Cross-sectional High-resolution Transmission Electron Microscopy Research Articles

Interfaces between solution-derived LiMn2O4 thin films and MgO and Au/MgO substrates

Spinel LiMn2O4 thin films have been prepared on MgO(110) and Au/MgO(110) substrates by a chemical solution deposition method. The interfaces between film and substrate were characterized by means of high-resolution transmission electron microscopy (HREM) as well as x-ray diffraction. Cross-sectional HREM observation revealed that LiMn2O4 films grew epitaxially on the MgO(110) and Au/MgO(110) substrates. In the LiMn2O4/MgO system, misfit dislocations formed to accommodate the lattice strain at the LiMn2O4/MgO interface. In the LiMn2O4/Au/MgO system, Au grew epitaxially on the MgO substrate with its surface facetted along {111} planes, probably because the surface energy of this plane is relatively low. The formation of these facets is considered to have a favorable effect on the growth of {111} planes of LiMn2O4 when deposited on the Au film.

Crystalline silicon thin films with porous Si backside reflector

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) was chosen to deposit a thin Si film on a PS multilayer structure at low temperature and high deposition rate. This technique allows one to deposit a Si film with epitaxial quality on the top of PS without destroying its multi-layer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epi-layers of 10 μm are grown at very high deposition rates (approx. 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the PS/epi-Si interface and spreads through the whole epi-layer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

Diffusion induced amorphization in the crystalline SrTiO 3 thin films grown on Si (1 0 0) investigated by combinatorial method

Interface structures of SrTiO 3 (STO)/Si were investigated systematically using combinatorial method with growth temperature gradient in pulse laser deposition (PLD) and cross-sectional high-resolution transmission electron microscopy (HRTEM). Besides SiO 2 interfacial layer, another amorphized STO layer which lying on the SiO 2 interfacial layer was also observed in the crystalline STO thin films grown on Si (1 0 0) at growth temperatures above 600 °C. From the growth condition dependence studies on the formation of amorphized STO layers and the electron energy loss spectroscopy measurements, the origin of the amorphization was concluded as the diffusion of Si from substrate. This is the first observation of a diffusion induced amorphization phenomenon in the crystalline STO thin films grown on Si (1 0 0). Our results show that at higher growth temperatures, the interface structures of STO/Si are dominated by the diffusion of Si from the Si substrates.

HRTEM observation of CdSe/ZnSe SQWs grown on vicinal GaAs substrate

Microscopic structural properties of CdSe/ZnSe single quantum wells grown on vicinal GaAs substrate are investigated by cross-sectional high-resolution transmission electron microscope. The interface of ZnSe buffer and GaAs substrate, the surface of top ZnSe, and CdSe well are observed. At the ZnSe/GaAs interface, the lattice structure is strongly smeared, indicating that structures other than ZnSe and GaAs are formed. Stacking faults are generated at this region. A distinct step and terrace structure is observed at the surface of top ZnSe. The averaged terrace width coincides with the nominal value of 5° vicinal surface. Well layer is tilted with regard to the (2 0 0) lattice plane by 5°, and no misfit dislocation is found. Strain mapping of the obtained lattice image is performed.

Investigation of initial growth process for GaN heteroepitaxial layers grown on Si(0 0 1) and Si(1 1 1) substrates by ECR-assisted MBE

High luminescence efficiency zincblende-GaN (β-GaN) crystal islands were preferentially grown on Si at initial growth process, particularly on non-pretreated (0 0 1)-oriented Si. The initial growth process was strongly influenced by substrate treatment before growth, namely substrate surface state. It was confirmed by a high-resolution cross-sectional transmission electron microscopy that both wurtzite- and β-GaN crystal grains were epitaxially grown by mixing in the plane on a 2.5 nm-thick amorphous SiN X interface layer, which was instantly formed within 10 s during substrate nitridation or/and initial growth process.

Crystalline silicon thin films with porous Si backside reflector (abstract only)

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) is chosen to deposit a thin Si film on a PS multilayer structure at low temperature and a high deposition rate. This technique allows one to deposit a Si film with an epitaxial quality on the top of PS without destroying its multilayer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epilayers of 10 μm are grown at very high deposition rate (around 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the interface PS/epi-Si and spreads through the whole epilayer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

Self-assembled epitaxial and polycrystalline magnetic nickel nanocrystallites

Nanocrystalline nickel particles were embedded in amorphous alumina and crystalline TiN matrices using a pulsed-laser deposition process to investigate the effect of texturing on magnetic properties of nickel nanocrystallites. The crystalline quality of both the matrix and magnetic particles were investigated by cross-sectional high-resolution transmission electron microscopy. The embedded Ni nanocrystals were found to be epitaxial in case of TiN matrix and polycrystalline in Al2O3 amorphous matrix. The Ni nanocrystals on TiN/Si grow epitaxially because the TiN acting as a template grows epitaxially on Si substrate via domain epitaxy. On the other hand, Ni nanocrystals in Al2O3 matrix are polycrystalline because of the amorphous nature of the alumina matrix. Magnetization versus temperature measurements have shown that the blocking temperature, above which the samples lose magnetization-field (M–H) hysteretic behavior, of Ni–TiN sample (∼190 K) is significantly higher than that of Ni–Al2O3 sample (∼30 K) with a similar size distribution of embedded magnetic particles. A comparison of the values of coercivity (Hc) of the two samples, measured from M–H data, indicates that epitaxial Ni nanocrystals also exhibit significantly higher coercivity than polycrystalline Ni particles. The high values of TB and Hc of Ni–TiN samples with respect to TB of Ni–Al2O3 samples are believed to be associated with preferred alignment of nanocrystallites.

In situ studies of the role of excess Ga on the growth morphology of thin GaN layers

Homoepitaxial GaN growth and heteroepitaxial growth of GaN on 6H-SiC(0 0 0 1) is studied in situ by low energy electron microscopy and low energy electron diffraction with the goal to elucidate the dominant influence of excess Ga on the growth morphology on substrates with different surface preparation. High-resolution cross-sectional transmission electron microscopy is used to determine the shape and internal structure of the GaN crystals. The results clearly show that (0 0 0 1)-face-terminated growth can be obtained only with Ga-rich conditions and that it is possible with significant excess Ga to reverse the microfaceting appearing in the very early stages of growth. (0 0 0 1)-face-terminated growth is discussed in terms of quasi-liquid phase epitaxy from an adsorbed Ga layer.

Cubic boron nitride thin film heteroepitaxy

In this study we investigate the possibility of nucleating nanocrystalline cubic boron nitride (c-BN) thin films directly onto suitable substrates without the soft turbostratic BN (t-BN) interlayer that is usually observed. This would open a path to the epitaxial growth of c-BN films which is essential particularly for practicable applications in electronic devices. Appropriate substrates are required to exhibit a lattice that matches the c-BN crystallite structure, survives the ion bombardment imperative for c-BN film formation, and is not disturbed by the development of a heterogeneous interface layer. In accordance with these criteria, monocrystalline AlN is selected and employed as a potential substrate for direct c-BN film growth using mass selected ion beam deposition. A detailed examination of the BN/AlN interface microstructure by cross-sectional high-resolution transmission electron microscopy reveals that the AlN crystallinity is indeed retained, with no amorphous layer next to the BN film as commonly observed on Si substrates. Nanocrystalline BN grains with the cubic, and, more frequently, with the wurtzitic structure are found in direct contact with certain regions of the rugged AlN substrate, covering about one-third of its entire surface with no mediating t-BN or other interface layer. The c-BN and w-BN growth areas are textured and exhibit definite preferential orientation relationships with the faceted AlN substrate surface. The consequences of these findings for the understanding of the role of the t-BN interlayer in c-BN film nucleation are discussed.

Coexistence behavior of the CuPtB-type and the CuAu–I-type ordered structures in highly strained CdxZn1−xTe/GaAs heterostructures

Ordered structures in highly strained CdxZn1−xTe/GaAs heterostructures epitaxial layers grown on (001)GaAs substrates were investigated by using selected area electron diffraction pattern (SADP) and cross-sectional high-resolution transmission electron microscopy (HRTEM) measurements. The SADP results showed two sets of (1/2 1/2 1/2) superstructure reflections with symmetrical intensities along the [110] axis, and the corresponding HRTEM images indicated a doublet periodicity in the contrast of the {111} lattice planes. Two structures, one corresponding to the CuPtB-type ordering for each direction of the doublet periodicity on the {111} lattice planes along the [110] axis and the other corresponding to superstructure spots related to the CuAu–I type ordering were observed in the SADP. The doublet periodicity of 200 lattice fringes, associated with the CuAu–I-type ordered structure was also observed in the HRTEM image, and many antiphase boundaries were observed in ordered regions. The formation of the two ordered structures in the CdxZn1−xTe epilayers might originate from the minimization of the relaxation energy due to the high strain effect resulting from the large lattice mismatch between the CdxZn1−xTe epilayer and the GaAs substrate. These results provide important information on the microstructural properties for improving the efficiencies of CdxZn1−xTe-based optoelectronic devices operating in the blue-green spectral region.

Stabilization of zinc-blende cubic AlN in AlN/W superlattices

AlN/W superlattices with bilayer periods of 3.5–7 nm were grown on MgO (001) by magnetron sputtering. For AlN thicknesses lAlN⩽1.5 nm, a cubic phase of AlN was observed using high-resolution cross-sectional transmission electron microscopy and x-ray diffraction (XRD). Pole-figure XRD scans showed a reflection that matched the theoretically predicted interplanar spacing of zinc-blende phase (Zb-AlN), and that could not be explained by either the wurtzite or rocksalt structures. The stabilization of zb–AlN is explained as a result of good interfacial matching between W(100) and zb–AlN(011). When lAlN was increased above 1.5 nm, XRD scans showed a rapid decrease in satellite peak intensities, indicating a degradation of the layered structure, and the appearance of a wurtzite structure.

Crystallography and structural evolution of LiNbO3 and LiNb1−xTaxO3 films on sapphire prepared by high-rate thermal plasma spray chemical vapor deposition

The structure and the crystallography of lithium niobate and lithium niobate–tantalate thin films (0.2–1.0 μm in thickness) with the tantalum composition range of 0 ≤ x ≤ 0.5 grown on (0001) sapphire substrate by thermal plasma spray chemical vapor deposition have been studied by means of cross-sectional high-resolution transmission electron microscopy and x-ray diffraction. The tantalum composition in the films shows a minor effect on the rocking curve full width at half maximum values. The narrowest rocking curve width was obtained for the LiNb0.5Ta0.5O3 film to be as low as 0.25° θ. The films are under compressive strain along the c direction; c- and a-axis lattice parameters are correspondingly smaller and higher than those of the bulk single crystal. Under optimized growth conditions, the LiNbO3 and LiNb1−xTaxO3 films are 97% c-axis oriented. The film out-of-plane orientation changes from the [0001] to the [0112] direction by either decreasing the growth rate or increasing the substrate temperature. Particular attention has been paid to the orientation of individual grains in the partly c-axis-oriented films. The results demonstrate that their orientations are not random and specific orientation relationships are preferred for the film nucleation. The surface of as-received sapphire substrate reveals polishing defects with the well-defined surface ledges of 1–2 nm in height with smooth terraces of 25 nm in width. In the case of columnar growth, the terrace width becomes a limiting factor controlling the lateral crystallite size in the film. Finally, the film growth mechanism is discussed.

The formation of dislocations in the interface of GeSi/low-temperature Si buffer grown on Si (0 0 1)

We have studied the mechanism of strain relaxation of Ge 0.3Si 0.7 alloy layers grown on low-temperature (LT) Si buffers by high-resolution X-ray diffraction and transmission electron microscopy (TEM). X-ray rocking-curve analyses show that the strain relaxation of an alloy layer is quite inhomogeneous at its early stage, and becomes homogeneous when the layer approaches becomes fully relaxed. High-resolution cross-section TEM analyses indicate that stacking faults have formed in the LT buffer near the interface of LT-Si/GeSi and have separated the mismatch dislocations in the interface of GeSi/LT-Si into Shockley partials. We propose that this could be due to the aggregation of the vacant defects in the LT-Si layer. The mechanism of this aggregation is the osmotic force created by the vacancy super-saturation in the LT-Si layer and/or the tension stress propagating from the interface of GeSi/LT-Si.

Growth, structural, and magnetic properties of high coercivity Co/Pt multilayers

Electron beam evaporated Co/Pt multilayers {[Co(tCo nm)/Pt(1 nm)]10, 0.2<tCo<2 nm} with perpendicular magnetic anisotropy and room temperature coercivities Hc=2–15 kOe are studied as a function of growth temperature TG. Hysteresis loops and magnetic force microscopy (MFM) indicate changes in the magnetization reversal mechanism along with a sharp increase in coercivity for TG≳230–250 °C. Films grown at TG<230 °C (tCo=0.2–0.4 nm) show micrometer size magnetic domains and rectangular hysteresis indicating magnetization reversal dominated by rapid domain wall motion following nucleation at Hn∼Hc. Films grown at TG>250 °C show fine-grained MFM features on the sub-100-nm length scale indicating reversal dominated by localized switching of small clusters. High-resolution cross-sectional transmission electron microscopy (TEM) with elemental analysis shows columnar grains extending throughout the multilayer stack. Co depletion and structural defects at the grain boundaries provide a mechanism for exchange decoupling of adjacent grains, which may result in the high coercivities observed.

Characterization of Silicon Nitride Thin Films on Si and Overlayer Growth of Si and Ge

Crystalline silicon nitride (SiNx) thin films on Si(111) and amorphous SiNx films on Si(001) have been obtained after NH3 or NO exposure at T≈1175 K. The crystallinity of the film on Si(111) has been verified with high-resolution cross-sectional transmission electron microscopy (TEM) and scanning tunneling microscopy. The thickness of the SiNx film is 3–6 atomic layers. When compared with the known phases of Si3N4, our SiNx film is relatively close to β-Si3N4, but it could be a new phase of silicon nitride. Si or Ge forms 3D islands initially when deposited on both crystalline and amorphous SiNx films, and most of the islands are not aligned with the Si substrates. However, on SiNx/Si(111), the islands aligned with the Si substrate grow faster than other islands, so that the overlayer gradually grows into a (111)-oriented columnar film. On SiNx/Si(001), the overlayer films remain polycrystalline in later stages of growth.

Cross-sectional high-resolution transmission electron microscopy observation of Si(1 1 3) 3×2 structure

Atomic structure of Si(1 1 3) 3×2 reconstructed surface was examined by high-resolution transmission electron microscopy (HRTEM). Cross-sectional atomic structure of the Si(1 1 3) 3×2 was observed along [ 1 ̄ 1 0] direction at specimen temperature of about 800 K inside an ultrahigh vacuum transmission electron microscope. Simulated images for some probable structure models were compared with the experimental HRTEM images. It was found that, if void area in Ranke’s model is filled by adatom so that a row of adatoms is created, the simulated HRTEM image shows good agreement with the observed one. The structure model recommended by Dabrowski et al. [Surf. Sci. 331–333 (1995) 1022; Phys. Rev. Lett. 73 (1994) 1660] was not able to explain the HRTEM images obtained in the present work.

Varying implant dose rate for defect reduction in laser thermal processing

Laser thermal processing (LTP) of ion implanted silicon involves melting and recrystallizing an implantation induced amorphous layer containing dopants as a method of producing ultra shallow highly doped junctions in silicon. After LTP there can exist a high concentration of extended defects in the recrystallized region. These defects are commonly in the form of stacking faults and microtwins that propagate from throughout the region that had been amorphous prior to laser melting. In order to determine the origin of these extended defects, the effect of the dose rate of the silicon preamorphization implant was studied. A 10 keV 1×10 15 cm −2 Si + implant was done into silicon at dose rates between 0.06 and 0.48 mA/cm 2. High-resolution cross-sectional transmission electron microscopy (HR-XTEM) results show the roughness of the amorphous crystalline interface increases with increasing dose rate up to 0.24 mA/cm 2 then decreases because of dynamic annealing. Quantitative plan-view TEM results of the defect density after LTP processing at 0.75 J/cm 2 laser power show a direct correlation between the amorphous crystalline interface roughness and the final defect density. Reduction in amorphous/crystalline interface roughness prior to laser thermal processing results in a dramatic reduction of LTP recrystallization defects.

Room-temperature epitaxial growth of CeO2(001) thin films on Si(001) substrates by electron beam evaporation

Epitaxial growth of CeO2(001) thin films on Si(001) substrates was achieved by electron beam evaporation. Reflection high-energy electron diffraction and cross-sectional high-resolution transmission electron microscopy established the formation of an epitaxial CeO2(001)/Si(001) structure with a cube-on-cube configuration. The epitaxial structure had to be formed at a temperature below 200 °C with a Si(001)-2×1, 1×2 reconstructed surface, and it could be formed even at room temperature. In order to improve the crystallinity, homoepitaxial growth conditions at a higher temperature with a high oxygen flow rate were also investigated. Homoepitaxy of ceria grown on a 5-nm-thick initial layer was demonstrated by θ/2θ-scan and φ-scan of x-ray diffraction.

Characterization of AlGaN/GaN structures on various substrates grown by radio frequency-plasma assisted molecular beam epitaxy

The structural properties and surface morphology of AlGaN/GaN structures grown on LiGaO2 (LGO), sapphire, and hydride vapor phase epitaxy (HVPE)-grown GaN templates are compared. AlGaN grown on LGO substrates shows the narrowest x-ray full width at half maximum (FWHM) for both symmetric 〈00.4〉 and asymmetric 〈10.5〉 reflections. Atomic force microscopy (AFM) analysis on AlGaN surfaces on LGO substrates also show the smoothest morphology as determined by grain size and rms roughness. The small lattice mismatch of LGO to nitrides and easily achievable Ga-polarity of the grown films are the primary reasons for the smoother surface of AlGaN/GaN structure on this alternative substrate. Optimizations of growth conditions and substrate preparation results in step flow growth for an AlGaN/GaN structure with 300 A thick Al0.25Ga0.75N on 2.4 μm thick GaN. A high III/V flux ratio during growth and recently improved polishing of LGO substrates aids in promoting two dimensional step flow growth. The GaN nucleation layer directly on the LGO substrate showed no evidence of mixed phase cubic and hexagonal structure that is typically observed in the nucleation buffer on sapphire substrates. Cross-sectional high-resolution transmission electron microscopy (HRTEM) was performed on an AlGaN/GaN heterostructure grown on LGO. The atomic arrangement at the AlGaN/GaN interface was sharp and regular, with locally observed monolayer and bilayer steps.

Phase evolution in aluminum nitride thin films on Si(100) prepared by radio frequency magnetron sputtering

Aluminum nitride (AlN) thin films were deposited on Si(100) substrate using radio frequency (r.f.) magnetron sputtering. AlN/Si interfaces were analyzed by selected-area electron diffraction, cross-sectional high-resolution transmission electron microscopy and Auger electron spectroscopy. The effects of increasing deposition times on the resultant AlN phase and composition, especially in the interface regions, were discussed. AlN film, deposited at r.f. power of 200 W and substrate temperature of 250°C for 1 min, consists of two distinct layers of different phases. At the film/substrate interface area a 3.3-nm thick amorphous AlN with a small amount of oxygen was formed. A polycrystalline AlN phase oriented preferentially to the c-axis was observed in the rest of the film. As deposition time was increased to 2 min, the thickness of the amorphous AlN layer was decreased to 1.5 nm, indicating that the amorphous phase was crystallized during the increased deposition time. Lower substrate temperature of 150°C also certified the trend of the decrease in the amorphous phase with increased deposition time. A heating effect on the substrate by bombardment of energetic neutralized and ions in the deposition ambient and the diffusion of the interface oxygen to the growing film would promote the surface activation for the crystallization.