Sequential tunnelling in GaAs/AIGaAs superlattices grown by molecular beam epitaxy
Date of Issue1998
School of Electrical and Electronic Engineering
Sequential tunnelling in tight-binding superlattices has been studied experimentally and theoretically. The observed negative differential conductance (NDC) effect is attributed to the formation of high-field domains (HFDs) during the electron sequential tunnelling process through the superlattice. In a variably spaced superlattice, regardless of the biasing direction (forward or reverse bias), the I-V characteristics at low temperature showed strong current oscillations with increasing voltage separation between the adjacent main peaks as the magnitude of the bias voltage is increased. Our results unambiguously show that the high-field domain formation in a superlattice should take place first at the region of the superlattice where the widest well is located, i.e., where the wave function is the most localised. In a conventional superlattice with identical periods, the sequential tunnelling of electrons has been observed from both I-V and C-V measurements. The extension of the high-field domain region associated with the sequential tunnelling process is observed as a sharp increase in the capacitance due to an accumulation of electrons in the well of the boundary period between the high-field domain and the low-field regions. The sharp increase in the capacitance occurs at the onset of the NDC process, and corresponds to a sharp reduction in the current in the I-V characteristic. The space charge accumulation in the superlattice during the off-resonance state is deduced from the C-V characteristic. The coexistence of the high- and low-field regions has been probed by the photoluminescence (PL) technique, which shows a Quantum Confined Stark Effect (QCSE)-based red-shift of the transitions from higher excited states to the first heavy hole level. In this conventional superlattice, the HFD formation is found to start at the anode due to the electron screening effect. The hysteresis in the current was observed in the I-V characteristics of a conventional superlattice, and is attributed to the instability of the electric field distribution within the superlattice. The temperature dependence of the sequential tunnelling process is also discussed. The sequential tunnelling process was observed in both types of tight-binding superlattice structures with ohmic contact and Schottky contacts. A model for the sequential tunnelling in Schottky diodes is discussed. A simplified theoretical model was proposed to account for the observation of NDC in tight-binding superlattice structures by taking into account the coupling between the adjacent wells in the HFD region. Overall good qualitative agreement between our experimental observations and theoretical simulations was achieved.
DRNTU::Engineering::Electrical and electronic engineering::Semiconductors