Atomic, strain and domain dynamics in ferroelectrics under cyclic electric field

Structural dynamics is a subject of a recent interest in solid state physics and a challenge for modern X-ray crystallography. Time-dependent response of solids to an external perturbation on atomic, mesoscopic and macroscopic length scales is the key to understanding many physical properties. We present the data-acquisition system (Gorfman et al., 2013) (DAQ) for X-ray diffraction study of repetitive micro- or millisecond dynamics in a broad range of materials (Gorfman, 2014) under cyclic perturbation. The DAQ operates on the principle of a multichannel analyser: it collects pulses from a single-photon-counting detector and resorts them between 10000 channels. Each channel corresponds to a certain time delay relative to the beginning of a latest perturbation cycle. The width of a channel (temporal resolution of the experiment) can be as small as 10 ns. We investigated atomic, strain and domain dynamics in SrxBa1-xNb2O6 single crystals: SBN50 (x=0.5, uniaxial ferroelectric) and SBN61(x=0.61, uniaxial relaxor ferroelectric). The experiments questioned the reason for large piezoelectric effect in uniaxial ferroelectrics where 180⁰ (inversion) domains are present, while non-180⁰ domains are absent. We applied triangular electric field (Figure 1) of variable frequencies and variable strengths. The time and electric field dependence of a set of Bragg rocking curves were followed: intensities, positions and peak widths were analysed giving access to atomic, strain and domain dynamics under external electric field. Figure 1. Left: contour plot of a -5 9 7 Bragg rocking curves from SBN50 single crystal collected under dynamically applied sub-coercive electric field. Right: dynamics of the peak position as a function of applied electric field, showing non-linear and hysteretic behaviour.

X-ray investigation of polytype distribution in InAs nanowires during MBE growth

The monolithic integration of III-V semiconductors with Si is the ideal way to combine the superior optoelectronic properties of the compound semiconductors with the mature Si technology. This integration can be realized by growing epitaxially dislocation-free III-V NWs on Si substrates either in the vapor-liquid-solid (VLS) or in the vapor-solid (VS) mode associated with the presence or absence, respectively, of group-III liquid droplets on the NW tips [1]. In this work, we investigate the correlation between the growth mode and the forming polytypes in InAs NWs grown on Si(111). The growth was performed in the molecular beam epitaxy chamber of beamline 11XU at Spring8 [2], while the structural dynamics was probed by in situ x-ray diffraction. Specifically, the time evolution of the formation of wurtzite (WZ) and zincblende (ZB) polytypes was monitored during the NW growth. Despite the As-rich growth conditions, a spontaneous build-up of liquid In on Si was found to be present in the nucleation phase, where the InAs nuclei mainly grow in the WZ phase with low number of stacking faults. Shortly after the nucleation, the liquid In is consumed by the excessive As, and the growth continues in the VS mode with an increasing density of stacking faults forming in the NW crystal. The time evolution of the liquid Indium signal (Fig. (a)) correlates well with the time evolution of wurzite growth rate (Fig (b)). The latter saturates at a time where the liquid indium disappers, i.e. where the VLS changes into the VS mode, whereas the zinc-blende polytypes grow almost continuous in both VLS and VS growth mode. The dynamics of stacking faults density was determined quantitatively by ex-situ X-ray diffraction measuring thestacking fault induced increase of the peak width of wurtzite reflections at InAs nanowire samples of different length ; i.e. growth time [3].