Browsing by Author "Chao, KA"

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A self-consistent investigation of the semimetal-semiconductor transition in InAs/GaSb quantum wells under external electric fields
(2004-07-07)
Lapushkin, I
;
Zakharova, A
;
Yen, ST
;
Chao, KA
;
電子工程學系及電子研究所
We investigate the phase transitions in InAs/GaSb quantum wells sandwiched between two wide-gap AlSb barrier layers under an external electric field perpendicular to interfaces. The Schrodinger and the Poisson equations are solved self-consistently to derive the subband dispersions, the potential profile, the electron charge distribution in the InAs layer, and the hole charge distribution in the GaSb layer. The Burt-Foreman envelope function theory and the scattering matrix method are used to solve the Schrodinger equation in the framework of the eight-band k (.) p model, including the spin-splitting of subbands in our calculation. We have found that in a thick InAs/GaSb quantum well, which has been investigated experimentally by Cooper et al (1998 Phys. Rev. B 57 11915), under low external electric fields, two electron levels stay below the highest hole level at zero in-plane wavevector k(parallel to) = 0. Then, the anticrossings of electron and hole levels produce several minigaps in the inplane dispersions, inside which the states of other subbands exist. As a result, the system is in a sernimetal phase. With increasing external electric field, the semimetal phase changes to semiconductor phase with only one hybridization gap. When all electron levels become higher than the hole levels at higher electric fields, the system has a semiconducting gap.
Direct experimental evidence of the hole capture by resonant levels in boron doped silicon
(2005)
Yen, ST
;
Tulupenko, V
;
Cheng, ES
;
Dalakyan, A
;
Lee, CP
The variation of hole population for the localized and resonant states of B-doped Si excited by sequences of short electric-field pulses has been investigated by the technique of time-resolved spectroscopy. The capture of holes by resonant levels is verified. A new spectral line earlier theoretically predicted was experimentally observed for the first time.
Effects of lattice mismatch and bulk anisotropy on interband tunneling in broken-gap heterostructures
(2005-03-15)
Zakharova, A
;
Yen, ST
;
Nilsson, K
;
Chao, KA
;
電子工程學系及電子研究所
We have studied the effects of bulk anisotropy and the strain induced by lattice mismatch on the interband tunneling in broken-gap single-barrier InAs/AlSb/GaSb heterostructures and double-barrier InAs/AlSb/GaSb/InAs/AlSb/GaSb heterostructures. We have used the eight-band k.p model and the scattering matrix method, combined with the Burt envelope function theory, to calculate the interband transmission coefficients through the broken-gap heterostructures. We have found a noticeable anisotropy of the transmission coefficients when the magnitude of the in-plane wave vector increases to around 0.25 nm(-1). We have also found that the strain and the bulk anisotropy of quasiparticle dispersion produce additional peaks in the tunneling probability. For the double-barrier resonant-tunneling structures we discover a large spin splitting of the resonant-tunneling peaks caused by the lack of inversion symmetry. A strong influence of the strain induced by lattice mismatch appears in the current-voltage characteristics of the studied broken-gap heterostructures. In InAs/AlSb/GaSb structures the interband tunneling processes into the heavy-hole states contribute mainly to the peak current density if the sample is grown on InAs, but if the sample is grown on GaSb the interband tunneling processes into the light-hole states become the main contribution to the peak current density. As a result, the structure grown on GaSb has a much larger peak current density. This phenomenon was observed experimentally. (C) 2005 American Institute of Physics.
Evidence for capture of holes into resonant states in boron-doped silicon
(2004-11-01)
Yen, ST
;
Tulupenko, VN
;
Cheng, ES
;
Chung, PK
;
Lee, CP
The variation of hole population in the resonant states of B-doped Si excited by sequences of short electric-field pulses has been investigated by the technique of time-resolved step-scan far-infrared spectroscopy. From the variation of the p(3/2) absorptions, we find that the hole population in the ground state decreases continuously with the sequential electric pulses, as a result of the breakdown delay and hole accumulation in long-lived excited states. The measured time-varying spectra of the p(1/2) series have been analyzed and attributed to a significant variation of the hole population in the resonant states. We have also observed a new absorption line at 676 cm(-1) which is probably caused by the electric-field induced mixing of the resonant states. (C) 2004 American Institute of Physics.
Growth of high-quality Ge epitaxial layers on Si(100)
(2003-05-15)
Luo, GL
;
Yang, TH
;
Chang, EY
;
Chang, CY
;
Chao, KA
A method of growing high-quality epitaxial Ge layers on a Si(100) substrate is reported. In this method, a 0.8 mum Si0.1Ge0.9 layer was first grown. Due to the large lattice mismatch between this layer and the Si substrate, many dislocations form near the interface and in the lower part of the Si0.1Ge0.9 layer. A 0.8 mum Si0.05Ge0.95 layer and a 1.0 mum top Ge layer were subsequently grown on the Si0.1Ge0.9 layer. The formed interfaces of Si0.05Ge0.95/Si0.1Ge0.9 and Ge/Si0.05Ge0.95 can bend and terminate the upward-propagated dislocations very effectively. The in situ annealing process was also performed for each individual layer. Experimental results show that the dislocation density in the top Ge layer can be greatly reduced, and the surface is very smooth, while the total thickness of the structure is only 2.6 mum.
Hybridization of electron, light-hole, and heavy-hole states in InAs/GaSb quantum wells
(2001-12-15)
Zakharova, A
;
Yen, ST
;
Chao, KA
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
We have investigated the hybridization of the electron states. the light-hole states and the heavy-hole states in InAs/GaSb broken-gap quantum wells. This effect is profound Mien the InAs layer and the GaSb layer are sufficiently thick such that the electron level lies below the heavy-hole level and the light-hole level at zone center. To calculate the dispersions and the wave functions in these structures we have applied the scattering matrix algorithm to the eight-band k.p model. We have found a hybridization gap as large as 20 meV resulting from the anticrossing of the electron and the light-hole dispersion curves. A multiple anticrossing of the electron states. the light-hole states and the heavy-hole states may occur when the heavy hole level lies in the hybridization gap produced by the electron states and the light-hole states. This unusual hybridization of the three subbands. which behaves differently for the "spin-up" and the "spin-down" states. has been investigated in details around the anticrossing point, While the electronlike and light holelike states mix strongly, the heavy holelike state may remain unperturbed.
Landau level structures and semimetal-semiconductor transition in strained InAs/GaSb quantum wells
(2004-03-01)
Zakharova, A
;
Yen, ST
;
Chao, KA
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Using Burt's envelope function theory and the scattering matrix method, we investigate the hybridized electron-hole Landau levels in strained InAs/GaSb quantum wells sandwiched between wide-gap AlSb barrier layers under electric and a quantizing magnetic fields applied perpendicular to interfaces. At zero magnetic field, in the structures studied here, the lowest electron level in the InAs layer lies below the highest heavy-hole level in the GaSb layer. With increasing magnetic field, the electron levels move up and the heavy-hole levels move down, producing anticrossings and gaps in the Landau level structures. We have found that the Landau level structures depend strongly on the lattice-mismatched strain and the applied voltage. As a result, in the region before anticrossings, the g factor of the lowest electron Landau level has a larger value for the quantum well structure grown on GaSb than that for the structure grown on InAs, while in the region after anticrossings the situation reverses for the g factor. Under low magnetic field, the difference between the electron g factors for the structures grown on different substrates is found to be as large as 10 for zero bias and decreases significantly with increasing bias. When all electron levels become higher than hole levels at high magnetic fields, the semimetal-semiconductor transition occurs. The critical magnetic field B-c for the phase transition in structures grown on InAs is found to be lower than that in structures grown on GaSb. It is also obtained that a positive voltage biased across the InAs/GaSb well essentially decreases B-c. Therefore, for a fixed magnetic field, the semimetal-semiconductor transition can be controlled by a bias.
Six-band k center dot p calculation of spin-dependent interband tunneling in strained broken-gap heterostructures under a quantizing magnetic field
(2005-09-01)
Zakharova, A
;
Nilsson, K
;
Chao, KA
;
Yen, ST
;
電子工程學系及電子研究所
We investigate spin-dependent interband magnetotunneling processes in strained broken-gap resonant tunneling structures made from InAs, AlSb, and GaSb, which are promising materials for quantum devices. InAs/AlSb/GaSb/InAs/AlSb/GaSb double-barrier structures grown on both InAs and GaSb are considered. Transmission coefficients for interband tunneling processes from individual eigenstates in the InAs emitter as well as current-voltage characteristics were calculated using a six-band k center dot p model and the scattering matrix method. We predict that due to lattice-mismatch induced strain, the interband tunneling current density for the structure grown on InAs can be one or two orders of magnitude less than that for the structure grown on GaSb. Furthermore, as a consequence of interband magnetotunneling, structures grown on different substrates yield different spin polarization of the tunneling current. It is obtained that the current spin polarization can be greater than 90%. These resonant tunneling structures can be used as spin filters in the rapidly growing field of spintronics.
Spin Hall effect on edge magnetization and electric conductance of a 2D semiconductor strip
(2005-09-30)
Mal'shukov, AG
;
Wang, LY
;
Chu, CS
;
Chao, KA
;
電子物理學系
The intrinsic spin Hall effect on spin accumulation and electric conductance in a diffusive regime of a 2D electron gas has been studied for a 2D strip of a finite width. It is shown that the spin polarization near the flanks of the strip, as well as the electric current in the longitudinal direction, exhibit damped oscillations as a function of the width and strength of the Dresselhaus spin-orbit interaction. Cubic terms of this interaction are crucial for spin accumulation near the edges. As expected, no effect on the spin accumulation and electric conductance have been found in case of Rashba spin-orbit interaction.
Spin polarization of an electron-hole gas in InAs/GaSb quantum wells under a dc current
(2006-03-01)
Zakharova, A
;
Lapushkin, I
;
Nilsson, K
;
Yen, ST
;
Chao, KA
We have investigated the spin polarization of electrons and holes in InAs/GaSb broken-gap quantum wells under nonequilibrium conditions when a dc electric field is applied parallel to interfaces. The existence of a nonzero asymmetric part of the quasiparticle distribution function caused by the dc current and the spin-split of the electron-hole hybridized states generates a finite spin polarization in both the InAs layer and the GaSb layer. With a very weak asymmetry of the distribution function, our self-consistent calculation yields about 1% spin polarization for electrons in the InAs layer and holes in the GaSb layer. The signs of these spin polarizations depend on the widths of the layers, changing the widths drives a phase transition in the electron-hole gas between the hybridized semiconducting phase and the normal semiconducting phase. In the hybridized semiconducting phase, the spin polarizations in both the InAs layer and the GaSb layer have the same sign. Crossing the phase boundary, the electron spin polarization and the total spin polarization in the InAs/GaSb quantum well can change their signs.
Spin relaxation dynamics of quasiclassical electrons in ballistic quantum dots with strong spin-orbit coupling
(2004-12-01)
Chang, CH
;
Mal'shukov, AG
;
Chao, KA
;
物理研究所
;
Institute of Physics
We performed path integral simulations of spin evolution controlled by the Rashba spin-orbit interaction in the semiclassical regime for chaotic and regular quantum dots. The spin polarization dynamics have been found to be strikingly different from the D'yakonov-Perel' (DP) spin relaxation in bulk systems. Also an important distinction has been found between long time spin evolutions in classically chaotic and regular systems. In the former case the spin polarization relaxes to zero within relaxation time much larger than the DP relaxation, while in the latter case it evolves to a time independent residual value. The quantum mechanical analysis of the spin evolution based on the exact solution of the Schrodinger equation with Rashba SOI has confirmed the results of the classical simulations for the circular dot, which is expected to be valid in general regular systems. In contrast, the spin relaxation down to zero in chaotic dots contradicts what has to be expected from quantum mechanics. This signals on the importance of quantum effects missed in the semiclassical simulations for long time spin dynamics.
Spin-current generation and detection in the presence of an ac gate
(2003-12-01)
Mal'shukov, AG
;
Tang, CS
;
Chu, CS
;
Chao, KA
;
電子物理學系
We predict that in a narrow gap III-V semiconductor quantum well or quantum wire, an observable electron spin current can be generated with a time-dependent gate to modify the Rashba spin-orbit coupling constant. Methods to rectify the so generated ac current are discussed. An all-electric method of spin-current detection is suggested, which measures the voltage on the gate in the vicinity of a two-dimensional electron gas carrying a time-dependent spin current. Both the generation and detection do not involve any optical or magnetic mediator.
Strain-induced coupling of spin current to nanomechanical oscillations
(2005-09-02)
Mal'shukov, AG
;
Tang, CS
;
Chu, CS
;
Chao, KA
;
電子物理學系
We propose a setup which allows us to couple the electron spin degree of freedom to the mechanical motions of a nanomechanical system not involving any of the ferromagnetic components. The proposed method employs the strain-induced spin-orbit interaction of electrons in narrow gap semiconductors. We have shown how this method can be used for detection and manipulation of the spin flow through a suspended rod in a nanomechanical device.
Strain-induced semimetal-semiconductor transition in InAs/GaSb broken-gap quantum wells
(2002-08-15)
Zakharova, A
;
Yen, ST
;
Chao, KA
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
We investigate the hybridization of the electron, heavy-hole and/or light-hole dispersion relations in strained InAs/GaSb quantum wells. In the considered structures, the lowest electron level lies below several hole levels at zero in-plane wave vector k(parallel to), so that the anticrossings of subbands produce gaps in the in-plane dispersions. To calculate the electronic band structures of such quantum wells grown on different substrates, we use the eight-band k.p model and the scattering matrix method. We have found that the order of levels at the zone center (k(parallel to)=0), gap positions and magnitudes can change due to the lattice-mismatched strain. Strain can also enhance the hybridization of electron and light-hole states at k(parallel to)=0 considerably. In the structure with a thick InAs layer grown on GaSb, we have obtained a negative indirect gap in the in-plane dispersion resulting from the anticrossing of electronlike and highest heavy-hole-like subbands. If the substrate is InAs, the gap becomes direct and positive. This phenomenon can be treated as strain-induced semimetal-semiconductor phase transition.