Chemical Lattice Imaging Research Articles

Atomic-scale characterization of perovskite superlattice using chemical lattice imaging and spatially resolved electron energy-loss spectroscopy

The layered structure of a perovskite tricolor superlattice {SrTiO3(STO)∕LaAlO3(LAO)∕La0.6Sr0.4MnO3(LSMO)}n on an STO substrate is investigated using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Conventional TEM imaging is not effective for thickness evaluation, in which both LAO and LSMO layers show the same image contrast because of the similarity in their mean atomic numbers. We observe 001 lattice fringes, which are chemically sensitive in the case of the perovskite structure ABO3, since the 001 structure factor is proportional to the difference between A- and B-site atomic scattering factors. The thickness of each layer is evaluated with an accuracy of one unit cell. We also apply spatially resolved EELS to analyze the depth profile of Ti, Mn, and La contents.

High-resolution transmission electron microscopy studies of quantum wire structures grown on submicrometre gratings of V-grooves

We present an extensive characterization of a quantum wire (QWR) structure using transmission electron microscopy (TEM). The structure consisted of a single GaAs layer in between AlGaAs barriers, grown on a GaAs substrate patterned with a submicrometre grating of V-grooves. For reference we also studied other QWR, as well as, quantum well (QW) samples, fabricated under similar conditions. We used bright field and dark field imaging to study the overall structure, high-resolution TEM to study the layer thickness and the interface quality, and chemical lattice imaging to study the compositional variations across the interfaces. In the QWR sample, there were mainly two distinctly different areas of the QW: on the (100) planes between the V-grooves, the QW was flat, whereas the QW on the near side walls of the V-grooves had a flat lower interface and a saw-tooth shaped upper interface. The QWRs at the bottom of the V-grooves were crescent shaped. We also observed a fundamental difference in growth of the GaAs and the AlGaAs on the side wall, where the AlGaAs formed straight interfaces, determined by high-index planes, whereas the GaAs tended to form alternating low-index planes giving a saw-tooth appearance of the GaAs QW.

Investigation of interface intermixing and roughening in low-temperature-grown AlAs/GaAs multiple quantum wells during thermal annealing by chemical lattice imaging and x-ray diffraction

The effect of thermal annealing on the interface quality in undoped, AlAs/GaAs multiple quantum well (MQW) structures grown at a low substrate temperature (310 °C) by molecular beam epitaxy has been investigated using chemical lattice imaging and high resolution x-ray diffraction. The low-temperature-grown MQW is of high crystalline quality comparable to the standard-temperature-grown MQW. However, significant interface roughening and intermixing occurs at the quantum well heterointerface when the structures are annealed beyond 700 °C. The effective activation energy for interdiffusion is estimated as 0.24±0.07 eV. The structural properties observed here suggest that the excess arsenic associated with the low-temperature growth substantially enhances the diffusion of column III vacancies across an interface, which leads directly to intermixing of Al and Ga.

Quantitative compositional mapping of InxGa1-XAs/GaAs quantum wells by annular dark field imaging

Annular dark field imaging in the scanning transmission electron microscope (STEM) exhibits both high resolution and Z-contrast. It is intrinsically quantitative since image data can be recorded directly from linear detectors into digital memory. Annular dark field imaging has been used, along with energy filtered imaging to correct for sample thickness variation, to map out the In concentration in InxGa1-xAs quantum wells with near atomic resolution and sensitivity. This approach is similar to “chemical lattice imaging”, which maps out composition variation using a conventional transmission electron microscope image and a vector pattern recognition algorithm.The quantum wells were grown by molecular-beam epitaxy (MBE). Figure 1 shows a typical high resolution annular dark field image of a 50 Å wide nominal In0.3Ga0.7As/GaAs quantum well. The linescan in figure 2 gives the actual numbers making up the image. Barring contaminants and lattice imperfections, the change in intensity with position is caused by two things: variation of In concentration and thickness.

High-resolution transmission electron microscopy of heterostructures

Chemical information on a near-atomic scale may be obtained from high-resolution transmission electron microscope (HREM) images. This requires first a distinct contrast difference in the adjacent semiconductor materials in a particular projection direction for a wide range of objective lens defoci and specimen thicknesses for the given microscope parameters (acceleration voltage, spherical aberration constant). Second, the image must contain Fourier components which behave almost linear with layer composition. The contrast formation in the systems Al/GaAs and Si/Ge, where such conditions may be found and which are representative for amplitude and phase contrast respectively, is discussed. The method of chemical lattice imaging is applied to Al x Ga 1− x As structures and reveals differences in the abruptness of GaAs on Al x Ga 1− x As and Al x Ga 1− x As on GaAs interfaces. Selected area diffraction systems with high accuracy.

Characterization of GaAs/AlGaAs quantum wells and quantum wires by chemical lattice imaging

In this study a quantum well/quantum wire (QW/QWR) structure grown on a grating of V-grooves has been characterized by a technique related to chemical lattice imaging. This technique makes it possible to extract quantitative information from high resolution images.The QW/QWR structure was grown on a GaAs substrate patterned with a grating of V-grooves. The growth rate was approximately three monolayers per second without growth interruption at the interfaces. On this substrate a barrier of nominally Al0.35 Ga0.65 As was deposited to a thickness of approximately 300 nm using metalorganic vapour phase epitaxy . On top of the Al0.35Ga0.65As barrier a 3.5 nm GaAs quantum well was deposited and to conclude the structure an additional approximate 300 nm Al0.35Ga0.65 As was deposited. The GaAs QW deposited in this manner turns out to be significantly thicker at the bottom of the grooves giving a QWR running along the grooves. During the growth of the barriers an approximately 30 nm wide Ga-rich region is formed at the bottom of the grooves giving a Ga-rich stripe extending from the bottom of each groove to the surface.

Mapping the composition of materials at the atomic level

This paper briefly outlines how transmission electron microscopic lattice imaging techniques can be used to map the composition of crystalline materials at the atomic level.Under appropriate conditions, a conventional lattice image is a map of the sample structure, because the dominant reflections used to form lattice images are relatively insensitive to compositional changes in the sample. Such reflections may be termed “structural”. In many cystalline materials, compositional changes occur by atomic substitution on a particular subset of lattice sites. In these systems, compositional changes are accompanied by the appearance of reflections, which we name “chemical”. Such reflections, for example the (200) in the zinc-blende structure, owe their existence to chemical differences between the various atomic species present on the different lattice sites. For fundamental reasons these reflections are often weak; they come about because of incomplete cancellation of out of phase contributions from different sublattices. “Chemical lattice imaging” exploits dynamical scattering to maximize the intensity of such reflections, and uses the objective lens as a bandpass filter to enhance their contribution to the image.

Chemical lattice imaging of a Ni-based superalloy

Abstract We demonstrate the application of quantitative chemical lattice imaging techniques to a metallic system. The first results establish that the chemical interface between γ and γ′ phases of a nickel-based superalloy can be quantitatively identified.

Chemical mapping of materials at the atomic level

Under appropriate conditions, a conventional lattice image is a map of the sample structure, because the dominant reflections used to form lattice images are relatively insensitive to compositional changes in the sample. However, certain reflections, such as the (200) in the zinc-blende structure, owe their existence to chemical differences between the various atomic species present on the lattice sites. Such chemical reflections arc sensitive to compositional changes in the sample. For fundamental reasons such reflections are often weak; they come about because of incomplete cancellation of out of phase contributions from different sublattices. “Chemical lattice imaging” exploits dynamical scattering to maximize the intensity of such reflections, and uses the objective lens as a bandpass filter to enhance their contribution to the image.

Quantitative chemical mapping: Spatial resolution

Quantitative Chemical Mapping is an electron microscopic technique capable of revealing compositional variations in crystalline materials. It combines chemical lattice imaging which maps the sample composition, with vector pattern recognition, which quantifies the local information content of the image to measure the local sample composition. Here we briefly address the spatial resolution of this technique, assuming complete familiarity with its theoretical underpinnings.In chemical imaging, we are concerned with the way that a compositional inhomogeneity is imaged under conditions appropriate for chemical sensitivity, and how the pattern recognition algorithm extracts information from a chemical lattice image. The problem can be formulated as follows. Given a “chemical impulse” of a specific shape, such as a column of Al atoms imbedded in GaAs (approximating a δ-function), an abrupt interface (a θ-function), or a diffuse interface (e.g., with an error function profile), what is the shape of the impulse on the analyzed chemical image? Or, alternatively, what region of the sample contributes to the information content of an image unit cell? By reciprocity, these two formulations are equivalent.

Quantitative chemical lattice imaging: theory and practice

We describe how the composition of materials may be quantitatively mapped with near-atomic resolution and sensitivity. The procedure combines chemical lattice imaging (which sensitively records the compositional information) with vector pattern recognition (which efficiently extracts and quantifies the local information content). We describe the theoretical underpinnings which allow the local information content to be related to the local sample composition, and the procedure by which this may be realized in practice. Single and double atom substitutions in individual atomic columns of semiconductors can thus be detected at about 60% and 90% confidence levels, respectively. Potential complications due to nonlocal effects (Fresnel fringing, dynamical scattering), geometrical sample imperfections (thickness changes, surface roughness), radiation damage, and photographic nonlinearities are shown to be negligible.

Diffusion in multilayers, studied by quantitative chemical mapping at the atomic level

Using chemical lattice imaging in combination with vector pattern recognition, we study diffusion and interdiffusion at single interfaces in mulilayered semiconductors quantitatively with atomic plane resolution. 20 periods of 50Å C-doped GaAs/50Å undoped GaAs/50Å Al0.4Ga0.6As grown by Molecular Beam Epitaxy(MBE) at 600C was examined to study interdiffusiuori across single GaAs/AlGaAs interfaces as a function of temperature (650C to 750C), depth of interface beneath the surface, and doping . After annealing in bulk form, cross-sectional TEM samples were prepared chemically. Chemical lattice images in the <100> zone axis obtained at an accelerating voltage of 400KV with a JEOL 4000-EX high resolution transmission electron microscope were analyzed by a digital pattern recognition method to obtain composition profiles at each interface before and after annealing.Figure 1 is a chemical lattice image of a C:GaAs/AlGaAs interface. The compositional information in the sample is contained in the local patterns that make up such images. The analysis of such chemical images by the pattern recognition yields quantitative composition profiles across single interfaces. The composition profiles shown in Figure 2 refer to a single interface at a depth of 300Å, before and after annealing at 700C for one hr.

Does luminescence show semiconductor interfaces to be atomically smooth?

Luminescence spectra from quantum wells are routinely interpreted in terms of atomically smooth and atomically abrupt interfaces. Here we show that this interpretation is inconsistent with photoluminescence, photoluminescence excitation, and quantitative microscopic (chemical lattice imaging) results. We argue that the discussion of interfacial roughness in terms of ‘‘an island size’’ is too naive. A full characterization of an interface requires the description of a ‘‘roughness spectrum,’’ specifying the amplitude of the interfacial corrugation versus corrugation wavelength over the relevant length scale.

Low temperature interdiffusion in the HgCdTe/CdTe system, studied at near-atomic resolution

By combining chemical lattice imaging and vector pattern recognition we determine, as a function of annealing temperature, the composition of individual atomic planes across each HgCdTe/CdTe interface of a multiquantum well stack. The resultant composition profiles, which directly reveal the chemical change across each interface at near atomic resolution, are analyzed in terms of linear and nonlinear diffusion theory, to deduce the interdiffusion coefficient and its activation energy. We find the interdiffusion coefficient to be nonlinear, and a sensitive function of the interface depth beneath the surface.

Atomically Abrupt and Smooth Heterointerfaces: An Optical Investigation

ABSTRACTLuminescence spectra from quantum wells are routinely interpreted in terms of atomically smooth and atomically abrupt interfaces. Here we show that this interpretation is inconsistent with photoluminescence, photoluminescence excitation, and quantitative microscopic (chemical lattice imaging) results. We argue that the discussion of interfacial roughness in terms of “an island size” is too naive. A full characterization of an interface requires the description of a “roughness spectrum”, specifying the amplitude of the interfacial corrugation vs corrugation wavelength over the relevant length scale.

Stability of Multilayered Semiconductor Systems

ABSTRACTWe have studied the atomic scale relaxation of multilyered semiconductor systems through interdiffusion, and its implications for layer and hence device stability. Our technique combines chemical lattice imaging and vector pattern recognition to measure interdiffusion coefficients as small as 10-20 cm2/s at single AiGaAs/GaAs interfaces. Our results can be summarized as follows. (a) The interdiffusion coefficient and hence layer stability depend strongly on the distance from the top surface. (b) The strong influence of the surface stems from its role as the source of rative point defects (interstitials, vacancies) involved in the diffusion process. (c) The study of the initial transients in the diffusion process can directly yield the migration and formation energies of these defects as a function of their charge state. (d) The non-linear nature of the diffusion process can have serious consequences for the low temperature stability of multilayered materials and devices. We also discuss the implications of our results for the design and fabrication of stable multilayered systems.

Semiconductor interfaces: Abruptness, smoothness, and optical properties

The combination of chemical lattice imaging and digital pattern recognition now allows a quantitative determination of the compositional change across individual semiconductor heterointerfaces at near-atomic resolution. Results show previous conclusions regarding interfacial abruptness and smoothness not to be justified, and provoke detailed questions on the way interfacial structure influences the optical properties of multilayered structures.

Chemical mapping of semiconductor interfaces at near atomic resolution.

We combine chemical lattice imaging with digital pattern recognition to map, at near-atomic resolution, the compositional change across GaAs/AlGaAs interfaces of the highest optical quality. These maps quantify the information content of each unit cell of the lattice image. Our results show that state-of-the-art GaAs/AlGaAs interfaces contain substantial atomic roughness on scales finer than suggested by optical measurements.

Direct Observation of Intermixing in GAAS/AIAS Multilayers After Very Low-Dose Ion-Implantation

ABSTRACTWe combine chemical lattice imaging and digital vector pattern recognition to study quantitatively, kinetic intermixing in GaAs/AlAs multilayers. We thus obtain, with atomic plane resolution and near-atomic sensitivity, composition profiles across each interface of a multilayer stack before and after ion-implantation. Our results show significant intermixing even when only one 320 keV Ga+ ion is implanted at 77 K into each 2000 A2 area of the interface. This corresponds to an incident ion dose of 5×l012/cm2.The intermixing is not uniform along the interface. At each interface, we observe more intensely intermixed regions, whose widths correspond to those created by the damage track of a single implanted ion, as expected from Monte-Carlo simulations. It thus appears that we can directly image intermixing due to single energetic ions implanted into the multilayered GaAs/AlAs structure.

Interfacial Stability and Interdiffusion Examined at the Atomic Level

ABSTRACTUsing chemical lattice imaging in combination with vector pattern recognition, we obtain quantitative profiles of the chemical change across single interfaces with atomic plane resolution. We thus study interdiffusiuon across single GaAs/AlGaAs interfaces as a function of temperature, depth of interface beneath the surface, and doping. Since our technique is sensitive to interdiffusion coefficients as small as 10−20 cm2/s, we can study atomic level changes at a single interface at the low temperatures used for many device processing steps (∼700C). Our results show interdiffusion, and hence the layer stability depend not only on temperature and doping, but also on the distance of the interface from the surface. The implications of these results for the stability of multilayered structures are discussed.