Ti Alloyed α -Ga 2 O 3: Route towards Wide Band Gap Engineering

The suitability of Ti as a band gap modifier for α-Ga2O3 was investigated, taking advantage of the isostructural α phases and high band gap difference between Ti2O3 and Ga2O3. Films of (Ti,Ga)2O3 were synthesized by atomic layer deposition on sapphire substrates, and characterized to determine how crystallinity and band gap vary with composition for this alloy. We report the deposition of high quality α-(TixGa1−x)2O3 films with x = 3.7%. For greater compositions the crystalline quality of the films degrades rapidly, where the corundum phase is maintained in films up to x = 5.3%, and films containing greater Ti fractions being amorphous. Over the range of achieved corundum phase films, that is 0% ≤ x ≤ 5.3%, the band gap energy varies by ∼270 meV. The ability to maintain a crystalline phase at low fractions of Ti, accompanied by a modification in band gap, shows promising prospects for band gap engineering and the development of wavelength specific solar-blind photodetectors based on α-Ga2O3

Role of ALD Al2O3 Surface Passivation on the Performance of p-Type Cu2O Thin Film Transistors

High-performance p- type oxide thin film transistors (TFTs) have great potential for many semiconductor applications. However, these devices typically suffer from low hole mobility and high off-state currents. We fabricated p-type TFTs with a phase-pure polycrystalline Cu2O semiconductor channel grown by atomic layer deposition (ALD). The TFT switching characteristics were improved by applying a thin ALD Al2O3 passivation layer on the Cu2O channel, followed by vacuum annealing at 300 degrees C. Detailed characterization by transmission electron microscopy-energy dispersive X-ray analysis and X-ray photoelectron spectroscopy shows that the surface of Cu2O is reduced following Al2O3 deposition and indicates the formation of a 1-2 nm thick CuAlO2 interfacial layer. This, together with field-effect passivation caused by the high negative fixed charge of the ALD Al2O3, leads to an improvement in the TFT performance by reducing the density of deep trap states as well as by reducing the accumulation of electrons in the semiconducting layer in the device off-state.Peer reviewe

Rapid Vapor-Phase Deposition of High-Mobility p-Type Buffer Layers on Perovskite Photovoltaics for Efficient Semi-Transparent Devices

Perovskite solar cells (PSCs) with transparent electrodes can be integrated with existing solar panels in tandem configurations to increase the power conversion efficiency. A critical layer in semi-transparent PSCs is the inorganic buffer layer, which protects the PSC against damage when the transparent electrode is sputtered on top. The development of n-i-p structured semi-transparent PSCs has been hampered by the lack of suitable p-type buffer layers. In this work we develop a p-type CuOx buffer layer, which can be grown uniformly over the perovskite device without damaging the perovskite or organic hole transport layer. The CuOx layer has high hole mobility (4.3 ± 2 cm2 V-1 s-1), high transmittance (>95%), and a suitable ionization potential for hole extraction (5.3 ± 0.2 eV). Semi-transparent PSCs with efficiencies up to 16.7% are achieved using the CuOx buffer layer. Our work demonstrates a new approach to integrate n-i-p structured PSCs into tandem configurations, as well as enable the development of other devices that need high quality, protective p-type layers.EPSRC Department Training Partnership studentship (No: EP/N509620/1), as well as Bill Welland. T.N.H. acknowledges funding from the EPSRC Centre for Doctoral Training in Graphene Technology (No. EP/L016087/1) and the Aziz Foundation. W.-W.L. and J.L.M.-D. acknowledge support from the EPSRC (Nos.: EP/L011700/1, EP/N004272/10), and the Isaac Newton Trust (Minute 13.38(k)). M.N. and J.L.M.-D. acknowledge financial support from EPSRC (No. EP/P027032/1). S. D. S. acknowledges support from the Royal Society and Tata Group (UF150033). R.L.Z.H. acknowledges support from the Royal Academy of Engineering under the Research Fellowship scheme (No.: RF\201718\1701), the Centre of Advanced Materials for Integrated Energy Systems (EPSRC Grant No. EP/P007767/1), the Isaac Newton Trust (Minute 19.07(d)), and the Kim and Juliana Silverman Research Fellowship at Downing College, Cambridge

Low-temperature thermal and plasma-enhanced atomic layer deposition of metal oxide thin films

Atomic layer deposition (ALD) is a method for thin film fabrication with atomic level precision. This thesis focuses on low-temperature thermal and plasma- enhanced ALD and presents results on thin film growth by these techniques with examples of common ALD materials: Al2O3, ZnO and TiO2. As an example of limitations of the thermal ALD the nucleation and growth of Al2O3 and ZnO films on different grades of poly(methyl methacrylate) (PMMA) are presented, showing that the initiation of the growth is strongly dependent on both the deposited material and the substrate. A potential application of the ALD ZnO films in polymer surface functionalization is demonstrated by changing in the surface wettability by means of UV-illumination. To overcome the nucleation delay in thermal ALD, room-temperature PEALD of ZnO films was demonstrated. It is shown that the growth and properties of the films depend on the PEALD reactor configuration and the plasma conditions therein. The plasma species interactions shown to be beneficial to the film growth were observed to damage the polymer substrates, the severeness depending on the polymer material. The last part of this thesis describes plasma mode transitions in capacitively- and inductively-coupled plasmas, that are typically used in PEALD processing. In addition to the mode transition induced changes in the plasma parameters, the contribution of the different plasma species to the growth and properties of ZnO and TiO2 films are demonstrated and discussed

Atomic layer deposition of functional multicomponent oxides

Advances in the fabrication of multicomponent oxide thin films are crucial to prepare specific compositions with precise structures and controlled interfaces. This will enable the investigation of novel phenomena and development of new devices and applications. Atomic layer deposition (ALD) has flourished over the last decades in fabrication of conformal thin films and nanostructures with atomic-scale control. Nonetheless, the scenario of deposition of complex oxides with desired properties has proven to be challenging. In this article, we scrutinize the basics of the precursor and process design for ALD followed by a review on the major achievements in the synthesis of doped and complex oxides identifying several relevant examples that are foreseen to have direct technological applications. Finally, current challenges and perspectives on ALD complex oxides are given.M.C. would like to acknowledge the Spanish MINECO [“Severo Ochoa” Programme for Centres of Excellence in R&D Grant Nos. SEV-2015-0496 and MAT2017-83169-R (AEI/FEDER, EU)]. M.N. acknowledges funding from the EPSRC, Grant No. EP/P027032/1.Peer reviewe

Forming-free and non-linear resistive switching in bilayer HfO_x/TaO_x memory devices by interface-induced internal resistance

Resistive switching memory devices with tantalum oxide (TaOx) and hafnium oxide (HfOx) mono- and bilayers were fabricated using atomic layer deposition. The bilayer devices with Ti and TiN electrodes show non-linear switching characteristics, and can operate without requiring an initial electroforming step. The insertion of the HfOx layer induces the switching behaviour on single layer TaOx that shows Zener diode-like characteristics, with conductivity depending on the top electrode metal. The electronic conductivity mechanism study shows Schottky emission at low voltage regime followed by tunneling at higher applied bias, both indicating interface-dominated conduction. The switching mechanism study is supported by X-ray photoelectron spectroscopy characterization of the films that show a formation of TaOxNy and TaNx species at the oxide-electrode interface. This interfacial layer serves as a high resistivity barrier layer enabling the homogeneous resistive switching behavior.</p

Development of a microfluidic design for an automatic lab-on-chip operation

Simple and easy to use are the keys for developing lab-on-chip technology. Here, a new microfluidic circuit has been designed for an automatic lab-on-chip operation (ALOCO) device. This chip used capillary forces for controlled and precise manipulation of liquids, which were loaded in sequence from different flowing directions towards the analysis area. Using the ALOCO design, a non-expert user is able to operate the chip by pipetting liquids into suitable inlet reservoirs. To test this design, microfluidic devices were fabricated using the programmable proximity aperture lithography technique. The operation of the ALOCO chip was characterized from the flow of red-, blue- and un-dyed deionized water. Experimental result indicated that red water, which filled first the analysis area, was substituted entirely with blue water. Controlled sequential flows of these water in the ALOCO device are demonstrated in this paper.peerReviewe

Nickel oxide thin films grown by chemical deposition techniques: Potential and challenges in next‐generation rigid and flexible device applications

Funder: Aziz FoundationFunder: Downing College, CambridgeFunder: Isaac Newton Trust; Id: http://dx.doi.org/10.13039/501100004815Abstract: Nickel oxide (NiO x ), a p‐type oxide semiconductor, has gained significant attention due to its versatile and tunable properties. It has become one of the critical materials in wide range of electronics applications, including resistive switching random access memory devices and highly sensitive and selective sensor applications. In addition, the wide band gap and high work function, coupled with the low electron affinity, have made NiO x widely used in emerging optoelectronics and p‐n heterojunctions. The properties of NiO x thin films depend strongly on the deposition method and conditions. Efficient implementation of NiO x in next‐generation devices will require controllable growth and processing methods that can tailor the morphological and electronic properties of the material, but which are also compatible with flexible substrates. In this review, we link together the fundamental properties of NiO x with the chemical processing methods that have been developed to grow the material as thin films, and with its application in electronic devices. We focus solely on thin films, rather than NiO x incorporated with one‐dimensional or two‐dimensional materials. This review starts by discussing how the p‐type nature of NiO x arises and how its stoichiometry affects its electronic and magnetic properties. We discuss the chemical deposition techniques for growing NiO x thin films, including chemical vapor deposition, atomic layer deposition, and a selection of solution processing approaches, and present examples of recent progress made in the implementation of NiO x thin films in devices, both on rigid and flexible substrates. Furthermore, we discuss the remaining challenges and limitations in the deposition of device‐quality NiO x thin films with chemical growth methods. imag

Room-temperature plasma-enhanced atomic layer deposition of ZnO : Film growth dependence on the PEALD reactor configuration

Room-temperature plasma-enhanced atomic layer deposition (PEALD) of ZnO was studied by depositing the films using diethylzinc and O2 plasma from inductively-coupled plasma (ICP) and capacitively-coupled plasma (CCP) plasma source configurations. The CCP-PEALD was operated using both remote and direct plasma. It was observed that the films deposited by means of remote ICP and CCP were all highly oxygen rich, independently on plasma operation parameters, but impurity (H, C) contents could be reduced by increasing plasma pulse time and applied power. With the direct CCP-PEALD the film composition was closer to stoichiometric, and film crystallinity was enhanced. The ZnO film growth was observed to be similar on silicon, polycarbonate and poly(methyl methacrylate) substrates, but changes in polymer surface morphology indicate plasma-induced damage during the deposition due to exposure to ion bombardment when direct plasma was applied.peerReviewe

Nucleation and growth of ZnO on PMMA by low-temperature atomic layer deposition

ZnO films were grown by atomic layer deposition at 35 °C on poly(methyl methacrylate) substrates using diethylzinc and water precursors. The film growth, morphology, and crystallinity were studied using Rutherford backscattering spectrometry, time-of-flight elastic recoil detection analysis, atomic force microscopy, scanning electron microscopy, and x-ray diffraction. The uniform film growth was reached after several hundreds of deposition cycles, preceded by the precursor penetration into the porous bulk and island-type growth. After the full surface coverage, the ZnO films were stoichiometric, and consisted of large grains (diameter 30 nm) with a film surface roughness up to 6 nm (RMS). The introduction of Al 2O3 seed layer enhanced the initial ZnO growth substantially and changed the surface morphology as well as the crystallinity of the deposited ZnO films. Furthermore, the water contact angles of the ZnO films were measured, and upon ultraviolet illumination, the ZnO films on all the substrates became hydrophilic, independent of the film crystallinity.peerReviewe