PHASE TRANSITION AND ULTRAFAST CARRIER DYNAMICS AND TRANSPORT IN TWO-DIMENSIONAL SEMICONDUCTORS

Xin Tao

Single-crystal In2Se3 thin layers were fabricated, for the first time, using mechanical exfoliation, and the studies of crystalline-crystalline (α → β) phase transformations as well as the corresponding changes of the electrical properties in these thin layers. The results show that, in contrast to bulk single crystals, the β phase can persist in single-crystal thin layers at room temperature (RT) and has an electrical resistivity about 1-2 orders of magnitude lower than the α phase. Furthermore, we find that the temperature of the α → β phase transformation increases by as much as 130 K with the layer thickness decreasing from ~ 87 nm to ~ 4 nm. For these In2Se3 thin layers, the accessibility of the β phase at RT, with distinct electrical properties than the α phase, provides the basis for multi-level phase-change memories in a single material system. Carrier dynamics in single-crystal In2Se3 thin layers with various thicknesses was studied by femtosecond optical pump-probe reflectivity and ultrafast photocurrent measurements. The results suggest that, in thinner (thicker) layers, the carrier recombination dynamics is dominated by three-carrier (bimolecular) Auger process. The Auger time constant was found to decrease with deceasing layer thickness. Surface states were suggested to be the origin of the transition between different Auger processes as the layer thickness varies. Single-crystal two-dimensional (2D) CuInSe2 with various thicknesses were synthesized, for the first time, by a solid-state chemical reaction between Cu and single-crystal exfoliated In2Se3 nanosheets. The transient optical reflectivity, obtained from femto-second optical pump-probe measurements on single CuInSe2 nanosheets, suggest that the hot carrier cooling process dominates the carrier dynamics within a few ps following the optical excitation. Spatially resolved pump-probe measurements, coupled to simple model calculations, were used to obtain the ambipolar hot carrier diffusion coefficient in single nanosheets. The dependence of the hot carrier diffusion coefficient on the nanosheet thickness provides insight into the limiting mechanisms of hot carrier transport, and can be used to gauge the possibility of efficient hot carrier collection in nanostructured CuInSe2 solar cells.

PHASE TRANSITION AND ULTRAFAST CARRIER DYNAMICS AND TRANSPORT IN TWO-DIMENSIONAL SEMICONDUCTORS

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