ICMCTF2016 Session C3: Transparent Conducting Oxides and Related Inorganic and Organic Materials

In this presentation, we will give an overview of our work on H ion implantation in ZnO [3-5]. Single crystal ZnO substrates were implanted with 100 keV H^- with doses of ranging from 1×10¹⁶ to 1×10¹⁷ cm^-2 at room temperature. Microstructures of H implanted ZnO were characterised by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Optical characteristics of H implanted ZnO was studied by cathodoluminescence. The trend existing as a function of implantation dose was studied in detail by all techniques. XRD results show that H implantation causes the formation of a deformed layer with a larger c-parameter than the ZnO substrate. This layer was also observed in N implanted ZnO [6]. The surface energies of different planes were extracted from a reverse Wulff construction of the equilibrium shape of these nanovoids formed within a H environment [5]. A preliminary report of our recent findings on Zn implantation will also be presented.

Transparent electrodes are widely used in various optoelectronic devices as thin film photovoltaics and leds. In this paper, we propose to develop a new kind of transparent electrodes based on transparent conductive oxide (TCO) grids. One of the most promising applications of this new TCO is their integration in semi-transparent thin film solar cells. Indeed, such new TCOs have the capability to be more transparent in the visible spectral range while both keeping a suitable resistivity and enhancing the light trapping in the device. In this context, TCO grids were elaborated with a low cost process consisting in three steps: deposition of microspheres by a langmuir blodget process, reactive ion etching of microspheres array and TCO deposition (ITO, ZnO:Al or ITZO) before chemical etching of microspheres. The aim of this study is to propose a high figure of merit TCO considering resistivity, total transmission and diffuse transmission in the visible wavelength range. The resistivity of TCO grids show values lower than 5.10^-4 ohm.cm and the total transmission and haze factor in transmission is also at high level in the visible wavelength range at respectively 88% and 23% at 550nm. These transparent electrodes were then integrated in thin a-Si:H-based semi-transparent solar cells. Grid-form electrodes were designed to have a high transmission in the visible wavelength range and a high diffusive properties in the absorption wavelength range of a-Si:H. The improvement of light management in the solar cell induced by TCO grids allows depositing low a-Si:H thicknesses (<100nm) without lowering cell efficiency. As a consequence, the total transmission of light through such semi-transparent solar cell increased (>50% in the visible range) with the same conversion efficiency (3%).

NiO films have been found to be a useful material that can be employed in many devices to enhance their performance, such as AlGaN/GaN heterostructure field-effect transistors, p-type thin-film transistors, etc, and shows promise as a hole-transporting layer in perovskite solar cells recently [1]. However, stoichiometric NiO is an insulator with high electrical resistivity (r) of 10¹³ Ω·cm. It is well known that the existence of nickel vacancies will cause NiO material to exhibit p-type characteristics. Our previous study has reported that NiO films with good p-type conduction were achieved by reactive magnetron sputtering using higher O₂ partial pressure [2]. Kim et. al. [3] also reported that increasing the O₂ ratio during the deposition process accelerates the non-stoichiometry, which creates more nickel vacancies and Ni³⁺ ions, leading to the enhancement of p-type conduction. Therefore, we utilized reactive sputtering in a pure O₂ gas atmosphere at various pressures to deposit the NiO films, and the effect of O₂ working pressure on the electrical and optical properties of the films was investigated.

In this work, the NiO films were deposited on glass substrates using a Ni target reacting with various pure O₂ pressures by three dimensional physical vapor deposition (3D-PVD) system. It was found that the resistivity of NiO films continuously increases from 9.16 x 10^-3 to 3.20 Ω·cm as the O₂ working pressure is increased from 1 to 15 mTorr, however the oxygen content in the films decreases significantly . The Hall measurements for NiO films deposited at various O₂ working pressures all show p-type conduction. Optical Emission Spectroscopy (OES) was employed to analyze the change of the plasma constituents during deposition of the films at the various O₂ pressures, and it shows that the inten sity of oxygen ions in the sputtering chamber decreases with increasing oxygen working pressure. We believe that excess oxygen atoms react with Ni atoms of the Ni target on the surface area at a higher o xygen working pressure. Therefore, when the O₂ working pressure increases, the reductions of oxygen content and Ni³⁺ ions in the films lead to a decrease in the nickel vacancies and hole concentration, which results in the increase of resistivity in the films. On the other hand, both carrier mobility and transmittance of the films rise as higher oxygen working pressure was introduced during the deposition of the NiO films.

In this contribution, we report on ionized magnetron sputtering of amorphous oxide semiconductors (AOS). The main goal of this study was to explore this approach for reducing the growth temperature of Zn-Sn-O (ZTO).

AOS are very interesting materials, combining optical transparency with high electron mobility, suitable for transparent electronics. Annealing is typically required to improve the electronic properties of AOS thin films which limits the use of temperature sensitive substrates such as plastic foils. In High Power Impulse Magnetron Sputtering (HiPIMS), high ionization of the sputtered material may be achieved enhancing the non-thermal energy input to the substrate.

We have carried out reactive HiPIMS depositions of ZTO thin films from an alloy Zn-Sn sputtering target. Our main focus was on the effect of deposition conditions on the film properties and we compared the results to In-Ga-Zn-O (IGZO) deposited by RF sputtering from an oxide target. All films were deposited in the same deposition system without substrate heating and characterized with respect to optical transparency, microstructure, electron mobility and resistivity. The best as-deposited films had electron mobility up to 13, and 9 cm²∙V^-1∙s^-1 for IGZO and ZTO, respectively. We have observed strong lateral gradients of the films properties which we attributed to the enhanced incorporation of sputtered atomic oxygen. The sputtered oxygen promotes formation of the oxide at lower oxygen partial pressures. The reactive process used for ZTO depositions provides additional freedom in the control of particle flux to the substrate and thus also film growth. Surprisingly, no significant effect of the ionization and plasma density was observed. The deposition pressure had the most pronounced impact on the electronic properties for either material.

Full Face Erosion (FFE) circular planar cathodes have recently been introduced as an alternative for R&D rotatable development. The technology has proven to facilitate suitable and relevant research that could be applied to the sputtering of Indium Tin Oxide (ITO) from rotatable targets. The rotating plasma on the FFE circular magnetron sources can simulate some of the benefits of the rotatable cylindrical cathode technology, such as a clean target and the ability to maintain clean anode in a dual cathode sputtering. Initial results on ITO deposition have shown that comparative Transparent Conductive Oxide (TCO) properties can be obtained when comparing FFE planar and cylindrical rotatable cathodes.

The industrial field of TCO magnetron sputtering has been seeing a rapid transition from large area planar to large area rotatable cathodes. Most R&D labs, however, have equipment that would require a large investment in order to implement relevant rotatable cathode technology. In many cases the downsizing on cathode diameter and cathode length has large implications in the relevance of the research itself. In addition, R&D labs find it very difficult to justify the large target consumable bill associated with the TCO rotatable cathode targets. The process of qualifying new target compositions is also cost problematic when it comes to a cylindrical rotatable option.

New plasma sprayed compositions are helping in bringing the costs of TCO’s, like ITO, rotatable targets down. Plasma spray techniques also bring benefits for the preparation of special target compositions.

The aim of this paper is to present comparative data for traditional sintered ceramic ITO material and plasma sprayed targets for both FFE and rotatable cylindrical targets and to demonstrate the usefulness of the FFE approach as an R&D evaluation tool for rotatable cylindrical development.