X-ray Bragg Ptychography on a Single InGaN/GaN Core-Shell Nanowire

Fast nanoscale imaging of strain in a multi-segment heterostructured nanowire with 2D Bragg ptychography

Developing semiconductor devices requires a fast and reliable source of strain information with high spatial resolution and strain sensitivity. This work investigates the strain in an axially heterostructured 180 nm-diameter GaInP nanowire with InP segments of varying lengths down to 9 nm, simultaneously probing both materials. Scanning X-ray diffraction (XRD) is compared with Bragg projection ptychography (BPP), a fast single-projection method. BPP offers a sufficient spatial resolution to reveal fine details within the largest segments, unlike scanning XRD. The spatial resolution affects the quantitative accuracy of the strain maps, where BPP shows much-improved agreement with an elastic 3D finite element model compared with scanning XRD. The sensitivity of BPP to small deviations from the Bragg condition is systematically investigated. The experimental confirmation of the model suggests that the large lattice mismatch of 1.52% is accommodated without defects.

3D Bragg Coherent Diffraction Imaging of Extended Nanowires: Defect Formation in Highly Strained InGaAs Quantum Wells

InGaAs quantum wells embedded in GaAs nanowires can serve as compact near-infrared emitters for direct integration onto Si complementary metal oxide semiconductor technology. While the core-shell geometry in principle allows for a greater tuning of composition and emission, especially farther into the infrared, the practical limits of elastic strain accommodation in quantum wells on multifaceted nanowires have not been established. One barrier to progress is the difficulty of directly comparing the emission characteristics and the precise microstructure of a single nanowire. Here we report an approach to correlating quantum well morphology, strain, defects, and emission to understand the limits of elastic strain accommodation in nanowire quantum wells specific to their geometry. We realize full 3D Bragg coherent diffraction imaging (BCDI) of intact quantum wells on vertically oriented epitaxial nanowires, which enables direct correlation with single-nanowire photoluminescence. By growing In_0.2Ga_0.8As quantum wells of distinct thicknesses on different facets of the same nanowire, we identified the critical thickness at which defects are nucleated. A correlation with a traditional transmission electron microscopy analysis confirms that BCDI can image the extended structure of defects. Finite element simulations of electron and hole states explain the emission characteristics arising from strained and partially relaxed regions. This approach, imaging the 3D strain and microstructure of intact nanowire core-shell structures with application-relevant dimensions, can aid the development of predictive models that enable the design of new compact infrared emitters.

4th generation synchrotron source boosts crystalline imaging at the nanoscale

New 4^th-generation synchrotron sources, with their increased brilliance, promise to greatly improve the performances of coherent X-ray microscopy. This perspective is of major interest for crystal microscopy, which aims at revealing the 3D crystalline structure of matter at the nanoscale, an approach strongly limited by the available coherent flux. Our results, based on Bragg ptychography experiments performed at the first 4^th-generation synchrotron source, demonstrate the possibility of retrieving a high-quality image of the crystalline sample, with unprecedented quality. Importantly, the larger available coherent flux produces datasets with enough information to overcome experimental limitations, such as strongly deteriorated scanning conditions. We show this achievement would not be possible with 3^rd-generation sources, a limit that has inhibited the development of this otherwise powerful microscopy method, so far. Hence, the advent of next-generation synchrotron sources not only makes Bragg ptychography suitable for high throughput studies but also strongly relaxes the associated experimental constraints, making it compatible with a wider range of experimental set-ups at the new synchrotrons.

Revealing nano-scale lattice distortions in implanted material with 3D Bragg ptychography

Small ion-irradiation-induced defects can dramatically alter material properties and speed up degradation. Unfortunately, most of the defects irradiation creates are below the visibility limit of state-of-the-art microscopy. As such, our understanding of their impact is largely based on simulations with major unknowns. Here we present an x-ray crystalline microscopy approach, able to image with high sensitivity, nano-scale 3D resolution and extended field of view, the lattice strains and tilts in crystalline materials. Using this enhanced Bragg ptychography tool, we study the damage helium-ion-irradiation produces in tungsten, revealing a series of crystalline details in the 3D sample. Our results lead to the conclusions that few-atom-large 'invisible' defects are likely isotropic in orientation and homogeneously distributed. A partially defect-denuded region is observed close to a grain boundary. These findings open up exciting perspectives for the modelling of irradiation damage and the detailed analysis of crystalline properties in complex materials.

On the use of the scattering amplitude in coherent X-ray Bragg diffraction imaging

Lens-less imaging of crystals with coherent X-ray diffraction offers some unique possibilities for strain-field characterization. It relies on numerically retrieving the phase of the scattering amplitude from a crystal illuminated with coherent X-rays. In practice, the algorithms encode this amplitude as a discrete Fourier transform of an effective or Bragg electron density. This short article suggests a detailed route from the classical expression of the (continuous) scattering amplitude to this discrete function. The case of a heterogeneous incident field is specifically detailed. Six assumptions are listed and quantitatively discussed when no such analysis was found in the literature. Details are provided for two of them: the fact that the structure factor varies in the vicinity of the probed reciprocal lattice vector, and the polarization factor, which is heterogeneous along the measured diffraction patterns. With progress in X-ray sources, data acquisition and analysis, it is believed that some approximations will prove inappropriate in the near future.

Strain mapping inside an individual processed vertical nanowire transistor using scanning X-ray nanodiffraction

Semiconductor nanowires in wrapped, gate-all-around transistor geometry are highly favorable for future electronics. The advanced nanodevice processing results in strain due to the deposited dielectric and metal layers surrounding the nanowires, significantly affecting their performance. Therefore, non-destructive nanoscale characterization of complete devices is of utmost importance due to the small feature sizes and three-dimensional buried structure. Direct strain mapping inside heterostructured GaSb-InAs nanowire tunnel field-effect transistor embedded in dielectric HfO2, W metal gate layers, and an organic spacer is performed using fast scanning X-ray nanodiffraction. The effect of 10 nm W gate on a single embedded nanowire with segment diameters down to 40 nm is retrieved. The tensile strain values reach 0.26% in the p-type GaSb segment of the transistor. Supported by the finite element method simulation, we establish a connection between the Ar pressure used during the W layer deposition and the nanowire strain state. Thus, we can benchmark our models for further improvements in device engineering. Our study indicates, how the significant increase in X-ray brightness at 4th generation synchrotron, makes high-throughput measurements on realistic nanoelectronic devices viable.

Revealing misfit dislocations in InAs x P1-x -InP core-shell nanowires by x-ray diffraction

InAs _x P_1-x nanowires are promising building blocks for future optoelectronic devices and nanoelectronics. Their structure may vary from nanowire to nanowire, which may influence their average optoelectronic properties. Therefore, it is highly important for their applications to know the average properties of an ensemble of the nanowires. Structural properties of the InAs _x P_1-x -InP core-shell nanowires were investigated using the coplanar x-ray diffraction performed at a synchrotron facility. Studies of series of symmetric and asymmetric x-ray Bragg reflections allowed us to determine the 26% ± 3% of As chemical composition in the InAs _x P_1-x core, core-shell relaxation, and the average tilt of the nanowires with respect to the substrate normal. Based on the x-ray diffraction, scanning, and transmission electron microscopy measurements, a model of the core-shell relaxation was proposed. Partial relaxation of the core was attributed to misfit dislocations formed at the core-shell interface and their linear density was estimated to be 3.3 ± 0.3 × 10⁴ cm^-1.

Coherent Bragg imaging of 60 nm Au nanoparticles under electrochemical control at the NanoMAX beamline

Nanoparticles are essential electrocatalysts in chemical production, water treatment and energy conversion, but engineering efficient and specific catalysts requires understanding complex structure-reactivity relations. Recent experiments have shown that Bragg coherent diffraction imaging might be a powerful tool in this regard. The technique provides three-dimensional lattice strain fields from which surface reactivity maps can be inferred. However, all experiments published so far have investigated particles an order of magnitude larger than those used in practical applications. Studying smaller particles quickly becomes demanding as the diffracted intensity falls. Here, in situ nanodiffraction data from 60 nm Au nanoparticles under electrochemical control collected at the hard X-ray nanoprobe beamline of MAX IV, NanoMAX, are presented. Two-dimensional image reconstructions of these particles are produced, and it is estimated that NanoMAX, which is now open for general users, has the requisites for three-dimensional imaging of particles of a size relevant for catalytic applications. This represents the first demonstration of coherent X-ray diffraction experiments performed at a diffraction-limited storage ring, and illustrates the importance of these new sources for experiments where coherence properties become crucial.

Investigation of Cavity Enhanced XEOL of a Single ZnO Microrod by Using Multifunctional Hard X-ray Nanoprobe

The multifunctional hard X-ray nanoprobe at Taiwan Photon Source (TPS) exhibits the excellent ability to simultaneously characterize the X-ray absorption, X-ray excited optical luminescence (XEOL) as well as the dynamics of XEOL of materials. Combining the scanning electron microscope (SEM) into the TPS 23A end-station, we can easily and quickly measure the optical properties to map out the morphology of a ZnO microrod. A special phenomenon has been observed that the oscillations in the XEOL associated with the confinement of the optical photons in the single ZnO microrod shows dramatical increase while the X-ray excitation energy is set across the Zn K-edge. Besides having the nano-scale spatial resolution, the synchrotron source also gives a good temporal domain measurement to investigate the luminescence dynamic process. The decay lifetimes of different emission wavelengths and can be simultaneously obtained from the streak image. Besides, SEM can provide the cathodoluminescence (CL) to be a complementary method to analyze the emission properties of materials, we anticipate that the X-ray nanoprobe will open new avenues with great characterization ability for developing nano/microsized optoelectronic devices.

Complete structural and strain analysis of single GaAs/(In,Ga)As/GaAs core–shell–shell nanowires by means of in-plane and out-of-plane X-ray nanodiffraction

Typically, core–shell–shell semiconductor nanowires (NWs) made from III–V materials with low lattice mismatch grow pseudomorphically along the growth axis, i.e. the axial lattice parameters of the core and shell materials are the same. Therefore, both the structural composition and interface strain of the NWs are encoded along directions perpendicular to the growth axis. Owing to fluctuations in the supplied growth species during molecular beam epitaxy (MBE) growth, structural parameters such as local shell thickness, composition and strain may differ between NWs grown onto the same substrate. This requires structural analysis of single NWs instead of measuring NW ensembles. In this work, the complete structure of single GaAs/(In,Ga)As/GaAs core–shell–shell NW heterostructures is determined by means of X-ray nanodiffraction using synchrotron radiation. The NWs were grown by MBE on a prepatterned silicon (111) substrate with a core diameter of 50 nm and an (In,Ga)As shell thickness of 20 nm with a nominal indium concentration of 15%, capped by a 30 nm GaAs outer shell. In order to access single NWs with the X-ray nanobeam being incident parallel to the surface of the substrate, a single row of holes with a separation of 10 µm was defined by electron-beam lithography to act as nucleation centres for MBE NW growth. These well separated NWs were probed sequentially by X-ray nanodiffraction, recording three-dimensional reciprocal-space maps of Bragg reflections with scattering vectors parallel (out-of-plane) and perpendicular (in-plane) to the NW growth axis. From the out-of-plane 111 Bragg reflection, deviations from hexagonal symmetry were derived, together with the diameters of probed NWs grown under the same conditions. The radial NW composition and interface strain became accessible when measuring the two-dimensional scattering intensity distributions of the in-plane 2{\overline 2}0 and 22{\overline 4} reflections, exhibiting well pronounced thickness fringes perpendicular to the NW side planes (truncation rods, TRs). Quantitative values of thickness, composition and strain acting on the (In,Ga)As and GaAs shells were obtained via finite-element modelling of the core–shell–shell NWs and subsequent Fourier transform, simulating the TRs measured along the three different directions of the hexagonally shaped NWs simultaneously. Considering the experimental constraints of the current experiment, thicknesses and In content have been evaluated with uncertainties of ±2 nm and ±0.01, respectively. Comparing data taken from different single NWs, the shell thicknesses differ from one to another.

Structural Changes in a Single GaN Nanowire under Applied Voltage Bias

GaN nanowires (NWs) are promising building blocks for future optoelectronic devices and nanoelectronics. They exhibit stronger piezoelectric properties than bulk GaN. This phenomena may be crucial for applications of NWs and makes their study highly important. We report on an investigation of the structure evolution of a single GaN NW under an applied voltage bias along polar [0001] crystallographic direction until its mechanical break. The structural changes were investigated using coherent X-ray Bragg diffraction. The three-dimensional (3D) intensity distributions of the NWs without metal contacts, with contacts, and under applied voltage bias in opposite polar directions were analyzed. Coherent X-ray Bragg diffraction revealed the presence of significant bending of the NWs already after metal contacts deposition, which was increased at applied voltage bias. Employing analytical simulations based on elasticity theory and a finite element method (FEM) approach, we developed a 3D model of the NW bending under applied voltage. From this model and our experimental data, we determined the piezoelectric constant of the GaN NW to be about 7.7 pm/V in [0001] crystallographic direction. The ultimate tensile strength of the GaN NW was obtained to be about 1.22 GPa. Our work demonstrates the power of in operando X-ray structural studies of single NWs for their effective design and implementation with desired functional properties.

Measuring Three-Dimensional Strain and Structural Defects in a Single InGaAs Nanowire Using Coherent X-ray Multiangle Bragg Projection Ptychography

III-As nanowires are candidates for near-infrared light emitters and detectors that can be directly integrated onto silicon. However, nanoscale to microscale variations in structure, composition, and strain within a given nanowire, as well as variations between nanowires, pose challenges to correlating microstructure with device performance. In this work, we utilize coherent nanofocused X-rays to characterize stacking defects and strain in a single InGaAs nanowire supported on Si. By reconstructing diffraction patterns from the 21̅1̅0 Bragg peak, we show that the lattice orientation varies along the length of the wire, while the strain field along the cross-section is largely unaffected, leaving the band structure unperturbed. Diffraction patterns from the 011̅0 Bragg peak are reproducibly reconstructed to create three-dimensional images of stacking defects and associated lattice strains, revealing sharp planar boundaries between different crystal phases of wurtzite (WZ) structure that contribute to charge carrier scattering. Phase retrieval is made possible by developing multiangle Bragg projection ptychography (maBPP) to accommodate coherent nanodiffraction patterns measured at arbitrary overlapping positions at multiple angles about a Bragg peak, eliminating the need for scan registration at different angles. The penetrating nature of X-ray radiation, together with the relaxed constraints of maBPP, will enable the in operando imaging of nanowire devices.