Scanning Reflection High-energy Electron Diffraction Research Articles

High throughput oxide lattice engineering by parallel laser molecular-beam epitaxy and concurrent x-ray diffraction

A laser molecular-beam epitaxy (LMBE) system for the fabrication of atomically controlled oxides superlattices and an x-ray diffractometer that measures spatially resolved x-ray diffraction spectra have been developed based on the concept of combinatorial methodology. The LMBE chamber has two moving masks, an automated target stage, a substrate heating laser, and an in situ scanning reflection high-energy electron diffraction system. The x-ray diffractometer with a curved monochromator and two-dimensional detector is used for rapid concurrent x-ray diffraction intensity mapping with the two axes of the detector corresponding to the diffraction angle and a position in the sample.

Combinatorial synthesis and luminescent characteristics of RECa 4O(BO 3) 3 epitaxial thin films

A series of RECa 4O(BO 3) 3 (RECOB) thin films (RE=Rare earth elements) was fabricated on ultrasmooth GdCOB (0 1 0) substrates by laser molecular beam epitaxy (MBE) method to explore the optimal growth conditions and to evaluate the luminescent properties. The growing surface of the film on a temperature gradient substrate was diagnosed with scanning reflection high energy electron diffraction (S-RHEED) system for quickly specifying optimum growth temperature and its dependence of rare earth element. The lowest temperature for epitaxial film growth was defined by the appearance of streak pattern in RHEED and it reflected the lattice matching with the substrate. TbCOB and EuCOB emitted respectively very bright green (543 nm) and red (611 nm) light, indicating that they are promising new phosphors. It is noteworthy that we could detect a remarkably strong red emitting region in the EuCOB–ScCOB–PrCOB ternary composition spread library.

Temperature-gradient epitaxy under in situ growth mode diagnostics by scanning reflection high-energy electron diffraction

We have developed a parallel film growth method on a temperature-gradient substrate to quickly control and optimize the film growth mode. A continuous-wave neodymium-doped yttrium–aluminum–garnet laser heating was used to achieve a stable temperature gradient covering a 300 °C range of temperatures over a distance of 11 mm. The growth mode was determined by time-resolved scanning reflection high-energy electron diffraction. Transition from layer-by-layer to step-flow growth by the deposition temperature was observed during La0.5Sr0.5MnO3 film growth on a single SrTiO3 substrate, proving a powerful tool not only for investigating the growth dynamics but also for seeking the optimized deposition conditions in one run of experiment.

Combinatorial molecular beam epitaxy synthesis and characterization of magnetic alloys

We have employed combinatorial approach to explore molecular beam epitaxy synthesis and characterization of multicomponent magnetic alloys. Structural evolution was studied in situ using scanning reflection high-energy electron diffraction and magnetic properties were probed using magneto-optic imaging and SQUID magnetometry. Results from a ternary system of CoMnGe are presented. These alloys exhibit a rich variety of magnetic and magneto-optic behavior, including a robust phase near Co 0.4Mn 0.4Ge 0.2 with high-spin polarization, high T c and high magneto-optic response. The observed magnetic responses show strong correlation with structural transformations such that structural ordering and symmetry change can give rise to enhanced magnetic and magneto-optic interactions. The observed magnetism in these alloys and their compatibility with Ge substrates make them potential candidates for spin electronic transport studies and applications.

Combinatorial Synthesis and Characterization of CoMnGe Alloys

AbstractTernary alloys of CoMnGe have been synthesized and characterized using combinatorial molecular beam epitaxy techniques. Structural evolution was studied using real-time scanning reflection high-energy electron diffraction, and magnetic properties were probed using magnetooptic Kerr effect imaging and SQUID magnetometry. Growth and properties on several substrate orientations were explored. These alloys exhibit a rich variety of magnetic and magnetooptic behavior, including a robust phase near Co2MnGe with high spin polarization, Tc, and magnetooptic response. The observed magnetic responses show strong correlation with structural transformations, such that structural ordering leads to enhanced magnetism. The observed magnetic behavior and the alloys’ compatibility with Ge and Si substrates make them potential candidates for spin electronic transport studies and applications.

Micro-Four-Point Probes in a UHV Scanning Electron Microscope for In-Situ Surface-Conductivity Measurements

For in-situ measurements of surface conductivity in ultrahigh vacuum (UHV), we have installed micro-four-point probes (probe spacings down to 4 μm) in a UHV scanning electron microscope (SEM) combined with scanning reflection–high-energy electron diffraction (RHEED). With the aid of piezoactuators for precise positioning of the probes, local conductivity of selected surface domains of well-defined superstructures could be measured during SEM and RHEED observations. It was found that the surface sensitivity of the conductivity measurements was enhanced by reducing the probe spacing, enabling the unambiguous detection of surface-state conductivity and the influence of surface defects on the electrical conduction.

MICRO-FOUR-POINT PROBES IN A UHV SCANNING ELECTRON MICROSCOPE FOR IN-SITU SURFACE-CONDUCTIVITY MEASUREMENTS

For in-situ measurements of surface conductivity in ultrahigh vacuum (UHV), we have installed micro-four-point probes (probe spacings down to 4 μm) in a UHV scanning electron microscope (SEM) combined with scanning reflection–high-energy electron diffraction (RHEED). With the aid of piezoactuators for precise positioning of the probes, local conductivity of selected surface domains of well-defined superstructures could be measured during SEM and RHEED observations. It was found that the surface sensitivity of the conductivity measurements was enhanced by reducing the probe spacing, enabling the unambiguous detection of surface-state conductivity and the influence of surface defects on the electrical conduction.

Morphological instability in InAs/GaSb heterostructures

Using transmission electron microscopy, reflection electron microscopy, and atomic force microscopy, we have examined InAs layers grown on (1 0 0)GaSb using molecular beam epitaxy at temperatures ranging from 400 to 500°C and at growth rates ranging from 0.03 to 0.09 monolayers/s. We have found that for lower growth rates and higher temperatures, nominally 8 monolayer InAs layers experience long-wavelength (>100 nm) small amplitude (∼1 nm) surface undulations. From linear stability theory, we find that the appearance of these undulations is consistent with large cation surface diffusion lengths that have been measured by other researchers using scanning reflection high-energy electron diffraction. It also appears, however, that there is no kinetic critical thickness for morphological instability of the InAs layer.

Epitaxial growth in Cu/Si(001)2 × 1 at high temperatures

The growth mechanism of Cu/Si(001)2 × 1 has been studied by means of scanning RHEED (reflection high energy electron diffraction) microscopy and scanning Auger electron spectroscopy. We obtained the following experimental results: (1) At room temperature, an intermixing layer of Cu with the Si surface is formed and no defined interface structure is observed. (2) At high temperatures ( T ∼ 500 °C), anisotropic growth of islands is observed on Si(001)2 × 1 and the long axis of the islands corresponds to the direction normal to the dimmer row. The distribution of the anisotropic islands is closely correlated with the domain structure of 2 × 1 and 1 × 2. (3) The islands are composed of Cu-silicide and the interface regions between the islands and the substrate are rich in Si. (4) The small reduction of the Si Auger signal with increasing Cu coverage (0–100 ML) upon deposition at 500 ° C is explained by three-dimensional nucleation of islands with a low density of on the Si clean surface 1 × 1 structure. Island formation occurs in a modified mode of Volmer-Weber growth. Lastly, the nucleation mechanism of the Cu-silicide islands is discussed.

Microprobe AES Combined With Scanning RHEED Microscope

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.