Mg thin films with Al seed layers for UV plasmonics

We demonstrate that the inclusion of aluminum (Al) seed layers allows for the growth of finer grain, smoother magnesium (Mg) films, allowing for improved plasmonic response in the UV spectral range. We deposit Al seed layers and Mg films using DC/RF magnetron sputtering. For Al seed layer thicknesses of 2, 5, 10 and 15 nm, we measure the corresponding optical constants over the wavelength range of 250 to 600 nm using a variable angle spectroscopic ellipsometer and compare the results to a reference 100 nm thick Mg film deposited without a seed layer. The optical constants of Mg depend on the seed layer thicknesses and the surface morphology of Mg films. The surface morphology of the Mg films are characterized using scanning electron microscopy, atomic force microscopy and x-ray diffraction and the surface oxide layer on Mg is examined using x-ray photoelectron spectroscopy. We observe that the presence of the seed layer improves the LSP figure of merit in the UV spectral range of 250-400 nm. From the perspective of plasmonics applications, we find that the best localized surface plasmon resonance (LSP) figures of merit are observed for the 10 nm thick Al seed layer. In fact, the LSP figure of merit of the 100 nm Mg film with and without the Al seed layer is found to be greater than Al films in the spectral range of 250-400 nm, confirming earlier findings with thicker Mg films.