Zinc blende GaAsSb nanowires grown by molecular beam epitaxy

We report the growth of GaAsSb nanowires (NWs) on GaAs(111)B substrates by Au-assisted molecular beam epitaxy. The structural characteristics of the GaAsSb NWs have been investigated in detail. Their Sb mole fraction was found to be about?25%. Their crystal structure was found to be pure zinc blende (ZB), in contrast to the wurtzite structure observed in GaAs NWs grown under similar conditions. The ZB GaAsSb NWs exhibit rotational twins around their [111]B growth axis, with twin-free segments as long as 500?nm. The total volumes of GaAsSb segments with twinned and un-twinned orientations, respectively, were found to be equal by x-ray diffraction analysis of NW ensembles.     

Temperature conditions for GaAs nanowire formation by Au-assisted molecular beam epitaxy

Molecular beam epitaxial growth of GaAs nanowires using Au particles as a catalyst was investigated. Prior to the growth during annealing, Au alloyed with Ga coming from the GaAs substrate, and melted. Phase transitions of the resulting particles were observed in?situ by reflection high-energy electron diffraction (RHEED). The temperature domain in which GaAs nanowire growth is possible was determined. The lower limit of this domain (320?°C) is close to the observed catalyst solidification temperature. Below this temperature, the catalyst is buried by GaAs growth. Above the higher limit (620?°C), the catalyst segregates on the surface with no significant nanowire formation. Inside this domain, the influence of growth temperature on the nanowire morphology and crystalline structure was investigated in detail by scanning electron microscopy and transmission electron microscopy. The correlation of the nanowire morphology with the RHEED patterns observed during the growth was established. Wurtzite GaAs was found to be the dominant crystal structure of the wires.     

Zn(1-x)MnxTe diluted magnetic semiconductor nanowires grown by molecular beam epitaxy

It is shown that the growth of II-VI diluted magnetic semiconductor nanowires is possible by the catalytically enhanced molecular beam epitaxy (MBE). Zn(1-x)MnxTe NWs with manganese content up to x=0.60 were produced by this method. X-ray diffraction, Raman spectroscopy, and temperature dependent photoluminescence measurements confirm the incorporation of Mn(2+) ions in the cation substitutional sites of the ZnTe matrix of the NWs.     

Catalyst-free GaN nanorods grown by metalorganic molecular beam epitaxy

High-density GaN nanorods with outstanding crystal quality were grown on c-sapphire substrates by radio-frequency plasma-assisted metalorganic molecular beam epitaxy under catalyst- and template-free growth condition. Morphological and structural characterization of the GaN nanorods was employed by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy (HRTEM). These results indicate that the rod number density can reach 1/spl times/10/sup 10/ cm/sup -2/ and the nanorods are well-aligned with preferentially oriented in the c-axis direction. Meanwhile, no metallic (Ga) droplet was observed at the end of the rods, which is the intrinsic feature of vapor-liquid-solid method. Nanorods with no traces of any extended defects, as confirmed by TEM, were obtained as well. In addition, optical investigation was carried out by temperature- and power-dependent micro-photoluminescence (/spl mu/-PL). The PL peak energies are red-shifted with increasing excitation power, which is attributed to many-body effects of free carriers under high excitation intensity. The growth mechanism is discussed on the basis of the experimental results. Catalyst-free GaN nanorods presented here might have a high potential for applications in nanoscale photonic devices.     

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.     

Metal contacts to n-type AlGaAs grown by molecular beam epitaxy

We report for the first time the barrier heights of Cu, Ni, Ag, Ti on etched n-type Al_0.33Ga_0.67As and their dependence on annealing temperature with I–V and C–V techniques. The barrier heights of Al and Au, measured for comparison, are 0.96 and 1.06 eV, respectively, in excellent agreement with the results reported previously. The barrier heights of the Cu, Ni, Ag and Ti/n---Al_0.33Ga_0.67As diodes are found to be 1.08, 0.90, 0.87 and 0.87 eV, respectively. It is observed that the barrier heights for Al, Au, Cu and Ti contacts monotonically decrease with annealing temperature. For the Ag and Ni contacts, however, they become higher after being annealed at 473 K for 10 min and become lower thereafter, accompanied by a change of their ideality factors in opposite direction. The barrier heights extrapolated from C–V measurements for all metals studied are higher than that deduced from I–V data, and become higher after annealing at high temperatures, indicating the existence of a thin oxide layer at interface and broadening of the oxide after annealing. Our results can be qualitatively explained by the quality of contact and defects created at the semiconductor surface due to interdiffusion.     

MnTe and ZnTe grown on sapphire by molecular beam epitaxy

We report on growth of MnTe layers by molecular beam epitaxy on Al₂O₃ substrates and of ZnTe layers on hybrid MnTe/Al₂O₃ substrates. The aim of our work was to prepare hexagonal phases of epitaxial thin films of these two materials. In the case of MnTe, the hexagonal NiAs-type phase was prepared by depositing the film directly on Al₂O₃ substrates. On the other hand, the crystal structure of ZnTe layers grown on hybrid MnTe/Al₂O₃ substrates was found to depend on the layer thickness: layers thinner than 0.05 μm grew in a metastable hexagonal wurtzite structure, but with further increases of the thickness, the cubic zinc blende phase of ZnTe tended to appear. The structural properties of MnTe and ZnTe layers were characterized by high energy electron and X-ray diffraction methods. Electrical properties of MnTe films were assessed by the Hall effect measurements. The topography and microstructure were analyzed by atomic force microscope. The Néel temperature and magnetic domains structure of antiferromagnetic hexagonal MnTe layers were obtained from neutron experiments.     

Self-catalyzed growth of GaAs nanowires on cleaved Si by molecular beam epitaxy

Self-assembled GaAs nanowires have been grown on Si by molecular beam epitaxy without the use of any outside metal catalyst. The growth occurs on Si facets obtained by the cleavage of Si(100) substrates. The growth has been obtained with or without Ga pre-deposition. In both cases two kinds of nanowires have been obtained. The wires of the first type clearly present a Ga droplet at their free end and have a lattice structure that is wurtzite for wide regions beneath the Ga droplet. The second type, in contrast, ends with pyramidally shaped GaAs and has a crystal lattice that is mainly zincblende with only a few and small wurtzite regions, if any. The Ga-ended nanowires are longer than the others and thinner on average. The experimental findings suggest that the two types of nanowires grow after different growth processes.     

GaAs:Mn nanowires grown by molecular beam epitaxy of (Ga,Mn)as at MnAs segregation conditions

GaAs:Mn nanowires were obtained on GaAs(001) and GaAs(111)B substrates by molecular beam epitaxial growth of (Ga,Mn)As at conditions leading to MnAs phase separation. Their density is proportional to the density of catalyzing MnAs nanoislands, which can be controlled by the Mn flux and/or the substrate temperature. After deposition corresponding to a 200 nm thick (Ga,Mn)As layer the nanowires are around 700 nm long. Their shapes are tapered, with typical diameters around 30 nm at the base and 7 nm at the tip. The wires grow along the 111 direction, i.e., along the surface normal on GaAs(111)B and inclined on GaAs(001). In the latter case they tend to form branches. Being rooted in the ferromagnetic semiconductor (Ga,Mn)As, the nanowires combine one-dimensional properties with the magnetic properties of (Ga,Mn)As and provide natural, self-assembled structures for nanospintronics.     

AlN/GaN superlattices grown by gas source molecular beam epitaxy

AlN/GaN superlattices with layer thickness between 0.5 and 20 nm have been grown. The substrates were (6H)-SiC(0001) and Al₂O₃(0001) (sapphire). The growth was performed using a modified gas source molecular beam epitaxy (MBE) technique. Standard effusion cells were used as sources of aluminum and gallium, and a small, MBE-compatible, electron cyclotron resonance plasma source was used to activate nitrogen gas prior to deposition. Auger, X-ray, and transmission electron microscopy studies confirmed the existence of well-defined layers. High resolution electron microscopy revealed pseudomorphic behavior between the two materials for layers thinner than 6 nm. By contrast, completely relaxed individual layers of GaN and AlN with respect to each other were present for bilayer periods above 20 nm. Cathodoluminescence showed a shift in the emission peak of up to 0.7 eV. The observed emission energy shifts were used to estimate the band discontinuities.     

Au-catalysed free-standing wurtzite structured InAs nanosheets grown by molecular beam epitaxy

Nano Research - In this study, we report the growth of free-standing InAs nanosheets using Au catalysts in molecular beam epitaxy. Detailed structural characterizations suggest that wurtzite...     

NH3-MBE生长极化场二维电子气材料

介绍了用ＮＨ３－ＭＢＥ技术在蓝宝石Ｃ面上外延的高质量的ＧａＮ单层膜以及ＧＮ／ＡｌＮ／ＧａＮ极化感应二维电子气材料。外延膜都是Ｎ面材料。形成的二维电子气是“倒置二维电子气”。ＧａＮ单层膜的室温电子迁移率为３００ｃｍ＾２／Ｖｓ。二维电子气材料的迁移率为６８０ｃｍ＾２／Ｖｓ（ＲＴ）和１７００ｃｍ＾２／Ｖｓ（７７Ｋ）,相应的二维电子气的面密度为３．２＊１０＾１３ｃｍ＾－２（ＲＴ）和２．６ｘ１０＾１３ｃｍ＾     

Ordered arrays of silicon nanowires produced by nanosphere lithography and molecular beam epitaxy

Because of their importance in fundamental research and possible applications in nanotechnology and nanoelectronics, semiconductor nanowires have attracted much interest. In addition to the growth itself, the control of the size and location is an essential problem. Here we show the growth of ordered arrays of vertically aligned silicon nanowires by molecular beam epitaxy using prepatterned arrays of gold droplets on Si(111) substrates. The ordered arrays of gold particles were produced by nanosphere lithography.     

Au stabilization and coverage of sawtooth facets on Si nanowires grown by vapor-liquid-solid epitaxy

Si nanowires grown in UHV by Au-catalyzed vapor-liquid-solid epitaxy are known to exhibit sidewalls with {112}-type orientation that show faceting. To understand the origin of the faceting, Au induced faceting on Si(112) surfaces was studied in situ by spot-profile-analyzing low-energy electron diffraction. With increasing Au coverage at 750 degrees C, the Si(112) surface undergoes various morphological transformations until, at a critical Au coverage of about 3.1 x 10 (14) atoms/cm (2), a phase consisting of large (111) and (113) facets forms, similar in structure to the nanowire sidewalls. This phase is stable at larger Au coverages in equilibrium with Au droplets. We suggest that Si nanowire surfaces exhibit this structure, and we derive the Au coverage on the two types of facets.     

High quality AlGaAs layers grown by molecular beam epitaxy at low temperatures

Low-temperature (600 °C) molecular beam epitaxy (MBE) growth of AlGaAs has been studied. It was found that the quality of AlGaAs grown at low temperatures can be as good as that grown at high temperatures (>700 °C) if the source materials and the growth chamber are very clean. The threshold currents of Al_0.6Ga_0.4As/Al_0.15Ga_0.85As/Al_0.6Ga_0.4As double heterostructure (DH) lasers grown at low temperatures and high temperatures are almost the same. The material quality can be further improved with a proper amount of indium doping. Photoluminescence (PL) linewidths of 3.1 meV and 1.7 meV have been measured for Indoped Al_0.42Ga_0.58As and Al_0.18Ga_0.82As at 4 K, respectively. They are the narrowest linewidths for the MBE-grown AlGaAs with comparable Al contents at any growth temperature. With a proper amount of In doping, double-barrier resonant tunnelling diodes have also shown improved peak-to-valley current ratios.     

Deep UV light emitting diodes grown by gas source molecular beam epitaxy

We report the structural, electrical, and optical properties of deep UV light emitting diodes (LEDs) grown by gas source molecular beam epitaxy with ammonia on sapphire and AlN/sapphire template substrates. AlN/sapphire substrates were grown by stress controlled hydride vapor phase epitaxy (HVPE). LEDs based on n- and p-type AlN/Al_xGa_1?xN (0.05 ≤ x ≤ 0.08) superlattices are demonstrated operating to wavelengths as short as 250 nm. We report a significant enhancement in the cathodoluminescence intensities (by factor of ～100) and photoluminescence lifetimes in the Al_xGa_1?xN/Al_yGa_1?yN superlattices consisting of well material grown in the three dimensional mode. We interpret these observations in terms of formation of quantum well/quantum dot active regions.     

Enhanced radiative efficiency in GaN quantum dots grown by molecular beam epitaxy

Self-assembled GaN quantum dots (QDs), grown on AlN by molecular beam epitaxy, were investigated by time-resolved photoluminescence spectroscopy. We investigate the emission mechanism in GaN QDs by comparing the carrier recombination dynamics in single and multiple period QDs. At 100 K, the PL decay time in single period QD structures is considerably shorter than in stacked QDs. Compared to single period QDs, the room temperature PL efficiency is considerably enhanced in 20 period QDs due to the reduction in nonradiative recombination processes.     

Morphology of self-catalyzed GaN nanowires and chronology of their formation by molecular beam epitaxy

GaN nanowires are synthesized by plasma-assisted molecular beam epitaxy on Si(111) substrates. The strong impact of the cell orientation relative to the substrate on the nanowire morphology is shown. To study the kinetics of growth, thin AlN markers are introduced periodically during NW growth. These markers are observed in single nanowires by transmission electron microscopy, giving access to the chronology of the nanowire formation and to the time evolution of the nanowire morphology. A long delay precedes the beginning of nanowire formation. Then, their elongation proceeds at a constant rate. Later, shells develop on the side-wall facets by ascending growth of layer bunches which first agglomerate at the nanowire foot.