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

Tuesday, April 26, 2016 1:50 PM in Room Sunset
Tuesday Afternoon

Time Period TuA Sessions | Abstract Timeline | Topic C Sessions | Time Periods | Topics | ICMCTF2016 Schedule

Start Invited? Item
1:50 PM Invited C3-2 New Insights into Ion Implantation in Zno
Jennifer Wong-Leung, Keng Chan (Australian National University, Australia); Lasse Vines (University of Oslo, Norway); Cuong Ton-That, Sumin Choi (University of Technology Sydney, Australia); Matthew Phillips (University of Technology, Sydney, Australia); Li Li (Australian National University, Australia); Bengt Svensson (University of Oslo, Norway); Chennupati Jagadish (Australian National University, Australia)
ZnO is a promising wide bandgap semiconductor for optoelectronic devices operating in the ultraviolet region. However, the realisation of ZnO based electronic devices is still hindered by the difficulty to achieve reproducible and stable p-type ZnO. The role of H in ZnO has received wide attention since Van de Walle [1] postulated that H acts as a shallow donor and may be the reason for the persistent high n-type conduction observed in native ZnO substrates. The high conductivity layer in ZnO formed by H implantation has been characterised by secondary mass ion spectrometry and scanning spreading resistance microscopy [2]. However, details of the microstructure of this layer are still not well understood.

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.

2:30 PM C3-4 TCO Grids Development for Semi-Transparent Thin Film Solar Cells
Cédric Ducros, Fabrice Emieux, Alexandre Pereira (CEA Grenoble, France)

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%).

2:50 PM C3-5 Microstructures and Optoelectronic Properties of Nickel Oxide Films Deposited by Reactive Magnetron Sputtering at Various Working Pressures of Pure Oxygen Environment
Sheng-Chi Chen, Chao-Feng Lu (Ming Chi University of Technology, Taiwan, Republic of China); Chao-Kuang Wen (National Taiwan University, Taiwan, Republic of China); Shih-Wen Hsu (Ming Chi University of Technology, Taiwan, Republic of China); Tung-Han Chuang (National Taiwan University, Taiwan, Republic of China)

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.

3:10 PM C3-6 Control of Carrier Transport in Polycrystalline Ga-doped ZnO Transparent Conductive Films
Tetsuya Yamamoto, Hisao Makino, Junichi Nomoto (Kochi University of Technology, Japan)
We have been developing a deposition technology to control carrier concentration (N) together with the improvement of carrier transport due to the enhanced carrier mobility in intragrains and little contribution of grain boundary (GB) scattering to carrier transprt for highly transparent conductive Ga-doped ZnO (GZO) polycrystalline films. 200-nm-thick GZO films were deposited on glass substrates by ion plating with dc arc discharge. The substrate temperature was 200 °C. To acheive the GZO films, we investigated the effects of oxygen (O2) gas flow rates (OFRs) and discharge current (ID) on the structural, optical and electrical properties. The ID was added between the plasma gun and resource pellets, a sintered ZnO with the Ga2O3 contents of 4.0 wt.%. During the film growth, O2 gas was introduced into the deposition chamber to control oxygen-related point defects. We found that OFR is the main factor limiting N and the contribution of GB scattering to the carrier transport, whereas ID is the dominant factor to limit the carrier mobility in the intragrains. By optimizing both ID and OFRs to reduce the electrical resistivity with high carrier transport, we have achieved GZO films with the electrical resistivity of 1.96 μΩm with N of 1.25 E1021 cm-3 and Hall mobility of 25.4 cm2/Vs. Analysis of out-of-plane XRD and GIXRD measurement results show that the GZO films exhibited well-defined (0001) orientation, leading to little contribution of GB scattering to the carrier transport. We found the improved Hall mobility as a result. It implies the reduced density of chemisorbed O species at the GBs.
3:30 PM C3-7 Reactive Magnetron Sputtering of Zn-Sn-O Amorphous Oxide Semiconductors
Tomas Kubart (Uppsala University, Angstrom Laboratory, Sweden); Lucie Prusakova (University of West Bohemia, Czech Republic); Asim Aijaz (Uppsala University, Angstrom Laboratory, Sweden)

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.

3:50 PM C3-8 Comparative Study of Sintered and Plasma Sprayed Indium Tin Oxide Coatings Deposited Using Full Face Erosion Planar and Rotatable Cathodes
Frank Papa, Victor Bellido-Gonzalez, Alex Azzopardi (Gencoa Ltd., USA)

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.

Time Period TuA Sessions | Abstract Timeline | Topic C Sessions | Time Periods | Topics | ICMCTF2016 Schedule