Showing papers on "Sputter deposition published in 2007"

Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these

Abstract: Electrochromic (EC) materials are able to change their optical properties, reversibly and persistently, by the application of an electrical voltage. These materials can be integrated in multilayer devices capable of modulating the optical transmittance between widely separated extrema. We first review the recent literature on inorganic EC materials and point out that today's research is focused on tungsten oxide (colouring under charge insertion) and nickel oxide (colouring under charge extraction). The properties of thin films of these materials are then discussed in detail with foci on recent results from two comprehensive investigations in the authors' laboratory. A logical exposition is obtained by covering, in sequence, structural features, thin film deposition (by sputtering), electronic band structure, and ion diffusion. A novel conceptual model is given for structural characteristics of amorphous W oxide films, based on notions of defects in the ideal amorphous state. It is also shown that the conduction band density of states is obtainable from simple electrochemical chronopotentiometry. Ion intercalation causes the charge-compensating electrons to enter localized states, implying that the optical absorption underlying the electrochromism can be described as ensuing from transitions between occupied and empty localized conduction band states. A fully quantitative theory of such transitions is not available, but the optical absorption can be modeled more phenomenologically as due to a superposition of transitions between different charge states of the W ions (6+, 5+, and 4+). The Ni oxide films were found to have a porous structure comprised of small grains. The data are consistent with EC coloration being a surface phenomenon, most likely confined to the outer parts of the grains. Initial electrochemical cycling was found to transform hydrated Ni oxide into hydroxide and oxy-hydroxide phases on the grain surfaces. Electrochromism in thus stabilized films is consistent with reversible changes between Ni hydroxide and oxy-hydroxide, in accordance with the Bode reaction scheme. An extension of this model is put forward to account for changes of NiO to Ni2O3. It was demonstrated that electrochromism is associated solely with proton transfer. Data on chemical diffusion coefficients are interpreted for polycrystalline W oxide and Ni oxide in terms of the lattice gas model with interaction. The later part of this review is of a more technological and applications oriented character and is based on the fact that EC devices with large optical modulation can be accomplished essentially by connecting W-oxide-based and Ni-oxide-based films through a layer serving as a pure ion conductor. Specifically, we treat methods to enhance the bleached-state transmittance by mixing the Ni oxide with other oxides characterized by wide band gaps, and we also discuss pre-assembly charge insertion and extraction by facile gas treatments of the films, as well as practical device manufacturing and device testing. Here the emphasis is on novel flexible polyester-foil-based devices. The final part deals with applications with emphasis on architectural “smart” windows capable of achieving improved indoor comfort jointly with significant energy savings due to lowered demands for space cooling. Eyewear applications are touched upon as well.

Novel top‐gate zinc oxide thin‐film transistors (ZnO TFTs) for AMLCDs

Abstract: — High-performance top-gate thin-film transistors (TFTs) with a transparent zinc oxide (ZnO) channel have been developed. ZnO thin films used as active channels were deposited by rf magnetron sputtering. The electrical properties and thermal stability of the ZnO films are controlled by the deposition conditions. A gate insulator made of silicon nitride (SiNx) was deposited on the ZnO films by conventional P-CVD. A novel ZnO-TFT process based on photolithography is proposed for AMLCDs. AMLCDs having an aperture ratio and pixel density comparable to those of a-Si:H TFT-LCDs are driven by ZnO TFTs using the same driving scheme of conventional AMLCDs.

The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

Abstract: This study addresses the material properties of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films and their application as front contacts in silicon thin-film solar cells. Optimized films exhibit high conductivity and transparency, as well as a surface topography with adapted light-scattering properties to induce efficient light trapping in silicon thin-film solar cells. We investigated the influence on the ZnO:Al properties of the amount of alumina in the target as well as the substrate temperature during sputter deposition. The alumina content in the target influences the carrier concentration leading to different conductivity and free carrier absorption in the near infrared. Additionally, a distinct influence on the film growth of the ZnO:Al layer was found. The latter affects the surface topography which develops during wet-chemical etching in diluted hydrochloric acid. Depending on alumina content in the target and heater temperature, three different regimes of etching behavior have been i...

Luminescence mechanism of ZnO thin film investigated by XPS measurement

Abstract: The effects of annealing environment on the luminescence characteristics of ZnO thin films that were deposited on SiO2/Si substrates by reactive RF magnetron sputtering were investigated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). An analysis of the O 1s peak of ZnO film revealed that the concentration of oxygen vacancies increased with the annealing temperature from 600 °C to 900 °C under an ambient atmosphere. The PL results demonstrated that the intensity of green light emission at 523 nm also increased with temperature. Under various annealing atmospheres, the analyses of PL indicated that only one emission peak (523 nm) was obtained, indicating that only one class of defect was responsible for the green luminescence. The green light emission was strongest and the concentration of oxygen vacancies was highest when the ZnO film was annealed in ambient atmosphere at 900 °C. The results in this investigation show that the luminescence mechanism of the emission of green light from a ZnO thin film is associated primarily with oxygen vacancies.

High performance thin film transistors with cosputtered amorphous indium gallium zinc oxide channel

Abstract: The authors report the fabrication of high performance thin film transistors (TFTs) with an amorphous indium gallium zinc oxide (a-IGZO) channel, which was deposited by cosputtering using a dual IGZO and indium zinc oxide (IZO) target. The effect of the indium content on the device performance of the a-IGZO TFTs was investigated. At a relatively low IZO power of 400W, the field-effect mobility (μFE) and subthreshold gate swing (S) of the a-IGZO TFTs were dramatically improved to 19.3cm2∕Vs and 0.35V/decade, respectively, compared to those (11.2cm2∕Vs and 1.11V/decade) for the TFTs with the a-IGZO channel (reference sample) prepared using only the IGZO target. The enhancement in the subthreshold IDS-VGS characteristics at an IZO power of 400W compared to those of the reference sample was attributed to the reduction of the interface trap density rather than the reduction of the bulk defects of the a-IGZO channel.

Toughness of hard nanostructured ceramic thin films

Abstract: This article reports on the investigation of cracking of hard, 3–5 μm thick Zr–Cu–O, Zr–Cu–C, Ti–Cu–C and Si–Me–N (Me = Ta, Zr, Mo, W) magnetron sputtered nanostructured films using microindentation measurements. Main aim of this investigation is to determine the interrelationships between the cracking of film, its structure and mechanical properties and to assess the toughness of thin film. Correlations between the formation of cracks, the mechanical properties of film and substrate, structure of film and macrostress σ generated in the film during its growth were investigated in detail. It was found that the resistance of the film to cracking increases with increasing ratio Hf3/Ef⁎2. It was found that (1) the correct assessment of toughness of the thin film requires to investigate the system thin film/substrate as one unit because mechanical properties of the substrate play a decisive role in the formation of cracks, (2) the strongest parameter influencing the formation of cracks is the film structure and its macrostress σ and (3) nanostructured films with X-ray amorphous structure and small compressive macrostress (σ ≈ − 0.1 GPa) are very stable against the cracking even at high values of the film hardness Hf exceeding 20 GPa.

Interface microstructure engineering by high power impulse magnetron sputtering for the enhancement of adhesion

Abstract: An excellent adhesion of hard coatings to steel substrates is paramount in practically all application areas. Conventional methods utilize Ar glow etching or cathodic arc discharge pretreatments that have the disadvantage of producing weak interfaces or adding droplets, respectively. One tool for interface engineering is high power impulse magnetron sputtering (HIPIMS). HIPIMS is based on conventional sputtering with extremely high peak power densities reaching 3kWcm−2 at current densities of >2Acm−2. HIPIMS of Cr and Nb was used to prepare interfaces on 304 stainless steel and M2 high speed steel (HSS). During the pretreatment, the substrates were biased to Ubias=−600V and Ubias=−1000V in the environment of a HIPIMS of Cr and Nb plasma. The bombarding flux density reached peak values of 300mAcm−2 and consisted of highly ionized metal plasma containing a high proportion of Cr1+ and Nb1+. Pretreatments were also carried out with Ar glow discharge and filtered cathodic arc as comparison. The adhesion was ev...

Cholesterol biosensor based on rf sputtered zinc oxide nanoporous thin film

Abstract: Cholesterol oxidase (ChOx) has been immobilized onto zinc oxide (ZnO) nanoporous thin films grown on gold surface. A preferred c-axis oriented ZnO thin film with porous surface morphology has been fabricated by rf sputtering under high pressure. Optical studies and cyclic voltammetric measurements show that the ChOx∕ZnO∕Au bioelectrode is sensitive to the detection of cholesterol in 25–400mg∕dl range. A relatively low value of enzyme’s kinetic parameter (Michaelis-Menten constant) ∼2.1mM indicates enhanced enzyme affinity of ChOx to cholesterol. The observed results show promising application of nanoporous ZnO thin film for biosensing application without any functionalization.

Bipolar resistive switching in polycrystalline TiO2 films

Abstract: Bipolar resistive switching was found in thin polycrystalline TiO2 films formed by the thermal oxidation of sputtered Ti films. With a Ag top electrode, TiO2 film, and Pt bottom electrode, bistable resistive switching with a low operating voltage and a good uniformity was observed repeatedly without an initial electrical “forming” process. This switching phenomenon might be described as the formation and rupture of a filamentary conductive path consisting of a chain of Ag atoms. The temperature dependence of the switching voltage is discussed in terms of interstitial ionic diffusion of Ag in the TiO2 matrix.

Resistance switching in the metal deficient-type oxides: NiO and CoO

Abstract: The resistance switching properties in Pt∕Ni–O∕Pt and Pt∕Co–O∕Pt synthesized by the magnetron sputtering have been investigated. The oxygen partial pressure during sputtering and the post-thermal process are crucial to forming of the trilayer. By investigating x-ray photoemission spectroscopy spectra, the increase of initial resistance in Ni–O was caused by the variation of the stoichiometry, while that in the Co–O was accompanied by the phase transformation between CoO and Co3O4. The resistance switching in Pt∕Co–O∕Pt and Pt∕Ni–O∕Pt exhibits the analogous electrode area and temperature dependences. As a result of the I-V measurements at the elevated temperature, the assistance of Joule heating in the reset process is implied.

Solution-phase deposition and nanopatterning of GeSbSe phase-change materials.

Abstract: Chalcogenide films with reversible amorphous-crystalline phase transitions have been commercialized as optically rewritable data-storage media, and intensive effort is now focused on integrating them into electrically addressed non-volatile memory devices (phase-change random-access memory or PCRAM). Although optical data storage is accomplished by laser-induced heating of continuous films, electronic memory requires integration of discrete nanoscale phase-change material features with read/write electronics. Currently, phase-change films are most commonly deposited by sputter deposition, and patterned by conventional lithography. Metal chalcogenide films for transistor applications have recently been deposited by a low-temperature, solution-phase route. Here, we extend this methodology to prepare thin films and nanostructures of GeSbSe phase-change materials. We report the ready tuneability of phase-change properties in GeSbSe films through composition variation achieved by combining novel precursors in solution. Rapid, submicrosecond phase switching is observed by laser-pulse annealing. We also demonstrate that prepatterned holes can be filled to fabricate phase-change nanostructures from hundreds down to tens of nanometres in size, offering enhanced flexibility in fabricating PCRAM devices with reduced current requirements.

Properties of titanium nitride films prepared by direct current magnetron sputtering

Abstract: Titanium nitride (TiN) thin films of different thickness were deposited by direct current (dc) magnetron sputtering under conditions of various N2 concentrations (05–34%) The electrical, optical, structural, compositional and morphological properties of the films were studied and the results were discussed with respect to N2 concentration and thickness of the films At low N2 concentration of 05% (of the total sputtering pressure 11 Pa), golden coloured stoichiometric TiN films were obtained and with increase in the N2 concentration non-stoichiometric TiNx phases resulted However, irrespective of the N2 concentration, the TiN stoichiometry in the films increased with increase in the film thickness In the surface of the films the presence of nitride (TiN), oxynitride (TiOxNy) and oxide (TiO2) phases were observed and the quantity of these phases varied with the N2 concentration and thickness The films of lower thickness were found to be amorphous and the crystallinity was observed in the films with increase in the thickness The crystalline films showed reflections corresponding to the (1 1 1), (2 0 0) and (2 2 0) orientation of the cubic TiN and also features associated with TiNx phases The transmission spectra of the films revealed the typical characteristics of the TiN films ie a narrow transmission band, however, the width varied with thickness, in the wavelength range of 300–600 nm and exhibited low transmission in the infrared region The TiN films deposited at low N2 concentration of 05% showed smooth and uniform morphology with densely packed crystallites With increase in N2 concentration various characteristics such as needle type crystallization, bubble precipitates and after bubble burst morphologies were observed in the films However, at higher N2 concentration conditions, uniformity developed in the films with increase in thickness

Room temperature deposited indium zinc oxide thin film transistors

Abstract: Depletion-mode indium zinc oxide (IZO) channel thin film transistors were fabricated on glass substrates from layers deposited at room temperature using rf magnetron sputtering. The threshold voltage was in the range from −5.5to−6.5V depending on gate dielectric (SiO2) thickness and the drain current on-to-off ratio was ∼105. The maximum field effect mobility in the channel was ∼4.5cm2V−1s−1, lower than the Hall mobility of ∼17cm2V−1s−1 in the same layers, suggesting a strong influence of scattering due to trapped charges at the SiO2-IZO interface. The low deposition and processing temperatures make these devices suitable for applications requiring flexible substrates.

Effect of oxygen partial pressure on the electrical and optical properties of highly (200) oriented p-type Ni1−xO films by DC sputtering

Abstract: Thin films of NiO (bunsenite) with (200) preferential orientation were synthesized on glass substrates by direct current sputtering technique in Ar+O2 atmosphere. Nanostructural properties of the NiO films were investigated by X-ray diffraction and also by atomic force microscopic (AFM) studies. Electrical and optical properties of the deposited films were investigated as a function of different partial pressure of oxygen in the sputtering gas mixture during deposition. The films showed p-type electrical conduction and the conductivity depends on the partial pressure of oxygen. The electrical conductivity (σRT) was found to be .0615 S cm−1 for films deposited with 100% O2 and its value sharply decreased with the decrease the partial pressure of O2; for example σRT for 50% O2 was 6.139 × 10−5 S cm-1. The mechanism of the origin of p-type electrical conductivity in the NiO film is discussed from the viewpoint of nickel or oxygen vacancies, which generate holes and electrons respectively. X-ray photoelectron spectroscopic studies supported the above argument. Corresponding optical properties showed that the transparency decreases with increasing oxygen partial pressure and the bandgap also decreases.