On the origin of the 2.8 eV blue emission in p-type GaN:Mg : A time-resolved photoluminescence investigation
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Internet Journal Nitride Semiconductor Research
On the origin of the 2.8 eV blue emission in p-type GaN:Mg : A time-resolved photoluminescence investigation F.Shahedipour1 and B.W.Wessels1 1Department
of Materials Science and Engineering and Materials Research Center,
(Received Tuesday, June 19, 2001; accepted Friday, August 17, 2001)
The decay dynamics of the 2.8 eV emission band in p-type GaN was investigated using timeresolved photoluminescence spectroscopy. The luminescence intensity decays non-exponentially. The decay dynamics were consistent with donor-acceptor pair recombination for a random distribution of pair distances. Calculations using the Thomas-Hopfield model indicated that recombination involves deep donors and shallow acceptors.
1 Introduction In order to make GaN p-type, the material is doped with Mg to a concentration of greater than 1 x 1019 cm-3. At this dopant concentration a strong blue luminescence ba nd peaked at 2.8 eV is observed [1] [2] [3] [4]. Two models have emerged to describe the blue band luminescence, whose origin remains controversial. One model attributes the photoluminescence to donor-acceptor pair (DAP) recombination where the donor is s hallow and the acceptor is deep [4] [5] [6]. Alternatively, it has been proposed that DAP luminescence involves a deep donor and a shallow acceptor [1] [7]. Excitation intensity studies of the blue luminescence give strong support for the latter model [7]. The deep donor has been tentatively attributed to a nitrogen vacancy or nitrogen vacancy complex [8] [9]. Time dependent photoluminescence of this band has been initially studied. Nonexponential time decay has been observed that is attributed to a DAP transition [10] [11]. In the present study, transient photoluminescence spectroscopy was used to determine the nature of the defects responsible for the blue luminescence band in p-type GaN. From the time decay measurements and using the model of Thomas and Hopfield (T-H) for pair recombination, the acceptor involved is shown to be effective-mass-like, giving support to the deep donor-shallow acceptor model for this band. 2 Experimental Results and Discussion For this study p-type GaN:Mg epitaxial layers were grown by atmospheric pressure metal organic chemical vapor deposition. Layers were 2µm thick with 20 nm thick GaN buffer layers. All the as-grown layers were
semi-insulating. A post-deposition annealing in a nitrogen ambient at 700°C for 30 minutes was used to activate the Mg acceptors. Low temperature photoluminescence (PL) measurements (16 K) were used to characterize the defects in these materials. A CW He-Cd laser and a pulsed nitrogen laser (4 nsec pulse, 20 Hz repetition and a photon energy of 3.69 eV) were used for steady-state and time resolved PL measurements, respectively. A Spex grating monochromator with Hamamatsu photomultiplier tube was used, along with a series of neutral density filters to attenuate the excitation density. A closed cycle cryostat was employed for obtaining sample temperature in the range of 15K-300K.
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