Utilization of Amorphous Silicon Carbide (a-Si:C:H) as a Resistive Layer in Gas Microstrip Detectors
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much smaller than that of electrons, the ions tend to pile up near the anode. The accumulation of the positive charges modifies the electric field profile and decreases the avalanche gain. This charging-up effect can be reduced by introducing an additional electrode, whose potential is somewhere between those of the drift electrode and the anode, between the The ions two adjacent anodes. drift electrode accumulated around the anode are swept out towards this intermediate electrode, namely cathode, through the potential lines drawn in Fig. 1. Positive ions landing on the surface of the substrate do not move if the cathode anode substrate is insulating. The substrate thus requires a certain conductivity to neutralize the positive charges depositing on the surface layer (a-Si:C: surface. At the same time, the surface (insulator) 95 pm conductivity should not be too high, sub2ra0 (islto)H because then the leakage current 2oo ýun between the anode and cathode is large enough to produce excessive noise Fig. 1 Schematic diagram of the cross section above the signal level. It has been of a gas microstrip detector. 523 Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
shown empirically that the surface resistivity must be within the range of 1012 - 1016 Q/1J to provide a stable avalanche gain.[1] Therefore, finding a suitable substrate material with resistivity in the above range has been of primary interest. Among the strategies to achieve the desired resistivity are ion-implantation of insulating materials and sputter-coating of thin semiconducting films.[2-4] Both good long-term stability and high rate capability have been obtained by these methods. However, ion-implantation is much more expensive than ordinary thin-film coating process. Moreover, charging-up of electrons at the insulating substrate during implantation makes it difficult to control the uniformity and dosage accurately. Sputtering of conducting material is relatively easy and cheap, but it is hard to change the resistivity in a broad range since the resistivity is controlled mostly by varying the thickness. APPLICATION OF a-Si:H TO SURFACE COATING Hydrogenated amorphous silicon (a-Si:H) and its alloys are good candidates for a semiconducting substrate for gas microstrip detectors (GMDs) due to their good radiation hardness
and the feasibility of depositing over a wide area at low cost. Not only is the bulk resistivity of a-Si:H layer in a suitable range for a GMD (-109 Qlcm), it can also be easily controlled over a wide range by doping or alloying in the gas phase. However, as reported by Savard et al.,[5] intrinsic a-Si:H layers showed variation in surface resistivity and gain with the backplane voltages (Vb). They attributed such a behavior to the undepleted carriers in a-Si:H, i.e., nonbonding valence electrons (dangling bonds). We also observed that it takes several hours to reach the equilibrium value of the surface resistivity. Another feature of a-Si:H film is its light sensitivity. The resistivity decreases by 3 or 4 orders o
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