Work Function Extraction of Indium Tin Oxide Films from MOSFET Devices

Abstract

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 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 i...

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“…The Ag electrode on the rear side is in full contact, where the charge transport is primarily perpendicular to the ITO film. In addition, the work function of ITO can reach as high as ≈5 eV, [ 53 ] which is larger than that of Ag (4.26 eV), [ 54 ] making the electrode more favorable for hole transport. However, in our laboratory, the MoO X film has to be taken out from the thermal evaporator for the next step sputtering process, during which water and gas in the air will inevitably affect the MoO X film, resulting in the increase of oxygen vacancies and the decrease of hole selectivity.…”

Fluorination of an N,N,N′,N′-Tetraphenylbenzidine Derivative as a Dopant-Free Hole-Transporting Material for Moisture-Resistant Perovskite Solar Cells

“…Thin films of the HTM were spin-coated with the TPB-2F and TPB-2H solutions (100 mg/mL in chloroform) at 1000 rpm for 60 s. After drying in a vacuum chamber at an ambient temperature for 10 min, gold electrodes of about 100 nm thickness were thermally evaporated on top of the HTM films in vacuum. Since the obtained HOMO levels of TPB-2F and TPB-2H are about −5.36 eV and −5.10 eV, which are close to the Fermi level of the vacuum-coated gold film with the reported work function ranging from −5.1 to −5.47 eV, and lower than the Fermi level of the ITO electrode with the work function of about −4.8 eV, , a hole-injection contact is established from the gold layer toward the TPB-2F and TPB-2H films without any potential barrier for current flow through the sample. It is well known that the SCLC ( I ) is given by the Mott–Gurney law − as described in the following formula: where μ is the charge carrier mobility, U is the applied voltage across the sample from gold to the ITO electrodes, U bi is the built-in potential ascribed to the work function difference between the two electrodes, d is the thickness of the TPB-2H and TPB-2F films, and C is the sample capacitance.…”

“…The calculated energy levels of the two molecules are consistent with the experimental values. about −4.8 eV, 24,25 a hole-injection contact is established from the gold layer toward the TPB-2F and TPB-2H films without any potential barrier for current flow through the sample. It is well known that the SCLC (I) is given by the Mott−Gurney law 26−28 as described in the following formula:…”

12.42% Monolithic 25.42 cm2 Flexible Organic Solar Cells Enabled by an Amorphous ITO‐Modified Metal Grid Electrode

“…[ 56 ] Meanwhile, the work function of all these α‐ITO films was estimated to be about 4.61 eV based on the Kelvin probe force microscopy results (Figure S7, Supporting Information), which is in agreement with value of 4.2–5 eV as reported previously. [ 57 ] With the work function of Ag/Cu electrode and α‐ITO films, the energy level of the devices was drawn as Figure 2b. The current density versus voltage ( J–V ) characteristics and external quantum efficiencies (EQE) spectra are exhibited in Figure 2c,d and the corresponding performance parameters are listed in Table 1 .…”