In Figure we give the top SEM views of one sample, before (Figure a) and after (Figure b) removing the silver film. One can see in Figure a, the silver dendrites formed during the etching process. In Figure b, we give a tilted SEM view of the SiNWs after removing the Ag film. In Figure , we give cross-sectional SEM images of SiNWs films prepared at 10, 20, 30, 40, 50, 60, 70, 80, and 90 min etching times. As reported in Figure , the mean length of the film varies from 21 to 38 μm. From Figure , we notice that the etching velocity is similar for 10, 20, and 30 min. However, it is more important for 40 and 50 min, where the maximum length is reached. When the sample is etched at greater durations, the length decreases by 10 μm from its maximum value and seems to be stabilized at values around 28 μm. This was attributed to the fact that when the etching process started, the HF solution etches the silicon substrate leading to an increase of the SiNWs’ length. After reaching the maximum length at 50 min, SiNWs themselves are etching by the HF solution as observed in the SEM image corresponding to 60 min.
Top SEM views of one sample. (a) Before, and (b) after removing the silver film.
Cross-section SEM images of formed SiNWs at different durations.
Variation of the length of SiNWs films vs. etching time.
The total reflectivity in the 250 to 1,250 nm spectral range of formed films has very low values, less than 1% in the UV domain and a maximum of 8% in the visible and near-infrared regions (Figure ). Generally, radiations with small wavelengths (UV domain) are absorbed at small depth and this absorption depends on the surface morphology. In the case of the non-treated sample, the reflectivity in the UV region is greater than 50% as shown in the inset of Figure . However, values of the reflectivity of SiNWs films in the UV domain are in the 0.5% to 1.5% range, which is unusual in silicon, even with texturized morphology and/or porous silicon. This was attributed to the important internal surface area of SiNWs. From curves in Figure , we remark that in the fully used spectral range, the SiNWs film elaborated at 50 min has the minimum value of the total reflectivity. The small values of reflectivity are attributed to the multiple reflections of incident photons which are important when the length of SiNWs film is important (38 μm during 50 min).
Total reflectivity spectra of all SiNWs films in the 250 to 1,250 nm wavelength range. The inset is the total reflectivity of untreated silicon surface.
For the electronic characterization, we use the LBIC technique at the He-Ne wavelength. A schematic illustration of the LBIC technique is given in Figure . LBIC measurements were performed on metal-insulator-semiconductor structures formed on SiNWs films. Typical LBIC profiles are given in Figure . We notice that we obtained the same shape of LBIC profiles for samples etched during 30, 50, 60, 70, 80, and 90 min. However, for samples etched during 10, 20, and 40 min, the LBIC profiles have approximately the same shape of that one performed on the MIS diode without SiNWs (0 min). Using the LBIC measurements (ILBIC
), we determine the effective values of L
. To carry out L
values, we fit the LBIC theoretical expression given in Equation 1 [16
] to the measured LBIC profiles.
Schematic illustration of the LBIC technique.
Typical normalized LBIC profiles. Vertical dash line corresponds to the metal position on the top of the MIS structure.
Obtained values of the effective diffusion length are plot in Figure . We remark that the obtained L values can be divided into two domains: red and green regions in Figure . The red region corresponds to the L values obtained for samples prepared during 0, 10, 20, and 40 min. The green region corresponds to samples prepared at 30, 50, 60, 70, 80, and 90 min.
Obtained values of the diffusion length vs. etching time.
To understand why L values change from a sample to another, we use the cross-section SEM images of Figure and the total reflectivity of films given in Figure . In Figure , we remark that durations 10, 20, and 40 min lead to inhomogeneous SiNWs films. However, the cross-section SEM images of samples prepared during 30, 50, 60, 70, 80, and 90 min show homogeneous films. In addition, taken into account that at the used wavelength in the LBIC investigations (He-Ne: 633 nm), the corresponding values of the total reflectivity (Figure ) cannot explain, for example, why the L value of the sample prepared at 90 min is greater than that one prepared at 30 min. Consequently, we attribute these variations not to the total reflectivity, but to the carriers’ trapping at surface defects. For this purpose, we consider the schemes given in Figure . Thus, when the MIS diode contains homogeneous SiNWs, a great amount of photo-generated electrons by the laser beam can reach the top metal contact. However, when SiNWs are not homogeneous, surface recombination at small wires reduces the LBIC current value, leading to a decrease in the effective diffusion length.
Schematic illustration of the MIS diode when the SiNWs film is homogeneous and not.