Ab Initio Study of Electronic Transport in Cubic-HfO2 Grain Boundaries

In polycrystalline materials the grain boundaries (GBs) are particularly important as they can act as a sink for atom defects and impurities, which may drive structural transformation of the materials and consequently modify their properties. Characterising the structure and properties of GBs is critical for understanding and controlling material property. Here, we investigated how GBs can modify the structural, electronic, and transport properties of the polycrystalline material . In general, grain boundaries are considered to be detrimental to the physical stability and electronic transport in . Anyway, studying by first principles the two most stable and common types of GBs, the tilt and the twist, we found substantial differences on the impact they have on the material properties. In fact, while tilt defects create channels of different sizes and shapes in hafnia along which the electronic transport is stronger in relation to leakage current through GBs, twist defects create a sort of amorphous structure that tends to resemble the bulk and which is independent of the number of rotated planes/atoms

Doped and codoped silicon nanocrystals: The role of surfaces and interfaces

Si nanocrystals have been extensively studied because of their novel properties and their potential applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. These new properties are achieved through the combination of the quantum confinement of carriers and the strong influence of surface chemistry. As in the case of bulk Si the tuning of the electronic, optical and transport properties is related to the possibility of doping, in a controlled way, the nanocrystals. This is a big challenge since several studies have revealed that doping in Si nanocrystals differs from the one of the bulk. Theory and experiments have underlined that doping and codoping are influenced by a large number of parameters such as size, shape, passivation and chemical environment of the silicon nanocrystals. However, the connection between these parameters and dopant localization as well as the occurrence of self-purification effects are still not clear. In this review we summarize the latest progress in this fascinating research field considering free-standing and matrix-embedded Si nanocrystals both from the theoretical and experimental point of view, with special attention given to the results obtained by ab-initio calculations and to size-, surface- and interface-induced effects

First Principle Studies of B and P Doped Si Nanocrystals

The properties of n- and p-doped silicon nanocrystals obtained through ab initio calculations are reviewed here. The aim is the understanding of the effects induced by substitutional doping on the structural, electronic and optical properties of free-standing and matrix-embedded Si nanocrystals. The preferential positioning of the dopants and their effects on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through total-energy considerations. The localization of the acceptor and donor related levels in the band gap of the Si nanocrystals, together with the impurity activation energy, are discussed as a function of the nanocrystals size. The dopant induced differences in the optical properties with respect to the undoped case are presented. Finally, the case of B and P co-doped nanocrystals is discussed showing that if carriers are perfectly compensated, the Si nanocrystals undergo a minor structural distortion around the impurities inducing a significant decrease of the impurities formation energies with respect to the single doped case. Due to co-doping, additional peaks are introduced in the absorption spectra, giving rise to a size-dependent red shift of the absorption spectra

Defects and strain enhancements of second-harmonic generation in Si/Ge superlattices

International audienceStarting from experimental findings and interface growth problems in Si/Ge superlattices, we have investigated through ab initio methods the concurrent and competitive behavior of strain and defects in the second-harmonic generation process. Interpreting the second-harmonic intensities as a function of the different nature and percentage of defects together with the strain induced at the interface between Si and Ge, we found a way to tune and enhance the second-harmonic generation response of these systems. (C) 2014 AIP Publishing LLC

Engineering Silicon Nanocrystals: Theoretical study of the effect of Codoping with Boron and Phosphorus

We show that the optical and electronic properties of nanocrystalline silicon can be efficiently tuned using impurity doping. In particular, we give evidence, by means of ab-initio calculations, that by properly controlling the doping with either one or two atomic species, a significant modification of both the absorption and the emission of light can be achieved. We have considered impurities, either boron or phosphorous (doping) or both (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupy nearest neighbor sites near the surface. In addition, such systems present band-edge states localized on the impurities giving rise to a red-shift of the absorption thresholds with respect to that of undoped nanocrystals. Our detailed theoretical analysis shows that the creation of an electron-hole pair due to light absorption determines a geometry distortion that in turn results in a Stokes shift between adsorption and emission spectra. In order to give a deeper insight in this effect, in one case we have calculated the absorption and emission spectra going beyond the single-particle approach showing the important role played by many-body effects. The entire set of results we have collected in this work give a strong indication that with the doping it is possible to tune the optical properties of silicon nanocrystals.Comment: 14 pages 19 figure

Optoelectronic interconnects for integrated circuits - Achievements 1998 – 2001 - Silicon based interconnects

Silicon is the leading semiconductor in the microelectronic industry.Not only silicon is dominating the past. Its technology is predicted to improve with the increasing demand for higher complexity integrated circuits and to maintain its role for a long time to go. At the same time the enormous progress of communication technologies in the last years has increased the demand for efficient and low-cost optoelectronic functions. For several present and future applications, photonic materials - in which light can be generated, guided, modulated, amplified, and detected - need to be integrated with standard electronic circuits in order to combine the information-processing capabilities of electronics data transfer and the speed of light. In particular, chip-to-chip or even intrachip optical communications all require the development of efficient optical functions and their integration with state-of-the-art electronic functions.The use of optical interconnects among devices is indeed a solution to what is known as the "interconnects bottleneck". The total length of interconnecting conductors within a chip is continuously increasing towards values of several km in 1 cm2. This produces problems related to the Joule effect, heat dissipations and slow down of the system related to resistance and capacity of the system. The replacement of electrical interconnects with optical ones would solve the problem. However Si is characterized by an indirect bandgap and by a weak electro-optic effect. It is therefore not suitable, in its bulk form, for the implementation of fundamental optical functions such as light sources and modulators. Several approaches need hence to be investigated. The European Commission focussed a research program on the interconnects problem launching several projects within the Microelectronic Advanced Research Initiative (MEL-ARI).In the case of Si optoelectronics we are faced with a material problem to circumvent the physical inability of bulk silicon to emit light. Several strategies have been considered and explored. They can be divided in three different groups: (i) low dimensional systems, (ii) emitting impurities and (iii) semiconductor silicides. For low dimensional systems the cases of porous silicon, silicon nanocrystals, Si/CaF2 and Si/SiO2 quantum wells have been explored. In all of these cases nanometer sized silicon is embedded within an insulating host. The quantum confinement has several effects: it increases the radiative probability, it decreases the non-radiative recombination routes, it increases the energy of emission. Indeed intense room temperature photoluminescence can be achieved. The main problem here is the carrier injection to achieve electroluminescence. Several routes have been followed and several devices operating at room temperature were fabricated. The challenges here are to efficiently inject carriers in a semi-insulating material and to have sufficiently low operating voltages. The most promising case of light emitting impurities is that of Er in Si. Room temperature operating devices were fabricated. A great effort was spent in comprehending the basic physical mechanisms. The main advantage here is that standard technology based on single crystal silicon can be used introducing erbium as a dopant. Particularly interesting is the light emitting transistor in which erbium, introduced in the collector-base region, is excited through electrons injected from the emitter. The third approach of semiconducting silicides is based on the observation that these silicides can be grown on Si and should have a direct bandgap. Indeed, luminescent devices have been fabricated with beta-FeSi2 precipitates formed by ion implantation in a Si diode. The quantum efficiency obtained from these devices is however not yet sufficiently high. Though none of the approaches is at a stage ready for application, this European effort can be considered extremely successful. In fact, this booklet reports a picture of the state-of-the-art in Si-based optoelectronics with major improvements with respect to the past both in terms of materials properties, understanding and device performances. Still some work is needed to obtain real applications; nevertheless the obtained results represent a solid basis for future developments

First-principles optical properties of Si/CaF2 multiple quantum wells

The optical properties of Si/CaF2 multiple quantum wells are studied ab initio by means of the linear-muffin-tin-orbital method. In particular, we investigate the dependence of the optoelectronic properties on the thickness of the Si wells. We find that below a well width of similar to 20 Angstrom, new transitions appear in the optical region with an evident polarization dependence. The oscillator strength of these transitions shows a dramatic increase as the width of the Si well decreases. A comparison is made with recent experimental work on similar systems. Our results show that quantum confinement and passivation are necessary in order to have photoluminescence in confined silicon-based materials

Engineering quantum confined silicon nanostructures: ab-initio study of the structural, electronic and optical properties

Chapter V

Optical properties of Si/CaF2 superlattices

We present a first-principle theoretical study of the dielectric functions of Si/CaF2 superlattices. In particular, we investigate how the optical response depends on the thickness of the Si layers. Our results show that for very thin Si slabs (well width less than ~20 Å) optical excitation peaks are present in the visible range. These peaks are related to strong transitions between localized states. Moreover, the static dielectric costant is considerably reduced. From the comparison made with recent experimental data on similar systems we conclude that the quantum confinement, a good surface passivation and the presence of localized states are the key ingredients in order to have photoluminescence in confined silicon based systems

First principles optical properties of low dimensional silicon structures

The book provides a snapshot of the state of the art and points out directions for future research in such different subjects as photonic band gap crystals, semiconductor quantum dot and wire lasers, silicon optoelectronics, carbon-based nanostructure physics, polymer based nano-composite and quantum wires, DNA nano-technology and silicon bio-compatibility, nano-scale optical characterisation, spray and cluster deposition, self assembly, imprint technology, quantum computing and quantum dot-based computation