Evaluation of an alternative technique for the fabrication of direct detector X-ray imagers: spray pyrolysis of lead iod
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Evaluation of an alternative technique for the fabrication of direct detector X-ray imagers: spray pyrolysis of lead iodide and mercury iodide
J. F. Condeles, J. C. Ugucioni and M. Mulato Departamento de Física e Matemática, Faculdade de Filosofia Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, 14040-901, Ribeirão Preto, SP, Brazil Abstract This work discusses the new use of an old deposition technique: spray pyrolysis. The objective is the deposition of thin films of lead iodide and mercury iodide and their future use as photoconductors in medical X-ray digital imagers. We discuss the main advantages and limitations of the deposition process comparing both materials. The final thin films are structurally investigated using X-ray diffraction and microscopy. The deposition technique seems to be very promising for the future development of large area radiation detectors. INTRODUCTION Amorphous silicon based X-ray imagers have been studied in the last 15 years by some universities and industrial laboratories [1]. Some prototypes have even come to the market already [2]. For these applications a phosphor material must be used on top of the a-Si:H pixelated active matrix imager. The phosphor is responsible for the conversion of the X-ray photons into photons with wavelengths in the visible range, which in turn are absorbed by the pi-n a-Si:H photodetector. This method is called indirect. On the other hand, the direct method is based on the use of a photoconductor that directly converts the X-ray photons into electric charge, which is stored in each pixel. A single microcrystalline silicon TFT is also used for the addressing of each pixel in this case [3,4]. Among the many material candidates for the direct detection system, lead iodide (PbI2) and mercury iodide (HgI2) are very promising. They have large atomic numbers and also large energy band gaps [5-7]. The PbI2 is a lamellar semiconductor material with a hexagonal structure made of a plane of lead atoms between two planes of iodine atoms. Its melting point is 408oC, it has a dielectric constant equal to 21, a density of 6.2 g/cm3 and forbidden band gap of 2.34 eV. Its atomic number is ZPb = 82 and ZI = 53. HgI2 is also a semiconductor with a wide gap (2.13-2.20eV) and high atomic number (ZHg = 80; ZI =53)[6,7]. When solid, this material may have three phases [8-10]: an α-HgI2 (tetragonal and red), β-HgI2 (orthorrombic and yellow) and orange-HgI2 (with a mixed structure). The αphase is the most promising for radiation detectors [1,2,11], and its phase transition to the β-HgI2 is observed at a temperature of 1270C at a pressure of 1atm [8,9]. Its melting point is 251ºC [9]. Films and crystals of HgI2 have been grown by thermal evaporation [12], screen printing [11,12], physical vapor deposition (PVD) [11], and another ones. The growth of large volumes of both materials has already been studied in the past for the detection of high-energy photons for nuclear applications. Nevertheless, few studies have been published regarding the fabrication o
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