Single electron charging and single electron tunneling in sub-micron AlGaAs/GaAs double barrier transistor structures

Transport phenomena in a specially optimized vertical asymmetric sub-micron Al0.28Ga0.72As/GaAs double barrier structure with modulation-doped barriers is investigated by applying a bias to a special Schottky side gate, which allows the effective area of the conducting channel to be finely ''tuned''. For a sufficiently small device, the electrical properties of the controllable quantum dot bounded by well defined heterostructure barriers and an adjustable side wall potential are expected to be governed by single electron charging, and single electron resonant tunneling. Single electron transistor (SET) operation is possible because the number of electrons in the dot, n, can be varied one-by-one with the side gate. The drain current flowing through the conducting channel in response to a small drain voltage is strongly modulated by the gate voltage close to ''pinch-off'' as n approaches zero, and oscillations in the drain current persist up to about 25 K. A gate modulated zero current region of Coulomb blockade, step-like features, and resonances exhibiting negative differential resistance are clearly observed at low bias. Although this technology is very promising for the realization of SET operation at temperatures well above 4.2 K, the temperature dependence of the Coulomb blockade is an important limitation. We describe the evolution of the conductance oscillations and the degradation of Coulomb blockade with temperature from 0.3 K up to 25 K, and propose that co-tunneling in a small system containing just a ''few'' electrons in which the zero-dimensional energy level spacing is significant and comparable to the Coulomb charging energy is important, and is likely to be more complex than that documented for large planar dot structures containing ''many'' electrons.