Project full title: " Nanowires for solid state lighting "

Project acronym: NWs4LIGHT Grant agreement no: 280773

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D2.4 Report on the use of X-ray nano-beams to the NW-LEDs.

D. Dzhigaev1, 2, T. Stankevic3, F. Seiboth4, A. Shabalin1, M. Rose1, M. N. Strikhanov2, G. Falkenberg1, C. G. Schroer4, A. Mikkelsen5, R. Feidenhans’l3 and I. A. Vartanyants1, 2

1Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, D-22607 Hamburg, Germany 2National Research Nuclear University, “MEPhI”, 115409 Moscow, Russia

3Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark 4Insitute of Structural Physics, Technische Universität of Dresden, D-01062 Dresden, Germany

5Lund University, Synchrotron Radiation Research, 22100 Lund, Sweden

Introduction. In recent years a number of promising developments within information technology,

optoelectronics, and energy harvesting are connected with an application of one-dimensional

(1D) crystals in the form of nanowires (NWs) [1,2]. Several material characteristics are

responsible for the unique NW functionality; among these strain plays an important role.

PETRA III synchrotron radiation source provides a stable, bright, and coherent x-ray beam,

which can be focused down to around hundred nanometers [3] to match the size of single

NWs. Measurements of the diffracted signal from an isolated sample allow to determine strain

in a single NW by mapping of the 3D reciprocal space in the vicinity of the selected Bragg

reflection. Our main aim was to investigate strain field distribution inside the NWs with the

help of x-ray Bragg ptychography (XBP), coherent x-ray diffraction imaging (CXDI), and

diffraction mapping (DM) of the scattered intensity [4, 5, 6, 7].

Figure 2: SEM image of the forest of GaN/InGaN NWs.

Figure 1: (a) SEM image of the forest of InP NWs. (b) SEM image of the single InP NW.

Project full title: " Nanowires for solid state lighting "

Project acronym: NWs4LIGHT Grant agreement no: 280773

2

Samples

We used two kind of NWs as samples. A single free standing NW made of InP (see

Fig. 1), and GaN/InGaN core-shell NW (see Fig. 2). InP NW were grown in metal organic

vapor phase epitaxy (MOVPE). Gold particles were used as a catalyst for the growth process.

The size of the selected NW in our measurement was about 100 nm in diameter and 7.5 µm in

length. The second sample was a single GaN/InGaN core-shell NW, which has an application

in a new generation of light emitting diodes (LED). It consists of the GaN core about 392 nm

in diameter and InGaN shell about 8 nm thick. The length of the NW was 2 µm. The NWs

were grown on the silicon substrate using the metal organic chemical vapor deposition

(MOCVD). A single NW was isolated at the tip of the substrate in order to access it with the

nano-focused beam.

Experiment

We performed our experiments at the nano-focusing end-station of the P06 beamline

at the PETRA III synchrotron source in Hamburg, Germany [8]. The experimental set-up and

scanning scheme for the experiment with the InP NW are shown in Fig. 3. For the second

sample the experimental set-up is shown in Fig. 4. Two perpendicularly positioned

nanofocusing lenses (NFLs) based on parabolic compound refractive x-ray lenses (CRLs)

were used to obtain a nano-sized focus of the incident x-ray beam with the 15.25 keV energy.

The Pilatus 300K hybrid-pixel detector (Dectris, Switzerland) with the pixel size of the

172x172 𝜇𝑚2 was used as the detector.

Project full title: " Nanowires for solid state lighting "

Project acronym: NWs4LIGHT Grant agreement no: 280773

3

Bragg ptychography and CXDI measurements

To determine the size and shape of the incident x-ray beam a tantalum test object in the form

of the Siemens star was used. The detector was positioned 2 m downstream from the sample

in transmission geometry. We applied extended ptychographical algorithm (ePIE) [9,10] for

Figure 3: Experimental set-up for 3D XBP experiment. (a) Single InP NW were mounted on a moving sample stage as a sample. The detector was placed downstream to measure single (111) Bragg peak (2𝜃𝐵). (b) Scanning scheme for 3D XBP, every dot is a scanning point.

Figure 4: Experimental set-up for 3D XBP experiment. Single GaN/InGaN core-shell NW was mounted on a moving sample stage as a sample. The detector was placed downstream to measure single (001) Bragg peak at 17° (2θB) relative to the optical axis of incoming x-ray beam. Arrow shows direction of rotation of the NW for angular scans during 3D XBP.

Figure 5: Reconstruction of the Siemens star and illumination (see insets) at the sample position. Amplitudes (a) and phases (b).

a b

Project full title: " Nanowires for solid state lighting "

Project acronym: NWs4LIGHT Grant agreement no: 280773

4

the reconstruction of the probe function using a Siemens star (see Fig. 5).

This knowledge of the illumination function was necessary for the successful reconstruction

of the ptychographical data and was used as initial guess in the phase retrieval procedure.

The InP NW was scanned on the raster grid with the acquisition time of 200 seconds

per position [11]. In addition, we performed an angular scan at one position on the sample in

order to obtain three-dimensional intensity distribution of the Bragg peak in reciprocal space.

The measured diffraction patterns and results of reconstruction from XBP and CXDI data are

shown in Fig. 6 and Fig. 7, respectively.

Figure 6: Diffraction maps and results of ptychographical reconstruction. (a) Diffraction map of the raster scan: (i) region of the InP NW, (ii) region of the Bragg peak splitting, (iii) region of the gold particle. (b) Reconstructed amplitude of the object function. (c) Reconstructed <𝑢𝑧> component of the displacement field vector in the NW. The scale bar is equal to 100 nm. Color bars show normalized values of the amplitude in (b) and displacement in Å in (c).

Figure 7: Results of 3D CXDI reconstruction. (a) 3D distribution of intensity in reciprocal space in the vicinity of the (111) Bragg peak. (b) 3D reconstruction of the object with the plane showing transverse distribution of the phase inside of the wire. (c) Reconstructed amplitude of the object. (d) Projected <𝑢𝑧> component of the displacement vector field. Scale bars are equal to 100 nm. Color bars show normalized values of the amplitude in (c) and lattice displacement in Å in (d).

Project full title: " Nanowires for solid state lighting "

Project acronym: NWs4LIGHT Grant agreement no: 280773

5

For the GaN/InGaN sample the detector

was positioned 2.1 m downstream from the

sample in the Bragg condition of (001)

reflection. The NW was scanned on the 2D

raster grid consisted of 11×11 illumination

positions with the step size of 50 nm and

acquisition time of 75 seconds per position. At

each position the sample was rotated in the range

of ±0.5° around the center of the Bragg peak (see

Fig. 4) with an angular step size of 0.1°.

Ptychographical data (see Fig. 8 (a, b))

were reconstructed with the help of the ePIE

algorithm. The reconstructed 2D projection of

the NW’s electron density from the central

angular position shows high spatial resolution of

5.9 nm (see Fig. 8 (c)). The main issue during

this experiment was the stability of the

experimental setup, because during long scans

the drifts of the sample were observed. Further

processing of the diffraction patterns from the

angular scans at each position will allow

determining the 3D electron density and 3D

distribution of the strain field in GaN core.

Diffraction mapping measurements

Diffraction mapping is a technique based on scanning the small forest of NWs or a

single NW in the nano-focused beam while measuring the intensity and position of the Bragg

peaks. This approach was applied to study the strain distribution in the GaN/InGaN sample,

which was described above. The sample was brought in the Bragg condition and scanned in

Figure 8: Measured data and the first result.

(a) Diffraction map of the 2D XBP scan for the central angular position. (b) Zoomed diffraction pattern from the “pyramid” top of the NW (outlined with the red line in (a)). (c) 2D amplitude of the reconstructed object function of the NW.

Project full title: " Nanowires for solid state lighting "

Project acronym: NWs4LIGHT Grant agreement no: 280773

6

the beam with 30 nm steps while measuring two Bragg peaks — (10-10) and (10-11). Due to

heterogeneous composition and strain only some parts of the NW were in the Bragg

condition, therefore the measurements were repeated rotating the NW around its vertical axis.

Thus, different parts of the NW, having different lattice constants were measured individually.

Four different parts were imaged by mapping the intensity of the Bragg peaks (see Fig. 9).

Moreover, positions of the Bragg peaks were used to calculate strain and tilt of the crystal

planes.

From Fig. 9 we can see that the core of the NW is strained non-uniformly with large

strain gradient at the base. We have found that the NW is also tilted by 0.65 degrees with

respect to the substrate (see Fig. 9 (a)). This could be caused by dislocations at the interface

between the NW and the substrate. Next we can see the outer part of the core which was

grown radially, where the strain is clearly different in the outer and central parts. Further we

can see an intensity profile consisting of three vertical stripes, which can be attributed to the

edges of the NW. Finally, we can see the InGaN shell. The shell is strained asymmetrically.

The left side is fully strained to match the underlying GaN core, whereas the right side is

Figure 9: Measurement results show four distinct regions in the nanowire. Intensity, strain and tilt maps are shown for the inner core (a), outer core (b), edges (c) and shell (d).

Project full title: " Nanowires for solid state lighting "

Project acronym: NWs4LIGHT Grant agreement no: 280773

7

almost fully relaxed. This asymmetry may be due to unequal growth conditions, causing also

larger amount of InGaN accumulate in the upper part of the NW.

In addition, a small forest of a

InGaN/GaN single quantum well NWs was

investigated. The beam was scanned across

approximately 30 NWs and several diffraction

spots were measured. Though the NWs were

overlapping while the scanning they can be still

distinguished from the diffraction mapping scan

due to random tilting of the NWs. Structural

inhomogeneities appear as asymmetric smearing

of the Bragg peaks. By measuring the intensity

of different parts of the Bragg peak we can map

different parts of the NWs, such as the quantum well and core separately. This is shown in

Fig. 10 where the In-rich quantum wells can be seen separately from the GaN cores due to the

larger lattice constant.

References

Y. Li, F. Qian, J. Xiang, Ch. Lieber, M. Mater. Today 9, 18 (2006). L.-E. Wernersson, C. Thelander, E. Lind, L. Samuelson, Proc. IEEE 98, 2047 (2010). D. Dzhigaev, et al., J. Phys.: Conf. Ser. 499, 012020 (2014). D. Dzhigaev, et al., (2014) (in preparation). M. A. Pfeifer et al., Nature 442, (2006). I. A. Vartanyants, O.M. Yefanov, Chapter 12 in the book: X-ray Diffraction. Modern

Experimental Techniques. Edited by O. H. Seeck and B. M. Murphy, Pan Stanford Publishing (in print) (http://arxiv.org/abs/1304.5335) (2013)

P. Godard et al., Nature Comm. 2, 568 (2011). C. G. Schroer et al., Nucl. Instrum. Meth. A 616 93 (2010). P. Thibault et al., Ultramicroscopy 109, 338-343 (2009). A. M. Maiden, J. M. Rodenburg, Ultramicroscopy 109, 1256-1262 (2009) D. Dzhigaev, et al., Photon Science Annual Report (http://photon-

science.desy.de/annual_report/files/2012/20122530.pdf) (2012).

Figure 10. Diffraction map of a small forest of nanowires. Different parts of the nanowires have different lattice constants due to composition and strain; therefore they contribute to different parts of the Bragg peaks. By mapping the scattered intensities we can see which parts of the nanowires deviate from expected strain or