Thin Solid Films 403 – 404 (2002) 363–367 0040-6090/02/$ - see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0040-6090 Ž 01 . 01515-2 Memory effects in highly resistive p–i–n heterojunctions for optical applications R. Schwarz , P. Louro *, Yu. Vygranenko , M. Fernandes , M. Vieira , M. Schubert a,b a, a a a c Electronics and Communications Department, ISEL, P-1949-014 Lisboa, Portugal a Departamento de Fısica, IST, P-1096 Lisboa, Portugal b ´ Institut fur Physikalische Elektronik, IPE, Universitat Stuttgart, D-70569 Stuttgart, Germany c ¨ ¨ Abstract Large area p–i–n diode structures based on amorphous hydrogenated silicon can be used as single element image sensors where the information is read out by a scanning laser beam. A high sensitivity is reached with silicon–carbon alloy contact layers. The higher defect density in the large band gap material is usually a problem for efficient carrier collection in solar cell applications. When used as an image sensor, however, the charge stored in deep defects represents an easy way to realize short-term image storage. In the case of a p-(Si:H) y i-(Si:H) y n-(Si C :H) sensor structure we have measured a memory effect of approximately x 1yx 1% after several minutes of image projection. Metastable sensor degradation is observed in accordance with the Staebler–Wronski effect. Fast degradation of sensor performance — corresponding to 90% erasable image storage capability — was studied in an unalloyed structure using a Nd:YAG laser system. The response can be modeled by a stretched exponential decay with parameters depending on the laser pulse energy.  2002 Elsevier Science B.V. All rights reserved. Keywords: Diode structures; Amorphous hydrogenated silicon; Scanning laser beam 1. Introduction Recently, detailed characterization was done on thin film image sensors composed of a p–i–n structure based on hydrogenated amorphous silicon (a-Si:H) and depos- ited onto ZnO-coated glass substrates. Particularly low minimal image intensities were achieved in structures where the slightly doped contact layers consisted of hydrogenated amorphous silicon–carbon films (a- Si C :H). Such laser scanned photodiode (LSP) image x 1yx sensors were characterized with respect to optical and electrical parameters of the individual layers w1x, I–V and C–V characteristics under illumination of the devices w2x, and spectral response under various bias conditions w3x. It is to be expected that the well-known degradation of optoelectronic properties of a-Si:H under illumination, known as the Staebler–Wronski effect (SWE) w4x, will also affect the image sensor response. In fact, the higher * Corresponding author. the material quality, the larger this effect should be. As an example, the efficiency of an a-Si:H solar cell decreased to 50% of its initial level after light soaking under 500 mW y cm intensity during 1000 h w5x. 2 However, the degradation should be reversible either by long-term relaxation or by thermal annealing. This metastable effect can be exploited in a positive way for image storage. In an earlier work Paasche et al. measured a charge storage time of several days in a homogeneous SiC thin film w6x. The details of sensor degradation, image storage, and sensor recovery will actually be much more complex than what is known from single layer behavior. In a heterostructure image sensor we also have to deal with interface quality, band offsets, potential drops across the wide-band gap contact layers, and screening of the electric field due to charges accumulated in deep defects. This work describes the small reduction in sensor response of a p-(Si:H) y i-(Si:H) y n-(Si C :H) sensor x 1yx structure after illumination with a square-shaped white light image of some 0.5 mW y cm intensity. The stored 2