Journal
PHYSICAL REVIEW B
Volume 80, Issue 17, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.80.174105
Keywords
II-VI semiconductors; polymorphism; semiconductor thin films; solid-state phase transformations; stoichiometry; surface energy; vibrational modes; wide band gap semiconductors; zinc compounds
Funding
- Science Foundation Ireland: PI [06/IN.1/I92]
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Stoichiometric B4 thin films have formally divergent surface energies, which arise from the intrinsic dipole of the unit cell. Previous density functional theory studies have predicted that below a critical thickness this results in relaxation to the nonpolar planar h-MgO structure. The calculations presented here demonstrate that h-MgO-structured ZnO thin films are themselves unstable with respect to further relaxation to the d-BCT structure, which restores near-tetrahedral local coordination while minimizing the surface dipole. Although the B4 -> h-MgO relaxation is disfavored for slabs thicker than 20 layers, d-BCT is predicted to be the favored polymorph for slabs up to 54 layers. Nudged elastic band calculations and vibrational analysis indicate that the h-MgO -> d-BCT relaxation is spontaneous at nonzero temperatures.
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