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In this study, we analyzed the influence of indium supply on the growth dynamics of gold-catalyzed InGaAs nanowires by in situ synchrotron X-ray diffraction. A high In/Ga supply ratio results in strong size inhomogeneity of Au particles and interrupts the nanowire growth at a certain point of time. Based on the experimental results, we discussed the state of Au catalysts with high indium content during the nanowire growth. We found that a growth temperature below the eutectic temperature is essential to avoid the growth interruption and maintain the nanowire growth. The high In/Ga ratio necessitates accurate size control of Au particles before growth for further improvement of the nanowire growth.

In-situ X-ray diffraction was used to investigate initial stages of GaN growth on graphene-covered SiO2 substrates with or without AlN buffer layers by plasma-assisted molecular beam epitaxy. The in-plane lattice parameter of GaN remained almost constant during growth without AlN buffer layers, confirming weak interactions between GaN and graphene. On the substrate with AlN buffer layers, the GaN lattice parameter continuously changed from AlN value to GaN value, indicating that the AlN islands act as seeds for GaN growth. Besides, GaN has preferential orientations epitaxial to graphene. Graphene can be used as a template of GaN growth on amorphous substrates.

The evolution of polytypism during GaAs nanowire growth was investigated by in situ X-ray diffraction. The growth of nanowires was found to start with the formation of zincblende structure, followed by the growth of wurtzite structure. The growth process was well reproduced by a simulation based on a layer-by-layer nucleation model. The good agreement between the measured and simulated results confirms that nucleation costs higher energy for the stackings changing the crystal structure than for those conserving the preceding structure. The transition in prevalent structure can be accounted for by the change of local growth conditions related to the shape of a triple phase line rather than by the change in supersaturation level, which quickly reaches steady state after starting growth.

Synchrotron X-ray diffraction has been applied to the in situ monitoring of the molecular beam epitaxial growth of self-assembled InAs/GaAs(001) quantum dots (QDs). As well as the strain distribution inside QDs, the lateral and vertical size of QDs was determined as a function of growth time. In combination with post-growth atomic force microscopy, the evolution of the total volume of QDs was evaluated. It was found that the QD volume increases at a similar rate over the temperature range of 450–480 °C. A significant mass transport from the wetting layer and the substrate was confirmed.

Multilayer structures comprised of a central layer of the binary transition metal pnictide MnSb and outer layers of standard semiconductors have been grown using molecular beam epitaxy (MBE). The growth of the GaAs overlayers was characterized using a combination of reflection high energy electron diffraction (RHEED) and in situ surface x-ray diffraction (SXRD). It was found that a GaAs overlayer can be grown via MBE onto a MnSb(1 101)/GaAs(001) virtual substrate. For both GaAs(111)A and InGaAs(111)A starting substrates, it was found that the GaAs overlayer altered the final strain state of the central MnSb layer causing a change from tensile strain to compressive strain in the out of plane direction.

Surface x-ray diffraction was employed, in situ, to measure the GaSb(001)-(1 × 5) and (1 × 3) surface phases under technologically relevant growth conditions. We measured a large set of fractional-order in-plane diffraction peaks arising from the superstructure of the surface reconstruction. From the data we calculated two-dimensional (2D) Patterson functions, the peaks of which represent inter-atomic distances weighted by the number of electrons in the individual atoms. For the (1 × 3) phase we obtained good agreement between our data and the β(4 × 3) model proposed in recent experimental and theoretical work. Our measurements on the Sb-rich (1× 5) phase provide evidence that the structure under growth conditions is, in fact, different from that of the models previously suggested on the basis of scanning tunneling microscopy (STM). We discuss reasons for this discrepancy as well as the identified structural elements for these reconstructions, which include surface relaxations and subsurface rearrangement.

The surface structure of GaAs(0 0 1)-c(4 × 4) was investigated by synchrotron surface X-ray diffraction which is sensitive to both in-plane and out-of-plane structures. The atomic coordinates and Debye–Waller factors for up to the sixth layer from the surface are given. The resultant atomic coordinates were compared with those given by a first-principles calculation. Among a variety of heterodimer models that were examined by theoretical calculation, our data fit best a three-heterodimer model where three Ga–As heterodimers are present in one unit cell. The preference for the formation of heterodimers is discussed in relationship to the transition process from the 2 × 4 to the c(4 × 4) structures.

Molecular-beam epitaxial growth of InAs/GaAs(0 0 1) quantum dots was investigated by grazing-incidence X-ray diffraction using a diffractometer directly coupled to an molecular-beam epitaxy (MBE) apparatus. The use of synchrotron radiation and a two-dimensional X-ray detector enabled reciprocal space mapping at a rate of 9.6 s per frame. The X-ray diffraction images revealed that the strain and shape of the quantum dots change significantly during the sequential growth process consisting of island formation, growth interruption and GaAs encapsulation.

The GaAs(0 0 1)-( 2 × 4 ) reconstructed surface was investigated by in situ surface X-ray diffraction. X-ray diffraction patterns were measured with increasing substrate temperature within the β phase of GaAs(0 0 1)-( 2 × 4 ) in a constant As flux of 5 × 1 0 − 7 Torr. At relatively low temperatures up to 545 ∘C, the observed X-ray diffraction patterns agree well with the β 2( 2 × 4 ) surface. However, a different X-ray diffraction pattern was obtained at temperatures close to the α phase, while the 2 × 4 periodicity still persisted. This change is explained by partial As-dimer desorption which results in a mixture of the β 2( 2 × 4 ) and α 2( 2 × 4 ) structures.

The dynamics of the molecular-beam-epitaxy (MBE) of GaAs (0 0 1) was investigated with a surface X-ray diffractometer coupled to an MBE chamber. The diffracted intensity of X-rays was measured in real time during growth. The temporal oscillation of the intensity for the growth at 550°C was explained well by the distributed-growth model, while the intensity for the growth at 435°C showed a complicated behavior arising from the changes in the surface reconstruction and the morphology. A method for separating the dynamics of the morphology from that of the reconstruction is presented.

The effect of substrate orientation on strain relaxation mechanisms of an InGaAs layer grown on vicinal GaAs substrates was investigated by in situ X-ray diffraction (XRD). The crystallographic tilt and indium segregation in the InGaAs layer were altered depending on the miscut direction and angle. In the case of the substrate tilted 6° toward the [110] direction, one type of misfit dislocations was formed preferentially rather than other types, especially in the rapid relaxation phase. While in the case of the substrate tilted 6° toward the direction, no anisotropies during relaxation were observed. The present finding indicates that the appropriate use of vicinal substrates may lead to a novel method of improving the crystal quality of heterolayers.

Organopalladium species ({Pd}) immobilized on an Sterminated GaAs substrate (S/GaAs) effectively catalyzes C-C bond formation in the Mizoroki-Heck reaction with cycle durability. However, the immobilizing mechanism of {Pd} is unknown. In this study, we deposited Pd(OCOCH3)2 on S/GaAs in two different methods, namely dry-physical vapor-deposition and wetchemical deposition, and compared the catalytic activities in the Mizoroki-Heck reaction. Also, S-termination and {Pd}-immobilization on GaAs grains were performed by the wet-chemical method to monitor the change in the surface chemical structure during the preparation process with diffuse reflectance Fourier transform infrared spectroscopy (FT-IR). FT-IR measurements implied that the immobilization of catalytic active {Pd} was related to the OH groups on the S-terminated surface. {Pd}-S/GaAs prepared dryphysically showed poor catalytic activity, because {Pd} was not immobilized under absence of OH groups. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The growth of five layers of InAs quantum dots embedded in GaAs matrices was investigated by in situ synchrotron X-ray diffraction. Time-resolved X-ray diffraction revealed the evolution of the strains and height of quantum dots during the entire growth process including the nucleation of islands and encapsulation with GaAs at various substrate temperatures. Comparisons of in situ X-ray results with postgrowth photoluminescence spectra showed a clear correlation between the structural and optical properties.

We have investigated the growth by molecular beam epitaxy (MBE) of GaAs on MnSb. MnSb epilayers on GaAs(111), GaAs(001) and In0.5Ga0.5As(111) of thickness ∼ 50 nm were used as substrates for GaAs films. The MBE growth of GaAs was monitored in situ using synchrotron X-ray diffraction and reflection high energy electron diffraction. We show data on strain relaxation and growth mode, and discuss the epitaxial relationship between GaAs(001) and MnSb(11‾01). Schematic of the molecular beam epitaxy/X-ray diffraction in situ experiment and typical 2D detector data, with GaAs(111) and MnSb(0001) diffraction features.

In situ real-time x-ray diffraction measurements during In 0.12 Ga 0.88 As/GaAs(001) epitaxial growth are performed for the first time to understand the strain relaxation mechanisms in a lattice-mismatched system. The reciprocal space maps of 004 diffractions are obtained, and allow us to evaluate the evolution of residual strain and crystal quality simultaneously as a function of layer thickness. In the time evolution, we have classified five thickness ranges, and deduced the dominant dislocation behavior in each phase.

The formation and evolution of rotational twin (TW) domains introduced by a stacking fault during molecular-beam epitaxial growth of GaAs on Si (111) substrates were studied by in situ x-ray diffraction. To modify the volume ratio of TW to total GaAs domains, GaAs was deposited under high and low group V/group III (V/III) flux ratios. For low V/III, there was less nucleation of TW than normal growth (NG) domains, although the NG and TW growth rates were similar. For high V/III, the NG and TW growth rates varied until a few GaAs monolayers were deposited; the mean TW domain size was smaller for all film thicknesses.

The polytypism of GaAs nanowires was investigated by in situ X-ray diffraction under different growth conditions. The growth of nanowires was found to start with the formation of the zincblende structure, followed by the growth of the wurtzite structure. The observed growth process of nanowires was well reproduced by simulations based on a layer-by-layer nucleation model. At a low growth temperature and a high growth rate, stacking faults were found to be frequently introduced owing to the reduction in energy barrier. As a result, the zincblend and wurtzite segments in nanowires were highly fragmented and the wurtzite structure was formed in the early stage of growth.

We report an anomalous lattice deformation of GaN layers grown on SiC(0001) by molecular beam epitaxy. The evolution of the lattice parameters during the growth of the GaN layers was measured by in situ synchrotron X-ray diffraction. The lattice parameters in the directions parallel and normal to the surface showed significant deviation from the elastic strains expected for lattice-mismatched films on substrates up to a thickness of 10 nm. The observed lattice deformation was well explained by the incorporation of hydrostatic strains due to point defects. The results indicate that the control of point defects in the initial stage of growth is important for fabricating GaN-based optoelectronic devices.

The evolution of strain inside InAs/GaAs(001) quantum dots during annealing after deposition was studied by in situ grazing‐incidence X‐ray diffraction. The use of synchrotron radiation and two‐dimensional detector enabled measurements at a temporal resolution of 9.6 s. The temperature dependence of the structural change during annealing can be interpreted in terms of the miscibility of InAs and GaAs.

Direct measurements on the growth of InAs quantum dots (QDs) and various cap layers during molecular beam epitaxy are performed by in situ X-ray diffraction (XRD). The evolution of strain induced both in the QDs and cap layers during capping is discussed based on the XRD intensity transients obtained at various lattice constants. Transients with different features are observed from those obtained during InGaAs and GaAs capping. The difference observed is attributed to In-Ga intermixing between the QDs and the cap layer under limited supply of In. Photoluminescence (PL) wavelength can be tuned by controlling the intermixing, which affects both the strain induced in the QDs and the barrier heights. The PL wavelength also varies with the cap layer thickness. A large redshift occurs by reducing the cap thickness. The in situ XRD observation reveals that this is a result of reduced strain. We demonstrate how such information about strain can be applied for designing and preparing novel device structures.

In situ structure analysis of GaAs(001)-c(4 x 4) has been carried out by synchrotron surface x-ray diffraction, which is sensitive to the three-dimensional structure and the atomic species. On the basis of 98 independent in-plane diffractions and 11 fractional-order rod profiles, the atomic coordinates and thermal vibration parameters were determined. X-ray diffraction results show the buckling of surface dimers and a strain field extending up to the sixth layer from the surface. An anomalous diffraction technique has been employed to specify the atomic species of the surface dimers. It has provided direct evidence of the formation of Ga-As heterodimers.

Synchrotron X-ray reflectivity and grazing incidence diffraction were applied to study the structures of phospholipid Langmuir-Blodgett films under controlled humidity. Distearoylphosphatidylethanolamine (DSPE) and DSPE-PEO lipopolymers, in which a poly(ethylene oxide) chain with n = 8, 17, or 45 EO units is covalently linked to the polar headgroup of DSPE, were used. When the relative humidity was changed from below 2 to 97%, the phosphate headgroup in a DSPE film was hydrated with a concomitant thickness increase of 4.2 A and an electron density decrease. The swelling of the monolayer was found to be a reversible process without any significant roughening of the film. Subtle differences in the film thickness could be detected for DSPE monolayers transferred under various lateral pressures. The degree of lateral ordering of the alkyl chains in DSPE monolayers increased considerably when the humidity was raised. In the case of DSPE-PEO with eight EO units, the hydration of the monolayer was also found to be reversible, but the water uptake was larger due to the presence of the hydrophilic polymer interlayer, which is located between the substrate and the DSPE moieties. Under high humidity, the lipopolymer monolayer with n = 17 exhibited a well-defined, layered structure similar to that of DSPE-PEO with n = 8. In the case of n = 45, however, there are indications for a significant intermixing of the polymer with the DSPE moieties, probably resulting in the formation of hydrophobic nanodomains of closely packed alkyl chains that are immersed into the hydrated polymer.

In-situ monitoring of the crystal structure formation during Ga-assisted GaAs nanowire growth on Si(111) substrates has been performed in a combined molecular beam epitaxy growth and x-ray characterization experiment. Under Ga rich conditions, we show that an increase in the V/III ratio increases the formation rate of the wurtzite structure. Moreover, the response time for changes in the structural phase formation to changes in the beam fluxes is observed to be much longer than predicted time scales of adatom kinetics and liquid diffusion. This suggests that the morphology of the growth interface plays the key role for the relative growth structure formation rates.

In-plane asymmetric strain relaxation in lattice-mismatched InGaAs/GaAs(001) heteroepitaxy is studied by in situ three-dimensional X-ray reciprocal space mapping. Repeating crystal growth and growth interruptions during measurements allows us to investigate whether the strain relaxation is limited at a certain thickness or saturated. We find that the degree of relaxation during growth interruption depends on both the film thickness and the in-plane directions. Significant lattice relaxation is observed in rapid relaxation regimes during interruption. This is a clear indication that relaxation is kinetically limited. In addition, relaxation along the [110] direction can saturate more readily than that along the [1̄10] direction. We discuss this result in terms of the interaction between orthogonally aligned dislocations.

Self-assembly of high-density InAs quantum dots (QDs) on Sb-irradiated GaAs buffer layers was observed in-situ by a time-resolved X-ray diffraction (XRD) technique using a combination of XRD and molecular beam epitaxy. Evolution of dot height and lattice constant was analyzed during InAs QD growth and subsequent growth interruption (GI), and as a result, dislocated giant dots due to coalescence and coherent dots were separately evaluated. An Sb-irradiated GI (Sb-GI) method to be applied after InAs growth was attempted for the suppression of coalescence. Using this method, the XRD intensity of giant dots decreased, and the photoluminescence intensity of InAs QDs was enhanced. High-density InAs QDs without giant dots were produced by using the combination of the QD growth on the Sb-irradiated GaAs buffer layers and the Sb-GI.

In situ real-time X-ray diffraction measurements during In0.12Ga0.88As/GaAs(001) epitaxial growth are performed for the first time to understand the strain relaxation mechanisms in a lattice-mismatched system. The high resolution reciprocal space maps of 004 diffraction obtained at interval of 6.2 nm thickness enable transient behavior of residual strain and crystal quality to be observed simultaneously as a function of InGaAs film thickness. From the evolution of these data, five thickness ranges with different relaxation processes and these transition points are determined quantitatively, and the dominant dislocation behavior in each phase is deduced.

Surface x-ray diffraction has been employed, in situ, to measure InAs(001)-(2×4) surface phases under technologically relevant growth conditions. For the As-rich (2×4) phase, the authors obtain good agreement between the data and the β2(2×4) surface reconstruction model. Comparison of our measurements on the (2×4) phase measured close to the metal-rich phase transition to models from density functional theory suggests a mixture of α2(2×4) and β2(2×4) surface structures present on the surface.

The molecular beam epitaxial (MBE) growth and postgrowth annealing of InAs/GaAs(001) quantum dots were investigated by grazing-incidence X-ray diffraction using a diffractometer integrated with an MBE apparatus. The use of synchrotron radiation and a two-dimensional X-ray detector enabled X-ray diffraction measurements on the internal strains and height of dots at a rate of less than 10 s per frame. Mass transport by surface diffusion was found to play a major role in the nucleation of three-dimensional islands of InAs/GaAs(001).

The structure of a Te layer formed on a Au(111) substrate by underpotential deposition (UPD) in an electrolytic solution has been studied using in situ surface X-ray diffraction technique. The measurements were carried out for a series of samples which were kept at UPD potential for 4 to 59 h. The results revealed that the Te UPD layer is unstable. The top layer is analyzed to consist of the UPD Te atoms and Au atoms which diffuse from the Au(111) substrate. Also, the Te UPD layer does not have the structure with periodicity reported in previous works, such as (√3 x √3)R30° after ample time elapses. Stripping voltammetry for the Te UPD layer shows that the interaction between Te and Au increases with time, supporting the finding that the top layer is a mixture of Te and Au.

The structure of a δ-doped crystal, Si(800Å)/Ge(1ML)/Si(001), grown by molecular beam epitaxy has been studied by surface X-ray diffraction and X-ray standing-wave method. The positions of the Ge layer and the Si capping layer are determined with respect to the Si substrate. In the analysis, the roughnesses of the interface and the surface are considered. The distance between the bottommost capping layer and the topmost substrate layer is 2.068±0.002 in units of the interlayer spacing of Si(004), while the distance between the Ge layer and the topmost Si substrate layer is 1.058±0.001. The former value corresponds to an averaged structure where the macroscopic elastic theory is applicable to the evaluation of the strain and the latter value corresponds to a microscopic structure where the bulk bond length is conserved. Coexistence of the two kinds of structures is made possible by the interfacial roughness.