Combinatorial Insights into Doping Control and Transport Properties of Zinc Tin Nitride

TL;DR

This study employs a combinatorial sputtering approach to optimize growth conditions for ZnSnN2, revealing key insights into its doping, bandgap tuning, and transport properties relevant for photovoltaic applications.

Contribution

It introduces a systematic exploration of growth parameters for ZnSnN2, identifying conditions for low carrier density, large grain size, and bandgap tunability, advancing understanding of its fundamental properties.

Findings

Optimal growth temperature at 230°C with Zn/(Zn+Sn)=0.60 yields large grains and low carrier density.

Evidence of Burstein-Moss shift affecting the apparent bandgap in Sn-rich compositions.

Carrier density can be tuned by adjusting cation composition, indicating defect complex formation.

Abstract

ZnSnN2 is an Earth-abundant analog to the III-Nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. Despite these desirable properties, discrepancies in the fundamental bandgap and degenerate \emph{n}-type carrier density have been prevalent issues in the limited amount of literature available on this material. Using a combinatorial RF co-sputtering approach, we have been able to explore a growth-temperature-composition space for Zn(1+x)Sn(1-x)N(2) over the ranges 35-340 degrees C and 0.30-0.75 Zn/(Zn+Sn). In this way, we were able to identify an optimal set of deposition parameters for obtaining as-deposited films with wurtzite crystal structure and carrier density as low as 1.8 x 10^(18) cm^(-3). Films grown at 230 degrees C with Zn/(Zn+Sn) = 0.60 were found to have the largest grain size overall (70 nm diameter on average) while also exhibiting low carrier density (3 x 10^(18) cm^(-3)) and high mobility (8.3 cm^(2) V^(-1) s^(-1)). Furthermore, we report evidence of a Burstein-Moss shift widening the apparent bandgap as cation composition becomes increasingly Sn-rich, and tunable carrier density as a function of cation composition (lower carrier density for higher Zn content), which suggests the formation of defect complexes. Collectively, these findings provide important insight into the fundamental properties of the Zn-Sn-N material system, and also highlight the potential to utilize ZnSnN2 for photovoltaics.