Double-crystal X-ray Diffractometry Research Articles

OMVPE Growth of Epitaxial InSb Thin Films Using a Novel Group V Source Compound

ABSTRACTTo date, OMVPE has failed to grow epitaxial InSb at temperatures below about 400 °C. Therefore, we have studied InSb deposition using the novel group V source triisopropylantimony (TIPSb) with trimethylindium (TMIn), comparing this with results in the same reactor using trimethylantimony (TMSb). We have grown InSb on both GaAs and on high-resistivity p-type InSb substrates, exploring the effects of temperature, substrate, V/ill ratio, and thickness on film properties. Our best results with TMSb and TMIn were obtained using a V/ill ratio of 8 and growth temperature of 450 °C. The unintentionally doped films exhibited good crystallinity and were n-type about lx1015 cm-3 down to 5 K, with 77 K mobilities of 90,000 and 253,00°Cm2/V-sec for InSb on GaAs and on InSb, respectively. These results are comparable to the best published MBE data. While InSb films could not be grown well at temperatures below 425 °C using TMSb, specular epilayers were obtained with TIPSb at temperatures as low as 300 °C, though with a low deposition rate. Our best conditions with TIPSb were with a Vill ratio of about 12 and a substrate temperature of 350 °C. The results of variable temperature and variable magnetic field Hall, double crystal x-ray diffractometry, Nomarski microscopy, Auger electron spectroscopy and scanning electron microscopy will be presented.

Anisotropic and inhomogeneous strain relaxation in pseudomorphic In0.23Ga0.77As/GaAs quantum wells

The structural properties of pseudomorphic In0.23Ga0.77As/GaAs single quantum wells are investigated with x-ray double-crystal diffractometry. Anisotropic tilting of lattice planes along dislocations and anisotropic reflectivity of fully relaxed domains coexisting with strained domains are reported for the first time. Due to the anisotropic strain relaxation the crystal symmetry changes from tetragonal in the fully strained case to monoclinic in a partially relaxed quantum well.

Rapid heteroepitaxial growth of Ge films on (100) GaAs by pulsed supersonic free-jet chemical beam epitaxy

Very high epitaxial growth rates (0.25 μm/s), not attainable by conventional vapor phase film growth methods, have been obtained using a pulsed supersonic free jet to supply digermane (Ge2H6) for thermal decomposition at a heated (100)GaAs surface. Double-crystal x-ray diffractometry and scanning electron microscopy show that highly planar, smooth, and commensurate epitaxial Ge films are grown on (100)GaAs using gas pulses of 250 ms duration at 1–3 atm (0.1–0.3 MPa) pressure for substrate temperatures of 480–680 °C. It is believed that the very high growth rates effectively ‘‘freeze’’ the Ge-GaAs interface resulting in minimal interfacial diffusion and reduced incorporation of impurities from the surrounding growth environment.

Heteroepitaxial growth of Ge films on (100) GaAs by pyrolysis of digermane

Pyrolysis of high-purity digermane (Ge2 H6 ) has been used to grow epitaxial Ge films of high crystalline quality on (100) GaAs substrates in a low-pressure environment. X-ray double-crystal diffractometry shows that fully commensurate, coherently strained epitaxial Ge films can be grown on (100) GaAs at digermane partial pressures of 0.05–40 mTorr for substrate temperatures of 380–600 °C. Amorphous films also were deposited. Information about the crystalline films surface morphology, growth mode, and microstructure was obtained from scanning electron microscopy, cross-section transmission electron microscopy, and in situ reflectivity measurements. The amorphous-to-crystalline transition temperature and the morphology of the crystalline films were both found to depend on deposition conditions (primarily the incidence rate of Ge-bearing species and the substrate temperature). Epitaxial growth rates using digermane were found to be about two orders of magnitude higher than rates using germane (GeH4 ) under similar experimental conditions.

Lattice-mismatch-generated dislocation structures and their confinement using superlattices in heteroepitaxial GaAs/InP and InP/GaAs grown by chemical beam epitaxy

We report a detailed transmission electron microscopy and x-ray double-crystal diffractometry study of the lattice-mismatch-induced defect structures in InP grown on (100) GaAs substrate and vice versa by chemical beam epitaxy. A rough estimate of the dislocation densities in GaAs on InP is 2×1010 cm−2 at the interface and 5×107 cm−2 at the surface of the epilayer. The corresponding values in InP on GaAs are slightly lower as expected for the compressive stress state for InP. The majority of the dislocations lie on the {111} slip planes with 1/2 [110]- and 1/2 [101]-type Burgers vectors. A cross-grid-type interfacial misfit dislocation array is not observed. Instead, a complicated dislocation structure near the interface, consisting of overlapping pyramidal dislocation tangles (PDT) similar to those observed previously in InGaAs on InP caused by interfacial misfit particles, is presented. The interfacial dislocations form a cellular structure in GaAs on InP and a random structure in InP on GaAs. A Moiré fringe spacing study of InP on GaAs indicates a localized change in composition at the interface, possibly due to As incorporation or GaAs/InP intermixing. The formation of PDT defects and the variation in composition at the interface suggest a breakdown of layer-by-layer growth in the initial stage of growth which results in island nucleation. A dislocation mechanism for the PDT formation is also proposed. All epilayers prepared by chemical beam epitaxy (CBE) without two-stage growth are specular. X-ray rocking curve linewidth measurement shows a general reduction in the linewidth with increasing growth temperature and is insensitive to the substrate misorientation. Fine surface morphology revealed by Nomarski interference microscopy shows no correlation with x-ray linewidth. Results on the reduction of dislocation by varying growth temperature, substrate misorientation angle, and using AlGaAs/GaAs superlattice barriers for dislocation propagation are presented and their effectiveness are discussed. A realistic scheme to achieve an unwarped wafer with low dislocation density is proposed.

Double crystal X-ray diffraction characterization of multilayer heteroepitaxial structures with submicron layers

The method of double crystal X-ray diffractometry is used to determine parameters of multilayer epitaxial heterostructures consisting of layers of various composition and thickness from dozens of nanometers to a few microns. Computer simulation is carried out in a semikinematical approximation. The sufficient influence of thin epitaxial layers both, on oscillation picture and the main peak form of the rocking curves is shown. The minimum thickness that influences the X-ray rocking curve form and determines the method sensibility is about 10 nm. [Russian Text Ignore]

A study of the distribution of hydrogen and strain in proton-bombarded liquid-encapsulated Czochralski-grown GaAs by double-crystal x-ray diffraction and secondary ion mass spectrometry

Proton bombardment is an effective method of providing electrical and optical isolation for devices in GaAs integrated circuits. In this study, n-type LEC GaAs has been implanted with protons at doses of 10 15 and 10 16 ions cm −2 at an energy of 190 keV. Double-crystal X-ray diffractometry of the as-implanted material reveals complex rocking curves featuring high intensity damage-related peaks. These have been analysed by comparison with simulated rocking curves which were calculated using the dynamic theory of X-ray diffraction with assumed strain profiles. This simulation-matching technique has enabled the determination of the strain distributions for the two doses studied. For the 10 15 cm −2 dose the peak strain is 3.8 × 10 −4 and for the 10 16 cm −2 dose it is 2.2 × 10 −3 with the peak strain for both samples occurring at a depth of 1.6 μm. Secondary ion mass spectrometry (SIMS) profiles confirm that the peak in the hydrogen concentration occurs close to the peak in strain. A further feature of the strain profiles in the as-implanted sample is that a close match with the experimental data is only obtained when a highly strained layer close to the surface is put into the simulation. Although this was unexpected, it was possible to substantiate its existence by etching studies. Subsequent annealing of the damage over the temperature range 150–500 °C (less than 200 °C in air, and more than 200 °C in arsine) has enabled modifications to the strain profile as a function of annealing temperature to be determined. There is no change at 150 °C but above this temperature the level of strain reduces as the annealing temperature increases and the structural damage recovers between 400 and 500 °C which approximates to the optimum annealing temperature required to achieve good isolation properties. SIMS profiles of the annealed samples show that hydrogen is not mobile at 150 °C but above 150 °C it moves down the strain gradient towards the surface. The data are discussed in the light of the currently perceived model of radiation damage annealing in GaAs, i.e. there are two recovery stages at about 200 °C and 400 °C.

Thermal Strain Study of Almost Lattice-Matched Epitaxial InuGa1-uAs1-vP1-v Films on InP(100) Substrates

AbstractSingle layers of ~ 0.5µm thick InuGa1-uAs1-vPv (0.52 < u < 0.63 and 0.03 < v < 0.16) were grown epitaxially on InP(100) substrates by liquid phase epitaxy at ~ 630°C. The compositions of the films were chosen to yield a constant banndgap of ~ 0.8 eV (λ = 1.55 µm) at room temperature. The lattice mismatch at room temperature between the epitaxial film and the substrate varies from - 4 × 10-3 to + 4 × 10-3. The strain in the films was characterized in air by x-ray double crystal diffractometry with a controllable heating stage from 23°C to ~ 700°C. All the samples have an almost coherent interfaces from 23°C to about ~ 330°C with the lattice mismatch accomodated mainly by the tetragonal distortion of the epitaxial films. In this temperature range, the x-ray strain in the growth direction increases linearly with temperature at a rate of (2.0 ± 0.4) × 10-6/°C and the strain state of the films is reversible. Once the samples are heated above ~ 300°C, a significant irreversible deterioration of the epitaxial films sets in.

Estimation of percentage relaxation in Si/Si1-xGex strained-layer superlattices

A series of Si/Si1-xGex strained-layer superlattice structures has been studied by X-ray double-crystal diffractometry, Raman spectroscopy and transmission electron microscopy. The periodicity of the superlattices, the alloy composition and the degree of relaxation have been measured. The precisions of the three techniques are discussed and the results critically compared.

Characterization of porous silicon layers by means of X-ray double-crystal diffractometry.

(111) and (100) p-type silicon wafers of 0.55 to 33 Ωcm resistivities were anodized in 50 wt% HF solution and formed porous silicon layers were investigated by using X-ray double-crystal diffraction techniques. The porous layer is shown to be a monolithic single crystal, and its lattice spacing is slightly larger than that of the unanodized substrate, resulting in elastic bending of the wafer. The difference in lattice spacing between the porous layer and the substrate was nearly identical independent of wafer orientation and wafer resistivity. The crystal lattice of porous silicon layer is found to be elastically distorted predominantly in the direction normal to the wafer surface. The lattice spacing of porous layer increased slightly with time and the crystalline quality degraded. The results suggest that stresses, generated by the growth of native oxide layers on pore surfaces, are responsible for the elastic lattice distortion of porous layer. It is also shown tht the crystalline quality of the porous layer produced on the (111) wafers appears superior to that of the (100) wafers for all the wafer resistivity studied.

Asymmetries in dislocation densities, surface morphology, and strain of GaInAs/GaAs single heterolayers

The dislocation densities, surface morphology, and strain of Ga1−xInxAs/GaAs epitaxial interfaces as a function of indium composition and layer thickness have been investigated by transmission electron microscopy, medium energy ion blocking, and double-crystal x-ray diffractometry. The electron microscopy shows that in the thinnest dislocated films (90 and 160 nm, x=0.07) 60° α dislocations form first in one 〈110〉 direction at the interface. Surprisingly, however, an asymmetry in residual layer strain is not detected in these samples, suggesting that the dislocations have the same Burgers vector or are evenly distributed between two Burgers vectors. Orthogonal arrays of dislocations are observed in films thicker than 300 nm (60° and edge-type, x=0.07). In this case, dislocation densities in each 〈110〉 direction are equal to within experimental error while an asymmetry in in-plane strain is measured (18% and 30% for x=0.07, 300, and 580 nm thick, respectively). An unequal distribution of Burgers vectors of 60° or edge-type dislocations is considered responsible for the strain asymmetry in these thicker samples.

Lattice distortions induced by carbon in silicon

Abstract X-ray double-crystal diffractometry has been used to investigate the lattice distortions in silicon crystals containing carbon impurities. Precise lattice-spacing measurements have been carried out with a double-source double-crystal transmission technique. A linear correlation between the absolute value of lattice spacing and the carbon concentration was observed. X-ray double-crystal reflection topography was used to measure residual minute lattice strains also as a function of the carbon atom number density. A similar relationship was found. A comparison of the results of the lattice-spacing measurements and of the topographic investigations indicates that the lattice consists of consecutive layers which are alternately more and less carbon enriched. The observed lattice deformations arise from the substitutional exchange of silicon and carbon atoms.

Effect of AlN Buffer Layer on AlGaN/α-Al2O3 Heteroepitaxial Growth by Metalorganic Vapor Phase Epitaxy

AlxGa1-xN(0≦x≦0.4) films grown on sapphires are found to be composed of many mosaic crystallites with various orientations by means of double-crystal X-ray diffractometry. This fluctuation of orientation can be considerably reduced and the surface morphology (smoothness and uniformity) of the film can be remarkably improved by the preceding deposition of the AlN buffer layers. A phenomenological model, which can explain the effect of the AlN buffer layer on the surface morphology and the crystalline quality of AlGaN films, is proposed.

Reconstruction mechanisms in ion-implanted yttrium iron garnet films

Yttrium iron garnet films implanted with Ne+ ions at two different doses have been analyzed by x-ray double-crystal diffractometry and conversion electron Mössbauer spectroscopy after thermal treatment at different temperatures. The strain profile, paramagnetic fraction, hyperfine fields of both the paramagnetic and magnetic fraction, and the full width at half-maximum of the Mössbauer peaks have been measured for the different temperatures. The reconstruction mechanism of the crystalline lattice has been studied. It appears that at least two different mechanisms are involved with two activation energies. The first one, which starts at nearly 150–200 °C and is completed at nearly 500–600 °C, is on a short-range scale and is driven by oxygen diffusion, while the second one, which has a higher activation energy, is on a long-range scale. There is clear evidence that, at least for the lower dose implanted sample, the reconstruction mechanism is not an epitaxial regrowth.

Crystal quality investigation of InGaAs/InP and InGaAlAs/InP single heterostructures grown by molecular-beam epitaxy

X-ray double-crystal diffractometry has been used to assess the crystal quality of InGaAs/InP single heterostructures grown by molecular-beam epitaxy. Experiments performed on step etched samples have indeed shown that the width of the epilayer Bragg peak as a function of the epilayer thickness is in good agreement with the theoretical predictions for perfect crystals. Finally, preliminary observations of InGaAlAs/InP heterostructures have shown very narrow Bragg peaks and this demonstrates the good crystal quality of the quaternary layers.

Double-crystal x-ray investigations of semi-insulating (Ga,Fe) double-doped InP substrates

We report further x-ray double-crystal diffractometry experiments on the structural perfection of the semi-insulating (Ga, Fe) double-doped LEC-grown InP substrates. Series of x-ray reflection topographs were taken at different settings on the rocking curves using the double-crystal arrangement in parallel setting. Variations in lattice-plane spacings and lattice-plane orientations induced by Ga inhomogeneity were determined. (In, Ga)As layers grown by molecular-beam epitaxy on (001) oriented (Ga,Fe) double-doped InP substrates were also characterized.

X-ray double-crystal characterization of molecular beam epitaxially grown Si/Si1−xGex strained-layer superlattices

We report the first study of Si/Si1−xGex strained-layer superlattices using x-ray double-crystal diffractometry. Symmetric rocking curves (400) were used to determine the structural parameters and strain distribution. All the superlattices were found to have excellent crystalline quality and uniformity both in thickness and alloy composition. Kinematical simulations of rocking curves revealed that most of the misfit between Si and Si1−xGex is accommodated by lattice deformation of the alloy layers. A small tensile strain (∼0.05%) in the growth direction was also found in Si and was attributed to the difference in thermal expansion coefficients of Si and Si1−xGex.

Molecular beam epitaxial growth and structural characterization of ZnS on (001) GaAs

The effect of surface nucleation processes on the quality of ZnS layers grown on (001) GaAs substrates by molecular beam epitaxy is reported. Reflection high energy electron diffraction indicated that nucleation at high temperatures produced more planar surfaces than nucleation at low temperatures, but the crystalline quality as assessed by X-ray double crystal diffractometry is relatively independent of nucleation temperature. A critical factor in layer quality was the initial roughness of the GaAs surfaces.

Extremely low threshold InGaAsP/InP DFB laser by MOCVD/LPE hybrid process

The InP and InGaAsP layers grown by low pressure MOCVD were characterized by a double crystal X-ray diffractometry and 4.2 K photoluminescence measurements. It was confirmed that these layers were of high quality and the hetero-interfaces were very sharp. The growth characteristics on corrugated substrates were investigated for the application to 1.3 μm InGaAsP/InP distributed feedback (DFB) laser diodes. These devices exhibited extremely low threshold currents of 3.8 mA in CW mode at 20 ° C for a normal cavity length of 300 μm. The typical values of the differential efficiency (η) and the characteristic temperature (To) were 0.25 mW/mA and 53 K, respectively. Single longitudinal mode operation was attained up to 25 mW light output. These results show a great potential of MOCVD for the fabrication of InGaAsP/InP optical devices.

Annealing induced strain in 31P+-implanted Si (111) and Si (100)

The thermal annealing of (100) and (111) oriented Si crystals after high dose implantation with 80 keV 31P+ ions was followed by double-crystal x-ray diffractometry. Analysis of the rocking curves shows that at higher temperature annealings after regrowth at 630 °C, P atoms may leave their substitutional position at temperatures higher than 700 °C, depending upon the implantation dose and surface orientation.