Work Function Extraction of Indium Tin Oxide Films from MOSFET Devices

Recent commercialization has increased the research interest in transparent conducting oxides like indium tin oxide being implemented in display technologies and sensors. A wide range of values (4.2–5 eV) for the work function of ITO films are reported in literature. In this paper, we present an approach to extract the work function of indium tin oxide films from MOSFET devices. RF sputtered indium tin oxide is used as a transparent gate electrode to fabricate n-MOSFET. For the fabrication of the MOSFET, a four-level mask is used. Electrical characterization is performed on these MOSFET devices. We obtained work function value in the range between 4.62–4.81 eV using this technique. © The Author(s) 2018. 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: 10.1149/2.0081803jss]

Indium tin oxide (ITO) is highly conductive wide bandgap semiconductor and exhibits excellent light transmission characteristics in visible and infrared spectrum. 1 The high electrical conductivity is due to contribution of oxygen vacancies and substitutional tin (Sn). Due to these unique properties, they are used as passive elements such as transparent electrodes in light-emitting devices (LED), 2 solar cells 3 and liquid crystal displays. 4 ITO is also used in organic electroluminescent devices like organic-LED (OLED) as an anode or a hole-injecting electrode due to its characteristic high work function ( ). 5 The work function of ITO also plays a central role in determining the efficiency and performance of the OLEDs and organic photovoltaics (OPVs) through control of hole injection process. 6,7 ITO is commonly regarded as high work function electrode. In the past, ITO has been used as hole collecting electrode in OPVs, or hole injecting electrode in OLEDs. Currently, OPVs or OLEDs are gaining popularity for being implemented with an inverted structure for improved air stability. The work function of ITO is reduced to facilitate electron collection or injection in such inverted structures.
In many such applications, the work function has a significant impact on the device performance as it affects the energy barrier height at the heterojunction interface. 8 Additionally, applications like thermionic emission and Schottky effect require work function to act as a critical factor to determine the amount of current that can be emitted from a hot cathode. 9,10 Furthermore, determination of work function is of great importance to understand wide range of surface phenomenon in numerous applications utilizing indium tin oxide films. Hence, the work function of ITO is of critical importance. Work function measurements are broadly divided into two categories, absolute and relative measurements. Relative measurements involve probing methods such as Kelvin probe method, which makes use of contact potential difference between the sample and the reference electrode. 11 Absolute measurements employ photoemission, thermionic emission, field emission etc, in order to eject the surface electrons and are extensively characterized to obtain the absolute work function values. The problems with these methods are that they are expensive and involve complex experimental setups. Work function of ITO has been reported in the range of 4.2-5 eV. [12][13][14] However, all these values were measured in air with respect to reference electrode or using the tunneling characteristic between ITO/semiconductor heterojunction, which might have introduced a lot of anomaly in the measurements. 14,15 Some of the other techniques reported which are used to measure the work function of ITO are ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy. 8  In this work, we present a unique technique to measure work function of ITO films by fabricating two identical MOSFET devices. One of the MOSFET has aluminum (Al) metal as gate contact, and transparent ITO forms the gate metal for the second MOSFET. From the threshold voltage equations of both the devices, work function of ITO is extracted.

Experimental
For fabrication of n-MOSFET transistor, p-type silicon (100) was used. The silicon wafer was cut into two equal pie shaped samples, each sample fits up to ten MOSFET devices on it. On these wafers, two identical sets of MOSFETs, but with different gate metals were fabricated under similar process conditions. One MOSFET had Al as gate contact and the other had ITO as gate contact electrode. These MOSFETS will be referred as MOSFET/Al and MOSFET/ITO henceforth. Al was used as source and drain (S/D) contacts for both kind of devices. The wafers were chemically cleaned with standard procedure using acetone, methanol followed by deionized water. During MOSFET device fabrication, four levels of lithography masks were employed. Following is the brief outline of MOSFET device fabrication. Wet oxidation was performed to grow a layer of 4500 Å thick silicon dioxide. This was achieved at 1100 • C for 45 minutes through steam bubbler and nitrogen as carrier gas.
Source and drain diffusion.-The first level mask was used to create S/D wells with the help of negative photoresist (PR). After defining wells in the oxide layer, phosphorus was pre-deposited into silicon at 950 • C for 15 minutes followed by wet oxidation at 1100 • C for 20 minutes to form n-type S/D regions.
Contact windows and metallization.-The second level mask was used to create contact windows at S/D. Al film was thermally evaporated. Third level mask was used to pattern Al in such a way that it remains confined only to S/D contact regions. Al deposited on rest of the sample is etched away using Al etch solution (16 parts of phosphoric acid, 1-part nitric acid, 1-part acetic acid, 20 parts DI water).  Figure 1 shows cross section of the fabricated MOSFETs, one having Al gate electrode and the other with ITO gate electrode.

Results and Discussion
I-V Characteristics of both MOSFETs are shown in Figure 2. These were achieved using the Tektronix 576 curve tracer by measuring the drain current (I d ) against drain source voltage (V ds ) as function of different gate voltages (V gs ). The threshold voltage for both the MOSFETs are found by the plotting √ I d vs V gs characteristics as shown in Figure 3. The threshold voltage is extracted by extrapolating the linear part of curve. The threshold voltage for MOSFET with Al gate contact is given by the following equation 7 The threshold voltage for MOSFET with ITO gate contact is given by Where ms = metal -silicon. In the above equations, ms(Al) and ms(ITO) are the work function differences of Al/silicon and ITO/silicon interfaces respectively. F is the fermi potential, Qi is   19 Du et al. have reported the work function of RF magnetron sputtered ITO in preparation of ITO/SiO x /n-Si solar cells at substrate temperature of 250 • C is 4.99 eV using UPS. 20 The work function of DC sputtered ITO film was reported as 4.68 eV using UPS. 21 The work function of thermally evaporated ITO films is reported in the range 4.60-4.75 eV. 15 The work function calculated using n-MOSFET device fits in the range of those reported by other work function extraction techniques. The reported technique can be implemented to extract the work function of ITO films used in metal/insulator/metal type devices, solar cells, OLEDs, OPVs and thin film transistors. Further, surface modification techniques can be used to fine tune the work function of ITO based organic optoelectronic devices to enhance the performance. 7,22,23

Conclusions
Work function of RF sputtered ITO films is obtained from electrical properties of MOSFET devices. ITO is used as transparent conducting