Optical properties of silicon rich silicon nitride (SixNyHz) from first principles

The real and imaginary parts of the complex refractive index of SixNyHz have been calculated using density functional perturbation theory. Optical spectra for reflectivity, adsorption coefficient, energy-loss function (ELF), and refractive index, are obtained. The results for Si3N4 are in agreement with the available theoretical and experimental results. To understand the electron energy loss mechanism in Si rich silicon nitride, the influence of the Si doping rate, of the positions of the dopants, and of H in and on the surface on the ELF have been investigated. It has been found that all defects, such as dangling bonds in the bulk and surfaces, increase the intensity of the ELF in the low energy range (below 10 eV). H in the bulk and on the surface has a healing effect, which can reduce the intensity of the loss peaks by saturating the dangling bonds. Electronic structure analysis has confirmed the origin of the changes in the ELF. It has demonstrated that the changes in ELF is not only affected by the composition but also by the microstructures of the materials. The results can be used to tailor the optical properties, in this case the ELF of Si rich Si3N4, which is essential for secondary electron emission application

Special Issue "50th Anniversary of the Kohn-Sham Theory-Advances in Density Functional Theory"

The properties of many materials at the atomic scale depend on the electronic structure, which requires a quantum mechanical treatment. The most widely used approach to make such a treatment feasible is density functional theory (DFT), the advances in which were presented and discussed during the DFT conference in Debrecen. Some of these issues are presented in this Special Issue

Low pressure chemical vapor deposition of silicon nitride films from tridimethylaminosilane

In this study amorphous stoichiometric silicon nitride films were synthesized by low pressure chemical vapor deposition (LPCVD) using tri(dimethylamino) silane (TDMAS) and ammonia (NH3). The growth kinetics were determined as a function of temperature in the range of 650 - 900 °C, total pressure in the range of 0.15 - 0.60 Torr, and NH3/TDMAS flow ratio in the range of 0 - 10. At constant condition of pressure (0.5 Torr), TDMSA flow rate (10 sccm) and NH3 flow rate (100 sccm), the deposition rate of as-deposited silicon nitride films was found to follow an Arrehnius behavior in the temperature range of 650 - 800 °C with an activation energy of 41 ± 3 kcal mol-1. The film characterizations including compositional, structural, physical, optical and mechanical properties were determined by using XPS, RBS, X-ray diffraction, Nanospec interferometry, Ellipsometer, FTIR, UV Visible, as well as other techniques. The results demonstrated the feasibility of using TDMAS in the synthesis of high quality silicon nitride films by LPCVD

The formation of silicon nitride from trisilylamine and ammonia

Silane gas has been used for three decades as a precursor for plasma enhanced chemical vapour deposition processes but is unsustainable in the longer term due to the extremely hazardous nature of the compound. Alternative precursor materials have been proposed but have proved to be largely incompatible with the chemistry of the deposition process or the requirements of semiconductor process technology. One compound with the chemical and technological potential as a precursor for silicon nitride deposition is trisilylamine. Calculation of the Gibbs free energy change for the formation of silicon nitride from the reaction of trisilylamine and ammonia demonstrates that the reaction IS thermodynamically feasible as is the reaction involving silane with ammonia. The standard molar enthalpy of formation for trisilylamine was obtained from a semiempirical molecular orbital calculation while the standard molar entropy of formation was determined from spectroscopic data in the absence of a calorimetric value. Thermodynamic properties have been calculated for a range of aminated species using semi-empirical methods and entropy vs. molecular weight equations. These species are potential intermediates in a plasma discharge of trisilylamine and ammonia, with their successive combination leading to the deposition of a film of silicon nitride. Thermodynamic values for reactions involving the formation, propagation and termination of radical species of trisily lamine and ammonia have been determined and a mechanism is proposed for the deposition of silicon nitride films by plasma enhanced chemical vapour deposition. These results indicate that there is no thermodynamic barrier to the use of trisilylamine as a precursor with ammonia gas for the plasma enhanced deposition of silicon nitride films

Development and analyses of innovative thin films for photovoltaic applications

In solar cell current research, innovative solutions and materials are continuously requested for efficiency improvements. Si-based technology rules over 95% of the market, with silicon heterojunction (SHJ) solar cell reaching 26.7% record efficiency. Nonetheless, hydrogenated amorphous silicon (a-Si:H) layers employed in the structure still have challenges, resolvable with oxygen/nitrogen inclusion. In parallel, new technologies based on different materials still lack in the market due to stability issues or low efficiencies. However, a preliminary study of their properties creates a deeper knowledge exploitable in photovoltaic application. In this perspective, we investigated both innovative Si-based materials (nanocrystalline and amorphous silicon oxy-nitride and oxide thin films, nc-SiOxNy, a-SiOxNy and a-SiOx, respectively) and innovative materials (perovskite lanthanum-vanadium oxide LaVO3 thin films, indium gallium nitride InxGa1-xN and aluminium indium gallium nitride AlxInyGa1-x-yN layers) for solar cell concepts. Different deposition conditions have been employed to extract their influence on compositional, optical, and electrical properties. The study on nc-SiOxNy layers by conductive atomic force microscopy (c-AFM) and surface photovoltage (SPV) has allowed to clarify O, N, and B content, and annealing treatment role on microscopic transport properties. On a-SiOx and a-SiOxNy layers, by spectral ellipsometry, Fourier transform infrared spectroscopy, photoconductance decay and SPV, we can conclude that moderate insertions of O/N in a-Si:H lead to a decrease of optical parasitic absorption, preserving the passivation quality of the layers. The measurements by AFM and Kelvin probe force microscopy on LaVO3 have clearly shown that it is a poor charge-transport medium, thus not suitable for photovoltaic applications. The analysis on InGaN and AlGaInN by SPV measurements has shown how low In content, Si doping and no misfit dislocations in InGaN/GaN structure cause less recombination processes at the interface, whereas, the strain relaxation (tensile and compressive) with the formation of pinholes produces better interfaces in the AlGaInN/GaN samples

Highly transparent and highly passivating Silicon nitride for solar cells

This thesis concerns the optimisation and application of Silicon nitride (SiNx) films for silicon solar cells. Systematic and comprehensive studies of SiNx properties are undertaken to advance (i) the technology of SiNx synthesised by plasma enhanced chemical vapour deposition (PECVD), and (ii) the understanding of recombination at SiNx-passivated silicon surfaces. We examine the film properties of SiNx prepared by a microwave/radio-frequency dual-mode PECVD reactor. It is shown that there is no universal correlation between surface recombination and (i) bulk structural properties such as chemical bond densities, and (ii) bulk optical properties such as refractive index and extinction coefficient. Results of this study repudiate the common perception that surface recombination decreases as SiNx becomes Si-rich. The finding introduces the potential to independently control the optical and surface recombination properties of SiNx. This is of great importance for the industrial application of SiNx films to photovoltaic cells, as it allows the front surface transmission to be maximised while still attaining outstanding surface passivation. We attain a low and relatively constant surface recombination over a wide range of SiNx refractive indices. Notably, the behaviour is observed on several types of silicon surface surfaces-planar, textured, p-type, n-type, diffused and undiffused-with direct relevance to most silicon solar cell structures. The results confirm that the trade-off between the optical transmission and surface recombination is circumvented. In specific, we attain a highly transparent and highly passivating SiNx film. The value of this film is demonstrated on an n-type interdigitated back contact solar cell with no front surface diffusion, which makes the cell highly susceptible to the front surface passivation. On such a cell, the optimum SiNx developed in this thesis enables a conversion efficiency of 24.4 +/- 0.5% under standard testing conditions (25 Celsius degrees, AM1.5G spectrum). Besides the significant improvement in optical transmission and surface passivation, the results of this thesis also advance the current understanding of recombination at SiNx-passivated silicon surfaces. It is found that an increase in recombination of the textured surfaces is related to the presence of vertices and/or edges of the pyramids rather than to the presence of {111}-orientated facets. Furthermore, this thesis demonstrates that the increase in recombination introduced by (i) a lower pressure, leading to a higher refractive index, (ii) a higher NH3:SiH4 ratio, leading to a lower refractive index, and (iii) the vertices and/or edges of the pyramids, is primarily attributable to an increase in interface defect density rather than a decrease in SiNx charge density. In addition, we hypothesise that the increase in interface defect density is caused by an ion bombardment of the silicon surface at a lower pressure, and by an excessive incorporation of NHb radicals into the SiNx film network at a higher NH3:SiH4 ratio. The satisfactory resolution of the trade-off between optical transmission and surface passivation, and the improved understanding of recombination at SiNx-passivated silicon surfaces, represent significant contributions to the science and technology of silicon solar cells