Growth and optical investigations of high quality individual CdTe/(Cd,Mg)Te core/shell nanowires.

CdTe nanowires with the average diameter of only 40 nm coated with (Cd,Mg)Te shells are grown using Au-catalyzed vapor-liquid-solid growth mechanism in a system for molecular beam epitaxy. High optical quality of individual nanowires is revealed by means of low temperature cathodoluminescence and micro-luminescence. It is found that, the optical emission spectrum consists mostly of the near band edge emission without any significant contribution of defect related luminescence. Moreover, the importance of surface passivation with (Cd,Mg)Te coating shells is demonstrated.

Nanoscale morphology of multilayer PbTe/CdTe heterostructures and its effect on photoluminescence properties.

We study nanoscale morphology of PbTe/CdTe multilayer heterostuctures grown by molecular beam epitaxy on hybrid GaAs/CdTe (100) substrates. Nominally, the structures consist of 25 repetitions of subsequently deposited CdTe and PbTe layers with comparable thicknesses of 21 and 8 nm, respectively. However, the morphology of the resulting structures crucially depends on the growth temperature. The two-dimensional layered, superlattice-like character of the structures remains preserved only when grown at low substrate temperatures, such as 230 °C. The samples grown at the slightly elevated temperature of 270 °C undergo a morphological transformation to structures consisting of CdTe and PbTe pillars and columns oriented perpendicular to the substrate. Although the pillar-like objects are of various shapes and dimensions these structures exhibit exceptionally strong photoluminescence in the near infrared spectral region. At the higher growth temperature of 310 °C, PbTe and CdTe separate completely forming thick layers oriented longitudinally to the substrate plane. The observed topological transformations are driven by thermally activated atomic diffusion in the solid state phase. The solid state phase remains fully coherent during the processes. The observed topological transitions leading to the material separation in PbTe/CdTe system could be regarded as an analog of spinodal decomposition of an immiscible solid state solution and thus they can be qualitatively described by the Cahn-Hillard model as proposed by Groiss et al (2014 APL Mater. 2 012105).

Activation of an intense near band edge emission from ZnTe/ZnMgTe core/shell nanowires grown on silicon.

The absence of luminescence in the near band edge energy region of Te-anion based semiconductor nanowires grown by gold catalyst assisted molecular beam epitaxy has strongly limited their applications in the field of photonics. In this paper, an enhancement of the near band edge emission intensity from ZnTe/ZnMgTe core/shell nanowires grown on Si substrates is reported. A special role of the use of Si substrates instead of GaAs substrates is emphasized, which results in an increase of the near band edge emission intensity by at least one order of magnitude accompanied by a simultaneous reduction of the defect related luminescence. A possible explanation of this effect relies on the presence of Ga-related deep level defects in structures grown on GaAs substrates, which are absent when Si substrates are used. Monochromatic mapping of the cathodoluminescence clearly confirms that the observed emission originates, indeed, from the ZnTe/ZnMgTe core/shell nanowires, whereas individual objects are studied by means of microphotoluminescence.

Influence of substrate nitridation temperature on epitaxial alignment of GaN nanowires to Si(111) substrate.

An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.

TEM characterization of MBE grown CdTe/ZnTe axial nanowires.

CdTe/ZnTe axial nanowires were successfully fabricated by molecular beam epitaxy with the use of Au nano-catalysts and vapour-liquid-solid growth mechanism. Nanowires had zinc-blende structure with numerous stacking faults in the bottom ZnTe part and near perfect crystalline structure in the top CdTe part. Energy dispersive X-ray spectroscopy (EDXS) and lattice fringe spacing analysis revealed nonabrupt nature of hetero-interface, whose width was estimated to be 50-70 nm for the nanowires having a diameter in the range from 40 to 50 nm.

ZnTe-ZnO core-shell radial heterostructures grown by the combination of molecular beam epitaxy and atomic layer deposition.

ZnTe-ZnO core-shell radial heterostructures were grown using a new method of combining molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Zinc telluride nanowires (core) were grown on a GaAs substrate using gold catalyzed vapor-liquid-solid mechanism. An atomic layer deposition technique using diethyl zinc and deionized water as precursors was applied for zinc oxide shell formation. The core-shell ZnTe-ZnO heterostructures thus obtained were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and photoluminescence measurements.

TEM determination of directions of (Ga,Mn)As nanowires grown by MBE on GaAs(001) substrates.

The structure of GaMnAs nanowires (NW) with nominal Mn concentration of up to 7 at% was investigated by transmission electron microscopy. The (Ga,Mn)As NW were grown on epiready GaAs(001) n-type wafers by molecular beam epitaxy. The crystal structure of the NW was determined to be zinc-blende. NW with Mn concentrations lower than 5 at% grow along the 111 direction. NW with higher Mn concentrations grow along the 110 direction and reveal a branching structure. The main nanowire and branches grow along the 110 directions belonging to only one {111} plane.

Influence of the Si cap layer on the SiGe islands morphology.

Transmission electron microscopy (TEM), atomic force microscopy (AFM), and EDX methods were used to study morphology and chemical composition of SiGe/Si(001) islands grown at 700 degrees C and covered at 550 degrees C and 700 degrees C by Si layers of different thickness. The samples were grown in ultra high vacuum chemical vapor deposition process (UHVCVD) controlled with in situ reflection of high-energy electron diffraction (RHEED). The islands transformed from initial pyramid and dome shapes to lens shape for 1.4 nm and 3.7 nm cap layer thickness at 550 degrees C and 700 degrees C, respectively. An increase of lateral to vertical ratio was observed during the transformation. For the higher depositing temperature the ratio was bigger and was increasing continuously with cap layer thickness. Also, with increasing Si cap layer thickness, the Ge concentration was decreasing, which was more observable for higher capping temperature.

Wurtzite (Ga,Mn)As nanowire shells with ferromagnetic properties.

(Ga,Mn)As having a wurtzite crystal structure was coherently grown by molecular beam epitaxy on the {1100} side facets of wurtzite (Ga,In)As nanowires and further encapsulated by (Ga,Al)As and low temperature GaAs. For the first time, a truly long-range ferromagnetic magnetic order is observed in non-planar (Ga,Mn)As, which is attributed to a more effective hole confinement in the shell containing Mn by the proper selection/choice of both the core and outer shell materials.

Coexistence of optically active radial and axial CdTe insertions in single ZnTe nanowire.

We report on the growth, cathodoluminescence and micro-photoluminescence of individual radial and axial CdTe insertions in ZnTe nanowires. In particular, the cathodoluminescence technique is used to determine the position of each emitting object inside the nanowire. It is demonstrated that depending on the CdTe deposition temperature, one can obtain an emission either from axial CdTe insertions only, or from both, radial and axial heterostructures, simultaneously. At 350 °C CdTe grows only axially, whereas at 310 °C and 290 °C, there is also significant deposition on the nanowire sidewalls resulting in radial core/shell heterostructures. The presence of Cd atoms on the sidewalls is confirmed by energy dispersive X-ray spectroscopy. Micro-photoluminescence study reveals a strong linear polarization of the emission from both types of heterostructures in the direction along the nanowire axis.

Zn(1-x)Mg(x)Te nanowires grown by solid source molecular beam epitaxy.

This paper reports on the epitaxial growth of single-crystalline ternary Zn(1-x)Mg(x)Te nanowires covering a broad compositional range of molar fraction 0≤x≤0.75. The nanowires were grown on (100), (110), and (111) GaAs substrates using a vapor-liquid-solid mechanism. Solid source molecular beam epitaxy and an Au-based nanocatalyst were used for these purposes. The composition of nanowires can be adjusted by changing the ratio of Mg to Zn molecular beam fluxes. Electron microscopy images show that the nanowires are smooth and slightly tapered. The diameters of the obtained nanowires are from 30 to 70 nm and their length is around 1 µm. X-ray diffraction analysis and transmission electron microscopy reveal that the nanowires have a zinc-blende structure throughout the whole range of obtained compositions, and have a [Formula: see text] growth axis. The Raman measurements reveal both the expected splitting and shift of phonon lines with increasing Mg content, thus proving the substitutional incorporation of Mg into metallic sites of the ZnTe lattice.

Homogenous indium distribution in InGaN/GaN laser active structure grown by LP-MOCVD on bulk GaN crystal revealed by transmission electron microscopy and x-ray diffraction.

We present transmission electron microscopy (TEM) and x-ray quantitative studies of the indium distribution in In(x)Ga(1-x)N/GaN multiple quantum wells (MQWs) with x = 0.1 and 0.18. The quantum wells were grown by low-pressure metalorganic chemical vapour deposition (LP-MOCVD) on a bulk, dislocation-free, mono-crystalline GaN substrate. By using the quantitative TEM methodology the absolute indium concentration was determined from the 0002 lattice fringe images by the strain measurement coupled with finite element (FE) simulations of surface relaxation of the TEM sample. In the x-ray diffraction (XRD) investigation, a new simulation program was applied to monitor the indium content and lateral composition gradients. We found a very high quality of the multiple quantum wells with lateral indium fluctuations no higher than Δx(L) = 0.025. The individual wells have very similar indium concentration and widths over the whole multiple quantum well (MQW) stack. We also show that the formation of 'false clusters' is not a limiting factor in indium distribution measurements. We interpreted the 'false clusters' as small In-rich islands formed on a sample surface during electron-beam exposure.

Piezoelectric field around threading dislocation in GaN determined on the basis of high-resolution transmission electron microscopy image.

A new method of determining the piezoelectric field around dislocations from high-resolution transmission electron microscopy images is presented. In order to determine the electrical potential distribution near a dislocation core, we used the distortion field, obtained using the geometrical phase method and the non-linear finite element method. The electrical field distribution was determined taking into account the inhomogeneous strain distribution, finite geometry of the sample and the full couplings between elastic and electrical fields. The results of the calculation for a transmission electron microscopy thin sample are presented.

An approach to the systematic distortion correction in aberration-corrected HAADF images.

Systematic distortion has been analysed in high-angle annular dark-field (HAADF) images which may be caused by electrical interference. Strain mapping techniques have been applied to a strain-free GaAs substrate in order to provide a broad analysis of the influence of this distortion on the determination of local strain in the heterostructure. We have developed a methodology for estimating the systematic distortion, and we correct the original images by using an algorithm that removes this systematic distortion.

Nanometric scale investigation of local strain in GaInAs islands by high resolution and analytical TEM.

The continuous displacement field within elastically relaxed GaInAs islands was calculated from digitized HREM images of [110] cross sections of In0.35Ga0.65As layers grown on GaAs by molecular beam epitaxy. Experimental maps of the deformations parallel to the interface (epsilonx) and along the growth direction (epsilonz) were drawn and compared with the ones calculated via the finite element method. It was found that epsilonx exp was systematically higher than epsilonx calc and the significant maximum observed for epsilonz exp within the island could not be found for epsilonz calc. These discrepancies were attributed to a variation of the chemical composition in the island. The maps showing the indium concentration gradient drawn from HREM and FE calculations were compared to quantitative profiles for indium concentration obtained by nanometric X-ray microanalysis in TEM. The measured gradient within the island backs our assumption.