J. Buckley

Experimental and theoretical study of electrode effects in HfO 2 based RRAM

In this work, the impact of Ti electrodes on the electrical behaviour of HfO2-based RRAM devices is conclusively clarified. To this aim, devices with Pt, TiN and Ti electrodes have been fabricated (see Fig. 1). We first provide several experiments to clearly demonstrate that switching is driven by creation-disruption of a conductive filament. Thus, the role of TiN/Ti electrodes is explained and modeled based on the presence of HfOx interfacial layer underneath the electrode. In addition, Ti is found responsible to activate bipolar switching. Moreover, it strongly reduces forming and switching voltages with respect to Pt-Pt devices. Finally, it positively impacts on retention. To support and interpret our results we provide physico-chemical measurements, electrical characterization, ab-initio calculations and modeling.

Novel Si-based nanowire devices: Will they serve ultimate MOSFETs scaling or ultimate hybrid integration?

Both CMOS scaling and NEMS sensor devices scaling converge to the same type of sub 100 nm objects. This opens new fields of application for IC chips integrating both complex signal treatment and very highly sensitive sensing functionalities.

Ferrocene and porphyrin monolayers on Si(100) surfaces: preparation and effect of linker length on electron transfer.

The missing link: Ferrocene and porphyrin monolayers are tethered on silicon surfaces with short (see picture, left) or long (right) linkers. Electron transfer to the silicon substrate is faster for monolayers with a short linker.Ferrocene and porphyrin derivatives are anchored on Si(100) surfaces through either a short two-carbon or a long 11-carbon linker. The two tether lengths are obtained by using two different grafting procedures: a single-step hydrosilylation is used for the short linker, whereas for the long linker a multistep process involving a 1,3-dipolar cycloaddition is conducted, which affords ferrocene-triazole-(CH(2))(11)-Si or Zn(porphyrin)-triazole-(CH(2))(11)-Si links to the surface. The modified surfaces are characterized by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Cyclic voltammetry experiments show that the redox activity of the tethered ferrocene or porphyrin is maintained for both linker types. Microelectrode capacitor devices incorporating these modified Si(100) surfaces are designed, and their capacitance-voltage (C-V) and conductance-voltage (G-V) profiles are investigated. Capacitance and conductance peaks are observed, which indicates efficient charge transfer between the redox-active monolayers and the electrode surface. Slower electron transfer between the ferrocene or porphyrin monolayer and the electrode surface is observed for the longer linker, which suggests that by adjusting the linker length, the electrical properties of the device, such as charging and discharging kinetics and retention time, could be tuned.

Investigation of Hybrid Molecular/Silicon Memories With Redox-Active Molecules Acting as Storage Media

In this paper, a physical investigation of hybrid molecular/Si memory capacitor structures is proposed, where redox-active molecules act as storage medium. Fc and ZnAB3P porphyrin were grafted on (100) Si with both a direct bond and a chemical linker in order to investigate the electron transfer properties of the molecule/Si system. The chemical structures of the molecular layers were analyzed with X-ray photoelectron spectroscopy. Cyclic voltammetry and impedance spectroscopy were also performed on capacitor structures in order to characterize the charge transfer between the redox molecules and the Si and the effect of an organic linker on its rate. To explain our results, an original electrical model of molecule/Si memory structures is proposed. Calculated data are compared to impedance results for the cases of Fc either directly grafted or with linker. The model allows us to give a theoretical confirmation of the influence of the linker over the redox energy and on the time constant of the molecular capacitor. Finally, density functional theory calculations provide an in-depth physical insight of the effect of the linker over the Fc redox energy. The results obtained in this paper show the strong impact of the engineering of the redox molecules and their linker on the electron transfer properties.

Thorough investigation of Si-nanocrystal memories with high-k interpoly dielectrics for sub-45nm node Flash NAND applications

In this paper we show for the 1st time that Silicon nanocrystal (Si-ncs) memories with high-k (HfO2, Al2O3 and HfAlO) interpoly dielectrics (IPD) can offer excellent behaviour in the Fowler-Nordheim regime, with great relevance for future sub-45 nm NAND memory generations. We significantly advance the state-of-the-art by showing a strict correlation between the different IPD properties and the performance obtained on memory transistors down to 90 nm gate lengths. In particular the results demonstrate that HfAlO IPDs combine the fast p/e and good 105 cycles endurance behaviour of HfO2 and the long retention of Al2O3 with no activation up to 125degC Then, in order to boost the memory window, we also integrated a hybrid Si-nc/SiN layer floating gate, with a HfAlO based IPD. It is shown that a 6V DeltaVth can be achieved, with good retention and cycling behaviours.

Current Conduction Model for Oxide-Based Resistive Random Access Memory Verified by Low-Frequency Noise Analysis

A conduction model consisting of two parallel resistances from a highly conductive filament region and a uniform leakage oxide region is proposed in this brief to represent the current conduction in the filament-type switching resistive random access memory cell. Low-frequency noise analysis of current fluctuation at different resistance states has been employed to verify its efficiency. It is found that, in the low-resistance regime, filament resistance dominates current conduction and noise varies as a power law of resistance, whereas in the high-resistance regime, uniform oxide leakage is the major source of conduction, giving rise to a nearly constant noise level.

Low-frequency noise in oxide-based (TiN/HfOx/Pt) resistive random access memory cells

In this brief, low-frequency noise (LFN) characteristics are studied in oxide-based (TiN/HfOx/Pt) resistive random access nemory cells having different dimensions. It is confirmed that, for the low resistance state (LRS), current conduction is localized without an area dependence, whereas for the high resistance state, it is a uniform leakage current throughout the whole device area. An LFN model is proposed for the filamentary LRS based on the carrier number fluctuation approach, allowing physical analysis of filament characteristics and the surrounding trap concentration.

From Atomistic to Device Level Investigation of Hybrid Redox Molecular/Silicon Field-Effect Memory Devices

In this paper, an extensive investigation of hybrid molecular/Si field-effect memories is presented, where redox ferrocene (Fc) molecules play the role of the memory charge storage nodes. Engineering of the organic linkers between Fc and Si is achieved by grafting Fc with different linker lengths. The study shows a clear correlation between results from atomistic computational density functional theory, electrochemical measurements (cyclic voltammetry) and electrical data obtained by a detailed study on capacitors and pseudo-MOS devices. Physical-chemical analyses (atomic force microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy), corroborate the quality of molecular layers on devices.

Bistable molecules development and Si surface grafting: two chemical tools used for the fabrication of hybrid molecule/Si CMOS component

For the past ten years, there has been considerable interest dedicated to the miniaturisation of CMOS devices. The research axes followed to obtain scalable devices are numerous as the possibilities offered by both technological (top-down) and new (bottom-up) approaches are studied. Concerning the latter approach molecular electronics is a growing field of interest. Notably, the fabrication of hybrid molecule/Si structures paves the way for development of devices with electrical performances that can be tuned thanks to the molecular properties initially targeted. Here we present the recent results obtained in the two approaches we follow in order to develop new hybrid molecule/Si memory elements. The first axis focuses on the development of specific molecules that could allow a fine tuning of the memory retention characteristic. The second axis deals with the integration of redox molecules inside capacitive cells and the study of their electrical properties. The capacitance of such components clearly shows that an effect of charge transfer is observed only when redox active molecules (porphyrins) are grafted on Si.

Comparison between front- and back-gating of Silicon Nanoribbons in real-time sensing experiments

Field-effect transistors (FETs) with open gate structures such as Silicon Nanoribbons (SiNRs) are promising candidates to become general platforms for ultrasensitive, label-free and real-time detection of biochemical interactions on surface. This work proposes and demonstrates the viability of a solution for integrating Ag/AgCl reference electrodes with the microfluidics. A comparison between different polarization schemes is carried out with an analysis of the respective advantages and disadvantages.