Measurement of mobility-lifetime products in amorphous semiconductors

Abstract

Charge-carrier drift-mobility-lifetime (μτ) product, the range of the carrier, is one of the most important electronic properties of a semiconductor material for device applications. The determination of μτ is, therefore, of fundamental importance in the characterization of amorphous semiconductors and has been the key issue in a number of recent papers. This paper describes two experimental techniques for μτ product measurements and their application to a-Se and Cl-doped Se:0.35%As electro-radiographic films. Xerographic measurements involve corona charging the surface of an amorphous semiconductor film to a voltage Vo. The film is then exposed to a highly absorbed step illumination at the end of which the residual potential, VR1, on the surface is measured. VR1/Vo is then related to μτ. In the interrupted field time-of-flight (IFTOF) measurement technique, during the flight of the photoinjected holes through the specimen, the applied field is removed at time T1 for an interruption duration of ti and then reapplied and the remaining holes extracted. The fractional recovered photocurrent at the end of the interruption time, ti was found to follow the deep-trapping kinetics described by i(T1 + ti)/i(T1) = exp (−ti/τ) where τ is the charge-carrier lifetime. By interrupting the time-of-flight photocurrent at different locations, the technique allows for the determination of the trapping time τ as a function of location across the specimen and hence the examination of sample inhomogeneities. Xerographic residual potential and IFTOF experiments carried out on a range of a-Se and Cl-doped a-Se:0.35%As electroradiographic films show that there is a good correlation between the trapping time determined from xerographic measurements and that determined from IFTOF. The results are discussed in terms of carrier-trapping processes in amorphous semiconductors. The capture cross section of deep hole traps in a-Se and Cl-doped a-Se:0.35%As is determined from the measurement of the hole-mobility-lifetime product and the saturated cycled-up residual potential. Application of the ballistic and diffusional models of Street (Philos. Mag. B, 49, L15 (1984)) indicate that the hole-capture radius in a-Se is 2–3 Å (1 Å = 10−1 m) and that the chemical modification of a-Se by combinational doping with 0.35%As and 10–20 ppm Cl does not affect the basic capture process. The two experimental techniques described represent the most meaningful methods of charge transport and trapping characterization in amorphous semiconductors.