Enhanced Dark Current Suppression of Amorphous Selenium Detector With Use of IGZO Hole Blocking Layer
We examined the potential application of indium gallium zinc oxide (IGZO) as a hole-blocking layer for an amorphous selenium (a-Se)-based detector to reduce the dark current and improve the sensitivity of the detector. By employing a thin layer of IGZO (375 nm), the dark current of an a-Se detector remains below 1 pA/mm2 up to electric fields as high as 60 V/μm. The measured dark current at different electric fields is comparable to the thermal generation currents in a-Se, thus demonstrating the good hole-blocking properties of IGZO. The detector's photo response was characterized using a blue light-emitting diode at different electric fields. A factor of three improvement in external quantum efficiency was observed by increasing the electric field of the detector from 10 to 50 V/μm.
Investigation of Hole-Blocking Contacts for High-Conversion-Gain Amorphous Selenium Detectors for X-Ray Imaging
In this paper, we investigated different organic and inorganic hole-blocking contacts for amorphous selenium (a-Se)-based photodetectors: <formula formulatype="inline"><tex Notation="TeX">$\hbox{CeO}_{2}$</tex></formula>, <formula formulatype="inline"><tex Notation="TeX">$\hbox{TiO}_{2}$</tex></formula>, perylene tetracarboxylic bisbenzimidazole (PTCBI), and polyimide (PI). <formula formulatype="inline"><tex Notation="TeX">$\hbox{CeO}_{2}$</tex></formula> has previously been used as a blocking layer for high-gain a-Se devices. <formula formulatype="inline"><tex Notation="TeX">$\hbox{TiO}_{2}$</tex> </formula> has similar properties to <formula formulatype="inline"><tex Notation="TeX">$\hbox{CeO}_{2}$</tex></formula>. PTCBI has a higher ionization potential compared to a-Se and has a very low hole mobility. PI is a common insulator in the semiconductor industry. It was found that an 800-nm PI layer reduces the dark current by more than two orders of magnitude in comparison with 30 nm of <formula formulatype="inline"><tex Notation="TeX">$\hbox{CeO}_{2}$</tex></formula>, 20 nm of <formula formulatype="inline"><tex Notation="TeX">$\hbox{TiO}_{2}$</tex></formula>, and 50 nm of PTCBI. No significant charge trapping was found in the devices consisting of an 800-nm PI layer. Unlike previously reported inorganic hole-blocking contact technology, PI layers further benefit from a simple spin coating fabrication step before evaporation of a-Se. Photodetector samples incorporating the PI layer are tested at high electric fields, and gains reaching 4.4 were observed at an electric field <formula formulatype="inline"><tex Notation="TeX">$>\hbox{80}\ \hbox{V}/\mu \hbox{m}$</tex></formula>. We conclude that using a PI layer is a promising step in the development of high-conversion-gain detectors for emerging applications in large-area medical diagnostic imaging, crystallography, and nondestructive test.
Speci fi cation Test for Panel Data Models with Interactive Fixed E ff ects ∗
In this paper, we propose a consistent nonparametric test for linearity in a large dimensional panel data model with interactive fixed effects. Both lagged dependent variables and conditional heteroskedasticity of unknown form are allowed in the model. We estimate the model under the null hypothesis of linearity to obtain the restricted residuals which are then used to construct the test statistic. We show that after being appropriately centered and standardized, the test statistic is asymptotically normally distributed under both the null hypothesis and a sequence of Pitman local alternatives by using the concept of conditional strong mixing that was recently introduced by Prakasa Rao (2009). To improve the finite sample performance, we propose a bootstrap procedure to obtain the bootstrap -value. A small set of Monte Carlo simulations illustrates that our test performs well in finite samples. An application to an economic growth panel dataset indicates significant nonlinear relationships between economic growth, initial income level and capital accumulation.