Diffraction Space Mapping Research Articles

Ultrahigh B doping(<~1022cm−3)during Si(001) gas-source molecular-beam epitaxy: B incorporation, electrical activation, and hole transport

Si(001) layers doped with B concentrations C{sub B} between 1x10{sup 17} and 1.2x10{sup 22} cm{sup -3} (24 at %) were grown on Si(001)2x1 at temperatures T{sub s}=500-850 degree sign C by gas-source molecular-beam epitaxy from Si{sub 2}H{sub 6} and B{sub 2}H{sub 6}. C{sub B} increases linearly with the incident precursor flux ratio J{sub B{sub 2}}{sub H{sub 6}}/J{sub Si{sub 2}}{sub H{sub 6}} and B is incorporated into substitutional electrically active sites at concentrations up to C{sub B}{sup *}(T{sub s}) which, for T{sub s}=600 degree sign C, is 2.5x10{sup 20} cm{sup -3}. At higher B concentrations, C{sub B} increases faster than J{sub B{sub 2}}{sub H{sub 6}}/J{sub Si{sub 2}}{sub H{sub 6}} and there is a large and discontinuous decrease in the activated fraction of incorporated B. However, the total activated B concentration continues to increase and reaches a value of N{sub B}=1.3x10{sup 21} cm{sup -3} with C{sub B}=1.2x10{sup 22} cm{sup -3}. High-resolution x-ray diffraction (HR-XRD) and reciprocal space mapping measurements show that all films, irrespective of C{sub B} and T{sub s}, are fully strained. No B precipitates or misfit dislocations were detected by HR-XRD or transmission electron microscopy. The lattice constant in the film growth direction a{sub (perpendicular} {sub sign)} decreases linearly withmore » increasing C{sub B} up to the limit of full electrical activation and continues to decrease, but nonlinearly, with C{sub B}>C{sub B}{sup *}. Room-temperature resistivity and conductivity mobility values are in good agreement with theoretical values for B concentrations up to C{sub B}=2.5x10{sup 20} and 2x10{sup 21} cm{sup -3}, respectively. All results can be explained on the basis of a model which accounts for strong B surface segregation to the second-layer with a saturation coverage {theta}{sub B,sat} of 0.5 ML (corresponding to C{sub B}=C{sub B}{sup *}). At higher C{sub B} (i.e., {theta}{sub B}>{theta}{sub B,sat}), B accumulates in the upper layer as shown by thermally programmed desorption measurements, and a parallel incorporation channel becomes available in which B is incorporated into substitutional sites as B pairs that are electrically inactive but have a low charge-scattering cross section. (c) 2000 The American Physical Society.« less

Influence of annealing oxygen pressure on in-plane epitaxy and twinning in (110)YBa2Cu3O7−δthin films

The effects of annealing oxygen partial pressure on the in-plane epitaxy and twin formation in (110)-oriented ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) thin films grown on (110) ${\mathrm{SrTiO}}_{3}$ substrates by pulsed laser deposition have been studied by high-resolution x-ray diffraction, using diffraction-space mapping with a four-circle diffractometer. These films are found to have a structure in which the crystalline a and b axes are tilted about $0.5\ifmmode^\circ\else\textdegree\fi{}$ away from those of the substrates, while keeping the (110) planes parallel to each other. A decrease of in situ annealing oxygen pressure induces a reduction in twinning, which is finally suppressed in the film annealed at an oxygen pressure of 600 mTorr. Our results suggest that twin formation during a tetragonal-to-orthorhombic transformation is controlled by competition between the uniaxial force exerted at the interface by the vicinal substrate surface and the thermodynamic force originating from the difference in the lattice parameters for the a and b axes of orthorhombic YBCO.

Study of interfaces in GaInSb/InAs quantum wells by high-resolution X-ray diffraction and reciprocal space mapping

The electrical and optical properties of advanced epitaxial structures, such as quantum wells and superlattices are strongly influenced by the quality of their interfaces. The GaInSb/InAs system is particularly important because of its promise for use in infrared detection in the 8–14 μm ranges. In this material system the two compounds do not share a common anion, and different bond types exist at the interface depending upon the growth parameters. We have looked at single quantum-well structures grown with solid source molecular beam epitaxy such that a distinct interface type would be found in each sample. X-ray rocking curves and full dynamical simulations were performed for each quantum well structure. Quantum wells with three types of interfaces were grown and analyzed; random interfaces, Sb-like interfaces, and As-like interfaces formed with a one monolayer group III deposition followed by a five second group V soak. With the exception of the interfaces, all three SQWs have nominally the same structure; 5000 Å of GaSb buffer layer grown upon a GaSb substrate, a 150 Å quantum well with 35 Å Ga 0.75In 0.25Sb barriers and a final GaSb cap layer of 50 Å thickness. Well-resolved Pendellösung fringes were found in all samples indicating high quality in the epitaxial layers and interfaces. The SQW with the As-like interfaces had the highest degree of quality as evidenced by persistence of the fringes.

Phase contrast imaging and diffraction-space mapping of periodic inversion domains in LiNbO3 with advanced x-rays

This paper presents some recent advances in structural characterization of periodic inversion domains in ferroelectric nonlinear optical crystals using novel techniques such as diffraction-space mapping, diffuse scattering imaging and phase-contrast imaging. The great advantages of using both highly coherent hard x-ray photons delivered by the third-generation synchrotron source and the well-defined x-ray beam provided by state-of-the-art laboratory diffractometer are addressed in terms of investigation of periodically domain-inverted LiNbO3 crystals. The microstructural characteristics of the periodic inversion domains in LiNbO3 are discussed, with an indication of their differences from naturally-occurring inversion domains.

Crystal structure of LPE-grown LiNb 1− xTa xO 3 epitaxial films

LiNb 1− x Ta x O 3 epitaxial films, which were grown on LiNbO 3 substrates at various growth temperatures by the liquid-phase epitaxial method, were characterized using X-ray rocking curves, diffraction-space maps and absolute lattice constants. The films had relaxed structures regardless of the growth temperature, and the density of the dislocations due to the lattice mismatch between the films and the substrates increased with increasing growth temperature. Therefore, the crystallinity of the films became poor with increasing growth temperature.

Sensitivity of Bragg surface diffraction to analyze ion-implanted semiconductors

A special case of the x-ray multiple diffraction phenomenon, the Bragg surface diffraction (BSD), has been investigated under lattice damage due to ion implantation in GaAs (001) samples. The BSD profile is very sensitive to the diffraction regime (dynamical or kinematical) and provides information regarding crystalline perfection and lattice strains in both directions—parallel and perpendicular—to the sample surface. Results from grazing-incidence x-ray diffraction and reciprocal space mapping are also reported.

Strain relaxation onset in [formula omitted] multiple-quantum wells investigated by high-resolution X-ray diffraction and atomic force microscopy

In this work we analyze the strain relaxation onset in In 0.08Ga 0.92As GaAs multiple-quantum wells (MQWs) grown by low-pressure metal organic chemical vapour deposition. The strain field determination and the structural characterization of the epitaxial layers and heterointerfaces are performed by high-resolution X-ray diffraction and reciprocal space mapping, while the surface morphology is analyzed by atomic force microscopy (AFM). An accurate determination of the strain status of the MQWs indicates a pseudomorphic growth in the epilayers. Nevertheless, the reciprocal space measurements show a distinct intensity profile of the diffuse scattering around each satellite peak. The diffuse intensity is very much elongated along the in-plane direction and the elongation is found to increase with the well layer thickness. Furthermore, in some cases lateral lobes appear within the elongated peaks which indicate an enhanced lateral ordering of the interfaces. AFM images of the heterostructure surface reveal the presence of monolayer steps. The step front distance is found to be related to the in-plane ordering measured by the X-ray experiments. In addition, we show that AFM and X-ray measurements combined to each other can provide a deeper insight in the strain relaxation phenomenon as well as direct information on the vertical correlation of the heterointerfaces.

Crystal structure of LPE-grown LiNbO 3 epitaxial films

LPE (liquid-phase epitaxial)-grown LiNbO 3 films with various thicknesses on LiNbO 3 substrates were characterized using diffraction-space maps and absolute lattice constants. The Li content in the films was estimated from the absolute lattice constants. The films had strained structures regardless of the film thickness. The Li content increased as the film thickness increased to about 15-μm, and it was constant at film thicknesses thicker than about 15-μm. It was found that the crystallinity of the films with thicknesses ranging from about 15-μm to about 25-μm was better than that of the substrates, because the composition was nearly stoichiometric and the films had a small composition gradient, few dislocations and little thermal stress.

High resolution x-ray diffraction studies of short-period CdTe/MnTe superlattices

We have investigated the elastic properties of epitaxial MnTe layers using triple axis high resolution x-ray diffraction and reciprocal space mapping. A series CdTe/MnTe superlattices (SLs) grown by molecular beam epitaxy and nearly strain compensated, were deposited on [001] Cd1−xZnxTe substrates. In order to obtain the MnTe content of these SLs without an a priori knowledge of the elastic properties of cubic MnTe, annealing experiments were performed to interdiffuse the individual layers into a mixed Cd1−xMnx Te alloy layer. For a precise analysis of the data, it was found to be important to determine the in-plane strain of the superlattice layers using reciprocal space maps around symmetric and asymmetric reciprocal lattice points. The value for the Poisson ratio of zinc-blende MnTe was determined to be ν=C11/2C12=0.77±0.15.

The orientation dependence of the hydrogen distribution within Mo/V (110) and (001) multilayered artificial superlattices

We have investigated the hydrogen distribution within multilayered Mo/V (110) and (001) artificial superlattices. The hydrogen concentration, determined by the nuclear resonance reaction, is found to decrease with decreasing layer thickness. The decrease is attributed to an interface region with reduced hydrogen content. The extension of this interface region is found to be two monolayers in Mo/V (110), compared to the previously reported three monolayers in Mo/V (001) superlattices. In the (110) oriented samples the relative amount of H in the interface region decreases with increasing average H content (H/V - 0.5). This effect is shown to be related to the orientation of the hydrogen planes with respect to the boundaries implied by the Mo/V interfaces. The hydrogen induced volume change, obtained by conventional x-ray diffraction (XRD) and reciprocal space mapping, is found to be smaller for the (110) samples than for the (001) oriented samples at room temperature.

X-ray diffraction solutions to heteroepitaxial growth problems

Modern X-ray diffraction equipment offers a wide choice of methods for understanding the problems involved in the heteroepitaxial growth of a wide range of materials. The basic double axis rocking curve continues to play a key role in the characterisation of heteroepitaxial strained layer structures. The strength of the technique arises from the ability to accurately simulate the diffraction process using the Takagi-Taupin formulation of dynamic theory. There is an increasing use of diffraction space mapping for the less perfect heteroepitaxial systems with large lattice mismatches and different crystal structures, e.g. superconductors, gallium nitride on sapphire. Detailed studies of the state of relaxation of pseudomorphic systems can be achieved using the triple crystal diffractometer. The triple axis diffractometer can also be used to measure the unit cell distortion in good quality non-pseudomorphic systems. Where layers are less perfect and where layers are very thin lower resolution diffraction space mapping can provide useful information with a large reduction in data collection time.

Characterization Of Si-Doped GaN on (00.1) Sapphire Grown by MOCVD

AbstractWe investigated structural, electrical and optical properties and their relationships for MOCVD grown Si-doped GaN with different doping levels. The changes in structural properties such as strain, crystallinity and strain relaxation as a function of carrier concentrations were measured by high-resolution triple-axis X-ray diffraction and reciprocal space mapping technique. As the carrier concentration of the samples increases from 1.6×1018 to 9.5×1018/cm3 Hall mobility decreases from 222 to 170cm2/Vsec, and peak intensity ratio of band edge to yellow luminescence in photoluminescence spectra decreases too. The variation of mosaic spread along [00.1] direction determined from full width at half maximum(FWHM) of GaN (00.2) peaks shows good agreement with the variation in Hall mobility. In addition, dislocation density determined from FWHM of (10.2) peaks is shown to be related to the variation in the intensity of yellow luminescence in PL spectra.

Periodic domain inversion in poled via high-resolution x-ray topography and diffraction-space mapping

Periodic domain inversion produced by electron beam writing on the face of a hydrothermally grown crystal has been investigated by high-resolution multiple-crystal topography and diffraction-space mapping. Periodic contrast, with spacings which are consistent with the lateral dimensions of the inversion domains, has been observed by choosing a high spatial resolution mode for topography. Via reciprocal-space mapping, the scattering features along and have been extracted and shown, by combination with high angular resolution topography, to relate to the lattice tilting and strain induced during the domain inversion process. The origins of the diffraction contrast in the domain inverted regions, and the factors affecting the diffraction images of the domains are discussed.

Applications of multiple-crystal diffractometry

The elucidation of the structure of semiconductor multilayers can be adequately determined by X-ray methods but the interpretation is not always straightforward. In this paper we introduce the idea of full three-dimensional diffraction-space mapping to obtain information on the three-dimensional structure of imperfect materials. We also stress the importance of this method for the interpretation of the data from high-resolution X-ray diffractometry. The presence of defects and diffraction effects can create significant changes to the diffraction pattern that require a more complete analysis than that obtained from simple profiles. These subtle influences can in general only be understood by diffraction-space mapping. Interpretation of diffraction-space maps from the high-resolution multiple-crystal multiple-reflection diffractometer permits the use of three extra very powerful tools. The first is multiple-crystal topography so that the diffraction-space intensity features can be related to lateral contrast on the photographic emulsion, the second is the accurate determination of lattice parameters and the third is the simulation of the diffraction shapes using dynamical theory.

Effect of substrate miscut on the structural properties of InGaAs linear graded buffer layers grown by molecular-beam epitaxy on GaAs

We investigated the effect of substrate misorientation on the structural properties of an InGaAs linearly compositionally graded buffer layer and an AlGaAs/InGaAs superlattice grown by molecular-beam epitaxy on both on-axis and 2° miscut (001) GaAs substrates. Triple axis x-ray diffraction and reciprocal space maps were used to determine the relaxation and tilt of the buffer layer and superlattice with respect to each other and to the substrate. A tilt of 0.17° between the buffer layer and substrate was observed from (004) reciprocal space maps of the heterostructure grown on an off-axis substrate, while none was observed for the heterostructure grown on the on-axis substrate. The layer tilt axis for the miscut substrate, which coincided with a 〈110〉 direction, was 32° from the miscut axis. (224) glancing exit reciprocal space maps showed the majority of the buffer layer to be relaxed, with the top portion corresponding to a strained denuded zone free of misfit segments. Intersubband measurements showed no relation between device performance and the 2° miscut substrate, even though the two samples exhibited different superlattice peak widths.

High-resolution diffraction-space mapping and topography

The importance of collecting the distribution of scattered X-rays in two dimensions with the “right probe” will be addressed. The data-collection method will be briefly covered and how this greatly assists the interpretation of structural features giving rise to the distributed X-ray scattering. The combination of diffraction-space mapping with multiple crystal topography will also be presented to show how any region of scattering can be related to lateral structural changes or crystal imperfections. The simulation of the diffraction profiles of structures with defects will be addressed as well as the interpretation of “unusual and strange” diffraction features observed in high resolution, which yield further useful information on the materials under study.

Diffraction Space Mapping of Heteroepitaxial Layers

Abstract The acceptance angle of the detector of a double axis diffractometer is designed such that all of the diffracted beam is recorded for a rocking curve. When this acceptance angle is reduced, as in the triple axis diffractometer, two dimensional diffraction data can be recorded. In the resulting diffraction space maps each diffraction feature has a shape and a position from which the unit cell dimensions of a heteroepitaxial layer can be derived as well as information about relative tilts, curvature, lattice parameter variations and defect densities. Applications of diffraction space mapping using high and low resolution optics are discussed

Determining the lattice relaxation in semiconductor layer systems by x-ray diffraction

This paper illustrates the procedure for extracting structural information available from x-ray diffraction space mapping and topography. The methods of measuring, the residual strain, macroscopic tilts, microscopic tilts and their lateral dimensions, and the strain field disruption emanating from the interfacial defects are presented. Partially relaxed thick InGaAs layers on GaAs substrates were studied and it was concluded that the relaxation and macroscopic tilting were anisotropic, the microscopic tilting reduced with thickness, and the interfacial disruption did not continue to increase with increasing relaxation. A ‘‘mosaic grain growth’’ model is postulated to account for the diminishing microscopic tilt spread and increasing topographic contrast with layer thickness.

Characterization of quantum wells by X-ray diffraction

Measurements of the intensity distribution in diffraction space yield information far beyond that available by simple profiles. This paper briefly reviews the parameters obtainable from quantum well structures, when using conventional double-crystal and slit-based diffractometers, and shows how further structural parameters can be extracted by diffraction space mapping. To interpret subtle features a ' delta -function like' diffraction probe has been used to be sure of the origin of some of the features observed. The influence of layer tilting and lattice relaxation on the diffraction process can lead to imprecise fits, but from a diffraction space map, and including the diffuse scattering, further valuable information is extracted. The interpretation of these features has required additional modification to dynamical theory and the use of multiple-crystal topography. Examples of some unusual diffraction effects with possible explanations are also given.

The Evolution of Strain Relaxation Close to The Critical Thickness

ABSTRACTHigh resolution X-ray diffraction space mapping has been used to follow the change in the distribution of residual strain and localised relaxation in low mismatched epitaxial layers. Using this new technique, we have obtained a series of diffraction space maps of partially relaxed epitaxial layers of In.1Ga.9As on GaAs. The layers have different thicknesses and hence different degrees of strain relaxation. The diffuse scatter close to the Bragg peaks provides information about the imperfect and distorted regions in the structure and this has allowed us to examine the extent and distribution of residual strain close to the dislocations. We have followed the evolution of local relaxation, which is confined initially to regions around isolated dislocations, through to the case of overlapping dislocation strain fields, leading to a more homogeneous strain field distribution and microscopic and macroscopic tilting of the layers.