Cross-sectional High-resolution Electron Microscopy Research Articles

Gold films epitaxially grown by diffusion at the 3C–SiC/Si interface

A thin layer of gold grown epitaxially in a 3C–SiC/Si interface is observed using conventional (CTEM) and high-resolution electron microscopy (HREM) methods. This interlayer was formed after an extended thermal treatment, at 500°C for 500 h, of a 3C–SiC/Si system on which Au was deposited on the surface of SiC for the formation of a Schottky diode. Misfit dislocations are observed in both SiC/Au and Au/Si interfaces having Burgers vectors b= 1 2 〈1̄ 1 0〉 parallel to the interfaces. Such an epitaxy is made in spite of the very high misfit in the Au/Si interface. The net of the misfit dislocations is visible only by cross-section HREM observations. The position of the extra half-planes of the interfacial dislocations is illustrated. An explanation for the mechanism of the Au mass transport through the unaffected SiC film is given. This is attributed to pipe diffusion through the partial dislocations bounding the stacking faults and inversion domain boundaries that pre-exist in 3C–SiC. The same diffusion mechanism allows the transport of Si from the 3C–SiC/Si interface to the top surface of SiC on which islands of Si and Au are formed.

Electrically conducting oxide thin films of (Sr,Ca)RuO 3 and structural compatibility with (Ba,Sr)TiO 3

Electrically conducting oxide thin films, Ca xSr 1−xRuO 3, where x varies from 1 to 0, were prepared by radio frequency (RF) magnetron sputtering, and their structural compatibility with (Ba,Sr)TiO 3 thin films was investigated. It was found that both materials crystallize into the perovskite structure, and the lattice parameter for Ca xSr 1−xRuO 3 can be tuned to that of (Ba,Sr)TiO 3 by adjusting the Ca/Sr ratio, so that the compatibility between the two materials at the interface is increased. Structural, chemical, and electrical properties of the thin films and the heterostructures were characterized. Cross-sectional high-resolution electron microscopy (HREM) revealed that the (Ba,Sr)TiO 3 thin film was grown epitaxially on the (Ca,Sr)RuO 3. The potential utility of Ca xSr 1−xRuO 3 as a bottom electrode for (Ba,Sr)TiO 3 is suggested.

Interface Characterization of AlN/TiN/MgO(001) Prepared by Molecular Beam Epitaxy

The AlN/TiN/MgO(001) interfaces, prepared by molecular beam epitaxy, have been characterized by cross-sectional high-resolution electron microscopy (HREM). The thin TiN buffer layer, with the thickness of 40 nm, is epitaxially grown on the MgO(001) substrate. Owing to the same structure-type as well as the small mismatch of their lattice parameters, the growth is governed by the parallel orientation relationship of (001)TiN||(001)MgO, (010)TiN||(010)MgO, and (111)TiN||(111)MgO. Two kinds of processes of the hexagonal AlN epitaxial growth on the as-received TiN(001), differed by the (0001)AlN plane parallel to, and the (1012) plane approximately parallel to the MgO substrate surface, respectively, are identified, and within them, several cases are classified which are based on the consideration of crystallographic symmetry. Theoretical calculations based on the geometrical model that was recently proposed and applied to a number of epitaxial systems have been carried out to rationalize these observations.

Mo/SiO 2 multilayers for soft x-ray optical applications

The optimal reflectivity of soft-x-ray multilayers made of new pairs of materials was theoretically calculated in the wavelength range of 2.0–4.5 nm. Molybdenum and silicon dioxide were then selected for “high index” and “low index” layer, respectively, in the multilayer system. The microstructures and composition profiles of multilayers of molybdenum and silicon dioxide were investigated by means of low-angle x-ray diffraction, cross-sectional high resolution electron microscopy, an energy-filtering transmission electron microscope elemental mapping, and Auger electron spectrometry. The results show that no diffusion of Si into Mo layers has occurred, and only slight diffusion of O into Mo layers are seen. Sharp and relatively smooth interfaces have formed. With increasing number of layers, the interfacial roughness was propagated through the multilayer stack and low-frequency roughness increases while the high-frequency roughness decreases.

Characterization of the YbBa2Cu3O7-y and YBa2Cu3O7-y thin superconducting films prepared by chemical solution deposition on MgO(001) substrate.

Thin superconducting films of the YbBa2Cu3O7-y (Yb123) and YBa2Cu3O7-y (Y123), prepared by post-deposition annealing of the metal naphthenates gels spin-coated on MgO(001) substrate, have been characterized by cross-sectional high-resolution electron microscopy and X-ray diffraction. It was found that the c-axis Yb123 films were epitaxially grown on the MgO(001) substrate at the temperature range from 700 degrees C to 775 degrees C in a gas mixture containing Ar and O2 with the oxygen partial pressure of p(O2) = 10(-4) atm. In contrast to the Yb123 films, it was found that Y123 films could be derived at a wider temperature range from 750 degrees C to 950 degrees C. Randomly oriented Y123 films were also grown on the MgO(001) substrate besides the majority of in-plane c-axis oriented growth.

Raman scattering and high resolution electron microscopy studies of metal-organic chemical vapor deposition-tungsten disulfide thin films

Metal-organic chemical vapor deposition (MOCVD) using tungsten hexacarbonyl (W(CO) 6) and H 2S as precursors was employed to prepare tungsten disulfide (WS 2) thin films on Si substrates. Various degrees of preferred orientation as characterized by the WS 2 crystallites with their basal planes grown parallel to the substrate ( c(=)) and non-parallel to the substrate ( c(∥)) were obtained. Raman scattering was applied to study the effects of preferred orientation in WS 2 thin films on the Raman processes. As the fraction of c(∥) crystallites increases in the thin film structure, a large second order Raman peak appeared on a shoulder of the A 1g mode to the lower wavenumbers. Based on the analysis previously provided (C. Sourisseau, F. Cruege, M. Fouassier, Chem. Phys. 150 (1991) 281), this peak was characterized by two phonon-coupling originating from longitudinal acoustic (LA) and transverse acoustic (TA) phonons at the K point of the Brillouin zone. Cross-section high resolution electron microscopy (HREM) revealed that the WS 2 thin films grew with c(=) near the interface, followed by the formation of non-parallel crystallite ( c(∥)) co-mixing with c(=). Transition from c(=) to c(∥) in the microstructure occurred smoothly, showing curvature in the lattice image. Occurrence of the localized curvature in the thin film microstructure seems to be responsible for the enhancement of the two phonon-coupling process.

Microstructure characterization of YBa 2Cu 3O 7− y thin film derived from the metal (Y, Ba, and Cu) naphthenates gels coated on the SrTiO 3 (100) substrate

By postdeposition annealing at temperature range from 650–850°C in the gas mixture containing O 2 and Ar with the oxygen partial pressure of p(O 2)=10 −4 atm, thin films of YBa 2Cu 3O 7− y (Y123) have been derived from the metal (Y, Ba, Cu) naphthenates gels spin-coated on the SrTiO 3 (100) substrate. The film derived at 700°C has been characterized by means of cross-sectional high resolution electron microscopy (HREM) so as to clarify the details of the microstructures in the derived films. The epitaxial c-axis, a-axis and then nonepitaxial Y123 polycrystal layers were observed successively from the lower to the upper part of the film. The c-axis Y123 thin film, thickness range from 10 nm to 60 nm, was epitaxially formed on the substrate and the a-axis island-shaped grains generated from the c-axis film. The 90° oriented a- and c-axis Y123 grains were found to form meandering grain boundaries; another 90° domain was formed from two differently oriented a-axis grains in which their b- and c-axis are alternatively parallel to each other. A small amount of poly-crystallized Y123 embedded in the amorphous were observed in the upper part of the film.

Formation of CuPt-type ordered (Cd, Zn)Te at CdTe/ZnTe interface

A CuPt-type ordered CdZnTe has been observed at CdTe/ZnTe interface sequentially grown by organometallic vapor-phase epitaxy on (0 0 1)GaAs at 400°C. Selected area electron diffraction, cross-sectional high-resolution electron microscopy, and computer image simulation have been employed to characterize the ordered structure in CdZnTe. Between CdTe and ZnTe, we observed a relatively uniform layer with dark contrast band image by conventional transmission electron microscopy. Through [1 1 0] projection, selected area electron diffraction pattern showed two sets of { 1 2 1 2 1 2 } extra spots with symmetrical intensity and corresponding high-resolution images showed doubling periodicity in contrast on {1 1 1} lattice planes. The ordered structure has been identified as CuPt-type ordered CdZnTe from electron diffraction, high-resolution images, and computer image calculations. Through [1 1 0] projection, it was observed that there were two variants for ordering according to each direction of doubling periodicity on {1 1 1} lattice planes, i.e., on (1 1 ̄ 1) and on ( 1 ̄ 1 1) . Each variant was observed to have its own domain structure of which size ranged from 10 to 20 nm and the two variants were observed with equal probability.

Interface structures of heteroepitaxially grown Pr123/Y123 and Pr123/Nd123 crystals by liquid phase epitaxy

PrBa 2Cu 3O 7− δ /YBa 2Cu 3O 7− δ (Pr123/Y123) and PrBa 2Cu 3O 7− δ /NdBa 2Cu 3O 7− δ (Pr123/Nd123) epitaxially grown crystals were fabricated by Liquid Phase Epitaxy (LPE) and the interface structures were investigated by cross-sectional high-resolution electron microscopy (HREM). In the case of Pr123/Y123, crystals were grown on the (001) Y123 surface of a Y123 substrate, and in the case of Pr123/Nd123, crystals were grown on the (010) Nd123 surface of a Nd123 substrate. For Pr123/Y123 and Pr123/Nd123, misfit dislocation spacings of 31 nm and 0.7 mm resulted from the lattice mismatch of a-axis were observed, respectively, and no interdiffusion layer was detected within the region of 10 nm apart from the growth interface. The misfit dislocation at the interface was introduced so as to completely release the elastic strains.

Effects of growth temperature and surface treatment on growth orientation and interface structure during molecular beam epitaxy of CdTe on (0 0 1)GaAs

The effects of growth temperature and substrate surface treatment on the growth orientation and interface structure during molecular beam epitaxy of CdTe on (0 0 1)GaAs have been investigated using cross-sectional high-resolution electron microscopy. It has been observed that the substrate preheating at 600°C for 5 min without conventional chemical etching is sufficient to remove the native oxide layer. For the growth temperature near 300°C, (1 1 1) growth was dominant at lower growth temperatures whereas (0 0 1) growth at higher temperatures. At a certain temperature, 290°C in this experiment, the growth orientation was not determined only by growth temperature, but seemed to be determined by substrate surface treatments prior to growth, such as chemical etching and substrate preheating at 600°C. It has been observed that the chemical etching caused etch pits and roughening of substrate surface and that it enhanced the (0 0 1) nucleation and growth in the etch pits, so that the (0 0 1) growth with chemical etching could happen at lower growth temperatures than those without chemical etching.

Microstructure of Josephson junctions in relation to their properties

The microstructure of various types of Josephson junctions is investigated with cross-sectional (CS) high-resolution electron microscopy (HREM). Special specimen preparation techniques, to facilitate the CS investigation, are discussed in the first part. Examples are shown of specially produced arrays of junctions and of the focused ion beam thinning technique allowing us to investigate specific measured junctions. Ramp-type junctions with (PrBCGaO) barrier layers are discussed in more detail in the second part. The microstructure of PrBCGaO (0.1 < x < 1.0) thin films, barrier layers and bulk material is studied by HREM. Thin barrier layers with contain a Ga-rich intergrowth, never observed before in YBCO-type structures. Ga-rich inclusions in single thin films exhibit a similar structure. In addition, diffusion of Ga in the ion-etched substrate surface region was observed. The observed segregation of Ga from the PrBCGaO barrier layer explains the observed junction properties.

Microstructural characterization of high-efficiency Cu(In,Ga)Se 2 solar cells

Microstructures of Cu(In,Ga)Se 2 (CIGS) thin-film solar cells with a MgF 2/ITO/ZnO/CdS/ CIGS/Mo/glass structure were studied by a cross-sectional high resolution and analytical transmission electron microscopy (TEM). The Mo back contact, CIGS absorber and CdS buffer layer were deposited by rf-magnetron sputtering, physical vapor deposition (PVD) and chemical bath deposition (CBD), respectively. The interfaces between the CIGS absorber and the Mo back contact and between the CdS buffer layer and the CIGS absorber layer were investigated in detail. There were two layers at the CIGS/Mo interface. One was a MoSe 2 layer and the other was an amorphous layer. The thickness of the interface layers depends on the deposition conditions of the Mo layers. The CdS-layer deposited by CBD was a mixture of cubic and hexagonal phases with large number of stacking faults. The {1 1 1} plane of cubic or {0 0 0 1} plane of hexagonal CdS tends to be parallelto the {1 1 2} plane of CIGS.

Atomic process of epitaxial growth of gold on magnesium oxide studied by cross-sectional time-resolved high-resolution electron microscopy

Gold was vacuum deposited on a (001) surface of magnesium oxide inside a high-resolution electron microscope. The atomic process of epitaxial growth was observed cross sectionally in situ with time-resolved high-resolution transmission-electron microscopy. Various types of growth phenomena, such as nucleation center formation, structural fluctuations, shape modulation, secondary nucleation, and coalescence, were analyzed in real space at a spatial resolution of 0.2 nm and a time resolution of 1/60 s. It was found that a central corner of a cluster is truncated and constructed repeatedly during the growth.

Structure-Property Relationships in Sputtered Magneto-Optic Multilayers

One technology for high-density information storage employs magneto-optic thin films. Key material properties include a large magneto-optic Kerr rotation of polarized light (especially in the blue wavelength range), high perpendicular magnetic anisotropy, good reflectivity and a large magnetic coercivity. These can be achieved in nano-scale magnetic multilayers (e.g., Co-Pt) in which one of the constituent layers is ferromagnetic. Naturally the magnetic properties can be manipulated to a great extent by the preparation conditions, and the microstructure is most readily revealed by cross-section high-resolution electron microscopy (HREM). In our work, extremely high quality multilayers were fabricated by sputtering using a variety of inert gases, and most notably the influence of interface sharpness on properties was assessed. Intermetallic compounds, which are essentially “natural” multilayers, were also extensively studied.Figure 1 shows magneto-optic Kerr rotation hysteresis loops for multilayers sputtered in 5 mtorr of either argon or the heavier xenon gas.

Misfit accommodation mechanisms at moving reaction fronts during topotaxial spinel-forming thin-film solid-state reactions: A high-resolution transmission electron microscopy study of five spinels of different misfits

Abstract Thin films of five different spinels MgAl2O4, MgCr2O4, MgIn2O4, TiMg2O4 and MgFe2O4 were grown by topotaxial solid state reactions on MgO(001) substrates. The spinels widely differed in their lattice parameter resulting in a variation in sign and amount of lattice misfit to the substrate. The films were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy/selected-area electron diffraction and energy-dispersive X-ray microanalysis before the spinel-MgO reaction fronts were investigated in detail by cross-sectional high-resolution electron microscopy. For almost vanishing misfit (TiMg2O4-MgO and MgFe2O4-MgO) the films were exactly cube-to-cube oriented to the substrate, with the spinel-MgO reaction front completely coherent. During the reaction, a network of cation antiphase boundaries formed in these films. For non-vanishing misfit, semicoherent reaction fronts occurred, with different interfacial defects forming, depending on sign and amount of the s...

The submicroscopic structure of reaction fronts in solid-solid reactions and its correlation with reaction mechanism and kinetics

Experimental investigations of the atomic structure of topotaxial solid-solid reaction fronts were performed. XRD, TEM, SAED, EDS, RBS and specifically cross-sectional high-resolution electron microscopy (HRTEM) were applied to solidsolid reaction interfaces of a number of topotaxial model systems. The reaction kinetics of a suicide-forming reaction was shown to depend on the orientation of the silicide NiSi 2. HRTEM revealed a corresponding difference in the atomic structure of the reaction front. The reaction mechanism of spinel-forming reactions was found to be determined by the spinel/oxide lattice misfit which at the moving reaction front has to be steadily accommodated. Sign and degree of the lattice misfit at the reaction front were systematically varied in the experiments by studying a series of different spinels, viz. Mg 2TiO 4, MgFe 2O 4, MgAl 2O 4, MgCr 2O 4 and MgIn 2O 4, revealing interfacial dislocations of different types. Respective results are discussed in terms of a model featuring the interplay between kinetic reaction barriers, the atomic mechanism of the solid state reaction and the properties of the interfacial defects (ledges, dislocations) persisting at the moving reaction front.

Atomic and Electronic Structure of V/MgO Interface

Thin films of vanadium were deposited on the (001) surface of a MgOsubstrate by molecular beam epitaxy (MBE) and the V/MgO interface wasinvestigated by cross-sectional high resolution electron microscopy(HREM) and electron energy loss spectroscopy (EELS). In order todetermine the location of atoms at the interface, computersimulations were performed for four possible models, and bestmatching between the experimental and simulated images was obtainedfor the model where the V atoms are located directly on top of the Mgatoms at the interface. Interface bonding mechanism was investigatedby a first principles molecular-orbital (MO) calculation using thediscrete-variational (DV)-Xα method for a modelcluster of the interface, i.e., (Mg9O9V5).The V-3d band was located in between the band-gap of MgO, and nearly empty Mg-3sp orbitals werefound to overlap with the V-3d band. The Mg-3sp and V-3d hybridizedin a bonding manner, thereby generates strong covalent bondingbetween V and Mg. Nearly filled O-2p orbitals were also found tohybridize with the V-3d orbitals in an antibonding manner. The bondoverlap population of the V–O bond was approximately four timessmaller than that of the V–Mg bond when the bond-length was thesame. The near edge structure of EELS specific to the interface wasobtained using a V/MgO multilayer specimen at both Mg-K and O-Kedges. Comparison between the experimental and theoretical spectra bythe present MO calculation clearly found the presence of hybridizedorbitals of V-3d with Mg-3p.

Cross-sectional time-resolved high-resolution electron microscopy of epitaxial growth of Au on MgO

Vapor phase epitaxial growth techniques are indispensable for production of thin film electric devices. Various structural analyses have been attempted to evaluate the epitaxial growth. Conventional transmission electron microscopy (CTEM) is a most useful method. In particular, it is known that a plan-view time-resolved CTEM of in-situ vacuum-deposition in a microscope can analyze each process of epitaxial growth. The nucleation in vacuum-deposition was also in-situ observed by a time-resolved high resolution electron microscopy (TRHREM). However many unresolved problems still remain in the studies of the epitaxial growth because it is difficult to observe the epitaxial interfaces less than a few nanometer under appropriate conditions. Much more advanced techniques are required for electron microscopy to obtain detailed information.In the present study, a TRHREM for the cross-sectional observation was developed to elucidate the epitaxial growth process in vacuum-deposition.Gold (Au) was vacuum-deposited on (001) surfaces of the magnesium oxide (MgO) substrates at room temperature in a specimen chamber of a 200-kV high-resolution electron microscope (JEOL, JEM2010).

Full coverage of ultra-thin SrTiO 3 layer in a-YBa 2Cu 3O 7− x/SrTiO 3/ a-YBa 2Cu 3O 7− x sandwiched thin film

Microstructures of an ultra-thin SrTiO 3 layer sandwiched in between two a-axis oriented YBa 2Cu 3O 7− x ( a-YBCO) thin films on a SrTiO 3 substrate have been studied by cross-sectional high-resolution electron microscopy. The ultra thin (3 nm) homogenous SrTiO 3 layer has grown on a surface of a-YBCO thin film with higher coverage. The surface energy of a-YBCO thin film is higher than that of c-axis oriented YBCO ( c-YBCO) thin film. Therefore, a-YBCO offers a more chemically active surface allowing the SrTiO 3 layer to fully cover the a-YBCO surface. The presence of a shallow gradient in the slope of the surface, rather than the absolute height variation, was found to be key to obtain a pin-hole free junction. Pin-holes mainly occur on steep steps, caused by a 90° domain boundary or a nucleation of a c-YBCO pyramid at the bottom of an a-YBCO film.

Temperature-mediated phase selection during growth of GaN on (111)A and (1̄1̄1̄)B GaAs substrates

GaN layers having the zinc blende and wurtzite structures can be selectively deposited on (111)A and (1̄1̄1̄)B GaAs substrates by varying the growth temperature. Using the growth temperature as a variable, layers having the two structures have been sequentially deposited. The as-grown structures have been examined by cross-sectional high resolution electron microscopy. Results indicate that the two phases once formed are structurally stable in the temperature range examined. Furthermore, the transition from GaN (zinc blende) to GaN (wurtzite) is sharp, whereas a faulted region is observed during the reverse transition. Arguments have been developed to rationalize these observations.