Showing papers by "J. Carstensen published in 2003"

Pores in III–V Semiconductors

Abstract: The paper reviews electrochemically etched pores in III-V compound semiconductors (GaP, InP, GaAs) with emphasis on nucleation and formation mechanisms, pore geometries and morphologies, and to several instances of self-organization. Self-organization issues include the formation of single-crystalline two-dimensional hexagonal arrays of pores with lattice constants as small as 100 nm found in InP, synchronized and unsynchronized diameter oscillations coupled to current and voltage oscillations, and pore domain formation. The findings are discussed in relation to pores observed in silicon. Some novel porperties of the porous layers obtained in III-V compounds are briefly described.

Self-organized growth of single crystals of nanopores

Abstract: 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.

Organic and aqueous electrolytes used for etching macro- and mesoporous silicon

Abstract: The aim of this work is to explain the influence of the solvent molecules on silicon dissolution using several hydrofluoric acid containing organic electrolytes. The investigations focus on the morphology of the mesopores and macropores and the electrochemical valence of the overall reaction. Some basic properties of the electrolyte (polarity, tendency to form silicon oxide, and the H donor properties) were found to dominate the macropore formation in p-type silicon. For macropore formation the correct balance between the two main dissolution paths (direct silicon dissolution and dissolution via an anodic oxide) is very important. For mesopores in highly doped p- and n-type silicon the direct dissolution is dominant. The consequences of mixing organic and aqueous solutions are discussed.

Porous III–V compound semiconductors: formation, properties, and comparison to silicon

Abstract: Pore formation in n-type III–V semiconductors will be discussed and compared to pore formation in silicon. While by now many different kinds of pores were produced in silicon, the “pore zoology” in III–Vs was rather limited until recently. This paper will briefly review the specific pore morphologies in some compound semiconductors, nucleation and formation mechanisms, the relation to comparable Si pores (including some new observation in Si), and the particularly striking features that pores in IIIsemiconductors exhibit many features of self organization and on occasion peculiar luminescence properties.

Single crystalline 2D porous arrays obtained by self organization in n‐InP

Abstract: Self organization is a rather common phenomenon during pore formation in III-V semiconductors. The so called tetrahedron-like pores, the domains of crystallographically oriented pores in n-GaAs, or the macroscopic voltage oscillations in n-InP at high constant current densities are examples of a self organization process. In this paper we will discuss two-dimensional arrays of pores in n-InP with the unique property that they may form a single crystal as a result of a self organization process. The reasons for this long range order and its dependence on the etching conditions will be discussed.

A comparison of pores in silicon and pores in III–V compound materials

Abstract: This paper compares the morphologies of porous silicon and porous III–V compounds and discusses their growth mechanisms. Looking into the fine structure of pores, in silicon (meso)pores with intercalating octahedra are prevalent, while in III–V compounds chains of tetrahedra are observed. The anisotropic properties of pore growth in all materials can be understood considering the role of surface states in the band gap and the anisotropy of the passivation of these states which leads to an “aging” effect of the surface. In III–V compounds, and for the first time in silicon, the strong interaction of pores during their growth has been found. In InP as well as in silicon these interactions lead to self-induced diameter modulations. To achieve this effect in Si requires the etching conditions to be comparable to those in the III–V compounds.

Electrochemical pore etching in Ge

Abstract: 61.43.Gt, 68.37.Hk, 81.65.Cf, 82.45.Vp Nucleation and growth of electrochemically obtained pores on (111) and (100) oriented n-Ge in different electrolytes was investigated. On rough surfaces pore density increases as the current density increases, whereas on smooth surfaces the situation is inverse, i.e., the pore density increases as the current density decreases. The macropores show strong anisotropic features with a cone-like shape. This can be under-stood if the passivation of the pore walls in Ge is less pronounced as in the case of Si or III–V com-pounds, but strongly anisotropic.

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

Abstract: 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

Abstract: 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.

Voltage oscillations – an emergent property at high density pore growth

Abstract: The paper addresses macroscopic voltage oscillations observed during anodic etching of pores in n-InP [1] and GaP [2]. These oscillations always occur concurrently with the modulation of pore diameters which are synchronized on large areas of the samples. The observed macroscopic voltage oscillations represent a kind of pattern formation of the system at high pore density which does not occur when the pore density is low and no interaction between pores is present. The voltage oscillations obtained at different temperatures and current densities are analyzed in time and space, including Wavelet transformations. A model of these macroscopic voltage oscillations will be presented, which is based on the basic principles of the current burst model [3] developed for Si and which can be successfully applied to III-V compounds, too.

Edge shunt passivation in silicon solar cells by chemical etching investigated by lock-in thermography and CELLO

Abstract: Shunts in solar cells are often localized at the edges. Some of these edge shunts have a linear (ohmic) behavior and some of them have a non-linear (diode-like) one. At least the linear edge shunts, which are the most dangerous ones, can be removed by local etching. Two proven nondestructive techniques for shunt imaging are lock-in thermography and CELLO. Lock-in thermography detects local heat sources under forward or reverse bias in the dark, whereas CELLO measures the small signal electrical response to a pulsed laser beam at different voltages. These two techniques are compared with respect to their ability to investigate the influence of edge etching procedures on the shunt activity.