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.     

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.     

Self-assembled Al(x)Ga(1-x)N nanorods grown on Si(001) substrates by using plasma-assisted molecular beam epitaxy

Hexagonal Al(x)Ga(1-x)N nanorods were grown by plasma-assisted molecular beam epitaxy (PAMBE) on Si(001) substrates. The Al mole fraction was determined from x-ray diffraction (XRD) measurement and its value was varied from 0 to 15. It is found that, under group III-rich conditions, the growth rate of the Al(x)Ga(1-x)N nanorods decreases and the diameter increases due to the possibility of incorporation of aluminium and gallium. In order to study structural and optical properties, x-ray diffraction and cathodoluminescence (CL) measurements were carried out. The Al content (x) is calculated from these measurements and their values are compared.     

Diameter-dependent composition of vapor-liquid-solid grown Si(1-x)Ge(x) nanowires

Diameter-dependent compositions of Si(1-x)Ge(x) nanowires grown by a vapor-liquid-solid mechanism using SiH(4) and GeH(4) precursors are studied by transmission electron microscopy and X-ray energy dispersive spectroscopy. For the growth conditions studied, the Ge concentration in Si(1-x)Ge(x) nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below approximately 50 nm. The size-dependent nature of Ge concentration in Si(1-x)Ge(x) NWs is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth.     

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.     

Nanostructured InSiAs solid solution grown by molecular beam epitaxy on the Si(001) surface

The possibility of obtaining a new nanostructured material, an InSiAs solid solution, by molecular beam epitaxy on the Si(001) surface is reported. It is demonstrated that, during simultaneous deposition of indium, silicon, and arsenic, nanometer-size islands with a rectangular base and the sides oriented along the {110} directions can form when the layer thickness exceeds 35 nm. After depositing a 100 nm thick layer of the solid solution, islands with the side ratio d₁₁₀ /d_{1[`1]0</sub> = 1.35d_{110} /d_{1\bar 10} = 1.35 and a typical lateral size of d<sub>1[`1]0} ~ 35 ±10d_{1\bar 10} \sim 35 \pm 10 nm are obtained.     

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.     

Properties of Hg1−x Zn x Te solid solutions

Hg_1–x ZN _x Te alloys were prepared by inter-diffusion at high temperatures using mixtures of HgTe and ZnTe compounds in the form of compressed powders. The lattice constant was determined using X-ray diffraction measurements and was found to exhibit a linear dependence onx over the entire composition range. Diffused reflectance measurements as a function of wavelength yielded values of the energy bandgap that agree closely with those reported previously on single crystals. The room temperature electrical conductivity and thermoelectric power were measured as a function ofx. Intrinsic conduction was found to dominate for 0<x<0.3. In this composition range the thermoelectric power was negative due to the high ratio of electron to hole mobilities.     

Modeling of InAs-InSb nanowires grown by Au-assisted chemical beam epitaxy

Interesting phenomena during the Au-assisted chemical beam epitaxy of InAs-InSb nanowire heterostructures have been observed and interpreted within the framework of a theoretical model. An unusual, non-monotonous diameter dependence of the InSb nanowire growth rate is demonstrated experimentally within a range of deposition conditions. Such a behavior is explained by competition between the Gibbs-Thomson effect and different diffusion-induced material fluxes. Theoretical fits to the experimental data obtained at different flux pressures of In and Sb precursors allow us to deduce some important kinetic coefficients. Furthermore, we discuss why the InAs nanowire stem forms in the wurtzite phase while the upper InSb part has a pure zinc blende crystal structure. It is hypothesized that the 30° angular rotation of nanowire when passing from InAs to the InSb part is driven by the lowest surface energy of (1100) wurtzite and (110) zinc blende facets.     

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.     

Growth of germanium nanowires on silicon(111) substrates by molecular beam epitaxy

Heteroepitaxial growth of Ge nanowires was carried out on Si(111) substrates by MBE. Au seeds were used as precursor for the VLS growth of the nanowires. Even if the Au droplets do not act as catalyst for the dissociation of gas, they are local preferential areas where the energetic barrier of Ge nucleation is lowered compare to the remaining non activated surface. Two sets of Au seeds were used as precursors for the VLS process. The first set have an average diameter of 125 nm and the second of 25 nm. In-situ RHEED monitoring showed a Au wetting layer between these seeds before the nanowires growth as well as at the end of the Ge nanowires growth. It means that the wetting layer acted as a surfactant from the Si(111) surface to the Ge grown layer between the nanowires. Analysis of SEM images brought the fact that the diffusion of gold from the droplets on the surface and the sidewalls of the nanowires via the Ostwald ripening is a key parameter of the growth of the nanowires.     

Photoluminescence and optical properties of Mg x Zn1−x Te alloys

The photoluminescence and optical properties of Mg _x Zn_1–x Te alloys have been studied in the composition range 0<x<0.48. The results are discussed taking into account the formation of band tails due to the alloying effect or random distribution of impurities. The role of residual defects in magnesium-rich alloys is emphasized. On the other hand, a preliminary investigation showed that it is possible to incorporate lithium by high-temperature diffusion in Mg _x Zn_1–x Te alloys without altering the magnitude or the homogeneity of the magnesium concentration. Evidence is obtained for an increased quantum efficiency after lithium doping.     

Synthesis and raman scattering from Zn(1-x)Mn(x)S diluted magnetic semiconductor nanowires

The growth of Diluted Magnetic Semiconducting (DMS) Zn(1-x)Mn(x)S (0 < or = x < 0.6) nanowires (NWs) using a three-zone furnace and two solid sources is reported. The approach is generally applicable to many binary and ternary NW systems that grow by the Vapor-Liquid-Solid growth mechanism. Mn concentration was controlled by the temperature of the Mn source. The Zn/Mn ratio was found to determine the crystalline structure, i.e., wurtzite or zinc blende. High-resolution transmission electron microscopy measurements revealed highly crystalline single phase NWs. The vibrational properties of the DMS NWs with different Zn/Mn ratios were studied by correlating their Raman scattering spectra with the composition measured by Energy Dispersive X-Ray Spectroscopy (EDS). We find that the transverse optical (TO) phonon band disappears at the lowest Mn concentrations, while the longitudinal optical (LO) phonon band position was found insensitive to x. Three additional Raman bands were observed between the ZnS q = 0 TO and LO phonons when Mn atoms were present in the NWs. These bands are similar to those reported previously for bulk Zn(1-x)Mn(x)S and their origin is still controversial.     

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.     

Growth of Zn(1_x)Mg(x)O and Zn(1_x)Cd(x)O nanowires and the application in light emitting devices

Magnesium and Cadmium doped ZnO nanowires were successfully grown by Chemical Vapor deposition method in a tube furnance. Photoluminescence spectra show that the band gap of ZnO nanowire has been tuned from 4.00 eV to 2.08 eV by Magnesium and Cadmium doping. Transmission Electron Microscopy and X-ray diffraction characterization analysis indicate that most of the formed nanowires are single crystalline with good quality. Zn(1-x)Cd(x)O nanowire sample was used for heterojunctional light emitting diode fabrication. Electroluminescence measurement yields a strong emission peak at 553 nm from the Zn(1-x)Cd(x)O nanowire.     

Size dependent magnetization and high-vacuum annealing enhanced ferromagnetism in Zn(1-x)Co(x)O nanowires

Diameter controllable ZnO nanowires have been fabricated by thermal evaporation (vapor transport) with various sizes of gold nanoparticles as catalysts. Diluted magnetic semiconductor (DMS) Zn(1-x)Co(x)O nanowires were then made by high energy Co ion implantation. The as-implanted and the argon-annealed Zn(1-x)Co(x)O nanowires displayed weak ferromagnetism while the high-vacuum annealed nanowires exhibited strong ferromagnetic ordering at room temperature. Size dependent behavior has been observed in the magnetic field and temperature dependences of magnetization. The shrinkage of the nanowire diameter reduced the spontaneous magnetization as well as the hysteresis loops. Field cooled and zero-field cooled magnetization and coercivity measurements were performed between 2 and 300 K to study the evolution of magnetism from the weak to the strong ferromagnetic states. In particular, superparamagnetic features were observed and shown to be intrinsic characteristics of the DMS Zn(1-x)Co(x)O nanowires. The room-temperature spontaneous magnetization of individual Zn(1-x)Co(x)O nanowires was also established by using magnetic force microscope measurements.     

Fabrication, structural characterization and photoluminescence of single-crystal Zn(x)Cd(1-x)S zigzag nanowires

Large-scale synthesis of ternary Zn(x)Cd(1-x)S zigzag nanowires was achieved in a one-step metal-organic chemical vapour deposition (MOCVD) process with co-fed single precursors of ZnS and CdS. Their morphologies, structures and optical properties were characterized and confirmed by scanning electron microscopy, high-resolution transmission electron microscopy, x-ray spectroscopy, and photoluminescence. The Zn(x)Cd(1-x)S zigzag nanowires are single crystalline, with axis [001], by changing the growth direction from [Formula: see text] to [Formula: see text]. Regarding the formation of zigzag nanowires, we suggest that the shear strain and slight fluctuation of the reaction conditions may be the major factors that make the nanowires change growth direction. In addition, because of the lower temperature and versatility, this new fabrication method might present a new and facile way to form other ternary nanomaterials. Furthermore, the green emission of the nanowires may have potential applications in electronic/optical nanodevices.     

气态源分子束外延生长重碳掺杂p型InGaAs研究

采用气态源分子束外延(GSMBE)技术,以四溴化碳(CBr4)作为碳杂质源,系统研究了InP衬底上碳掺杂p型InGaAs材料的外延生长及其特性,在AsH3压力5.33×104Pa,生长温度500℃条件下获得了空穴浓度高达1×1020/cm3、室温迁移率为45cm2/Vs的重碳掺杂p型In0.53Ga0.47As材料。研究了CBr4和AsH3束流强度以及生长温度等生长条件对碳掺杂InGaAs外延层组份、空穴浓度和迁移率的影响,并对不同生长条件下的氢钝化效应进行了分析。     

InAs/GaAs nanostructures grown on patterned Si(001) by molecular beam epitaxy

The potential benefit from the combination of the optoelectronic and electronic functionality of III-V semiconductors with silicon technology is one of the most desired outcomes to date. Here we have systematically investigated the optical properties of InAs quantum structure embedded in GaAs grown on patterned sub-micron and nanosize holes on Si(001). III-V material tends to accumulate in the patterned sub-micron holes and a material depletion region is observed around holes when GaAs/InAs/GaAs is deposited directly on patterned Si(001). By use of a 60?nm SiO(2) layer and patterning sub-micron and nanosize holes through the oxide layer to the substrate, we demonstrate that high optical quality InAs nanostructures, both quantum dots and quantum wells, formed by a two-monolayer InAs layer embedded in GaAs can be epitaxially grown on Si(001). We also report the power-dependent and temperature-dependent photoluminescence spectra of these structures. The results show that hole diameter (sub-micron versus nanosize) has a strong effect on the structural and optical properties of GaAs/InAs/GaAs nanostructures.