The anodic pore formation is optimized to be self-organized with an almost perfectly hexagonally close packed structure as shown in Figure a. The pore walls have a mean thickness of around 190 nm. The typical aspect-ratio of the pores is between 3,000 and 3,700 after electrochemical pore formation. The pore shape is trapezoidal, as depicted in Figure a. As illustrated in Figure b, the curro-pores grow straight in <100> direction.
SEM images of electrochemically etched curro-pores in InP. (a) Top view and (b) cross-sectional view.
Figure a–d shows that the previously etched curro-pore array can be opened by photoelectrochemical/photochemical etching. This membrane fabrication process has several advantages. Firstly, the resulting membrane has a great surface homogeneity, as illustrated in Figure a and b. The second advantage is that no underetching near the O-ring occurs, as depicted in Figure d. The underetching is prevented by choosing photochemical dissolution of the mesoporous layer with blue light. Blue light is used, because it is absorbed very surface near and, thus, surface near dissolution of the mesoporous layer is highly enhanced compared to isotropic dissolution. The third big advantage is that the membrane thickness is freely adjustable from nearly wafer thickness (up to almost 500 µm) to less than 100 µm. This allows the aspect-ratio of the pore structures to be smaller than 1,000, which is especially important for a homogenous coating of pore walls with an Al2O3 interlayer by ALD as discussed later. The free adjustability of the membrane thickness is a direct consequence of the great surface homogeneity and the absence of underetching phenomena. Fourthly, the membrane fabrication process is semi self-limiting as the dissolution rate drastically decreases as soon as the curro-pore array is reached. The pore openings at the membrane back-side can be modified from cone-like to straight by adjusting the photocurrent due to the applied illumination. A small photocurrent, e.g., results into cone-like pore openings, as depicted in Figure b.
SEM images of the membrane back-side. (a) Top-view and (b) highly magnified top-view. Optical microscopy images of the membrane in cross-section (c) in the center of the membrane, and (d) near the O-ring.
The pore structure after 48 h of post-etching is shown in Figure a and b. Compared to the only electrochemically etched pores, the cathodically post-etched pores are now rectangular with 90° angles and completely straight and nearly equi-distant pore walls. The mean pore wall width decreased from around 190 nm for the only electrochemically etched samples down to around 80 nm after 48 h of cathodic post-etching. The post-etching is homogenous over the complete pore length. This can be seen when comparing the cross-sectional view of the pore structure before post-etching (Figure b) and after post-etching (Figure b). The electrical characterization of the cathodically post-etched sample exhibits a completely ohmic behavior and an increase in the resistivity to 57 Ω
cm. Compared to the bulk InP resistivity of 0.019 Ω
cm, this is an increase by a factor of 3,000. The piezoelectric properties of the porous InP sample have been investigated using a double beam laser interferometer. A linear dependence between the applied voltage and the measured displacement is found - as expected for piezoelectric, but not for ferroelectric materials. The d14
component is measured to be around
pm/V, which is in the same order of magnitude as sputtered PZT thin films [10
] and by a factor of 30 larger compared to bulk InP [2
]. By luck, the maximum magnitude of the piezoelectric effect is in the <100> direction, which is exactly the optimal growth direction of the curro-pores in (100) oriented InP wafers.
SEM images of the membrane structure after 48 h of post-etching under cathodic bias. (a) Top view and (b) cross-sectional view in the middle of the membrane.
For the magnetoelectric composite device, the galvanic filling with a magnetostrictive metal, such as Ni, is needed. The post-etched InP membranes can be (and already have been) coated with a thin Al2O3 interlayer by ALD to prevent ohmic contacts between the piezoelectric and magnetostrictive component. Therefore, the galvanic deposition process of Ni can be developed and optimized in AAO membranes with similar pore dimensions because they are cheap and commercially easily available. Afterwards, this Ni deposition process is most probably directly applicable to Al2O3 coated InP membranes, because the interface electrolyte/Al2O3 is identical, and the pore dimensions are similar. The ALD deposition process of Al2O3 is capable to coat membrane structures with an aspect-ratio larger than 1,500.
Figure a shows the bottom part of a AAO membrane completely filled with Ni. One can see that the Ti/Au plating base adheres very well to the membrane. The Ni nanowires directly start growing from the plating base. The nanowires are solid and do not show any voids on optical inspection. Some of the Ni nanowires are broken or missing due to the sample cleavage. In Figure b, the middle part of the filled membrane is shown. The nanowires are still solid without any voids. They tend to be easily mechanically deformable, as indicated in Figure b by the twisty Ni nanowire in the middle of the picture. In order to show the possibility of filling the membrane completely with Ni, the membrane is overfilled resulting in an approximately 7 µm thick closed Ni film on top of the membrane surface. This layer is not caused by single Ni nanowires reaching the top surface faster than others, starting the formation of a solid Ni layer at the surface. It rather seems that all nanowires reach the surface at the same time because there are no empty pores visible, as shown in Figure c. The Ni nanowires remain solid even at the surface and do not exhibit any voids, as already seen in the previous parts of the membrane.
SEM images of AAO membranes with Ni. (a) Bottom part of the membrane including the plating base, (b) middle part, and (c) top of the membrane including overfilled Ni layer.