Asymmetric skew X-ray diffraction at fixed incidence angle: application to semiconductor nano-objects

Abstract

A procedure for obtaining three-dimensionally resolved reciprocal-space maps in a skew X-ray diffraction geometry is described. The geometry allows tuning of the information depth in the range from tens of micrometres for symmetric skew diffraction down to tens of nanometres for strongly asymmetric skew geometries, where the angle of incidence is below the critical angle of total external reflection. The diffraction data are processed using a rotation matrix formalism. The whole three-dimensional reciprocal-space map can be measured by performing a single azimuthal rotation of the sample and using a two-dimensional detector, while keeping the angle of incidence and the X-ray information depth fixed (FIXD method). Having a high surface sensitivity under grazing-incidence conditions, the FIXD method can be applied to a large variety of Bragg reflections, particularly polar ones, which provide information on strain and chemical composition separately. In contrast with conventional grazing-incidence diffraction, the FIXD approach reveals, in addition to the lateral (in-plane) components, the vertical (out-of-plane) component of the strain field, and therefore allows the separation of the scattering contributions of strained epitaxial nanostructures by their vertical misfit. The potential of FIXD is demonstrated by resolving the diffraction signal from a single layer of InGaN quantum dots grown on a GaN buffer layer. The FIXD approach is suited to the study of free-standing and covered near-surface nano-objects, as well as vertically extended multilayer structures.

Radial Growth of Self-Catalyzed GaAs Nanowires and the Evolution of the Liquid Ga-Droplet Studied by Time-Resolved in Situ X-ray Diffraction

“…in-situ structure characterization of the growing ensemble of self-catalyzed GaAs NW, we monitor the evolution of the phase-selective Bragg reflections (311) of GaAs zinc-blende (ZB), (220) of GaAs twinned zinc-blende (TZB) and(10.3) of GaAs wurtzite (WZ) 49,50. …”

“…For in situ structure characterization of the growing ensemble of self-catalyzed GaAs NW, we monitor the evolution of the phase-selective Bragg reflections (311) of GaAs zinc-blende (ZB), (220) of GaAs twinned zinc-blende (TZB) and (10.3) of GaAs wurtzite (WZ). 49,50 Figure 2a illustrates the scanning geometry (for further details see Supporting Information (SI)). For this set of reflections, scattering from ZB, TZB, and WZ is well separated in reciprocal space, in contrast to the symmetric (111) reflection where the signals of WZ and ZB can overlap.…”

“…For in situ structure characterization of the growing ensemble of self-catalyzed GaAs NW, we monitor the evolution of the phase-selective Bragg reflections (311) of GaAs zinc-blende (ZB), (220) of GaAs twinned zinc-blende (TZB) and (10.3) of GaAs wurtzite (WZ). , Figure a illustrates the scanning geometry (for further details see Supporting Information (SI)). For this set of reflections, scattering from ZB, TZB, and WZ is well separated in reciprocal space, in contrast to the symmetric (111) reflection where the signals of WZ and ZB can overlap. , In the right panel of Figure b, the time-evolution of the integrated intensities of 2D-cuts through the centers of those three phase-selective Bragg reflections (illustrated in the reciprocal space map (RSM) in the left panel) are shown after renormalization to correct their structure-factor differences and subtraction of the contribution from parasitic GaAs islands to the scattering signal. , We take the integrated scattered intensity of a phase-selective Bragg reflection as a measure for the volume of the respective phase in the crystal.…”

X-ray topography of subsurface crystal layers

“…There are a large number of different X-ray techniques that allow us to study thin subsurface layers with a thickness of about 0.1 mm (Afanas'ev & Melkonyan, 1983;Afanas'ev et al, 1989;Authier, 2001;Bowen & Tanner, 1998;Imamov et al, 1989;Novikov et al, 1995). It is also highly efficient to use asymmetric reflections followed by rotation of the sample around the diffraction vector (Grigoriev et al, 2016). But no less effective is the diffraction scheme for which the angle between the reflecting plane and the crystal surface is slightly larger than the value of the Bragg angle (Fodchuk & Kshevetsky, 1992;Fodchuk et al, 1995aFodchuk et al, , 2009Swiatek & Fodchuk, 2016):…”

X-ray investigation of strained epitaxial layer systems by reflections in skew geometry

“…Pietsch & Borchard, 1987) or to perform reciprocal-space mapping in different skew geometries (see e.g. Grigoriev et al, 2016). For laboratory-based diffractometers, skew geometries were probably first discussed, and consequently first used for the evaluation of the structure of InGaAs/InGaAs strained layer superlattices, by Nakashima (1992) using a double-crystal diffractometer without a receiving slit, and by Hö che & Hö fer (1992) as a method of reducing the penetration depth without increasing the halfwidth of the rocking curve as in asymmetric coplanar diffraction.…”