1-octadecene monolayers on Si(111) hydrogen-terminated surfaces: effects of substrate doping

We have studied the electronic properties, in relation to their structural properties, of monolayers of 1-octadecene attached on a hydrogen-terminated (111) silicon surface. The molecules are attached using the free-radical reaction between C=C and Si-H activated by an ultraviolet illumination. We have compared the structural and electrical properties of monolayers formed on silicon substrate of different types (n-type and p-type) and different doping concentrations from low-doped (~1E14 cm-3) to highly doped (~1E19 cm-3) silicon substrates. We show that the monolayers on n-, p- and p+ silicon are densely packed and that they act as very good insulating films at a nanometer thickness with leakage currents as low as ~1E-7 A.cm-2 and high quality capacitance-voltage characteristics. The monolayers formed on n+-type silicon are more disordered and therefore exhibit larger leakage current densities (>1E-4 A.cm-2) when embedded in a silicon/monolayer/metal junction. The inferior structural and electronic properties obtained with n+-type silicon pinpoint the important role of surface potential and of the position of the surface Fermi level during the chemisorption of the organic monolayers.Comment: 33 pages, 8 figures, to be published J. Appl. Phy

Toward a better understanding of the doping mechanism involved in Mo(tfd-COCF3)3_3)_3 doped PBDTTT-c

In this study, we aim to improve our understanding of the doping mechanism involved in the polymer PBDTTT-c doped with(Mo(tfd-COCF3)3. We follow the evolution of the hole density with dopant concentration to highlight the limits of organic semiconductor doping. To enable the use of doping to enhance the performance of organic electronic devices, doping efficiency must be understood and improved. We report here a study using complementary optical and electrical characterization techniques, which sheds some light on the origin of this limited doping efficiency at high dopant concentration. Two doping mechanisms are considered, the direct charge transfer (DCT) and the charge transfer complex (CTC). We discuss the validity of the model involved as well as its impact on the doping efficiency.Comment: Accepted manuscript, J. Appl. Phy

Impact of dopant species on the interfacial trap density and mobility in amorphous In-X-Zn-O solution-processed thin-film transistors

Alloying of In/Zn oxides with various X atoms stabilizes the IXZO structures but generates electron traps in the compounds, degrading the electron mobility. To assess whether the latter is linked to the oxygen affinity or the ionic radius, of the X element, several IXZO samples are synthesized by the sol-gel process, with a large number (14) of X elements. The IXZOs are characterized by XPS, SIMS, DRX, and UV-spectroscopy and used for fabricating thin film transistors. Channel mobility and the interface defect density NST, extracted from the TFT electrical characteristics and low frequency noise, followed an increasing trend and the values of mobility and NST are linked by an exponential relation. The highest mobility (8.5 cm2/Vs) is obtained in In-Ga-Zn-O, and slightly lower value for Sb and Sn-doped IXZOs, with NST is about 2E12 cm2/eV, close to that of the In-Zn-O reference TFT. This is explained by a higher electronegativity of Ga, Sb, and Sn than Zn and In, their ionic radius values being close to that of In and Zn. Consequently, Ga, Sb, and Sn induce weaker perturbations of In-O and Zn-O sequences in the sol-gel process, than the X elements having lower electronegativity and different ionic radius. The TFTs with X = Ca, Al, Ni and Cu exhibited the lowest mobility and NST > 1E13 cm2/eV, most likely because of metallic or oxide clusters formation

Non-Arrhenius conduction due to the interface-trap-induced disorder in X-doped amorphous InXZnO thin-film transistors

Thin film transistors, with channels composed of In-X-Zn oxides, IXZO, with X dopants: Ga, Sb, Be, Mg, Ag, Ca, Al, Ni, and Cu, were fabricated and their I-V characteristics were taken at selected temperatures in the 77K<T<300K range. The low field mobility, mu, and the interface defect density, Nst were extracted from the characteristics for each of the studied IXZOs. At higher T the mobility follows the Arrhenius law with an upward distortion, increasing as T was lowered, gradually transforming into the exp [-(T0/T)1/4] variation. We showed that mu(T, Nst) follows mu0exp[-Eaeff(T,Nst)/kT], with T-dependent effective activation energy Eaeff(T, Nst) accounts for the data, revealing a linear correlation between Eaeff and Nst at higher T. Temperature variation of Eaeff(T, Nst) was evaluated using a model assuming a random distribution of conduction mobility edge Ec values in the oxides, stemming from spatial fluctuations induced by disorder in the interface traps distribution. For a Gaussian distribution of Ec, the activation energy Eaeff(T, Nst) varies linearly with 1/T, which accounts satisfactorily for the data obtained on all the studied IXZOs. The model also shows that Eaeff(T, Nst) is a linear function of Nst at a fixed T, which explains the exponential decrease of mu with NST

Negative Differential Resistance, Memory and Reconfigurable Logic Functions based on Monolayer Devices derived from Gold Nanoparticles Functionalized with Electro-polymerizable Thiophene-EDOT Units

We report on hybrid memristive devices made of a network of gold nanoparticles (10 nm diameter) functionalized by tailored 3,4(ethylenedioxy)thiophene (TEDOT) molecules, deposited between two planar electrodes with nanometer and micrometer gaps (100 nm to 10 um apart), and electropolymerized in situ to form a monolayer film of conjugated polymer with embedded gold nanoparticles (AuNPs). Electrical properties of these films exhibit two interesting behaviors: (i) a NDR (negative differential resistance) behavior with a peak/valley ratio up to 17, and (ii) a memory behavior with an ON/OFF current ratio of about 1E3 to 1E4. A careful study of the switching dynamics and programming voltage window is conducted demonstrating a non-volatile memory. The data retention of the ON and OFF states is stable (tested up to 24h), well controlled by the voltage and preserved when repeating the switching cycles (800 in this study). We demonstrate reconfigurable Boolean functions in multiterminal connected NP molecule devices.Comment: Full manuscript, figures and supporting information, J. Phys. Chem. C, on line, asap (2017

Physical Study by Surface Characterizations of Sarin Sensor on the Basis of Chemically Functionalized Silicon Nanoribbon Field Effect Transistor

Surface characterizations of an organophosphorus (OP) gas detector based on chemically functionalized silicon nanoribbon field-effect transistor (SiNR-FET) were performed by Kelvin Probe Force Microscopy (KPFM) and ToF-SIMS, and correlated with changes in the current-voltage characteristics of the devices. KPFM measurements on FETs allow (i) to investigate the contact potential difference (CPD) distribution of the polarized device as function of the gate voltage and the exposure to OP traces and, (ii) to analyze the CPD hysteresis associated to the presence of mobile ions on the surface. The CPD measured by KPFM on the silicon nanoribbon was corrected due to side capacitance effects in order to determine the real quantitative surface potential. Comparison with macroscopic Kelvin probe (KP) experiments on larger surfaces was carried out. These two approaches were quantitatively consistent. An important increase of the CPD values (between + 399 mV and + 302 mV) was observed after the OP sensor grafting, corresponding to a decrease of the work function, and a weaker variation after exposure to OP (between - 14 mV and - 61 mV) was measured. Molecular imaging by ToF-SIMS revealed OP presence after SiNR-FET exposure. The OP molecules were essentially localized on the Si-NR confirming effectiveness and selectivity of the OP sensor. A prototype was exposed to Sarin vapors and succeeded in the detection of low vapor concentrations (40 ppm).Comment: Paper and supporting information, J. Phys. Chem. C, 201

A Silicon Nanowire Ion-Sensitive Field-Effect-Transistor with elementary charge sensitivity

We investigate the mechanisms responsible for the low-frequency noise in liquid-gated nano-scale silicon nanowire field-effect transistors (SiNW-FETs) and show that the charge-noise level is lower than elementary charge. Our measurements also show that ionic strength of the surrounding electrolyte has a minimal effect on the overall noise. Dielectric polarization noise seems to be at the origin of the 1/f noise in our devices. The estimated spectral density of charge noise Sq = 1.6x10-2 e/sqr(Hz) at 10 Hz opens the door to metrological studies with these SiNW-FETs for the electrical detection of a small number of molecules.Comment: One file including paper (with 3 figures) and supplementary information (with 5 figures). Submitte