Have you characterized your intrinsic carrier concentration or electrical conductivity?  IV-VI materials tend to have high intrinsic defect concentrations, usually stemming from non-stoichiometry (IV:VI ratio).  You may wish to check what your baseline process is producing and optimize that in one of two ways:

1. Adjust growth conditions to minimize intrinsic conductivity and carrier concentration.  This will make it easier to observe the impact of extrinsic dopants.

2. Adjust growth conditions to achieve the desired conductivity and majority carrier type using intrinsic carriers.

Can you share more information about your growth technique.  This could help other provide more details on how to proceed.

by increasing the doping concentration of metal material

You can improve the conductivity of a p-type alpha-SnS sample by either increasing the doping density or increasing the mobility of charge carriers. (Also SnS is highly anisotropic interms of conductivity along different crystal planes therefore orientation of samples may also influence conductivity).  Annealing in H2S at ~300C has been proven to increase the crystalline size of SnS samples and thus result in a decreased resesitivity, however, authors report that this step increases the concentration of charge carriers and not necessarily increase their mobility. I wouldn't suggest annealing in H2S as its highly toxic and only do this if you have the correct ventilation and adequate gas scrubber system. I have also tried annealing SnS in a sealed furnace consisting of elemental chalcogens (Ch) this often leads to the oxidation of the SnS surface forming SnCh2 phases.  I would either suggest changing the deposition parameters and optimize your conditions, a suggested publication Doi:10.1016/j.cap.2015.03.026. 

Or follow the thermal evaporation process optimized here for high efficiency SnS solar cells Doi: 10.1002/adma.201402219,

I would like that you see see this paper, it may be of help to you:Structual and thermoelectric properties of undoped IV-VI epitaxial films alloyed with tin,

at the link:http://home.agh.edu.pl/~ets2004/proceedings/Koening.PDF,

The concept of doping is by alloying. You can add Pb. to substitute a fraction of Sn.

wish you success

Hi Reddy,

I think Post annealing in RTP  in a vacuum in the ambient environment of Ar/N2 gas phenomenon is quite simple and effective in this regard.

Thank you very much for the kind information and suggestions!

Dr N K Reddy