Molecular Beam Epitaxy Technique Research Articles

Optimization of exchange bias in Co/CoO magnetic nanocaps by tuning deposition parameters

In the present work, we report exchange bias tuning by varying thin film deposition parameters such as synthesis method and underlying layer patterning. The patterned substrates for this study were prepared by self-assembly of polystyrene (PS) latex spheres (~530nm) on Si (100) substrate. The desired magnetic nanocaps composed of CoO/Co bilayer film on these patterned substrates were prepared by molecular beam epitaxy technique under ultra-high vacuum conditions. For this, a Co layer of ~10nm thickness was deposited on the substrates and then oxidized in-situ to form CoO/Co/PS in-situ oxidized film or ex-situ in ambiance which also gives CoO/Co/PS naturally oxidized film. Simultaneously, reference thin films of Co (~10nm) were also prepared on plane Si substrate and similar oxidation treatments were performed on them respectively. The magnetic properties studied using SQUID technique revealed higher exchange bias (~1736Oe) in the in-situ oxidized Co/PS film as compared to that in naturally oxidized Co/PS film (~1544Oe) and also compared to the reference film. The observed variations in the magnetic properties are explained in terms of surface patterning induced structural changes of the deposited films and different oxidation methods.

Millimeter-scale layered MoSe2 grown on sapphire and evidence for negative magnetoresistance

Molecular beam epitaxy technique has been used to deposit a single layer and a bilayer of MoSe2 on sapphire. Extensive characterizations including in-situ and ex-situ measurements show that the layered MoSe2 grows in a scalable manner on the substrate and reveals characteristics of a stoichiometric 2H-phase. The layered MoSe2 exhibits polycrystalline features with domains separated by defects and boundaries. Temperature and magnetic field dependent resistivity measurements unveil a carrier hopping character described within two-dimensional variable range hopping mechanism. Moreover, a negative magnetoresistance was observed, stressing a fascinating feature of the charge transport under the application of a magnetic field in the layered MoSe2 system. This negative magnetoresistance observed at millimeter-scale is similar to that observed recently at room temperature in WS2 flakes at a micrometer scale [Zhang et al., Appl. Phys. Lett. 108, 153114 (2016)]. This scalability highlights the fact that the underlying physical mechanism is intrinsic to these two-dimensional materials and occurs at very short scale.

Optical quenching mechanism in InAs quantum dots in an Al0.95Ga0.05As matrix

InAs quantum dots (QDs) were grown grown in an Al0.95Ga0.05As matrix by using the molecularbeam epitaxy technique. Photoluminescence (PL) measurements were made as functions of the magnetic fields and the temperature. Two prominent PL transitions were observed from QDs and defects in the matrix layer at 5 K. In magnetic fields, the transition from QDs does not change its spectral shape at magnetic fields up to 15 T, whereas the defect-related transition shows a blue-shift at magnetic fields above 8 T. By varying the temperature from 5 K to room temperature, the transition from QDs persists up to ~ 200 K and the defects-related transition quenches quickly near 70 K. The activation energies obtained by using an Arrhenius fitting of the PL intensities indicate that the excitons dissociated by thermal energy transfer into higher energy levels.

Effect of surface passivation on the electronic properties of GaAs nanowire:A first-principle study

Crystal structures of GaAs nanowires prepared by employing molecular beam epitaxy technique are often dominated by the wurtzite (WZ) phase.Recently,Galicka et al.found that the WZ GaAs nanowires grown along the[0001]direction in smaller size are energetically more favorable than other nanowires with the zinc blende phase grown along a specific direction (2008 J.Phys.:Condens.Matter 20 454226).The native nanowire usually has abundant unsaturated surface dangling bonds (SDBs) inducing significant surface states,leading to electrons accumulating at the nanowire surface. Thus the electrical property of the nanowire is very sensitive to the surface condition.However,surface passivation can effectively remove the surface states from the SDBs,and optimize the device performance.In this paper,using the first-principle calculations in combination with density function theory,we investigate the effect of surface passivation on the electronic structure of the GaAs nanowires grown along the[0001]direction.Various passivation species (hydrogen (H),fluorine (F),chlorine (Cl) and bromine (Br)) with different coverage ratios are considered.The GaAs nanowires hydrogenated with different locations and coverage ratios display different electronic properties.It is found that the GaAs native nanowire with a smaller diameter shows a semiconductor characteristic with indirect band gap,which originates from the fact that at smaller diameter,the surface stress becomes more remarkable,and then leads to surface atomic reconstruction.After passivation,the indirect band gap is translated into the direct band gap.For the GaAs nanowire with an As SDB hydrogenated,one deep donor level is located in the gap,and its band structure shows an n-type characteristic.For the GaAs nanowire with a Ga SDB hydrogenated,one shallow acceptor level is located in the gap,and its band structure shows a p-type characteristic.For the GaAs nanowire with a Ga-As dimer hydrogenated, its band structure shows an intrinsic semiconductor characteristic.For the GaAs nanowire with all of the Ga SDBs hydrogenated,the band structure shows a metallic characteristic.The band gap of the GaAs nanowire gradually increases as the hydrogen passivation ratio increases.For 50% hydrogen passivation,the band gap for the symmetrical passivation is slightly bigger than that for the half-side passivation.For the F-,Cl-and Br-passivation,the band gap decreases compared with for H-passivation.This is due to the fact that the ability of passivating atoms to compensate for surface atoms is weak,thereby reducing the band gap.The mechanism for the surface passivation is the suppression of surface states by the ability of the passivating atoms to compensate for surface atoms.These results show that the electronic properties of GaAs nanowires can be modulated by surface passivation,which is helpful for using GaAs nanowires as components and interconnections of nanoscale devices.

Infrared reflection spectra of the films of topological insulator Pb1-xSnxSe on the substrates ZnTe/GaAs

Infrared reflectivity spectra of the 700 nm thick topological insulator Pb 1-x Sn x Se films grown by the molecular beam epitaxy technique on ZnTe/GaAs substrates were studied. Using dispersion analysis of reflectivity spectra plasmon and phonon parameters for the samples under study were obtained.

Tuning the effective band gap and finding the optimal growth condition of InN thin films on GaN/sapphire substrates by plasma assisted molecular beam epitaxy technique

InN thin films are grown on GaN/sapphire substrates with varying the nitrogen plasma power in plasma assisted molecular beam epitaxy (PA-MBE) system. In order to evaluate the effect of nitrogen plasma power on the different properties of the InN films, several characterization viz. x-ray diffraction, atomic force microscopy, photoluminescence measurement, infra-red spectroscopy and Hall measurement were performed. Two interesting phenomena observed from the measurements are described in this paper. Firstly, it is found from both the photoluminescence and infrared spectroscopy that only by varying the nitrogen plasma power (thus the III/V ratio), one can fine tune the optical absorption edge, i.e., the effective band gap of InN from ∼0.72 eV to ∼ 0.77 eV. Secondly, it is inferred that the film grown with stoichiometric condition (III/V ∼ 1) exhibits the best structural and electrical properties.

Self-Assembled Formation of Well-Aligned Cu-Te Nano-Rods on Heavily Cu-Doped ZnTe Thin Films

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 materialThe online version of this article (doi:10.1186/s11671-016-1741-x) contains supplementary material, which is available to authorized users.

Growth and characterization of charge carrier spatially confined SrMnO3/La0.7Sr0.3MnO3/SrMnO3 trilayers

(SrMnO3)x/(La0.7Sr0.3MnO3)y/(SrMnO3)z (x,y,z=number of unit cells) trilayers have been grown using a Reflection High Energy Electron Diffraction calibrated layer-by-layer molecular beam epitaxy technique. X-Ray Reflectivity and X-Ray Diffraction measurements confirm the structural quality and the abruptness of the interfaces. Electrical transport property analysis as a function of temperature show effects related to the spatial confinement of the charge carriers induced by the layering. These results are important in view of future developments of oxide based heterostructures for innovative quantum devices.

Synthetic hybrid Co2FeGe/Ge(Mn) superlattice for spintronics applications

Herein, we propose and provide evidence for the experimentally and theoretically promising route of exploring spintronics materials with high performance via a synthetic hybrid of half-metallic ferromagnet and diluted magnetic semiconductor. Crystalline and well-ordered [Co2FeGe/Ge(Mn)]n superlattice, which is free of secondary phase separation, were prepared by the hybridization of end members, Co2FeGe and Ge(Mn), using the molecular beam epitaxy technique. Besides comparable magnetic properties with respect to the Co2FeGe films, the superlattice sample exhibits superior properties in electric conductivity and spin polarization, thus enhancing in favor of the spin injection efficiency. These results demonstrate the high feasibility of spintronics materials with low saturation magnetization, small coercivity, high Curie temperature, and high spin injection efficiency through the proposed route in this work.

P-type single-crystalline ZnO films obtained by (N,O) dual implantation through dynamic annealing process

Single-crystalline ZnO films were grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy technique. The films have been implanted with fixed fluence of 120 keV N and 130 keV O ions at 460 °C. Hall measurements show that the dually-implanted single-crystalline ZnO films exhibit p-type characteristics with hole concentration in the range of 2.1 × 1018–1.1 × 1019 cm−3, hole mobilities between 1.6 and 1.9 cm2 V−1 s−1, and resistivities in the range of 0.353–1.555 Ω cm. The ZnO films exhibit (002) (c-plane) orientation as identified by the X-ray diffraction pattern. It is confirmed that N ions were effectively implanted by SIMS results. Raman spectra, polarized Raman spectra, and X-ray photoelectron spectroscopy results reflect that the concentration of oxygen vacancies is reduced, which is attributed to O ion implantation. It is concluded that N and O implantation and dynamic annealing play a critical role in forming p-type single-crystalline ZnO films.

Novel Quaternary Dilute Magnetic Semiconductor (Ga,Mn)(Bi,As): Magnetic and Magneto-Transport Investigations

Magnetic and magneto-transport properties of thin layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor grown by the low-temperature molecular-beam epitaxy technique on GaAs substrates have been investigated. Ferromagnetic Curie temperature and magneto-crystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Post-growth annealing treatment of the layers has been shown to enhance the hole concentration and Curie temperature in the layers. A significant increase in the magnitude of magneto-transport effects caused by incorporation of a small amount of Bi into the (Ga,Mn)As layers, revealed in the planar Hall effect (PHE) measurements, is interpreted as a result of enhanced spin-orbit coupling in the (Ga,Mn)(Bi,As) layers. A two-state behaviour of the planar Hall resistance at zero magnetic field provides its usefulness for applications in non-volatile memory devices.

Current transport and deep levels in p-Si/n-Zn0.9Mg0.1O/n-ZnO heterojunction

The paper describes the deep level transient spectroscopy (DLTS) measurements that were performed in order to investigate defects in p-Si/n-Zn0.9Mg0.1O/n-ZnO heterojunction (HJ). The HJ structure was grown by molecular beam epitaxy (MBE) technique on a p-type Si(111) substrate at 500 °C. The DLTS studies had been preceded by the current-voltage (I-V) and capacitance-voltage (C-V) measurements. The I-V measurements show the rectifying behavior of the HJ and indicate that the low forward bias (less than 0.6 V) mechanism of current transport is dominated by recombination-tunneling whereas at higher voltage space charge limited current (SCLC) governs the current transport. The I-V and C-V measurements yield donor concentration of ND = 5·1015 cm−3 in the Zn0.9Mg0.1O layer. This value is two orders of magnitude less than the acceptor concentration, NA = 2·1017 cm−3 in the Si substrate. Therefore it has been assumed that the depletion region extends mainly on the Zn0.9Mg0.1O side of the HJ and the DLTS signal has its origin in this layer and at the HJ interface. The DLTS measurements performed within the temperature range of 20–400 K revealed the presence of three electron traps, with activation energies of 0.06 eV, 0.38 eV, and 0.39 eV respectively. The latter two are ascribed to the defects in the Zn0.9Mg0.1O layer, while the trap with the activation energy of 0.06 eV is located either in the Zn0.9Mg0.1O layer or at the interface with Si.

Recent studies of oxide-semiconductor heterostructures using aberration-corrected scanning transmission electron microscopy

Abstract

Investigation of room-temperature wafer bonded GaInP/GaAs/InGaAsP triple-junction solar cells

We report on the fabrication of III–V compound semiconductor multi-junction solar cells using the room-temperature wafer bonding technique. GaInP/GaAs dual-junction solar cells on GaAs substrate and InGaAsP single junction solar cell on InP substrate were separately grown by all-solid state molecular beam epitaxy (MBE). The two cells were then bonded to a triple-junction solar cell at room-temperature. A conversion efficiency of 30.3% of GaInP/GaAs/InGaAsP wafer-bonded solar cell was obtained at 1-sun condition under the AM1.5G solar simulator. The result suggests that the room-temperature wafer bonding technique and MBE technique have a great potential to improve the performance of multi-junction solar cell.

Incorporation of pervasive impurities on HVPE GaN growth directions

High crystallinity thick films with low free carrier concentration (≤1×1015/cm3) and low compensation are required for many GaN-based electronic device applications. It has been demonstrated that low pressure chemical vapor and molecular beam epitaxy techniques can reproducibility deposit homoepitaxial films with low residual impurity concentrations. However, their typical slow growth rates prevent their utilization for thick film growth. Presently, hydride vapor phase epitaxy is the sole method that can deposit films with residual impurity concentrations ≤5×1016/cm3 at hundreds of microns per hour growth rate. It is crucial to verify if this method can reproducibly deliver thick free-standing GaN films of high crystalline quality with exceptionally low and uniform free carrier concentration. X-ray diffraction, Raman scattering, and low temperature photoluminescence experiments were carried out on a number of samples prepared by dicing a free-standing wafer into several pieces perpendicular and parallel to the major growth directions; namely, c-plane {0001}, a-plane {11−20}, and m-plane {1−100}. SIMS depth profiles were employed to identify and quantify the concentration of the pervasive impurities. Spatial maps of a Raman line sensitive to free-carrier concentration were measured to determine the spatial distribution of the net impurity concentration. The reduced concentration of un-compensated shallow donors was also verified by low temperature electron paramagnetic resonance.

An atomic scale study of surface termination and digital alloy growth in InGaAs/AlAsSb multi-quantum wells

An atomic scale study has been performed to understand the influence of the (As,Sb) shutter sequences during interface formation on the optical properties of InGaAs/AlAsSb quantum wells. Our cross-sectional scanning tunneling microscopy results show that the onset of the Sb profile is steep in the Sb-containing layers whereas an appreciable segregation of Sb in the subsequently grown Sb free layers is observed. The steep rise of the Sb profile is due to extra Sb that is supplied to the surface prior to the growth of the Sb-containing layers. No relation is found between the (As,Sb) termination conditions of the Sb-containing layers and the resulting Sb profiles in the capping layers. Correspondingly we see that the optical properties of these quantum wells are also nearly independent on the (As,Sb) shutter sequences at the interface. Digital alloy growth in comparison to conventional molecular beam epitaxy growth was also explored. X-ray results suggest that the structural properties of the quantum well structures grown by conventional molecular beam epitaxy techniques are slightly better than those formed by digital alloy growth. However photoluminescence studies indicate that the digital alloy samples give rise to a more intense and broader photoluminescence emission. Cross-sectional scanning tunneling microscopy measurements reveal that lateral composition modulations present in the digital alloys are responsible for the enhancement of the photoluminescence intensity and inhomogeneous broadening.

Epitaxial growth and magnetic properties of ultraviolet transparent Ga2O3/(Ga1-xFex)2O3 multilayer thin films.

Multilayer thin films based on the ferromagnetic and ultraviolet transparent semiconductors may be interesting because their magnetic/electronic/photonic properties can be manipulated by the high energy photons. Herein, the Ga2O3/(Ga1−xFex)2O3 multilayer epitaxial thin films were obtained by alternating depositing of wide band gap Ga2O3 layer and Fe ultrathin layer due to inter diffusion between two layers at high temperature using the laser molecular beam epitaxy technique. The multilayer films exhibits a preferred growth orientation of crystal plane, and the crystal lattice expands as Fe replaces Ga site. Fe ions with a mixed valence of Fe2+ and Fe3+ are stratified distributed in the film and exhibit obvious agglomerated areas. The multilayer films only show a sharp absorption edge at about 250 nm, indicating a high transparency for ultraviolet light. What’s more, the Ga2O3/(Ga1−xFex)2O3 multilayer epitaxial thin films also exhibits room temperature ferromagnetism deriving from the Fe doping Ga2O3.

Perspective: Rapid synthesis of complex oxides by combinatorial molecular beam epitaxy

The molecular beam epitaxy (MBE) technique is well known for producing atomically smooth thin films as well as impeccable interfaces in multilayers of many different materials. In particular, molecular beam epitaxy is well suited to the growth of complex oxides, materials that hold promise for many applications. Rapid synthesis and high throughput characterization techniques are needed to tap into that potential most efficiently. We discuss our approach to doing that, leaving behind the traditional one-growth-one-compound scheme and instead implementing combinatorial oxide molecular beam epitaxy in a custom built system.

Induced Superconductivity and Engineered Josephson Tunneling Devices in Epitaxial (111)-Oriented Gold/Vanadium Heterostructures.

We report a unique experimental approach to create topological superconductors by inducing superconductivity into epitaxial metallic thin film with strong spin-orbit coupling. Utilizing molecular beam epitaxy technique under ultrahigh vacuum conditions, we are able to achieve (111) oriented single phase of gold (Au) thin film grown on a well-oriented vanadium (V) s-wave superconductor film with clean interface. We obtained atomically smooth Au thin films with thicknesses even down to below a nanometer showing near-ideal surface quality. The as-grown V/Au bilayer heterostructure exhibits superconducting transition at around 3.9 K. Clear Josephson tunneling and Andreev reflection are observed in S-I-S tunnel junctions fabricated from the epitaxial bilayers. The barrier thickness dependent tunneling and the associated subharmonic gap structures (SGS) confirmed the induced superconductivity in Au (111), paving the way for engineering thin film heterostructures based on p-wave superconductivity and nano devices exploiting Majorana Fermions for quantum computing.

Tunable inverse topological heterostructure utilizing(Bi1−xInx)2Se3and multichannel weak-antilocalization effect

In typical topological insulator (TI) systems the TI is bordered by a non-TI insulator, and the surrounding conventional insulators, including vacuum, are not generally treated as part of the TI system. Here, we implement the first material system where the roles are reversed, and the TSS form around the non-TI (instead of the TI) layers. This is realized by growing a layer of the tunable non-TI $(Bi_{1-x}In_{x})_{2}Se_{3}$ in between two layers of the TI $Bi_2Se_3$ using the atomically-precise molecular beam epitaxy technique. On this tunable inverse topological platform, we systematically vary the thickness and the composition of the $(Bi_{1-x}In_{x})_{2}Se_{3}$ layer and show that this tunes the coupling between the TI layers from strongly-coupled metallic to weakly-coupled, and finally to a fully-decoupled insulating regime. This system can be used to probe the fundamental nature of coupling in TI materials and provides a tunable insulating layer for TI devices.