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Notes: Double-crystal and triple-axis x-ray diffractometry was used to characterize in detail the strain and composition of short period Si6Ge4, Si8Ge8, Si9Ge6, and Si17Ge2 strained-layer superlattices (SLSs), grown by molecular-beam epitaxy. Nominally strain-symmetrized superlattices, intended to be free standing from underlying buffer layers and the substrate, grown on rather thin (20 nm thick) SiGe alloy buffers with constant Ge content (Si6Ge4 and Si8Ge8) are compared to those grown on 1.3-μm-thick step-graded SiGe alloy buffers (Si6Ge4 and Si9Ge6). Due to the much higher instrumental resolution offered by triple-axis diffractometry (Δ2aitch-theta=12 arcsec) buffer and SLS peaks are clearly separated from each other, which overlap in corresponding double-crystal-diffractometry measurements (Δ2aitch-theta in the range of 180 arcsec to 2°). The lattice constants parallel and perpendicular to the [001] growth direction are determined independently from each other and thus precise strain data of the buffers and the SLS constituting layers were extracted from two-dimensional reciprocal space maps around (004) and (224) reciprocal lattice points (RELPs). The only fitting parameter in a dynamical simulation of conventional rocking curves is then the relative thickness ratio of the Si and Ge layers in the superlattice. The strain relaxation process and the principle of tailoring the strain status in the electronically active layers are shown to be different in structures with single-step and step-graded buffers. For these superlattice samples the RELPs originating from the zeroth-order SLS reflection show significant mosaic broadening (full width at half-maximum of 1100–1300 arcsec perpendicular to the ω/2aitch-theta scan direction in reciprocal space). In contrast, the corresponding RELPs from a pseudomorphic SLS with a much higher average Si content (Si17Ge2), but consisting just of 10 periods, which was grown directly on a Si buffer layer deposited on (001)-oriented Si substrate, are not broadened (full width at half-maximum of 14 arcsec). Besides the determination of strain and composition, examples for the interpretation of diffuse x-ray scattering are given, conveniently measured by reciprocal space mapping.

Notes: X-ray reciprocal space mapping around the symmetrical (004) Bragg reflection and a kinematical x-ray diffraction model were employed in order to determine the geometry and the structural perfection of surface corrugations or quantum wires. This method was used for the analysis of (001) Cd1−xZnxTe surface corrugations fabricated by holographic lithography and subsequently reactive ion etched with typical periods of 500 nm. Comparison of the measurement and simulation provides conclusive information on etching depth, wire period, wire width, and the inclination of the side walls. Furthermore, the analysis yields a parameter that contains information on side wall roughness, shape fluctuations and, in principle, the crystallographic damage caused by the reactive ion etching process. Due to the high resolution of triple axis diffractometry small strain gradients are observable in the damaged region. © 1994 American Institute of Physics.

Notes: Molecular beam epitaxial growth of PbTe/EuTe superlattices on (111) BaF2 was studied using reflection high energy electron diffraction (RHEED). The surface reconstructions of EuTe, its nucleation and critical layer thickness on (111) PbTe were investigated by in situ RHEED. From these studies, the optimum conditions for the growth of PbTe/EuTe superlattices were derived. PbTe/EuTe superlattice (SL) samples were investigated by high resolution x-ray diffraction. Triple axis x-ray diffractometry was employed to characterize the structural parameters of a PbTe/EuTe SL sample (87.6 PbTe monolayers/5 monolayers EuTe) deposited on a 4 μm thick PbTe buffer layer. Reciprocal space maps around the (222) and (264) Bragg reflections are used to analyze the strain status of the SL layers with very high precision. Using reciprocal space mapping, the small strain gradient present within the SL along growth direction can be determined quantitatively. The full width at half-maximum (FWHM) values of (222) SL x-ray diffraction peaks along and perpendicular to the [111] growth direction were measured. It was found that along the [111] direction the FWHM's increase with satellite number, which is most probably caused by either lateral superlattice thickness variations of 0.4% for the x-ray spot size of 1×8 mm2 or a thickness variation in growth direction of 1.5%. The broadening of the main superlattice peak perpendicular to the [111] growth direction indicates the presence of mosaicity in the superlattice layers, which obscures due to its magnitude the measurement of lateral interface roughness.

Notes: An x-ray diffraction study of the growth of Pb1−xMnxTe/PbTe (x=0.022–0.037) multi-quantum well (MQW) structures of period lengths 205–950 A(ring) is presented. In order to obtain a reliable Mn diffusion coefficient at the growth temperature T=270 °C, computed diffusion profiles are used as input for the calculation of diffractograms using kinematical diffraction theory. The satellites of the symmetric (222) Bragg reflections are compared to measurements. The analysis is based on a numerical solution of the diffusion equation which considers explicitly the effect that the boundary at the surface of the growing epitaxial layer moves during growth. Thus for the as-grown MQWs Mn interdiffusion is considered to occur already during the growth process. In addition, the Mn diffusion profiles of one sample which is isothermally annealed 4, 16, and 64 min at T=270 °C, are simulated using fixed boundary conditions. From comparison to both boundary conditions, the Mn diffusion coefficient of 8×10−17 cm2/s at T=270 °C is determined within an accuracy of a factor two. The results are well adapted to our special growth conditions, but the numerical model is applicable in generality. It uses only two essential input parameters: the diffusion coefficient and the growth rate.

Notes: High-resolution x-ray reciprocal space mapping was employed to determine the in-depth strain distribution of Si1−xGex films with linear composition gradings between 4.2% and 15% Ge per μm, and thicknesses between 0.4 and 1.7 μm. The variation of grading and thickness parameters of the samples provides a complete picture of the overall relaxation behavior of linearly graded epilayers. The x-ray data show a top layer of grading-dependent residual strain whereas the lower parts of the films are completely and/or partly relaxed with respect to the Si substrate. © 1995 American Institute of Physics.

Notes: X-ray diffraction in thin layers containing small randomly placed defects is described by means of the kinematical diffraction theory and optical coherence formalism. The method enables us to calculate both the diffracted intensity and its angular distribution, so that it can be used for simulating double crystal and triple crystal x-ray diffractometry experiments. The theory has been applied to experimental data obtained from diffractometry measurements of an epitaxial ZnTe layer with mosaic structure after several steps of chemical thinning. A good agreement of the theory with experiments has been achieved.

Notes: Periodic arrays of 150 and 175 nm-wide GaAs–AlAs quantum wires and quantum dots were investigated, fabricated by electron beam lithography, and SiCl4/O2 reactive ion etching, by means of reciprocal space mapping using triple axis x-ray diffractometry. From the x-ray data the lateral periodicity of wires and dots, and the etch depth are extracted. The reciprocal space maps reveal that after the fabrication process the lattice constant along the growth direction slightly increases for the wires and even more so for the dots. © 1995 American Institute of Physics.

Notes: Strain and microstructure of porous silicon on (001) wafers with different porosity were investigated by triple axis x-ray diffractometry using an instrumental resolution of 12 arcsec. The Bragg diffraction peaks arising from the porous Si contain information both on the mean strain (1.29–2.95×10−3) and on strain gradients (0.70–1.42×10−3) in these samples. In specimens with a porosity of 60% the pores are shown to be elongated over about 200 nm along the [001] direction, and over 50 nm in directions parallel to the growth plane. It is demonstrated that the correlation function for the pores has an extension along the [001] direction which is about a factor of 4 larger than along the [010] direction. From measurements of the intensity distribution of diffuse x-ray scattering a crystallographical damage in the silicon skeleton can be excluded. Regions of the porous layer near the interface to air are shown to be tensilely strained both along and perpendicular to the substrate normal.

Notes: Double crystal and triple axis x-ray diffractometry was used to characterize the structural properties of short period Si6Ge4 superlattices grown by molecular beam epitaxy on either a thin (20 nm) single step Si0.6Ge0.4 alloy buffer or on a thick step-graded Si1−xGex(0〈x〈0.4, 700 nm thick) buffer followed by a 550 nm Si0.6Ge0.4 layer. Reciprocal space maps around the (004) and (224) reciprocal lattice points yield direct information on the strain status of the buffer and superlattice layers. For the thick step-graded buffer indeed all layers with different Ge content are fully relaxed and thus the growth of an almost freestanding superlattice is possible.

Notes: Triple axis x-ray diffractometry was employed for the structural characterization of a 100 period Si9Ge6 superlattice grown by molecular beam epitaxy on a thick step-graded SiGe alloy buffer. From the distribution of diffusely scattered intensity around reciprocal lattice points the correlation function of the deformation field due to structural defects has been calculated using kinematical theory of x-ray diffraction. From the extension of the correlation function it turns out that on the average the entire superlattice (0.2 μm thick) scatters coherently along growth direction, whereas laterally the coherently scattering regions are extended only over about 40 nm.