Molecular Beam Epitaxy Technique Research Articles

Controllable epitaxy of quasi-one-dimensional topological insulator α-Bi4Br4 for the application of saturable absorber

Bismuth bromide (α-Bi4Br4) can demonstrate various exotic topological states, including higher-order topological insulator with hinge states and quantum spin Hall insulator with helical edge states. To date, α-Bi4Br4 nanowires can be obtained by using the exfoliation method from the bulk. However, it is still a great challenge to efficiently prepare α-Bi4Br4 nanowires suitable for potential applications, e.g., saturable absorber in ultrafast pulsed fiber lasers. Here, we report the controllable growth of α-Bi4Br4 thin films consisting of nanowires via molecular beam epitaxy technique. We show that the morphology of the α-Bi4Br4 nanowires depends on the growth temperature and BiBr3 flux. In addition, we also achieve α-Bi4Br4 nanowires on NbSe2 and gold substrates. Furthermore, we performed the saturable absorption property of α-Bi4Br4 thin films with a modulation depth of 21.58% and mode-locking at 1556.4 nm with a pulse width of 375 fs in the pulsed fiber lasers. Those results demonstrate the synthesis of quasi-1D topological material α-Bi4Br4, which is expected to be used for the fundamental research of topological physics and potential applications in optical devices.

High sensitivity and fast response infrared detector fabricated with the Bi2Se3-PbSe heterojunction

A heterojunction photodetector based on the photogating effect is fabricated by combining the novel topological insulator Bi2Se3 with the traditional narrow-gap PbSe. The Bi2Se3-PbSe heterojunction is constructed by depositing PbSe thin film on surface of Bi2Se3 with the molecular beam epitaxy technique. Large lattice mismatch between cubic PbSe and rhombohedral Bi2Se3 is relaxed through the van der Waals epitaxial growth process, leading to an atomically abrupt and smooth Bi2Se3-PbSe heterointerface. Benefiting from the prominent photosensitivity in narrow-gap PbSe and the ultrafast charge transport in topological insulator Bi2Se3, high photosensitivity and fast response are simultaneously observed in the as-fabricated photodetectors. Responsivity and detectivity are 31.4 AW−1 and 1.7 × 1011 cmHz0.5 W−1 respectively. Rising and falling time are 240 μs and 500 μs respectively. In this structure, the appropriate band alignment of Bi2Se3-PbSe heterojunction and the photogating effect could synergistically utilize the advantages of both Bi2Se3 and PbSe, suggesting a potential solution to broaden the application prospect for both sides.

Theory of MBE Growth of Nanowires on Reflecting Substrates

Selective area growth (SAG) of III-V nanowires (NWs) by molecular beam epitaxy (MBE) and related epitaxy techniques offer several advantages over growth on unpatterned substrates. Here, an analytic model for the total flux of group III atoms impinging NWs is presented, which accounts for specular re-emission from the mask surface and the shadowing effect in the absence of surface diffusion from the substrate. An expression is given for the shadowing length of NWs corresponding to the full shadowing of the mask. Axial and radial NW growths are considered in different stages, including the stage of purely axial growth, intermediate stage with radial growth, and asymptotic stage, where the NWs receive the maximum flux determined by the array pitch. The model provides good fits with the data obtained for different vapor–liquid–solid and catalyst-free III-V NWs.

Research progress of two-dimensional quantum spin Hall insulator in monolayer 1<i>T'</i>-WTe<sub>2</sub>

Quantum spin Hall effect, usually existing in two-dimensional (2D) topological insulators, has topologically protected helical edge states. In the year 2014, there was raised a theoretical prediction that monolayer transition metal dichalcogenides (TMDs) with 1<i>T'</i> phase are expected to be a new class of 2D quantum spin Hall insulators. The monolayer 1<i>T'</i>-WTe<sub>2</sub> has attracted much attention, because it has various excellent characteristics such as stable atomic structures, an obvious bandgap opening in the bulk of monolayer 1<i>T'</i>-WTe<sub>2</sub>, and tunable topological properties, which paves the way for realizing a new generation of spintronic devices. In this review, we mainly summarize the recent experimental progress of the 2D quantum spin Hall insulators in monolayer 1<i>T'</i>-WTe<sub>2</sub>, including the sample preparation via a molecular beam epitaxy technique, the detection of helical edge states and their response on external magnetic fields, as well as the modulation of more rich and novel quantum states under electron doping or strain. Finally, we also prospect the future applications based on monolayer 1<i>T'</i>-WTe<sub>2</sub>.

Гетероструктура квантово-каскадного детектора частотного диапазона 2.5 ТГц

The design of the heterostructure of a 2.5 THz range quantum-cascade detector is proposed and heterostructure is grown by molecular-beam epitaxy technique. To optimize the thicknesses of the layers of the heterostructure cascades, a numerical method for iterative solution of the Schrodinger ¨ −Poisson equation in the k · p formalism was used. The grown heterostructure of the quantum-cascade detector showed a high structural perfection, confirmed by the small values of the average FWHM of the high-order satellite peaks on the X-ray diffraction rocking curves, which were (8.3 ± 0.5)′′. Analysis of dark-field images obtained by transmission electron microscopy showed that the total thickness of the layers in the cascade is (137.3 ± 6.9) nm, which corresponds to the calculated thickness of the layers in the cascade of the heterostructure of the quantum-cascade detector.

Heterostructures of quantum-cascade lasers with nonselective overgrowth by metalorganic vapour phase epitaxy

The possibility of fabrication of 4.6 μm spectral range quantum-cascade laser heterostructures by molecular-beam epitaxy technique with non-selective overgrowth by the metalorganic vapour-phase epitaxy is shown. The active region of the laser was formed on the basis of a heteropair of In0.67Ga0.33As/In0.36Al0.64As solid alloys. The waveguide claddings are formed by indium phosphide. The results of surface defects inspection and X-ray diffraction analysis of quantum-cascade laser heterostructures allow to conclude that the structural quality of the heterostructures is high and the estimated value of the root mean square surface roughness does not exceed 0.7 nm. Lasers with four cleaved facets exhibit lasing at room temperature with a relatively low threshold current density of the order of 1 kA/cm2. Keywords: superlattices, quantum-cascade laser, epitaxy, indium phosphide.

Heterostructure of a 2.5 THz range quantum-cascade detector

The design of the heterostructure of a 2.5 THz range quantum-cascade detector is proposed and heterostructure is grown by molecular-beam epitaxy technique. To optimize the thicknesses of the layers of the heterostructure cascades, a numerical method for iterative solution of the Schrodinger--Poisson equation in the k· p formalism was used. The grown heterostructure of the quantum-cascade detector showed a high structural perfection, confirmed by the small values of the average FWHM of the high-order satellite peaks on the X-ray diffraction rocking curves, which were (8.3±0.5)n. Analysis of dark-field images obtained by transmission electron microscopy showed that the total thickness of the layers in the cascade is (137.3±6.9) nm, which corresponds to the calculated thickness of the layers in the cascade of the heterostructure of the quantum-cascade detector. Keywords: superlattices, quantum-cascade detector, epitaxy, gallium arsenide

Heterostructures of quantum-cascade lasers with nonselective overgrowth by metalorganic vapour phase epitaxy

The possibility of fabrication of 4.6 μm spectral range quantum-cascade laser heterostructures by molecular-beam epitaxy technique with non-selective overgrowth by the metalorganic vapour-phase epitaxy is shown. The active region of the laser was formed on the basis of a heteropair of In0.67Ga0.33As/In0.36Al0.64As solid alloys. The waveguide claddings are formed by indium phosphide. The results of surface defects inspection and X-ray diffraction analysis of quantum-cascade laser heterostructures allow to conclude that the structural quality of the heterostructures is high and the estimated value of the root mean square surface roughness does not exceed 0.7 nm. Lasers with four cleaved facets exhibit lasing at room temperature with a relatively low threshold current density of the order of 1 kA/cm2. Keywords: superlattices, quantum-cascade laser, epitaxy, indium phosphide.

Epitaxial growth, intrinsic point defects and electronic transport optimization of MnTe films

The NiAs-type MnTe compound is one of important and environmental friendly p-type thermoelectric materials for generating intermediate temperature powern. The low hole concentration in the pristine MnTe greatly restricts its thermoelectric performance. However, the systematic experimental studies of thermoelectric materials are still lacking so far. In this work, MnTe thin films are grown by molecular beam epitaxy (MBE) technique, and their intrinsic point defect structures are characterized by scanning tunneling microscope (STM). Through the regulation of the intrinsic point defects, the electrical transport performances of MnTe films are remarkably improved. The results show that Mn vacancies (V<sub>Mn</sub>) and Te vacancies (V<sub>Te</sub>) are the dominant intrinsic point defects in MnTe film. With the increase of the substrate temperature (<i>T</i><sub>sub</sub>) and the decrease of the Mn:Te flux ratio, the hole concentration in MnTe film increases greatly, reaching a maximum value of 21.5 × 10<sup>19</sup> cm<sup>–3</sup>, which is one order of magnitude higher than that of the intrinsic MnTe bulk. This is attributed to the significantly increased concentration of p-type V<sub>Mn</sub> in MnTe film, and thus leads the conductivity (<i>σ</i>) and power factor (<i>PF</i>) to increase remarkably. Finally, the MnTe film grown at <i>T</i><sub>sub</sub> = 280 ℃ and Mn∶Te = 1∶12 obtains the maximum <i>PF</i> of 1.3 μW·cm<sup>–1</sup>·K<sup>–2</sup> at 483 K in all grown films. This study clarifies the characteristics of intrinsic point defects and their relationship with the electrical transport properties of MnTe based compounds, which provides an importantguidance for further optimizing their thermoelectric performances.

Real-space characterization of tetragonal CuO epitaxial films

It is widely believed that the undoped Mott insulator is crucial to understand the origin of high-TC superconductivity and the complex phase diagram in cuprates. Tetragonal CuO, the simplest structure among cuprates, is regarded as an archetypal model to explore the ground state physics of Mott insulator. Here, we report on the epitaxial growth and surface characterization of tetragonal CuO thin film by combining ozone-assisted molecular beam epitaxy and multi-analytical techniques. In situ scanning tunneling microscopy measurement reveals two sets of Cu2+ square lattices with a reversed contrast of their apparent heights between neighboring domains. A spectroscopy study demonstrates an upper bound of the charge transfer gap as 3.68 eV, and quantitatively agrees with the previous photoemission and x-ray photoelectron results. Our work will deepen the understanding of high-energy electronic structures in parent cuprates.

Josephson Effect in MgB2/Pd/Nb Trilayer Josephson Junctions

This paper reports fabrication techniques and results of MgB2/Pd/Nb trilayer Josephson junctions. The MgB2 bottom electrode was co-evaporated by molecular beam epitaxy (MBE) technique from both magnesium and boron sources at a low substrate temperature ~ 300 °C, while the interlayer and the top niobium electrode (Pd/Nb bilayer) were deposited ex-situ using RF sputtering. The junctions exhibited and Josephson effect as well as a modulation of the critical current in a magnetic field applied in a direction normal to the junction plane. Fractional and integer Shapiro steps were observed at voltages corresponding to the frequency of the applied microwave radiation field. The products of the junctions compare well with the previously reported values. The results suggest that it should be possible to fabricate all-MgB2 and MgB2 as one of the electrodes Superconductor/Normal/Superconductor (SNS), Superconductor/Insulator/Superconductor (SIS) or even Superconductor/Ferromagnet/Superconductor (SFS) tunnel junctions with interesting characteristics and for various applications.
 Keywords: MgB2; all-MgB2; Josephson Tunnel junctions; trilayer devices; Niobium

The adhesion of L-methionine amino acid through Dip Pen Nanolithography on silver thin films grown by Molecular Beam Epitaxy technique

The present work explore the fabrication of four silver thin films with controlled thicknesses of 30 nm, 60 nm, 90 nm and 120 nm, by using ultra-high vacuum (UHV) molecular beam epitaxy (MBE) technique. The peculiarities of their transformations depending on substrate quality, roughness and shape profile types were analyzed by Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDX) and Atomic Force Microscopy (AFM) scanning approaches. Topographical characteristics and structure modifications were highlighted depending on the deposition thicknesses. Within the increase of silver layers, Ag particles started to form a more uniform, clustered layer on the glass substrate. The improved regularity of Ag particles inside clusters defines better the cluster profiles, emphasizing a preferred orientation and arrangement of Ag particles. For the thicker layers of 90 nm and 120 nm, a well-ordered particles arrangement is present in a much higher amount, unifying the clusters with preferred regular orientations. This phenomenon has conducted to the formation of cluster islands, thus increasing the surface roughness. The reactivity of Ag thin films toward L-methionine amino acid was explored by Dip Pen Nanolithography (DPN). A clear dependence between the formation of distinct surface properties and the ease of direct-write patterning was demonstrated. An optimum linkage preference of molecular L-methionine for the most flat surface of 30 nm growth has been emphasized, while for the other substrates the writing process is critically affected by the surface roughness.

Capacitive sensor based on GaN honeycomb nanonetwork for ultrafast and low temperature hydrogen gas detection

Passive devices are highly desired for the detection of H2 gas due to its high safeness and low energy consumption. In this study, GaN with honeycomb nanonetwork structure (GaN-HN) epitaxially grown on Si wafer by molecular beam epitaxy technique was utilized for the fabrication of capacitive H2 gas sensors, on which 10 nm thick Pd layer was deposited as the sensing electrode. XRD, SEM, and AFM results show that the cubic phase Pd is uniformly coated on the surface of GaN-HN and maintains an interesting structure of honeycomb nanonetwork. It is found that the response time decreases gradually with the increase of the operating temperature and it reaches a steady value ∼3 s at temperatures higher than 50 °C. Ultrafast response time of 1.3 s towards 1000 ppm H2 was observed for the Pd-GaN/Si capacitive sensor operated at the temperature of 70 °C. The low limit of detection (LOD) is also evaluated by experiment and IUPAC definition, 500 ppb and 440 ppb, respectively. Moreover, the capacitive sensor exhibits ignorable response to CH4, NO2 and common VOCs gases. The long-term stability and humidity effect on H2 gas detection were also studied. Schottky junction capacitance of Pd/GaN might be dominated by the hydrogen dipole layer. The proposed Pd-GaN/Si capacitive sensor investigated here exhibits ultrafast response speed, low operating temperature, high selectivity and stability, making it attractive for applications in timely H2 leakage detection.

Temperature-dependent characteristics of GeSn/Ge multiple-quantum-well photoconductors on silicon.

Temperature-dependent characteristics of GeSn/Ge multiple-quantum-well (MQW) photoconductors (PCs) on silicon substrate were investigated. The high quality GeSn/Ge MQW epitaxial structure was grown on a silicon substrate using low temperature molecular beam epitaxy techniques with atomically precise thickness control. Surface-illuminated GeSn/Ge MQW PCs were fabricated using complementary metal-oxide-semiconductor-compatible processing and characterized in a wide temperature range of 55-320 K. The photodetection range was extended to λ=2235nm at T=320K due to bandgap shrinkage with Sn alloying. Measured spectral responsivity was enhanced at reduced temperatures. These results provide better understanding of GeSn/Ge MQW structures for efficient short-wave infrared photodetection.

Epitaxy Induced Highly Ordered Sm2Co17-SmCo5 Nanoscale Thin-Film Magnets.

Utilizing the molecular beam epitaxy technique, a nanoscale thin-film magnet of c-axis-oriented Sm2Co17 and SmCo5 phases is stabilized. While typically in the prototype Sm(Co, Fe, Cu, Zr)7.5-8 pinning-type magnets, an ordered nanocomposite is formed by complex thermal treatments, here, a one-step approach to induce controlled phase separation in a binary Sm-Co system is shown. A detailed analysis of the extended X-ray absorption fine structure confirmed the coexistence of Sm2Co17 and SmCo5 phases with 65% Sm2Co17 and 35% SmCo5. The SmCo5 phase is stabilized directly on an Al2O3 substrate up to a thickness of 4 nm followed by a matrix of Sm2Co17 intermixed with SmCo5. This structural transition takes place through coherent atomic layers, as revealed by scanning transmission electron microscopy. Highly crystalline growth of well-aligned Sm2Co17 and SmCo5 phases with coherent interfaces result in strong exchange interaction, leading to enhanced magnetization and magnetic coupling. The arrangement of Sm2Co17 and SmCo5 phases at the nanoscale is reflected in the observed magnetocrystalline anisotropy and coercivity. As next-generation permanent magnets require designing of materials at an atomic level, this work enhances our understanding of self-assembling and functioning of nanophased magnets and contributes to establishing new concepts to engineer the microstructure for beyond state-of-the-art magnets.

Structural and ultraviolet photo-detection properties of laser molecular beam epitaxy grown GaN layers using solid GaN and liquid Ga targets

GaN epitaxial layers have been grown on sapphire (0001) substrate by laser molecular beam epitaxy technique using laser ablation of polycrystalline GaN solid and liquid Ga metal targets in the ambient of nitrogen plasma. In-situ reflection high energy electron diffraction and exsitu atomic force microscopy observations revealed that GaN growth using solid target yields rough surface under three-dimensional growth mode while a flat surface GaN is obtained using liquid Ga target. From X-ray rocking curve measurement, it is also observed that the GaN layer grown using solid GaN target has a relatively better structural quality. X-ray photoelectron spectroscopy confirmed Ga–N bond formation and near-stoichiometric composition of the GaN epilayers. The influence of threading dislocation density on the ultraviolet (UV) photoresponse properties of GaN layers have been studied using metal-semiconductor-metal (MSM) based device structure. It is found that the GaN MSM structure obtained using solid GaN target with lesser screw and dislocation densities exhibits a higher responsivity with fast response and recovery time.

Structural defects in MBE-grown CdTe-basing heterojunctions designed for photovoltaic applications

Structural defects in the p-ZnTe/i-CdTe/n-CdTe single-crystalline heterojunctions designed for photovoltaic applications have been investigated by transmission electron microscopy (TEM) and deep-level transient spectroscopy (DLTS). Lattice parameters and misfit strain in the undoped cadmium telluride (CdTe) absorber layers of the heterojunctions, grown by the molecular-beam epitaxy technique on two different substrates, GaAs and CdTe, have been determined with high-resolution x-ray diffractometry. A dense network of misfit dislocations at the lattice-mismatched CdTe/GaAs and ZnTe/CdTe interfaces and numerous threading dislocations and stacking faults have been shown by the cross-sectional TEM imaging of the heterojunctions. The DLTS measurements revealed five deep-level traps in the heterojunctions grown on GaAs and only three of them in the heterojunctions grown on CdTe. Four of the traps have been attributed to the electronic states of extended defects, presumably dislocations, on the grounds of their logarithmic capture kinetics of charge carriers. Two of these traps, displaying the largest values of their capture cross-section and the properties characteristic of bandlike electronic states, have been ascribed to the core states of dislocations. It is argued that they are most likely responsible for decreased lifetime of photo-excited carriers resulting in a low energy conversion efficiency of solar cells based on similarly grown heterojunctions.

A study of the GaN/Si(111) epitaxial structures grown by PA MBE via coalescence overgrowth of GaN nanocolumns

The GaN/Si(111) epitaxial structures were synthesized by coalescence overgrowth of GaN nanocolumns using the plasma-assisted molecular beam epitaxy (PA-MBE) technique. Such epitaxial structures can be used as a buffer layer for obtaining high quality GaN epilayers. Structural, electrical and chemical properties of these samples were studied. It was demonstrated that KOH etching of the grown GaN/Si(111) samples results in the separation of the GaN epilayer from the substrate.

Monolithic Light Reflector‐Nanowire Light Emitting Diodes

AbstractThe development of small size, ultracompact photonic devices is required for a wide range of applications, such as next‐generation automobile displays, micro‐light‐emitting diode (µ‐LED) TVs, smart phones and watches, virtual reality, and augmented reality. Here, for the first time, a new approach is reported to dramatically reduce the size of photonic device chips by using a monolithic light reflector‐nanowire LED system. Moreover, the vertical cavity nanowire structure for the surface‐lighting emission is developed by selective area epitaxy that can precisely control the gaps and diameters of the GaN nanowires. The light reflection is provided by high quality Al metal reflector which is directly deposited by molecular beam epitaxy (MBE) technique. The selective area growth shows uniform GaN heterostructure arrays with excellent crystal quality and desired aspect ratio. Furthermore, an advanced selective area etching process is introduced to fabricate Al mirrors between nanowire arrays. Significantly improved light efficiency is observed at ≈550 nm spectral wavelength from the monolithic Al light reflector‐vertical surface emitting nanowire LED system. To break the “green gap” bottleneck in III‐nitride photonics, the approach will open a new route toward monolithic, ultrasmall, electrically pumped nanoscale light emitters for next‐generation photonic and electronic devices.

Experimental estimation of the spin diffusion length in undoped p-Ge on Fe3Si using vertical spin-valve devices

Using vertical spin-valve devices, we experimentally investigate the room-temperature spin diffusion length in an undoped p-Ge layer grown on ferromagnetic Fe3Si. Because low-temperature molecular beam epitaxy techniques enable us to grow vertically stacked and all-epitaxial CoFe/Ge/Fe3Si trilayers on Si(111), we can intentionally vary the thickness (tGe) of the intermediate undoped p-Ge layer during the growth. With decreasing tGe, the magnitude of the spin signals gradually increases at room temperature. From the analysis based on the model by Fert and Jaffrès, the room-temperature spin diffusion length in the undoped p-Ge grown on Fe3Si is experimentally estimated to be ∼8.4 nm, much shorter than those reported in previous works on commercial p-Ge substrates.