X-ray Rocking Curve Analysis Research Articles

Defect formation and diffusion mechanism in ion-assisted molecular-beam epitaxy.

A simple moving boundary diffusion model has been used to characterize defect incorporation kinetics during ion-assisted molecular-beam epitaxy. The model permits analysis of the dependence of the final defect concentration on the growth rate, defect diffusivity, defect production range, and the shape of defect depth distribution. The results indicate a linear dependence of the final defect concentration on the ion-to-atom flux ratio which is in the growth-rate-limited regime of the model. Comparison between the model and the film strains measured by x-ray rocking curve analyses has been made and reveals that the thermal spike energy deposited by the bombarding ions during epitaxial growth has a significant effect on the apparent activation energy of the defect migration. A transition temperature above which the defect migration is thermally activated and below which the defect migration is cascade assisted can be defined. The experimentally observed temperature dependence of the defect concentration can be attributed to cascade-assisted diffusion of the defects. Comparison between the model and the multisite multiply activated migration model for low-energy dopant incorporation has also been made. The results show the similarity between the defect incorporation and dopant incorporation which gives a unified view of both processes.

Precipitation of as in low-temperature MBE-grown gaas

Recent work, on As rich GaAs buffer layers grown at low temperatures by molecular beam epitaxy (MBE), shows that this material has electronic and materials properties that differ from GaAs grown at higher temperatures. Among the interesting properties of these buffer layers are reduced recombination times and high breakdown voltages useful in power field effect transistors (FETs). We have used TEM, in conjunction with double crystal x-ray rocking curves, in studying GaAs grown and annealed at a variety of temperatures. X-ray rocking curve analysis has separated LT (low temperature) GaAs growth into three ranges of growth temperature: the “low-range” below 215°C, the “mid-range” from 215 to 340°C, and the “high-range” above 340°C. The low-range material is amorphous, as suggested by the presence of only the bulk reflections in the x-ray diffractometei profile and from HRTEM images of the MBE-grown buffer layer. The mid-range is single crystal and defect-free, although larger in lattice parameter than stoichiometric GaAs by as much as 0.08%.

Lattice tilt and dislocations in compositionally step-graded buffer layers for mismatched InGaAs/GaAs heterointerfaces

The strain relaxation and dislocation densities of a compositionally step-graded InGaAs/GaAs buffer layer have been investigated. The structural results were correlated with the electrical properties of an In0.3Ga0.7As/In0.29Al0.71As modulation doped heterostructure grown on top. The buffer was a three step InxGa1−xAs structure each layer 300 nm thick and each In composition step x=0.1. X-ray rocking curve analysis measured a tilt about an in-plane <110≳ axis of about 0.3° per step. At the top interface of the buffer the density of misfit dislocations evaluated by electron microscopy was comparable to the number required to account for this tilt. Thus, the 60° burgers vectors for the majority of dislocations along one in-plane line direction was restricted to a maximum of two rather than the usual four possibilities. The electron mobility in the two-dimensional electron gas channel showed a small assymetry (4%) in the two in-plane <110≳ directions and was highest in the direction parallel to the tilt axis. The sheet electron concentration was 1.2×1012/cm2 with peak mobilities of 9300 and 31 000 cm2/V s at room temperature and 77 K, respectively.

Anomalous refractive index change and recovery of electro-optic coefficient r33 in proton-exchanged LiTaO3 optical waveguides after annealing

An increase of surface refractive index and a recovery of the r33 electro-optic coefficient have been observed for proton-exchanged LiTaO3 optical waveguides after annealing. On the basis of the results of nuclear reaction analysis, x-ray rocking curve analysis, and infrared-absorption spectroscopy, a qualitative model to explain these phenomena is proposed. The evolutions of the index profile and the r33 coefficient after proton exchange and annealing are considered to be caused by the movement of protons in the crystal lattice and the interdiffusion of protons and Li ions. The condition required for the fabrication of waveguides with good electro-optic performance is also shown.

High-temperature epitaxy of diamond in a turbulent flame

Diamond has been grown epitaxially on 1.5 mm diameter, natural diamond seed crystals at temperatures of 1200–1300 °C in a premixed, turbulent oxyacetylene flame. During a typical 1 h deposition, a polyhedral-shaped single crystal was observed to grow on top of a 〈100〉 oriented cylindrical seed crystal. The growth surface is composed of both {100} terraces and {110} ridges, arranged into well-formed pyramidal shaped structures with very long range order. Raman analyses show a lack of non-diamond carbon and a 1332 cm −1 peak which is indistinguishable from natural type IIa diamond. Low-temperature photoluminescence measurements indicate a greatly reduced level of localized radiative defects. Laue X-ray diffraction measurements have confirmed the epitaxial nature of the deposit, and preliminary X-ray rocking curve analyses are presented. This is the first report of the high-temperature epitaxial growth of diamond in a turbulent flame.

Growth and characterization of AlxGa1−xAs Bragg reflectors by LP-MOCVD

Using a moderate-sized LP-MOCVD production reactor, we have demonstrated the growth of Bragg reflector structures that reflect nearly 100% of the light at the design wave-length of 850 nm. The designs studied consist of alternating layers of Al0.10Ga0.90As and Al0.85Gao0.15As, with up to 30 periods. Characterization data show the structures to be highly reproducible and uniform, with good agreement between data obtained by double-crystal x-ray rocking curve analysis, cross-sectional transmission electron microscopy, and optical-reflectance spectroscopy. GaAs solar cells utilizing Bragg reflectors have been fabricated and characterized.

Optimization of MBE of CdTe/CdTe: Refinement in Structural Quality Evaluation of MBE Grown (111) CdTe

ABSTRACTA detailed optimization of MBE growth of CdTe on CdTe (111)β using white beam synchrotron x-ray topography, and x-ray rocking curve analysis has been carried out. Defect structures in (111)5 substrates and epilayers are imaged with white beam topography and, for the first time, direct observation of stress induced microtwin growth in CdTe epilayers has been made. X-ray rocking curve analysis of CdTe substrates is shown to be relatively insensitive to defect structures consisting of dislocations and inclusions. On the other hand rocking curve analysis of (111) epilayers reveals a sensitivity to the presence of microtwin. The defect microstructure of a CdTe (111)Bepilayer with the narrowest rocking curve FWHM value published to date (9.4 arc-sec) will be discussed.

Rapid isothermal processing of strained GeSi layers

A cold-wall rapid thermal processor was used to study the oxidation and annealing properties of Ge/sub x/Si/sub 1-x/ strained layers. The dry oxidation rate of Ge/sub x/Si/sub 1-x/ was found to be the same as that of Si, while the wet oxidation rate was found to be higher than that of Si, and the oxidation rate increases with the Ge concentration (up to 20% in this study). A high fixed oxide charge density (>5*10/sup 11//cm/sup 2/) and interface trap level density (>10/sup 12//cm/sup 2/-eV) at the oxide interface have been determined from capacitance-voltage measurements. Using techniques such as X-ray rocking curve analysis and I-V and C-V measurements of the p-n heterojunction it was found that the degradation of electronic properties of metastable Ge/sub x/Si/sub 1-x/ strained layers during rapid thermal annealing are related to the formation of structural defects at the heterointerfaces.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Rapid Structural Defect Mapping of Bulk II-VI Semiconductors Using White-Beam Synchrotron Topography and X-ray Rocking Curve Analysis

ABSTRACTNondestructive characterization techniques for substrates, epilayers, and device structures are becoming increasingly important in the semiconductor industry. Synchrotron-based white-beam x-ray topography, x-ray rocking curve measurements, and etch pit density were used to map the defect structure in a variety of CdTe and CdZnTe single crystal substrates, which are important for IR detector applications involving HgCdTe. Defects such as low angle grain boundaries have been successfully correlated using topography, rocking curves, and etch pit density, and twins have been observed using topography and rocking curves. The effectiveness of white-beam synchrotron topography for rapid and nondestructive defect analysis and substrate screening is discussed.

Vapor-Phase Growth of Epitaxial and Bulk ZnSe

ABSTRACTEpitaxial and bulk ZnSe of good structural perfection have been grown by vapor-phase techniques. Epitaxial undoped ZnSe layers were grown by metal-organic chemical vapor deposition (MOCVD) on GaAs {100} substrates in a horizontal-flow quartz reactor chamber. Conventional pyrolytic growth at 450°C was used, with diethyl zinc (DEZn) and diethyl selenium (DESe) reactants transported in hydrogen carrier gas.Layers with smooth surface morphology and very good crystal structure were obtained, with no evidence of gas-phase pre-reaction. Sharp electron channeling patterns produced in the scanning electron microscope (SEM) indicated that {100}-oriented ZnSe layers were grown on (100) GaAs surfaces, as expected. X-ray rocking-curve analysis with a silicon four-crystal monochrom-ator gave full-width at half-maximum (FWHM) line widths of 165 to 180 arc-sec for layers 2.5 to 3.0 μm thick, better than values for MOCVD-grown ZnSe/GaAs reported to date in the literature known to us. Cathodoluminescence (CL) imaging in the SEM showed significant defect substructure in the layers, probably due to lattice-misfit dislocations in the interface region. Bright blue CL emission from the layers was observed in the SEM at both 77K and room temperature, indicating the dominance of radiative recombination in the material.Bulk ZnSe crystals were grown using a physical vapor transport technique. The crystals, a few millimeters on a side, had fully developed crystal facets, predominantly {110(-oriented. X-ray rocking-curve analysis gave FWHM values of about 19 arc-sec, indicating excellent structural perfection.

The synthesis of superconducting Tl–Ca–Ba–Cu–oxide films by the reaction of spray deposited Ca–Ba–Cu–oxide precursors with Tl2O vapor in a two-zone reactor

Superconducting Tl–Ca–Ba–Cu–oxide films have been prepared on yttria stabilized zirconia substrates via the reaction of Tl2O vapor with precursor Ca–Ba–Cu–oxide films prepared by the spray pyrolysis of a solution of the metal nitrates. The vapor reaction process, evaluated in both air and oxygen ambients, was carried out in a two-zone reactor which permitted the independent control of the temperatures of the sample and of a boat containing thallium oxide. Sample temperatures of 865–905 °C and boat temperatures of 775–870 °C were investigated. X-ray diffraction analysis of the best samples, which were prepared in an oxygen ambient at sample temperatures of 895–900 °C and boat temperatures of 805–810 °C, revealed the presence of a highly oriented Tl2Ca2Ba2Cu3O10+y phase with a trace of the Tl2CaBa2Cu2O8+y phase. FWHM's of ∼2°θ were obtained in x-ray rocking curve analyses of the oriented 2223 phase. Tc (0) values of 95–105 K were measured for the better samples, and a zero field critical current density of 28 800 amps/cm2 was measured at 77 K for the best sample. SEM micrographs taken of the highest Jc samples reveal a highly textured structure consisting of platelike crystals.

Assessment of mismatched epitaxial layers by X-ray rocking curve measurements and simulations

In the design of heteroepitaxial systems the knowledge of composition, strain and thickness of the various layers involved is of great importance. The determination of these structural parameters is among the applications most common in high-resolution X-ray diffraction. Numerical simulation of rocking curves has emerged as an important aid in the determination of the relevant structural parameters. Based upon the dynamical theory of X-ray diffraction a FORTRAN program has been written to simulate and refine structural parameters from rocking curve data. X-ray rocking curve analysis has been used to develop and optimize MOCVD-growth of Ga x In 1- x P/GaAs heteroepitaxial systems. We observed superlattice peaks in samples of low misfit. With the aid of computer simulations we attribute these to unintentional periodic compositional fluctuations (Ga/In) of the order of 1%.

Self-consistent determination of the perpendicular strain profile of implanted Si by analysis of x-ray rocking curves

Results of a determination of strain perpendicular to the surface and of the damage in (100) Si single crystals irradiated by 250-keV Ar+ ions at 77 K are presented. Double-crystal x-ray diffraction and dynamical x-ray diffraction theory are used. Trial strain and damage distributions were guided by transmission electron microscope observations and Monte Carlo simulation of ion energy deposition. The perpendicular strain and damage profiles, determined after sequentially removing thin layers of Ar+-implanted Si, were shown to be self-consistent, proving the uniqueness of the deconvolution. Agreement between calculated and experimental rocking curves is obtained with strain and damage distributions which closely follow the shape of the trim simulations from the maximum damage to the end of the ion range but fall off more rapidly than the simulation curve near the surface. Comparison of the trim simulation and the strain profile of Ar+-implanted Si reveals the importance of annealing during and after implantation and the role of complex defects in the final residual strain distribution.

Characterization of beryllium-implanted GaAsP/GaP strained-layer superlattice structure

A dynamic model is presented based on Takagi's theory of dynamic diffraction of a distorted crystal, which incorporates depth-dependent strain and spherically symmetric gaussian distribution of random displaced atoms. The analysis of X-ray rocking curves using this model provides damage and strain depth distributions for ion-implanted strained-layer superlattices. This model has been used to interpret an experimental logarithmic (400) X-ray diffraction intensity vs. 2θ angle profile of a 75 keV beryllium-implanted GaAsP/GaP strained-layer superlattice structure reported in the literature.

Structural, electrical and optical characterization of singlecrystal ErAs layers grown on GaAs by MBE

A detailed study is presented of the structural, electrical, and optical properties of ErAs films grown on GaAs by molecular beam epitaxy (MBE). ErAs layers 1500A thick were grown successfully over a relatively wide range of substrate temperatures (420-580° C), although overgrowth of GaAs on ErAs was found to be difficult. In-situ reflection highenergy electron diffraction (RHEED), x-ray diffraction, and Rutherford backscattering (RBS) measurements all indicate single crystal growth. Analysis of X-ray rocking curves reveals that, over the range of substrate temperatures studied, strain due to the lattice mismatch between ErAs and GaAs is completely inelastically relieved in the 1500A thick ErAs layers. Variable-temperature Hall measurements reveal metallic behaviour in all samples, with no pronounced dependence on substrate temperature. Spectrally narrow (0.6 meV) intra 4f-shell transitions of Er3+ (4f11), at 1.54 μm, have been observed in ErAs epitaxial layers both in absorption (by Fourier transform infra-red spectroscopy, FTIR) and in emission (by cathodoluminescence). The crystal-field splittings observed in the FTIR spectra are consistent with the cubic(O h)symmetry expected for the Er lattice site in unstrained ErAs, in good agreement with the x-ray analyses.

Misfit strain relaxation in GexSi1−x/Si heterostructures: The structural stability of buried strained layers and strained-layer superlattices

The critical thickness-strain relationships for buried strained layers and strained-layer superlattices (SLSs) are derived using an energy balance model. Relaxation of the entire heterostructure and individual strained layers by both 60° type a/2〈011〉 and pure edge dislocations is considered. GexSi1−x/Si heterostructures designed to investigate the stability regimes predicted by the model were grown by molecular-beam epitaxy. The extent of relaxation and the detailed dislocation structure were assessed in annealed structures by x-ray rocking curve analysis, transmission electron microscopy, and Nomarski microscopy of etched samples. Comparison of metastable as-grown and post-growth annealed microstructures revealed the evolution of misfit dislocation structure as equilibrium was approached on annealing in the temperature range 600–900 °C. The predominant relaxation mechanism for most GexSi1−x/Si heterostructures was by 60° a/2 〈011〉 misfit dislocations at the first strained-layer/substrate interface. However, for SLSs with strained layers exceeding their individual critical thickness, pairs of dislocations and prismatic loops on (011) planes were observed entrained within the GexSi1−x layers. The agreement between experimentally observed relaxation behavior and the critical thickness versus strain predictions of the energy balance model is remarkable. Superlattices and buried strained layers were found to be only slightly more stable than an uncapped strained layer of the equivalent strained-layer thickness. Interdiffusion was observed at annealing temperatures above 800 °C; annealing at 900 °C for 30 min was sufficient to eliminate the strain and composition modulation in a superlattice with a 55-nm period and x=0.25. The relative influence of the various strain relaxation mechanisms is discussed for the alloy range 0&amp;lt;x&amp;lt;1 and for geometries with strained-layer dimensions varying from 0.8 to ∼500 nm.

Improvement of Inp-GaAs-Si Quality by Thermal-Cycle Growth

ABSTRACTSingle-crystal films of InP have been deposited on GaAs, GaAs-coated Si, and InP substrates by metalorganic chemical vapor deposition (MOCVD). Defect-reduction schemes involving various thermal annealing recipes have been developed and characterized. Material quality has been assessed by a variety of methods including transmission electron microscopy, X-ray rocking curve analysis, low-temperature photoluminescence, lifetime measurements, Hall-effect measurements, electrochemical profiling, and Nomarski microscopy. The use of either a thermal-cycle-growth or a thermal-cycle-annealing process leads to heteroepitaxial InP film quality which is significantly improved over that of its as-grown state, with the thermal-cycle growth appearing to be the more effective technique.

Growth of high quality single crystals of YB 66

Single crystals of YB 66 have been grown by the indirect heating floating zone method. The effects of growth rate and feed rod composition on the crystal quality were examined. The optimum growth rate was 25 mm/h. The congruent composition was confirmed to be [B]/[Y] = 62. Preliminary X-ray rocking curve analysis showed that the full width half maximum of the rocking curve of the best crystal was less than 12 min.

Characterization of ground-based and space-based mercuric iodide single crystals by means of X-ray topography and diffractometry

X-ray topography and rocking curve analysis have been used in order to characterize the crystalline quality of α-HgI 2 single crystals grown by physical vapor transport on the ground and in a microgravity environment during the SL3 mission of Spacelab.

MODIFICATION OF ELECTRONIC MATERIALS WITH MeV IONS

MeV ion beams often allow unique opportunities for the modification of the electronic properties of materials. In this talk I shall discuss three such examples from our recent work. The first is the production of deeply buried SiO2 layers formed in Si by MeV oxygen implantation for a variety of implantation and annealing conditions. The resulting material has been characterized by XTEM. The second involves the modification of InP by N implantation and GaAs by O implantation. In these cases we have used XTEM, x-ray rocking curve analysis, ion channeling, and nuclear reaction profiling to study the structural damage and its behavior under thermal annealing. The third example shows how the resistivity of thin amorphous carbon films can be changed over a range of 10^4 by MeV ion bombardment. The mechanism for this phenomenon is shown to be closely related to that proposed for MeV ion induced desorption.