On the Connection of Anisotropic Conductivity to Tip Induced Space Charge Layers in Scanning Tunneling Spectroscopy of p-doped GaAs

The electronic properties of shallow acceptors in p-doped GaAs{110} are investigated with scanning tunneling microscopy at low temperature. Shallow acceptors are known to exhibit distinct triangular contrasts in STM images for certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to identify their energetic origin and behavior. A crucial parameter - the STM tip's work function - is determined experimentally. The voltage dependent potential configuration and band bending situation is derived. Ways to validate the calculations with the experiment are discussed. Differential conductivity maps reveal that the triangular contrasts are only observed with a depletion layer present under the STM tip. The tunnel process leading to the anisotropic contrasts calls for electrons to tunnel through vacuum gap and a finite region in the semiconductor.Comment: 11 pages, 8 figure

Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field

The effect of an external electric field F on the excitonic photoluminescence (PL) spectra of a symmetric coupled double quantum well (DQW) is investigated both theoretically and experimentally. We show that the variational method in a two-particle electron-hole wave function approximation gives a good agreement with measurements of PL on a narrow DQW in a wide interval of F including flat-band regime. The experimental data are presented for an MBE-grown DQW consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers. The bias voltage is applied along the growth direction. Spatially direct and indirect excitonic transitions are identified, and the radius of the exciton and squeezing of the exciton in the growth direction are evaluated variationally. The excitonic binding energies, recombination energies, oscillator strengths, and relative intensities of the transitions as functions of the applied field are calculated. Our analysis demonstrates that this simple model is applicable in case of narrow DQWs not just for a qualitative description of the PL peak positions but also for the estimation of their individual shapes and intensities.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B

A multipurpose experimental facility for advanced X-ray Spectrometry applications

Ponencia presentada en la European Conference on X-Ray Spectrometry (EXRS). 2014Motivation, Ultra High Vacuum Chamber (UHVC) project: To support/enhance the training of scientists/engineers from developing countries in the operation of synchrotron radiation instrumentation; To provide beam time access for R&D projects and hands-on training in SR-XRS based techniques; To promote networking and knowledge sharing; To increase the quality and the competitiveness of the developing countries to apply beam time proposals at SR facilities; To contribute in the further development of XRS techniques in applications with socioeconomicalrelevance (characterization of energy storage/conversion materials, environmental, biological and biomedical applications)Fil: Leani, Juan José. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía y Física; Argentina.Fil: Leani, Juan José. Nuclear Science and Instrumentation Laboratory, IAEA Laboratories; Austria.Física Atómica, Molecular y Química (física de átomos y moléculas incluyendo colisión, interacción con radiación, resonancia magnética, Moessbauer Efecto.

Depth resolved scanning tunneling spectroscopy of shallow acceptors in gallium arsenide

Scanning tunneling spectroscopy (STS) at 8 K is used to study single shallow acceptors embedded near {110}-surfaces ill gallium arsenide (GaAs). At appropriate bias voltages the circularly symmetric contrast normally observed for charged defects evolves into a pronounced triangular shaped protrusion. Comparing dopants at different depths under the surface, we find a linear shift of the associated conductivity maximum along (112) directions. Comparative Studies of Carbon and Zinc acceptors in a modulation-doped heterostructure reveal that both dopants act similarly. The experimental findings Suggest that the highly anisotropic features induced by acceptors resemble a bulk property of the GaAs crystal prominently demonstrating its Ziricblende symmetry

Diamagnetic shift of disorder-localized excitons in narrow GaAs/AlGaAs quantum wells

A correlation between the diamagnetic shift and transition energy of disorder-localized excitons is observed in magnetomicrophotoluminescence (mu PL) on narrow GaAs/Al0.3Ga0.7As quantum wells (QW's). The QW's were grown by molecular-beam epitaxy without growth interruption at the interfaces. mu PL spectra were obtained in a confocal setup with the magnetic field applied normal to the QW plane. The lowest-energy exciton states have the smallest diamagnetic coefficients; the exciton diamagnetic shift in the localized exciton tail of the QW emission spectra increases by a factor of 2 as a function of transition energy. The positive correlation between diamagnetic shift and emission energy reveals exciton localization by short-range correlated interface disorder

Probing semiconductor gap states with resonant tunneling

Tunneling transport through the depletion layer under a GaAs {110} surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry

Temperature dependence of indirect-exciton luminescence in in-plane magnetic field

We report on a magneto-luminescence on a double quantum well subject to an in-plane magnetic field. The attention is paid to the properties of interwell excitons, which are indirect in the real space and which become indirect in the reciprocal space as well when a finite in-plane magnetic field is applied. Such indirect exciton states become optically inactive unless some relaxation mechanisms of their momentum appear. The experiment is carried out on a sample where, as reported previously, the radiative recombination of indirect excitons is possible due to their localization or via collisions with structural defects. The experimental data presented here, measured at various temperatures, favour the latter mechanism which is less sensitive to the system temperature in comparison with the former one. (c) 2008 Elsevier B.V. All rights reserved

Luminescence of double quantum wells subject to in-plane magnetic fields

We report on photoluminescence (PL) measurements of a symmetric GaAs/AlGaAs double quantum well (DQW) in high magnetic fields. For this study, a selectively contacted p-delta n-DQW-delta n-p structure was chosen, allowing an independent tuning of the electron density in the DQW and thus a creation of a two-dimensional electron gas. Our attention was focused on phenomena in in-plane magnetic fields, where the field-induced depopulation of the antibonding subband observable in the PL spectra as a so-called N-type kink was predicted by Huang and Lyo (HL) [Phys. Rev. B 59, 7600 (1999)]. Whereas the equivalent behavior has been observed several times in the electric transport measurements and a proper theoretical description has been found, to the best of our knowledge, no PL experiment in a direct comparison with the theoretical model developed by HL has ever been published. We carried out a self-consistent calculation based on their model and achieved a good agreement with our experimental results. Additionally, the influence of the excitonic interaction on the PL spectra, not taken into account by HL, is also discussed. This enables us to explain small deviations from the HL theory. The interpretation of the in-plane magnetic field measurements is supported by the experiment with the magnetic field in the perpendicular orientation that allows a sufficiently accurate estimation of the electron density in the DQW. Distinctive renormalization effects of DQW subbands at various electron densities are also observed and discussed