2506 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 5, OCTOBER 2006 Comparison of Mercuric Iodide and Lead Iodide X-Ray Detectors for X-Ray Imaging Applications G. Zentai, Member, IEEE, L. Partain, Member, IEEE, R. Pavlyuchkova, C. Proano, M. Schieber, Member, IEEE, K. Shah, P. Bennett, L. Melekhov, and H. Gilboa Abstract—Mercuric iodide (HgI ) and lead iodide (PbI ) mate- rials have been investigated for several years as direct converter layers for digital x-ray imaging applications. A difficult challenge of both lead iodide and mercuric iodide is the higher than desired leakage currents. These currents are influenced by different factors such as applied electrical field, layer thickness, layer density, elec- trode structure, material purity and by the deposition parameters. Minimizing the leakage current must also be achieved without ad- versely affecting charge transport, which plays a large role in gain and is influenced by these parameters. Other challenges relate to increasing film thickness without degrading electrical properties. This paper compares some imagers as the result of optimization process. We deposited the above materials on flat panel thin film tran- sistor (TFT) arrays with 127 um pixel pitch. The imagers were eval- uated for both radiographic and fluoroscopic imaging. Modulation Transfer Function (MTF) was measured as a function of the spa- tial frequency. The MTF data were compared to values published in the literature for indirect detector (CsI). Image lag characteris- tics of mercuric iodide appear adequate for fluoroscopic rates. The structure and x-ray diffraction data of the two materials were com- pared to explain the difference in image lag between them. Index Terms—Lead compounds, medical imaging, mercury compounds, polycrystalline semiconductors, thin film devices, x-ray image sensors, , x-ray imaging. I. INTRODUCTION W IDE band gap semiconductors are finding new applica- tion areas in the development of digital x-ray imaging technologies [1]–[5]. Higher resolution is possible with such direct detectors since the blurring of light spreading is elimi- nated. Light spreading is a major issue in the photodiode - phos- phor type “indirect” detectors. A common detector design of di- rect detectors incorporates a semiconductor converter layer de- posited directly upon a thin film transistor array that provides both pixelation and switching capability. Transistor arrays fab- ricated from amorphous silicon (a-Si) have reached coverage Manuscript received November 9, 2004; revised September 27, 2005. G. Zentai, L. Partain, R. Pavlyuchkova, and C. Proano are with Ginzton Technology Center, Mountain View, CA 94043 USA (e-mail: george. zentai@varian.com; larry.partain@varian.com; raisa.pavlyuchkova@varian. com; cesar.proano@varian.com). M. Schieber is with the School of Applied Science and Technology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, and with Real Time Radiography, Jerusalem Technology Park, Jerusalem 91487, Israel (e-mail: schieber@vms.huji.ac.il). K. Shah and P. Bennett are with Radiation Monitoring Devices, Watertown, MA 02472 USA (e-mail: kshah@rmdinc.com; pbennett@rmdinc.com). L. Melekhov and H. Gilboa are with Real Time Radiography, Jerusalem 91487, Israel (e-mail: leonid.melekhov@realtimeradiography.com; haim. gilboa@realtimeradiography.com). Digital Object Identifier 10.1109/TNS.2006.880975 areas of 43 cm 43 cm and pixelation below 100 m pitch [5], [6]. A key feature of the materials most prominently being studied is their relatively low processing temperatures. Phys- ical vapor deposition (PVD) is easily applicable because the materials’ suitably high vapor pressures can provide the growth rates needed to reach reasonable film thickness. The advantages of photoconductor layers for digital radiog- raphy X-ray detectors have been demonstrated in previous pa- pers [1]–[4]. Direct detector materials need to have several at- tributes, including high x-ray absorption, high charge collection, low dark current, good uniformity, high spatial resolution, low signal retention (image lag) just to mention a few, and these are difficult to achieve in a single material. Higher sensitivity and lower bias voltage required for PbI and HgI films show their superiority as compared to a-Se. Both films are more stable at elevated and at low temperatures than a-Se because a-Se requires temperature control not only during operation but also during shipment. Deposition of polycrystalline CdZnTe (CZT) on a large area is difficult and direct deposition onto TFT arrays is not possible because of the very high processing temperature ( 600 C). There have been experiments to deposit CZT on separate substrate but the connection of the pixelated TFT matrix readout array to the CZT layer was very problematic on high pixel count large area arrays [7]. II. SAMPLE PREPARATION Lead iodide films are produced through a vacuum deposi- tion process of the material in synthesized form. High purity material is commercially obtained and subsequently melted in an evacuated ampoule and zone refined horizontally. The de- position takes place at a pressure ranging from to Torr and the substrate is heated within the range of 150–230 C. The upper limit of this temperature range is given by the a-Si array ‘safe processing limit’. Evaporation rates up to 20 m/hr and durations of several hours are possible, resulting in films with thickness over 200 m. PbI films can be fabricated with widely varying densities, from roughly 3 to 6 g/cm . This is achieved through variations in deposition parameters, post de- position treatments and annealing. A general trend is for denser films to have higher leakage currents; this trend is not readily evident for sensitivity. The system for Physical Vapor Deposited (PVD) mercuric io- dide coating is based upon a glass reactor where polycrystalline mercuric iodide is deposited from vapor phase under reduced pressure. Highly purified mercuric iodide powder is loaded into 0018-9499/$20.00 © 2006 IEEE