Rocking Curve Width Research Articles

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.

Rocking curve imaging for diamond radiator crystal selection

The combination of low atomic number, high crystal packing density, and very high Debye temperature makes diamond the best material for use as a bremsstrahlung radiator in the coherent bremsstrahlung (CB) process, a process that is uniquely suited for generating highly polarized high-energy photon beams for photonuclear experiments. The crystal quality of the diamond radiator has a vital effect on the polarization and other properties of the photon beam and the best large-area diamond monocrystals currently available, both natural and synthetic, contain many defects that can degrade their performance as CB radiators. The diamonds used for this study were synthetic type Ib samples produced through the HPHT process by the firm Element Six. They were examined using the double crystal rocking curve imaging method in a synchrotron X-ray beam. Dislocation densities were calculated from the measured rocking curve peak position maps in the way proposed by Ferrari et al. [1]. It is shown that dislocation is one major defect that affects the rocking curve width in local regions. The most significant contribution to the whole-crystal rocking curve width for thin crystals is the systematic variation of the peak position across its surface. This is interpreted in terms of a large-scale bending of the entire crystal. Data supporting this interpretation are presented, and possible explanations for the bending and methods for its mitigation are discussed.

Bias and temperature dependent charge transport in high mobility cobalt-phthalocyanine thin films

The temperature dependent current-voltage (J-V) characteristics of highly-oriented cobalt phthalocyanine films (rocking-curve width=0.11°) deposited on (001) LaAlO3 substrates are investigated. In the temperature range 300–100 K, charge transport is governed by bulk-limited processes with a bias dependent crossover from Ohmic (J∼V) to trap-free space-charge-limited conduction (J∼V2). The mobility (μ) at 300 K has a value of ∼7 cm2 V−1 s−1 and obeys Arrhenius-type (ln μ∼1/T) behavior. However, at temperatures <100 K, the charge transport is electrode-limited, which undergoes a bias dependent transition from Schottky (ln J∼V1/2) to multistep-tunneling (conductivity varying exponentially on the inverse of the square-root of electric field).

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.

Influence of crystal quality of underlying GaN buffer on the formation and optical properties of InGaN/GaN quantum dots

A study of InGaN quantum dots (QDs) grown on two different GaN templates—GaN growth using a conventional two-step approach and growth using our recently developed high temperature (HT) AlN as a buffer—is reported. The HT AlN buffer leads to a significant reduction in the dislocation density, particularly screw dislocations, in subsequently deposited GaN. This reduction is confirmed by a significant decrease in the (0002) x-ray diffraction rocking curve width. The GaN on the HT AlN buffer leads to a high density (1010/cm2) of InGaN QDs, whereas in contrast InGaN QDs on the conventional GaN layer grown using the two-step approach have a much smaller density (∼108/cm2). Furthermore, the carrier lifetimes for the QDs on the GaN/HT AlN have been found to be up to nine times longer than those for the QDs on the conventional GaN.

Homoepitaxial growth and characterization of ZnO(0001) thin films grown by metalorganic chemical vapor epitaxy

AbstractWe study the properties of Zn‐face ZnO(00.1) layers grown on the Zn‐face of high quality ZnO(00.1) single crystal wafers by metalorganic chemical vapour epitaxy. The layers are grown directly on the ZnO substrates without any treatment of the substrate before starting the growth. The growth temperature is varied between 600–1050 °C while the reactor pressure is kept at 50 Torr. Atomic force microscopy reveals monolayer steps with a step length of 170 nm. The root‐mean‐square roughness is 0.1 nm over a 20×20 μm2 area. A two‐dimensional step morphology is only seen for growth temperatures above 950 °C. Layers grown below 950 °C exhibit very rough three dimensional surface morphologies. The full width at half maximum for x‐ray rocking curves is 22 arcsec and 36 arcsec across the (00.2) and the (20.1) reflections, respectively. This is comparable to the rocking curve widths of the as‐received substrates. The peaks of the ZnO epitaxial thin film and the substrate are indistinguishable in reciprocal space maps. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Homoepitaxy of ZnO on bulk and thin film substrates by low temperature metal organic chemical vapor deposition using tert-butanol

Homoepitaxial growths of ZnO were performed by low pressure metal organic chemical vapor deposition (MOCVD) on both bulk hydrothermal and thin film buffer layers of ZnO grown by pulsed laser deposition on c-sapphire substrates. The surface roughness, photoluminescence, and x-ray diffraction (0002) peak omega rocking-curve line widths for the bulk and thin film substrates were similar prior to MOCVD growth. After MOCVD growth, it was found that the surface morphology, crystallographic quality, and optical properties of the ZnO layers were far superior to those typically observed for heteroepitaxial growth of ZnO layers on c-sapphire by MOCVD. The MOCVD layers were comparable for growths on both the bulk substrate and the thin film buffer layer. Furthermore, no significant impact of carrier gas was observed. The quality of the ZnO layer appeared to be correlated with the surface morphology and the crystalline quality of the substrate rather than being dependent on whether the substrate was in bulk or thin film form.

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 <1 nm), phase-pure material with lower x-ray rocking curve widths (Δω<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.

Improved crystalline quality nonpolar a-GaN films grown by hydride vapor phase epitaxy

Nonpolar a-GaN films were grown by hydride vapor phase epitaxy on r-sapphire. As the thickness of the grown films was increased from 50 to over 250μm, the width of the double-crystal rocking curve for the (11–20) reflection decreased from 2000to500arcsec, indicating a strong decrease in the dislocation density. Microcathodoluminescence mapping of the thick films suggests that the dislocation density is ∼102cm−2, which is more than two orders of magnitude lower than for thin films. The authors also observe a corresponding decrease in the density of residual donors from 1020to1018cm−3, with respective mobility increase from (10–20) to 150cm2∕Vs. The luminescence spectra of the thin films show the presence of intense defect bands attributed in literature to stacking faults. These bands are very strongly suppressed in thick films, which also show about an order of magnitude higher band edge luminescence intensity.

Molecular beam epitaxy and structural anisotropy of m-plane InN grown on free-standing GaN

This study reports on the growth of high-quality nonpolar m-plane [11¯00] InN films on free-standing m-plane GaN substrates by plasma-assisted molecular beam epitaxy. Optimized growth conditions (In/N ratio ∼1 and T=390–430 °C) yielded very smooth InN films with undulated features elongated along the [112¯0] orientation. This directionality is associated with the underlying defect structure shown by the anisotropy of x-ray rocking curve widths parallel to the [112¯0] (i.e., 0.24°–0.34°) and [0001] (i.e., 1.2°–2.7°) orientations. Williamson–Hall analysis and transmission electron microscopy identified the mosaic tilt and lateral coherence length and their associations with different densities of dislocations and basal-plane stacking faults. Ultimately, very low band gap energies of ∼0.67 eV were measured by optical absorption similar to the best c-plane InN.

DuMond analysis of bending in single crystals by Laue diffraction using σ–π polarization geometry

A DuMond analysis of X-ray diffraction patterns has been carried out in the case of a combined σ–π polarization configuration, obtained using a setup with a double-crystal monochromator in reflection (Bragg) geometry and an analyser in transmission (Laue) geometry. The derived analytical expressions allow the characterization of the bending of the analyser and the quantitative estimation of the curvature radius and its sign from the width of the crystal rocking curves. The theoretical analysis is applied to the case of a thin, accidentally bent, Si crystal.

Comparison of MBE Growth of InSb on Si (001) and GaAs (001)

We describe the epitaxial growth of InSb films on both Si (001) and GaAs (100) substrates using molecular-beam epitaxy and discuss the structural and electrical properties of the resulting films. The complete 2 μm InSb films on GaAs (001) were grown at temperatures between 340°C and 420°C and with an Sb/In flux ratio of approximately 5 and a growth rate of 0.2 nm/s. The films were characterized in terms of background electron concentration, mobility, and x-ray rocking curve width. Our best results were for a growth temperature of 350°C, resulting in room-temperature mobility of 41,000 cm2/V s. For the growth of InSb on Si, vicinal Si(001) substrates offcut by 4° toward (110) were used. We investigated growth temperatures between 340°C and 430°C for growth on Si(001). In contrast to growth on GaAs, the best results were achieved at the high end of the range of T S = C, resulting in a mobility of 26,100 cm2/V s for a 2 μm film. We also studied the growth and properties of InSb:Mn films on GaAs with Mn content below 1%. Our results showed the presence of ferromagnetic ordering in the samples, opening a new direction in the diluted magnetic semiconductors.

High optical quality ZnO epilayers grown on sapphire substrates by reactive magnetron sputtering of zinc target

Zinc oxide (ZnO) epilayers were grown on (0 0 0 1) sapphire substrates by reactive sputtering of a zinc target at substrate temperatures of 300 and 600 °C. High-resolution X-ray diffraction, UV–visible and photoluminescence (PL) measurements were carried out to obtain information about epitaxy, micro-structure and optically active defects in these epilayers. Though the epilayer deposited at 600 °C showed a slightly smaller crystallite size along the growth direction as compared to that deposited at 300 °C, it was much superior in terms of other micro-structural parameters. It exhibited significantly small values of micro-strain (2×10 −4), rocking curve width (∼0.13°), mosaic twist (0.35°), and screw (6.6×10 8 cm −2)- and edge (2.9×10 11 cm −2)-type dislocation densities. Absorption and PL studies showed the high optical quality of the ZnO epilayer deposited at 600 °C, which exhibited a narrow (full-width at half-maximum—FWHM∼96 meV) and intense band edge luminescence at room temperature. The micro-structural parameters and the sharp PL peak show that the reactively sputtered ZnO epilayer grown at 600 °C is comparable in epitaxial and optical quality with ZnO grown by other epitaxial processes.

Basal Plane Stacking-Fault Related Anisotropy in X-ray Rocking Curve Widths of m-Plane GaN

A model of basal plane stacking faults as boundaries between incoherently scattering domains in m-plane GaN films is reviewed. m-Plane GaN films are analyzed with a modified version of the Williamson–Hall analysis in order to determine the length-scale of coherent scattering and tilt-mosaic contribution to X-ray rocking curve widths for the primary in-plane directions. This analysis shows that basal plane stacking faults are the predominant source of rocking-curve width anisotropy in the m-plane films, and indicate that the modified Williamson–Hall analysis can be used as a non-destructive technique for measuring basal plane stacking fault densities in m-GaN films.

X-ray characterization of dislocation density asymmetries in heteroepitaxial semiconductors

We demonstrate an x-ray rocking curve method which allows detection of an asymmetry in the dislocation densities in an heteroepitaxial (001) zinc blende semiconductor layer. These dislocations exist on two types of slip systems with their misfit dislocation line segments oriented along either a [1−10] direction (type A) or a [110] direction (type B). An imbalance in the densities of dislocations on these slip systems produces an observable azimuthal variation in the rocking curve width for symmetric x-ray reflections. An approximate quantitative model allows the estimation of the dislocation densities on the two types of slip systems.

Enhanced x-ray resolving power achieved behind the focal circles of Cauchois spectrometers

Maintaining high resolving power is a primary challenge in hard x-ray spectroscopy of newly developed bright and transient x-ray sources such as laser-produced plasmas. To address this challenge, the line widths in x-ray spectra with energies in the 17 keV to 70 keV range were recorded by positioning the detectors on and behind the focal circles of Cauchois type transmission-crystal spectrometers. To analyze and understand the observed line widths, we developed a geometrical model that accounts for source broadening and various instrumental broadening mechanisms. The x-ray sources were laboratory Mo or W electron-bombarded anodes, and the spectra were recorded on photostimulable phosphor image plates. For these relatively small x-ray sources, it was found that when the detector was placed on or near the focal circle, the line widths were dominated by the effective spatial resolution of the detector. When the detector was positioned beyond the focal circle, the line widths were determined primarily by source-size broadening. Moreover, the separation between the spectral lines increased with distance behind the focal circle faster than the line widths, resulting in increased resolving power with distance. Contributions to line broadenings caused by the crystal thickness, crystal rocking curve width, geometrical aberrations, and natural widths of the x-ray transitions were in all cases smaller than detector and source broadening, but were significant for some spectrometer geometries. The various contributions to the line widths, calculated using simple analytical expressions, were in good agreement with the measured line widths for a variety of spectrometer and source conditions. These modeling and experimental results enable the design of hard x-ray spectrometers that are optimized for high resolving power and for the measurement of the x-ray source size from the line widths recorded behind the focal circle.

Optimization of a Bonse–Hart instrument by suppressing surface parasitic scattering

An improved crystal design for a Bonse-Hart ultra-small-angle scattering instrument is presented. Defects at the diffracting surfaces of the conditioning crystals were found to be at the origin of diffuse scattering that enhances the intensity in the wings of the rocking curves by several orders of magnitude. In order to improve the performance of the instrument, the monolithic channel-cut crystals were replaced by pairs of separate polished and deeply etched crystals. These crystals were mounted on a mechanical stage that allows very precise parallel alignment of the crystals to within a tiny fraction of the rocking curve width (sub-microrad range). By using these double-crystal set-ups, the parasitic background scattering was reduced by more than an order of magnitude. The steps to achieve the optimum surface quality of the crystals as well as the precision mechanical design for their parallel alignment are described. Significant improvement of the signal-to-background ratio and the available wavevector range of the instrument make it suitable for studying the microstructure and dynamics of dilute and weakly scattering soft-matter systems. This development also has potential applications in X-ray optics such as low-background and tunable monochromators and collimators.

Characterizing the thickness dependence of epitaxial GaN grown over GaN nanocolumns using X-ray diffraction

The crystal quality, as determined by X-ray diffraction, of GaN epitaxial films synthesized by MOCVD overgrowth of oriented GaN nanocolumns on Si(1 1 1) is studied as a function of film thickness and compared with control films grown on sapphire substrate. Samples were characterized using a three-beam X-ray diffraction technique from which depth-dependent rocking-curve widths (for the (0 1 1¯ 3)/(0 1¯ 12¯) crystal plane) are obtained as functions of depth. Through the examination of four samples prepared under conditions spanning key growth parameters it is found that optimizing for overgrowth at higher substrate temperatures can lead to coalesced films with significantly narrower rocking-curve widths. The results indicate that initially disconnected crystal domains stemming from individual columns do merge under the right conditions and that the coalesced crystal improves with increasing thickness. Overgrowth with the substrate temperature at 1000 °C (the highest of the study) resulted in a GaN overgrowth with properties superior to those of the control sample, a standard high-quality 2 μm MOCVD GaN thin-film grown on sapphire substrate.