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 of p-type or n-type semiconductor, howsoever flat its surface may be, will have roughness on its surface much larger than the interatomic crystal spacing `(~~2 "to" 3 Å)`. When such slabs of p-type and n-type semiconductors are brought physically incontact, a continuous contact at the atomic level will not be possible at the junction. Due to it, the junction will behave as a discontinuity for the flowing charge carrier.