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Materials Science at Oregon State University

Growth and Characterization of Metastable Heterostructural Alloys: a Novel Method for Materials Optimization

Date: Thursday, Apr 14th
Presenter: Bethany Matthews, OSU Physics


Alloying has been a common way to tune and manipulate material properties. Up to now, studies of alloys have primarily focused on homogeneous alloys, resulting in materials which are typically stable or have low enthalpies of mixing. However, heterogeneous alloys, often highly metastable with high enthalpies of mixing, offer the additional contribution of a structural change, at some composition, to alter the properties. The study of heterogeneous alloys allows for better understanding of metastability and the benefits obtainable from metastable states. This type of alloying can be applied to anisotropic materials with correspondingly anisotropic properties like orthorhombic SnS, widely considered for applications in PV devices due to high absorption and suitable carrier concentration. SnS has carrier mobilities which are excellent in-plane, but are quite low out-of-plane, and thus lower inter-layer current, rendering it unsuitable for solar cell application. By alloying with an isotropic material, the properties would become isotropic, including the mobility.
Here we demonstrate analysis of alloying of heterogeneous materials: orthorhombic SnX and cubic CaX (X=S,Se) and wurtzite ZnO and cubic MnO. The alloys were deposited as thin films by pulsed layer deposition or sputtering. Deposition parameters were explored to obtain solubility limits. The optical and structural properties were measured and analyzed as a function of composition. The compositional regions of structural transition were investigated with particular interest as properties were predicted to change much more dramatically.