Surface roughening during low temperature Si(100) epitaxy

I n situ real space images and reflection high energy electron diffraction (RHEED) data for the initial stages of molecular beam epitaxy growth of highly strained InxGa1−xAs (0.3≤x≤0.6) on GaAs(100) are reported. RHEED patterns, specular intensity oscillations, and surface lattice constant data are recorded simultaneously during growth. From the RHEED analysis we divide the film evolution into two stages, corresponding to a largely unrelaxed and relaxed surface lattice constant. Scanning tunneling microscopy images of the surface of the film as it appeared during growth reveal that the initial phase has a rippled morphology. With further growth the film evolves to a distinctive three-dimensional island microstructure. Both of these characteristic morphologies are interpreted as manifestations of strain in the growing film.

Low-dimensional carbon-based materials, in particular two-dimensional graphenic carbon structures, have been produced from single-walled carbon nanotube disruption using high-shear mixing and/or treatments in sulfonitric acid mixtures at both room and high temperature. Among other two-dimensional graphenic carbon structures, colloidal dispersions of graphenic nanoflakes have been obtained. Different structural arrangements, resulting from the reorganization of carbon because of the disruption procedures applied, were observed through selected area electron diffraction (SAED) and through reflection high-energy electron diffraction (RHEED) analyses coupled to transmission and scanning electron microscopy observations. Such combined investigations in the real and reciprocal space provided structural information at the nanoscale on the clustering of graphene layers in nanoplatelets or/and on their assembly into highly ordered (single-crystal) nanosheets. Furthermore, a different carbon phase exhibiting an orthorhombic cell withCmmasymmetry has been detected by SAED and RHEED analyses. In addition, a variety of self-assemblies of hexagonal basal planes have been observed to occur as the result of their different rotational and/or translational stacking faults. Overall, the reported results contribute to define the conditions for a controlled self-assembly of graphene-based structures with tailored dimensions, which is an important technological challenge, as their structure at the nanoscale dramatically affects their electrical properties.

Surface morphology evolution of epitaxially grown CeO2(110) layers on Si(100) substrates is studied using atomic force microscopy (AFM) and reflection high energy electron diffraction (RHEED). The surface has a faceted structure; a stripe-appearance and triangular-shape in plan- and cross-sectional views, respectively. AFM measurements clarify that as the layer thickness increases, the cross-sectional shape changes from a gable roof shape toward trapezoidal, which is consistent with RHEED analyses. The width of the facet monotonically increases with the layer thickness, while its height saturates at ∼5 nm above 600 nm in thickness, which means that the surface approaches smooth morphology. Ion channeling analyses indicate that the thicker the layer, the better the crystalline quality at the surface.

Growth and surface structure of vanadium oxide on anatase (0 0 1)

The reaction of CH4 on a Si(111)-7×7 surface is investigated by reflection high-energy electron diffraction (RHEED) analysis, quadrupole mass spectroscopy (QMS) and scanning electron microscopy/energy dispersive analysis of X-ray (SEM/EDAX). The RHEED patterns during CH4 exposure indicate the evolution of structures such as δ-7×7, 1×1, √3×√3 and SiC for various exposures at temperatures from room temperature (RT) up to approximately 800 °C. A mass spectrum shows that CH4 decomposition products are primarily CH3, CH4, and H2 at a tungsten filament temperature of 1500 °C. A SEM image of the SiC formed at 825 °C and 7920 L shows that the surface is rugged and consisted of hills and valleys. On the basis of the EDAX measurements, it is determined that the SiC layers are C-rich at the hill and Si-rich at the valley. It is found that the SiC layer is generated only after the formation of the √3×√3 structure.

Epitaxial thin GaN films (∼60 nm) have been grown on a-plane sapphire substrates at different growth temperatures (500–700 °C) using laser molecular beam epitaxy (LMBE). The effect of growth temperatures on the structural and optical properties of GaN layers grown on low temperature (LT) GaN buffer on prenitridated a-sapphire have been studied systematically. The in situ reflection high energy electron diffraction pattern revealed the three-dimensional epitaxial growth of GaN films on a-sapphire under the adopted growth conditions. The full width at half maximum (FWHM) value of x-ray rocking curves (XRCs) along GaN (0002) and (10-12) planes decreases with increasing growth temperature. The FWHM values of (0002) and (10-12) XRC for the 700 °C grown GaN film are 1.09° and 1.08°, respectively. Atomic force microscopy characterization showed that the grain size of GaN increases from 30–60 to 70–125 nm with the increase in growth temperature as GaN coalescence time is shorter at high temperature. The refractive index value for the dense GaN film grown at 600 °C is obtained to be ∼2.19 at the wavelength of 632 nm as deduced by spectroscopic ellipsometry. Photoluminescence spectroscopy confirmed that the epitaxial GaN layers grown on a-sapphire at 600–700 °C possess near band edge emission at ∼3.39 eV, close to bulk GaN. The GaN growth at 700 °C without a buffer still produced films with better crystalline and optical properties, but their surface morphology and coverage were inferior to those of the films grown with LT buffer. The results show that the growth temperature strongly influences the structural and optical quality of LMBE grown epitaxial GaN thin films on a-plane sapphire, and a growth temperature of >600 °C is necessary to achieve good quality GaN films.

Surface micro-structure analysis is very important for surface study in material science. Observations of surface atomic steps and reconstructed structures have been made using several techniques: reflection high energy electron microscopy (RHEEM), low energy electron reflection microscopy (LEERM) and low energy electron diffraction microscopy (LEEDM).In the present experiment, observations of surface micro-structures have been made using a scanning type reflection high energy electron diffraction (RHEED) microscopy. This technique has certain advantages of easy combinations with multiple surface analyzing techniques such as scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS).A schematic diagram of the scanning RHEED microscope combined with the microprobe AES is shown in Fig. 1. RHEED patterns are observed on the fluorescent screen through a viewing port. To observe the micro-structure (scanning RHEED image or dark field image), a particular diffraction spot is selected by means of the other small fluorescent screen with an aperture.

Biaxially texture magnesium oxide (MgO) films have been grown using ion beam assisted deposition (IBAD). IBAD-MgO films can easily to grow in the preferred direction and to form biaxial texture fast by the effect of ion beam assisted current, where the ability of preferred growth is directly controlled by the ion beam assisted current. An in situ reflection high energy electron diffraction (RHEED) has often been used to monitor the texture development of MgO film during the IBAD process. Here, we developed novel numerical analysis methods for RHEED patterns based on gray level. We constructed corresponding gray scale images and binary images from RHEED patterns. The area and brightness of selected diffraction spots, such as (004) diffraction spots, were measured based on the gray scale images. Further, rich information on the whole RHEED binary images, such as high-brightness pixels proportion of gray value higher than 50% gray level, can be obtained. Results from these qualitative analyses of RHEED patterns were coincided to the texture evolution obtained by phi scan rocking curves. This indicates these kinds of analysis methods based on gray level are effective for RHEED patterns.

Epitaxial growth of anatase TiO 2 thin films on LaAlO 3(100) prepared using pulsed laser deposition

Epitaxial BaTiO3 (BTO) thin films were deposited by molecular beam epitaxy on (001) silicon using an approximately 5 ML thick SrTiO3 (STO) as an intermediate buffer layer. In situ reflection high-energy electron diffraction (RHEED) was employed to quantitatively determine strain relaxation from the change in the in-plane lattice spacing. The crystalline quality, composition, and surface morphology of the BTO thin films were characterized by a combination of x-ray diffraction (XRD), atomic force microscopy, and x-ray photoelectron spectroscopy. RHEED analysis indicates that the initial growth of BTO was pseudomorphic. Strain relaxation occurred when the thickness reached a critical value of 10 MLs or 4nm. The lattice spacing approached the bulk BTO value for films with 30nm thickness. The BTO layer grows via a two dimensional growth mode. XRD measurement indicates a rocking curving width of the BTO (002) peak on Si as low as 0.9° has been achieved. Strain relaxation of the BTO films grown on different substrates including MgO, MgO/STO buffered Si, and STO buffered Si are compared.

RHEED (reflection high energy electron diffraction) is a technique widely used in molecular beam epitaxy (MBE) and for the characterization of surfaces in other contexts. Despite its widespread use, however, the interpretation of RHEED has largely been limited to the use of positions of the reflections with no attempt made to use the intensities in the patterns.The reasons for this are related to the complexity of RHEED computations. There are several methods which can be used to calculate RHEED intensities from flat surfaces. However, in MBE (and other crystal growth methods) where RHEED is most often used, the surfaces are rough. Of the dynamical methods for calculating RHEED intensities from flat surfaces, only the modified Cowley-Moodie multislice method with the edge patching algorithm can be applied to rough surfaces without making severe approximations. This method is computationally intensive because the ensemble average over the statistical parameters of the surface cannot be done before the dynamical calculations.

Reflection high energy electron diffraction study of CoSi2/Si multilayer structures

By use of in situ reflection high-energy electron diffraction (RHEED), the heteroepitaxial growth of strained-layer EuTe on PbTe(111) was investigated. From the determination of the in-plane EuTe surface lattice constant as a function of the layer thickness, the onset of EuTe strain relaxation (at critical layer thickness h c ) was obtained. Time-dependent intensity measurements of different features in the RHEED patterns indicate that the critical layer thickness essentially coincides with an abrupt roughening of the EuTe surface. In spite of the observation that for certain growth conditions a change in the damping of the RHEED intensity oscillations occurs at h c , generally, the damping behavior does not lead to the correct critical layer thickness

A scanning tunnelling microscopy study of the deposition of Si on GaAs(001); Implications for Si δ-doping

We present two methods to access reciprocal space with reflection high-energy electron diffraction (RHEED) during substrate rotation. The extraction of an arbitrary number of still frames from a continuously changing RHEED pattern is realized by triggering the substrate rotation and it allows analysis of quasistatic RHEED patterns that are updated every revolution. At the same time, the intensity along a line parallel to the shadow edge can be used to reconstruct a planar cut through the reciprocal lattice similar to a low-energy electron diffraction pattern. This RHEED pattern directly reveals the symmetry of the surface reconstruction and its changes during the deposition process.