Influence of design parameters on dark current of vertically integrated a-Si:H diodes.
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D1.2.1
Influence of design parameters on dark current of vertically integrated a-Si:H diodes. C. Miazza1, N.Wyrsch1, G.Choong1, S. Dunand1, C. Ballif1, A. Shah1, N. Blanc2, F. Lustenberger2, R. Kaufmann2, M. Despeisse3, P. Jarron3 1 Institut de Microtechnique, Université de Neuchâtel, Breguet 2, 2000 Neuchâtel, Switzerland, 2 CSEM SA, Badenerstrasse 569, P.O. Box, 8048 Zurich, Switzerland, 3 CERN, CERN Meyrin, 1211 Geneva 23, Switzerland. Abstract: Image and particle sensors based on thin film on CMOS (TFC) technology, where a-Si:H detectors are vertically integrated on top of a CMOS chip, basically provide high sensitivity and low dark current densities (Jdark). However, as shown in previous work and as confirmed by the actual measurements, Jdark values depend on the topology of the chip and on the detector structure used. The present paper describes a systematic study carried out, both with test structures on glass and also with a dedicated CMOS test chip designed by CERN. The increase in Jdark is shown to be related to border effects, and especially on the detailed structure of the pixel periphery. In all cases, lower Jdark are obtained when one uses metal-i-p instead of n-i-p configuration detectors. Transferring these results to the standard TFC sensors used by them, the authors have obtained values of Jdark as low as 20 pA/cm2 at -1 V reverse bias. Introduction: The vertical integration of a thin-film a-Si:H detector on top of a dedicated CMOS integrated circuit is an attractive solution to enhance the performances of the resulting monolithic sensors. This approach is useful in the field of visible light imaging [1, 2], particle detection and X- or γ-ray detection [3, 4, 5]. For monolithic image sensors a high sensitivity (> 50 V/ (µJ/cm2)) together with potentially low dark current densities (Jdark) can be achieved ( 90 %). This fact combined with the high quantum efficiency (QE(λ)) of a-Si:H in the visible spectral range improves the final sensitivity [6]: S w (λ ) =
FF . A pix λ q . .QE (λ ) C int hc 0
[V/ (µJ/cm2)]
(1)
where Apix is the whole pixel area, Cint the integration capacitance, λ the wavelength, q the elementary charge, hc0 is the product of Plank constant and the light velocity in vacuum. In order to reach an enhanced dynamic range and the possibility of very low light level detection a very low Jdark values is a key issue [7]. This is also the case in the field of particle detection and X- or γ-ray sensors. Here, depending on the application, one needs to be able to detect as low as 60 to 80 electron/hole charge pairs generated per micron of material thickness. Therefore, it is crucial to understand the origin of Jdark and the mechanisms influencing it in a TFC configuration, so as to be able to fabricate vertically integrated monolithic sensors for these applications.
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In the case of a-Si:H n-i-p photodiodes the lower limit for Jdark is given by the thermally generated charge in the intrinsic layer. The density of thermally generated current depends on the defect density and the
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