Reflection High-energy Electron Diffraction Images Research Articles

Fabrication of highly oriented Mn4N/Pt epitaxial bilayer structure on MgO(001) for spintronics applications

Magnetization control by electric current or spin current is an attractive technology with potential applications in memory and logic devices, and Mn4N has attracted much attention in recent years as a candidate material for current-induced magnetization switching and magnetic domain wall motion, as well as a skyrmion material. For these applications, a bilayer structure of Mn4N and heavy metals must be formed with high crystalline quality. In this study, we have achieved the epitaxial growth of Mn4N thin films on MgO(001) with a Pt underlayer. The reflection high-energy electron diffraction images and x-ray diffraction ω-rocking curves show that the Mn4N films fabricated on the Pt underlayers exhibit higher crystalline quality than the Mn4N films directly formed on the MgO. Furthermore, anomalous Hall effect loops with bump-like structures were observed. This is considered to be due to the topological spin texture. The Mn4N/Pt epitaxial bilayer structure has potential for use in spin–orbit torque applications and is a candidate configuration for topological spin textures.

Monitoring MBE Substrate Deoxidation via RHEED Image-Sequence Analysis by Deep Learning

Reflection high-energy electron diffraction (RHEED) is a powerful tool in molecular beam epitaxy (MBE), but RHEED images are often difficult to interpret, requiring experienced operators. We present an approach for automated surveillance of GaAs substrate deoxidation in MBE reactors using deep-learning-based RHEED image-sequence classification. Our approach consists of an nonsupervised autoencoder (AE) for feature extraction, combined with a supervised convolutional classifier network. We demonstrate that our lightweight network model can accurately identify the exact deoxidation moment. Furthermore, we show that the approach is very robust and allows accurate deoxidation detection for months without requiring retraining. The main advantage of the approach is that it can be applied to raw RHEED images without requiring further information such as the rotation angle, temperature, etc.

Structural Properties of TaAs Weyl Semimetal Thin Films Grown by Molecular Beam Epitaxy on GaAs(001) Substrates

Thin crystalline layers of TaAs Weyl semimetal are grown by molecular beam epitaxy on GaAs(001) substrates. The (001) planes of the tetragonal TaAs lattice are parallel to the GaAs(001) substrate, but the corresponding in-plane crystallographic directions of the substrate and the layer are rotated by 45°. In spite of a substantial lattice mismatch (about 19%) between the GaAs(001) substrate and TaAs epilayer, no misfit dislocations are observed at the GaAs(001)/TaAs(001) interface. Only stacking fault defects in TaAs are detected by transmission electron microscopy. Thorough X-ray diffraction measurements and analysis of the in situ reflection high-energy electron diffraction images indicate that TaAs layers are fully relaxed already at the initial deposition stage. Atomic force microscopy imaging reveals the columnar structure of the layers, with lateral (parallel to the layer’s surface) columns about 20 nm wide and 200 nm long. Both X-ray diffraction and transmission electron microscopy measurements indicate that the columns share the same orientation and crystalline structure.

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.

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.

Machine learning analysis of perovskite oxides grown by molecular beam epitaxy

Reflection high-energy electron diffraction (RHEED) is a ubiquitous in situ molecular beam epitaxial (MBE) characterization tool. Although RHEED can be a powerful means for crystal surface structure determination, it is often used as a static qualitative surface characterization method at discrete intervals during a growth. A full analysis of RHEED data collected during the entirety of MBE growths is made possible using principle component analysis (PCA) and k-means clustering to examine significant boundaries that occur in the temporal clusters grouped from RHEED data and identify statistically significant patterns. This process is applied to data from homoepitaxial SrTiO$_{3}$ growths, heteroepitaxial SrTiO$_{3}$ grown on scandate substrates, BaSnO$_{3}$ films grown on SrTiO$_{3}$ substrates, and LaNiO$_{3}$ films grown on LaAlO$_{3}$ substrates. This analysis may provide additional insights into the surface evolution and transitions in growth modes at precise times and depths during growth, and that video archival of an entire RHEED image sequence may be able to provide more insight and control over growth processes and film quality.

Monitoring the growth of III-nitride materials by plasma assisted molecular beam epitaxy employing diffuse scattering of RHEED

AlGaN alloys find important applications in UV emitters and detectors, as well as in high-power high-frequency electronics. While reflection high energy electron diffraction (RHEED) is a standard technique for in situ monitoring of the growth of AlGaN alloys by plasma assisted molecular beam epitaxy, this paper investigates a new mode of its application. During the growth of AlGaN alloys, the ratio of the group III (Al + Ga) to group V (active nitrogen) adatoms critically controls the materials property of AlGaN films and is optimal within a very narrow window of operation. Moreover, this ratio is dependent in a complex fashion on various growth parameters, including substrate temperature, and is difficult to determine quantitatively in real time. This paper provides a method to estimate that important parameter. This can be carried out through the capture of the RHEED image from the fluorescent screen using an inexpensive video camera setup and a simple analysis procedure. While most RHEED analyses focus on the diffraction pattern, e.g., the line spacing, this work quantifies diffused scattering of the electron beam from a metallic layer that forms on the top of the growth surface during deposition under excess group III conditions, which is typically employed during growth. Two alternate methods for data analysis have been explored and compared. The results indicate that this process can qualitatively trace the variation of the thickness of the thin metallic layer, and hence the group III to group V flux ratio, for different substrate temperatures. This technique being simple, fast, and cost-effective can be incorporated into standard MBE systems for real-time in situ characterization of AlGaN alloys.

RHEED oscillations in spinel ferrite epitaxial films grown by conventional planar magnetron sputtering

Real-time in situ reflection high energy electron diffraction (RHEED) observations of Fe3O4, γ-Fe2O3, and (Co,Fe)3O4 films on MgO(001) substrates grown by a conventional planar magnetron sputtering was studied. The change in periodical intensity of the specular reflection spot in the RHEED images of three different spinel ferrite compounds grown by two different sputtering systems was examined. The oscillation period was found to correspond to the 1/4 unit cell of each spinel ferrite, similar to that observed in molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) experiments. This suggests that the layer-by-layer growth of spinel ferrite (001) films is general in most physical vapor deposition (PVD) processes. The surfaces of the films were as flat as the surface of the substrate, consistent with the observed layer-by-layer growth process. The observed RHEED oscillation indicates that even a conventional sputtering method can be used to control film thickness during atomic layer depositions.

Van der Waals Epitaxy of GaSe/Graphene Heterostructure: Electronic and Interfacial Properties.

Stacking two-dimensional materials in so-called van der Waals (vdW) heterostructures, like the combination of GaSe and graphene, provides the ability to obtain hybrid systems that are suitable to design optoelectronic devices. Here, we report the structural and electronic properties of the direct growth of multilayered GaSe by molecular beam epitaxy on graphene. Reflection high-energy electron diffraction images exhibited sharp streaky features indicative of a high-quality GaSe layer produced via a vdW epitaxy. Micro-Raman spectroscopy showed that, after the vdW heterointerface formation, the Raman signature of pristine graphene is preserved. However, the GaSe film tuned the charge density of graphene layer by shifting the Dirac point by about 80 meV toward lower binding energies, attesting to an electron transfer from graphene to GaSe. Angle-resolved photoemission spectroscopy (ARPES) measurements showed that the maximum of the valence band of the few layers of GaSe are located at the Γ point at a binding energy of about -0.73 eV relative to the Fermi level (p-type doping). From the ARPES measurements, a hole effective mass defined along the ΓM direction and equal to about m*/m0 = -1.1 was determined. By coupling the ARPES data with high-resolution X-ray photoemission spectroscopy measurements, the Schottky interface barrier height was estimated to be 1.2 eV. These findings allow a deeper understanding of the interlayer interactions and the electronic structure of the GaSe/graphene vdW heterostructure.

Big-data reflection high energy electron diffraction analysis for understanding epitaxial film growth processes.

Reflection high energy electron diffraction (RHEED) has by now become a standard tool for in situ monitoring of film growth by pulsed laser deposition and molecular beam epitaxy. Yet despite the widespread adoption and wealth of information in RHEED images, most applications are limited to observing intensity oscillations of the specular spot, and much additional information on growth is discarded. With ease of data acquisition and increased computation speeds, statistical methods to rapidly mine the data set are now feasible. Here, we develop such an approach to the analysis of the fundamental growth processes through multivariate statistical analysis of a RHEED image sequence. This approach is illustrated for growth of La(x)Ca(1-x)MnO(3) films grown on etched (001) SrTiO(3) substrates, but is universal. The multivariate methods including principal component analysis and k-means clustering provide insight into the relevant behaviors, the timing and nature of a disordered to ordered growth change, and highlight statistically significant patterns. Fourier analysis yields the harmonic components of the signal and allows separation of the relevant components and baselines, isolating the asymmetric nature of the step density function and the transmission spots from the imperfect layer-by-layer (LBL) growth. These studies show the promise of big data approaches to obtaining more insight into film properties during and after epitaxial film growth. Furthermore, these studies open the pathway to use forward prediction methods to potentially allow significantly more control over growth process and hence final film quality.

Enhancement of spin-asymmetry by L21-ordering in Co2MnSi/Cr/Co2MnSi current-perpendicular-to-plane magnetoresistance devices

Co 2 MnSi / Cr / Co 2 MnSi (001)-fully epitaxial current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices were fabricated via an UHV magnetron sputtering system. The relationship between the degree of chemical ordering in Co2MnSi (CMS) and the CPP-GMR characteristics was investigated systematically against the annealing temperature of the devices. X-ray diffraction profiles and reflection high-energy electron diffraction images indicated that annealing improved L21-ordering. The MR ratio also increased upon annealing and the maximum MR ratio of 5.2% and ΔRA of 6.5 mΩ μm2 were achieved by annealing at 400 °C. These results indicate that promoting the degree of L21-ordering in CMS enhances the bulk and/or interface spin-asymmetry coefficients.

A real-time quantitative assessment of self-assembled quantum dots by reflection high-energy electron diffraction

A method for in-situ quantitative characterization of quantum dots during growth is provided using the readily available reflection high-energy electron diffraction (RHEED). RHEED patterns of uncapped self-assembled InAs/GaAs quantum dots are investigated theoretically and experimentally. Previously predicted intensity fringes along chevron tails of quantum dot's RHEED diffraction spots are observed experimentally. Post-growth atomic force microscopic images and theoretical RHEED images of the same are obtained parallel to experimental data. The bounding facets of the quantum dots are determined using the angle between the chevrons. The size (height) of the quantum dots is determined using the periodicity of intensity fringes along the chevrons during growth.

Effect of residual hydrogen and temperature on initial growth stages and properties of GaAs 1−xN x

In this work we investigate the evolution of the growth mode and the dependence of the structural and optical properties of III–V dilute nitrides with the growth temperature and the presence of hydrogen. GaAsN epilayers were fabricated on (0 0 1) GaAs substrates by rf nitrogen plasma-assisted chemical beam epitaxy. Various amounts of hydrogen were introduced independently in the growth chamber during the GaAsN epilayer fabrication. Reflection high-energy electron diffraction (RHEED) was used to monitor the evolution of the surface reconstruction during growth, while the interaction of the residual hydrogen with the rf plasma source was monitored, simultaneously and in real time, using optical spectroscopy. The presence of hydrogen and slight variations of the growth temperature resulted in noticeable changes in the surface reconstruction during the initial stages of GaAsN fabrication. For sufficiently high amounts of hydrogen, the RHEED image at the onset of GaAsN growth was characterized by the appearance of three-dimensional features, indicating an increased surface disorder; however, after the subsequent growth of few monolayers, two-dimensional RHEED was recovered for all samples regardless of the amount of residual hydrogen. In addition, samples fabricated under excess hydrogen exhibited an apparent nitrogen content reduction, as indicated by high-resolution X-ray diffraction analysis, and a noteworthy blue shift and signal degradation of their low-temperature photoluminescence spectra compared to samples that were fabricated under lower hydrogen partial pressures.

Effect of strain anisotropies on RHEED patterns of quantum dots

The strain distributions and of reflection high-energy electron diffraction (RHEED) patterns of uncapped pyramidal shape InAs Stranski–Krastanov quantum dots fabricated on GaAs(0 0 1) substrate are investigated theoretically. Three-dimensional strain anisotropy is computed with an atomistic elasticity approach, using inter-atomic Keating potentials and the strain energy is minimized using the conjugate gradient numerical method. RHEED images are predicted in the framework of the kinematical theory, by taking into account the refraction of the electron beam at the quantum dot/vacuum interface. Clear correlation between RHEED image features and quantum dot structural properties is established showing that even rather small strain anisotropies imply noticeable changes in RHEED images stressing the potential of RHEED in detecting and deciphering real-time strain distributions in quantum dots.

RHEED metrology of Stranski–Krastanov quantum dots

Reflection high-energy electron diffraction (RHEED) patterns of uncapped pyramidal shaped InAs Stranski–Krastanov quantum dots fabricated on GaAs (0 0 1) substrate are investigated theoretically. Clear correlations between features in the RHEED images and quantum dot structural properties, as quantum dot facet orientation and quantum dot height, are established. In particular, it is shown that lateral facet orientation and dot heights can be directly extracted from the characteristic chevron angles and predicted intensity oscillations along the chevron tails, respectively.

Room-temperature epitaxial growth of AlN on atomically flat MgAl2O4 substrates

The authors have grown AlN films on atomically flat MgAl2O4 (111) substrates at room temperature (RT) by pulsed-laser deposition. The in situ reflection high-energy electron diffraction image exhibits a streaky pattern, and the root-mean-square value of the surface roughness as measured by atomic force microscopy is as low as 0.45nm, which indicates that single-crystalline AlN grows epitaxially on MgAl2O4 with a smooth surface, even at RT. Electron backscattered diffraction and x-ray diffraction measurements reveal that AlN {11−20} shows a clear sixfold symmetry without 30° rotational domains, and the epitaxial relationship is AlN [11−20]‖MgAl2O4 [0−11]. Grazing incidence angle x-ray reflectivity characterization indicates that the interfacial reaction between AlN and MgAl2O4 is completely suppressed in the case of RT growth and the heterointerface for the RT-AlN∕MgAl2O4 structure is abrupt and thermally stable.

Epitaxial growth of GaN on copper substrates

We have grown GaN films on Cu(111) substrates using pulsed laser deposition. We have found that GaN(0001) grows epitaxially on Cu(111) when employing low temperature grown AlN buffer layers with an in-plane epitaxial relationship of AlN[112¯0]‖Cu[11¯0] Reflection high-energy electron diffraction images have exhibited sharp streaky patterns, indicating that GaN grows with a flat surface. Electron backscattering diffraction observations have revealed that neither 30° rotational domains nor cubic phase domains exist in the GaN films. Spectroscopic ellipsometry measurements have shown that the heterointerfaces in the GaN∕AlN∕Cu structure are abrupt. The epitaxial growth of GaN on Cu substrates is likely to raise the power limit for future light emitting and electron devices due to the high thermal conductivity of Cu.

Kinematical Analysis of the Evolution of Reflection High Energy Electron Diffraction Patterns of Quantum Dot Heterostructures: Correlation with Strain and Anisotropy

ABSTRACTThe strain distributions and of reflection high energy electron diffraction (RHEED) patterns of uncapped pyramidal shape InAs Stranski-Krastanov quantum dots fabricated on GaAs(001) substrate are investigated theoretically. The three dimensional strain anisotropy is computed with an atomistic elasticity approach, using inter-atomic Keating potentials and the strain energy is minimized using the conjugate gradient numerical method. RHEED images are predicted in the framework of the kinematical theory, by taking into account the refraction of the electron beam at the quantum dot/vacuum interface. Clear correlation between RHEED image features and quantum dot structural properties is established. The study stresses the potential of RHEED for future experimental real-time (during growth) detections and deciphering of strain anisotropies in quantum dots.

Influence of the substrate surface termination on the properties of bcc-cobalt films: GaAs(1 1 0) versus Sb/GaAs(1 1 0)

Reflection high energy electron diffraction (RHEED) and synchrotron radiation photoemission spectroscopy (SRPES) have been used to investigate the evolution of the crystallinity and interface reaction of Co thin films deposited on Sb/GaAs(1 1 0). The RHEED images have allowed us to qualitative observe an improvement of the Co bcc crystal structure compared to the deposition on the bare substrate. Additionally, the use of high energy resolution synchrotron radiation photoemission spectroscopy has shown that the interdiffusion process behaves differently from that observed for the non-passivated surface.