ZnO nanowires grown on Al2O3-ZnAl2O4 nanostructure using solid-vapor mechanism

We present Al2O3-ZnAl2O4-ZnO nanostructure, which could be a prominent candidate for optoelectronics, mechanical and sensing applications. While ZnO and ZnAl2O4 composites are mostly synthesized by sol-gel technique, we propose a solid-vapor growth mechanism. To produce Al2O3-ZnAl2O4-ZnO nanostructure, we conduct ZnO:C powder heating resulting in ZnO nanowires (NWs) growth on sapphire substrate and ZnAl2O4 spinel layer at the interface. The nanostructure was examined with Scanning Electron Microscopy (SEM) method. Focused Ion Beam (FIB) technique enabled us to prepare a lamella for Transmission Electron Microscopy (TEM) imaging. TEM examination revealed high crystallographic quality of both spinel and NW structure. Epitaxial relationships of Al2O3-ZnAl2O4 and ZnAl2O4-ZnO are given.Comment: Conference: 13th Polish-Japanese Joint Seminar on Micro and Nano Analysi

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

Amphoteric Be in GaN

We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of BeGa in the positron annihilation signals in Be-doped GaN, while the emergence of VGa at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.Peer reviewe

Determination of built-in electric fields in quaternary InAlGaN heterostructures.

International audienceA study of internal electric field contribution to the light emission mechanism of InAlGaN based multiquantum wells was performed. To determine the magnitude of the built-in electric field we employed: (i) theoretical estimation of the piezoelectric and spontaneous polarizations, (ii) analysis of the emission energy as a function of the quantum well width, (iii) hydrostatic pressure experiments, and finally (iv) measurements of photoluminescence decay. Performed calculations gave high magnitude of the built-in electric field. On the contrary, independently of the quantum well width the pressure shift of the light emission energy and the photoluminescence decay time showed almost constant values. These observations are interpreted as evidence of a lack of the built-in electric field in the used quaternary quantum wells. Possible reasons for the controversies between theory and experiment are suggested

Time-resolved spectroscopy of (Al,Ga,In)N based quantum wells: Localization effects and effective reduction of internal electric fields.

International audienceWe investigate the microscopic mechanisms of the highly intense light emission from (Al,Ga,In)N/(Al,Ga,In)N quantum wells (QW’s). We concentrate on the competition between radiative and nonradiative recombination of electron-hole pairs for Al0.12Ga0.84In0.04N/Al0.30Ga0.69In0.01N multiple quantum wells of various widths. These nominal compositions correspond to the maximization of the luminescence intensity. By using time-resolved photoluminescence, we verify that such particular compositions of the quaternaries are capable of minimizing the magnitude of the longitudinal electric field which usually appears in such hexagonal group-III nitride based QW’s. We find that the radiative lifetime is nearly independent of the well width, whereas it should show an exponential dependence in the case of a strong electric field. We discuss our results via a theoretical estimation of the internal electric field. We also analyze the role played by local potential fluctuations in the quaternary alloy on the localization of carriers, by measuring the change of the photoluminescence energy, intensity, and dynamics versus the temperature

Towards purely radiative recombination at room-temperature in nonpolar AlGaN/GaN quantum wells

International audienceIn contrast to GaN-based structures grown along [0001], nonpolar growth allows for the realization of thick quantum wells (QWs) with an optimal overlap between electron and hole wavefunctions. However, nonpolar GaN layers grown on non-lattice-matched substrates exhibit high densities of defects. Then, even at low temperature (T), the carrier recombination is mainly nonradiative.Here, we present the dynamics of charge carriers in AlGaN/GaN QWs of various widths and Al-contents grown on the a-facet of GaN crystals. The QW dislocation density and the exciton diffusion length at 300K are 2.105 cm-2 and 100 nm, respectively. Therefore, dislocations play a minor role in the carrier dynamics at 300K. For all samples, we observe an increasing QW PL lifetime with T, indicating the absence of nonradiative phenomena in the low-T range. Purely radiative recombination is observed up to 240K for a 7 nm thick QW. This suggests the possibility of realizing UV emitters with narrow emission and high internal quantum efficiency at 300K.The observed decrease in QW PL lifetime in the high-T range arises from the thermal escape of carriers from the QW to the AlGaN barriers and their subsequent nonradiative recombination. The efficiency of nonradiative recombination at 300K is thus reduced by limiting the thermal escape of carriers from the QW. This is obtained by growing thick QWs rather than increasing the barrier Al-content, which is important from point of view of defect generation

Nonlinear emission properties of an optically anisotropic GaN-based microcavity

The pronounced optical in-plane anisotropy of a suitably designed nonpolar GaN-based microcavity, mainly inherited from the valence-band complexity of wurtzite semiconductors, allows the coexistence of two different light-matter coupling regimes along orthogonal polarization planes. When increasing the excitation power density, a transition to a nonlinear coherent emission is observed under nonresonant optical pumping for both polarization directions at carrier densities remaining well below the exciton-saturation level. Their differing nature is experimentally revealed by means of polarization-resolved Fourier space and near-field imaging. While one polarization direction exhibits all specificities of the strong light-matter coupling regime and features polariton lasing at high injection, the other one lacks a straightforward interpretation due to its complex coupling regime. Possible mechanisms are addressed and evaluated

Efficient radiative recombination and potential profile fluctuations in low-dislocation InGaN∕GaN multiple quantum wells on bulk GaN substrates.

International audienceWe investigated the relation between structural properties and carrier recombination processes in InGaN∕GaNmultiple quantum well(MQW) structures with quantum well widths of 3 and 9nm, grown by metal-organic chemical-vapor deposition on bulk GaN crystals. Quantum barriers of the samples are heavily n-type doped in order to effectively screen the large polarization-induced electric fields which commonly occur in hexagonal InGaN∕GaN quantum structures. High thermal stability in these structures, reflected by strong photoluminescence(PL) even above 400K, is attributed to a combination of low-dislocation densities and potential profile fluctuations in the InGaN∕GaNquantum wells. The role of potential profile fluctuations is further investigated by time-resolved photoluminescence and cathodoluminescence(CL) mapping. Comparison of both samples shows that the sample with 3‐nm-wide QWs exhibits (i) a larger width of the PL peak in the temperature range of 8–420K, (ii) a higher amplitude of potential profile fluctuations as measured by CL mapping, and (iii) higher radiative and nonradiative PL recombination times. At the same time a much weaker drop of PL intensity with temperature is recorded on the sample with 9‐nm-wide QWs. Our results show that, contrary to intuitive expectation, a decrease of the potential profile fluctuation amplitude can be helpful in enhancing the radiative recombination efficiency, particularly at high temperatures