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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: X-ray reciprocal space mapping has been used to investigate the strain status of microgun-pumped blue and blue-green laser structures. The devices exploit graded-index, separate confinement Zn1−xCdxSe/ZnSe heterostructures grown on InGaAs or GaAs substrates by molecular-beam epitaxy. The location of the reciprocal lattice point of the ZnSe buffer layer within a normally forbidden region of reciprocal space indicates that the ZnSe buffer layer is unusually strained, with an appreciable biaxial tensile strain despite the smaller lattice parameter of the III–V substrate relative to ZnSe. We associate such a phenomenon with the presence of the highly strained laser structure coupled with preferential strain relaxation at the II–VI/III–V heterointerface. © 1996 American Institute of Physics.

Notes: Molecular beam epitaxy of PbTe on BaF2 (111) is studied using reflection high-energy electron diffraction (RHEED). The influence of growth parameters (substrate temperature and growth rate) on surface kinetics and the steady-state growth surface morphology is investigated employing dynamical RHEED measurements (RHEED oscillations). For a well adjusted stoichiometric PbTe beam flux composition, two-dimensional layer-by-layer growth can be achieved from substrate temperatures as high as 410 °C down to temperatures below 95 °C, with a maximum number of 230 RHEED oscillations observed at substrate temperatures in the 160 °C range. At temperatures above 400 °C, the growth kinetics start to be modified by PbTe reevaporation from the layer surface. The dependence of the RHEED oscillations on substrate temperature and growth rate indicates the importance of adatom surface diffusion for the surface morphology developed under steady-state growth conditions, and for all growth conditions, a close correlation between steady-state growth surface step density and damping of RHEED oscillations is observed. Furthermore, it is shown that even very small changes in the beam flux composition have a dramatic influence on the RHEED intensity oscillations as well as the surface processes involved in the growth. With only a small additional Te2 flux used for the growth, an abrupt growth mode transition from layer-by-layer to step flow growth is induced. This is the first evidence that PbTe molecules impinging on the layer surface do not dissociate upon adsorption, but remain in a molecular state until incorporated in the crystal lattice. © 1995 American Institute of Physics.

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: We report neutron diffraction studies of magnetic correlations in MBE-grown (111) EuTe/PbTe multilayers. Bulk EuTe is a type-II fcc antiferromagnet. This structure consists of ferromagnetic (FM) sheets of spins on (111)-type planes which are coupled antiferromagnetically to one another. In EuTe/PbTe the lattice mismatch strain selects an arrangement of the FM sheets parallel to the multilayer plane. Thus, in the case of an odd number of EuTe monlayers, there should be a nonzero magnetic moment—in other words, such superlattices are expected to behave as ferrimagnets, not anitferromagnets. This has been confirmed by recent SQUID studies. As suggested by the above model, the dipolar fields arising from the lack of total moment compensation may introduce interlayer magnetic coupling, even though in this system there are no carriers that can transfer magnetic interactions across the nonmagnetic spacers through the RKKY mechanism, which is usually the case in coupled magnetic multilayered systems. Our experiments have indeed revealed clear magnetic superlattice peak patterns in several samples. The results of measurements in high external fields (up to 6 T) bring further support for the dipolar nature of the observed interlayer coupling. However, in some contrast to the expected scenario, the most pronounced coupling effects were seen in the case of an even number of monolayers. Possible explanations of this fact (e.g., higher-order terms in dipolar fields, or structural imperfections) are discussed.

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: A theory has been developed describing x-ray diffuse scattering from misfit dislocations in epitaxial layers. This approach has been used for explaining the origins of diffuse x-ray scattering from SiGe layers with linearly graded Ge content. The distribution of the diffusely scattered intensity in reciprocal plane measured by triple-axis x-ray diffractometry has been compared with theoretical predictions and a good agreement has been achieved. It is demonstrated that the main part of the diffusely scattered intensity originates from random strains caused by misfit dislocations at the substrate–epilayer interface or in the relaxed part of the compositionally graded layers. The contribution of the threading dislocation segments to the diffuse scattering is rather small. © 1995 American Institute of Physics.

Notes: Mn-VI/II-VI superlattices belong to the family of artificial multilayered structures composed from zinc-blende II-VI semiconducting compounds and Mn chalcogenides using MBE and ALE. While all naturally existing crystals of MnTe have the NiAs (hexagonal) structure, the zinc-blende form is a fcc Heisenberg antiferromagnet with dominant nearest neighbor interactions. The magnetic properties of such frustrated systems are strongly influenced by lattice mismatch strain. For example, tensile strain produces an incommensurate helical antiferromagnetic phase with an in-plane axis. The strain and therefore helical period increases with thickness of CdTe, but in the presently studied samples is weak enough to allow investigation of the onset of incommensurate helical effects. In previously studied MBE-grown Mn-VI/Zn-VI systems, the nonmagnetic spacers were too thick to allow interlayer coupling. We report neutron diffraction studies of new MnTe/CdTe superlattices prepared by ALE with extremely thin CdTe spacers (from 2 to 6 monolayers). The results indicate the formation of spin helices, consistent with the tensile nature of strain in the MnTe layers. In addition, the widths of the AF diffraction peaks and the presence of satellite peaks clearly indicate that the magnetic interactions propagate through the CdTe spacers, introducing coherence between the helices in different MnTe layers. The nature of the interaction responsible for this transfer is not yet clearly understood. Some possible mechanisms are discussed.