H. Münder

Electrodeposition of metals into porous silicon

Abstract The electroless and cathodic electrodeposition of metals (Au, Cu, Ni) into porous silicon (PS) is studied. The electrochemically modified PS layers are analysed by X-ray photoelectron spectroscopy and sputter depth profiling. The electroless deposition oxidizes PS simultaneously. For this reaction a new concept of injection current multiplication is proposed. After cathodic metal deposition the pores are filled with metal quantitatively without oxidation of PS.

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.

Closed-chamber chemical vapor deposition : new cyclic method for preparation of microcrystalline silicon films

A new cyclic chemical vapor deposition (CVD) process for low-temperature preparation of microcrystalline silicon and its alloys is proposed. The cycle includes an a-Si:H layer deposition step and a hydrogen-radical treatment step. The H-treatment step is carried out under closed-chamber CVD (CC-CVD). It provides conservation of Si mass at an equilibrium between H-etching and redeposition. Thus, films of high crystallinity can be achieved. The advantages are a high deposition rate, high reactive gas utilization and precise control of the film structure. In situ monitoring of the plasma emission spectrum has been used to investigate the CC-CVD process features. The films are characterized by Raman spectroscopy, scanning electron microscopy, temperature-dependent dark conductivity, and infrared transmission spectroscopy.

Formation techniques for porous silicon superlattices

Abstract Porosity superlattices (SLs) are a new type of Si-based heterostructures which exhibit a periodical variation of the porosity in depth. These structures have been investigated by transmission electron microscopy. Different formation techniques for porous Si SLs will be presented: SLs on p-type doped Si were formed by periodic variation of the formation current density or by using periodically doped Si substrate layers. An influence of the substrate quality on the interface roughness has been found. On n-type Si the illumination intensity has been periodically changed during the etching process which leads to a periodical variation in the macropore radii. An explanation for this dependence is suggested.

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

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).

Characterisation of porous silicon layers by spectroscopic ellipsometry

Abstract The influence of the microscopic structure of porous silicon layers on the dielectric function is determined by spectroscopic ellipsometry. The investigated layers were formed on high and low p-type doped substrates. Their microscopic structure was changed by varying the current density in the electrochemical formation process. The measured dielectric function was found to be extremely sensitive on the microscopic structure. New features occur in the measured dielectric function which are characteristic for the silicon skeleton in the porous silicon layers.