Structural optical and electrical properties of a transparent conductive ITO/Al–Ag/ITO multilayer contact

Abstract

Indium tin oxide (ITO) is a widely used material for transparent conductive oxide (TCO) films due to its good optical and electrical properties. Improving the optoelectronic properties of ITO films with reduced thickness is crucial and quite challenging. ITO-based multilayer films with an aluminium–silver (Al–Ag) interlayer (ITO/Al–Ag/ITO) and a pure ITO layer (as reference) were prepared by RF and DC sputtering. The microstructural, optical and electrical properties of the ITO/Al–Ag/ITO (IAAI) films were investigated before and after annealing at 400 °C. X-ray diffraction measurements show that the insertion of the Al–Ag intermediate bilayer led to the crystallization of an Ag interlayer even at the as-deposited stage. Peaks attributed to ITO(222), Ag(111) and Al(200) were observed after annealing, indicating an enhancement in crystallinity of the multilayer films. The annealed IAAI film exhibited a remarkable improvement in optical transmittance (86.1%) with a very low sheet resistance of 2.93 Ω/sq. The carrier concentration increased more than twice when the Al–Ag layer was inserted between the ITO layers. The figure of merit of the IAAI multilayer contact has been found to be high at 76.4 × 10−3 Ω−1 compared to a pure ITO contact (69.4 × 10−3 Ω−1). These highly conductive and transparent ITO films with Al–Ag interlayer can be a promising contact for low-resistance optoelectronics devices. Introduction Transparent conducting oxides (TCO) thin films have been receiving much attention regarding their use as contacts in several optoelectronic devices such as LEDs [1], solar cells [2] and flat panel displays [3]. Indium tin oxide (ITO) is the most commonly used TCO for industrial and laboratory applications due to its excellent optical and electrical properties [4,5]. It is a wide-bandgap material (3.6–4.0 eV) with low electrical resistivity. ITO contains the rare and expensive metal indium, which Beilstein J. Nanotechnol. 2020, 11, 695–702. 696 is reflected in the market value of the material [6]. Hence, a reduction of the ITO consumption is desirable. ITO films with smaller thickness would result in high optical transmittance in the visible region. However, the resistivity would increase, which is an issue [4,7,8]. Therefore, the search for new material compositions and structures of ITO-based films to enhance the performance in optoelectronic devices is of importance. The inclusion of a thin metal film between a top and a bottom ITO layer to form a multilayer structure has been explored recently for efficient photoelectric devices [7]. The multilayer structure not only improves the conductivity of the contact but also make the device cost-effective since less indium metal is needed [9-11]. The insertion of a metal layer reduces the transparency of the ITO electrode due to opaqueness of the metal, but selecting an optimal metal thickness can effectively decrease the reflection from the metal film and thus enhance the transmittance. Furthermore, it gives room for controlling the transparency in the visible region of the light spectrum [9,12,13]. However, the quality of both metal and ITO layers determines the optical and electrical performance of the multilayer structures [4]. Embedding a thin metal film between ITO layers coupled with annealing enhances the photoresponse and the rectification properties of the ITO device. Single or double metal thin films of Ag, Al, Ti, Au, Cr, or Ni have been embedded between ITO layers [4,8,14-17]. Free electrons in the metal/ITO materials accelerate the separation of charge carriers and hence improve the transport from the lower to the upper part of the device [9]. The good adhesion, low resistivity, and the stability against oxidation and corrosion of Al films make them suitable for application in optical and electronic devices [18-20]. The low resistivity and relatively high transmittance (compared to other metals) in the visible region of Ag thin films at room temperature led to the wide use of Ag layers in ITO multilayer contacts [21-24]. However, Ag thin films agglomerate upon annealing due to low adhesion, which degrades the quality of the films [25]. This issue can be overcome by adding a thin layer of Al, Au, Pd, or Cr to the Ag film to improve the adhesion [4,25,26]. Optical and electrical properties of ITO films are enhanced by post-deposition annealing especially at high temperatures [7]. Gulen et al. [27] exposed pure ITO films deposited by sputtering to heat treatment at temperatures of 100–700 °C. An improvement of the microstructural, optical and electrical properties of the film annealed at 400 °C was observed. Similarly, a significant enhancement of the optoelectronic properties of an ITO/Ag(Cr)/ITO multilayer film was achieved by annealing at 500 °C [4]. Further treatments beyond 500 °C resulted in the degradation of the film structure due to the appearance of metallic nanoparticles on the surface of the multilayer [4,28]. Furthermore, Cho et al. estimated a figure of merit of 12.28 × 10−4 Ω−1 for a 5.07 nm thick intermediate Al film after annealing at 200 °C [29]. Rapid thermal annealing of ITO/Ag/ ITO films by Joeng et al. [28] led to an improvement in transmittance for films annealed at 300 °C. The lowest sheet resistance and resistivity values were obtained after annealing at 500 °C, but with reduced optical transmittance. Also, a durability test of an ITO sandwich electrode with Ag alloy against heat treatment at 450 °C was carried out by Roh et al. [30]. An appreciable durability and stability of the Ag films was observed. In the present work, the structural, optical and electrical properties of an Al–Ag bilayer between ITO layers (ITO/ Al–Ag/ITO) are examined. Moreover, annealing was carried out at 400 °C with an ITO/Al–Ag/ITO (IAAI) multilayer film and a pure ITO film for comparison. Results and Discussion Figure 1 shows the X-ray diffraction (XRD) patterns for as-deposited and annealed IAAI multilayer films. The as-deposited film shows an amorphous structure of the top ITO layer with a strong Ag(111) diffraction peak, showing that the Ag intermediate layer is crystalline, comparable to the work of Kim et al. [31]. There is no diffraction peak of the Al film, which is consistent with the work of Cho et al. [29]. The IAAI film becomes polycrystalline upon annealing at 400 °C. Strong diffraction peaks of ITO(222), Ag(111) and ITO(440) were observed after annealing. The appearance of diffraction peaks of ITO(222), Ag(111) and Al(200) in the annealed film indicate an enhanced crystallinity of the film. Diffraction peaks of In2O3 appear to be dominant without any traces of SnO2, Sn or SnO peaks. Figure 1: XRD spectra of as-deposited and annealed IAAI films. During deposition of the IAAI films, the kinetic energy of the sputtered atoms arriving at the substrate is low, which leads to the amorphous structure. The kinetic energy of the Ag atoms is Beilstein J. Nanotechnol. 2020, 11, 695–702. 697 Table 1: Elemental composition of the IAAI films before and after annealing on the silicon substrate. O (wt %) Al (wt %) Ag (wt %) Si (wt %) In (wt %) Sn (wt %) as-deposited IAAI film 12.20 0.35 5.70 69.46 11.04 1.25 annealed IAAI film 10.01 0.55 5.81 71.15 11.17 1.31 higher and the Ag film crystallizes already during deposition. During annealing the adatoms gain additional energy resulting in an increased mobility. This enables grain growth and crystallization [32-34]. The grain sizes of as-deposited and annealed IAAI films were calculated using the Scherrer equation,