P. Barate

Evaluation of InGaPN and GaAsPN materials lattice-matched to Si for multi-junction solar cells

We compare the potentiality of bulk InGaPN and GaAsPN materials quasi-lattice-matched to silicon (Si), for multi-junction solar cells application. Bandgaps of both bulk alloys are first studied by a tight-binding model modified for nitrogen incorporation in diluted regimes. The critical thicknesses of those alloys are then calculated for various compositions. For the same lattice-mismatch and nitrogen amount, the bandgap of bulk GaAsPN is found to be closer to the targeted gap value of 1.7 eV for high efficiency tandem solar cell. GaPN and GaAsPN epilayers are then grown by molecular beam epitaxy on GaP substrate and studied by photoluminescence and X-ray diffraction. A GaAsPN bulk alloy emitting light at 1.77 eV at room temperature is obtained, demonstrating promising properties for further use in III-V/Si photovoltaic multijunction solar cells.

Electrical spin injection into InGaAs/GaAs quantum wells: A comparison between MgO tunnel barriers grown by sputtering and molecular beam epitaxy methods

An efficient electrical spin injection into an InGaAs/GaAs quantum well light emitting diode is demonstrated thanks to a CoFeB/MgO spin injector. The textured MgO tunnel barrier is fabricated by two different techniques: sputtering and molecular beam epitaxy. The maximal spin injection efficiency is comparable for both methods. Additionally, the effect of annealing is also investigated for the two types of samples. Both samples show the same trend: an increase of the electroluminescence circular polarization (Pc) with the increase of annealing temperature, followed by a saturation of Pc beyond 350 °C annealing. Since the increase of Pc starts well below the crystallization temperature of the full CoFeB bulk layer, this trend could be mainly due to an improvement of chemical structure at the top CoFeB/MgO interface. This study reveals that the control of CoFeB/MgO interface is essential for an optimal spin injection into semiconductor.

All optical method for investigation of spin and charge transport in semiconductors: Combination of spatially and time-resolved luminescence

A new approach is demonstrated for investigating charge and spin diffusion as well as surface and bulk recombination in unpassivated doped semiconductors. This approach consists in using two complementary, conceptually related, techniques, which are time-resolved photoluminescence (TRPL) and spatially resolved microluminescence (μ PL) and is applied here to p + GaAs. Analysis of the sole TRPL signal is limited by the finite risetime. On the other hand, it is shown that joint TRPL and μ PL can be used to determine the diffusion constant, the bulk recombination time, and the spin relaxation time. As an illustration, the temperature variation of these quantities is investigated for p + GaAs.

L-valley electron spin dynamics in GaAs

However,alltheseexperimentswereperformedwith optical excitation energies close to the band gap (typ-ically 1.5–2 eV in GaAs), yielding the photogeneration ofspin-polarized electrons in the valley. In addition to itsfundamental aspect, the understanding of the electron spindynamics in the upper valleys is crucial for devices basedon electrical injection such as spin light-emitting diodes(LEDs)

Fabrication of an InGaAs spin filter by implantation of paramagnetic centers

We report on the selective creation of spin filtering regions in non-magnetic InGaAs layers by implantation of Ga ions by Focused Ion Beam. We demonstrate by photoluminescence spectroscopy that spin dependent recombination (SDR) ratios as high as 240% can be achieved in the implanted areas. The optimum implantation conditions for the most efficient SDR are determined by the systematic analysis of different ion doses spanning four orders of magnitude. The application of a weak external magnetic field leads to a sizable enhancement of the SDR ratio from the spin polarization of the nuclei surrounding the polarized implanted paramagnetic defects.

Optical orientation of electron spins in GaAs L-valleys

We report on optical orientation experiments in GaAs epilayers with excitation energies in the 3 eV region, leading the photo-generation of spin-polarized electrons in the satellite L valley. From both continuous-wave and time resolved measurements we show that a significant fraction of the electron spin memory can be conserved when the electron is scattered from the L to the Γ valley following an energy relaxation of several hundreds of meV. A typical L-valley electron spin relaxation time of 200 fs is deduced, in agreement with theoretical calculations.