Photoluminescence Quenching Spectroscopy of Trapmediated Er 3+ Excitation Mechanisms in Er-Implanted GaN
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The third site pumped by a broad, below-gap absorption band, attributable to an Er-related trap and the high-concentration center pumped by direct 4f shell absorption were not observed with abovegap excitation. These results indicate that excitation of Er 3, luminescence in crystalline semiconductor hosts by either optical or electrical injection of electron-hole pairs may be dominated by trap-mediated carrier capture and energy transfer processes which don't excite a large fraction of the optically active Er3" sites. These trap-mediated processes also control the thermal quenching of Er3' emission in semiconductors. In the present work we report two-source PL quenching experiments on the four Er3" PL spectra observed in Er-implanted GaN. In these experiments, Er + PL spectra are excited by mechanically chopped above-gap light (the pump light), while a second, unmodulated source (the quenching light) is applied simultaneously with its wavelength tuned to one of the site-selective, below-gap absorption bands identified in our previously reported PLE studies. The unmodulated quenching light selectively quenches (diminishes) the modulated PL spectrum corresponding to the wavelength-selected below-gap absorption band, thereby enabling the contributions of each of the four Er sites to the above-gap excited PL spectrum to be isolated. All four sites are found to contribute to the above-gap excited Er31 PL spectrum, with relative efficiencies determined by the carrier capture cross sections and concentrations of the defects or traps which mediate the excitation of each Er site. By scanning the wavelength of the unmodulated quenching light it is also possible to obtain site-selective optical absorption spectra for these defects or traps. These experiments demonstrate a direct connection between the traps which mediate the above-gap excitation of the Er3' PL and the extrinsic absorption bands responsible for the below-gap, wavelength-selective excitation of the PL spectra from the various Er sites. 667 Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
EXPERIMENTAL PROCEDURE The GaN films were grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD). Er atoms were implanted with a dosage of 4 x 10"' ions/cm2 at 280 keV, and the implanted films were annealed at 900'C for 30 minutes under flowing N 2 gas. The PL experiments were carried out at 6 K with the samples mounted in a Janis Supervaritemp cryostat. Above-gap excitation for the PL spectroscopy was provided by mechanically chopped 325 nm light from a HeCd laser, while an Ar ion laser, Ti:Sapphire laser, and a Xe lamp- double monochromator system, all in unmodulated operation, were used as below-gap quenching sources. Photoluminescence was analyzed by a I-m single grating monochromator and detected by a liquid nitrogen-cooled Ge photodetector. The output of the Ge detector was monitored by a lock-in amplifier to provide synchronous, phase sensitive detection of the PL signal modulated at the mechanical chopper frequency (-80 Hz).
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