In this paper, we study the metal-catalyzed synthesis of Si nanowires (Si-NWs) in a plasma based chemical vapor deposition system. In these deposition systems due to the high efficiency of precursor molecule dissociation, both uncatalyzed and catalyzed growth mechanisms can take place. The first one gives rise to the formation of the quasi one-dimensional (1D) Si-NWs, while the second one to a continuous two-dimensional (2D) Si layer over the substrate or on the nucleated Si-NWs. The Si-NWs formation is then the result of the competition between these two processes. The control parameters ruling these two contributions are here explored. Samples with different weights of 1D and 2D growth are deposited and characterized by using a plasma based chemical vapor deposition apparatus operating at T < 400 °C. It is found that the main control parameter of these processes is the plasma power through the distribution of the precursor dissociation products. By properly tuning the power, Si-NWs with 1 × 1010 cm−2 of density, up to 1 μm long and without uncatalyzed growth are obtained. The optical functionality of the samples, grown with different 1D/2D contributions, is investigated and it is demonstrated that the uncatalyzed layer produces a total reflectance as high as ∼40%, similar to that found in a planar Si wafer, while the highly dense Si-NWs, without the uncatalyzed deposition, produce a total reflectance of ∼15%.