| 1. |
- Chayanun, Lert, et al.
(författare)
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Direct Three-Dimensional Imaging of an X-ray Nanofocus Using a Single 60 nm Diameter Nanowire Device
- 2020
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 20:11, s. 8326-8331
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Chayanun, Lert
(författare)
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Nanowire devices for X-ray detection
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High spatial resolution X-ray microscopy has become a dedicated tool to study nanocrystals and nanostructure devices in recent years. In general, the spatial resolution of X-ray microscopy depends on the spot size of the X-ray beam and the pixel size of X-ray detectors. High-resolution X-ray detection ideally requires a minimal active region with a sufficient thickness for the X-ray absorption, which leads to nanowire-shaped structures. This thesis made use of semiconductor nanowires to create a single-pixel X-ray detector at nanoscale resolution. The basic interaction between X-rays and nanowire devices can best be investigated by choosing a sample geometry where the nanowire is oriented in-plane with the substrate and orthogonal to the beam. X-ray beam induced current (XBIC), which is the physical process used in X-ray detectors, was used as the primary method to investigate the electrical response from nanowire devices. Different aspects of the XBIC process were investigated in two nanowire materials, InP and InGaP, with two types of doping profiles, n-i-n and p-i-n.The spectrally resolved XBIC measurements shed light on the underlying XBIC signal generation process in nanowire devices, showing that the XBIC signal originates at the atomic level with photoelectric absorption. Then, the X-ray flux variation revealed that the n+-i-n+ doped InGaP nanowire devices were affected by charge trapping leading to photogating and photodoping effects. In contrast, both kinds of p-i-n doped nanowire devices illustrated a linear response as function of the X-ray photon flux, which makes this doping profile more suitable for X-ray detectors. The XBIC measurements of this thesis could reveal the spatially resolved charge collection efficiency (CCE) or internal quantum efficiency (IQE) of the nanowire device. This result emphasizes the key ability of XBIC to be used in the development of nanowire solar cells. Furthermore, calculations based on the finite element method (FEM) was used to get a better understanding of the XBIC results.Although the in-plane nanowire devices can be used for understanding of the XBIC process at the nanoscale, they are not ideal for X-ray detection. The spatial resolution is still limited by the length of the active region, and the diameter of the nanowire limits the absorbing length. A novel fabrication process was therefore developed for a single vertical nanowire device where standing as-grown nanowires were turned into single pixel devices. With this configuration, the incident X-rays can be absorbed along the nanowire axis instead of the diameter. The nanowire used for this device is a p-i-n doped InP nanowire with a diameter of 60 nm as pixel size. Unlike the horizontal NW devices, the flux variation XBIC measurement reveals a sub-linear behaviour.The vertical nanowire device was used to make a high-resolution 3D image of a 90 nm nanofocused X-ray beam by scanning the device in different planes along the beam. The measurements reveal details of the intensity distribution that agree well with calculations based on ptychography. Instead of the nanowire diameter, the spatial detection was limited to about 100 nm due to the stability of the measurement system and X-ray absorption in the top contact. In the future, the device design with as-grown nanowires could scale up into multi-pixel array detectors operating much like conventional X-ray detectors.
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| 3. |
- Hammarberg, Susanna, et al.
(författare)
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High resolution strain mapping of a single axially heterostructured nanowire using scanning X-ray diffraction
- 2020
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Ingår i: Nano Research. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 13:9, s. 2460-2468
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Tidskriftsartikel (refereegranskat)abstract
- Axially heterostructured nanowires are a promising platform for next generation electronic and optoelectronic devices. Reports based on theoretical modeling have predicted more complex strain distributions and increased critical layer thicknesses than in thin films, due to lateral strain relaxation at the surface, but the understanding of the growth and strain distributions in these complex structures is hampered by the lack of high-resolution characterization techniques. Here, we demonstrate strain mapping of an axially segmented GaInP-InP 190 nm diameter nanowire heterostructure using scanning X-ray diffraction. We systematically investigate the strain distribution and lattice tilt in three different segment lengths from 45 to 170 nm, obtaining strain maps with about 10−4 relative strain sensitivity. The experiments were performed using the 90 nm diameter nanofocus at the NanoMAX beamline, taking advantage of the high coherent flux from the first diffraction limited storage ring MAX IV. The experimental results are in good agreement with a full simulation of the experiment based on a three-dimensional (3D) finite element model. The largest segments show a complex profile, where the lateral strain relaxation at the surface leads to a dome-shaped strain distribution from the mismatched interfaces, and a change from tensile to compressive strain within a single segment. The lattice tilt maps show a cross-shaped profile with excellent qualitative and quantitative agreement with the simulations. In contrast, the shortest measured InP segment is almost fully adapted to the surrounding GaInP segments. [Figure not available: see fulltext.].
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