1. Recombination Dynamics in Nitride Heterostructures: role of the piezoelectric field vs carrier localization A.Vinattieri, M.Colocci, M.Zamfirescu Dip.Fisica- INFM-LENS, Firenze In collaboration with F.Rossi, N.Armani, C.Ferrari IMEM-CNR, Parma A.Reale, A.Di Carlo, P.Lugli INFM-Dip.Fisica, Univ.Roma Tor-Vergata Recombination Dynamics is ruled by charge accumulation in the well and loss of carriers from the ground level induced by both radiative and non- radiative recombinations processes. The measured PL decays show a time dependence that is controlled by an interplay between radiative and non-radiative recombination processes. The PL decays are affected by carrier trapping and detrapping mechanisms, depending on the lattice temperature Conclusions: Motivation Why GaN, AlN, InGaN are interesting ? Tunable bandgap in the UV-visible spectral range High radiative efficiency (Blue-visible lasers) Radiation hardness (UV detectors) Major problems Poor material quality even for epitaxially grown material High dislocation density ( typically 10 9 -10 10 cm -2 ) Highly strained material when grown on Sapphire and SiC: Huge built- in electric field (MV/cm) The built-in field causes the Quantum -Confined Stark Eggect, dramaticalyy reducing the oscillatorstrength. Thereforeemission shifths to lowe energy and radiative rate bencomes smaller. PL-time integrated spectra and CL results TR-PL decays vs detection energy Temperature Analysis – PL intensity TR-PL – peak shift To investigate the effect of built-in field on the carrier dynamics we can “ probe” the oscillator Strength by photoluminescence experiments in different excitation conditions. High excitation density, easily reached in Cathodoluminescence experiments, can induce complete screening of the internal fields for the narrower wells. Time-resolved Photoluminescence experiments show the recovery of the built-in field as the photoinjected carriers recombine. Therefore the main features observed in PL spectra are nicely described by the interplay between polarization field, charge screening and radiative and non- radiative recombinations. Different aspects of the recombination dynamics can be addressed by different Experimental conditions: excitation density, stationary or pulsed excitation, lattice temperature PL spectra and decays can be nicely reproduced in the framework of a model by coupling a self-consistent solution of Schrödinger and Poisson equations to determine the electronic states in the nanostructure with a rate equation model to account for time-dependent effects of charge re-arrangement. screened by optical pumping Unscreened potential profile CB VB CB When the well is wider, the Stark shift becomes more important. Increasing the excitation density a blue shift is observed Sample 02Sample 03 Sample 04 PL temporal responses evaluated in a spectral window of 8 meV as a function of the detection energy. The wider QW (Sample 02) presents the more pronounced spectral shift, together with a weaker PL intensity. Both phenomena are a consequence of the fast destruction of the screening of internal polarization field. The oscillator strength of the optical transitions is quickly reduced because of wavefunction separation. An inverse Quantum Confined Stark Effect is observed No major change in the PL FWHM is observed, except at early times The faster decay on the high energy side of the PL spectra is due to two different processes; (i) free carrier relaxation towards lower energies ;(ii) Red- shift of the peak energy due to the recovery of the built-in field. EMEM TR-PL for different detection energy Non-resonant excitation exhibit nonmonotonic dependence of PL intensity. At high temperature detrapping in the barriers contribute to increase QW carriers, and thus recombinations. Temperature Analysis S Shape of PL shift Peak energy presents S-like shift because of trapping-detrapping mechanism, activated by temperature.Such process is nicely described by a thermally populated inhomogeneously broadened band (  broadening parameter). High Resolution TEM characterization #MQW01#MQW02#MQW04 Experimental setup Where do carriers localize? Roughness due to interface fluctuations. Alloy inhomogeneities: indium clusterization as seen in NSOM PL spectra