P.E. Simmonds

Time-resolved magneto-photoluminescence measurements of the integer QHE, fractional QHE and extreme quantum limit

Low-temperature, time-resolved magneto-photoluminescence (TRPL) measurements of 2D electron-free-va-lence-hole recombination have been carried out in the IQHE, FQHE and magnetically induced electron solid regimes of a GaAs/AlGaAs single heterojunction. Clear differences, not present at elevated temperatures, are found in the measured PL decay time (~ 1 ns) for components of multiple-peak structure observed beyond v = 15 used previously to construct a phase diagram associated with the Wigner solid. The new TRPL results and earlier PL data, taken together, are consistent with a mechanism in which the spectral feature associated with crystallisation derives from the trapping of excitons within a few Bohr radii (z separation) of the heterointerface and their subsequent (x,y) localisation by the presence of a 2D electron system with a degree of spatial order at this interface.

Central-cell corrections for Si and S in GaAs in a strong magnetic field

The central-cell correction has been determined experimentally for the two donor impurities S and Si in GaAs. Data have been obtained for magnetic fields to 39 T, corresponding to γ≈6. The observed behavior is in good agreement with theory. The analysis permits accurate evaluation of zero-field central-cell corrections, yielding 0.110 and 0.059 meV for S and Si, respectively.

Photoluminescence and magneto-transport of wide InGaAsInP modulation doped quantum wells

Abstract A photoluminescence (PL) study of 700A wide modulation doped InGaAsInP quantum wells in magnetic fields up to 10 Tesla, is reported. PL from the lowest electron sub-bands, localised in the space charge potentials at the two sides of the QW, and from higher delocalised sub-bands extending across the whole width of the QW, is observed. The performance of simultaneous Quantum Hall Effect (QHE), Shubnikov-de-Haas and PL measurements is found to be crucial to obtain a reliable understanding of the behaviour of the PL spectra in magnetic field. In particular the QHE provides direct information on the number of filled sub-bands and Landau levels in a complicated system of total carrier density ∼1012 cm−2, containing several populated electronic sub-bands.

High-field photoluminescence in GaAs single heterojunctions: Mapping of an optically determined phase boundary correlated with the electron liquid-solid transition

Abstract Bandgap magneto-photoluminescence is used to study the optical signature of a high-quality, low-density GaAs-AlGaAs heterojunction in the integer QHE, fractional QHE and Wigner regimes. In the extreme quantum limit an intense, new spectral feature is observed to emerge for v 1 5 , the temperature dependence of which is used to define two characteristic temperatures, Tc1 and Tc2. The boundary formed by the lower mapping, Tc1, in the (B, T)-plane correlates well with the electron liquid-solid transition established by other techniques.

Low temperature magneto-photoluminescence investigations of the 2D hole system in p-type GaAs—AlGaAs heterojunctions

Abstract Bandgap magneto-photoluminescence (PL) has been used to investigate the optical signature of high quality p-type GaAs—Al x Ga 1− x As single heterojunction samples in the quantum Hall effect (QHE) regime. The PL spectra show a series of sharp lines, with a number of bulk features attributed to free exciton emission and acceptor-related bound excitons evident. Significantly, however, one line within the complex PL spectrum, at slightly lower energy to the free exciton, becomes a dominant feature at low temperatures and has an intensity maximum which exhibits superlinear behaviour with excitation power that correlates with the v = 1 QHE. This line is assigned to an excitonic process at the heterointerface reminiscent of 2D excited subband recombination in n-type structures.

Magneto-photoluminescence studies of a 2D electron system: Signatures of the fractional quantum Hall effect and Wigner solid

Abstract The anticipated transition from a liquid to a magnetically-induced Wigner solid (MIWS) ground state in a two-dimensional electron system is investigated by bandgap photoluminescence (electron-valence band hole recombination) in two GaAs single heterojunctions of different densities and intersubband spacings. For Landau level filling factors ν 1 5 the photoluminescence spectra are dominated by a new line, the temperature dependence of which is used to map a phase boundary which is situated close to the boundary associated with a pinned MIWS phase mapped by a variety of other techniques. A weak peak, split off to lower energy from the main PL emission, may also be of relevance to the MIWS. Comparison is made with optical data that probes this transition by electron-neutral acceptor recombination in Be δ-doped samples. In our higher density sample (∼ 10 11 cm −2 ), a temperature-dependent laser-induced 2D electron density depletion effect observed at high magnetic fields and low temperatures is investigated systematically. It has also been possible to study 2D electron to acceptor-bound hole recombination in this sample associated with residual, low concentration carbon impurity levels as a further probe of the QHE regime. Additionally, a definitive optical signature of the ν= 1 3 FQHE hierarchy is reported.

Many body resonant polaron interaction in quantum wells containing high densities of free carriers

Strong resonant polaron coupling phenomena in the photoluminescence spectra of modulation doped quantum wells are reported. The polaron interaction arises between hole quasi-particles in otherwise filled electron Landau levels, and LO phonons of the quantum well material.

Optical Spectroscopy of Double Barrier Resonant Tunneling Structures

Recent applications of optical spectroscopy to the study of double barrier resonant tunneling structures are discussed. Particular topics emphasized are the determination of charge build up in the quantum well active region on resonance, determination of the nature of the tunneling processes and the detection of electroluminescence emission resulting from impact ionisation in double barrier structures.

Time-resolved spectroscopy in the electron solid regime

Abstract Time-resolved magneto-photoluminescence is used to probe the low-temperature ground states of electrons confined to two dimensions. Clear differences are found in measured decay times (∼1ns) for components of multiple-peak spectra observed in the electron solid regime consistent with exciton localisation by a Wigner lattice.

Magneto-optics of wide modulation doped quantum wells: Landau level pinning phenomena

Landau level pinning phenomena in the photoluminescence spectra of an asymmetric, wide modulation doped quantum well are reported. The processes are driven by the need to maintain a self-consistent potential and charge distribution under conditions of high field Landau level quantization.