Reflection High-energy Electron Diffraction Patterns Research Articles

Structural and surface characterizations of 2D β-In2Se3/3D β-Ga2O3 heterostructures grown on c-Sapphire substrates by molecular beam epitaxy

Integrating two-dimensional (2D) layered materials with wide bandgap β-Ga2O3 has unveiled impressive opportunities for exploring novel physics and device concepts. This study presents the epitaxial growth of 2D β-In2Se3/3D β-Ga2O3 heterostructures on c-Sapphire substrates by plasma-assisted molecular beam epitaxy. Firstly, we employed a temperature-dependent two-step growth process to deposit Ga2O3 and obtained a phase-pure (2¯01)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(\\overline{2 }01)$$\\end{document} β-Ga2O3 film on c-Sapphire. Interestingly, the in-situ reflective high-energy electron diffraction (RHEED) patterns observed from this heterostructure revealed the in-plane ‘b’ lattice constant of β-Ga2O3 ~ 3.038Å. In the next stage, for the first time, 2D In2Se3 layers were epitaxially realized on 3D β-Ga2O3 under varying substrate temperatures (Tsub) and Se/In flux ratios (RVI/III). The deposited layers exhibited (00l) oriented β-In2Se3 on (2¯01)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(\\overline{2 }01)$$\\end{document} β-Ga2O3/c-Sapphire with the epitaxial relationship of [112¯0]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$[11\\overline{2 }0]$$\\end{document} β-In2Se3 || [010] β-Ga2O3 and [101¯0]\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$[10\\overline{1 }0]$$\\end{document} β-In2Se3 || [102] β-Ga2O3 as observed from the RHEED patterns. Also, the in-plane ‘a’ lattice constant of β-In2Se3 was determined to be ~ 4.027Å. The single-phase β-In2Se3 layers with improved structural and surface quality were achieved at a Tsub ~ 280 °C and RVI/III ~ 18. The microstructural and detailed elemental analysis further confirmed the epitaxy of 2D layered β-In2Se3 on 3D β-Ga2O3, a consequence of the quasi-van der Waals epitaxy. Furthermore, the β-Ga2O3 with an optical bandgap (Eg) of ~ 5.04 eV (deep ultraviolet) when integrated with 2D β-In2Se3, Eg ~ 1.43eV (near infra-red) can reveal potential applications in the optoelectronic field.

MBE-grown tetragonal FeTe consisting of c-axis-aligned nanocrystals

Tetragonal FeTe grown on c-plane sapphire by the molecular beam epitaxy technique is found to result in a new structural phase consisting of c-axis-aligned nanocrystals. Their reflection high-energy electron diffraction patterns display two sets of streaks simultaneously at all rotation angles of the sample. High-resolution x-ray diffraction studies confirm that the nanocrystals are tetragonal FeTe with their c-axes aligned to the growth direction. Atomic force microscopy imaging reveals that further growth of these nanocrystals involves a cannibalism process resulting in nanocrystal pillars with sizes of about 0.5–1 µm. The temperature-dependent resistance of these thin films displays an overall semiconducting behavior, however, with a non-measurable state or jumps and falls depending on their nominal thickness, which can be attributed to the thermal responses of the nanocrystals during cooling and heating processes. This discovery provides an approach to form inhomogeneous heterostructures with all possible twisted angles.

Identification of Kikuchi lines in electron diffraction patterns collected in small-angle geometry.

It is demonstrated that Kikuchi features become clearly visible if reflection high-energy electron diffraction (RHEED) patterns are filtered using digital image processing software. The results of such pattern transformations are shown for SrTiO3 with mixed surface termination for data collected at different azimuths of the incident electron beam. A simplified analytical approach for the theoretical description of filtered Kikuchi patterns is proposed and discussed. Some examples of raw and filtered patterns for thin films are shown. RHEED patterns may be treated as a result of coherent and incoherent scattering of electron waves. The effects of coherent scattering may be considered as those occurring due to wave diffraction by an idealized crystal and, usually, only effects of this type are analysed to obtain structural information on samples investigated with the use of RHEED. However, some incoherent scattering effects mostly caused by thermal vibrations of atoms, known as Kikuchi effects, may also be a source of valuable information on the arrangements of atoms near the surface. Typically, for the case of RHEED, Kikuchi features are hidden in the intensity background and researchers cannot easily recognize them. In this paper, it is shown that the visibility of features of this type can be substantially enhanced using computer graphics methods.

Effect of sub-grain boundaries in CdZnTe on HgCdTe film grown by molecular beam epitaxy

HgCdTe films grown by molecular beam epitaxy (MBE) are essential for creating high-performance infrared focal plane arrays (IRFPAs) like dual band detectors, high operating temperature (HOT) detectors, and avalanche photodiode (APD) detectors. CdZnTe is recognized as the optimal substrates for growing high crystal quality HgCdTe due to its lattice matching, which is adjusted by selecting the Zn mole fraction. However, defects like grain boundaries, twins, inclusions, dislocations, and sub-grain boundaries that often occur in CdZnTe substrates can significantly reduce the crystal quality of HgCdTe. HgCdTe grown on CdZnTe with these defects usually no longer suit for high quality device fabrication. Although the impact from various defects have been extensively studied in HgCdTe grown on CdZnTe, but the sub-grain boundaries is still not yet involved. In our study, to better understand the effect of sub-grain boundaries in CdZnTe on HgCdTe films, CdZnTe substrates with and without sub-grain boundary were selected to growth MWIR HgCdTe films under optimized condition. Results indicated that etch pit density (EPD) of CdZnTe substrates with sub-grain boundary is approximately 3–5 times higher than boundary free areas. But HgCdTe grown on these substrates bears more than an order of magnitude increase in dislocation proliferation compared to the boundary free region. Dislocation generation resulting from dislocation penetration from substrates, mismatch between HgCdTe and CdZnTe, and sub-grain boundaries were investigated and compared. The intense dislocation proliferation in HgCdTe may be mainly due to twins formed during the initial growth stage of HgCdTe films, rather than dislocation penetration from substrates, as revealed by reflection high-energy electron diffraction (RHEED) pattern monitoring. Therefore, screening for sub-grain boundaries in CdZnTe before HgCdTe growth is necessary.

Surface Atomic Arrangement of Aluminum Ultra-Thin Layers Grown on Si(111).

Surface atomic arrangement and physical properties of aluminum ultrathin layers on c-Si(111)-7 × 7 and hydrogen-terminated c-Si(111)-1 × 1 surfaces deposited using molecular beam epitaxy were investigated. X-ray photoelectron spectroscopy spectra were collected in two configurations (take-off angle of 0° and 45°) to precisely determine the surface species. Moreover, 3D atomic force microscopy (AFM) images of the air-exposed samples were acquired to investigate the clustering formations in film structure. The deposition of the Al layers was monitored in situ using a reflection high-energy electron diffraction (RHEED) experiments to confirm the surface crystalline structure of the c-Si(111). The analysis of the RHEED patterns during the growth process suggests the settlement of aluminum atoms in Al(111)-1 × 1 clustered formations on both types of surfaces. The surface electrical conductivity in both configurations was tested against atmospheric oxidation. The results indicate differences in conductivity based on the formation of various alloys on the surface.

Electric-field control of magnetization reversal at room temperature in SmFeO3 single-phase multiferroic thin film

Rare-earth orthoferrite SmFeO3 (SFO) is a promising single-phase multiferroic material because of simultaneous coexistence of magnetic and ferroelectric orders at room temperature (RT). However, the spin structures within this bulk material are antiferromagnetic arrangements, and the magnetoelectric (ME) coupling effect is weak. Thus, the realization of electric control of magnetization reversal at RT remains challenging. In this work, SFO thin films with different thicknesses of 30 nm, 60 nm and 90 nm were constructed onto LaNiO3 (LNO)-buffered LaAlO3 (LAO) substrates by pulsed laser deposition (PLD) method. The X-ray diffraction (XRD) θ-2θ scans and reflection high-energy electron diffraction (RHEED) patterns demonstrate the SFO is perfect growth along the LAO (00 l) direction. Besides, the magnetic measurement results indicate that all the thin films are induced obvious RT ferromagnetism by compressive strain of substrates. More importantly, when providing a pair of opposite polarization voltages of± 2.5 V for the 90 nm thin film, the ferroelectric and induced ferromagnetic domains at the same local area can realize synchronous reversal, which shows that the SFO thin film under compressive strain is a rare ferromagnetic-based ME multiferroic material at RT. This kind of material will produce wide application prospect in future electric-write magnetic-read high-density memories and other spintronics devices.

Realistic simulation of reflection high-energy electron diffraction patterns for two-dimensional lattices using Ewald construction

Reflection high-energy electron diffraction (RHEED) is a powerful tool for characterizing crystal surface structures. However, the setup geometry leads to distorted and complicated patterns, which are not straightforward to link to the real-space structures. A program with a graphical user interface is provided here to simulate the RHEED patterns. Following the Ewald construction in the kinematic theory, we find out the exact geometric transformation in this model that determines the positions of diffraction spots. The program can deal with many forms of surface structures, including surface reconstructions or domains. The simulations exhibit great agreement with the experimental results in various cases. This program will benefit the structure analysis in thin-film growth and surface science studies.

Indium incorporation at InxGa1-xN relaxed self-assembled nanostructures on Si substrates

In this work, we report on self-assembling of InxGa1-xN nanostructures under strong nitrogen-rich conditions on Si (111) substrates by molecular beam epitaxy. In-situ analysis of the reflection high-energy electron diffraction patterns allowed to follow the strain evolution of the films as well as to monitor the morphological changes related to the In incorporation. Scanning electron microscopy and atomic force microscopy images of the samples evidenced the growth of different nanostructures such as nanocolumns and nanowalls depending on the In content. X-ray diffraction (XRD) measurements allowed to determine the indium concentration while Rutherford backscattering spectrometry (RBS) was used to obtain the composition and thickness of the films and to explore the way in which In and Ga atoms are incorporated in the InxGa1-xN ternary alloy. XRD and RBS complementary analysis suggests that phase separation, occurring at the higher indium concentration, induces both the In and Ga rich phases segregation towards the external surface of the growths. Therefore, the columnar surface arrangement observed on the extreme binary compounds is transformed into separated nanowalls as the indium content increases. The probable origin of this morphology change is discussed. A shift towards lower energies of the low temperature photoluminescence spectra was observed for increased In concentration and a value of the bowing band gap parameter of about 3 was estimated using the extrema binary compounds and the intermediated indium concentration samples.

Application of machine learning to reflection high-energy electron diffraction images for automated structural phase mapping

We have developed a phase mapping method based on machine learning analysis of reflection high-energy electron diffraction (RHEED) images. RHEED produces diffraction patterns containing a wealth of static and dynamic information and is commonly used to determine the growth rate, the growth mode, and the surface morphology of epitaxial thin films. However, the ability to extract quantitative structural information from the RHEED patterns that appear during film growth is limited by the lack of versatile and automated analysis techniques. We have created a deep learning-based analysis method for automating the identification of different RHEED pattern types that occur during the growth of a material. Our approach combines several supervised and unsupervised machine learning techniques and permits the extraction of quantitative phase composition information. We applied this method to the mapping of the structural phase diagram of ${\mathrm{Fe}}_{\mathrm{x}}{\mathrm{O}}_{\mathrm{y}}$ thin films grown by pulsed laser deposition as a function of growth temperature and oxygen pressure close to the hematite-magnetite phase boundary. The in situ RHEED-based mapping method produces results that are qualitatively similar to postsynthesis x-ray diffraction analysis.

Multiclass classification of reflection high-energy electron diffraction patterns using deep learning

Reflection high-energy electron diffraction (RHEED) is widely used because it allows in-situ observations of the surface behavior of a sample during molecular beam epitaxy (MBE) growth. In particular, RHEED patterns have been used for the calibration of growth conditions because they change considerably depending on the sample temperature, amount of material supplied, and supply ratio. However, RHEED pattern analysis depends on the expertise of the operator and there is also a time limit; therefore, it cannot be readily applied to real-time feedback control. In our previous study, we reported a model that acquires RHEED images of MBE GaAs grown on a GaAs substrate via machine learning using a convolutional neural network. However, the model that classifies the two patterns was a binary classification model and extensibility via data addition was limited. In this study, we report a classification model that can classify three or more classes. This suggests the possibility of a generalized RHEED pattern classifier.

Skill-Agnostic analysis of reflection high-energy electron diffraction patterns for Si(111) surface superstructures using machine learning

Skill-Agnostic analysis of reflection high-energy electron diffraction patterns for Si(111) surface superstructures using machine learning

Growth and characterization of quaternary-alloy ferromagnetic semiconductor (In,Ga,Fe)Sb

We study the growth and properties of the quaternary-alloy ferromagnetic semiconductor (In0.94−x,Gax,Fe0.06)Sb (x = 5%–30%; the Fe concentration is fixed at 6%) grown by low-temperature molecular beam epitaxy. Reflection high-energy electron diffraction patterns, scanning transmission electron microscopy lattice images, and x-ray diffraction spectra indicate that the (In0.94−x,Gax,Fe0.06)Sb layers have a zinc blende crystal structure without any other second phase. The lattice constant of the (In0.94−x,Gax,Fe0.06)Sb films changes linearly with the Ga concentration x, indicating that Ga atoms substitute In atoms in the zinc-blende structure. We found that the carrier type of (In0.94−x,Gax,Fe0.06)Sb can be systematically controlled by varying x, being n-type when x ≤ 10% and p-type when x ≥ 20%. Characterization studies using magnetic circular dichroism spectroscopy indicate that the (In0.94−x,Gax,Fe0.06)Sb layers have intrinsic ferromagnetism with relatively high Curie temperatures (TC = 40–120 K). The ability to widely control the fundamental material properties (lattice constant, bandgap, carrier type, and magnetic property) of (In0.94−x,Gax,Fe0.06)Sb demonstrated in this work is essential for spintronic device applications.

Improved growth quality of epitaxial ZnTe thin films on Si (111) wafer with ZnSe buffer layer

To achieve high-quality and low-cost ZnTe epitaxial films, a low-temperature (LT, 200 °C) ZnSe buffer layer with a thickness of 5 nm was grown on Si (111) wafers prior to the epitaxy of ZnTe layers by molecular beam epitaxy. Reflection high-energy electron diffraction patterns reveal that ZnTe epilayers grown at 335 °C with a thickness of 100 nm on the LT-ZnSe buffer layer present a completely two-dimensional growth mode. Film surfaces present smooth and flat morphology with the lowest surface roughness of ∼2.2 nm at a Te/Zn beam equivalent pressure ratio of 1.6. X-ray diffraction θ-2θ scanning indicates that the out-of-plane structure of ZnTe films is highly preferred with the (111) growth orientation of ZnSe/Si. X-ray diffraction φ scanning further confirms that the in-plane structure of ZnTe epilayer is also a single-crystal orientation growth. Thus, high-quality ZnTe (111) single-crystal epilayer with the lowest full-width at half-maximum of ∼200 arc sec and the smallest dislocation density of ∼1.12 × 108 cm−2 was grown on Si (111) wafers. In addition, the ZnSe buffer layer could alleviate the residual stress effectively for the heteroepitaxy of Si wafers. The most balanced residual stress for ZnTe/ZnSe/Si with σx of ∼96.5 MPa and σy of ∼94.2 MPa was obtained, which is essential for achieving high-performance and good mechanical properties of ZnTe-based devices.

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.

Surface and interface structures of epitaxial Sb2Se3 on mica

Sb2Se3 thin film is an emerging photon absorber used in solar cells. We report the study of surface and interface structures of Sb2Se3(120) film grown on mica substrate by a high-rate vapor transport method. The interface epitaxial relationship between Sb2Se3 and mica examined by the cross- sectional TEM images and diffraction patterns along the [001] and [10]2¯ directions of Sb2Se3 reveal a rectangular structure with lengths of 4.03 ± 0.1 Å and 5.29 ± 0.1 Å, consistent with the [120] out-of-plane direction of Sb2Se3 bulk lattice parameters. In contrast, the two-dimensional reciprocal space map (2D map) constructed from azimuthal reflection high-energy electron diffraction (ARHEED) patterns from the surface exhibits a decorated hexagonal structure. This surface structure emerges from six epitaxial orientation domains/rods and each domain has a rectangular unit mesh of 3.94 ± 0.09 Å and 26.95 ± 1.16 Å along the [001] and [10]2¯ directions. The 26.95 Å is consistent with the unit mesh of the outermost layer of the Sb2Se3(120) domains/rods. Our 2D map reveals surface information that are not easily observed by other diffraction techniques.

Classification of in situ reflection high energy electron diffraction images by principal component analysis

The reflection high-energy electron diffraction (RHEED) method is widely used for the in situ observation of molecular beam epitaxy (MBE). This is because the RHEED pattern dynamically changes according to the growth conditions, such as surface temperature and material supply. However, to date, the RHEED pattern has been categorized and recognized based on the experience of the researcher. In this study, we investigated the classification of RHEED pattern datasets without using labeling by the principal component analysis method that reduces the dimensionality of the data. The RHEED images were successfully classified during the MBE growth of GaAs, demonstrating that unsupervised learning can be used to recognize RHEED patterns.

Layer-by-layer epitaxial growth of monoclinic SrIrO3 thin films on (111)-oriented SrTiO3 through interface engineering

When transition metal oxides is grown along (111) direction, a variety of intriguing phenomena is expected to arise. One of important system, SrIrO3, crystallizes into monoclinic hexagonal-based 6M structure when grown on (111)-oriented perovskite SrTiO3, and is expected to host novel topological states. However, the growth of high quality SrIrO3 on (111)-oriented substrate is still a challenge. We report on high-quality layer-by-layer epitaxial growth of monoclinic SrIrO3 on SrTiO3(111) substrate by inserting the CaTiO3 buffer layer. The scanning transmission electron microscopy image shows perfect epitaxial growth of monoclinic SrIrO3. Reflection high-energy electron diffraction and low-energy electron diffraction patterns exhibit smooth film surface with reconstruction. Inserting CaTiO3 buffer layer proves to be an effective way of interface engineering in realizing layer-by-layer growth by reducing the lattice mismatch. The study paves way for the future research on novel quantum states in monoclinic SrIrO3 films and heterostructures.

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.

Critical thickness as a function of the indium molar fraction in cubic InXGa1-XN and the influence in the growth of nanostructures

The critical thickness for the relaxation of cubic (β) InXGa1-XN layers grown over (002) β-GaN/MgO, with different indium content, particularly of 17, 44, and 70% have been determined experimentally. The layers were grown by plasma-assisted molecular beam epitaxy. In all the samples studied, the critical thickness (hc) of the pseudomorphic layer was measured with a frame-by-frame analysis of reflection high energy electron diffraction (RHEED) patterns. The growth mode transition during layer growth was identified via RHEED patterns, from layer by layer or 2D to 3D growth mode by gradually transitioning from a streaky to a spotty pattern. The experimental hc value of β-InXGa1-XN on β-GaN was compared to values calculated from the Fisher model. For the indium concentration of x = 0.44, the self-assembling epitaxial nanostructures were successfully grown in the Stranski–Krastanov growth mode with a critical thickness of 1.8 ± 0.7 nm. The β-In0.44Ga0.56N nanostructures showed a variation in morphology and density depending on the deposition time; self-assembled nanodots and nano-bars were confirmed by atomic force microscopy. Photoluminescence spectra of β-In0.44Ga0.56N nanostructures showed emissions associated with quantum confinement; these peaks indicate a blue shift with respect to the bulk.

Effect of indium accumulation on the growth and properties of ultrathin In(Ga)N/GaN quantum wells

In this work, we present the investigation of InN/GaN multiple-quantum-well (MQW) growth by plasma-assisted molecular beam epitaxy using in-situ reflection high-energy electron diffraction (RHEED) to monitor the growth process. The analysis of the RHEED intensity and pattern transitions identified an indium surface accumulation even with a nominal thickness of InN as small as 0.5 monolayer (ML). This result explicitly shows that, even at low growth temperatures of ~550 °C, not all of the supplied indium is incorporated into the quantum well (QW). Moreover, the residual indium can become incorporated into the GaN matrix on either side of the QW. Both QW thickness and the photoluminescence (PL) emission energy showed a self-regulating behavior. The apparent thickness did not exceed 2 MLs even when the deposited InN thickness is as large as 5 MLs. The PL emission shows a continuous redshift with the deposited InN from ~370 nm for 0.5 ML until it saturates at ~423 nm for >2 ML. Based on the observed growth phenomena, a qualitative growth model was developed to explain the self-limited epitaxial growth of ultrathin In(Ga)N/GaN QWs.