A. Dussaigne

In surface segregation in InGaN/GaN quantum wells

We investigate both theoretically and experimentally the effects of the In surface segregation in InGaN/GaN quantum wells (QWs). It is shown that this phenomenon induces a blue-shift of the QW photoluminescence (PL) energy, which does not depend on the QW width, at least for well thicknesses larger than 1.5 nm. The oscillator strength of the QW optical transitions decreases when the segregation process increases due to the spatial separation of the electron and hole pairs by the internal electric field. The surface segregation phenomenon has been studied by reflection high-energy electron diffraction in the case of molecular beam epitaxy growth with NH3 as the nitrogen source. Evidence for surface segregation is given by comparing PL results and data deduced from a careful analysis of the growth rate variation of GaN deposited on an In covered surface

Injection dependence of the electroluminescence spectra of phosphor free GaN-based white light emitting diodes

Multicolor, multi-quantum well light emitting diodes have been fabricated by molecular beam epitaxy by inserting quantum wells of various widths in the active region. The In content of the wells is 15%-20% and the color is governed by carrier confinement and the Stark effect. Combining a proper number of blue and yellow quantum wells allows to obtain monolithic white LEDs. The electroluminescence spectra of the diodes have been studied. At low injection, the luminescence intensity varies quadratically with the injection current, showing that the electroluminescence originates from the depleted region of the diode, and that non-radiative recombination paths exist. However, for higher injection currents, the luminescence efficiency of the wells situated near the n-side of the junction starts to vary linearly with the current, and this is accompanied by the appearance of GaN electroluminescence. We show that this is due to the entering of these wells into the neutral region of the diode, explaining the injection dependence of the color of these multicolor LEDs.

Exciton localization on basal stacking faults in a-plane epitaxial lateral overgrown GaN grown by hydride vapor phase epitaxy

We present a detailed study of the luminescence at 3.42 eV usually observed in a-plane epitaxial lateral overgrowth (ELO) GaN grown by hydride vapor phase epitaxy on r-plane sapphire. This band is related to radiative recombination of excitons in a commonly encountered extended defect of a-plane GaN: I1 basal stacking fault. Cathodoluminescence measurements show that these stacking faults are essentially located in the windows and the N-face wings of the ELO-GaN and that they can appear isolated as well as organized into bundles. Time-integrated and time-resolved photoluminescence, supported by a qualitative model, evidence not only the efficient trapping of free excitons (FXs) by basal plane stacking faults but also some localization inside I1 stacking faults themselves. Measurements at room temperature show that FXs recombine efficiently with rather long luminescence decay times (360 ps), comparable to those encountered in high-quality GaN epilayers. We discuss the possible role of I1 stacking faults in...

High-Al-content crack-free AlGaN/GaN Bragg mirrors grown by molecular-beam epitaxy

We report on the growth by molecular-beam epitaxy on 2 in. sapphire substrates of crack-free AlxGa1−xN/GaN distributed Bragg reflectors (DBRs) with high-Al composition (x=0.5). This is achieved by introducing a thick AlN interlayer and strain mediating AlyGa1−yN layer between the substrate and DBR. The relatively larger refractive index ratio between Al0.5Ga0.5N and GaN permits one to obtain a quite large spectral stopband width (49 nm) and a high reflectance value (69%) for only eight mirror periods.

Dependence of the Mg-related acceptor ionization energy with the acceptor concentration in p-type GaN layers grown by molecular beam epitaxy

Hall effect and capacitance-voltage C(V) measurements were performed on p-type GaN:Mg layers grown on GaN templates by molecular beam epitaxy with a high range of Mg-doping concentrations. The free hole density and the effective dopant concentration NA−ND as a function of magnesium incorporation measured by secondary ion mass spectroscopy clearly reveal both a magnesium doping efficiency up to 90% and a strong dependence of the acceptor ionization energy Ea with the acceptor concentration NA. These experimental observations highlight an isolated acceptor binding energy of 245±25 meV compatible, at high acceptor concentration, with the achievement of p-type GaN:Mg layers with a hole concentration at room temperature close to 1019 cm−3.

Two-color GaN/AlGaN quantum cascade detector at short infrared wavelengths of 1 and 1.7 μm

A two-color GaN-based quantum cascade detector is demonstrated. This photodetector operates simultaneously at a peak wavelength of 1.7 and 1 μm at room temperature without any external voltage. These peaks correspond, respectively, to the e1e2 and e1e3 intersubband absorption of the active GaN quantum well. The extractor has been designed to allow for efficient transfer of electrons from both the e2 and e3 states to the next period. The 1 μm detected wavelength is the shortest value reported for an intersubband semiconductor based detector.

High doping level in Mg-doped GaN layers grown at low temperature

We have studied the properties of Mg-doped GaN epilayers grown by molecular beam epitaxy (MBE) with ammonia as nitrogen source. GaN p-n homojunctions has been developed to determine the optoelectronic characteristics of the junctions as a function of the p-type GaN growth conditions. It is shown that the electrical characteristics strongly depend on the Mg flux and the growth temperature. As a result, the junction characteristics have been drastically improved and state of the art MBE-grown p-type layers have been obtained: the hole concentration, the mobility, and the resistivity are 1×1018cm−3, 9cm2∕Vs, and 0.75Ωcm, respectively. These characteristics lead to an increase of the homojunction light emitting diode (LED) optical output power by two orders of magnitude. To further assess the quality of these MBE-grown p-type layers, we have prepared a hybrid LED which consists of an InGaN∕GaN quantum well active structure grown by metal organic vapor phase epitaxy followed by a p-type region grown by MBE. An...

Photoluminescence energy and linewidth in GaN/AlN stackings of quantum dot planes

We analyze the room temperature photoluminescence properties of several multilayer stackings of GaN/AlN quantum dots. We report drastic differences of emission energies and linewidths between continuous wave and time-resolvedphotoluminescence experiments. In continuous wave experiments, the screening of internal electric fields by accumulation of e-h pairs in quantum dot planes induces a blue-shift as well as an unexpected narrowing of the emission line, when the laser intensity is increased. Under intense, pulsed excitation, in time-resolvedphotoluminescence, a substantial blue-shift is induced, due to the partial cancelation of the quantum confined Stark effect. When the system is again free to relax, we observe a time-dependent red-shift of the line, which maintains a fairly constant width. We attribute the observed behavior of energies and linewidths to the intricate contributions of the in-plane distribution of dot sizes and of the depth-dependent decrease of the degree of excitation of the different planes. We support our interpretations by the use of a model based on a self-consistent solution of the Schrodinger and Poisson equations within the envelope function approximation.

Low-temperature time-resolved cathodoluminescence study of exciton dynamics involving basal stacking faults in a-plane GaN

Time-resolved cathodoluminescence at 27 K has been performed on a-plane GaN grown by epitaxial lateral overgrowth. We detail the relaxation and recombination mechanisms of excitons [free or bound to neutral donors, or bound to I1-type basal stacking faults (BSFs)] in relation to the local density in BSFs. We describe the slow exciton capture rate on isolated BSFs by a diffusion model involving donors via a hopping process. Where BSFs are organized into bundles, we relate the shorter rise time to intra-BSF localization processes and the multiexponential decay to the type-II band alignment of BSFs in wurtzite GaN.

Control of the polarity of GaN films using an Mg adsorption layer

The polarity of GaN epilayers grown by molecular beam epitaxy is controlled using Mg. This is achieved by simultaneously exposing the surface to Mg and NH3 fluxes during growth interruption. Reflection high-energy electron diffraction (RHEED) indicates the formation of a Mg3N2, layer. Overgrowing GaN on this surface leads to a polarity inversion either from Ga to N or N to Ga. The change of the polarity is followed in situ by RHEED via surface reconstructions of the GaN surface. The polarity inversion is further confirmed by convergent beam electron diffraction experiments. Finally, high-resolution transmission electron microscopy images show different interface morphologies between Ga/N and N/Ga polarity boundaries. The control of the GaN polarity opens the way for novel periodic polarity structures dedicated to non-linear optics