2. SEM images of different SiNW structures 3.Results and discussion Fabrication and Characterization of silicon nanowires on Si (001) substrates Jinzhao Pan, Meiqi Guo, Zilong Wu, Zhenyang Zhong State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, PRC. 1. Introduction Silicon nanowires (NWs) have been considered as building blocks of next generation for novel electronic devices, biological/chemical sensors, optoelectronic devices, renewable energy devices due to their unique electrical and optical properties, as well as their compatibility with the sophisticated Si technology. Varieties of methods have been exploited to fabricate Si NWs, including vapor-liquid-solid(VLS) growth method, self-assembly on miscut Si (001) substrates, and etc. Recently, metal-assisted chemical etching method was developed as a top-down method, which has attracted much attention for advantages on low-cost and easy control of either diameter, length or orientation. Studies on fabrication progresses of SiNWs have been reported, according to which one can change SiNWs’ feature by changing substrate orientation, concentration of oxidant, solution temperature, etching time. 2.2 Separated SiNWs 2.3 ordered SiNWs 2. Sample preparation Fig 1. Fabrication sketch of ordered SiNWs. 2. SEM images of different SiNW structures 2.1 Disordered SiNWs Fig 2. SEM image of disordered SiNWs fabricated by chemical etch method assisted by a 20nm thick Ag layer. The scale bar is 5μm. Fig 3. SEM images of separated SiNWs. Fig 4. SEM images of ordered SiNWs. 3.Results and discussion Fig 6. Reflectivity of disordered SiNWs compared with bare Si. In Figure 5, steps are found on the surface of SiNWs. As the second prefer orientation of SiNWs on Si(001) substrate is <111> direction, the steps can be explained that: with the presence of external electrical field energy barrier for hole injection from gold film to the lateral sides of SiNWs is reduced, it becomes comparable to the energy needed for etching down, then etching along <111> directions become possible. The steps come out when etching direction changes from [111] to [1-11], the angel measured on the Fig 5 fits our explanation well. Reflectivity measurements are made on disordered SiNWs. As shown in Fig 6 samples with nanowire structure exhibit a significant decay in reflectivity over all measured wavelength, especially in shorter wavelength area. It possible to use SiNWs, prepared by our method, as antireflective layers in optoelectronic devices, such as solar cells. Fig 5. SEM image of separated SiNWs with electrical field existance druing etching. The inset shows the step on SiNW surface. The scale bar is 1μm. 4.Conclusion In summary, disordered , separated and ordered SiNWs on Si(001) substrates have been fabricated through metal-assisted chemical etching, SEM images of all SiNWs show that the existence of external electrical field have influence on the preference of etching direction, which gives a potential control of SiNWs orientation by controlling applied electrical field. Also, low reflectivity is confirmed for disordered SiNWs.