Malte Leisner

New Applications of Electrochemically Produced Porous Semiconductors and Nanowire Arrays

The growing demand for electro mobility together with advancing concepts for renewable energy as primary power sources requires sophisticated methods of energy storage. In this work, we present a Li ion battery based on Si nanowires, which can be produced reliable and cheaply and which shows superior properties, such as a largely increased capacity and cycle stability. Sophisticated methods based on electrochemical pore etching allow to produce optimized regular arrays of nanowires, which can be stabilized by intrinsic cross-links, which serve to avoid unwanted stiction effects and allow easy processing.

Pores in n-Type InP: A Model System for Electrochemical Pore Etching

The growth mechanism of currentline-oriented pores in n-type InP has been studied by Fast-Fourier-Transform Impedance Spectroscopy (FFT IS) applied in situ during pore etching and by theoretical calculations. Several pore growth parameters could thus be extracted in situ that are otherwise not obtainable. These include the space-charge-region (SCR) width, the SCR potential, the capacitance at the pore tips, and the avalanche breakdown field strength. It could be demonstrated that the system adjusts itself in such a way that the potential across the space-charge-region at the pore tips is kept constant within a certain bandwidth of the applied potential. This provides for a constant field strength at the pore tips, ensuring that avalanche breakdown occurs, generating the necessary holes for the electrochemical dissolution of InP.

Smoothening the Pores Walls in Macroporous n-Si

Our work reports on the etching of very smooth pores in n-Si with backside illumination. From the point of view of the so- called current burst model we explain the origin of the roughness on macropore walls. By trying different types of electrolytes at very low or moderate pH, electrolytes with very small dissolution rates of SiO2, or etching at very low temperatures, we show that the roughness of the walls can be controlled. Finally we propose to use a viscous electrolyte which lead to a roughness of only 9 nm on a 800 x 800 nm2 area which is a factor 5 smaller than the roughness observed for aqueous electrolytes.

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.

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.

Impedance Spectroscopy as a Powerful Tool for Better Understanding and Controlling the Pore Growth Mechanism in Semiconductors

This work shows new results toward a better understanding of macropore growth in semiconductors by using in situ FFT impedance spectroscopy. A new interpretation of the voltage impedance is proposed. In particular, the pore quality could be quantified for the first time in situ, especially by extracting the valence of the electrochemical process. The study paves the way toward an automatized etching system where the pore etching parameters are adjusted in situ during the pore etching process.