Reflection High-energy Electron Diffraction Investigation Research Articles

In situ x-ray and electron scattering studies of oxide molecular beam epitaxial growth

We perform in situ synchrotron x-ray ray diffraction (SXRD)/reflection high energy electron diffraction (RHEED) studies on the growth of complex oxide thin films by molecular beam epitaxy. The unique deposition chamber, located at the Advanced Photon Source, allows the preparation of complex oxide samples with monolayer precision and facilitates the formation of direct correlations between in situ x-ray studies and the more prevalent RHEED investigations. Importantly, because SXRD and RHEED probe different atomic-scale processes during thin film synthesis, their concomitant use enables the extraction of details concerning growth behavior that one cannot determine from either probe alone. We describe the results of such in situ studies on the epitaxial growth of perovskite LaNiO3 on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001). We find that during the earliest stages of growth, the RHEED and x-ray signals do not agree with each other, demonstrating that while regular RHEED oscillations may imply high quality growth, the film–substrate interface can undergo significant changes during deposition due to the occurrence of interdiffusion at the growth temperature.

A RHEED/MBE-STM investigation of the static and dynamic InAs(001) surface

We report here the temperature-dependent incorporation kinetics of dimeric arsenic in InAs(001) homoepitaxy, using reflection high-energy electron diffraction (RHEED). Surface reconstructions, in combination with the RHEED investigation have provided insight into the growth of InAs(001), developing an accurate method of controlling the V:III ratio, which has been utilised to probe the low temperature epitaxial growth of indium arsenide epitaxial layers.

In situ observation of low temperature growth of Ge on Si(1 1 1) by reflection high energy electron diffraction

In this paper we investigate the initial stages of epitaxial growth of Ge on Si(111) and the impact of growth temperature on strain evolution in situ by reflection high energy electron diffraction (RHEED) for temperatures between 200°C and 400°C. The change in surface morphology from a flat wetting layer to subsequent islanding that is characteristic for Stranski–Krastanov growth is monitored by spot intensity analysis. The corresponding critical layer thickness is determined to 3.1<dc<3.4ML. In situ monitoring of the strain relaxation process reveals a contribution of the Si(111) 7×7-surface reconstruction to the strain relaxation process. High resolution transmission electron microscopy confirms that the Ge islands exhibit a high degree of structural perfection and an ordered interfacial misfit dislocation network already at a growth temperature of 200°C is established. The temperature dependency of island shape, density and height is characterized by atomic force microscopy and compared to the RHEED investigations.

A RHEED investigation of self‐assembled GaN nanowire nucleation dynamics on bare Si and on Si covered with a thin AlN buffer layer

Abstractmagnified imageBased on reflection high energy electron diffraction (RHEED) experiments, we show that the nucleation delay of GaN nanowire growth by plasma‐assisted molecular beam epitaxy on nitrided Si(111) and on AlN/Si(111) exhibit similar behaviors in function of the substrate temperature and of the Ga flux. On bare Si(111), a finite delay, estimated to be one minute in our system and under our set of growth conditions, is necessary to form and subsequently amorphize crystalline β‐Si3N4 before GaN can start to form. Although the amorphization time is found to be independent of the substrate temperature, this process adds a supplemental delay within the overall delay in the growth of GaN nanowires on bare Si(111), compared to growth on AlN/Si(111) or on amorphous silicon nitride on Si(111). (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Complementary RHEED and XRD investigation of β-FeOOH and α-Fe203 nanoparticles

Hydrothermal synthesis has been used systematically to produce α-Fe203 nanoparticles, via an intermediate β-FeOOH phase, as a function of temperature, time and surfactant concentration. The nanoparticles were examined using the complementary techniques of X-ray diffractometry (XRD) and reflection high-energy electron diffraction (RHEED). The relative merits of XRD and RHEED are reported on, supported by conventional transmission electron microscopy, to appraise the reaction products.

Real-time RHEED investigation of indium segregation in InGaAs layers grown on vicinal GaAs(001) substrates

The surface segregation of indium atoms was investigated in situ and in real time by reflection high-energy electron diffraction (RHEED) during molecular-beam epitaxy of InGaAs layers on misoriented GaAs(001) substrates. The strong damping of the RHEED oscillations was quantitatively related to the strength of the segregation process that was found to decrease with increasing miscut angle.

Growth of ultrathin cobalt films on Fe(001) studied by reflection high-energy electron diffraction and x-ray diffraction

Thin layers of the body-centered-cubic (bcc) cobalt (Co) phase with (001) orientation have been grown on single-crystalline Fe(001). Reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD) have been used to investigate the surface and the bulk crystal structure of the deposited Co films. The RHEED investigation shows that the bcc Co structure can be stabilized up to about 8 atomic monolayers (at. ML) and that the bcc Co surface is more atomically rough as compared to Fe but improves with increased growth temperature. RHEED patterns from thicker Co films indicate a transformation from the bcc structure to a hexagonal close packed (hcp). The XRD investigation shows that this hcp phase is textured and that the films contain a small amount of Co with a face-centered-cubic stacking sequence.

Preparation and structural analysis of Fe 2+ xTi 1− x thin films in the C14 Laves phase stability range

We report the epitaxial growth of single phase (0 0 1)-oriented thin films of Fe 2+ x Ti 1− x in the C14 Laves phase stability range using molecular beam epitaxy (MBE). The growth was studied by reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD). The onset temperature for epitaxial growth and the temperature range for improved crystalline coherence were identified. From X-ray reflectometry analysis the rms roughness was estimated to 0.5 nm for typical film thicknesses of 22 nm. As revealed by scanning tunneling microscopy (STM), this roughness is discrete and due to step edges corresponding to the full c-axis length of Fe 2Ti. The epitaxial growth implies an extended range of homogeneity as compared to bulk samples. A strong dependence of the growth behavior on the composition within the range of homogeneity was observed exhibiting increased crystalline coherence at higher iron concentrations. Furthermore, the volume increase of the unit cell with the Ti concentration is compatible to Vegard's law. Evidence for an iron-induced surface reconstruction was found in RHEED investigations.

Self-organized formation processes of CdSe quantum dots studied by reflection high-energy electron diffraction

Initial stages of the growth of CdSe layers on ZnSe and formation processes of CdSe quantum dots (QDs) are studied by in situ reflection high-energy electron diffraction (RHEED). The growth process of the CdSe layer at the initial stage is divided into five stages: (1) stable two-dimensional (2D) growth, (2) metastable 2D growth, (3) metastable growth just before the onset of QD formation, (4) QD formation, and (5) coalescence of QDs. There is a critical growth stage at which the formation of QDs is initiated. It is demonstrated by the RHEED investigation that the formation of the QD is associated with the release of strain energy. Because the strain energy stored in the dislocation-free 2D layer cannot be fully released as long as the surface is flat, in order to decrease the strain energy, the development of the islands occurs, competing with smaller increase of the surface energy.

In situ monitoring of the growth of oxide thin films at high oxygen pressure

Pulsed laser deposition is a well established deposition technique for oxide thin films and heterostructures. The knowledge and control of the growth processes of oxide materials are important for the fabrication of high-quality epitaxial and heteroepitaxial thin-film structures. Because of the high oxygen pressure during the growth process standard reflection high-energy electron diffraction (RHEED) analysis is not possible. To overcome this problem we have used a two-stage differential pumping system. In this way, the electron path in the high-pressure oxygen atmosphere within the deposition chamber has been reduced considerably. The specular beam intensity of the RHEED system was sufficiently high to allow the observation of intensity oscillations during the growth of oxide materials such as SrTiO3 and La2/3Ca1/3MnO3. In addition to the RHEED investigation the surface morphology of the deposited films has been characterized in situ by ultrahigh-vacuum atomic force microscopy.

In situ RHEED, AFM, and REM investigations of the surface recovery of MBE-grown GaAs(0 0 1)-layers during growth interruptions

The evolution of the surface morphology of homoepitaxial, MBE-grown GaAs(0 0 1)-layers was studied during growth interruptions by in situ RHEED (reflection high-energy electron diffraction) as well as by ex situ AFM (atomic force microscopy) and REM (reflection electron microscopy). The recovery of the RHEED intensity during annealing was correlated with a significant decrease of the densities of steps, islands and holes, seen in AFM and REM images, obtained from quenched samples. While islands seem to be dissolved by Ostwald ripening, the densities of steps and holes were found to approach saturation values. The appearance of anisotropic shapes of islands and holes, and characteristic changes during annealing were attributed to different mechanisms governing surface kinetics. It is concluded that, while diffusion of monomers dominate the growth periods, formation and diffusion of GaAs molecules is responsible for the changes of anisotropy during annealing under As pressure.

Interface structure of (001) and(113)AGaAs/AlAssuperlattices

The interfaces of short-period GaAs/AlAs superlattices grown on GaAs (001) and $(113)A$ surfaces are investigated by means of reflection high-energy-electron diffraction (RHEED) and Raman spectroscopy. RHEED investigations during growth of the heterostructures reveal an intermixed normal (AlAs-on-GaAs) interface while the inverted one is seen to be abrupt. We measured the Raman shifts of the GaAs-like confined optical phonons. In [001] grown superlattices, we determine the thickness of the intermixed region at the normal interface by comparison of the phonon wave vectors with the dispersion curve. This procedure is also applied to the [113] grown samples, where a splitting of the confined ${\mathrm{LO}}_{3}$ modes confirms the presence of an interface corrugation. We determine the height of the corrugation to be 3.4 \AA{} (two bilayers) at one interface while the other one is intermixed. This model is used for a reinterpretation of previously published Raman data [da Silva et al., Phys. Rev. B 53, 1927 (1996)] leading to a better agreement between experiment and theory.

RHEED investigations on the “pseudomorphic” growth of cobalt on Cu(001)

Using an advanced reflection high energy electron diffraction (RHEED) system we found oscillations of the in-plane lattice spacing on the monolayer scale during deposition of Co on Cu(001) [J. Fassbender et al., Phys. Rev. Lett. 75 (1995) 4476]. Here we present in detail the influence of the deposition rate on these oscillations and therefore on the growth process. With increasing deposition rate an enhanced variation of the in-plane lattice spacing occurs which is interpreted as a transition from a layer-by-layer to a more three-dimensional growth mode only for the first two monolayers. An unexpected linear behavior between the deposition rate and the maximum in-plane relaxation is found.

The NO-induced (1 × 2)→(1 × 1) structural transition of Pt(110): a quantitative RHEED investigation

The adsorption of NO on Pt(110) and its induced (1 × 2)→(1 × 1) structural transition was studied with quantitative reflection high-energy electron diffraction (RHEED). Time/exposure series of rocking curve sets were recorded during on-going adsorption processes. A direct evaluation focusing on the development of reflection intensities and rocking curve shapes in dependence on the NO exposure allowed dosages to be found for the most rapid structural change and for saturation. 10 sets of rocking curves from room temperature adsorption were quantitatively evaluated using an automated fitting procedure in conjunction with dynamic RHEED theory to determine the values of a comprehensive set of structural parameters for the substrate and the adsorbate. We found considerable changes of the Pt relaxations just prior to the transition from the (1 × 2) missing-row reconstruction into the (disordered) (1 × 1) structure. As long as the substrate is (1 × 2) reconstructed, NO adsorbs on bridge sites on the first Pt layer and on hollow sites on the (111) microfacets while in the case of the (1 × 1) structure first-layer top sites are occupied too. As soon as the transition begins, the rearrangement of the first Pt layer provides for additional vacant bridge and top sites in the second layer. These are then occupied while the hollow sites loose their population.

In situ RHEED and AFM investigation of growth front morphology evolution of Si(001) grown by UHV-CVD

We have successfully demonstrated the applicability of in-situ reflection high-energy electron diffraction (RHEED) to investigate the atomic-scale growth mechanisms in chemical vapor deposition (CVD). Measurements of surface structure and morphology of Si(100) films have been made in-situ by differentially pumped RHEED and compared ex-situ with atomic force microscopy (AFM). We use RHEED to examine the film morphology and structure, such as to distinguish two-dimensional growth (2D) from three-dimensional growth, and for the latter the formation of facets, twins, and polycrystalline morphology, all of which have been observed during CVD grown Si from SiH 4. In the temperature range of 550 to 650°C, the silicon epitaxial growth was found to be initiated either by the formation of three-dimensional (3D) islands, which coalesce and result in two-dimensional (2D) growth, or by the formation of 2D islands that continue to grow in a 2D mode. The 2D phase consists of a disordered 2 × 1 reconstruction and a c(4 × 4) reconstruction. Atomic force microscopy (AFM) measurements performed ex-situ on the epitaxial films confirm the RHEED observations and give the morphology evolution at various growth conditions.

Reflection high-energy electron diffraction study on the SrTiO 3 surface structure

This paper reports a reflection high-energy electron diffraction investigation of the surface structures of the (001) and (110) crystal faces of SrTiO 3 given various surface treatments. The as-polished surface has a (1×1) unreconstructed structure on both the (001) and (110) crystal faces. The vacuum annealed surface has a two-domain (2×1) superstructure on the (001) face and a (4×4) structure on the (110) face. The oxygen annealed surface has two superstructures on the (001) face: a two-domain (√13×√13-R33.7°) structure and a c(6×2) structure, and a (4×2) or a (4×4) structure on the (110) face. Of these surfaces, the oxygen annealed surface has the highest quality.

Sb as an indium-absorptive barrier in ZnSe/InP(100) heterostructures

Antimony layers, sandwiched between a ZnSe/InP(100) heterostructure interface, can be used as a barrier preventing outdiffusion of In into ZnSe epilayers. Through core level photoemission spectroscopy and reflection high-energy electron diffraction investigations, we found that the Sb layer must be annealed in order to prevent In outdiffusion. Barrier action occurs by means of InSb formation. A single pseudomorphic monolayer was insufficient to prevent In from outdiffusing. A thicker Sb layer greatly reduced In outdiffusion but through misfit dislocation the initially strained InSb film relaxed to its bulk lattice constant and did not allow for subsequent pseudomorphic epitaxy of strained ZnSe overlayers.

RHEED investigation of Ge surface segregation during gas source MBE of [formula omitted] heterostructures

Ge segregation during silicon gas source molecular beam epitaxy (Si-GSMBE) has been studied by in situ growth rate measurements using reflection-high-energy-electron diffraction (RHEED) intensity oscillations. Growth rate of Si on Si 1 − x Ge x gradually decreases to the Si homoepitaxial growth rate, which is attributed to Ge surface segregation at the growth interface. This segregation has been modelled using a mass balance equation and it has been found that the observed growth rate enhancement can be used as a direct measure of the Ge segregation. Using this novel in situ technique, concentration dependence of Ge segregation was studied, and it was found that the segregation decay curve is nonlinear, resulting in two segregation regimes dependent upon the Ge concentration, consistent with previous studies. Temperature dependence studies reveal that surface hydrogen, which is produced by the dissociation of Si 2H 6 and GeH 4 on the surface during growth, may act as a growth controlling surfactant, and comparison with solid source growth results suggests that it significantly suppresses the Ge segregation, leading to a more precise control of the interface. Finally, the thermal stability of the segregated surfaces was examined. Growth interruption and annealing during Si overlayer growth on Si 1 − x Ge x resulted in a small increase in the surface Ge concentration, which may be ascribed to the outdiffusion effect of Ge from the near surface region.

Atomic layer controlled doping by reflection high-energy electron diffraction during epitaxial growth

The detailed observation of dynamical behaviors of reflection high-energy electron diffraction (RHEED) patterns during the adsorption processes of Li, Se, and Zn is carried out. It is found that the RHEED intensity variation reflects the Li surface coverage during Li adsorption process on a Se-covered surface. This fact enables one to control the doping of Li ‘‘in situ.’’ A new method for atomic-layer controlled doping of ZnSe layers with lithium is proposed based on the RHEED investigations. The method would allow the incorporation of Li atoms on Zn sites of ZnSe by monitoring the RHEED patterns and intensities, by which the formation of Li interstitials would be suppressed. Photoluminescence spectrum shows the growth of high-quality p-type layers.

Reaction mechanisms of tertiarybutylarsine on GaAs (001) surfaces and its relevance to atomic layer epitaxy and chemical beam epitaxy

We explore the use of tertiarybutylarsine (TBAs) as an alternative arsine source in atomic layer epitaxy (ALE) of GaAs. X-ray photoelectron spectroscopy (XPS), reflection high energy electron diffraction (RHEED), and reflectance difference spectroscopy (RDS) are used to characterize the surface reactions of TBAs on GaAs (001) Ga-rich surfaces. At a substrate temperature of 320° C and an exposure level of 90 L of TBAs, AsHx (x = 1 or 2) is thought to be the adsorbed arsenic species. As the substrate temperature increases, As-rich surfaces are readily obtained with improved RHEED 2 x 4 patterns. No carbon related species are observed throughout the TBAs exposure experiments between 320° C and 540° C. It is suggested that AsHx is the adsorbed species after TBAs decomposes on surface Ga atom. Interactions between AsHx pairs form arsenic atoms by H2 release. RDS allows anin-situ real time study of TBAs on GaAs (001) Ga-rich surfaces. It is found that the RDS results are consistent with those obtained from XPS and RHEED investigations and can provide information on the rates of reactions and the extent of surface reconstruction simultaneously. Implications for the growth of GaAs by atomic layer epitaxy and chemical beam epitaxy using TBAs are discussed.