Session CC - Condensed Matter I.
MIXED session, Friday morning, November 15
Henry Oliver E, Holiday Inn
The temperature dependence of capacitance and AC conductance for GaAs/AlGaAs multiquantum well (MQW) structures with 4,8,16 and 32 wells has been studied at different bias and frequency ranges. Data indicates that low temperature conductance is mostly due to thermally assisted tunneling (TA) through the first excited state. At high temperature thermionic emission (TE) current is dominant. Activation energies for TA and TE are 120\pm15 meV and 170\pm10 meV respectively. Capacitance changes from the geometrical value at low temperature to the capacitance corresponding to a single period of the structure at high temperature. Applied bias voltage reduces effective barrier height and prevents accumulation of charge in quantum wells. At biases above 150 mV (for 4 well sample) TE current dominates over the whole temperature region with energy activation of 140\pm10 meV. As the bias is increased the single period capacitance (saturation) drops until only the geometrical capacitance is observed at a bias of 400 mV. An equivalent circuit is proposed to explain the behavior observed. -Supported in part by NSF under grant# ECS94-12248