Gold-thickness-dependent Schottky barrier height for charge transfer in metal-assisted chemical etching of silicon
© Zuo et al.; licensee Springer. 2013
Received: 4 March 2013
Accepted: 16 April 2013
Published: 26 April 2013
Large-area, vertically aligned silicon nanowires with a uniform diameter along the height direction were fabricated by combining in situ-formed anodic aluminum oxide template and metal-assisted chemical etching. The etching rate of the Si catalyzed using a thick Au mesh is much faster than that catalyzed using a thin one, which is suggested to be induced by the charge transport process. The thick Au mesh in contact with the Si produces a low Au/Si Schottky barrier height, facilitating the injection of electronic holes from the Au to the Si, thus resulting in a high etching rate.
Silicon nanowires (SiNWs) have attracted significant research interest because of their unique properties and potential applications as building blocks for advanced electronic devices [1, 2], biological and chemical sensors [2–4], and optoelectronic devices  as well as photovoltaic devices [2, 6, 7]. Metal-assisted chemical etching has attracted increasing attention in the recent years because of its simplicity and low cost coupled with its excellent control ability on the structural and electrical parameters of the resulting SiNWs [8–13]. In metal-assisted chemical etching, the formation rate of SiNWs, i.e., the etching rate of Si substrate, is controlled by the mass transfer process of the reagent, including the by-product, and by the charge transfer process during the Si etching [13, 14].
The crystallographic orientation and the doping properties of the Si substrate, the type and the structure of a noble metal, the component and the concentration of the etching solution, temperature, illumination, and so on were reported to have a substantial effect on the etching rate [11, 12, 14–17]. In the present study, the thickness of the Au catalyst film, which is a new control dimension, was found to affect the etching rate of Si during the fabrication of SiNWs by a method that combines the anodic aluminum oxide (AAO) template and the metal-assisted chemical etching. The aforementioned method results in the formation of large-area, vertically aligned SiNW arrays with a uniform diameter along the height direction. Furthermore, the method shows better control on the diameter, spacing, and density of SiNW arrays.
Results and discussion
Structure of the patterned Si substrate
Structure of the SiNW arrays
Effect of Au mesh thickness on the etching rate
Mechanism for difference in the etching rate
The charge transfer between the Si and the Au would be heavily affected by the Au/Si Schottky barrier height (see Figure 7b). It has been reported that the size of the metal has an important effect on the surface band bending of Si [13, 14]. The Schottky barrier height of the semiconductor/metal contact is said to increase with the decrease of the feature size of the metal [13, 21, 22]. Based on the results and discussions above, the thickness of the Au mesh, and not the lateral size, can be suggested as the factor that determines the Au/Si Schottky barrier height, considering the continuous property of the Au mesh. The barrier height ΦB decreases with the increase of the thickness of the Au mesh. Therefore, electronic holes can be easily injected from the thick Au mesh into the Si substrate underneath the Au because of the reduced barrier height compared with that of the thin Au mesh, thus, resulting in a high etching rate.
According to the model of charge transfer through the Schottky barrier, ideally, the Fermi energy level determined by the doping type and doping level of the Si substrate and the work function of the noble metal will determine the surface band bending of Si, thus affecting the hole injection from the noble metal to the Si and, furthermore, affecting the etching rate. In fact, n-doped Si was found to be etched faster than p-doped Si [17, 23], and the etching rate decreases with increasing dopant concentration for both n- and p-doped Si [11, 17, 24]. Meanwhile, Li et al. reported that the etching rate showed only small variation for a Au-coated p+, p−, and n+ Si substrate and a Pt-coated Si was etched faster compared with a Au-coated Si . Obviously, abovementioned experiment results cannot be accounted for only by the charge transfer through an ideal Schottky barrier. A rigorous model should consider the full process of charge transfer including the generation of holes, diffusion in the metal, going through the Schottky barrier, as well as diffusion in the Si substrate, which involved the catalytic activity of the noble metal for oxidant (affecting the generation rate of holes), the surface state of Si, the diffusion of holes from the etching front to off-metal areas or to the sidewall of the formed structure (especially in a heavily doped Si, resulting in the formation of a porous structure), etc. [14, 17]. However, this has not been done so far, and it needs to be further explored.
Metal-assisted chemical etching of Si allows fabricating large-area SiNWs with predetermined doping type and doping level. By utilizing the AAO template, the diameter, spacing, and areal density of nanowires can be further controlled through optimizing the anodizing conditions. Moreover, the SiNWs fabricated by this method are well-discrete and vertically aligned, which is critical for subsequent coating of other layers in device fabrication. Therefore, this technique is very promising for device fabrication based on SiNW array, for instance, SiNW radial p-n junction solar cells .
In conclusion, combining the AAO template and the metal-assisted chemical etching process results in large-area, vertically aligned SiNWs with a uniform diameter along the height direction. The thickness of the Au film was found to affect the etching rate of Si, which might be caused primarily by the charge transfer process. A thick Au mesh that comes in contact with Si reduces the Au/Si Schottky barrier height, which facilitates the injection of electronic holes from the Au mesh into the Si, thereby resulting in a high etching rate of Si. This method provides a simple and low-cost approach to the control of the doping type, doping level, diameter, spacing, areal density of SiNW arrays, etc. Well-discrete and vertically aligned SiNW array fabricated by this method is very promising for device applications based on SiNW arrays.
This work is partly supported by the National Natural Science Foundation of China under grant nos. 61106011 and 51172109 and the Anhui Province Natural Science Foundation under grant no. 1308085QF109.
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