Micro- and nano-structures in silicon studied by DLTS and scanning probe methods

Presently there is a high interest in silicon-based optical devices that would render possible the development of fully silicon-based optoelectronics. Being an indirect gap semiconductor, silicon is poorly efficient as light emitter since radiative emission is limited by carrier recombination at non-radiative centers. One of the possible approaches to enhance the radiative emission from Si is the controlled introduction of micro- (dislocations) or nano- (nanocrystals) structures, which, providing quantum confinement of free carriers, prevent their diffusion towards non-radiative channels. Dislocations introduced in silicon by plastic deformation and Si nanocrystals embedded in amorphous silicon matrix have been investigated by junction spectroscopy and scanning probe microscopy methods

Opto-electronic behavior of silicon used for ULSI

The Laboratory of Chemistry and Physics of Semiconductor (LCPS) is active and internationally known since the early eighties in the field of growth and characterization of semiconductors, with major interests about silicon based materials. In this frame the electrical, structural and optical properties of silicon have been studied in details, with the simultaneous development of growth processes and characterization apparatuses. In particular, the liquid phase epitaxy technique has been applied to grow erbium-doped silicon and the LBIC (Light beam induced current) technique has been developed for the study of extended defects in ULSI and solar silicon

Dislocations, extended defects and interfaces at nanoparticles as effective sources of room temperature photo- and electro-luminescence in silicon and silicon-germanium

In view of demonstrating the technical feasibility of silicon based, high efficiency room temperature light emitting devices (LED), the primary objectives of this Project are: 1. to find practical approaches for the creation of suitable dislocation structures or of alternative structures (like misfit dislocations in Si-Ge heterostructures, dislocation loops at Si/SiO2 interfaces and Si nanocrystals in amorphous silicon) such to enhance the quantum efficiency of the dislocation-related luminescence (DRL) or of the intrinsic emission of silicon at room temperature in view of applications for silicon-based LEDs. To this purpose, several processes of dislocation generation will be employed, in addition to the conventional plastic deformation at high temperature 2. to obtain a clear understanding of the physics of the dislocation-assisted light emission, of the correlations between the dislocation structure and their emission properties, of the role of carbon, oxygen and other light impurities like hydrogen and nitrogen as well as of metallic impurities on their emission properties and of the correlation between device surface quality, configuration and luminescence yield 3. to carry out theoretical investigations on the correlation between core structure of extended defects and optical and electronic properties of dislocations About the first topic, the photo-(PL) and electro-luminescence (EL) emission of dislocations generated by different means will be studied in clean conditions. Different dislocation structures will be generated by special deformation procedures, by ion implantation or by the injection of self-interstitials or vacancies generated in a silicon matrix during the growth process of a precipitate having a different molar volume of the matrix, as it happens with SiO2, SiC and Er-oxide in heat treated carbon loan and carbon doped Cz silicon, carbon- implanted FZ silicon and Er-implanted Cz silicon. Also Si-Ge heterostructures, interesting because potentially compatible with conventional microelectronic processes will be studied, before and after strain relaxation. The second topic will be treated by studying the effect of all the mentioned impurities on clean dislocations prepared by plastic deformation of Fz or Cz silicon, whose cleanliness will be tested by photoluminescence (PL), Deep Level Transient Spectroscopy (DLTS) and lifetime mapping measurements. The effect of impurities on the light emission features of oxygen precipitates will be also studied. Eventually, the correlation between dislocation structure and optical properties will be studied using different computational methods, as the scc-DFTB (self consistent charge functional based tight binding method) and the AIMPRO (ab initio modeling program) http://intas.mater.unimib.it

Nanocrystalline silicon films for photovoltaic and optoelectronic applications

The primary aim of this research project is to develop computational tools capable of assisting the design of a new nc-Si growth process with a Low Energy variant of a Plasma Enhanced Chemical Vapour Deposition (LEPECVD) reactor, addressed at the deposition of nc-Si films for both photovoltaic and optoelectronic applications. This objective is for many aspects really at the frontier of the today knowledge of multiphase materials, and for this reason requires the involvement of different theoretical and experimental tools and expertises. An LEPECVD reactor is actually in full use in one of the partner’s laboratories and has been already demonstrated to be a very powerful tool for high growth rate, high quality epitaxial silicon and silicon-germanium films. The modelling activities include Molecular dynamics (MD) and abinitio calculations applied to the simulation of the growth of nc-Si grains in a amorphous silicon (a- Si) matrix, to the evaluation of the best a-Si/ nc-Si ratio and the elastic/plastic effects consequent to the presence of nanocrystals of silicon in the a-Si matrix and to the presence of a grain boundary phase, which could be responsible of unwanted carrier recombination processes. The computational tools will be also used to evaluate the band offset vs microstructure and strain, in view of the fine-tuning of the optoelectronic properties. As the computer modelling could not be granted for a complete forecasting of the role of process parameters on the local nanostructural aspects and associated physical properties, additional theoretical studies on quantum confinement will be carried out. Such theoretical studies will be based on the results of systematic measurements of optoelectronic properties of nc-Si. The development of the computer modelling and of theoretical studies on quantum confinement will be paralleled, from the very early stage of the Project, by nc-Si growth experiments and by state of the art morphological, microstructural, compositional, electrical and optoelectronic characterization