Morphology of silicon (111) during electrochemical etching in NH4F electrolytes

The n-type silicon with (111) orientation and resistivity of 8-12 .źcm was electrochemically etched. The results obtained by electrochemical etching of silicon in 0.1M NH4F (pH4.5), 0.1M NH4F (pH4.0) and 0.2M NH4F (pH4.0) electrolytes, indicates that slight increase of the NH4F concentration by 0.1M results in faster silicon dissolution. In slightly reactive 0.1M NH4F (pH4.5), we can observe initial stage of silicon dissolution on the inside edges of atomic terraces. Flat atomic terrace under electrochemical etching undergoes to mesa-type structures surrounded by strongly corrugated areas. The diameter and heights of the mesas are 150-200 nm and 5-7 nm, respectively. The mesa-type structures are the remnants of the atomic terraces and theirs formation proceed independently on the electrolyte concentration. The mesas appear faster at higher concentration and faster disappear by pits formation. During increase of etching time, increase of pore size, more in the case of diameter than depth, because pits coagulation appears.