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

Temperature-dependence of exciton radiative recombination in (Al,Ga)N/GaN quantum wells grown on a-plane GaN substrates

This article presents the dynamics of excitons in a-plane (Al,Ga)N/GaN single quantum wells of various thicknesses grown on bulk GaN substrates. For all quantum well samples, recombination is observed to be predominantly radiative in the low-temperature range. At higher temperatures, the escape of charge carriers from the quantum well to the (Al,Ga)N barriers is accompanied by a reduction in internal quantum efficiency. Based on the temperature-dependence of time-resolved photoluminescence experiments, we also show how the local disorder affects the exciton radiative lifetime at low temperature and the exciton non-radiative lifetime at high temperature.We acknowledge financial support from the Swiss National Science Foundation through Project No. 129715 and from the Polish National Science Center (Project DEC-2011/ 03/B/ST3/02647). The work was partially supported by the European Union within European Regional Development Fund through Innovative Economy Grant No. POIG.01.01.02-00-008/08. P.C. also acknowledges financing from the European Union Seventh Framework Program under grant agreement No. 265073

INFLUENCE OF THE MULTIDRUG TRANSPORTER INHIBITORS ON THE ACTIVITY OF KV1.3 VOLTAGE-GATED POTASSIUM CHANNELS

Using the whole-cell patch-clamp technique, the influence of selected multidrug resistance modulators, both plant-derived compounds and derivatives on the activity of voltage-gated potassium channels Kv1.3 was investigated. Twelve compounds with phenolic and terpenic structures were tested: the stilbenes piceatannol (1) and its tetramethoxy (2) and tetracetoxy (3) derivatives, the flavonoids naringenin (4) and its methylated derivatives: naringenin-4',7-dimethylether (5) and naringenin-7-methylether (6), and aromadendrin (7), the coumarins esculetin (13) and scopoletin (9) and ent-abietane diterpenes, helioscopinolide B (10) and its 3 beta-acetoxy derivative (11) and helioscopinolide E (12). The studies were performed on a model system with Kv1.3 channels endogenously expressed in human T lymphocytes. Obtained data provide evidence that compounds 2, 5 and 6 applied at 30 mu M inhibited the amplitude of recorded currents to 31%, 4% and 29% of its control value, respectively. On the other hand, compounds 3, 4, 7-12 (at 30 mu M) and compound 1 (at 40 mu M) did not affect significantly the channel activity. These results indicate that some methoxy-derivatives of the tested compounds are effective inhibitors of Kv1.3 channels. Since the inhibition of Kv1.3 channels may inhibit the proliferation of prostate, breast and colon cancer cells expressing these channels, the channel inhibitors may exert an anti proliferative action. This action combined with a simultaneous modulation of the multidrug resistance may be significant for a potential application of these compounds in cancer chemotherapy.. - Polish Ministry of Research and University Education [N 301 2549 33]; Science and Technology Foundation (FCT), Portugal. - The Authors would like to express best thanks to the colleague from the Biophysics Department of Wroclaw Medical University - Dr. Andrzej Pola, for a kind help in providing blood samples for isolation of T lymphocytes. This work was supported by the Polish Ministry of Research and University Education (Ministerstwo Nauki i Szkolnictwa Wyzszego) Grant no N 301 2549 33 (A.T. and K.M.) and by the Science and Technology Foundation (FCT), Portugal (N.D. and M-J.U.F.)

Band Structure and Refractive Index of Gallium Nitride Under Pressure

The effect of hydrostatic pressure on direct gap and refractive index of GaN is investigated up to 5.5 GPa. Band structure of GaN is calculated by Linear Muffin-Tin Orbitals (LMTO) method for different values of pressure. Resulting pressure coefficient of the main gap and of the refractive index are in a good agreement with the experimental ones

Piezoelectric field and its influence on the pressure behavior of the light emission from GaN/AlGaN strained quantum wells

We have studied the influence of hydrostatic pressure on the light emission from a strained GaN/AlGaN multiquantum well system. We have found that the pressure coefficients of the photoluminescence peak energies are dramatically reduced with respect to that of GaN energy gap and this reduction is a function of the quantum well thickness. The decrease of the light emission pressure coefficient may be as large as 30% for a 32 monolayer (8 nm) thick quantum well. We explain this effect by the hydrostatic-pressure-induced increase of the piezoelectric field in quantum structures. Model calculations based on the k x p method and linear elasticity theory reproduce the experimental results well, demonstrating that this increase may be explained by small anisotropy of the wurtzite lattice of GaN and a specific interplay of elastic constants and values of the piezoelectric tensor. (C) 2001 American Institute of Physics

Prevention and treatment of post-partum depression: a controlled randomized study on women at risk

Background. Research is needed to evaluate the efficacy of prevention and treatment for post-partum depression. Method. Subjects were screened with the Edinburgh Post-natal Depression Scale (EPDS) at the obstetric clinic. Mothers at risk ( N = 258) (EPDS scores [ges ]9) were randomly assigned to a prevention/treatment group or a control group. The prevention group received one cognitive-behavioural prevention session during hospitalization. At 4 to 6 weeks post-partum, subjects were screened again with the EPDS, after drop-out rates (refusals plus no return of the second EPDS) of 25.4% (33/130) in the intervention group and 10.9% (14/128) in the control group. Mothers with probable depression (EPDS scores [ges ]11) were assessed using the Hamilton Depression Rating Scale (HDRS) and the Beck Depression Inventory (BDI). Mothers with major depression continued in the treatment group ( N = 18) or in the control group ( N = 30). Treated subjects received a cognitive-behavioural programme of between five and eight weekly home-visits. Results. Compared with the control group, women in the prevention group had significant reductions in the frequency of probable depression (30.2% v. 48.2%). Recovery rates based on HDRS scores of <7 and BDI scores of <4 were also significantly greater in the treated group than in the control group. Conclusions. The study suggests that this programme for prevention and treatment of post-partum depression is reasonably well-accepted and efficacious

Large built-in electric field and its influence on the pressure behavior of the light emission from GaN/AlGaN strained quantum wells

Influence of hydrostatic pressure on the light emission from strained GaN/AlGaN multi-quantum-well systems has been studied, Pressure coefficients of the photoluminescence peak energies show a strong reduction with respect to that of the GaN energy ? ap and this reduction is a function of the quantum-well thickness. The decrease of the light emission pressure coefficient may be almost 40% for a 32 monolayer (8 nm) thick quantum well in comparison with a 4 monolayer well. We explain this effect by a hydrostatic-pressure-induced increase of the piezoelectric field in the GaN/AlGaN quantum structures

The Role of Internal Electric Fields in III-N Quantum Structure

Binary nitrides: of wurtzite GaN, AlN, InN, and their solid solutions represent a family of semiconductors of crucial importance for modern optoelectronics. Strained quantum wells, like GaN/AlGaN and specially InGaN/GaN, form active layers of the light emitters working in green-UV part of the spectrum. The operation of these devices strongly depends on the emission spectra of considered quantum structures which are greatly influenced by the presence of built-in electric fields. The electric field acting via quantum confined Stark effect in the mentioned structures changes the energies and intensity of the emitted light. The effect can lead to the spectral shift of a photo- and electroluminescence by many hundreds of meV. In this review we will briefly cover the influence of internal electric fields on both optical and electrical properties of nitride based heterostructures and quantum wells. We would like to draw reader's attention to the usefulness of high-pressure investigation in the study of electric fields in nitrides and to show how the interpretation of these experiments influences the way we calculate the electric fields in the quantum structures