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].