Observation of atomic-like transition in Sapphire crystal by deep UV photoluminescence spectroscopy
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.200
Observation of atomic-like transition in Sapphire crystal by deep UV photoluminescence spectroscopy Nikesh Maharjan and Mim Lal Nakarmi* Department of Physics, Brooklyn College and the Graduate Center of the City University of New York, Brooklyn, NY 11210, USA
*Email: [email protected]
ABSTRACT
Deep UV photoluminescence (PL) spectroscopy was employed to study optical properties of a sapphire substrate sample. The sample was photo-excited by the third harmonic laser of a Ti:sapphire pulse laser at wavelength ~ 266 nm which is a below bandgap excitation. In the low temperature (12 K) PL measurement, we observed two sharp atomic-like emissions in the ultraviolet region with peaks at 3.361 eV and 3.315 eV with spectral line-width of 0.85 and 3.30 nm respectively, in the PL spectrum. We performed temperature and power-dependent PL measurements of the sample and observed that the emission peak positions did not change with changing excitation power and sample temperature. We also performed X-ray photoelectron spectroscopy for chemical composition analysis of the sample to explore the origin of the atomic-like emission that could be used for single photon sources for quantum information technology. We will discuss a possible electronic transition and its origin in sapphire.
INTRODUCTION Sapphire (Al2O3) has been widely popular as a substrate to grow thin film materials due to availability of high-quality research grade wafers at reasonably price. Nitride based devices such as light emitting diodes (LEDs) are commercially grown on sapphire substrates using metal organic vapor deposition technique although there is a large lattice mismatch nitrides, for example, 16% with gallium nitride (GaN). In order to overcome the lattice mismatch between GaN and sapphire substrate with (0001) plane, a thin buffer layer is grown at low temperature before growing epitaxial layer at high
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temperature [1]. High crystalline quality aluminum nitride (AlN) epitaxial layer can be grown on sapphire substrate by inserting buffer layer using three-step growth or pulse deposition methods to reduce dislocation[2,3] for UV LEDs. Because of its ultra-wide bandgap ~ 9.4 eV [4], sapphire is also used as transparent optical window in optical measurements. In an experiment of photoluminescence (PL) study of zinc oxide (ZnO) nanopowders, we built a sample holder using a piece of sapphire wafer as optical window so that laser can pass into the sample for photoexcitation. While performing PL measuring at low temperature, we observed two sharp emission peaks in the PL spectrum near the band-edge of ZnO. The peak position was not expected for ZnO and the sharp atomic-like emission with linewidth of less than 1 nm was in fact a surprise.
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