John E. Northrup | Searcharxiv

arXiv2011

Structural origin of gap states in semicrystalline polymers and the implications for charge transport

We quantify the degree of disorder in the π-π stacking direction of crystallites of a high performing semicrystalline semiconducting polymer with advanced X-ray lineshape analysis. Using first principles calculations, we obtain the density of states of a system of π-π stacked polymer chains with increasing amounts of paracrystalline disorder. We find that for an aligned film of PBTTT the paracrystalline disorder is 7.3%. This type of disorder induces a tail of trap states with a breadth of ~100 meV as determined through calculation. This finding agrees with previous device modeling and provides physical justification for the mobility edge model.

cond-mat.mtrl-sci↗

arXiv1998

Clean and As-covered zinc-blende GaN (001) surfaces: Novel surface structures and surfactant behavior

We have investigated clean and As-covered zinc-blende GaN (001) surfaces, employing first-principles total-energy calculations. For clean GaN surfaces our results reveal a novel surface structure very different from the well-established dimer structures commonly observed on polar III-V (001) surfaces: The energetically most stable surface is achieved by a Peierls distortion of the truncated (1x1) surface rather than through addition or removal of atoms. This surface exhibits a (1x4) reconstruction consisting of linear Ga tetramers. Furthermore, we find that a submonolayer of arsenic significantly lowers the surface energy indicating that As may be a good surfactant. Analyzing surface energies and band structures we identify the mechanisms which govern these unusual structures and discuss how they might affect growth properties.

cond-mat↗