Total Reflection Angle X-ray Spectroscopy Research Articles

Adsorption processes of Se on the GaAs(111)A–(2x2) surface

The adsorption processes of Se on the GaAs(111)A surface have been systematically studied using reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and total-reflection-angle X-ray spectroscopy (TRAXS). We have found that a reconstructed structure of (2 3 ×2 3 ) –R30° is formed on the Se-adsorbed GaAs(111)A. A structure model has been proposed for the GaAs(111)A– (2 3 ×2 3 ) –R30°–Se surface, which consists of two Se trimers located at a hollow site of the GaAs(111)A surface and three Ga vacancies per unit cell. The proposed structure model sufficiently explains experimental data from RHEED, XPS, and STM, and satisfies electron counting requirements.

Structure and composition of the ZnSe(001) surface during atomic-layer epitaxy

Atomic-layer epitaxy (ALE) processes of ZnSe on GaAs(001) have been studied using reflection high-energy electron diffraction (RHEED), total reflection-angle x-ray spectroscopy, and x-ray photoelectron spectroscopy. We have obtained direct evidence that the growing surface of ZnSe(001) changes its chemical composition during ALE growth, which corresponds to the alternate formation of the Se-stabilized $(2\ifmmode\times\else\texttimes\fi{}1)$ and Zn-stabilized $c(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructions. The rocking-curve analysis of RHEED have been used in structure analysis for these reconstructed surfaces: the $(2\ifmmode\times\else\texttimes\fi{}1)$ surface has the Se-dimer structure, in agreement with previous studies. On the other hand, we have found that the $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface has the Se-vacancy structure, contrary to the previously proposed Zn-vacancy structure. The growth rate of ZnSe has been estimated to be about 0.5 bilayer per ALE cycle, which is consistent with the formation of these surface structures.

Atomic layer epitaxy processes of ZnSe on GaAs(0 0 1) as observed by beam-rocking reflection high-energy electron diffraction (RHEED) and total-reflection-angle X-ray spectroscopy (TRAXS)

Atomic layer epitaxy (ALE) processes of ZnSe on GaAs(0 0 1) have been studied by using reflection high-energy electron diffraction and total-reflection-angle X-ray spectroscopy in real time. We have obtained direct evidence that the growing film of ZnSe(0 0 1) changes its surface chemical composition during ALE growth, which corresponds to the alternate formation of the Se-stabilized (2×1) and Zn-stabilized c(2×2) reconstructions. The growth rate of ZnSe has been estimated to be about 0.5 BL per ALE cycle, which can be consistent with the previously proposed structure models for these reconstructed surfaces.

SURFACE STRUCTURES AND SURFACE CONDUCTANCE DURING METAL ADSORPTION ON SEMICONDUCTORS

Epitaxial growth modes for "two-step deposition" processes of metals on a Si(111) surface were investigated. Depth distributions of composition during the growth were analyzed by using RHEED-TRAXS (total reflection angle X-ray spectroscopy). We first made [Formula: see text]-(Ag, Au, Ga), [Formula: see text]-Sn and (4×1)-In structures, and then second metals (Ag, Au, Sn, Ga and In) were deposited on these surfaces at room temperature. Growth processes observed are classified into five growth modes: ordinary growth (O), alloying growth (A), substitution atom growth (S), particle formation growth mode (P) and floating atom growth (F). During the growth processes, we measured also surface conductivities which showed interesting behaviors. These results can be partly understood considering the growth modes, atomic arrangement, surface composition, Fermi level pinning and band bending, etc.

Determination of the topmost atomic layer of NdBa 2Cu 3O y thin films by developed total reflection angle X-ray spectroscopy (TRAXS)

The surface termination layer of c-axis oriented NdBa 2Cu 3O y thin films deposited by laser ablation process has been studied by a developed TRAXS method, described as glancing incidence-exit X-ray scattering (GIEXS). Using the SrTiO 3 substrate terminated with a well defined surface, coated with island-like gold layer, we have demonstrated the effectiveness of GIEXS to distinguish the surface atomic layer from their underlying layered bulk crystal. Applying this method to examine the surface atomic layer of NdBa 2Cu 3O y thin films cleaned with atomic oxygen beam, we have found that the surface of NdBa 2Cu 3O y thin films is predominantly terminated with CuO chain plane.

In-Situ Rheed-Traxs Monitoring Alloy Composition of the Surface During RF-MBE Growth of GaInN and AIGaN

AbstractIn-situ reflection high energy electron diffraction total reflection angle X-ray spectroscopy (RHEED-TRAXS) was performed to monitor alloy composition at the surface during growth of nitrides by RF-MBE for the first time. TRAXS signal of the GaLα line is found to be more sensitive to the composition at the surface than the GaKαline. A difference in the composition of layer adsorbed on the surface and the solid alloy layer has been identified.

Atomic Depth Distribution Analysis of In and Ga on Si(111) during Epitaxial Growth and New Surfactant-Mediated Epitaxy

Atomic depth distribution and growth modes of In and Ga on a Si(111) surface were studied by a method using reflection high-energy electron diffraction (RHEED) and total reflection angle X-ray spectroscopy (TRAXS). Indium was deposited on a Si(111)-√3×√3-Ga(1 ML) surface at room temperature. After 2 ML and 6 ML of In deposition, the peak positions and the shapes were similar in the glancing angle dependence of GaK emission. By the analysis of these measurements, the growth mode was concluded to be as follows. First, two layers of In grew on the Ga layer in an ordinary growth mode. After the third layer, however, In grew under the third Ga layer, resulting in a layer structure of (In2Ga)Inm (m=1,2,3 ...). On other Ga-covered Si(111) surfaces, In grew in an ordinary growth mode.

Atomic depth distribution and growth modes of Ga on Si(111)-2√3 × 2√3-Sn and √3 × √3-Ag

We analyzed depth distribution of composition and growth modes during Ga deposition on Si(111)-2√3 × 2√3-Sn and √3 × √3-Ag structures at room temperature. Applying RHEED-TRAXS (total reflection angle X-ray spectroscopy), the dependence of the X-ray (GaK, SnL, AgL, and SiK) emission on the glancing angle of the electron beam was measured. When 1 ML of Ga was deposited on the 2√3 × 2√3-Sn structure, the mixing of Ga and Sn atoms was clearly observed, and a quasi-√3 × √3-(Sn,Ga) structure was formed. After further Ga deposition, a liquid-like film of GaSn alloy was formed. When 1 ML of Ga was deposited on the √3 × √3-Ag surface, the structure was destroyed and a Ga layer was formed on the Si(111) surface, and Ag atoms which dissociated from the √3 × √3-Ag structure formed particles on the underlying Ga layer. For further deposition, a liquid-like Ga film grew, and Ag particles were buried in the Ga film without being destroyed. This alloying and separation of metals in atomic scale is discussed referring to the bulk phase diagrams of binary alloys.

Development of an Ultrahigh Vacuum High Resolution Scanning Transmission Electron Microscope

A 100 kV ultrahigh vacuum high resolution scanning transmission electron microscope (UHV-HR-STEM) has been developed for the purpose of analyzing micro-volume element clean surfaces by means of total reflection angle X-ray spectroscopy (TRAXS). The instrument is provided with a thermal field emission electron gun (T-FEG) with a build-up W(100) emitter. Since the objective lens is designed to minimize the spherical aberration coefficient (Cs) of the objective pre-field to Cs = 0.52 mm, the highest theoretical resolution of 0.17 nm is achieved. Also, the specimen stage was incorporated into a small space in the objective polepiece. A clean UHV of 2.2 × 10 −8 Pa was achieved in the specimen environment by means of a dry pump system

Atomic depth distribution analysis of Ag and Au on Si(111) during epitaxial growth by total reflection angle X-ray spectroscopy

When an electron beam is incident upon a surface, the electron trajectories near the surface in the sample change drastically depending upon the change in glancing angle θ g. The X-ray emission from surface atoms changes markedly depending on their atomic depth distribution. Thus, by measuring the θ g dependence of the X-ray emission, the composition in depth direction near the surface can be estimated. This method is very sensitive and a depth resolution better than 1 ML can be attained. By applying this method, it was found that when Ag was deposited on a Si(111)-α-√3 × √3-Au structure at room temperature, Ag grew on Au in the normal fashion; on the other hand, when 1 ML of Au was deposited on a Si(111)-√3 × √3-Ag structure at room temperature, the sites of Ag and Au atoms exchanged with each other and Ag atoms moved to the topmost layer of the surface. At further Au deposition, Au atoms moved continuously below the Ag layer, resulting in a condition as if Ag atoms were floating. These are new epitaxial growth modes, wherein the former should be called the “substitution atom growth mode” and the latter the “floating atom growth mode”. These phenomena were confirmed also by UHV-SEM observation. The results are further explained by Monte Carlo electron trajectory simulation.

Atomic depth distribution analysis of Ag and Au on Si(111) during epitaxial growth by total reflection angle X-ray spectroscopy

Atomic depth distribution analysis of Ag and Au on Si(111) during epitaxial growth by total reflection angle X-ray spectroscopy

In situ observation of Ba oxide layer at the interface between Y1Ba2Cu3O7−x film and MgO substrate

The very first stage of the growth of c-axis oriented Y1Ba2Cu3O7−x films prepared by coevaporation of metallic constituents on MgO(100) was examined in situ by new surface sensitive x-ray spectroscopy. Using reflection high-energy electron diffraction and total reflection-angle x-ray spectroscopy, the Ba oxide layer of Y1Ba2Cu3O7−x was found to grow at the interface between MgO(100). It was further observed that the coverage and the flatness of this Ba–oxide layer was higher than that of Y and Cu–oxide layers.

Theoretical Study on Total-Reflection-Angle Effect of Fluorescent X-Rays Emitted from Atoms Deposited on Surfaces

Theoretical analysis is performed for the angular distribution of fluorescent X-rays in a total-reflection-angle X-ray spectroscopy (TRAXS) experiment. The angular dependence of fluorescent X-ray intensity is caused by the interference effect between the direct beam and that reflected by the substrate surface. The calculation results agree with the experimental results reported by Hasegawa et al. [Jpn. J. Appl. Phys. 24 (1985) L387].

In-situ RHEED-TRAXS analysis during the preparation of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films

Reflection high-energy electron diffraction (RHEED) with total-reflection-angle X-ray spectroscopy (TRAXS) has been applied to the chemical and structure analysis during preparation of YB/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) films. The characteristic X-rays of YL alpha , BaL alpha , and CuK alpha emitted even from 0.8-AA-thick YBCO were clearly observed by TRAXS. In addition, the surface sensitivity of this method was found to be comparable to or higher than that of Auger electron spectroscopy. From the RHEED observation, it was revealed that the lattice spacing drastically changes from 4.09 AA to 3.8 AA at a mean thickness of less than one unit cell of YBCO at the early growth stage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Epitaxial growth of metals on Si and Ge investigated by RHEED-TRAXS and UHV-SEM

Abstract In the study of epitaxy, it is very important to investigate the surface composition during the growth process. In RHEED experiments, strong X-rays are excited from the sample surface hit by the electron beam. The intensity of these adsorbate characteristic X-rays changes drastically with change of the take-off angle (θt) and it becomes maximum at a critical angle (θc) corresponding to total reflection of the X-rays. Applying this phenomenon, surface element analysis is possible with high sensitivity. This method is called RHEED-TRAXS (total reflection angle X-ray spectroscopy). The sensitivity of this surface element analysis is generally comparable to that for AES but for heavy atoms it is higher. When the deposited metal atoms are confined to the surface, the glancing angle (θg) dependence of the emitted X-rays is expressed by I/sin θg. Thus, by measuring the θg dependences, the method gives also good information on the in-depth composition. Applying the RHEED-TRAXS method we have studied in situ the adsorption, desorption, and epitaxial growth processes of Au, Ag and Au-Ag alloys on clean Si(111) surfaces. We have developed a new UHV-SEM which has a high resolution of about 5 A. It was found that by using the UHV-SEM we could observe directly domain contrasts from two-dimensional surface structures such as 7 × 7, 5 × 2-Au, and √3 × √3-Ag which are formed by Ag and Au deposition on Si(111). These contrasts correspond to two-dimensional element mapping at the surfaces.

Epitaxial growth of metals on Si and Ge investigated by RHEED-TRAXS and UHV-SEM

In the study of epitaxy, it is very important to investigate the surface composition during the growth process. In RHEED experiments, strong X-rays are excited from the sample surface hit by the electron beam. The intensity of these adsorbate characteristic X-rays changes drastically with change of the take-off angle (θ t) and it becomes maximum at a critical angle (θ c) corresponding to total reflection of the X-rays. Applying this phenomenon, surface element analysis is possible with high sensitivity. This method is called RHEED-TRAXS (total reflection angle X-ray spectroscopy). The sensitivity of this surface element analysis is generally comparable to that for AES but for heavy atoms it is higher. When the deposited metal atoms are confined to the surface, the glancing angle (θ g) dependence of the emitted X-rays is expressed by I/sin θ g. Thus, by measuring the θ g dependences, the method gives also good information on the in-depth composition. Applying the RHEED-TRAXS method we have studied in situ the adsorption, desorption, and epitaxial growth processes of Au, Ag and Au-Ag alloys on clean Si(111) surfaces. We have developed a new UHV-SEM which has a high resolution of about 5 Å. It was found that by using the UHV-SEM we could observe directly domain contrasts from two-dimensional surface structures such as 7 × 7, 5 × 2-Au, and √3 × √3-Ag which are formed by Ag and Au deposition on Si(111). These contrasts correspond to two-dimensional element mapping at the surfaces.

Advances in high Tc superconducting thin film research at srl-istec

Thin film research at SRL is focussed on in two major area: 1. (1) understanding the mechanisms of film growth of oxide superconductors, and 2. (2) developing superconducting oxide films with correct chemical composition and perfect crystal structure. The oxide films are being fabricated by evaporation, pulsed laser ablation, rf sputtering, and metalorganic chemical vapor deposition. Key features of these systems include oxidation using O + beams, in-situ monitoring of film surface structure by reflection high energy election diffraction (RHEED), in-situ examining of chemical composition by total-reflection-angle X-ray spectroscopy (TRAXS), and analysis of ablated species using time of flight mass spectroscopy.

Promotion of epitaxial growth of Ge by Ag and Pb deposited on a clean Ge(111) surface

Epitaxial growth of Ge on Ag and Pb covered Ge(111) surfaces is investigated by reflection high-energy electron diffraction (RHEED) intensity oscillation, RHEED spot profile analysis and total reflection angle X-ray spectroscopy. The Ag and Pb atoms are segregated at the surface during the Ge growth even at room temperature. Furthermore, the optimum temperature for duration of strong RHEED oscillation is significantly lowered on the Ag covered Ge(111) surface. It was found that the spot width of RHEED changes periodically during the growth. Effective activation energies for surface diffusion estimated from the specular spot width of RHEED are 2.6 eV on clean Ge(111) and 0.95 eV on Ge(111)-Ag surfaces. It is considered that this energy difference effectively causes promotion of surface diffusion of a Ge atom leading to low temperature epitaxy.

Investigation of InP surface under arsenic pressure using RHEED-TRAXS

The surface of InP under arsenic pressure is investigated by reflection high-energy electron diffraction total reflection angle X-ray spectroscopy (RHEED-TRAXS). A Si(Li) solid-state detector with a thin parylene window is used as an X-ray detector. The sensitivities of Ga Lα and As Lα lines on an InP wafer were better than 1 monolayer (ML). From the surface of the InP wafer under arsenic pressure at 470 ° C, 2 ML of As were detected before cleaning the InP substrate surface, and 3 ML of As at 520 ° C within 10 min. It was considered that a few ML of InAs were grown while cleaning the InP surface under arsenic pressure, replacing phosphorus by arsenic.

Initial growth stage of InAs/GaAs studied by RHEED-TRAXS method

RHEED-TRAXS (total reflection angle X-ray spectroscopy in RHEED experiments) technique has been focused on an observation of In desorption processes from GaAs(100) substrate surfaces. Results show that In desorption energy sensitively changes along with arsenic partial pressure, P As 4 . Furthermore, at 3.0×10 −8 Torr of P As 4 , ITDS (isothermal desorption spectroscopy) curves have a refraction point below the In Lα line intensity corresponding to one monolayer of InAs. RHEED intensity measurements indicate that the refraction point corresponds to the change of the growth manner, i.e. a single layer growth occurs followed by three-dimensional island growth, the Stranski-Krastanov growth mode, at this P As 4 . On the other hand, at 1.0×10 −7 Torr of P As 4 , InAs grows with the Volmer-Weber growth mode.