MBE Growth and Properties of ZnTe- and CdTe-Based Nanowires (original) (raw)
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Growth and Properties of ZnMnTe Nanowires
Acta Physica Polonica A, 2007
Catalytically enhanced growth of ZnMnTe diluted magnetic semiconductor nanowires by molecular beam epitaxy is reported. The growth is based on the vapor-liquid-solid mechanism and was performed on (001) and (011)-oriented GaAs substrates from elemental sources. X-ray diffractometry, scanning and transmission electron microscopy, atomic force microscopy, photoluminescence spectroscopy, and Raman scattering were performed to determine the structure of nanowires, their chemical composition, and morphology. These studies revealed that the obtained ZnMnTe nanowires possess zinc-blende structure, have an average diameter of about 30 nm, typical length between 1 and 2 µm and that Mn 2+ ions were incorporated into substitutional sites of the ZnTe crystal lattice.
Catalytic growth of ZnTe nanowires by molecular beam epitaxy: structural studies
Nanotechnology, 2007
ZnTe nanowires were grown by molecular beam epitaxy on GaAs substrates of three different orientations: (100), (110), and (111)B. The catalyst droplets were produced through in situ annealing of a previously deposited Au layer and by forming the eutectic alloy with Ga from the substrate. The influence of substrate orientation and growth parameters on the properties of nanowires was investigated using scanning and transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. The growth process was based on the vapour-liquid-solid mechanism and the contribution of the diffusion-induced effect in this mechanism was confirmed by correlating the length and the diameter of the produced nanowires. The nanowires had diameters ranging from 30 to 70 nm and lengths between 1 and 2 μm. The growth axis of the nanowires was 111 and the nanowires grew along 111 directions of the substrate, independent of the substrate orientation used. The nanowires had stacking faults at the bottom and those grown at optimal conditions possessed perfect cubic structure near the top.
Nanowire growth and sublimation: CdTe quantum dots in ZnTe nanowires
Physical Review Materials
The role of the sublimation of the compound and of the evaporation of the constituents from the gold nanoparticle during the growth of semiconductor nanowires is exemplified with CdTe-ZnTe heterostructures. Operating close to the upper temperature limit strongly reduces the amount of Cd present in the gold nanoparticle and the density of adatoms on the nanowire sidewalls. As a result, the growth rate is small and strongly temperature dependent, but a good control of the growth conditions allows the incorporation of quantum dots in nanowires with sharp interfaces and adjustable shape, and it minimizes the radial growth and the subsequent formation of additional CdTe clusters on the nanowire sidewalls, as confirmed by photoluminescence. Uncapped CdTe segments dissolve into the gold nanoparticle when interrupting the flux, giving rise to a bulb-like (pendant-droplet) shape attributed to the Kirkendall effect.
Optical properties of single ZnTe nanowires grown at low temperature
Applied Physics Letters, 2013
Optically active gold-catalyzed ZnTe nanowires have been grown by molecular beam epitaxy, on a ZnTe(111) buffer layer, at low temperature (350 • C) under Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per µm 2 ). The crystalline structure is zinc blende as identified by transmission electron microscopy. All nanowires are tapered and the majority of them are 111 oriented. Low temperature microphotoluminescence and cathodoluminescence experiments have been performed on single nanowires. We observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to the exciton energy in bulk ZnTe. This shift is attributed to the strain induced by a 5 nm-thick oxide layer covering the nanowires, and this assumption is supported by a quantitative estimation of the strain in the nanowires.
Selective growth of ZnSe and ZnCdSe nanowires by molecular beam epitaxy
Nanotechnology, 2005
Controlled growth of ZnSe and ZnCdSe nanowires is demonstrated by molecular beam epitaxy using Au or Ag catalyst films in the temperature range 400-550 • C. The highest density of small-diameter (10 nm), highly-crystalline ZnSe nanowires is achieved by using Au at 400 • C. Direct growth onto transmission electron microscope grids clearly indicates a tip-growth regime. Pre-patterning of the catalyst film allows highly selective ZnSe deposition as probed by photoluminescence and Raman spectroscopy. In similar conditions, the addition of Cd vapour in the MBE reactor allows the synthesis of ZnCdSe ternary nanowires.
Temperature-Dependent Growth Direction of Ultrathin ZnSe Nanowires
Small, 2007
Semiconductor nanowires (NWs) are promising candidates for applications in nanoscale electronic and optoelectronic devices. In recent years, much effort has been devoted to synthesizing NWs with controlled morphology and structure using various approaches, for example, laser-assisted chemical vapor deposition (CVD), [1] oxide-assisted CVD, [2] thermal CVD, [3-8] metal-catalyzed molecular beam epitaxy (MBE) [9-12] and chemical beam epitaxy (CBE). [13] Among these techniques, the metal-catalytic (also called the vapour-liquid-solid (VLS) [14]) growth method offers a number of advantages. For example, it can produce freestanding NWs with fully controlled diameters because the nucleation sites and the diameters of the NWs can be wellcontrolled by the preformed metal catalysts. For most semiconductor NWs with large diameters, the growth directions are generally specific and easily controlled. However, ultrathin NWs often display various growth directions. The lattice orientation in ultrathin semiconductor NWs is important because it may affect the optical and transport properties of the NW. [15, 16] In previous studies, several growth directions, such as < 111 > , [1] < 112 > , [2, 10] and < 110 > , [6, 10, 11, 15] have frequently been observed in ultrathin semiconductor NWs grown by different techniques. The mechanisms for the formation of NWs with varied orientations are not fully understood, mainly due to a lack of appropriate characterization. Theoretical analyses have shown that the growth directions of NWs may depend on the synthesis conditions, surface states (e.g., the presence of oxygen or impurities), and defect structures (e.g., surface steps and dislocations). [17] We have reported the synthesis of II-VI semiconductor NWs based on Au-catalyzed VLS MBE. [10] The MBE technique provides an ideal clean growth environment in which the atomic structure, impurity, and doping state can be easily controlled. We observed that ultrathin ZnSe (or ZnS)
Structural characterization of the epitaxially grown core–shell ZnTe/ZnMgTe nanowires
Radiation Physics and Chemistry, 2013
We report the method of the epitaxial growth of the core-shell ZnTe/ZnMgTe nanowires. The morphology and the crystal structure of several samples grown in different processes have been studied by scanning electron microscopy, high resolution transmission electron microscopy and X-ray diffraction methods. It was shown that the ZnMgTe shell growth was clearly epitaxial with a good crystal quality. The average lattice spacing of the ZnTe cores and ZnMgTe shells have been calculated and Mg content in the shells has been estimated. It was documented that growing the shell lattice mismatched to the core induces the strain in the core. The model of the strain creation mechanism has been proposed. The presence of a shell with a larger energy gap than that of the core results in a strong emission in the spectral region near the band edge.
Selected optical properties of core/shell ZnMnTe/ZnO nanowire structures
physica status solidi (b), 2011
We report here on selected optical properties of core/shell Zn 1Àx Mn x Te/ZnO nanowires (NWs). The Zn 1Àx Mn x Te cores of the investigated structures (containing from 3.5 to 7% of Mn) were grown by the MBE technique using gold-catalyzed vaporliquid-solid mechanism. Polycrystalline ZnO shells coating Zn 1Àx Mn x Te NWs were overgrown by the atomic layer deposition (ALD) method. Core/shell NWs were then examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman scattering, and photoluminescence (PL) measurements. The lines characteristic of the ZnO band edge emission and the intra-Mn 2þ transition were observed in the PL spectra. Modification of the Raman spectra of as-grown Zn 1Àx Mn x Te samples induced by the ZnO shell is demonstrated and discussed.
Crystal Research and Technology, 2009
Axial heterostructure nanowires (NWs) of ZnTe/CdTe were grown by vapour-liquid-solid growth realized in a molecular beam epitaxial chamber. By alternative supply of Zn or Cd and constant Te the heterostructure was generated. The liquid phase is provided by a Au-based eutectic droplet which stays at the tip of the NW during the entire growth. For structural and chemical characterization by TEM the NWs were harvested from the substrate and transferred to a holey carbon film. The NWs exhibit an expansion of the diameter correlated with the interface region between ZnTe and CdTe. Idiomorphic growth of the CdTe is evident from electron diffraction experiments. The growth rate of CdTe appears to be smaller compared to that of ZnTe at the same temperature. Both, quantitative high-resolution TEM and energy dispersive X-ray spectroscopy line scans reveal a smeared ZnTe/CdTe interface along about 200 nm. The smearing is due to both, the liquid catalyst which buffers the supply of Cd instead of Zn at the liquid/solid interface and to the strain which is induced by the lattice mismatch. It forces the system to consume the remnant Zn for the NW growth in favour of Cd. Dedicated to Prof. Wolfgang Neumann on the occasion of his 65 th birthday
Acta Physica Polonica A
In this work we report on the atomic structures, elemental distribution, defects and dislocations of three types of semiconductor nanowires: ZnTe, CdTe, and complex ZnTe/(Cd,Zn)Te core/shell hetero-nanowires grown by a molecular beam epitaxy on (111) Si substrate using a vapor-liquid-solid mechanism. The structural properties and the chemical gradients were measured by transmission electron microscopy methods. The nanowires reveal mainly sphalerite structure, however wurtzite nanowires were also observed.