Keynote Speaker
Assoc. Prof. Christopher Mellor
Spectroscopic ellipsometry as a characterisation technique for vdW materials
One of the earliest applications of ellipsometry was to measure the thickness of thin dielectric layers on a bulk substrate, most notably silicon dioxide on silicon. Later, spectroscopic ellipsometry provided important insights into the dielectric functions of semiconductors including gallium arsenide and silicon. Research on van der Waals (vdW) materials involves the study of materials that are sometimes are only one atom or unit cell thick. Initially such samples were exclusively produced by the exfoliation of flakes from bulk crystals. Such flakes, e.g. molybdenum disulfide, can be smaller than 50 microns x 50 microns in area, making imaging spectroscopic ellipsometry a powerful method of measuring the optical properties of the flakes. In recent years researchers have been directly growing vdW materials onto substrates using semiconductor epitaxial growth techniques. In Nottingham we are growing vdW materials using molecular beam epitaxy (MBE). For example, high temperature molecular beam epitaxy (HT-MBE) has been used to grow hexagonal boron nitride (hBN) on both highly oriented pyrolytic graphite (HOPG) and sapphire substrates [1]. The HT-MBE hBN can be grown with controlled amounts of dopants e.g. carbon [2]. By measuring the optical constants of exfoliated flakes of hBN using an Accurion EP4 imaging spectroscopic ellipsometer we can compare the properties of large areas ( ~ cm2) of hBN, grown by HT-MBE on sapphire, with those of flakes exfoliated from bulk grown material. We find that the exfoliated flakes have sharper band edge transitions than HT-MBE material and that ellipsometry can be used as a fast, non-destructive, characterisation technique to assist in the optimisation of growth parameters. iSE can also measure the local optical properties of HT-MBE grown hBN in the presence of inhomogeneities introduced onto the substrate. More recently MBE has been employed to grow gallium selenide (GaSe) on sapphire [3]. A potential application for GaSe is as a deep ultra-violet (DUV) photodetector. Spectroscopic ellipsometry can provide both the dielectric functions of the grown layer, which can be compared to theoretical predictions, and the extinction coefficients of the films in the DUV, helping to inform the photodetector design.
References:
1. T.S. Cheng, et al, J. Vac. Sci. Technol. B 36 (2) 02D103 (2018).
2. N. Mendelson, et al, Nature Materials 20 (3) 321 (2021).
3. M. Shiffa, et al, Small 20 (7) 2305865 (2024).