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

HIGH PRESSURE FREEZE-OUT OF ELECTRONS IN UNDOPED GaN CRYSTAL. PROOF OF EXISTENCE OF RESONANT DONOR STATE (NITROGEN VACANCY)

We investigated free carriers related opticał absorption in GaN in hydrostatic pressures up to 30 GPa. The disappearance of this absorption at pressures close to 18 GPa was explained by trapping electrons resulting from the shift of nitrogen vacancy related donor level into the GaN energy gap at high pressure. We estimated the energetic position of this level at atmospheric pressure to be about 0.8 eV above the conduction band minimum

Light Induced Recovery of EL2 Defect from the Metastable Configuration

The measurements: of changes in a magnitude of ΕL2 characteristic infrared absorption were used to investigate a phenomenon of light induced recovery of the defect from its metastable state in semi-insulating (SI) and n-type GaAs. At a temperature of 12K illumination with photons of energy 1.45 eV caused partial recovery for both SI and n-type samples. For n-type samples partial recovery occurred also after irradiation with photons of energy lower than 0.73 eV

Effect of the Built-In Strain on the In-Plane Optical Anisotropy of m-Plane GaN/AlGaN Quantum Wells

We study theoretically the influence of the anisotropic biaxial strain originating from the lattice mismatch between the m-plane GaN/AlGaN quantum wells structure and the substrate on the optical anisotropy of such systems. It is demonstrated that the oscillator strengths for optical transitions with polarization of light parallel and perpendicular to the crystal axis c strongly depend on strain to such an extent that, by increasing the concentration of Al in the substrate from x = 0 to x = 0.5 one can change the polarization of the emitted light with respect to the c-axis by 90 degrees

Pressure Dependence of Exciton Binding Energy in GaN/Alx\text{}_{x}Ga1−x\text{}_{1 - x}N Quantum Wells

We present a theoretical study of excitons in GaN/Al$\text{}_{x}$Ga$\text{}_{1 - x}$N wurtzite (0001) quantum wells subjected to hydrostatic pressure. Our results show that the combined effect of pressure induced changes in band structure and piezoelectric field leads to reduction of the exciton binding energy. This subtle effect is described quite accurately by our multiband model of excitons in quantum wells

Effect of growth polarity on vacancy defect and impurity incorporation in dislocation-free GaN

Contains fulltext : 32448.pdf (publisher's version ) (Open Access)We have used positron annihilation, secondary ion mass spectrometry, and photoluminescence to study the point defects in GaN grown by hydride vapor phase epitaxy (HVPE) on GaN bulk crystals. The results show that N polar growth incorporates many more donor and acceptor type impurities and also Ga vacancies. Vacancy clusters with a positron lifetime tau(D) = 470 +/- 50 ps were found near the N polar surfaces of both the HVPE GaN layers and bulk crystals. (C) 2005 American Institute of Physics