In-Operando Optical Spectroscopy of Field-Effect-Gated Al-Doped ZnO

Transparent conductive oxides (TCO) have the unique characteristics of combining optical transparency with high electrical conductivity; such a property makes them uniquely alluring for applications in visible and infrared photonics. One of their most interesting features is the large sensitivity of their optical response to the doping level. We performed the active electrical manipulation of the dielectric properties of aluminum-doped ZnO (AZO), a TCO-based on Earth-abundant elements. We actively tuned the optical and electric performances of AZO films by means of an applied voltage in a parallel-plate capacitor configuration, with SrTiO₃ as the dielectric, and monitored the effect of charge injection/depletion by means of in-operando spectroscopic ellipsometry. Calculations of the optical response of the gated system allowed us to extract the spatially resolved variations in the dielectric function of the TCO and infer the injected/depleted charge profile at the interface.

Doping-Dependent Optical Response of a Hybrid Transparent Conductive Oxide/Plasmonic Medium

Understanding the interaction between plasmonic nanoparticles and transparent conductive oxides is instrumental to the development of next-generation photovoltaic, optoelectronic, and energy-efficient solid-state lighting devices. We investigated the optical response of hybrid media composed of gold nanoparticles deposited on aluminum-doped zinc oxide thin films with varying doping concentration by spectroscopic ellipsometry. The dielectric functions of bare AZO were addressed first, revealing doping-induced effects such as the band gap shift and the appearance of free carriers. In the hybrid media, a blue-shift of the localized surface plasmon resonance of Au NPs as a function of increasing Al doping of the substrate was observed, ascribed to the occurrence of a charge transfer between the two materials and the doping-dependent variation of the polarizability of the substrate.

Demonstration of Near Edge X-ray Absorption Fine Structure Spectroscopy of Transition Metals Using Xe/He Double Stream Gas Puff Target Soft X-ray Source

A near 1-keV photons from the Xe/He plasma produced by the interaction of laser beam with a double stream gas puff target were employed for studies of L absorption edges of period 4 transitional metals with atomic number Z from 26 to 30. The dual-channel, compact NEXAFS system was employed for the acquisition of the absorption spectra. L_1-3 absorption edges of the samples were identified in transmission mode using broadband emission from the Xe/He plasma to show the applicability of such source and measurement system to the NEXAFS studies of the transition metals, including magnetic materials.

Vapor-Phase Molecular Doping in Covalent Organosiloxane Network Thin Films Via a Lewis Acid-Base Interaction for Enhanced Mechanical Properties

Incorporating inorganic components in organosiloxane polymer thin films for enhanced mechanical properties could enable better durability and longevity of functional coatings for a multitude of applications. However, molecularly dispersing the inorganic dopants while preserving the cyclosiloxane rings represents a challenge for cross-linked organosiloxane networks. Here, we report a molecular doping strategy using vapor-phase infiltration. On the basis of the proper Lewis acid-base interaction between diethyl zinc (DEZ) and cyclotrisiloxane rings, we achieved a complete infiltration of the organometallic precursors and well-distributed Zn-OH terminal groups formed in the initiated chemical vapor deposited poly(1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane) (PV3D3) films. X-ray photoelectron spectroscopy and nanoscale infrared spectroscopy together with density functional theory simulation reveal that the formation of a Lewis acid-base adduct rather than a ring-opening process is possibly involved in anchoring DEZ in the cross-linked network of PV3D3. Because of the incorporation of Zn-OH components, the organic-inorganic hybrid films obtained via our vapor-phase molecular doping exhibit a 10.2% larger elastic modulus and 67.0% higher hardness than the pristine PV3D3. Unveiling the reaction mechanisms between organometallic precursors and cross-linked organic networks provides new insights for expanding the vapor-phase processing strategies for engineering hybrid materials at the nanoscale.

ZnO Thin Films Growth Optimization for Piezoelectric Application

The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O2 is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. Subsequent annealing at 773 K further improves the film quality. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode.

Design strategy and interface chemistry of ageing stable AZO films as high quality transparent conducting oxide

In this work, an attempt has been made to produce high quality Al-doped ZnO (AZO) transparent conductors by pulsed direct current (DC) magnetron sputtering under varied process conditions, namely sputtering power, sputtering pressure, doping level, and deposition time. The idea is to develop stable AZO films with excellent electro-optical properties, which can replace indium tin oxide (ITO) as transparent conducting oxide. The films are deposited on glass and polyethylene terephthalate (PET) substrates and characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS) for material properties. The electronic and optical properties are investigated using UV-Vis-NIR absorption spectroscopy, photoluminescence spectroscopy, four probe resistivity and Hall measurements, which facilitate the design strategy to achieve promising electro-optical properties. AZO films under optimized process conditions possess sheet resistance below 5 Ω/sq. and maximum visible light transmittance close to 90%. The lowest value of electrical resistivity is found to be 2.1 × 10^-4 Ω·cm. The combination of low electrical resistivity and high optical transparency leading to the figure of merit of the order of 10^-2 Ω^-1 is critically dependent on the process conditions of sputtering as well as doping contents. High free carrier concentration in AZO films leads to the Burstein-Moss shift and this observed blue shift of the absorption spectrum as compared to that of ZnO with native defects depends on the doping contents and the process conditions. This work further explores the role of interface chemistry to achieve high stability of AZO films under ambient oxidizing conditions. The adsorption kinetics of cationic rhodamine 6G and anionic methyl orange at AZO surface with varying adsorption sites and ageing studies on the sheet resistance reveal that the excellent stability of AZO films is delicately linked to the process conditions as well as the content of the dopant material used in the preparation of the AZO sputtering targets. The findings of the study confirm that the sheet resistances of selective AZO films undergo no change and fixed at 4.5 Ω/sq. even for ageing period of over six months.

Electronic properties of Ag-doped ZnO: DFT hybrid functional study

Studying the possibility of a p-type conduction mechanism in the Ag-doped ZnO can clarify persisting ambiguities in the related materials and devices.