zener breakdown only occurs in zener diodes with a volatge below 5-6V. above which avalanche breakdown occurs. zener beakdown occurs in highly doped semiconductor junctions where the depletion region with is very small. the breakdown occurs due to high electric field across the depletion region (E=V/w) since the depletion width w is very small the electric field is very high which leads to rupture of covalent bonds which leads to carrier generation which is a cascading effect and leads to zener breakdown. Avalance breakdown is in lightly doped junctions in which the minority carriers strike the atoms releasing more carriers and thus cascading this effect leading to avalanche break down.
u can refer to some good books on semiconductors by authors like M.S.Tyagi, S.M.Sze, Ben G. Streetman. we used to study these in our graduation and masters.