High-resolution Triple-axis X-ray Diffraction Research Articles

Mg composition dependent strain analysis in nonpolar a-plane MgxZn1−xO films

Nonpolar a-plane (112¯0) MgxZn1−xO (a-MgxZn1−xO) films are deposited on (011¯2) r-sapphire substrates using metalorganic chemical vapor deposition with varying Mg composition (x from 0 to 0.25). Unit cell parameters with Mg composition are determined by high-resolution triple-axis x-ray diffraction. In-plane strain along the c-axis [0001] and m-axis [11¯00] in the films is anisotropic and increases with increasing Mg composition. The in-plane strain anisotropy changes with Mg composition in a-MgxZn1−xO. Calculations are carried out to determine the influence of Mg content on the residual interfacial strain.

Strain relaxation and surface morphology of high indium content InAlAs metamorphic buffers with reverse step

Low-temperature step-graded high indium content InAlAs (In% = 0.75) metamorphic buffer layers with reverse step layer grown on GaAs substrate by molecular beam epitaxy are investigated in this paper. The composition and the strain relaxation of the top InAlAs layer are determined by high-resolution triple-axis X-ray diffraction measurements, which show that the top InAlAs layer is nearly fully relaxed and the growth parameters for these samples have little influence on the strain relaxation ratio. Surface morphology is observed by reflection high-energy electron diffraction pattern and atomic force microscopy. The surface morphology is found to depend strongly on both the growth temperature and the As flux. Compared with other samples, the sample growth under the optimized conditions has the smallest value of root mean square surface roughness. Furthermore, the ω − 2 θ and ω scans of the triple-axis X-ray diffraction and transmission electron microscopy result also show the sample grown under the optimized conditions has good crystalline quality.

Low temperature step-graded InAlAs/GaAs metamorphic buffer layers grown by molecular beam epitaxy

Low-temperature step-graded InAlAs metamorphic buffer layers on GaAs substrate grown by molecular beam epitaxy were investigated. The strain relaxation and the composition of the top InAlAs layer were determined by high-resolution triple-axis x-ray diffraction measurements, which show that the top InAlAs layer is nearly fully relaxed. Surface morphology was observed by reflection high-energy electron diffraction pattern and atomic force microscopy. Under a selected range of growth parameters, the root mean square surface roughness of the sample grown at 380 °C is 0.802 nm, which has the smallest value compared with those of other samples.Furthmore, The ω-2θ and ω scans of the triple-axis x-ray diffraction, and photoluminescence show the sample grown at 380 °C has better crystalline quality. With decreasing As overpressure at this growth temperature, crystalline quality became poor and could not maintain two dimensional growth with increasing overpressure. The carrier concentrations and Hall mobilities of the InAlAs/ InGaAs/GaAs MM-HEMT structure on low-temperature step-graded InAlAs metamorphic buffer layers grown in optimized conditions are high enough to make devices.

Crystal Growth and Defect Characterization of AlN Single Crystals

In this paper, we report results from seeded AlN PVT growth experiments carried out using SiC seeds. The purpose of the experiments was to understand the morphology and crystalline quality of PVT AlN crystals grown under the crystal growth environments investigated. AlN single crystal films of 120-650μm in thickness were grown and freestanding AlN single crystal pieces up to 4×5mm2 were obtained. Surface morphologies and crystal defects in these AlN single crystals were studied using optical microscopy. Selected AlN single crystals were studied using a high-resolution triple-axis X-ray diffraction technique and a Synchrotron White Beam X-ray Diffraction Topography technique. Defects identified in AlN crystals are cracks, dislocations, grain boundaries and crystallite inclusions.

Effects of postgrowth rapid thermal annealing on InAlAs∕InGaAs metamorphic high-electron-mobility transistor grown on a compositionally graded InAlAs∕InGaAlAs buffer

Effects of postgrowth rapid thermal annealing (RTA) on structural and electrical properties of an In0.52Al0.48As∕In0.52Ga0.48As metamorphic high-electron-mobility transistor (MHEMT) structure grown on a GaAs substrate utilizing a compositionally graded InAlAs∕InGaAlAs buffer layer were investigated. High-resolution triple-axis x-ray diffraction, photoluminescence, and van der Pauw–Hall measurements were used for the investigation. While the RTA improved the structural property of the MHEMT, it degraded the channel mobility of the MHEMT due to defect-assisted impurity redistribution.

Synchrotron white beam x-ray topography (SWBXT) and high resolution triple axis diffraction studies on AlN layers grown on 4H- and 6H-SiC seeds

ABSTRACTFor nitride based devices such as LEDs, high power FETs and laser diodes, single crystal substrates of AlN are highly desirable. While the sublimation technique is suitable for growing bulk AlN crystals, appropriate seeds are also necessary for growing large diameter oriented boules. 4H- and 6H-SiC substrates which are readily available commercially can potentially be implemented as seeds for bulk AlN growth. However, issues regarding SiC decomposition at high temperatures, thermal expansion mismatch, single crystal growth, etc. need to be addressed. Towards this end, a series of growth experiments have been carried out in a resistively heated reactor using on and off-axis 4H- and 6H-SiC substrates as seeds for AlN growth from the vapor phase. Several hundred microns thick AlN layers have been grown under different growth conditions. Synchrotron white beam x-ray topography (SWBXT) has been used to map the defect distribution in the grown layers and high resolution triple axis x-ray diffraction (HRTXD) experiments were carried out to record reciprocal space maps from which tilt, mismatch and strain data can be obtained. These results are analyzed with respect to the growth conditions in order to gain a better understanding of this growth process.

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.

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.

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.

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.

X-ray diffraction analysis of SiGe/Si heterostructures on sapphire substrates

Si/Si 1−x Ge x heterostructures on improved silicon-on-sapphire substrates were grown epitaxially by ultrahigh vacuum chemical vapor deposition for application as p-channel field effect transistors. High-resolution triple-axis x-ray diffraction was used to analyze these structures quantitatively and to evaluate their thermal stability. Outdiffusion of Ge from the strained Si1−xGex quantum well was observed after annealing at 850 °C. The amount of outdiffusion was comparable to that observed in Si1−xGex structures on bulk Si wafers.

Analysis Of Sige Fet Device Structures On Silicon-on-sapphire Substrates by X-Ray Diffraction

AbstractSi/Si1-xGex, heterostructures on improved silicon-on-sapphire substrates were grown epitaxially by ultra-high vacuum chemical vapor deposition for application as p-channel field effect transistors. High-resolution triple-axis x-ray diffraction was used to analyze these structures quantitatively and to evaluate the effects of device fabrication processes on them. Out-;diffusion of Ge from the Si1-xGex, quantum well was observed after fabrication as was the change in thickness of the Si cap layer due to wafer cleaning and gate oxidation at 875 °C

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.

Dislocation nucleation barrier in SiGe/Si structures graded to pure Ge

High-resolution triple-axis x-ray diffraction measurements were used to study strain relaxation in the individual layers of a SiGe/Si structure step-graded to pure Ge. The tilt of each layer is explained by extending the model previously proposed for obtaining the nucleation activation energy of dislocations to account for the reduced miscut of the growth surface as the sample relaxes and the variation in the materials properties with alloy composition.

Hole energy levels and intersubband absorption in modulation-doped Si/Si1-xGex multiple quantum wells.

A theoretical and experimental study of intersubband transitions in modulation-doped p-type Si/SiGe quantum wells is presented for SiGe wells with widths between 26 and 65 \AA{} and Ge contents in the range from 19% to 50%. The SiGe multiple quantum wells are pseudomorphically strained with an in-plane lattice constant equal to the lattice constant of the Si substrate. Calculations of the in-plane dispersion of the quantum-well states are performed within the envelope-function approach, with full inclusion of the degeneracy and warping of the three topmost bulk valence bands described by the strain-dependent Luttinger-Kohn Hamiltonian. Many-body effects such as the Hartree potential and the exchange-correlation interaction are taken into account in a self-consistent manner. The transmission spectra are finally calculated with a dielectric simulation for the multilayer stack. Using the structural parameters determined by high-resolution triple-axis x-ray diffraction, the results of the calculation are in excellent agreement with the observed intersubband absorption that occurs between 480 and 1830 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the different samples. As long as the excited states are confined to the SiGe wells, the full widths of the absorption lines are only 20 meV. This value represents the narrowest absorption line so far observed in p-type SiGe quantum wells.

Intersubband absorption in modulation doped p-type Si 1-xGe x quantum wells: theory and experiment

A study of intersubband infrared absorption in modulation doped p-type Si/SiGe quantum wells is presented for SiGe wells with thicknesses between 22 Å and 64 Å and Ge contents in the range from 23% to 58%. The peak positions of the absorption lines are observed between 500 cm -1 and 2200 cm -1. Depending on the barrier height (i.e., on the Ge content of the wells), the heavy-hole states excited by the infrared radiation are either localized in the wells or strongly mixed with barrier-bound states and therefore delocalized. The shape of the absorption line correspondingly changes from a narrow Lorentz line to a rather broad absorption band. Using the structural parameters determined by high-resolution triple-axis x-ray diffraction, the results of a self-consistent Luttinger-Kohn type envelope function approach with the explicit inclusion of the strain in the quantum wells are in excellent agreement with the measured spectra.

Intersubband Absorption in Modulation-Doped p-Type Si/Si1-xGex Quantum Wells: A Systematic Study

A study of intersubband infrared (IR) absorption in modulation-doped p-type Si/SiGe quantum wells is presented for SiGe wells with widths between 26 Å and 36 Å and Ge contents in the range from 19% to 50%. Absorption lines between 725 cm-1 and 1830 cm-1 are observed. Depending on the barrier height (i.e., on the Ge content of the wells), the heavy-hole subbands excited by the IR radiation are either localized in the wells or strongly mixed with barrier-bound states and therefore delocalized. The shape of the absorption line correspondingly changes from a narrow Lorentz line to a rather broad absorption band. Using the structural parameters determined by high-resolution triple-axis X-ray diffraction, the results of a self-consistent calculation are in excellent agreement with the measured absorption spectra.