Changes in structure and conduction type upon addition of Ir to ZnO thin films

Zn-Ir-O (Zn/Ir ≈ 1/1) thin films have been reported to be a potential p-type TCO material. It is, however, unknown whether it is possible to achieve p-type conductivity at low Ir content, and how the type and the magnitude of conductivity are affected by the film structure. To investigate the changes in properties taking place at low and moderate Ir content, this study focuses on the structure, electrical and optical properties of ZnO:Ir films with iridium concentration varying between 0.0 and 16.4 at.%. ZnO:Ir thin films were deposited on glass, Si, and Ti substrates by DC reactive magnetron co-sputtering at room temperature. Low Ir content (up to 5.1 at.%) films contain both a nano-crystalline wurtzite-type ZnO phase and an X-ray amorphous phase. The size of the crystallites is below 10 nm and the lattice parameters a and c are larger than those of pure ZnO crystal. Structural investigation showed that the film's crystallinity declines with the iridium concentration and films become completely amorphous at iridium concentrations between 7.0 and 16.0 at.%. An intense Raman band at approximately 720 cm− 1 appears upon Ir incorporation and can be ascribed to peroxide O22– ions. Measurable electrical conductivity appears together with a complete disappearance of the wurtzite-type ZnO phase. The conduction type undergoes a transition from n- to p-type in the Ir concentration range between 12.4 and 16.4 at.%. Absorption in the visible range increases linearly with the iridium concentration.VMTKC project 18, agreement No. 1.2.1.1/16/A/005; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Conductivity in transparent oxide semiconductors

Despite an extensive research effort for over 60 years, an understanding of the origins of conductivity in wide band gap transparent conducting oxide (TCO) semiconductors remains elusive. While TCOs have already found widespread use in device applications requiring a transparent contact, there are currently enormous efforts to (i) increase the conductivity of existing materials, (ii) identify suitable alternatives, and (iii) attempt to gain semiconductor-engineering levels of control over their carrier density, essential for the incorporation of TCOs into a new generation of multifunctional transparent electronic devices. These efforts, however, are dependent on a microscopic identification of the defects and impurities leading to the high unintentional carrier densities present in these materials. Here, we review recent developments towards such an understanding. While oxygen vacancies are commonly assumed to be the source of the conductivity, there is increasing evidence that this is not a sufficient mechanism to explain the total measured carrier concentrations. In fact, many studies suggest that oxygen vacancies are deep, rather than shallow, donors, and their abundance in as-grown material is also debated. We discuss other potential contributions to the conductivity in TCOs, including other native defects, their complexes, and in particular hydrogen impurities. Convincing theoretical and experimental evidence is presented for the donor nature of hydrogen across a range of TCO materials, and while its stability and the role of interstitial versus substitutional species are still somewhat open questions, it is one of the leading contenders for yielding unintentional conductivity in TCOs. We also review recent work indicating that the surfaces of TCOs can support very high carrier densities, opposite to the case for conventional semiconductors. In thin-film materials/devices and, in particular, nanostructures, the surface can have a large impact on the total conductivity in TCOs. We discuss models that attempt to explain both the bulk and surface conductivity on the basis of bulk band structure features common across the TCOs, and compare these materials to other semiconductors. Finally, we briefly consider transparency in these materials, and its interplay with conductivity. Understanding this interplay, as well as the microscopic contenders for providing the conductivity of these materials, will prove essential to the future design and control of TCO semiconductors, and their implementation into novel multifunctional devices

Coloration mechanism in proton intercalated electrochromic hydrated NiOy and Ni1–xVxOy thin films

Electrochromic ͑EC͒ films of nickel oxide, with and without vanadium, were prepared by reactive dc magnetron sputtering. They were characterized by electrochemical and optical measurements and studied by X-ray photoelectron spectroscopy ͑PES͒ using synchrotron radiation. The films were analyzed under as-deposited conditions and after bleaching/coloration by insertion/ extraction of protons from a basic solution and ensuing charge stabilization. Optical measurements were consistent with a coloration process due to charge-transfer transitions from Ni 2+ to Ni 3+ states. The PES measurements showed a higher concentration of Ni 3+ in the colored films. Moreover, two peaks were present in the O 1s spectra of the bleached film and pointed to contributions of Ni͑OH͒ 2 and NiO. The changes in the O 1s spectra upon coloration treatment indicate the presence of Ni 2 O 3 in the colored film and necessitated an extension of the conventional model for the mechanism of EC coloration. The model involves not only proton extraction from nickel hydroxide to form nickel oxyhydroxide but also participation of NiO in the coloration process to form Electrochromic ͑EC͒ materials are able to change their optical properties reversibly and persistently upon charge insertion/ extraction under the application of an external voltage. 1 Materials that color upon charge insertion/extraction are called cathodic/ anodic. These materials can be implemented in energy-saving and comfort-enhancing architectural "smart windows," and other application areas include mirrors with variable specular reflectance, nonemissive information displays, surfaces with variable thermal emittance, eyewear, etc. 1-4 EC technology has been used for some niche applications for several years. However, the situation is now changing and electrochromism will reach wider applications. 5 Thin films of Ni oxide possess anodic electrochromism, as discovered in the mid-1980s 6-9 and subsequently studied in a large number of investigations. 12 Several varieties of reactively sputter-deposited EC hydrated nickel-based oxide thin films have been reported. 3,20 The addition of V facilitates magnetron sputtering but has little influence on the EC properties, as further discussed below. Electrochromism in Ni oxide is surprisingly poorly understood despite the large amount of prior work and the technological importance of this material. We therefore initiated a concerted effort for which earlier results, focused on proton transport, were reported in a companion paper. ͓1͔ This model is elaborated and given further support below. Detailed studies of the coloration mechanism call for a surface-sensitive analysis method, and in particular X-ray photoelectron spectroscopy ͑PES͒, as employed in the present investigation, is a powerful tool. Experimental Film deposition.-EC Ni oxide-based thin films were deposited by reactive dc magnetron sputtering from 5 cm diameter targets of Ni and NiV 0.08 ͑purity 99.95%͒. The proportions of the gases in the plasma were approximately 96% Ar, 2% O 2 , and 2% H 2 for sputtering of Ni 1−x V x O y films and 79.2% Ar, 4.8% O 2 , and 16% H 2 for sputtering of NiO y . The total sputter pressure was 30 mTorr and the sputtering power was 200 W. The target-substrate distance was 13 cm. More details about the deposition were given elsewhere. 20 The substrates used for optical and electrochemical measurements were glass plates precoated with a layer of indium tin oxide ͑ITO͒ ͑i.e., In 2 O 3 :Sn͒, 24,25 with a resistance/square of 60 ⍀. Graphite substrates were employed for composition determinations using Rutherford backscattering spectrometry ͑RBS͒. Typical film thicknesses, recorded by surface profilometry across a masked edge, were 200 nm. Physical techniques.-RBS was used to determine the elemental composition of the films 21 by analyzing the backscattered yield upon bombardment with 2.0 MeV ␣ particles. Spectrophotometric measurements were conducted on films that had been withdrawn from the electrolyte ͑see below͒ and cleaned in deionized water. Specifically, we measured normal transmittance T and near-normal reflectance R by use of a Perkin-Elmer doublebeam spectrophotometer operating in the 300 Ͻ Ͻ 2500 nm wavelength range, employing an integrating sphere. Barium sulfate served as a reference for the reflectance measurements

Coloration Mechanism in Proton-Intercalated Electrochromic Hydrated NiOy and Ni1-xVxOy Thin Films

Electrochromic (EC) films of nickel oxide, with and without vanadium,   were prepared by reactive dc magnetron sputtering. They were   characterized by electrochemical and optical measurements and studied   by X-ray photoelectron spectroscopy (PES) using synchrotron radiation.  The films were analyzed under as-deposited conditions and after   bleaching/coloration by insertion/extraction of protons from a basic   solution and ensuing charge stabilization. Optical measurements were consistent with a coloration process due to charge-transfer transitions   from Ni2+ to Ni3+ states. The PES measurements showed a higher   concentration of Ni3+ in the colored films. Moreover, two peaks were   present in the O 1s spectra of the bleached film and pointed to contributions of Ni(OH)(2) and NiO. The changes in the O 1s spectra   upon coloration treatment indicate the presence of Ni2O3 in the colored   film and necessitated an extension of the conventional model for the   mechanism of EC coloration. The model involves not only proton   extraction from nickel hydroxide to form nickel oxyhydroxide but also participation of NiO in the coloration process to form Ni2O3