Optimization of the a -SiC p -layer in a -Si:H-based n-i-p Photodiodes
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Optimization of the a-SiC p-layer in a-Si:H-based n-i-p photodiodes Y. Vygranenko1, A. Sazonov2, G. Heiler3, T. Tredwell3, M. Vieira1,4, A. Nathan5 1 Electronics, Telecommunications and Computer Engineering, ISEL, Lisbon, 1950-062, Portugal 2 Electrical and Computer Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada 3 Carestream Health Inc., Rochester, NY, 14652-3487, USA 4 CTS-UNINOVA, Quinta da Torre, 2829-516, Caparica, Portugal 5 London Centre for Nanotechnology, UCL, London, WC1H 0AH, United Kingdom
ABSTRACT Our work is aimed at enhancing the external quantum efficiency (EQE) of n-i-p photodiodes by reducing the absorption losses in the p-layer and the recombination losses in the p-i interface. We have applied boron-doped and undoped hydrogenated amorphous silicon carbon alloy (a-SiC:H) grown in hydrogen-diluted, silane-methane plasma to both the p-layer and undoped buffer layer, thus tailoring the p-i interface. The current-voltage, capacitance-voltage, and spectral-response characteristics of fabricated photodiodes are correlated with the doping level, optical band gap, and deposition conditions for a-SiC:H layers. The optimized device exhibits a leakage current of about 110 pA/cm2 at the reverse bias of 5 V, and a peak value of 89% EQE at a wavelength of 530 nm. At shorter wavelengths, the EQE decreases down to 56% at a 400 nm wavelength. Calculations of transmission/reflection losses at the front of the photodiode show that observed short-wavelength sensitivity enhancement can be attributed to improved separation of electronhole pairs in the p-layer depletion region. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) p-i-n photodiodes are commonly used as pixel sensors in digital radiographic flat-panel imaging detectors [1]. Photodiode performance is one of the factors limiting the signal-to-noise ratio and image quality. In particular, a high sensor sensitivity in the visible spectral range is required to provide an efficient optical coupling with conventional phosphors such as CsI:Tl or Gd2O2S:Tb [2]. One of the approaches to minimize the absorption losses in the p-layer is to use an a-Si1-xCx:H alloy having a wider band gap than a-Si:H. This approach has been widely used for solar cells by engineering the heterojunction a-SiC:H/a-Si:H p-i interface in order to accommodate the band offset, to optimize the electric field profile in this region, to passivate the interface defects, and to reduce recombination [3–5]. One of the findings is that the implementation of a thin buffer layer of intrinsic a-SiC:H results in a higher open circuit voltage and fill factor [6]. The similar interface design has been applied for a-Si:H-based n-i-δi-p photodiodes. In this device, a thin (~4 nm) undoped a-SiC:H buffer (δi) significantly reduces the reverse dark current and recombination losses at the p-i interface [7]. A 220°C deposition process has been developed for n-i-δi-p photodiode arrays [8]. In this paper, we report on blue-enhanced n-i-δi-p photodiodes deposited at a substrate temperatu
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