Using Xcrysden you can generate the slabs but not is POSCAR format. you can save as xsf file which can be opened with VESTA then save in POSCAR format.

You can use Modelview Software to generate any face of your bulk system. Modelview is under licence and it's cost about 600 euros. please, send an e-mail to: [email protected] .

You may try ASE Gui for that purpose. But in case of small surfaces, you can prepare it manually by understanding symmetry.

CIF2Cell can do this (see attached link). It is free and can solve this and any other cell setup problem that you might have.

Drawbacks:

1) you in general have to figure out how to construct your supercell map yourself, although there are some helpful tools for aligning some cells (see the manual for an example).

2) you have to start from a CIF file.

http://sourceforge.net/projects/cif2cell/

Castep (or Wien2k) can also be used to generate the structure file you wanted. However, you have to change the format of the file to adapt VASP

Please any can you tell me if any free software is available ......? for designing the surface model of structures 

Virtual NanoLab is good

Material Studio but is is only commercially available, not sure if VESTA will do the job,

VESTA can do this

There is also atomsk which is quite powerful

Avogadro is free and good