Analysis of Silicon Dioxide Interface Transition Region in MOS Structures S. Markov * † , N. Barin † , C. Fiegna † , S. Roy * , E. Sangiorgi † , and A. Asenov * * Dept. Electronics and Electrical Eng., Univ. of Glasgow, G11 7SW, UK {s.markov|s.roy|a.asenov}@elec.gla.ac.uk † ARCES-IUNET, University of Bologna, v. Venezia 52, Cesena 47023, Italy {nbarin|cfiegna|esangiorgi}@arces.unibo.it Abstract We study the Si(100) inversion layer quantisation, capacitance and tunnelling charac- teristics in the case of a gradual band gap transition at the Si/SiO 2 interface. A linear band gap transition of 0.5 nm at the SiO 2 side results in nearly 20% redistribution of carriers from the 2-fold to the 4-fold degenerate valley, due to the greater wave-function penetration and sub-band level lowering for the 4-fold valley. The gate capacitance is enhanced by up to 12% for a 1.0 nm nominal oxide thickness, and the direct tunnelling current density increases by an order of magnitude. 1 Introduction The exact physical arrangement of atoms and the chemical composition at the common boundary between SiO 2 and Si remains disputable, but the energy levels of the elec- tronic states that directly affect the electronic properties of the interface, are determined to a good agreement [1, 2, 3, 4, 5]. Additional electronic states, energetically aligned closer to the Si conduction band, are found in the first 2 - 6 ˚ A from the end of the or- dered Si structure. This implies a gradual formation of a sufficiently large band gap in the SiO 2 [2, 3], and stipulates a gradual transition of the dielectric permittivity [4]. The effect of a gradual interface band-gap transition on the electron mobility and energy quantisation in metal-oxide-semiconductor (MOS) structures has been briefly addressed by Stern [6], who predicted lowering of the sub-band energies, and suggested part of the scattering usually attributed to surface roughness is due to the charge in the tran- sitional oxide. Recently Watanabe accounted for the interface transition region (TR) when estimating the tunnelling oxide effective mass and effective oxide thickness [7]. However, a more detailed study of the MOS inversion layer with consideration of the TR is still missing. Here we obtain a semi-quantitative estimate for the effect of grad- ual interface band-gap transition on the inversion layer quantisation, capacitance and tunnelling characteristics. SIMULATION OF SEMICONDUCTOR PROCESSES AND DEVICES Vol. 12 Edited by T. Grasser and S. Selberherr - September 2007 149