M. G. Berger

Porosity superlattices: a new class of Si heterostructures

Porosity superlattices have been investigated by transmission electron microscopy, photoluminescence and reflectance spectroscopy. The superlattices were formed on p-type doped Si using two different techniques. Firstly, for homogeneously doped substrates we have periodically varied the formation current density and thereby the porosity. Secondly, the current density was kept constant while etching was performed on periodically doped Si layers. For the first type of superlattices the layer thicknesses were determined by transmission electron microscopy. The results are in good agreement with the values calculated from the etching rate and time. For both types of superlattices, reflectance and photoluminescence spectra show strong modulation due to the periodicity of the superlattice.

Dielectric filters made of PS: advanced performance by oxidation and new layer structures

Copyright (c) 1997 Elsevier Science S.A. All rights reserved. For the formation of PS dielectric filters a detailed calibration of the etch rates and refractive indices is required. The effective dielectric function of PS was determined for different substrate doping levels as a function of the anodization current density by fitting reflectance spectra. Based on these results a number of different dielectric filters were realized. For device applications a thermal oxidation step is necessary to reduce aging effects which occur as a result of the native oxidation of PS. In addition, thermal oxidation results in a qualitatively improve filter performance due to a reduced absorption in the PS layers. Therefore the dielectric functions of PS oxidized in dry O 2 at temperatures up to 950 °C were determined. A continuous variation of the porosity and hence the refractive index with depth was used to realize so-called rugate filters. This type of interference filter allows the design of structures with more complex reflectance or transmittance characteristics than structures consisting of discrete single layers.

Investigation and design of optical properties of porosity superlattices

Abstract We have investigated the optical properties of porosity superlattices and complex multilayer systems. Type II superlattices reveal a more complex layer structure than expected from the substrate doping levels. Type I layer systems have been used to form highly reflective layer systems and Fabry-Perot filters.

Analysis of the depth homogeneity of p-PS by reflectance measurements

Abstract We have investigated changes in the etch rate of p-PS with increasing etching time as well as changes of the porosity of buried layers with depth. These effects can be attributed to the influence of chemical etching and variations in the electrolyte composition with depth. To study these changes, first the porosities of layers above and below layers with different thicknesses were determined by a fit of the reflectance spectra of these layer systems using the effective medium theory. Secondly we have measured oscillations in the reflectance during the formation of PS layers caused by the increasing layer thickness. Using these experimental results we are able to give a functional description of the changes in the optical thickness with depth. In addition, the influence of the chemical etching and changes of the HF concentration on the optical thickness can be estimated. As a result a method for changing the current with depth can be given, which can be used to minimize porosity gradients.

Formation of porous silicon on patterned substrates

Abstract Application of porous silicon in device structures requires the formation of micron-size porous areas. Therefore, selective area anodization on photolithographically patterned p-doped substrates was investigated. As shown in this work, porosity and layer thickness vary from the edge to the middle of the structures. This inhomogeneity strongly depends on the doping level of the substrate and the lateral size of the structure. When organic photoresists are used, an anisotropic undercutting of up to several 10 μm occurs at the edge of the structures. This can largely be reduced by using thermally treated Si 3 N 4 deposited by plasma-enhanced chemical vapour deposition as a masking layer. In this case an isotropic undercutting of the masking layer is observed permitting fabrication of porous silicon structures in the μm range by photolithography.

An Extended Quantum Model for Porous Silicon Formation

Porous silicon formed by anodization of a p-type silicon substrate is characterized by a distribution of crystallites with diameters smaller than about 100 A. The corresponding size distributions obtained from Raman measurements show distinct peaks which are explained for the first time by the tunneling of holes through crystallite barriers during the formation process of porous silicon. The theoretical description is based on quantum mechanical calculations of the tunneling probability of the holes through small crystallites into the electrolyte. This tunneling probability shows oscillations as a function of crystallite size which are comparable to the structures observed in the size distributions. The calculations presented provide a deeper understanding of these size distributions and of the basic formation mechanism of porous silicon

Color-sensitive photodetector based on porous silicon superlattices

Color-sensitivity of Si photodiodes was achieved by integrating porous silicon (PS) Bragg reflectors and Fabry–Perot filters. The PS was formed in the p+-type part of the p+n junction which required illumination of the samples during anodization. The optimal illumination power density turned out to be a compromise: high power densities are necessary to enable high anodization current densities, but this results in a degraded filter performance. The PS layers had no significant influence on the electrical characteristics of the photodiodes, but as expected they strongly modified the spectral response. The results are in good agreement with the reflectance spectra of the filters.

A non-destructive study of the microscopic structure of porous Si

Abstract The microscopic structure of porous Si films formed on different p-type doped substrates has been investigated by Raman spectroscopy. From a detailed line shape analysis of the Raman phonon peak, nanocrystal size distributions are obtained. These distribution functions depend on different sample parameters like the porosity and the doping level of the substrate, but are also influenced by the current density during the formation process. On thick samples, a change in the microscopic structure with depth is observed. This is due to a further chemical thinning close to the surface and to the limitation of the diffusion of reactive species from the deeper lying regions through the pores.

Investigation of different oxidation processes for porous Si by XPS

Abstract The oxidation behaviour of porous Si films formed on p-type doped substrates has been investigated. On freshly prepared porous Si no oxygen is found. During the native oxidation suboxides are formed. For low p-doped samples SiO 2 is formed after 1000 h, whereas on highly doped samples only Si 2 O is found. Part of the difference between low and high doped substrates is probably due to different orientations of the inner surfaces. It seems that the orientation of the inner surfaces of porous Si films formed on p + -doped substrates is more preferentially (111) than it is the case for the lower doped samples. For the anodic oxidation process also suboxide formation is found which results in the formation of SiO 2 .

Investigation of different oxidation processes for porous silicon studied by spectroscopic ellipsometry

Abstract In this paper we investigate the oxidation of porous silicon by O3, H2O2, and, for comparison, in normal air. Such an oxidation may serve as passivation for porous silicon in applications in order to prevent devices from degradation. The changes in the dielectric function caused by this oxidation was monitored by spectroscopic ellipsometry. Application of both H2O2 and O3 resulted in a significant lowering of the values of the imaginary part of the dielectric function as expected when oxidizing the inner surfaces of these layers. For a multilayer structure we show that ozone treatment of this structure indeed passivates that sample against further oxidation in air as studied over an extended period of time (3 months).