Sammendrag
Nitride semiconductors possess a variety of properties, such as a high dielectric breakdown voltage and bandgap energies ranging from ultraviolet to infrared, favorable for scientific development and application in efficient tandem solar cells. The ideal bandgap for top cells has been calculated to be ∼ 1.75 eV in combination with Si bottom cells, in a two-terminal device. However, some challenges related to materials phase separation and lattice mismatch, as well as cost barriers related to the choice of rare materials and initial growth process, pose problems for consumer implementation. Thin films of ZnGexSn1−xN2 (where x = 0, 0.428 or 0.736) were grown on P:ZnO by reactive co-sputtering of metallic Zn, Sn and Ge targets employing high-power impulse magnetron sputtering (HiPIMS) and conventional RF sputtering. These materials were selected due to their earth-abundance and non-toxic properties. Structural properties were characterized by electron diffraction (ED) and chemical properties were analyzed with energy dispersive X-ray spectroscopy (EDS), both in transmission electron microscopy (TEM). In the present work, high resolution TEM and ED were used in combination to study the atomic structure and strain effects of thin films, which were grown along the [001] direction, perpendicular to the surface of the substrate, and a higher applied Ge target power predictably yields a higher Ge incorporation. A higher Ge content in the alloy is found to produce a more polycrystalline structure, in comparison to pristine ZnSnN2. Additionally, ED indicates that the film structure without Ge has a significant lattice mismatch with the ZnO substrate (a = (0.336 ± 0.004) nm for ZnSnN2, a = (0.319 ± 0.001) nm for ZnO, mismatch ∼ 5.3%) and the induced strain is relaxed through formation of misfit dislocations. In contrast, films with higher Ge incorporation (a = (0.328 ± 0.005) nm for x = 0.74, a = (0.331 ± 0.005) nm for x = 0.43) exhibit a better lattice matching with the substrate (a = (0.326 ± 0.001) nm for ZnO, mismatch ∼ 0.6% and ∼ 1.2% respectively).
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