A.S.G. Thornton

Observation of spin splitting in single InAs self-assembled quantum dots in AlAs

Using magneto-tunneling spectroscopy, we observe the Zeeman spin splitting of the ground state of a single InAs quantum dot grown within AlAs. We obtain values for the g factor of different quantum dots between +0.52±0.08 and +1.6±0.2, with magnetic field applied in the plane of the dot. This value for the g factor is considerably different from that of bulk InAs (g=−14.8), and we explain this using a simple three band k⋅p calculation. Using the spin split states of the dot as a probe, we observe the complete spin polarization of the emitter accumulation layer.

Many-body effects in a quantised 2DES probed by discrete-level tunnelling spectroscopy

Abstract We use the discrete state of a self-assembled InAs quantum dot for tunnelling spectroscopy of the local density of states of a 2DES over the full-energy range from the Fermi energy, EF, to the subband edge. In a magnetic field, B, applied parallel to the current we observe the formation of Landau levels and measure their width as a function of B. When the dot state is resonant with EF, we observe a Fermi-edge singularity in the current which enables us to study many-body effects in a magnetic field.

Magnetotunnelling and Photoluminescence Spectroscopy of Self-Assembled InAs Quantum Dots

Self-assembled InAs quantum dots (QDs) in AlAs and GaAs matrices are investigated by tunnelling and optical spectroscopy. Tunnelling through an individual QD in the AlAs barrier of a n-i-n single-barrier device is used to probe the properties of both the emitter two-dimensional electron gas (2DEG) and the QD. The Landau fan of the 2DEG is mapped at magnetic field parallel to the tunnelling current, B\VertI, as well as the spin splitting in the QD at B⊥I. An electron g-factor in the QD is determined as g=1.2±0.5. In the photoluminescence spectrum of InAs QDs in B up to 23 T, a strong anisotropy in the diamagnetic shift is found. The spatial extent of the carrier wave function in the dot is estimated as 60 A. Under hydrostatic pressure up to 8 kbar, the pressure coefficient for the dot emission line is (9.1±0.2) meV/kbar, about 20% smaller than for the Γ-point bandgap in bulk GaAs.

Fermi edge singularities in high magnetic fields

Abstract The current voltage characteristics of a single barrier tunnelling device which contains self-organized InAs quantum dots within the barrier are studied in zero and strong magnetic field conditions (both perpendicular and parallel to the interfaces). It is observed that in all cases there are large peaks whenever the energy of the bound state of a dot approaches the chemical potential of the 2d electron system formed at an interface. These are identified as thermally broadened Fermi-edge singularities which arise from many-body contributions to the tunnelling amplitude. We note that the standard theory of such singularities appears to work well in situations when the spin states of the dot are Zeeman split but is qualitatively incorrect in zero field. In the quantum Hall regime the peak height is an oscillatory function of magnetic field in agreement with theoretical predictions.

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 across the device changes the energy of the dot ground state relative to the 2DES. For magnetic field, B, applied parallel to the current, we observe peaks in the current‐voltage characteristic, I(V), corresponding to the formation of Landau levels (LLs) in the 2DES although the lowest energy levels are not well resolved at low B due to the eAect of the quasiparticle lifetime. At higher B, at filling factor ma 1 and beyond, we observe a number of eAects. First, we observe directly the exchange enhancement of the Land e g-factor; the lower energy spin polarised LL moves to lower energy with increasing B. Second, close to ma 1, the current from the lowest LL is suppressed although the current is restored as the temperature, T, is increased from 100 mK to 2 K. Finally, for m6 1, reproducible fine structure appears in I(V), which is very sensitive to both B and T. ” 1998 Elsevier

Chaos in quantum wells and analogous optical systems

Abstract The quantized states of a 60 nm wide potential well in a large tilted magnetic field are investigated using scaled field resonant tunnelling spectroscopy. In contrast to previous experiments on this type of system, the tunnelling characteristics are measured by changing both the magnetic field strength B and the applied bias voltage V such that V/B2 is approximately constant. This ensures that the classical phase space for electrons in the potential well has the same mixed stable-chaotic character for all fields. As a consequence of this scaling, each closed orbit in the potential well produces many periodic resonant peaks in plots of d2I/dB2 versus B. This type of scaled field experiment can be used to probe quantum states corresponding to dynamical regimes which are inaccessible to fixed field resonant tunnelling studies. We also consider analogies between the electron orbits and light rays in gradient refractive index lenses.

High Pressure as a Tool to Study Electron Localization

We have used high pressure to investigate resonant tunnelling in a single-barrier, n-i-n GaAs/ AlAs/GaAs diode with an embedded layer of InAs self-assembled quantum dots (SAQD). We have obtained convincing evidence for resonant tunnelling through individual r-valley-related electron states that we associate with the SAQD. The tunnel current through a SAQD was used as a local probe of a localized phase of a two-dimensional electron system in the accumulation layer of the diode. We have found evidence that at low densities, the localized electrons form relatively large, high-density clusters.

Optical and Resonant Tunnelling Spectroscopy of Self-Assembled Quantum Dot Systems

The electronic properties of self-assembled quantum dots have been studied by means of optical and tunnelling spectroscopy. The effect of confining barrier composition and design on the thermal behaviour of the dot optical properties is reported and exploited for the realisation of quantum dot based lasers. Tunnel current spectroscopy through a single discrete quantum dot state is used to investigate the electronic properties of an adjacent two-dimensional electron gas, including the Landau level density of states and many-body enhanced g-factor in the presence of a magnetic field.