Luminescence from Erbium-Doped Gallium Nitride Thin Films
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Favennec et al. studied the dependence of the emission intensity of the Er 3+ ions on the bandgap of the host semiconductor and on the sample temperature [7]. Several different semiconductors were implanted with Er' ions and the emission intensity was measured at different temperatures. It was found that the intensity decreased at higher temperatures. This thermal quenching of the emission intensity was more severe for the smaller bandgap materials, such as Si and GaAs. The wide bandgap II-VI compounds, such as ZnTe and CdTe, exhibited the least temperature dependence. Since the wider bandgap semiconductors lead to less thermal quenching of the Er emission, the III-V nitride alloys appear to be especially promising host materials for rare earth doping. These alloys have a bandgap ranging from 1.9 eV for InN to 3.4 eV for GaN and 6.2 eV for AIN. However, due to a lack of a lattice matched substrate, present III-V nitride epilayers contain a high density of dislocation defects and various impurity elements. Nevertheless, very encouraging results have been obtained with Er-doped III-V nitride materials. Erbium Doping Methods Several different methods have been used for incorporating Er atoms into III-V semiconductor materials, mainly ion implantation and epitaxial growth. Each method presents certain advantages as well as difficulties. Apparently, there are no reports of Er incorporation into these semiconductor materials during bulk growth or by diffusion. Ion implantation has been widely used in processing integrated electronic circuits and optoelectronic devices. Because this method is a non-equilibrium process, it is not limited by solubility constraints or by surface chemistry. Wilson et al. [8] were the first to introduce Er into GaN and AIN materials using ion implantation. They observed strong infrared luminescence centered at 1.54 pm. However, coimplantation with 0 and subsequent furnace annealing was needed to achieve this luminescence. Subsequently, several other research groups used ion implantation to dope III-N semiconductors with Er atoms [9,10,11 ]. Three different methods of epitaxial growth have been used successfully for doping III-N semiconductors with Er atoms: gas source Gen II metal-organic molecular beam epitaxy (MOMBE) [12]; hydride vapor phase epitaxy (HVPE) [11], and solid source molecular beam epitaxy (MBE) [13]. However, with each of these techniques, there have been difficulties incorporating Er atoms into the epilayers and obtaining optically active centers. The maximum concentration of Er in these epilayers has been on the order of 1019 cm-3 . Nevertheless, high quality epilayers, doped with Er ions, have been achieved and good luminescence characteristics have been observed. Optical Excitation Spectroscopy Photoluminescence (PL) spectroscopy has been the main optical technique used to characterize the emission of Er-doped III-N semiconductor materials. This technique involves optical excitation of the Er ions and measurement of the spectrum of the light emission as a function of intens
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