Showing papers by "Peter H. Beton published in 1995"

Manipulation of C60 molecules on a Si surface

Abstract: We have used the tip of a scanning tunneling microscope to position individual C60 molecules on a Si(111) surface. It is possible to form simple patterns of molecules at room temperature using this technique.

Electron-concentration-dependent quantum-well luminescence: Evidence for a negatively charged exciton

Abstract: The quantum-well electroluminescence of p-i-n GaAs/AlAs double-barrier resonant tunneling diodes has been investigated. The bias-dependent tunability of the resonance conditions for electron and hole tunneling allows for a continuous change of the relative electron and hole concentrations in the quantum well. As the electron concentration is increased, the quantum-well emission line due to heavy-hole free exciton recombination is replaced by a new excitonic line, 2 meV lower in energy. This line is attributed to a negatively charged exciton ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$. The magnetic field and temperature dependence of the quantum-well emission have been used to characterize this transition.

Measuring the Probability Density of Quantum Confined States

Abstract: We show that it is possible to measure the probability density of a quantum-confined state using resonant magnetotunneling. We have measured the probability densities of the lowest three bound states formed in a semiconductor nanostructure and show that they are eigenstates of a parabolic potential.

Landau-level populations and slow energy relaxation of a two-dimensional electron gas probed by tunneling spectroscopy

Abstract: The tunnel current in a double-barrier GaAs/AlAs heterostructure device is investigated in the voltage range close to the threshold for resonant tunneling and as a function of magnetic field applied parallel to the current direction. Additional peaks in the current are observed that correspond to tunneling transitions from the emitter-accumulation layer to the quantum well with a change of Landau quantum number. Analysis of the data indicates that the electron population in the two lowest Landau levels of the emitter-accumulation layer is far from thermal equilibrium. The inter-Landau-level transition time between these levels is estimated to be in excess of 100 ns at fields \ensuremath{\ge}4 T.

Atomic scale modifications of GaAs using a scanning tunneling microscope

Abstract: We have created atomic scale modifications on the GaAs(111)B surface by applying voltage pulses between the tip of a scanning tunneling microscope and a GaAs sample under ultrahigh vacuum conditions. A voltage pulse of 5 V (sample negative) for 25 ms results in the creation of a disordered region (approximately, 3 nm×3 nm in area) of As trimers. In addition, surface stacking faults are formed which extend over distances of order 10 nm and terminate on surface defects. A pulse with the same parameters, but opposite polarity, creates a nanometer scale crater. We argue that the smallest features are formed by electric field induced diffusion (for negative bias pulses) or field desorption (positive bias pulses).

Resonant tunnelling through a single impurity in high magnetic fields: Probing a two-dimensional electron gas on a nanometre scale

Abstract: Tunnelling through highly localised impurity states in the quantum well of a resonant tunnelling diode allows us to study the properties of the two-dimensional electron gas formed at the emitter barrier of the device. In high magnetic fields, the electrons form quantum dots in the disordered potential due to unscreened donors in the depleted collector contact.

A novel approach in fabrication and study of laterally quantum-confined resonant tunnelling diodes

Abstract: A novel fabrication process, based on selective wet etching and photolithography, has been developed to produce quantum wire resonant tunnelling diodes contacted via a free-standing GaAs bridge Current-voltage characteristics of quantum wire resonant tunnelling diodes have been studied at 300 mK in the presence of a magnetic field oriented either parallel to or perpendicular to the current For the smallest device, in the presence of a magnetic field oriented perpendicular to the current and parallel to the wire, we are able to deduce the probability density of the lowest three bound states from the magnetic field dependence of the current and show that the one-dimensional confining potential is close to parabolic In the presence of a magnetic field oriented parallel to the current a continuous transition from electrostatic (at low field) to magnetic (at high field) confinement is observed

Effect of a parallel magnetic field on the resonant-tunneling current through a quantum wire.

Abstract: The effect of a magnetic field applied parallel to the current through quantum confined GaAs/AlAs resonant-tunneling diodes with submicrometer lateral dimensions is studied theoretically. A tunneling current peak associated with an intersubband transition is predicted for intermediate magnetic field. This results from a difference in the degree of confinement in the emitter and the well. The results are compared with experiments.

Creation and annihilation of positively and negatively charged excitons in GaAs quantum wells

Abstract: The electro- and photoluminescence of p-i-n and n-i-n GaAs double-barrier resonant tunnelling diodes have been investigated for various concentrations of free carriers (either holes or electrons) in the quantum well (QW). For these structure it is possible to change the relative electron or hole density quasi-continuously with applied bias voltage. At electron densities well below that for exciton unbinding, the formation of negatively charged excitons (X - ) is demonstrated. In asymmetric QW structures the electron densities can be increased above the critical density and exciton unbinding is observed in the luminescence spectra. We also report the observation of the positively charged exciton (X 2 + ) in the luminescence spectra of a GaAs quantum well. The conditions for forming X 2 + are compared with those needed to see the negatively charged exciton (X - ), the heavy-hole free exciton (X) and free carrier recombination.

Some Recent Developments in Quantum Transport in Mesoscopic Structures and Quantum Wells

Abstract: This paper describes some recent developments in our understanding of quantum transport phenomena in mesoscopic semiconducting structures. Two types of structure are considered: ultra-small wires of a degenerate semiconductor in the presence of a strong magnetic field and small-area resonant tunneling diodes. There are already some excellent review articles on universal conductance fluctuations and quantum interference effects in mesoscopic metallic and semiconducting structures in the linear transport regime. Therefore this subject is dealt with only briefly in the following section, which also highlights some recent work on the magnetoresistance of semiconducting n + GaAs wires in the high magnetic field regime. The paper then goes on to describe the manifestation of mesoscopic effects in small-area resonant tunneling devices, which are inherently non-linear conductors, exhibiting strong deviations from Ohm’s Law behavior.

Cretion and annihilation of negatively charged excitons in GaAs quantum wells

Abstract: The electroluminescence of p-i-n GaAs double-barrier resonant tunnelling diodes has been investigated for various concentrations of free carriers (either holes or electrons) in the quantum well (QW). For these structures it is possible to change the relative electron or hole density quasi-continuously with applied bias-voltage. New low-temperature excitonic recombinations are identified in the electroluminescence spectra. Continuous variation of the carrier density in the quantum well from a hole-rich into an electron-rich environment leads to the observation of positively charged excitons (X 2 +), neutral excitons (X), negatively charged excitons (X −) and finally an unbound electron-hole recombination. An increased temperature causes the dissociation of the charged excitons in favour of the neutral heavy-hole free excitonX. In high magnetic fields the unbound electron-hole recombination is transferred into an excitonic recombination.

Resonant magnetotunneling via quantum confined states

Abstract: We have measured the low temperature current-voltage characteristics ( I ( V )) of GaAs/A1As resonant tunnelling diodes with sub-micron lateral dimensions. Additional peaks in I ( V ) are observed due to resonant tunnelling via one-dimensional quantum wire states. In the presence of a magnetic field oriented perpendicular to the current and parallel to the wire the peaks show a complex splitting evolving into a regular series at high field with upt to 20 resonances. For the smallest device we are able to deduce the probability density of the lowest three bound states from the magnetic field dependence of the current and show that the confining potential is close to parabolic. For a magnetic field which is perpendicular to both the current and the wire a much weaker dependence on magnetic field is observed confirming the one-dimensional nature of our device. Finally, in the presence of a field oriented parallel to the current a continuous transition from electrostatic (at low field) to magnetic confinement (at high field) is observed.