Browsing by Author "Yen, ST"

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980 nm InGaAs/AlGaAs quantum well lasers with extremely low beam divergence
(1996)
Yen, ST
;
Lin, G
;
Liu, DC
;
Tsai, CM
;
Lee, CP
A novel cladding structure for semiconductor quantum-well lasers with small beam divergence and low threshold current
(1996-09-01)
Yen, ST
;
Lee, CP
;
交大名義發表
;
電子工程學系及電子研究所
;
National Chiao Tung University
A novel cladding structure is proposed and analyzed for semiconductor quantum-well lasers ttl achieve a small vertical-beam divergence and a low threshold current density simultaneously, This cladding structure is designed to guide a wide optical mode but with a high peak intensity in quantum wells. The wide expansion of the optical mode results in a small beam divergence. In addition, the high optical intensity in quantum wells causes a low threshold current density, This novel cladding structure is optimized. The result shows that this type of cladding structures can achieve a beam divergence as small as 14.6 degrees while the threshold current density remains small.
A novel cladding structure for semiconductor quantum-well lasers with small beam divergence and low threshold current
(1996-09-01)
Yen, ST
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
A novel cladding structure is proposed and analyzed for semiconductor quantum-well lasers ttl achieve a small vertical-beam divergence and a low threshold current density simultaneously, This cladding structure is designed to guide a wide optical mode but with a high peak intensity in quantum wells. The wide expansion of the optical mode results in a small beam divergence. In addition, the high optical intensity in quantum wells causes a low threshold current density, This novel cladding structure is optimized. The result shows that this type of cladding structures can achieve a beam divergence as small as 14.6 degrees while the threshold current density remains small.
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 doping in the active region of 630-nm band GaInP-AlGaInP tensile-strained quantum-well lasers
(1998-09-01)
Yen, ST
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
We theoretically analyze 630-nm band GaInP-AIGaInP tensile-strained quantum-well (QW) lasers with doping in the active region. The radiative current can be significantly reduced by introducing n-type doping in the active region. However, this advantage is reduced by the increase of the leakage current, As a result, the threshold current is reduced and the emission wavelength is shortened for multiquantum-well (MQW) lasers by n-type doping. But for single-quantum-well (SQW) lasers, because of the large increase of the leakage current, the threshold current increases with n-type doping.
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.
Extremely small vertical far-field angle of InGaAs-AlGaAs quantum-well lasers with specially designed cladding structure
(1996-12-01)
Lin, G
;
Yen, ST
;
Lee, CP
;
Liu, DC
;
電子工程學系及電子研究所
We report on a very small vertical far-field angle achieved by lasers with a specially designed structure. For an InGaAs-AlGaAs quantum-well laser with a 2.5-mu m-wide ridge waveguide, the far-field pattern has a vertical far-field angle of 13 degrees and a lateral far-field angle of 8 degrees. Meanwhile, the threshold current remains acceptably low (approximate to 36 mA for a 500-mu m-long cavity), The slope efficiency of the L-I characteristic is high (>0.9 W/A) compared to that of the conventional laser.
Extremely small vertical far-field angle of InGaAs-AlGaAs quantum-well lasers with specially designed cladding structure
(1996-12-01)
Lin, G
;
Yen, ST
;
Lee, CP
;
Liu, DC
;
電子工程學系及電子研究所
We report on a very small vertical far-field angle achieved by lasers with a specially designed structure. For an InGaAs-AlGaAs quantum-well laser with a 2.5-mu m-wide ridge waveguide, the far-field pattern has a vertical far-field angle of 13 degrees and a lateral far-field angle of 8 degrees. Meanwhile, the threshold current remains acceptably low (approximate to 36 mA for a 500-mu m-long cavity), The slope efficiency of the L-I characteristic is high (>0.9 W/A) compared to that of the conventional laser.
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.
Resonant hydrogenic impurity states and 1s-2p(0) transitions in coupled double quantum wells
(2003-10-15)
Yen, ST
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Calculations of the 1s and resonant 2p(0) states of a shallow donor in double-quantum-well structures are performed. The variational method is used to calculate the localized part of the impurity states, taking into account the effect of intersubband mixing. The resonance coupling of the 2p(0) state with the first subband is then dealt with using the Green function technique. The results show that for an asymmetric double-quantum-well structure the 1s state has a maximum binding energy as the donor is around the center of the wider well while the 2p(0) state has in general a maximum binding energy as the donor is in the narrower well. The resonant coupling of the 2p(0) state is stronger for the structures with a stronger intersubband mixing, where the 2p(0) energy level is closer to the first subband bottom. The resonance-induced broadening of the 2p(0) state can be as large as 6 meV, corresponding to a lifetime of similar to0.1 ps. The resonance in general causes a negligibly small blueshift but can give a redshift of the order of 1 meV when the resonance is strong. A phase transition of the 2p(0) state can occur from the resonant nature to the bound nature by modulation of the interwell coupling. The 1s-2p(0) transition energy is also calculated. The possibility of population inversion between the 1s and 2p(0) states is discussed.
Semiconductor lasers with unconventional cladding structures for small beam divergence and low threshold current
(1996-10-01)
Yen, ST
;
Lee, CP
;
交大名義發表
;
電子工程學系及電子研究所
;
National Chiao Tung University
Quantum well semiconductor lasers with a novel cladding design to achieve small output beam divergence and low threshold current, simultaneously are discussed. Cladding structures that cause expansion in the optical mode, while maintaining a good confinement factor, are reviewed. The design criteria and compromises are discussed in detail. The new structures offer much improved beam divergence along with low threshold current compared with conventional graded-index separate confinement heterostructure (GRINSCH) lasers. An optimized structure with the new cladding design can yield a far-field angle as low as 14.6 degrees and a low threshold current density of 180 A cm(-2).
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 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.
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.
Theoretical analysis of 630-nm band GaInP-AlGaInP strained quantum-well lasers considering continuum states
(1997-03-01)
Yen, ST
;
Lee, CP
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
GaInP-AlGaInP strained quantum-well lasers with emission wavelength at 630-nm band are theoretically analyzed in detail and then optimized, The valence band structure of quantum wells is obtained by evaluating the 6x6 Luttinger-Kohn Hamiltonian including the coupling among the heavy hole, the light hole, and the spin-orbital split-off hole bands, The effect of optical transition from/to continuum states not confined to the quantum well is studied, It is found that the optical transition from/to the continuum states is serious as the band gap of the confining layers is close to the quasi-Fermi level separation, leading to considerable radiative current, This radiative current is undesirable since the corresponding optical transition does not contribute significantly to the threshold gain, The gain-radiative current characteristic is therefore poor for confining layers containing a low Al content. To avoid unreasonable gain/absorption, the non-Markovian convolution lineshape is used instead of the conventional Lorentzian lineshape. The leakage current is high for single quantum-well lasers with wide bandgap confining layers, It can be reduced by increasing the quantum-well number, the dopant concentration, and the band gap of cladding layers, The calculated threshold current agrees wed with the observation, The band gap shrinkage due to the carrier-carrier interaction is considered to obtain an emission wavelength consistent with the experimental result.
Theoretical analysis of 630-nm band GaInP-AlGaInP strained quantum-well lasers considering continuum states
(1997-03-01)
Yen, ST
;
Lee, CP
;
交大名義發表
;
電子工程學系及電子研究所
;
National Chiao Tung University
GaInP-AlGaInP strained quantum-well lasers with emission wavelength at 630-nm band are theoretically analyzed in detail and then optimized, The valence band structure of quantum wells is obtained by evaluating the 6x6 Luttinger-Kohn Hamiltonian including the coupling among the heavy hole, the light hole, and the spin-orbital split-off hole bands, The effect of optical transition from/to continuum states not confined to the quantum well is studied, It is found that the optical transition from/to the continuum states is serious as the band gap of the confining layers is close to the quasi-Fermi level separation, leading to considerable radiative current, This radiative current is undesirable since the corresponding optical transition does not contribute significantly to the threshold gain, The gain-radiative current characteristic is therefore poor for confining layers containing a low Al content. To avoid unreasonable gain/absorption, the non-Markovian convolution lineshape is used instead of the conventional Lorentzian lineshape. The leakage current is high for single quantum-well lasers with wide bandgap confining layers, It can be reduced by increasing the quantum-well number, the dopant concentration, and the band gap of cladding layers, The calculated threshold current agrees wed with the observation, The band gap shrinkage due to the carrier-carrier interaction is considered to obtain an emission wavelength consistent with the experimental result.
Theoretical investigation on semiconductor lasers with passive waveguides
(1996-01-01)
Yen, ST
;
Lee, CP
;
交大名義發表
;
電子工程學系及電子研究所
;
National Chiao Tung University
Semiconductor lasers with vertically integrated passive waveguides are theoretically studied using the coupled mode theory and exact calculation. Formulas for the threshold current density and the far-field patterns are derived. The physical concepts of the modulation of the beam divergence by passive waveguides are given, The exact caluclated results show that the beam divergence can be greatly improved by paying a price of only a slight increase of the theshold current density, The operation mode selection is discussed. Attention is also paid to the appearance of side lobes for very narrow far-field patterns, Discussions are given for device design,