Thin-film Photodiode with an a-Si:H/nc-Si:H Absorption Bilayer
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Thin-film Photodiode with an a-Si:H/nc-Si:H Absorption Bilayer Y. Vygranenko1, 2*, M. Vieira1,2, A. Sazonov3 1
Electronics, Telecommunications and Computer Engineering, ISEL, 1949-014 Lisbon, Portugal CTS-UNINOVA, 2829-516 Caparica, Portugal 3 Electrical and Computer Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada 2
ABSTRACT We report on the fabrication and characterization of n+-n-i-Gi-p thin-film photodiodes with an active region comprising a hydrogenated nanocrystalline silicon (nc-Si:H) n-layer and a hydrogenated amorphous silicon (a-Si:H) i-layer. The combination of wide- and narrow-gap absorption layers enables the spectral response extending from the near-ultraviolet (NUV) to the near-infrared (NIR) region. Moreover, in the low-bias range, when only the i-layer is depleted, the leakage current is significantly lower than that in the conventional nc-Si:H n+-n-p+ photodiode deposited under the same deposition conditions. Device with the 900nm/400nm thick n-i-layers exhibits a reverse dark current density of 3 nA/cm2 at 1V. In the high-bias range, when the depletion region expands within the n-layer, the magnitude of the leakage current depends on electronic properties of nc-Si:H. The density of shallow and deep states, and diffusion length of holes in the n-layer have been estimated from the capacitance-voltage characteristics and from the bias dependence of the long-wavelength response, respectively. To improve the quantum efficiency in the NIR-region, we have also implemented a Cr / ZnO:Al back reflector. The observed long-wavelength spectral response is about twice as high as that for a reference photodiode without ZnO:Al layer. Results demonstrate the feasibility of the photodiode for low-level light detection in the NUV-to-NIR spectral range. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) photodiodes are conventionally used in flat-panel digital X-ray image sensors due to large-area capability and high sensitivity in the visible spectral range [1]. A similar technology based on hydrogenated nanocrystalline silicon (nc-Si:H) can be developed for imaging applications. The optical bandgap of highly crystalline nc-Si:H films is close to the bandgap of crystalline silicon thus allowing to extend the spectral response in the near-infrared (NIR). However, the absorption coefficient of nc-Si:H is low in the NIRrange, i.e., the development of thin-film photodiodes requires a light management to increase the effective optical path within the absorber [2]. Another technological issue is associated with the complex microstructure of this heterogeneous material, which determines the electronic properties and effects the device performance. In particular, defects at grain boundaries, voids, and other structural defects are the source of excessive leakage current limiting the lowest detectable light level. In this paper, we report on the fabrication and characterization of nc-Si:H-based photodiodes of different configurations. The discussed technological developments are aimed to minimize the leak
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