  Investigations on II-Sn-N2 (II = Mg, Ca) have been started very recently
compared to the intense research of the Zn-IV-N2 (IV = Si, Ge, Sn). In this
work, we perform a comprehensive study of cation disorder in stoichiometric
MgSnN2 and crystal structure characteristic, doping behavior of
off-stoichiometric Mg1+xSn1-xN2 (x = -0.8, -0.6, -0.5, -0.4, -0.2, 0.2, 0.4,
0.5, 0.6, 0.8) by using the cluster expansion method and first principles
calculations. It is found that cation disorder in stoichiometric MgSnN2 induces
a band gap reduction because of a violation of the octet rule. Moreover, the
local disorder, namely forming (4,0) or (0,4) tetrahedra, would lead to an
appreciable band gap reduction and hinder the enhancement of the optical
absorption. An off-stoichiometric Mg/Sn ratio can strongly affect the
morphology of Mg1+xSn1-xN2 samples due to the higher ionicity of the Mg-N bonds
in comparison with Zn-N bonds. Furthermore, Mg1+xSn1-xN2 compounds show an
ambipolar self-doping behavior, i.e., Mg-rich Mg1+xSn1-xN2 show p-type doping
while Sn-rich ones exhibit n-type doping owing to the formation of
acceptor-type antisite defect MgSn or donor-type antisite defect SnMg,
respectively.
