Formation of macropores in $n$-Si upon anodization in an organic electrolyte

The photoelectrochemical etching of solar-grade $n$-type silicon in a 4% solution of HF in dimethylformamide is experimentally studied. The pore morphology, porosity, effective valence, and etching rate are examined in relation to the applied voltage, illumination intensity of the sample's backside, and process duration. It is found that the anodization of $n$-Si in an organic electrolyte substantially differs from that in aqueous solutions. This is manifested in that, at a voltage exceeding the threshold value, in the so-called breakdown mode, macropores with vertical walls begin to multiply and branch intensively due to the appearance of side pores. The appearance of secondary pores is accompanied by an increase in porosity, a decrease in the propagation velocity of the porous front deeper into the substrate, and rapid transition to the electropolishing mode. In the breakdown mode at a low illumination level, a fractal structure of macropores propagating along certain crystallographic directions is observed: $\langle$100$\rangle$ and along the previously unobserved $\langle$111$\rangle$. It is demonstrated that the morphology of macropores can be controlled in the course of anodization by passing from one mode to another upon changing the external parameters: voltage or illumination. It is shown that using an organic electrolyte makes it possible to obtain macroporous membranes with a porosity substantially exceeding that of macroporous membranes formed in aqueous electrolytes under the same conditions.