Reflection High-energy Electron Diffraction Intensity Research Articles

Dynamic change in the surface and layer structures during epitaxial growth of Si on aSi(111)−7×7surface

In order to investigate the dynamic process during growth of a Si layer on the $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7$ surface held at 380 \ifmmode^\circ\else\textdegree\fi{}C, the rocking curve of reflection high-energy electron diffraction (RHEED) is continuously measured at 0.5\ifmmode^\circ\else\textdegree\fi{} to 6\ifmmode^\circ\else\textdegree\fi{} at intervals of 0.05\ifmmode^\circ\else\textdegree\fi{} to the glancing angle of the incident electron beam which takes 18 sec. At the initial growth stage, the multilayer islands are grown on the native $7\ifmmode\times\else\texttimes\fi{}7$ surface with broader Bragg peaks in the rocking curve than those from the native surface. The sharpness of the Bragg peak is subsequently recovered after the thickness of the Si layer reaches 3 BL $(1\mathrm{B}\mathrm{L}=0.31\mathrm{}\mathrm{nm}),$ at which the growth transforms to layer-by-layer growth. The comparison of the measured rocking curve with the calculated one based on the dynamical theory of RHEED intensity is also performed by optimizing each atomic position in the growing layer so as to minimize the difference between both curves. The space of the double layer of the (111) plane in the multilayer islands expands and is restored to the normal spacing after the growth mode transforms to the layer-by-layer mode. The broadening of the Bragg peaks at the initial growth stage relates to the rearrangement process of a stacking-fault layer in the $7\ifmmode\times\else\texttimes\fi{}7$ structure on the substrate surface.

Atomic hydrogen-cleaned GaAs(100) negative electron affinity photocathode: Surface studies with reflection high-energy electron diffraction and quantum efficiency

The quantum efficiency of a vicinal GaAs(100) negative electron affinity (NEA) photocathode is studied and correlated to the surface morphology. Cleaning of a GaAs(100) vicinal surface by atomic hydrogen and by heating are investigated using reflection high-energy electron diffraction (RHEED). After atomic hydrogen cleaning at 500 °C, the GaAs surface exhibits a streaky (2×4)-reconstructed RHEED pattern. When the GaAs(100) surface is activated to NEA by the alternate deposition of cesium and oxygen, a quantum efficiency of ∼9% is measured. The photocathode quantum efficiency correlates with the out-of-phase RHEED intensity measured before activation. After the quantum efficiency decreases with operating time, further atomic hydrogen exposure also produces a (2×4) pattern. Surfaces prepared or revived by atomic hydrogen produce brighter out-of-phase electron diffraction patterns and, when activated to NEA, higher quantum efficiency compared to those that are heat cleaned.

Plasma-assisted molecular beam epitaxy for ZnO based II–VI semiconductor oxides and their heterostructures

Plasma-assisted molecular beam epitaxy of ZnO epilayers and MgZnO/ZnO heterostructures on Al2O3(0001) substrates is described. A thin MgO layer is employed as a buffer for ZnO. The influence of the buffer on the initial growth of ZnO is discussed with the corresponding reflection high-energy electron diffraction (RHEED) studies. We found that the MgO buffer promotes the lateral growth of ZnO, which results in two-dimensional growth. A 3×3 reconstruction is observed and the RHEED intensity oscillations are recorded. The RHEED oscillations have been used in situ to monitor and control the growth of MgZnO/ZnO heterostructures. MgZnO/ZnO single-quantum-well structures have been grown and studied by cathodoluminescence.

Wide range temperature dependence of reflection high-energy electron diffraction rocking curve from a Si(111)7×7 surface

In order to determine the Debye parameter for the surface layer of Si crystal, we have measured the rocking curves of reflection high-energy electron diffraction (RHEED) from a Si(111)-7×7 surface at temperatures extending over a wide range, 293–1073 K. From the analysis of the dynamical calculation of RHEED intensity taking account of the thermal diffuse scattering, the adatoms and rest atoms largely shifted upward compared with the atomic positions derived by Brommer et al. using ab initio molecular-dynamics scheme [Phys. Rev. Lett. 68, 1355 (1992)] and the optimum Debye temperature has been estimated at 420 K, which is much lower than that of bulk Si (505–658 K). Since the Debye temperature is considered to be an averaged value in a few surface layers, the difference means that the vibrational amplitude of surface atom is large compared to bulk atom.

Temperature dependence of RHEED oscillation in homoepitaxial growth of SrTiO 3(100) films on stepped substrates

Homoepitaxial growth of SrTiO 3 (STO) films on STO(100) substrates with unit-cell-high steps manifests itself by a marked change in the intensity oscillations of the specular reflectivity of reflection high-energy electron diffraction (RHEED) oscillations with increasing temperature. Atomic force microscopy (AFM) revealed a high density of growing islands distributed all over the edges and terraces of the pre-existing steps at lower deposition temperatures. In response, the RHEED intensity exhibits an ‘overdamped’ profile with no oscillation. At higher deposition temperatures ( T s≈500°C), larger (≈40 nm) but fewer islands were observed to form preferentially on the step edges with a gradually recovered RHEED intensity, followed by RHEED oscillations after a few monolayers of coverage. At even higher growth temperatures, immediate periodic oscillations in RHEED intensity were observed. However, the temperature dependence of the oscillation period indicates that it may not be directly related to a layer-by-layer growth mode, as is generally believed.

Growth control of MnAs on GaAs(001) by reflection high-energy electron diffraction

The MnAs(1̄100) surface is investigated during growth by reflection high-energy electron diffraction (RHEED). (1×2), (1×1), (2×1) and (4×1) RHEED patterns have been observed by varying the growth conditions, indicating various stoichiometry dependent reconstructions. A phase diagram showing the dependence of the reconstructions on the growth parameters is presented. RHEED intensity oscillations have been found, evidencing layer-by-layer growth of MnAs.

In-plane lattice spacing oscillatory behaviour during the two-dimensional hetero- and homoepitaxy of metals

The present paper is devoted to the analysis of in-plane lattice spacing oscillations (IPLSOs) during two-dimensional hetero- and homoepitaxial growth on (001) surfaces of cubic metals using reflection high-energy electron diffraction (RHEED). Our aim was to check systematically the agreement between experimental results and theoretical predictions. We have selected the most significant heteroepitaxial systems B/ A with a misfit positive (V/Fe, Mn/Fe), negative (Ni/Fe, Ni/V, Co/Cu) or close to zero (Fe/Cu, Co/Ni). The phase relationships between the different oscillating physical parameters [RHEED intensity, full-width at half maximum (FWHM) of diffraction streaks and in-plane lattice spacing] are examined, as well as the azimuthal dependence of the elastic relaxation process. A general good agreement between experimental data and theory was observed. However, for some systems (Ni/Fe, V/Fe), we prove a strong anisotropy of the phenomenon along the [10] and [11] azimuths of the square surface lattice. We also focus on the case of homoepitaxial growth and search for a convincing demonstration of IPLSOs in this specific case. The influence of substrate temperature and azimuthal dependance are also investigated. Surprisingly, very large relaxation effects are observed and seem to be correlated to the occurrence of surface reconstruction and/or surface contamination. Finally, an anisotropic character was again observed on V and Nb (001) surfaces. The morphology of the two-dimensional islands during growth or/and anisotropic surface reconstructions could explain such anisotropy.

Layer-by-layer growth of ZnO epilayer on Al2O3(0001) by using a MgO buffer layer

By introducing a thin MgO buffer, layer-by-layer growth of ZnO epilayers on Al2O3(0001) substrates is achieved by plasma-assisted molecular beam epitaxy. The MgO buffer is very effective on the improvement of surface morphology during the initial growth stage, which eventually leads to an atomically flat surface. As a result, (3×3) surface reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Structural analysis indicates that the twin defect with a 30° in-plane crystal orientation misaligned is completely eliminated, while the total dislocation density is reduced. Free exciton emissions at 3.3774 eV (XA) and 3.383 eV (XB) are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.

Reflection high-energy electron diffraction intensity oscillations during growth of GaN(0001)A by plasma-assisted molecular beam epitaxy

Reflection high-energy electron diffraction (RHEED) intensity oscillations have been observed during radio-frequency plasma-assisted molecular beam epitaxy of GaN on its (0001)A face. The starting A (Ga) face was prepared by growing a micrometer-thick GaN layer directly on a low-index 6H–SiC(0001) substrate at 650°C. RHEED intensity oscillations are measured with substrate temperatures less than 550°C in both Ga-limited and N-limited growth conditions. In the N-limited condition, an initial transient high-frequency oscillation is observed before it reaches a steady-state frequency. If the Ga flux is subsequently stopped while keeping the N flux unchanged, a few extra oscillations are recorded. Scanning tunneling microscopy images of surfaces quenched during growth show triangular-shaped islands, verifying a two-dimensional growth mode. At substrate temperatures greater than 550°C, neither island nucleation nor intensity oscillation is observed, suggesting a step-flow growth mode.

Using neural networks to construct models of the molecular beam epitaxy process

This paper presents the systematic characterization of the molecular beam epitaxy (MBE) process to quantitatively model the effects of process conditions on film qualities. A five-layer, undoped AlGaAs and InGaAs single quantum well structure grown on a GaAs substrate is designed and fabricated. Six input factors (time and temperature for oxide removal, substrate temperatures for AlGaAs and InGaAs layer growth, beam equivalent pressure of the As source and quantum well interrupt time) are examined by means of a fractional factorial experiment. Defect density, X-ray diffraction, and photoluminescence are characterized by a static response model developed by training back-propagation neural networks. In addition, two novel approaches for characterized reflection high-energy electron diffraction (RHEED) signals used in the real-time monitoring of MBE are developed. In the first technique, principal component analysis is used to reduce the dimensionality of the RHEED data set, and the reduced RHEED data set is used to train neural nets to model the process responses. A second technique uses neural nets to model RHEED intensity signals as time series, and matches specific RHEED patterns to ambient process conditions. In each case, the neural process models exhibit good agreement with experimental results.

TOPMOST SURFACE AND GROWTH MECHANISM OF BaTiO3 THIN FILM GROWN BY LAS ER MOLECULAR BEAM EPITAXY

The BaTiO3 thin film was grown epitaxially on SrTiO3(001) substrate by laser molecular beam epitaxy (laser-MBE). The film growth process w as monitored by in situ reflection high-energy electron diffraction (RHEED), the regular RHEED intensity oscillation from the 0th-Bragg reflection shows an unit cell layer-by-layer growth mode. The crystalline structure and the surface morp hology of the laser-MBE BaTiO3 film were characterized by X-ray diffr action (XRD) and by atomic force microscopy (AFM), respectively. The XRD and AFM results show that laser-MBE BaTiO3 film exhibit the tetragonal c-axi s oriented structure and an atomically smooth surface with a root mean square su rface roughness of 016nm. The topmost surface of the film was studied by angl e-resolved X-ray photoelectron spectroscopy (ARXPS), indicating the laser-MBE Ba TiO3 film is predominantly terminated with TiO2 atomic pla ne. The topmost surface of laser-ablated BaTiO3 film was also analyze d. The film growth mechanism was investigated in atomic scale.

Observation of CaF2 Growth on Si(111)7*7 Surface by RHEED.

Thin film growth of CaF2 on Si(111)7×7 depending on the substrate temperature has been studied by Reflection High-Energy Electron Diffraction (RHEED). In this study, three aspects such as solid-phase epitaxy, vapor-phase epitaxy and electron stimulated desorption (ESD) effect were investigated. In the solid-phase epitaxy, a 7×1 surface structure has been newly found at a very narrow temperature range of 840-850oC. In the vapor-phase epitaxy, RHEED intensity oscillations were observed at the substrate temperature range of 100-450oC, which suggests a layer by layer growth due to 2-dimensional nuclei. In the range of 500-700oC, the oscillation disappears but the reflection intensity is kept at a high level, which suggests a step-flow growth mode. It was judged from the change of Kikuchi pattern that CaF2 thin films grown at the substrate temperature range of 200-700oC were all of type-B; however, that grown at 100oC was of type-A. Since the ESD effect on the CaF2 surface is generally serious, the effect is also examined.

RHEED Intensity Oscillations for the Stoichiometric Growth of SrTiO3 Thin Films by Reactive Molecular Beam Epitaxy

The growth of high quality multicomponent oxide thin films by reactive molecular beam epitaxy (MBE) requires precise composition control. We report the use of in situ reflection high-energy electron diffraction (RHEED) for the stoichiometric deposition of SrTiO3 (1 0 0) from independent strontium and titanium sources. By monitoring changes in the RHEED intensity oscillations as monolayer doses of strontium and titanium are sequentially deposited, the Sr:Ti ratio can be adjusted to within 1% of stoichiometry. Furthermore, the presence of a beat frequency in the intensity oscillation envelope allows the adjustment of the strontium and titanium fluxes so that a full monolayer of coverage is obtained with each shuttered dose of strontium or titanium. RHEED oscillations have also been employed to determine the doping concentration in barium- and lanthanum-doped SrTiO3 films.

The Effect of Magnesium Oxide Buffer on the Epitaxial Growth of Zinc Oxide on Sapphire

ABSTRACTZnO single crystal thin film is grown on Al2O3(0001) substrate by plasma-assisted molecular beam epitaxy employing a thin MgO buffer layer. Using reflection high-energy electron diffraction (RHEED), we investigated the surface morphology evolution during the buffer and ZnO growth. We found that a few nanometers thick MgO layer deposited on Al2O3(0001) substrate strongly influences the subsequent growth of ZnO by promoting lateral epitaxial growth, which eventually leads to an atomically flat surface. As a result, (3×3) surface reconstruction of ZnO is observed and RHEED intensity oscillations are recorded. Structural investigations indicate that MgO with rock-salt crystallographic structure forms on the Al2O3(0001) surface as a template between the substrate and ZnO epilayer. Above that, the ZnO epilayer grows with little strain. The mosaicity in the ZnO film is suppressed by more than two orders as indicated by both symmetric and asymmetric X-ray rocking curves. The twin defect with a 30° in-plane crystal orientation difference is completely eliminated. Free exciton emissions at 3.3774 eV (XA) and 3.383 eV (XB) are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.

Indium incorporation during the growth of InGaN by molecular-beam epitaxy studied by reflection high-energy electron diffraction intensity oscillations

The indium incorporation in hexagonal InGaN grown by plasma assisted molecular-beam epitaxy is studied by means of reflection high-energy electron diffraction (RHEED) intensity oscillations. It is demonstrated that, in addition to being incorporated in the alloy, indium acts as a surfactant, significantly changing the gallium incorporation. This surfactant effect has to be taken into account to allow for a precise in situ determination of the alloy composition. The indium concentrations determined in situ by RHEED intensity oscillations were found to be in good agreement with ex situ results of Rutherford backscattering spectroscopy. This method also allows us to directly assess the maximum In incorporation as a function of the substrate temperature.

Effects of Built-in Polarization Field on the Optical Properties of AlGaN/GaN Quantum Wells

AlGaN/GaN quantum well (QW) structures were grown entirely by molecular-beam epitaxy on c-plane sapphire substrates. Reflection high-energy electron diffraction intensity oscillations, which testify a smooth growth front surface at the molecular monolayer scale, were used to precisely measure in situ both the growth rate and the Al content in AlGaN alloys. High-qualily AlGaN/GaN QWs with thicknesses varying from 10 to 80 Angstrom were achieved. Their photoluminescence (PL) linewidth is 10 to 30 meV, and the Stokes shift, determined by temperature-dependent FL, also ranges between 10 to 30 meV. The sample geometry and the Al composition in the barriers were varied in order to assess the built-in electric Grid present in wurtzite nitride-based structures. The magnitude of the deduced electric field points out the role of spontaneous polarization.

GaN on Si(111): From Growth Optimization to Optical Properties of Quantum Well Structures

In this paper, we report on the growth of GaN films and AlGaN/GaN multiple quantum wells (MQWs) on Si(111) substrates by molecular beam epitaxy using ammonia as nitrogen source. A smooth layer-by-layer two-dimensional growth mode allows to obtain reflection high-energy electron diffraction intensity oscillations during GaN, AlN and AlGaN growth. Low temperature photoluminescence (PL) spectra of thick nearly relaxed GaN films are dominated by an intense excitonic emission at 3.471 eV having a full width at half maximum of 5 meV. On the other hand, the PL spectra of thick strained GaN films exhibit a broad band edge emission at 3.455 eV having a full width at half maximum of 15 meV. As revealed by transmission electron microscopy, AlGaN/GaN MQW structures with sharp interfaces are achieved on Si(111) substrates and each QW exhibits a clearly resolved PL emission peak.

GaN on Si(111): From Growth Optimization to Optical Properties of Quantum Well Structures

In this paper, we report on the growth of GaN films and AlGaN/GaN multiple quantum wells (MQWs) on Si(111) substrates by molecular beam epitaxy using ammonia as nitrogen source. A smooth layer-by-layer two-dimensional growth mode allows to obtain reflection high-energy electron diffraction intensity oscillations during GaN, AlN and AlGaN growth. Low temperature photoluminescence (PL) spectra of thick nearly relaxed GaN films are dominated by an intense excitonic emission at 3.471 eV having a full width at half maximum of 5 meV. On the other hand, the PL spectra of thick strained GaN films exhibit a broad band edge emission at 3.455 eV having a full width at half maximum of 15 meV. As revealed by transmission electron microscopy, AlGaN/GaN MQW structures with sharp interfaces are achieved on Si(111) substrates and each QW exhibits a clearly resolved PL emission peak.

NO2-assisted molecular-beam epitaxy ofFe3O4,Fe3−δO4,andγ−Fe2O3thin films on MgO(100)

We report on the molecular beam epitaxial growth of single-crystalline, stoichiometric ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ and $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ films on MgO(100), using ${\mathrm{NO}}_{2}$ as the oxidizing agent. M\ossbauer spectroscopy on ${}^{57}\mathrm{Fe}$ probe layers is used to determine accurately the stoichiometry of the films. It is found that also all intermediate nonstoichiometric ${\mathrm{Fe}}_{3\ensuremath{-}\ensuremath{\delta}}{\mathrm{O}}_{4}$ phases can be obtained. The formation of the metastable compound $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ clearly demonstrates the large oxidizing power of ${\mathrm{NO}}_{2}.$ Although the shape anisotropy dictates that the zero-field magnetization direction should lie entirely in the plane of the film, this is never observed. Stoichiometric ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ has large out-of-plane components and only in the case of highly oxidized ${\mathrm{Fe}}_{3\ensuremath{-}\ensuremath{\delta}}{\mathrm{O}}_{4}$ does the magnetization approach the film plane. Upon further oxidation to stoichiometric $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3},$ however, it rotates back, and finally becomes almost completely perpendicular to the plane of the film. Furthermore, in the case of (near-) stoichiometric ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4},$ the magnetizations of the A and B sublattices are not completely coupled antiparallel. On average, the magnetization of the B site ions is $4\ifmmode^\circ\else\textdegree\fi{}$ closer to the film plane than the magnetization of the A site ions. All the as-grown films exhibit a $(\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})\mathrm{R}45\ifmmode^\circ\else\textdegree\fi{}$ surface reconstruction, independent of the stoichiometry. Using simple electrostatic considerations, we propose three possible surface terminations: a half-filled A layer, a B layer with oxygen vacancies and a B layer with hydroxyl groups. Upon annealing, the $(\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2})\mathrm{R}45\ifmmode^\circ\else\textdegree\fi{}$ reconstruction irreversibly transforms to a $3\ifmmode\times\else\texttimes\fi{}1$ reconstruction, caused by Mg outdiffusion from the substrate. Strong reflection high-energy electron diffraction intensity oscillations give direct, unambiguous evidence that ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ has a two-dimensional layer-by-layer growth mode over the entire temperature range studied, i.e., from 273 to 723 K, guaranteeing atomically flat surfaces and interfaces in multilayer structures. The largest oscillations are obtained on ex situ cleaved, UHV-annealed MgO(100) substrates, or on in situ annealed ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}/\mathrm{MgO}(100)$ films. Deposition above $\ensuremath{\sim}700\mathrm{K}$ is accompanied by rapid Mg outdiffusion.

Step edge barriers versus step edge relaxation in GaAs:Sn molecular beam epitaxy

Sn strongly segregates to the surface during MBE growth. We have previously proposed a microscopic mechanism for this process in which Sn can incorporate into either a substitutional site or into a surface interstitial site. These two sites compete with each other resulting in two main effects: (1) strain causes an initial surface roughening that has a characteristic length of 1000 nm and a corrugation height of about 5 ML and (2) a subsequent enhanced layer growth mode due to the strain driven formation of small islands. As growth proceeds, 2D islands nucleate on terraces, grow in size, and then coalesce at layer completions. In situ STM and ex situ AFM on quenched GaAs:Sn surfaces show that above descending step edges during growth there are zones denuded of islands. The focus of this paper is to consider the implications and causes of these denuded zones. We examine whether these denuded zones could be due to (1) the removal of step edge barriers or (2) an inhibition of nucleation due to strain. During growth on vicinal surfaces in the presence of Sn, the specular reflection high-energy electron diffraction (RHEED) intensity oscillates at a frequency which can be up to 15% faster than the monolayer growth rate. We propose that the relative change in the apparent growth rate is proportional to the ratio of the denuded length to the terrace length. We further demonstrate that the incoherent addition of oscillations in the diffracted intensity with different frequencies gives rise to the very strong beats observed during growth. The results are compared to Monte Carlo calculations. Of the mechanisms considered we conclude that only nucleation inhibited by strain can explain the data.