A. B. Henriques

Anisotropy of electron and hole g-factors in (In,Ga)As quantum dots

The g-factor tensors of electron and hole in self-assembled (In,Ga)As/GaAs quantum dots are studied by time-resolved ellipticity measurements in a three dimensional vector magnet system. Both g-factor tensors show considerable deviations from isotropy. These deviations are much more pronounced for the hole than for the electron and are described by different anisotropy factors, which can even have opposite signs.

Optical second harmonic generation in the centrosymmetric magnetic semiconductors EuTe and EuSe

Spectroscopy of the centrosymmetric magnetic semiconductors EuTe and EuSe reveals spin-induced optical second harmonic generation (SHG) in the band gap vicinity at 2.1-2.4 eV. The magnetic field and temperature dependence demonstrates that the SHG arises from the bulk of the materials due to a novel type of nonlinear optical susceptibility caused by the magnetic dipole contribution combined with spontaneous or induced magnetization. This spin-induced susceptibility opens access to a wide class of centrosymmetric systems by harmonics generation spectroscopy.

Sharp lines in the absorption edge of EuTe and Pb0.1Eu0.9Te in high magnetic fields

The optical absorption spectra in the region of the transition energies of epitaxial layers of EuTe and Pb0.1Eu0.9Te, grown by molecular beam epitaxy, were studied using circularly polarized light, in the Faraday configuration. Under ?+ polarization a sharp symmetric absorption line (full width at half-maximum 0.041?eV) emerges on the low energy side of the band-edge absorption, for magnetic field intensities greater than 6?T. The absorption line shows a huge red shift (35?meV?T?1) with increasing magnetic field. The peak position of the absorption line as a function of magnetic field is dominated by the d?f exchange interaction of the excited electron and the Eu2+ spins in the lattice. The d?f exchange interaction energy was estimated to be JdfS = 0.15 ? 0.01?eV. In Pb0.1Eu0.9Te the same absorption line is detected, but it is broader, due to alloy disorder, indicating that the excitation is localized within a finite radius. From a comparison of the absorption spectra in EuTe and Pb0.1Eu0.9Te the characteristic radius of the excitation is estimated to be ??.

Anisotropy of the cyclotron mass in superlattices containing two populated minibands

The cyclotron mass in periodically delta-doped InP was studied, using the temperature dependence of the Shubnikov-de Haas (SdH) effect in tilted magnetic fields. The samples had two populated minibands, E1 and E2, both of which contributed with oscillatory components to the SdH spectrum. When the magnetic field is tilted from the direction parallel to the axis of the superlattice, the cyclotron mass associated with E1 electrons increases, as expected for a quasi-two-dimensional system. In contrast, the cyclotron mass of E2 electrons decreases. This decrease is due to Bragg reflections of electrons by the superlattice, which lead to shorter orbits when the magnetic field is tilted. It is estimated that in periodically delta-doped semiconductors the cyclotron mass can decrease at most by a factor of three when the magnetic field is rotated by π/2.

Dispersion of electron g-factor with optical transition energy in (In,Ga)As/GaAs self-assembled quantum dots

The electron spin precession about an external magnetic field was studied by Faraday rotation on an inhomogeneous ensemble of singly charged, self-assembled (In,Ga)As/GaAs quantum dots. From the data the dependence of electron g-factor on optical transition energy was derived. A comparison with literature reports shows that the electron g-factors are quite similar for quantum dots with very different geometrical parameters, and their change with transition energy is almost identical.

Delta-doping superlattices in multiple quantum wells

Abstract The quantum confined Stark effect has been extensively used for amplitude modulation. One way of improving the performance of multiple quantum well structures to be used in light modulation at high bit rates is by increasing the Stark shift for a given externally applied voltage. GaAs/AlGaAs multiple quantum well structures containing an nipi delta-doping superlattice, where the n-type doping is inserted in the quantum wells and the p-type in the barriers, are expected to double the Stark shift, according to Batty and Alsopp (Electron. Lett. 29 (1993) 2066). Such structures have been studied in detail to evaluate their potential for use in the fabrication of optical modulators. It has been observed that the required balance between n- and p-type doping levels is not trivial to achieve due to the presence of interface hole traps whose population depends on the quantum well doping concentration. It is estimated that for undoped quantum wells around 15% of the holes provided by the p-doping are trapped at the interfaces. Photoluminescence measurements, supported by calculations, point out that even though an indirect transition between electrons in the quantum wells and holes in the barriers is present at low temperatures at energies below the quantum well fundamental transition energy, at room temperature such a transition is absent and the observed optical emission occurs at essentially the same energy as that of an equivalent undoped structure.

Strong luminescence from Tamm states in modulation-doped superlattices

Calculations are presented for the photoluminescence spectrum of InP/In0.53Ga0.47As superlattices doped with Si. When doping is confined to the inner barriers, the photoluminescence is dominated by transitions between Tamm states in the electron and valence bands, which contribute with an optical band at energies higher than the In0.53Ga0.47As band gap. These results are in good agreement with experimental observations.

Single and periodically Si -doped InP grown by LP-MOVPE

Single and periodically Si -doped InP layers were grown by LP-MOVPE at . A full width at half maximum of 32 ? was obtained for the net charge concentration profile for a sample with a peak net charge concentration of . Numerical simulations showed that the impurities are localized over less than three InP monolayers. No dopant diffusion or segregation was observed. The periodic structures, grown with barriers varying from 100 to 300 ?, all had nearly the same carrier sheet concentration per dopant layer and impurity localization characteristics.

Magnetic ordering of EuTe∕PbTe multilayers determined by x-ray resonant diffraction

In this work we use resonant x-ray diffraction combined with polarization analysis of the diffracted beam to study the magnetic ordering in EuTe∕PbTe multilayers. The presence of satellites at the (12 12 12) magnetic reflection of a 50 repetition EuTe∕PbTe superlattice demonstrated the existence of magnetic correlations among the alternated EuTe layers. The behavior of the satellites intensity as T increases toward the Neel temperature TN indicates that these correlations persist nearly up to TN and suggests the preferential decrease of the magnetic order parameter of external monolayers of each EuTe layer within the superlattice.

nipi delta-doping superlattices for amplitude modulation

GaAs/AlGaAs multiple quantum well structures containing an nipi delta-doping superlattice, where the n-type doping is inserted in the quantum wells and the p-type in the barriers, have been studied in detail to evaluate their potential for use in the fabrication of amplitude modulators. It is shown that C is an adequate p-type dopant for such structures, however, little exibility is found in the growth conditions, in particular for the V to III uxes ratio, for obtaining such layers. It is also observed that the required balance between n and p type doping levels is not trivial to be achieved due to the presence of interface hole traps whose population depends on the quantum well doping concentration. In addition, the observed photoluminescence near-edge emission at room temperature occurs at essentially the same energy as that of an equivalent undoped structure. Finally, no deep level emissions are observed which could deteriorate the device performance.