On the other hand, if the dominant
mass transfer path is path II, a low etching rate for the thick Au mesh can also be inferred because of the large diffusion distance along the vertical direction. However, the present study shows that the thick Au mesh induces a high etching rate, and the SiNWs in the same sample have almost identical heights, especially for the SiNW arrays with large heights (see Figures 4b and 5d). The observations contradict the predictions for both models. Therefore, the mass transfer process can be concluded as a non-dominant factor with regard to the different etching rates. Figure learn more 7 Schematic of the reagent and
by-product diffusion paths and diagram of the Au/Si Schottky contact. (a) Schematic of two possible diffusion paths of the reagent and by-product during the metal-assisted chemical etching process. (b) Energy band diagram of the Au/Si Schottky contact; Φ B is the CDK phosphorylation barrier height for the electronic holes injected from the Au into the Si. The difference in the etching rates is naturally attributed to the charge transfer process. An oxidation-reduction reaction is well accepted to occur during the etching of the Si in a solution containing HF and H2O2[14, 20]. The Entospletinib nmr H2O2 is preferentially reduced at the noble metal surface, thereby generating electronic holes h+ according to reaction 1 (cathode reaction) : (1) At the anode, the generated electronic holes are injected into the Si substrate in contact with the metal, Baricitinib leading to the oxidation and then to the dissolution of the Si underneath the metal according to reaction 2 : (2) 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 Φ B 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.