Reflection High-energy Electron Diffraction Research Articles

(Invited) Thin Epitaxial Single-Crystal Si on Sige Followed By in-Situ Deposition of High-k Dielectrics – Novel Gate Stacks for Achieving Extremely Low Dit and Highly Reliable SiGe MOS

Silicon-germanium (SiGe) is replacing silicon (Si) as the channel layer for the p-type metal-oxide-semiconductor (pMOS) in the aggressively scaled complementary MOS (CMOS) technology. Many research efforts involving SiGe gate stacks have resulted in the attainment of low interfacial trap densities (Dits). Si-passivated SiGe gate stacks are the most promising method to achieve both low Dit and high device reliability. However, regardless of the growth methods, most publications reported the pileup of Ge atoms on top of the grown Si even after extensive treatments. The subsequent process in forming gate stacks, including oxidation, inevitably caused the segregated Ge to form GeOx, leading to degradation of the interfacial quality of the gate stacks.We have used molecular beam epitaxy (MBE) to grow thin single-crystal Si of 0.79nm thickness (six MLs) epitaxially on epi-Si1-xGex(001) under ultra-high vacuum (UHV). Different from its counterpart of chemical vapor deposition (CVD), MBE enables epi-Si growth on Ge at growth temperatures below 300 °C. The measurements using reflection high-energy electron diffraction (RHEED), high-resolution synchrotron radiation X-ray diffraction (HR-XRD), and the scanning transmission electron microscopy with high-angle annular-dark-field (STEM-HAADF) have revealed high crystallinity of the epi-Si, the Si1-xGex layers and abrupt interfaces of the high-k/epi-Si/Si1-xGex layers. Moreover, our unique capability of epi growth/analysis enabled the study of the electronic surface structures of epi-Si/epi-Ge using in-situ high-resolution synchrotron radiation photoemission spectroscopy (SRPES) via a portable sample transfer station under UHV. We have used SRPES to demonstrate that the as-grown Si1−xGex(001)-2×1 surfaces with Ge content ranging from 0.1 to 0.9 are all terminated with buckled Ge–Ge dimers, and the surface electronic structure is similar to that of epi-Ge(001)-2×1. The direct deposition of epi-Si on as-grown SiGe removed the top buckled Ge-Ge dimers, which diffused into the epi-Si and some segregated to the top of the Si. As a result, the topmost layer beneath the epi-Si was 1 ML of Ge, and the interfaces of high-k/epi-Si/Si1−xGex are similar to those of the high-k/epi-Si/Ge. Again, the hetero-interfaces of our gate dielectrics are different from those grown using CVD.The well-controlled interfaces have enabled the achievement of low interfacial trap densities (Dit) of (3 - 6) × 1011 eV-1cm-2 in these high-k/epi-Si/Si1-xGex samples. The minimum Dit values remained at 3 × 1011 eV-1cm-2 regardless of the Ge content, demonstrating the effective passivation of the low-temperature deposited epi-Si. The limited Ge segregation during the low-temperature growth of epi-Si may attribute to the attainment of the low Dit values. We have benchmarked the minimum Dit (Dit, min) of our work with other state-of-the-art SiGe gate stacks. The Dit,min values from the other efforts increased with increasing Ge content, which may be caused by the GeOx formation that was difficult to control in the high-Ge-content (HGC) Si1-xGex.The effective charge sheet densities for the Si1-xGex gate stacks were extracted via examination of capacitance-voltage (C-V) hysteresis with decreasing stress voltage in the accumulation region of the MOS capacitors. We have attained very high acceleration factors of 8-12, indicating high reliability of the HfO2/epi-Si/Si1-xGex pMOS gate stacks.Furthermore, we have used a scavenging method, consisting of a cyclic process of an O2 exposure followed by a UHV annealing, to eliminate the segregated and diffused Ge in epi-Si before high-k deposition. Upon O2 exposure at room temperature, both the Si and Ge surface atoms are simultaneously oxidized to give rise to four Si charge states and Ge suboxides, respectively. The subsequent in-situ annealing at 500 °C under UHV moved the oxygen atom in the Ge suboxides to bond with the nearby Si atom. The annealing also caused the diffused Ge inside the epi-Si to segregate to the surface. The processes of O2 exposure followed by annealing were repeated three times resulting in an oxidized Si/Ge surface having only the four Si oxidized states without GeOx, but with a minimal amount of segregated Ge.

Consideration of the Intricacies Inherent in Molecular Beam Epitaxy of the NaCl/GaAs System.

The high cost of substrates for III–V growth can be cost limiting for technologies that require large semiconductor areas. Thus, being able to separate device layers and reuse the original substrate is highly desirable, but existing techniques to lift a film from a substrate have substantial drawbacks. This work discusses some of the complexities with the growth of a water-soluble, alkali halide salt thin film between a III–V substrate and overlayer. Much of the difficulty stems from the growth of GaAs on an actively decomposing NaCl surface at elevated temperatures. Interestingly, the presence of an in situ electron beam incident on the NaCl surface, prior to and during GaAs deposition, affects the crystallinity and morphology of the III–V overlayer. Here, we investigate a wide range of growth temperatures and the timing of the impinging flux of both elemental sources and high energy electrons at different points during the growth. We show that an assortment of morphologies (discrete islands, porous material, and fully dense layers with sharp interfaces) and crystallinity (amorphous, crystalline, and highly textured) occur depending on the specific growth conditions, driven largely by changes in GaAs nucleation which is greatly affected by the presence of the reflection high energy electron diffraction beam.

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.

Thickness-dependent structural phase transition and self-intercalation of two-dimensional ferromagnetic chromium telluride thin films

The quest for next generation spintronic devices has promoted the exploration of ferromagnetism in a two-dimensional (2D) limit which enriches the family of 2D materials. Here, we realized the molecular beam epitaxial growth of atomically flat chromium telluride (CrTex) films on Si(111) substrates in the 2D limit and discovered a thickness-dependent structural phase transition with self-intercalation during the growth. Combining the in situ reflection high-energy electron diffraction, scanning tunneling microscopy, x-ray photoemission spectroscopy, and ex situ x-ray diffraction, we found that the first layer of CrTex films formed in a CrTe2 crystalline phase as a buffer layer for further growth. Afterward, the chromium atoms began to intercalate into the layers of CrTe2, and the Cr3Te4 phase dominated the following growth over the second layer. Subsequent superconducting quantum interference device measurements verified the ferromagnetism in the chromium telluride film down to one layer limit. Our results provide important information on the structural phase transition during the growth of CrTex films, which would be an ideal platform for studying ferromagnetism in 2D systems, and the growth of high-quality CrTex films on Si substrates would benefit the further applications of 2D ferromagnetic films.

Stoichiometric Growth of Monolayer FeSe Superconducting Films Using a Selenium Cracking Source

As a novel interfacial high-temperature superconductor, monolayer FeSe on SrTiO3 has been intensely studied in the past decade. The high selenium flux involved in the traditional growth method complicates the film’s composition and entails more sample processing to realize the superconductivity. Here we use a Se cracking source for the molecular beam epitaxy growth of FeSe films to boost the reactivity of the Se flux. Reflection high-energy electron diffraction shows that the growth rate of FeSe increases with the increasing Se flux when the Fe flux is fixed, indicating that the Se over-flux induces Fe vacancies. Through careful tuning, we find that the proper Se/Fe flux ratio with Se cracked that is required for growing stoichiometric FeSe is close to 1, much lower than that with the uncracked Se flux. Furthermore, the FeSe film produced by the optimized conditions shows high-temperature superconductivity in the transport measurements without any post-growth treatment. Our work reinforces the importance of stoichiometry for superconductivity and establishes a simpler and more efficient approach to fabricating monolayer FeSe superconducting films.

The Effects of Substrate Temperature on the Growth, Microstructural and Magnetic Properties of Gadolinium-Containing Films on Aluminum Nitride

To facilitate future novel devices incorporating rare earth metal films and III-V semiconductors on Si substrates, this study investigates the mechanisms of growth via molecular beam epitaxy of gadolinium (Gd) on aluminum nitride (AlN) by determining the impact of substrate temperature on microstructure. The Gd films underwent extensive surface analysis via in situ reflective high energy electron diffraction (RHEED) and ex-situ SEM and AFM. Characterization of the surface features of rare earth metal films is important, as surface geometry has been shown to strongly impact magnetic properties. SEM and AFM imaging determined that Gd films grown on AlN (0001) from 80 °C to 400 °C transition from wetting, nodular films to island–trench growth mode to reduce in-plane lattice strain. XRD and Raman spectroscopy of the films revealed that they were primarily comprised of GdN, Gd and Gd2O3. The samples were also analyzed by a vibrating sample magnetometer (VSM) at room temperature. From the room temperature magnetic studies, the thick films showed superparamagnetic behavior, with samples grown between 240 °C and 270 °C showing high magnetic susceptibility. Increasing GdN (111) 2θ peak position and single-crystal growth modes correlated with increasing peak magnetization of the thin films, suggesting that lattice strain in single-crystal films was the primary driver of enhanced magnetic susceptibility.

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

Molecular beam epitaxy of antiperovskite oxides

Antiperovskites, or inverse perovskites, have recently emerged as a material class with a plethora of promising electronic properties. This Perspective describes the molecular beam epitaxy (MBE) growth of oxide antiperovskites Sr3PbO and Sr3SnO. We show that MBE offers great potential in regard to not only growing antiperovskites with high structural quality but also providing a means for seamless integration with advanced characterization techniques, including x-ray photoelectron spectroscopy, low-energy electron diffraction, reflection high-energy electron diffraction, and scanning tunneling microscopy, to facilitate the analyses of their intrinsic properties. The initial results point toward the feasibility of atomically controlled antiperovskite growth, which can open doors to study topological and correlated electronic states in an electronic environment quite distinct from what is available in conventional complex oxides.

Growth and characterization of (Sc2O3)x(Ga2O3)1−x by molecular beam epitaxy

(Sc2O3)x(Ga2O3)1−x was grown by molecular beam epitaxy at low temperatures (100 °C) using a variety of growth sequences to avoid surface segregation of Ga. Continuous and digital growth techniques always produced Ga segregation. This surface segregation was attributed to the stronger bond between the Sc and O compared to the Ga and O. A digital growth technique (alternate opening of Sc and Ga shutters with the O shutter open continuously during the growth) was unsuccessful in eliminating this effect. The segregation was eliminated using a growth technique in which the Ga shutter was closed for a set amount of time toward the end of the growth while the O and Sc shutters remained open. Characterization with reflection high energy electron diffraction, x-ray diffraction, and transmission electron microscopy revealed the growth of a fine-grained polycrystalline film under these conditions. A third growth technique was used that involved closing the Ga shutter for a set amount of time toward the end of the growth while the O and Sc shutters were open continuously. This technique was successful in depositing a uniform film. However, the breakdown field was only 1.40 MV/cm (at 1 mA/cm2). The addition of Ga to Sc2O3 diminished the insulating properties of the film. These initial experiments indicate that phase segregation is likely to be a major issue with most growth techniques and that alloying Ga2O3 with elements other than Sc, such as Gd or Al, might be a more successful approach.

Detailed surface studies on the reduction of Al incorporation into AlGaN grown by molecular beam epitaxy in the Ga-droplet regime

A better understanding of the molecular beam epitaxy (MBE) grown AlGaN layers is crucial for the development of UV emitters based on III-nitrides. In this work, the growth kinetics of AlGaN structures deposited by plasma-assisted MBE under Ga-rich conditions on AlGaN/AlN/sapphire templates is studied in detail. The surface quality of the layers and the Al concentration was investigated in situ by reflection high-energy electron diffraction, atomic force microscopy and X-ray photoelectron spectroscopy. The Al content in the grown AlGaN layers was also determined by high-resolution X-ray diffraction, while optical properties were examined by photoluminescence and cathodoluminescence. Obtained results reveal that an increase of the Ga flux influences the Al incorporation probability only when stepping into the Ga-droplet regime. This leads to a decrease of the Al concentration in AlGaN layers, but to a different extent in the bulk and at the surface.

Mg‐Facilitated Growth of Cubic Boron Nitride by Ion Beam‐Assisted Molecular Beam Epitaxy

Trace amounts of Mg deposited on a diamond (100) substrate surface facilitate the growth of cubic boron nitride (c‐BN) by ion beam‐assisted molecular beam epitaxy. Fourier transform infrared spectroscopy indicates that films grown with Mg are cubic, while those without Mg are either hexagonal BN or lacking measurable cubic or hexagonal signatures. Initiating the growth with 0.005 monolayer equivalent of Mg is sufficient to yield epitaxial films with >99% c‐BN. Reflection high energy electron diffraction, electron energy loss spectroscopy, and X‐Ray photoelectron spectroscopy indicate the surface of the film to be sp2‐bonded BN, consistent with the results of other groups. High‐resolution scanning transmission electron microscopy reveals c‐BN with a high density of stacking faults and twinning. A model is proposed by which Mg locally diminishes the energy barrier to dissociation of the as‐deposited sp2‐bonded BN, facilitating the nucleation of c‐BN.

Direct observation of spin polarization in epitaxial Fe3O4(001)/MgO thin films grown by magnetron sputtering

We obtained epitaxial single-crystal Fe3O4(001)/MgO(001) thin films by magnetron sputtering. The high quality of the grown Fe3O4 films was confirmed by reflection high-energy electron diffraction and x-ray photoelectron spectroscopy. Atomic magnetic properties of Fe3O4(001)/MgO(001) were investigated using vibrating sample magnetometry and x-ray magnetic circular dichroism. The values of saturation magnetization and magnetic moment are 407 ± 5 emu/cm3 (3.26 ± 0.04 μB/(f.u.)) and 3.31 ± 0.15 μB/(f.u.), respectively, in the Fe3O4 film as thin as 5 nm, which are close to the bulk values. The spin polarization was directly measured using spin-resolved photoemission spectroscopy. The measured spin polarization has a maximum value of −42% ± 3%, which is comparable to the theoretical value for the (2 × 2)R45° reconstructed Fe3O4(001) surface. Furthermore, the film thickness-dependent measurements indicate that the anti-phase boundaries significantly decrease the spin polarization rather than the lattice mismatch. Our results demonstrate that epitaxial Fe3O4(001)/MgO thin films grown by magnetron sputtering have desired magnetic properties, facilitating the potential application of Fe3O4-based spintronic devices.

Sim-trhepd-rheed – Open-source simulator of total-reflection high-energy positron diffraction (TRHEPD) and reflection high-energy electron diffraction (RHEED)

The present paper reports sim-trhepd-rheed (STR), an open-source simulator of total-reflection high-energy positron diffraction (TRHEPD) and reflection high-energy electron diffraction (RHEED) experiments which are used for atom-scale surface structure determination of a material. Diffraction data obtained by these experiments are analyzed by comparison with numerical simulations. The STR simulator is used for the analysis of experimental diffraction data by simulating the rocking curve from a given trial surface structure by solving the partial differential equation of the dynamical quantum diffraction theory for positron or electron wavefunctions. Using the obtained surface structure, electronic structure, and other physical quantities can be evaluated through first-principles calculations. For this purpose, a utility software was also developed in order to realize a first principles calculation with the Quantum ESPRESSO suite. Program summaryProgram title: sim-trhepd-rheedCPC Library link to program files:https://doi.org/10.17632/89vpfvp84k.1Developer's repository link:https://github.com/sim-trhepd-rheed/sim-trhepd-rheed/Code Ocean capsule:https://codeocean.com/capsule/2323181Licensing provisions: GNU General Public License v3.0Programming language: Fortran 90, Python 3External routines/libraries: BLAS library, LAPACK libraryNature of problem: Partial differential equation in dynamical quantum diffraction theory.Solution method: Numerical solution by the multi-slice method for the partial differential equation.

Engineering the stoichiometry of a TiO2-rich SrTiO3(001) surface

We investigate the stoichiometry of a TiO2-rich SrTiO3(001) surface by depositing less than one monolayer of SrO under typical epitaxy conditions. We find that this deposition yields atomically flat islands on the step-and-terrace surface. All these islands have a height of exactly one full SrTiO3 unit cell, instead of the expected half unit cell. Reflection high-energy electron diffraction indicates that a reconstruction occurs on the bare surface, which disappears with SrO deposition. To explain these results, we propose a simple model with two key elements: (i) a TiO2 double layer must intrinsically be present on the as-prepared SrTiO3(001) surface, and (ii) with deposited SrO, this top layer rearranges itself to form islands with a height of one unit cell.

In-situ estimation of emission wavelength of embedded InAs QDs using RHEED intensity measurements

We propose a method for estimating the emission wavelength of self-assembled InAs quantum dots (QDs) embedded in a capping layer using reflection high-energy electron diffraction (RHEED) intensity measurements. As QDs are capped with a Ga(In)As layer, the RHEED intensity of QDs decays, and the decay degree depends on the growth conditions of capping. The RHEED intensity decay was analyzed, and a relationship between the decay degree and photoluminescence peak energy of embedded QDs was determined. This relationship can be used to predict the emission wavelength of embedded QDs during capping, which is particularly useful for QD-based optical device fabrication.

In-situ study of InAs quantum dots encapsulated in asymmetric (Al)GaAs confinement barriers

In this work the self-assembling of InAs quantum dots (QDs) within asymmetric barriers of (Al)GaAs is studied via reflection high energy electron diffraction (RHEED). A comparative study between the AlGaAs/InAs/GaAs interfaces and its mirror-like heterostructure GaAs/InAs/AlGaAs showed significant differences in the self-assembling and capping of the QDs. The critical thickness of InAs QDs results was proven to be larger when it is grown on AlGaAs alloys, compared with the deposition on GaAs layers. This change is explained by the reduced mobility of In atoms on the Al-containing surfaces, for which the QDs density is increased due to the strain relieve. Through the in-situ analysis of diffusion parameters, it is concluded that the mobility of In atoms decreases the mass transport of 2D and 3D precursors that conduces to the self-assembling of the QDs nanoislands, modifying the rate at which the QDs are formed. Further, during the first stages of QDs capping it is observed that the III-V materials intermixing plays a predominant role. The nanoislands are less affected when are covered by AlGaAs in comparison with the GaAs capping, preserving the QDs morphology and avoiding materials alloying. By following the RHEED intensity behavior during the QDs capping, a model was proposed to obtain quantitative parameters for the smoothing process. High resolution x ray diffraction (HRXRD) measurements show the composition of sharp interfaces for the AlGaAs/InAs/GaAs heterostructure. Lastly, numerical simulations were performed to evaluate the strain changes using the experimental information as input data.

Canonical approach to cation flux calibration in oxide molecular-beam epitaxy

Molecular-beam epitaxy (MBE) is the gold standard for the epitaxial growth of complex oxides with the best material properties as determined by respective figures of merit. Unfortunately, once more than one cation is involved in the material desired, MBE growth often becomes plagued by difficulties in stoichiometry control. Instead of relying on a quartz crystal microbalance to measure the fluxes of the individual molecular beams, which lacks accuracy, or reflection high-energy electron diffraction oscillations of the targeted multication oxide in layer-by-layer growth, which lacks general applicability, here, we describe a canonical approach based on the growth of films of the constituent binary oxides or metals individually for cation flux calibration. This method can calibrate the flux of each molecular beam with an absolute accuracy of \ifmmode\pm\else\textpm\fi{}1%. After describing the growth parameters of binary oxides or metals enabling the individual fluxes of 39 elements of the periodic table to be determined, we demonstrate the efficacy of this approach by applying it to the growth of the quaternary ferromagnetic metal ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ to achieve films with transport properties rivalling the best reported using thin-film growth techniques providing stoichiometric transfer.

Correlation between Plasmon Excitations and Wavefields under Reflection High-Energy Electron Diffraction Condition

Correlation between Plasmon Excitations and Wavefields under Reflection High-Energy Electron Diffraction Condition

Combinatorial synthesis of heteroepitaxial, multi-cation, thin-films via pulsed laser deposition coupled with in-situ, chemical and structural characterization

Combinatorial synthesis via a continuous composition spread is an excellent route to develop thin-film libraries as it is both time- and cost-efficient. Creating libraries of functional, multicomponent, complex oxide films requires excellent control over the synthesis parameters combined with high-throughput analytical feedback. A reliable, high-throughput, in-situ characterization analysis method is required to meet the crucial need to rapidly screen materials libraries. Here, we report on the combination of two in-situ techniques—(a) Reflection high-energy electron diffraction (RHEED) for heteroepitaxial characterization and a newly developed compositional analysis technique, low-angle x-ray spectroscopy (LAXS), to map the chemical composition profile of combinatorial heteroepitaxial complex oxide films deposited using a continuous composition spread method via pulsed laser deposition. This is accomplished using a unique state-of-the-art combinatorial growth system with a fully synchronized four-axis mechanical substrate stage without shadow masks, alternating acquisition of chemical compositional data using LAXS at various different positions on the sim 41 mm times 41 mm range and sequential deposition of multilayers of SrTiO_3 and hbox {SrTi}_{0.8}hbox {Ru}_{0.2}hbox {O}_3 on a 2-inch (50.8 mm) hbox {LaAlO}_3 wafer in a single growth run. Rutherford backscattering spectrometry (RBS) is used to calibrate and validate the compositions determined by LAXS. This study shows the feasibility of combinatorial synthesis of heteroepitaxial, functional complex oxide films at wafer-scale via two essential in-situ characterization tools—RHEED for structural analysis or heteroepitaxy and LAXS for compositional characterization. This is a powerful technique for development of new films with optimized heteroepitaxy and composition.