Can we take one slab of p - type semiconductor and physically
join it to another n - type semiconductor to get p - n junction?

Explanation

No! Any slab, howsoever flat, will have roughness much larger than the inter-atomic crystal spacing `(~2 " to" " 3 " Å )` and hence continuous contact at the atomic level will not be possible. The junction will behave as a discontinuity for the flowing charge carriers.