Solid State Phenomena Vols. 156-158

Abstract: In this paper the new method for determination of luminescent centers concentration are discussed. While the possibility of electron traps determination and definition of its activation energy are suggested. The cathodoluminescent (CL) method was used. The determination of luminescent centers concentration in silicon oxide is based on the measurements of dependences of CL intensity on electron beam current. The presence and energy of activation of electron traps were studied by measurement of rise time and decay of luminescent band during the stationary irradiation of silica by electron beam.

Abstract: This paper reports a detailed study of the electrical activation and the surface morphology of 4H-SiC implanted with different doping ions (P for n-type doping and Al for p-type doping) and annealed at high temperature (1650–1700 °C) under different surface conditions (with or without a graphite capping layer). The combined use of atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning capacitance microscopy (SCM) allowed to clarify the crucial role played by the implant damage both in evolution of 4H-SiC surface roughness and in the electrical activation of dopants after annealing. The high density of broken bonds by the implant makes surface atoms highly mobile and a peculiar step bunching on the surface is formed during high temperature annealing. This roughness can be minimized by using a capping layer. Furthermore, residual lattice defects or precipitates were found in high dose implanted layers even after high temperature annealing. Those defects adversely affect the electrical activation, especially in the case of Al implantation. Finally, the electrical properties of Ni and Ti/Al alloy contacts on n-type and p-type implanted regions of 4H-SiC were studied. Ohmic behavior was observed for contacts on the P implanted area, whilst high resistivity was obtained in the Al implanted layer. Results showed a correlation of the electrical behavior of contacts with surface morphology, electrical activation and structural defects in ion-implanted, particularly, Al doped layer of 4H-SiC.

Abstract: This paper reviews the current status of graphene transistors as potential supplement to silicon CMOS technology. A short overview of graphene manufacturing and metrology methods is followed by an introduction of macroscopic graphene field effect transistors (FETs). The absence of an energy band gap is shown to result in severe shortcomings for logic applications. Possibilities to engineer a band gap in graphene FETs including quantum confinement in graphene Nanoribbons (GNRs) and electrically or substrate induced asymmetry in double and multi layer graphene are discussed. Novel switching mechanisms in graphene transistors are briefly introduced that could lead to future memory devices. Finally, graphene FETs are shown to be of interest for analog radio frequency applications.

Abstract: This study presents ionized impurity impacts on silicon nanowire MOS transistors. We calculate the current characteristics with a self-consistent three-dimensional (3D) Green’s function approach and show the effects of both acceptor and donor impurities on the physical electron properties. In particular, we emphasize that the presence of a donor induces different transport phenomena according to the applied gate bias. Our results show that the influence of a single impurity strongly depends on its position and induces high transistor performance variability with current modifications from 50% to two orders of magnitude.

Abstract: We develop a theory for scaling properties of quantum transport in nano-field effect transistors. Our starting point is a one-dimensional effective expression for the drain current in the Landauer-Büttiker formalism. Assuming a relatively simple total potential acting on the electrons the effective theory can be reduced to a scale-invariant form yielding a set of dimensionless control parameters. Among these control parameters are the characteristic length l and -width w of the electron channel which are its physical length and -width in units of the scaling length . Here is the Fermi energy in the source contact and is the effective mass in the electron channel. In the limit of wide transistors and low temperatures we evaluate the scale-invariant i-v characteristics as a function of the characteristic length. In the strong barrier regime, i. e. for long-channel behavior is found. At weaker barriers source-drain tunneling leads to increasingly significant deviations from the long-channel behavior.

Abstract: In the present paper we discuss effects due to high-energy ion bombardment of SiO2 layers with embedded Si nanocrystals (NCs), such as the formation of new Si NCs in such layers, amorphization of previously existing NCs, modification of NC size distribution, and modification of optical and electrical properties of NCs. These effects are identified as resulting from anisotropic strain - anisotropic heating in NCs-SiO2 layers under ion irradiation.

Abstract: Scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) have been used to investigate Si+-implanted amorphous silicon dioxide layers and the formation of Si nanoclusters. The microstructure of the Si doped silica films was studied by energy filtered transmission electron microscopy (EFTEM) in a 200 kV FEI Tecnai F20 TEM. The samples were amorphous, thermally grown 500 nm SiO2 layers on Si substrate doped by Si+ ions with an energy of 150 keV up to an atomic dopant fraction of about 4 at%. A thermal post-annealing leads to formation of silicon clusters with sizes 1-5 nm and concentrations of about 1018 cm-3. Respective cathodoluminescence spectra in the near IR region indicate such structural changes by appearance of an additional band at 1.35 eV as well as additional emission bands in the visible green-yellow region.

Abstract: The role of multi-pulse feedback in self-organized nanostructure (ripples) formation on silicon surface upon femtosecond laser ablation is investigated. For irradiation at constant intensity and pulse repetition rate, the previously postulated feedback effect of accumulated dose with in¬creasing number of pulses is confirmed and investigated in detail: both the modified surface area as well as the complexity and feature size of generated nanostructures increase with accumulated dose. More interestingly, at constant total incident dose (number of pulses times pulse energy) accumu¬lation and feedback depend strongly on temporal pulse separation. The feedback becomes increas¬ingly weaker with increasing time intervals between successive pulses, involving times up to one second and more before individual pulses act independently. In a first attempt to model this long-lived coupling, we find that conduction band electrons, produced by the preceding laser pulse, can provide, indeed, such feedback by facilitating coupling of subsequent pulses for substantial delays. However, the achieved time span of about a millisecond is still significantly shorter than observed experimentally.

Abstract: The set of quantum confinement levels in SiGe quantum wells (QW) was observed in the temperature range from 80 to 300 K by means of charge deep-level transient spectroscopy (Q-DLTS) and transport measurements. These observations proved possible due to a passivation of structure surface with organic monolayer deposition. Si/SiGe/Si structures with different Ge contents in SiGe layer were studied. The confined levels in passivated structures became apparent through DLTS measurements as various activation energies in temperature dependence of the rate of carrier emission from QW. It was found that the recharging of SiGe QWs and carrier emission accomplish due to thermo-stimulated tunneling. The steps in the current-voltage characteristics originated from direct tunneling via the confined states were found to determine the current flow at high fields.

Abstract: This paper summarises results on the design, fabrication and characterisation of one-dimensional (1D) Photonic Crystals (PCs) for silicon micro-photonics. Anisotropic and photo-electrochemical etching were used to obtain silicon wall arrays with a high aspect ratio. The characteristics of these wet etching techniques, including their advantages and disadvantages are considered. Optical reflection and transmission spectra of the photonic structures fabricated were characterised by Fourier Transform Infra-Red (FTIR) micro-spectroscopy over a wide spectral range of =1.5-14.5m. These measurements reveal that side-wall roughness impacts the optical properties of 1D PCs. Problems associated with Photonic Band-Gap (PBG) tuning in periodic structures infiltrated with nematic liquid crystals are discussed. A design of a composite 1D PC on an SOI platform for electro-tuning is proposed. The structure was fabricated and tuning due to an electro-optical effect with E7 liquid crystal filler was demonstrated.