Claude Weisbuch

Proposal for an efficient source of polarized photoelectrons from semiconductors

Abstract It is expected that a cesiated semiconductor such as GaAs excited with circularly polarized photons of energy close to the band gap would be a very bright and intense source of polarized electrons. Under easily attainable experimental conditions it is predicted that one could obtain a current of 10−3–10−4 A with an electronic polarization close to 50 per cent.

Optical detection of conduction electron spin resonance in InP

Abstract Conduction electron spin resonance is optically detected in InP at 1.7 K. The measured value | g ∗ | = 1.26 ± 0.05 is in very good agreement with theoretical predictions and with the only other experimental determination available.

Spin-dependent recombination and optical spin orientation in semiconductors

Abstract Spin-dependent recombination is observed in Ga 0.6 Al 0.4 As at 77°K on the intensity of the donor-acceptor pairs photoluminescence. The lifetime is enhanced by a factor 2.3 when photocreated electrons and recombination centers are spin polarized by optical pumping with circularly polarized light. Optical orientation and spin-dependent recombination lead to a steady-state electronic spin polarization as large as 70%.

Resonant Raman Scattering by excitonic polaritons in semiconductors

Abstract We report results on Resonant Raman Scattering (RRS) mediated by excitonic polaritons in a high-purity semiconductor (CdTe) at low temperatures. For the first time ingoing and outgoing resonances at the n = 1, 2, 3 exciton states are detected on the one and two LO-phonon RRS. The transformation at resonance of sharp Raman peaks into well-thermalized exciton luminescence bands is observed for every ingoing, outgoing or intermediate state resonance.

Experimental evidence of nanometer-scale localized recombination due to random In fluctuations in InGaN/GaN quantum wells (Conference Presentation)

In nitride ternary alloys, natural compositional disorder induces strong electronic localization effects. We present a new experimental approach which allows a direct probing at nanometer scale of disorder-induced localization effects in InGaN/GaN quantum wells (QWs). In this experiment, samples are p-type heterostructures incorporating an InGaN/GaN QW nearby the surface. The electrons are locally injected from a scanning tunneling microscope (STM) tip into the conduction band of the thin cladding top GaN layer and captured in the InGaN QW where they radiatively recombine. The injected current is maintained constant by the STM feedback loop and the injection electron energy is controlled by the bias voltage applied to the tip-sample tunnel junction. The luminescence onset voltage coincides with electron injection above the bottom of the conduction band in the bulk GaN (beyond the band bending region). Thereby, scanning the tip allows the high-resolution mapping of the luminescence process in the InGaN QW. Spatial fluctuations of the luminescence peak energy and linewidth are observed on the scale of a few nanometers, which are characteristic of disorder-induced carrier localization. A model based on the so-called localization landscape theory is developed to take into account the effect of alloy disorder into simulations of the structure properties. The localization landscape notably describes an effective confining potential, whose basins and crests define the localization regions of carriers. This theory accounts well for the observed nanometer scale carrier localization and the energy-dependent luminescence linewidth observed for the quantum electron states in the disordered energy band.

Carrier localization induced by alloy disorder in nitride devices: theory and experiments (Conference Presentation)

We present a model of carrier distribution and transport accounting for quantum localization effects in disordered semiconductor alloys. It is based on a recent mathematical theory of quantum localization which introduces a spatial function called localization landscape for carriers. These landscapes allow us to predict the localization of electron and hole quantum states, their energies, and the local densities of states. The various outputs of these landscapes can be directly implemented into a drift-diffusion model of carrier transport and into the calculation of absorption/emission transitions. This model captures the two major effects of quantum mechanics of disordered systems: the reduction of barrier height (tunneling) and lifting of energy ground states (quantum confinement), without having to solve the Schrodinger equation. Comparison with exact Schrodinger calculations in several one-dimensional structures demonstrates the excellent accuracy of the approximation provided by the landscape theory [1]. This approach is then used to describe the absorption Urbach tail in InGaN alloy quantum wells of solar cells and LEDs. The broadening of the absorption edge for quantum wells emitting from violet to green (indium content ranging from 0% to 28%) corresponds to a typical Urbach energy of 20 meV and is closely reproduced by the 3D sub-bandgap absorption based on the localization landscape theory [2]. This agreement demonstrates the applicability of the localization theory to compositional disorder effects in semiconductors. [1] M. Filoche et al., Phys. Rev. B 95, 144204 (2017) [2] M. Piccardo et al., Phys. Rev. B 95, 144205 (2017)

High-Efficiency Nitride-Base Photonic Crystal Light Sources

The research activities performed in the framework of this project represent a major breakthrough in the demonstration of Photonic Crystals (PhC) as a competitive technology for LEDs with high light extraction efficiency. The goals of the project were to explore the viable approaches to manufacturability of PhC LEDS through proven standard industrial processes, establish the limits of light extraction by various concepts of PhC LEDs, and determine the possible advantages of PhC LEDs over current and forthcoming LED extraction concepts. We have developed three very different geometries for PhC light extraction in LEDs. In addition, we have demonstrated reliable methods for their in-depth analysis allowing the extraction of important parameters such as light extraction efficiency, modal extraction length, directionality, internal and external quantum efficiency. The information gained allows better understanding of the physical processes and the effect of the design parameters on the light directionality and extraction efficiency. As a result, we produced LEDs with controllable emission directionality and a state of the art extraction efficiency that goes up to 94%. Those devices are based on embedded air-gap PhC - a novel technology concept developed in the framework of this project. They rely on a simple and planar fabrication process that is very interesting for industrial implementation due to its robustness and scalability. In fact, besides the additional patterning and regrowth steps, the process is identical as that for standard industrially used p-side-up LEDs. The final devices exhibit the same good electrical characteristics and high process yield as a series of test standard LEDs obtained in comparable conditions. Finally, the technology of embedded air-gap patterns (PhC) has significant potential in other related fields such as: increasing the optical mode interaction with the active region in semiconductor lasers; increasing the coupling of the incident light into the active region of solar cells; increasing the efficiency of the phosphorous light conversion in white light LEDs etc. In addition to the technology of embedded PhC LEDs, we demonstrate a technique for improvement of the light extraction and emission directionality for existing flip-chip microcavity (thin) LEDs by introducing PhC grating into the top n-contact. Although, the performances of these devices in terms of increase of the extraction efficiency are not significantly superior compared to those obtained by other techniques like surface roughening, the use of PhC offers some significant advantages such as improved and controllable emission directionality and a process that is directly applicable to any material system. The PhC microcavity LEDs have also potential for industrial implementation as the fabrication process has only minor differences to that already used for flip-chip thin LEDs. Finally, we have demonstrated that achieving good electrical properties and high fabrication yield for these devices is straightforward.