Triple-axis X-ray Research Articles

High-quality InAsyP1−y step-graded buffer by molecular-beam epitaxy

Relaxed, high-quality, compositionally step-graded InAsyP1−y layers with an As composition of y=0.4, corresponding to a lattice mismatch of ∼1.3% were grown on InP substrates using solid-source molecular-beam epitaxy. Each layer was found to be nearly fully relaxed observed by triple axis x-ray diffraction, and plan-view transmission electron microscopy revealed an average threading dislocations of 4×106 cm−2 within the InAs0.4P0.6 cap layer. Extremely ordered crosshatch morphology was observed with very low surface roughness (3.16 nm) compared to cation-based In0.7Al0.3As/InxAl1−xAs/InP graded buffers (10.53 nm) with similar mismatch and span of lattice constants on InP. The results show that InAsyP1−y graded buffers on InP are promising candidates as virtual substrates for infrared and high-speed metamorphic III–V devices.

Strain evolution in hydrogen-implanted silicon

The structural changes that accommodate wafer splitting after hydrogen implantation of silicon and the transfer of split layers to a handle substrate were investigated using triple axis x-ray diffraction, to monitor the strain in the implanted layer, and atomic force microscopy. Silicon substrates (004) were implanted with a hydrogen dose that ranged from 5×1015 cm−2 to 8×1016 cm−2 and energies of either 30 kV or 140 kV. The changes in the implanted layer properties were investigated after annealing at 150–300°C for short times. For annealing temperatures up to 150°C, the strain-induced implant profile did not change appreciably nor did the surface roughness increase, indicating that, for these implant conditions, the implant is stable. Annealing at 200°C or higher for 10 min or more led to increased surface roughness and a change to the implant profile, although blister formation did not occur. Higher surface roughness is not conducive to successful wafer bonding, so these measurements helped to determine the annealing sequence that is appropriate for bonding. In this case, hydrophobic bonded wafers were successfully fabricated with the transferred layer showing similar structural properties as a thin epitaxial film of the same thickness. The use of x-ray scattering for this technique can be easily transferred to other materials for which other techniques, such as infrared absorption, have not been developed.

X-ray characterization of bulk AIN single crystals grown by the sublimation technique

Bulk AlN single crystal boules have been grown using the sublimation technique and several substrates have been prepared from them. Microstructural characterization of these substrates has been performed using synchrotron white beam X-ray topography (SWBXT) and high-resolution triple axis X-ray diffraction. Our study has revealed that AlN single crystal boules grown by the sublimation technique can possess a high structural quality with dislocation densities of 800–1000/cm 2 and rocking curves with a full-width at half-maximum of less than 10 arcsec. The distribution of dislocations is inhomogeneous with large areas of the wafer free from dislocations. Inclusions are also observed (density of the order of 10 5/cm 3) and their distribution is also inhomogeneous.

Damage Mechanisms of Ion Implanted Bulk (0001) ZnO Single Crystals.

ABSTRACTCrystalline damage created by ion-implantation of dopant impurities in ZnO (0001) substrates was characterized as a function of atomic mass of implanted species using triple-axis (2θ-ω/ω) x-ray diffraction and Rutherford backscattering (RBS). The former revealed the presence of implantation-induced strain through the broadening of the isometric and asymmetric 2θ-ω reflections. However, RBS indicated that the damage introduced during implantation of these ions was insufficient to transform the crystalline lattice into a completely amorphous state. Additional XRD characterization as a function of annealing temperature of the implanted materials showed a reduction in the broadening of the isometric reflections, indicating that structural recovery of implanted ZnO crystals can be achieved.

Reciprocal space mapping of GaN xAs 1 −x grown by RF plasma-assisted solid source molecular beam epitaxy

We present the change in diffused scattering intensity and crystal truncation rod in X-ray reciprocal space mapping (RSM) of GaN x As 1− x grown by a radio frequency (RF) nitrogen plasma source in a solid source molecular beam epitaxy system. The RSM results are discussed in relation to variation in the low-temperature photoluminescence (PL) efficiency and peak energy. The X-ray RSM plots were recorded using a high-resolution triple-axis X-ray diffractometer on GaNAs samples annealed from 550 to 800°C. The RSM plots show higher diffused scattering around the GaNAs and GaAs (0 0 4) reflection point for the as-grown GaN x As 1− x sample with x=1.3%, indicating a high incorporation rate of N atoms and N complexes as interstitials, compared to a 5 μm-thick unintentionally doped GaAs sample. A reduction in diffused scattering around the GaN x As 1− x and GaAs (0 0 4) Bragg reflection point was observed with sharp termination of the crystal surface in the sample annealed at ∼700°C for 10 min. This is in good agreement with the observed improved PL efficiency, due to annihilation of interstitial point defects.

Synchrotron white beam X-ray topography and high resolution triple axis X-ray diffraction studies of defects in SiC substrates, epilayers and device structures

Synchrotron white beam X-ray topography and high resolution triple axis X-ray diffraction studies of defects in SiC substrates, epilayers and device structures

Relaxed InAsP layers grown on step graded InAsP buffers by solid source MBE

AbstractSi-doped InAsxP1-x layers with As mole fractions ranging from 0.05 to 0.50 were grown on InAsxP1-x step-graded buffer layers on InP substrates by solid source molecular beam epitaxy. The growth parameters consisted of a P:In flux ratio of 7:1, a growth temperature of ∼ 485°C, a growth rate of 2.2 Å/s, and an As:In flux ratio of 0.37-2.36 for varying As mole fractions. The As mole fraction and the layer relaxation were determined using triple axis x-ray diffraction measurements. Near complete relaxation (>93%) was achieved for all Si-doped InAsxP1-x epilayers. The structural morphology indicated that the InAsxP1-x graded buffer layers were effective in relieving the lattice mismatch strain as evidenced by a well-developed crosshatch morphology and low rms surface roughness. The electron concentration, mobility, and Si donor activation energy for each InAsxP1-x composition were determined using temperature dependent Hall measurements. At a constant electron carrier concentration of %3.5×1016 cm-3, the 300 K carrier mobility increased from 2700 to 4732 cm2/V-sec with increasing As mole fraction from 0.05 to 0.50.

Study of impurity segregation, crystallinity, and detector performance of melt-grown cadmium zinc telluride crystals

A review of growth methods used to produce Cd 1− x Zn x Te (CZT) (0.0< x<0.20) crystals for radiation detector applications is presented. Most of the results emphasize the high-pressure Bridgman (HPB) method. For selected melt-grown HPB ingots, the liquid/solid segregation coefficients of some impurities were measured. The correlation of the impurity content and nuclear detector performance will be discussed. Extended defects and surface and bulk crystallinity were measured using triple and double axis X-ray diffraction techniques (TAD and DAD XRD), X-ray topography, and infrared microscopy. X-ray diffraction maps and IR images were generated and compared to gamma-ray detector tests to correlate macroscopic defects with the nuclear detector responses. Defects states of CZT were also investigated using low-temperature photoluminescence spectroscopy. Comparisons between the material and detector properties for different CZT growth methods will be discussed.

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.

A high resolution triple axis X-ray diffraction analysis of radiation damage in lysozyme crystals

We have used high-resolution triple axis X-ray diffraction analyses to monitor the effects of radiation damage by Cu Kα X-rays on the defect structure in hen egg white lysozyme crystals. At long irradiation times (greater than 45 h of X-ray exposure) we observed the expected decrease in peak intensity and increase in the angular extent of the peak breadth. In contrast, the initial stages of irradiation showed relatively complex changes in both the peak breadth and the intensity: during the period from 25 to 45 h of irradiation the angular breadth of the intensity (both the full width at half maximum and the full width at 1% of the maximum intensity) decreased to a minimum value. Some possible reasons for this complex behavior are discussed.

Comparison of cadmium zinc telluride crystals grown by horizontal and vertical Bridgman and from the vapor phase

Characterization studies of Cd 1− x Zn x Te (0< x<0.24) crystals, CZT, grown by high pressure vertical Bridgman (HPVB), low pressure (LPB) vertical modified Bridgman (VB), horizontal modified Bridgman (HB), and physical vapor deposition (PVD) methods were performed. For selected melt-grown ingots, the liquid/solid segregation coefficients of some of the impurities were established. For most of the crystals, the surface and the bulk crystallinity were determined using triple and double axis X-ray diffraction techniques (TAD and DAD XRD). X-ray topography maps were also used to study macroscopic defects. The difference in properties of CZT grown by these methods are discussed.

Doping-induced losses in AlAs/GaAs distributed Bragg reflectors

We have studied n- and p-type doping-induced performance degradation of AlAs/GaAs distributed Bragg reflectors (DBRs) for applications in vertical cavity lasers (VCLs). Based on high-accuracy optical reflectance and triple-axis x-ray diffraction measurements on a variety of differently doped DBR structures grown by metalorganic vapor-phase epitaxy, a fitting procedure was employed to extract the doping-dependent optical loss. A striking observation is that the reflectance of these DBRs is much more sensitive to n- than p-type doping incorporation. While in the latter case the loss can be well accounted for by intervalence-band and free-carrier absorption, additional loss mechanisms must be considered for n-type DBRs. We relate the losses to doping-enhanced interdiffusion effects resulting in increased interface scattering. These findings should have important consequences for the design of VCLs, demonstrating the importance of reduced n-type doping concentrations and/or growth temperatures, or the application of alternative device concepts, e.g., employing intracavity contacts.

Triple-axis x-ray diffraction analyses of hen egg-white lysozyme crystals

We have used high-resolution triple-axis x-ray diffraction analyses to monitor the defectstructure in tetragonal crystals of hen egg-white lysozyme as a function of x-ray irradiation time.At long irradiation times we observed the expected decrease in peak intensity and increase in theangular extent of the peak breadth. In contrast, the initial stages of irradiation showed relativelycomplex changes in both the peak breadth and the intensity; in fact, during the period from 25 to45 h of irradiation the angular breadth of the intensity (both the full-width at half-maximum andthe full-width at 1% of the maximum intensity) decreased to a minimum value. We have foundthat the unambiguous analysis of defects at high angular resolution is complicated by the fact thatthe diffraction characteristics of protein crystals apparently lie at the confluence of the kinematic(ideally imperfect) and dynamic (ideally perfect) treatments of diffraction.

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.

Analysis of Radiation Damage in Lysozyme Crystals with High Resolution Triple Axis X-Ray Diffraction

ABSTRACTHigh resolution triple axis X-ray diffraction has been used to monitor the effects of X-ray radiation damage in hen egg white lysozyme crystals. At long irradiation the expected decrease in peak intensity and increase in the angular extent of the peak breadth was seen. In contrast, both the intesi-ties and peak breadths exhibited more complex behavior during the initial stages of irradiation. Possible reasons for these observations are discussed.

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.

Anomalous strains in the cubic-phase GaN films grown on GaAs (001) by metalorganic chemical vapor deposition

Strains in cubic GaN films grown on GaAs (001) were measured by a triple-axis x-ray diffraction method. Residual strains in the as-grown epitaxial films were in compression, contrary to the predicted tensile strains caused by large lattice mismatch between epilayers and GaAs substrates (20%). It was also found that the relief of strains in the GaN films has a complicated dependence on the growth conditions. We interpreted this as the interaction between the lattice mismatch and thermal mismatch stresses. The fully relaxed lattice constants of cubic GaN are determined to be 4.5038±0.0009 Å, which is in excellent agreement with the theoretical prediction of 4.503 Å.

Triple-axis X-ray diffraction analyses of lysozyme crystals.

High-resolution triple-axis X-ray diffraction techniques have been used to monitor the defect structure in hen egg-white lysozyme crystals. Analyses from the (440), (12;0) and (160) reflections showed significant differences in the intensity distribution around the respective reciprocal-lattice points. This work suggests that X-ray diffraction analytical methods developed primarily for relatively perfect inorganic crystals can be successfully applied to structurally defective macromolecular crystals. The analysis of defects at high angular resolution is complicated, however, by the observation that protein crystals lie at the convergence of the kinematic (ideally imperfect) and dynamic (ideally perfect) treatments of diffraction.

Misfit dislocation formation in p/p + silicon vapor-phase epitaxy

We investigated the misfit dislocation nucleation in lightly boron-doped silicon epitaxial layers deposited by vapor-phase epitaxy at 1080°C–1150°C on heavily boron-doped substrates. The substrate resistivity was 4 mΩ cm which produced a misfit of 1.5×10 −4 and the layer thickness was 10 μm. An orthogonal array of 〈1 1 0〉-type dislocations was observed at the interface, as determined by g· b contrast criteria using Lang transmission X-ray topography. The misfit dislocations were observed only around the wafer periphery and the misfit dislocation length was shorter in regions where both orthogonal 〈1 1 0〉 segments existed. The density of the misfit segments was determined at the interface by defect-etching. In regions where the misfits were observed, triple-axis X-ray diffraction measurements determined that the layers were measurably relaxed. The distribution of defects indicates that the wafer periphery possesses heterogeneous nucleation sites for misfit dislocation nucleation.

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).