Communication—Anomalous Shift of the Capacitance-Voltage Characteristics Obtained for p-MOS Capacitors with a Tin-Doped Indium Oxide Gate

Properties of low-threshold silicon p-MOS capacitors with tin-doped indium oxide (ITO) gates have been investigated. A shift of the capacitance-voltage (C-V) characteristics after annealing the capacitors in an oxygen atmosphere was observed. An anomalous shift of the C-V characteristics is discussed based on the possible presence of negative charge at the ITO-silicon dioxide interface that comes from the diffusion of indium atoms into the silicon dioxide at annealing temperatures above 400◦C. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0011607jss] All rights reserved.

It is well known that the thermal oxidation of the silicon surface generates a non-mobile positive space charge region within the oxide layer adjacent to the silicon-silicon dioxide interface that has a very substantial effect on the silicon properties at the interface and has severely limited the development of low-threshold p-MOS devices.
In this work we report an anomalous shift of the current-voltage (C-V) characteristics obtained for p-MOS capacitors with transparent conducting ITO gates deposited by sputtering technique with a posterior annealing in oxygen atmosphere. To the extent of our knowledge there is not such report in the literature. Our technological approach allows for obtaining low-threshold p-MOS capacitors that can be useful for different applications.

Experimental
The p-MOS capacitors were fabricated on an n-type 3-5 -cm silicon wafer. The SiO 2 layer, with a thickness around 70 nm, was grown at 1000 • C. Then, the wafer was cut in two parts for the fabrication of capacitors with aluminum and ITO gates. An Al layer was deposited at the back of both SiO 2 -Si structures, and on the SiO 2 film of the wafer used for capacitors with the Al gate, followed by a short post-annealing in forming gas at 450 • C for obtaining an ohmic contact. On the other part of the wafer, the ITO film was deposited on the SiO 2 layer by dc magnetron sputtering in pure argon atmosphere, with a power of 100 W, at a 3 mTorr working pressure; we use a ceramic target (AJA International, Inc.) with 90 wt% In 2 0 3 and 10 wt% of SnO 2 . Fused silica substrates were also used during the same deposition process together with the SiO 2 -Si structures for optical measurements. A photolithography process was applied for etching the Al and ITO layers in order to obtain the MOS capacitors with a gate area of 2 × 3 mm 2 . The half wafer containing the ITO-SiO 2 -Si capacitors was divided in several parts, which together with the ITO films on fused silica substrates, were subjected to a post-annealing process in oxygen atmosphere, at different temperatures (from 200 to 450 • C), and for 1 hour at atmosphere pressure.

Results
The electrical and optical properties of the ITO films after the post-annealing in oxygen are summarized in Table I. As-deposited the ITO films in pure argon atmosphere were amorphous showing a poor transparency that may be due to the composition of the sputtered target, whose black color leads to the conclusion that the target may contain foreign phases as metallic tin, indium and/or blue/black sub oxides of tin and indium that can be caused by the reducing atmosphere used during the target fabrication. Considerable improvement of the film properties was observed after the annealing in the oxygen z E-mail: amalik@inaoep.mx atmosphere. 1 This annealing allows for obtaining transparent stoichiometric ITO films, whose conductivity is determined by the tin atoms as well as the presence of oxygen vacancies. The best combination of these dopants, tin and oxygen vacancies, allows to fabricate highly transparent and conducting polycrystalline ITO films after annealing at temperatures from 250 to 350 • C. The measured optical bandgap depends on the carrier concentration due to the well-known Burstein-Moss (BM) effect in degenerated semiconductors. 2 Figure 1 shows the dependence of carrier density and optical bandgap on the annealing temperature.
Annealing in oxygen at temperatures above 300 • C leads to a decay of the oxygen vacancies, and thus to a reduction of the carrier density. Hence, the dependence of the optical bandgap with an increasing annealing temperature can be explained by the BM effect. The relative shift of the optical bandgap ( E BM ) with respect to the optical bandgap of undoped indium oxide (around 3.7 eV 3 ), as a function of the twothird power of the carrier density for the ITO films annealed at different temperatures, is shown in Figure 2. Our results agree well with those published in the literature. 4 Figure 3 shows a comparison of the quasi-static C-V characteristics of the p-MOS capacitor with ITO gate annealed in oxygen at different temperatures, with the C-V characteristics of the Al-SiO 2 -Si capacitor. Figure 4 shows the threshold voltage for the capacitors with ITO gates annealed at different temperatures and the flatband difference ( FB) for ITO-SiO 2 -Si capacitors relative to the flatband of the capacitor with Al-gate.   value of the ITO work function would be around 5.5 eV; this value is very doubtful, even though it has been reported for ITO film surfaces that have been subjected to a treatment in oxygen or ozone plasma. 6,7 Normally the work function should be in the 4.1-4.8 eV range. [8][9][10][11] Moreover, a very sharp change of the flatband value in the 400-450 • C temperature range requires further attention.

Discussion
In this regards, we want to bring up the model of metal-induced negative charge at the ITO/SiO 2 interface. At annealing temperatures   above 350 • C, indium can diffuse into the silicon oxide from the nonstoichiometric amorphous as-deposited ITO film with indium excess. The indium diffusivity into silicon dioxide has been reported to be sufficiently high. 12,13 Moreover, an abnormal shift of C-V characteristics has been reported for Au-SiO 2 -Si capacitors after applying a low-temperature (up 400 • C) treatment in nitrogen atmosphere, and that shift was explained as due to diffusion of gold atoms into the silicon dioxide. 14 If trivalent indium atoms replace tetravalent silicon atoms in the basic Si 4 O 8 tetrahedron, a negatively charged (InSi 3 O 8 ) − ion can be formed. In the case that two indium atoms occupy the position of silicon atoms in the tetrahedron, a double negatively charged (In 2 Si 3 O 8 ) 2− ion may be formed. This fixed negative charge at the ITO/SiO 2 interface can balance a positive charge at the SiO 2 /Si interface, thus explaining the considerable shift of the C-V characteristics in the capacitors with ITO gates annealed at high temperatures.
Experimental evidence of negative charge in silicon dioxide grown on n-type silicon surfaces rinsed with RCA alkaline solution contaminated with trivalent (as indium) aluminum or iron is reported in. [15][16][17] This charge has the value of (3-6) × 10 11 cm −2 . 16 Using the well known equations for a p-MOS capacitor, 18 the negative charge necessary to obtain a zero threshold voltage for capacitors fabricated on silicon substrates with a donor density of 5 × 10 14 cm −3 and a fixed positive charge of 5 × 10 10 cm −2 at the SiO 2 /Si interface, needs to be 1.5 × 10 11 cm −2 . This value agrees with that reported for Fe or Al contaminated SiO 2 . Of course, SIMS analysis is mandatory to confirm our hypothesis.

Summary
We report experimental results obtained for silicon p-MOS capacitors with ITO-gates annealed. The post-annealing of silicon p-MOS capacitors with ITO gate at temperatures below 400 • C in oxygen atmosphere leads to a shift of the C-V characteristics which can be explained as due to the shift of the Fermi level in the degenerated semiconductor ITO film For annealing temperatures above 400 • C, the hypothesis of the presence of negative charge at the ITO-SiO 2 interface built during the annealing of non-stoichiometric as-deposited ITO film is discussed. Our reported results are important for overcoming the influence of the fixed positive charge in silicon dioxide in the development of low-threshold p-MOS devices.