Light Generating Carrier Recombination and Impurities in Wurtzite GaN/Al 2 O 3 Grown by MOCVD
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U. KAUFMANN, M. KUNZER AND C. MERZ, I. AKASAKI* AND H. AMANO* Fraunhofer-Institute for Applied Solid State Physics, Tullastrasse 72, D-79108 Freiburg, Germany *Department of Electrical Engineering, Meijo University, Nagoya 468, Japan ABSTRACT We have studied by photoluminescence (PL) and optically detected magnetic resonance (ODMR) undoped, n-doped and p-doped thin wurtzite GaN layers grown by metal-organic chemical vapor deposition on sapphire substrates. From the PL data for free excitons an accurate value of the free A-type exciton binding energy and a more accurate estimate for the hole effective mass is deduced. The localization energies of the Mg and the Zn neutral acceptor bound excitons are found to be in good agreement with Haynes' rule. A sharp emission line, assigned to free electron recombination at a 116 meV shallow acceptor, together with three additional weak zero-phonon-lines (ZPLs), assigned to distant donor-acceptor (DA) pairs, are reported for the first time. The chemical nature of this acceptor and that of three residual donors, inferred from the DA pair ZPLs, is discussed. The effects of strain in thin GaN layers on a dissimilar substrate like sapphire are emphasized with respect to the energetic position of narrow PL lines. The ODMR data obtained for undoped, Mg-doped and Zn-doped GaN layers provide insight into the recombination mechanisms responsible for the broad yellow (2.25 eV), the violet (3.15 eV) and the blue (2.8 eV) PL bands, respectively. The ODMR results for Mg and Zn also show that these acceptors do not behave effective mass like and indicate that the acceptor hole is mainly localized in the nearest neighbor shell surrounding the acceptor core. INTRODUCTION Samples of gallium nitride, GaN, were synthesized already in 1959 and the highest energy optical emission from this material was found to be peaked near 3.44 eV at 90 K [1]. Twelve years later when crystals grown by hydride vapor phase epitaxy [2] were available it was established that wurtzite GaN is a direct band gap semiconductor with Eg =3.50 eV at low temperatures in thick (Ž100 jin) quasi-bulk layers [3]. Thus, the potential of GaN for short wavelength optoelectronic emitters was obvious at an early time but the failure to achieve p-type conductivity during the early seventies delayed this development for decades. During the late eighties GaN layers with high crystalline quality and good surface morphology were prepared by metal-organic vapor phase epitaxy [4]. The real breakthrough was achieved in 1989 with the discovery that p-type conductivity can be enforced in Mg doped GaN [5]. Since then the progress in the technology of GaN based p-n junction light emitting diodes (LEDs) was breath taking. Blue [6,7,8], green [9], and even yellow [10] high-brightness LEDs based on GaN and its alloys with In and Al have been realized and are or will soon be available commercially. In view of the above technological successes it is surprising that the light generating carrier recombination processes in GaN as well as the residual im
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