Thermoelectric and micro-Raman measurements of carrier density and mobility in heavily Si-doped GaN wires

International audienceCombined thermoelectric-resistivity measurements and micro-Raman experiments have been performed on single heavily Si-doped GaN wires. In both approaches, similar carrier concentration and mobility were determined taking into account the non-parabolicity of the conduction band. The unique high conductivity of Si-doped GaN wires is explained by a mobility µ=56 cm2 /V s at a carrier concentration n = 2.6 10^20 /cm 3. This is attributed to a more efficient dopant incorporation in Si-doped GaN microwires as compared to Si-doped GaN planar layers. (c) 2013 AIP Publishing LLC

Exciton recombination dynamics in a-plane (Al,Ga)N/GaN quantum wells probed by picosecond photo and cathodoluminescence.

International audienceWe present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs

Thermal carrier emission and nonradiative recombinations in nonpolar(Al,Ga)N/GaN quantum wells grown on bulk GaN.

International audienceWe investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320K range. We first show that the effective lifetime of QW excitons tau increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing tau depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs

Thermal carrier emission and nonradiative recombinations in nonpolar (Al,Ga)N/GaN quantum wells grown on bulk GaN

We investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320 K range. We first show that the effective lifetime of QW excitons s increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing s depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs

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

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: I-1 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 I-1 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 I-1 stacking faults in the overall recombination mechanism of excitons

Intrinsic dynamics of weakly and strongly confined excitons in nonpolar nitride-based heterostructures

Both weakly and strongly confined excitons are studied by time-resolved photoluminescence in a nonpolar nitride-based heterostructure grown by molecular beam epitaxy on the a-facet of a bulk GaN crystal, with an ultralow dislocation density of 2 × 105 cm-2. Strong confinement is obtained in a 4 nm thick Al0.06Ga0.94N/GaN quantum well (QW), whereas weakly confined exciton-polaritons are observed in a 200 nm thick GaN epilayer. Thanks to the low dislocation density, the effective lifetime of strongly confined excitons increases between 10 and 150 K, proving the domination of radiative recombination processes. Above 150 K the QW emission lifetime diminishes, whereas the decay time of excitons in the barriers increases, until both barrier and QW exciton populations become fully thermalized at 300 K. We conclude that the radiative efficiency of our GaN QW at 300 K is limited by nonradiative recombinations in the barriers. The increase of exciton-polariton coherence lengths caused by low dislocation densities allows us to observe and model the quantized emission modes in the 200 nm nonpolar GaN layer. Finally, the low-temperature phonon-assisted relaxation mechanisms of such center-of-mass quantized exciton-polaritons are described

Diodes électroluminescentes blanches monolithiques

The aim of this work was to study and fabricate monolithic white light emitting diodes (LEDs) grown by molecular beam epitaxy (MBE) using NH3 as nitrogen source. The method proposed at the lab consist in inserting in the active zone of the LED some (Ga,In)N/GaN quantum wells emitting in the blue and yellow range. Even if the In content of the InxGa1-xN alloy is limited at 20-25%, the presence of an internal electric field permits to obtain wave lengths in all the visible spectrum. Unfortunately, this electric field decreases the oscillator strength of the quantum well for well width of more than 2 nm. The radiative efficiency of quantum wells emitting in the yellow is thus low compared to a quantum well emitting in the blue. Therefore, the yellow emission is a problem and we have had to try to counterbalance the electric field effects in order to increase the quantum efficiency of thick wells. In an other way, the power of MBE LEDs is lower than the EPVOM ones. One of the differences between the two growth techniques is the growth of p-type GaN. Different parameters have been tested in order to determine the right growth conditions to obtain good optoelectronic quality of p-type GaN layer. To finish, these different refinings will be checked by elaborating LEDs and in particular white monolithic LEDs with large spectra.Ce travail concerne la croissance par épitaxie sous jets moléculaires avec NH3 comme source d'azote de diodes électroluminescentes (DELs) blanches monolithiques. La méthode proposée au laboratoire consiste à insérer dans la zone active de la DEL des puits quantiques (Ga,In)N émettant dans le bleu et le jaune. Bien que la concentration en In de l'alliage InxGa1-xN soit limitée à 20-25%, la présence d'un champ électrique interne dans les hétérostructures nitrures permet d'obtenir des longueurs d'onde balayant tout le spectre visible. Malheureusement, ce champ électrique diminue aussi la force d'oscillateur de puits quantiques de largeur de plus de 2nm. Ainsi, l'efficacité radiative de puits quantiques émettant dans le jaune est faible comparé à un puits quantique émettant dans le bleu. L'émission dans le jaune pose donc problème et il nous a fallu tenté de contrebalancer les effets du champ électrique afin d'accroître le rendement quantique des puits larges. D'autre part, la puissance des DELs EJM est faible par rapport à celles épitaxiées en EPVOM. Une des grandes différences entre ces deux techniques est la croissance du GaN de type p. Différents paramètres ont été testés afin de déterminer les conditions de croissance adéquates à l'épitaxie du GaN de type p de bonne qualité optoélectronique. Finalement, ces différents affinages ont été vérifiés par l'élaboration de DELs et en particulier d'une DEL blanche monolithique à large spectre

Diodes électroluminescentes blanches monolithiques

Ce travail concerne la croissance par épitaxie sous jets moléculaires avec NH3 comme source d azote de diodes électroluminescentes (DELs) blanches monolithiques. La méthode proposée au laboratoire consiste à insérer dans la zone active de la DEL des puits quantiques (IN,Ga)N émettant dans le bleu et le jaune. Bien que la concentration en In de l alliage InxGa1-xN soit limitée à 20-25%, la présence d un champ électrique interne dans les hétérostructures nitrures permet d obtenir des longueurs d ondes balayant tout le spectre visible. Malheureusement, ce champ électrique diminue aussi la force d oscillateur de puits quantiques de largeur de plus de 2 nm. Ainsi, l efficacité radiative de puits quantiques émettant dans le jaune est faible comparée à un puits quantique émettant dans le bleu. L émission dans le jaune pose donc problème et il nous a fallu tenter de contrebalancer les effets du champ électrique afin d accroître le rendement quantique des puis larges. D autre part, la puissance des DELs EJM est faible par rapport à celles épitaxiées en EPVOM. Une des grandes différences entre ces deux techniques est la croissance du GaN de type p. Différents paramètres ont été testés afin de déterminer les conditions de croissance adéquates à l épitaxie du GaN de type p. Différents paramètres ont été testés afin de déterminer les conditions de croissance adéquates à l épitaxie du GaN de type p de bonne qualité optoélectronique. Finalement, ces différents affinages ont été vérifiés par l élaboration de DELs et en particulier d une DEL blanche monolithique à large spectre.This work deals with the molecular beam epitaxy (MBE) with NH3 as nitrogen source growth of white monolithic electroluminescent diode (LEDs). The method proposed at the lab consist in inserting in the active zone of the LED some (In,Ga)NGaN quantum wells emitting in the blue and yellow. Even if the In concentration of the InxGa1-xN alloy is limited at 20-25%, the presence of an internal electric field permit to obtain wave lengths in all the visible spectrum. Unfortunately, this electric field decreases the oscillator strength of the quantum well with thickness of more than 2 nm. The radiative efficiency of quantum wells emitting in the yellow is thus low compared to a quantum well emitting in the blue. So, the yellow emission is a problem and we have had to try to counterbalance the electric field effects in order to increase the quantum efficiency of thick wells. In an other way, the power of MBE LEDs is lower than the EPVOM ones. One of the difference between the two growth techniques is the growth of p-type GaN. Different parameters have been tested in order to determine the right growth conditions for the good optoelectronic quality p-type GaN epitaxy. To finish, these different refinings will be checked by elaborate LEDs and in particular white monolithic LEDs with large spectra.NICE-BU Sciences (060882101) / SudocSudocFranceF