Toshio Hada

Structures and electrical properties of zinc oxide films prepared by low pressure sputtering system

Abstract Polycrystalline zinc oxide films with and without admixed aluminum and copper were prepared on amorphous substrates by using a co-sputtering process. The crystallographic orientation and electrical properties varied with both sputtering gas pressure and contents of admixed aluminum or copper. In a high sputtering gas pressure a normal orientation with c -axis perpendicular to the film surface was predominant, whereas, in a low sputtering gas pressure a parallel orientation with c -axis parallel to the film surface was predominant. The admixed aluminum inhibited growth of the normal orientation, but the admixed copper enhanced a growth of the parallel one. The energy gap of zinc oxide films was observed to be about 3.3 eV at room temperature. In the low sputtering gas pressure, the admixed aluminum forms a donor level at about 0.08 eV below the conduction band and produces an increase in the electrical conductivity by over three orders of magnitude. However, the admixed copper forms deep-lying traps for conduction electrons at about 2.5 eV below the conduction band and reduces the conductivity by over three orders of magnitude.

High rate deposition of thick piezoelectric ZnO films using a new magnetron sputtering technique

ZnO films were prepared on glass substrates with a new reactive magnetron sputtering technique using a zinc target. A solenoid coil around a bell jar was used to cause an increase in the parallel component of the leakage lines to the target surface. Highly oriented ZnO films (c‐axis orientation) were fabricated on the glass substrates. The deposition rate increases with increasing discharge current, and it attains 15 μm/h for (O2 50%:Ar50%), 11 μm/h for (81.5%:18.5%), and 9 μm/h for 100% oxygen. These films were obtained at a 500‐mA discharge current and a 0.2‐Torr total gas pressure. Resistivities of the films were 106–109 Ω cm. A piezoelectric coupling constant of thick ZnO films (∼30 μm) was obtained by measuring a frequency response of transducer impedance near the fundamental resonance frequency of the interdigital electrode. Experimental results were about 80% of the theoretical values.

DC reactive magnetron sputtered ZnO films

Abstract This paper is concerned with a new reactive magnetron sputtering technique using a zinc target and 100% oxygen gas. A solenoid coil around a bell jar was used to cause an increase in the parallel component of the leakage magnetic lines to the target surface. Highly oriented ZnO films ( c -axis orientation) were fabricated on glass substrates at very high deposition rates (10 μm/h). The quality of the ZnO film was determined by X-ray diffraction, reflection electron diffraction, and scanning electron microscopy (SEM). Well-oriented ZnO films were obtained at relatively high oxygen pressures from 0.2 to 0.4 Torr. This sputtering system is more convenient than other existing sputtering systems.

High Rate Deposition of ZnO Film Using Improved DC Reactive Magnetron Sputtering Technique

This paper concerns with a new reactive magnetron sputtering technique using a zinc target and pure oxygen gas. A solenoid coil around a bell jar was used to improve an increase in the parallel component of the leakage magnetic lines to the target surface. Highly oriented ZnO films (c-axis orientation) were fabricated on glass and on (111) gold substrates. Deposition rate reached 10 µm/h at 500 mA. The quality of the ZnO film was determined by X-ray diffraction, reflection electron diffraction (RED), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Deviation σ on the glass substrate is 1.67°. Excellent ZnO films were obtained at relatively high oxygen gas pressures, 0.25 Torr on glass and 0.55 Torr on (111) gold substrate. Low temperature sputtering is possible for small discharge current.

A Current Instability in TiO2 Thin Film

The current instability is observed in the sputtered TiO2 thin films of Au-TiO2-Ti sandwich system under the application of positive bias on Au electrode. The process of the present current instability is studied in relation to the hydrogen ion conduction in the TiO2 thin films. As a result, the process is conjectured as follows: 1) dissociation of adsorbed water at Au-TiO2 interface, 2) injection of H+ into TiO2 at the positive bias, 3) migration of H+ through TiO2 under the applied field. Dissociation of water results from a coarse texture of the film surface. From the current response at the recovery of the current instability, the diffusion activation energy of H+ in the TiO2 films is estimated at 0.07 eV which is in good agreement with the value in TiO2 single crystal.

A lattice attenuation in CdS measured by the ultrasonic injection method

This paper concerns the use of Brillouin scattering measurements to study lattice attenuation in semiconducting CdS crystals. Measured acoustic fluxes are produced by the acoustoelectric domains. In order to measure the attenuation constant, an ultrasonic injection method is applied. A sample is divided into two parts; one part is used as a generation region of the acoustic flux [region (1)] and the other part is used as a propagation region of the injected flux [region (2)]. The acoustic attenuations of various frequencies are measured at region (2). The lattice attenuation is greatly affected by the rise time of an applied pulse in region (1). It is clear that acoustic flux in the acoustic domain which originates from the amplification of the pure thermally excited acoustic flux attenuates as f2 and is in accordance with the Akhieser loss. On the other hand, a shock‐excited acoustic flux attenuates as f∼1.5.

Photoacoustic Measurement of CdS by Transparent Transducer Method

This paper is concerned with the theoretical and experimental study of photoacoustic (PA) spectroscopy using a transparent transducer. When the light beam is illuminated from the sample surface and a PA signal is detected by the back transducer, a peak and a valley are mixed in the PA spectra. When the light is illuminated through the transparent window, the heat at the surface is directly detected by the transducer and there is no geometrical limitation to detect PA signal. A PA spectra of a block sample and the different PA spectra for various sample treatments are demonstrated.

High Rate Deposition of Thick Piezoelectric ZnO and AlN Films Using a New Magnetron Sputtering Technique

ZnO and AlN films were prepared on glass substrates with a new reactive magnetron sputtering technique using a metal target. Highly oriented ZnO films (c-axis orientation) were fabricated on the glass substrates. Resistivities of the films were 106–109 Ω cm for ZnO and 109–1011 Ω cm for AlN. An electromechanical coupling constant of thick ZnO films (~30 µm) was obtained by measuring a frequency response of transducer impedance near the fundamental resonance frequency of the interdigital electrode. For AlN, the c-axis orientation from substrate normal to parallel can be controlled by adding H2 gas to N2 gas.

Optical-transmission modulation in CdS caused by acoustoelectric domain propagating along c-axis

Abstract When an acoustoelectric domain in a piezoelectric semiconductor traverses the monochromatic light near the intrinsic absorption edge, absorbed and transmitted optical modulations are observed. In order to reveal the mechanisms, we apply an electric field parallel to the c -axis of the CdS single crystal. In this case the electric field dependences of the off-axis angle are different for a high field domain and an acoustic domain. Comparing the electric field dependence of the off-axis angle of the optical modulation with that of the high field domain or the acoustic domain, we conclude that the transmitted and absorbed optical modulations are caused by the acoustic domain. Further, in order to establish the nature and mechanism of the optical modulation in case of E || c , we observe the spectral dependences of the absorbed optical modulation and find that there are two kinds of light absorption; one is due to pure shift in the absorption edge and the other is due to broad tail for low energy side. In case of e || c ( e : polarization of incident light) both types of modulation exist at the same time and in case of e ⊥ c only the latter exists. And it is speculated that the transmitted optical modulation is due to the effect of induced birefringence caused by acoustic strain.

Frequency Spectrum of Acoustic Domain Caused by Variable Rise-Time Pulse

This paper is concerned with the behavior of acoustic domain in CdS with use of a variable rise-time of applied pulse. For a rise-time pulse slower than 6 µs, amplified fluxes are from thermal phonons. The peak and width of the flux distributions can be well explained by the small-signal theory. Signal shapes by Brillouin scattering become a trapezoid when the shock is relaxed. The lattice attenuation measured by the ultrasonic injection technique is proportional to the square of acoustic frequency (0.8 GHz to 2 GHz)