Double Crystal X-ray Diffraction Research Articles

Lattice Strain Relaxation and Compositional Control in As-Rich GaAsP/(100)GaAs Heterostructures Grown by MOVPE.

The fabrication of high-efficiency GaAsP-based solar cells on GaAs wafers requires addressing structural issues arising from the materials lattice mismatch. We report on tensile strain relaxation and composition control of MOVPE-grown As-rich GaAs1-xPx/(100)GaAs heterostructures studied by double-crystal X-ray diffraction and field emission scanning electron microscopy. Thin (80-150 nm) GaAs1-xPx epilayers appear partially relaxed (within 1-12% of the initial misfit) through a network of misfit dislocations along the sample [011] and [011-] in plane directions. Values of the residual lattice strain as a function of epilayer thickness were compared with predictions from the equilibrium (Matthews-Blakeslee) and energy balance models. It is shown that the epilayers relax at a slower rate than expected based on the equilibrium model, an effect ascribed to the existence of an energy barrier to the nucleation of new dislocations. The study of GaAs1-xPx composition as a function of the V-group precursors ratio in the vapor during growth allowed for the determination of the As/P anion segregation coefficient. The latter agrees with values reported in the literature for P-rich alloys grown using the same precursor combination. P-incorporation into nearly pseudomorphic heterostructures turns out to be kinetically activated, with an activation energy EA = 1.41 ± 0.04 eV over the entire alloy compositional range.

Effect of Rapid Thermal Annealing on the Emission Properties in InGaAsSb/AlGaAsSb Multiple Quantum Wells

An effect of rapid thermal annealing (RTA) on emission properties of InGaAsSb/AlGaAsSb multiple quantum wells (MQWs) grown by molecular‐beam epitaxy is systematically investigated by photoluminescence (PL) spectra. The emission from free‐exciton and localized carriers is confirmed in as‐grown InGaAsSb/AlGaAsSb MQWs using temperature and excitation power‐dependent PL spectra. The results of RTA for different times at a fixed temperature of 350 °C are analyzed. The PL intensity of InGaAsSb/AlGaAsSb MQWs is increased after an RTA process, and the peak position is slightly blueshifted at 300 K. Low‐temperature PL spectra show improved emission of free‐exciton and deteriorated localized carriers recombination at an annealing time of 60 s. Double crystal X‐ray diffraction (DCXRD) measurements are also carried out to study the crystalline quality before and after annealing. These results indicate that a suitable RTA treatment is effective in reducing the densities of nonradiative recombination centers, which implies an improved optical property and crystalline quality of quantum well structures. This study provides valuable understanding of recombination processes and improvement of optical properties by RTA treatment in InGaAsSb/AlGaAsSb MQWs.

Influence of chamber pressure on the crystal quality of homo-epitaxial GaN grown by radical-enhanced MOCVD (REMOCVD)

III-nitride was grown by radical-enhanced metalorganic chemical vapor deposition (REMOCVD) with a very high-frequency electric source 100 MHz, using nitrogen and hydrogen as a source gas free from ammonia gas. Applying radio frequency (RF) power at the top electrode generates activated nitrogen, hydrogen, and other nitrogen species. Homoepitaxial gallium nitride (GaN) growth was studied as a function of chamber pressure by REMOCVD. The grown GaN was characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and double crystal X-ray diffraction (XRD). Ga radicals and N radicals were detected by optical emission spectroscopy (OES) as a function of chamber pressure. The V/III ratio changes with the N2*/Ga* ratio, and the step flow growth of GaN was achieved under the chamber pressure of 300 Pa.

A Comparison of Defects Between InAs Single Crystals Grown by LEC and VGF Methods

InAs single crystals grown by the liquid-encapsulated Czochralski (LEC) method and vertical gradient freezing (VGF) are studied by low-temperature photoluminescence spectroscopy, infrared transmission and reflectance spectroscopy, double-crystal x-ray diffraction and Hall effect measurement, respectively. A properly controlled etching solution is used to reveal beautiful square dislocation etch pits in the crystals. In addition to extremely low dislocation density, the concentration of native defects in the VGF-InAs single crystals is much lower than that in LEC-InAs, giving VGF-InAs better electrical and optical properties. The nature of the defects in InAs single crystals is discussed by considering the variation in stoichiometry and environment during the crystal growth processes.

Effect of proton doping and heat treatment on the structure of single crystal silicon

The quality and structural perfection of single crystal silicon have been studied using double-crystal X-ray diffraction after hydrogen ion implantation and thermal annealing used in a number of semiconductor technologies. The fundamental difference of this approach is the possibility to rapidly obtain reliable experimental results which were confirmed using X-ray topography. Data have been presented for the condition of the damaged layer in n-type silicon single crystals (r = 100 W × cm) having the (111) orientation and a thickness of 2 mm after proton implantation at energies E = 200, 300 and 100 + 200 + 300 keV and dose D = 2 × 1016 cm-2 and subsequent heat treatment in the T = 100–900 °C range. Using the method of integral characteristics we have revealed a nonmonotonic dependence of the integral characteristics of the damaged layer, i.e., the mean effective thickness Leff and the mean relative deformation Da/a, on the annealing temperature, the maximum deformation being observed for ~300 °C. The results have allowed us to make a general assessment of the damaged layer condition after heat treatment.

Electrical conduction of C-implanted InAs single crystal

Carbon-ion-implanted InAs was investigated using double-crystal x-ray diffraction (DCXRD), Hall measurement and infrared absorption (IR) analysis. Multiple implantation were made at 0.1–0.4 MeV with 6.0 × 1012–2.0 × 1013 ions cm−2. After rapid thermal annealing at 300 °C for 20 s, the implantation-induced damage was removed substantially, indicating the recovery of crystallinity. The results of Hall measurement reveal strong electrical compensation and low conductivity in the implanted layer of the sample, suggesting the formation of acceptor CAs. In contrast, the lowest IR transmittance is observed in the 300 °C annealed sample, implying the existence of acceptor with significant concentration. The implanted layer turned to n-type after annealing at 400 °C with the increasing transmittance. After annealing at temperature of 500 °C, the decreasing carrier concentration and the increasing transmittance is attributed to the competition between the decomposition of C–H complexes and the formation of donor centers C–C.

Development of a Synthesis Technique and Characterization of High-Quality Iron Borate FeBO3 Single Crystals for Applications in Synchrotron Technologies of a New Generation

A flux growth technique for synthesizing FeBO3 single crystals of high structural perfection was developed. The high structural quality of the synthesized FeBO3 crystals was confirmed by means of double-crystal X-ray diffraction analysis both in Laue and Bragg geometries. The diffraction rocking curves taken locally and integrally over the surface are in excellent agreement with the calculated curves. Some macrodefects were revealed by X-ray topography in the crystal volume. However, the local defects do not prevent the use of defect-free regions of the crystal for synchrotron Mossbauer experiments.

Preparation and Properties of Sapphire by Edge-defined Film-fed Growth (EFG) Method with Different Growth Directions

Sapphire, belonging to hexagonal crystal system, is typically anisotropic which makes it direction-sensitive. To research the effects of growth directions on properties of sapphire, c-[0001] seed (c-sapphire) and a-[11-20] seed (a-sapphire) were used to prepare sapphire by edge-defined film-fed growth (EFG) method. The samples were analyzed through lattice integrity, dislocation and corrosion performance by double-crystal XRD, OM, AFM, SEM and EDX. It was shown that the lattice integrities of two growth-direction crystals were both well due to the small FWHM values. While the average densities of dislocation in c-sapphire and a-sapphire were 9.2×103 and 3.9×103 cm-2 respectively, the energy of dislocation in c-sapphire was lower than that in a-sapphire. During Strong Phosphoric Acid (SPA) etching, the surface of c-sapphire basically kept smooth but in a-sapphire there were many point-like corrosion pits where aluminum and oxygen atoms lost by 2:1. Our work means that it will be promising for growing c-[0001] seed sapphire by EFG if aided by parameter optimization.

Chemical Mechanical Polishing and Direct Bonding of YAG

Current limitations in both single crystal and polycrystalline (ceramic) solid state laser technologies for high power applications stem from thermal effects that cause degradation in both lasing efficiency and beam quality. YAG and Y2O3 have favorable material properties for producing these high power lasers. We demonstrate chemical mechanical polishing (CMP) processes for YAG and Y2O3 resulting in smooth surfaces, (< 1 nm RMS roughness), defect free, and subsequently suitable for direct bonding. The CMP process has been used in conjunction with surface activation to form single crystal YAG-YAG and polycrystalline Y2O3-Y2O3 bonded elements showing a proof of concept for the fabrication of composite laser elements. YAG single crystals were successfully polished with a 70-nm colloidal silica solution containing NaOH (pH 9.9). X-ray photoelectron spectroscopy measurements provided insights to the chemical impact of the NaOH. After NaOH exposure, YAG showed complete removal of Al from tetrahedral sites and a change in the Y3/2 and Y5/2 peak area ratios, indicating that the surface of YAG was sufficiently modified to allow the silica particles to abrade the byproduct surface layer. The final surface roughness after polishing was measured at 0.1 nm RMS roughness with no scratches deeper than 0.3 nm. YAG single crystals were also polished with a 70-nm Al2O3 slurry containing NaOCl (pH 11.4), however AFM measurements showed that the surface produced, 0.5 nm RMS roughness with no scratches deeper than 6.0 nm, was not as good as polishing with the colloidal silica slurry which was attributed to the effect of the chemical action of NaOH with the YAG surface. Polishing polycrystalline Y2O3 required a slurry with less aggressive chemical reactions, so the NaOCl/Al2O3 slurry was used to successfully polish the substrates to RMS roughness values of 0.5 nm and scratches no deeper than 1.0 nm. Triple axis diffraction rocking curves and double crystal x-ray diffraction imaging were used to demonstrate that the CMP process also removed subsurface damage that was present in the as-supplied material. The triple axis technique was also successfully employed for the first time to demonstrate that the polycrystalline Y2O3 subsurface damage was also significant reduced after the CMP process. After CMP and surface treatment, YAG-YAG and Y2O3-Y2O3 were bonded together and annealed at 1425 °C with no applied pressure. Cross section and plan view high resolution transmission electron images of the YAG-YAG bonded interface revealed a defect free bonding interface. Light transmission measurements showed that 90% of the interface area was bonded at room temperature contact and was further strengthened with annealing at 1425 °C for 72 hours. These conditions possess a much lower thermal budget than those previously shown to be required to bond YAG without a CMP step. The CMP and surface treatment processes that have been developed are applicable towards the creation of solid state laser composite elements and enable further progress and development for high power laser applications.

Epitaxial Lateral Overgrowth of InP on Nanopatterned GaAs Substrates by Metal–Organic Chemical Vapor Deposition

Epitaxial lateral overgrowth (ELO) of 1.8-μm InP films was performed on nanopatterned GaAs (001) substrates via metal–organic chemical vapor deposition. Parallel SiO2 trenches with a nano-scale width (∼ 170 nm) and various seed line orientations of [110], [410], [010] and [4–10] were first adopted to optimize the growth in our work. Scanning electron microscopy, atomic force microscopy and double-crystal x-ray diffraction were used to characterize the as-grown InP/GaAs heterostructures. The results reveal that the surface morphology and crystalline quality are strongly affected by the seed line orientations, and both of these parameters are optimized under the orientation of the [4–10] direction. A low average full width at half maximum of 285.1 arcsec and a root mean square surface roughness of 2.95 nm are obtained for the 1.8-μm InP film. To clearly observe the initial coalescence stage during the growth processes, thinner (500 nm) ELO InP films were also grown. We found that the seed line orientation has a significant effect on the lateral growth rate and affects the quality of the final InP/GaAs epilayer.

Enhancement of UV photoluminescence in ZnO tubes grown by metal organic chemical vapour deposition (MOCVD)

We have synthesized ZnO tubes, without any catalyst, on Al2O3 (001) substrates by metal organic chemical vapor deposition (MOCVD) at different growth temperatures, pressures and Zn-O ratios. The results confirm that the growth temperature, reactor pressure and Zn-O ratio play important roles in the formation of hexagonal ZnO tubes. Scanning electron microscopy (SEM) images indicated that, at a growth temperature of 500 °C, reactor pressure of 75 Torr and Zn-O ratio of 10/75 are an appropriate condition to obtain hexagonal and well-aligned ZnO tubes. High-resolution, double crystal XRD analysis shows a symmetric ω-scan rocking curve, with a full width at half maximum (FWHM) of 1.1°. It confirms the ZnO tubes have highly c-axis orientation on Al2O3 substrate. Subsequently, room temperature photoluminescence (PL) studies confirm that the enhancement of UV emissions from ZnO tubes is due to increasing Zn-O ratios.

The Effect of Buffer Types on the In0.82Ga0.18As Epitaxial Layer Grown on an InP (100) Substrate.

In0.82Ga0.18As epitaxial layers were grown on InP (100) substrates at 530 °C by a low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique. The effects of different buffer structures, such as a single buffer layer, compositionally graded buffer layers, and superlattice buffer layers, on the crystalline quality and property were investigated. Double-crystal X-ray diffraction (DC-XRD) measurement, Raman scattering spectrum, and Hall measurements were used to evaluate the crystalline quality and electrical property. Scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope (TEM) were used to characterize the surface morphology and microstructure, respectively. Compared with the In0.82Ga0.18As epitaxial layer directly grown on an InP substrate, the quality of the sample is not obviously improved by using a single In0.82Ga0.18As buffer layer. By introducing the graded InxGa1−xAs buffer layers, it was found that the dislocation density in the epitaxial layer significantly decreased and the surface quality improved remarkably. In addition, the number of dislocations in the epitaxial layer greatly decreased under the combined action of multi-potential wells and potential barriers by the introduction of a In0.82Ga0.18As/In0.82Al0.18As superlattice buffer. However, the surface subsequently roughened, which may be explained by surface undulation.

Influences of ultrathin amorphous buffer layers on GaAs/Si grown by metal–organic chemical vapor deposition

In this work, a technique for the growth of GaAs epilayers on Si, combining an ultrathin amorphous Si buffer layer and a three-step growth method, has been developed to achieve high crystalline quality for monolithic integration. The influences of the combined technique for the crystalline quality of GaAs on Si are researched in this article. The crystalline quality of GaAs epilayer on Si with the combined technique is investigated by scanning electron microscopy, double crystal X-ray diffraction (DCXRD), photoluminescence, and transmission electron microscopy measurements. By means of this technique, a 1.8-µm-thick high-quality GaAs/Si epilayer was grown by metal–organic chemical vapor deposition. The full-width at half-maximum of the DCXRD rocking curve in the (400) reflection obtained from the GaAs/Si epilayers is about 163 arcsec. Compared with only using three-step growth method, the current technique reduces etch pit density from 3 × 106 cm−2 to 1.5 × 105 cm−2. The results demonstrate that the combined technique is an effective approach for reducing dislocation density in GaAs epilayers on Si.

Defect analysis of the LED structure deposited on the sapphire substrate

Transmission electron microscope (TEM) and double-crystal X-ray diffraction (DCXRD) measurements have been performed to investigate dislocations of the whole structure of the LED layers deposited on both the conventional (unpatterned sapphire substrate, UPSS) and patterned sapphire substrates (PSS). TEM results show that there exists a dislocation-accumulated region near the substrate/GaN interface, where the dislocation density is much higher with the UPPS than that with the PSS. It indicates that the pattern on the substrate surface is able to block the formation and propagation of dislocations. Further analysis discloses that slope of the pattern is found to suppress the deposition of GaN, and thus to provide more spaces for the epitaxially lateral overgrowth (ELO) of high temperature GaN, which significantly reduces the number of the initial islands, and minimizes dislocation formation due to the island coalescence. V-defect incorporating the threading dislocation is detected in the InGaN/GaN multi-quantum wells (MQWs), and its propagation mechanism is determined as the decrease of the surface energy due to the incorporation of indium. In addition, temperature dependence of dislocation formation is further investigated. The results show that dislocation with the screw component decreases monotonously as temperature goes up. However, edge dislocation firstly drops, and then increases by temperature due to the enhanced thermal mismatch stress. It implies that an optimized range of the growth temperature can be obtained to improve quality of the LED layers.

Highly Sensitive Acetone Sensor Based on Pd/AlGaN/GaN Resistive Device Grown by Plasma-Assisted Molecular Beam Epitaxy

Highly sensitive acetone sensing performance of Pd/AlGaN/GaN resistive devices in the temperature range of 100 °C-250 °C and in the detection range of 100-1000 ppm was reported. A plasma-assisted molecular beam epitaxy was used to grow the AlGaN/GaN heterostructure on Si (111) substrate. Structural characterization of the grown epilayers was performed through double-crystal X-ray diffraction whereas atomic force microscopy was used to obtain the roughness of the sensing surface. Resistive mode configuration of the sample was tested toward acetone in the detection range of 100-1000 ppm and in the temperature range of 100 °C-250 °C. The optimum temperature was found to be 150 °C with response magnitude ~95% for the acetone concentration of 1000 ppm. The sensor response time and recovery time were found to be in the range of ~18-44 s and ~25-109 s, respectively. The cross-sensitivity of the device with other interfering species such as butanone, benzene, toluene, and xylene attributed to good acetone selectivity of the devices. Acetone sensing as well as current transport of the Pd/AlGaN/GaN devices was illustrated with effect including Langmuir adsorption-desorption kinetics and Schottky barrier height between Pd/AlGaN interfaces.

X-ray analysis of multilayer In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As HEMT heterostructures with InAs nanoinsert in quantum well

In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As HEMT heterostructures on InP substrates with elastically strained InAs insert in combined quantum well (QW) have been investigated using a combination of X-ray methods: double-crystal X-ray diffraction, X-ray reflectivity, and reciprocal space mapping. This approach has provided detailed complementary information about the layered and real crystal structures of the samples. The data obtained have made it possible to perform structural analysis of the multilayer systems and compare their characteristics with specified technological parameters, due to which the HEMT growth technology can be corrected and improved.

Impurity Free Vacancy Disordering (IFVD) of InGaAs/AlGaAs Quantum Well Laser Structures

Impurity free vacancy disordering (IFVD) has been used to investigate the atomic interdiffusion of InGaAs/InGaAs quantum well laser structures using double-crystal X-ray diffraction (DCXRD) and photoluminescence measurements. X-ray measurements showed that some part of the carbon was electrically activated after annealing without a dielectric capping layer, but not after annealing with a TiO2 capping layer. For a SiO2 capping layer, the tensile peak was still observed after annealing which is comparable to the samples annealed without capping layer. Photoluminescence results showed that a large energy shift was observed when the samples were coated with SiO2. A negligible photoluminescence shift was observed after annealing when the samples coated with TiO2.

Temperature dependence of the character of AlN nucleation layer grown on SiC substrates by MOCVD

The AlN layers were grown by metal-organic vapor phase epitaxy (MOVPE) on 3-inch (0001) 4HSiC, and the effects of growth temperature were investigated by atomic force microscopy, Raman scattering spectra and high-resolution X-ray diffraction measurements. The results showed that surface roughness was reduced by increasing the temperature for AlN layer. The results of X-ray double crystal diffraction showed that the AlN films with higher temperature exhibited smaller (004) and (105) rocking curve width, and while growth temperature reached 1120 °C, the full-width at half-maximum (FWHM) of the ω rocking curve (RC) was 420 arcsecond.

Hot Wall Epitaxy(HWE)법에 의한 BaIn&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;4&lt;/sub&gt; 단결정 박막 성장과 광전도 특성

A stoichiometric mixture of evaporating materials for <TEX>$BaIn_2S_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, <TEX>$BaIn_2S_4$</TEX> mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were <TEX>$620^{\circ}C$</TEX> and <TEX>$420^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by double crystal X-ray diffraction (DCXD). The carrier density and mobility of <TEX>$BaIn_2S_4$</TEX> single crystal thin films measured from Hall effect by van der Pauw method are <TEX>$6.13{\times}10^{17}cm^{-3}$</TEX> and <TEX>$222cm^2/v{\cdot}s$</TEX> at 293 K, respectively. The temperature dependence of the energy band gap of the <TEX>$BaIn_2S_4$</TEX> obtained from the absorption spectra was well described by the Varshni's relation, <TEX>$E_g(T)=3.0581eV-(3.9511{\times}10^{-3}eV/K)T^2/(T+536K)$</TEX>. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$BaIn_2S_4$</TEX> have been estimated to be 182.7 meV and 42.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$BaIn_2S_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{24}$</TEX>-exciton peaks for n = 24. A stoichiometric mixture of evaporating materials for <TEX>$BaIn_2S_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, <TEX>$BaIn_2S_4$</TEX> mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were <TEX>$620^{\circ}C$</TEX> and <TEX>$420^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by double crystal X-ray diffraction (DCXD). The carrier density and mobility of <TEX>$BaIn_2S_4$</TEX> single crystal thin films measured from Hall effect by van der Pauw method are <TEX>$6.13{\times}10^{17}cm^{-3}$</TEX> and <TEX>$222cm^2/v{\cdot}s$</TEX> at 293 K, respectively. The temperature dependence of the energy band gap of the <TEX>$BaIn_2S_4$</TEX> obtained from the absorption spectra was well described by the Varshni's relation, <TEX>$E_g(T)=3.0581eV-(3.9511{\times}10^{-3}eV/K)T^2/(T+536K)$</TEX>. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$BaIn_2S_4$</TEX> have been estimated to be 182.7 meV and 42.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$BaIn_2S_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{24}$</TEX>-exciton peaks for n = 24.

Influence of relative positions between RF coil and crucible on sapphire crystals by edge-defined film-fed growth (EFG) technique

To obtain the stable temperature field required for growing sapphire crystals, the influence of relative positions between RF coil and crucible on the performances of sapphires produced by edge-defined film-fed growth (EFG) technique was investigated. For comparison, the crucible was located at the top (case A) and the middle (case B) of the RF coil, respectively. Furthermore, the lattice integrities were studied by the double-crystal X-ray diffraction, and the dislocations were observed under the optical microscope and atomic force microscope after corroding in molten KOH at 390 °C. The crystals in case B exhibit better lattice integrity with smaller full width at half maximum of 29.13 rad·s, while the value in case A is 45.17 rad·s. The morphologies of dislocation etch pits in both cases show typical triangular symmetry with smooth surfaces. However, the dislocation density of 2.8×104 cm-2 in case B is only half of that in case A, and the distribution is more uniform, compared to the U-shaper in case A.