Measurement of strain in InGaN/GaN nanowires and nanopyramids

Transition from elastic to plastic strain release in core-shell nanowires revealed by in-plane x-ray diffraction

We investigate the strain evolution and relaxation process as function of increasing lattice mismatch between the GaAs core and surrounding In _x Ga_1-x As shell in core-shell nanowire heterostructures grown on Si(111) substrates. The dimensions of the core and shell are kept constant whereas the indium concentration inside the shell is varied. Measuring the [Formula: see text] and [Formula: see text] in-plane Bragg reflections normal to the nanowire side edges and side facets, we observe a transition from elastic to plastic strain release for a shell indium content x > 0.5. Above the onset of plastic strain relaxation, indium rich mounds and an indium poor coherent shell grow simultaneously around the GaAs core. Mound formation was observed for indium contents x = 0.5 and 0.6 by scanning electron microscopy. Considering both the measured radial reflections and the axial 111 Bragg reflection, the 3D strain variation was extracted separately for the core and the In _x Ga_1-x As shell.

Combining Nanofocused X-Rays with Electrical Measurements at the NanoMAX Beamline

The advent of nanofocused X-ray beams has allowed the study of single nanocrystals and complete nanoscale devices in a nondestructive manner, using techniques such as scanning transmission X-ray microscopy (STXM), X-ray fluorescence (XRF) and X-ray diffraction (XRD). Further insight into semiconductor devices can be achieved by combining these techniques with simultaneous electrical measurements. Here, we present a system for electrical biasing and current measurement of single nanostructure devices, which has been developed for the NanoMAX beamline at the fourth-generation synchrotron, MAX IV, Sweden. The system was tested on single InP nanowire devices. The mechanical stability was sufficient to collect scanning XRD and XRF maps with a 50 nm diameter focus. The dark noise of the current measurement system was about 3 fA, which allowed fly scan measurements of X-ray beam induced current (XBIC) in single nanowire devices.

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

The future of solid-state lighting can be potentially driven by applications of InGaN/GaN core-shell nanowires. These heterostructures provide the possibility for fine-tuning of functional properties by controlling a strain state between mismatched layers. We present a nondestructive study of a single 400 nm-thick InGaN/GaN core-shell nanowire using two-dimensional (2D) X-ray Bragg ptychography (XBP) with a nanofocused X-ray beam. The XBP reconstruction enabled the determination of a detailed three-dimensional (3D) distribution of the strain in the particular nanowire using a model based on finite element method. We observed the strain induced by the lattice mismatch between the GaN core and InGaN shell to be in the range from -0.1% to 0.15% for an In concentration of 30%. The maximum value of the strain component normal to the facets was concentrated at the transition region between the main part of the nanowire and the GaN tip. In addition, a variation in misfit strain relaxation between the axial growth and in-plane directions was revealed.

Influence of Substrate Microstructure on the Transport Properties of CVD-Graphene

We report the study of electrical transport in few-layered CVD-graphene located on nanostructured surfaces in view of its potential application as a transparent contact to optoelectronic devices. Two specific surfaces with a different characteristic feature scale are analyzed: semiconductor micropyramids covered with SiO2 layer and opal structures composed of SiO2 nanospheres. Scanning tunneling microscopy (STM) and scanning electron microscopy (SEM), as well as Raman spectroscopy, have been used to determine graphene/substrate surface profile. The graphene transfer on the opal face centered cubic arrangement of spheres with a diameter of 230 nm leads to graphene corrugation (graphene partially reproduces the opal surface profile). This structure results in a reduction by more than 3 times of the graphene sheet conductivity compared to the conductivity of reference graphene located on a planar SiO2 surface but does not affect the contact resistance to graphene. The graphene transfer onto an organized array of micropyramids results in a graphene suspension. Unlike opal, the graphene suspension on pyramids leads to a reduction of both the contact resistance and the sheet resistance of graphene compared to resistance of the reference graphene/flat SiO2 sample. The sample annealing is favorable to improve the contact resistance to CVD-graphene; however, it leads to the increase of its sheet resistance.

Mapping of Strain Fields in GaAs/GaAsP Core-Shell Nanowires with Nanometer Resolution

We report the nanoscale quantification of strain in GaAs/GaAsP core-shell nanowires. By tracking the shifting of higher-order Laue zone (HOLZ) lines in convergent beam electron diffraction patterns, we observe unique variations in HOLZ line separation along different facets of the core-shell structure, demonstrating the nonuniform strain fields created by the heterointerface. Furthermore, through the use of continuum mechanical modeling and Bloch wave analysis we calculate expected HOLZ line shift behavior, which are directly matched to experimental results. This comparison demonstrates both the power of electron microscopy as a platform for nanoscale strain characterization and the reliability of continuum models to accurately calculate complex strain fields in nanoscale systems.

Structural Properties of wurtzite InP-InGaAs nanowire core-shell heterostructures

We report on growth and characterization of wurtzite InP-In(1-x)Ga(x)As core-shell nanowire heterostructures. A range of nanowire structures with different Ga concentration in the shell was characterized with transmission electron microscopy and X-ray diffraction. We found that the main part of the nanowires has a pure wurtzite crystal structure, with occasional stacking faults occurring only at the top and bottom. This allowed us to determine the structural properties of wurtzite In(1-x)Ga(x)As. The InP-In(1-x)Ga(x)As core-shell nanowires show a triangular and hexagonal facet structure of {1100} and {101̅0} planes. X-ray diffraction measurements showed that the core and the shell are pseudomorphic along the c-axis, and the strained axial lattice constant is closer to the relaxed In(1-x)Ga(x)As shell. Microphotoluminescence measurements of the nanowires show emission in the infrared regime, which makes them suitable for applications in optical communication.