Browsing by Author "Lu, Chi-Ken"

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Effect of defect-enhanced molecular oxygen adsorption on the imbalance of hole versus electron mobility in conjugated polymers
(2007-05-01)
Lu, Chi-Ken
;
Pi, Shu-Ting
;
Meng, Hsin-Fei
;
物理研究所
;
Institute of Physics
The generally observed higher hole mobility relative to electron mobility in conjugated polymers is explained with the defects and adsorbed molecular oxygen. Adsorption of the extrinsic molecular oxygen leads to that electrons are bound more tightly than holes by the traps in the originally symmetric electronic system. Hence, the mobility imbalance emerges from the asymmetric binding energies. Besides, the defects are the favored adsorption sites because the intermolecular attraction is enhanced due to stronger induced dipole-dipole interaction when gap defect levels appear.
Hole doping by molecular oxygen in organic semiconductors: Band-structure calculations
(2007-06-01)
Lu, Chi-Ken
;
Meng, Hsin-Fei
;
物理研究所
;
Institute of Physics
The sensitive effect of O-2 adsorption on the electronic properties of organic semiconductors is investigated by band structure calculation. O-2 can actually p-dope the host materials even without illumination, i.e., a ground state property, in the circumstance of saturated coverage. Due to hybridization between O-2 and polymer, Fermi level of the oxygenated system is pinned at the nearly half-filled oxygen band and overlap with host valence band. The doping depends critically on the ionization potential. Each O-2 can dope more than 0.1 hole in dark and a full charge-transfer excitation around 2.3 eV photon energy is predicted.
Population inversion in a p-doped quantum well with reduced photon energy
(2006-07-01)
Lu, Chi-Ken
;
Meng, Hsin-Fei
;
Han, Pin
;
物理研究所
;
Institute of Physics
We study a multilayer silicon-germanium quantum well structure doped with acceptor impurities for resonant-state lasers capable of emitting photons of energy below 4 meV (1 THz). Unlike previous proposals on terahertz lasers in doped silicon-germanium quantum wells, the emitted photon energy does not need to exceed the acceptor binding energy, which is tens of meV. The energy constraint is relaxed by placing the acceptor impurity levels and the quantum well subband continuum in separate layers of different germanium compositions. We calculate the nonequilibrium behaviors of the holes in detail and demonstrate that population inversion between strain-split impurity levels can be built for sufficiently high acceptor densities under the application of a moderate dc electric field at about ten kelvins.