Time-of-flight measurement of longitudinal carrier transport in a-Si:H/a-SiN_x: H multilayer structures and gap state distribution in a-Si:H layers

The time-of-flight technique has been employed to study the electron transport longitudinal to multilayer structures of amorphous silicon/silicon nitride (a-Si:H/a-SiN_x: H). The transient photocurrent show a clear kink corresponding to the electron transit time which is the same as in bulk a-Si:H. An increase in the barrier layer thickness (1.3-4.5 nm) at a constant well layer width (3.5 nm) causes and exponential reduction in the electron drift velocity, while no appreciable change in the electron transport is observed when the well layer width is varied from 2.5 to 5.2 nm at a fixed barrier layer thickness (1.3 nm). This invariance indicates, definitely, the dominant role of electron tunneling through the barrier layers rather than surmounting potential barriers of a-SiN_x:H layers. However, the drift velocity changes exponentially with increasing applied field possibly due to decrease in the demarcation level. The observed phenomena can be well explained on the basis of combined processes of tunneling through the barrier layer and multiple trapping in the well. The temperature dependence of the drift velocity measured as a function of applied field is analyzed to elucidate the gap state distribution in a-Si:H well layers.