| 1. |
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| 2. |
- Darakchieva, Vanya, et al.
(författare)
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Free electron behavior in InN : On the role of dislocations and surface electron accumulation
- 2009
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 94:2, s. 022109-
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Tidskriftsartikel (refereegranskat)abstract
- The free electron behavior in InN is studied on the basis of decoupled bulk and surface accumulation electron densities in InN films measured by contactless optical Hall effect. It is shown that the variation in the bulk electron density with film thickness does not follow the models of free electrons generated by dislocation-associated nitrogen vacancies. This finding, further supported by transmission electron microscopy results, indicates the existence of a different thickness-dependent doping mechanism. Furthermore, we observe a noticeable dependence of the surface electron density on the bulk density, which can be exploited for tuning the surface charge in future InN based devices.
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| 3. |
- Darakchieva, Vanya, et al.
(författare)
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Unravelling the free electron behavior in InN
- 2008
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Ingår i: Optoelectronic and Microelectronic Materials and Devices, 2008. - : IEEE. - 9781424427161 ; , s. 90-97
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Konferensbidrag (refereegranskat)abstract
- Precise measurement of the optical Hall effect in InN using magneto-optical generalized ellipsometry at IR and THz wavelengths, allows us to decouple the surface accumulation and bulk electron densities in InN films by non-contact optical means and further to precisely measure the effective mass and mobilities for polarizations parallel and perpendicular to the optical axis. Studies of InN films with different thicknesses, free electron densities and surface orientations enable an intricate picture of InN free electron properties to emerge. Striking findings on the scaling factors of the bulk electron densities with film thickness further supported by transmission electron microscopy point to an additional thickness dependent doping mechanism unrelated to dislocations. Surface electron accumulation is observed to occur not only at polar but also at non-polar and semi-polar wurtzite InN, and zinc blende InN surfaces. The persistent surface electron density shows a complex behavior with bulk density and surface orientation. This behavior might be exploited for tuning the surface charge in InN.
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| 4. |
- Harati Zadeh, Hamid, 1964-, et al.
(författare)
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Photoluminescence study of Si-doped GaN/Al0.07Ga0.93N multiple quantum wells with different dopant position
- 2004
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:25, s. 5071-5073
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Tidskriftsartikel (refereegranskat)abstract
- The Si-doped GaN/Al0.07Ga0.93N multiple quantum wells (MQW) were investigated, using photoluminescence (PL) and time-resolved (PL) measurements. The influence of Si doping on the emission energy and recombination dynamics of the MWQs were also investigated, with different dopant position in the wells. It was observed that the redshifted emission of the MQWs was attributed to the self-energy shift of the electron states due to the correlated motion of the electrons exposed to the fluctuating potential of the donor ions. It was also observed that the PL decay time of the sample was ∼760 ps, at low temperature.
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| 5. |
- Monemar, Bo, 1942-, et al.
(författare)
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Defect related issues in the "current roll-off" in InGaN based light emitting diodes
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91, s. 181103-1-181103-3
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Tidskriftsartikel (refereegranskat)abstract
- Defect related contributions to the reduction of the internal quantum efficiency of InGaN-based multiple quantum well light emitting diodes under high forward bias conditions are discussed. Screening of localization potentials for electrons is an important process to reduce the localization at high injection. The possible role of threading dislocations in inducing a parasitic tunneling current in the device is discussed. Phonon-assisted transport of holes via tunneling at defect sites along dislocations is suggested to be involved, leading to a nonradiative parasitic process enhanced by a local temperature rise at high injection.
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| 6. |
- Monemar, Bo, et al.
(författare)
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Radiative recombination in In0.15Ga0.85N/GaN multiple quantum well structures
- 1999
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Ingår i: MRS Internet Journal of Nitride Semiconductor Research. - 1092-5783. ; 4:16
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature (< 800 degrees C) and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with delay time after pulsed excitation, but also with donor doping and with excitation intensity. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations, originating from segregation effects in the InGaN material, from interface roughness, and the strain fluctuations related to these phenomena, play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples, and appear to be important for all studied growth temperatures for the InGaN layers. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence peak up to 0.2 eV is observed for a Si donor density of 2 x 10(18) cm(-3) in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 Angstrom) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed.
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| 7. |
- Wongmanerod, S, et al.
(författare)
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Many-body effects in highly p-type modulation-doped GaAs/AlxGa1-xAs quantum wells
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:4, s. 2794-2798
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Tidskriftsartikel (refereegranskat)abstract
- Many-body effects have been optically investigated for modulation-doped quantum wells at high acceptor densities. The observed band-gap shrinkage, up to approximate to 20 meV, is consistent with calculations based on the Hartree and random-phase approximations including the finite well width effect. A recombination near the Fermi edge with light-hole character is strikingly enhanced at high acceptor densities. An interpretation based on carrier-carrier interaction is proposed. Finally, the exciton is found to be quenched for hole densities higher than approximate to 2x10(12) cm(-2).
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| 8. |
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| 9. |
- Araujo, C. M., et al.
(författare)
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Electrical resistivity, MNM transition and band-gap narrowing of cubic GaN : Si
- 2002
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Ingår i: Microelectronics Journal. - 0026-2692. ; 33:4, s. 365-369
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Tidskriftsartikel (refereegranskat)abstract
- The electrical resistivity of the Si-donor cubic GaN is investigated theoretically at low temperature. The critical impurity concentration, N-c, for the metal-nonmetal transition is estimated in three different ways: from using the generalized Drude approach (GDA) for the resistivity; from the vanishing of the chemical potential calculated using the dielectric function model with a Lorentz-Lorenz correction; from finding the crossing point between the energy in the insulating and metallic states. The bandgap narrowing (BGN) has been determined theoretically and experimentally above the MNM transition, The experimental data have been obtained with photoluminescence measurements. Theoretical and experimental results are in rough agreement in the range of impurity concentration of interest.
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| 10. |
- Beyer, Jan, et al.
(författare)
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Strong suppression of spin generation at a Fano resonance in a semiconductor nanostructure
- 2012
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We observe remarkable, complete suppression of spin generation under optical excitation in a thin InAs/GaAs wetting layer close to the light-hole excitonic resonance, leading to zero electron spin polarization as monitored by adjacent InAs quantum dots. The suppression is attributed to efficient spin relaxation/scattering at the Fano resonance between the light-hole exciton states and the heavy-hole continuum of the wetting layer. The complete suppression is found to remain effective up to temperatures exceeding 100 K.
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| 11. |
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| 12. |
- Boström, Mathias, et al.
(författare)
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Atmospheric water droplets can catalyse atom pair break-up via surface-induced resonance repulsion
- 2013
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 101:4, s. 43002-
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Tidskriftsartikel (refereegranskat)abstract
- We present the theory for a retarded resonance interaction between two identical atoms near a dielectric surface. In free space the resonance interaction between isotropically excited atom pairs is attractive at all atom-atom separations. We illustrate numerically how this interaction between oxygen, sulphur, hydrogen, or nitrogen atom pairs may turn repulsive near water droplets. The results provide evidence of a mechanism causing excited state atom pair breakage to occur in the atmosphere near water droplets.
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| 13. |
- Boström, Mathias, et al.
(författare)
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Casimir attractive-repulsive transition in MEMS
- 2012
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 85:11, s. 377-
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Tidskriftsartikel (refereegranskat)abstract
- Unwanted stiction in micro-and nanomechanical (NEMS/MEMS) systems due to dispersion (van der Waals, or Casimir) forces is a significant hurdle in the fabrication of systems with moving parts on these length scales. Introducing a suitably dielectric liquid in the interspace between bodies has previously been demonstrated to render dispersion forces repulsive, or even to switch sign as a function of separation. Making use of recently available permittivity data calculated by us we show that such a remarkable nonmonotonic Casimir force, changing from attractive to repulsive as separation increases, can in fact be observed in systems where constituent materials are in standard NEMS/MEMS use requiring no special or exotic materials. No such nonmonotonic behaviour has been measured to date. We calculate the force between a silica sphere and a flat surface of either zinc oxide or hafnia, two materials which are among the most prominent for practical microelectrical and microoptical devices. Our results explicate the need for highly accurate permittivity functions of the materials involved for frequencies from optical to far-infrared frequencies. A careful analysis of the Casimir interaction is presented, and we show how the change in the sign of the interaction can be understood as a result of multiple crossings of the dielectric functions of the three media involved in a given set-up.
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| 14. |
- Boström, Mathias, et al.
(författare)
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Casimir force between atomically thin gold films
- 2013
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:2, s. 43-
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Tidskriftsartikel (refereegranskat)abstract
- We have used density functional theory to calculate the anisotropic dielectric functions for ultrathin gold sheets (composed of 1, 3, 6, and 15 atomic layers). Such films are important components in nano-electromechanical systems. When using correct dielectric functions rather than bulk gold dielectric functions we predict an enhanced attractive Casimir-Lifshitz force (at most around 20%) between two atomically thin gold sheets. For thicker sheets the dielectric properties and the corresponding Casimir forces approach those of gold half-spaces. The magnitude of the corrections that we predict should, within the today's level of accuracy in Casimir force measurements, be clearly detectable.
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| 15. |
- Boström, Mathias, et al.
(författare)
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Casimir-Lifshitz interaction between ZnO and SiO2 nanorods in bromobenzene turns repulsive at intermediate separations due to retardation effects
- 2012
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 85:4, s. 044702-
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Tidskriftsartikel (refereegranskat)abstract
- We consider the interaction between a ZnO nanorod and a SiO2 nanorod in bromobenzene. Using optical data for the interacting objects and ambient we calculate the force (from short-range attractive van der Waals force to intermediate-range repulsive Casimir-Lifshitz force to long-range entropically driven attraction). The nonretarded van der Waals interaction is attractive at all separations. We demonstrate a retardation-driven repulsion at intermediate separations. At short separations (in the nonretarded limit) and at large separations (in the classical limit) the interaction is attractive. These effects can be understood from an analysis of multiple crossings of the dielectric functions of the three media as functions of imaginary frequencies.
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| 16. |
- Boström, Mathias, et al.
(författare)
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Comment on "Calculation of the Casimir force between imperfectly conducting plates"
- 2000
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 61:4
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- In a recent paper [Phys. Rev. A 59, R3149 (1999)] Lamoreaux reported calculations of the Casimir force. The experimentally found permittivity was used in the calculations. Large deviations were found between numerically evaluated forces and forces derived from a series expanded plasma model. We would like to comment on a few results presented in this work. First, we claim that important features of the imaginary component of the permittivity of copper, presented in Fig. 1(a) are due to the interpolation procedure and are not caused by physical phenomena. These features influence the calculated permittivity for imaginary frequencies, which is the quantity used to calculate the Casimir attraction. Second, we discuss the extrapolation procedure used for low frequencies. The results depend substantially on how this extrapolation is performed.
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| 17. |
- Boström, Mathias, et al.
(författare)
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Enlarged molecules from excited atoms in nanochannels
- 2012
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 86:1, s. 014701-
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Tidskriftsartikel (refereegranskat)abstract
- The resonance interaction that takes place in planar nanochannels between pairs of excited-state atoms is explored. We consider interactions in channels of silica, zinc oxide, and gold. The nanosized channels induce a dramatically different interaction from that in free space. Illustrative calculations for two lithium and cesium atoms demonstrate that there is a short-range repulsion followed by long-range attraction. The binding energy is strongest near the surfaces. The size of the enlarged molecule is biggest at the center of the cavity and increases with channel width. Since the interaction is generic, we predict that enlarged molecules are formed in porous structures, and that the molecule size depends on the size of the nanochannels.
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| 18. |
- Boström, Mathias, et al.
(författare)
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Entropy of the Casimir effect between real metal plates
- 2004
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Ingår i: Physica A. - : Elsevier BV. - 0378-4371 .- 1873-2119. ; 339:01-Feb, s. 53-59
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Tidskriftsartikel (refereegranskat)abstract
- We first derive the zero-and-finite-temperature dispersion-forces in terms of changes in the energy of the electromagnetic normal modes of the system. We then use this to evaluate the entropy of the electromagnetic normal modes for the "Casimir system" consisting of two metal plates. We demonstrate that this entropy obeys the Nernst heat theorem. (C) 2004 Elsevier B.V. All rights reserved.
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| 19. |
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| 20. |
- Boström, Mathias, et al.
(författare)
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Fractional van der Waals interaction between thin metallic films
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 61:3, s. 2204-2210
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Tidskriftsartikel (refereegranskat)abstract
- The van der Waals (vdW) interaction between thin metallic films varies with separation as the separation to a fractional power. This is in contrast to the usual integer-power separation dependence between objects such as atoms, dielectric films, or thick metallic films. We have calculated the free energy of attraction between sheets of gold, silver, copper, beryllium, and tungsten numerically using experimentally found dielectric functions. The results are compared with the corresponding analytical results obtained using simple model dielectric functions. We have investigated how thin the metallic films must be in order for the fractional vdW interaction to be present. To our knowledge, fractional vdW interaction has not yet been confirmed experimentally.
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| 21. |
- Boström, Mathias, et al.
(författare)
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Lithium atom storage in nanoporous cellulose via surface-induced Li-2 breakage
- 2013
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 104:6, s. 63003-
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a physical mechanism that enhances a splitting of diatomic Li-2 at cellulose surfaces. The origin of this splitting is a possible surface-induced diatomic-excited-state resonance repulsion. The atomic Li is then free to form either physical or chemical bonds with the cellulose surface and even diffuse into the cellulose layer structure. This allows for an enhanced storage capacity of atomic Li in nanoporous cellulose.
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| 22. |
- Boström, M., et al.
(författare)
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Non-perturbative theory of dispersion interactions
- 2015
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 90:3
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Tidskriftsartikel (refereegranskat)abstract
- Some open questions exist with fluctuation-induced forces between extended dipoles. Conventional intuition derives from large-separation perturbative approximations to dispersion force theory. Here, we present a full non-perturbative theory. In addition, we discuss how one can take into account finite dipole size corrections. It is of fundamental value to investigate the limits of validity of the perturbative dispersion force theory.
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| 23. |
- Boström, Mathias, et al.
(författare)
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Repulsive van der Waals forces due to hydrogen exposure on bilayer graphene
- 2012
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 85:1, s. 012508-
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Tidskriftsartikel (refereegranskat)abstract
- We consider the effect of atomic hydrogen exposure to a system of two undoped sheets of graphene grown near a silica surface (the first adsorbed to the surface and the second freestanding near the surface). In the absence of atomic hydrogen, the van der Waals force between the sheets is attractive at all separations, causing the sheets to come closer together. However, with the addition of atomic hydrogen between the sheets, the long-range van der Waals interaction turns repulsive at a critical concentration. The underlying triple layer structure (SiO(2)-atomic hydrogen gas-air) gives rise to a long-range repulsion that at large-enough separations dominates over the more rapidly decaying attraction between the two-dimensional undoped graphene sheets (and between the outer graphene sheet and SiO(2)). This may be an avenue to tune the separation between two graphene sheets with the gas concentration. The doping of the graphene layers increases the attractive part of the interaction and hence reduces the net repulsive interaction.
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| 24. |
- Boström, Mathias, et al.
(författare)
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Resonance interaction induced by metal surfaces catalyzes atom-pair breakage
- 2013
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : American Physical Society. - 1050-2947 .- 1094-1622. ; 87:4, s. 044701-
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Tidskriftsartikel (refereegranskat)abstract
- We present the theory for retarded resonance interaction between two identical atoms at arbitrary positions near a metal surface. The dipole-dipole resonance interaction force that binds isotropically excited atom pairs together in free space may turn repulsive close to an ideal (totally reflecting) metal surface. On the other hand, close to an infinitely permeable surface it may turn more attractive. We illustrate numerically how the dipole-dipole resonance interaction between two oxygen atoms near a metal surface may provide a repulsive energy of the same order of magnitude as the ground-state binding energy of an oxygen molecule. As a complement we also present results from density-functional theory.
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| 25. |
- Boström, Mathias, et al.
(författare)
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Retardation-enhanced van der Waals force between thin metal films
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 62:11, s. 7523-7526
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Tidskriftsartikel (refereegranskat)abstract
- We recently investigated the van der Waals force between thin metal films. Under certain conditions this force decrease with separation to a fractional power. In the present work we use optical data of metals and the zero-temperature Lifshitz formalism to demonstrate a retardation effect. The retarded attraction between thin metal films may be larger than the nonretarded attraction. This property is related to a comparatively weak retardation dependence of the energy that originates from the transverse magnetic modes. At separations where the transverse electric modes give a significant contribution, the net effect can actually be an increased attraction. This effect vanishes with increasing film thickness and with increasing dissipation.
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