[Show abstract][Hide abstract] ABSTRACT: The pseudo-MOSFET (Ψ-MOSFET) method is extended for the electrical characterization of heavily doped (1019–1020 cm−3) SOI wafers with 10–40 nm film thickness. The field-effect modulation is small and does not enable the formation of an inversion channel. Only accumulation and depletion-controlled volume conduction modes are activated by increasing the back-gate voltage to 40 V. An updated model describing the conduction regimes for heavily doped SOI wafers is derived. This model provides a simple method for parameters extraction such as surface and volume mobility and doping level. Four-point probe and Hall effect measurements fully validate our Ψ-MOSFET results. It is found that high-dose implantation results in good redistribution and electrical activation of impurities, without affecting the quality of the buried oxide and Si–SiO2 interface.
[Show abstract][Hide abstract] ABSTRACT: We tentatively present possible architectures of Silicon On Insulator (SOI) transistors for the final stages of the scaling of silicon microelectronics. The scaling trends for conventional partially depleted and fully depleted SOI MOSFETs are critically examined. A ground plane can considerably attenuate short-channel effects. The manufacturability of extremely thin MOSFETs is demonstrated. Based on quantum calculations, we discuss the merits of double-gate transistors with volume inversion.
International Journal of High Speed Electronics and Systems 04/2012; 10(01).
[Show abstract][Hide abstract] ABSTRACT: We study scanning gate microscopy (SGM) in open quantum rings obtained from buried semiconductor InGaAs/InAlAs heterostructures. By performing a theoretical analysis based on the Keldysh-Green function approach we interpret the radial fringes observed in experiments as the effect of randomly distributed charged defects. We associate SGM conductance images with the local density of states (LDOS) of the system. We show that such an association cannot be made with the current density distribution. By varying an external magnetic field we are able to reproduce recursive quasi-classical orbits in LDOS and conductance images, which bear the same periodicity as the Aharonov-Bohm effect.
[Show abstract][Hide abstract] ABSTRACT: We combine scanning gate microscopy (SGM) experiments and simulations to demonstrate imaging of the electron local density of states within open quantum rings (QRs). SGM is based on a weak electrostatic perturbation of the electron system by a charged tip, which alters the transmission of electrons through the system. When the QRs are in the ballistic and coherent regime of transport, conductance fringes are observed in SGM images when the tip scans over the QR area as well as in its vicinity. Comparing our results to quantum mechanical simulations of transport in realistic QRs, we demonstrate that the fringes observed over the QR area are directly connected to the local density of states inside the QR . Moreover, the magnetic field dependence of the fringes observed in the vicinity of the QRs indicates that they originate from the electrostatic Aharonov-Bohm effect, and correspond to iso-phase lines for electrons . From these results, one can expect to design new kinds of quantum nanodevices based on a precise spatial control of electron interferences and trajectories.  F. Martins et al., PRL 99,136807 (2007).  B. Hackens et al., Nat. Phys. 2, 826 (2006).
[Show abstract][Hide abstract] ABSTRACT: We study the relationship between the local density of states (LDOS) and the conductance variation $\Delta G$ in scanning-gate-microscopy experiments on mesoscopic structures as a charged tip scans above the sample surface. We present an analytical model showing that in the linear-response regime the conductance shift $\Delta G$ is proportional to the Hilbert transform of the LDOS and hence a generalized Kramers-Kronig relation holds between LDOS and $\Delta G$. We analyze the physical conditions for the validity of this relationship both for one-dimensional and two-dimensional systems when several channels contribute to the transport. We focus on realistic Aharonov-Bohm rings including a random distribution of impurities and analyze the LDOS-$\Delta G$ correspondence by means of exact numerical simulations, when localized states or semi-classical orbits characterize the wavefunction of the system.
[Show abstract][Hide abstract] ABSTRACT: Combining scanning gate microscopy (SGM) experiments and simulations, we demonstrate low temperature imaging of the electron probability density |Psi|(2)(x,y) in embedded mesoscopic quantum rings. The tip-induced conductance modulations share the same temperature dependence as the Aharonov-Bohm effect, indicating that they originate from electron wave function interferences. Simulations of both |Psi|(2)(x,y) and SGM conductance maps reproduce the main experimental observations and link fringes in SGM images to |Psi|(2)(x,y).
[Show abstract][Hide abstract] ABSTRACT: The influence of the electric field on the radiative recombination of an electron-hole pair in a silicon quantum dot is assessed by means of a variational calculation. In contrast with III-V devices, in the case of silicon the use of SiO2 as a matrix makes possible the application of very large electric fields, which should indeed have a considerable impact on the radiation lifetime. For a distribution of crystallites, a growing electric field should lead to a quenching of the radiative recombination in the larger dots and to a noticeable shift of the spectrum.
[Show abstract][Hide abstract] ABSTRACT: Traditionally, the understanding of quantum transport, coherent and ballistic1, relies on the measurement of macroscopic properties such as the conductance. While powerful when coupled to statistical theories, this approach cannot provide a detailed image of "how electrons behave down there". Ideally, understanding transport at the nanoscale would require tracking each electron inside the nano-device. Significant progress towards this goal was obtained by combining Scanning Probe Microscopy (SPM) with transport measurements2-7. Some studies even showed signatures of quantum transport in the surrounding of nanostructures4-6. Here, SPM is used to probe electron propagation inside an open quantum ring exhibiting the archetype of electron wave interference phenomena: the Aharonov-Bohm effect8. Conductance maps recorded while scanning the biased tip of a cryogenic atomic force microscope above the quantum ring show that the propagation of electrons, both coherent and ballistic, can be investigated in situ, and even be controlled by tuning the tip potential.
[Show abstract][Hide abstract] ABSTRACT: Light-emitting electrochemical cells with low current threshold can be realized through mixing conjugated polymers and molten salts. Current drive capability is proportional to the overall interface perimeter of the planar, discotic molten salt domains inserted into the polymer matrix. Electric force microscopy indicates that this interface perimeter exhibits a specific dependence on the molten salt content in the active layer, with a well-defined maximum. We show that this maximum corresponds to an optimal current drive.
The Journal of Physical Chemistry B 09/2006; 110(31):15049-51. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: When the tip-sample distance is small, Kelvin probe force microscopy is affected by parametric amplification. This is due to the fact that the electric force has two components; the higher one having a frequency exactly twice as high as the lower. The oscillation amplitude may substantially depart from what is usually expected. Those phenomena are analytically modeled and experimentally shown, and the optimal parameter values which must be used for voltage detection are established.
[Show abstract][Hide abstract] ABSTRACT: Recent advances in scanning gate microscopies (SGM) have made it possible to image electron flow in nanostructures, using the tip as a moving gate (M.A. Topinka et al., Science 289, 2323 (2000)). Our experiment focuses on quantum rings (QR), patterned from a two-dimensional electron gas (2DEG) buried close to the sample surface (25 nm). We can therefore reach a very small tip-sample distance, thereby improving the imaging resolution. Furthermore, the absence of metallic gates on the top of our sample allows to probe the electron flow directly inside the QR. The SGM images, obtained at 4.2 K, reveal striking periodic oscillations of the resistance along the QR circumference as the tip scans over the sample surface. These oscillations respect the radial symmetry of the QR, and their periodicity is much larger than the Fermi wavelength. We analyze the influence of the tip-induced perturbation on the 2DEG by changing the tip voltage (both in accumulation and depletion regimes) and the tip-sample distance.
[Show abstract][Hide abstract] ABSTRACT: ilole groups are known to present a high electron affinity. Initially, copolymn. of siloles with fluorene was aimed at improving electron injection into the polymer layer and so improving the electroluminescent properties of org. light-emitting diodes (OLED's) made from fluorene. But it also provides the ability to turn the light emission color to the green part of the spectrum and to stop the well-known spectral shift degrdn. occurring in fluorene-based materials. In this paper we report the synthesis and the characterization of 1,1-dimethyl-2,5-bis(fluoren-2-yl)-3,4-diphenylsilole 4, and of two sol. conjugated random copolymers derived from 9,9-ditetradecylfluorene and 1,1-dialkyl-2,5-diphenylsilole, where the alkyl group is either Me (11a) or n-hexyl (11b). Silole 4 crystallizes in the triclinic P-1 space group with a = 9.8771(8), b = 10.6240(10), c = 16.585(2) .ANG., a = 95.775(8), b = 97.025(7), and g = 111.738(8)°. The results obtained with this mol., operating in a single-layer OLED (luminance »450 Cd/m2 at 12 V; hmax = 0.2 Cd/A), give evidences for the complementarity of the silole and the fluorenyl moieties in the improvement of the charge injection processes when compared with 1,1-dimethyl-2,3,4,5-tetraphenylsilole. The results obtained from org. light-emitting electrochem. cells (LEC's) made from silole-fluorene copolymers 11a, 11b and molten salts show an improvement of both the device lifetime and the spectral stability when compared with polyfluorene. To explain devices performances elec. characterization data and at. force microscope (AFM) imaging were combined.
[Show abstract][Hide abstract] ABSTRACT: We use low temperature scanning gate microscopy to study the electron dynamics inside Aharonov-Bohm (AB) rings in the phase-coherent regime. Our samples are prepared by etching from high-mobility two-dimensional electron systems 25 nm below the surface (B. Hackens et al., Phys. Rev. Lett. 94, 146802 (2005)). The bias applied on the AFM tip, as it is scanned over the AB ring, induces a local perturbation of the electric potential experienced by the electrons, thereby affecting the ring resistance. In particular, we observe in detail the effect of small variations of the perpendicular magnetic field in different regimes (up to B=6 T). As we plot these resistance variations as a function of the AFM tip position, we observe resistance modulations which have the spatial symmetry of the ring, and an amplitude equal to that of the AB oscillations. This strongly suggests that our resistance maps are closely linked to the periodic modulations of the electron probability density. Finally, we also performed simulations of the electron wavefunctions in AB rings, as well as of the electron transmission through the AB rings. Including the effect of the perturbating potential of the tip in such simulations, we get valuable informations which help to explain our experimental results.
[Show abstract][Hide abstract] ABSTRACT: We propose to use a parametric amplification regime for small charge or potential difference detection in electric force microscopy. First we give a simple method to accurately estimate the instability domains of the oscillating system. Then we establish general and fully analytical expressions of the parametric amplification gain, and discuss the optimal parameter values which must be used for voltage or charge detection. We show that even in conventional Kelvin probe force microscopy the parametric effect should be taken into account.
Physical Review B 05/2005; 71(20):205404. · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mixing molten salts with luminescent conjugated polymers provides the ability to lower the current threshold of organic electroluminescence (EL) devices. However, this process generally results in unwanted premature electrical aging. We used dynamic atomic force microscopy (AFM) in the electric force detection regime to study the phase microseparation occurring in the salt-polymer blend. We found that in the spin-coated layers, the molten salt most often forms discotic, roughly self-organized sub-microdomains. Their size and density strongly depend on the polymer side chains and overall molecular weight, on the molten salt nature, and on their respective concentrations in solution. We show that the diode current is injected into the vicinity of the interface between the salt and polymer domains, since it is proportional to the perimeter of the salt domains per surface unit, as estimated from the AFM images. The electrical aging is attributed to the degradation of the interface between the polymer and salt domains. This is further corroborated by a number of electrical data.
Journal of Applied Physics 12/2004; 96(12):7219-7224. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A specific layer transfer process is developed so as to obtain multi-layer devices based on spin-coated polymers. This process is put to good use for embedding fine evaporated Ag layers between polyfluorene (PF) films with cyano side groups. The resulting structures, which are a stack with sequence Ag/PF/Ag/PF/Ag, exhibit reproducible electrical bistability. The difference between the “off” and “on” currents may extend up to eight orders of magnitude. Hence this effect might be used for producing memory devices.
[Show abstract][Hide abstract] ABSTRACT: SiC power MESFETs are studied for applications to power amplification in the microwave domain. A small-signal model is explained. It provides the intrinsic bias-dependent parameters of the devices, such as G<sub>m</sub>, C<sub>gs</sub>, C<sub>ds</sub>, C<sub>gd</sub>, and the current gain cut-off frequency f<sub>T</sub>. It is shown that f<sub>T</sub> and G<sub>m</sub> decrease with the drain to source bias V<sub>DS</sub>, because of self-heating effects. The model also includes temperature dependence and is in good agreement with experiments. This is a first step to non-linear modelling of SiC MESFETs.
[Show abstract][Hide abstract] ABSTRACT: The operation of 1-3 nm thick SOI MOSFETs, in double-gate (DG) mode and single-gate (SG) mode (for either front or back channel), is systematically analyzed. Strong interface coupling and threshold voltage variation, a large influence of substrate depletion underneath the buried oxide, the absence of drain current transients, and degradation in electron mobility are typical effects in these ultra-thin MOSFETs. The comparison of SG and DG configurations demonstrates the superiority of DG-MOSFETs: ideal subthreshold swing and remarkably improved transconductance (consistently higher than twice the value in SG-MOSFETs). The experimental data and the difference between SG and DG modes is explained by combining classical models with quantum calculations. The key effect in ultimately thin DG-MOSFETs is volume inversion, which primarily leads to an improvement in mobility, whereas the total inversion charge is only marginally modified.
IEEE Transactions on Electron Devices 04/2003; · 2.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have studied the well known spectral shift of polyfluorene blue light emitters, using either electrochemical cells or conventional organic light emitting diodes. From the recording of the electroluminescence spectra as a function of “stress” time, we conclude that the degradation mainly depends on the number of electrons which have passed through the conducting polymer, rather than on the hole current magnitude or the dissipated power.
The European Physical Journal Applied Physics 01/2003; · 0.71 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The paper investigates the characteristics of microwave transmission in coplanar waveguides (CPWs) fabricated on various silicon carbide substrates. Propagation constant and characteristic impedance measurements were performed at frequencies from 0.1 to 10 GHz. Different propagation modes are observed as a function of frequency and substrate resistivity. In the case of the low-loss structure, a quasi-TEM equivalent circuit model was developed from the available process parameters, taking into account the effects of electromagnetic fields in CPW structures.