X-ray Rocking Curve Width Research Articles

Cold Seeded Epitaxy and Flexomagnetism in GdAuGe Membranes Exfoliated From Graphene/Ge(111).

Remote and van der Waals epitaxy are promising approaches for synthesizing single crystalline membranes for flexible electronics and discovery of new properties via extreme strain; however, a fundamental challenge is that most materials do not wet the graphene surface. We develop a cold seed approach for synthesizing smooth intermetallic films on graphene that can be exfoliated to form few nanometer thick single crystalline membranes. Our seeded GdAuGe films have narrow X-ray rocking curve widths of 9-24 arc seconds, which is 2 orders of magnitude lower than their counterparts grown by typical high temperature methods, and have atomically sharp interfaces observed by transmission electron microscopy. Upon exfoliation and rippling, strain gradients in GdAuGe membranes induce an antiferromagnetic to ferri/ferromagnetic transition. Our smooth, ultrathin membranes provide a clean platform for discovering new flexomagnetic effects in quantum materials.

Conducting surface layers formed by hydrogenation of O-implanted β-Ga2O3

Lightly n-type β-Ga2O3 grown by Halide Vapor Phase Epitaxy (HVPE) on heavily n-type doped β-Ga2O3 substrate was implanted with 1 MeV O ions to a fluence of 1016 cm−2. The film remained β-polymorph and showed no broadening of the x-ray rocking curve width after irradiation even though the calculated number of primary defects was very high. The implanted region was characterized by a strong compensation, likely due to the presence of a high density of split Ga vacancy acceptors. Treatment of the irradiated film in dense hydrogen plasma at 330 °C for 0.5 h led to the formation of a conducting surface layer about 0.5 µm-thick with carrier density 1017 cm−3, a suppression of the signal due to Fe acceptors in Deep Level transient Spectroscopy (DLTS) and a strong enhancement of DLTS peak caused by centers at Ec-0.74 eV (so called E2 * traps). The mechanism appears to be that hydrogen plasma treatment leads to creation of a high number of donor states due complexing of hydrogen with Ga vacancies and to passivation of Fe acceptors with hydrogen donors.

Heteroepitaxial growth and microwave plasma annealing of DC reactive sputtering deposited TiZrN film on Si (100)

Epitaxial TiZrN films were successfully grown on Si (100) substrates at high temperature by using DC magnetron reactive sputtering with a Ti0.68Zr0.32 alloy target. Annealing of a grown TiZrN/Si specimen was performed by applying microwave plasma with a gas mixture of N2 and H2 to improve the film quality. The X-ray rocking curve (XRC) width of (200)TiZrN reflection can reach a minimum value of 1.18°. The heteroepitaxial relationship of TiZrN with Si is {001}TiZrN//{001}Si and <110>TiZrN//<110>Si. The (200)TiZrN XRC width of a 20 nm-thick TiZrN film reduces from 1.99° to 0.85° by microwave plasma annealing. As a result of the improvement of the film quality, the annealed film exhibits a much lower resistivity (28 μΩ‧cm) than that of the as-deposited one (44 μΩ‧cm). The surface morphologies of the TiZrN films are smooth with the surface roughness in the range of 1–2 nm.

Возможности характеризации кристаллических параметров структур Cd-=SUB=-x-=/SUB=-Hg-=SUB=-1-x-=/SUB=-Te на подложке из GaAs методом генерации на отражение второй гармоники зондирующего излучения

Abstract Comparative results of computational simulation and experiment are provided, concerning the azimuthal angle dependency of the registered signal second harmonic reflected from CdxHg1–xTe structures and GaAs substrates at the normal incidence of probing laser radiation and azimuthal rotation of its polarization plane. The study of (013) GaAs substrate and buffer layers of CdTe|ZnTe|GaAs has revealed that the deviations from the (013) surface orientation with regard to crystal physics angles ϴ and φ are ~1–3° for GaAs substrates and up to 8° for buffer layers of CdTe|ZnTe|GaAs, while the amplitude of the signal second harmonic reflected from buffer layers can well be considered as inversely related to the full width of X-ray rocking curves at half-maximum. Experimental data show that the values of non-linear susceptibility tensor components for CdxHg1–xTe crystalline structure significantly exceed those for CdTe and GaAs.

Reduced dislocation density and residual tension in AlN grown on SiC by metalorganic chemical vapor deposition

Crack-free AlN films with threading dislocation density (TDD) below 109 cm−2 are needed for deep-UV optoelectronics. This is typically achieved using pulsed lateral overgrowth or very thick buffer layers (&amp;gt;10 μm), a costly and time-consuming approach. A method for conventional metalorganic chemical vapor deposition growth of AlN/SiC films below 3 μm with greatly improved quality is presented. Focusing on substrate pretreatment before growth, we reduce average film stress from 0.9 GPa (tension) to −1.1 GPa (compression) and eliminate cracking. Next, with optimized growth conditions during initial deposition, AlN films with x-ray rocking curve widths of 123 arc-sec (0002) and 304 arc-sec (202¯1) are developed, and TDD is confirmed via plan view transmission electron microscopy (TEM) to be 2 × 108 cm−2. Film stress measurements including x-ray 2θ-ω, reciprocal space mapping, and curvature depict compressively stressed growth of AlN on 4H-SiC due to lattice mismatch. The thermal expansion coefficient mismatch between AlN and SiC is measured to be Δα=αAlN−αSiC=1.13×10−6 °C−1 and is found to be constant between room temperature and 1400 °C. TEM confirms the existence of dense misfit dislocation (MD) networks consistent with MD formation near SiC step edges and low MD density regions attributed to nearly coherent AlN growth on SiC terraces. These low-TDD, crack-free AlN/SiC buffers provide a platform for deep-UV optoelectronics and ultrawide bandgap electronics.

Strain Relaxation in GaSb/GaAs(111)A Heteroepitaxy Using Thin InAs Interlayers.

We have systematically studied the strain relaxation processes in GaSb heteroepitaxy on GaAs(111)A using thin InAs interlayers. The growth with 1 ML- and 2 ML-InAs leads to formation of an InAsSb-like layer, which induces tensile strain in GaSb films, whereas the GaSb films grown with thicker InAs layers (≥3 ML) are under compressive strain. As the InAs thickness is increased above 5 ML, the insertion of the InAs layer becomes less effective in the strain relaxation, leaving residual strain in GaSb films. This leads to the elastic deformation of the GaSb lattice, giving rise to the increase in the peak width of X-ray rocking curves.

Impact of impurities and crystal defects on the performance of CVD diamond detectors

Radiation detectors based on diamond are highly favored for particle physics research due to the superior radiation hardness. In this work, we investigate the influence of impurities and crystalline imperfections on the charge collection efficiency (CCE) of single crystal diamond. Seventeen (17) ultra-low fluorescent diamond samples grown by microwave plasma chemical vapor deposition method from IIa Technologies PTE LTD are pre-selected for this study. The measured CCE of all samples using 241Am (α-particles) as ionizing source are analyzed together with the concentration of trace impurities and crystalline imperfection in the crystal. The amounts of impurities are quantified from integrated fluorescence intensity arising from the nitrogen vacancies (NV) created during different CVD growth process conditions. The crystal imperfections are assessed by X-ray rocking curves from X-ray topography images obtained at the Cornell High Energy Synchrotron Source. The CCE decays rapidly as the intensity of NV (INV), phonon sideband approaches that of diamond 2nd order Raman peak which follows the relation: CCEINV=100/1+INV1.052. The energy resolution, ΔE/E (ratio of the energy spectrum width to the most probable peak) highly correlates with broader rocking curve width distribution. Prime novelty statementThis work provides an understanding on the most important factors that contribute to degradation of charge collection efficiency (CCE) in diamond based detectors and sensors. The CCE decays rapidly as the intensity of nitrogen vacancy phonon sideband approaches that of diamond 2nd order Raman peak which follows the relation: CCEINV=100/1+INV1.052. The energy resolution, that is the ratio of the energy spectrum width to the most probable peak, highly correlates with broader X-ray rocking curve width distribution.

Structural anisotropic properties of a-plane GaN epilayers grown on r-plane sapphire by molecular beam epitaxy

Heteroepitaxial non-polar III-Nitride layers may exhibit extensive anisotropy in the surface morphology and the epilayer microstructure along distinct in-plane directions. The structural anisotropy, evidenced by the “M”-shape dependence of the (112¯0) x-ray rocking curve widths on the beam azimuth angle, was studied by combining transmission electron microscopy observations, Raman spectroscopy, high resolution x-ray diffraction, and atomic force microscopy in a-plane GaN epilayers grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). The structural anisotropic behavior was attributed quantitatively to the high dislocation densities, particularly the Frank-Shockley partial dislocations that delimit the I1 intrinsic basal stacking faults, and to the concomitant plastic strain relaxation. On the other hand, isotropic samples exhibited lower dislocation densities and a biaxial residual stress state. For PAMBE growth, the anisotropy was correlated to N-rich (or Ga-poor) conditions on the surface during growth, that result in formation of asymmetric a-plane GaN grains elongated along the c-axis. Such conditions enhance the anisotropy of gallium diffusion on the surface and reduce the GaN nucleation rate.

GaN Power Schottky Diodes with Drift Layers Grown on Four Substrates

We have examined the performance of gallium nitride (GaN) high-power Schottky diodes fabricated on unintentionally doped (UID) metalorganic chemical vapor deposition (MOCVD) films grown simultaneously on four substrates ranging in threading dislocation density from 5 × 103 cm - 2 to 1010 cm - 2. The substrates were an intentionally doped and a UID freestanding hydride vapor phase epitaxy substrate, an MOCVD GaN template grown on a sapphire wafer, and a bulk GaN substrate grown via an ammonothermal method. Capacitance–voltage (C–V) results showed the carrier concentration was ∼2 × 1016 cm−3 for films grown on each of the four substrates. With that doping level, the theoretical breakdown voltage (Vb) is ∼1600 V. However, measured Vb for the devices tested on each of the four substrates fell short of this value. Also, the breakdown voltages across each of the four substrates were not substantially different. This result was especially surprising for films grown on bulk GaN substrates, because of their superior crystal quality, as determined from their x-ray rocking curve widths. Simple probability calculations showed that most of the diodes tested on the bulk substrate did not cover a single threading dislocation. Although optimization of edge-termination schemes is likely to improve Vb, we believe that point defects, not threading dislocations, are the main reason for the reduced performance of these devices.

Effect of Epilayer Tilt on Dynamical X-ray Diffraction from Uniform Heterostructures with Asymmetric Dislocation Densities

In this work we extend the dynamical theory of Bragg x-ray diffraction to account for a tilted, asymmetrically defected, uniform-composition epitaxial layer atop a (001) substrate. In a zincblende semiconductor there are eight active slip systems, within which two distinct types of dislocations exist. These two types are distinguished by their misfit segments, which are oriented along either the [110] or \([1\bar{1}0]\) direction. The two threading dislocation densities can be measured by observing the variation of the x-ray rocking curve width with the incident beam azimuth. However, the tilting of the epilayer also has a measurable and potentially conflicting effect on the rocking curve as a function of azimuth. First, the peak position varies by (nominally) twice the layer’s absolute tilt within a full azimuthal rotation. Second, the tilting of the layer affects the epilayer rocking curve width. Through use of the modified dynamical diffraction theory, we show that the peak width’s azimuthal dependence on tilt is of only second order, so that the layer misorientation with respect to the substrate need not be considered for the purpose of determining the two dislocation populations by x-ray diffraction. Dynamical simulations were performed and compared with experimental measurements for a ZnSe/GaAs(001) structure grown by photoassisted metalorganic vapor-phase epitaxy, and in this way the two dislocation density populations were found to be DA = 1.6 × 108 cm−2 and DB = 2.0 × 108 cm−2.

High quality AlN grown on double layer AlN buffers on SiC substrate for deep ultraviolet photodetectors

High quality AlN epilayers were grown on SiC substrates using double layer AlN buffers growth method by metal organic chemical vapor deposition and exploited as active deep ultraviolet optoelectronic materials through the demonstration of AlN Schottky barrier photodetectors. The grown AlN epilayers have smooth surfaces, low etch-pit density, narrow width of x-ray rocking curves, and strong band edge photoluminescence emission with low impurity emissions. AlN Schottky photodetectors are shown to possess outstanding features including extremely low dark current and high breakdown voltage.

GaN-Based Light-Emitting Diode With Sputtered AlN Nucleation Layer

The crystal quality, electrical, and optical characteristics of GaN-based light-emitting diodes (LEDs) were improved using a sputtered AlN nucleation layer. Replacing the in situ AlN nucleation layer with the sputtered AlN nucleation layer reduced the (002) and (102) X-ray rocking curve widths of the GaN layer from 318.0 to 201.1 and 412.5 to 225.0 arcsec, respectively. The -20-V reverse leakage current of the LEDs with the sputtered AlN nucleation layer is about three orders less than that of the LEDs with the in situ AlN nucleation layer. In addition, the LEDs with sputtered AlN nucleation layer could sustain more than 60% passing yield on the ESD test of under a -600-V machine mode, whereas the LEDs with the in situ AlN nucleation layer sustained less than 40% passing yield. Moreover, the 20-mA output power of the LEDs with the sputtered AlN nucleation layer also improved by approximately 5.73% compared with that of the LEDs with the in situ AlN nucleation layer.

Characterization of a sandwich-type large CVD single crystal diamond particle detector fabricated using a lift-off method

Crystallinity of a freestanding large (size: ~9×7mm) undoped single crystal (SC) diamond film grown by plasma chemical vapor deposition (CVD) and radiation response of a sandwich-type radiation detector made from this SC plate to α particles were investigated. The freestanding SC film was formed by elimination of a substrate using a so-called “lift-off method” after the growth of a thick-film on a pre-ion-implanted substrate. The formed SC plate has a high optical transparency, indicating no remarkable optical absorption seen in the wavelength from ultraviolet to near-infrared region. In the energy spectra of 5.486MeV α-particles, relatively sharp peaks were observed for both hole and electron transits. For the hole transit, the peak resolution of the pulse height distribution and charge collection efficiency (CCE) were ~0.7% and 98%, respectively. On the other hand, for the electron transit, the peak resolution and CCE obtained were ~4.4% and 89%, respectively. In the photoluminescence spectrum at room temperature, a strong sharp peak around 235nm corresponding to free exciton and a weak visible band mainly related to nitrogen-vacancy complex were observed. The spectral width of the (004) plane X-ray rocking curve was 16.5″. The micrograph taken using crossed-polarizers demonstrated that there are some white regions and four-petaled patterns, originating from internal strain and most likely threading dislocations, respectively.

Reduction of structural defects in a-plane GaN epitaxy by use of periodic hemispherical patterns in r-plane sapphire substrates

Using hemisphere-shaped patterned r-plane sapphire substrates, high quality nonpolar (112¯0) a-plane gallium nitride (GaN) films have been obtained with superior structural characteristics to films grown on conventional (unpatterned) r-plane sapphire. This is evidenced by reduced x-ray rocking curve widths, smaller in-plane crystallographic anisotropy, and smoother surface morphology. Observations by transmission electron microscopy and cathodoluminescence reveal that the defect density is remarkably reduced in regions above the patterned hemispheres. The growth of a-plane GaN on the patterned substrates proceeds first by a relatively fast growth of the film on the flat surface region, and then a gradual lateral overgrowth over the protruding hemispheres, where the direct epitaxial growth is severely retarded.

Studies on the effect of ammonia flow rate induced defects in gallium nitride grown by MOCVD

Gallium nitride (GaN) epitaxial layers were grown with different V/III ratios by varying the ammonia (NH 3) flow rate, keeping the flow rate of the other precursor, trimethylgallium (TMG), constant, in an MOCVD system. X-ray rocking curve widths of a (1 0 2) reflection increase with an increase in V/III ratio while the (0 0 2) rocking curve widths decrease. The dislocation density was found to increase with an increase in ammonia flow rate, as determined by hot-wet chemical etching and atomic force microscopy. 77 K photoluminescence studies show near band emission at 3.49 eV and yellow luminescence peaking at 2.2 eV. The yellow luminescence (YL) intensity decreases with an increase in V/III ratio. Positron annihilation spectroscopy studies show that the concentration of Ga-like vacancies increases with an increase in ammonia flow rate. This study confirms that the yellow luminescence in the GaN arises due to deep levels formed by gallium vacancies decorated with oxygen atoms.

Chloride vapor-phase epitaxy of gallium nitride at a reduced source temperature

Gallium nitride (GaN) layers on sapphire substrates have been grown by hydride-chloride vaporphase epitaxy (HVPE) at a Ga-source temperature reduced from 890°C (standard value) to 600°C. All epilayers are transparent and have smooth surfaces. The structural quality of layers was evaluated by measuring the X-ray rocking curve width (FWHM) using a triple-crystal X-ray diffractometer. For the best samples, this quantity amounted to 8 arc min. Analysis of the dependence of the crystal structure quality and photoluminescent properties of GaN layers on the Ga-source temperature showed that a decrease in this parameter to 600°C did not significantly affect the HVPE growth of GaN despite a considerable change in the vapor phase composition (ratio of GaCl and GaCl3 concentrations).

Highly piezoelectric AlN thin films grown on amorphous, insulating substrates

AlN thin films were grown by reactive sputtering on amorphous SiO2 thin films. Film texture, x-ray rocking curve width, mechanical stress, and the clamped piezoelectric constant d33,f were studied as a function of rf bias power and substrate roughness. A high d33,f of 5.0 pm/V was achieved at low substrate roughness and low mechanical AlN film stress. Increasing substrate roughness and stress leads to a deterioration of d33,f, which is correlated with a higher density of opposite polarity grains detected by piezoresponse force microscopy. Extrapolating to 100% uniform polarity, a d33,f of 6.1 pm/V is derived as highest possible value, probably corresponding to the d33,f=e33/c33E of perfect single crystalline material. Growth mechanisms are proposed and underlined by high resolution transmission electron microscopy to explain the observed phenomena.

Growth of InSb epilayers and quantum wells on Ge(001) substrates by molecular beam epitaxy

InSb epilayers and InSb/Al0.20In0.80Sb quantum well structures were grown on Ge(001) substrates by molecular beam epitaxy. Epilayers grown using a two-step process, which involved different temperatures, were characterized in situ using reflection high energy electron diffraction and studied ex situ using high-resolution x-ray diffraction, Nomarski optical microscopy, and Hall-effect measurements. The narrowest x-ray rocking curve width for 2.0- and 5.0-μm-thick InSb epilayers were 250 and 173 arc sec, respectively. Electron mobilities in the 5.0-μm-thick InSb epilayer and the InSb/Al0.20In0.80Sb single quantum well at room temperature were 34 500 and 8600 cm2/V s, respectively, which are the highest values for these films on Ge(001) substrates reported to date.

In adlayer mediated molecular beam epitaxial growth and properties of a-plane InN on freestanding GaN

The role of the In adlayer on the morphological and structural properties of nonpolar a-plane InN films was elucidated during the plasma-assisted molecular beam epitaxy on freestanding GaN. Reflection high energy electron diffraction during In adsorption experiments on a-plane InN surfaces revealed a stable In adlayer coverage of ∼2 ML. This In adlayer-mediated growth was responsible for achieving atomically smooth surfaces (rms roughness of &amp;lt;1 nm), phase-pure material with lower x-ray rocking curve widths (Δω&amp;lt;0.5°), lower crystal mosaic tilt/twist, and decreased stacking fault densities, compared to N-rich conditions. The photoluminescence peak emission and band gap energy of the a-plane InN films were ∼0.63 and ∼0.7 eV, respectively.

Effect of substrate roughness on c-oriented AlN thin films

(001)-textured AlN thin films as needed for bulk acoustic wave devices exhibit large mechanical stress variations as a function of growth substrate properties. We studied the relationship between stress and the surface morphology of a thermally oxidized silicon substrate that was modified by a thin amorphous silicon layer. A rms roughness of 0.1–1.1 nm of the latter resulted in an increase in mechanical stress in the subsequently sputtered AlN thin film going from −700 to +200 MPa. At the same time, the x-ray rocking curve width of AlN increased from 1.3° to 2.3°. The roughness of the Si interlayer was controlled by the Ar sputter pressure. Interestingly, the maximal roughness is obtained at an intermediate pressure. This is explained by an interplay of nucleation and diffusion phenomena governed by the kinetics of impinging atoms and ions. The Si interlayer was essential to avoid cracking of membranes exhibiting mixed Pt and SiO2 surfaces below the AlN film.