MESOSCOPIC EFFECTS IN RESONANT-TUNNELING DIODES

We have investigated resonant tunnelling in GaAs/(AlGa)As heterostruct ures which have been fabricated into square mesas 6 x 6 mum. A delta-l ayer of donors (n approximately 2 x 10(9) cm-2) has been incorporated at the centre of the quantum well which is 9 nm wide. The I(V) charact eristics show a feature at approximately 70 mV, which is below the thr eshold for the main resonance and is due to resonant tunnelling throug h single donor states in the well. This feature is also present in lar ge area mesas. At lower biases and at low temperatures we see a new se t of resonances which, although they occur in all small area mesas, di ffer in detail between devices with regard to their strength and bias position. The form of the low-bias structure is strongly dependent on temperature, T, below 4 K where several very sharp steps appear, becom ing sharper as T is decreased. We have also investigated the dependenc e of the new structure on magnetic field, B, parallel to the current d irection. We attribute the new features to tunnelling through potentia l fluctuations on the mesoscopic scale due to donor clustering.