Acta Physica Polonica A

Acta Physica Polonica A
Vol. 98	No 3	September 2000

SEMICONDUCTOR FERROMAGNETIC STRUCTURES

T. Story

Institute of Physics, Polish Academy of Sciences
Al. Lotników 32/46, 02-668 Warsaw, Poland

Ferromagnetic semiconductor structures such as superlattices or trilayers form a new class of magnetic systems composed entirely of semiconductor materials. The examples are Ga_1-xMn_xAs-AlGaAs with the ferromagnetic layer of GaMnAs semimagnetic (diluted magnetic) semiconductor or EuS-PbS with the ferromagnetic member (EuS) of the family of europium chalcogenides. We discuss the spectrum of perspective ferromagnetic semiconductor materials, the effect of size and stress on magnetic properties of ultrathin semiconductor ferromagnetic layers, and the effect of interlayer exchange in all-semiconductor systems.

PACS numbers: 75.50.Pp, 75.70.Cn

APPLICATION OF GaN PRESSURE
GROWN CRYSTALS FOR EPITAXY
OF GaN-BASED STRUCTURES

I. Grzegory

High Pressure Research Center, Polish Academy of Sciences
Soko/lowska 29/37, 01-142 Warsaw, Poland

The results obtained with the use of the pressure grown GaN single crystalline substrates allow to draw the following conclusions important for the construction of In-free UV light emitting diodes and lasers and InGaN-based high power blue lasers: 1. The application of the pressure grown GaN single crystalline substrates allows to grow near dislocation free layer structures by both metal organic chemical vapor deposition and MBE. 2. The elimination of dislocations leads to highly efficient UV emission from GaN and GaN/AlGaN quantum wells which is impossible for strongly dislocated structures grown on sapphire. 3. At high excitations (i.e. in lasers) dislocations are effective nonradiative recombination centers also in the InGaN containing structures, therefore the elimination of these defects is crucial for better performance of blue lasers. 4. The analysis of microstructural and optical properties of the InGaN containing dislocation free structures shows that the main mechanisms of carrier localization in InGaN are not related with the nm scale compositional fluctuations in InGaN. In the paper, the optical and structural properties of the near dislocation free GaN-based structures leading to the above conclusions are discussed.

PACS numbers: 81.10.-h, 81.05.Dz, 73.61.Ey, 78.66.Fd

POLARIZATION INDUCED EFFECTS
IN AlGaN/GaN HETEROSTRUCTURES

O. Ambacher

Walter Schottky Institute, TU-Munich, Am Coulombwall, 85748 Garching, Germany

Two-dimensional hole and electron gases in wurtzite GaN/Al_xGa_1-xN/GaN heterostructures areinduced by strong polarization induced effects. The sheet carrier concentration and the confinement of the two-dimensional carrier gases located close to one of the AlGaN/GaN interfaces are sensitive to a high number of different physical properties such as polarity, alloy composition, strain, thickness, and doping. We have investigated the structural quality, the carrier concentration profiles, and electrical transport properties by a combination of high resolution X-ray diffraction, Hall effect, and C-V profiling measurements. The investigated heterostructures with N- and Ga-face polarity were grown by metalorganic vapor phase or plasma induced molecular beam epitaxy covering a broad range of alloy compositions and barrier thickness. By comparison of theoretical and experimental results we demonstrate that the formation of two-dimensional hole and electron gases in GaN/AlGaN/GaN heterostructures both rely on the difference of the polarization between the AlGaN and the GaN layer. In addition the role of polarity on the carrier accumulation at different interfaces in n- and p-doped heterostructures will be discussed in detail.

PACS numbers: 77.65.Ly, 77.70.+a, 77.84.-s

DILUTED MAGNETIC III-V SEMICONDUCTORS

A. Twardowski

Institute of Experimental Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland

During recent years diluted magnetic semiconductors based on III-V compounds have been of considerable interest. In this respect we review the basic properties of these materials, which are nearly exclusively Mn-based systems, such as GaMnAs, InMnAs, GaMnSb, and GaN:Mn. We discuss the nature of Mn impurity. Different Mn centers are considered and experimental pieces of evidence suggesting the dominating role of Mn (d⁵) configuration are given. Then we analyze s, p-d exchange, together with resulting magnetooptical properties (in particular absorption edge slitting for heavily p-type GaMnAs). The coupling between Mn ions (d-d exchange) and ferromagnetic ordering observed in InMnAs and GaMnAs is the next subject. Some mechanisms responsible for this ordering are presented. Finally we discuss transport properties and some selected problems of quantum structures based on III-V diluted magnetic semiconductors.

PACS numbers: 71.55.Eq, 71.70.Gm, 75.50.Pp, 78.20.Ls

2D ELECTRON GAS IN NON-UNIFORM MAGNETIC FIELDS

B.L. Gallagher^a, V. Kubrak^a, A.W. Rushforth^a, A.C. Neumann^a, K.W. Edmonds^a, P.C. Main^a, M. Henini^a, C.H. Marrows^b, B.J. Hickey^b, S. Thoms^c and D.E. Dahlberg^d

^a School of Physics and Astronomy, University of Nottingham
Nottingham NG7 2RD, U.K.
^b Department of Physics, University of Leeds, Leeds LS2 9JT, U.K.
^c Nanoelectronics Research Center, University of Glasgow, Glasgow G12 8QQ, U.K.
^d Department of Physics, University of Minnesota, MN 55455, USA

We present a review of some of our recent experimental work on hybrid ferromagnet/semiconductor devices in which 2D electrons propagate through magnetic barriers, periodic magnetic modulations, and random magnetic fields.

PACS numbers: 73.20.Dx, 73.50.Fq, 73.23.Ad, 73.50.Jt

HYDROGEN AND ITS COMPLEXES IN SILICON

L. Dobaczewski^a, K. Bonde Nielsen^b, K. Gosciński^a and O. Andersen^b

^a Institute of Physics, Polish Academy of Sciences
Al. Lotników 32/46, 02-668 Warsaw, Poland
^b Institute of Physics and Astronomy, University of Aarhus, Aarhus, Denmark

In this study the technique of Laplace transform (high resolution) deep level transient spectroscopy combined with the uniaxial stress method has been used to study a symmetry and the defect reconfiguration kinetics (the stress induced alignment) of some forms of hydrogen-related centres. We have confirmed the trigonal symmetry of the defect related to the isolated bond centred hydrogen. When hydrogen decorates the vacancy-oxygen pair (the A centre) the apparent defect orthorhombic symmetry is not lowered as a result of a very high hydrogen jumping rate between two unsaturated broken bonds of the vacancy. We also show that the stress-induced defect alignment in some cases can be related to the same microscopic mechanism of the hydrogen motion as it is for the diffusion process.

PACS numbers: 68.55.Ln, 71.55.Cn, 73.40.Lq

HIGH-PRESSURE STUDIES OF SEMICONDUCTORS IN THE FAR-INFRARED: DONOR STATES IN QUASI-2D

B.A. Weinstein^a, J.G. Tischler^a, R.J. Chen^b, H.A. Nickel^a, B.D. McCombe^a, A.B. Dzyubenko^c and A. Sivachenko^d

^a Department of Physics, SUNY Buffalo, NY, 14260-1500, USA
^b Brimrose Corporation of America, Baltimore, Md., 21236, USA
^c General Physics Institute, RAS, Moscow, Russia
^d Weizman Institute, Rehovot, Israel

We review recent experimental advances by the Buffalo group in performing far-infrared magnetospectroscopy under fine tuning of applied high hydrostatic pressure. Experiments are reported for the effects of pressure on Si donors in modulation doped GaAs/AlGaAs quantum wells. We clearly observe pressure-mediated competition between free (i.e., Landau level) and bound electron states - the latter arising from both neutral (D⁰) and charged (D^-) donor species. With increasing pressure, there is a progression of the observed spectra from being dominated by cyclotron resonance and the D^- singlet (or singlet-like bound magnetoplasmon) transitions, to showing the D⁰ 1s -> 2p⁺ line. The main reason for this evolution is the decrease in electrons due to the crossover of the Si levels associated with the Gammap (well) and X (barrier) conduction minima. Indeed, for pressures above 30 kbar the Gammap(well)-X(barrier) crossover quenches all the transitions. However, we find strong evidence that electrons are independently lost to a trap, which becomes active several kbar below this crossing. A possible candidate for this trap is residual Se impurities in the barriers. We present the results of detailed numerical calculation which are found to agree very well with the measured field dependencies of the cyclotron resonance, D⁰ and D^- transition energies. In the sample with the highest doping, a new transition is observed for fields and pressures above 7.5 T and 5 kbar. Reasons for this apparent anomaly are discussed.

PACS numbers: 73.20.Hb, 73.20.Dx, 78.66.Fd

SELF-ASSEMBLED PARALLEL MESOSCOPIC Pb WIRES ON VICINAL Si(111)

M. Jalochowski

Institute of Physics, Maria Curie-Sklodowska University
Pl. M. Curie-Sklodowskiej 1, 20-031 Lublin, Poland

The work presents a novel method of production of mesoscopic metallic wires on semiconducting surfaces. Making use the self-assembly phenomenon, arrays of extremely long and perfectly parallel mesoscopic Pb-wires on vicinal Si(111) substrates are formed and studied in UHV conditions. Before deposition of Pb a uniform distribution of monoatomic steps and terraces was induced by formation of Au chains running along step edges. The wires growing on the substrates held at temperatures close to the room temperature reach up to 8 ľ length. A reflection high electron energy diffraction experiment shows that the wires laying on Si(533) along the step edges have triangular cross-section determined by (111) and (100) facets of Pb. Scanning tunneling microscopy images collected at low temperatures have enabled us to determine details of the wires shape and morphology of the substrate. The width of the wires was approximately equal to 60~nm whereas their height was about 10 nm. The observed strong growth anisotropy is attributed to step edge barriers and high Pb mobility on the smooth Si(111) narrow terraces that form vicinal surfaces and the anisotropic strain due to large misfit between Pb and Si lattices.

PACS numbers: 68.35.Bs, 68.55.Jk, 68.65.+g, 81.15.Ef, 61.16.Ch

DETECTION OF SINGLE SUBMILLIMETER-WAVE PHOTONS USING QUANTUM DOTS

S. Komiyama^a, O. Astafiev^b, V. Antonov^b and T. Kutsuwa^a

^a Department of Basic Science, University of Tokyo
Komaba 3-8-1, Meguro-ku, Tokyo, Japan
^b Japan Science and Technology Corporation (JST)
Kawaguchi, Saitama 332-0012, Japan

Single-photon detection in a range of submillimeter waves (lambda = 0.17-0.20 mm) is demonstrated by using lateral semiconductor quantum dots fabricated on a high-mobility GaAs/AlGaAs single heterostructure crystal. When a submillimeter photon is absorbed by the quantum dot while it is operated as a single-electron transistor, it switches on (or off) the conductance through the quantum dot. An incident flux of 0.1 photons/s on an effective detector area, (0.1 mm)², is detected with a 1 ms time resolution. The effective noise equivalent power is roughly estimated to reach on the order of 10^-22 W/Hz^1/2, a value superior to the ever reported best values of conventional detectors by a factor more than 10^4.

PACS numbers: 72.20.Ht, 73.40.Hm, 76.40.+b

ELECTRONIC PROPERTIES OF InAs/GaAs SELF-ASSEMBLED QUANTUM DOT STRUCTURES AND DEVICES STUDIED BY PHOTOCURRENT SPECTROSCOPY

D.J. Mowbray^a, P.W. Fry^a, M.S. Skolnick^a, I.E. Itskevich^b, L. Harris^a, A.D. Ashmore^a, J.J. Finley^a, L.R. Wilson^a, K.L. Schumacher^a, J.A. Barker^c, E.P. O'Reilly^c, M. Al-Khafaji^d, A.G. Cullis^d, M. Hopkinson^d, J.C. Clark^d and G. Hill^d

^a Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, U.K.
^b School of Engineering, University of Hull, Cottingham Road, Hull HU6 7RX, U.K.
^c Department of Physics, University of Surrey, Guildford GU2 5XH, U.K.
^d Department of Electronic and Electrical Engineering, University of Sheffield
Sheffield S1 3JD, U.K.

The power of photocurrent spectroscopy to study the electronic properties of InAs/GaAs self-assembled quantum dots is described. From comparison of results from different samples it is shown that photocurrent provides a direct means to measure absorption spectra of quantum dots. Studies in high electric field enable the electron-hole vertical alignment to be determined. Most surprisingly this is found to be opposite to that predicted by all recent predictions. Comparison with theory shows that this can only be explained if the dots contain significant amounts of gallium, and have a severely truncated shape. The nature of the ground and excited state transitions, carrier escape mechanisms from dots, in-plane wave function anisotropies and the modal gain of a quantum dot laser are determined.

PACS numbers: 73.61.Ey, 78.66.Fd, 42.55.Px, 73.50.Pz

STIMULATED SCATTERING OF MICROCAVITY POLARITONS

J. Bloch, P. Senellart, V. Thierry-Mieg and J.Y. Marzin

Laboratoire de Microstructures et de Microélectronique, 196 av. Henri Ravéra
92 225 Bagneux Cedex, France

We report on cw optical experiments performed in a semiconductor microcavity containing a single quantum well in the strong coupling regime. Angularly resolved photoluminescence measurements under non-resonant excitation show the collapse of a relaxation bottleneck as the excitation power is increased. As a result, the emission close to k_parallel=0 presents a non-linear behavior. In a two-beam experiment we resonantly inject polaritons at k_parallel=0 and show that relaxation from states with large in-plane wave vector toward k_parallel=0 is stimulated by the polariton final state population.

PACS numbers: 71.36.+c, 71.35.Lk, 78.30.Fs, 42.55.Sa

BREAKDOWN OF ROTATIONAL SYMMETRY
AT SEMICONDUCTOR INTERFACES:
A MICROSCOPIC DESCRIPTION
OF VALENCE SUBBAND MIXING

S. Cortez, O. Krebs and P. Voisin

Laboratoire de Physique de la Matičre Condensée, Ecole Normale Supérieure
24 rue Lhomond, Paris, France

The recently discovered in-plane optical anisotropy of [001]-grown quantum wells offers a new theoretical and experimental insight into the electronic properties of semiconductor interfaces. We first discuss the coupling of X and Y valence bands due to the breakdown of rotation inversion symmetry at a semiconductor hetero-interface, with special attention to its dependence on effective parameters such as the valence band offset. The intracell localization of Bloch functions is explained from simple theoretical arguments and evaluated numerically from a pseudo-potential microscopic model. The role of envelope functions is then considered, and we discuss the specific case of non-common atom interfaces. Experimental results and applications to interface characterization are presented. These calculations give a microscopic justification, and establish the limits of the heuristic "H_BF" model.

PACS numbers: 78.20.Fm, 78.20.Jq

IF PAN Home Page ACTA Home Page