the answer to your question is SP 2 becoz the nitrogens lone pair are delocalized in  the d orbitals of silicon

chk this as a proof

Title:	Molecular structure of nitrogen trichloride as determined by electron diffraction
Author(s):	Burgi, H. B. Stedman, Donald H. Bartell, Lawrence S.
Issue Date:	Oct-1971
Publisher:	Elsevier
Citation:	Burgi, H. B., Stedman, D., Bartell, L. S. (1971/10)."Molecular structure of nitrogen trichloride as determined by electron diffraction." Journal of Molecular Structure 10(1): 31-38. <http://hdl.handle.net/2027.42/33551>
Abstract:	Nitrogen trichloride was found to have a bond length of rg = 1.759 +/- 0.002 A and a Cl-N-Cl angle of 107.1 +/- 0.5[deg]. The bond angle is larger than that found in NF3, consistent with the (recently revised) trends displayed by the trihalides of phosphorus and arsenic, but much lower than the 120[deg] angle reported for the isoelectronic molecule N(SiH3)3. Moreover, a comparison between selected compounds reveals that the N-Cl bond length is appreciably greater, relatively, than the N-Si bond length. Accordingly, the bond angles and bond lengths suggest a greater reluctance of the nitrogen lone pairs to delocalize onto Cl than onto SiH3 groups. Mean amplitudes of vibration of NCl3 were derived both from the diffraction data and from recently published infrared and Raman frequencies. The values agree within the estimated uncertainties.
Persistent URL (URI):	http://hdl.handle.net/2027.42/33551 http://www.sciencedirect.com/science/article/B6TGS-44BMWGK-5 0/2/6299ff3e7ec099f3128edf8fdea410b2
Other Identifiers:	10.1016/0022-2860(71)87058-8
Appears in Collections:	Interdisciplinary and Peer-Reviewed

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you can confirm this on the following link

http://deepblue.lib.umich.edu/handle/2027.42/33551