Charge collection efficiency in the presence of non-uniform carrier drift mobilities and lifetimes in photoconductive detectors

Abstract

We consider the charge collection efficiency (CCE) for semiconductors in which the charge transport parameters, the drift mobility μ, and the carrier lifetime τ have spatial dependence, i.e., μ = μ(x) and τ = τ(x), where x is the distance from the radiation receiving top electrode toward the rear electrode. The small signal carrier packet drift analysis (CPDA) is re-examined, and the CCE efficiency for electrons and holes is formulated in terms of μ(x)τ(x)F(x), where F is the field. We use two model mobility and lifetime variations that are linear and exponential and then calculate and compare CCE determined from the CPDA equation, numerical solution of the continuity equation and Monte Carlo simulations as a function of the parameters characterizing the linear and exponential changes. The use of standard CCE equations for nonuniform samples is extensively examined, and errors are quantified by introducing a spatial average (SA) ⟨τ(x)⟩, average inverse (AI) ⟨1/τ(x)⟩, a new effective lifetime, and a kth order average. The SA lifetime works best when τ(x) monotonically decreases with x and AI works best when τ(x) monotonically increases with x. Stabilized a-Se x-ray photoconductors were considered as a practical application of this work. Both hole and electron lifetimes decrease in a-Se upon x-ray irradiation. Using the empirical equations derived recently for τh(x) and τe(x) as a function of dose D(x) in the sample, the CCE for two a-Se samples corresponding to a low-end device quality and the “best” was determined as a function of applied field.

Enhanced Hole Transport in Ni/Y₂O₃/n-4H-SiC MOS for Self-Biased Radiation Detection

“…Recent advancement in crystal growth technology has facilitated regular production of 250 μm thick single crystalline 4H-SiC epitaxial layers with ultralow point defect density 10 11 cm −3 and micropipe defect density <1 cm −2 [7]. Semiconductor single crystals with low defect density is crucial to electronic devices for achieving superior transport properties [8].…”

The Effect of Fractionation during the Vacuum Deposition of Stabilized Amorphous Selenium Alloy Photoconductors on the Overall Charge Collection Efficiency

“…The examination of Table 3 shows that, under positive bias, the lowest error for HCE comes from using the HRAI, that is, the average of the inverse hole range. According to the CE calculations in [ 6 ], if the hole range increases in the direction of drift of photoinjected holes, as in the present case (see Figure 6 ), then the range to use in Equation (4) is that from Equation (7), HRAI. This is indeed the result in the present case for 20 keV radiation with an error of −11.3%, but the errors from using HRSA and HRAI are comparable under 30 keV radiation.…”

“…It can still be used under high radiation intensities with errors quantified in [ 22 ]. Equation (4) can also be used for a semiconductor in which the carrier range varies monotonically (either simply increasing or decreasing along x ) if one defines appropriately averaged carrier ranges [ 6 ].…”

“…If the radiation-receiving electrode is biased negatively, then the hole and electron ranges need to be interchanged, since they would be drifting in opposite directions to those shown in Figure 7 a,c,d. A derivation based on free carrier dynamics and the assumptions behind Equation (8) are discussed in [ 6 ]. In this work, Equation (8) is integrated numerically with the electron and hole ranges r e ( x ) and r h ( x ), varying with the distance from the radiation receiving electrode, as given in Figure 6 .…”

“…Second, using the data on the As content vs. distance into film, the spatial variation of the hole and electron ranges (the drift mobility and lifetime products) is constructed. Third, using the carrier range profiles, the charge carrier CE is calculated based on the formulation summarized in [ 6 ]. This CE is compared with the corresponding case of a uniform composition a-Se film, assuming an average As content throughout the film.…”