Influence of the roughness of molybdenum back electrode on the photodiode characteristics under He–Ne illumination

Abstract

Molybdenum thin films, about 300 nm thick, were RF sputtered on silicon wafers used as a substrate, in order to produce thin film diode back electrodes. The Mo films were treated with reactive ion etching (RIE) for different times (20 s, 40 s, 60 s and 120 s), producing surfaces with different degrees of roughness. Atomic force microscopy (AFM) showed a pyramidal structure of the etched surfaces. It was found that the roughness is strongly related to the RIE time. The change of the reflection coefficient of the etched Mo surfaces was measured with respect to the non-etched (as-is) Mo surface. The results showed dependence of the relative change of the surface reflection with the RIE time. Such prepared molybdenum surfaces of different roughness were used as back electrodes for preparation of hydrogenated amorphous silicon p-i-n diodes, prepared by the plasma-enhanced chemical vapour deposition method (PECVD). Molybdenum coating (150 nm thick) by RF sputtering was also used to produce top electrodes. The final diode structure was Mo/p-i-n/Mo. The grid structure of the top electrode was produced by a photolithography technique. Thirteen various front electrode grid patterns were designed to permit the He–Ne laser beam spot of 7 mW and about 0.7 mm cross-section. The laser beam penetrated through the p-i-n structure, covering the entire diode active area. The short circuit current (Isc) was measured for 13 diode grid shapes of each sample, including the diodes on the flat (untreated) back Mo electrode. The results showed that the average Isc depends on the RIE time. The dark and He–Ne light characteristics showed changes with the RIE time. From the measured IV characteristics, the serial resistance (Rs) was found to increase with the RIE time, except for the 20 s treated RIE sample. Results of Isc and Rs suggested that, besides the light scattering mechanism resulting from the surface roughness, there is another contributing factor to the photodiode performance enhancement, and that is the surface of the pi junction; the greater the roughness, the larger the junction area.