Porous silicon preparation and measurement details 13
Preparation of porous silicon
In the initial stage of anodization, the dissolution of Si atoms creates indentations on the exposed Si surface. In the initial phase of the dissolution process, the transport of holes and F ions to the solid/liquid interface is influenced by the electrical resistance of the c-Si wafers or the electrolyte.
Characterization measurement details
- UV-Vis-NIR specular reflectance spectroscopy
- Fourier transform infrared spectroscopy
- Photoluminescence spectroscopy
- Raman spectroscopy
The measured reflectance spectra of the PS films are then corrected for any spectral dependence of the reference mirror. The FTIR specular reflectance spectra of the PS layer presented in the following sections of this thesis are all corrected for the spectral dependence of the reference mirror.
When performing the algebra of Eq. 3.6), the reflectance of the film-substrate system will be given as The procedure makes it necessary to consider two adjacent interference fringes in the specular reflection of the material. The spectral characteristics are strongly influenced with the changes in the crystalline structure of the material [1-3, 26].
Theoretical formulation for the determination of refractive
Waves at rough interfaces
½ is generally a complex number; the real part corresponds to the fraction of coherent radiation in the regular direction of propagation and the imaginary part corresponds to the surface diffraction. In such systems, the intensity equivalent ½ is denoted as the interface modulation coefficient, which corresponds to the intensity of coherent radiation in the direction of normal propagation. 1-) would mean both loss of coherence and size in the regular direction of propagation at the interface.
Waves in inhomogeneous medium
Mathematical expressions for specular reflectance and
When performing the algebra of Eq. 3.3), the permeability of the film-substrate system would be given as The optical and scattering parameters for the simulation of the spectra are given in Sect. The optical and scattering parameters for the simulation of the spectra are given in Sect.
Interference conditions
Determination of interference fringe order, thickness and
- Determination of interference fringe order, m
- Determination of film thickness, d
- Waves in PS media
- Waves at PS interfaces
- Mathematical expression for the specular reflectance of
An alternative method which is only applicable in an optically thin region of the spectrum, i.e. the region of lower interference orders is given below. In the case of PS, for the air/PS film interface, specular reflectance r12' and transmittance t12' for an electromagnetic wave, with due consideration of the fractional coverage of Si and voids and the surface roughness of the constituents (in this case, the roughness is only for Si), is given by With these considerations, the specular reflectance of the PS film-Si substrate system at an angle of incidence from the normal, for a plane wave, is now given.
Numerical simulations
The numerical values of the optical parameters (n, k and d) and the scattering due to inhomogeneity and interface roughness for the simulation of the spectra are given below. The inputs for the determination of d and the results of the iterations are summarized in Table 3.1. It is observed in Table 3.1 that the results of the iterations oscillate, eventually converging to a value very close (~ 1% error) to the actual thickness of the film.
Summary
To improve the accuracy in determining d, it would be sufficient for the iteration process to end with d2, which is the closest estimate of d.
A closer view of the extreme interference goes towards an optically thin region in the measured specular reflectance of PS fabricated on c-Si wafer with resistivity 0.1-0.5 cm under 25 mA/cm2 current density for 2 min. The effect on the microstructure is simply a consequence of the effect of anodizing current density on the PS formation kinetics. This results in the blue-shift of the PL peak position with the increase of the excitation energy.
Studies on growth rate and refractive index of porous
High B-doping concentration wafers
As a first step in the determination procedure, the interference pattern in the measured reflectance spectrum is fitted through Eq. At this stage of the analysis, the mere similarity of the interference pattern in the measured spectrum is sufficient. After the determination of m for the measured spectrum, the RHS of Eq. 3.11), i.e. the product of m and the corresponding wavelength, which has been scaled down by a factor of 2, is plotted against the corresponding wavelength and is shown as in Fig.
Low B-doping concentration wafers
As a first step in the determination of neff and d, the order of the interference fringe is determined for each interference fringe in the specular reflection similar to the case of PS prepared on wafers with high B-doping concentration. A closer view of the interference fringes that are considered for determining the interference order is shown in Fig. The interference fringes are labeled with the interference fringe orders determined through Method II of Chapter 3.
Effects of anodization conditions on growth rate and effective
- Effects of anodization time on growth rate and
- Effects of anodization current density on growth rate
- Effects of resistivity on growth rate and effective
Summary
In the literature, there is even a detailed study on the choice of EMT for the determination of the refractive index depending on the porosity of the material [45]. This controversial scenario in the PL of PS has motivated to identify the origin of the phenomenon in PS. The variations of effective refractive index and growth rate under anodizing conditions are explained based on the effects of anodizing conditions on the microstructure of PS.
A schematic of the setup used for the preparation of PS is shown in Figure. Depending on the fit and non-fit of the measured interference pattern with EMT, the determination is made.
Experimental results on photoluminescence of PS
- Photoluminescence spectra
- Photoluminescence excitation spectra
Figures 5.1 and 5.2 show the PL spectra of the representative PS prepared under different anodizing conditions at different excitations. Since the PL spectra have already been corrected for the excitation photon flux, Eq (5.1) is independent of the excitation flux. The dependent parameters for the PLE spectra are the same as for the PL spectra and can be given by Eq.
Theoretical simulation of optical absorption
Equation (5.2) is that, although at normal incident conditions, the spectral characteristics of optical absorption are not significantly affected even when averaged over all oblique angles of incidence (e.g. 0 to /2) with equal ratios of s and p polarizations . . 5.2) in its simplest form is sufficient for the simulation of the optical absorption spectrum. So in general the reflection with all possible oblique angles of incidence or at least the normal incident reflection as in Eq. 5.2) shall be used for the simulation of the optical absorption spectrum in the analysis of the PLE spectrum for solids. Theoretical simulations of optical absorption spectra by bulk c-Si optical constants for different thicknesses and zero reflection conditions are shown in Fig.
Comparison of simulated optical absorption with PLE
However, above 3.4 eV, which is the onset of strong absorption, the spectra show a clear dependence on the thickness and reflectance of the film.
Structure of PS
The core of Si, the region between the pores, in PS is defect-free and also protected by the surrounding covalently bonded Si atoms from attack by chemical species. To summarize, PS is described as c-Si structure with impregnated pores of various shapes and sizes that would make labyrinth penetration in crystalline matrix. Therefore, PS is not only a composition of Si and voids, but also composed of certain chemical compounds such as hydrides and oxides of Si.
Si atoms in the pore walls are exposed to the environment and are vulnerable to attack by reactive chemical species such as oxygen, hydrogen, etc. Depending on the available Si surfaces, which is controlled by the anodization conditions and reactive chemical species present in the environment, the partial presence of chemical compounds in the pore walls varies. In the discussions on the structure of PS (Section 5.4), it has been observed that the Si bonds in the pore walls are passivated by reactive chemical species such as oxygen, hydrogen, etc.
Understanding the experimental data
- Broad PL spectrum of PS
- Excitation energy dependent peak position
- Dissimilarities between PLE and absorption
- Luminescence enhancement with anodization current
Now it is to understand the blue-shift of the PL peak position with the excitation energy. This is precisely the reason, in Eq. 5.5), the temperature is explicitly shown as a function of , the energy of excitation. There is another feature in the PLE of efficient luminescent PS - the downward slope of the spectra above 3.4 eV gradually flattens as the emission energy increases.
Summary
The spectral lineshape of bulk c-Si of wafers with high B doping concentration is intrinsically asymmetric, and the asymmetry increases in PS with the increasing anodization current density. This observation supports that Si in PS retains the optical properties of the respective bulk c-Si wafer. 6.9, it is deduced that Si in PS retains the RS properties of the respective bulk c-Si wafer.
Studies on Raman scattering in porous silicon 87
Low B-doping concentration wafers
Figures 6.4 and 6.5 show the measured single-phonon RS PS prepared on wafers with low B doping concentration using different anodization current densities and their. The decrease in RS intensity with increasing anodization current density can be understood as a consequence of the decrease in the number of Si atoms per unit volume. Therefore, the RS intensity decreased with the anodizing current density, as shown in Fig.
High B-doping concentration wafers
In Chapter 5 it was only shown that the spectral characteristics of the optical absorption of Si regions in PS are the same as those of the bulk c-Si for all anodizing current densities and not the strength of the optical absorption. The asymmetric line shape and the peak position are also very strongly dependent on the wavelength. While it is possible that the Si regions in PS could have the optical properties of the respective bulk c-Si wafer, this has not yet been explicitly shown.
One-phonon RS peak intensity with B-doping
Fano effect
6.11(b) and (c), is mainly due to intervalence band electronic RS [37], and increases with the B doping concentration. These variations in the line shape asymmetry with the B doping concentrations are similar to those observed in bulk c-Si wafers. The line shape asymmetry in the RS of PS is fitted with the Fano function of Eq.
Summary
It is an obvious fact that the surface atoms are more abundant in PS than in the c-Si wafer. With the increase of anodizing current density, the number of surface atoms in PS increases from the c-Si wafer value, reaches a critical value and then slowly decreases. The variations in q for PS fabricated on cm-resistivity c-Si wafer with the anodization current density are comparable to that of the number of surface atoms when recorded under an excitation wavelength of 632.8 nm.
For this purpose, only simple multilayer structures of the form alternating high and low refractive index layers are considered. The cavity wavelength is in the middle of the reflectance band in the simulation, while it is. The measured specular reflection spectra of the PS multilayer structures are compared with the theoretical simulations using T-matrix method.
Porous silicon multilayer structures 109
Choice of refractive index and thickness
PS multilayer structures
- Dielectric Bragg mirror
- Single microcavity
However, neither the center of the reflection band nor the width of the measured spectra clearly match the simulations. In addition, the thicknesses of the layers manufactured with both high and low anodizing current densities and the refractive index of the layers manufactured with a low anodizing current density are influenced. The discrepancy in the reflection bandwidth between the measurement and the simulation is a clear indication that the interfacial refractive index of PS layers is different from the bulk refractive index.
Summary
The high refractive index layers are prepared with 5 mA/cm2 anodization current density for 20 sec. and the low refractive index layers are prepared with 75 mA/cm2 anodization current density for 3 sec. These differences in the cavity between the measurement and the simulation are due to the changes in the bulk refractive index of low refractive index layers and the thicknesses of both high and low refractive index layers with the anodizing process as seen in the case of DBMs. Before preparing the multilayer structures, the high and low refractive index layers prepared with 5 and 75 mA/cm2 anodizing current density, respectively, are characterized for the refractive index and growth rate with the procedure described in Chapter 3.
This chapter presents these conclusions and the future prospects of the studies conducted in the dissertation. The mathematical steps in determining the interference fringe order m and thickness d for the case of transmission geometry are given here. To determine is compensated.
Conclusions and future scopes 121
Future scopes
