X-ray Rocking Curve Measurements Research Articles

Single crystal growth of complex layered oxychalcogenide by melt-solidification method

We have successfully grown single crystals of layered mixed-anion compound Sr2ZnCu2Se2O2 by a simple melt-growth method. Single crystals of the layered oxychalcogenides Sr2ZnCu2Se2O2 were grown by melt-solidification of stoichiometric raw materials. Plate-like single crystals were obtained, and the maximum size of crystals was 2.5 × 1.4 mm2. Thermal analysis result indicates simple melting-solidification-like behavior, and the size of crystals increases significantly above the melting point revealed by this measurement. X-ray rocking curve (XRC) measurement results of the single crystal showed no peak splitting, with a narrow full width half maximum of 30 and 97 arcsec in (105) and (0012) peaks, respectively. The electrical conductivity of single crystals on the a(b) plane was 4.34 × 10−1 S/cm at room temperature. Obtained single crystals were transparent, and the absorption edge was observed around 560 nm in the transmittance measurement. Although layered oxychalcogenide Sr2ZnCu2Se2O2 has a complex composition and structure, this compound exhibits congruent melting behavior in a sealed environment. This feature enables us to grow single crystals of mixed-anion compounds easily by the melt-growth method.

Growth of thick GaN films on 2-inch ScAlMgO4 substrates by halide vapor phase epitaxy

Thick GaN bulks up to 5.2 mm were grown on 2-inch ScAlMgO4 (SAM) substrates by halide vapor phase epitaxy. Free-standing GaN substrates were fabricated by spontaneous separation during the cooling process. The crystalline quality and orientation of GaN for each film thickness were evaluated by X-ray Rocking Curve (XRC) measurements on the (0002) and (10–12) planes. The FWHM of the XRC (0002) and (10–12) diffraction peaks for the 2.9-mm-thick GaN free-standing bulk were 39 and 51 arcsec, respectively, and the dark spot density measured by cathodoluminescence mapping was 1.5 × 106 cm−2. In addition, the radius of curvature of its free-standing GaN bulk was 5.0 m. The impurity concentrations originating from the SAM almost reached detection limits of the SIMS measurements at a GaN film thickness of around 10 µm and more. The crystalline quality of GaN grown on the SAM substrates was comparable to that of GaN grown on sapphire substrates, indicating that the SAM substrate can replace conventional sapphire substrate because of the ease of fabrication of free-standing GaN bulk.

Formation of quaternary [formula omitted] epilayers driven by thermally induced interdiffusion between spinel [formula omitted] epilayer and [formula omitted] substrate

Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate β–(AlxGa1−x)2O3 layer (β - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.

Preparation and Investigation of Micro-Transfer-Printable Single-Crystalline InP Coupons for Heterogeneous Integration of III-V on Si

New requirements for high-frequency applications in wireless communication and sensor technologies need III-V compound semiconductors such as indium phosphide (InP) to complement silicon (Si)-based technologies. This study establishes the basis for a new approach to heterogeneous integration of III-V on Si aimed at the transfer of single-crystalline InP coupons on Si via micro-transfer printing (μTP). The InP coupons will act as high-quality virtual substrates that allow selective homo-epitaxy. We present the chemical-mechanical polishing-based preparation and structural characterization of µm-thin (001) InP platelets, starting from high-quality 4-inch bulk crystals and micro-patterning into transferable coupons of several hundred µm2. The obtained InP platelets exhibit the desired thickness—below 10 ± 1 µm—and low surface roughness—<0.3 nm—on both sides, meeting the precondition for µTP and epitaxy. X-ray rocking curve measurements provide accurate spatial maps of the total strain, which indicate small strain variations in the µm-thin InP sample. Rocking curve mappings of the (0 0 4) reflection reveal half-widths below 16 arcsec in the majority of the sample area after thinning that is similar to commercially available InP bulk substrates. Pole figure measurements show no evidence of stress-induced micro-twinning or stacking faults. Overall, minor indications of crystal quality degradation in the product platelets, compared with the bulk samples, were detected.

Mixed Organic Cations Promote Ambient Light-Induced Formation of Metallic Lead in Lead Halide Perovskite Crystals.

One major concern toward the performance and stability of halide perovskite-based optoelectronic devices is the formation of metallic lead that promotes nonradiative recombination of charge carriers. The origin of metallic lead formation is being disputed whether it occurs during the perovskite synthesis or only after light, electron, or X-ray beam irradiation or thermal annealing. Here, we show that the quantity of metallic lead detected in perovskite crystals depends on the concentration and composition of the precursor solution. Through a controlled crystallization process, we grew black-colored mixed dimethylammonium (DMA)/methylammonium (MA) lead tribromide crystals. The black color is suggested to be due to the presence of small lead clusters. Despite the unexpected black coloring, the crystals show higher crystallinity and less defect density with respect to the standard yellow-colored DMA/MAPbBr3 crystals, as indicated by X-ray rocking curve and dark current measurements, respectively. While the formation of metallic lead could still be induced by external factors, the precursor solution composition and concentration can facilitate the formation of metallic lead during the crystallization process. Our results indicate that additional research is required to fully understand the perovskite precursor solution chemistry.

High crystallinity N-polar InGaN layers grown on cleaved ScAlMgO4 substrates

We have grown high-crystallinity InGaN layers on ScAlMgO4 (SAM) substrates using metalorganic vapor-phase epitaxy. We have prepared atomically flat SAM substrates by cleaving them along the c-plane and have utilized direct InGaN growth without any low-temperature buffer layer. The resulting InGaN layer has a distinct hexagonal hillock morphology and remarkable crystalline quality. The x-ray rocking curve measurements showed that (0002̄) and (10–1–2) peaks full widths at half-maximum are as good as 384 and 481 arcsec, respectively. The calculated threading dislocations densities are as low as 2.9 × 108 and 1.6 × 109 cm−2 in the case of screw-type and edge-type dislocations, respectively.

Correlation of Crystal Defects with Device Performance of AlGaN/GaN High-Electron-Mobility Transistors Fabricated on Silicon and Sapphire Substrates

Herein, the performance of AlGaN/GaN high-electron-mobility transistor (HEMT) devices fabricated on Si and sapphire substrates is investigated. The drain current of the AlGaN/GaN HEMT fabricated on sapphire and Si substrates improved from 155 and 150 mA/mm to 290 and 232 mA/mm, respectively, at VGS = 0 V after SiO2 passivation. This could be owing to the improvement in the two-dimensional electron gas charge and reduction in electron injection into the surface traps. The SiO2 passivation resulted in the augmentation of breakdown voltage from 245 and 415 V to 400 and 425 V for the AlGaN/GaN HEMTs fabricated on Si and sapphire substrates, respectively, implying the effectiveness of SiO2 passivation. The lower transconductance of the AlGaN/GaN HEMT fabricated on the Si substrate can be ascribed to the higher self-heating effect in Si. The X-ray rocking curve measurements demonstrated that the AlGaN/GaN heterostructures grown on sapphire exhibited a full-width half maximum of 368 arcsec against 703 arcsec for the one grown on Si substrate, implying a better crystalline quality of the AlGaN/GaN heterostructure grown on sapphire. The AlGaN/GaN HEMT fabricated on the sapphire substrate exhibited better performance characteristics than that on the Si substrate, owing to the high crystalline quality and improved surface.

Direct Current Reactive Sputtering Deposition and Plasma Annealing of an Epitaxial TiHfN Film on Si (001)

Deposition of a heteroepitaxial TiHfN film with a growth rate of about 1 μm/h was successfully achieved on a Si (001) substrate at a temperature above 700 °C by direct current magnetron reactive sputtering of a Ti0.6Hf0.4 (in atomic fraction) target with an Ar/N2 gas mixture. Annealing of the as-deposited TiHfN/Si sample at a temperature above 1000 °C using microwave plasma with H2/N2 gas was performed to further improve the TiHfN film’s quality. X-ray diffraction results show that the heteroepitaxial TiHfN film on Si exhibits a cube-on-cube relationship as {001}TiHfN//{001}Si and <110>TiHfN//<110>Si. X-ray rocking curve measurements show that the full width at half maximum of (200)TiHfN is 1.36° for the as-deposited TiHfN film, while it is significantly reduced to 0.53° after microwave plasma annealing. The surface morphologies of the as-deposited and annealed TiHfN films are smooth, with a surface roughness of around ~2 nm. Cross-sectional scanning/transmission electron microscopy (S/TEM) shows a reduction in defects in the annealed film, and X-ray photoelectron spectroscopy shows that the film composition remains unchanged. Additionally, S/TEM examinations with atomic resolution illustrate domain matching epitaxy (DME) between TiHfN and Si at the interface. The TiHfN films have good electrical conducting properties with resistivities of 40–45 μΩ·cm.

Heteroepitaxy of (1 0 0)-oriented rutile GeO2 film on c-plane sapphire by pulsed laser deposition

• Wide bandgap GeO 2 film was grown on c-plane sapphire by pulsed laser deposition. • The crystalline GeO 2 film has a pure rutile structure with a good (1 0 0) orientation. • The (1 0 0)-oriented rutile GeO 2 film has a smooth surface and wide bandgap of 5.12 eV. Rutile germanium oxide (r-GeO 2 ) film has been directly grown heteroepitaxially on c-plane sapphire by pulsed laser deposition. The chemical composition, structural, surface morphology, optical properties, and crystal quality of the film have been systematically investigated. Raman spectroscopy and X-ray diffraction results show that the GeO 2 film is the rutile structure with a good (1 0 0) orientation. The X-ray rocking curve measurement shows that the full width at half maximum of r-GeO 2 (2 0 0) peak is 24 arc min. Atomic force microscope and transmittance measurements indicate that the transparent r-GeO 2 film has a surface roughness of 12.2 nm and a wide bandgap of 5.12 eV. We believed that these results will greatly facilitate the development of wide-bandgap r-GeO 2 film for application in electronic devices.

Tuning the p-type doping of GaN over three orders of magnitude via efficient Mg doping during halide vapor phase epitaxy

The precise control of Mg concentration ([Mg]) in p-type GaN layers from 2.3 × 1016 to 2.0 × 1019 cm−3 was demonstrated by halide vapor phase epitaxy (HVPE) on n-type GaN (0001) freestanding substrates. [Mg] in GaN layers could be controlled well by varying the input partial pressure of MgCl2 formed by a chemical reaction between MgO solid and HCl gas under the thermodynamic equilibrium condition. In the sample with [Mg] of 2.0 × 1019 cm−3, a step-bunched surface was observed because the surface migration of Ga adatoms was enhanced by the surfactant effect of Mg atoms. The samples show high structural qualities determined from x-ray rocking curve measurements. The acceptor concentration was in good agreement with [Mg], indicating that almost all Mg atoms act as acceptors. The compensating donor concentrations in the samples were higher than the concentrations of Si, O, and C impurities. We also obtained the Mg acceptor level at a sufficiently low net acceptor concentration of 245 ± 2 meV. These results show that the HVPE method is promising for fabricating GaN vertical power devices, such as n-channel metal–oxide–semiconductor field-effect transistors.

Semipolar (112̅2) AlGaN-Based Solar-Blind Ultraviolet Photodetectors with Fast Response

The high-quality semipolar (112̅2) AlGaN epitaxial films have been obtained on m-plane sapphire by metal-organic chemical vapor deposition. X-ray rocking curve measurements show the full-width at half-maximums of semipolar (112̅2)-oriented AlGaN films are 0.357° and 0.531° along [112̅3̅]AlGaN and [11̅00]AlGaN, respectively. The fabricated semipolar AlGaN metal-semiconductor-metal solar-blind ultraviolet (UV) photodetector (PD) exhibits a high responsivity of 1842 A/W. The fast response and reliability of the UV PD are ensured via fast switching with a rise and decay time of 90 ms and 53(720) ms, respectively. The UV PD exhibits a significant reduction in the dark current, that is, from 100 μA to 780 fA at 10 V, using a simple wet chemical etching to modify the surface properties of materials. The photo-to-dark-current ratio value of the etched UV PD reaches 4 orders of magnitude higher than the unetched UV PD under 270 nm illumination. These are attributed to the fact that KOH wet etching assists in eliminating the surface states and reconstructing the surface oxides. This work might provide a new potential for the development of solar-blind UV PDs with high performance.

Growth and characterization of heteroepitaxial (001) and (111) diamond on Ir/sapphire structures

We report the comparative study on the growth and characterization of heteroepitaxial (001) and (111) diamond on Ir/(112¯0) and (0001) sapphire using direct current plasma chemical vapor deposition, in order to understand differences in heteroepitaxial behavior in both crystal directions as a preliminary step for large-sized high quality heteroepitaxial (111) diamond growth. An Ir deposition and bias-enhanced nucleation treatment were followed by 60, 190, and 1500 s diamond growth, under the same process conditions. The growth behaviors, morphological, and structural properties of the two crystal directions were greatly different. The growth rates of (001) and (111) diamond were 500 and 60 nm/min, respectively. The epitaxial relationships of (001) and (111) diamond were confirmed to be (001) diamond//(001) Ir//(112¯0) sapphire and (111) diamond//(111) Ir//(0001) sapphire. Further, in-plane relationship of (001) diamond was [1¯10] diamond//[1¯10] Ir//[11¯00] sapphire. In the case of (111) diamond growth, 180°-rotated twins and penetration twins in diamond were observed. The atomic force microscopy and X-ray rocking curve measurements exhibited that the surface roughness and the crystalline quality of (111) diamond were inferior to those of (001) diamond. Consequently, the growth conditions of Ir and diamond in [111] direction need to be considerably modified for suppressing twins and surface roughening and obtaining a high growth rate and high crystalline quality.

X-ray diffraction analysis and X-ray topography of high-quality ScAlMgO4 substrates

X-ray diffraction analysis through high-resolution X-ray rocking curve measurement and X-ray topography was performed on Czockralski (CZ) -grown ScAlMgO4 (SAM) substrates. An extremely narrow rocking curve width of 0.002° (7.2 arcsec) was obtained, and the three-dimensional (3D) configuration of dislocations in the interior of the SAM crystal structure was analyzed through 3D section topography. Burgers vectors of dominating dislocations with slip planes of (0001) were determined to be parallel to the a-axis of the SAM crystal, and the dislocations were found to form clusters.

Growth of bulk single crystal ScAlMgO4 boules and GaN films on ScAlMgO4 substrates for GaN-based optical devices, high-power and high-frequency transistors

Single crystal growth of ScAlMgO4 boules with 10 mm up to 4 in. (c.a. 101.6 mm) in diameter by Czochralski technique was demonstrated. Some high quality ScAlMgO4 single crystal boules with 2-inches (c.a. 50.8 mm) in diameter were successfully grown without dislocation, which was revealed by X-ray rocking curve and X-ray topography measurements for the c-plane wafers. After a GaN template was formed on a substrate cut out from a grown ScAlMgO4 boule by using metalorganic vapor phase epitaxy, a thick GaN film was epitaxially grown on the template using hydride vapor phase epitaxy (HVPE). At the cooling stage after the growth in the HVPE process, the thick grown GaN film was separated by itself from the ScAlMgO4 substrate due to high cleavability of ScAlMgO4, which makes it possible the reuse of the substrate. The free-standing GaN grown on the reused ScAlMgO4 substrate showed the same quality as a GaN fabricated on a fresh ScAlMgO4 substrate. The reuse of ScAlMgO4 substrates is beneficial for saving the GaN films fabrication cost. The fabrication of a free-standing GaN substrate with 2 in. in diameter and 1 mm in thickness on ScAlMgO4 substrates was demonstrated to be more high-yield than the fabrication on a sapphire substrate. The growth method shown in this study is very promising, and it opens an efficient way to obtain GaN free-standing wafers and to reduce the production cost of GaN wafers.

Single crystal growth of small-angle-grain-boundary-free Mg2Si via vertical Bridgman method

Mg2Si single crystal was grown via vertical Bridgman method using pyrolytic boron nitride (pBN) crucibles coated with boron nitride (BN). No stress was imposed from the liquid/ solid interface (molten Mg2Si/ BN-coated crucible surface) during the solidification leading to a small angle grain boundary free crystal. As a result of X-ray rocking curve measurement of the cut wafer, in-plane high crystallinity was confirmed and a sharp single peak with FWHM of 44–51 arcsec was measured. The typical electron concentration of grown crystal was 6.6 × 1017 cm−3 and Hall mobility was 280 cm2/Vs. The high electron concentration was attributed to the contamination of crystal by boron from BN coating source, confirmed by glow discharge mass spectroscopic (GDMS) analysis.

Structural and ultraviolet photo-detection properties of laser molecular beam epitaxy grown GaN layers using solid GaN and liquid Ga targets

GaN epitaxial layers have been grown on sapphire (0001) substrate by laser molecular beam epitaxy technique using laser ablation of polycrystalline GaN solid and liquid Ga metal targets in the ambient of nitrogen plasma. In-situ reflection high energy electron diffraction and exsitu atomic force microscopy observations revealed that GaN growth using solid target yields rough surface under three-dimensional growth mode while a flat surface GaN is obtained using liquid Ga target. From X-ray rocking curve measurement, it is also observed that the GaN layer grown using solid GaN target has a relatively better structural quality. X-ray photoelectron spectroscopy confirmed Ga–N bond formation and near-stoichiometric composition of the GaN epilayers. The influence of threading dislocation density on the ultraviolet (UV) photoresponse properties of GaN layers have been studied using metal-semiconductor-metal (MSM) based device structure. It is found that the GaN MSM structure obtained using solid GaN target with lesser screw and dislocation densities exhibits a higher responsivity with fast response and recovery time.

On the thickness scaling of ferroelectricity in Al0.78Sc0.22N films

Thickness scaling on ferroelectric properties of sputter-deposited poling-free Al0.78Sc0.22N films has been examined. The c-axis oriented films were confirmed by X-ray rocking curve measurements with a film as thin as 10 nm. Ferroelectric-type hysteresis and poling-free behaviors are observed from the capacitance measurements, even with a thickness of 20 nm. The remnant polarization (P r) shows a gradual degradation when the thickness is less than 35 nm. The switching (SW) cycle test reveals a wake-up effect for the film, especially for thick films of over 35 nm. A longer SW cycle of over 105 times can be obtained with thinner Al0.78Sc0.22N films around 20 nm at the cost of P r.

Role of DMSO concentration in the crystallization process of MAPbBr3 perovskite single crystal films

Halide perovskite single-crystal films have attracted significant attention due to their extraordinary performance in various optoelectronic applications such as solar cells, light-emitting devices, photodetectors, and X-ray detectors. However, the surface quality and crystallinity of perovskite single crystal films (PSCFs) prepared by the popular space-confined method limits the applications in the high-performance devices. Herein, we demonstrate a simple method to improve the crystallinity of MAPbBr3 PSCFs by introducing dimethylsulfoxide (DMSO) as the coordination solvent into the precursor solutions. When increasing the DMSO concentration in the perovskite precursor solutions, the precipitation of lead bromide on the surface of PSCFs is suppressed. The best MAPbBr3 PSCF obtained at 2.5% DMSO concentration has a full width at half maximum (FWHM) of 0.0042° in the X-ray rocking curve measurements, which is significantly improved compared with the best published results. The mechanism how DMSO concentration affect the crystallization process and the formation of defects is also discussed. The study provides an effective route to fabricate perovskite single crystal films and inspires further explorations on the production of high-performance optoelectronic devices.

Crystal growth, finite element analysis and oxygen annealing of ZnO bulk crystal grown by low-temperature-gradient chemical vapor transport

The growth of high-quality ZnO bulk crystals is an outstanding challenge, and the nature of grown-in defects in crystals is far from well understood. In this work, a specially designed low-temperature-gradient CVT growth system was developed based on finite element analysis. High-quality ZnO single crystals were successfully grown. The supersaturation front in the growth system was controlled, and internal stress in the crystal was reduced by introducing a radiation baffle and a special seed pedestal. The structural, electrical, and optical properties of ZnO crystals were studied before and after oxygen annealing. The scanning electron microscope (SEM) and optical microscope (OM) results indicate the stable growth is achieved under the Zn-rich growth condition. The X-ray rocking curve measurements reveal the high crystalline quality of the grown crystal with the full-width at half-maximum (FWHM) value of 17 arcsec for the (004) reflection. The time-of-flight secondary ion mass spectroscopy (ToF-SIMS) results prove the high purity of grown crystals and reveal the variations of trace impurity concentrations in crystals. When annealed in oxygen at 1000 °C, the optical band gap (Eg) changes from 2.50 eV to 3.15 eV. The Raman analysis indicates that oxygen vacancies (Vo) in the as-grown crystal disappear after annealing. The Hall measurements show that the room temperature (RT) mobilities before and after annealing are 197 cm2V−1c−1 and 207 cm2V−1c−1, respectively. The RT carrier concentration decreases by about two orders of magnitude to 5.45 × 1015 cm−3, while the RT resistivity increases by about one order of magnitude to 5.57 Ω cm after annealing. The positron annihilation lifetimes (PALS) results prove the presence of Zn vacancies (Vzn) after annealing. The origins of as-grown ZnO green luminescence (GL) and the temperature-dependent blue shifts of the GL peak positions are explained based on Zni and Vo defects. The fine structures of annealed ZnO GL are found to be related to Vzn native defects rather than Cu impurity.

Development of polarity inversion in a GaN waveguide structure for modal phase matching

In this work, we report on the fabrication of a GaN/AlGaN waveguiding structure dedicated to modal phase matching, where GaN waveguide has planar polarity inversion. First, we optimized the growth conditions for the AlGaN epilayer. Second, on top of the AlGaN epilayer, we fabricated the waveguiding structure starting with the growth of the Ga-polar GaN epilayer followed by atomic layer deposition (ALD) of an Al $$_{2}$$ O $$_{3}$$ layer, then, continuing with the growth of N-polar GaN epilayer. We tested several layer thicknesses, but with 20 nm we managed to inverse the GaN polarity from Ga to N. To confirm the N-polarity, we etched the GaN epilayer surface in aqueous KOH solution. We performed out-of-plane (0002) and in-plane (11–20) X-ray diffraction and rocking curve measurements to estimate the crystalline quality of the AlGaN epilayer, Ga- and N-polar GaN epilayer. Atomic force microscopy measurement lets us evaluate the epilayer surface morphology and roughness. Optical and scanning electron microscopy inspection revealed characteristic hexagonal N-polar GaN epilayer surface. We used high-resolution transmission electron microscopy to investigate the crystallinity and orientation of the Ga- and N-polar GaN epilayer, also the Al $$_{2}$$ O $$_{3}$$ ALD layer, the interface quality of the waveguide structure.