High-energy Electron Diffraction Intensity Research Articles

Изучение кинетики сближения ступеней поверхности Si(100)

In this work, the convergence kinetics investigations of the SA and SB steps on Si(100) substrates with inclination 0.5o and 0.1o were carried out. Analysis of the time dependence of reflection high-energy electron diffraction (RHEED) intensity was used to establish the growth kinetics character on vicinal Si(100) surfaces. It is shown that, in a Si flow at the growth rate of 0.37 ML/s, the step convergence velocity has a decreasing exponential dependence with the temperature increase. It is determined that the single-domain surface formation velocity increases with an increase in the terrace width on the surface, which may be due to the partial participation of growth due to the formation of two-dimensional islands. Above a temperature of 650°C, the dominant growth mode is due to the step movement and the single-domain surface formation velocity decreases with an increase in the terrace width. Thus, the single-layer step convergence is determined by both the MBE growth conditions and the Si(100) substrate orientation. The convergence of SA and SB steps of the Si(100) surface is explained by the slowdown of the step SA motion, which is associated with complex permeability mechanisms and a kink formation of steps. It is assumed that the reason for the slowdown of the step convergence with increasing temperature is an increase in the kink density at the SA step, which reduces the step SA permeability coefficient.

Study of Si(100) surface step convergence kinetics

In this work, the convergence kinetics investigations of the SA- and SB-steps on Si(100) substrates with inclination 0.5o and 0.1o were carried out. Analysis of the time dependence of reflection high-energy electron diffraction (RHEED) intensity was used to establish the growth kinetics character on vicinal Si(100) surfaces. It is shown that, in a Si flow at the growth rate of 0.37 ML/s, the step convergence velocity has a decreasing exponential dependence with the temperature increase. It is determined that the single-domain surface formation velocity increases with an increase in the terrace width on the surface, which may be due to the partial participation of growth due to the formation of two-dimensional islands. Above a temperature of 650oC, the dominant growth mode is due to the step movement and the single-domain surface formation velocity decreases with an increase in the terrace width. Thus, the single-layer step convergence is determined by both the MBE growth conditions and the Si(100) substrate orientation. The convergence of SA- and SB-steps of the Si(100) surface is explained by the slowdown of the step SA-motion, which is associated with complex permeability mechanisms and a kink formation of steps. It is assumed that the reason for the slowdown of the step convergence with increasing temperature is an increase in the kink density at the SA-step, which reduces the step SA-permeability coefficient. Keywords:: Molecular-beam epitaxy, reflection high-energy electron diffraction, surface, terraces, steps, kinks.

A C++ code for simulations of RHEED intensity oscillations within the kinematical approximation

This paper proposes a new software implementation for the rapid development and deployment of computer simulations of reflection high-energy electron diffraction (RHEED) intensity oscillations during the growth of thin epitaxial films. The calculations are based on the kinematical approximation. In this model the RHEED intensity oscillations from growing layers is considered as an effect of changes of surface roughness. New version program summaryProgram Title: growth22CPC Library link to program files:https://doi.org/10.17632/zkpwhrrfng.1Licensing provisions: GNU General Public License 3Programming language: C++Journal reference of previous version: Computer Physics Communications 170 (2005) 265–286Does the new version supersede the previous version?: YesReasons for the new version:The previous version of the program was written in Object Pascal [1]. This programming language is no longer popular in the scientific community. Responding to user's feedback, the current distribution of the software has been implemented in C++. This version implements improvements for ergonomics, computational performances and code readability. In addition, the output generated by the growth22 (saved in the coverageProfiles.dat file) can be used directly by program [2].Summary of revisions:The growth22 is written in standard C++, therefore one can use the GNU g++ compiler or any ANSI C++ compliant compiler to build the executable program. The code is distributed in the form of one growth22.cpp source file. A Table 1 lists the retired Object Pascal procedures and shows replacements by C++ functions. The present version of the program enables the user to easily manage the input data. Input data are loaded from a manually prepared inputData.dat text file where the user provides information on the parameters of the simulation, i.e. the growth model, the number of growing layers, the upper limit of growth time interval, the values of the phenomenological parameters that measure the net rate of transfer from one layer to the next, and the values of the growth rate for each layer. The user can edit this file as per the requirement of the problem domain. Details of the parameters in the sample inputData.dat file are described in the readme.pdf file provided with the software.The nature of problem:Generally, in order to explain the experimental data two theories—kinematical and dynamical—associated with high-energy electron diffraction in epitaxial layers are used. The dynamical theories take into account multiple scattering within the crystal and are used whenever diffraction from a perfect epitaxial layers is considered [3]. On the other hand, the kinematical theory treats the scattering from each plane in the crystal sample as being independent of others [1,4].In the kinematical theory of RHEED it is assumed that electron wave scatters from atom in the surface layer only once and then interferes with waves scattered on other atoms. This model can be used to describe diffraction effects for small angles of incidence, when the electron beam does not penetrate the deeper layers of the crystal. In the kinematical model the intensity oscillations of specularly reflected electron beam from growing layer is considered as an effect of changes of surface roughness.In this paper we present a time-efficient computer model for kinematical calculations of RHEED intensity oscillations during the growth of thin epitaxial films.The method of solution:According to kinematical approximation the diffracted-beam intensity is given by [4]:(1)I(S‖→=0,Sz)=|∑n=0∞(θn−θn+1)exp(−iSznd)|2 where S→ is the scattering vector, d is the interlayer spacing and n is an integer. θn−θn+1 is the exposed layer coverage which is treated as the atomic distribution function gives the position of surface atoms. For Sz=π/d Eq. (1) simplifies to:(2)I=|∑n=0∞(θn−θn+1)cos⁡(nπ)|2.Variation of the coverage ratio θn of the nth surface layer is modelled by a set of non-linear differential equations [1,4]. The Cash-Karp fifth-order Runge-Kutta method [5] with error estimation and adaptive stepsize control was used for solving initial value problem for non-linear differential equations [6]. The absolute and relative error tolerances and number of integration intervals are fully user-configurable.The proposed code approaches the modelling of RHEED from two different points of view, i.e. the data and functional perspectives. The data perspective (i.e. calculating the layer coverages as a function of their growth time) is used to characterize the data sources of the program whereas the functional perspective includes services provided by the program. It should be noted that the perspective of the data can be expressed by a completely different model introduced by the user.The coverage evolution curves for the diffusive growth model, corresponding kinematical diffracted intensity and total roughness of the surface are shown in Fig. 1. During the numerical calculations showed in Fig. 1 it was assumed that the number of growing layers (numLayers) equals 15, the upper limit of growth time interval (tMax) equals 12, the growth rate equals 1 MLs−1 (for all layers), and the values of the parameter kn (that determines the amount of interlayer diffusion) changing according to the linear relation:(3)kn=k1−(k1−kN)n−1N−1, where N is the total number of growing layers.

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.

Multi-cycle RHEED oscillation under nitrogen supply in alternative source supply AlN growth by rf-MBE

An alternative source supply sequence was applied to aluminum nitride (AlN) growth by rf-plasma-assisted molecular-beam epitaxy on silicon carbide (SiC) substrates with a high-quality AlN template layer. Under a nitrogen-only source supply after aluminum source supply, multi-cycle oscillations of reflection high-energy electron diffraction (RHEED) intensity were observed, which were due to layer-by-layer growth of the AlN layer. The RHEED oscillation under nitrogen-only supply indicates that excess aluminum exists on the surface as a few-monolayer-thick wetting layer along with microdroplets. By repeating the sequence, a 105 nm thick high-quality AlN layer was coherently grown on a SiC substrate without aluminum droplets.

Changes of RHEED intensity with hydrogen position on metal surface

Based on the multiple scattering theory, reflection high-energy electron diffraction (RHEED) intensities are calculated from H-adsorbed Ni(111) surfaces by explicitly taking the scattering from H atoms into account. Systematic RHEED intensity changes in the rocking curves and the azimuthal plots are obtained by changing the H position both in the surface normal and parallel directions. In the rocking curves, the peak and dip positions as well as their intensities distinctly change under some diffraction conditions. In the azimuthal plots, the peak widths show systematic variations as a function of H height. Changes in RHEED intensities are also obtained from the surfaces with subsurface H as a function of the H depth. The intensity changes as a function of H position are different from those as a function of surface relaxation of the topmost Ni layer. The results show that RHEED should be a promising tool to extract the structural information of H on metal surfaces.

Fast Phase Retrieval from Reflection High Energy Electron Diffraction Intensities during Growth

Fast Phase Retrieval from Reflection High Energy Electron Diffraction Intensities during Growth

Structural and magnetic properties of perovskite SrMnO3 thin films grown by molecular beam epitaxy

The structural and magnetic properties of the epitaxial (0 0 1) SrMnO3 (SMO) thin film grown on (0 0 1) SrTiO3 (STO) substrate by molecular beam epitaxy (MBE) are reported. In-situ reflection high energy electron diffraction (RHEED) intensity oscillation is utilized to precisely adjust the stoichiometry by tuning the atom source shuttered time during the growth. The surface of the thin film with obvious steps and terraces is observed to be atomically flat. The epitaxial single crystal SMO thin film which is grown along the direction of (0 0 1) STO substrate is confirmed to be four-fold symmetric along the 〈0 0 1〉 azimuth by X-ray diffraction φ scan. The XRD reciprocal space map indicates that the SMO thin film is effectively strained in the present thickness and is typical tetragonal crystal structure. It is found that the present SMO thin film is ferromagnetic. The factors, namely Mn3+-Mn4+ double exchange coupling, oxygen vacancies and the strain, lead the SMO thin films to produce ferromagnetism.

Monitoring surface roughness during film growth using modulated RHEED intensity oscillations

Separation of the high- and low-frequency components of Reflection High-Energy Electron Diffraction (RHEED) intensity oscillations during pulsed deposition allows the extraction of a signal that is in phase with the cyclic surface morphology evolution during layer-by-layer growth. Similar to a biased impedance measurement in electricity, the periodic modulation of surface roughness induced by the pulsed deposition probes the differential response of the growth front to changes in step density. This signal does not follow the complex variation of the RHEED oscillation phase with diffraction conditions and surface reconstruction and therefore allows a direct detection of monolayer completion. Off-Laue Circle oscillations show promise to probe the surface morphology evolution at sharply defined in-plane spatial frequencies.

Real-time algorithm to determine the period and phase of Reflection High-Energy Electron Diffraction (RHEED) intensity oscillations during epitaxial growth

We present and discuss an algorithm that reliably determines the period and phase of RHEED intensity oscillations. Based on multiple running averages of the data, this algorithm is not affected by damping or rapid arbitrary changes of the average intensity on a time scale larger than the growth oscillations. It is therefore suitable for a general and precise determination of the deposited layer thickness, at least in homoepitaxy. An additional pre-filtering running average allows the removal of the periodic modulation induced in pulsed laser deposition (PLD).

Algorithms for determining the phase of RHEED oscillations

Oscillations of reflection high-energy electron diffraction (RHEED) intensities are computed using dynamical diffraction theory. The phase of the oscillations is determined using two different approaches. In the first, direct, approach, the phase is determined by identifying the time needed to reach the second oscillation minimum. In the second approach, the phase is found using harmonic analysis. The two approaches are tested by applying them to oscillations simulated using dynamical diffraction theory. The phase of RHEED oscillations observed experimentally is also analysed. Experimental data on the variation of the phase as a function of the glancing angle of incidence, derived using the direct method, are compared with the values computed using both the direct and harmonic methods. For incident-beam azimuths corresponding to low-symmetry directions, both approaches produce similar results.

Hybrid molecular beam epitaxy for the growth of stoichiometric BaSnO3

Owing to its high room-temperature electron mobility and wide bandgap, BaSnO3 has recently become of significant interest for potential room-temperature oxide electronics. A hybrid molecular beam epitaxy (MBE) approach for the growth of high-quality BaSnO3 films is developed in this work. This approach employs hexamethylditin as a chemical precursor for tin, an effusion cell for barium, and a radio frequency plasma source for oxygen. BaSnO3 films were thus grown on SrTiO3 (001) and LaAlO3 (001) substrates. Growth conditions for stoichiometric BaSnO3 were identified. Reflection high-energy electron diffraction (RHEED) intensity oscillations, characteristic of a layer-by-layer growth mode were observed. A critical thickness of ∼1 nm for strain relaxation was determined for films grown on SrTiO3 using in situ RHEED. Scanning transmission electron microscopy combined with electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy confirmed the cube-on-cube epitaxy and composition. The importance of precursor chemistry is discussed in the context of the MBE growth of BaSnO3.

Theoretical analysis of reflection high-energy electron diffraction (RHEED) and reflection high-energy positron diffraction (RHEPD) intensity oscillations expected for the perfect layer-by-layer growth.

Predictions from two theoretical models, allowing one to determine the phase of intensity oscillations, are compared for reflected beams of electrons and positrons. Namely, results of the precise dynamical calculations are compared with results obtained using a simplified approach. Within the simplified model, changes in the specularly reflected beam intensity, expected to occur during the deposition of new atoms, are described with the help of interfering waves and the effect of refraction, and respective approximate analytical formulas are employed to determine the phase of the oscillations. It is found that the simplified model is very useful for understanding the physics ruling the appearance of intensity oscillations. However, it seems that the model with the realistic potential is more suitable for carrying out interpretations of experimental data.

Direct growth of metallic TiH2 thin films by pulsed laser deposition

We report the layer-by-layer growth and electronic properties of (111)-oriented, nearly stoichiometric TiH2 films (δ-phase) pulsed-laser-deposited on α-Al2O3(0001) using a TiH2 ceramic target. The content of the δ-phase increased as the decomposition to the Ti metal was suppressed at low temperatures. Moreover, long-lasting oscillations of reflection high-energy electron diffraction intensity were observed during the initial growth of the δ-phase film. The film showed metallic conductivity down to low temperatures. The results of Ti 2p–3d resonant photoemission spectroscopy and Hall measurement were consistent with those of the conducting electrons residing in the Ti 3d states.

Complex oxide growth using simultaneous in situ reflection high-energy electron diffraction and x-ray reflectivity: When is one layer complete?

During layer-by-layer homoepitaxial growth, both the Reflection High-Energy Electron Diffraction (RHEED) intensity and the x-ray reflection intensity will oscillate, and each complete oscillation indicates the addition of one monolayer of material. However, it is well documented, but not well understood, that the phase of the RHEED oscillations varies from growth to growth and thus the maxima in the RHEED intensity oscillations do not necessarily occur at the completion of a layer. We demonstrate this by using simultaneous in situ x-ray reflectivity and RHEED to characterize layer-by-layer growth of SrTiO3. We show that we can control the RHEED oscillation phase by changing the pre-growth substrate annealing conditions, changing the RHEED oscillation phase by as much as 137°. In addition, during growth via pulsed laser deposition, the relaxation times between each laser pulse can be used to determine when a layer is complete, independent of the phase of the RHEED oscillation.

Computer studies on reflection high-energy electron diffraction from the growing surface of Ge(001)

The results of calculations of reflection high-energy electron diffraction intensities, measured at different stages of the homoepitaxial growth of Ge(001), are described. A two-dimensional Bloch wave approach was used in calculations of the Schrödinger equation with a one-dimensional potential. The proportional model was used for partially filled layers,i.e.the scattering potential was taken to be proportional to the coverage and the potential of the fully filled layer. Using such an approach, it was shown that it is possible to obtain valuable information for the analysis of experimental data. The results of these calculations were compared with data for off-symmetry azimuths from the literature, and satisfactory agreement between the theoretical and experimental data was found. Also assessed was whether developing more advanced models (i.e.going beyond the proportional model), to make a more detailed account of the diffuse scattering, might be important in achieving a fully quantitative explanation of the experimental data.

Structural and electrical characterization of InN, InGaN, and p-InGaN grown by metal-modulated epitaxy

InN, high indium content InGaN, and Mg-doped InGaN were grown by metal modulated epitaxy (MME). Transient reflection high-energy electron diffraction intensities were analyzed during the growth of InN and found to be similar to that previously reported for GaN and AlN. The x-ray diffraction rocking curve and background electron concentration of InN grown by MME were found to be respectable in comparison to recent reports in literature. InGaN alloys grown by MME were also investigated, and a method for detecting indium surface segregation was demonstrated. It was found that the shutter modulation scheme could be modified to prevent phase separation by indium surface segregation, and a range of single-phase InGaN samples with indium contents throughout the miscibility gap were grown. Using the discovered method of suppressing phase separation, several p-InxGa1 − xN samples were grown with indium contents from x = 0 to 0.22. A maximum hole concentration of 2.4 × 1019 cm−3 was detected by Hall effect characterization, demonstrating feasibility of these p-InGaN layers for use in several device applications.

Epitaxial Structures of Band-Gap-Engineered α-(CrxFe1-x)2O3 (0 ≤x≤1) Films Grown on C-Plane Sapphire

Solid solutions of α-Cr2O3 and α-Fe2O3 are novel oxide semiconductors exhibiting narrower band gaps than those of the end members. We have investigated the heteroepitaxial growth and crystalline structure of pulsed-laser deposited α-(CrxFe1-x)2O3 films on c-plane sapphire substrates. Under the growth condition optimized for α-Fe2O3, reflection high-energy electron diffraction intensity oscillations were observable for both α-Fe2O3 and α-Cr2O3, enabling us to control film thickness in a layer-by-layer fashion. The composition dependence of the epitaxial structures, including phase purity, orientation, strain, and in-plane rotation domain, was characterized by X-ray diffraction to reveal a defect-free composition range (0 ≤x ≤0.50). The films with 0.60 ≤x ≤1 were found to have 180°-rotation domains. The absorption spectra of α-(Cr0.50Fe0.50)2O3 solid solution (corundum) and superlattice (nominally composed ilmenite) films are compared with those of end members and an α-Fe2O3/α-Cr2O3 bilayer film to elucidate the origin of the band-gap narrowing.

Coherent growth of superconducting TiN thin films by plasma enhanced molecular beam epitaxy

We have investigated the formation of titanium nitride (TiN) thin films on (001) MgO substrates by molecular beam epitaxy and radio frequency acitvated nitrogen plasma. Although cubic TiN is stabile over a wide temperature range, superconducting TiN films are exclusively obtained when the substrate temperature exceeds 710 °C. TiN films grown at 720 °C show a high residual resistivity ratio of approximately 11 and the superconducting transition temperature (Tc) is well above 5 K. Superconductivity has been confirmed also by magnetiztion measurements. In addition, we determined the upper critical magnetic field (μ0Hc2) as well as the corresponding coherence length (ξGL) by transport measurements under high magnetic fields. High-resolution transmission electron microscopy data revealed full in plane coherency to the substrate as well as a low defect density in the film, in agreement with a mean-free path length ℓ ≈ 106 nm, which is estimated from the residual resistivity value. The observations of reflection high energy electron diffraction intensity oscillations during the growth, distinct Laue fringes around the main Bragg peaks, and higher order diffraction spots in the reciprocal space map suggest the full controlability of the thickness of high quality superconducting TiN thin films.

Observation and control of the surface kinetics of InGaN for the elimination of phase separation

The growth of InGaN alloys via Metal-Modulated Epitaxy has been investigated. Transient reflection high-energy electron diffraction intensities for several modulation schemes during the growth of 20% InGaN were analyzed, and signatures associated with the accumulation, consumption, and segregation of excess metal adlayers were identified. A model for shuttered, metal-rich growth of InGaN was then developed, and a mechanism for indium surface segregation was elucidated. It was found that indium surface segregation only occurs after a threshold of excess metal is accumulated, and a method of quantifying this indium surface segregation onset dose is presented. The onset dose of surface segregation was found to be indium-composition dependent and between 1 and 2 monolayers of excess metal. Below this surface threshold off excess metal, metal-rich growth can occur without indium surface segregation. Since at least 2 monolayers of excess metal will accumulate in the case of metal-rich, unshuttered growth of InGaN at the low temperatures required to suppress thermal and spinodal decomposition, this study reveals that some form of modulation must be employed to maintain this adlayer thickness. These theories were applied in the growth of InGaN with varying compositions using Metal-Modulated Epitaxy. Single-phase, high-quality InGaN films with compositions throughout the miscibility gap with root mean square roughnesses less than 0.8 nm were obtained, demonstrating the feasibility of shuttered, metal-rich InGaN growth.