Coherent diffraction imaging of a single epitaxial InAs nanowire using a focused x-ray beam

The cross section of an InAs nanowire with a diameter of 150 nm epitaxially grown on a 111-oriented InP substrate was characterized using a combination of x-ray scanning microdiffraction and coherent diffraction imaging. Using an x-ray beam focused by a Fresnel zone plate, we were able to scan in real space and hence localize single nanowires on the substrate in the as-grown epitaxial state. For one single nanowire, the three-dimensional coherent intensity distribution in reciprocal space was mapped around the (111) InAs reflection. Using phase retrieval algorithms, the cross section of the wire was reconstructed with a spatial resolution of 8 nm along one direction.

Figure 1

(Color online) (a) Sketch of the setup. A focused x-ray beam is Bragg diffracted by a single InAs nanowire of 150 nm diameter. A 3D calculation of the expected scattered intensity close to the InAs (111) reflection is shown (isointensity surface). (b) Distribution of InAs wires on a selected sample area obtained by scanning x-ray diffraction microscopy (linear intensity scale $I_t$). It shows the projection of several wires along the direction of the incident x-ray beam. The red ellipse indicates the corresponding size of the beam.

Figure 2

(Color online) (a) 3D RSM measured around the (111) InAs Bragg reflection. An isointensity surface is shown. The Bragg reflection from the InP substrate appears at higher $q_z$ values, out of the plotted data range. (b) 2D cut in the $q_x{q}_y$ plane of the RSM shown in (a). The intersection with the CTR is indicated by a circle. Missing data on the left side of the pattern are due to missing frames at scanning points where the CCD intercepts the substrate Bragg reflection.

Figure 3

Retrieved complex-valued electron density—(a) magnitude and (b) phase—of the wire section obtained from the data shown in Fig. 2b. The dashed square in (a) indicates the support region used in the reconstruction algorithm. Scales in both (a) and (b) are linear.

Figure 4

Retrieved complex-valued electron density—(a) magnitude and (b) phase—of the wire section obtained from the data shown in Fig. 2b in which the regions of missing data have been replaced by the center-symmetric values of the diffraction pattern. The dashed square in (a) indicates the support region used in the reconstruction algorithm. Scales in both (a) and (b) are linear.