Molecular Beam Epitaxy Research Articles

Recent Advances in the Growth and Compositional Modelling of III–V Nanowire Heterostructures

Nanowire heterostructures offer almost unlimited possibilities for the bandgap engineering and monolithic integration of III–V photonics with Si electronics. The growth and compositional modelling of III–V nanowire heterostructures provides new insight into the formation mechanisms and assists in the suppression of interfacial broadening and optimization of optical properties. Different models have been proposed in the past decade to calculate the interfacial profiles in axial nanowire heterostructures mainly grown by molecular beam epitaxy and metal–organic vapour phase epitaxy. Based on various assumptions, existing models have different sets of parameters and can yield varying results and conclusions. By focusing on deterministic models based on classical nucleation theory and kinetic growth theory of III–V ternary monolayers in nanowires, we summarize recent advancements in the modelling of axial heterostructures in III–V nanowires, describe and classify the existing models, and determine their applicability to predictive modelling and to the fitting of the available experimental data. In particular, we consider the coordinate-dependent generalizations of the equilibrium, nucleation-limited, kinetic, and regular growth models to make interfacial profiles across axial heterostructures in different III–V nanowires. We examine the factors influencing the interfacial abruptness, discuss the governing parameters, limitations, and modelling of particular material systems, and highlight the areas that require further research.

Memristor Array Based on Wafer-Scale 2D HfS2 for Dual-Mode Physically Unclonable Functions.

Conventional Si-based physically unclonable functions (PUFs) take advantage of the unique variations in the fabrication processes. However, these PUFs are hindered by limited entropy sources and susceptibility to noise interference. Here we present a memristive device based on wafer-scale (2-in.) two-dimensional (2D) hafnium disulfide (HfS2) grown by molecular beam epitaxy (MBE). The polycrystalline HfS2 thin film can offer enhanced entropy sources for PUF applications, such as lattice defects, which can facilitate the random formation of conductive filaments and result in significant device-to-device (D2D) variations. Our proposed PUF design seamlessly integrates two distinct operating modes within a single circuit module. First, a reconfigurable and highly secure mode 1, and second, an ultrareliable mode 2, both with near-ideal Entropy (∼1.0), normalized Hamming distance (∼0.5) and correlation coefficient (∼0.0). Additionally, a predictive Fourier regression model further confirms the unpredictable nature of our dual-mode PUF, with an average prediction accuracy of ∼50%.

Tuning the Magnetism in Ultrathin CrxTey Films by Lattice Dimensionality

Abstract2D magnetic crystals with atomic thickness exhibit intriguing physical properties, which have attracted considerable research interest in the related materials’ family, both in fundamental research and in developing spintronic devices. The recent discovery of some non‐van der Waals 2D magnetic crystals expands the systems. Nevertheless, the relationship between the dimensionality of microscopic magnetic exchange interactions and macroscopic magnetic properties at the 2D limit remains to be fully elucidated. Here, we have fabricated mono‐phased continuous ultrathin CrTe2 and Cr3Te4 films by molecular beam epitaxy and elucidated the diverse magnetism tuned by the dimensionality of exchange interactions by a joint study of spin‐polarized scanning tunneling microscopy, magnetization, magneto‐transport measurements, and density functional theory calculations. The transition from a zigzag‐antiferromagnetic order in the monolayer CrTe2 to a ferromagnetic (FM) order in the second‐layer CrTe2 is confirmed, which is driven by their varied in‐plane lattice constants induced change of 2D exchange interactions. A robust FM state with large perpendicular magnetic anisotropy in Cr3Te4 is observed, originating from its strong 3D exchange interactions. The observed evolution of magnetism demonstrates that the dimensionality of magnetic exchange interactions strongly influences magnetism even at the 2D limit.

Colossal Strain Tuning of Ferroelectric Transitions in KNbO3 Thin Films.

Strong coupling between polarization (P) and strain (ɛ) in ferroelectric complex oxides offers unique opportunities to dramatically tune their properties. Here colossal strain tuning of ferroelectricity in epitaxial KNbO3 thin films grown by sub-oxide molecular beam epitaxy is demonstrated. While bulk KNbO3 exhibits three ferroelectric transitions and a Curie temperature (Tc) of ≈676K, phase-field modeling predicts that a biaxial strain of as little as -0.6% pushes its Tc >975K, its decomposition temperature in air, and for -1.4% strain, to Tc >1325K, its melting point. Furthermore, a strain of -1.5% can stabilize a single phase throughout the entire temperature range of its stability. A combination of temperature-dependent second harmonic generation measurements, synchrotron-based X-ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal a single tetragonal phase from 10 K to 975K, an enhancement of ≈46% in the tetragonal phase remanent polarization (Pr),and a ≈200% enhancementin its optical second harmonic generation coefficients over bulk values. These properties in a lead-free system, but with properties comparable or superior to lead-based systems, make it an attractive candidate for applications ranging from high-temperature ferroelectric memory to cryogenic temperature quantum computing.

Epitaxial growth of Ge1−x Sn x thin film with Sn composition of 50% and possibility of Ge-Sn ordered bonding structure formation

Abstract We examined the epitaxial growth of Ge1−x Sn x thin film with x~0.50 on GaSb(111) substrate using a low-temperature molecular beam epitaxy. X-ray diffraction and Raman scattering spectroscopy analyses revealed that the Ge1−x Sn x thin film with x~0.50 with no strain was successfully grown on GaSb(111). Furthermore, in the Raman analysis, Ge1−x Sn x with x~0.50 showed a sharper Ge-Sn peak compared to Ge-Ge and Sn-Sn peaks, and an improved thermal stability than Ge1−x Sn x with a smaller Sn composition. This suggests that the Ge1−x Sn x with x~0.50 would have an ordered Ge-Sn bonding like a zinc blend system, that is significantly different from Si1−x Ge x case.

Molecular beam epitaxy and band structures of type-II antiferromagnetic semiconductor EuTe thin films

The rare-earth Eu-based compounds with a unique half-filled 4f orbital have attracted an amount of research interest recently. Here, we synthesized EuTe(001) single-crystal thin films on SrTiO3(001) substrate via molecular beam epitaxy (MBE). The scanning tunneling microscopy and x-ray diffraction results indicate that the grown EuTe thin films orientated as EuTe[100]//SrTiO3[110] in plane. In the angle-resolved photoemission spectroscopic (ARPES) measurements, the grown EuTe films show a semiconductive band structure with the valence band maximum lying on the center point of the Brillouin zone. The bandgap size of EuTe was further identified by the optical transmission spectra as 2.2 eV. The antiferromagnetic transition temperature of the grown EuTe film is 10.5 K measured by a superconductive quantum interference device (SQUID). Our results provide important information on the fundamental electronic structures for the further research and applications of the Eu-based compounds.

Broadband cavity-enhanced optical flux monitoring

This work describes a new type of sensor for growth process monitoring named broadband cavity-enhanced optical flux monitoring sensor (BBCE-OFM). Like existing optical flux monitoring (OFM) solutions, it relies on absorption spectroscopy. However, the implementation of an optical cavity reduces the measurement uncertainty, enabling efficient operation even at very low growth rates. Using the BBCE-OFM sensor mounted in our solid-source oxide molecular beam epitaxy reactor, we achieved an uncertainty of ±2% on the measurement of Sr and Ti growth rates in SrTiO3 at around 1 Ml/min, to be compared to the ±16% obtained in the same conditions using a conventional OFM setup. Furthermore, our sensor architecture, based on an echelle monochromator and LEDs replacing the hollow cathode lamps used in standard OFM sensors, is more robust against drift.

Reduction of the Threading Dislocation Density in GaSb Layers Grown on Si(001) by Molecular Beam Epitaxy

AbstractThe monolithic integration of III‐V semiconductors on Si emerges as a promising approach for realizing photonic integrated circuits. However, the performance and reliability of epitaxially grown devices on Si are hampered by the threading dislocation density (TDD) generated during the growth. In this study, the efficiency of a structure, combining III‐Sb‐based insertion layers and thermal annealing is evaluated, on the reduction of the emerging TDD in GaSb buffer layers grown on Si(001) substrates by molecular beam epitaxy. the impact of the thickness, composition, and number of the insertion layers is extensively explored. Then a detailed study of the annealing cycles with different conditions is conducted. A record TDD in the low 107 cm−2 for a 2.25 µm GaSb buffer grown on Si(001) is ultimately demonstrated.

Boosting the two-dimensional electron gas density of Al0.20Ga0.80N/GaN heterostructures by regrowth of an epitaxial Sc0.18Al0.82N layer

Sc-alloyed AlN epilayers were grown on commercial GaN/sapphire templates by molecular beam epitaxy (MBE) to validate the Sc composition of 18% and in-plane lattice matching with GaN. As-grown Sc0.18Al0.82N/GaN heterostructure exhibits a two-dimensional electron gas (2DEG) density of 3.58 × 1013 (2.96 × 1013) cm−2 and a mobility of 241 (295) cm2⋅V−1⋅s−1 at 300 (77) K, demonstrating the feasibility of employing its high spontaneous polarization toward high 2DEG density and highlighting detrimental impurity scattering due to the regrowth interface. Implementation of AlN impurity blocking layers boosts 2DEG mobility to 1290 (8730) cm2 V−1⋅s−1at 300 (77) K. In addition, we have engineered a surface-treatment strategy to selectively decompose the 2.5-nm-thick GaN cap layer of commercial GaN(2.5 nm)/Al0.20Ga0.80N(22 nm)/GaN heterostructure epi-wafers at high temperature prior to MBE regrowth of Sc0.18Al0.82N to eliminate impurity incorporation at the regrowth interface. Regrowth of a Sc0.18Al0.82N layer on the pristine Al0.20Ga0.80N surface increases 2DEG density from 7.89 × 1012 to 9.57 × 1012 cm−2, together with a slight reduction in mobility from 2160 to 1970 cm2⋅V−1⋅s−1 at 300 K, reducing the sheet resistance by 10%. Our calculation implies that it is practical to boost 2DEG density to over 2.0 × 1013 cm−2 by thinning Al0.20Ga0.80N down to 4 nm.

Germanium surface segregation in highly doped Ge:β-Ga2O3 grown by molecular beam epitaxy observed by synchrotron radiation hard x-ray photoelectron spectroscopy

We present a study of Ge segregation at the surface of highly germanium-doped gallium oxide (2.5 × 1020 cm−3 nominal doping level) grown by molecular beam epitaxy. We probed the dopant concentration as a function of depth by hard x-ray photoelectron spectroscopy and standard laboratory photoemission spectroscopy. We notably found that there is germanium segregation within the top 2 nm where its concentration is 3 times the nominal doping level. This increased dopant concentration leads to a threefold enhancement of surface conductivity. The results suggest a reliable method for delta doping for power electronics applications.

Two‐Dimensional Layered Topological Semimetals for Advanced Electronics and Optoelectronics

AbstractDue to the outstanding physical properties and the integrated characteristics, such as the gapless band structure, high state density, high conductivity, dangling‐bond‐free surface, and tunable Fermi level, two‐dimensional layered topological semimetals (2D LTSMs) exhibit a promising application prospect for including electrode contact and terahertz detection. Despite the surging in attention, a comprehensive strategy is still crucial to meet the necessary conditions for the practical application of 2D LTSMs. According to the fermion types and the band structures, 2D LTSMs can be divided into Dirac semimetals, Weyl semimetals, and nodal‐line semimetals. This review comprehensively encapsulates the intrinsic properties, typical synthesis methods, and applications in electronics and optoelectronics about these 2D LTSMs. To establish the fundamental theory, typical crystal structures, and physical properties of different 2D LTSMs are summarized at first. The effective synthesis strategies including exfoliation, molecular beam epitaxy, and vapor deposition are analyzed systematically. Finally, the article emphasizes the exploration of applications, significant challenges, and promising development directions of 2D LTSMs, which will drive 2D LTSMs to become the promisingly leading technology for next‐generation electronic and optoelectronic systems.

Molecular Beam Epitaxy Growth of Atomically Flat GaAs(111)A by As2

Molecular Beam Epitaxy Growth of Atomically Flat GaAs(111)A by As₂

High-quality InAs homoepitaxial layers grown by molecular beam epitaxy

The growth conditions for InAs homoepitaxy by molecular beam epitaxy were comprehensively studied across a broad spectrum of substrate temperatures, As2/In flux ratios, and growth rates. It was found that the surface morphology and overall quality of the InAs layers were significantly influenced by these parameters. Optimal conditions, including a lower growth temperature, reduced As2 flux, and slower growth rate, were pivotal in achieving high-quality InAs layers. Two primary characterization techniques, differential interference contrast microscopy and atomic force microscopy, were employed to evaluate the material quality. High-quality InAs homoepitaxial layers were successfully grown at a substrate temperature of 455 °C and a growth rate of 0.33 monolayers per second (ML/s). These layers exhibited a remarkably low defect density of approximately 300 defects per square centimeter, which is over an order of magnitude lower than previously reported, and a notably low root-mean-square roughness of 0.116 nm. At a growth rate of 0.33 ML/s, the growth temperature range for InAs homoepitaxial layers was found to be quite broad, whereas the As2/In flux ratio remained within a narrow range. This study underscores the critical role of precise control over growth parameters in the molecular beam epitaxy process for producing high-quality InAs homoepitaxial layers.

Polarity controlled ScAlN multi-layer transduction structures grown by molecular beam epitaxy

We report on the molecular beam epitaxial growth and characterization of polarity-controlled single and multi-layer Scandium Aluminum Nitride (ScAlN) transduction structures grown directly on ScAlN templates deposited by physical vapor deposition (PVD) on Si(001) substrates. It is observed that direct epitaxial growth on PVD N-polar ScAlN leads to the flipping of polarity, resulting in metal (M)-polar ScAlN. By effectively removing the surface impurities, e.g., oxides, utilizing an in situ gallium (Ga)-assisted flushing technique, we show that high quality N-polar ScAlN epilayers can be achieved on PVD N-polar ScAlN templates. The polarity of ScAlN is confirmed by utilizing polarity-sensitive wet chemical etching and atomic-resolution scanning transmission electron microscopy. Through interface engineering, i.e., the controlled formation or removal of surface oxides, we have further demonstrated the ability to epitaxially grow an alternating tri-layer piezoelectric structure, consisting of N-polar, M-polar, and N-polar ScAlN layers. Such multi-layer, polarity-controlled ScAlN structures promise a manufacturable platform for the design and development of a broad range of acoustic and photonic devices.

Metal-sown selective area growth of high crystalline quality InAsSb nanowires and networks by molecular-beam epitaxy

Metal-sown selective area growth of high crystalline quality InAsSb nanowires and networks by molecular-beam epitaxy

Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film.

Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X = Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems.

Flexible Near-Infrared Plasmon-Polaritons in Epitaxial Scandium Nitride Enabled by van der Waals Heteroepitaxy.

Van der Waals heteroepitaxy refers to the growth of strain- and misfit-dislocation-free epitaxial films on layered substrates or vice versa. Such heteroepitaxial technique can be utilized in developing flexible near-infrared transition metal nitride plasmonic materials to broaden their photonic and bioplasmonic applications, such as antifogging, smart windows, and bioimaging. Here, we show the first conclusive experimental demonstration of the van der Waals heteroepitaxy-enabled flexible semiconducting scandium nitride (ScN) thin films exhibiting near-infrared, low-loss epsilon-near-zero, and surface plasmon-polariton resonances. Deposited on fluorophlogopite-mica substrates with molecular beam epitaxy, polaritonic ScN heterostructures mark the first semiconducting nitride to exhibit plasmon resonance at the 1100-1250 nm spectral range, inside the biological transmission window. Interestingly, optical properties of such ScN exhibit remarkable stability even after bending more than 100 times. Creating low-cost and high-quality flexible yet refractory plasmonic ScN heterostructures for the near-infrared spectral range will advance flexible optics and bioplasmonic devices for practical applications.

Optimization of MBE-grown GaSb buffer on GaAs substrates for infrared detectors

The aim of this work was to improve the quality of the GaSb buffer layers on GaAs substrates using the molecular beam epitaxy (MBE) technology. The high quality of the GaSb buffer layers is one of the most important elements enabling the synthesis of good quality of type II superlattices (T2SL) structures for infrared applications. The main challenges in this regard are: compensation of the difference in lattice constants between GaAs and GaSb and obtaining the highest achievable surface quality of the final GaSb layer. In the literature, many authors describe different techniques to obtain the best quality of a GaSb buffer layer. In this work, we present the results of HRXRD, AFM, TOF-SIMS, SEM, and Nomarski optical microscope measurements obtained for 2 μm thick GaSb buffer layers. The GaSb layers are made according to different techniques and these results are compared with a GaSb buffer construction technique according to our own technology. During the processes, we also obtained an unintentional structure of one of the buffer layers, which allowed us to obtain very good results in terms of surface structure and crystallographic quality where FWHM in ωRC scan was equal to 138 arcsec and RMS 0.20 nm proving that there is still a lot of work to be done in this area.

Route to Enhancing Remote Epitaxy of Perovskite Complex Oxide Thin Films.

Remote epitaxy is taking center stage in creating freestanding complex oxide thin films with high crystallinity that could serve as an ideal building block for stacking artificial heterostructures with distinctive functionalities. However, there exist technical challenges, particularly in the remote epitaxy of perovskite oxides associated with their harsh growth environments, making the graphene interlayer difficult to survive. Transferred graphene, typically used for creating a remote epitaxy template, poses limitations in ensuring the yield of perovskite films, especially when pulsed laser deposition (PLD) growth is carried out, since graphene degradation can be easily observed. Here, we employ spectroscopic ellipsometry to determine the critical factors that damage the integrity of graphene during PLD by tracking the change in optical properties of graphene in situ. To mitigate the issues observed in the PLD process, we propose an alternative growth strategy based on molecular beam epitaxy to produce single-crystalline perovskite membranes.

Carrier Scattering Analysis in AlN/GaN HEMT Heterostructures with an Ultrathin AlN Barrier

Experimental AlN/GaN heterostructures (HSs) with an ultrathin AlN barrier were obtained using molecular beam epitaxy with plasma activation of nitrogen. The layer resistance of the optimized structures was less than 230 Ω/¨. The scattering processes that limit the mobility of two-dimensional electron gas in undoped AlN/GaN HSs with an ultrathin AlN barrier have been studied. It is shown that in the ns range characteristic of AlN/GaN HEMT HSs (ns 1 × 1013 cm–2), a noticeable contribution to the scattering of charge carriers is made by the roughness of the heterointerface.