Effect of Annealing on the X-ray Detection Properties of Nano-sized Polycrystalline Lead Oxide Films

Cho, Sung-Ho; Kang, Sang-Sik; Kwon, Chul Soon; Yun, Min; Park, Ji-Koon; Park, Sung-Kwang; Choi, Jang-Yong; Mun, Chi-Woong; Lee, Hyungwon; Nam, Sang-Hee

doi:10.1143/jjap.47.7393

Cited by 8 publications

(6 citation statements)

References 9 publications

(10 reference statements)

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“…This is expected to reduce an amount of ionized centres in PbO thus improving drift mobilities of electrons and holes as well as suppressing recombination. The improvement of the X-ray sensitivity [14] and alteration in resistivity [15] of the PbO samples observed after the thermal annealing in atmosphere of oxygen support our finding.…”

supporting

confidence: 89%

Self-passivation of vacancies in α -PbO

Berashevich¹,

Rowlands²,

Reznik³

2013

EPL

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We introduce a self-passivation of single lead (Pb) and oxygen (O) vacancies in the α-PbO compound through formation of a Pb-O vacancy pair. The preferential mechanism for pair formation involves initial development of the single Pb vacancy which, by weakening the covalent bonding, sets up the crystal lattice for an appearance of the O vacancy. Binding of the Pb and O vacancies occurs through the ionization interactions. Since no dangling bonds appear at the Pb-O pair site, this defect has a minor effect on the electronic properties. In such, vacancy self-passivation offers a practical way to improve the transport properties in thermally grown PbO layers.Polycrystalline Lead Oxide (PbO) is one of the few photoconductive materials with a long-more than 60 years -history of employment in imaging devices. Although PbO is extensively used as a photoconductor, very little is known about the electronic properties and charge transport in this material. It is generally believed that transport in PbO is controlled by trapping in defects and that trapping is a cause of low mobility-time product [1]. However, the nature of defects is not fully understood. Emerging applications of PbO in the direct conversion flat panel radiation medical imaging detectors revived the interest in studying defects in this material as defects can significantly affect imaging performance [2].Our recent modeling of the native point defects in α-PbO [3] has shown that thermally deposited PbO layers should contain a significant concentration of single vacancies due to their moderate formation energies. Single vacancies are amphoteric defects appearing in the different charge states. In the neutral charge state, the O vacancy (V O(0) )) holds two electrons and forms the deep donor level. The neutral Pb vacancy (V Pb(0) )) is filled with holes, and is a shallow acceptor. It was established that these vacancies prefer to appear doubly ionized, V Pb(2−) and V O(2+) , acting as compensating centres to each other. Indeed, two compensating vacancies have a lower formation energy than neutral ones, such asThe fact that vacancies prefer to appear in charge states suggests that we have to consider the ionization interactions between them and the formation of a neutral vacancy pair V Pb-O instead of two separate compensating vacancies. This should further decrease the formation energy, approximately speaking, the free energy required to insert a defect in a lattice is reduced by the energy liberated due to ionization of the donor and acceptor. Indeed, the formation of the defect complexes in favor over the single defects is often observed during material deposition [4]. In this work, we present our study of the formation mechanism of V Pb-O pair in PbO layers and its effect on the electronic properties. Analysis of the formation of VPb-O vacancy pair was performed using the density functional theory (DFT) available in the Wien2k package [5] which utilizes the fullpotential augmented plane-wave method. The PerdewBurke-Ernzerhof parameterization [6] of the gen...

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supporting

confidence: 89%

Self-passivation of vacancies in α -PbO

Berashevich¹,

Rowlands²,

Reznik³

2013

EPL

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“…In order to calculate W ± from XPM measurements, the signal is integrated and the total collected charged is compared against the exposure to the PbO detector 13 , 17 . For the 3.75 s X-ray pulse at F = 4 V/μm, we obtained W ± ≈ 12 eV/ehp, which is larger than theoretically predicted by Klein rule ( eV) 13 but it is in a good agreement with the previously reported value of 9.9 eV/ehp, obtained under similar experimental conditions (5 s X-ray pulse, F = 3.5 V/μm 13 ).…”

Section: Resultsmentioning

confidence: 99%

“…In addition, the X-ray charge yield of PbO, although higher than that of a-Se, was still lower than the theoretically predicted one. This resulted in relatively high electron-hole pair creation energy W ± The evaluation of W ± was previously performed with an X-ray induced photocurrent method (XPM) 13 , 17 . In this technique, the charge carriers are generated with a relatively long X-ray pulse, while constant bias voltage is applied to the detector to extract the generated carriers.…”

Section: Introductionmentioning

confidence: 99%

Characterization of polycrystalline lead oxide for application in direct conversion X-ray detectors

Semeniuk

Grynko

DeCrescenzo

et al. 2017

Sci Rep

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While polycrystalline lead oxide (poly-PbO) is known to be one of the most promising photoconductors for utilization in X-ray detectors, its major performance parameters such as charge yield and mobility-lifetime product (μτ) are still not well established and require further investigation. Combining the conventional X-ray induced photocurrent and pulse height spectroscopy techniques we examine the X-ray photogeneration and recombination processes in poly-PbO. The measurements indicate that the amount of energy required to release a single electron hole pair W ± (inverse of charge yield) strongly depends on applied electric field and at 10 V/μm reaches ~20 eV/ehp. Fitting the measured pulse height spectra with the Hecht formula provided μτ for holes and electrons to be 4.1 × 10−8 cm2/V and 10−9 cm2/V, respectively. Obtained μτ values combined with recently reported mobility values of charge carriers in PbO suggest a new direction towards improvement of PbO technology by incorporation of Frisch grid or X-ray transistor architectures.

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“…The grey hashed area represents the acceptable range for dark current in an FPXI. Data have been taken from various sources, including the following: a-Se ( i -layer and n-i-p ) from [ 15 ], a-Se ( n-i ) from [ 118 ], HgI 2 (PVD at 0.25 V/μm and SP) from [ 51 ], HgI 2 (PVD at 0.4 V/μm) from [ 122 ], PbI 2 (PVD) from [ 56 ], PbI 2 (SP) from [ 48 ], Cd 0.95 Zn 0.05 Te from [ 58 ], PbO (PVD) from [ 59 ], PbO (SP) from [ 123 ], PbBr 2 and HgBr 2 from [ 124 ] and BiI 3 from [ 125 ].…”

Section: Figurementioning

confidence: 99%

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

Kasap

Frey

Belev

et al. 2011

Sensors

400

326

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In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.

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Effect of Annealing on the X-ray Detection Properties of Nano-sized Polycrystalline Lead Oxide Films

Cited by 8 publications

References 9 publications

Self-passivation of vacancies in α -PbO

Self-passivation of vacancies in α -PbO

Characterization of polycrystalline lead oxide for application in direct conversion X-ray detectors

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

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