First-Principles Defect Thermodynamics of MgSnN2 : Origin and Suppression of Unintentional n-Type Conductivity

Sammanfattning

The current development of sustainable technologies has increased the interest in new sustainable alternatives to established III–nitride semiconductor materials. Earth abundant and non-toxic ternary II–IV–N2 semiconductors are currently being studied for this role. In this thesis, the recently developed semiconductor MgSnN2 is studied using ab initio density functional theory. This is motivated by the suitable band gap of the material for the so-called “green-gap” spectral region, making it promising for optoelectronic applications. A current challenge for the material is the unintentionally high electron concentration in the conduction band, unintentionally rendering the semiconductor n–type, which results in less favorable electronic properties than desired. This thesis addresses this problem by searching for the defects most likely responsible for this behavior and by investigating potential ways to suppress them. The study reveals that within the band gap, there is no competing native defect capable of suppressing the dominant donor defect SnMg, which is identified as the primary origin of the excess electrons observed in MgSnN2. Different growth conditions were investigated to suppress the concentration of this defect, where the strongest suppression was obtained under Sn–poor conditions at growth relevant temperatures around 600 K. However, even under these conditions, the reduction remained limited to approximately a factor of 2.2 due to the self–consistent charge neutrality response of the dominant doubly ionized Sn+2Mg donor defect, which fundamentally limits the achievable suppression. The results therefore indicate that native chemical potential engineering alone is insufficient to fully suppress the unintentional n–type conductivityin MgSnN2, making stable equilibrium p–type doping through both native and extrinsic defect engineering highly challenging.