New spectroscopic data of erbium ions in GaN thin films
Introduction
Visible emission of the RE3+ ions in Wide Band Gap Semiconductors (WBGS) are being intensively investigated. Among the wide-bandgap semiconductors, III-V nitrides seem to be the most promising host materials for RE3+ doping due to their energy band gap, which makes them transparent to the visible emission of RE3+. Being chemically and thermally stable too, they are good candidates for optoelectronic and photonic applications. GaN is the most popular and new methods of synthesis (MOCVD and SSMBE) have allowed an increase in the incorporation of RE3+ into this material. Recent results obtained for RE3+-doped GaN are very promising since GaN doped with Eu3+, Tm3+ and Er3+ have been used to realize full-color electroluminescent devices [1]. Despite several studies of the spectroscopic properties of different RE3+ ions in GaN thin films, no clear correlation between the efficiency of the visible RE3+ luminescence and the interaction of RE3+ with the host has been established [2], [3], [4]. A complete understanding of the spectroscopic properties of the RE dopants in the semiconductor (excitation schemes and luminescence efficiency) and their relationship to growth processes is needed in order to improve the performance of current RE3+ doped GaN devices. In this paper, new results on the spectroscopic analysis of the Er3+ ions in GaN thin films are presented.
Section snippets
Experimental
The GaN:Er3+ thin films were grown using the Solid State Solid Source Molecular Beam Epitaxy (SSMBE) method on a p-type (1 1 1) Si substrate after deposition of an AlN buffer layer. The complete synthetic procedure is described in [5]. The Er3+ concentration of the GaN:Er3+ thin films, measured with Rutherford Backscattering Spectroscopy (RBS) and Secondary Ion Mass Spectroscopy (SIMS), ranges from 0.025 up to 11.2 at.%.
The Er3+luminescence was excited either by an OPO pumped by the third harmonic
Luminescence at room temperature
Excitation of the Er3+ ions within the GaN band gap (to the or multiplets) yields a luminescence spectrum dominated at room temperature by emission in the visible range. This is easily assigned to radiative transitions from the and multiplets to the ground state one. Weak transitions from the lower multiplets (, , ) are observed, too (Fig. 1). Emission from the multiplet shows a rather high efficiency compared to that observed from the and
Identification of the emitting centers
The occurrence of several centers for Er3+ ions and excitation of the multiplet within the GaN band gap and out of the absorption range of the states of the 4f11 configuration has already been reported for the Er-implanted GaN thin films [6], [7]. The excitation spectrum consisted of an unstructured broad band extending from 460 to 530 nm. The present additional excitation lines are well within this broad band and the shape of the C2 and C3 emission spectra are very similar
Visible emission of the C1 center
The C1 center can be considered as the main center. The time dependence of the luminescence at room temperature deviates from a simple exponential behavior irrespective of the Er3+ concentration (Fig. 3). For low Er3+ concentrations, the decay profile can be fitted to two exponential functions explained by the presence of two Er3+ sites. For high Er3+ concentrations, the experimental decay curves cannot be fitted by only two exponentials indicating that other processes such as
Crystal field strength analysis
Based on the emission spectra recorded for different C1 excited multiplets, the energies of the Stark components and the overall splitting of the ground multiplet were deduced. From the overall splitting of the 4fN multiplets, it is possible to deduce the crystal field (CF) strength using a scalar field parameter Nv which allows a convenient comparison between different host materials [15]. The overall splitting of the ground multiplet (299 cm−1) gives 1557 cm−1 for the Nv value that is
Conclusions
Based on site-selective excitation spectroscopy, three kinds of sites were identified for the Er3+ ions in GaN. The main center was concluded to be Er located on the Ga site. The majority of the Er3+ ions are in this center. The two other sites were assigned to Er3+-related defects and ascribed to Er3+ species in interstitial positions. A non-exponential decay profile and the concentration dependence of the luminescence for the main center indicate that the optical relaxation involves
Acknowledgements
This material is based upon work partially supported by the European Research Office of the US Army under contract No N62558-02-M-5113. Mrs Gardant is acknowledged for her assistance.
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