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

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

On Stress-Induced Polarization Effect in Ammonothermally Grown GaN Crystals

The results of basic ammonothermal crystallization of gallium nitride are described. The material is mainly analyzed in terms of the formation of stress (called stress-induced polarization effect) and defects (threading dislocations) appearing due to a stress relaxation process. Gallium nitride grown in different positions of the crystallization zone is examined in cross-polarized light. Interfaces between native ammonothermal seeds and new-grown gallium nitride layers are investigated in ultraviolet light. The etch pit densities in the seeds and the layers is determined and compared. Based on the obtained results a model of stress and defect formation is presented. New solutions for improving the structural quality of basic ammonothermal gallium nitride crystals are proposed

On Stress-Induced Polarization Effect in Ammonothermally Grown GaN Crystals

The results of basic ammonothermal crystallization of gallium nitride are described. The material is mainly analyzed in terms of the formation of stress (called stress-induced polarization effect) and defects (threading dislocations) appearing due to a stress relaxation process. Gallium nitride grown in different positions of the crystallization zone is examined in cross-polarized light. Interfaces between native ammonothermal seeds and new-grown gallium nitride layers are investigated in ultraviolet light. The etch pit densities in the seeds and the layers is determined and compared. Based on the obtained results a model of stress and defect formation is presented. New solutions for improving the structural quality of basic ammonothermal gallium nitride crystals are proposed

Semiconductor nitride light sources: 2014 Nobel prize in physics to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura

Podstawy fizyczne działania półprzewodnikowych przyrządów emitujących światło, tzn. diod laserowych i elektroluminescencyjnych zostały omówione w zwięzły sposób. Historyczny rozwój badań nad zastosowaniem półprzewodnikowych źródeł światła, w tym źródeł światła czerwonego został zarysowany. Trudności i rozwój półprzewodnikowych źródeł światła w ostatnim półwieczu został omówiony. Przełomowy wkład Isamu Aksaki, Hiroshi Amano i Shuji Nakamuro w rozwój badan i technologii diod opartych o azotki metali grupy III, który doprowadził do powstania źródeł światła niebieskiego, zielonego i fioletowego w latach 90. XX w., i został uhonorowany Nagrodą Nobla z fizyki w 2014 r., został szczegółowo opisany. Dalszy rozwój dziedziny do chwili obecnej i jej perspektywy zostały zarysowane. Rola polskich ośrodków naukowych w tej dziedzinie, znacząca w skali światowej, została również lakonicznie omówiona.Physical foundations of semiconductor light emitting devices, i.e. laser diodes (LDs) and light emitting diodes (LEDs), and history of studies concerning application of semiconductor light sources, including sources of red light, are shortly outlined. The seminal contributions of Isamu Akasaki, Hiroshi Amano and Shuji Nakamura to the investigations and applications of the group III metal nitrides that led to emergence of blue, green and violet light sources in the last decade of XX century, honored by 2014 Nobel Prize in Physics, are presented in detail. Further developments in this area up to the present day and future perspectives, including the important role played therein by Polish research institutions, are also sketched

Cw and time-resolved spectroscopy in homoepitaxial GaN films and GaN-GaAlN quantum wells grown by molecular beam epitaxy.

International audienceWe have grown GaN films and GaN–AlGaN quantum wells (QWs) on homoepitaxial substrates, by molecular beam epitaxy using ammonia. Both the GaN film and the QW are found to have superior excitonic recombination properties which are extremely promising for the development of indium free ultra-violet lasers based on nitrides

Localization Effects in InGaN/GaN Double Heterostructure Laser Diode Structures Grown on Bulk GaN Crystals.

International audienceWe demonstrate a double heterostructure (DH) nitride laser diode (LD) with an untypically wide 9.5 nm InGaN active region instead of the commonly used narrow InGaN quantum wells. Structures were grown on bulk GaN, which ensures low dislocation densities and therefore low concentrations of nonradiative recombination centers. The efficient screening of polarization induced electric fields in the structures under investigation, which contained heavily (1×1019 cm-3) Si doped barriers, was demonstrated by means of hydrostatic pressure dependent photoluminescence measurements on a simplified sample with identical active region. Since the detrimental separation of carriers by electric fields becomes more pronounced for wider InGaN active regions, efficient screening of polarization induced electric fields is essential for the investigated heterostructures. Optical and electrical parameters of this LD were comparable to those of comparable devices with typical QWs of ~4 nm as active region. We observed a high thermal stability of the photoluminescence intensity and, via time-resolved photoluminescence, a relatively temperature-independent radiative decay time. These observations support the significance of carrier localization phenomena for the radiative recombination processes of the investigated structure. The implications of these results for DH LD structures are discussed

Optical properties of GaN epilayers and GaN/AlGaN quantum wells grown by molecular beam epitaxy on GaN(0001) single crystal substrate.

International audienceGaN epilayers and GaN/AlGaN quantum wells(QWs) were grown by molecular beam epitaxy on GaN(0001) single crystal substrates. Transmission electron microscopy(TEM) was used to assess the crystal quality of the homoepitaxial layers. A dislocation density of less than 105 cm−2 is deduced from TEM imaging. Low temperature (1.8 K) photoluminescence(PL) of homoepitaxial GaN reveals PLlinewidths as low as 0.3 meV for bound excitons. The PL integrated intensity variation between 10 and 300 K is compared to that observed on a typical heteroepitaxialGaN/Al2O3 layer. A 2 nm thick GaN/Al0.1Ga0.9NQW has been studied by time-resolved and continuous wave PL. The decay time is close to a purely radiative decay, as expected for a low defect density. Finally, the built-in polarization field measured in a homoepitaxialQW is shown to be comparable to that measured on heteroepitaxialQWsgrown either on sapphire or silicon substrates