X-ray diffraction solutions to heteroepitaxial growth problems

Modern X-ray diffraction equipment offers a wide choice of methods for understanding the problems involved in the heteroepitaxial growth of a wide range of materials. The basic double axis rocking curve continues to play a key role in the characterisation of heteroepitaxial strained layer structures. The strength of the technique arises from the ability to accurately simulate the diffraction process using the Takagi-Taupin formulation of dynamic theory.

There is an increasing use of diffraction space mapping for the less perfect heteroepitaxial systems with large lattice mismatches and different crystal structures, e.g. superconductors, gallium nitride on sapphire. Detailed studies of the state of relaxation of pseudomorphic systems can be achieved using the triple crystal diffractometer. The triple axis diffractometer can also be used to measure the unit cell distortion in good quality non-pseudomorphic systems. Where layers are less perfect and where layers are very thin lower resolution diffraction space mapping can provide useful information with a large reduction in data collection time.     

Growth of ZnTe layers on (111) GaAs substrates by metalorganic vapor phase epitaxy

ZnTe layers were grown on (111) GaAs substrates by metalorganic vapor phase epitaxy using dimethylzinc and diethyltelluride as the source materials. X-ray diffraction analysis revealed that epitaxial ZnTe layers can be obtained on (111) GaAs substrates. X-ray rocking curves, Raman spectroscopy, and photoluminescence measurements showed that the crystal quality of ZnTe layers depends on the substrate temperature during the growth. A high-crystalline quality (111) ZnTe heteroepitaxial layer with strong near-band-edge emission at 550 nm was obtained at a substrate temperature of 440 °C.     

Analysis of epitaxial laterally overgrown silicon structures by high resolution x‐ray rocking curve imaging

Spatially resolved rocking curve imaging has been used to analyze laterally overgrown silicon layers grown by liquid phase epitaxy. We were able to study both the overgrown layer as well as the strain fluctuations of the Si substrate underneath by means of a tabletop x‐ray topographic setup. The strain‐field analysis reveals relative changes of the lattice parameter up to 3.5×10^‐6 in the silicon substrate underneath the overgrown layer in particular regions and a down bending of both wings of the epitaxial overgrown layers. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

General orientational characteristics of heteroepitaxial layers of AIIBVI semiconductors on sapphire and semiconductor substrates with diamond and sphalerite structures (AIIIBV)

X-ray reflection and transmission diffractometric studies of nonisostructural heteroepitaxy showed that the spatial rotation of the crystal lattice of the grown heteroepitaxial layer with respect to the substrate lattice about the crystallographic direction common for both lattices is the most general characteristic feature of their mutual crystallographic orientation. This feature leads to the coincidence of the 〈0001〉 polar axis of the heteroepitaxial layer of the A^IIB^VI compounds with one of the 〈111〉 polar axes of the cubic substrate. The 30° and 90° rotations in the plane of heteroepitaxy and the vernier-type conjugating of the planar nets of the heteroepitaxial layer and the substrate are the particular cases of their mutual crystallographic orientations.     

Preparation and Characterization of MBE-grown Si/CaF2 Heterostructures

Growth and fabrication of silicon-on-insulator structures, based on heteroepitaxial growth of insulating films on Si, is an area of research that has rapidly developed in recent years. Thin CaF₂ films and Si/CaF₂ multilayers were prepared on {111}-oriented Si substrates by molecular beam epitaxy (MBE) and were investigated by RHEED and RBS. For epitaxial growth the Si substrates were cleaned using the ultraviolet/ozone surface cleaning method which is an effective tool to remove contaminants from the surface by low-temperature in-vacuo preheating. The growth of CaF₂ on such Si{111} substrates provides epitaxial layers with a high structural perfection. When this layer system, however, is used as a substrate for epitaxial Si growth the Si layers show always twin formation. Si layers without twins could be obtained only after deposition of thin amorphous Si buffer layers at room temperature, immediately followed by Si growth at 700 °C.     

Determination of lattice parameters at thin epitaxial layers by RHEED

A simple method is described for measurement of lattice parameters of thin heteroepitaxial layers by RHEED technique using the substrate as a calibration substance. Results are presented for special cases of I–III–VI₂ chalcopyrite type semiconductor epilayers on GaAs substrates. The accuracy of the method is in the order of about 5 · 10⁻³.     

Size Effect and Processes of Interdiffusion in Multilayer Films

The electrophysical properties of multilayer structures consisting of the alternate Co and Ni layers or Co and Cr layers obtained in high vacuum (˜ 10⁻³ Pa) were investigated. The size dependences of the temperature coefficient of the resistance (TCR) of the films were obtained experimentally and then compared with the R. Dimmich theoretical model. The analysis shows that the main reason of the difference between theoretical and experimental data of TCR is the interdiffusion of atoms from one layer to another.     

Application of X-ray diffraction methods in the study of micrometer-sized porous Si layers

An X-ray analysis of porous silicon layers (Sb-doped n ⁺-Si(111)) obtained by anodic oxidation for different times with a current of 50 mA/cm² is performed by the methods of double-crystal rocking curves and total external reflection. A nondestructive method for monitoring the stationary process of the formation of micrometer-sized porous silicon layers and estimating their porosity and thickness is proposed. The parameters obtained for porous silicon layers with a thickness of ～6 μm are confirmed by the joint processing of diffraction curves for the 111 and 333 reflections on the basis of the developed model of dynamic scattering from layers while taking into account the strain profiles Δd(z)/d, the static Debye-Waller factor f(z), and the porosity P(z). The advantages and drawbacks of the proposed method are discussed.     

X-ray diffraction study of short-period AlN/GaN superlattices

The structure of short-period hexagonal GaN/AlN superlattices (SLs) has been investigated by X-ray diffraction. The samples have been grown by metalorganic vapor-phase epitaxy (MOVPE) in a horizontal reactor at a temperature of 1050°C on (0001)Al₂O₃ substrates using GaN and AlN buffer layers. The SL period changes from 2 to 6 nm, and the thickness of the structure varies in a range from 0.3 to 1 μm. The complex of X-ray diffraction techniques includes a measurement of θ-2θ rocking curves of symmetric Bragg reflection, the construction of intensity maps for asymmetric reflections, a measurement and analysis of peak broadenings in different diffraction geometries, a precise measurement of lattice parameters, and the determination of radii of curvature. The thickness and strain of separate SL layers are determined by measuring the θ-2θ rocking curves subsequent simulation. It is shown that most SL samples are completely relaxed as a whole. At the same time, relaxation is absent between sublayers, which is why strains in the AlN and GaN sublayers (on the order of 1.2 × 10^?2) have different signs. An analysis of diffraction peak half-widths allows us to determine the densities of individual sets of dislocations and observe their change from buffer layers to SLs.     

Mechanism of epitaxial ferrite-spinel layer formation on magnesium oxide substrate

The initial stage of small gap heteroepitaxial growth of magnesium-manganese ferrite films during chemical gas transportation reactions on orientation (100) magnesium oxide substrate was studied. Film growth would begin with origination of parallely oriented three-dimensional nuclei, their shapes at lower (1170 K) and at appear (1420 K) film synthesis temperature limits being different. After little islands had grown together and continuous layer formed the secondary nuclei would appear, their shapes resembling cut-up pyramids. Continuous layer minimum thickness was 0.15 μm. The growing together of little islands results in film block structure. Block sizes could be as big as 300–400 μm if films grow at the speed of 2 μm/min, the density of islands being 10⁶ cm⁻². Linear density of unconformity dislocations at film-substrate boundary would be (3.7 ÷ 7.4) 10³ cm⁻¹. It would depend on magnesium content for a film. Elastic stresses in films would come to 7 · 10⁸ ÷ 3 · 10⁹ dyne/cm². X-ray topography pictures of block structures and of accumulations of admixtures at block boundaries are presented.     

GaP:Mg layers grown on GaN by MOCVD

We have investigated the growth of magnesium-doped GaP (GaP:Mg) layers on GaN by metalorganic chemical vapor deposition. The hole carrier concentration increased linearly from 0.8×10¹⁸ to 4.2×10¹⁸ cm⁻³ as the Bis(cyclopentadienyl) magnesium (Cp₂Mg) mole flow rate increased from 1.2×10⁻⁷ to 3.6×10⁻⁷ mol/min. However, the hole carrier concentration decreased when the CP₂Mg mole flow rate was further increased. The double crystal X-ray diffraction (DCXRD) rocking curves showed that the GaP:Mg layers were single crystalline at low CP₂Mg molar flow. However, the GaP:Mg layers became polycrystalline if the CP₂Mg molar flow was too high. The decrease in hole carrier concentration at high CP₂Mg molar flow was due to crystal quality deterioration of the GaP layer, which also resulted in the low hole mobility of the GaP:Mg layer.     

A Comparison of Different Multiple-Crystal Diffractometer Arrangements to Measure the Reflection Curve of SiGe Layers on Si Substrates

Five different double- and triple-crystal diffractometer (DCD, TCD) arrangements are used to measure the reflection curve of a heteroepitaxial structure consisting of a 41 nm thick Si_1-xGe_x layer (x = 0.327) on an Si-100 substrate. The advantages and disadvantages of these arrangements including different monochromator/collimator and analyzer set-ups are discussed. It is shown that even for this perfect heteroepitaxial layer TCD measurements provide the best results especially in the reproduction of deep minima of the intensity oscillations in the reflection curve.     

Energy band-gap in elastic-strained heteroepitaxial layers

The effect of internal elastic stress on energy band-gap shift in the A³B⁵ heteroepitaxial layers was investigated. The possibilities to get higher accuracy of divergent beam X-ray stress determination method, designed in the scanning electron microscope (SEM), was presented. X-ray topography method in the SEM with X-ray magnification from 2 up to 10 was for the first time described. The epilayer photoluminescence (PL) peak energy shift as a function of internal elastic stress was measured on the GaInP/GaAs, GAInAsP/InP heterostructures and the influence of different factors on the accuracy of the results was discussed.     

Structure of porous surface layers of single-crystal GaAs(001) wafers from data of X-ray diffractometry and reflectometry

The surface morphology and structure parameters of the surface layers of the single-crystal GaAs(001) wafers subjected to He⁺ ion implantation (E = 300 keV, D = 10¹⁶ atoms/cm²) and subsequent electrochemical etching in a 0.5 M H₂SO₄ aqueous solution are investigated by X-ray reflectometry and double-crystal X-ray diffractometry. The strain and amorphization profiles over the layer thickness are determined from X-ray diffraction data. The density of surface layers, their thickness, and the changes of the surface relief upon etching are evaluated from the reflectometric data. The experimental parameters of the studied layers are compared with the theoretical distributions of implanted impurities and intrinsic point defects, which are calculated by the Monte Carlo method. A correlation is revealed between the layer thickness and the depth of the maximum defect concentration. It is found that the electrochemical etching predominantly occurs in strongly amorphized regions of the surface layer and does not lead to a change in the total layer thickness. The results of X-ray diffraction investigations are confirmed by scanning electron microscopy.     

Simultaneous analysis of double-crystal X-ray rocking curves from a set of crystallographic planes

A technique of simultaneous analysis of rocking curves from different crystallographic planes of multilayer semiconductor heterostructures is developed and implemented. By the example of the GaAs-In_xGa_1?x As/GaAs(001) heterostructure with a quantum well and the rocking curves from the (004), (113), and (115) planes, it is demonstrated that simultaneous analysis makes it possible to derive both more reliable and more exhaustive information on the thickness, strain, and degree of amorphization of individual layers and interfaces in this heterostructure. The procedure developed can be used effectively to analyze arbitrary experimental data sets obtained by different X-ray methods sensitive to the general set of structural parameters.     

OD approach to the polytypism in CdP2

The structures of CdP₂ are described as stacking sequences of layers of CdP₄ tetrahedra. On the basis of the OD theory of Dornberger-Schiff, the stacking possibilities are derived and convenient stacking notations proposed. The idealized layer structure is given, the percentage of α-CdP₂ for any stacking sequence defined. The symbol of the OD groupoid family is P11(n)2m · (4) x, y with parameters x = y = 1/2 and (4⁺), (4⁻) as the two alternative transformations from layer to layer. The net constants are a = b ≈ 5.29 Å; the “thickness” of a layer is c₀ ≈ 4.95 Å.     

TEM study of defects in AlxGa1−xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al_xGa_1?xN layers with various Al mole fractions (x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al_xGa_1?xN layer (x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed.The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al_xGa_1?xN layer with both polarities.The lattice relaxation of metal-polar Al_0.2Ga_0.8N was observed, while N-polar Al_0.2Ga_0.8N did not relax. The dislocation densities in the N-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers were 5.5×10⁹ cm^?2 and 9×10⁹ cm^?2, respectively, and in metal-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers these were 1×10¹⁰ cm^?2 and 6×10⁹ cm^?2, respectively.Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.     

Effect of dislocation loops in macroscopically dislocation-free GaP substrates on the perfection of homo-epitactic deposits

Substrates and epitactic layers of GaP are etched under optimized conditions with the defect-revealing, preferential R-C etchant and subsequently examined using high-magnification (about 2500X) optical microscopy techniques. It is possible then to distinguish two new phenomena, viz. etch pits characteristic of dislocation loops in the substrate, and inclined dislocation dipoles in the layer. For layers grown on dislocation-free substrates we find that (i) the surface densities of both defects are equal (～5 × 10⁵cm^-2), and (ii) the average diameter of the dislocation loops in the substrate is roughly the same as the average distance between the two dislocations of the dislocation dipoles (0.5?1μm). Hence the perfection of these layers is determined by interfacial dislocation loops. Because the density of dislocation loops is only about a factor of two lower in highly (～1 × 10⁵ cm^-2) dislocated substrates, growth on these substrates results in layers which have even slightly lower dislocation density than layers grown on dislocation-free substrates. In the former case also single dislocations in the substrate propagate into the layer. Minority-carrier lifetime data indicate that minority-carrier recombination at dislocations is a restrictive factor for the luminescence quality of layers grown both on dislocation-free and highly-dislocated substrates.     

Strain analysis of InGaN/GaN multi quantum well LED structures

Five period InGaN/GaN multi quantum well (MQW) light emitting diode (LED) structures were grown by a metalorganic chemical vapor deposition (MOCVD) system on c‐plane sapphire. The structural characteristics as a strain‐stress analysis of hexagonal epilayers MQWs were determined by using nondestructive high resolution x‐ray diffraction (HRXRD) in detail. The strain/stress analysis in AlN, GaN, and InGaN thin films with a variation of the In molar fraction in the InGaN well layers was conducted based on the precise measurement of the lattice parameters. The a‐ and c‐lattice parameters of the structures were calculated from the peak positions obtained by rocking the theta axis at the vicinity of the symmetric and asymmetric plane reflection angles, followed by the in‐plane and out‐of‐plane strains. The biaxial and hydrostatic components of the strain were extracted from the obtained a‐ and c‐direction strains values.     

X-ray measurement of deformation and dislocation density in semiconductor strained layers

Double-crystal X-ray diffraction is commonly used to measure the misfit strain and relaxation of epitaxial semiconductor layers. In this paper, a framework is developed which links the measured parameters Δd / d and Δø to the deformation tensor of a semicoherent layer. Isotropic elasticity theory and the Frank-Bilby equation are used to derive an analytical expression for this deformation. By combining X-ray measurements of different planes, it is possible to obtain the misfit strain and details of the misfit dislocation array in a strained layer grown on a substrate of arbitrary orientation. In (001) layers, it is shown that the misfit strain and relaxation can be found from just six rocking curves, although the most accurate measurements require twelve rocking curves.