Determination of the stacking fault density in highly defective single GaAs nanowires by means of coherent diffraction imaging

Abstract

Coherent x-ray diffraction imaging is used to measure diffraction patterns from individual highly defective nanowires, showing a complex speckle pattern instead of well-defined Bragg peaks. The approach is tested for nanowires of 500 nm diameter and 500 nm height predominately composed by zinc-blende (ZB) and twinned zinc-blende (TZB) phase domains. Phase retrieval is used to reconstruct the measured 2-dimensional intensity patterns recorded from single nanowires with 3.48 nm and 0.98 nm spatial resolution. Whereas the speckle amplitudes and distribution are perfectly reconstructed, no unique solution could be obtained for the phase structure. The number of phase switches is found to be proportional to the number of measured speckles and follows a narrow number distribution. Using data with 0.98 nm spatial resolution the mean number of phase switches is in reasonable agreement with estimates taken from TEM. However, since the resolved phase domain still is 3-4 times larger than a single GaAs bilayer we explain the non-ambiguous phase reconstruction by the fact that depending on starting phase and sequence of subroutines used during the phase retrieval the retrieved phase domain host a different sequence of randomly stacked bilayers. Modelling possible arrangements of bilayer sequences within a phase domain demonstrate that the complex speckle patterns measured can indeed be explained by the random arrangement of the ZB and TZB phase domains.

Correlated Nanoscale Analysis of the Emission from Wurtzite versus Zincblende (In,Ga)As/GaAs Nanowire Core–Shell Quantum Wells

“…In the transition region close to the ZB interface (cyan), the diffraction spreads to high q y due to the interference between stacking variations that are smaller than the size of the projected beam. [41][42][43] This structure was observed in all eight NWs measured by nanoXRD. Indeed, a high density of short, alternating segments of WZ and ZB stacking, including stacking faults (SFs) and twins, have been reported for the transition from the ZB to the WZ phase.…”

Characterization of individual stacking faults in a wurtzite GaAs nanowire by nanobeam X-ray diffraction

“…Once the Bragg diffraction is found, the typical speckle pattern around the WZ diffraction maximum is recorded by a two-dimensional (2D) MaxiPix detector with 516  516 pixels of 55 mm  55 mm size mounted at a distance of 728 mm from the sample. The speckle pattern is the result of interference between the WZ segments separated by SFs (Favre-Nicolin et al, 2010;Davtyan et al, 2016). A threedimensional (3D) reciprocal-space map of the diffracted intensity can be recorded by combining a series of 2D diffraction patterns collected at different angles of incidence.…”

“…It is evident from the figure that nw1 shows more ordered and simple oscillations along Q z with about three periods superimposed, while the structure of the oscillations of nw2 is more complex. This implies that fewer defects occur in the illuminated part of nw1 than in nw2, since NWs with many defects do not exhibit a clear periodicity in the speckle patterns (Davtyan et al, 2016).…”

“…In the paper by Favre-Nicolin et al (2010) a reversed MonteCarlo fitting procedure has been used for the reconstruction of the ZB/WZ sequence, and a standard fitting method was used by Biermanns et al (2012) for the determination of the lengths and positions of ZB and WZ segments. A direct phaseretrieval method was used by Davtyan et al (2016) for the determination of the stacking fault density in a single NW. However, this work also demonstrates that the result of the phase-retrieval algorithm might be ambiguous when high defect densities are present.…”

Norm-Minimized Scattering Data from Intensity Spectra

“…The key ingredients are the restricted positive eigenvalues of (III. 15) bounded from above by the spectral norm ||½ m+1 || 2 = 1, cf. (B.1).…”