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.     

Ferromagnetism and ferromagnetic resonance in Mn and As co-implanted Si and GaAs

Ferromagnetism persisting above 375 K and anisotropic ferromagnetic resonance (FMR) spectra have been detected for the first time in Si co-implanted with Mn and As and annealed under appropriate conditions. For comparison, semi-insulating GaAs samples have been implanted with the same ions and subsequently annealed. They also exhibit ferromagnetism with a Curie temperature well in excess of 375 K. High-resolution transmission electron microscopy (HR TEM) performed on the samples with the best magnetic characteristics has shown the presence of nanoclusters due to the segregation of the implanted species in both Si and GaAs. The angular dependence of the FMR spectra reveals the existence of magnetic entities with the hard magnetization axes aligned along the four equivalent 〈1 1 1〉 crystal axes. The spectra are very similar in Si and GaAs, indicating that hexagonal MnAs clusters might be formed in Si.     

Vertically standing Ge nanowires on GaAs(110) substrates

The growth of epitaxial Ge nanowires is investigated on (100), (111) B and (110) GaAs substrates in the growth temperature range from 300 to 380?°C. Unlike epitaxial Ge nanowires on Ge or Si substrates, Ge nanowires on GaAs substrates grow predominantly along the [Formula: see text] direction. Using this unique property, vertical [Formula: see text] Ge nanowires epitaxially grown on GaAs(110) surface are realized. In addition, these Ge nanowires exhibit minimal tapering and uniform diameters, regardless of growth temperatures, which is an advantageous property for device applications. Ge nanowires growing along the [Formula: see text] directions are particularly attractive candidates for forming nanobridge devices on conventional (100) surfaces.     

Comparisons on the properties of (Zn1-xCox)2-W type barium hexaferrite thin films prepared on the Si (100) and (111) substrates

The (Zn_1-xCo_x)₂-W type barium hexaferrite thin films have been prepared by a radio frequency magnetron sputtering method on the Si (100) and the Si (111) substrates respectively. With increasing the annealing temperatures (800, 850, 900, 950, and 1000 °C), the Ba(CoZn)₂Fe₁₆O₂₇ phases emerge from the amorphous matrix. The hexaferrite thin films on Si (111) substrates have a larger saturation magnetic field (636.6 kA/m) than those on Si (100) substrates (159.1 kA/m). The magnetic hysteresis measurements show that they exhibit an isotropic behavior for thin films deposited on both substrates. Films on the Si (111) substrates are magnetically harder than those on the Si (100) substrates.     

Growth of GaAs Nanowires on Si Substrates Using a Molecular Beam Epitaxy

Au-catalyzed GaAs nanowires were grown on Si substrates by vapor-liquid-solid growth method using a molecular beam epitaxy (MBE). The MBE growth could produce controlled crystalline orientation and uniform diameter along the wire axis of the GaAs nanowires by adjusting growth conditions including growth temperature and V/III flux ratio. Growths of GaAslang001rang as well as GaAslang111rang nanowires were observed by transmission electron microscopy and scanning electron microscopy. Epitaxially grown GaAslang111rang nanowires on a Si(111) substrate were verified through x-ray diffraction out-of-plane 2thetas/omega-scans. A strong room-temperature photoluminescence (PL) was observed from the epitaxially grown GaAslang111rang nanowires on a Si(100) substrate. Results of low-temperature (10 K) PL measurements and current-sensing atomic force microscopy indicated that the GaAs nanowires on a Si substrate were unintentionally doped with Si     

Morphology- and orientation-controlled gallium arsenide nanowires on silicon substrates

GaAs nanowires were epitaxially grown on Si(001) and Si(111) substrates by using Au-catalyzed vapor-liquid-solid (VLS) growth in a solid source molecular beam epitaxy system. Scanning electron microscopy analysis revealed that almost all the GaAs nanowires were grown along <111> directions on both Si substrates for growth conditions investigated. The GaAs nanowires had a very uniform diameter along the growth direction. X-ray diffraction data and transmission electron microscopy analysis revealed that the GaAs<111> nanowires had a mixed crystal structure of the hexagonal wurtzite and the cubic zinc-blende. Current-voltage characteristics of junctions formed by the epitaxially grown GaAs nanowires and the Si substrate were investigated by using a current-sensing atomic force microscopy.     

Room Temperature Ferromagnetism of Ge/MnAs Digital Alloys

The magnetic properties of Ge/MnAs digital alloys on GaAs (001) substrates, grown by molecular beam epitaxy, were investigated using a Quantum Design SQUID magnetometer. The Ge (1 nm)/MnAs (0.15 nm) digital alloy showed ferromagnetism up to 334 K with a coercive field of 576 Oe at room temperature, as determined from temperature-dependent magnetization and hysteresis loop measurements. The ferromagnetic and antiferromagnetic phases of a Ge₇/(Mn_0.5As_0.5)₁ superlattice, investigated using the full-potential linearized augmented plane wave (FLAPW) method yielded a ferromagnetic ground state with a high spin magnetic moment of Mn (3.45 _B) which induces only very small magnetic moments on its neighboring As or Ge atoms.     

Study of MnAs as a Spin Injector for GaAs-Based Semiconductor Heterostructures

We report studies of two key factors involved in the use of MnAs as a spin injector, namely, Mn diffusion and spin injection properties when it is grown on GaAs-based semiconductor heterostructures. Depth profile of Mn concentration in a sample consisting of a MnAs film grown on GaAs is investigated with time-of-flight secondary ion mass spectroscopy. The result shows that Mn diffusion into GaAs is negligible, unlike the case of Ga_1?x Mn _x As/GaAs, in which Mn diffusion is significant. Meanwhile, robust spin injection is observed with MnAs as a spin injector for a GaAs-based light emitting diode, persisting up to room temperature.     

Growth of GaInAs/AlInAs heterostructure nanowires for long-wavelength photon emission

We investigated the growth of GaInAs/AlInAs heterostructure nanowires on InP(111)B and Si(111) substrates in a metalorganic vapor phase epitaxy reactor. Au colloids were used to deposit Au catalysts 20 and 40 nm in diameter on the substrate surfaces. We obtained vertical GaInAs and AlInAs nanowires on InP(111)B surfaces. The GaInAs nanowires capped with GaAs/AlInAs layers show room-temperature photoluminescence. The peak exhibits a blue-shift when the Ga content in the core GaInAs nanowire is increased. For the GaInAs/AlInAs heterostructure growth, it is possible to change the Ga content sharply but Al also exists in the GaInAs layer regions. We also found that the ratios of Ga and Al contents to In content tend to increase and the axial growth rate to decrease along the nanowire toward the top. We were also able to make vertical GaInAs nanowires on Si(111) surfaces after a short growth of GaP and InP.     

A Mask Based on a Si Epitaxial Layer for the Self-Catalytic Nanowire Growth on GaAs(111)B and GaAs(100) Substrates

Technical Physics Letters - GaAs nanowires (NWs) were generated on the surface of GaAs(111)B and GaAs(100) substrates from molecular fluxes by the self-catalytic growth method. A mask for NW growth...     

Molecular composition and orientation of interstitial versus surface silicon oxides for Si(111)/SiO2 and Si(100)/SiO2 interfaces using FT-IR and X-ray photoelectron spectroscopies

This work represents a characterization study of silicon oxide on Si(111) and Si(100) surfaces intended for use as substrates in organic light-emitting diodes (OLEDs) on chip devices. Samples have been prepared using either native oxide formation or thermal oxidation, and they have also been treated for activation of hydroxyl groups on their surface. Both Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) have been used in order to understand the molecular orientation as well as the chemical composition of the various oxide types formed during these different oxidation processes. These spectroscopic studies reveal the formation of two different types of oxides on these substrates. The first type is a thin oxide layer on the surface, whereas the second type, called interstitial, is found deeper in the substrate. Specifically, it was found that the Si(100) substrate forms a randomly oriented interstitial oxide, whereas the presence of a lower quantity but more oriented interstitial oxide was found for the Si(111) substrate. In addition, we report for the first time the position of the impurity oxygen for Si(111) substrates at 1122 cm(-1). Finally, in both Si(100) and Si(111) substrates, the thin (<15 A) silicon oxide layers are oriented and appear to contain silicon atoms of similar oxidation states. In contrast, both the thicker surface film (100 A) as well as the interstitial oxide produced by the thermal oxidation procedure show random orientation and relative uniformity. Overall these orientation studies clearly show that the formation process of surface oxides in different substrates clearly creates species that are oriented differently with respect to the surface.     

Structural and magnetic properties of MnAs/GaAs ferromagnetic semiconductor nanocomposite material

Self-organized (Ga,Mn)As nanoclusters, embedded in GaAs and formed during thermal annealing of Ga_1-xMn_xAs layer at 500 °C or 600 °C, were studied using Transmission Electron Microscopy (TEM) and Magnetic Force Microscopy (MFM). We found that 10–20 nm large NiAs-type hexagonal MnAs nanocrystals gave magnetic contrast in MFM images, whereas smaller zinc-blende nanoinclusions were not visible by means of this technique. Theoretical simulations showed that MFM contrasts reflect interaction between magnetic tip and many randomly distributed MnAs nanocrystals.     

Magnetic Interactions in Granular Paramagnetic–Ferromagnetic GaAs: Mn/MnAs Hybrids

The magneto-transport properties of granular paramagnetic–ferromagnetic GaAs:Mn/MnAs hybrids have been studied. Positive and negative magnetoresistivity can be observed in the paramagnetic matrix as well as in the GaAs:Mn/MnAs hybrid system. The experimental results and the respective relevant interactions are discussed. A phenomenological explanation from a microscopic point of view is presented. It is suggested that the paramagnetism of the matrix plays a major role in the magneto-resistive behavior of the hybrids, in contrast to diamagnetic–ferromagnetic hybrids where the magnetoresistance (MR) is dominated by the ferromagnetism of the clusters.     

热丝CVD法低温制备的多晶硅薄膜质量对衬底依赖性的研究

以SiH4和H2作为反应气体,采用HWCVD的方法分别在石英玻璃、AZO、Si（100）和Si（111）衬底上制备了多晶硅薄膜。利用X射线衍射（XRD）,拉曼（Raman）光谱和傅里叶红外（FT-IR）吸收光谱研究了不同衬底对多晶硅薄膜的择优取向、晶化率和应力的影响,用SEM观察了多晶硅薄膜的表面形貌。研究发现在4种衬底上生长的多晶硅薄膜均为（111）择优取向。单晶硅片对多晶硅薄膜有很强的诱导作用,并且Si（111）的诱导作用优于Si（100）的诱导作用。AZO对多晶硅薄膜生长也有一定的诱导作用。通过计算薄膜晶态比,得到除以石英为衬底的样品外,其它3种样品的晶态比均在90%以上,尤其以单晶硅片为衬底的样品更高。石英玻璃、AZO和Si（100）上生长的多晶硅薄膜中均存在压应力。     

Control of GaP and GaAs nanowire morphology through particle and substrate chemical modification

We demonstrate two very different morphologies for GaP and GaAs nanowires grown by Au-assisted MOVPE on Si(111) substrates: rodlike wires and tapered wires with sharp tips. We show that the morphology is related to the stability of the particles at the wire tips during growth, and we propose that the mechanism of this effect is diffusion of Au away from the tip. Diffusion occurs, leading to tapered wires, only if there is a clean Si surface to act as a reservoir for the Au. Furthermore, the presence of indium in the particles, even at background levels from previous growth runs, inhibits the migration of Au. These results demonstrate a dramatic example of the sensitivity of wire morphology to substrate and particle chemistry, which could provide an important tool to tune nanowire morphology through particle alloying or surface treatment.     

Oriented In<Subscript>3–<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>4</Subscript> films on the (100) surface of Si,GaAs, and InP single crystals

Spray pyrolysis of aqueous solutions, with thiourea as a sulfurizing agent, has been used to grow cubic indium sulfide films on single-crystal substrates: In_3–x S₄(111)/Si(100), In_3–x S₄(111)/GaAs(100), and In_3–x S₄(111)/InP(100). The lattice parameter a of the sulfide has been shown to increase with an increase in the lattice parameter of the substrate, whereas the film grown on GaAs has the highest lattice strain (as assessed from X-ray diffraction line broadening). The films have a constant phase composition and (111) texture, in contrast to films grown on glassy substrates by the same method.     

Structure and magnetic properties of Fe epitaxial thin films prepared by UHV rf magnetron sputtering on GaAs single-crystal substrates

Fe thin films were prepared on GaAs single-crystal substrates of (100)_B3, (110)_B3, and (111)_B3 orientations by ultra high vacuum rf magnetron sputtering. The effects of substrate orientation and substrate temperature on the film growth, the structure, and the magnetic properties were investigated. On GaAs(100)_B3 substrates, Fe(100)_bcc single-crystal films are obtained at 300 °C, whereas Fe films consisting of bcc(100) and bcc(221) crystals epitaxially grow at room temperature (RT). Fe(110)_bcc and Fe(111)_bcc single-crystal films are respectively obtained on GaAs(110)_B3 and GaAs(111)_B3 substrates at RT-300 °C. The in-plane lattice spacings of these Fe epitaxial films are 0-9% larger than the out-of-plane lattice spacings due to accommodation of lattice mismatch between the films and the substrates. The film strain is decreased by employing an elevated substrate temperature of 300 °C. The in-plane magnetization properties are reflecting the magnetocrystalline anisotropy of bulk bcc-Fe crystal.     

Transport Properties of Hybrids with Ferromagnetic MnAs Nanoclusters and Their Potential for New Magnetoelectronic Devices

Granular hybrid structures containing ferromagnetic nanoclusters embedded in a semiconducting matrix are an interesting class of materials as their properties can be tuned in a wide range. Hybrids are a promising alternative to dilute magnetic semiconductors in the field of spintronics and magnetoelectronics, because the nanoclusters can show ferromagnetic behavior even at room temperature. In this review, it is focused on the rather well investigated dilute magnetic semiconductor (Ga,Mn)As with MnAs inclusions. Different preparation methods are presented which were developed over the last two decades in order to obtain MnAs nanoclusters in the semiconducting matrix and to tune the structural and magnetic properties of these clusters. Recent results on the influence of the nanoclusters on the hybrids' transport properties as well as first approaches to use hybrids with a random nanocluster distribution in new spintronic devices are discussed. In addition, the perspective of using single MnAs nanoclusters as well as ordered arrangements of a few nanoclusters in new planar magnetoelectronic devices is illustrated.     

GaAS/Si(100)异质外延4°偏角衬底对外延层结晶质量的提高(英文)

用低压金属有机物化学汽相沉积法(MOCVD)在Si(100)无偏角和Si(100)4°偏角衬底上外延生长GaAs层。异质外延采用两步生长法,并分别优化了两种衬底上的非晶低温缓冲层的生长条件。用X射线双晶衍射(XRD)和透射电子显微镜(TEM)对两种衬底上的GaAs外延层进行了结构表征,其中Si(100)4°偏角衬底上1.8μm厚GaAs的(004)面XRD衍射半高全宽338 arcsec,同比在无偏角衬底上的半高全宽为494arcsec,TEM图片显示4°偏角衬底上外延层中的位错密度大大降低。     

The role of substrate accommodation in the (111)- to (100)-state transition of gold particles on alkali halides

Annealing experiments on Au particles formed by vapour diffusion into shielded regions above air-cleaved NaCl and KCl substrates and subsequent nucleation has revealed significant differences in the ability of the substrates to cause reorientation of small crystallites by migration (causing (111)-azimuthal alignment) or internal atomic relaxation to the (100) epitaxial configuration. The observed phenomena support a recent theory governing epitaxial behaviour in the vacuum-deposited f.c.c. metal-alkali halide system. Strong evidence is obtained that secondary twinning on a {111}-based Au crystallite with 〈110〉_Au//〈100〉_subst is an intermediate stage in the atomic relaxation process encountered in the (111)- to (100)-state transition.