Cross-sectional High-resolution Transmission Electron Microscopy Research Articles

Measurements of thermally induced nanometer-scale diffusion depth of Pt∕Ti∕Pt∕Au gate metallization on InAlAs∕InGaAs high-electron-mobility transistors

Platinum diffusion in InAlAs was investigated utilizing a Pt∕Ti∕Pt∕Au gate contact on an In0.52Al0.48As∕In0.53Ga0.47As∕InP high-electron-mobility transistor (HEMT) structure. Capacitance-voltage measurements on large gate field-effect transistors and high-resolution cross-sectional transmission electron microscopy enabled the measurement of Pt diffusion depth with nanometer-scale accuracy. A continuous increase in Pt diffusion depth was observed at an annealing temperature of 250 °C with increasing time. After a 40 min anneal, a diffusion depth of 8 nm was measured. Such a deep Pt diffusion in a HEMT structure not only changes device parameters but also constitutes a serious reliability problem during device operation.

Partial encapsulation of Pd particles by reduced ceria-zirconia

Direct observation of metal-oxide interfaces with atomic resolution can be achieved by cross-sectional high-resolution transmission electron microscopy (HRTEM). Using this approach to study the response of a model, single-crystal thin film automotive exhaust-gas catalyst, Pd particles supported on the (111) ceria-zirconia (CZO) surface, to a redox cycle, we have found two distinct processes for the partial encapsulation of the Pd particles by the reduced CZO surface that depend on their relative crystallographic orientations. In the case of the preferred orientation found for Pd particles on CZO, Pd(111)[110]∕∕CZO(111)[110], a flat and sharp metal/oxide interface was maintained upon reduction, while ceria-zirconia from the adjacent surface tended to accumulate on and around the Pd particle. In rare cases, Pd particles with other orientations tended to sink into the oxide support upon reduction. Possible mechanisms for these encapsulation processes are proposed.

HRTEM analysis of the nanostructure of porous silicon

The nanometric structure of porous silicon makes this material to be very suitable for its use in many different fields, including optoelectronics and biological applications. In the present work, the structure of porous silicon was investigated in detail by means of cross-sectional high-resolution transmission electron microscopy and digital image processing, together with electron energy loss spectroscopy. The structure of the Si/porous silicon interface and that of the silicon nanocrystals that compose porous silicon have been analyzed in detail. A strong strain contrast in the Si/porous silicon interface caused by high stresses was observed. Accordingly, dislocation pairs are found to be a possible mechanism of lattice matching between porous silicon and the Si substrate. Finally, high relative concentration of oxygen in the porous silicon layer was observed, together with low relative electron concentration in the conduction band when compared to Si.

Assessment of Chemical Solution Synthesis and Properties of Gd2Zr2O7 Thin Films as Buffer Layers for Second-Generation High-Temperature Superconductor Wires

Chemical solution processing of Gd2Zr2O7 (GZO) thin films via sol-gel and metalorganic decomposition (MOD) precursor routes have been studied on textured Ni-based tape substrates. Even though films processed by both techniques showed similar property characteristics, the MOD-derived samples developed a high degree of texture alignment at significantly lower temperatures. Both precursor chemistries resulted in exceptionally dense, pore-free, and smooth microstructures, reflected in the cross-sectional and plan-view high-resolution scanning and transmission electron microscopy studies. On the MOD GZO buffered Ni–3at.% W (Ni–W) substrates with additional CeO2/YSZ sputtered over layers, a 0.8-μm-thick YBa2Cu3O7−δ (YBCO) film, grown by an ex situ metalorganic trifluoroacetate precursor method, yielded critical current, Ic (77 K, self-field), of 100 A/cm width. Furthermore, using pulsed-laser deposited YBCO films, a zero-field superconducting critical current density, Jc (77 K), of 1 × 106 A/cm2 was demonstrated on an all-solution, simplified CeO2(MOD)/GZO(MOD)/Ni–W architecture. The present study establishes GZO buffers as a candidate material for low-cost, all-solution coated conductor fabrication.

Interface-dominated growth of a metastable novel alloy phase

A new D023 metastable phase of Cu3Au was found to grow at the interfaces of Au/Cu multilayers deposited by magnetron sputtering. The extent of formation of this novel alloy phase depends upon an optimal range of interfacial width primarily governed by the deposition wattage of the direct current magnetron used. Such interfacially confined growth is utilized to grow a ∼300-nm-thick Au/Cu multilayer with thickness of each layer nearly equal to the optimal interfacial width which was obtained from secondary-ion mass spectrometry (SIMS) data. This growth technique is observed to enhance the formation of the novel alloy phase to a considerable extent. The SIMS depth profile also indicates that the mass fragment corresponding to Cu3Au occupies the whole film while x-ray diffraction (XRD) shows almost all the strong peaks belonging to the D023 structure. High-resolution cross-sectional transmission electron microscopy shows the near-perfect growth of the individual layers and also the lattice image of the alloy phase in the interfacial region. Vacuum annealing of the alloy film and XRD studies indicate stabilization of the D023 phase at ∼150 °C. The role of interfacial confinement, the interplay between interfacial strain and free energy, and the hyperthermal species generated during the sputtering process are discussed.

Thermal response of Ru electrodes in contact with SiO2 and Hf-based high-k gate dielectrics

A systematic experimental evaluation of the thermal stability of Ru metal gate electrodes in direct contact with SiO2 and Hf-based dielectric layers was performed and correlated with electrical device measurements. The distinctly different interfacial reactions in the Ru∕SiO2, Ru∕HfO2, and Ru∕HfSiOx film systems were observed through cross-sectional high-resolution transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy with electron-energy-loss spectra, and energy dispersive x-ray spectra analysis. Ru interacted with SiO2, but remained stable on HfO2 at 1000°C. The onset of Ru∕SiO2 interfacial interactions is identified via silicon substrate pitting possibly from Ru diffusion into the dielectric in samples exposed to a 900°C∕10-s anneal. The dependence of capacitor device degradation with decreasing SiO2 thickness suggests Ru diffuses through SiO2, followed by an abrupt, rapid, nonuniform interaction of ruthenium silicide as Ru contacts the Si substrate. Local interdiffusion detected on Ru∕HfSiOx samples may be due to phase separation of HfSiOx into HfO2 grains within a SiO2 matrix, suggesting that SiO2 provides a diffusion pathway for Ru. Detailed evidence consistent with a dual reaction mechanism for the Ru∕SiO2 system at 1000°C is presented.

Nucleation of diamond by pure carbon ion bombardment—a transmission electron microscopy study

A cross-sectional high-resolution transmission electron microscopy (HRTEM) study of a film deposited by a 1 keV mass-selected carbon ion beam onto silicon held at 800 °C is presented. Initially, a graphitic film with its basal planes perpendicular to the substrate is evolving. The precipitation of nanodiamond crystallites in upper layers is confirmed by HRTEM, selected area electron diffraction, and electron energy loss spectroscopy. The nucleation of diamond on graphitic edges as predicted by Lambrecht et al. [W. R. L. Lambrecht, C. H. Lee, B. Segall, J. C. Angus, Z. Li, and M. Sunkara, Nature, 364 607 (1993)] is experimentally confirmed. The results are discussed in terms of our recent subplantation-based diamond nucleation model.

Epitaxial Growth of Fe/MgO/Fe Heterostructures on SrTiO3(001) Substrates by Magnetron Sputtering

We report on the preparation and structural properties of MgO/Fe bilayers (a MgO thin film on an Fe thin film), Fe/MgO bilayers (an Fe thin film on a MgO thin film), and Fe/MgO/Fe trilayers, all grown by magnetron sputtering on SrTiO3(001) substrates. We investigated the structural properties of these heterostructures with X-ray diffraction (XRD) (θ–2 θ scan, rocking curve and pole figure measurements), atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM). Single-crystalline epitaxial growth was confirmed by the X-ray pole figure measurements for both the MgO/Fe and Fe/MgO bilayers, which were the basic layer components for the Fe/MgO/Fe trilayer. The cross-sectional HRTEM clearly indicated epitaxial growth of the Fe/MgO/Fe trilayer structures on the SrTiO3(001) substrates with excellent flat interfaces having roughness of less than a few angstroms.

Structural and photoluminescence characteristics of molecular beam epitaxy-grown vertically aligned In 0.33Ga 0.67As/GaAs quantum dots

In this paper, we present the results of structural and photoluminescence (PL) studies on vertically aligned, 20-period In 0.33Ga 0.67As/GaAs quantum dot stacks, grown by molecular beam epitaxy (MBE). Two different In 0.33Ga 0.67As/GaAs quantum dot stacks were compared. In one case, the In 0.33Ga 0.67As layer thickness was chosen to be just above its transition thickness, and in the other case, the In 0.33Ga 0.67As layer thickness was chosen to be 30% larger than its transition thickness. The 2D–3D growth mode transition time was determined using reflection high-energy electron diffraction (RHEED). Structural studies were done on these samples using high-resolution X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM). A careful analysis showed that the satellite peaks recorded in X-ray rocking curve show two types of periodicities in one sample. We attribute this additional periodicity to the presence of an aligned vertical stack of quantum dots. We also show that the additional periodicity is not significant in a sample in which the quantum dots are not prominently formed. By analyzing the X-ray rocking curve in conjunction with RHEED and PL, we have estimated the structural parameters of the quantum dot stack. These parameters agree well with those obtained from XTEM measurements.

Synthesis and memory effect study of Ge nanocrystals embedded in LaAlO3 high-k dielectrics

A floating gate memory structure utilizing Ge nanocrystals embedded in LaAlO3 (LAO) high-k dielectric films has been fabricated by pulsed-laser deposition. A cross-sectional high-resolution transmission electron microscopy study revealed that the floating gate structure contains 5-nm-diam spherelike Ge nanocrystals embedded in amorphous LAO. A significant memory effect with a very high density of charge storage up to 2×1013∕cm2 in the Ge nanocrystals and a maximum flat band voltage shift of 3.2 V have been achieved for the trilayer structure of LAO(8nm)∕Ge∕LAO(3nm)∕Si. The memory structure utilizing the Ge nanocrystals grown in 1 min showed excellent charge retention characteristics, whereas the decay in memory capacitance after 104s of stress under a flat band voltage was only 8%. These results suggest that this memory structure utilizing Ge nanocrystals embedded in a LAO dielectric offers a high potential for the further scaling of floating gate memory devices. In addition, the effects of Ge growth time, and thus the size and density of the Ge nanocrystals, to the charge storage and charge retention characteristics were also studied.

The influence of an extrinsic interfacial layer on the polarization of sputtered BaTiO3 film

As an origin of degradation of remnant polarization in Pt∕BaTiO3∕Pt capacitor structure, an interfacial layer formed at the interface of BaTiO3 film and a Pt bottom electrode is considered. BaTiO3 films were deposited on two types of bottom electrodes (La0.5Sr0.5CoO3 and Pt) by the radio frequency magnetron sputtering method and both capacitors showed a microstructural similarity with strong preferred orientations. However, a Pt∕BaTiO3∕La0.5Sr0.5CoO3 capacitor exhibited a saturated hysteresis loop with the remnant polarization (2Pr) of 6μC∕cm2, and for the Pt∕BaTiO3∕Pt structure, the polarization-voltage curve revealed a linear dielectric characteristic. From a cross-sectional high-resolution transmission electron microscope analysis of the Pt∕BaTiO3∕Pt capacitor showing the linear dielectric property, an interfacial layer with an amorphous structure as well as a multidomain structure in the interior of the BaTiO3 film were observed. It is concluded that the interfacial layer might help degradation of polarization and its origin can be classified as being extrinsic.

Nanoconstricted structure for current-confined path in current-perpendicular-to-plane spin valves with high magnetoresistance

We have successfully observed a nanoconstricted structure for current-confined-path (CCP) effect in current-perpendicular-to-plane-giant-magnetoresistance (CPP-GMR) spin valves. By inserting an AlCu nano-oxide layer (NOL) formed by ion-assisted oxidation (IAO) between a pinned layer and a free layer, the MR ratio was increased while maintaining a small area resistance product (RA). The cross-sectional high-resolution transmission electron microscopy image of the sample with RA=380mΩμm2, ΔRA=16mΩμm2, and MR ratio=4.3% showed that an amorphous oxide layer is a main part of the NOL that blocks the electron conduction perpendicular to plane. Some parts of the NOL are punched through crystalline, metallic channels having a diameter of a few nanometers, which are thought to work as nanoconstricted electron conduction paths between the pinned layer and the free layer. Nano-energy-dispersive-x-ray-spectrum analysis also showed that Cu is enriched in the metallic channels, whereas Al is enriched in the amorphous oxide region, indicating that the metallic channel is made of Cu and the oxide is made of Al2O3. The nanoconstricted structure with good segregation between the metallic channel and the oxide layer enables us to realize a large MR ratio in CCP-CPP spin valves.

Morphology dependence of the dielectric properties of epitaxial BaTiO3 films and epitaxial BaTiO3/SrTiO3 multilayers

Epitaxial BaTiO3 films and epitaxial BaTiO3/SrTiO3 multilayers were grown by pulsed laser deposition on vicinal surfaces of (001)-oriented Nb-doped SrTiO3 (SrTiO3 : Nb) single-crystal substrates. Atomic force microscopy was used to investigate the surface topography of the deposited films. The morphology of the films, of the BaTiO3/SrTiO3 interfaces, and of the column boundaries was investigated by cross-sectional high-resolution transmission electron microscopy. Measurements of the dielectric properties were performed by comparing BaTiO3 films and BaTiO3/SrTiO3 multilayers of different numbers of individual layers, but equal overall thickness. The dielectric loss saturates for a thickness above 300 nm and linearly decreases with decreasing film thickness below a thickness of 75 nm. At the same thickness of 75 nm, the thickness dependence of the dielectric constant also exhibits a change in the linear slope both for BaTiO3 films and BaTiO3/SrTiO3 multilayers. This behaviour is explained by the change observed in the grain morphology at a thickness of 75 nm. For the thickness dependence of the dielectric constant, two phenomenological models are considered, viz. a ‘series-capacitor’ model and a ‘dead-layer’ model.

Nonpolar a-plane GaN film on Si(100) produced using a specially designed lattice-matched buffer: A fresh approach to eliminate the polarization effect

Using pulsed laser deposition, nonpolar a-plane GaN thin films were grown on Si(100) substrates coated with a nonpolar MnS(100) plane buffer layer. The film showed an epitaxial relationship of GaN(112¯0)∥MnS(100)∥Si(100) with an in-plane alignment of GaN[1¯100]∥MnS[010]∥Si[010]. The high-resolution cross-sectional transmission electron microscopy image of the GaN∕MnS interface showed an abrupt atomic interface. A strong band-edge emission from the GaN film was observed at 364.3nm (=3.4eV) in cathodoluminescence measurements at 30K. This result in controlling the growth plane provides GaN films free of polarization effects in the direction of film growth, which favor the integration of optoelectronic devices combined with silicon.

Growth of a-plane ZnO Thin Films on r-plane Sapphire by Plasma-assisted MBE

ABSTRACTZnO thin films have been epitaxially grown on r-plane sapphire by RF-plasma-assisted molecular beam epitaxy. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies indicate that the epitaxial relationship between ZnO and r-plane sapphire is (1120)ZnO // (1102)sapphire and [0001]ZnO // [1101]sapphire. Atomic force microscopy measurements reveal islands extended along the sapphire [1101] direction. XRD omega rocking curves for the ZnO (1120) reflection measured either parallel or perpendicular to the island direction suggest the defect density anisotropy along these directions. Due to the small lattice mismatch along the ZnO [0001] direction, few misfit dislocations were observed at the ZnO/Al2O3 interface in the high-resolution cross-sectional TEM image with the zone axis along the ZnO [1100] direction.

Carbon nanotube/metal interface studied by cross-sectional transmission electron microscopy

We present the first results on the interface between carbon nanotube (CNT) and metal by cross-sectional high-resolution transmission electron microscopy and spatially resolved electron energy loss spectroscopy on Fe-filled CNTs. The iron core is mainly single crystalline $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}$. Under specific orientation relationships, a semicoherent CNT/Fe interface forms, where local lattice mismatch is accommodated by ``interface dislocations.'' A $0.2--0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ energy shift of the $\ensuremath{\pi}*$ peak of the carbon $K$-edge towards higher values is found at the interface. The results prove experimentally an interfacial bonding between the CNT wall and Fe.

Electrical and materials characterization of GSMBE grown Si1−x−yGexCy layers for heterojunction bipolar transistor applications

This paper reports on detailed materials and electrical characterization of strain-compensated Si1−x−yGexCy (x ⩽ 11%, y ⩽ 1%) layers synthesized by gas source molecular beam epitaxy (GSMBE). Materials assessment included the application of spectroscopic ellipsometry (SE), x-ray diffraction (XRD) and high-resolution cross-section transmission electron microscopy (HRXTEM). An almost fully strain-compensated Si0.88Ge0.11C0.01 alloy with high substitutional carbon incorporation (XRD) and well-defined structure with coherent interfaces (HRXTEM) was produced. An increase of C content (up to 1.6%) resulted in a tensile strained SiGeC alloy and degraded electrical characteristics. The minority-carrier lifetime is a parameter strongly dependent on mid-gap energy band levels, which determine the leakage currents in heterojunction bipolar transistors, and it was assessed here using the capacitance-transient technique. Metal–oxide–semiconductor (MOS) test capacitors were fabricated on Si/SiGe/Si and Si/SiGeC/Si structures using low-temperature plasma oxidation for dielectric growth. The strain-free Si0.88Ge0.11C0.01 layers exhibit a minority-carrier lifetime, τg, of ∼0.1–0.2 µs which constitutes degradation of the carrier lifetime by two orders of magnitude compared to control SiGe layers. The obtained range of τg is nevertheless sufficient for the successful application of epitaxial strain-compensated SiGeC layers in high performance heterojunction bipolar transistors (HBTs).

Nanostructured TiC/a-C coatings for low friction and wear resistant applications

Closed-field unbalanced and balanced magnetron sputtering was used to deposit nanocrystalline TiC (nc-TiC)/amorphous carbon (a-C) nanocomposite coatings with hydrogenated or hydrogen-free a-C matrix, respectively. The contents of Ti and C in the coatings have been varied over the full range of interest (7–45 at.% Ti) by changing the flow rate of acetylene gas or the locations of substrates relative to the center of C/TiC targets. Different levels of bias and deposition pressure were used to control the nanostructure. The nanocomposite coatings exhibit hardness of 5–35 GPa, hardness/ E-modulus ratio up to 0.15, wear rate of 4.8×10 −17 m 3/N m lap, friction coefficient of 0.04 under dry sliding and strong self-lubrication effects. The nanostructure and elemental distribution in the coatings have been characterized with cross-sectional and planar high-resolution transmission electron microscopy (HRTEM) and energy-filtered TEM. The influence of the volume fraction and size distribution of nc-TiC on the coating properties has been examined.

Formation of silicon on plasma synthesized aluminum nitride structure by ion cutting

The application of silicon-on-insulator (SOI) substrates to high-power integrated circuits is hampered by self-heating effects due to the poor thermal conductivity of the buried SiO2 layer. We propose to replace the buried SiO2 layer in SOI with a plasma synthesized AlN thin film to mitigate the self-heating penalty. The AlN films synthesized on silicon by metal plasma immersion ion implantation and deposition exhibit outstanding surface topography and excellent insulating characteristics. Using a modified direct bonding process in conjunction with hydrogen-induced layer transfer, a silicon-on-AlN structure has been successfully fabricated. Cross-sectional high-resolution transmission electron microscopy, x-ray photoelectron spectroscopy, and spreading resistance profiling results reveal a uniform buried AlN layer beneath a single crystal Si overlayer. The interfaces between the top Si layer, buried AlN layer, and Si substrate are smooth and sharp. Moreover, the use of relatively thick buried AlN layer bodes well for SOI wafers in high voltage and high power applications.

Formation of silicon-on-aluminum nitride using ion-cut and theoretical investigation of self-heating effects

We propose to replace the buried SiO 2 layer in silicon-on-insulator (SOI) with a plasma-synthesized AlN thin film to mitigate the self-heating penalty. The AlN films synthesized on Si by metal plasma immersion ion implantation–deposition (Me-PIIID) exhibit outstanding surface topography and excellent insulating characteristics. Using direct bonding process and the hydrogen-induced layer transfer method, a silicon-on-AlN (SOAN) structure has been successfully fabricated. Cross-sectional high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) depth profiles and spreading resistance probe (SRP) reveal that a uniform buried AlN layer is under a single crystal Si overlayer. The interfaces between the top Si layer, buried AlN layer, and Si substrate are smooth and sharp. In addition, the new SOAN device has been verified in two-dimensional device simulation and demonstrates that the self-heating penalty of SOI can indeed be reduced using SOAN substrates.