Growth, disorder, and physical properties of ZnSnN2

Abstract

We examine ZnSnN2, a member of the class of materials contemporarily termed “earth-abundant element semiconductors,” with an emphasis on evaluating its suitability for photovoltaic applications. It is predicted to crystallize in an orthorhombic lattice with an energy gap of 2 eV. Instead, using molecular beam epitaxy to deposit high-purity, single crystal as well as highly textured polycrystalline thin films, only a monoclinic structure is observed experimentally. Far from being detrimental, we demonstrate that the cation sublattice disorder which inhibits the orthorhombic lattice has a profound effect on the energy gap, obviating the need for alloying to match the solar spectrum.

Charge-neutral disorder and polytypes in heterovalent wurtzite-based ternary semiconductors: The importance of the octet rule

“…This reduction in band gap is even greater than the approximately 1 eV difference reported by Feldberg et al 15 Similar trends were reported in the comparison of the calculated band gaps and total energies of the ordered and disordered ternary zincblendebased ZnSnP 2 system, with larger band gaps and smaller energies of formation for the two ordered, octet-rulepreserving CuAu and chalcopyrite structures, compared to an ordered octet-rule-violating structure, and to a structure with Sn and Zn atoms placed randomly on the cation sublattice. 44 For the ZnSnN 2 system, the defect calculations by Chen et al 20 also provide some insight, although less directly, into the reductions in band gaps that can be expected from violations of the octet rule.…”

“…It was suggested that controlling the degree of this disorder would allow the controlled variation of the band gap from 1 to 2 eV. 15 However, measurement of the band gap by photoluminescence excitation spectroscopy 19 of material that looked wurtzitic in x-ray diffraction gave a gap in close agreement with accurately predictive quasiparticle band structure calculations for perfectly ordered P na2 1 crystals, using experimentally determined lattice parameters. 13,26 The main question we address in this paper is how to reconcile these seemingly contradictory observations.…”

“…5(a). None of the superlattice peaks or expected peak splittings have been observed for ZnSnN 2 , 15,17,19 while superlattice peaks and the expected peak splittings for P na2 1 ordering have been reported for ZnGeN 2 . 29 Because the calculated ratios of the lattice parameters are closer to the ratios for the ideal wurtzite lattice for ZnSnN 2 than for ZnGeN 2 , as shown later in Table I, the splittings of the degeneracies in the diffraction peaks due to distortions from the idealized wurtzite structure are expected to be larger for ZnGeN 2 than for ZnSnN 2 .…”

“…19, and in the other reports of x-ray diffraction measurements of ZnSnN 2 . 15,17 Figure 8(a) shows the dependence of the intensities of the superstructure peaks on the number of layers, relative to the P na2 1 (221) peak, which is the strongest peak common to the P na2 1 , P mc2 1 and wurtzite crystal structures. This result illustrates that the measurement of the diffraction spectrum as a function of the number of layers should be a powerful method to probe the disorder.…”

“…13 The synthesis of ZnSnN 2 in particular has fueled interest in its use for solar photovoltaics, especially because this material is composed solely of earthabundant elements. [14][15][16][17][18][19][20][21] As is the case for the zincblendebased ternaries, the type of ordering and the possibility of order-disorder transitions in the wurtzite-based ternaries are of fundamental as well as practical interest. For ZnGeN 2 and ZnSnN 2 the parent binary materials are the III-nitrides, and the parent binary phase is wurtzite.…”