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  • 1. Akabori, M.
    et al.
    Hidaka, S.
    Iwase, H.
    Yamada, S.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics.
    Realization of In0.75Ga0.25As two-dimensional electron gas bilayer system for spintronics devices based on Rashba spin-orbit interaction2012In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 112, no 11, p. 113711-Article in journal (Refereed)
    Abstract [en]

    Narrow gap InGaAs two-dimensional electron gas (2DEG) bilayer samples are fabricated and confirmed to have good electronic qualities as well as strong Rashba-type spin-orbit interactions (SOIs). The 2DEG systems are realized by molecular beam epitaxy in the form of wide quantum wells (QWs) with thicknesses tQW∼40-120nm modulation doped in both the upper and lower InAlAs barriers. From the Hall measurements, the overall mobility values of μe ∼15 m2/V s are found for the total sheet electron density of ns ∼8 × 1011/cm2, although the ns is distributed asymmetrically as about 1:3 in the upper and lower 2DEGs, respectively. Careful low temperature magneto-resistance analysis gives large SO coupling constants of α ∼20 × 10 -12eV m as well as expected electron effective masses of m*/m0 ∼0.033-0.042 for each bilayer 2DEG spin sub-band. Moreover, the enhancement of α with decrease of tQW is found. The corresponding self-consistent calculation, which suggests the interaction between the bilayer 2DEGs, is carried out and the origin of α enhancement is discussed.

  • 2.
    Ekenberg, Ulf
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Gvozdic, D. M.
    Analysis of electric-field-induced spin splitting in wide modulation-doped quantum wells2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 20, p. 205317-Article in journal (Refereed)
    Abstract [en]

    We analyze the proper inclusion of electric-field-induced spin splittings in the framework of the envelope function approximation. We argue that the Rashba effect should be included in the form of a macroscopic potential as diagonal terms in a multiband approach rather than the commonly used Rashba term dependent on k and electric field. It is pointed out that the expectation value of the electric field in a subband is sometimes not unique because the expectation values can even have opposite signs for the spin-split subband components. Symmetric quantum wells with Dresselhaus terms and the influence of the interfaces on the spin splitting are also discussed. We apply a well established multiband approach to wide modulation-doped InGaSb quantum wells with strong built-in electric fields in the interface regions. We demonstrate an efficient mechanism for switching on and off the Rashba splitting with an electric field being an order of magnitude smaller than the local built-in field that determines the Rashba splitting. The implications of our findings for spintronic devices, in particular the Datta-Das spin transistor and proposed modifications of it, are discussed.

  • 3.
    Ekenberg, Ulf
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101). University of Belgrade, Serbia.
    Gvozdíc, Dejan M.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101). University of Belgrade, Serbia.
    Spin splitting in modulation-doped semiconductor quantum wells2009In: Quantum Wells: Theory, Fabrication and Applications, Nova Science Publishers, Inc., 2009, p. 385-425Chapter in book (Refereed)
    Abstract [en]

    We review different ways to achieve a spin splitting of two-dimensional electron and hole subbands with the combination of inversion asymmetry and spin-orbit interaction. In particular we focus on novel mechanisms to achieve a substantial spin splitting with a small applied bias across the sample. We discuss the proper inclusion of electric-field-induced spin splittings in the framework of the envelope function approximation and argue that the Rashba effect should be included in the form of a macroscopic potential as diagonal terms in a multiband approach rather than commonly used terms dependent on k and electric field. One of our findings is that the expectation values of the electric field can differ substantially and even have opposite signs for the spin-split components of a subband. Thus the frequent assignment of one expectation value to a subband is sometimes not appropriate. We also discuss symmetric quantum wells with Dresselhaus terms and the influence of the interfaces on the spin splitting. Our approach is applied to wide modulation-doped n-type InGaSb quantum wells with strong built-in electric fields in the interface regions. We demonstrate an efficient mechanism for switching on and off the Rashba splitting with an electric field being an order of magnitude smaller than the local built-in field that determines the Rashba splitting. For a slightly asymmetric quantum well we demonstrate a reversal of the spin direction in a spin subband in two steps as the in-plane wave vector is increased a little. Our most significant results pertain to the superefficient Rashba effect for holes. With a careful design of doping profile and strain we find that the wave vector splitting for hole subbands can be made several thousand times stronger than for electrons at the same electric field. The implications of our findings for spintronic devices, in particular the Datta-Das spin transistor and proposed modifications of it, are discussed.

  • 4.
    Ekenberg, Ulf
    et al.
    KTH, Superseded Departments, Physics.
    Mauritz, O.
    Control of spontaneous spin splitting in an asymmetric quantum well with the use of strain and/or magnetic field2001In: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 10, no 03-jan, p. 81-85Article in journal (Refereed)
    Abstract [en]

    Spin-orbit coupling combined with inversion asymmetry gives rise to spin splitting even in the absence of an applied magnetic field. The size of this spin splitting can be controlled by changing the degree of asymmetry using a gate voltage. We present here other less obvious ways of controlling the spontaneous spin splitting in a two-dimensional hole gas, where these effects are particularly large. Applying moderate stress can easily decrease the spin splitting by an order of magnitude. The mechanism is the strain-induced energy shift of the heavy-hole and light-hole subbands, which diminishes the degree of band mixing, which is found to be strongly correlated to the spin splitting. An applied magnetic field causes an additional Zeeman splitting, but we find that a magnetic field of 1T can be sufficient to practically erase the difference between a symmetric quantum well (without subband splitting) and an asymmetric quantum well. We have simulated Shubnikov-de Haas oscillations and found that two periodicities in 1/B can occur even for one filled spin-degenerate hole subband.

  • 5.
    Gustafsson, Oscar
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Berggren, Jesper
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Photonics (Closed 20120101).
    Hallén, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Hammar, Mattias
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Höglund, L.
    Karim, A.
    Noharet, B.
    Wang, Q.
    Gromov, A.
    Almqvist, S.
    Zhang, A.
    Acreo, Sweden.
    Junique, S.
    Andersson, J. Y.
    Asplund, C.
    von Würtemberg, R. Marcks
    Malm, H.
    Martijn, H.
    Long-wavelength infrared quantum-dot based interband photodetectors2011In: Infrared physics & technology, ISSN 1350-4495, E-ISSN 1879-0275, Vol. 54, no 3, p. 287-291Article in journal (Refereed)
    Abstract [en]

    We report on the design and fabrication of (Al)GaAs(Sb)/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits including long carrier lifetime due to the type-II band alignment, suppressed Shockley-Read-Hall generation-recombination due to the relatively large-bandgap matrix material, inhibited Auger recombination processes due to the tensile strain and epitaxial simplicity. Metal-organic vapor-phase epitaxy was used to grow multiple (Al)GaAs(Sb) QD layers on InAs substrates at different QD nominal thicknesses, compositions, doping conditions and multilayer periods, and the material was characterized using atomic force and transmission electron microscopy, and Fourier-transform infrared absorption spectroscopy. Dot densities up to 1 x 10(11) cm(-2), 1 x 10(12) cm(-2) and 3 x 10(10) cm(-2) were measured for GaAs, AlGaAs and GaAsSb QDs, respectively. Strong absorption in GaAs, AlGaAs and GaAsSb multilayer QD samples was observed in the wavelength range 6-12 mu m. From the wavelength shift in the spectral absorption for samples with varying QD thickness and composition it is believed that the absorption is due to an intra- valance band transition. From this it is possible to estimate the type-II inter-band transition wavelength, thereby suggesting that (Al)GaAs(Sb) QD/InAs heterostructures are suitable candidates for LWIR detection and imaging.

  • 6.
    Gustafsson, Oscar
    et al.
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Karim, Amir
    Berggren, Jesper
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Wang, Qin
    Reuterskiöld-Hedlund, Carl
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Ernerheim-Jokumsen, Christopher
    KTH.
    Soldemo, Markus
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Weissenrieder, Jonas
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Persson, Sirpa
    Almqvist, Susanne
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics.
    Noharet, Bertrand
    Asplund, Carl
    Göthelid, Mats
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Andersson, Jan Y.
    Hammar, Mattias
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Photoluminescence and photoresponse from InSb/InAs-based quantum dot structures2012In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 20, no 19, p. 21264-21271Article in journal (Refereed)
    Abstract [en]

    InSb-based quantum dots grown by metal-organic vapor-phase epitaxy (MOVPE) on InAs substrates are studied for use as the active material in interband photon detectors. Long-wavelength infrared (LWIR) photoluminescence is demonstrated with peak emission at 8.5 mu m and photoresponse, interpreted to originate from type-II interband transitions in a p-i-n photodiode, was measured up to 6 mu m, both at 80 K. The possibilities and benefits of operation in the LWIR range (8-12 mu m) are discussed and the results suggest that InSb-based quantum dot structures can be suitable candidates for photon detection in the LWIR regime.

  • 7. Gvozdic, D. M.
    et al.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Efficient switching of Rashba spin splitting in wide modulation-doped quantum wells2007In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, no 5Article in journal (Refereed)
    Abstract [en]

    The authors demonstrate that the size of the electric-field-induced Rashba spin splitting in an 80 nm wide modulation-doped InGaSb quantum well can depend strongly on the spatial variation of the electric field. In a slightly asymmetric quantum well it can be an order of magnitude stronger than for the average uniform electric field. For even smaller asymmetry spin subbands can have wave functions and/or expectation values of the spin direction that are completely changed as the in-plane wave vector varies. The Dresselhaus effect [Phys. Rev. 100, 580 (1955)] can give an anticrossing at which the spin rapidly flips.

  • 8. Gvozdic, D. M.
    et al.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT).
    Strong dependence of spin direction and wave function localization on In-plane wave vector in wide modulation-doped quantum wells2007In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 61, no 1, p. 273-277Article in journal (Refereed)
    Abstract [en]

    An important mechanism in spintronics is spin-splitting induced by structure and/or bulk inversion asymmetry. These effects are frequently assumed to depend on two parameters usually denoted by α and β respectively, and α is assumed to be proportional to some average electric field. We here demonstrate that the spatial dependence of the electric field gives very important effects absent in simpler models. These effects are particularly clear in wide modulation-doped quantum wells where there are two weakly interacting electron gases in the interface regions. Using an 8 × 8 k . p matrix approach we obtain anticrossings between interacting subbands at which the spin direction and/or wave function localization are found to be strong functions of the in-plane wave vector.

  • 9. Gvozdic, D. M.
    et al.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Superefficient electric-field-induced spin-orbit splitting in strained p-type quantum wells2006In: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 73, no 6, p. 927-933Article in journal (Refereed)
    Abstract [en]

    We investigate theoretically the efficiency of the Rashba effect, i.e. the spin-orbit splitting resulting from an electric field. In contrast to previous studies, where the carriers have usually been taken to be electrons, we focus on holes and are able to demonstrate remarkable improvements of the effect by several orders of magnitude. We also show that the frequently-neglected lattice-mismatch between GaAs and AlGaAs can be used to further enhance the efficiency of the wave vector splitting mechanism. The Rashba effect is the fundamental mechanism behind the Datta-Das spin transistor and we find that for a small electric field of 2 kV/cm the spin precession length becomes only 36nm.

  • 10. Gvozdic, D. M.
    et al.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Superiority of p-type spin transistors2006In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T126, p. 21-26Article in journal (Refereed)
    Abstract [en]

    The spintronic device that has probably stimulated the most research interest is the Datta-Das spin transistor. The mechanism behind it, called the Rashba effect, is that an applied voltage gives rise to a spin splitting. We demonstrate that the relevant spin splitting in k-space can be made more than three orders of magnitude larger for holes than for electrons at the same electric field. This is partly achieved by utilizing the frequently neglected lattice-mismatch between GaAs and AlGaAs. We design heterostructures where this efficient Rashba effect should show up. Compared to present transistors, we conclude that electron-based spin transistors will have problems in becoming competitive but hole-based ones are much more promising.

  • 11. Gvozdic, D. M.
    et al.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Thylén, Lars
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics.
    Comparison of performance of n- and p-type spin transistors with conventional transistors2005In: Journal of Superconductivity, ISSN 0896-1107, E-ISSN 1572-9605, Vol. 18, no 3, p. 349-356Article in journal (Refereed)
    Abstract [en]

    A spintronic device that has stimulated much research interest is the Datta-Das spin transistor. The mechanism behind it called the Rashba effect is that an applied voltage gives rise to a spin splitting. We propose ways to optimize this effect. The relevant spin splitting in k-space is predicted to increase with electric field at a rate that is more than two orders of magnitude larger for holes than for electrons. Furthermore, the almost negligible lattice-mismatch between GaAs and AlGaAs can be used to further enhance the advantage of hole-based spin transistors. Compared to present transistors we conclude that electron-based spin transistors will have problems to become competitive but hole-based ones are much more promising.

  • 12.
    Gvozdic, Dejan
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Strong enhancement of Rashba effect in strained p-type quantum wells2005In: Physics of Semiconductors, Pts A and B / [ed] Menendez, J; VanDeWalle, CG, MELVILLE: AMER INST PHYSICS , 2005, Vol. 772, p. 1423-1424Conference paper (Refereed)
    Abstract [en]

    One of the most studied spintronic devices is the spin transistor proposed by Datta and Das. The mechanism behind this transistor is the Rashba effect: The inversion asymmetry caused by the gate voltage gives rise to a spin splitting. We show that the relevant spin splitting in k-space is typically two orders of magnitude larger in unstrained p-type quantum wells compared to n-type quantum wells. We also show that further order-of-magnitude improvement can be obtained by utilizing the frequently ignored lattice-mismatch between GaAs and AlGaAs.

  • 13.
    Gvozdic, Dejan M.
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Beyond the Rashba model2006In: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 32, no 02-jan, p. 458-461Article in journal (Refereed)
    Abstract [en]

    We analyze some common approximations made in connection with the Rashba effect, where a macroscopic electric field gives rise to a spin splitting. We demonstrate that the size of the Rashba splitting is not given by the expectation value of the electric field or some other average electric field, as is commonly assumed. Instead we find that the local electric field near an interface of a wide asymmetric modulation-doped quantum well can give rise to a Rashba splitting that is an order of magnitude larger than expected from the average electric field. The localization of the wave functions of the spin subbands can be quite sensitive to the parallel wave vector. Clear deviations from an energy-independent wave vector splitting occur when nonparabolicity is taken into account.

  • 14.
    Gvozdic, Dejan M.
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Investigation of the super-efficient Rashba effect by simulation of Shubnikov-de Haas oscillations in a two-dimensional hole gas2006In: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 34, no 02-jan, p. 377-380Article in journal (Refereed)
    Abstract [en]

    The Shubnikov-de Haas effect is frequently used for two-dimensional systems to determine individual subband populations, e.g. when the subbands are split by the Rashba effect. We have previously shown that the Rashba effect can give a wave vector splitting for holes that is up to three orders of magnitude larger than for electrons at the same electric field. To reach the optimum we have made a careful design of a modulation-doped quantum well with a top gate in which the negative differential Rashba effect is utilized. From the calculated hole Landau levels we determine the density of states at the Fermi energy and demonstrate a clear difference between the symmetric case without bias and the asymmetric case with a gate voltage of 100 meV, where the spin subband populations differ by a factor 3.

  • 15. Gvozdic, Dejan M.
    et al.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Rapid spin flip in a spin subband at an anticrossing region in a slightly asymmetric modulation-doped quantum well2008In: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 40, no 6, p. 2081-2083Article in journal (Refereed)
    Abstract [en]

    The spin splitting due to structure and bulk inversion asymmetry is calculated for electron subbands in wide slightly asymmetric InGaSb quantum wells. At anticrossings, rapid spin flips in two steps are found as the in-plane wave vector along the [1 1] direction is increased by 0.002 nm(-1). First the y-component and then the x-component is flipped. A change of bias of about 1 meV across the quantum well is sufficient move the Fermi level across the anticrossing region.

  • 16. Gvozdić, D. M.
    et al.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Novel mechanism for rapid spin flip with increasing in-plane wave vector in slightly asymmetric modulation-doped quantum wells2008In: PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIENCE/INTERNATIONAL CONFERENCE ON NANOSCIENCE AND TECHNOLOGY, 2008, Vol. 100, no PART 5Conference paper (Refereed)
    Abstract [en]

    We calculate the electric-field-induced spin splittings in wide slightly asymmetric modulation-doped quantum wells. When spin subbands are anticrossing we demonstrate twostep spin flips as the in-plane wave vector along the [11] direction is increased by 0.002 nm-1. At the beginning of this interval the y-component flips, at the end the x-component. Simultaneously the energy separation stays roughly constant below 1-eV and the wave functions are interchanged. A bias change of about 1 meV is sufficient to move the Fermi level from below to above the anticrossing region.

  • 17.
    Gvozdić, Dejan M.
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101). University of Belgrade, Serbia.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101).
    Novel mechanism for order-of-magnitude enhancement of rashba effect in wide modulation-doped quantum wells2007In: Physics of Semiconductors: 28th International Conference on the Physics of Semiconductors - ICPS 2006, American Institute of Physics (AIP), 2007, p. 1371-1372Conference paper (Refereed)
    Abstract [en]

    The Rashba effect leading to subband splitting in quantum wells is frequently taken to be proportional to some average electric field. We here show that taking the spatial variation of the electric field into account gives important effects. In particular we demonstrate that one can apply a moderate electric field to a wide modulation-doped quantum well and get an order-of-magnitude enhancement of the Rashba splitting characteristic of the built-in interface field. For small asymmetry the wave function localization and spin projection of a subband can be rapid functions of the in-plane wave vector.

  • 18. Holmstrom, P.
    et al.
    Thylén, Lars
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Ekenberg, Ulf
    KTH, Superseded Departments, Physics.
    Proposal of an optical modulator based on resonant tunneling and intersubband transitions2001In: IEEE Journal of Quantum Electronics, ISSN 0018-9197, E-ISSN 1558-1713, Vol. 37, no 2, p. 224-230Article in journal (Refereed)
    Abstract [en]

    We propose and analyze an optical modulator based on intersubband transitions. The absorption is modulated by modulating the carrier density in the ground state of a quantum well (QW). Electrons are injected resonantly into this subband from a QW reservoir subband through a single barrier. When the two states are tuned out of resonance, the electrons are rapidly evacuated by means of the optical held, A waveguide based on surface plasmons is assumed in order to have a high optical mode overlap. Calculations are performed for a cascaded structure with four periods, assuming InGaAs-InAlAs QWs, The considered modulator structure operates at lambda =6.0 mum and is RC limited to 27 GHz, An extinction ratio of it is obtained with a low applied voltage of 0.6 V, At larger applied voltages, the absorption is bistable, Absorption at shorter/longer wavelengths can be obtained by using materials with a larger/smaller conduction band offset. We also assess resonant tunneling from a 2-D electron gas reservoir into an array of quantum dots and compare it to the 2-D-2-D tunneling resonance.

  • 19.
    Holmström, Petter
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Jänes, Peter
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Thylen, Lars
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Efficient electroabsorption for mid-infrared wavelengths using intersubband transitions2008In: PROCEEDINGS OF THE 17TH INTERNATIONAL VACUUM CONGRESS/13TH INTERNATIONAL CONFERENCE ON SURFACE SCIENCE/INTERNATIONAL CONFERENCE ON NANOSCIENCE AND TECHNOLOGY / [ed] Johansson LSO, Andersen JN, Gothelid M, Helmersson U, Montelius L, Rubel M, Setina J, Wernersson LE, Bristol: IOP PUBLISHING LTD , 2008, Vol. 100Conference paper (Refereed)
    Abstract [en]

    We have demonstrated efficient intersubband (IS) electroabsorption in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapor phase epitaxy (MOVPE). An absorption modulation of 2300 cm(-1) at lambda=5.7 mu m due to Stark shift of the IS resonance was achieved with a low applied voltage swing of +/-0.5 V in a multipass waveguide structure. Two useful wavelength ranges of lambda approximate to 5.4-5.8 mu m and 6.3-6.6 mu m were obtained by considering the two flanks of the IS resonance. Based on the experimental results it is estimated that an electroabsorption modulator with a low peak-to-peak voltage of V-PP = 0.9 V can yield a modulation speed of f(3dB) = 120 GHz with the present material by using a strongly confining surface plasmon waveguide of 30 mu m length.

  • 20.
    Holmström, Petter
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Jänes, Peter
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Ekenberg, Ulf
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Thylén, L.
    Design of intersubband optical modulators2002In: Proceedings of the 26th International Conference on the Physicsof Semiconductors (26th ICPS), Edinburgh, Scotland, 2002, 2002, p. P126-Conference paper (Refereed)
  • 21.
    Holmström, Petter
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Jänes, Peter
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Thylén, Lars
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Efficient infrared electroabsorption with 1 V applied voltage swing using intersubband transitions2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 93, no 19, article id 191101Article in journal (Refereed)
    Abstract [en]

    We have demonstrated efficient intersubband electroabsorption in InGaAs/InAlGaAs/InAlAs step quantum wells grown by metal-organic vapor-phase epitaxy. An absorption modulation of 6 dB (Delta alpha=2300 cm(-1)) at lambda similar to 5.7 mu m due to Stark shift of the intersubband resonance was achieved at a low applied voltage swing of +/- 0.5 V in a multipass waveguide structure. The interface intermixing was estimated by comparing experimental and theoretical Stark shifts. It is predicted that the present material in a strongly confining surface plasmon waveguide can yield an electroabsorption modulator with a peak-to-peak voltage of V-pp=0.9 V and modulation speed of f(3dB)approximate to 130 GHz.

  • 22.
    Holmström, Petter
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Jänes, Peter
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Thylén, Lars
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Infrared modulator at 6 um with 1-V applied voltage swing using  intersubband transitions in step quantum wells grown by MOVPE2007Conference paper (Refereed)
  • 23.
    Holmström, Petter
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Jänes, Peter
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Thylén, Lars
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Strong electroabsorption using intersubb and transitions in InGaAs/InAlGaAs/InAlAs step quantum wells2006In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118Article in journal (Other academic)
  • 24.
    Jänes, Peter
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Holmström, Petter
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Ekenberg, Ulf
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    A high-speed intersubband modulator based on quantum interference in double quantum wells2002In: IEEE Journal of Quantum Electronics, ISSN 0018-9197, E-ISSN 1558-1713, Vol. 38, no 2, p. 178-184Article in journal (Refereed)
    Abstract [en]

    Calculations on a modulator based on quantum interference in AlGaAs/GaAs asymmetric double quantum wells (QWs) are performed. The modulation of the absorption is based on the anti-crossing behavior of the two lowest states in the coupled wells. At anti-crossing, the oscillator strengths of the transitions from these two lowest states to a higher state are changed in opposite directions. The width of the barrier between the wells should be thick enough to allow a large change in oscillator strength with applied field, yet thin enough so that the absorption peaks of the transitions are resolved. The QWs are designed so that one absorption peak has only a small energy shift for the transition used for modulation while the absorption varies rapidly with the applied voltage. A complete structure including a surface plasmon waveguide is proposed enabling calculations of modal absorption. Parameters important for the performance of the modulator are then determined. An extinction ratio of 10 dB at a wavelength of 8.4 mum is predicted for a device length of 18 mum and a peak-to-peak voltage of 0.9 V. The resistance-capacitance-limited 3-dB bandwidth is 130 GHz. The predicted performance compares very favorably with present interband modulators based on the quantum-confined Stark effect.

  • 25. Liu, X. Y.
    et al.
    Fälth, J. F.
    Andersson, T. G.
    Holmström, Petter
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Jänes, Peter
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ekenberg, Ulf
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Thylén, Lars
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Structural and optical properties of GaN/AlN multiple quantum wells for intersubband applications2005In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 278, no 1-4, p. 397-401Article in journal (Refereed)
    Abstract [en]

    GaN/AIN multiple quantum well structures of 1, 5 and 20 periods were grown by molecular beam epitaxy (MBE). To investigate structural parameters, symmetrical scan (0002) and reciprocal space mapping in the vicinity of the GaN (10 15) plane were made by X-ray diffraction (XRD). The layer thickness, composition and relaxation were determined and gave good agreement with simulated results. The 20 period multiple quantum well (MQW) sample exhibited an intersubband resonance at 360 meV, which corresponds well to the structure data determined by XRD.

  • 26. Malkova, N.
    et al.
    Ekenberg, Ulf
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Spin properties of quantum wells with magnetic barriers. I. A k center dot p analysis for structures with normal band ordering2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 15Article in journal (Refereed)
    Abstract [en]

    The electronic band-edge spectrum of magnetic semiconductor quantum wells containing a diluted magnetic semiconductor as one of the constituents is studied within the envelope-function formalism. The effects of Mn d electrons are explicitly included in a k.p Hamiltonian which in the first approximation of perturbation theory is shown to be reduced to an effective Kane model. The sp-d hybridization leads to a spin-splitting effect. The results are applied to the system Cd1-xMnxTe/CdTe. The spin-splitting effect is studied as a function of external magnetic field, well width, valence band offset and fraction of the magnetic atoms. The numerical results are in accord with experimental data.

  • 27. Malkova, N.
    et al.
    Ekenberg, Ulf
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Spin properties of quantum wells with magnetic barriers. II. Inverted band ordering and spin polarized interface states2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 15Article in journal (Refereed)
    Abstract [en]

    The electronic band-edge spectrum of the interface states in the magnetic semiconductor quantum wells based on narrow-gap semiconductors with mutually inverted band arrangement is studied within the envelope-function formalism. Interface states are shown to appear in these structures in the case of overlapping bulk bands of the constituents. The hybridization between the bare sp-electron states and the d states of the Mn atoms leads to spin splitting. The spin-splitting effect of the interface states as a function of external magnetic field, well width, band offsets, and fraction of the magnetic atoms, is studied. One essential result is that one can design a structure where the states localized at the interfaces only have one spin direction. The results give evidence of the perspective for using the magnetic semiconductor structures in spin electronics.

  • 28. Mauritz, O.
    et al.
    Ekenberg, Ulf
    KTH, Superseded Departments, Physics.
    Magnetic-field-induced suppression of spontaneous spin-splitting of hole subbands2000In: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 6, no 04-jan, p. 779-782Article in journal (Refereed)
    Abstract [en]

    The spin-splitting in the valence band in an InxGa1-xAs/InxGa1-xAsyP1-y quantum well is investigated theoretically using a 6 x 6 Luttinger-Kohn Hamiltonian. We compare the Landau levels in a perpendicular magnetic field with the corresponding results for the subband dispersion;. It is shown that the asymmetry of the quantum well has a very small impact on the Landau level splitting for B > 1 T in sharp contrast to the subbands in the absence of a magnetic field. The significance of our findings on the interpretation of Shubnikov-de Haas experiments is discussed.

1 - 28 of 28
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