Improving the Morphological and Optical Properties of Sputtered Indium Tin Oxide Thin Films by Adopting Ultralow-Pressure Sputtering

Abstract

The morphological and optical properties of indium tin oxide ͑ITO͒ thin films deposited by an ultralow-pressure dc magnetron sputtering ͑ULPS͒ method followed by postannealing treatment at 250°C are reported. The surface roughness of the film ͑R rms : 0.5 nm͒ deposited using ULPS was about 5 times lower than that of the film ͑R rms : 2.7 nm͒ sputtered using a pressure of 6.7 ϫ 10 −1 Pa. ITO thin films with a low resistivity of 3.7 ϫ 10 −4 ⍀ cm were also achieved using a continuous two-step deposition process, in which the initial layer was deposited using ULPS and then the final layer was deposited with a SP of 6.7 ϫ 10 −1 Pa, without the use of any other additional steps. Both the ULPS and continuous two-step deposition methods were found to be effective for producing ITO thin films with enhanced morphologies that make them suitable for use in display devices.Indium tin oxide ͑ITO͒ thin films are highly degenerate, widebandgap semiconductors ͑E g : 3.4-4.3 eV͒. They have a low electrical resistivity due to their high carrier concentration, and the location of their Fermi level is above the lower edge of the conduction band. 1-3 ITO films also exhibit a high level of transmission in the visible near-infrared regions of the electromagnetic spectrum. Due to these unique properties, ITO thin films have been widely studied for their potential use in the optoelectronics industry and display devices, such as solar-control windshield glass, solar cells, organic light-emitting displays ͑OLEDs͒, flexible display devices, and transparent, flexible electronic circuit boards. [3][4][5] Smooth surface morphology with high electrical conductivity is the crucial factor for the current driven devices such as OLEDs. So far, several methods have been employed to prepare ITO thin films. Examples include spray pyrolysis, reactive evaporation, pulsed laser deposition, conventional magnetron sputtering, cesium-ion-induced magnetron sputtering, and ion-beam assisted sputtering. 6-11 Among these deposition methods, magnetron sputtering is preferable, because it allows easy process control for the device structure and the production of display devices with large areas. However, one of the practical drawbacks of using sputtered ITO thin films for display device applications is their rough surface morphology. This makes them unsuitable for use as the semiconducting layer or transparent electrode of display devices. For example, the quality of the interface between the ITO electrode and the organic emitting layer is crucial to the performance of OLEDs. 12-15 Using a rough ITO layer as the anode may cause undesired electrical shorts or decrease the contact area between the emitting layers and the anode and even form bubbles to deteriorate the color quality and lifetime of the emitting area. 12,16 These functional properties of ITO thin films are strongly dependent on their microstructures, which are influenced by the deposition method and processing conditions used to make them.In this study, ITO thin films with smooth surface morphologies w...

Improvement in the performance of ZnO thin film transistors by using ultralow-pressure sputtering

“…We speculated that the dense structure of the ULPSZnO film with very narrow columns may have originated from the increasing kinetic energy of the sputtered particles and the restriction of growth of the columnar structure. 18 Figure 4 shows the measured XRR data for the ZnO films deposited by CSP and ULPS. The critical angle for the total external reflection for the ULPS-deposited ZnO film was larger than that for the CSP, indicating that the ULPS-ZnO film ͑5.21 g / cm 3 ͒ was denser than the CSP one ͑4.70 g / cm 3 ͒.…”

Improving the Performance of Tin Oxide Thin-Film Transistors by Using Ultralow Pressure Sputtering

“…The higher density of the ULPS-deposited SnO x films compared to CSP was attributed to the enhanced kinetic energy of the sputtered particles. 11,20 The energy loss of a sputtered particle as it passes through the sputtering gas can be estimated from the following equation of Meyer et al 21…”

“…[8][9][10] Recently, Huh et al reported the strong dependence of the electrical properties of indium-doped tin oxide ͑ITO͒ films on the sputtering pressure and thus the kinetic energy of the sputtered particles. 11 This study reports the fabrication of TFTs with a SnO x film as the channel layer by using ultralow pressure sputtering ͑ULPS͒, with a sputtering pressure lower than 1.3 ϫ 10 −1 Pa, compared to higher than 6.7 ϫ 10 −1 Pa for a conventional sputtering pressure ͑CSP͒. As the sputtering pressure was decreased from 6.7 ϫ 10 −1 to 6.7 ϫ 10 −2 Pa, the N d of the SnO x thin film was dramatically reduced 100-fold from 10 19 to 10 17 cm −3 , resulting in a transistor performance with a reasonable mobility and on-and off-current ratio ͑I on/off ͒.…”

Forming ITO/Ag Hole-Array/ITO Multilayers for Near Infrared Transparent Conducting Electrodes and Filters

“…[1][2][3] Currently, tin-doped indium oxide (ITO) is normally adopted as typical material of TCEs due to its high optical transparency and low electrical resistivity. 4 However, demand for improving the performance of photonic devices requires the development of electrodes (or filters) with better electrical and mechanical (or flexible) properties. Because of its low optical loss and high conductivity, thin Ag films have been commonly incorporated to form oxide/metal/oxide (OMO) multilayers, which exhibit low resistivity and high visible transmittance.…”