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

(NO) No,it is not possible. Any slab,no matter how flat it is,will have surface roughness much larger than the inter-atomic crystal spacing $(\sim 2$ to $3 \mathring{A})$.
Because of this roughness,continuous contact at the atomic level cannot be achieved.
The junction would behave as a discontinuity for the flowing charge carriers,preventing the formation of a proper $p-n$ junction.