Systematic study of FIB-induced damage for the high-quality TEM sample preparation.

Nowadays, a focused Ga ion beam (FIB) with a scanning electron microscopy (SEM) system has been widely used to prepare the thin-foil sample for transmission electron microscopy (TEM) or scanning TEM (STEM) observation. An establishment of a solid strategy for a reproducible high-quality sample preparation process is essential to carry out high-quality (S)TEM analysis. In this work, the FIB damages introduced by Ga+ beam were investigated both experimentally and stopping and range of ions in matter (SRIM) simulation for silicon (Si), gallium nitride (GaN), indium phosphide (InP), and gallium arsenide (GaAs) semiconductors. It has been revealed that experimental investigations of the FIB-induced damage are in good agreement with SRIM simulation by defining the damage as not only "amorphization" but also "crystal distortion". The systematic evaluation of FIB damages shown in this paper should be indispensable guidance for reliable (S)TEM sample preparation.

Atomic-Scale Multimodal Characterization of Self-Assembled InAs/InGaAlAs Quantum Dots.

Self-assembled quantum dots (QDs) are potential candidates for photoelectric and photovoltaic devices, because of their discrete energy levels. The characterization of QDs at the atomic level using a multimodal approach is crucial to improving device performance because QDs are nanostructures with highly correlated structural parameters. In this study, scanning transmission electron microscopy, geometric phase analysis, and atom probe tomography were employed to characterize structural parameters such as the shape, strain, and composition of self-assembled InAs-QDs with InGaAlAs spacer layers. The measurements revealed characteristic AlAs-rich regions above the QDs and InAs-rich regions surrounding the QD columns, which can be explained by the relationship between the effect of strain and surface curvature around the QD. The methodology described in this study accelerates the development of future QD devices because its multiple perspectives reveal phenomena such as atomic-scale segregations and allow for more detailed discussions of the mechanisms of these phenomena.

An Automated Site-Specific Tip Preparation Method for Atom Probe Tomography Using Script-Controlled Focused Ion Beam/Scanning Electron Microscopy.

The automation of the atom probe tomography (APT) tip preparation using a focused ion beam (FIB) with a scanning electron microscopy (SEM) dual-beam system will certainly contribute to systematic APT research with higher throughput and reliability. While our previous work established a method to prepare tips with a specified tip curvature and taper angle automatically, by using script-controlled FIB/SEM, the technique has been expanded to automated "site-specific" tip preparation in the current work. The improved procedure can automatically detect not only the tip shape but also the interface position in the tip; thus, the new function allows for control of the tip apex position. In other words, automated "site-specific" tip preparations are possible. The details of the automation procedure and some experimental demonstrations, that is, a Pt cap on Si, InGaN-based MQWs, and a p-n junction of GaAs, are presented.

Robust Preparation of Sub-20-nm-Thin Lamellae for Aberration-Corrected Electron Microscopy.

Aberration-corrected scanning transmission electron microscopy (STEM) has been advancing resolution, sensitivity, and microanalysis due to the intense demands of atomic-level microstructural investigations. Recent STEM technologies require preparing a thin lamella whose thickness is ideally below 20 nm. Although focused-ion-beam/scanning-electron-microscopy (FIB/SEM) is an established method to prepare a high-quality lamella, nanometer-level controllability of lamella thickness remains a fundamental problem. Here, the robust preparation of a sub-20-nm-thin lamella is demonstrated by FIB/SEM with real-time feedback from thickness quantification. The lamella thickness is quantified by back-scattered-electron SEM imaging in a thickness range between 0 and 100 nm without any reference to numerical simulation. Using real-time feedback from the thickness quantification, the FIB/SEM terminates thinning a lamella at a targeted thickness. The real-time feedback system eventually provides 1-nm-level controllability of the lamella thickness. As a proof-of-concept, a near-10-nm-thin lamella is prepared from a SrTiO3 crystal by our methodology. Moreover, the lamella thickness is controllable at a target heterointerface. Thus, a sub-20-nm-thin lamella is prepared from a LaAlO3 /SrTiO3 heterointerface. The methodology offers a robust and operator-independent platform to prepare a sub-20-nm-thin lamella from various materials. This platform will broadly impact aberration-corrected STEM studies in materials science and the semiconductor industry.

Development of automated tip preparation for atom probe tomography by using script-controlled FIB-SEM.

Atom probe tomography (APT) has become a popular technique for microstructural analysis of a wide range of alloys and devices over the past two decades owing to the employment of laser-assisted field evaporation and the development of site-specific tip preparation using a focused ion beam (FIB) with a scanning electron microscopy (SEM) system. In laser-assisted field evaporation, laser irradiation conditions largely influence mass resolution; therefore, recent commercial APT instruments allow strict control of the analysis conditions. However, the mass resolution is affected not only by the laser condition but also by the thermal conductivity of the material and the tip shape. In addition, it is also important to keep the tip shape constant in order to obtain tomography data with good reproducibility since the analytical volume highly depends on the tip shape. In this study, we have developed a method to fabricate the tip with the desired shape automatically by using a script-controlled FIB-SEM system, which has traditionally depended on the skill of the FIB-SEM operator. The tip shape was then intentionally changed by using this method, and its effect on the APT data is also discussed.

Atomic Diffusion of Indium through Threading Dislocations in InGaN Quantum Wells.

The compositional and structural investigations of threading dislocations (TDs) in InGaN/GaN multiple quantum wells were carried out using correlative transmission electron microscopy (TEM) and atom probe tomography (APT). The correlative TEM/APT analysis on the same TD reveals that the indium atoms are diffused along the TD and its concentration decreases with distance from the InGaN layer. On the basis of the results, we directly observed that the indium atoms originating from the InGaN layer diffuse toward the epitaxial GaN surface through the TD, and it is considered to have occurred via the pipe diffusion mechanism induced by strain energy relaxation.

Stable field-emission from a CeB6 nanoneedle point electron source.

A single CeB6 nanoneedle structure has been fabricated using a focused ion beam (FIB) and its field emission characteristics have been evaluated. A converged electron beam has been obtained, attributed to its sharpened tip with a radius of curvature of about 10 nm. Combined with its low work function, the required electric field is as low as 1.6 V nm-1 to generate a field emission current of 50 nA. The most outstanding feature of the CeB6 nanoneedle emitter is its excellent current stability that enabled continuous emission for 16 hours with a fluctuation of 1.6% and without deterioration even in a vacuum of 10-7 Pa. The stable field-emission is attributed to the nanometric tip radius that led to reduction in gas adsorption and desorption. In addition, the downward dipolar structure on the emission surface is also beneficial for making the surface inert. These performance factors make CeB6 a practical field-emission point electron source for microscopy applications.

A stable LaB6 nanoneedle field-emission point electron source.

A material with a low work function exhibiting field-emission of electrons has long been sought as an ideal point electron source to generate a coherent electron beam with high brightness, long service life, low energy spread, and especially stable emission current. The quality and performance of the electron source are now becoming limiting factors for further improving the spatial resolution and analytical capabilities of the electron microscope. While tungsten (W) is still the only material of choice as a practically usable field emission filament since it was identified more than six decades ago, its electron optical performance remains unsatisfactory, especially the poor emission stability (>5% per hour), rapid current decay (20% in 10 hours), and relatively large energy spread (0.4 eV), even in an extremely high vacuum (10-9 Pa). Herein, we report a LaB6 nanoneedle structure having a sharpened tip apex with a radius of curvature of about 10 nm that is fabricated and finished using a focused ion beam (FIB) and show that it can produce a field emission electron beam meeting the application criteria with a high reduced brightness (1010 A m-2 sr-1 V-1), small energy spread (0.2 eV), and especially high emission stability (<1% fluctuation in 16 hours without decay). It can now be used practically as a next-generation field-emission point electron source.

X-ray diffraction and in situ pressurization of dentine apatite reveals nanocrystal modulus stiffening upon carbonate removal.

Bone-like materials comprise carbonated-hydroxyapatite nanocrystals (c-Ap) embedding a fibrillar collagen matrix. The mineral particles stiffen the nanocomposite by tight attachment to the protein fibrils creating a high strength and toughness material. The nanometer dimensions of c-Ap crystals make it very challenging to measure their mechanical properties. Mineral in bony tissues such as dentine contains 2~6 wt.% carbonate with possibly different elastic properties as compared with crystalline hydroxyapatite. Here we determine strain in biogenic apatite nanocrystals by directly measuring atomic deformation in pig dentine before and after removing carbonate. Transmission electron microscopy revealed the platy 3D morphology while atom probe tomography revealed carbon inside the calcium rich domains. High-energy X-ray diffraction in combination with in situ hydrostatic pressurization quantified reversible c-Ap deformations. Crystal strains differed between annealed and ashed (decarbonated) samples, following 1 or 10 h heating at 250 °C or 550 °C respectively. Measured bulk moduli (K) and a-/c-lattice deformation ratios (η) were used to generate synthetic Ksyn and ηsyn identifying the most likely elastic constants C33 and C13 for c-Ap. These were then used to calculate the nanoparticle elastic moduli. For ashed samples, we find an average E11=107 GPa and E33 =128 GPa corresponding to ~5% and ~17% stiffening of the a-/c-axes of the nanocrystals as compared with the biogenic nanocrystals in annealed samples. Ashed samples exhibit ~10% lower Poisson's ratios as compared with the 0.25~0.36 range of carbonated apatite. Carbonate in c-Ap may therefore serve for tuning local deformability within bony tissues. STATEMENT OF SIGNIFICANCE: Carbonated apatite nanoparticles, typical for bony tissues, stiffen the network of collagen fibrils. However, it is not known if the biogenic apatite mechanical (elastic) properties differ from those of geologic mineral counterparts. Indeed the tiny dimensions and variable carbonate composition may have strong effects on deformation resistance. The present study provides experimental measurements of the elastic constants which we use to estimate Young's moduli and Poisson's ratio values. Comparison between ashed and annealed dentine samples quantifies the properties of both carbonated and decarbonated apatite nanocrystals. The results reveal fundamental attributes of bony mineral and showcase the additive advantages of combining X-ray diffraction with in situ hydrostatic compression, backed by atom probe and transmission electron microscopy tomography.

A controllable and efficient method for the fabrication of a single HfC nanowire field-emission point electron source aided by low keV FIB milling.

A single hafnium carbide (HfC) nanowire field-induced electron emitter with a sharp tip apex is fabricated by Pt deposition and focused ion beam (FIB) milling. The structure of the electron emitter is characterized by scanning transmission electron microscopy (STEM) and atom probe tomography (APT). The HfC nanowire is single-crystalline with a thin oxide layer on its tip surface. The field emission properties are determined by using both in situ transmission electron microscopy (TEM) and a field-emission probe in a high-vacuum chamber. A high current of 173 nA was obtained at a low extraction voltage of 631 V with an emission gap of 5 mm. The emission current is stable at 60 nA for 100 min with a fluctuation of 0.7%. The deduced work function was 3.1 eV. It is suggested that the implanted Ga ions and the oxide layer induce more downward dipoles that are beneficial for lowering the work function and creating a stable surface. When the low keV FIB processing is applied, it takes within 30 minutes to finish a HfC nanowire emitter, establishing an efficient procedure for the preparation of nanowire emitters. These results provide a controllable and fast production method for the fabrication of single nanowire field-emission point electron sources.

Investigation of nanoscale voids in Sb-doped p-type ZnO nanowires.

While it has multiple advantageous optoelectronic and piezoelectric properties, the application of zinc oxide has been limited by the lack of a stable p-type dopant. Recently, it was discovered that antimony doping can lead to stable p-type doping in ZnO, but one curious side effect of the doping process is the formation of voids inside the nanowire. While previously used as a signifier of successful doping, up until now, little research has been performed on these structures themselves. In this work, the effect of annealing on the size and microstructure of the voids was investigated using TEM and XRD, finding that the voids form around a region of Zn7Sb2O12. Furthermore, using Raman spectroscopy, a new peak associated with successful doping was identified. The most surprising finding, however, was the presence of water trapped inside the nanowire, showing that this is actually a composite structure. Water was initially discovered in the nanowires using atom probe tomography, and verified using Raman spectroscopy.