A. S. Plaut

Graphene growth on h-BN by molecular beam epitaxy

Abstract The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms.

Luminescence experiments on acceptor δ-doped GaAs-AlGaAs single heterojunctions with optically tunable electron concentration

Abstract We have investigated radiative recombination of 2D-electrons with holes bound to acceptors from a δ-layer, positioned at a well defined distance from the interface, in an n-type GaAs-AlGaAs single heterojunction. Using magnetotransport and magnetooptics we demonstrate that under continuous photoexcitation by the laser light the concentration of 2D-electrons in these heterojunctions can be strongly decreased as compared to the value which is realised in the dark after illumination and that this is accompanied by an enhanced electron mobility.

Formation of GaAsN nanoinsertions in a GaN matrix by metal-organic chemical vapour deposition

Coherent ultrathin GaAsN insertions are formed in a GaN matrix by predeposition of an ultrathin GaAs layer on a GaN surface, followed by annealing in an NH3 atmosphere and overgrowth with GaN. During the overgrowth, most of the As atoms are substituted by N, with a dense array of coherent GaAsN nanodomains with lateral sizes of about 3-4 nm formed in the GaN matrix. We report a green luminescence due to GaAsN insertions, surviving at high observation temperatures and excitation densities.

Spin relaxation in optically excited quantum wells

Interband excitation of GaAs with circularly polarised light produces a spin-polarised population of photoexcited carriers. The equilibrium degree of spin polarisation under cw excitation depends on the balance of spin relaxation and recombination rates, and can be monitored directly through the degree of polarisation of recombination radiation. We have measured the polarisation at 2 K of recombination radiation following band edge excitation for a variety of GaAs/AlGaAs quantum well samples of carrier content ranging from ≈ 108 cm -2 up to ≈ 2×1011 cm-2 in which values of recombination time have been measured, and have thereby determined the spin relaxation times. Two remarkable results emerge; (i) the spin relaxation time increases from 30 ps to > 8 ns as the carrier concentration is increased; and (ii) for degenerate n-type quantum wells the recombination radiation is close to 100% circularly polarised, there being strong evidence for a long spin memory in the electron Fermi sea. The first result may be explained in terms of a spin relaxation mechanism involving electron-hole exchange; the second has, at present, no clear theoretical interpretation.

Magneto-optics of electron-gases confined in GaAs quantum dots

Photoluminescence spectroscopy has been used to probe the occupied electron states below the Fermi energy of zero-dimensional electron systems in both zero and finite magnetic fields. The arrays of modulation-doped quantum dots investigated were fabricated by reactive-ion etching of Al0.3Ga0.7As/GaAs heterojunctions with a δ-layer of Be present in the GaAs at 200 A from the interface in order to improve luminescence efficiency. We show that the low magnetic field dispersion (B < 8 T) of the acceptor recombination line is directly related to the magnetic field dependence of the total ground state energy of interacting electrons in the quantum dots.

Optical nonlinearity in GaAs quantum dots

We have measured the optical saturation intensity of GaAs quantum dots and have found it to be 50 W/cm2; more than an order of magnitude smaller than that reported for GaAs quantum wells. Compared to such quantum wells, our quantum dots also show a larger amount of saturation, again by more than an order of magnitude. We find that the saturation intensity of our quantum dots depends exponentially on the photoexcitation energy, with greater intensities required for photon energies closer to the bottom of the quantum dot confinement potential.

Magneto-optics of quantum wires grown on V-grooved substrates

Abstract AlGaAs/GaAs undoped quantum wires (QWRs) grown by organometallic chemical vapour deposition on V-grooved substrates have been studied by photoluminescence (PL) and photoluminescence excitation (PLE) in zero and finite magnetic fields. From the study of the diamagnetic shift of the peaks as a function of wire width and orientation with respect to the magnetic field, we have deduced one-dimensional effective masses for the lowest subband transitions.

Counting molecular-beam grown graphene layers

We have used the ratio of the integrated intensity of graphenes Raman G peak to that of the silicon substrates first-order optical phonon peak, accurately to determine the number of graphene layers across our molecular-beam (MB) grown graphene films. We find that these results agree well both, with those from our own exfoliated single and few-layer graphene flakes, and with the results of Koh et al. [ACS Nano 5, 269 (2011)]. We hence distinguish regions of single-, bi-, tri-, four-layer, etc., graphene, consecutively, as we scan coarsely across our MB-grown graphene. This is the first, but crucial, step to being able to grow, by such molecular-beam-techniques, a specified number of large-area graphene layers, to order.

THE CIRCULAR PHOTOGALVANIC EFFECT IN TWO-DIMENSIONAL HOLE GASES IN PERPENDICULAR MAGNETIC FIELD

We have measured the circular photogalvanic effect in a GaAs two-dimensional hole gas in zero and perpendicular magnetic field. The increase of the photoinduced voltage in very small magnetic field, when excited at fixed photon energy, demonstrates behavior reminiscent of that seen in anomalous Hall measurements on ferromagnets. The spectral dependence of the photoinduced voltage in magnetic field exhibits a series of maxima and minima that oscillate around zero volts. When plotted, the energy positions of these voltage peaks form a clear Landau-level-like fan chart.

Observation of many-body interactions of electrons in coupled double quantum wells

We have investigated, by resonant inelastic light-scattering, the finite in-plane wavevector dispersion of the collective excitations of electron bilayers in AlGaAs/GaAs double quantum wells. As the wavevector is increased the plasmon mode first becomes Landau damped and then at larger wavevectors it re-appears as it emerges from one of the two maxima of the continuum of single-particle excitations in a two-dimensional system with two occupied subbands. These measurements enable quantitative determination of many-body interactions in dilute electron bi-layers.