Molecular Beam Epitaxy Growth Research Articles

Cross-sectional shape evolution of GaN nanowires during molecular beam epitaxy growth on Si(111).

We study the cross-sectional shape of GaN nanowires (NWs) by transmission electron microscopy. The shape is examined at different heights of long NWs, as well as at the same height for NWs of different lengths. Two distinct trends in the evolution of the cross-sectional shape along the NW length are observed. At the top, merging NWs develop common {11̄00} side facets. At the bottom, the NWs acquire roundish shapes. This observation is explained by the entirely different NW environments at the top and the bottom of the NWs. At the top, NWs are exposed to the Ga and N atomic fluxes giving rise to axial growth, resulting in the equilibrium growth shape with zero growth rate at the {11̄00} facets. At the bottom, NWs are shadowed from the impinging fluxes and are only annealed, allowing them to eventually approach the equilibrium crystal shape. The study of identical samples by grazing incidence small-angle X-ray scattering independently confirms these trends in the shape evolution of the sidewall facets.

Two-dimensional magnetic transition metal halides: molecular beam epitaxy growth and physical property modulation

The spontaneous magnetization of two-dimensional (2D) magnetic materials can be maintained down to the monolayer limit, providing an ideal platform for understanding and manipulating magnetic-related properties on a 2D scale, and making it important for potential applications in optoelectronics and spintronics. Transition metal halides (TMHs) are suitable 2D magnetic candidates due to partially filled d orbitals and weak interlayer van der Waals interactions. As a sophisticated thin film growth technique, molecular beam epitaxy (MBE) can precisely tune the growth of 2D magnetic materials reaching the monolayer limit. Moreover, combining with the advanced experimental techniques such as scanning tunneling microscopy, the physical properties of 2D magnetic materials can be characterized and manipulated on an atomic scale. Herein, we introduce the crystalline and magnetic structures of 2D magnetic TMHs, and show the 2D magnetic TMHs grown by MBE and their electronic and magnetic characterizations. Then, the MBE-based methods for tuning the physical property of 2D magnetic TMHs, including tuning interlayer stacking, defect engineering, and constructing heterostructures, are discussed. Finally, the future development opportunities and challenges in the field of the research of 2D magnetic TMHs are summarized and prospected.

Asymptotic stage of self-catalyzed growth of III-V nanowires by molecular beam epitaxy

A new analytic theory is developed for asymptotic stage of self-catalyzed growth of III-V nanowires (NWs) by molecular beam epitaxy (MBE), where NWs collect all group III atoms deposited from vapor. The shadowing NW length is derived which corresponds for the full shadowing of the substrate surface in MBE. The NW length and radius are derived depending on the effective deposition thickness and MBE growth parameters. It is shown that the NW length increases, and their length decreases with decreasing the array pitch and increasing the V/III flux ratio. Keywords: II-V nanowires, shadowing effect, length, radius, surface density, modeling

Асимптотическая стадия роста автокаталитических III-V нитевидных нанокристаллов методом молекулярно-пучковой эпитаксии

A new analytic theory is developed for asymptotic stage of self-catalyzed growth of III-V nanowires (NWs) by molecular beam epitaxy (MBE), where NWs collect all group III atoms deposited from vapor. The shadowing NW length is derived which corresponds for the full shadowing of the substrate surface in MBE. The NW length and radius are derived depending on the effective deposition thickness and MBE growth parameters. It is shown that the NW length increases, and their length decreases with decreasing the array pitch and increasing the V/III flux ratio.

Stripe phase in high-<i>T</i><sub>c</sub> superconductor FeSe/SrTiO<sub>3</sub>

The enhancement of superconductivity in one unit-cell FeSe grown on SrTiO<sub>3</sub> is an important discovery in high-temperature superconductivity. In this system, the crucial role of the SrTiO<sub>3</sub> substrate has been extensively studied. Its contribution mainly manifests in two aspects: charge transfer and interfacial electron-phonon coupling. However, study of the intrinsic properties of the FeSe thin film itself is still insufficient. In this article, we review the latest research progress of the mechanism of the enhancement of superconductivity in FeSe/SrTiO<sub>3</sub>, covering the newly discovered stripe phase and its relationship with superconductivity. By using scanning tunneling microscope and molecular beam epitaxy growth method, we find that the electrons in FeSe thin film tend to form stripe patterns, and show a thickness-dependent evolution of short-range to long-range stripe phase. The stripe phase, a kind of electronic liquid crystal state (smectic), originates from the enhanced electronic correlation in FeSe thin film. Surface doping can weaken the electronic correlation and gradually suppress the stripe phase, which can induce superconductivity as well. More importantly, the remaining smectic fluctuation provides an additional enhancement to the superconductivity in FeSe film. Our results not only deepen the understanding of the interfacial superconductivity, but also reveal the intrinsic uniqueness of the FeSe films, which further refines the mechanism of superconductivity enhancement in FeSe/SrTiO<sub>3</sub>.

GaAsBi: From Molecular Beam Epitaxy Growth to Devices

GaAsBi has been researched as a candidate material for optoelectronic devices for around two decades. Bi‐induced localized states induce a rapid rising of the valence band edge through a band anticrossing interaction, which has a profound effect on the bandgap and the spin–orbit splitting. The band engineering possible, even with just a few percent bismuth, makes GaAsBi an attractive material for THz emitters, telecommunication lasers, and low noise photodetectors, among other devices. There has been substantial progress in some of these areas; however, progress toward many of the potential applications of GaAsBi has been hindered by device quality issues, brought about by the low substrate temperatures necessary for the growth of GaAsBi with sufficiently large Bi fractions. This review, presents an overview of the applications for which GaAsBi has been advocated and the key results in these areas. The molecular beam epitaxy growth and postgrowth processing of GaAsBi are then explored as well as the novel techniques that have been suggested to improve material quality.

Monolayer 1T-NbSe2 as a 2D-correlated magnetic insulator.

Monolayer group V transition metal dichalcogenides in their 1T phase have recently emerged as a platform to investigate rich phases of matter, such as spin liquid and ferromagnetism, resulting from strong electron correlations. Newly emerging 1T-NbSe2 has inspired theoretical investigations predicting collective phenomena such as charge transfer gap and ferromagnetism in two dimensions; however, the experimental evidence is still lacking. Here, by controlling the molecular beam epitaxy growth parameters, we demonstrate the successful growth of high-quality single-phase 1T-NbSe2. By combining scanning tunneling microscopy/spectroscopy and ab initio calculations, we show that this system is a charge transfer insulator with the upper Hubbard band located above the valence band maximum. To demonstrate the electron correlation resulted magnetic property, we create a vertical 1T/2H NbSe2 heterostructure, and we find unambiguous evidence of exchange interactions between the localized magnetic moments in 1T phase and the metallic/superconducting phase exemplified by Kondo resonances and Yu-Shiba-Rusinov–like bound states.

InAs/GaAs quantum dot laser epitaxially grown on on-axis (001) GaAsOI substrate.

Quantum dot (QD) laser as a light source for silicon optical integration has attracted great research attention because of the strategic vision of optical interconnection. In this paper, the communication band InAs QD ridge waveguide lasers were fabricated on GaAs-on-insulator (GaAsOI) substrate by combining ion-slicing technique and molecular beam epitaxy (MBE) growth. On the foundation of optimizing surface treatment processes, the InAs/In0.13Ga0.87As/GaAs dot-in-well (DWELL) lasers monolithically grown on a GaAsOI substrate were realized under pulsed operation at 20 °C. The static device measurements reveal comparable performance in terms of threshold current density, slope efficiency and output power between the QD lasers on GaAsOI and GaAs substrates. This work shows great potential to fabricate highly integrated light source on Si for photonic integrated circuits.

Molecular beam epitaxial growth of GaAs/GaNAsBi core–multishell nanowires

GaAs/GaNAsBi/GaAs core–multishell nanowires were grown using molecular beam epitaxy on Si(111) substrates. The formation of the 20 nm wide GaNAsBi shell with a regular hexagonal structure was observed. The shell is estimated to contain approximately 1.5% N and 2.6% Bi and has a compressive lattice mismatch of less than 0.2% with GaAs layers. The strain mediation by the introduction of both N and Bi suppresses the crystalline deformation, resulting in the clear formation of the GaNAsBi shell. Thus, we obtained room-temperature photoluminescence with the maximum position at approximately 1300 nm from the GaAs/GaNAsBi/GaAs core–multishell nanowires.

Optimization of the Growth of the Van der Waals Materials Bi2Se3 and (Bi0.5In0.5)2Se3 by Molecular Beam Epitaxy

The naturally existing chalcogenide Bi2Se3 is topologically nontrivial due to the band inversion caused by strong spin-orbit coupling inside the bulk of the material. The surface states are spin polarized, protected by the time-inversion symmetry, and thus robust to the scattering caused by non-magnetic defects. A high purity topological insulator thin film can be easily grown via molecular beam epitaxy (MBE) on various substrates to enable novel electronics, optics, and spintronics applications. However, the unique surface state properties have historically been limited by the film quality, which is evaluated by crystallinity, surface morphology, and transport data. Here we propose and investigate different MBE growth strategies to improve the quality of Bi2Se3 thin films grown by MBE. In addition, growths of topological trivial insulator (Bi0.5In0.5)2Se3 (BIS) are also investigated. BIS is often used as a buffer layer or separation layer for topological insulator heterostructures. Based on the surface passivation status, we have classified the substrates into two categories, self-passivated or unpassivated, and determine the optimal growth mechanisms on the representative sapphire and GaAs, respectively. Growth temperature is a crucial control parameter for the van der Waals epitaxy for both types of substrates. For Bi2Se3 on GaAs, the surface passivation status determines the dominant growth mechanism.

Plasma assisted molecular beam epitaxial growth of GaN with low growth rates and their properties

A systematic investigation on PA-MBE grown GaN with low growth rates (less than 0.2 µm/h) has been conducted in a wide growth temperature range, in order to guide future growth of sophisticated fine structures for quantum device applications. Similar to usual growths with higher growth rates, three growth regions have been revealed, namely, Ga droplets, slightly Ga-rich and N-rich 3D growth regions. The slightly Ga-rich region is preferred, in which GaN epilayers demonstrate optimal crystalline quality, which has been demonstrated by streaky RHEED patterns, atomic smooth surface morphology, and very low defect related yellow and blue luminescence bands. The growth temperature is a critical parameter to obtain high quality materials and the optimal growth temperature window (~ 700–760 °C) has been identified. The growth rate shows a strong dependence on growth temperatures in the optimal temperature window, and attention must be paid when growing fine structures at a low growth rate. Mg and Si doped GaN were also studied, and both p- and n-type materials were obtained.

Influence of pulsed Al deposition on quality of Al-rich Al(Ga)N structures grown by molecular beam epitaxy

Surfaces of AlN epitaxial layers grown by molecular beam epitaxy (MBE) in continuous mode were analyzed by means of X-ray photoelectron spectroscopy, atomic force microscopy and reflection high-energy electron diffraction without breaking the ultra-high vacuum. Our studies show the presence of the Al-Al and N-N bonds on the AlN samples, as well as a rather rough 3D-like structure. Significant improvement of the surface quality was achieved by employing a low flux Al deposition in pulsed mode at the end of the MBE growth (still under a nitrogen background pressure). Subsequently, on the obtained AlN buffer layers, Al0.85Ga0.15N/GaN multiple quantum wells (MQWs) were grown to show the influence of the buffer layers quality on the entire structure properties. The photoluminescence studies indicate that the introduction of pulsed low flux Al deposition improves optical properties of the grown MQWs layers, i.e., the relative intensity between the QW emission and the low energy emission is clearly in favor of pulsed Al deposition.

WSe2 growth on hafnium zirconium oxide by molecular beam epitaxy: the effect of the WSe2 growth conditions on the ferroelectric properties of HZO

Direct integration of transition metal dichalcogenides on a ferroelectric such as hafnium zirconium oxide (HZO) using an industrially scalable technique is important for realizing various ferroelectric-based device architectures. The interface formed due to the processing conditions during direct deposition is the focus of the current study. In this work, molecular beam epitaxy (MBE) is used to directly deposit WSe2 on HZO substrates, and the effects of the MBE growth conditions, specifically high temperature and a high Se flux, are examined. Anneals of HZO under a Se flux, which serve to replicate the conditions during actual WSe2 deposition, result in the crystallization of amorphous as-deposited HZO substrates and incorporation of Se into the HZO. The crystallinity and composition of the HZO substrates affect the degree of Se incorporation. Some of the Se found in the HZO is an adsorbed layer that can be thermally desorbed, but it also has a chemisorbed component fully incorporated within the HZO lattice. Measurement of the electrical properties of the HZO films did not provide evidence that the incorporated Se was detrimental to the functionality of the HZO as a ferroelectric layer.

High thermal conductivity and thermal boundary conductance of homoepitaxially grown gallium nitride (GaN) thin films

Gallium nitride (GaN) has emerged as a quintessential wide band-gap semiconductor for an array of high-power and high-frequency electronic devices. The phonon thermal resistances that arise in GaN thin films can result in detrimental performances in these applications. In this work, we report on the thermal conductivity of submicrometer and micrometer thick homoepitaxial GaN films grown via two different techniques (metal-organic chemical vapor deposition and molecular beam epitaxy) and measured via two different techniques (time domain thermoreflectance and steady-state thermoreflectance). When unintentionally doped, these homoepitaxial GaN films possess higher thermal conductivities than other heteroepitaxially grown GaN films of equivalent thicknesses reported in the literature. When doped, the thermal conductivities of the GaN films decrease substantially due to phonon-dopant scattering, which reveals that the major source of phonon thermal resistance in homoepitaxially grown GaN films can arise from doping. Our temperature-dependent thermal conductivity measurements reveal that below 200 K, scattering with the defects and GaN/GaN interface limits the thermal transport of the unintentionally doped homoepitaxial GaN films. Further, we demonstrate the ability to achieve the highest reported thermal boundary conductance at metal/GaN interfaces through in situ deposition of aluminum in ultrahigh vacuum during molecular beam epitaxy growth of the GaN films. Our results inform the development of low thermal resistance GaN films and interfaces by furthering the understanding of phonon scattering processes that impact the thermal transport in homoepitaxially grown GaN.

Molecular beam epitaxy growth of monolayer hexagonal MnTe2 on Si(111) substrate* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11604366, 11634007, 21872099, and 22072102) and the National Natural Science Foundation of Jiangsu Province, China (Grant No. BK 20160397). F. S. L. acknowledges support from the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No.

Monolayer MnTe2 stabilized as 1T structure has been theoretically predicted to be a two-dimensional (2D) ferromagnetic metal and can be tuned via strain engineering. There is no naturally van der Waals (vdW) layered MnTe2 bulk, leaving mechanical exfoliation impossible to prepare monolayer MnTe2. Herein, by means of molecular beam epitaxy (MBE), we successfully prepared monolayer hexagonal MnTe2 on Si(111) under Te rich condition. Sharp reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) patterns suggest the monolayer is atomically flat without surface reconstruction. The valence state of Mn4+ and the atom ratio of ([Te]:[Mn]) further confirm the MnTe2 compound. Scanning tunneling spectroscopy (STS) shows the hexagonal MnTe2 monolayer is a semiconductor with a large bandgap of ∼ 2.78 eV. The valence-band maximum (VBM) locates at the Γ point, as illustrated by angle-resolved photoemission spectroscopy (ARPES), below which three hole-type bands with parabolic dispersion can be identified. The successful synthesis of monolayer MnTe2 film provides a new platform to investigate the 2D magnetism.

Growth and Structure of Strong Pockels Material Strontium Barium Niobate on SrTiO3 and Si by Molecular Beam Epitaxy

Silicon Photonics In article number 2100111, Alexander A. Demkov and co-workers present the molecular beam epitaxy growth of a strontium barium niobate (SBN) thin film on Si (001). The crystalline structure of the film is analyzed through x-ray diffraction, reflective high energy electron diffraction, and scanning transmission electron microscopy. SBN, known for its very high Pockels coefficient, along with LiNbO3 and BaTiO3, is a promising candidate for applications in integrated Si photonics.

Suppressing carrier density in (Bi x Sb1− x )2Te3 films using Cr2O3 interfacial layers

Band structure engineering and interfacial buffer layers have been demonstrated as effective means to tune the Fermi level in topological insulators (TIs). In particular, the charge compensated compound (Bi x Sb1−x )2Te3 (BST) plays a critical role in the molecular beam epitaxy growth of magnetic TIs. Here we introduce a strategy of exploiting epitaxial Cr2O3 as a buffer layer and amorphous Cr2O3 as a capping layer in the growth of BST films. These films exhibit carrier density lower than 1012 cm−2 over a wide range of Bi contents and higher mobility than BST films directly grown on Al2O3 substrate, shedding light on the importance of interfacial layers for TI films and paving a new avenue to the application of magnetic BST films.

Antimony segregation in an InAs/InAs1−xSbx superlattice grown by metalorganic chemical vapor deposition

Metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are two versatile growth techniques that can readily produce multilayer structures with atomic-level precision control, which have found broad applications in technology. However, compared to MBE, MOCVD growth involves the surface reaction of metal-organic precursor compounds, which changes during film deposition. Consequently, a thorough investigation on the chemical profile layer-by-layer is critical for optimizing MOCVD film performance. Here, we examine Sb segregation in an MOCVD-grown InAs/InAs1−xSbx superlattice by analyzing composition and lattice strain at atomic resolution using scanning transmission electron microscopy and compare with the previously reported MBE growth results. Our findings show a different Sb profile along the growth direction in MOCVD, with the segregation coefficient being higher at the InAsSb-on-InAs interface (0.807 ± 0.021) than at the InAs-on-InAsSb interface (0.695 ± 0.009), giving rise to asymmetric composition and lattice strain profiles unlike those obtained with MBE. Furthermore, we obtain direct evidence of Sb clusters with size of ∼1–3 nm and Sb ordering within the InAs1−xSbx layer, which is largely absent in the reported MBE growth. These findings demonstrate the concurrent interplay between surface segregation, surface reconstruction, and surface reaction that is unique to MOCVD growth with broad implications on preparing Sb-containing quantum materials.

Even-Odd Layer-Dependent Anomalous Hall Effect in Topological Magnet MnBi2Te4 Thin Films.

Recently, MnBi2Te4 has been demonstrated to be an intrinsic magnetic topological insulator and the quantum anomalous Hall (QAH) effect was observed in exfoliated MnBi2Te4 flakes. Here, we used molecular beam epitaxy (MBE) to grow MnBi2Te4 films with thickness down to 1 septuple layer (SL) and performed thickness-dependent transport measurements. We observed a nonsquare hysteresis loop in the antiferromagnetic state for films with thickness greater than 2 SL. The hysteresis loop can be separated into two AH components. We demonstrated that one AH component with the larger coercive field is from the dominant MnBi2Te4 phase, whereas the other AH component with the smaller coercive field is from the minor Mn-doped Bi2Te3 phase. The extracted AH component of the MnBi2Te4 phase shows a clear even-odd layer-dependent behavior. Our studies reveal insights on how to optimize the MBE growth conditions to improve the quality of MnBi2Te4 films.

Backside Illuminated “Ge-on-Si” NIR Camera

In this paper, a near-infrared “Ge-on-Si” camera with backside illumination, is presented. The camera consists of a photonic chip, a readout chip, and a commercial microcontroller, which is connected to a laptop via USB. The functionality of two-dimensional ( 10×10 pixels) as well as one-dimensional ( 64×1 pixel) “Ge-on-Si” pixel arrays with a pitch of 25 μm are demonstrated. The photonic chip consists of basic CMOS technology process in combination with a molecular beam epitaxy growth process. With this technology and the backside illumination a high fill-factor of 74% is achieved. Further the “Ge-on-Si” detectors are operated under zero bias operation in order to minimize the dark current and thus to maximize the signal-to-noise ratio. This enables dark currents better than 100 pA to be achieved in each pixel. A linear behavior between photocurrent and optical power of the “Ge-on-Si” detectors over a large dynamic range for optical powers between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> W and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> W is shown. With relatively thin Ge absorption layers of only 400 nm, optical responsivities of 0.36 A/W at a wavelength of 1310 nm could be achieved. The functionality of the “Ge-on-Si” camera is demonstrated by the movement of a single mode fiber over the photonic chip at a wavelength of 1310 nm.