Shoji Yamada

Electrochromic properties of sputtered nickel‐oxide films

Electrochromic properties of nickel‐oxide films have been investigated. Films were prepared by rf sputtering from a compressed powder NiO target in an atmosphere of an Ar‐O2 mixture. The deposition rate of the nickel oxide decreased when the total pressure of the sputtering atmosphere was increased, but was independent of the oxygen content. The films obtained can be electrochemically colored and bleached in a 1‐N NaOH electrolyte solution, and are colored anodically. The colored film is dark brown, and the bleached one is transparent and colorless. The maximum coloration efficiency is obtained with the films prepared at the total pressure of 0.04 Torr, and is 36 cm2/C at a wavelength of 640 nm. Coloration efficiency is independent of oxygen content in the sputtering atmosphere.

Absorption bands of electrochemically-colored films of WO3, MoO3 and MocW1−cO3

Optical absorption bands of electrochemically-colored films of amorphous WO3, MoO3 and MocW1-cO3 were investigated. The peak photon energy of the bands depends not only on the amount of injected charges but also on the Mo content. In the WO3 films, the absorption band originates from the intervalence trasfer of electrons. In MoO3 and mixed films, however, the absorption bands are considered to consist of two different components. The first is the intervalence trasfer band and the second is a component of absorption at higher energies. The second component is considered to be related to Mo atoms. The area under the absorption curve increases linearly with increasing density of injected charges in the WO3 film. In the film with large c, the absorption tends to saturate.

Preparation conditions of sputtered electrochromic WO3 films and their infrared absorption spectra

Abstract Tungsten oxide films were prepared by rf sputtering in an argon-oxygen atmosphere from W and WO3 targets. To bring about reversible electrochromic (EC) characteristics, as-deposited films require an aging process (i.e. cycles of injection/ejection of charge carriers). The infrared absorption band at around 3300 cm−1 increases during the aging process, and it is assigned as OH stretching vibrations of absorbed water. By coloration after aging, the 3300 cm−1 band decreases, and a new band appears at 2400 cm −1. The latter band is considered to be to the stretching mode of radicals incorporated in the WO3 matrix. At low coloration levels, the 2400 cm−1 band increases slightly with injected charge, and a coloration mechanism other than the usual double injection model may be considered. The coloration efficiency depends on the preparation conditions. Its maximum value is the same for films prepared from W and WO3 targets, and is 60 cm2/C at a wavelength of 600 nm. When a tungsten target is used, the substrate temperature is low and the deposition rate is high compared with a WO3 target.

Recombination Radiation as Possible Mechanism of Light Emission from Reverse-Biased p-n Junctions under Breakdown Condition

There are two models, recombination and bremsstrahlung models, proposed for the interpretation of visible light emission from reverse-biased semiconductor junctions under breakdown conditions. The emission spectra are calculated on the basis of these two models and compared with the published experimental results on Si and Ge p-n junctions. Experimental spectra can be interpreted well using the model of recombination between conduction-band electrons and valence-band holes. The spectra calculated on the basis of the bremsstrahlung model cannot be fitted to the experimental ones.

Preparation and aging of sputtered tungstic oxide films

Conditions for the preparation of electrochromic tungstic oxide films with high efficiency for coloration have been investigated. Tungstic oxide films were deposited on glass substrates by rf sputtering in Ar‐O2 mixture from a compressed powder WO3 target. As‐deposited films require an ‘‘aging process,’’ in which the density of charges extracted in bleaching is smaller than that injected in coloring. It is considered that a part of the protons introduced during the aging combine with bonds of unstable oxygen contained in the as‐deposited films. After the aging period, sputtered films have reversible coloration and bleaching cycles. Maximum electrochromic coloration efficiencies are 0.09 and 0.04 cm2/mC at wavelengths of 800 and 500 nm, respectively. They are obtained when the sputtering atmosphere is 0.04–0.05‐Torr total pressure and 5% oxygen content. At any rate, substrate temperature should be kept at 100 °C or less.

Hydrogen Etching of Silicon Carbide

The etch rate of SiC crystals with hydrogen was investigated as a function of the reaction temperature, the hydrogen flow velocity and the hydrogen partial pressure in the H2-Ar mixture. An etching reaction mechanism and calculated expressions for the etch rate have been developed based on thermodynamical considerations. The experimental results are well explained from the approximate expression at the etch rate region higher than about 3 µ/min.

Temperature dependence of photoluminescence of ZnS-ZnSe superlattices

Abstract Undoped and In-doped ZnSe and ZnS-ZnSe SL films were prepared on GaAs (100) substrate by hot wall epitaxy and their photoluminescence was measured at temperatures ranging from 4.2 to 300 K. It was found that the photoluminescence associated with the band edge and with the D(In)-A pair shifted with temperature in a similar way to that of the band gap of the bulk. Photoluminescence associated with deep level defects is discussed in relation to strain and shallow levels. Moreover ZnS-ZnSe SLs with various thickness of ZnS layer (barrier layer) were prepared and the photon energies of their PL were analysed using the Kronig-Penney model to discuss band offsets Δ E c and Δ E v .

Effect of Tl additive on conductivity and absorption coefficient in glassy As2Se3

Abstract Effects of addition of Tl on dc and ac conductivities and optical absorption coefficient of glassy As 2 Se 3 were investigated. The dc conductivity measured at temperatures 230–350 K was linear in the Arrhenius plot. The dc conductivity decreased with increasing Tl content up to 0.1 at.% and increased at a content >0.1 at.%, and the activation energy for the dc conduction increased with increasing Tl content up to 0.1 at.% and decreased at a content >0.1 at.%. The ac conductivity measured at 77–350 K in the frequency range between 30 Hz and 1 MHz depended linearly on the frequency, but hardly on the temperature. The ac conductivity was independent of Tl content up to 0.3 at.%, but it increased with increasing Tl content at >0.3 at.%. It was assumed, for both undoped and Tl-doped As 2 Se 3 samples, that the bipolaron hopping mechanism was predominant in the ac conduction. The optical absorption coefficient obeyed the Urbach rule, and the Urbach tail shifted to lower energy with Tl content > 0.1 at.%.

Properties of solid-state electrochromic cells using Ta 2 O 5 as electrolyte

Solid-state electrochromic (EC) cells (ITO/WO3/Ta2O5/ITO), where ITO is indium-tin-oxide, were fabricated. Individual films were prepared by rf sputtering. The conduction mechanism in the Ta2O5 electrolyte was considered from the measurement of ac conductivity. Absorption spectra, coloration efficiency and memory characteristics of the EC cells were investigated. These results are compared with those of EC cells with liquid electrolytes.

Electrochromic properties of rf‐sputtered tungstic oxide film prepared from a W metal target

Tungstic oxide film is prepared from a W metal target by the rf sputtering method and its electrochromic characteristics are compared with that from WO3 target. Maximum coloration efficiency found for both kinds of films are equal and about 0.06 cm2 /mC at the wavelength of 600 nm, which is higher than that of typical evaporated films. Films from the W metal target, for the maximum coloration efficiency, can be prepared at a broader oxygen range of sputtering atmosphere than that of a WO3 target. It is found that films from a W metal target can be prepared at higher input power than that of a WO3 target. Therefore, the W metal target is suitable to get films with required thickness and EC characteristics.