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Universidade de Aveiro, Departamento de Fisica, Aveiro, Portugal Thomson-CSF, Orsay, France CHREA-CNRS, Valbonne, France 1. INTRODUCTION GaN, its alloys, QWs and MQWs have gained an important place among shortwavelength optical emitters and high temperature electronic devices [1,2]. The performance of such devices is limited by the presence of native and impurity defects. The understanding of the optical properties of the basic material allows us to improve its quality and thus increase the performance of these materials. In non intentionally doped (nid) hexagonal good quality GaN layers grown on sapphire, 6H-SiC or Si, free exciton (FXC, FXB, FXA), donor bound exciton (DX), acceptor bound exciton (AX) and donor-acceptor pair (DAP) transitions have been reported by several authors [3, and references therein]. Besides these typical emissions, emission lines in the range 3.3 - 3.44 eV have been observed in nid and intentionally doped hexagonal GaN layers. However the nature of these recombinations is not completely clarified. Some authors assigned them to a superposition of LO phonon assisted transitions of DX and FX [3-8], excitons bound to neutral donors with deeper donor levels [5], band to impurity transitions and/or free to bound emission involving oxygen [9-1 I], DAP transitions [12-141, shallow bound excitons of cubic phases [15, excitons bound to structural defects [16-20] and Zn related recombinations [21]. In this work we analyse the luminescence between 3.36 eV and 3.41 eV of nid hexagonal GaN samples grown on sapphire. We found sample dependent emission lines with no DAP behaviour. From the data we are able to identify different kinds of recombination processes in the same spectral region. 2. EXPERIMENT Hexagonal GaN layers of ca. 2 gim where grown by MOCVD on (0001) sapphire substrates. Steady state (SS) photoluminescence (PL) is excited by the 325 nm line of a He-Cd laser. The luminescence signal was dispersed by a SPEX 1704 Im monochromator and detected by a Hamamatsu photomultiplier. Excitation intensity was varied by neutral density (ND) filters. Time resolved (TR) measurements were carried out with a pulsed Xe lamp as an excitation source and a boxcar system for detection. The samples were mounted on a cold finger of a closed-cycle He cryostat and the temperature of the samples was varied between 12-300K.

419 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society

Samples are oxygen free and present a background concentration of approximately 5x 1017cm-'. 3. RESULTS Fig. I shows SSPL and TRPL of an oxygen free sample (sample A) where besides the excitonic emissions at 3.47 eV, DAP recombination at 3.27 eV and a yellow band (YB) recombination a set of lines can be observed in the range between 3.3 and 3.41 eV, namely at 3.400 eV, 3.342 and 3.328 eV (Fig. 2). 101

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Fig. 2: Temperature dependent PL spectra of sample A

Fig. I: SS and TRPL of