Atomic Layer Molecular Beam Epitaxy Research Articles

Atomic layer molecular beam epitaxy of kagome magnet RMn6Sn6 (R = Er, Tb) thin films

Kagome lattices have garnered substantial interest because their band structure consists of topological flat bands and Dirac cones. The RMn6Sn6 (R = rare earth) compounds are particularly interesting because of the existence of the large intrinsic anomalous Hall effect (AHE), which originates from the gapped Dirac cones near the Fermi level. This makes RMn6Sn6 an outstanding candidate for realizing the high-temperature quantum AHE. The growth of RMn6Sn6 thin films is beneficial for both fundamental research and potential applications. However, most of the studies on RMn6Sn6 have focused on bulk crystals, and the synthesis of RMn6Sn6 thin films has not been reported so far. Here, we report the atomic layer molecular beam epitaxy growth, structural and magnetic characterizations, and transport properties of ErMn6Sn6 and TbMn6Sn6 thin films. It is especially noteworthy that TbMn6Sn6 thin films have out-of-plane magnetic anisotropy, which is important for realizing the quantum AHE. Our work paves the avenue toward the control of the AHE using devices patterned from RMn6Sn6 thin films.

An atomically tailored chiral magnet with small skyrmions at room temperature

Creating materials that do not exist in nature can lead to breakthroughs in science and technology. Magnetic skyrmions are topological excitations that have attracted great attention recently for their potential applications in low power, ultrahigh density memory. A major challenge has been to find materials that meet the dual requirement of small skyrmions stable at room temperature. Here we meet both these goals by developing epitaxial FeGe films with excess Fe using atomic layer molecular beam epitaxy (MBE) far from thermal equilibrium. Our atomic layer design permits the incorporation of 20% excess Fe while maintaining a non-centrosymmetric crystal structure supported by theoretical calculations and necessary for stabilizing skyrmions. We show that the Curie temperature is well above room temperature, and that the skyrmions have sizes down to 15 nm as imaged by Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The presence of skyrmions coincides with a topological Hall effect-like resistivity. These atomically tailored materials hold promise for future ultrahigh density magnetic memory applications.

Atomic layer epitaxy of kagome magnet Fe3Sn2 and Sn-modulated heterostructures

Magnetic materials with kagome crystal structure exhibit rich physics, such as frustrated magnetism, skyrmion formation, topological flat bands, and Dirac/Weyl points. Until recently, most studies on kagome magnets have been performed on bulk crystals or polycrystalline films. Here, we report the atomic layer molecular beam epitaxy synthesis of high-quality thin films of topological kagome magnet Fe3Sn2. The structural and magnetic characterization of Fe3Sn2 on epitaxial Pt(111) identifies highly ordered films with c-plane orientation and an in-plane magnetic easy axis. Studies on the local magnetic structure by anomalous Nernst effect imaging reveal in-plane oriented micrometer size domains. Superlattice structures consisting of Fe3Sn2 and Fe3Sn are also synthesized by atomic layer molecular beam epitaxy, demonstrating the ability to modulate the sample structure at the atomic level. The realization of high-quality films by atomic layer molecular beam epitaxy opens the door to explore the rich physics of this system and investigate novel spintronic phenomena by interfacing Fe3Sn2 with other materials.

High-temperature interface superconductivity in bilayer copper oxide films by pulsed laser deposition

In a seminal work, Gozar et al. reported on the high-temperature interface superconductivity in bilayers of insulating La2CuO4 and metallic La2−xSrxCuO4 (x=0.45). An interesting question to address is how general and robust this interface superconductivity is. In the past, the cuprate bilayers were grown in a unique atomic-layer molecular beam epitaxy system, with a Sr doping range of x≤0.47, and the atomically flat interface was thought to be indispensable. Here, we have fabricated bilayers of La2CuO4 and La2−xSrxCuO4 by pulsed laser deposition. We have tried to extend the nominal doping range of Sr from the previous maximum of 0.47 to the present 1.70 (the nominal Sr content in the targets). X-ray diffraction result indicates that our La2−xSrxCuO4 films with x≤0.60 have very high crystalline quality; but the film crystalline structure degrades gradually with further increasing x, and finally the structure is fully lost when x reaches 1.40 and higher. Although the film quality scatters dramatically, our experiments show that there exists superconductivity for bilayers in nearly the entire over-doped Sr range, except for a non-superconducting region at x∼0.80. These observations demonstrate that the interface superconductivity in copper oxides is very general and robust.

Germanium Incorporation in Silicon Carbide Epitaxial Layers Using Molecular Beam Epitaxy on 4H-SiC Substrates

Ternary (Si1-xCy)Gex+y solid solutions were grown on Si-face 4H-SiC applying atomic layer molecular beam epitaxy at low temperatures. The grown layers consist of twinned 3C-SiC revealed by cross section electron microscopy. The germanium was incorporated on silicon lattice sites as revealed by atomic location by channeling enhanced microanalysis transmission electron microscopy studies. The Ge concentration of the grown 3C-(Si1-xCy)Gex+y heteroepitaxial layers decreases with increasing growth temperatures, but exceeds the solid solubility limit.

Spectrum and phase mapping across the epitaxial γ-Al2O3/SrTiO3 interface

Epitaxial heterostructures of γ−Al2O3/SrTiO3, grown by atomic layer deposition (ALD) and molecular beam epitaxy, have been characterized by advanced electron microscopy techniques, including aberration-corrected negative-Cs imaging, electron-energy-loss near-edge fine-structure analysis, and off-axis electron holography. Analysis of two-dimensional spectrum maps from samples that previously showed highly conductive interfacial layers revealed partial reduction of the Ti oxidation state in the SrTiO3 layer from Ti4+ to Ti3+, which was confined to within ∼1–2 unit cells of the interface. Electron holography of an ALD-grown sample revealed a phase profile within the SrTiO3 layer that rose sharply over a distance of about 1 nm moving away from the interface. Taken together, these results suggest a strong connection between reduction of oxidation state, which could be caused by oxygen vacancies and the quasi-two-dimensional electron gas present at the γ−Al2O3/SrTiO3 interface.

Complex conductance of ultrathin La2–xSrxCuO4 films and heterostructures

We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal (La1.55Sr0.45CuO4) and a cuprate insulator (La2CuO4), in which each layer is just one unit cells thick. We have studied the magnetic field and temperature dependence of the complex sheet conductance, σ(ω), of these films. Experiments have been carried out at frequencies between 2 and 50 MHz using the single-spiral coil technique. We found that: (i) the inductive response starts at ΔT = 3 K lower temperatures than Re σ(T), which in turn is characterized by a peak close to the transition, (ii) this shift is almost constant with magnetic field up to 14 mT; (iii) ΔT increases sharply up to 4 K at larger fields and becomes constant up to 8 T; (iv) the vortex diffusion constant D(T) is not linear with T at low temperatures as in the case of free vortices, but is rather exponential due to pinning of vortex cores, and (v) the dynamic Berezinski–Kosterlitz–Thouless (BKT) transition temperature occurs at the point where Y=(lω/ξ+)2=1. Our experimental results can be described well by the extended dynamic theory of the BKT transition and dynamics of bound vortex–antivortex pairs with short separation lengths.

Magnetic field, frequency and temperature dependence of complex conductance of ultrathin La1.65Sr0.45CuO4∕La2CuO4 films and the organic superconductors κ-(BEDT-TTF)2Cu[N(CN)2]Br

We used atomic-layer molecular beam epitaxy (ALL-MBE) to synthesize bilayer films of a cuprate metal ([Formula: see text], LSCO) and a cuprate insulator ([Formula: see text], LCO), in which interface superconductivity occurs in a layer that is just one-half unit cell thick. We have studied the magnetic field and temperature dependence of the complex sheet conductance, [Formula: see text], of these films, and compared them to [Formula: see text]-(BEDT-TTF)[Formula: see text] single crystals. The magnetic field [Formula: see text] was applied both parallel and perpendicular to the 2D conducting layers. Experiments have been carried out at frequencies between 23 kHz and 50 MHz using either two-coil mutual inductance technique, or the LC resonators with spiral or rectangular coils. The real and the imaginary parts of the mutual-inductance [Formula: see text] between the coil and the sample were measured and converted to complex conductivity. For [Formula: see text] perpendicular to the conducting layers, we observed almost identical behavior in both films and [Formula: see text]-Br single crystals: (i) the transition onset in the inductive response, [Formula: see text](T) occurs at a temperature lower by 2 K than in [Formula: see text], (ii) this shift is almost constant with magnetic field up to 8 T; (iii) the vortex diffusion constant [Formula: see text] is exponential due to pinning of vortex cores. These results can be described by the extended dynamic theory of the Berezinski–Kosterlitz–Thouless (BKT) transition and dynamics of bound vortex–antivortex pairs with short separation lengths.

Magnetic field and temperature dependence of complex conductance of ultrathin La1.65Sr0.45CuO4/La2CuO4films

We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal (La${}_{1.55}$Sr${}_{0.45}$CuO${}_{4}$) and a cuprate insulator (La${}_{2}$CuO${}_{4}$) film, in which each layer is just one unit cell thick. We have studied the magnetic field and temperature dependences of the complex sheet conductance, \ensuremath{\sigma}(\ensuremath{\omega}), of these films. Experiments have been carried out at frequencies between 2 and 20 MHz using the single-spiral coil technique. We found that (i) the inductive response, ${L}_{k}^{\ensuremath{-}1}$($T$), starts at 1.7 K lower temperatures than Re\ensuremath{\sigma}($T$), which in turn is characterized by a peak close to the transition, (ii) this shift is almost constant with magnetic field up to 8 T; (iii) the vortex diffusion constant $D$($T$) is not linear with $T$ at low temperature, as in the case of free vortices, but rather is exponential due to pinning of vortex cores, and (iv) the Berezinski-Kosterlitz-Thouless (BKT) transition temperature occurs at the point where $Y$ = (${l}_{\ensuremath{\omega}}$/${\ensuremath{\xi}}_{+}$)${}^{2}$ = 1. Our experimental results can be described well by the extended dynamic theory of the BKT transition and the dynamics of bound vortex-antivortex pairs with short separation lengths.

Raman scattering in InAs/AlGaAs quantum dot nanostructures

We report on Raman scattering experiments on InAs/AlxGa1−xAs quantum dot heterostructures with 0≤x≤0.6. The samples were prepared by using molecular beam epitaxy (MBE) and atomic layer MBE for the growth of different layers. For x>0, we detected several lines originating from the AlxGa1−xAs alloy. These can be related to scattering from GaAs-like and AlAs-like phonons with q≅0, and weaker scattering from disorder-activated phonons with q≠0. In particular, we identified a line at ∼250 cm−1 as due to disorder-activated longitudinal optical phonons in the alloy. This conclusion is different than the attribution of this line to scattering from dots and, consequently, we do not recognize the possibility of deriving any information about the actual composition of the dots from an analysis of this line as proposed by other authors.

(lnP)5/(Ga0.47In0.53As)5 superlattice confined 1.5 µm multiquantum well laser grown by all-solid source atomic layer molecular beam epitaxy.

Room temperature laser emission near 1.55 µm is obtained in compressive strained multiquantum well separate confinement heterostructure grown at 340°C by solid source Atomic Layer Molecular Beam Epitaxy, where (lnP)5/(Ga0.47In0.53As)5, lattice-matched short period superlattices have been used as pseudoquaternary barrier to confine Ga0.27In0.73As wells. These preliminary results show that solid source Atomic Layer Molecular Beam Epitaxy is well adapted to fabricate advanced optoelectronic components including pseudoquaternary material.

Optical Properties of Self-assembled InAs Quantum-dot Superluminescent Diodes

We report broad-band superluminescent diodes (SLDs) with different active layers using a selfassembled InAs quantum-dot (QD) structure grown by using atomic layer molecular beam epitaxy. The photoluminescence and the electroluminescence measurements show a relationship between the emission properties and different activelayer structures of the QD-SLD These results explain the possibility of QD-based SLDs exceeding the performance of quantumwell-based SLDs.

Initial stages of the autocatalytic oxidation of the InAs(0 0 1)-(4 × 2)/ c(8 × 2) surface by molecular oxygen

The initial stages of oxidation of the In-rich InAs(0 0 1)-(4 × 2)/ c(8 × 2) surface by molecular oxygen (O 2) were studied using scanning tunneling microscopy (STM) and density functional theory (DFT). It was shown that the O 2 dissociatively chemisorbs along the rows in the [1 1 0] direction on the InAs surface either by displacing the row-edge As atoms or by inserting between In atoms on the rows. The dissociative chemisorption is consistent with being autocatalytic: there is a high tendency to form oxygen chemisorption sites which grow in length along the rows in the [1 1 0] direction at preexisting oxygen chemisorption sites. The most common site size is about 21–24 Å in length at ∼25% ML coverage, representing 2–3 unit cell lengths in the [1 1 0] direction (the length of ∼5–6 In atoms on the row). The autocatalysis was confirmed by modeling the site distribution as non-Poisson. The autocatalysis and the low sticking probability (∼10 −4) of O 2 on the InAs(0 0 1)-(4 × 2)/ c(8 × 2) are consistent with activated dissociative chemisorption. The results show that is it critical to protect the InAs surface from oxygen during subsequent atomic layer deposition (ALD) or molecular beam epitaxy (MBE) oxide growth since oxygen will displace As atoms.

Improvement of InAs quantum dots optical properties in close proximity to GaAs(0 0 1) substrate surface

In this work we demonstrate a growth process for obtaining high optical emission efficiency InAs/GaAs(0 0 1) quantum dots (QD) formed at short distance to the interface with the GaAs substrate. In particular, after an initial exposure of the substrate surface to long times of atomic hydrogen flux ( t H up to 45 min) followed by a posterior growth of a GaAs buffer layer by atomic layer molecular beam epitaxy, both steps at low substrate temperature ( T S=450 °C), an enhancement of InAs QD optical emission efficiency is obtained, even at close proximity (3.5 nm) to the substrate interface. This process fulfils the strict requirements in terms of substrate temperature and buffer layer thickness (distance from the QD to the substrate interface) for its possible use as an optimal regrowth protocol on previously patterned GaAs substrates.

Room temperature Si δ-growth on Ge incorporating high-K dielectric for metal oxide semiconductor applications

A low temperature Al2O3∕4 monolayer amorphous Si gate stack process was demonstrated on p-type Ge wafers using atomic layer deposition and molecular beam epitaxy. Multifrequency capacitance-voltage (C-V) and current-voltage (I-V) characteristics showed excellent electrical properties of the Pt∕Al2O3∕4 ML Si∕Ge metal oxide semiconductor capacitor. No kinks from 1MHzto4kHz and a leakage current density of 2.6×10−6A∕cm2 at 1V with an equivalent oxide thickness of 2.5nm. The interface characterization using a conductance method showed that interface trap density at the near midgap was 8×1012eV−1cm−2 and a mean capture cross section of holes was extracted to be 10−16cm2.

Optical and structural properties ofIn0.5Ga0.5As quantum dots with different numbers of stacks grown by atomic layer molecular beamepitaxy: vertical realignment of weakly coupled quantum dots

We have investigated optical and structural properties of variousIn0.5Ga0.5As quantum dot (QD) structures as a function of stacking number, grown by atomic layer molecularbeam epitaxy (ALE). We found that the excitation power and temperature dependence ofwell-separated two emissions from 10 stacked InGaAs QD samples are different from those ofother QDs with the stacking numbers of 1, 3 and 5. Although the GaAs spacer thickness is∼35 nm at which the strain field penetration can be ignored for Stranski–Krastanovmode-grown InAs QDs, our ALE-grown QDs are influenced by the strain field penetrationdue to the larger size of ALE QDs. From transmission electron microscopic images, weobserved that the ten stacked QD sample has a complex QD size distributionpredominantly due to vertical size variation with stacking and that upper stacked (5–10)QD layers are vertically realigned due to the merged strain field penetration betweenlaterally coupled QDs.

Quantum dot strain engineering of InAs∕InGaAs nanostructures

We present a complete study both by experiments and by model calculations of quantum dot strain engineering, by which a few optical properties of quantum dot nanostructures can be tailored using the strain of quantum dots as a parameter. This approach can be used to redshift beyond 1.31μm and, possibly, towards 1.55μm the room-temperature light emission of InAs quantum dots embedded in InGaAs confining layers grown on GaAs substrates. We show that by controlling simultaneously the lower confining layer thickness and the confining layers’ composition, the energy gap of the quantum dot material and the band discontinuities in the quantum dot nanostructure can be predetermined and then the light emission can be tuned in the spectral region of interest. The availability of two degrees of freedom allows for the control of two parameters, which are the emission energy and the emission efficiency at room temperature. The InAs∕InGaAs structures were grown by the combined use of molecular beam epitaxy and atomic layer molecular beam epitaxy; their properties were studied by photoluminescence and photoreflectance spectroscopies and by atomic force microscopy; in particular, by means of photoreflectance not only the spectral features related to quantum dots were studied but also those of confining and wetting layers. The proposed approach has been used to redshift the room-temperature light emission wavelength up to 1.44μm. The optical results were analyzed by a simple effective-mass model that also offers a rationale for engineering the properties of structures for efficient long-wavelength operation.

Emission wavelength trimming of self-assembled InGaAs/GaAs quantum dots with GaAs/AlGaAs superlattices by rapid thermal annealing

The effects of rapid thermal annealing on InGaAs quantum dots grown by atomic layer molecular beam epitaxy to the structural transformation and optical properties are investigated. No misfit dislocation was observed from either the as-grown or annealed dots. The size and composition of the quantum dots become more uniform upon annealing mainly from the height fluctuation as predicted by the theoretical model. Large bandgap blue shifts, resulted from the In and Ga interdiffusion, were observed with the preservation of three-dimensional carrier confinement. The GaAs/AlGaAs superlattice was found to minimize the defect diffusion and dot interdiffusion during the high-temperature epitaxial overgrowth.

Self-assembled InAs quantum wire lasers on (001)InP at 1.6μm

In this work, the authors present results on the growth by atomic layer molecular beam epitaxy and characterization of lasers with one and three stacked layers of InAs quantum wires (QWRs) as active zone and aluminum-free waveguides on (001) InP substrates. The separated confinement heterostructure consists of n-p InP claddings and a waveguide formed by short period superlattices of (InP)5∕(GaInAs)4 lattice matched to the InP substrate. The optimum growth conditions (substrate temperature and As and P pressures) have been determined to obtain waveguides with a flat surface in order to get a uniform QWR distribution. Lasing emission is observed at a wavelength of ∼1.66μm up to 270K from 15×3000μm2 devices, with a threshold current density at that temperature of 2kA∕cm2.

In As/GaAs quantum dot lasers with dots in an asymmetric [formula omitted] quantum well structure

We report the advantages of using InAs/GaAs quantum dots (QDs) having In x Ga 1 - x As asymmetric strain-released layers (ASRL) over the conventional InAs/GaAs QDs in long wavelength operation. Atomic layer molecular beam epitaxy was used to enhance the uniformity of InAs QDs in an In x Ga 1 - x As quantum well structure. The red shift as large as ∼ 50 nm could be achieved by varying the thickness and indium composition of the In x Ga 1 - x As ASRL. We observed the longest wavelength of 1288 nm produced by the InAs/GaAs QD with ASRL by photoluminescence (PL). However, the stimulated emission gave the longest wavelength of 1206 nm, blue-shifted as large as ∼ 82 nm from the PL peak at room temperature, which is attributed to the optical transitions via higher sub-band levels of the InAs/GaAs QDs.