Cross-sectional Scanning Transmission Electron Microscopy Research Articles

Nanoscale structural and electrical properties of graphene grown on AlGaN by catalyst-free chemical vapor deposition

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C3H8 flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq−1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles.

Fabrication process and failure analysis for robust quantum dots in silicon

We present an improved fabrication process for overlapping aluminum gate quantum dot devices on Si/SiGe heterostructures that incorporates low-temperature inter-gate oxidation, thermal annealing of gate oxide, on-chip electrostatic discharge (ESD) protection and an optimized interconnect process for thermal budget considerations. This process reduces gate-to-gate leakage, damage from ESD, dewetting of aluminum and formation of undesired alloys in device interconnects. Additionally, cross-sectional scanning transmission electron microscopy (STEM) images elucidate gate electrode morphology in the active region as device geometry is varied. We show that overlapping aluminum gate layers homogeneously conform to the topology beneath them, independent of gate geometry and identify critical dimensions in the gate geometry where pattern transfer becomes non-ideal, causing device failure.

Effects of cyclic loading on subsurface microstructural changes of zirconia polycrystals in nanoscale mechanical processing

Yttria stabilized zirconia (YSZ) is an attractive biomaterial with excellent properties, but its deformation mechanism under repetitive loads is still unclear. In this study, the subsurface microstructural changes of YSZ polycrystals under repetitive loading/unloading were investigated by multi-cyclic nanoindentation. Raman spectroscopy analysis revealed that a tetragonal-to-monoclinic (t-m) phase transformation occurred, and multi-cyclic indentation promoted this transformation significantly. Cross-sectional scanning transmission electron microscopy showed that monoclinic variants were extensively generated beneath the free surfaces outside the indent planes, whereas grain refinement was dominant beneath the indenter tip. In addition, load-displacement curve analysis demonstrated that the t-m phase transformation occurred during unloading rather than loading, and that the phase transformation-induced volume expansion caused an incremental change of residual indent depth. The extension of monoclinic variants was promoted by repetitive shear deformation, resulting in significant pileups, as detected by atomic force microscopy. The effect of sample surface preprocessing on indentation behaviors was also investigated. Microcracking and surface spalling were confirmed around the multi-cyclic indents made on a diamond-turned YSZ sample. Based on the experimental results, a model of subsurface microstructure distribution and evolution was established. The findings from this study help clarify the historical effects in subsurface damaging mechanism of brittle polycrystalline materials caused by cyclic tool-workpiece interactions in nanoscale mechanical processing.

Screw dislocations on pyramidal planes induced by Vickers indentation in HVPE GaN

The dislocation structure around the Vickers indentation on a hydride vapor-phase epitaxy GaN wafer was observed by multi-photon photoluminescence imaging and cross-sectional scanning transmission electron microscopy. Straight dislocation lines parallel to the six equivalent directions, i.e. a rosette pattern, were observed in the subsurface. Below the rosette pattern, a flower-like pattern similar to dislocation loops expanded in the six equivalent directions. Under the flower-like pattern, an inverted triangular dark area composed of dislocations on pyramidal planes was observed. This structure did not vary with load (20–980 mN) or crack formation. The inverted triangular dark area contained screw dislocations with a slip system and an estimated density of 2 × 109 cm−2. The penetration depth of the screw dislocation on the pyramidal plane was found to be proportional to the length of the diagonal indentation line.

Structural Characterization of a Ga<sub>2</sub>O<sub>3 </sub>Epitaxial Layer Grown on a Sapphire Substrate Using Cross-Sectional and Plan-View TEM/STEM Analysis

Ga2O3 is a hopeful wide-band-gap semiconductor material for a next-generation power semiconductor. We performed crystal structure analysis on Ga2O3 film on sapphire substrate using cross-sectional transmission electron microscope (TEM) and atomic resolution plan-view scanning transmission electron microscopy (STEM). The TEM analysis suggested that the main Ga2O3 film is composed of κ-Ga2O3 or mixed crystal of κ-Ga2O3 and ε-Ga2O3. But, it is difficult to distinguish these two possibilities only by cross-sectional TEM. Contrast modulation of Ga atomic columns in the atomic resolution HAADF-STEM image showed that the main part of the Ga2O3 film was κ-Ga2O3 monolayer grown along the c-axis direction, and twins are formed.

Structural Analysis of CoFeB/MgO-Based Perpendicular MTJs With Junction Size of 20 nm by STEM Tomography

We have successfully observed the process-induced microstructure of the ultrafine CoFeB-MgO-based magnetic tunnel junctions with a perpendicular easy axis (p-MTJs) down to junction size of less than 20 nm using scanning transmission electron microscope (STEM) tomography combined with energy-dispersive X-ray spectroscopy (EDX). This made it possible to examine 3-dimensionally the precise structural information inside the entire MTJ. By this, the root cause of the change in the resistance of the MTJ was found to be the altered region consisting of Fe-dipping attached to the edge of the MgO and ultrathin TaO layer uniformly formed on the MTJ surface. This altered region is caused by etching during patterning of the MTJ, and the resistance value of the MTJ changes depending on the degree of adhesion of insulating TaO and Fe atoms at the edge of the MgO barrier layer. Furthermore, it was found that the unevenness of the MgO/CoFeB interface inside the MTJ was not a factor that caused the change in the thickness of the MgO barrier layer in the current process. Our results show that STEM tomography is an effective tool for failure analysis to clarify the relationship between the detailed structure of fine MTJs and magnetic properties. Unlike the conventional cross-sectional STEM observation, since the STEM tomographic observation includes the whole view of the MTJ, this enables higher resolution observation as the MTJ size decreases without overlooking the cause of failure inside the MTJ.

Reactive Sputtering Deposition of Epitaxial TiC Film on Si (100) Substrate

Epitaxial (100) TiC film deposition on Si (100) substrate by direct current magnetron reactive sputtering of a metallic Ti target with 3%–6% CH4 in Ar gas was investigated. X-ray diffraction and cross-sectional scanning transmission electron microscopy (STEM) reveal that epitaxial cubic TiC can be grown on the Si substrate by domain matching epitaxy in 5/4 ratio with the epitaxial relationship of TiC (100)[0 1 ¯ 1] // Si (100)[0 1 ¯ 1]. For sputtering with 3% and 4% CH4, the deposited films are found to consist of both TiC and metallic Ti phases. Increasing the CH4 flow ratio to 5% results in a deposited film completely consisting of TiC without metallic Ti phase. The crystallinity of the deposited TiC is also improved with increasing the CH4 ratio to 5%. X-ray photoelectron spectroscopy shows that the [C]/[Ti] atomic ratio in TiC is nearly close to 1 for growth with 5% CH4 flow ratio and above. The measured electrical resistivities of the deposited films also increase from 41 to 153 μΩ·cm with increasing the CH4 ratio from 3% to 6%. With film growth beyond 50 nm thickness, it is shown that some disoriented TiC grains are formed.

Chemical bonding at room temperature via surface activation to fabricate low-resistance GaAs/Si heterointerfaces

Bonding mechanism at room temperature (RT) in GaAs/Si heterointerfaces fabricated by surface-activated bonding (SAB) is examined using cross-sectional scanning transmission electron microscopy combined with low-temperature focused ion beam and time-of-flight secondary ion mass spectrometry. In the bonding process at RT, atomic intermixing at the interfaces, presumably due to the transient enhanced diffusion assisted by the point defects introduced in the surface activation process, is confirmed. The defect-assisted atomic diffusion at the interfaces, as well as the formation of atomically clean and activated surfaces, would be the key concept of SAB, by which we can create tough heterointerfaces at RT. Meanwhile, the defects on the activated surfaces would degrade the interface resistance. The degraded properties can be recovered by an appropriate annealing after the SAB processes, although the atomistic structure around the heterointerfaces would be modified during the annealing. By controlling SAB and subsequent annealing conditions, we can obtain low-resistance heterointerfaces via the optimization of the trade-off relationship between the chemical bonding strength and the electronic properties, determined by the activated surfaces before bonding.

Controllable Vacancy Redistributions in Embedded Thin Films Under Concentrated Electric Fields

The redistribution and accumulation of oxygen vacancies under electric field underpins both the functionality and degradation of electrical and electrochemical devices comprising oxide layers. Interfacial accumulations of oxygen vacancies form defect dipoles, significantly reduce the breakdown strength, increase the breakdown path area, yet facilitates charge transfer reactions. Confinement of both oxygen vacancy redistributions and their mobility remains critical to preserving function in oxide systems.Here we study one of the most important, yet random transport processes: the redistribution of oxygen vacancies in binary oxides under high local electric fields. The ability to identify or predefine the shape, size, and location of vacancy distributions is critical to controlling interfacial properties of functional oxides. We achieved superior uniformity in HfO2 films by embedding highly-ordered metal nanoisland (NI) arrays to act as electric field concentrators. Embedded films exhibit significant reduction in operating voltages while displaying enhanced uniformity of resistance states. This behavior is attributed to the concentration of electric fields along Pt and Ti NIs and their interactions with the surrounding oxide film matrix environment, which induce separate and distinct filamentary formation mechanisms that affect the stability. A comparison of the density and distribution of the oxygen vacancies responsible driving filament dynamics is made via combined electrostatic force microscopy (EFM) and conductive atomic force microscopy (c-AFM) studies. Finally, the morphological evolution of conducting vacancy filaments is enabled by three-dimensional (3-D) c-AFM nanotomography and cross-sectional scanning transmission electron microscopy – energy dispersive spectroscopy (STEM-EDS) to provide direct correlations of nanoisland-oxide interactions with overall switching performance. The application of these exciting results to other vacancy-mediated electronic and electrochemical processes will also be discussed.

Characterization of Nanoscopic Cu/Diamond Interfaces Prepared by Surface-Activated Bonding: Implications for Thermal Management

The microstructures of Cu/diamond interfaces prepared by surface-activated bonding at room temperature are examined by cross-sectional scanning transmission electron microscopy (STEM). A crystalline defect layer composed of Cu and diamond with a thickness of approximately 4.5 nm is formed at the as-bonded interface, which is introduced by irradiation with an Ar beam during the bonding process. No crystalline defect layer is observed at the 700 °C annealed interface, which is attributed to the recrystallization of the defect layer due to the high-temperature annealing process. Instead of the defect layer, a mating interface layer and a copper oxide layer are formed at the interface. The mating interface layer and the copper oxide layer play a role in relieving the residual stress caused by the difference between the thermal expansion coefficients of diamond and Cu. The thermal boundary resistance (TBR) of the as-bonded interface is measured to be 1.7 ± 0.2 × 10–8 m2·K/W by the time domain pulsed-light-heating thermoreflectance technique. These results indicate that the direct bonding of diamond and Cu is a very effective technique for improving the heat-dissipation performance of power devices.

Directly Linking Low-Angle Grain Boundary Misorientation to Device Functionality for GaAs Grown on Flexible Metal Substrates.

A new growth method to make highly oriented GaAs thin films on flexible metal substrates has been developed, enabling roll-to-roll manufacturing of flexible semiconductor devices. The grains are oriented in the <001> direction with <1° misorientations between them, and they have a comparable mobility to single-crystalline GaAs at high doping concentrations. At the moment, the role of low-angle grain boundaries (LAGBs) on device performance is unknown. A series of electron backscatter diffraction (EBSD) and cathodoluminesence (CL) studies reveal that increased doping concentrations decrease the grain size and increase the LAGB misorientation. Cross-sectional scanning transmission electron microscopy (STEM) reveals the complex dislocation structures within LAGBs. Most importantly, a correlative EBSD/electron beam-induced current (EBIC) experiment reveals that LAGBs are carrier recombination centers and that the magnitude of recombination is dependent on the degree of misorientation. The presented results directly link increased LAGB misorientation to degraded device performance, and therefore, strategies to reduce LAGB misorientations and densities would improve highly oriented semiconductor devices.

Van der Waals Heteroepitaxy of Te Crystallites/2H-MoTe2 Atomically Thin Films on GaAs Substrates by Using Metal-Organic Chemical-Vapor Deposition

Te crystallites/few atomic-layer MoTe2 films grown on (100) GaAs substrates by using metal-organic chemical — vapor deposition (MOCVD) were investigated. MOCVD provides high-quality films, controllable film thickness, composition uniformity, and scalability to wafer scale size via the pyrolysis process of metal-organic sources. We observed several tens of nanometers of Te crystallites/three atomic layers 2H phase MoTe2 on the GaAs substrates. This observation was confirmed by using Raman spectroscopy, X-ray photoemission spectroscopy (XPS), atomic force microscopy, cross-sectional scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. A two-dimensional transition-metal dichalcogenide deposited on GaAs by using the van der Waals epitaxy is a promising method for the development of future high-performance optoelectronic and electronic devices.

Formation mechanism and suppression of Ga-rich streaks at macro-step edges in the growth of AlGaN on an AlN/sapphire-template

Abstract The formation mechanism of Ga-rich streaks in the AlGaN layers formed at macro-step edges was investigated by analyzing AlGaN layers grown by metalorganic vapor phase epitaxy on AlN/sapphire-templates. Energy dispersive X-ray spectroscopy in the cross-sectional scanning transmission electron microscopy revealed that the macro-steps formed during AlN growth were maintained through the upper AlGaN layer. This can be explained by the less interaction between adjacent macro-steps due to the sufficiently smaller surface diffusion lengths of Ga and Al species than the inter-macro-step distance of submicron to a few microns. Moreover, atomic force microscopy indicated that the apparent smooth region between macro-steps involved many atomic steps and terrace width of a few ten nanometers which may be the actual scale of the diffusion lengths. At the macro-step edge, facets that are different from a c-plane are formed. The higher incorporation efficiency of Ga atoms on these facets compared with that of the c-plane likely led to the formation of Ga-rich streaks due to the less inter-surface diffusion between the facets and c-plane regions. We demonstrated AlGaN growth without Ga-rich streaks using the flattened AlN/sapphire-template by polishing; this resulted in the achievement of homogeneous emission energies in AlGaN-based quantum wells.

Interdiffusion during heteroepitaxial Au growth on Pd thin films by electroless Au plating (ELGP) at room temperature

Electroless Au plating (ELGP) on Pd thin films is performed. The ELGP time dependence on the surface morphology is investigated by a scanning electron microscope. Continuous nucleation and growth of hemispherical Au islands are observed as the plating time increases. Heteroepitaxial growth of the electroless plated Au on the Pd surface is confirmed by cross-sectional scanning transmission electron microscope images. Cross-sectional energy-dispersive X-ray spectroscopy reveals interdiffusion between the Pd atoms and the plated Au atoms at room temperature, which allows the formation of Au–Pd alloys. The heteroepitaxial Au growth on Pd is significant for reducing contact resistance of Au/Pd structures.

Effect of Mg shell on MgH2 dehydrogenation by morphological and mathematical analysis

Magnesium (Mg) formation during dehydrogenation of nanocrystalline magnesium hydride (MgH2), exhibiting a bimodal particle size distribution (4 ± 1 and 16 ± 8 μm) at 320 and 350°C, was investigated using X-ray diffraction patterns, cross-sectional scanning electron microscopy (SEM) and transmission electron microscopy. Magnesium hydride dehydrogenation consists of an initial period of slow hydrogen gas (H2) release (incubation period). Following incubation, hydrogen release increases significantly. SEM images of the dehydrogenating magnesium hydride particles are graphically divided into several concentric annuli. The radially outward flux of hydrogen released within the particles was estimated by developing a quantitative approach. During incubation, the absence of a magnesium shell necessitates slower hydrogen atom diffusion through magnesium hydride as depicted by the flux estimation. Beyond incubation, the enhanced hydrogen release is due to the presence of a magnesium shell. Thus, the effect of the absence/presence of a magnesium shell on magnesium hydride dehydrogenation was studied using the estimated fluxes.

Improved current-perpendicular-to-plane giant magnetoresistance outputs by heterogeneous Ag-In:Mn-Zn-O nanocomposite spacer layer prepared from Ag-In-Zn-O precursor

A precursor material Ag-In-Zn-O (AIZO) for the spacer layer of current-perpendicular-to-plane giant magnetoresistance devices has been developed. A pseudo-spin-valve structure with polycrystalline Co2(Mn0.6Fe0.4)Ge (CMFG) Heusler alloy ferromagnetic layers and a AIZO spacer precursor exhibited a large magnetoresistance ratio (ΔR/R) up to 54% at a resistance-area product (RA) of ∼0.075 Ω μm2 and a maximum output voltage of ∼18 mV. Cross-sectional scanning transmission electron microscopy observations revealed that the spacer layer was not uniform but had a heterogeneous Ag-In:Mn-Zn-O nanocomposite structure, which is considered to have formed by the oxidation of Mn diffused from the CMFG layers and by the reduction of In2O3 to metallic In alloyed with Ag. Due to the current-confinement effect through the fcc Ag-In metallic path (current-confined-path, CCP) with the average lateral size of ∼7 nm surrounded by a rocksalt Mn-Zn-O matrix, both RA and ΔR/R were enhanced compared to the case with the uniform AgSn alloy spacer layer. A Ag/IZO bilayer precursor also formed a similar Ag-In:Mn-Zn-O nanocomposite spacer with a lateral size of the Ag-In CCPs of ∼3 nm, and the device with the Ag/IZO precursor showed ΔR/R ∼ 30%. The difference in ΔR/R between the AIZO and Ag/IZO precursors can be explained by the matching of effective spin resistance between the CMFG ferromagnetic layers and the Ag-In CCP with different sizes. The resistance dispersion of the devices was also analyzed both experimentally and theoretically.

Effect of surface modification treatment of buffer layer on thermal tolerance of synthetic ferrimagnetic reference layer in perpendicular-anisotropy magnetic tunnel junctions

We improve thermal tolerance of a Co/Pt based synthetic ferrimagnetic (SyF) reference layer in a CoFeB/MgO-based perpendicular anisotropy magnetic tunnel junction by employing a surface modification treatment (SMT) using Ar plasma on a Pt buffer layer underneath the SyF layer. A higher perpendicular anisotropy and more robustness against annealing for the SyF reference layer is observed by the employment of SMT. From the cross-sectional scanning transmission electron microscope of the magnetic tunnel junction and atomic force microscopy on the Pt buffer layer, SMT has no significant effect on the surface roughness of the Pt buffer layer. On the other hand, X-ray diffraction analysis reveals that SMT increases the lattice spacing of the Pt buffer layer and shrinks that of the Co/Pt multilayer, which are maintained after annealing at 400 °C. Those structural changes could lead to the improvement of perpendicular anisotropy and thermal tolerance against annealing. A high-resolution Rutherford backscattering analysis reveals that Ar ions used for SMT penetrate to the Pt buffer layer, resulting in variation of the lattice spacing for the Pt buffer layer and Co/Pt multilayer. In addition, suppression of Fe diffusion from the CoFeB reference layer to the bottom Co/Pt multilayer is observed by using energy dispersive X-ray spectrometry, which also contributes to the higher thermal tolerance against annealing. The present observation reveals that the employment of SMT enables one to obtain both high perpendicular magnetic anisotropy and high robustness against annealing through crystallographic change of the Co/Pt multilayer and suppression of Fe diffusion from CoFeB layer into the Co/Pt multilayer.

Formation of graphene atop a Si adlayer on the C-face of SiC

The structure of the SiC(000-1) surface, the C-face of the {0001} SiC surfaces, is studied as a function of temperature and of pressure in a gaseous environment of disilane (Si2H6). Various surface reconstructions are observed, both with and without the presence of an overlying graphene layer (which spontaneously forms at sufficiently high temperatures). Based on cross-sectional scanning transmission electron microscopy measurements, the interface structure that forms in the presence of the graphene is found to contain 1.4 - 1.7 monolayers (ML) of Si, a somewhat counter-intuitive result since, when the graphene forms, the system is actually under C-rich conditions. Using ab initio thermodynamics, it is demonstrated that there exists a class of Si-rich surfaces containing about 1.3 ML of Si that are stable on the surface (even under C-rich conditions) at temperatures above about 400 K. The structures that thus form consist of Si adatoms atop a Si adlayer on the C-face of SiC, with or without the presence of overlying graphene.

Reactive sputtering for highly oriented HfN film growth on Si (100) substrate

Highly oriented cubic (100) HfN films were grown on Si (100) substrates by direct current magnetron reactive sputtering of a metallic Hf target in an Ar/N2 gas environment. The influence of N2 flow ratio on the (100) preferred orientation and crystallinity of the HfN films is investigated. X-ray diffraction shows that not only HfN but also orthorhombic HfSi2 forms in the sputtered films. Increasing the N2 flow ratio is unfavorable for the formation of HfSi2 while the deposition rate of HfN is decreased. X-ray diffraction and cross-sectional scanning transmission electron microscopy (STEM) reveal that epitaxial orthorhombic HfSi2 can form on the Si substrate, and (100) HfN is in epitaxy with the epitaxial HfSi2. As a result, a (100) oriented HfN film can grow on Si. The epitaxial relationships are shown to be HfN (100)[011¯] // HfSi2 (020)[001] // Si (100)[011¯] and HfN (100)[011¯] // HfSi2 (020)[100] // Si (100)[011¯]. Atomically resolved STEM images also show the bonding characteristics across the HfN/HfSi2 and HfSi2/Si interfaces.

Direct evidence of Mg diffusion through threading mixed dislocations in GaN p–n diodes and its effect on reverse leakage current

Mg diffusion is a common problem in GaN devices with p–n junctions. Although this impurity diffusion is reported to occur through threading dislocations (TDs), no direct evidence has yet been obtained. Therefore, we tried the direct observation of Mg diffusion by atom probe tomography (APT) analysis. The n-type drift layer of the fabricated p–n diode was exposed, and etch pits were formed on the drift layer to identify the TD position. The APT analysis around TDs was carried out by lifting out the drift layer around specific etch pits using a focused ion beam to include TDs. The relationship between the etch pit shape and the TD type was confirmed by cross-sectional scanning transmission electron microscopy observation. The APT analysis of two types of etch pits formed on the mixed dislocations was performed, and Mg diffusion was clearly observed through the mixed dislocations. In this work, we show direct evidence of Mg diffusion via mixed dislocations in GaN p–n diodes and its effect on reverse leakage current.