Magneto-tunnelling spectroscopy of a two-dimensional electron system

We use a single, InAs self-assembled quantum dot to probe the local density of states of a two-dimensional electron system (2DES) at all energies from the sub-band edge to the Fermi energy. The dot is incorporated as part of an AlAs barrier in a single barrier tunnel diode. Variation of the bias acr oss the device changes the energy of the dot ground state relative to the 2 DES. For magnetic field, B, applied parallel to the current, we observe pea ks in the current-voltage characteristic, I(V), corresponding to the format ion of Landau levels (LLs) in the 2DES although the lowest energy levels ar e not well resolved at low B due to the effect of the quasiparticle lifetim e. At higher B, at filling factor nu =1 and beyond, we observe a number of effects. First, we observe directly the exchange enhancement of the Lande g -factor; the lower energy spin polarised LL moves to lower energy with incr easing B. Second, close to nu = 1, the current from the lowest LL is suppre ssed although the current is restored as the temperature, T, is increased f rom 100 mK to 2 K. Finally, for nu less than or equal to 1, reproducible fi ne structure appears in I(V), which is very sensitive to both B and T. (C) 1998 Elsevier Science B.V. All rights reserved.