1. EXPERIMENTAL PROCEDURE EXPERIMENTAL PROCEDURE
Fabrication and characterization of free-standing porous silicon and alumina membranes M. Dimitropoulos, P. Kontou, M. Savranakis, A. Christoulaki, N. Spiliopoulos, D. L. Anastassopoulos, A. A. Vradis Solid State Laboratory, Department of Physics, University of Patras, GR 26504, Greece
INTRODUCTION
Porous materials are of scientific and technological importance due to the presence of controllable dimension voids at the nanometer scale. Research efforts in this field have been driven by the rapidly emerging applications such as biosensors, drug delivery, gas separation, energy storage and fuel cell technology. The research in this field offers exciting new opportunities for developing new strategies and techniques for the synthesis and applications of these materials. The purpose of this study is to fabricate free-standing porous alumina and silicon membranes in order to use them in various applications such as matrix for nanowires, fluid flow experiments and optoelectronic devices. After the fabrication, the membranes were characterized by Scanning Electron Microscopy.
POROUS ALUMINA
EXPERIMENTAL PROCEDURE
Anodization of an aluminum foil is carried out in sulfuric acid 0.3M and oxalic/phosphoric acid 0.3M under applied voltages of 20V and 40V respectively in a two-step anodization process [1]. The process takes place into an electrochemical cell. Subsequently, the membranes’ barrier layer is dissolved in a solution of phosphoric acid 5%wt. in a double tank cell in order to obtain a free-standing membrane [2].
Figure 4: Current vs time for barrier layer dissolution.
Figure 3: Current vs anodization time for applied voltage of 40V.
For the barrier layer dissolution process, during the first step we observe a minus electrical conductivity which is steady (1) until the barrier layer starts the dissolution process (2). The increase of the current indicates a rapid rate of barrier layer destruction (3,4). The steady state in the last step shows a higher electrical conductivity which means the total destruction of the barrier layer (5). The procedure is the same for both 20V and 40V.
Figure 2: Double tank cell for PA barrier layer dissolution.
Figure 1: Electrochemical cell for PA anodization. (1) Ni cathode electrode, (2) aluminum foil, (3) Cu plate, (4) spring loaded electric contact, (5) Power supply, (6) stirrer, (7) electrolyte, (8) Cold Plate, (9) mercury thermometer.
RESULTS
From the PA anodization we can observe three characteristic stages of PA formation. During the first step a layer of dense aluminum oxide is created and therefore the current is decreased. In the next step the creation of the first pores starts and an increase in the current is observed. The current reaches a steady state value in the third step indicating the development of the pores with a steady growing rate.
Figure 5: (a) Top View of PA, (b) Cross Section View of PA
Images of PA were taken by Scanning Electron Microscopy for constant applied voltage of 20V and 40V. The porous alumina membranes can be considered as porous medium with a number of parallel cylindrical nanotubes penetrating throughout its whole thickness.
POROUS SILICON
EXPERIMENTAL PROCEDURE
RESULTS
The anodization graph presents the anodizing voltage as a function of time during silicon anodization in galvanostatic mode under constant current density of 60 mA/cm2 . During the first seconds of the process the voltage is increasing until the pore creation is initiated. The peak in the voltage corresponds to the pore opening.
Anodization of a silicon wafer is carried out in various concentrations of HF:H2O:CH3CH2OH electrolyte solution [3] and current densities. The procedure takes place inside a double tank electrochemical cell. Electrical contact is achieved with an electrolytic backside contact [4]. Subsequently, when the PS sample is formed a second etching with increased current density takes place which is necessary to detach the PS layer from the Si substrate to form the membrane. The second etching process led to the electropolishing of the interface region between the PS layer and the Si substrate. As a result, the PS layer is completely detached from its substrate.
Figure 8: (a) Top View of Psi (straight pores), (b) Cross section of Psi, (c) Top View of Psi (branched pores).
There is a high flexibility in porous structures that can be created through electrochemical etching on PS, with altering the conditions of the process such as current density, HF concentration, wafer doping, illumination (n-type silicon). The porous media varies from straight and smooth pores to 3D and branched with pores sizes varying from a few tens of nanometers to micrometers, sometimes on the same membrane.
Figure 6: Double tank cell for PSi electrochemical etching.
Figure 7: Anodization voltage vs time for constant current density of 60 mA/cm2
CONCLUSIONS
The pore diameter is related to the applied voltage and the membrane’s thickness depends on the duration of the second anodization.
The temperature of the electrolyte affects the speed of the anodization procedure.
The diversity of porous media that can be achieved from PS electrochemical etching can provide not only regular applications such as solar cells and biosensors but also many simulation applications such as oil and mineral extraction.
REFERENCES
[1]A.Christoulaki et al., J.Appl.Electrochem (2014) 44,
[2] M. Lillo et al., Journal of Membrane Science (2009) 327,
[3] Michael J. Sailor, Porous Silicon in Practice: Preparation, Characterization and Applications (2012) 4,
[4]Leigh Canham, Properties of Porous Silicon (1997) 1.2 , 12-14