Yutaka Kishimoto

Moisture-resistant ZnO transparent conductive films with Ga heavy doping

Moisture-resistant ZnO transparent conductive films were formed with Ga heavy doping by off-axis-type rf magnetron sputtering. The resistivity of 12.4wt% Ga-doped ZnO is 1.3×10−3Ωcm and changes by less than 3% over a 2000h reliability test at a temperature of 85°C and a humidity of 85%. The crystal structural analysis of the heavily Ga-doped ZnO films indicates that the c axis grows along various directions, which is quite different from the conventional c-axis oriented growth. The effect of heavy doping is discussed based on the crystal structural transformation and carrier compensation by excess Ga segregated in the film.

Bandwidth-tunable filter consisting of SAW resonators and BaSrTiO 3 varactors directly fabricated on a LiTaO 3 wafer

A prototyped bandwidth-tunable filter consisting of SAW resonators and BaSrTiO3 (BST) varactors is discussed in this paper. In general, the BST varactors require high deposition temperature (>600°C) in order to obtain good varactor performances. Thus special cared wafer integration process of BST varactors onto a 42°YX-LiTaO3 wafer was newly developed to prevent performance deterioration in SAW resonators. The bandwidth-tunable filter composed of ten pieces of both surface acoustic wave (SAW) resonators and BST varactors on the 42°YX-LiTaO3 substrate was successfully fabricated using the integration process. The measured center frequency of the filter is 1GHz, and the 3dB bandwidth is electrically tuned from 4.5 to 9.6MHz by adjusting a DC supply voltage from 0 to 10V.

Preparation of (K, Na)NbO 3 -CaTiO 3 film by RF magnetron sputtering

Lead-free piezoelectric (K, Na)NbO₃-CaTiO₃ (KNN-CT) films were prepared on Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering. Co-sputtering using a number of KNN-CT targets which vary in compositions of Na/(K+Na) ratio and amount of CaTiO₃ provided variations in compositions of KNN-CT films. The obtained KNN-CT films had compositions corresponding to the average compositions of the used targets represented by chemical formula of (1-n)(K_1-x, Na_x)NbO₃-nCaTiO₃ (x=0.460-0.617, n=0.000-0.065) and the compositional dependencies of their piezoelectric properties were studied. Most KNN-CT films with n>0.000 exhibited relatively high transverse piezoelectric coefficients |e₃₁ *|=|d₃₁|/s_11,p>6.0C/m² (where s_11,p is elastic compliance of the KNN-CT films), whereas n=0.000 |e₃₁ *|<;5.0C/m². In XRD analysis, (002) peak angle of these pseudocubic perovskite films increased with increase of n. From these results, we conclude that CaTiO₃ was successfully substituted as part of KNN and piezoelectric constant increased by effect of CaTiO₃. In addition, the KNN-CT films with x=0.587-0.617 and n=0.027-0.053 exhibited higher |e₃₁ *| than any other x and n. Moreover, by 750°C post-annealing process, |e₃₁ *| of the KNN-CT film with x=0.587 and n=0.040 was enhanced and the highest |e₃₁ *|=11.7C/m² was confirmed. This result of our experiment shows one of highest piezoelectric coefficient for lead free piezoelectric films.