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V13. MBE Growth and Optical Properties of III/V-II/VI Hybrid Core-Shell Nanowires

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V13. MBE Growth and Optical Properties of III/V-II/VI Hybrid Core-Shell Nanowires V13. MBE growth and optical properties of III/V-II/VI hybrid core-shell nanowires Alexander Pawlis and Mihail Ion Lepsa Peter Grünberg Instutite (PGI-9, PGI-10) JARA-Fundamentals for Information Technology Research Center Jülich, 52425 Contents 1. Introduction ........................................................................................................................ 2 2. Molecular beam epitaxy of semiconductor heterostructures .............................................. 3 2.1 Molecular beam epitaxy ................................................................................................... 3 2.2 MBE growth..................................................................................................................... 7 3. Growth, morphology and structural properties of GaAs/ZnSe core-shell nanowires ...... 12 3.1 Growth of self-catalyzed GaAs nanowires .................................................................... 12 3.2 Growth of ZnSe shell ..................................................................................................... 14 3.3 Structural properties of GaAs/ZnSe core/shell nanowires ............................................. 15 4. Theoretical basis for describing optical properties of semiconductor nanostructures ...... 17 4.1 Energy gap, band structure and effective mass approximation ..................................... 17 4.2 Electron-photon interaction, transition matrix and oscillator strength .......................... 19 4.3 Einstein coefficients, transition rates and radiative lifetime .......................................... 20 4.4 Band offset, heterostructures and 1-dim. confinement (quantum well)......................... 21 5. Photoluminescence properties of the GaAs/ZnSe core/shell nanowires .......................... 23 References ................................................................................................................................ 25 2 1. Introduction For electronic and optoelectronic applications, self-catalyzed grown semiconductor nanowires (NWs) especially from classical III/V compound semiconductors ((AlGa)As, InAs, InSb) have attracted much research interest in the last ten years. Measurements of the charge transport characteristics along the NW axis allows for studying quantum effects on the nanoscale as typically NW diameters are in the order of 10-100 nm. If those NWs are combined with certain contact designs, they provide a suitable base for a new class of modern devices such as field effect transistors or gate confined quantum dots. Regarding optical and optoelectronic applications, NWs are particularly suitable for nanolasers and single photon sources due to their high length-to-width aspect ratio, forming a build-in waveguide system. However, typical core-only NWs provide an extremely large surface/volume ratio. This leads to strong non-radiative surface recombination of carriers, which for optoelectronic applications drastically reduces the quantum efficiency of NW based devices. Therefore, suitable passivation techniques are required to enhance the optical quantum efficiency and to explore relevant quantum effects in the NWs. Efficient passivation is achieved by epitaxial co- verage of the core material with a shell material pro- viding a larger bandgap as the core and a small lattice mismatch (e.g. difference between the lattice para- meters of the core and shell material) to it. The most prominent core/shell NW system is GaAs/(Al,Ga)As. Here, the GaAs NWs are passivated by an epitaxial shell of (Al,Ga)As. The latter must be additionally capped with another GaAs layer to maintain chemical stability Fig. 1.1: Schematic drawing (right side) of a GaAs/ZnSe core/shell of the system, as (Al,Ga)As NW and calculation of the conduction band alignment (left side) rapidly oxidizes in air. Due between GaAs and ZnSe close to the interface (provided by Dr. N. to the improved surface Demarina). Considering a shell thickness of 70 nm, a core radius passivation, capped NWs of 35 nm and a shell n-type doping concentration of 5x10 17 cm -3, yield a noticeably enhanced electron wavefunctions (red, blue) are localized at the periphery of photoluminescence (PL) the GaAs core . emission. In time-resolved PL measurements of AlGaAs/GaAs NWs, carrier lifetimes up to multiple nanoseconds were demonstrated, which together with the enhanced emission intensity validates the passivation effect. A prominent alternative system to GaAs/AlGaAs is GaAs/ZnSe. The II/VI compound semiconductor ZnSe is also almost lattice matched with GaAs. Improved enhancement of the optical properties of a GaAs core NWs is possible since GaAs/ZnSe provides substantially larger band offsets than AlGaAs/GaAs. This results in improved carrier localization and balanced confinement of electrons and holes in the GaAs core. A schematic drawing of a GaAs/ZnSe core/shell NW and calculations of the conduction band alignment (see also Sec.4.4) of core and shell at the interface are shown in Fig. 1.1 Depending on the doping level of the shell, electron wave functions can be localized in the periphery of the GaAs core. 3 Additionally, the ZnSe shell provides a higher stability against oxidation without the need for additional GaAs capping. Finally, the extremely small lattice mismatch of 0.3 % between GaAs and ZnSe allows for engineering nearly perfect epitaxial interfaces between GaAs and ZnSe with extremely low dislocation densities. Although the radial interface between core and shell is nearly defect-free, the GaAs core itself can contain randomly stacked axial segments of wurtzite (WZ) and zincblende (ZB) crystal phase. This polymorphism (see also Sec.3.3) leads to twinning domains and stacking faults at the phase boundaries between the segments. In this course, the growth by molecular beam epitaxy (MBE) and optical properties of GaAs/ZnSe core-shell NWs are studied. Here, we introduce first the basics of the MBE growth with specific exemplification. In the following section the growth, morphology and structural properties of the GaAs/ZnSe core-shell NWs are considered. Then the theoretical basis for the understanding of the optical properties of the GaAs/ZnSe core-shell NWs are introduced. Finally, the PL properties of the GaAs/ZnSe core-shell NWs are discussed. 2. Molecular beam epitaxy of semiconductor heterostructures 2.1 Molecular beam epitaxy Molecular beam epitaxy is a refined form of physical vapor deposition for epitaxial growth of high quality semiconductor, metal and insulator thin films. The epitaxial growth refers to the deposition of a crystalline film on a crystalline substrate, the film being in registry with the substrate. The term epitaxy comes from the Greek roots epi, meaning "above", and taxis, meaning "in ordered manner". It can be translated "to arrange upon". Using MBE, high quality semiconductor layers are grown regarding purity, crystal phase, control of layer thickness and doping. The main characteristic features of the MBE are: - precise controlled atomic or molecular thermal beams react with a clean heated crystalline surface (substrate); - ultra high vacuum (UHV) conditions; - the beams are obtained by effusion from solid (sublimation) or liquid (evaporation) ultra pure material sources at high local temperature; - low growth rate (1 µm/h ~ 1 monolayer/s) which permit the surface migration of the impinging species resulting in the growth of atomically flat surfaces ; - the growth governed mainly by the kinetics of the surface processes , under conditions far from thermodynamic equilibrium; - ultra rapid shutters in front of the beam sources allowing nearly atomically abrupt transitions from one material to another and therefore, obtaining of abrupt interfaces . Historically, the basic concepts of the MBE growth process have been developed in 1958 by K. G. Günther at Siemens Research Laboratories (Erlangen) within so called ‘three temperature method’ . The foundation of the MBE was done in the mid-sixties, when the first semiconductor films were grown by J.R. Arthur and A.Y. Cho at Bell Laboratories while studying the interaction of Ga atoms and As 2 molecules with crystalline GaAs in UHV conditions. In the following, some of the characteristics of the MBE will be discussed. a) Vapor pressure It is well known that in a closed volume, a liquid or solid element is in equilibrium with the above gas phase (see Fig.2.1a). The corresponding pressure of the gas is called the saturated or equilibrium vapor pressure, peq . Increasing the temperature, the vaporization will be 4 Fig. 2.1: (a) Illustration of the vaporization of a liquid or solid material in a closed volume resulting in the saturated vapor pressure of the gas phase. (b) Phase diagram of a single element. stimulated and peq will increase. Each element has a characteristic phase diagram, which describes the coexistence of different physical states of the element. This is shown in Fig. 2.1b. Only at one point, the so called triple point (T) the three different physical states coexist. The MBE method makes use of the vaporization of solid or liquid materials in ultrahigh vacuum (UHV) conditions. The vapors are directed to a crystalline substrate as atom (molecular) beams where they condense in the solid state. In Fig. 2.2, the vapor pressure of different elements as function of temperature is shown. Depending on the element, the vaporization results in molecules or atoms and a certain peq is reached by sublimation or evaporation at different -3 temperatures. For example, a peq of 10 torr
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