Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article Determination of alumina bandgap and dielectric functions of diamond MOS by STEM-VEELS J. Cañas a, ⁎ , J.C. Piñero b , F. Lloret a , M. Gutierrez a , T. Pham c , J. Pernot c , D. Araujo a a Dpto. Ciencia de los Materiales, Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto Real, Cádiz, Spain b Dpto. Didáctica de la Matemática, Facultad de Ciencias de la Educación, Universidad de Cádiz, 11510 Puerto Real, Cádiz, Spain c Universite Grenoble Alpes, Institut NEEL, F-38042 Grenoble, France ARTICLE INFO Keywords: Diamond MOS STEM-VEELS Bandgap Dielectric functions Alumina ABSTRACT Alumina is a promising candidate for fabricating the gate of diamond metal oxide semiconductor field effect transistor (MOSFET) due to its outstanding nominal properties: A high gap of 8.8 eV and a high static dielectric constant of 9. However, such properties are strongly dependent on the synthesis. As gate oxides are usually very thin layers (5–50 nm), investigating its properties is not straightforward. Electron energy loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) methodology is reported in the nm-scale range. Monochromatic 60 keV electron beam is used to obtain the low energy loss spectrum in order to allow an accurate zero loss peak deconvolution and to avoid Cherenkov effect. The low energy loss spectrum is used to extract the bandgap along diamond-alumina interface and to perform Kramers-Kronig analysis to obtain the complex dielectric function of the Al 2 O 3 . High resolution electron microscopy (HREM) and STEM-EELS in- vestigations show that the oxide phase of our sample is γ alumina. Its measured bandgap is 6.8 eV and the dielectric functions yield a value of 3 for the high frequency dielectric constant. 1. Introduction Fabricating an efficient dielectric gate is a bottleneck to achieve a successful diamond metal oxide semiconductor field effect transistor (MOSFET). High κ oxides are used to fabricate the gate, which is a metal oxide semiconductor capacitor (MOS) structure, to enhance the capa- citance of the transistor. In order to avoid leakage currents through the gate, the oxide must present a barrier for carriers in its band alignment respect diamond [1]. This implies that a successful gate oxide must have a wide bandgap, ideally greater than diamond. Moreover, the oxide must not contain a high density of active defects in the bulk or states in the diamond/oxide interface because they can affect the transient behaviour of the capacitor [2]. Electrons from the metal can tunnel through oxide defects resulting in leakage currents. On the other hand, a high density of states in the diamond/oxide interface can pin the Fermi level getting charged under bias voltage [3,4]. In the last years, progress has been made in the deposition of these oxides and deposition techniques such as atomic layer deposition (ALD) or sput- tering deposition (SD) are used to fabricate the gate. Alumina is a promising candidate due to its outstanding properties in its α stable phase [5]. Even so, acquiring a monocrystalline oxide layer of ∼50 nm without active defects and with a proper interface with diamond is still an open problem. Depending on the synthesis conditions, alumina can also be obtained in its γ alumina phase [6–8]. While α stable phase has an hexagonal lattice, the γ phase is much more complex. It is nominally a spinel lattice, but as a transition phase, its stoichiometry can con- tinuously vary modifying slightly its lattice symmetry. As a result of its structural disorder, γ alumina is expected to have a lower gap than α- alumina [9]. Even so, gamma alumina-diamond MOS has demonstrated depletion and accumulation mode in p-type oxygen terminated dia- mond MOSFET [10]. Diamond surface also plays a crucial role as its electronic affinity determine the band offset configuration with the alumina. Oxygen- terminated diamond is the stable termination showing negative elec- tron affinity and insulator behaviour while hydrogen termination dis- plays surface conductivity. Surface oxygenation is known to displace diamond’s bands toward to lower energies resulting in a lower barrier for holes. Valence band offset of diamond/alumina interface has been studied by XPS yielding a type I junction when considering a gap of 7.4 eV for alumina [11]. However, the wide range of alumina’s bandgap values in literature increase the uncertainty and a systematic method for measuring their properties is required [8,12–15]. Valence electron energy loss spectroscopy in scanning transmission electron microscopy (STEM-VEELS) has the potential of measuring the bandgap and the https://doi.org/10.1016/j.apsusc.2018.06.163 Received 9 March 2018; Received in revised form 12 June 2018; Accepted 18 June 2018 ⁎ Corresponding author. E-mail address: jesus.canas@uca.es (J. Cañas). Applied Surface Science xxx (xxxx) xxx–xxx 0169-4332/ © 2018 Elsevier B.V. All rights reserved. Please cite this article as: Cañas, J., Applied Surface Science (2018), https://doi.org/10.1016/j.apsusc.2018.06.163