Influence of hydrogen peroxide on the photoanodization of $n$-Si in the breakdown mode

The electrochemical etching of $n$-Si (100) in an electrolyte composed of 4% HF solution in 30% hydrogen peroxide is experimentally studied at a voltage exceeding the breakdown voltage. The effect of the illuminance of the wafer back side on the porous-structure morphology and such parameters as porosity, effective valence, and pore growth rate are examined. The data obtained are compared with those for structures subjected to photoanodization in an aqueous electrolyte at the same HF concentration. It is found that the presence of hydrogen peroxide strongly changes the morphology of macropores, makes their diameter smaller, and raises by a factor of $\sim$2 the rate of growth deeper into the substrate. In the presence of H$_2$O$_2$, there appear inclined secondary pores oriented at an angle of 15$^{\circ}$–35$^{\circ}$ to the main channel axis and a number of breakthrough mesopores propagating in the $\langle$100$\rangle$ directions in the plane parallel to the sample surface. The effective valence of the electrochemical dissolution of silicon in the HF:H$_2$O$_2$ electrolyte at a low illumination level is close to unity and grows with increasing light intensity, always being smaller than 2.