Molecular Beam Epitaxy Growth Research Articles

Influence of strain on the indium incorporation in (0001) GaN

The incorporation of indium in GaN (0001) surfaces in dependence of strain is investigated by combining molecular-beam epitaxy (MBE) growth, quantitative transmission electron microscopy, and density-functional theory (DFT) calculations. Growth experiments were conducted on GaN, as well as on $30%\ifmmode\pm\else\textpm\fi{}2%$ partially relaxed $\mathrm{I}{\mathrm{n}}_{0.19}\mathrm{G}{\mathrm{a}}_{0.81}\mathrm{N}$ buffer layers, serving as substrates. Despite the only $0.6%$ larger in-plane lattice constant of GaN provided by the buffer layer, our experiments reveal that the In incorporation increases by more than a factor of two for growth on the $\mathrm{I}{\mathrm{n}}_{0.19}\mathrm{G}{\mathrm{a}}_{0.81}\mathrm{N}$ buffer, as compared to growth on GaN. DFT calculations reveal that the decreasing chemical potential due to the reduced lattice mismatch stabilizes the In--N bond at the surface. Depending on the growth conditions (metal rich or N rich), this promotes the incorporation of higher In contents into a coherently strained layer. Nevertheless, the effect of strain is highly nonlinear. As a consequence of the different surface reconstructions, growth on relaxed $\mathrm{I}{\mathrm{n}}_{x}\mathrm{G}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{N}$ buffers appears more suitable for metal-rich MBE growth conditions with regard to achieving higher In compositions.

Optimization of a carbon evaporator cell for MBE growth

A new design of carbon source using a S-shaped glassy carbon filament for p-doping and carbon deposition is presented. Due to the higher resistivity of the glassy carbon, lower current is needed to produce the same flux as in a source based on pyrolytic graphite filament. The source design is very compact and the use of water cooling is not mandatory. The results and simulations of the new design show a higher flux than the previously reported glassy carbon based source while avoiding hot spots that can cause an early filament degradation. Hole mobility vs doping level relation data on p-doped GaAs layers grown by molecular beam epitaxy using this new carbon source are shown.With this new C source GaAs (001) surface morphology flatness is preserved after depositing C, showing a reduced substrate heating radiation essential for operate in III-V solid source molecular beam epitaxy (MBE) systems.

Epitaxial registry and crystallinity of MoS2 via molecular beam and metalorganic vapor phase van der Waals epitaxy

Two-dimensional transition metal dichalcogenide (TMD) semiconductors have risen as an important material class for novel nanoelectronic applications. Molybdenum disulfide (MoS2) is the most representative TMD compound due to its superior stability and attractive properties for (opto-) electronic devices. However, the synthesis of single-crystalline and functional MoS2 across large-area substrates remains crucial for its successful integration in semiconductor industry platforms. Therefore, this work focuses on the study of MoS2 epitaxy via two well-established industry-compatible synthesis methods, promising for the large-area and single-crystalline integration of van der Waals (vdW) materials. These methods are molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) and have studied MoS2 quasi-vdW heteroepitaxy on reconstructed sapphire substrates and MoS2 vdW homoepitaxy on exfoliated MoS2 flakes. By examining the MoS2 structural properties using diffraction and spectroscopy techniques, the epitaxial relation and crystal quality are assessed, which reveals insights into the prevalence of inter- and intragrain defects such as grain boundaries and sulfur vacancies. The MBE method yields superior epitaxial MoS2 registry on both sapphire and MoS2 surfaces as compared to MOVPE, although inferior defectivity arises from the typical lower MBE growth temperature and chalcogen partial pressure. Moreover, both synthesis methods generate high densities of twinned MoS2 grain boundaries, which hamper defect-free integration. As a result, this challenging integration might become an important bottleneck for industrial TMD-based applications with a low tolerance for material defects.

Strain-driven quantum dot self-assembly by molecular beam epitaxy

Research into self-assembled semiconductor quantum dots (QDs) has helped advance numerous optoelectronic applications, ranging from solid-state lighting to photodetectors. By carefully controlling molecular beam epitaxy (MBE) growth parameters, we can readily tune QD light absorption and emission properties to access a broad portion of the electromagnetic spectrum. Although this field is now sufficiently mature that QDs are found in consumer electronics, research efforts continue to expand into new areas. By manipulating MBE growth conditions and exploring new combinations of materials, substrate orientations, and the sign of strain, a wealth of opportunities exist for synthesizing novel QD nanostructures with hitherto unavailable properties. As such, QDs are uniquely well positioned to make critical contributions to the development of future quantum technologies. In this tutorial, we summarize the history of self-assembled QDs, outline some examples of quantum optics applications based on QDs, discuss the science that explains the spontaneous formation of QDs, and provide recipes for successful QD growth by MBE for some of the most commonly used semiconductor materials systems. We hope that compiling this information in one place will be useful both for those new to QD self-assembly and for experienced researchers, ideally supporting the community’s efforts to continue pushing the boundaries of knowledge in this important field.

Precise control of atoms with MBE: from semiconductors to complex oxides

Molecular Beam Epitaxy (MBE) is a high-vacuum technique with atomic-layer control and precision. It is based on the chemical reaction of the atoms, molecules, or atomic clusters vaporized from the specific evaporation sources on the substrates. The molecular beam defines a unidirectional ballistic flow of atoms and/or molecules without any collisions amongst. In the late 1960s, MBE was initially developed for the growth of GaAs and (Al, Ga)As systems[1,2] due to the unprecedented capabilities and then was applied to study other material systems. MBE growth is conventionally performed in vacuum and ultra-high vacuum (UHV) (10-8–10-12 mbar) conditions.

Electronic and magnetic characterization of epitaxial VSe2 monolayers on superconducting NbSe2

There has been enormous recent interest in heterostructures of two-dimensional van der Waals materials. Integrating materials with different quantum ground states in vertical heterostructures is predicted to lead to novel electronic properties that are not found in the constituent layers. Here, we present direct synthesis of a superconductor-magnet hybrid heterostructure by combining superconducting niobium diselenide (NbSe2) with the monolayer vanadium diselenide (VSe2). Molecular-beam epitaxy growth in ultra-high vacuum yields clean and atomically sharp interfaces. Combining different characterization techniques and density-functional theory calculations, we investigate the electronic and magnetic properties of VSe2 on NbSe2. Low temperature scanning tunneling microscopy measurements show an absence of the typical charge density wave on VSe2 and demonstrate a reduction of the superconducting gap of NbSe2 on the VSe2 layer. This suggests magnetization of the VSe2 sheet, at least on the local scale. Our work demonstrates superconducting-magnetic hybrid materials with potential applications in future electronics devices.

High nitrogen flux plasma-assisted molecular beam epitaxy growth of InxGa1-xN films

Growth of efficient III-N based light emitting devices by plasma assisted molecular beam epitaxy has been elusive, even though the technique has attractive advantages in comparison to metal organic chemical vapor deposition. Modern high-flux radio frequency plasma systems could remedy this issue by enabling growth of InxGa1-xN at higher temperatures than previously possible, likely improving the material quality. In this work, active nitrogen fluxes of up to 3.5 μm/h GaN-equivalent growth rate were employed to grow InxGa1-xN alloys. InxGa1-xN growth rates of 1.3 μm/h were demonstrated at growth temperatures of 550 °C and 600 °C with maximum film compositions of In0.25Ga0.75N and In0.21Ga0.79N, respectively. A composition of In0.05Ga0.95N was observed in a film grown at 700 °C with smooth step-terrace morphology.

Crystal field effects on the photoemission spectra in Cr2O3 thin films: From multiplet splitting features to the local structure

Changes in the shape of X-ray photoemission (XPS) spectra can be related to changes in the local structure of a transition metal. By combining Crystal Field Multiplet calculations and well-controlled molecular beam epitaxy growth of α-Cr2O3(0001) thin films on α-Al2O3(0001) substrates, we prove that it is possible to link the features of Cr 2p XPS spectra with local distortions of CrO6 octahedra and d-orbitals reorganization. Hence, we show that the splitting of the Cr 2p3/2 envelope is related to the degeneracy of the t2g orbital triplet, which corresponds to a fully relaxed structure. Conversely, the broad unstructured Cr 2p3/2 envelope relies on splitting of t2g orbitals and it is the fingerprint of large trigonal distortions of CrO6 octahedra. Then, using the Cr 2p XPS as a structural tool for α-Cr2O3, we show that the Cr2O3 protective layer formed by oxidation of polycrystalline Ni30Cr alloy exhibits in-plane strains at early oxidation stages and grows preferentially along the c-axis.

Initial stage of MBE growth of MoSe2 monolayer

An atomically thin MoSe2 layer has been synthesized on mica using molecular beam epitaxy (MBE). The polymorphous of the MoSe2 layer depends on the coverage and the growth temperature. At low coverages and low growth temperature, 1T-MoSe2 forms in addition to a comparable quantity of 2H-MoSe2. The metastable 1T-MoSe2 transfers gradually to the stable 2H-MoSe2 before the completion of the first monolayer. The current result sheds some light on the complexity of the nucleation and growth of transition metal dichalcogenide (TMDC) monolayers and implies a possible route for a phase selective synthesis using MBE.

Formation of laterally ordered quantum dot molecules by in situ nanosecond laser interference

We demonstrate the growth and surface characterization of laterally ordered arrays of InAs quantum dot molecules (QDMs) on GaAs (100) substrates produced by a combination of in situ interferometric nanopatterning and molecular beam epitaxy growth. Four-beam ultraviolet laser interference is applied during the growth process resulting in the formation of quasi two-dimensional islands due to localized surface diffusion. With further InAs deposition, the edges of the islands are observed to act as preferential sites for the nucleation of InAs quantum dots. Well-ordered square arrays of lateral QDMs with a period of 300 nm and site occupancy ranging from single dot up to hexa-molecules are obtained by varying the InAs coverage from 1.55 ML to 1.75 ML.

Molecular beam epitaxy growth and surface structure of Sr1−xNdxCuO2 cuprate films

We report on the epitaxial growth and surface structure of infinite-layer cuprate Sr1-xNdxCuO2 films on SrTiO3(001) substrates by combining ozone-assisted molecular beam epitaxy and in situ scanning tunneling microscopy. Careful substrate temperature and flux control has been used to achieve single-phase, stoichiometric, and c-axis oriented films. The surface of the films is usually characterized by a mixed CuO2 surface and gridlike superstructure. The superstructure exhibits a periodicity of 3.47 nm that corresponds to a coincidence lattice between the overlayer peroxide SrO2 and underlying CuO2 plane, and gives rise to a conductance spectrum that is distinct from the Mott-Hubbard band structure of CuO2. At a higher Nd composition x > 0.1, a (2 x 2) surface characteristic of the hole-doped CuO2 emerges, which we ascribe to the intake of apical oxygens in the intervening Sr planes.

Classification of Reflection High-Energy Electron Diffraction Pattern Using Machine Learning

Reflection high-energy electron diffraction (RHEED) has wide application because it allows in situ observation of the sample surface behavior during molecular beam epitaxy growth. In particular, the RHEED pattern has been used as a milestone for growth condition calibration because it dynamically changes depending on the sample temperature, material supply rate, and supply ratio. However, RHEED pattern analysis depends on the accumulated know-how of the operator and has a time limitation; thus, its application to real-time feedback control is difficult. Moreover, with the conventional computerization method, it is difficult to correctly reflect and recognize the changes in RHEED due to changes in the observation conditions. On the other hand, the machine learning method using the convolutional neural network (CNN) recognizes feature points in the input database and is suitable for the classification of images with variability. In this study, we propose a measurement method for identifying the RHEED pattern of GaAs substrates during continuous rotation and build a data set of the growth conditions. A classification model is established by training the deep learning model using CNN, and is found to be more than 99% accurate. This is expected to be useful in the field of highquality III–V growth on GaAs.

Band engineering in epitaxial monolayer transition metal dichalcogenides alloy MoxW1−xSe2 thin films

The direct bandgap transition and spin–orbit-coupling-induced spin-splitting in monolayer transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) show great application potential in high-efficient optoelectronic devices and valleytronics and, thus, have attracted enormous research interest in recent years. Various MX2 monolayers usually show a distinct bandgap and spin-splitting size. Here, we realized the molecular beam epitaxial growth of monolayer MoxW1−xSe2 alloys with a controllable stoichiometric ratio x. Combining with the in situ angle-resolved photoemission spectroscopic and x-ray photoemission spectroscopic measurements, we determined the evolution of the valence band dispersion and the spin-splitting size with the change in the Mo ratio x. We found that the energy difference of both the valence band between the Γ and K points and the spin-splitting size at the K point reduce monotonically with the increasing Mo ratio x. The growth of MoxW1−xSe2 monolayer alloys and the method to control the stoichiometric ratio of Mo/W atoms provide an effective way to engineer the band structures in the two-dimensional MX2 materials.

Aluminum metallization of III–V semiconductors for the study of proximity superconductivity

Managing the interaction of materials with insertion layers and nonconventional molecular beam epitaxy growth conditions allows for interfaces that are more precise but requires judicious examination of the multiple possible design variables. Here, we show a comparison between As- and Sb-containing insertion layers between Al and two binaries with different group V elements and demonstrate that antimonide layers greatly improve the interface. In addition to depositing Al at extremely slow growth rates onto cold (below 0 °C) substrates, the reactivity is particularly minimized with AlSb insertion layers, which improves interface abruptness, preserves the underlying semiconductor layer’s crystalline properties, and produces flatter superconductor surfaces.

Molecular beam epitaxy growth of the highly conductive oxide SrMoO3

SrMoO3 is a promising material for its excellent electrical conductivity, but growing high-quality thin films remains a challenge. Here we synthesized epitaxial films of SrMoO3 using molecular beam epitaxy (MBE) technique under low oxygen-flow rate. Introduction of SrTiO3 buffer layers of 4–8 unit cells between the film and the (0 0 1)-oriented SrTiO3 or KTaO3 substrate was crucial to remove impurities and/or roughness of the film surface. The obtained film shows improved electrical conductivities as compared with films obtained by other techniques. The high quality of the SrMoO3 film is also verified by angle resolved photoemission spectroscopy (ARPES) measurements showing a clear Fermi surface.

Precise Arrays of Epitaxial Quantum Dots Nucleated by In Situ Laser Interference for Quantum Information Technology Applications.

Precisely ordered arrays of InAs quantum dots are formed on a nanoisland-structured GaAs (100) surface using in situ laser interference during self-assembled molecular beam epitaxial growth. Nanoislands induced by single-pulse four-beam laser interference act as preferential nucleation sites for InAs quantum dots and result in site occupation dependent on the size of nanoislands, the InAs coverage, and the laser parameters. By optimizing the growth and interference conditions, regular dense ordering of single dots was obtained for the first time using this in situ noninvasive approach. The photoluminescence spectra of the resulting quantum dot arrays with a period of 300 nm show good optical quality and uniformity. This technique paves the way for the rapid large-scale fabrication of arrays of single dots to enable quantum information technology device platforms.

Improved epitaxial growth of TbAs film on III–V semiconductors

In order to achieve high epitaxial quality of rocksalt TbAs, the authors studied the molecular beam epitaxy growth of TbAs films on zincblende (001) GaAs and (001) InP:Fe wafers. Despite the opposite strain condition of TbAs on these two substrates, mixed-orientation TbAs growth was observed on both substrates. However, the nucleation time and the continuing growth of the TbAs misoriented domains were influenced by the substrate type. By suppressing the growth of misoriented domains in the TbAs film, enhanced single-crystal orientation of TbAs grown on the (001) InP:Fe substrate was observed as compared to the (001) GaAs substrate. In addition, the cube-on-cube epitaxial arrangement of (001) TbAs with a thick film of up to ∼1150 nm is maintained on the (001) InP:Fe substrate but not on the (001) GaAs substrate. The improved TbAs film growth on the InP:Fe substrate exhibited enhanced optical properties when compared to that grown on the GaAs substrate, including a threefold reduction in the scattering rate. This largely improved optical property highlights the importance of increasing the epitaxial quality of TbAs films for future optoelectronic as well as other applications.

Influence of strain on the InAs1 – xSbx composition

The composition of III–V semiconductor alloys with multiple group V elements results from a complex interaction of each group V species with each other and with group IIIs. Molecular beam epitaxy growth conditions, such as the group V absolute fluxes and flux ratios, substrate temperature, group III growth rate, the presence of surfactants, and the alloy's strain state, all affect the composition of InAs1 – xSbx. These factors are codependent in a manner that is far from completely established. In this work, the authors examine how the sign and degree of strain affects the film’s composition. In this study, the authors show that InAs1 – xSbx alloys have some ability to resist the creation of strain by self-adjusting the incorporation of the group V elements that would otherwise be changed in an unfavorable way (by, for example, a substrate temperature change). This self-latching to the substrate lattice constant is beneficial, since it means that the fluxes may not have to be adjusted as precisely as otherwise needed.

New approach for the molecular beam epitaxy growth of scalable WSe2 monolayers

The search for high-quality transition metal dichalcogenides mono- and multi-layers grown on large areas is still a very active field of investigation. Here, we use molecular beam epitaxy to grow WSe2 on 15 × 15 mm large mica in the van der Waals regime. By screening one-step growth conditions, we find that very high temperature (>900 °C) and very low deposition rate (<0.15 Å min−1) are necessary to obtain high quality WSe2 films. The domain size can be as large as 1 µm and the in-plane rotational misorientation of 1.25°. The WSe2 monolayer is also robust against air exposure, can be easily transferred over 1 cm2 on SiN/SiO2 and exhibits strong photoluminescence signal. Moreover, by combining grazing incidence x-ray diffraction and transmission electron microscopy, we could detect the presence of few misoriented grains. A two-dimensional model based on atomic coincidences between the WSe2 and mica crystals allows us to explain the formation of these misoriented grains and gives insight to achieve highly crystalline WSe2.

Luminescence Properties of GaN/InxGa1−xN/InyGa1−yN Double Graded Structures (Zigzag Quantum Wells)

We have designed graded InGaN quantum well (QW) structures with the In composition increasing then decreasing in a zigzag pattern. Through polarization doping, this naturally creates a p–n junction. Separate structures are designed by varying the maximum In composition but maintaining a constant QW thickness. This is both in order to test the limits of the molecular beam epitaxy growth control in terms of the deposition source ramping rates and to determine the limits of the maximum In composition within a narrow QW. The composition, x, of InxGa1−xN is varied for all structures starting from a minimum of xmin = ∼ 3%, and increasing to different maximum compositions, xmax, then finally decreasing back to ∼ 3%. The samples are characterized for their structural and luminescent properties. The results of the photoluminescence studies on samples with xmax ranging up to 35% demonstrate broadband emission covering wavelengths between 380 nm and 700 nm. Additionally, although the general emission of structures with increasing xmax shifts to lower energy, the observed luminescence is comprised of several individual peaks which are currently being investigated as to their particular origin.