Triple-axis X-ray Diffractometry Research Articles

Analyses of determination conditions of n-GaN dislocation density by triple-axis X-ray diffraction

Dislocation densities of two hydride vapor phase epitaxy-grown hexagonal GaN samples, which are Si doped and unintentionally doped respectively, are determined by triple-axis X-ray diffractometry and van der Pauw variable temperature Hall-effect measurement. The dislocation densities of these two samples should be at the same level from the X-ray testing, the -FWHM (full width at half maximum) values of all corresponding reflections for these two samples are almost the same. But from the Hall-effect measurements, the dislocation density values should be different from each other remarkably, because the unintentionally doped sample belongs to Mott transition material, while the Si-doped one does not. This fact indicates that the X-ray testing is perhaps inaccurate under some conditions, although the triple-axis X-ray diffractometry is a highly suitable technique for discriminating different kinds of structural defects such as edge and screw dislocations that lead to characteristic broadening of symmetric and asymmetric Bragg reflection. The experimental result obtained so far (say, for hot-electron bolometer) shows that the dislocation density value from mobility fitting model is in good accordance with that from -FWHM fitting using Srikant method. The anomaly that the dislocation density from -FWHM fitting is much lower than that from mobility fitting for the same sample (sample 59#), indicates that dislocations located in grain boundary may not be tested by triple-axis X-ray diffractometry. According to mosaic model, the layer is assumed to consist of single crystallites, called mosaic blocks, which are assumed to be slightly misoriented with respect to each other. The out-of-plane rotation of the block perpendicular to the surface normal is of the mosaic tilt, and the in-plane rotation around the surface normal is of the mosaic twist. The average absolute values of tilt and twist angles are directly related to the FWHM values of the corresponding distributions of crystallographic orientations. So, the X-ray testing can determine the average orientation of the grains with the same interplanar distance, excluding the information about the grain boundary at which X-ray cannot interfere because of disdortion of lattice. The experimental results and calculation analyses indicate that the dislocation density value from Srikant model is accurate when the ratio of twist angle to tilt angle exceeds 2.0, or the magnitude of the lateral coherence length is larger than 1.5 m.

Growth and characterization of Ge nanostructures selectively grown on patterned Si

By utilizing different distribution of strain fields around the edges of oxide, which are dominated by a series of sizes of oxide-patterned windows, long-range ordered self-assembly Ge nanostructures, such as nano-rings, nano-disks and nano-dots, were selectively grown by ultra high vacuum chemical vapor deposition (UHV-CVD) on Si (001) substrates. High-resolution double-crystal symmetrical ω/2 θ scans and two-dimensional reciprocal space mapping (2D-RSM) technologies employing the triple axis X-ray diffractometry have been used to evaluate the quality and strain status of as-deposited as well as in-situ annealed Ge nanostructures. Furthermore, we also compare the quality and strain status of Ge epilayers grown on planar unpatterned Si substrates. It was found that the quality of all Ge epitaxial structures is improved after in-situ annealing process and the quality of Ge nano-disk structures is better than that of Ge epilayers on planar unpatterned Si substrates, because oxide sidewalls are effective dislocation sinks. We also noted that the degree of relaxation for as-deposited Ge epilayers on planar unpatterned Si substrates is less than that for as-deposited Ge nano-disk structures. After in-situ annealing process, all Ge epitaxial structures are almost at full relaxation whatever Ge epitaxial structures grew on patterned or unpatterned Si substrates.

High-resolution X-ray diffraction analysis of SnTe/Sn 1− xEu xTe superlattices grown on (1 1 1) BaF 2 substrates

A set of SnTe/Sn 1− x Eu x Te superlattice (SL) samples with increasing nominal Eu content x up to 0.28 was successfully grown on (1 1 1) BaF 2 substrates by molecular beam epitaxy. A complete structural characterization was performed by triple-axis X-ray diffractometry and reciprocal space mapping. The X-ray results showed that, despite the phase separation that normally occurs for unstrained Sn 1− x Eu x Te layers with x⩾0.02, an SL stack with homogeneous individual layers can be formed for SL samples with a nominal Eu content up to 0.16. No SL satellite peak structure could be identified for samples with x values higher than 0.24. The structural parameters of the individual layers that compose the SL were determined using a best-fit simulation procedure which compared the calculated X-ray spectra to the measured (2 2 2) ω/2Θ scans. The strain information used in the simulation was obtained from the reciprocal space maps measured around the (2 2 4) lattice point.

The fine structure of X-ray diffuse scattering in the vicinity of high-angle superlattice Bragg reflections.

Triple-axis X-ray diffractometry was used to study diffuse scattering from an AlAs/GaAs superlattice grown on an [001]-oriented GaAs substrate by molecular beam epitaxy. Reciprocal-space maps were obtained around the 002 reflection from the superlattice and its low-angle first-order satellite. The data obtained reveal quasi-Bragg diffuse-scattering sheets caused by conformal behavior of interfacial roughness as well as amplification of diffuse scattering when the incoming or outgoing angle is nearly equal to the Bragg angle of the superlattice or substrate. The observed features of diffuse-scattering fine structure are explained within the framework of the distorted-wave Born approximation. Nevertheless, this approximation is shown to be incorrect for quantitative analysis of diffuse scattering. In particular, the observed domination in intensity of the incoming Bragg features over the outgoing ones is shown to reflect the decay rate of the coherent X-ray field through the diffuse-scattering channel, which is not negligible relative to the coherent diffraction.

Formation of lateral-two-dimensional ordering in self-assembled InGaAs quantum dot on high index substrates

The in-plane strain relaxation properties on InGaAs/GaAs (3 1 1) B were investigated by means of reciprocal space mapping using triple axis X-ray diffractometry to study the origin of two-dimensional ordering of InGaAs quantum dots (QDs) on a GaAs (3 1 1) B substrate. From the X-ray data, the strain relaxation anisotropy in the InGaAs layer was observed to be larger on the GaAs (3 1 1) B substrate than on (0 0 1) . It is proposed that the large strain anisotropy is responsible for the QD ordering observed on the (3 1 1) B surface.

Relaxation of InGaN thin layers observed by X-ray and transmission electron microscopy studies

Double-crystal and triple-axis x-ray diffractometry and transmission electron microscopy are used to characterize the microstructure, strain, and composition of InGaN layers grown on GaN by metalorganic chemical vapor deposition (MOCVD). Three different samples with increasing In concentration have been studied, all grown on GaN deposited on sapphire either with GaN or AlN buffer layers. It was found that InGaN layers with nominal 28% and 40% InN content consist of two sub-layers; the first sub-layer is pseudomorphic with the underlying GaN with lower In content than nominal. The top sub-layer is fully relaxed with a high density of planar defects and In content close to the nominal value. This is in contrast to a common assumption applied to InGaN quantum wells that ‘quantum-dot like areas’ are formed with different In content. The sample with the nominal indium concentration of 45% does not exhibit any intermediate strained layer, is fully relaxed and the In concentration (43.8%) agrees well with the nominal value.

MOVPE growth of lattice-mismatched Al 0.88In 0.12As on GaAs (1 0 0) for space solar cell applications

We report, for the first time, the growth and characterization of lattice-mismatched Al 0.88In 0.12As on GaAs (1 0 0) as part of a program to develop a high-efficiency, triple-junction space solar cell. The expected practical efficiency of this cell is 31%. The proposed triple-junction cell consists of a 2.1 -eV Al 0.88In 0.12As top cell, a 1.6 -eV In 0.48Ga 0.52As 0.23P 0.77 middle cell, and a 1.2 -eV In 0.13Ga 0.87As bottom cell. The Al 0.88In 0.12As layers were grown on GaAs (1 0 0) wafers cut 6° off-axis, using metal organic precursors trimethylaluminum, trimethylindium and the hydride arsine in a horizontal, reduced-pressure metal organic vapor-phase epitaxy (MOVPE) reactor. Crystalline quality was evaluated through triple-axis X-ray diffractometry. Reciprocal lattice mapping showed that the Al 0.88In 0.12As layers were relaxed to a large extent, and were of high crystalline quality. Doping was performed using silane (n-type) and diethylzinc (p-type). Secondary ion mass spectrometry (SIMS) and electrochemical C– V profiling showed that it was possible to obtain abrupt doping profiles to the levels required for solar cell fabrication. Details of these results and those of dislocation density measurement using transmission electron microscopy (TEM) are described in this paper.

Defect characterization in epitaxial ZnO/epi-GaN/Al 2O 3 heterostructures: transmission electron microscopy and triple-axis X-ray diffractometry

Structural properties of epitaxial ZnO/epi-GaN/Al 2O 3 heterostructures have been studied by transmission electron microscopy and X-ray diffractometry for the first time. The majority of threading dislocations running along the growth direction of ZnO films are screw-type dislocations. At the interface region, a high density of dislocations located mainly on the basal plane was observed. These dislocations show intense dislocation reactions resulting in a rapid reduction of the dislocation density towards the upper region of the film. From X-ray rocking curve measurements, the full-width at half-maximum (FWHM) value of the (1 0 1) φ-scan is larger than that of the (0 0 2) ω-scan and increase with decreasing the ZnO film thickness. The increment of the width of the (1 0 1) φ-scan with decreasing the film thickness from 500 to 150 nm is much larger (0.13°) than that of the (0 0 2) ω-scan (0.04°). Different broadening in (1 0 1) φ-scan and (0 0 2) ω-scan rocking curves are explained in terms of the defect configuration as investigated by transmission electron microscopy (TEM).

Overgrowth of (In,Ga)(As,P) on rectangular-patterned surfaces using gas source molecular beam epitaxy

Integrated optical devices, such as planar waveguide-coupled Bragg-resonant filters, are an important component for the implementation of high density, narrow bandwidth optical filtering in wavelength division multiplexed optical communication systems. However, acceptable optical device performance of the filter is contingent upon the ability to maintain the integrity and profile of the as-fabricated rectangular-patterned Bragg resonators during epitaxial overgrowth of the cladding layer. Bragg gratings are defined in both InP substrates and InGaAsP epitaxial layers using X-ray lithography and reactive ion etching. The regrowth process is initiated by removing the native oxide with a low temperature (∼200°C) atomic hydrogen-assisted cleaning regimen that results in a grating that is unaltered from the desired as-etched rectangular-patterned profile. The techniques employed to preserve the grating profile are described, and the microstructural and optical quality of the structures are analyzed via scanning electron microscopy, triple axis X-ray diffractometry, and photoluminescence.

Analysis of lattice distortions in high-quality InGaAsP epitaxial overgrowth of rectangular-patterned InP gratings

The crystalline quality of InGaAsP deposited via gas-source molecular beam epitaxy on rectangular-patterned InP substrates has been investigated. Triple axis x-ray diffractometry measurements of InGaAsP deposited on planar InP substrates confirm that the pseudomorphic epilayer composition is uniform and of high-crystalline quality. Triple axis x-ray diffraction analysis of InGaAsP deposited on rectangular-patterned substrates reveals that the crystalline quality of the epilayer is not compromised by the presence of the grating. Symmetric [(004)] and asymmetric [(224)] reciprocal space maps of overgrown rectangular gratings indicate that both the InP and InGaAsP within the grating region exhibit an overall orthorhombic lattice distortion. Comparison of (004) diffraction scans with dynamical computer simulations suggests that the distortions are not related to detectable compositional grating. The magnitude of the orthorhombic strain, introduced by the rectangular-patterned grating, will alter the refractive index of both materials by amounts that are not expected to compromise the operation of optical devices.

Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray diffractometry

Abstract Important structural characteristics (correlation lengths of columnar crystallites, dislocation densities, angles of rotational disorder) of hexagonal GaN grown by metallorganic chemical vapour deposition on c-plane sapphire are determined by transmission electron microscopy and triple-axis X-ray diffractometry. GaN films exhibit an edge dislocation density in the range of 1011 cm−2, a tilt and twist angle of 0.1° and 1.3° and a columnar structure with a lateral and vertical correlation length of 150 and 1000 nm respectively. The determination of correlation lengths and dislocation densites from X-ray patterns was undertaken using two independent evaluation methods which are discussed in detail. It is also shown that triple-axis X-ray diffractometry is a highly suitable technique for the separation of different kinds of structural defects such as edge and screw dislocations that lead to a characteristic broadening of symmetric and asymmetric Bragg reflections. The correlation lengths and dislocat...

Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray diffractometry

Defect structure of epitaxial GaN films determined by transmission electron microscopy and triple-axis X-ray diffractometry

Preservation of rectangular-patterned InP gratings overgrown by gas source molecular beam epitaxy

The ability to preserve a grating profile during epitaxial overgrowth is vital to the design and operation of devices such as planar waveguide-coupled Bragg-resonant filters. Intentional preservation of rectangular-patterned InP gratings during gas source molecular beam epitaxial overgrowth has been accomplished. The use of a low temperature atomic hydrogen-assisted oxide removal technique alleviates the dependence on elevated temperatures for desorption of the native oxide, and is crucial to the preservation of the patterned profile. Confirmation of the overgrown grating fidelity is demonstrated via scanning electron microscopy and triple axis x-ray diffractometry.

The effect of substrate misorientation on the optical, structural, and electrical properties of GaN grown on sapphire by MOCVD

We report the growth and characterization of unintentionally doped GaN on both exact and vicinal (0001) sapphire substrates. The GaN heteroepitaxial layers are grown by metalorganic chemical vapor deposition on c-plane A12O3 substrates either on-axis or intentionally misoriented 2° toward the a-plane (1120) or 5 or 9° toward the m-plane (10 10). The samples are characterized by 300K photoluminescence, cathodoluminescence, and Hall-effect measurements as well as by triple-axis x-ray diffractometry to determine the effect of the misorientation on the optical, electrical, and structural properties of heteroepitaxial undoped GaN. Ten different sample sets are studied. The data reveal enhanced photo-luminescence properties, increased electron mobility, a reduced n-type background carrier concentration, and a somewhat degraded surface morphology and crystalline quality for the misoriented samples compared to the on-axis samples.

X-ray diffraction study of porous silicon layers etched on (111)-orientedp + substrate

Porous silicon (pSi) layers anodized on a (111)-orientedp+ substrate with a CVDp-type layer were examined by a double-crystal X-ray diffraction technique using a synchrotron source as well as by triple-axis X-ray diffractometry with a standard laboratory source. Several different techniques (SEM, TEM, PL, FTIR) were also employed to complement the X-ray experiments and in order to investigate more thoroughly the important pSi parameters. A range of samples, exhibiting various values for the wavelength of the maximum of the photoluminescence signal, were chosen for this study. Samples with porosity ranging from 20% to 85% and of varying layer thickness yielded unexpected results regarding the lattice parameter in the porous layer, which seems to be strongly influenced by the presence of the CVD layer itself. Mutual interaction between the pSi and CVD layers produced a relaxed state in which the porous layer seems to adopt the lattice parameter, in the perpendicular [111] direction, of the underlying CVD layer. The lattice parameter of the CVD layer itself (in the [111] direction) was relaxing in such a way that for the high porosity specimens it presents a larger difference from the substrate than for the low porosity capping pSi layers.

Complete p-type activation in vertical-gradient freeze GaAs co-implanted with gallium and carbon

High-resolution triple-axis x-ray diffractometry and Hall-effect measurements were used to characterize damage evolution and electrical activation in gallium arsenide co-implanted with gallium and carbon ions. Complete p-type activation of GaAs co-implanted with 5×1014 Ga cm−2 and 5×1014 C cm−2 was achieved after rapid thermal annealing at 1100 °C for 10 s. X-ray diffuse scattering was found to increase after rapid thermal annealing at 600–900 °C due to the aggregation of implantation-induced point defects. In this annealing range, there was ∼10%–72% activation. After annealing at higher annealing temperatures, the diffuse scattered intensity decreased drastically; samples that had been annealed at 1000 °C (80% activated) and 1100 °C (∼100% activated) exhibited reciprocal space maps that were indicative of high crystallinity. The hole mobility was about 60 cm2/V s for all samples annealed at 800 °C and above, indicating that the crystal perfection influences dopant activation more strongly than it influences mobility. Since the high-temperature annealing simultaneously increases dopant activation and reduces x-ray diffuse scattering, we conclude that point defect complexes which form at lower annealing temperatures are responsible for both the diffuse scatter and the reduced activation.

X-ray reciprocal space mapping of [formula omitted] heterostructures

The analysis of composition, strain, and strain relaxation in molecular beam epitaxy grown Si SiGe structures is described, based on the use of reciprocal space mapping techniques employing high resolution triple axis X-ray diffractometry.

Diffuse x-ray scattering from misfit dislocations in SiGe epitaxial layers with graded Ge content

A theory has been developed describing x-ray diffuse scattering from misfit dislocations in epitaxial layers. This approach has been used for explaining the origins of diffuse x-ray scattering from SiGe layers with linearly graded Ge content. The distribution of the diffusely scattered intensity in reciprocal plane measured by triple-axis x-ray diffractometry has been compared with theoretical predictions and a good agreement has been achieved. It is demonstrated that the main part of the diffusely scattered intensity originates from random strains caused by misfit dislocations at the substrate–epilayer interface or in the relaxed part of the compositionally graded layers. The contribution of the threading dislocation segments to the diffuse scattering is rather small.

Structural investigation of II–VI compound semiconductor quantum wires using triple-axis X-ray diffractometry

Corrugated CdZnTe substrates and quantum wires fabricated by dry etching techniques from (CdTe)/(MnTe) superlattices (SLs), which were grown by a combination of molecular beam and atomic layer epitaxy, were investigated with respect to their structural properties. The SLs were grown pseudomorphically on Cd 0.96Zn 0.04Te (001) oriented substrates. X-ray reciprocal space mapping around (004) and (115) reciprocal lattice points was used to measure the periods both along the growth and the lateral corrugation direction. The arrays were defined by holographic lithography and reactive ion etching with a typical period of about 400 nm. For the analysis of the corrugations, a kinematical X-ray diffraction model is used which yields their geometrical shape and also informations on the crystalline damage. Furthermore, it turns out that the reactive ion etching process causes an increase of the mean SL lattice constant of about 0.1% to 0.15% along the growth direction.

Structural investigations of GaAs/AlAs quantum wires and quantum dots by X-ray reciprocal space mapping

We have investigated periodic arrays of dry etched 150 nm and 175 nm wide, (110) oriented GaAs/AlAs quantum wires and quantum dots by means of reciprocal-space mapping using triple-axis X-ray diffractometry. From the X-ray data the lateral periodicity of wires and dots, the etch depth and the angle of misorientation of the wires with respect to the (110) direction are extracted. The reciprocal-space maps reveal that, after the fabrication process the lattice constant along the growth direction increases slightly for the wires and even more so for the dots.