Effect of Sputtering Atmosphere on ZnSnN₂ Thin Films Electrical and Optoelectronic Properties

ZnSnN2 is a member of ternary Zn-IV-N2 semiconductors, where IV = Si, Ge or Sn. According to theoretical calculations and preliminary experimental results, Zn-IV-N2 semiconductor materials afford same bandgap range than III-nitride semiconductors. It is also expected similar electronic and optical properties as for InGaN, for example, direct bandgaps and large optical absorption coefficients [1][2]. Based on first-principles calculations, ZnSnN2 becomes a promising material for photovoltaic. Exhibiting a bandgap value around 1.7 - 2 eV, it can provide an interesting alternative furnishing an inorganic thin film solution for tandem cell over silicon [3]. The most stable wurtzite-derived structure of bulk ZnSnN2 was given to be the orthorhombic Pna21 phase by calculating the total energy per unit cell, band structure and electronic density of states [4]. ZnSnN2 can be synthesized from various methods, in which sputtering technique tends to dominate. The primary advantages of using sputtering are high deposition rates, application for most materials, high-purity and adhesive films, ability to coat heat-sensitive substrates (glass substrates, plastic substrates) and automation capabilities [5]. It provides, moreover, a realistic way of integration with Si based bottom cells. By changing the experiment conditions, band gap can expand to 2 eV [6], n-type carrier concentration obtained 7.01x1017cm3 and high mobility receive up to 15.5 cm2/V.s [7]. In this work, ZnSnN2 thin films were produced on glass substrates by co-sputtering Zn and Sn targets at room temperature in a reactive atmosphere of argon, nitrogen and hydrogen. By changing the composition of this atmosphere, the optical and electrical properties of material are changed. The morphology and crystal structure were investigated by using SEM, AFM and X-ray diffraction. The optical band gap ranges from 1.8eV to 2.5 eV and was deduced from UV-VIS spectroscopy. The n-type carrier concentration was measured between 2.5e17 to 2.4e20 cm-3, and the highest carried mobility is 2.0 cm2/V.s. First photoconductive samples were fabricated and tested under solar simulator. Measurement gave a photocurrent value of 148mA/cm2. These results are promising ones for the potential application of ZnSnN2 material as a photovoltaic absorber. References: [1] A. Punya, W. R. L. Lambrecht, and M. Van Schilfgaarde, “Quasiparticle band structure of Zn-IV-N2 compounds,” Phys. Rev. B - Condens. Matter Mater. Phys., vol. 84, no. 16, pp. 1–10, 2011. [2] A. Punya., T. R. Paudel, and W. R. L. Lambrecht, “Electronic and lattice dynamical properties of II-IV-N2 semiconductors,” Phys. Status Solidi C, vol. 8, no. 7, pp. 2492–2499, 2011. [3] T. P. White, N. N. Lal, and K. R. Catchpole, “Tandem solar cells based on high-efficiency c-Si bottom cells: Top cell requirements for >30% efficiency,” IEEE J. Photovoltaics, vol. 4, no. 1, pp. 208–214, 2014. [4] L. Lahourcade, N. C. Coronel, K. T. Delaney, S. K. Shukla, N. A. Spaldin, and H. A. Atwater, “Structural and optoelectronic characterization of RF sputtered ZnSnN 2,” Adv. Mater., vol. 25, no. 18, pp. 2562–2566, 2013. [5] S. Swann, “Magnetron sputtering,” Phys. Technol., vol. 19, no. 6, pp. 67–75, 1988. [6] N. Feldberg et al., “Growth, disorder, and physical properties of ZnSnN2,” Appl. Phys. Lett., vol. 103, no. 4, 2013. [7] K. kumar Chinnakutti, V. Panneerselvam, and S. Thankaraj Salammal, “Tailoring optoelectronic properties of earth abundant ZnSnN2 by combinatorial RF magnetron sputtering,” J. Alloys Compd., vol. 772, pp. 348–358, 2019.