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In this article, we present a simple and low-cost experimental setup for thermoelectric effect measurements of thin film materials near room temperature, which can be used to determine their conductivity types. Bi2Te3 and Sb2Te3 thin films grown by the molecular beam epitaxy technique were used as the tested samples. Their Seebeck coefficients were determined to be $(-141\pm 1)\,{\mathrm{\mu V/K}}$ and $(39\pm 2)\,{\mathrm{\mu V/K}}$ , respectively, confirming that the former is an n-type material and the latter is a p-type material. A heterostructure composed of Sb2Te3 and Bi2Te3 was characterized by electrical transport measurements. Data fitting was carried out for its current–voltage characteristics with the Shockley diode model and a real diode model proposed by Cataldo et al. , and some physical parameters of the heterostructure were extracted, including its ideality factor and saturation current. Based on its rectifying current–voltage behavior, we confirm that the aforementioned heterostructure is a p-n junction, which echoes the contrast in the conductivity types of Sb2Te3 and Bi2Te3 as determined by the thermoelectric effect measurements.

How an interfacial superconductivity emerges during the nucleation and epitaxy is of great importance not only for unveiling the physical insights but also for finding a feasible way to tune the superconductivity via interfacial engineering. In this work, we report the nanoscale creation of a robust and relatively homogeneous interfacial superconductivity (TC ≈ 13 K) on the epitaxial FeTe surface, by van der Waals epitaxy of single-quintuple-layer topological insulator Bi2Te3. Our study suggests that the superconductivity in the Bi2Te3/FeTe heterostructure is generated at the interface and that the superconductivity at the interface does not enhance or weaken with the increase of the Bi2Te3 thickness beyond 1 quintuple layer (QL). The observation of the topological surface states crossing Fermi energy in the Bi2Te3/FeTe heterostructure with the average Bi2Te3 thickness of about 20 QL provides further evidence that this heterostructure may potentially host Majorana zero modes.

Recently, the development of efficient and non-noble metal electrocatalysts with good durability for the hydrogen evolution reaction (HER) has attracted increasing attention. The exotic and robust ...

Blue and ultraviolet radiation is well known to be associated with cataract formation and visual damage. Nowadays, children and teenagers are increasingly exposed to blue light and electromagnetic radiation with even shorter wavelength, which may cause damage to vision due to their frequent use of electronic display devices and participation in disco parties and concerts with illumination by UV light. We developed a Fe:Pd/ZnSe Schottky barrier photodetector that detects blue and UV light with excellent long-wavelength rejection power for green and red light of 4.24 and 5.65 orders, respectively. Its Schottky-barrier height and built-in potential are determined to be 2.48 and 1.17 eV, respectively. Together with the help of a lux meter, this simple and robust photodetector detects the blue light + UV content of different LED light bulbs with accuracy comparable to the much more sophisticated and expensive spectrometer. This compact and efficient blue + UV light detector can help us to increase the awareness of over-exposure to vision damaging radiation.

The surface orientation dependence on the hydrogen evolution reaction (HER) performance of topological crystalline insulator (TCI) SnTe thin films is studied. Their intrinsic activities are determined by linear sweep voltammetry and cyclic voltammetry measurements. It is found that SnTe (001) and (111) surfaces exhibit intrinsic activities significantly larger than the (211) surface. Density functional theory calculations reveal that pure (001) and (111) surfaces are not good electrocatalysts, while those with Sn vacancies or partially oxidized surfaces, with the latter as evidenced by X-ray photoelectron spectroscopy, have high activity. The calculated overall performance of the (001) and (111) surfaces with robust topological surface states (TSSs) is better than that of the lowly symmetric (211) surface with fragile or without TSSs, which is further supported by their measured weak antilocalization strength. The high HER activity of SnTe (001) and (111) is attributed to the enhanced charge transfer between H atoms and TSSs. We also address the effect of possible surface facets and the contrast of the HER activity of the available active sites among the three samples. Our study demonstrates that the TSSs and mirror symmetry of TCIs expedite their HER activity., 78 pages, 35 figures, 4 tables

The recent discovery of the interfacial superconductivity (SC) of the Bi2Te3/Fe1+yTe heterostructure has attracted extensive studies due to its potential as a novel platform for trapping and controlling Majorana fermions. Here we present studies of another topological insulator (TI)/Fe1+yTe heterostructure, Sb2Te3/Fe1+yTe, which also enjoys an interfacial two-dimensional SC. The results of transport measurements support that the reduction of excess Fe concentration of the Fe1+yTe layer not only increases the fluctuation of its antiferromagnetic (AFM) order but also enhances the quality of the SC of this heterostructure system. On the other hand, the interfacial SC of this heterostructure was found to have a wider-ranging TI-layer thickness dependence than that of the Bi2Te3/Fe1+yTe heterostructure, which is believed to be attributed to the much higher bulk conductivity of Sb2Te3 that enhances indirect coupling between its top and bottom topological surface states (TSSs). Our results provide the evidence of the interplay among the AFM order, itinerant carries from the TSSs and the induced interfacial SC of the TI/Fe1+yTe heterostructure system., 28 pages, 5 figures

Materials that have a perpendicular magnetic anisotropy (PMA) effect with a high Curie temperature are essential in spintronics applications. Cr2Te3 is a transition-metal chalcogenide that demonstr...

We report studies on the role of a ZnTe buffer in determining the crystalline phase of MBE-grown MnSe1−xTex alloy thin films. It was found that MBE growth of MnSe and MnTe directly onto a zincblende GaAs substrate usually result in their corresponding stable phases, which are rocksalt and hexagonal, respectively. A set of zincblende MnSe1−xTex alloy thin films with Te composition covering from 0.27 to 1 were fabricated on zincblende ZnTe buffer layers. We have addressed the lattice distortion issue due to thin film effect for these films. A combination of several structural characterizations demonstrates that a perfect lattice matched MnSe1−xTex/ZnTe heterostructure can indeed be realized. The results of this study pave the way for realizing a double-barrier MnSe1−xTex/ZnTe/MnSe1−xTex resonant tunneling diode structure with perfect lattice match as a candidate for a promising THz emitter.

How an interfacial superconductivity emerges during the nucleation and epitaxy is of great importance not only for unveiling the physical insights but also for finding a feasible way to tune the superconductivity via interfacial engineering. In this work, we report the nanoscale creation of a robust and relatively homogeneous interfacial superconductivity (

We discuss the optical analogy of reflected electron diffraction from a grating with non-uniform spacing at the nanoscale, and its relationship to another grating with uniform spacing. Two analytical approaches that offer a quantitative solution for the reflected diffraction from a grating are presented. A compact and low-cost experimental kit for demonstrating the physics behind the reflected diffraction from one-dimensional surface structures is described. As an example, the convolution theorem is used to explain the reflected diffraction from a sample consisting of two individual one-dimensional surface structures overlapping orthogonally. This study will broaden students' understanding of the physics behind diffraction.

A compact solid-state UV flame sensing system is developed and tested. We have addressed two approaches for obtaining additional six orders of long-wavelength rejection power on top of that of the built-in UV sensor based on a wide-bandgap II–VI Schottky-barrier structure. An op-amp-based amplification circuit with an average gain of 16 600 is used for signal amplification. It is demonstrated that the developed sensing system could detect a standard butane-air flame with excellent solar-blind characteristics, potentially capable for monitoring industrial boilers. Several feasible approaches were discussed for further improvement in the performance of the developed sensing system targeting its general usage in fire-safety applications.

We investigate the hydrostatic pressure dependence of interfacial superconductivity occurring at the atomically sharp interface between two non-superconducting materials: the topological insulator (TI) Bi2Te3 and the parent compound Fe1+yTe of the chalcogenide iron-based superconductors. Under pressure, a significant increase in the superconducting transition temperature Tc is observed. We interpret our data in the context of a pressure-induced enhanced coupling of the Fe1+yTe interfacial layer with the Bi2Te3 surface state, which modifies the electronic properties of the interface layer in a way that superconductivity emerges and becomes further enhanced under pressure. This demonstrates the important role of the TI in the interfacial superconducting mechanism.

A large bulk band gap and tunable Dirac carriers are desired for practical device applications of topological insulators. However, most known topological insulators are narrow gap materials and the manipulation of their Dirac surface states is limited by residual bulk charge carriers originating from intrinsic defects. In this study, via density functional theory based first-principles calculations, we predict that a layered hexagonal structure of Bi2S3 is stable, and it becomes a topological insulator under a moderate compressive pressure of about 5.3 GPa. Interestingly, we find that the strength of the spin–orbit interaction in Bi2S3 can be effectively enhanced by the applied pressure. This leads to an increased inverted band gap with pressure, which can reach 0.4 eV with a pressure of 13.7 GPa. Compared to Bi2Se3, intrinsic defects are suppressed in Bi2S3 under both cation- and anion-poor growth conditions. Our calculations predict a new Bi-based topological insulator, and also shed light on control over spin–orbit interactions in Bi2S3 and tuning of its topological properties.

To study the in situ doping effect upon monotonically increasing dopant concentration, a Bi2Te3 layer doped with Fe up to ~6.9% along the growth direction was fabricated by the molecular beam epitaxy (MBE) technique. Its resistance versus temperature curve displays a superconductivity transition at about 12.3 K. Detailed structural and chemical analysis via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) reveal that this layer consists of two types of unexpected Fe-Te nanostructures: one is FeTe thin layer formed near the surface, and the other is FeTe2 nanorod embedded in the Bi2Te3 layer. Based on the results of further electrical and magnetotransport studies, it is likely that the observed superconductivity originates from the interface between the FeTe nanostructure and the neighboring Bi2Te3 layer. We have addressed the formation mechanisms of the observed nanostructures, which is attributed to the strong reaction between Fe and Te atoms during the growth process. The findings of this study also provide an unusual approach to synthesizing nanostructures via heavy doping if the dopant element is strongly reactive with an element in the host matrix.

We investigated the dissipative regime of the Bi2Te3/FeTe topological insulator-chalcogenide interface superconductor at temperatures well below the Berezinski-Kosterlitz-Thouless transition. We observe a transition in the current-resistance and temperature-resistance curves that quantitatively agrees with the Likharev vortex-explosion phenomenon. In the limit of low temperatures and high current densities, we were able to demonstrate the regime of complete vortex-antivortex dissociation arising from current driven vortex-antivortex pair breaking.

A large bulk band gap and tunable Dirac carriers are desired for practical device applications of topological insulators. However, most known topological insulators are narrow gap materials and the manipulation of their Dirac surface states is limited by residual bulk charge carriers originating from intrinsic defects. In this study, via density functional theory based first-principles calculations, we predict that a layered hexagonal structure of Bi

We have developed an incandescent Mo source to fabricate large-area single-crystalline MoSe

Superconductivity is realized in Fe 1+ x Te ( x =0.1–0.2) thin films grown on topological insulator Bi 2 Te 3 by molecular beam epitaxy. A thorough analysis of the obtained transport data in terms of the vortex glass theory reveals the occurrence of a spontaneous vortex glass phase transition in the superconducting Fe 1+ x Te films. Such a phase transition is ascribed to the presence of excess Fe in Fe 1+ x Te, indicating the important role of excess Fe in the vortex dynamics of iron chalcogenide. Based on the transport data, a phase diagram of Fe 1+ x Te is also constructed.

Recently, a logarithmic decrease of conductivity has been observed in topological insulators at low temperatures, implying a tendency of localization of surface electrons. Here, we report quantum transport experiments on the topological insulator Bi2Te3 thin films with arrayed antidot nanostructures. With increasing density of the antidots, a systematic decrease is observed in the slope of the logarithmic temperature-dependent conductivity curves, indicating the electron-electron interaction can be tuned by the antidots. Meanwhile, the weak anti-localization effect revealed in magnetoconductivity exhibits an enhanced dominance of electron-electron interaction among decoherence mechanisms. The observation can be understood from an antidot-induced reduction of the effective dielectric constant, which controls the interactions between the surface electrons. Our results clarify the indispensable role of the electron-electron interaction in the localization of surface electrons and indicate the localization of surface electrons in an interacting topological insulator., 27 pages, article+supplemental materials, published in ACS Nano

This work focuses on the formation mechanism of a superconducting Fe1+xTe/Bi2Te3 bilayer fabricated through chemical reactions between a Bi2Te3 flux and an annealed Fe layer grown on a ZnSe(111)B buffer via the molecular beam epitaxy technique. The studies using energy-dispersive X-ray spectrometry and high-resolution transmission electron microscopy performed on a number of samples fabricated with different desired schemes provide evidence that several interfacial chemical reactions taking place at the Fe/ZnSe interface through the edges of voids of the annealed Fe layer contributed to the formation of the superconducting bilayer. We have also revealed that the bonding strength of the involved Fe compounds at the bottom interface of the annealed Fe layer seems to play an essential role.

Cu doping of ZnTe, which is an important semiconductor for various optoelectronic applications, has been successfully achieved previously by several techniques. However, besides its electrical transport characteristics, other physical and chemical properties of heavily Cu-doped ZnTe have not been reported. We found an interesting self-assembled formation of crystalline well-aligned Cu-Te nano-rods near the surface of heavily Cu-doped ZnTe thin films grown via the molecular beam epitaxy technique. A phenomenological growth model is presented based on the observed crystallographic morphology and measured chemical composition of the nano-rods using various imaging and chemical analysis techniques. When substitutional doping reaches its limit, the extra Cu atoms favor an up-migration toward the surface, leading to a one-dimensional surface modulation and formation of Cu-Te nano-rods, which explain unusual observations on the reflection high energy electron diffraction patterns and apparent resistivity of these thin films. This study provides an insight into some unexpected chemical reactions involved in the heavily Cu-doped ZnTe thin films, which may be applied to other material systems that contain a dopant having strong reactivity with the host matrix. Electronic supplementary material The online version of this article (doi:10.1186/s11671-016-1741-x) contains supplementary material, which is available to authorized users.

The discovery of two-dimensional superconductivity in Bi2Te3/FeTe heterostructure provides a new platform for the search of Majorana fermions in condensed matter systems. Since Majorana fermions are expected to reside at the core of the vortices, a close examination of the vortex dynamics in superconducting interface is of paramount importance. Here, we report the robustness of the interfacial superconductivity and 2D vortex dynamics in four as-grown and aged Bi2Te3/FeTe heterostructure with different Bi2Te3 epilayer thickness (3, 5, 7, 14 nm). After two years' air exposure, superconductivity remains robust even when the thickness of Bi2Te3 epilayer is down to 3 nm. Meanwhile, a new feature at ~13 K is induced in the aged samples, and the high field studies reveal its relevance to superconductivity. The resistance of all as-grown and aged heterostructures, just below the superconducting transition temperature follows the Arrhenius relation, indicating the thermally activated flux flow behavior at the interface of Bi2Te3 and FeTe. Moreover, the activation energy exhibits a logarithmic dependence on the magnetic field, providing a compelling evidence for the 2D vortex dynamics in this novel system. The weak disorder associated with aging-induced Te vacancies is possibly responsible for these observed phenomena., 21 pages, article+supplemental information, published in Scientific Reports

In the interfacial superconductor Bi2Te3/Fe1+yTe, two dimensional superconductivity occurs in direct vicinity to the surface state of a topological insulator. If this state were to become involved in superconductivity, under certain conditions a topological superconducting state could be formed, which is of high interest due to the possibility of creating Majorana fermionic states. We report directional point-contact spectroscopy data on the novel Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically sharp interface. Our data show highly unconventional superconductivity, which appears as complex as in the cuprate high temperature superconductors. A very large superconducting twin-gap structure is replaced by a pseudogap above ~12 K which persists up to 40 K. While the larger gap shows unconventional order parameter symmetry and is attributed to a thin FeTe layer in proximity to the interface, the smaller gap is associated with superconductivity induced via the proximity effect in the topological insulator Bi2Te3.

This paper describes a ZigBee-based wireless sensor network node for the ultraviolet (UV) detection of flame. The sensor node is composed of a ZnSSe UV photodetector, a current-sensitive front end including a high-gain current-to-voltage amplifier with 120 dB and a logarithm converter, and a transceiver operated at a 2.4-GHz industrial, scientific, and medical band. A passive photodetector is designed to have a cutoff at 360 nm and convert the UV emission of flame into picoamperes. Including mixed signal processing and ZigBee transmission, the speed of flame detection is as fast as 70 ms. The sensor node consumes only an average of 2.3 mW from a 3.3-V supply. The performance of a prototype sensor node was verified when the luminous flame was imaged onto the sensor node with different angles ranging from -30° to 30° and distances of 0.1, 0.2, and 0.3 m enabling effective fire safety applications.

Nanostructures of II–VI semiconductor materials could potentially offer novel and superior physical (in particular, optoelectronic) properties with respect to their bulk counterparts. Herein, we present our most recent research on several II–VI and related nanostructures grown by molecular beam epitaxy (MBE) technique. These include a ZnSe nanograting. This nanograting structure was realized at the surface of Fe/ZnSe bilayers grown on GaAs(001) substrates by thermal annealing. A model based on an Ewald construction is presented to explain its unusual reflection high-energy electron diffraction (RHEED) patterns. The formation mechanism of this one-dimensional (1D) nanostructure is possibly related to surface energy minimization, together with an Fe–Se exchange interaction and Fe-induced decomposition of several top ZnSe atomic layers during thermal annealing. Another nanostructure investigated was the ZnS Schottky barrier embedded with Fe quantum dots (QDs). Here, a Au/ZnS/Fe-QDs/ZnS/n +-GaAs(100) Schottky barrier structure containing five layers of spherical Fe quantum dots with a diameter of ~3 nm was fabricated. Its I–V characteristic measured from 5 K to 295 K displays negative differential resistance (NDR) for temperature ≤50 K. Staircase-like I–V characteristics were also observed at low temperature in some devices fabricated from this structure. Possible mechanisms that can account for the observed unusual I-V characteristic in this structure are presented. Finally, laterally grown Fe nanowires (NWs) on a ZnS surface were prepared. Under high growth/annealing temperature, two types of Fe NWs with specific orientations can be grown on the ZnS(100) surface. We propose a mean-field model that the torque exerted by type A Fe NWs could effectively turn the two components of type B Fe NWs slightly toward the ZnS [110] direction, leading to the observed misalignment of type B Fe NWs.

ZnS nanowires (NWs) were fabricated on sapphire by molecular beam epitaxy using Au-droplet catalyst. The NWs grown at 530 and 480 °C are in cubic phase while those grown at 430 °C consist of a minor portion in hexagonal phase. The catalyst heads were found to trap more constituent of the ZnS flux at lower growth temperature. The photoluminescence (PL) spectrum of the NWs grown at 430 °C shows a single peak at its band-edge emission while those grown at 480 and 530 °C display a double-peak feature. The possible origins of the observed PL peaks were discussed.

A Au/ZnS/Fe-quantum dots (QDs)/ZnS/n+-GaAs(1 0 0) Schottky-barrier structure containing five layers of spherical Fe QDs with diameter around 3 nm was fabricated by the molecular beam epitaxy technique. Its current–voltage (I–V) characteristics measured from 5 to 295 K display negative differential resistance (NDR) for temperature ⩽50 K, which is caused by the presence of Fe QDs. The highest peak-to-valley current ratio obtained at 5 K is as high as 15:1. Staircase-like I–V characteristic was also observed at low temperature in some devices fabricated from this structure. Possible mechanisms that can account for the observed unusual I–V characteristics in this structure were discussed.

Two types of self-assembled Fe nanowires (NWs) were grown by molecular beam epitaxy on ZnS(1 0 0) surface under high growth/annealing temperature. The type-A Fe NWs orient along the ZnS [1 1 0] direction with irregular shape, while the type-B Fe NWs orient along either the ZnS [1 8 0] or the [8 1 0] direction with seemingly straight shape. Detailed high-resolution transmission electron microscopy and selected area electron diffraction studies reveal that both types were single-crystalline with their elongated axis along the 〈1 0 0〉 direction family possibly due to the fact that the easy axis of Fe is along this direction. We have proposed a mean-field model to explain the slight misalignment of the type-B Fe NWs. The I–V characteristic of a single type-B Fe NW measured at room temperature displays a straight line nature corresponding to a resistivity of about 2.3×10−7 Ω m.

Nanotrenches are induced by thermal annealing Au droplets on ZnSe surfaces. High-resolution scanning electron microscopy studies of the nanotrench structures reveal that the preferred migration directions of the catalyst droplets are along the $$ {\left\langle {{\hbox{110}}} \right\rangle } $$ direction family. On a ZnSe(111)B surface, each of the trenches is along one of the six $$ {\left\langle {{\hbox{110}}} \right\rangle } $$ directions while on a nonvicinal ZnSe(100) surface, the trenches are along a pair of antiparallel $$ {\left\langle {{\hbox{110}}} \right\rangle } $$ directions. Based on the results obtained from atomic force microscopy surface profiling and electron energy-loss spectroscopy chemical analysis techniques, a model is proposed to describe the possible formation mechanisms of the␣observed nanotrenches. The highly parallel nanotrenches induced on the Au/ZnSe(100) structure as revealed in this study are potentially useful as a template for in situ fabrication of ordered one-dimensional nanostructures (such as nanowires) of many materials.

Ultrathin ZnS nanowires (NWs) were grown on a sapphire (0001) substrate at 430°C by the molecular-beam epitaxy (MBE) technique using self-assembled Au droplets as the catalyst. It was found that these NWs mainly consist of the cubic phase but a small portion was in the hexagonal phase. The analysis of the temperature-dependent band-edge (BE) emission of these NWs and that of a ZnS thin film revealed that the energy shift of the interband transition on temperature in ZnS is mainly attributed to the electron-phonon interactions. The observed blue shift of the BE emission of ZnS NWs could be quantitatively explained by the confinement of the excited excitons in the NW geometry.

The growth orientation and crystalline perfection of ZnSe nanowires (NWs) grown by the combination of molecular beam epitaxy (MBE) and vapor–liquid–solid (VLS) reaction was found dependent on the growth temperature. Transmission electron microscopy (TEM) observations reveal that the preferred growth orientation of NWs depends on both the size of NWs and the growth temperature. A growth temperature dependence of the thickness of the terminal growth zone under the metal/semiconductor interface is introduced to modify our previous proposed phenomenological model to explain these observations. Tapering growth (the size of the NWs becomes narrower towards the top) was observed for the samples grown at 400 °C, which can be attributed to the diffusion-induced VLS mode that can be activated at relatively low growth temperature but inhibited at higher growth temperature. A cubic to hexagonal phase transformation was observed on the tapered NWs and its formation mechanism was proposed.

We investigated current induced depairing in the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}/\mathrm{FeTe}$ topological insulator-chalcogenide interface superconductor. The measured depairing current density provides information on the magnetic penetration depth and superfluid density, which in turn shed light on the nature of the normal state that underlies the interfacial superconductivity.

Molecular beam epitaxy (MBE) via the vapor-liquid-solid (VLS) reaction was used to grow ZnSe nanowires (NWs) on (111), (100), and (110) oriented GaAs substrates. Through detailed transmission electron microscopy (TEM) studies, it was found that 〈111〉 orientation is the growth direction for NWs with size ≥30 nm, while NWs with size around 10 nm prefer to grow along the 〈110〉 direction, with a small portion along the 〈112〉 direction. These observations have led to the realization of vertical ZnSe NWs with size around 10 nm grown on a GaAs (110) substrate. An ordered ZnSe NW array fabricated on a GaAs (111) substrate with a novel prepatterning method associated with plasma etching shows a high degree of ordering and a good size uniformity of the as-grown NWs. The diameter of the NWs in the array is around 80 nm and most of them are found to orient vertically, but some tilt to one of the six possible directions of the 〈111〉 family.

We report on observation of simultaneous strong enhancements of second-harmonic generation (SHG) and third-harmonic generation (THG) in visible spectra region in a one-dimensional ZnSe-ZnMgS semiconductor photonic crystal. These enhancements come from phase matching and a high density of modes near the photonic band edge. Our result shows that the THG is due to direct /spl chi//sup (3)/ process instead of the cascaded two-step /spl chi//sup (2)/ process. Measured forward SH conversion efficiency close to 1% for only a few microns thick film demonstrates potential device applications. Theoretical calculations are in good agreement with the experimental measurements.

ZnSe nano-wires (NWs) were fabricated on GaAs(1 1 1) and GaAs(1 1 0) substrates by the molecular beam epitaxy (MBE) technique via the vapor–liquid–solid (VLS) reaction. The size dependence of NW growth orientation was studied by varying the Au catalyst size. It was revealed that 〈1 1 1〉 orientation is the preferred growth direction for NWs with size ⩾30 nm while NWs with size around 10 nm prefer to grow along the 〈1 1 0〉 direction, with a small portion along the 〈1 1 2〉 direction. A phenomenological model based on the principle that crystalline nucleation favors the minimum of the total system energy was proposed to explain these observations. An approach to achieve vertical NWs with size around 10 nm was demonstrated on a GaAs(1 1 0) substrate.

This study focused on the investigation of the thermal stability of Fe on ZnSe and ZnS matrix using secondary ion mass spectroscopy (SIMS). Sandwiched three layer source structures of ZnSe/Fe/ZnSe and ZnS/Fe/ZnS were grown on GaAs and GaP substrates, respectively, by the molecular beam epitaxy technique. The bottom II–VI layers and the Fe- sandwiched layers in these structures are single crystalline while the top II–VI layers are polycrystalline. Thermal annealing was conducted in a range covering from 320 to 550 °C. The SIMS depth profiles of the as-grown and annealed structures reveal that (1 0 0) oriented single crystalline Fe/ZnSe interface is thermally stable at temperature as high as 450 °C while its polycrystalline counterpart suffers from fast diffusion even at the growth temperature. In contrast, (1 0 0) oriented polycrystalline Fe/ZnS interface is quite stable at least up to 200 °C. For both ZnSe/Fe/ZnSe and ZnS/Fe/ZnS systems, (1 1 1) oriented structures were found to have lower thermal stability than (1 0 0) oriented ones. These results provide important findings towards the optimization of Fe-based tunneling magneto-resistance structures using a II–VI semiconductor barrier.