M. Sugiyama

Backreflection x‐ray standing waves and crystal truncation rods as structure probe for epilayer–substrate systems

Backreflection x‐ray standing waves (BRXSW) and crystal truncation rod (CTR) scatterings have been used to probe the structure of heteroepitaxial CaSrF2 crystals on GaAs(111)B substrate. Coupled with the film‐thickness information obtained from x‐ray Fresnel reflectivity data, the combined x‐ray standing wave and BRXSW data suggest one or both of the F and As layers missing from the heteroepitaxial interface. The CTR data support a missing F layer and interface (Ca, Sr) atoms occupying the T sites on GaAs(111) surface.

Surface and interface structures of S‐passivated GaAs(111) studied by soft x‐ray standing waves

Surface and interface structures of S‐passivated GaAs(111)A and (111)B with and without CaF2 overlayers have been investigated using the soft x‐ray standing‐wave technique. On the GaAs(111)A surface S atoms are located on top of the first layer Ga atoms, while on the GaAs(111)B surface S atoms replace the first layer As atoms. This is in agreement with the photoemission results. It is found that CaF2 deposition and post‐annealing does not change the position of S atoms. A well‐ordered S structure for S/GaAs(111)B is maintained, indicating a high stability of S—Ga bonds. This is in contrast to the low coherent fraction for the S interlayer atoms observed from the CaF2/S/GaAs(111)A system.

Initial stages of nanocrystal growth of compound semiconductors on Si substrates

Abstract we grew compound semiconductor nanocrystals on passivated Si surfaces, which were characterized by photoelectron spectroscopy and RHEED (reflection high energy electron diffraction). The Ga-deposited Si(111) surface with a 1×1 RHEED pattern was significantly converted into GaSb nanocrystals by Sb beam irradiation at 500°C. In order to grow better quality GaSb nanocrystals, Se-terminated Si surfaces were employed, and about 20 nm uniform GaSb nanocrystals were grown, whose crystallinity of good quality was verified by cross-sectional TEM. For InAs nanocrystals, hydrogen-terminated Si surfaces are suitable for growth at as low substrate temperature as 300°C.

Structure of fluoride/GaAs(111) heteroepitaxial interfaces

Abstract Atomic structure and stoichiometry of CaSrF2/GaAs(111) and CaF2/S/GaAs(111) heteroepitaxial interfaces have been determined using X-ray standing waves and X-ray scatterings along the surface normal at a synchrotron radiation source. CaSrF2 alloy films on the arsenic surface of GaAs(l 11) have a high crystalline order with a first F monolayer missing at the heteroepitaxial interface. (Ca, Sr) atoms are located in the T sites on top of the first-layer As atoms with little random disorder in the vertical direction, but these As atoms are likely to be missing in the examined samples. The AsGa double layers are subject to outward shifts in the interface region. CaF2 epilayers grown on a sulfide-treated GaAs(111) surface have also a good crystalline order in the vertical direction with a missing first F monolayer. The interlayer S atoms occupy slightly shifted top-layer As atom sites. The first-layer Ca atoms are placed at a short vertical height above the S monolayer, which disfavors the on-top sites for the adsorption position. The CaF2/S/GaAs(111) interface is found to be more rough than the CaSrF2/GaAs(111) interface.

X-ray standing-wave analysis of the (NH4)2Sx-treated GaAs(111)B surface

The position of sulfur atoms on an (NH4)2Sx-treated GaAs(111)B surface has been investigated by the X-ray standing-wave technique. S Kα fluorescence yields generated by (111) standing waves were measured using an ultrahigh vacuum goniometer system developed for angle-scan standing-wave experiments in an ultrahigh vacuum environment. The S Kα fluorescence angular yields demonstrate that the sulfur atoms located in nearly random positions before annealing occupy an average position of ∼ 0.2Aperpendicular to the GaAs(111) plane after annealing. The sulfur atoms at this position are considered to substitute with top As atoms on the (111)B surface judging from the bonding state of the sulfur atom obtained from S 2p photoemission spectra of the (NH4)2Sx-treated GaAs(111)B annealed surface.

X-RAY STANDING WAVE STUDY OF A SI-ADSORBED GAAS(001) SURFACE

The adsorption site of Si atoms deposited on a GaAs(001)‐(2×4) surface at 400 °C was investigated by back‐reflection x‐ray standing wave analysis using asymmetric (111) and (111) reflections. It was concluded that Si atoms occupy both As and Ga sites. The occupancy of Si atoms in the Ga site and the As site was estimated to be about 75% and 25%, respectively. The As coverage on a GaAs(001)‐(2×4) clean surface of 0.75 may be related to the occupancy of Si atoms in the Ga site.

A compact gas scintillation proportional counter for ultrasoft x rays

A gas scintillation proportional counter has been developed for measuring ultrasoft fluorescence x rays. This counter is high‐vacuum compatible for connecting to the ICF‐70 flange of a vacuum chamber. A Mylar window of 0.6‐μm thickness and 12‐mm diameter is utilized for the entrance of the ultrasoft x rays. The counter has a large entrance window of approximately 100 mm2 and an energy resolution high enough to separate C Kα and O Kα radiations. This counter is applicable to not only x‐ray fluorescence analysis, but also to fluorescence‐yield extended x‐ray absorption fine structure measurement of ultralow‐Z elements.

Interface structure and chemical bondings in Al/S‐passivated GaAs(111)

The position and chemical state of S atoms have been investigated for the Al/S‐passivated GaAs(111)A and (111)B samples using the soft x‐ray standing wave technique and synchrotron radiation photoemission spectroscopy. The x‐ray standing wave results indicate that the S atoms at the Al/S/GaAs(111) interface are significantly more disordered than the S atoms at the S/GaAs(111) surface. The position of the S atoms does not change upon the Al deposition. The synchrotron radiation photoemission spectra suggest that the Ga atoms bonded to S atoms before the Al deposition exchange with Al atoms upon deposition.

(NH 4 ) 2 S x -treated InP(100) surfaces studied by soft x-ray photoelectron spectroscopy

The sulfur chemical bonding state on (NH4)2Sx-treated InP(100) surfaces has been studied by S 1s core-level photoelectron measurements using synchrotron radiation soft x-rays. The change in the sulfur chemical bonding states caused by rinsing with water and annealing in vacuum after the (NH4)2Sx-treatment was observed clearly in the S 1s spectra. Four kinds of sulfur bonding states were resolved in the S 1s spectrum of the as-treated surface. Only one sulfur bonding state was detected on the surfaces with and without the water rinse after annealing, indicating that the InP surfaces were terminated by the S-In bond. The effect of rinsing the (NHj4)2Sx-treated InP surface is discussed by comparison with the (NH4)2Sx-treated GaAs surface.

Surface structures of (NH4)2Sx-treated GaAs studied by S K-edge X-ray absorption fine structure

Abstract Surface structures of (NH4)2Sx-treated GaAs(111)A, (111)B and (001) were investigated by S K-edge fluorescence yield X-ray absorption fine structure (XAFS) measurements using synchrotron radiation soft X-rays. The SGa bond length ordering on the three GaAs surfaces agrees well with XSW experimental results and theoretical calculations. The XAFS analyses confirm that the SGa bonds on the three GaAs surfaces have different bond lengths, supporting three kinds of adsorption sites on the three GaAs surfaces. However, the bond lengths on the GaAs(111)A and (111)B surfaces do not completely agree with those given by XSW experiments. This is considered to be caused by the surface roughness of the (NH4)2Sx-treated GaAs, rather than surface relaxation of the GaAs substrate.