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 Schrödinger equation. Comparison with exact Schrödinger calculations in several one-dimensional structures demonstrates the excellent accuracy of the approximation provided by the landscape theory . 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 . This agreement demonstrates the applicability of the localization theory to compositional disorder effects in semiconductors.
 M. Filoche et al., Phys. Rev. B 95, 144204 (2017)
 M. Piccardo et al., Phys. Rev. B 95, 144205 (2017)
We studied the dynamic of Fabry-Perot quantum cascade laser (QCL) devices operating in the mid-infrared range upon external optical injection from a tunable external cavity QCL (EC-QCL) operating in the same wavelength range. The studied lasers either had a symmetric (uncoated) or asymetric (by use of anti-reflective and highly-reflective coatings) cavity. Pulses from the EC-QCL were injected into the Fabry-Perot lasers below threshold and the reflected intensity by the laser cavity was measured as a function of the pump wavelength of the EC-QCL and of the driving current of the device QCL study. The resulting data was fitted with a theoretical model for the laser reflectivity taking into account the influence of the driving current on the waveguide group index, gain, and facet reflectivity to provide an accurate modeling of the laser parameters. Continuous wave light from the tunable pump source was also injected into the Fabry-Perot lasers at different driving currents above threshold while the emission spectra of the injected QCL were monitored. The study of the response of the studied QCLs to an external pump field provided an indirect measurement of the parametric gain of the active region. A theoretical modeling of the parametric gain of these QCLs based on the combined effect of population pulsations and population grating was derived and compared to the experimental results. The experiments performed during these studies overall improved the understanding of the dynamics of the population inversion of QCLs providing further insights into the formation of high-repetition-rate frequency combs recently demonstrated.
We discuss the unambiguous detection of Auger electrons by electron emission (EE) spectroscopy from a cesiated InGaN/GaN light-emitting diode (LED) under electrical injection. Electron emission spectra were measured as a function of the current injected in the device. The appearance of high-energy electron peaks simultaneously with the droop in LED efficiency shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process. A linear correlation was measured between the high energy emitted electron current and the “droop current” - the missing component of the injected current for light emission. We conclude that the droop originates from the onset of Auger processes. We compare such a direct identification of the droop mechanism with other identifications, most of them indirect and based on the many-parameter modeling of the dependence of the external quantum efficiency on the carrier injection.