ELECTRONIC TRANSPORT THROUGH SILICON (A-SI-H) BARRIERS IN HYDROGENATED AMORPHOUS-GERMANIUM (A-GE-H)

Films of a-Ge:H containing single and double barriers of a-Si:H with a total thickness of 10 nm are prepared by rf PCVD from germane and sil ane, respectively. The current-voltage characteristics of the sandwich structures are smooth and do not show any irregularities. The differe ntial conductivity sigma(E) of the structures is nearly step-like with values of 3 x 10(-8) and 10(-5) Ohm-1 cm(-1) for external fields belo w 200 and above 1000 V/cm, respectively. The ohmic conductivity (at lo wer fields) of the ultrathin a-Si:H barriers is identical to bulk a-Si :H and is 10(-10) Ohm(-1) cm(-1). This implies a discontinuity of the conduction band edge at the a-Si:H/a-Ge:H interface of 0.3 eV. For low fields and below 240 K, there is ohmic hopping with an activation ene rgy of 0.3 eV. The internal field in the barrier reaches very large va lues of 4 x 10(5) V/cm. For the first time, the observation of a vanis hing and a negative differential resistivity of the a-Si:H barriers ev en at room temperature is reported. It is concluded that there is no r eal quantum confinement in heterostructures based on amorphous semicon ductors.