M. Christophersen

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.

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.

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.

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.

Parameter Dependence of Pore Formation in Silicon within a Model of Local Current Bursts

The chemical reactions at the pore tip depend strongly on the choice of the electrolyte. They define the chemical transfer rate, properties and kinetics of the surface passivation, and ultimately the crystal orientation dependence of pore formation. In addition, there exists a number of processes which stabilize pore growth on length scales from several nm up to several 100 μm and determine, or at least influence, the size of pores: The potential distribution, carrier generation mechanisms and diffusion processes. We present a new model assuming a dissolution process which is inhomogeneous in time and in space (i.e. a local current burst). The time scales of the current bursts and the correlation length between these current bursts define additional time and length scales for the chemical dissolution processes at the silicon–electrolyte interface which support or even overrule the length scales of the stabilizing processes listed above. This allows to design an electrolyte (using e.g. diverse organic electrolytes and oxidizing and proton supplying ingredients) to optimize macropore growth in a wide range of materials and on length scales not possible in aqueous electrolytes.

Dependence of Macropore Formation in n‐Si on Potential, Temperature, and Doping

Subjecting illuminated n-type silicon wafers to anodic bias in an HF containing electrolyte results in the formation of macropo res under certain conditions. In this paper the formation of randomly nucleated macropores is studied as a function of the applied potential, the temperature, and the doping levels of the samples. A large number of micrographs was evaluated by computerized image processing and the data obtained are compared to predictions of pore formation models. It was found that the formation of randomly nucleated macropores involves a prolonged nucleation phase. Starting from a polished surface, first macropores occur after a certain amount of Si has been homogeneously dissolved. In this nucleation phase the thickness of the homogeneously dissolved Si depends strongly on the doping level and the temperature, but only weakly on the applied bias. In a second phase of st able pore growth, the density of pores is investigated as a function of temperature and anodic potential. For low-doped material a strong influence of the space-charge region on the average macropore density is observed in accordance with existing models; an increased anodic bias, e.g., decreases the density of pores. For highly doped silicon the situation reverses; increasing anodic bias increases the pore density, in contrast to predictions. The pore growth in this region is not very sensitive to the space-charge region but seems to be dominated by the chemical-transfer rate.

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.