SFB 410 II-VI Halbleiter Wachstumsmechanismen, niederdim. Strukturen ...
Am Hubland, 97074 Würzburg
Research foci (and basic equipment-based research projects):
Project A: Layer growth and growth mechanisms
Project A provides the extensive material base, which is of great importance for the entire SFB. Within the framework of three experimental subprojects, II-VI epitaxial layers and heterostructures are produced by molecular beam epitaxy, MBE, and are characterized by various methods. The fourth subproject is concerned with theoretical models of MBE growth.
Subproject A1 produces and analyses epitaxial layers and layer systems based on ZnSe as well as the growth of nano-scale structures by means of microscopic shadow masks. Thereby optical active structures, e.g. overlapping quantum wells, quantum wells whose lateral confinement is > kT as well as structures for magneto-transport investigations, are produced and analysed in co-operation with other subprojects. Another very important aspect of A1 is to supply the entire SFB with ZnSe based high quality, custom-made layers and layer structures, e.g. for micro-scale optical resonators.
Subproject A2 primarily produces and analyses Hg and Mn based heterostructures. The growth mechanisms of HgTe layers have been investigated, whose growth is comparatively close to thermodynamic equilibrium, and optimized HgTe/HgCdTe quantum wells and superlattices with custom-made band structures have been produced. Furthermore n and p modulation doped superlattices have been produced and optimized with regard to their crystalline quality and properties (doping, defect density and mobility). Central activities are the production of Mn based hetero-structures and metal gates, which are of great importance for magneto-optical and magneto-transport experiments.
Subproject A4 has conducted theoretical analyses of growth mechanisms by means of simple solid-on-solid models. By using Monte-Carlo simulations it will be determined to what extent the promising results can be reproduced on more complicated types of lattices and later on even on systems with several components. Of primary concern are fundamental microscopic processes and their influence on morphology and the dynamics of the growing surface. Anisotropy, diffusion and potential barriers play an important role and are to be examined by quantitative comparison with experiments concerning the relevance of their contributions
Subproject A5 is especially dedicated to the controlled injection of spin-polarised electrons in semiconductors, whereby the special characteristics of semi-magnetic II-VI materials have been exploited. For this purpose, ZnSe and BeTe based single and double barrier magnetic structures have been manufactured and analysed. Furthermore magnetic quantum dots in single barriers have been manufactured and analysed with regard to their transport behaviour. Finally magnetic II-VI semiconductors is being grown on IV semiconductors, in order to inject spin-polarised carrier in materials with very long spin life-times. In this endeavor subproject A5 is able to use the experience of subproject A3. As is the case for subprojects A1 and A2, A5 also supplies other SFB subprojects with high quality samples.
Projekt B: Surfaces, interfaces and defects
Project B covers experimental and theoretical investigations of the structural, electronic and lattice dynamic properties of II-VI surfaces and interfaces. The experimental work is predominantly done on samples, which are manufactured in project A, or on appropriately prepared single crystals. Because surface characteristics have a very significant impact regarding properties of interest to the SFB, this project consequently plays an important role.
Subproject B1 is concerned with the geometric structure of II-VI surfaces. By means of high resolution diffraction of slow electrons including a kinetic analysis of the diffraction profiles, material and preparation dependent reconstruction, terrace size, step properties, defect densities and structural parameter have been studied. The work of B1 is closely correlated with all subprojects of project A and with most subprojects of project B.
The above is complemented by subproject B2, where mainly the geometric structure, and stain in layers and nearby interfaces are analysed with the help of high resolution x-ray diffraction. The subproject is engaged on one hand with the characterization of selected samples which have been manufactured in project A and on the other hand with the method of depth resolved x-ray diffraction using extreme asymetric Bragg reflexes has been developed further by implementing parallel detection with spatial resolution and a diverging x-ray beam. Therefore the sample does not have to be moved during the measurement, so that in situ characterization can be accomplished during growth.
In subproject B3, investigations of electronic structures and chemical properties of II-VI surfaces and interfaces continue. For example valence band offsets and band gaps of various SFB relevant systems are most precisely defined by using electron spectroscopy, whereas fundamental method inherent problems should be clarified. Further focal points are an unambigious determination of surface bonds, analyses of modified heterostructure interfaces, factors which influence the band structure and Mn based heterostructures and nano-scale structures. Finally bulk band structure and buried interfaces are characterized using resonantly excited x-ray emission.
Subproject B4 is of exceptional importance for theoretical models of electronic and optical properties. On one hand improved functionals for the exchange and correlation potentials with respect to a description of II-VI semiconductors has been developed and implemented, and a time dependent density functional method has been employed for the calculation of optical data. On the other hand the interpretation of the experimental data is supported by accompanying theoretical computations which take ofrelevant effects (reconstruction, multiple scattering, background, electron hole correlation effects etc.) into consideration.
In subproject B7 the lattice dynamics of thin II-VI layers and hetero-layers on surfaces, monolayers and quantum dots is studied with the help of Raman scattering. Because many of these experiments can be performed only under UHV conditions the surface needs to be thoroughly characterized. Furthermore this project concentrates on the electronic band structure, which by means of electro- and photo-modulation spectroscopy, is analysed over a large energy range. Finally magnetic properties are studied with the aid of spin-flip Raman spectroscopy in regard to the
g-factor and the influence of their magnetic behaviour by the free charge carriers.
Subproject B8 broadens the x-ray analysis of II-VI materials and layer systems, especially in regard to their local properties. In particular the electronic bonds, e.g. in (Cd,Mg)Te and (Cd,Mn)Te, are analysed with respect to the local electron density distribution. The local geometric structure is defined by the pair distribution function, PDF, which results in an exact characterization of the inter-atomic distances, e.g., in the region of Mg or Mn.
Project C: Nano-scale structures
Project C consists of research groups which deal with the production and the characterization of nano-scale structures of II-VI semiconductors. These are manufactured with different chemical methods, either by lateral nano-scale structuring of MBE layers by means of photolithography, self organization or as nano-sized particles using different chemical methods.
Subproject C1 works on the development of semi-magnetic nano-scale structures based on II-VI semiconductors and their analysis by means of optical spectroscopy. A lateral modulation of the band gap is manufactured by means of selective diffusion. This is acomplished with either focused ion beam lithography or by selective annealing of quantum films through a mask, in order to produce quantum wires or dots, semi-magnetic double quantum dots, or ferromagnetic nano-scale structures. The magneto-optical analysis of single quantum dots represents a particular challenge, which are selected via a nano-sized mask.
In subproject C3, linear Raman and fs-resolved CARS spektroskopy are used to analyse quantum dots in a glass matrix, on thin films and on surfaces, as well as quantum wires and quantum islands. Of interest is a determination of the phonon phase relaxation as a function of the quantum dot size, composition, temperature, the separation of homogeneous and inhomogenous portions of the CARS signal and the T1 life time of the photons. Furthermore micro-scale Raman investigations on quantum wires and dots results in information about the size distribution and the strain in the islands or the wire size dependence on the electron phonon coupling coefficient.
In subproject C5 binary und ternary II-VI nano-scale particles with a small size distribution are synthesized and partially doped with magnetic atoms by means of optimized wet chemical methods. A successful analysis of the nano-scale particle surfaces and the coupling of the stabilzing molecules by means of high resolution synchrotron radiation shall be expanded to nano-scale particles manufactured with different preparation procedures. A much more detailed analysis of the physical properties of the particles shall be done using synchrotron radiation as well as by means of optical methods, in particular confocal optical microscopy. Another emphasis is on the production of homogeneous, ordered, nano-scale structured layers of nano-sized particles.
In subproject C6 new wet-chemical synthesis methods are being developed for the production of strongly fluorescent quantum dots (nano-sized particles) with well defined particle sizes mit the smallest possible size distribution. New biomimetic synthesis methods using phytochelatines for stabilisation, is to be used whereas the latter is derived from different peptides and has cysteine-mercaptan groups als nucleation sites for the growth of nano-sized CdS particles. These nano-sized particles are of great interest for the production of homogeneous layers (with C5) as well as for their analysis by means of optical and time resolved methods (with C1, D2, D4), Raman spectroscopy (with B7, C3) and several electron spectroscopic methods (with B3, C5).
In Subproject C7 II-VI micro-sized resonators with strong three dimensional photonic inclusion are being developed (with A1, C1) and thoroughly analysed. Because II-VI materials have exceptionally large excitonic binding energies and oscillator strenghts, they are predestinated as a system to analyse the exciton photon interaction in the strong coupling regime, as well as at high temperature and excitation levels Interesting aspects are energy and angle distribution of photon modes, Rabi splitting and the Purcell factor, e.g. as a function of the diameter of the structures.
Project D: Electronic, optical and magnetic properties
In project D experimental and theoretical investigations of electronic, optical and magnetic properties are conducted on epitaxically produced layers and nano-scale structures. The work is based on close collaboration with projects A and C, and is accomplished under consideration of the surface and interface characteristics determined in project B.
In subproject D1 computations of the band structure of narrow gap II-VI semiconductors and those relevant to transport theory are carried out, which are of great importance for various subprojects. In addition self-consistent calculations using the k.p method in the Hartree approximation is developed further, e.g. for the explicit consideration of external magnetic fields or for changes in the cargo carrier concentration in gated structures. For the interpretation of the anomalous SdH oscillations in p type quantum wells, a multiple band transport theory must be developed. For the investigations of semi-magnetic quantum structures in D3 und D6 a theoretical treatment with consideration of the spin flip scattering is very desirable.
In subproject D2 the successful magneto-optical investigations at semi-magnetic II-VI semiconductors are continued. The optical anisotropy is to be examined more exactly in order to analyse interfaces, for example, which are important for spin injection. Further investigations of the k dependence of the exchange constants in quantum dot systems have been expanded, whereby wet chemical synthesized quantum dots as well as quantum dots produced by organized growth have been employed. Finally the dynamic exchange energy between Mn and the 2D electron gas has been analysed in detail, in order to contribute to the understanding and to the optimisation of spin injection and detection of spin polarized currents.
In subproject D3 the electronic properties of Hg based, narrow gap II-VI compounds have been investigated by means of magneto-optical experiments. Of interest is the subband structure in two dimensional quantum well structures, whereby the space charge containing layers are of special importance. In close co-operation with D1 a detailed description of the band structure which takes many body effects into consideration is desired. Furthermore the realization and characterization of low dimensional model systems and the investigation of semi-magnetic thin layer structures is planned.
In subproject D4 temporally and spatially resolved optical spectroscopy is employed in order to analyse spin dynamics in wide gap, semi-magnetic II-VI nano-sized structures, whereby pump-probe experiments in the ps-fs regime in a magnetic field are carried out. In particular, aspects of spin injection in heterostructures and spin transport across simple heterojunctions, double barriers and coupled quantum well structures have been investigated. With regard to so called spin aligners or spin filters, the longitudinal or transversal spin relaxation is of particular interest. Finally the dependence of these phenomena on dimension is of great importance, e.g., the question of a spin bottleneck in a zero dimensional system.
In subproject D5 the theory of statistically distributed many body effects is applied on II-VI semiconductors, whereby transport mechanisms, in particular, are taken into account. For example, the very important interaction of the electron spins with the statistically oriented magnetic moments in a 2D electron gas has been thoroughly investigated. In addition, conductivity fluctuations are calculated as well as the influence of a lifting of the Parisi replica symmetry on the mesoscopic properties. Furthermore it is expected, that inelastic scattering on magnetic moments will lead to a metal-isolator transition under certain conditions, whereby a completely new theory of localization is possible due to the hierarchical order of the order parameter.
In subproject D6 quantum transport phenomena in HgTe/(Cd,Hg)Te quantum well structures have been investigated at low temperatures. One focal point is the transport properties in p type HgTe quantum wells, in which novel transport phenomena have been observed. Furthermore charge carrier concentration dependent experiments have been carried out in order to elucidate the symmetry dependent spin splitting of the valence band and different effective masses at the Fermi level. A second focal point is the study of spin glasses and Kondo systems, whereby micro-sized Hall bar structures are fabricated by means of nano-scale structuring. These structures can be employed as micro-sized magnetometers in order to investigate the anomalous Kondo effect and spin glass correlations.
Rewards and prizes: