Browsing by Author "Yen, S. T."

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Cyclotron masses and g-factors of hybridized electron-hole states in InAs/GaSb quantum wells
(2006-08-01)
Nilsson, K.
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Zakharova, A.
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Lapushkin, I.
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Yen, S. T.
;
Chao, K. A.
Using the eight-band k center dot p model and the Burt-Foreman envelope function theory to perform self-consistent calculations, we have studied the effect of electron-hole hybridization on the cyclotron masses m(*) and the effective g-factors g(*) of two-dimensional quasiparticles in InAs/GaSb quantum wells under a magnetic field applied perpendicular to the interfaces. We can modify the degree of hybridization by changing the InAs and/or GaSb layer width, or by inserting a thin AlSb barrier. While electron-light-hole hybridization dominates at both low and high fields, due to a sequence of anticrossings between electronlike and heavy-holelike levels, there is also an important contribution from heavy-hole states to the strong hybridization in the intermediate field range. The field-dependence of the hybridized energy eigenstates is manifested in the variations of m(*) and g(*). Characteristic discontinuous changes of both m(*) and g(*) appear at each anticrossing, resulting in a magnetic-field-driven oscillating behavior of these quantities for electronlike states of a given Landau level index. The electron g-factor can change sign when two eigenstates anticross. Hybridization of electron and hole states enhances the electron effective mass, and we have found a complicated dependence of this effect on the interaction strength. Without inserting an AlSb barrier, the strong interaction between the electronlike and the light-holelike states at low magnetic fields produces a large level repulsion, and hence relatively small effective masses and g-factors associated with these states. Intermediate interaction leads to weaker level repulsion and therefore very heavy electron cyclotron masses as well as large g-factors associated with the lowest Landau levels. A weak interaction only enhances the cyclotron masses of the electronlike states slightly. The hole effective masses change with both the magnetic field and the sample structure in a more complicated fashion.
Electric-dipole transitions between group-III acceptor states in uniaxially compressed Ge
(2010-09-01)
Wang, T. H.
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Yen, S. T.
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電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
We study the electric-dipole transitions between group-III acceptor states in Ge under stress along the [001] crystallographic direction in the effective mass approximation. We systematically investigate the cases of zero stress, infinitesimal stress, and finite stress including the low-stress and the high-stress regions. Our results show quantitative agreement with experimental data at zero stress and at infinitesimal stress. The relative intensities of infinitesimal-stress-induced components of transitions from the 1 Gamma(+)(8) state to the n Gamma(-)(8) states do not correlate significantly with the species of acceptors except for the transition to the 1 Gamma(-)(8) state. The oscillator strengths of some transitions are susceptible to the stress in the low-stress region (<0.3 kbar), and could be zero at a specific stress. The behaviours of the stress dependence of oscillator strengths for different transition lines are explained in terms of the compositions of the wavefunctions and the dipole matrix elements. In the high-stress region (greater than or similar to 3 kbar), the ground state is s-like, and only the transitions to the p-like states can have non-negligible oscillator strengths. The photon absorbed (emitted) and associated with each electric-dipole transition between the s-like and the p-like states is polarized either parallel or perpendicular to the stress direction. We also calculate the absorption spectra for Ge:Ga at liquid-helium temperature. The results are in good agreement with experiment.
Electronic structure analysis for group III acceptors in Ge under stress considering screening effect and central-cell correction
(2009-08-19)
Wang, T. H.
;
Yen, S. T.
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
We study theoretically the electronic structures of various group III acceptors in Ge under [001] stress, based on the effective-mass theory with a semi-empirical impurity potential which considers the q-dependent screening and the central-cell correction. An assignment is made for inter-level transition lines which were previously ignored or incorrectly assigned. In addition, our calculation can resolve crowding levels of final states of transition lines which have not been resolved by experimental techniques. The stress effect on the electronic structure can be understood by connecting with the composition of the states. Our results show that the binding energies decrease rapidly with the stress in the low-stress region, and for even-parity states they exhibit remarkable asymmetry in the stress dependence due to the large difference between the heavy-hole and the light-hole compositions. The acceptor states asymptotically approach a pure heavy-hole or light-hole state under high stress. In the limiting case of high stress, extra degeneracy appears. The central-cell correction may cause a significant chemical shift for even-parity states of nonisocoric acceptors. We also complete the assignment of the four line components into which the B line splits under stress. The newly assigned stress-dependent transition energies show excellent agreement with the experimental data for low stress. A justification is made for the applicability of our calculation scheme to the case of high stress.
Negative and positive electroluminescence from a compensated p-type germanium in terahertz frequencies
(2007-12-10)
Chung, P. K.
;
Lin, J. M.
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Yen, S. T.
;
Wang, T. H.
;
電子工程學系及電子研究所
Negative luminescence (NL) and positive luminescence in terahertz frequencies were observed from a compensated p-type Ge under various levels of pulse excitations at cryogenic temperatures. The NL spectra accompanying an S-type negative differential resistance phenomenon were attributed to the reduced emission transitions from the excited states to the ground states of the acceptors during the excitation. A possible excitation mechanism is also presented according to the measured data. (C) 2007 American Institute of Physics.