Structural and functional properties of Zn(Ge,Sn)N2 thin films deposited by reactive sputtering

Abstract

Semiconductor alloys ZnSnxGe1-xN2 have theoretical crystal structure and electronic structure similar to that of InGaN alloys. These promises of direct and tunable band gaps are very attractive to unlock a suite of functionality for these nitride semiconductors, namely for the use in long wavelength light emitters and light absorbers for solar cells. We report here a structural, electrical and optical investigation of sputtered ZnSnxGe1-xN2 films for 0 ≤ x ≤ 1 by gradually substituting germanium with tin. Compared to InGaN alloys which suffer from a miscibility gap and exhibit phase segregation beyond ~20% In, ZnSnxGe1-xN2 form advantageously a continuous alloy for 0 ≤ x ≤ 1. Its adjustable lattice parameter a (from 3.22 Å to 3.41 Å) according to Vegard's law as well as the linear variation of the vibration modes by Fourier transform infrared spectroscopy indicate that the ZnSnxGe1-xN2 alloying is achievable without phase separation. The single chemical environment measured by Mössbauer spectroscopy for Sn4+ ions, whatever Sn content in ZnSnxGe1-xN2, confirms the continuous nature of alloying. Samples exhibit semiconducting properties, including optical band gaps and electronic behaviors with temperature. The experimental observations show that the resistivity in ZnSnxGe1-xN2 alloys can cover several orders of magnitude from a “quasi-metallic” (for ZnSnN2) to a “quasi-insulating” (for ZnGeN2) behavior and that the band gap is tunable from 2.1 eV to 3.04 eV with a nearly linear dependence on the composition. Thus, ZnSnxGe1-xN2 materials offer a solution for bandgap tunability in nitride semiconductors, and may enable enhanced functionality such as efficient green and red light emitters and light absorbers for photosynthetic devices.