X-ray Crystal Truncation Rod Research Articles

Crystal quality and surface structure of sapphire and buffer layers on sapphire revealed by crystal truncation rod scattering

In the growth of group-III nitride crystals on sapphire substrates, insertion of a low-temperature-deposited thin buffer layer is very important. X-ray CTR (crystal truncation rod) measurement was conducted for sapphire substrate, sapphire of which surface was treated with nitridation process, and 100Å-thick AIN grown on sapphire. The X-ray CTR was observed for these samples. Theoretically calculated X-ray CTR spectra were shown for sapphire substrate and sapphire with thin grown layers. The calculation suggests that the X-ray CTR scattering measurement is a very powerful tool to investigate the structures of AlGaN crystals grown on sapphire substrate.

Thermal diffusion of Er atoms δ-doped in InP

Er σ-doped InP samples were investigated by X-ray CTR (crystal truncation rod) scattering measurement. The samples were prepared by OMVPE growth at 530°C, and after the growth annealed at 580, 630, and 650°C for 10–240 m. The distribution of Er atoms in InP was analyzed from the measured X-ray CTR spectra, and the diffusion coefficients of Er atoms in InP at 580, 630, and 650°C were obtained to be 1.2, 2.1 and 3.1×10 −18 cm 2 s −1. Such small diffusion coefficients suggest that Er atoms in InP are substitutional type impurity or form stable micro-crystals in InP.

Observation of composition in surface monolayers by X-ray scattering spectra caused by crystal truncation and interferences.

X-ray crystal-truncation-rod (CTR) scattering measurements using synchrotron radiation and an imaging plate can reveal the composition of a surface monolayer. Even when the composition is changed in only one atomic layer on the top surface, the X-ray CTR spectrum can change due to the differences in composition. X-ray CTR spectra are greatly enhanced by X-ray interference when a sample is designed properly. In this paper, it is shown by theoretical calculations and experiments for AlAs/GaAs samples grown by MBE that a 1 ML (monolayer)-thick AlAs layer embedded under 10 ML below the surface can enhance the modulation of an X-ray CTR spectrum.

Determination of composition distributions in InP/InGaAs/InP quantum-well structures by X-ray crystal truncation rod scattering and quantum levels

The distributions of As and Ga atoms in an InP/InGaAs/InP hetero-epitaxial layer were investigated on an atomic scale by X-ray CTR (crystal truncation rod) scattering. The relationship between PH 3-purge time (after the InGaAs layer was constructed) and the distributions of As and Ga atoms was studied. In samples which were designed to have a 1 ML-thick InGaAs layer, Ga atoms were not confined to 1 ML (monolayer). The energy levels of the InGaAs well layers were calculated using the measured As and Ga distributions. In order to compare them with the calculated result, a photoluminescence (PL) measurement was conducted and the measured energy levels agreed well with the calculation.

Microcrystallinity at SiO 2/Si(0 0 1) interfaces: An effect of annealing

By complementary use of X-ray crystal truncation rod (CTR) scattering and difference X-ray reflectivity (DXR), structural properties on ultra-thin thermal oxides grown on Si(0 0 1) substrates were obtained. A thin film which contains SiO 2 crystallites concentrated at the SiO 2/Si(0 0 1) interface revealed to have an interfacial thin layer characterized by high electron density, and vice versa. An effect of annealing in Ar gas has been understood consistently. The CTR scattering data indicated that probability of finding the crystallites in the oxide film remained, but the lattice constant of the crystallites increased along the direction perpendicular to the interface by the annealing. The decrease in the density calculated by this volumetric expansion agrees well with the decrease in average density of the interfacial thin layer observed by the DXR measurements.

Layer structure analysis of Er δ-doped InP by x-ray crystal truncation rod scattering

The layer structure and crystal structure of an Er δ-doped layer in InP are analyzed in a one monolayer (ML) level by an x-ray crystal truncation rod (CTR) scattering measurement using synchrotron radiation. The Er δ-doped InP sample is prepared by organometallic vapor phase epitaxy using trimethylindium, tertiarybutylphosphine, and tris(methyl-cyclopentadienyl)erbium as source materials. The analysis is made by comparing the measured CTR spectra and theoretically generated ones assuming four possible crystal structures for the Er δ-doped layers. We reveal that the Er atoms in InP form the rocksalt structure ErP. In the analysis of the crystal structure the x-ray interference plays a vital role. Er distribution in the δ-doped layer is clearly resolved in 1 ML level. It is shown that the Er atoms are well confined in 5 ML(15 Å) thickness. The total amount of Er atoms is 0.171 ML, which is quite close to 0.2 ML obtained by Rutherford backscattering measurement on the same sample.

Occupation site and distribution of δ-doped Er in InP measured by X-ray CTR scattering

Atomic level hetero-structure analysis of Er δ-doped InP grown by OMVPE was successfully made by the X-ray crystal truncation rod (CTR) scattering measurement using synchrotron radiation. It was shown that Er atoms in the δ-doped InP formed rocksalt structure ErP. Distributions of the Er atoms around the δ-doped layers were also revealed clearly on the atomic scale. The total amounts of incorporated Er increased exponentially as the Er supply time increased.

Distribution of As atoms in InP/InPAs (1 monolayer)/InP heterostructures measured by x-ray crystal truncation rod scattering

The interfaces of InP/InPAs(1 monolayer)/InP samples grown by organometallic vapor-phase epitaxy were investigated by the x-ray crystal truncation rod scattering measurement. The distribution of As atoms around the InPAs heterolayer was clearly revealed in the atomic scale from the measurement. It was shown that the distribution of As atoms into the layer under the InPAs layer was very small and that distribution of As atoms in the InP cap layer was, on the other hand, noticeable and the amount of As atoms was almost the same as that contained in the InPAs heterolayer. These results suggest that the extension of As atoms in the InP layer occurs due to the absorbed As atoms on the InPAs surface or As atoms remaining in the gas phase. Thus, to realize the abrupt InP/InPAs interface, the source-gas change sequence should be controlled to be very abrupt.

Structural transformations at [formula omitted] interfaces

Transformations of the atomic structure of the CaF 2 Si(111) interface during annealing have been studied by reflection high energy electron diffraction (RHEED) and X-ray crystal truncation rod (CTR) scattering. The surface morphology after annealing has been studied by atomic force microscopy (AFM). A conversion of the epitaxial relation of the film with respect to the substrate, from type-A (nonrotated) to type-B (with the axes of the film rotated by 180°C around the interface normal), is monitored by RHEED during annealing of the films. On the basis of RHEED, CTR and AFM data, a model of the conversion is suggested. In addition, a spontaneous transition of the type-B interface formed during the growth to the interface with another atomic arrangement has been studied with CTR. The possible role of point defect motion in the CaF 2 film during this transition is discussed.

Ｓｉ（１１１）面上ＣａＦ２薄膜の界面構造

The structure of the CaF2/Si (111) interface has been investigated with the use of X-ray crystal truncation rod scattering. The T -site which is just above the first layer Si atom is occupied by an interfacial Ca atom for a type-A interface as well as for a type-B interface. Structural parameters, describing the interface and the structure of the thin film have also been obtained and discussed thoroughly. Crystallinity of the type-A epilayer and that of the type-B epilayer has been compared. For type-B thin film, there is an evidence that a shortinterface-spacing-structure is not an equilibrium state, but can transform to a long-interfacespacing-structure. We are able to gain insight into a possibility that the type-A structure can also transform to a different structure.

Atomic level interface structure of InP/InPAs/InP measured by X-ray crystal truncation rod scattering

We conducted an X-ray crystal truncation rod (CTR) scattering measurement for samples of InP/InPAs/InP structure prepared by organometallic vapor phase epitaxy (OMVPE). The samples were 1 ML grown InAs capped by InP and InP exposed to AsH3 for 8 s. We were able to investigate the distributions of As atoms in the layer of each sample with atomic scale resolution. It was shown that in the sample intended to be 1 ML InAs, As atoms of only 0.355 ML InAs were contained, and in the sample exposed to AsH3 for 8 s, As atoms of as much as As 0.72 ML InAs were contained. The results of the X-ray CTR measurement agreed well with the result of the X-ray fluorescence yield measurement and the designed structures.

X-ray scattering from a rough surface and damaged layer of polished wafers

Theoretical and experimental investigations were performed to show the application of X-ray crystal truncation rod scattering combined with X-ray reflectivity in the measurements of surface roughness and near-surface damage of mechanochemically polished wafers. By fitting the measured crystal truncation rod curves it has been shown that polished wafers are divided into three parts-irregular steps on the surface, a damaged thin layer beneath the surface and a perfect bulk. The results show that the root mean square of the surface roughness of mechanochemically polished Fe-doped and/or S-doped InP wafers is one to two atom layers, and that the lateral correlation length of the surface roughness is about 3000-7500 AA. The thickness of the damaged region is found to be about 1000 atom layers.

Sm2+ Photoluminescence and X-Ray Scattering Studies of A- and B-Type Epitaxial CaF2 Layers on Si(111)

The crystalline quality and strain in molecular beam epitaxy (MBE)-grown CaF2:Sm/Si(111) structures with type-A (nonrotated) and type-B (rotated by 180° in the plane of the interface) epitaxial relationships were characterized by the bivalent samarium photoluminescence technique. It was found from the analysis of X-ray crystal truncation rod (CTR) scattering that the interface spacings along the [111] direction are (1.30±0.03)a0 for annealed samples with the type-A and (1.45±0.02)a0 with the type-B interface, where a0 is the Si(111) spacing. It was also confirmed that Ca atoms occupy T-sites at the interface (atop the atoms of the first Si layer) for the type-A interface. Both the photoluminescence and X-ray measurements indicate that the crystalline quality of type-A CaF2/Si(111) structures is inferior compared with that of type-B structures.

X-ray crystal truncation rod scattering from MBE grown (CaF 2-SrF 2)/Si(111) superlattices

Flouride CaF 2-SrF 2 superlattices (SLs) grown by molecular beam epitaxy have been studied by means of X-ray diffractometry for the first time. The diffraction patterns showed reasonably good crystalline quality of the SLs and a type-B epitaxial relation to the Si(111) substrate. From the analysis of the crystal truncation rod (CTR) profiles, based on the pseudomorphic model, it was obtained that despite the same high temperature (770°C) of formation of the CaF 2/Si(111) interface its structure depended on the growth temperature of the SLs. The shape of the CTR profiles confirmed the existence of the superlattice which consists of one or two monolayer thick SrF 2 layers. Some CaF 2/SrF 2-interface roughness was noticeable.

Effects of crystal surface roughness on x-ray CTR scattering

On the basis of a new formalism for the X-ray scattering from a crystal surface [J. Harada, Acta Cryst. A 48 (1992) 764] it is shown that if a crystal surface possesses some roughness, two kinds of effects are produced on the crystal truncation rod (CTR) scattering: the sharp CTR scattering reduces its intensity by a roughness damping factor (like a Debye-Waller factor), and diffuse scattering is generated. This is shown to be given by an intensity formula similar to that used for short-range-order diffuse scattering for a binary alloy system.

A new technique for the observation of X-ray CTR scattering by using an imaging plate detector

A new technique, using an Imaging Plate (IP) detector, was developed for the observation of X-ray crystal truncation rod (CTR) scattering. The use of an IP detector in conjunction with a synchrotron-radiation (SR) source is very effective for the observation of CTR scattering. The advantages and disadvantages of this technique are indicated by examples of the observation of CTR scattering from various samples: a naturally oxidized surface of an Si(111) wafer; MBE-grown GaAs/AlAs/GaAs on GaAs(001) substrate; Al-capped GaAs on an Si(111) substrate; and a cleavage NaCl(001) surface. It is also shown that it is possible to convert the observed intensity of the CTR scattering to an absolute scale if a stationary photograph is taken.

Characterization of the (0001) surface of ice I h crystal by crystal truncation rod scattering with the use of a synchrotron radiation source

An attempt was made to see whether the surface state of a single crystal of ice can or cannot be characterized by the X-ray crystal truncation rod (CTR) scattering technique. As a result, it was found that very well-defined CTR scattering, which provides valuable information about the surface state on a molecular level, can be observed, elongated from the 0002 Bragg point along the surface normal direction at 7°C below the melting point, if the surface is sublimated for approximately 1 h in a cold room after making mechanically flat the surface by using a Japanese plane. The result suggests that the surface prepared in this way is fairly flat on a molecular level.

New insight into the structure and growth of CaF2/Si(111)

We have used transmission electron microscopy and x-ray crystal truncation rod measurements to investigate thin (<50 Å) CaF2 films grown on Si(111) substrates by molecular beam epitaxy. The results indicate that CaF2/Si can be structurally as perfect as NiSi2/Si and CoSi2/Si, and that a reconstructed layer is present at the CaF2/Si(111) interface.

X-ray crystal truncation rod scattering: II. An effect of two-dimensional symmetry of the GaAs(001) surface

The X-ray scattering, CTR scattering, from the (001) surface of the ZnS type structure was formulated by introducing the structure factor for the spiral chain connecting one atom with another extended along the [001] direction from the (001) surface. On the basis of this formula it is shown that the the intensity of the CTR scattering elongated from the 111 Bragg point from the (001) surface of GaAs is different from that elongated from the 1 1 1 Bragg point, because of the two-fold rotational symmetry of the (001) surface of this crystal around the [001] axis, while the intensities are the same for both Bragg points, being subject to the bulk symmetry. This intensity difference was confirmed by the experimental observation, although the observed difference is not as large as expected from the theoretical prediction for the ideal (001) surface of GaAs. From the analysis of the CTR scattering the surface roughness of a mechano-chemically polished (001) surface of GaAs was estimated to extend in eight atomic sheets near the surface. The lattice spacing for the first layer of the surface was found to be shrunk from an ideal spacing by about 3% and that for the second layer by about 1%.

X-ray crystal truncation rod scattering II. An effect of two-dimensional symmetry of the GaAs(001) surface

X-ray crystal truncation rod scattering II. An effect of two-dimensional symmetry of the GaAs(001) surface