Nanowire devices for X-ray detection

Sammanfattning: 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.