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. 2020 Oct 21;20(11):8326–8331. doi: 10.1021/acs.nanolett.0c03477

Direct Three-Dimensional Imaging of an X-ray Nanofocus Using a Single 60 nm Diameter Nanowire Device

Lert Chayanun †, Lukas Hrachowina ‡, Alexander Björling §, Magnus T Borgström ‡, Jesper Wallentin †,*
PMCID: PMC7662902  PMID: 33084341

Abstract

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Nanoscale X-ray detectors could allow higher resolution in imaging and diffraction experiments than established systems but are difficult to design due to the long absorption length of X-rays. Here, we demonstrate X-ray detection in a single nanowire in which the nanowire axis is parallel to the optical axis. In this geometry, X-ray absorption can occur along the nanowire length, while the spatial resolution is limited by the diameter. We use the device to make a high-resolution 3D image of the 88 nm diameter X-ray nanofocus at the Nanomax beamline, MAX IV synchrotron, by scanning the single pixel device in different planes along the optical axis. The images reveal fine details of the beam that are unattainable with established detectors and show good agreement with ptychography.

Keywords: Nanowire, X-ray beam induced current, nanofocused X-rays, detector

X-ray analysis methods are vital for fields as diverse as clinical research, materials science, and nanoscience. X-ray sources have seen strong and diverse development in recent years, ranging from novel liquid–metal–jet lab sources,1 free electron lasers with unprecedented peak intensity,2 to the new fourth-generation diffraction limited synchrotrons.3 The focusing of X-rays has also advanced rapidly, reaching below 100 nm at many synchrotron beamlines and with demonstrations of focusing below 10 nm.4−6 X-ray detectors, however, still have limited spatial resolution. The first type of X-ray detectors uses scintillators7 to first convert X-ray photons to visible-light photons, which are subsequently collected by a conventional optical system. High spatial resolution scintillator detectors are limited by the Abbe diffraction limit of visible light to about 0.5 μm,8 and such systems require very thin scintillators with limited sensitivity. The second type of X-ray detectors, direct detectors, are based on semiconductors, in which the absorbed X-ray photons directly excite electrons that are measured using sophisticated electronics. Pixel detectors using this principle have a pixel size on the order of 50–200 μm.9−11 The same physical process can also be used as a method for high-resolution characterization of semiconductors, and then it is usually referred to as X-ray beam-induced current (XBIC).12−15

The ideal high-resolution detector pixel should have a minimal diameter, and at the same time be sufficiently thick along the optical axis to absorb most of the X-rays. These requirements naturally lead to a nanowire-shaped absorbing region. Semiconductor nanowires can be grown with almost unlimited length,16,17 and these structures have been developed for various next-generation electronic devices, such as light-emitting diodes (LEDs),18 transistors,19,20 lasers,21,22 photodetectors,23,24 and solar cells.25,26 X-ray detection has previously been shown in single nanowire devices.27,28 However, since the nanowires in those cases were oriented parallel with the substrate, orthogonal to the optical axis, the absorbing length was limited to the nanowire diameter while the spatial resolution was limited by the extent of the active region. Furthermore, this type of device is difficult to scale to multiple pixels.

Here, we demonstrate X-ray detection in a single nanowire oriented orthogonal to the substrate and parallel with the optical axis (Figure 1a,b). In this geometry, the spatial resolution is in principle limited by the nanowire diameter, 60 nm in the present study, while the absorption can occur along the nanowire length. This geometry utilizes the shape of the nanowire in an ideal way and forms a scalable basis for future array detectors. We use the device to make a direct 3D measurement of the X-ray nanofocus at the NanoMax beamline,29 situated at the first fourth-generation synchrotron MAX IV in Lund, Sweden.

Figure 1.

Figure 1

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Single nanowire device for X-ray detection. (a) Schematic of the vertical nanowire device where the top contact and the substrate are connected to the voltage source and the ampere meter, respectively. During the measurement, the nanowire device is oriented parallel with the optical axis. (b) SEM image of the top contact, where the tip of the 60 nm diameter nanowire can be observed at the center. Tilt 30°. (c) Current–voltage (I–V) characteristics in dark (solid line) and under green laser excitation (dashed line). (d) EBIC map of the as-grown nanowire with the EBIC signal in green and the secondary electron signal in red. The inset shows the profile of the EBIC signal along the axial nanowire in green. (e) Schematic of the experiment, where the nanowire device was 2D scanned at different planes around the X-ray nanofocus. (f) Overview XBIC map attained from the nanowire device, showing a strong signal from the nanowire.

To create the single nanowire devices, we first grew position controlled 60 nm diameter InP nanowires from Au seed particles. The nanowires were synthesized with a length of about 2 μm and were in situ doped with an axial p–i–n doping profile. Before device processing, some nanowires were characterized using electron beam-induced current (EBIC), as shown in Figure 1d. The EBIC profile along the axial direction of the nanowire, in the inset of Figure 1d, shows that the active region is about 1 μm long. After this, the as-grown nanowires were individually contacted as described in Methods. The current–voltage (I–V) characteristics of the nanowire device were measured under dark and light conditions, as shown in Figure 1c. The dark I–V characteristics reveals an ideality factor of about 14 from the fit at about 2 V, indicating a substantial contact resistance in this preliminary device. In addition, there is a considerable leakage current at reverse bias. Nevertheless, the dark current at zero bias is similar to the in-plane single contacted p–i–n doped InP nanowire devices that were studied using XBIC in our previous reports,15,30 and excitation with a green laser shows a clear photocurrent (Figure 1c).

For the X-ray experiments, the sample was connected to an amperemeter for the XBIC measurements30 and mounted on a high-resolution piezo-motor scanner. At the NanoMAX beamline, Kirkpatrick-Baez (KB) mirrors are used to focus the X-ray beam (Figure 1e).31 The standard method for nanofocused X-ray beam characterization is ptychographic imaging of a test sample, in our case a Siemens star, by measuring the transmitted beam in the far-field region and using phase retrieval. This method reconstructs not only the exit wave of the sample but also the complex field (phase and amplitude) of the beam at the sample, here located 435 μm downstream of the focal plane.32,33 The field of the beam at other planes along the optical axis can then be calculated by Fresnel propagation. Using this method, we found that the fwhm at the focus is 88 nm × 86 nm (vertical × horizontal) at the X-ray energy of 10 keV (Figure S2, SI).29

The nanowire device was scanned two-dimensionally across the nanofocused X-ray beam, as shown in Figure 1e, using a continuous scan horizontally and a vertical step scan. Figure 1f displays an overview XBIC map of the nanowire device with a 100 nm step size and a 0.1 s acquisition time. A strong signal can be observed from the nanowire in the center. There is also a significant signal generated by photoelectrons emitted from the metal top contact. The background signal is almost constant in the region around the peak signal from the nanowire (Figure S3, SI). Therefore, it was subtracted in the subsequent analysis, where the scan areas were smaller than the size of the top contact (<3 μm).

First, we investigated the device response as a function of the X-ray flux from Φ = 2.2 × 105 s–1 to Φ = 1.7 × 1010 s–1, using Si attenuators to change the flux. We show maps with 20 nm step size for a selected set of fluxes in Figure 2, with the full set shown in the SI (Figure S4). There is no significant influence of the X-ray photon flux on the size of the peak, as expected. We observe some variation in the shape of the central peak, which is presumably due to mechanical limitations of the experimental setup.

Figure 2.

Figure 2

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X-ray photon flux variation. (a–d) XBIC maps acquired with selected X-ray photon fluxes (all fluxes are shown in SI Figure S3). The average fwhm of the XBIC peaks is about 190 nm. (e) Maximum measured XBIC (red crosses) as a function of X-ray photon flux. The measured XBIC was fitted using a power-law, IXBIC = aΦb, with a = 3.24 × 10–18 and b = 0.60, shown as a red line. (f) Comparison between the measured horizontal XBIC profile (black solid line), the calculated X-ray flux profile using eq 2 (black dash-dotted line), the beam profile from ptychography at the focal plane (blue solid line), a box function representing the cross-section profile of the nanowire (blue dash-dotted line), and the convolution of the last two curves (red line).

To evaluate the sensitivity, the maximum XBIC signal at each X-ray photon flux was plotted as red crosses in Figure 2e. The XBIC signal, IXBIC, is ideally directly proportional to the X-ray flux, Φ

graphic file with name nl0c03477_m001.jpg 1

where q is the elementary charge, η is the photogenerated electron–hole pair yield per X-ray photon,34pabs is the X-ray absorption probability,15 and S is the carrier collection efficiency. The term S is closely related to the spatially dependent internal quantum efficiency (SIQE).15,35 With an X-ray energy of 10 keV, and assuming that the absorption occurs in the 1 μm of the active region along the nanowire axis, η and pabs are calculated to be 2.35 × 103 and 2.48 × 10–2, respectively. However, for the device here we instead observe a clear sublinear dependence. We have previously found both linear15,36 and nonlinear27 current-flux dependencies for XBIC measurements of nanowires. We could empirically fit the data well with a power-law equation IXBIC = aΦb, with a = 3.24 × 10–18 and b = 0.60, that is

graphic file with name nl0c03477_m002.jpg 2

Generally, we find that the XBIC signal is significantly lower than calculated from bulk values, as previously observed.15 The main reason is that secondary electrons and photons can escape from this nanostructured device before exciting electrons to the conduction band,13,15 which effectively leads to a lower yield η. Furthermore, the EBIC shows that carrier collection S is significantly below 100% for most of the nanowire length (Figure 1d).

The reason for the sublinear flux dependence is not clear. The generation of electron–hole pairs per absorbed photon, η, is likely constant since the escape of secondary photons and electrons should not be affected by the flux. Instead, we propose that the carrier collection S is reduced for higher fluxes. With increasing X-ray photon flux, the carrier generation rate is increased, leading to a higher density of generated carriers. One possibility is that this leads to increased Auger recombination.37 Another possibility is that the electrostatic potential and charge separation in these thin nanowires is dominated by surface traps that are gradually filled at higher fluxes.

An important question is what spatial resolution can be achieved with the 60 nm diameter nanowire device, since the X-ray focus is of comparable size to the nanowire diameter. The average fwhm of the XBIC peaks measured at different X-ray photon fluxes is about 190 nm in both the horizontal and the vertical directions (Figure S5, SI). However, the sublinear flux dependence discussed above enhances the low-intensity regions which increases the fwhm. By converting the XBIC map to the X-ray flux using eq 2, the fwhm of the peak is reduced to about 150 nm (Figure S5, SI). As a comparison, we calculated the convolution of the horizontal beam profile from ptychography with a 60 nm box function representing the nanowire, as shown in Figure 2f. The resulting peak has a fwhm of about 95 nm, significantly less than the measured peak width (Figure 2g). The measured peak could be broadened by emitted photoelectrons and X-ray fluorescence from the metal contact. In addition, the imperfect scanning due to the mechanical limitations of the stage at this high resolution, which can also be observed in the images themselves as discussed above, will increase the width of the XBIC peak.

The limited resolution of established X-ray detectors has prevented the direct measurement of the intensity distribution in a submicron X-ray focus. We previously demonstrated 3D imaging of the X-ray focus at the P10 beamline, PETRA-III, but the resolution was limited since the nanowire was oriented orthogonally to the beam.27 Therefore, we used the present nanowire device to map various planes along the nanofocused X-ray beam, to create a 3D measurement of the intensity. These maps were acquired at the maximum X-ray photon flux since the intensity of the beam is much lower away from the focal plane. The scans were done with a 20 nm step size and a 0.1 s acquisition time. Figure 3a shows two-dimensional XBIC maps from various planes along the beam, where the images on the left in the jet color scale come from propagation of the ptychographic reconstruction, and the images on the right in the hot color scale are the nanowire measurements. The focal plane is defined as z = 0, where negative positions are upstream toward the KB mirrors, and positive positions are downstream, as schematically illustrated in Figure 1e.

Figure 3.

Figure 3

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Three-dimensional imaging if an X-ray nanofocus. (a) XBIC maps (right columns, hot color scale) at several planes through the focus. The left columns (jet scale) show the calculated intensities from the propagation of a ptychographic reconstruction. (b) Intensity distribution of the beam detected at the far field (gray color scale). (c) Comparison between the peak intensity from ptychography (black solid line) and the measured X-ray photon flux (blue crosses) along the beam. The X-ray photon flux was calculated from the XBIC signal using eq 2.

The XBIC maps reveal details of the wavefield along the focusing beam which are impossible to achieve with regular detectors. Note that these XBIC maps were rotated to match the beam coordinates from ptychography. The images show some instability coming from the high-resolution scanning, which was done by moving the nanowire horizontally to the left, one line at a time upward, resulting in wavy features in the vertical direction. Overall, however, we observe good agreement with the ptychographic reconstructions of the beam. The beam profile shows an increasing number of peaks in both the upstream and downstream directions. The beam is more intense in the upper left part in the downstream direction and lower right part in the upstream direction. This asymmetric intensity can also be observed in the far-field, see Figure 3b, and it is characteristic for beams focused by KB mirrors.38 Likewise, the square beam profile in Figure 3b originates from the KB mirrors. Finally, we evaluated the maximum intensity along the optical axis as shown in Figure 3c. We find excellent quantitative agreement of the nanowire measurements with the ptychography.

In conclusion, we have demonstrated ultrahigh resolution X-ray detection in a single nanowire device. The entire scan area in these images is only slightly larger than the point spread function of the highest resolution established X-ray detectors available.7 The resolution is sufficient to directly image details of the intensity of the X-ray nanofocus, at a length scale that is out of reach of conventional X-ray detectors. The two aspects limiting the spatial resolution in our measurements is mechanical instability of the measurement system and the nanowire diameter. The first limitation could be addressed by a more sophisticated scanning system, for instance, using an interferometric stage,39 while the second one could be further improved with even thinner nanowires.

The sensitivity of the device is limited by both the primary X-ray absorption, which could be enhanced with longer nanowires, and the carrier collection. Ideally, the depletion region should be similar to the X-ray absorption length, which is 19 μm for InP with 10 keV X-rays, but our nanowire diode shows a relatively short depletion region in EBIC. We have previously demonstrated that high-quality diodes can be achieved by careful optimization in InP nanowire array solar cells.26 Better electrical characteristics would also allow reverse-biasing of the diode for further improved carrier collection. Finally, we note that this device design with as-grown nanowires, unlike the previously reported in-plane nanowires, is highly scalable and suitable for high-density array detectors.40,41

Methods

Nanowire Growth

Electron beam lithography (EBL) was used to create a structure for Au seed particles in SiN layer (∼70 nm) deposited on p-type InP (111) B substrate (Figure S1a–c). We used trimethyl indium (TMIn), and phosphine (PH3) precursor gases for InP nanowire synthesis in a metal organic chemical vapor phase deposition (MOCVD) process.42,43 Diethyl zinc (DEZn) and tetraethyl tin (TESn) were introduced into the chamber for the p- and n-doped segments, respectively. Furthermore, hydrogen chloride (HCl) was used to prevent radial growth.44,45 The molar fractions of these gases for the growth of each segment are listed in Table S1. The crystal quality of our nanowires should be similar to InP nanowires synthesized at similar conditions.46

Device Fabrication

The SiO2 passivation layer was deposited on the nanowires using atomic layer deposition (ALD) as shown in Figure S1d. An isolation layer using photoresist S1828 (Microposit) was then spin-coated on the substrate (Figure S1e). The thickness of the isolation layer was about 3 μm after the spin-coating. This isolation layer was too soft for the wire bonding when mounting onto a dual in-line pin (DIP) chip carrier. Therefore, UV lithography (UVL) was used to removed parts of the isolation layer allowing the bond pads to be deposited directly on the substrate (Figure S1f). Then, the thickness of the isolation layer was reduced by reactive ion etching (RIE) to get the nanowire tip on the top surface. After that, the sample was dipped in 1:10 buffered oxide etches (BOE) with deionized (DI) water to remove some parts of the passivation layer at the tip of nanowires. EBL was used to create a pattern for bond pads and top contacts (Figure S1g,h), which were deposited by Ti and Au with a thickness of 20 and 200 nm, respectively. Finally, the device was glued and wire bonded onto a chip carrier, which fits the XBIC measurement system at the NanoMAX beamline, MAX IV synchrotron, Lund, Sweden.30

Acknowledgments

We acknowledge MAX IV Laboratory for time on Beamline NanoMAX under Proposal 20200568. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496

Glossary

ABBREVIATIONS

EBIC

electron beam induced current

I–V

current–voltage

KB

Kirkpatrick-Baez

LED

light-emitting diode

SIQE

spatially dependent internal quantum efficiency

XBIC

X-ray beam induced current

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.0c03477.

  • Nanowire growth recipe; fabrication process flow for nanowire X-ray detectors; beam characterization with ptychography; XBIC profile with background from the top contact; XBIC maps with the full set of the flux variation (PDF)

Author Contributions

L.C. performed the data analysis and fabricated the nanowire devices. A.B. performed the ptychography analysis. L.H. and M.T.B. grew the nanowires. L.C. and L.H. performed the preliminary evaluation of the nanowire devices. L.C., A.B., and J.W. performed the measurement at the synchrotron radiation source. L.C. and J.W. wrote the paper with support of all the other authors.

Röntgen-Ångström Cluster, NanoLund, Marie Sklodowska Curie Actions, Cofund, Project INCA 600398, Swedish Research Council Grant 2015-00331, and European Research Council (ERC) Grant Agreement 801847.

The authors declare no competing financial interest.

Supplementary Material

nl0c03477_si_001.pdf (506.5KB, pdf)

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