OBSERVATION OF ZERO DIFFERENTIAL RESISTIVITY IN A-SI-H BARRIERS - ON THE ABSENCE OF QUANTUM CONFINEMENT IN AMORPHOUS-SEMICONDUCTORS FOR HIGH FIELDS

Single and double barriers of a-Si:H embedded in a-Ge:H with a total t hickness of 8 nm were prepared by rf PCVD and investigated by TEM. Tem perature and field dependent conductivity measurements suggest that tr ansport is determined by extended state and hopping conduction through the barrier. An equivalent circuit is used to explain the transport p henomena. The low field conductivity and its activation energy for the a-Si:H barriers are very similar to bulk a-Si:H. A value of 0.3 +/- 0 .1 eV for the discontinuity of the conduction band edge is concluded. The internal field in the barriers reaches large values of 4 X 10(5) V /cm. The current through the barriers depends exponentially on the fie ld and, for the highest fields, the differential resistivity of the a- Si:H barriers vanishes. Hence, the appearance of a quantum confinement in amorphous semiconductor heterostructures at higher fields is quest ionable.