INTRODUCTION TO THE PHYSICS OF QUANTUM DOTS

B. Kardynal Cambridge Research Laboratory, Toshiba Europe Ltd, 260 Cambridge Science Park, Cambridge CB4 0WE, UK

Quantum dots contain only a few well-defined energy levels for electron and/or holes as a result of the confinement of charge in all three spatial dimensions. Here, we describe both the application of photoluminescence spectroscopy and transport measurements to the characterisation of quantum dots and the novel phenomena that they exhibit. These include the Coulomb blockade, single electron tunnelling and single photon detection. The impact of quantum dots on future electronics and directions for future research, such as in quantum computing and cryptography, are addressed.

PACS numbers: 73.63.Kv, 78.67.Lt, 81.07.Ta

INTRODUCTION TO QUANTUM HALL EFFECTS

K.I. Wysokiński Institute of Physics, M. Curie-Skłodowska University, Radziszewskiego 10, 20-031 Lublin, Poland

The quantum Hall effect is a set of phenomena observed at low temperature in a two-dimensional electron gas subject to a strong perpendicular magnetic field. It manifests itself as a quantization of the nondiagonal elements (ρxy) of the resistivity tensor accompanied by simultaneous vanishing ρxx for ranges of the magnetic field. For the integer quantum Hall effect ρxy=h/νe2, where h is the Planck constant, e -- charge of an electron and ν is an integer, while for the fractional quantum Hall effect ν is a simple fraction. In spite of similar phenomenology deep and profound differences exist between these two effects. In the lecture the precision of the Hall quantization in the integer quantum Hall effect and briefly new types of quantum fluids observed in the fractional quantum Hall effect are discussed. Some recent theoretical and experimental discoveries connected with quantum Hall liquids are also mentioned.

PACS numbers: 73.43.--f, 73.40.--c, 73.50.Fq

Layered Semiconductors and Related Systems

H. Starnberg Department of Physics, Göteborg University and Chalmers University of Technology, 412 96 Göteborg, Sweden

The general properties of the layered transition metal dichalcogenides and the possibility to modify these materials by intercalation are reviewed. Examples are given of experimental results obtained by using angle-resolved photoelectron spectroscopy and very-low-energy electron diffraction. The possibility to use layered semiconductors as model systems in studies of e.g. Schottky barriers and surface photovoltage is exemplified by the Rb/WSe2 system. Attention is also paid to the use of van der Waals epitaxy in interface studies, and its possible practical applications. The potential of layered semiconductors like WSe2 in solar cell applications is also mentioned.

PACS numbers: 71.20.Nr, 71.20.Tx, 79.60.Bm, 79.60.Jv

Self-Compensating Incorporation of Mn in Ga1-xMnxAs

J. Masek and F. Maca Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Praha 8, Czech Republic

We consider hypothetical Ga7MnAs8, Ga16MnAs16 and Ga14Mn3As16 crystals with Mn in a substitutional, interstitial, and both positions. Spin-polarized full-potential linearized augmented plane wave calculations were used to obtain their electronic structure. We show that the interstitial Mn acts as a double donor and compensates the holes created by two Mn atoms in substitutional positions. This explains why the number of holes in Ga1-xMnxAs is much smaller than x. The presence of interstitial atoms may also be the reason for the lattice expansion with increasing content of Mn. The differences in electronic behavior of substitutional and interstitial Mn are discussed.

PACS numbers: 71.15.Ap, 71.20.Nr, 71.55.Eq, 75.50.Pp

FERMI LEVEL POSITION IN GaMnAs -- A THERMOELECTRIC STUDY

V. Osinniya, A. Jędrzejczaka, M. Arciszewskaa, W. Dobrowolskia, T. Storya and J. Sadowskia,b aInstitute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland bMAX-lab, Lund University, 22100 Lund, Sweden

Thermoelectric power was studied in the temperature range 100<=T<= 300 K in 0.3 -- 1μm thick ferromagnetic Ga1-xMnxAs epitaxial layers (0.015<=x<=0.06) in order to determine Fermi energy EF and carrier concentration p. For 0.015<=x<=0.05, at T=273K we find EF=275±50 meV and p=(2.5±0.5)×1020cm-3 (approximately Mn content independent). For x= 0.06, the Fermi energy decreases by about 100meV with the corresponding reduction of hole concentration to p=1.2×1020cm-3. At T=120K, these parameters vary between EF=380meV and p=3.5×1020cm-3 for x=0.015 to EF=110meV and p=5×1019cm-3 for x=0.06.

PACS numbers: 75.20.Ck, 75.30.Et, 73.50.--h

Interlayer Exchange Coupling in Short Period GaMnAs/GaAs Superlattices

W. Szuszkiewicz, E. Dynowska Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland B. Hennion, F. Ott Laboratoire Leon Brillouin, CE Saclay, 91191 Gif-sur-Yvette cedex, France M. Jouanne, J.F. Morhange Laboratoire des Milieux Desordonnes et Heterogenes, UPMC 4, pl. Jussieu, 75252 Paris, France M. Karlsteen and J. Sadowski Department of Experimental Physics, Chalmers University of Technology Göteborg, Sweden

The low dimensional structures containing III--V ferromagnetic semiconductors were intensively investigated in the last few years because of the variety of their potential applications. The aim of the present work was to study the interlayer exchange coupling in the short period GaMnAs/GaAs superlattices containing 16 or 8 GaMnAs monolayers. Samples with the magnetic layers corresponding to the mixed crystal composition with 5% or 6% of Mn were grown by MBE technique and characterized by both high-resolution X-ray diffraction and Raman scattering methods. Folded acoustic phonons were observed for the first time for this particular quantum system in the Raman scattering spectra. The interlayer exchange coupling in selected superlattices was investigated by both elastic neutron scattering (diffraction) and polarized neutron reflectivity measurements. Presence of the smooth interfaces and the stability of the superlattice period were confirmed by neutron reflectometry data. Ferromagnetic correlation of the magnetization vector in subsequent GaMnAs magnetic layers was demonstrated by both experimental methods.

PACS numbers: 61.12.--q, 75.25.+z, 78.30.Fs

ON THE KINETIC EXCHANGE IN AIIIBV DILUTED MAGNETIC SEMICONDUCTORS

J. Blinowskia and P. Kacmanb aInstitute of Theoretical Physics, Warsaw University, Hoża 69, 00-681 Warszawa, Poland bInstitute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland

In the case of II--VI diluted magnetic compounds it was shown that the kinetic exchange mechanism depends crucially on the electronic configuration of the magnetic ions and the charge transfer energies for the transitions from the band onto the magnetic ion and vice versa. In the present paper we analyze these effects in the family of III--V diluted magnetic semiconductors, showing that they affect considerably the predictions of the values of the Curie temperature and should be taken into account in the search for room temperature ferromagnetism in diluted magnetic semiconductors.

PACS numbers: 71.70.Gm, 75.50.Dd, 75.30.Et

MECHANISM OF RADIATIVE Mn2+ INTRA-SHELL RECOMBINATION IN BULK ZnMnS

V.Yu. Ivanov, M. Godlewski, S. Yatsunenko, A. Khachapuridze, M.S. Li and Z. Gołacki Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland

Origin of a fast component of the photoluminescence decay of Mn2+ intra-shell 4T1->6A1 transition is discussed based on the results of photoluminescence, photoluminescence kinetics and optically detected magnetic resonance experiments performed for bulk ZnMnS samples with about 1% Mn fraction. It is demonstrated that a fast component of the photoluminescence decay, reported previously for quantum dot structure and related to quantum confinement effects, is also observed in bulk samples and is related by us to very efficient spin cross-relaxation effects.

PACS numbers: 71.55.Gs, 76.30.Fc, 76.70.Hb, 78.55.Et

KERR EFFECT INVESTIGATIONS OF MAGNETIC INTERLAYER INTERACTIONS IN EuS--PbS MULTILAYERS

L. Kowalczyk, M. Chernyshova, T. Story Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland J.K. Ha Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands V.V. Voloubev and A.Yu. Sipatov Kharkov State Polytechnical University, 21, Frunze Str., 310002 Kharkov, Ukraine

Kerr magnetometry was employed to study the temperature dependence of magnetization and magnetic hysteresis loops in ferromagnetic EuS--PbS semiconductor multilayers in the temperature range T=3 -- 35K at low magnetic fields H<=150Oe. For EuS--PbS/KCl(100) structures with ultrathin non-magnetic PbS spacer of 1nm, we observed a maximum on the temperature dependence of magnetization at low fields H<=30Oe. For higher fields, we found for these structures a regular mean-field-like increase in magnetization with decreasing temperature. The same regular behavior was also found for EuS--PbS/KCl structures with thicker PbS spacer, as well as for all EuS--PbS/BaF2(111) multilayers independently of spacer thickness. For qualitative interpretation of these findings, we consider two magnetic contributions to the total energy of EuS--PbS multilayers: the Zeeman energy and the antiferromagnetic interlayer exchange coupling between ferromagnetic EuS layers via diamagnetic PbS spacer.

PACS numbers: 75.20.Ck, 75.30.Et

EFFECT OF HYDROGEN ON THE ELECTRONIC PROPERTIES OF GaAs1-yNy HETEROSTRUCTURES

M. Bissiri, V. Gaspari, G. Baldassarri H. v. H., F. Ranalli, A. Polimeni, M. Capizzi, A. Nucara INFM-Dip. di Fisica, Univ. di Roma "La Sapienza", 00185 Roma, Italy M. Geddo INFM-Dip. di Fisica, Univ. di Parma, 43010 Fontanini (Parma), Italy M. Fischer, M. Reinhardt and A. Forchel Universität Würzburg, Technische Physik, 97074 Würzburg, Germany

We have performed photoluminescence measurements in order to study the optical properties of hydrogenated GaAs1-yNy/GaAs heterostructures for y ranging from 0 to 0.03. Hydrogen irradiation leads to: (i) a progressive passivation of N-related recombination lines for low N content (y ~ 0.001); (ii) a sizable blue shift of the band gap in the "alloy" limit (y ~ 0.01). Thermal annealing restores the optical properties samples had before hydrogenation. These results can be accounted for by the formation of N---H+ complexes and demonstrate that hydrogen irradiation provides a powerful tool for the analysis of photoluminescence spectra of GaAs1-yNy.

PACS numbers: 78.66.Fd, 71.55.Eq, 78.55.Cr

HYDROGEN TUNING OF (InGa)(AsN) OPTICAL PROPERTIES

G. Baldassarri H. v. H., F. Ranalli, M. Bissiri, V. Gaspari, A. Polimeni, M. Capizzi, A. Nucara INFM-Dipartimento di Fisica, Universita degli Studi di Roma "La Sapienza", Piazzale A. Moro 2, 00185 Roma, Italy M. Geddo INFM-Dipartimento di Fisica, Universita di Parma, 43010 Fontanini (Parma), Italy M. Fischer, M. Reinhardt and A. Forchel Universität Würzburg, Technische Physik Am Hublaund, 97074 Würzburg, Germany

The effects of atomic hydrogen irradiation on the optical properties of (InGa)(AsN) single quantum wells were investigated by means of photoluminescence spectroscopy. For increasing hydrogen dose, the photoluminescence band peak energy of each nitrogen-containing sample blueshifts and for high hydrogen dose it reaches that of a corresponding nitrogen-free reference sample. This effect is accompanied by a broadening of the photoluminescence band line width and by a decrease in the photoluminescence efficiency. Thermal annealing at 550°C fully restores the original band gap value and the photoluminescence line width of the sample before hydrogenation. An interpretation of these phenomena is proposed in terms of an H perturbation of the charge distribution around the strongly electronegative N atoms, leading most likely to the formation of H--N complexes, and to an ensuing electronic passivation of nitrogen.

PACS numbers: 78.66.Fd, 71.55.Eq, 78.55.Cr

DYNAMICS OF PHOTOEXCITED CARRIERS IN GaInAs/GaAs QUANTUM DOTS

E. Ilczuka, K.P. Koronaa, A. Babińskia and J. Kuhlb aInstitute of Experimental Physics, Warsaw University, Hoża 69, 00-681 Warszawa, Poland bMPI für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany

We present photocurrent and time-resolved photoluminescence investigations of AlGaAs/GaInAs/GaAs structures containing GaInAs/GaAs self-assembled quantum dots. The high electrical field in those devices significantly influences carrier dynamics. The photocurrent spectra show a double peak with maxima at 1.40 and 1.47eV (at 80K). These maxima are due to the GaInAs wetting layer (higher) and the quantum dots (lower). The photoluminescence spectra comprise weak excitonic luminescence from GaAs at 1.504eV (at 80K) and stronger and broad emission from the Ga0.4In0.6As quantum dots. At 300K, the quantum dots emission has a lifetime of 1.1ns and has a maximum at an energy of 1.38eV. By analysis of both experiments, we can separate the influence of different radiative and nonradiative recombination processes. So, the tunneling rate: rT=0.5ns-1 and the radiative recombination rate in the quantum dots: rRQD=0.4ns-1 have been determined. The high tunneling probability (due to the influence of the built-in electric field) reveals that the tunneling effect is important for the recombination and transport processes in our structures.

PACS numbers: 72.40.+w, 78.47.+p, 78.67.--n

CARRIER DIFFUSION IN THE BARRIER ENABLING FORMATION OF CHARGED EXCITONS IN InAs/GaAs QUANTUM DOTS

K.F. Karlsson, E.S. Moskalenko, P.O. Holtz, B. Monemar Department of Physics and Measurement Technology, Linköping University, 581 83 Linköping, Sweden W.V. Schoenfeld, J.M. Garcia and P.M. Petroff Materials Department, University of California -- Santa Barbara, Santa Barbara, California 93106, USA

It is demonstrated that the photoluminescence spectra of single self-assembled quantum dots are very sensitive to the experimental conditions, such as excitation energy and crystal temperature. A qualitative explanation is given in terms of the effective diffusion of the photogenerated carriers, determined by the experimental conditions, which influence the capture probability and hence also the charge state of the quantum dots. This is proposed as a new tool to populate quantum dots with extra electrons in order to study phenomena involving charged excitons.

PACS numbers: 78.67.Hc, 71.35.--y, 78.55.Cr

FROM LOCALISED TO BALLISTIC EXCITONS IN GAAS QUANTUM WELLS

F. Pulizzi, P.C.M. Christianen, J.C. Maan High Field Magnet Laboratory, Research Institute for Materials University of Nijmegen, Toernooiveld 1, 6525ED Nijmegen, The Netherlands S. Eshlaghi, D. Reuter and A.D. Wieck Angewandte Festköperphysik, Ruhr-Universität Bochum, 44780 Bochum, Germany

The lateral motion of excitons in GaAs quantum wells is studied by means of spatially resolved photoluminescence. We show that at low temperatures (4.2K) the exciton motion evolves from localised excitons (zero mobility) in thin quantum wells to extremely high mobilities in wide wells. We find that for the widest quantum well investigated the observed motion cannot be explained by simple exciton diffusion and must be explained by the propagation of ballistic exciton polaritons.

PACS numbers: 71.35.--y, 71.36.+c

HIGH-PRESSURE MAGNETOTRANSPORT MEASUREMENTS OF RESONANT TUNNELLING VIA X-MINIMUM RELATED STATES IN ALAS BARRIER

M. Gryglas, J. Przybytek, M. Baj Institute of Experimental Physics, Warsaw University, Hoża 69, 00-681 Warsaw, Poland L. Eaves and M. Henini Department of Physics, University of Nottingham University Park, Nottingham NG7 2RD, UK

In this paper, we present the results of magnetotransport experiments performed on a single barrier GaAs/AlAs/GaAs heterostructures. Tunnel current was measured as a function of magnetic field for different values of bias voltage and hydrostatic pressure. We observed that the amplitude of the magnetooscillations of tunnel current quenched when the requirements for resonant tunnelling were met and it recovered in out-of-resonance conditions. This effect was observed both for tunnelling through donor states and through X-minimum related quasiconfined conduction band states. The fact that also in the latter case the amplitude was restored suggests that this process involved Xz subbands and took place without a participation of phonons (the so-called k||-conserving process).

PACS numbers: 73.43.Jn, 73.63.Hs

PERIODIC BEHAVIOR OF THE EXCITON OSCILLATOR STRENGTH WITH ALAS THICKNESS IN TYPE II GaAs/AlAs HETEROSTRUCTURES

C. Gourdon, D. Martins, P. Lavallard Groupe de Physique des Solides, Universites Paris 6 et Paris 7 CNRS UMR 75-88, Tour 23, 2 Place Jussieu, 75251 Paris cedex 05, France and E.L. Ivchenko Ioffe Physico-technical Institute, Russian Academy of Sciences 194021 St Petersburg, Russia

For a single GaAs/AlAs/GaAs type II pseudodirect double quantum well, as well as for superlattices it was predicted that the oscillator strength of the lowest optical transition has a periodic dependence on the number of AlAs monolayers. The oscillator strength depends on the coupling between the Γ and X electron states. We use samples containing a single GaAs/AlAs/GaAs double quantum well with thickness gradient to show experimental evidence of this effect. The results concerning the <&grl{Gamma}>-X coupling are obtained from the study of the ratio of photoluminescence intensities of the zero-phonon line and the phonon replica and from their time decay. They show the monolayer dependence of the Γ-X mixing potential. We extend the model describing the &Gammap;-X coupling for ideal interfaces in the frame of the envelope approximation to the case of non-abrupt interfaces and exciton localization. The amplitude of variation of the radiative recombination time due to the &Gammap;-X mixing is well reproduced within this model.

PACS numbers: 78.66.--w, 71.70.--d

INFLUENCE OF BUILT-IN ELECTRIC FIELD ON FORBIDDEN TRANSITIONS IN InxGa1-xAs/GaAs DOUBLE QUANTUM WELL BY THREE-BEAM PHOTOREFLECTANCE

G. Sęk, K. Ryczko, J. Misiewicz Institute of Physics, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland M. Bayer, T. Wang and A. Forchel Institute of Physics, University of Würzburg Am Hubland, 97074 Würzburg, Germany

Photoreflectance spectroscopy has been used to study optical transitions in In0.045Ga0.955As/GaAs double quantum well at 80K. The derivative nature of this contactless electromodulation technique allows for the observation of excited state transitions in the low-dimensional structure including the symmetry-forbidden ones. Excitonic symmetry-forbidden transitions can be observed due to the effect of mixing of heavy and light hole excitons and/or due to some asymmetry in the structure. We have shown that the built-in electric field in the region of double quantum well is weak enough (less than 0.5kV/cm) not to cause any significant energetic shift of features due to quantum confined Stark effect, on one hand. On the other hand, it is sufficient to change strongly the oscillator strength of forbidden transitions. To change the internal electric field, we have used photoreflectance in the three-beam mode with a third beam continuously illuminating the sample and causing changes of the built-in electric fields due to the photovoltage effect. This method works as a contactless forward bias and allows for a change of the field down to the flat band conditions. We have shown that changes of built-in electric field by amount of a few tenths of kV/cm can modify the intensity of forbidden transitions significantly. We show that, although the mixing of excitons is still important, a very weak built-in electric field can be dominant in the observation of forbidden excitonic transitions in double quantum well.

PACS numbers: 73.20.--r, 78.66.--w.

INTERSUBBAND PLASMON POLARITONS IN MULTIPLE QUANTUM WELL STRUCTURES

W. Ziętkowski and M. Załużny Institute of Physics, M. Curie-Skłodowska University, Pl. M. Curie-Skłodowskiej 1, 20-031 Lublin, Poland

Radiative intersubband plasmon polaritons in multiple quantum well structures, with arbitrary number of quantum wells, are discussed theoretically. The dispersion relations are derived modeling multiple quantum well structure by uniform uniaxial effective medium and using a full set of the Maxwell equations for macroscopic (local) electromagnetic fields. Numerical calculations are performed for a "GaAs--GaAs/AlGaAs multiple quantum well--air" structure corresponding to a typical total internal reflection geometry. Manifestation of these modes in total internal reflection spectra is also considered.

PACS numbers: 78.66.Fd, 73.20.Mf, 71.36.+c

NEGATIVE DIFFERENTIAL RESISTANCE AND SUPER-POISSONIAN SHOT NOISE IN A SYSTEM OF SINGLE ELECTRON TRANSISTORS

G. Michałek and B.R. Bułka Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznań, Poland

Currents and their fluctuations in two capacitively coupled single electron transistors were studied within the sequential tunneling approach. A special attention was focused on the effect of the negative differential resistance, which appears due to the Coulomb interactions of accumulated charges on both the single electron transistors. In this case large polarization fluctuations are activated, which results in a significant enhancement of the current shot noise.

PACS numbers: 73.23.Hk, 73.40.Gk, 73.50.Td

PHOTOLUMINESCENCE OF Er3+ NEAR 1.54μm IN SILICON-RICH SILICON OXIDE FILMS

D. Kuritsyna, V. Glukchanyuka, H. Przybylińskaa, A. Kozaneckia and W. Jantschb aInstitute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland bInstitut für Halbleiterphysik, J. Kepler Universität, 4040 Linz, Austria

Excitation of the intra- 4f-shell luminescence near 1.5μm in silicon-rich silicon oxide is studied. Silicon-rich silicon oxide was produced by high dose implantation of Si+ ions into SiO2 layers grown on silicon. Erbium doping was also performed using implantation of Er+ ions at an energy of 800keV. An evidence is presented that transfer of energy from defects related to excess silicon in silica is the dominant mechanism of excitation of Er3+ for optical pumping in the UV-blue wavelength range. Si-nanocrystals created by annealing at 1100°C rather compete for excitation with erbium than transfer energy to Er3+.

PACS numbers: 61.72.Ww, 78.55.Hx

Cd(Mg)Se SINGLE LAYERS AND CdSe/CdMgSe HETEROSTRUCTURES GROWN BY MOLECULAR BEAM EPITAXY ON InAs(001) SUBSTRATES

V.A. Kaygorodov, V.S. Sorokin St. Petersburg Electrotechnical University, Prof. Popov 5, St. Petersburg 197376, Russia I.V. Sedova, O.V. Nekrutkina, S.V. Sorokin, T.V. Shubina, A.A. Toropov and S.V. Ivanov Ioffe Physico-Technical Institute of RAS, Politekhnicheskaya 26, St. Petersburg 194021, Russia

We report on molecular beam epitaxy of CdSe/CdMgSe heterostructures on InAs(001) substrates and studies of their optical and structural properties. The CdMgSe energy gap versus composition dependence is determined. The zinc-blende MgSe band-gap energy and optical bowing parameter are estimated to be 4.05eV and 0.2eV, respectively. The CdSe quantum wells embedded into CdMgSe barriers demonstrate intense photoluminescence. Effective mass approximation calculations of electron--heavy hole optical transitions in CdSe quantum well are in a good agreement with the experimental data obtained.

PACS numbers: 78.66.Hf