Dear Sudhaama Sir Shankaracharya is supposed to be an "extension" of Shiva.He is not a avathar of Lord Shiva.Shiva had chastised him once which made him sing Manisha Panchakam.He was also a normal human being who was blessed by Shiva.He has composed slokas on all Gods/Godesses.He finally merged with shiva lingam, i think.