Sm-doped ZnO thin films grown by pulsed-laser deposition

Yu-Tso Liao^1*, Jyun-Han Chen¹, Shi-Yu Liu¹, Hsiang-Lin Liu¹, Fang-Yuh Lo¹, Ming-Yau Chern²

¹Department of Physics, National Taiwan Normal University, Taipei, Taiwan
²Department of Physics, National Taiwan University, Taipei, Taiwan

* Presenter:Yu-Tso Liao, email:g0981229367@gmail.com

Samarium-doped zinc oxide (Sm:ZnO) thin films are grown by pulsed-laser deposition (PLD) on c-oriented sapphire substrates with Sm concentration ranging from 1 to 10 atomic percent (at.%). The oxygen partial pressure is 3x10^-1 mbar and the substrate temperature is 525⁰C during deposition. The structural, optical, electrical, and magnetic properties are reported.
X-ray diffraction implies that Sm atoms are successfully incorporated into ZnO lattice. With increasing Sm content, the c-lattice constant decreases from 5.21 to 5.15 Å and the crystallite size decreases from 37.8 to 12.6 nm. Atomic force microscopy shows that all the samples having circular shape grain surface, and the surface roughness is between 3.6 and 22.4 nm. The optical properties are investigated by photoluminescence, transmission spectroscopy and ellipsometry. The results show that defects in Sm:ZnO thin films include zinc vacancy, zinc interstitials, and oxygen vacancy, and the defect density increases with increasing Sm content. Moreover, the transmittance in UV regime increases for Sm:ZnO. the optical band gap increases from 3.28 to 3.32 eV and the exciton binding energy decreases from 54.4 to 28.7 meV.
Resistivity of Sm:ZnO films increases with increasing Sm content, and Sm:ZnO becomes highly resistive for Sm content higher than 3 at.%. The resistances of Sm:ZnO films decreased when irradiated with 450, 532, and 658 nm laser, and the decreasing amplitude increases with increasing incident photon energy. The magnetic properties investigated by SQUID show that 1 at.% doping Sm:ZnO film exhibits ferromagnetic characteristics at low temperatures with an ordering temperature lower than 230 K. Other Sm:ZnO thin films show only paramagnetic characteristics. Room-temperature magneto-optical Faraday effect (MOFE) spectra showed only paramagnetic behavior for all the Sm:ZnO thin films, and the strength of MOFE is stronger for wavelengths between 340 and 380 nm, where the Verdet constant is around 20 rad/(mT*m) for photon.

Keywords: Samarium, Zinc oxide, Pulsed laser deposition, Thin film