H. Kim

Lattice and strain analysis of atomic resolution Z-contrast images based on template matching

A real space approach is developed based on template matching for quantitative lattice analysis using atomic resolution Z-contrast images. The method, called TeMA, uses the template of an atomic column, or a group of atomic columns, to transform the image into a lattice of correlation peaks. This is helped by using a local intensity adjusted correlation and by the design of templates. Lattice analysis is performed on the correlation peaks. A reference lattice is used to correct for scan noise and scan distortions in the recorded images. Using these methods, we demonstrate that a precision of few picometers is achievable in lattice measurement using aberration corrected Z-contrast images. For application, we apply the methods to strain analysis of a molecular beam epitaxy (MBE) grown LaMnO₃ and SrMnO₃ superlattice. The results show alternating epitaxial strain inside the superlattice and its variations across interfaces at the spatial resolution of a single perovskite unit cell. Our methods are general, model free and provide high spatial resolution for lattice analysis.

Kinetics of Si1−xGex(001) growth on Si(001)2×1 by gas-source molecular-beam epitaxy from Si2H6 and Ge2H6

Si1−xGex layers with x ranging from 0 to 0.30 were grown on Si(001)2×1 substrates at temperatures ranging from 450 to 950u2009°C by gas-source molecular-beam epitaxy (GS-MBE) from Si2H6 and Ge2H6. In the low-temperature surface-reaction-limited growth regime, the deposition rate RSiGe increases with increasing Ge concentration due to an enhancement in the hydrogen desorption rate resulting in a correspondingly higher steady-state dangling bond density. In the high-temperature impingement-flux-limited regime, where the steady-state hydrogen coverage approaches zero, RSiGe is controlled by the Si2H6 and Ge2H6 reactive sticking probabilities S which decrease with increasing Ge2H6 flux but are not strongly temperature dependent. SSi2H6 and SGe2H6 range from 0.036 and 0.28 on Si(001) to 0.012 and 0.094 during growth of Si0.82Ge0.18 at Ts=800u2009°C. In both growth regimes, large changes in RSiGe require only modest increases in incident Ge2H6 to Si2H6 flux ratios, JGe2H6/JSi2H6, due to Ge segregation which is strongly...

Electrochemically tunable thermal conductivity of lithium cobalt oxide

Using time-domain thermoreflectance, the thermal conductivity and elastic properties of a sputter deposited LiCoO2 film, a common lithium-ion cathode material, are measured as a function of the degree of lithiation. Here we report that via in situ measurements during cycling, the thermal conductivity of a LiCoO2 cathode reversibly decreases from ~5.4 to 3.7 W m(-1) K(-1), and its elastic modulus decreases from 325 to 225 GPa, as it is delithiated from Li1.0CoO2 to Li0.6CoO2. The dependence of the thermal conductivity on lithiation appears correlated with the lithiation-dependent phase behaviour. The oxidation-state-dependent thermal conductivity of electrolytically active transition metal oxides provides opportunities for dynamic control of thermal conductivity and is important to understand for thermal management in electrochemical energy storage devices.

Effects of H coverage on Ge segregation during Si1-xGex gas-source molecular beam epitaxy

The effects of H coverage θH on Ge segregation during Si1−xGex gas-source molecular beam epitaxy (GS-MBE) were investigated using D2 temperature programmed desorption (TPD). Si1−xGex films with x=0.01–0.30 were grown from Si2H6/Ge2H6 mixtures at Ts=450–800u2009°C, held at the growth temperature for 30 s, cooled to <200u2009°C, and then exposed to atomic deuterium until saturation coverage. D2 TPD spectra were fit using four peaks corresponding, in order of decreasing activation energy, to desorption from Si monodeuteride, Ge–Si mixed-dimer monodeuterides, Si dideuteride, and Ge monodeuteride. Steady-state Ge surface coverages were determined from the TPD data as a function of Ts and x. In contrast to solid-source MBE films grown in this temperature regime, Ge segregation during GS-MBE decreases with decreasing Ts due to the increasing H coverage. The results were well described by a model accounting for the Si/Ge site exchange and θH. The Ge segregation enthalpy varies from −0.28u2009eV at Ts⩾800u2009°C, where the steady...

Atomic resolution mapping of interfacial intermixing and segregation in InAs/GaSb superlattices: A correlative study

We combine quantitative analyses of Z-contrast images with composition analyses employing atom probe tomography (APT) correlatively to provide a quantitative measurement of atomic scale interfacial intermixing in an InAs/GaSb superlattice (SL). Contributions from GaSb and InAs in the Z-contrast images are separated using an improved image processing technique. Correlation with high resolution APT composition analyses permits an examination of interfacial segregation of both cations and anions and their incorporation in the short period InAs/GaSb SL. Results revealed short, intermediate, and long-range intermixing of In, Ga, and Sb during molecular beam epitaxial growth and their distribution in the SL.

Si(001):B gas-source molecular-beam epitaxy: Boron surface segregation and its effect on film growth kinetics

B-doped Si(001) films, with concentrations CB up to 1.7×1022u2009cm−3, were grown by gas-source molecular-beam epitaxy from Si2H6 and B2H6 at Ts=500–800u2009°C. D2 temperature-programed desorption (TPD) spectra were then used to determine B coverages θB as a function of CB and Ts. In these measurements, as-deposited films were flash heated to desorb surface hydrogen, cooled, and exposed to atomic deuterium until saturation coverage. Strong B surface segregation was observed with surface-to-bulk B concentration ratios ranging up to 1200. TPD spectra exhibited β2 and β1 peaks associated with dideuteride and monodeuteride desorption as well as lower-temperature B-induced peaks β2* and β1*. Increasing θB increased the area under β2* and β1* at the expense of β2 and β1 and decreased the total D coverage θD. The TPD results were used to determine the B segregation enthalpy, −0.53u2009eV, and to explain and model the effects of high B coverages on Si(001) growth kinetics. Film deposition rates R increase by ⩾50% with increa...

Dielectric functions and electronic structure of InAsxP1- x films on InP

The authors present room-temperature pseudodielectric function spectra ⟨e⟩ of InAsxP1−x films grown on (001) InP by chemical beam epitaxy. A wet-chemical etching procedure was used to remove overlayers and obtain the best approximation to the bulk dielectric responses e of the films. By line shape fitting, we determine the compositional dependences of the E1, E1+Δ1, E0′, E2, and E2′ critical-point energies. The results are in good agreement with calculations of the electronic structure done by the linear augmented Slater-type orbital method.

Structure of the Si(011)-(16 × 2) surface and hydrogen desorption kinetics investigated using temperature-programmed desorption

Abstract D 2 temperature-programmed desorption (TPD) was used to probe the structure of the Si(011)-(16 × 2) surface. Deuterium was adsorbed at 200°C to coverages θ D ranging up to complete saturation (approximately 1.1 ML) and the sample heated at 5°C s −1 . TPD spectra exhibited three second-order desorption peaks labelled β 2 , β∗ 1 and β 1 centered at 430, 520 and 550°C. Of the proposed models for the Si(011)-(16 × 2) reconstruction, the present TPD results as a function of θ D provide support for the adatom/dimer model with the β 2 peak assigned to D 2 desorption from the dihydride phase, while the β∗ 1 and β 1 peaks arise from adatom and surface-atom monohydride phases.

Ge(011)-c(8×10) surface structure and hydrogen desorption pathways: a temperature-programmed desorption and scanning tunneling microscopy study

Temperature-programmed desorption ( TPD) and scanning tunneling microscopy (STM ) were used to probe the atomic arrangement on clean Ge(011)-c(8◊10), as well as the desorption kinetics and pathways from hydrogenadsorbed surfaces. For the TPD measurements, the samples were heated at 2° Cs ’1 after adsorbing atomic deuterium at 100°C to coverages h D ranging up to saturation. Low-energy electron diVraction (LEED) and STM show that saturation deuterium coverage results in a (1◊1) structure with the surface composed of randomly distributed adatom islands. TPD spectra exhibit three second-order peaks corresponding to D 2 desorption from multideuterides, adatom monodeuterides and rest-atom monodeuterides. Desorption from the multideuteride phase (with an activation energy E a of 1.61 eV ) begins at 200°C and, by 270°C, only the D‐adatom and D‐rest-atom monodeuteride phases remain. D 2 begins to desorb from adatom sites (E a =1.76 eV ) above 230°C and from rest-atom sites (E a =1.83 eV ) above 240°C. From quantitative analyses of the TPD spectra, the adatom density on the clean surface is µ0.47. This high adatom density — similar to that of Si(111)-(7◊7), Si(011)-(16◊2) and Ge(111)-c(2◊8), all of which contain adatoms and rest atoms as primary building blocks — appears to rule out previously proposed models for the Ge(011)-c(8◊10) surface structure, for which the adatom density is 0.064.

Carbon incorporation pathways and lattice sites in Si1−yCy alloys grown on Si(001) by molecular-beam epitaxy

We use a combination of in situ and postdeposition experimental probes together with ab initio calculations of strain coefficients and formation energies associated with specific C configurations in the Si lattice to determine C incorporation pathways and lattice site distributions in fully coherent Si1−yCy alloy layers grown by molecular-beam epitaxy on Si(001) as a function of deposition temperature Ts (380u200a°C–680u200a°C) and C fraction y (0–0.026). Lattice strain and Raman spectroscopy measurements demonstrate that all C, irrespective of y, is incorporated into substitutional lattice sites in Si1−yCy(001) layers grown at Ts⩽480u200a°C. Increasing Ts⩾580u200a°C leads to strong C surface segregation, as shown by in situ angle-resolved x-ray photoelectron spectroscopy, yielding additional pathways for C incorporation. Photoluminescence measurements indicate that an increasing fraction of the incorporated C in the higher-temperature layers resides in dicarbon complexes. Reflection high-energy electron diffraction and ...