (C-V) characteristics of fabricated MOS NVM device shows a large flat band voltage shift (ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fb</sub> ) of around 8.00 V after applying a sweep voltage of -6 V to +6 V and then +6 V to -6 V in C-V response curves. Where, the control device shows negligible ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fb</sub> in comparison to MOS NVM device. The program and erase characteristics of the fabricated device have also been studied by Fowler-Nordheim tunneling, resulting in 2.31-V memory window for program/erase pulse of ±5 V. The retention capability has been studied by capacitance versus time (C-t) characteristics. This shows a fairly steady C-t response, indicating strong retention capability of fabricated device.

Two-dimensional semiconductors, such as transition-metal dichalcogenides (TMDs) and black phosphorous (BP), have found various potential applications in electronic and opto-electronic devices. However, several problems including low carrier mobility and low photoluminescence efficiencies still limit the performance of these devices. Interfacing 2D semiconductors with functional oxides provides a way to address the problems by overcoming the intrinsic limitations of 2D semiconductors and offering them multiple functionalities with various mechanisms. In this review, we first focus on the physical effects of various types of functional oxides on 2D semiconductors, mostly on MoS2 and BP as they are the intensively studied 2D semiconductors. Insulating, semiconducting, conventional piezoelectric, strongly correlated, and magnetic oxides are discussed. Then we introduce the applications of these 2D semiconductors/functional oxides systems in field-effect devices, nonvolatile memory, and photosensing. Finally, we discuss the perspectives and challenges within this research field. Our review provides a comprehensive understanding of 2D semiconductors/functional oxide heterostructures, and could inspire novel ideas in interface engineering to improve the performance of 2D semiconductor devices.

Vanadium oxide thin film deposited on Si by atomic layer deposition for non-volatile resistive switching memory devices

Combined theoretical and nanoscale experimental study of Pb(Ca,Ba)TiO3, Pb(Sr,Ba)TiO3, and Pb(Sr,Ca)TiO3 complex perovskite structures: An investigation of the ferroelectric and electronic properties

Modeling and simulation of low power ferroelectric non-volatile memory tunnel field effect transistors using silicon-doped hafnium oxide as gate dielectric

Non-volatile memory (NVM) devices using carbon dioxide (CO2) and nitrous oxide (N2O) plasma-assisted tunneling layers have outstanding electrical properties measured at 300 K. However, the retention characteristics of NVM devices using CO2 and N2O plasma-assisted tunneling oxides are different at 360 K. The memory window of an NVM device using CO2 tunneling oxide was about 2.01 V (81.7%) after 10 years at 360 K. However, there was retention charge loss in the NVM device using a N2O tunneling layer at 360 K due to the temperature instability under negative bias. For an actual NVM device using N2O tunneling oxide, the memory window was reduced to about 1.58 V (65.6%) after 10 years at 360 K.

Symmetry breaking and switchable thermal dielectric behaviors triggered by order-disorder phase transition in a neutral co-crystallized organic adduct

Ferroelectric nanoceramic materials have a huge potential in integrated electronics as passive components, nonvolatile ferroelectric memories, and optoelectronic devices. They are mainly utilized in the form of thin films in the recent past more than as powders. This chapter discusses the basics of nanocrystalline ferroelectrics and their synthesis routes. It focuses on sol-gel processing of prominent ferroelectric nanoceramics and their sintering methods. The chapter discusses the size effects in ferroelectric systems that have attracted a great deal of attention due to their potential application in the field of information technology. A laser technique called two-photon lithography is adopted to etch out a polymer into a 3D structure. Hydrothermal synthesis is also used to prepare BaTiO3, BiFeO3, BaSrTiO3, SrTiO3, PbTiO3, and PbZrTiO3 nanomaterials. Template synthesis involves fabrication of nanostructures using suitable template. This serves as a filling material for suitable polymeric precursors. The sol-gel method or precipitation, spray pyrolysis, or emulsion techniques are used to form ultrafine and uniform ceramic powders.

Substitutional charge disorder giving rise to quenched electric random-fields (RFs) is probably at the origin of the peculiar behavior of relaxor ferroelectrics, which are primarily characterized by their strong frequency dispersion of the dielectric response and by an apparent lack of macroscopic symmetry breaking at the phase transition. Spatial fluctuations of the RFs correlate the dipolar fluctuations and give rise to polar nanoregions in the paraelectric regime as has been evidenced by piezoresponse force microscopy (PFM) at the nanoscale. The dimension of the order parameter decides upon whether the ferroelectric phase transition is destroyed (e.g. in cubic PbMg1/3Nb2/3O3, PMN) or modified towards RF Ising model behavior (e.g. in tetragonal Sr1−x BaxNb2O6, SBN, x ≈ 0.4). Frustrated interaction between the polar nanoregions in cubic relaxors gives rise to cluster glass states as evidenced by strong pressure dependence, typical dipolar slowing-down and theoretically treated within a spherical random bond-RF model. On the other hand, freezing into a domain state takes place in uniaxial relaxors. While at T c non-classical critical behavior with critical exponents ρ ≈ 1.8, β ≈ 0.1 and α ≈ 0 is encountered in accordance with the RF Ising model, below T c ≈ 350 K RF pinning of the walls of frozen-in nanodomains gives rise to non-Debye dielectric response. It is relaxation- and creep-like at radio and very low frequencies, respectively.

Semiconducting III–V materials exhibiting piezoelectric properties are much sought after due to their potential applications in piezotronic and piezo-phototronic devices. Nanowires (NWs) of III–V semiconductors are particularly interesting in this respect due to the occurrence of the wurtzite (WZ) structure commonly associated with enhanced piezoelectric properties, as opposed to the zinc blende (ZB) structure that is typically observed in the bulk. However, performing direct measurements of the piezoelectric properties of III–V NWs using piezo-response force microscopy (PFM) is challenging, and the analysis and interpretation of such measurements is far from trivial. Here we present detailed finite element simulations of single GaAs NWs, with both WZ and ZB crystalline structure, scanned by an atomic force microscope tip in PFM mode, demonstrating the effect of the non-uniform electric field between the tip and NW, scan direction, as well as NW orientation on the resulting PFM signal. We also report PFM data from single GaAs and InP NWs with both ZB and WZ structure, grown by molecular beam epitaxy, based on a novel non-destructive intermittent contact PFM mode. We explain our experimental data in the framework of our simulations, and for the first time, extract an experimental value for the axial piezoelectric coefficient of WZ InP, pm V−1. The methods and analysis described here are particularly relevant for the investigation of piezoelectric properties in a wide range of semiconducting III–V NW systems.

Growth of Bi on the Ag(001) surface at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ has been studied. Low energy electron diffraction and scanning tunneling microscopy were used to reveal that the $\mathrm{Bi}∕\mathrm{Ag}(001)$ surface forms four distinct ordered structures upon deposition of Bi at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $c(2\ifmmode\times\else\texttimes\fi{}2)$, $c(4\ifmmode\times\else\texttimes\fi{}4)$, $p(4\ifmmode\times\else\texttimes\fi{}4)\text{\ensuremath{-}}p4g$, and $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R26.6\ifmmode^\circ\else\textdegree\fi{}$ phases. A reversible phase transition from the $c(2\ifmmode\times\else\texttimes\fi{}2)$ structure to $c(n\ifmmode\times\else\texttimes\fi{}4)$, with $n=6--7$, upon cooling to $\ensuremath{\sim}240\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ was observed. Another phase transition from the $c(4\ifmmode\times\else\texttimes\fi{}4)$ structure to $c(2\ifmmode\times\else\texttimes\fi{}2)$ upon heating was also observed. Angle-resolved photoelectron spectroscopy was used to study the evolution of the valence electronic structure. Several Bi derived surface states were observed along the high symmetry directions. A surface resonance band, split off from the Ag $5sp$ states, was observed along the edge of the Ag bulk band projected onto the (001) surface. This band is similar to that observed on $\mathrm{In}∕\mathrm{Cu}(001)$ and is considered to have a major contribution from the substrate states. $\mathrm{Bi}\text{\ensuremath{-}}6p$ derived bands, showing narrow bandwidth and periodicities coincident with the surface structure, were also observed.

We report the ferroelectric properties of single-crystalline BiFeO3 nanowires using piezoresponse force microscopy (PFM). The nanowires, synthesized by a hydrothermal approach, have a rhombohedral perovskite structure and a preferential growth of the (211) crystallographic plane perpendicular to the wire axis, as revealed by x-ray and electron diffraction investigations. PFM measurements reveal that the as-synthesized BiFeO3 nanowires, down to 40 nm in diameter, have components of spontaneous polarization along both the axial and radial directions, thereby demonstrating the ferroelectric nature of the wires. The results indicate that such ferroelectric BiFeO3 nanowires should provide promising opportunity for nanoscale nonvolatile memory devices.

Two-dimensional materials have drawn great attentions due to their atomic thin nature and enriched excellent physical and chemical properties. With their high mobility, suppressed short channel effect, enhanced gate control, additional valley degree of freedom and the ability to form various van der Waals heterostructures, 2D materials are believed to be promising candidates for the next generation electronics and optoelectronics. A great variety of 2D material based electronic and optoelectronic devices have been fabricated, including the field effect transistors, photo detectors, solar cells, sensors, nonvolatile memories, and other novel functional devices.