S. Langa

Self-organized growth of single crystals of nanopores

Self-organized single crystalline two-dimensional hexagonal arrays of pores in InP semiconductor compound are reported. We show that the self-arrangement of pores can be obtained on n-type substrates with (100) and (111) orientations. The long-range order in pore distribution evidenced in (100)InP samples proves to be favored by the so-called nucleation layer exhibiting branching pores oriented along 〈111〉 directions. The combination of long-range order with self-induced diameter oscillations is shown to be promising for nonlithographic growth of three-dimensional pore crystals.

Observation of crossing pores in anodically etched n-GaAs

Pores in GaAs in the micrometer range and oriented in 〈111〉 directions have been observed during the anodization of GaAs in aqueous HCl electrolytes. A direct evidence of pores intersection is presented which is a very promising feature for three-dimensional micro- and nanostructuring of III–V compounds for the production of photonic materials.

Formation of Porous Layers with Different Morphologies during Anodic Etching of n ‐ InP

Two different morphologies of porous layers were observed in (100)-oriented n-InP anodically etched in an aqueous solution of HCl. At high current density (60 mA/cm2) anodization leads to the formation of so-called current-line oriented pores. When the current density decreased to values lower than 5 mA/cm2 the morphology of the porous layers sharply changed and the pores began to grow along definite crystallographic directions.

Self-Induced Voltage Oscillations during Anodic Etching of n-InP and Possible Applications for Three-Dimensional Microstructures

Voltage oscillations were observed during anodic etching of 100 -oriented n-InP substrates in an aqueous solution of HCl at high constant current density. Under certain conditions, the oscillations lead to a synchronous modulation of the diameters of pores on large areas of the samples which indicates a correlation between the phases of the oscillations in the pores. These self-induced diameter oscillations may be useful for three-dimensional microstructuring of n-InP and thus for the design and fabrication of new photonic materials.

Correlation between morphology and cathodoluminescence in porous GaP

Porous layers fabricated by anodic etching of n-GaP substrates in a sulfuric acid solution were studied by electron microscopy and cathodoluminescence (CL) microanalysis. The morphology of porous layers was found to depend strongly upon the anodization conditions. When the etching process starts at the initial surface, “catacomb-like” pores and current-line oriented pores are introduced at low and high anodic current densities, respectively. After the initial development of either kind of pore, further anodization at the current density of about 1 mA/cm2 favors the propagation of pores along 〈111〉 crystallographic directions. The spatial and spectral distribution of CL in bulk and porous samples is presented. A comparative analysis of the secondary electron and panchromatic CL images evidenced a porosity induced increase in the emission efficiency.

Uniform and Nonuniform Nucleation of Pores during the Anodization of Si, Ge, and III-V Semiconductors

Morphology is one of the basic characteristics of porous layers. For electrochemically grown pores, morphology is strongly dependent on the starting phase of pore growth, the so-called nucleation phase. This paper addresses uniform and nonuniform nucleation of pores on the surface and consequently the development of pores into the bulk of the following semiconductor substrates: Si, Ge, and III-V compounds (GaAs, InP, and GaP). It was found that nonuniform nucleation can cause formation of domainlike porous structures in all investigated semiconductors. However, depending on the anisotropy of the substrate, these domains show significant differences between them. The particularities of each type of domains are discussed.

Formation of Tetrahedron-Like Pores during Anodic Etching of (100) Oriented n-GaAs

The morphology of porous layers obtained by electrochemical etching of (100) oriented n-GaAs substrates in an aqueous solution of HCl was studied. At low anodic current densities, up to 5 mA/cm 2 , pores in the form of triangular prisms grew along (111) crystallographic directions. For larger current densities the shape of the pores did not suffer any changes at the beginning of the process, while after a definite period of time the morphology of pores changed drastically to chains of tetrahedral voids with {111} facets.

Self-organized pore formation and open-loop control in semiconductor etching

Electrochemical etching of semiconductors, apart from many technical applications, provides an interesting experimental setup for self-organized structure formation capable, e.g., of regular, diameter-modulated, and branching pores. The underlying dynamical processes governing current transfer and structure formation are described by the current burst model: all dissolution processes are assumed to occur inhomogeneously in time and space as a current burst (CB); the properties and interactions between CBs are described by a number of material- and chemistry-dependent ingredients, like passivation and aging of surfaces in different crystallographic orientations, giving a qualitative understanding of resulting pore morphologies. These morphologies cannot be influenced only by the current, by chemical, material and other etching conditions, but also by an open-loop control, triggering the time scale given by the oxide dissolution time. With this method, under conditions where only branching pores occur, the ...

Selforganized formation of crystallographically oriented octahedral cavities during electrochemical pore etching

The parameter dependence of electrochemically etched pores in silicon is studied. Using HF containing organic electrolytes and backside illumination on moderately doped silicon, macropores and octahedrally shaped pores can grow simultaneously. All experimental results can be understood within the framework of the current burst model under the assumption that the system selforganizes and switches the pore morphologies to that mode which optimally consumes the available electronic holes in the reactions. These results can be used to control the pore growth and will be taken as an input for a Monte Carlo simulation to get a quantitative description of the etching processes.

Waveguide Structures Based on Porous Indium Phosphide

dTechnical University Hamburg-Harburg, 21073 Hamburg, Germany We researched the possibilities for engineering the morphology of porous structures in n-InP. Lithographic patterning of the sample surface before anodic etching was shown to modify considerably the electric field distribution which, in its turn, defined the direction of pore growth inside the specimen. We show that local formation of the nucleation layer combined with the possibility to introduce current-line oriented pores in a controlled manner represents a promising tool for manufacturing waveguide structures based on porous InP. Some results on simulation of the properties of these structures are presented.