Double-crystal X-ray Rocking Curve Research Articles

Characterization of a-plane GaN layers grown on patterned r-sapphire substrate by metal organic chemical vapor deposition

Non-polar a-plane GaN layers were grown on planar and hemispherical micro- and nano-patterned r-sapphire substrates (PSS) by metal organic chemical vapor deposition (MOCVD). Double Crystal X-ray Rocking Curve (DCXRC) and Transmission Electron Microscopy (TEM) analyses showed that the crystal quality of a-GaN grown on the micro-PSS was the best among three kinds of r-sapphire substrates, which was attributed to defect reduction through lateral overgrowth on the hemispherical patterns. The intensity of the near band-edge emission (NBE) at 3.4eV from a-GaN on the micro-PSS was increased by about 30 times compared with that of the conventional a-plane GaN on planar r-sapphire, while the yellow emission spectrum at 2.2eV was the lowest among the three samples. Comparison of the micro-photoluminescence mapping image and the cross-section TEM also showed high and low defect regions on the planar section and hemisphere, respectively.

Hot wall epitaxy법에 의한 MgGa2Se4단결정 박막 성장과 광학적 특성

단결정 성장을 위한 <TEX>$MgGa_2Se_4$</TEX> 다결정은 수평 전기로에서 합성하였으며, 결정구조는 rhombohedral이고 격자상수 <TEX>$a_0$</TEX>는 3.953 <TEX>${\AA}$</TEX>, <TEX>$c_0$</TEX>는 38.890 <TEX>${\AA}$</TEX>였다. <TEX>$MgGa_2Se_4$</TEX> 단결정박막은 HWE(Hot Wall Epitaxy) 방법으로 반절연성 GaAs(100)기판에 성장시켰다. 단결정박막의 성장 조건은 증발원의 온도 <TEX>$610^{\circ}C$</TEX>, 기판의 온도 <TEX>$400^{\circ}C$</TEX>에서 진행되었으며 성장 속도는 0.5 <TEX>${\mu}m/h$</TEX>였다. 단결정박막의 결정성은 이중 결정 x-선 회절곡선의 반폭치와 X-선 회절무늬의 <TEX>${\omega}-2{\theta}$</TEX>로부터 구하여 최적 성장 조건을 알 수 있었다. Hall 효과는 van der Pauw 방법에 의해 측정되었으며, 온도에 의존하는 운반자 농도와 이동도는 293 K에서 각각 <TEX>$6.21{\times}10^{18}/cm^3$</TEX>, 248 <TEX>$cm^2/v{\cdot}s$</TEX>였다. <TEX>$MgGa_2Se_4$</TEX>/SI(Semi-Insulated) GaAs(100) 단결정 박막의 광흡수 스펙트럼을 10 K에서 293 K까지 측정하였다. 광흡수 스펙트럼으로부터 구한 에너지 갭 <TEX>$E_g(T)$</TEX>는 varshni 공식 <TEX>$E_g(T)=E_g(0)=({\alpha}T^2/T+{\beta})$</TEX>을 잘 만족함을 알 수 있었다. 여기서 <TEX>$E_g(0)=2.34\;eV$</TEX>, <TEX>${\alpha}=8.81{\times}10^{-4}\;eV/K$</TEX>, <TEX>${\beta}=251\;K$</TEX>였다. A stoichiometric mixture of evaporating materials for <TEX>$MgGa_2Se_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. The crystal structure of these compounds has a rhombohedral structure with lattice constants <TEX>$a_0=3.953\;{\AA}$</TEX>, <TEX>$c_0=38.890\;{\AA}$</TEX>. To obtain the single crystal thin films, <TEX>$MgGa_2Se_4$</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>$610^{\circ}C$</TEX> and <TEX>$400^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by the double crystal X-ray rocking curve and X-ray diffraction <TEX>${\omega}-2{\theta}$</TEX> scans. The carrier density and mobility of <TEX>$MgGa_2Se_4$</TEX> single crystal thin films measured from Hall effect by van der Pauw method were <TEX>$6.21{\times}10^{18}\;cm^{-3}$</TEX> and 248 <TEX>$cm^2/v{\cdot}s$</TEX> at 293 K, respectively. The optical absorption of <TEX>$MgGa_2Se_4$</TEX> single crystal thin films was investigated in the temperature range from 10 K to 293 K. The temperature dependence of the optical energy gap of the <TEX>$MgGa_2Se_4$</TEX> obtained from the absorption spectra was well described by the Varshni's equation, <TEX>$E_g(T)=E_g(0)-({\alpha}T^2/T+{\beta})$</TEX>. The constants of Varshni's equation had the values of <TEX>$E_g(0)=2.34\;eV$</TEX>, <TEX>${\alpha}=8.81{\times}10^{-4}\;eV/K$</TEX> and <TEX>${\beta}=251\;K$</TEX>, respectively.

Effects of a ZnTe buffer layer on structural quality and morphology of CdTe epilayer grown on (001)GaAs by molecular beam epitaxy

The structural properties, crystalline quality and surface morphology of CdTe thin films without and with a ZnTe buffer layer grown on (001)GaAs by molecular beam epitaxy (MBE) have been studied. CdTe thin film directly prepared on GaAs substrate displays (111) orientation with an island growth mode, whereas the CdTe epilayers with a ZnTe buffer are (001)-oriented single-crystalline film with a two-dimensional (2D) growth mode. The morphology and surface root-mean-square (RMS) roughness of CdTe epilayers are also dramatically improved by using a ZnTe buffer. Furthermore, it is suggested that the high-temperature (HT) ZnTe buffer grown at 360 °C is more efficient for enhancing CdTe structural quality than the low-temperature (LT) one at 320 °C. The CdTe epilayer on the HT-ZnTe buffer shows a narrower full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (004) reflection and a smaller RMS roughness.

Growth and annealing of zinc-blende CdSe thin films on GaAs (0 0 1) by molecular beam epitaxy

CdSe thin films have been grown on GaAs (0 0 1) substrates by molecular beam epitaxy (MBE). The effects of substrate temperature and annealing treatment on the structural properties of CdSe layers were investigated. The growth rate slightly decreases due to the accelerated desorption of Cd from CdSe surface with an increase in the temperature. The sample grown at 260 °C shows a polycrystalline structure with rough surface. As the temperature increases over 300 °C, crystalline CdSe (0 0 1) epilayers with zinc-blende structure are achieved and the structural quality is improved remarkably. The epilayer grown at 340 °C displays the narrowest full-width at half-maximum (FWHM) from (0 0 4) reflection in double-crystal X-ray rocking curve (DCXRC) and the smallest root-mean-square (RMS) roughness of 0.816 nm. Additionally, samples fabricated at 320 °C were annealed in air for 30 min to study the films’ thermal stability. X-ray diffraction (XRD) results indicate that the zinc-blende structure remains unchanged when the annealing temperature is elevated to 460 °C, meaning a good thermal stability of the cubic CdSe epilayers.

Structural characteristics of ZnO films grown on (0001) or (11−20) sapphire substrates by atomic layer deposition

The structural characteristics of zinc oxide (ZnO) films deposited on the (0001)- or (11−20)-oriented sapphire substrates were investigated. ZnO films having low temperature ZnO buffer layers were prepared by atomic layer deposition using diethylzinc and nitrous oxide. The ZnO films were analyzed using cross-sectional transmission electron microscopy, high-resolution transmission electron microscopy and x-ray diffractometry. Cross-sectional high-resolution transmission electron microscopic observations showed the presence of extrinsic stacking faults along with Frankel partial dislocations in ZnO near the ZnO/sapphire heterointerface. Based on the results of cross-sectional transmission electron microscopic observations and double-crystal x-ray rocking curve measurements, it was found that buffer-layer annealing treatment tended to reduce the density of threading dislocations in a ZnO film efficiently so that the crystalline quality of the film was greatly improved.

Optimization of VI/II pressure ratio in ZnTe growth on GaAs(0 0 1) by molecular beam epitaxy

ZnTe epilayers were grown on GaAs(0 0 1) substrates by molecular beam epitaxy (MBE) at different VI/II beam equivalent pressure (BEP) ratios ( R VI/II) in a wide range of 0.96–11 with constant Zn flux. Based on in situ reflection high-energy electron diffraction (RHEED) observation, two-dimensional (2D) growth mode can be formed by increasing the R VI/II to 2.8. The Te/Zn pressure ratios lower than 4.0 correspond to Zn-rich growth state, while the ratios over 6.4 correspond to Te-rich one. The Zn sticking coefficient at various VI/II ratios are derived by the growth rate measurement. The ZnTe epilayer grown at a R VI/II of 6.4 displays the narrowest full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (0 0 4) reflection. Atomic force microscopy (AFM) characterization shows that the grain size enlarges drastically with the R VI/II. The surface root-mean-square (RMS) roughness decreases firstly, attains a minimum of 1.14 nm at a R VI/II of 4.0 and then increases at higher ratios. It is suggested that the most suitable R VI/II be controlled between 4.0 and 6.4 in order to grow high-quality ZnTe epitaxial thin films.

Substrate temperature dependence of ZnTe epilayers grown on GaAs(0 0 1) by molecular beam epitaxy

ZnTe thin films have been grown on GaAs(0 0 1) substrates at different temperatures with constant Zn and Te beam equivalent pressures (BEPs) by molecular beam epitaxy (MBE). In situ reflection high-energy electron diffraction (RHEED) observation indicates that two-dimensional (2D) growth mode can be established after around one-minute three-dimensional (3D) nucleation by increasing the substrate temperature to 340 °C. We found that Zn desorption from the ZnTe surface is much greater than that of Te at higher temperatures, and estimated the Zn sticking coefficient by the evolution of growth rate. The Zn sticking coefficient decreases from 0.93 to 0.58 as the temperature is elevated from 320 to 400 °C. The ZnTe epilayer grown at 360 °C displays the narrowest full-width at half-maximum (FWHM) of 660 arcsec from (0 0 4) reflection in double-crystal X-ray rocking curve (DCXRC) measurements. The surface morphology of ZnTe epilayers is strongly dependent on the substrate temperature, and the root-mean-square (RMS) roughness diminishes drastically with the increase in temperature.

Floating-zone growth and property characterizations of high-quality La2–xSrxCuO4 superconductor crystals

We have grown underdoped (x = 0.11, 0.12) and optimally doped (x = 0.16) La2–xSrxCuO4 single crystals by the traveling-solvent floating-zone technique. In order to prepare good quality cuprate crystals, we have made much effort to optimize the preparation procedures. For example, we haveadopted the sol–gel route to prepare a highly fine and homogeneous La2–xSrxCuO4 precursor powder for fabricating a very dense ceramic feed rod used for the floating-zone growth, and we have also used quite a slow growth rate. The high quality of the grown crystals has been verified by double-crystal x-ray rocking curves, with the full-width-at-half-maximum being only 113–150 arcseconds, which are the best data reported so far for La2–xSrxCuO4 crystals. The superconducting critical temperatures of the grown crystals are 30, 31 and 38.5 K for x = 0.11, 0.12 and 0.16 samples, respectively, according to magnetic measurements.

Nontrivial role of the resolution function in the formation of double-crystal X-ray rocking curves

A theoretical analysis of the shape of the resolution function of an X-ray diffractometer in the double-crystal nondispersive scheme for measuring rocking curves has been performed. It is shown that this function, along with the strong narrow central line (corresponding to the characteristic line to which the collimator crystal and slits in the experimental scheme are tuned) always contains an additional weak satellite, corresponding to the neighboring characteristic line. This feature manifests itself in experimental curves as an additional pseudopeak, whose intensity is comparable with the corresponding diffraction scattering intensity from a sample in the fundamental characteristic line, and whose shape and exact position are set by the resolution function in a specific experimental scheme. It is shown that the exact allowance for the angular and spectral distribution of incident radiation in the double-crystal scheme makes it possible to precisely determine the parameters of the structures studied.

Magnetic properties of Mn +-implanted and annealed Si 1− xGe x thin films grown on p-Si (100) substrates

Mn +-implanted and annealed Si 1− x Ge x thin films grown on p-Si (100) substrates were formed with the goal of producing (Si 1− x Ge x ) 1− y Mn y with a high ferromagnetic transition temperature ( T c ) . The double-crystal X-ray rocking curves and transmission electron microscopy images showed that the Mn-implanted and annealed Si 1− x Ge x thin films were single crystalline. The magnetization curves as functions of the magnetic field clearly showed that ferromagnetism existed in the Mn + -implanted and annealed Si 1− x Ge x thin films, and the magnetization curves as functions of the temperature showed that the T c value was above 300 K. These results indicate that the formed (Si 1− x Ge x ) 1− y Mn y thin films hold promise for potential applications in Si-based spintronic devices operating at room temperature.

Microstructure Evaluation and Wear Resistance Property of Al-Si-X/Al₂O₃Composite by the DisplacementReaction in Al-Mg Alloy Melt using High Energy Mechanical Milled Al-SiO₂-X Composite Powder

Single-crystal <TEX>$ZnIn_2S_4$</TEX> layers were grown on a thoroughly etched semi-insulating GaAs (100) substrate at <TEX>$450^{\circ}C$</TEX> with a hot wall epitaxy (HWE) system by evaporating a <TEX>$ZnIn_2S_4$</TEX> source at <TEX>$610^{\circ}C$</TEX>. The crystalline structures of the single-crystal thin films were investigated via the photoluminescence (PL) and Double-crystal X-ray rocking curve (DCRC). The temperature dependence of the energy band gap of the <TEX>$ZnIn_2S_4$</TEX> obtained from the absorption spectra was well described by Varshni's relationship, <TEX>$E_g(T)=2.9514\;eV-(7.24{\times}10^{-4}\;eV/K)T2/(T＋489K)$</TEX>. After the as-grown <TEX>$ZnIn_2S_4$</TEX> single-crystal thin films was annealed in Zn-, S-, and In-atmospheres, the origin-of-point defects of the <TEX>$ZnIn_2S_4$</TEX> single-crystal thin films were investigated via the photoluminescence (PL) at 10 K. The native defects of <TEX>$V_{Zn}$</TEX>, <TEX>$V_S$</TEX>, <TEX>$Zn_{int}$</TEX>, and <TEX>$S_{int}$</TEX> obtained from the PL measurements were classified as donor or acceptor types. Additionally, it was concluded that a heat treatment in an S-atmosphere converted <TEX>$ZnIn_2S_4$</TEX> single crystal thin films into optical p-type films. Moreover, it was confirmed that In in <TEX>$ZnIn_2S_4$</TEX>/GaAs did not form a native defects, as In in <TEX>$ZnIn_2S_4$</TEX> single-crystal thin films existed in the form of stable bonds.

Structural and surface characterization of large diameter, crystalline AlN substrates for device fabrication

In order to meet the need for higher quality nitride substrates, 2 in diameter boules of AlN have been developed and, from them, 2 in diameter substrates have been prepared with high crystalline quality. Double-crystal X-ray rocking curves indicate a full-width at half-maximum (FWHM) of 65 and 83 in on the symmetric (0 0 0 2) and the asymmetric (1 0 1¯ 4) lines, respectively. Etch pit density (EPD) measurements of dislocations are consistent with ∼10 3 cm −2 in the substrate. EPD measurements after homoepitaxial growth are typically ∼10 4 cm −2. Growth of Al x Ga 1− x N layers will increase the number of threading dislocations but graded buffer layers have been used to produce GaN layers with EPD of order 10 5 cm −2.

Mosaicity and Wafer Bending in SiC Wafers as Measured by Double and Triple Crystal X-Ray Rocking Curve and Peak Position Maps

Double and triple crystal rocking curve and peak position maps are constructed for a 4HSiC wafer for the symmetric (0 0 0 8) reflection in the normal position, the same reflection for a sample rotated 90º, and an asymmetric (1 23 6) reflection for the wafer in the normal position. These measurements were corrected for the ‘wobble’ in the instrument by scanning a 4” (1 1 1) Si wafer and assuming that the Si wafer was perfect and attributing the variations in the measurements to instrumental error. The x-ray measurements are correlated with a cross polar image, etch pit density map, white beam transmission x-ray topograph, and a laser light scan.

MBE HgCdTe on Si and GaAs substrates

This paper describes some recent results on the growth of HgCdTe on alternative substrates of GaAs and Si, and presents preliminary results on focal plane arrays (FPAs) applications. The quality of HgCdTe grown on Si was found to be identical to that grown on GaAs. The X-ray double-crystal rocking curve (XDRC) full-width at half-maximum (FWHM) values varied in a range of 55–75 arcsec (corresponding to the EPD values of 1–5×10 6 cm −2) for HgCdTe grown on Si or GaAs. The densities of surface defects were reduced to the levels below 300 and 500 cm −2 for HgCdTe grown on GaAs and on Si, respectively. A mean FWHM value of 64.1 arcsec with a standard deviation of 4.3 arcsec was obtained on HgCdTe (10 μm) grown on CdTe (8.6 μm)/Si, and a mean value of 53.9 arcsec with a standard deviation of 2.2 arcsec was obtained on HgCdTe (11.4 μm) grown on CdTe (12.3 μm)/GaAs. The maximum radical deviation of x values across the 3 in HgCdTe epilayers was less than 0.003, corresponding to a deviation of cutoff wavelengths at 80 K of less than 0.09 μm for MW and 0.25 μm for LW samples, respectively. The performance of FPAs fabricated on HgCdTe grown on Si was found to be similar to that on GaAs.

Influence of the trench depths of grooved GaN templates on the characteristics of overgrown AlGaN films

In this study, the properties of Al 0.18Ga 0.82N films grown on stripe-grooved GaN templates with different trench depths have been investigated by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), double crystal X-ray diffractometer (DCXRD), and photoluminescence (PL) measurement. Based upon the results of FESEM observations, a crack-free Al 0.18Ga 0.82N surface has been achieved using a GaN template with 1-μm-deep trenches. Moreover, from the observations of TEM, the density of threading dislocations in Al 0.18Ga 0.82N film was found to reduce during the lateral growth on the trench regions. DCXRD and PL both measurements were carried out to determine the quality of the Al 0.18Ga 0.82N films. The full-width at half-maxima of double crystal X-ray rocking curve and PL spectrum are the smallest, respectively, for the Al 0.18Ga 0.82N film grown on grooved GaN template having 1 μm-deep trenches. It is believed that the use of the grooved GaN templates effectively improves the quality of the overgrown Al 0.18Ga 0.82N films.

Heteroepitaxial growth of GaAs on Si by MOVPE using a-GaAs/a-Si double-buffer layers

In this work, a buffer structure of a-GaAs/a-Si double amorphous layers was used for the epitaxial growth of GaAs on Si substrate by metal-organic vapor phase epitaxy. The atomic force microscopy (AFM) images exhibited that the root-mean-square (RMS) value of the surface morphology was only 1.331 nm. The full-width at half-maximum (FWHM) of the double crystal X-ray rocking curve in the (4 0 0) reflection was about 102 arcsec. The top-surface etch-pit density (EPD) revealed by molten KOH etching was lower than 10 6 cm −2. From the material characterizations described above, the use of a-GaAs/a-Si buffer structure was found to be a very simple and effective approach for obtaining high-quality GaAs on Si for solar cell applications.

Seeded growth of AlN crystals on nonpolar seeds via physical vapor transport

Seeded growth of AlN single crystals was demonstrated in an induction-heated, high-temperature reactor via a physical vapor transport (PVT) process. AlN seeds were prepared from a self-seeded boule containing large single-crystalline grains. Seeded growth was interrupted several times in order to refill the AlN powder source, and a dedicated process scheme was used to ensure epitaxial growth on the seed surface, after prior exposure to air. The growth temperatures were in the range of 2200–2300°C, and the reactor pressure was in the range of 500–900 torr of UHP-grade nitrogen during each growth run. Under these growth conditions, a seed (10 mm diameter) expanded at an angle of 45°, and a larger single crystal up to 18 mm in diameter was obtained. The as-grown surface had three facets, of which facet (1120) was smooth and featureless while the other two, (4150) and (2570), showed serrated morphologies. The double-crystal x-ray rocking curve and glow discharge mass spectroscopy analysis confirmed that the grown crystal was of high crystalline quality with low impurity incorporation.

Effect of growth-interrupted method on quality of AlGaAs/GaAs multiple quantum wells prepared by MBE

AlGaAs/GaAs multi-quantum well (MQW) was prepared by molecular beam epitaxy(MBE) with growth-interrupted heterointerface (GIH) method and continuous growth (CG) method, respectively. The microstructures of the MQWs were characterized by double-crystal X-ray rocking curve (DCRC) and atomic force microscopy (AFM), and the photoluminescence (PL) properties of the MQWs were also studied. The MQWs grown with GIH method show that higher order satellite peaks of Pendellösung fringes are observed in DCRC, the roughness of surface is much reduced in AFM, and the full width at half maximum (FWHM) of exciton line is much narrower in PL. The results indicate that the GIH method reduces the monolayer growth step density at the heterointerface due to the migration of surface atoms for a few minutes growth interruption, and substantially improves the quality of AlGaAs/GaAs MQWs.

Direct growth of high-quality thick CdTe epilayers on Si (211) substrates by metalorganic vapor phase epitaxy for nuclear radiation detection and imaging

Direct growth of high-quality, thick CdTe (211) epilayers, with thickness up to 100 µm, on Si (211) substrates in a vertical metalorganic vapor phase epitaxy system is reported. In order to obtain homo-orientation growth on Si substrates, pretreatment of the substrates was carried out in a separate chamber by annealing them together with pieces of GaAs at 800–900°C in a hydrogen environment. Grown epilayers had very good substrate adhesion. The full-width at half-maximum (FWHM) value of the x-ray double-crystal rocking curve from the CdTe (422) reflection decreased rapidly with increasing layer thickness and remained between 140–200 arcsec for layers >18 µm. Photoluminescence measurement at 4.2 K showed high-intensity, bound excitonic emission and very small defect-related deep emissions, indicating the high crystalline quality of the grown layers. Furthermore, a CdTe/n+-Si heterojunction diode was fabricated that exhibited clear rectifying behavior.

Heteroepitaxy of CdTe on tilting Si(2 1 1) substrates by molecular beam epitaxy

CdTe(2 1 1)B epilayers were grown on 3 in Si(2 1 1) substrates which misoriented 0–10° toward [1 1 1] by molecular beam epitaxy (MBE). The relationship of X-ray double-crystal rocking curve (XRDCRC) FWHM and deflection angle from CdTe(2 1 1) to Si(2 1 1) was studied. For 4.2–4.5 μm CdTe, the best value of FWHM 83 arcsec was achieved while deflection angle is 2.76°. A FWHM wafer mapping indicated a good crystalline uniformity of 7.4 μm CdTe on tilting Si(2 1 1), with FWHM range of 60–72 arcsec. The shear strains of these epilayers were analyzed, using reciprocal lattice points of symmetric and asymmetric reflections measured by high-resolution multi-crystal multi-reflection X-ray diffractometer (HRMCMRXD). It was found that the shear strain angle γ ( 0 1 1 ¯ ) of epilayer is effectively reduced by using proper tilting Si(2 1 1) substrate. It was also proved that the lattice parameter of CdTe(2 1 1)B is affected by the shear strain and thermal strain.