Photon-Counting Detector (PCD) capable of resolving the energies of single X-ray photons is critical in medical imaging (e.g., Computed Tomography). A high count rate and negligible polarization is essential for a PCD. Besides, there has been a critical need to develop high-Z sensor for synchrotron X-ray facility. The very high X-ray fluxes (e.g., 1e6 – 1e12 photons/s/mm2) involved in both applications makes it very challenging for detector operation. Here, we demonstrate that our perovskite CsPbBr3 single crystal detectors have good performance for these applications.
Lead halide perovskites have been under intense research focus due to their potential for efficient solar cells and other devices. Among their outstanding optoelectronic properties is a long carrier lifetime which is difficult to reconcile within a simple direct band gap semiconductor picture. Recent studies point to Rashba band splitting, charge separation and the formation of a large polaron as possible explanations. Understanding band edge excitations and dynamics is therefore critical to developing a working model of these materials. Time-resolved THz spectroscopy (TRTS) is an ideal tool to interrogate these meV level excitations on a sub-100 fs time scale. Here we use an air-plasma based TRTS covering the 4 – 120 meV range to measure the THz differential reflectivity from a facet of a methylammonium lead triiodide single crystal following femtosecond optical excitation at the band edge. Full two–dimensional time/energy reflectivity maps reveal a beat between the THz probe light frequency and the crystal photoconductivity, which coherently oscillates at the LO phonon frequency and vanishes with a sub-picosecond lifetime. This indicates some population of carriers are impulsively formed upon excitation and are strong coupling to the polar lattice, in good agreement with the polaron picture. To our knowledge, this is the first observation of a coherent oscillation in the transport parameters in the lead halide perovskites.
Thallium based chalcogenide and halide semiconductors such as Tl4HgI6, TlGaSe2, Tl6SeI4 and Tl6SI4 are promising materials for room-temperature hard radiation detection. They feature appropriate band gaps, high mass densities and facile growth technology. However, these materials are being plagued by the Tl oxides impurity from Tl precursor or Tl containing binary precursors, which leads to problems including tube breakage, parasitic nucleation and detector performance deterioration. In this work, we present a facile way to chemically reduce Tl oxidations, and then eliminate oxygen impurity by adding high-purity graphite powder during synthesis and crystal growth. We also further investigated the reactivity between Tl oxides and graphite. The detector performance of Tl6SeI4 crystal was dramatically improved after lowering/removing the oxygen impurities. This result not only indicates the significance of removing oxygen impurity for improving detector performance. Our results suggest that the chemical reduction method we developed by adding carbon powder during synthesis is highly effective in substantially reducing oxygen impurities from Tl containing materials.
In this work, two Hg-based chalcogenides were investigated in detail to reveal their potential capability of radiation detection at room temperature (RT). Cs2Hg6S7, with a bandgap of 1.63 eV, which is designed by the dimensional reduction theory proposed by our group, were prepared and characterized. α-HgS, with a bandgap of ~2.10 eV, as a precursor used for the ternary compound synthesis, was also proposed and further investigated. For Cs2Hg6S7, the crystals tended to crystallize into needle form with small grains. Here, the conditions of Bridgman melt growth were optimized to obtain relatively large single crystals. The slight excess of Cs2S as a fluxing agent during growth was found to facilitate better crystallization and large grains. Interestingly, no inclusion or secondary phase was found in the as-grown single crystals. The improvement of bulk resistivity from ~10^6 Ωcm to 10^8 Ωcm was also achieved through the control of stoichiometry during crystal growth. For α-HgS crystals, both physical vapor transport and chemical vapor transport methods have been applied. By modifying the transport temperature and transport agent, single crystal with size about 3x1.5 mm^2 was grown with resistivity higher than 10^11 Ωcm. Photoluminescence (PL) revealed that multiple peaks observed in the 1.6-2.3 eV range and excitonic peak from for α-HgS single crystals were observed indicating good crystalline quality. Finally, the planar detectors for both crystals were tested under Co57 gamma ray source. Both of the crystals showed reasonable gamma ray response, while α-HgS crystals could respond at a relatively higher counting rate.
We have developed the detached Bridgman process for growth of CdZnTe crystals. Detachment of the solidification
interface from the growth ampoule results in a low density of dislocations in the grown material and large single crystal
grains. The detached Bridgman process also provides for direct control of the melt composition close to the growth front,
allowing for accurate control of both the density of the Te/Cd precipitates as well as the majority carrier concentration in
the grown material. The influence of melt-composition control and compensation by shallow and deep donors on
detector performance is presented.
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