H. Föll

Formation and application of porous silicon

All manifestations of pores in silicon are reviewed and discussed with respect to possible applications. Particular emphasis is put on macropores, which are classified in detail and reviewed in the context of pore formation models. Applications of macro-, meso-, and micropores are discussed separately together with some consideration of specific experimental topics. A brief discussion of a stochastic model of Si electrochemistry that was found useful in guiding experimental design for specific pore formation concludes the paper.

Processing of Three‐Dimensional Microstructures Using Macroporous n‐Type Silicon

A process for micromachining arbitrary structures with high aspect ratios in bulk silicon utilizing standard microelectronic processes is presented. It is based on electrochemical macropore formation on n-type silicon in electrolytes containing hydrofluoric acid. Very regular pore arrays with pore diameters and distances in the micrometer range and pore lengths of several hundred micrometers can be produced with this technique. Wafers with suitable prefabricated pore arrays are used as substrates for a micromachining process including anisotropic etching of very deep structures yielding straight walls to depths of as much as 150 μm.

Pore formation mechanisms for the Si-HF system

Abstract A model is presented with the potential to account for all processes of the reactive Si–liquid interface including, e.g. current oscillations, and the formation of nano-, meso, and macropores with their specific dependence on crystal orientation. The model assumes that current flow is spatially and temporally inhomogeneous-current thus flows in current ‘lines’ occurring in current ‘bursts’. The mean cycle time between correlated current bursts is mostly given by the kinetics of oxide dissolution and hydrogen passivation (which introduces a strong surface orientation dependence). Structure generation at the Si electrode (current oscillations in the time domain or pore formation in the space domain) under these assumptions is a self-organized process resulting from an interplay of synchronizing and desynchronizing mechanisms. Synchronizing mechanisms always couple the nucleation of a new current burst in a specific area to the history of that area, desynchronizing mechanisms may also depend on the interaction of current burst. Examples for synchronizing mechanisms are enhanced nucleation probabilities on (100) surfaces, response to local oxides from another current burst, or coupling of current bursts by space charge region effects. Desynchronization results, e.g. from quantum wire effects, or local reduction of reactants or potential by a current line. The model accounts qualitatively for most if not all observed phenomena, gives a number of quantitative relations, and makes numerous predictions.

Macroporous Silicon: A Two‐Dimensional Photonic Bandgap Material Suitable for the Near‐Infrared Spectral Range

We have fabricated macroporous silicon and proved its applicability as a two-dimensional photonic bandgap material in the near-infrared spectral range. Two different triangular lattices of circular air rods with lattice constants of 2.3 and 1.5 μm were etched at least 75 μm deep in an n-type silicon substrate by electrochemical pore formation in aqueous hydrofluoric acid. Photolithographic pre-patterning techniques and subsequent alkaline etching were used. In the case of the 1.5 μm lattice the photo-mask was modified so that series of etch pits were left out. These local perturbations of the initially regular lattice of air rods introduced photonic defects like waveguides. In the case of the 2.3 μm lattice we succeeded to micromechanically structure the macroporous layer to make 200 μm wide free-standing bars of porous material remain on the silicon substrate. These bars were used to measure the transmission of the photonic lattice dependent on the light polarization relative to the pore axes using FT infrared spectroscopy. The results excellently agree with theoretical calculations. The generation of KOH pits with lattice constants on a sub-μm length scale was demonstrated.

CELLO: an advanced LBIC measurement technique for solar cell local characterization

An advanced light beam-induced current measurement for solar cell local characterization, called CELLO, has been developed and tested on mono- and multi-crystalline Si solar cells. A solar cell is illuminated at near 1.5 AM light intensity, and is additionally subjected to intensity modulated scanning local illumination by a focused IR-laser. The linear response (current or potential) of the solar cell is measured for various fixed global conditions (different preset voltage or current values) during scanning. A large number of independent data with high spatial resolution are obtained. Applying an advanced fitting procedure to these data yields a set of local parameters for each point on the solar cell. This gives information on the spatial distribution of the photo current, the series and shunt resistance, the lateral diffusion of minority carriers, the quality of the back surface field, and even allows the calculation of local IV curves. The theoretical and experimental approach to this technique will be discussed, and the applicability of this new solar cell characterization tool will be demonstrated.

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.

Crystal orientation and electrolyte dependence for macropore nucleation and stable growth on p-type Si

Abstract Macropore formation in moderately doped p-type Si was studied in mostly galvanostatic experiments (2–10 mA cm −2 ) with various fluoride containing electrolytes and substrate orientations [(100), (511), (5 5 12), (111)] from the nucleation phase to the phase of stable pore growth. Macropores on p-type Si always grow anisotropically in 〈100〉- and 〈113〉-directions. The most important parameter of the electrolyte is its ability to supply oxygen and hydrogen. Whereas oxygen is necessary for smoothing the pore tips, hydrogen is the decisive factor for the anisotropic growth and the passivation of macropore side walls. Based on a better theoretical understanding of the electrode processes in general pore formation in particular, etching conditions could be optimized for the generation of macropores in p-type Si with better aspect ratios, better stability, and smaller diameters than those in n-type Si.

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.

A Model for Current‐Voltage Oscillations at the Silicon Electrode and Comparison with Experimental Results

The first consistent and complete model of current oscillations at the Si electrode is presented. The only basic assumption needed is an ionic breakthrough mechanism which is postulated to occur in thin oxides under oxidizing electrode conditions, leading to an enhanced and localized ion transport to the interface. Choosing reasonable values for three corresponding physical parameters and using a Monte Carlo simulation technique, first‐principle calculations yield quantitative data in excellent agreement with numerous experimental results, including the value of the current, surface roughness, the average oxide thickness, and the capacitance as a function of the phase of oscillations, and the frequency of the oscillations as a function of applied voltage, current density, etching rate or HF concentration, and temperature.

Crystal Orientation Dependence and Anisotropic Properties of Macropore Formation of p- and n-Type Silicon

The dependence of macropore morphology on the orientation of p- and n-type silicon samples was studied for various organic and aqueous electrolytes containing hydrofluoric acid. Scanning electron microscopy was used studying the morphology of the maeropores. The results show that the macropore formation in p- and n-type silicon is a strongly anisotropic process. Depending maeropores. The results show that the macropore formation in p- and n-type silicon is a strongly anisotropic process. Depending on the substrate orientation and preferred growth directions could be observed. The mierostructure of macropores was studied by analytical and high-resolution transmission electron microscopy. The surface of macropores in n- and p-type Si(001) shows {111} facets indicating that {111} planes are stabilized against further dissolution. Breakthrough pores show very specific anisotropic properties independent of the electrolyte. These pores consist of periodic arrangements of truncated octahedral voids with {111} walls strung up in directions. The erystal orientation dependence of pore formation reflects specific properties of the pore formation mechanism and one of the important electrolyte parameters is the ability to form an anodic oxide. Macropores formed in more strongly oxidizing electrolytes tend to have smoother macropore walls.