Yoshikazu Fujii

A compact ultrahigh vacuum x-ray diffractometer for surface glancing scattering using a rotating-anode source

A compact ultrahigh vacuum (UHV) x-ray diffractometer was designed and constructed using a small rotating-anode x-ray source of 18 kW. Its UHV specimen chamber is 200 mm high and 140 mm in diameter and can be evacuated up to 3×10−8 Pa. The x ray is incident into the chamber through a thin Be window, 0.2 mm thick and 30 mm in diameter. Diffracted and scattered x rays are taken out through another Be window, 0.4 mm thick and 60 mm in diameter. All the equipment, a rotating-anode x-ray source, an incident monochromator, and a two-circle diffractometer onto which an UHV specimen chamber and a conventional scintillation counter are mounted, are arranged on only one optical table, 70 cm wide and 90 cm in length. Configuration of the specimen holder in the chamber, which is designed for the surface glancing angle scatterings at present, can be controlled from outside of the vacuum. The apparatus is the best suited to in situ observations of growing crystal surfaces during the deposition. A preliminary experiment...

Charge state distributions of MeV He ions channeled through thin crystals with atomically clean surfaces

Charge state distributions of He ions transmitted through self-supporting SnTe and Au single crystals with atomically clean surfaces are measured for the energy range 0.5–2.0 MeV. Anomalous reduction of He + fraction is observed at planar channeling conditions. This is explained by the position dependent electron-loss and -capture probabilities of channeling He ions calculated from the classical Bohr and Bohr-Lindhard models. The effect of surface reconstruction on the charge state distribution is also discussed.

Improved x-ray reflectivity calculations for rough surfaces and interfaces

We have investigated the fact that the calculated x-ray reflectivity based on the Parratt formalism, coupled with the use of the Nevot-Croce representation of roughness, show a strange phenomenon where the amplitude of the oscillation due to interference effects increases for a rougher surface. Here, we propose that the strange result has its origin in a currently used equation due to a serious mistake in which the Fresnel transmission coefficient in the reflectivity equation is increased at a rough interface, and the increase in the transmission coefficient completely overpowers any decrease in the value of the reflection coefficient because of a lack of consideration of diffuse scattering. The mistake in Nevot and Croces treatment originates in the fact that the modified Fresnel coefficients were calculated based on the theory which contains the x-ray energy conservation rule at surface and interface. In their discussion, the transmission coefficients were replaced approximately by the reflection coefficients by the ignoring diffuse scattering term at the rough interface, and according to the principle of conservation energy at the rough interface also. The errors of transmittance without the modification cannot be ignored. It is meaningless to try to precisely match the numerical result based on a wrong calculating formula even to details of the reflectivity profile of the experimental result. Thus, because Nevot and Croces treatment of the Parratt formalism contains a fundamental mistake regardless of the size of roughness, this approach needs to be corrected. In the present study, we present a new accurate formalism that corrects this mistake, and thereby derive an accurate analysis of the x-ray reflectivity from a multilayer surface, taking into account the effect of roughness-induced diffuse scattering. The calculated reflectivity obtained by the use of this accurate reflectivity equation gives a physically reasonable result, and should enable the structure of buried interfaces to be analyzed more accurately.

Improvement of X-ray reflectivity calculations on a multilayered surface

X-ray reflectometry is a powerful tool for investigations of rough surface and interface structures. Now the X-ray reflectivity is calculated based on the Parratt formalism, accounting for the effect of roughness by the theory of Nevot-Croce. However, the calculated result showed a strange phenomenon in that the amplitude of the oscillation due to interference effects increases in the case of a specific roughness of the surface. We proposed that the strange result has its origin in a currently used an equation due to a serious mistake in which the Fresnel transmission coefficient in the reflectivity equation is increased at a rough interface, and the increase in the transmission coefficient completely overpowers any decrease in the value of the reflection coefficient because of a lack of consideration of diffuse scattering. In the present study, we present a new improved formalism that corrects this mistake, and thereby derive an accurate analysis of the X-ray reflectivity from a multilayer surface, taking into account the effect of roughness-induced diffuse scattering.

Position-dependent stopping of 12.5–30 keV He+ ions at crystal surface

Abstract Energy losses of 12.5–30 keV He + ions are measured at glancing-angle incidence on the (001) surface of SnTe. Position-dependent stopping powers of the surfaces for the ions are derived from the observed losses. It is shown that the stopping power is explained by the single collision of a neutral He atom with valence electrons outside the surface.

Comparison of Surface Roughness Estimations by X-ray Reflectivity Measurements and TEM observations

Estimations of surface and interfacial roughness by x-ray reflectivity measurements were compared with those from TEM and AFM observations. The x-ray reflectivity was calculated based on the Parratt formalism, accounting for the effect of roughness by the theory of Nevot-Croce. Estimated surface and interface roughnesses from the x-ray reflectivity measurements did not correspond to the TEM image observation results. This disagreement suggests that the modified Fresnel reflectance coefficient introduced by Nevot and Croce cannot be applied for a surface and interface having a large degree of roughness. In addition, the calculated result showed a strange phenomenon in that the amplitude of the oscillation due to interference effects increases in the case of a specific roughness of the surface. This strange result suggests that there is a serious problem in how the effect of the roughness is added to the Parratt formalism. The modification of Nevot and Croce for Fresnel coefficients has been used for only surface and interface reflection. However, the modification of the transmission coefficients has an important role when the roughness of the surface or interface is large, and the effect of diffuse scattering due to the roughness should not be ignored in the calculation. This is an essential problem regardless of the size of the roughness. An inaccurately addition of the Fresnel reflectance coefficient to the Parratt formalism causes this disagreement and strange result, and therefore the effect of roughness was properly incorporated into the theoretical formulation of the x-ray reflectivity. In this paper, we only describe these problems. Further investigation of this problem will be reported elsewhere where we propose a new formalism to derive the x-ray reflectivity for systems having surface and interfacial roughness.

Energy Losses of 12-32 keV H^+, He^+ and N^+ Ions at Glancing Angle Scattering from Clean Surfaces of Silicon Crystals

The energy spectra of scattered ions of 12–32 keV H + , He + and N + have been measured at glancing angle incidence on the clean (001) and (111) surfaces of Si. A similar formula of the position-dependent stopping power which has been derived for high energy light ions was used to analyze the present results of low energy ions.

Recent Developments in the X-Ray Reflectivity Analysis for Rough Surfaces and Interfaces of Multilayered Thin Film Materials

X-ray reflectometry is a powerful tool for investigations on rough surface and interface structures of multilayered thin film materials. The X-ray reflectivity has been calculated based on the Parratt formalism, accounting for the effect of roughness by the theory of Nevot-Croce conventionally. However, in previous studies, the calculations of the X-ray reflectivity often show a strange effect where interference effects would increase at a rough surface. And estimated surface and interface roughnesses from the X-ray reflectivity measurements did not correspond to the TEM image observation results. The strange result had its origin in a used equation due to a serious mistake in which the Fresnel transmission coefficient in the reflectivity equation is increased at a rough interface because of a lack of consideration of diffuse scattering. In this review, a new accurate formalism that corrects this mistake is presented. The new accurate formalism derives an accurate analysis of the X-ray reflectivity from a multilayer surface of thin film materials, taking into account the effect of roughness-induced diffuse scattering. The calculated reflectivity by this accurate reflectivity equation should enable the structure of buried interfaces to be analyzed more accurately.

A proposal of depth profile analysis method of strain distribution in surface layer using x-ray diffraction at small glancing angles of incidence

Diffracted x-rays at small glancing angle of incidence on surface materials were investigated as function of incidence angles for the studies of residual stresses of surface layers. The intensity of x-ray propagation in surface layer materials characterized by a complex refractive index that changes continuously with depth was derived, and with use of the result, an analyzing method for evaluating the depth profiles of the strain distribution in the surface layer was studied. The derived analyzing method can be applied to the residual stress distribution analysis of the surface layer materials of which densities change continuously in depth as multi thin films, compound plating layers. Now, the analyzing method for the depth profile of the strain distribution in the surface layer using x-ray diffraction at small glancing angles of incidence was discussed with correction on some errata of equations in the previous studies.

Improvement of X-ray reflectivity calculation on surface and interface roughness

In the conventional X-ray reflectivity (XRR) analysis for the estimation of multilayer surface, the reflectivity is calculated based on the Parratt formalism, accounting for the effect of roughness by the theory of Nevot-Croce. However, the calculated results have shown often strange behaviour due to the fact that the diffuse scattering at the rough interface was not taken into account in the equation. Then we developed new improved formalism to correct this mistake. In this study, we show applying of new improved formalism using a transmission electron microscope (TEM) observation result. The result of interfacial roughness by using the conventional XRR formulae showed large difference with the TEM result, and derived wrong structure of surface. While, the result by new improved formalism reproduce the TEM result well, but need appropriate parameters in transmission coefficient. It shows that new improved XRR formalism derives more accurate analysis of the XRR, but the reduced Fresnel coefficients with physical grounds in the reflectivity equation are need in further research.