Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S4: Surface Structure of Compound Semiconductors |
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Sponsoring Units: FIAP Room: LACC 515A |
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S4.00001: Scanning Tunneling Microscopy and Spectroscopy of Semiconductor Surfaces Invited Speaker: We discuss recent advances in our knowledge of the structure, electronic spectroscopy, and transport properties of semiconductors, obtained primarily through measurement with the scanning tunneling microscope (STM). For the wide band gap materials GaN and AlGaN, observation of various reconstructions together with first-principles theory have enabled the determination of many of the fundamental surface structures. The occurrence of incommensurate metallic layers on the surfaces, and their relevance for growth and control of thin film properties, will be described. Heterostructures of InGaP/GaAs have been studied in cross-section, permitting determination of the band offset between these materials. Detailed electrostatic simulations of the influence of the STM probe-tip on the measurement are used to provide an error bound on the result. Finally, for both Ge and SiC, spectroscopic measurements as a function of current reveal transport limitations in the conductance through surface states. Extension of the results to the determination of transport properties of other surfaces will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:42PM |
S4.00002: Invited Speaker: |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S4.00003: Atomic Structure of InGaAs Alloys Invited Speaker: The surface structure of a seemingly random alloy layer has a great impact on the compositional homogeneity and subsequent interface formation. For example, it has been suggested that random fluctuations in composition may initiate lateral composition that propagates through the remainder of the film. Our group studies the morphology and surface reconstruction of InxGa1-xAs alloy layers during growth and after annealing. Films of different compositions were grown by Molecular Beam Epitaxy on GaAs and InP to thicknesses less than the critical thickness for 3D islanding or misfit dislocation formation, and examined using in-situ Scanning Tunneling Microscopy and ex-situ Atomic Force Microscopy. The surface reconstruction of these layers is generally more disordered than those of their binary counterparts, and consists of different reconstruction domains. In particular, both surfaces show domains of a mixed-terminated (4x3) reconstruction, which is better ordered for the high In composition. In addition, there are pockets of a2(2x4) in the case of In0.27Ga0.73As/GaAs, and b2(2x4) in the case of In0.81Ga0.19As/InP. The coverage of both (2x4) reconstructions decreases during annealing, concomitant with a decrease in In surface concentration due to In desorption, suggesting that the (2x4) reconstructions are enriched in In compared to the (4x3)/(nx3). The coverage of different reconstructions also changes with film thickness, following changing surface composition and increasing strain energy. In the case of the In0.27Ga0.73As films, the In composition at the surface increases with film thickness and reaches a saturation level, in agreement with previous reports. The coverage of the (4x3) reconstruction reaches a saturation level at the same time, suggesting that a high and stable In concentration at the surface and/or a high strain energy favor a better ordered (4x3). The coverage of the a2(2x4) reconstruction increases initially with film thickness, then it decreases as the strain energy continues to increase, despite the fact that the surface reaches a stable composition. These results point out the importance of considering the effects of strain energy and inhomogeneous composition in the understanding of alloys surface structure. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:54PM |
S4.00004: Unified description of the formation and evolution of self-organized quantum dots in the InAs/GaAs(001) and Ge/Si(001) systems Invited Speaker: Self-organised semiconductor quantum dots, epitaxially grown on lattice-mismatched substrates, are promising candidates for the practical realisation of ``artificial atoms.'' Their peculiar tuneable properties open the way to novel applications in the fields of optoelectronics, single-electron and single-photon devices as well as quantum computation. However, a successful implementation requires a precise control over their shapes and sizes which, at present, is still an open problem. Its solution needs a basic understanding of the actual morphology of the quantum dots and of their further evolution during post-growth treatments. Here, by means of high-resolution scanning tunnelling microscopy, we investigate the model systems of self-organised quantum dots formed from single and binary semiconductor compounds, Ge grown on Si(001) substrates and InAs on GaAs(001), respectively. We demonstrate that for experimental conditions close to the thermodynamic limit (high substrate temperatures and low deposition rates) only two families of faceted and defect-free nanocrystals exist, small pyramids composed of four equivalent shallow facets, and lager multifaceted domes. The analogies between the two material systems extend also to the existence of hut-clusters and embryo islands that act as precursors for pyramid. A shape transition from pyramids to domes is seen to occur in both material systems. The transformation path, essentially consisting of the bunching of incomplete facets at the top of pyramids larger than a critical size, is precisely determined for Ge/Si(001) and explained in terms of surface diffusion processes only. These striking similarities further extend to the capping procedure that is needed in order to transform self-organised islands into true quantum dots. For both material systems we observe a backward dome to-pyramid transition accompanied by a strong height decrease. This complex phenomenology is rationalized in terms of intermixing processes driven by strain release. Our measurements suggest that the unified picture we are presenting for the prototype systems Ge/Si(001) and InAs/GaAs(001), extends, at least qualitatively, to a large number of material combinations that follow the Stranski-Krastanow growth mode. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:30PM |
S4.00005: Nanometer-Scale Structure and Properties of Dilute Semiconductor Alloys Invited Speaker: For many compound semiconductors, the addition of dilute concentrations of impurities leads to dramatic changes in the electronic, optical, and magnetic properties. For example, the introduction of a few percent nitrogen into GaAs leads to a band gap reduction of 100s of meV. Furthermore, the incorporation of a few percent manganese into GaAs enables a combination of semiconducting and ferromagnetic behavior. The resulting dilute semiconductor alloys are promising for several applications ranging from long-wavelength light-emitters and high efficiency solar cells to spin-electronics and spin-optoelectronics. In both cases, the nanometer-scale details of impurity incorporation are critical to understanding and controlling the observed properties. In this talk, I will discuss our recent investigations of the growth, nanometer-scale structure, and properties of dilute GaAsN and GaMnAs alloys, using nuclear reaction analysis and scanning tunneling microscopy, in conjunction with several other measurements. In GaAsN, we examine the role of surface reconstruction on the incorporation of nitrogen into substitutional vs. interstitial lattice sites, as well as the effect of nitrogen incorporation mechanisms on electronic and optical properties [1]. In the case of GaMnAs, we quantify clustering of Mn$_{Ga}$ and As$_{Ga}$ point defects, and its effect on electronic and magnetic properties [2]. [1] M. Reason, H. McKay, W. Ye, S. Hanson, V. Rotberg, and R.S. Goldman, ``Mechanisms of Nitrogen Incorporation in GaAsN Alloys,'' \textit{Appl. Phys. Lett. }\textbf{85}, 1692 (2004). [2] J.N. Gleason, M. Hjelmstad, V.D. Dasika, R.S. Goldman, S. Fathpour, S. Charkrabarti, and P.K. Bhattacharya, ``Nanometer-scale Studies of Point Defect Distributions in GaMnAs Alloys,'' \textit{Appl. Phys. Lett., }in press (January 3, 2005). [Preview Abstract] |
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