Nanoporous Silicon Thin Films Research Articles

STUDY ON BAND STRUCTURE OF NANOPOROUS SILICON THIN FILM

We investigate the energy band structure of nanoporous silicon thin film using first principles calculation based on density functional theory (DFT) with the generalized gradient approximation (GGA). The calculation results show that the band gaps of nanoporous silicon increase with increasing porosity, increase with decreasing the thickness of matrix layer, and almost independent of the thickness of pore layer. Moreover, the band structure of nanoporous silicon can be transformed from indirect to direct gap on thin films of (111) and (110) faces. It will be the guidance and reference for the fabrication of porous silicon optoelectronic devices.

Thermal conductivity model for nanoporous thin films

Nanoporous thin films have attracted great interest because of their extremely low thermal conductivity and potential applications in thin thermal insulators and thermoelectrics. Although there are some numerical and experimental studies about the thermal conductivity of nanoporous thin films, a simplified model is still needed to provide a straightforward prediction. In this paper, by including the phonon scattering lifetimes due to film thickness boundary scattering, nanopore scattering and the frequency-dependent intrinsic phonon-phonon scattering, a fitting-parameter-free model based on the kinetic theory of phonon transport is developed to predict both the in-plane and the cross-plane thermal conductivities of nanoporous thin films. With input parameters such as the lattice constants, thermal conductivity, and the group velocity of acoustic phonons of bulk silicon, our model shows a good agreement with available experimental and numerical results of nanoporous silicon thin films. It illustrates that the size effect of film thickness boundary scattering not only depends on the film thickness but also on the size of nanopores, and a larger nanopore leads to a stronger size effect of the film thickness. Our model also reveals that there are different optimal structures for getting the lowest in-plane and cross-plane thermal conductivities.

Study on Lattice Thermal Conductivity of Silicon Thin Film with Aligned Nano-pores

A frequency-dependent phonon Boltzmann transport equation solver is presented to numerically study the lattice thermal conductivity of nano-porous silicon thin film based on the Discrete Ordinates Method. We find that not only characteristic size and porosity, but also the shapes of both unit cell and pore do affect the lattice thermal conductivity. Therefore, we arranged a series of computational cases by the orthogonal design method to investigate the influence of geometric parameters Lx, Ly, ax/Lx and ay/Ly (Lx is unit cell length, Ly unit cell width, ax pore length and ay pore width). Furthermore, a non-linear regression model is established depending on the data obtained from those computational cases. The result shows that lattice thermal conductivity of nano-porous silicon thin film decreases obviously with the increase of ax/Lx and ay/Ly. Among these four geometric parameters, ay/Ly is the most significant factor while Lx and Ly have little effect on lattice thermal conductivity. The proposed regression model can offer useful suggestion for the fabrication of nano-porous silicon thin film with lower lattice thermal conductivity.

Cross-plane heat conduction in nanoporous silicon thin films by phonon Boltzmann transport equation and Monte Carlo simulations

The study on the heat conduction in nanoporous silicon structures has drawn much attention due to their significance for developing highly-efficient thermoelectric materials. In the nanostructures whose characteristic lengths are comparable to the phonon mean free path (MFP), ballistic transport and boundary scattering will lead to the size-dependence of effective thermal conductivity. In the present work, the cross-plane thermal transport along the pore axial direction in nanoporous silicon thin films (i.e. silicon thin films crossed by nanoscale cylindrical channels) is investigated by using the phonon Boltzmann transport equation (BTE) and the Monte Carlo (MC) simulations. It is found that the effective thermal conductivity varies not only with the porosity but also with the film thickness and pore radius. The smaller the film thickness or the pore radius, the smaller is the effective thermal conductivity for a given porosity. An analytical model for the cross-plane effective thermal conductivity of nanoporous silicon thin films is obtained on the basis of the phonon BTE. The model can capture the size effects due to the film thickness and the pore radius simultaneously, and be reduced to the Fourier’s law based model for macroporous materials. Good agreements have been achieved between the present model and the MC simulations. Our model can be useful for predicting and controlling phonon transport for thermoelectric devices.

Thermal conduction in nano-porous silicon thin film

Controlling the thermal conductivity of thermoelectric materials continues to be a goal for energy conversion applications. The Phonon Boltzmann Transport Equation is solved by using the Discrete Ordinates Method to numerically study the phonon thermal conductivity of nano-structured silicon thin film with pores in this study. The effects of the film thickness, film porosity, and porous structure are concerned. The numerical results show that the nano-pores are able to reduce the thermal conductivity of the silicon thin film sharply by the phonon boundary scattering, and the scattering boundary area has significant effect on the thermal conductivity. The method of local angle distribution between heat fluxes is introduced for the first time to optimize the pore placement for reducing the thermal conductivity.

Colored porous silicon as support for plasmonic nanoparticles

Colored nanoporous silicon thin films were employed as dielectric spacing layers for the enhancement of localized surface plasmon (LSP) polaritons. Upon formation of Au nanoparticles (Au-NPs) on these layers, a visible color change is observed due to multiple LSP resonance excitations. Far-field effects were assessed by angle-resolved reflectometry. Resonance enhancements, particularly for s-polarized light, account for the observed color change and are discussed in terms of effective medium and Mie scattering theory. Enhancements of the electric field strengths in the near-field and of the absorption in the substrate were deduced from finite difference time domain calculations and exceed considerably those of the non-porous Au-NP/Si interface. First results of improved photoelectrocatalytic hydrogen evolution at these interfaces are discussed. Samples were prepared by varied procedures of metal assisted etching and dry etching with XeF2. Structural and chemical properties were investigated by scanning electron and atomic force microscopy as well as energy dispersive x-ray analysis.

XPS study of palladium sensitized nano porous silicon thin film

Nano porous silicon (PS) was formed on p-type monocrystalline silicon of 2–5 Ω cm resistivity and (100) orientation by electrochemical anodization method using HF and ethanol as the electrolytes. High density of surface states, arising due to its nano structure, is responsible for the uncontrolled oxidation in air and for the deterioration of the PS surface with time. To stabilize the material PS surface was modified by a simple and low cost chemical method using PdCl2 solution at room temperature. X-ray photoelectron spectroscopy (XPS) was performed to reveal the chemical composition and the relative concentration of palladium on the nanoporous silicon thin films. An increase of SiO2 formation was observed after PdCl2 treatment and presence of palladium was also detected on the modified surface. I–V characteristics of Al/PS junction were studied using two lateral Al contacts and a linear relationship was obtained for Pd modified PS surface. Stability of the contact was studied for a time period of around 30 days and no significant ageing effect could be observed.