Characteristics of a-Si:H Schottky Photodiode by Using Mo Barrier Metal at Al/ITO Contact
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CHARACTERISTICS OF a-Si:H SCHOTTKY PHOTODIODE BY USING Mo BARRIER METAL AT Al/ITO CONTACT Yoshihiko Sakai, Hiroyuki Hotta, Tsutomu Hamada, Hisao Ito and Takashi Ozawa Electronic Imaging &Devices Research Laboratory, FUJI XEROX Co., Ltd. 2274 Hongo, Ebina-shi, Kanagawa 243-04, JAPAN ABSTRACT An ITO/a-Si:H Schottky Photodiode(PD) with Mo as a barrier metal at Al/ITO contact has been made and it shows its useful characteristics. The PD without Mo had high dark current proportional to the contact-via-hole area and quite a large resistance at the contact area. The XPS analysis of the Al/ITO interface indicated that At and 0 in ITO mutually diffused around the interface, resulting in the degradation of ITO/a-Si:H Schottky barrier. Most of the diffused Al formed aluminum oxide (A1203) at the Al/ITO interface, and which caused high resistance of the contact area. Inserting Mo barrier metal 500A thick, the dark current got less dependent on the contact area and decreased by 2/3 on 254x227/Am PD with 106x106pm contact-via-hole. The contact resistance also decreased significantly to 1/105 of those of the conventional ones. From the XPS analysis of interface, it was also found that Mo barrier metal prevented Al diffusion into the ITO and formation of aluminum oxide at the interface. INTRODUCTION Recently, page-width contact image sensors which need no reduction optics are being developed in order to make image input terminals smaller. A-Si:H1,2,3) is used as a major material of a photoelectric device for page width contact image sensors. Of the devices, there are two kinds of structure; planar and sandwich types. The planar type 4) has an advantage of high photocurrent by charge injection, but has high dark current which reduces photo/dark current ratio and photoresponse. The sandwich type is more a complex structure and process, but has very low dark current, high photo/dark current ratio, and fast photoresponsel, 2). Therefore, high S/N ratio and speed can be achieved with it. We have developed an image sensor that uses an ITO/a-Si:H/Cr sandwich-type photodiode with Schottky barrier5,6,7). The structure of this photodiode is the simplest of sandwich types. The photodiode of the developed image sensor, however, had an increase of dark current and contact resistance. Some factors increasing the dark current8) are known, for example, the influence of Al 9,10). Al has been used for interconnects in general, due to its low resistivity, in very large-scale integrated circuitsl 1).
These problems result in low S/N ratio and slow read rate of the image sensor, and which make it difficultto achieve higher resolution and higher speed. This paper describes the results of some experiments to identify the Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society
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sources of degradation and to eliminate them. From the viewpoint of influence to underlayer and process efficiency, Mo was selected among several materials as a diffusion barrier against Al. EXPERIMENT Conventional structure and characteristics The photodiode
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