Jürgen Carstensen

Growth Mode Transition of Crysto and Curro Pores in III-V Semiconductors

The formation of crystallographically oriented and current line oriented pores in n-type InP is reviewed and compared to other semiconductors in the light of some new results. A model for the formation of crystallographically oriented pores is presented that reproduced salient features of this pore type rather well. Impedance data together with their model-based evaluation are given and discussed. Some self-organization features of current line oriented pores and their possible relation to self-induced or externally triggered growth mode transitions between the two pore types conclude the paper.

Simulating Crystallographic Pore Growth on III-V Semiconductors

In this work, the growth of crystallographically oriented pores in III-V semiconductors has been investigated. Based on new and previous results a model for pore growth has been developed, which is mainly based on a stochastic branching probability of the pores. The stochastic nature of the model allowed to implement it as the core for Monte-Carlo-Simulations of pore growth. The simulations were able to reproduce the main features of crystallographically oriented pores, like uniform pore growth in ntype InP or the formation of domains on n-type GaAs and InP. The model is also capable of reproducing the logarithmic growth law for the pore depth, as well as the pore density oscillations with depth, as recently found.

Production of High Aspect Ratio Single Holes in Semiconductors

M.-D. Gerngros, H. Foll, A. Cojocaru, and J. Carstensen Institute for Materials Science, Christian-Albrechts-University of Kiel, Kaiserstr. 2, D-24143 Kiel, Germany In many areas of research exists a need for single small holes with diameters from a few nm to several µm and large aspect ratios. Ex-isting technologies for that are complex and rather limited. It is shown by a simple proof of principle that suitable single holes or specific arrays of some single holes can be made by first etching a very large number of small and deep holes or pores into semicon-ductors like Si or InP by established electrochemical means, fol-lowed by masking the desired holes and filling all others with, e.g., a metal in a galvanic process. The potential and limitations of this technique are discussed in some detail.

Porous InP as Piezoelectric Component in Magneto-Electric Composite Sensors

In this paper we focus on the production of an effective and cheap piezoelectric material for the application in a magneto-electric composite sensor. In this concept porous and piezoelectric InP serves as the matrix material. Its pores are then electrochemically filled with a magnetostrictive material. This arrangement of piezoelectric and magnetostrictive materials is chosen, because it allows for very large contact areas, good mechanical coupling between both components and thus high sensitivity to magnetic fields. InP, as all III-V semiconductors, is known to show piezoelectric behavior, since it has no inversion center due to its cubic crystal structure. Thus only the d14 component is a non-vanishing component of the piezoelectric modulus [1]. The piezoelectric effect of InP has been measured very rarely [2, 3] because even highly pure InP contains a lot of impurities, serving as doping centers, so that a sufficiently large number of free charge carriers will shortcircuit the charges induced by the piezoelectric effect. Our approach to overcome the short-circuiting of the polarization induced by the piezoelectric effect is to produce a closed packed pore array with overlapping space charge regions, where no free charge carriers are present. The formation of the necessary current-line pores in n-InP by electrochemical etching is a standard process by now [4]. In this work single crystalline (100) InP wafers doped with S and a doping level of 1.1x10 cm are used. Fig. 1a shows the resulting pore structure in top view after electrochemical etching and subsequent removal of the nucleation layer by mechanical polishing. The structure is optimized to produce a self-organized pore structure as densely packed as possible. The electrochemical etching of current-line pores into bulk InP already reduces the conductivity of the sample in comparison to bulk InP. Porous membranes already show piezoelectric behavior, but the “leakage” currents are still too high for the intended use. This is probably due to some areas where no space charge region is present and thus free charges still short-circuit the induced polarization. To overcome this problem the porous structure from Fig. 1a is chemically post-etched in an HF : HNO3 : EtOH : HAc solution for 48 h to reduce these areas, what is not possible electrochemically [5]. This process was optimized to be isotropic and self-limiting; it produces elliptical/circular pore shapes with a mean pore wall thickness of about 150 nm. The resulting pore structure is shown in Fig. 1b. The characteristic change in the pore geometry can be understood by considering the space charge region surrounding each pore, the resulting voltage drop across the space charge region and the crystal-orientation dependence of the electrochemical and chemical etching in InP. The chemical post-etching of the electrochemically etched samples reduces the leakage currents to a level low enough to use the piezoelectric properties. The piezoelectric response to deformation has been measured with a specialized interferometer (DBLI) from aixACCT. Fig. 1c shows the linear displacement vs. applied voltage of the being electrochemically etched and post-etched sample. The d14 component is found to be around a stunning |60| pm/V, about a factor of 30 larger than the values measured on bulk InP [6].

Dynamics of Macropore Growth in n-type Silicon Investigated by FFT In-Situ Analysis

The dynamics of macropore growth in n-type silicon were investigated by in-situ FFT impedance spectroscopy and transient analysis. In particular the response to fast growing pores to current density steps in the context of so-called anti-phase diameter oscillations was investigated. These pore growth mode allows for a very fast growth of deep macropores and could for the first time be stimulated by external current steps.

Open-loop-control of pore formation in semiconductor etching

Electrochemical etching of semiconductors gives rise to a wide variety of self-organized structures including fractal structures, regular and branching pores. The current-burst model and the aging concept are considered to describe the dynamical behavior governing the structure formation. Here the suppression of side-branching during pore growth is demonstrated by an open-loop-control method, resulting in pores with oscillating diameter.