Flexible a-Si : H Position-Sensitive Detectors E. FORTUNATO, L. PEREIRA, H. ÁGUAS, I. FERREIRA, AND R. MARTINS Contributed Paper Flexible and large area (5 mm 80 mm with an active length of 70 mm) position-sensitive detectors (PSDs) deposited onto poly- meric substrates (polyimide—Kapton VN) have been fabricated. The optimized structure presented is based on a heterojunction of amorphous silicon (a-Si : H)/ZnO : Al. The sensors were character- ized by spectral response, photocurrent dependence as a function of light intensity, and position detection measurements. The set of data obtained on one-dimensional PSDs based on the heterojunction show excellent performances with a maximum spectral response of 0.12 A/W at 500 nm and a nonlinearity of 10 over 70-mm length. The produced sensors present a nonlinearity higher than those ones produced on glass substrates, due to the different thermal coeffi- cients exhibited by the polymer and the a-Si : H film. In order to prove this behavior, it was measured the defect density obtained by the constant photocurrent method on a-Si : H thin films deposited on polymeric substrates and bent with different radii of curvature. Keywords—Amorphous silicon (a-Si : H), heterojunction, poly- imide substrates, position-sensitive detectors (PSDs), photodetec- tors, thin films. I. INTRODUCTION Currently, there is a growing interest in the production and characterization of lightweight, large area information displays, detectors, imaging sensors, and flexible/foldable electronics [1], [2]. These new demands motivate the au- thors to develop compatible processes based on thin-film technologies, and especially the amorphous silicon (a-Si : H) one, in order to produce position-sensitive detectors (PSDs) on inexpensive polymeric substrates. PSDs are single pho- todetectors that uses the lateral photoeffect [3] to detect in a continuous manner the position of an incident beam of light. Besides the inherent technology production facility of these sensors, one of the main advantages besides the low cost, when compared to other sensors, like charge-coupled devices (CCDs) or quadrant cells, is the high spatial resolu- tion obtained, which is not limited by the detector size or by dead regions among the elements. However, in applications where the control has to be made upon a rotating axis of a motor or if it is required the deter- Manuscript received September 20, 2004; revised January 10, 2005. The authors are with the Department of Materials Science, New University of Lisbon, Caparica 2829-516, Portugal (e-mail: elvira. fortunato@fct.unl.pt). Digital Object Identifier 10.1109/JPROC.2005.850302 mination of the angular position of the rotor of a motor, in a continuous manner, is of great importance to have a curved sensor with a cylindrical shape. The fabrication of a flexible PSD also leads to potential applications involving angle and position sensing. To do so, the sensing element should be de- posited on a flexible substrate. The production of flexible po- sition sensors introduces new challenges in addition to those produced on rigid substrates, such as glass [4]. First of all, the substrate flexibility can easily damage the film and induce some defects that may cause some deviations on the device performances. But other intrinsic properties of the polymeric substrate such as thermal conductivity and thermal expansion coefficient are different from those presented by the glass and may promote a mismatch between the deposited film and the substrate. So special care must be taken in order to reduce as far as possible the defects, namely, the residual stresses that are originated during the fabrication process [5]. In 2000 [6], the authors presented for the first time a one-dimensional flexible PSD deposited onto polymeric substrates with 20-mm length, using an a-Si : H nip homo- junction. As far as the present knowledge of the authors, no position sensors have been deposited on polymeric flexible substrates. The only devices deposited on flexible substrates using the a-Si : H technology are solar cells [7], [8] and thin-film transistors (TFTs) [9], [10]. Now, in this paper, an improved version of the previous one, with 80–mm length, based on a heterostructure between a-Si : H and ZnO : Al, will be presented. The absorption of the structure is not limited by the absorption of the polymer (for 600 nm, the transmission is around 60%, for a thickness of 75 m) and we can widen the energy range of detection, as well as improve the response time of the device. Besides, the sensor structure does not need to have photolithographic steps, so the total cost of the device is reduced. Fig. 1 shows a schematic drawing of the structure used, while Fig. 2 shows a photograph of a large area sensor deposited onto polymeric substrate, showing its flexibility. II. DEVICE FABRICATION The PSDs were fabricated on polyimide foils (Kapton type VN from Dupont) with a standard thickness of 75 m and with the following structure: polymer/Cr/ 0018-9219/$20.00 © 2005 IEEE PROCEEDINGS OF THE IEEE, VOL. 93, NO. 7, JULY 2005 1281