Atsushi Ashida

Magnetic and magneto-transport properties of ZnO:Ni films

Abstract Magnetic semiconductor (MS), Ni doped ZnO film was fabricated by pulsed laser deposition method on sapphire (0 0 0 1) substrates. Ni dissolves until 25 at % into ZnO by low temperature growth technique. Lattice constant c once increases with increasing Ni content and has maximum point at the Ni content of 5 at % , and then it suddenly decreases. In all the Ni content range, the films exhibits n-type conduction. With increasing the Ni content, the carrier density and mobility decrease. ZnO films with the Ni content range from 3 to 25 at % exhibit ferromagnetic behavior at 2 K . At 30 K , the magnetization against applied magnetic field shows superparamagnetic behavior and it maintains at least up to 300 K . To study the effect of carrier density, Al was additionally doped. The effect of carrier density and Ni content on the structure, magnetic and magneto-transport behaviors are described.

Electro-optic property of ZnO:X (X=Li,Mg) thin films

Abstract We have proposed an application of ZnO:X (X=Li,Mg,Ni,Al etc.) films for monolithic optical integrated circuits (OICs) (Mat. Res. Soc. Symp. Proc. 574 (1999) 317). Although non-doped ZnO has an electro-optic effect, it is only a Pockel’s effect. The electro-optic effect of Pb(Zr,Ti)O 3 (Jpn. J. Appl. Phys. 34 (1995) 5091) is superior to ZnO, because that is caused by a non-linear Kerr effect. Our group demonstrated that Li-doped ZnO (ZnO:Li) films exhibited ferroelectric behavior (Appl. Phys. A, in press). ZnO with ferroelectricity should have a non-linear electro-optic effect against the applied voltage. In this paper, to design the ZnO monolithic slab waveguide for electro-optical switch, the refractive indices of top and bottom electrode layers and core layer were investigated. Then, electro-optical property of ZnO:Li,Mg films was evaluated, and the possibility of applying to an optical switch was also discussed.

Direct piezoelectric properties of (100) and (111) BiFeO3 epitaxial thin films

The direct piezoelectric properties of BiFeO3 epitaxial thin films with different crystal orientation were investigated. Epitaxial films of (100) and (111) rhombohedral BiFeO3 fabricated using pulsed laser deposition showed rectangular hysteresis loops with remanent polarizations of 54 and 83 μC/cm2, respectively. Effective transverse piezoelectric coefficients (e31,f) of −3.5 and −1.3 C/m2 were obtained, for (100) and (111) films, respectively. Results suggest that the strong direct piezoelectric response of the (100) rhombohedral film results from the effects of the engineered-domain configuration.

The effects of Xe on an rf plasma and growth of ZnO films by rf sputtering

Sputter deposition of thin films using Xe gas instead of Ar is believed to suppress damage to the films caused by ion bombardment. To investigate the effects of Xe on ionic species in the rf plasma, and on the growth of ZnO thin films, rf plasmas generated using Xe and Ar sputtering gases were analyzed using Langmuir probe methods and optical emission spectroscopy (OES). The Langmuir probe results showed that the electron temperature of the Xe plasma was nearly half of that of the Ar plasma. In addition, the ion density in the Xe plasma was found to be greater than the ion density in the Ar plasma. OES data showed that a significant fraction of the ionic species in the Ar plasma was oxygen, which is thought to be the primary cause of damage to growing films. Ionic molecular oxygen (O2+) was observed only in the Ar plasma, and the concentration of excited atomic oxygen (O*) in the Ar plasma was ten times as high as that in the Xe plasma, for an oxygen partial pressure of 20%. In addition, ions in the Xe pl...

Influence of antiferromagnetic exchange interaction on magnetic properties of ZnMnO thin films grown pseudomorphically on ZnO (0001¯) single-crystal substrates

This study investigated the influence of antiferromagnetic exchange interaction on magnetic properties of ZnMnO thin films grown on ZnO (0001¯) single-crystal substrates. Using pulsed laser deposition, ZnMnO films with Mn concentrations as high as 14.1at.% were prepared. Investigations revealed that the lattice constant along the c axis of the ZnMnO films increases linearly with increasing Mn concentration. The lattice constant along the a axis is independent of the Mn concentration and is equal to that of the ZnO substrate, suggesting pseudomorphic growth of ZnMnO films. An atomic force microscopy image of the ZnMnO film grown at 640°C shows a step-and-terrace structure. An interference fringe around the ZnMnO 0004 symmetric diffraction image is clearly visible in the 2θ-ω x-ray diffraction profile, suggesting that the samples have a smooth surface and an abrupt interface. The intrinsic magnetic properties of these pseudomorphically grown ZnMnO films with smooth surfaces were evaluated. At temperatures g...

Ferroelectricity in Li-Doped ZnO:X Thin Films and their Application in Optical Switching Devices

We have proposed the application of ZnO:X (X=Li,Ni,Al etc.) films in monolithic optical integrated circuits (OICs). [rf1] To realize the optical switching device, dielectric properties of ZnO:Li deposited on SiO2/p-Si were evaluated in detail. From the results of the frequency dependence of the dielectric permittivity and the loss, and the temperature dependence of ac conductivity at various frequencies, the existence of mobile Li ion was confirmed. The pulsed C–V measurements [rf2] revealed that not only the mobile Li ion but also the ferroelectricity of ZnO:Li contributed to the hysteresis in the normal C–V behavior. To determine the processes assumed to occur in the switching device structure, a prototype of the waveguide structure was fabricated. Although the relationship between the refractive indices of the core and clad layers satisfied the required condition for propagation, several processes such as interdiffusion of doped ions, band alignment and/or rearrangement of space charge when applying the bias voltage were also revealed.

Electrical Characteristics of Controlled-Polarization-Type Ferroelectric-Gate Field-Effect Transistor

We propose a novel ferroelectric-gate field-effect transistor (FET) with a polar semiconductor channel, which is called a controlled-polarization-type ferroelectric-gate FET. The essential factor for this device is the stabilization of the spontaneous polarization of the ferroelectric film by the spontaneous polarization of a polar semiconductor, such as ZnO or GaN. We demonstrate two types of controlled-polarization-type ferroelectric-gate thin film transistors (TFTs), which are the ZnO/YMnO3/Pt bottom-gate-type structure and Pt/PZT/ZnO top-gate-type structure. Both structures show n-channel-type transistor operation with the memory effect originating from ferroelectricity. From the electrical characteristics of ferroelectric-gate TFTs, the effects of ferroelectricity, gate leakage current, space charge, and charge injection on the device operation are discussed.

Pulsed-Laser-Deposited YMnO3 Epitaxial Films with Square Polarization-Electric Field Hysteresis Loop and Low-Temperature Growth

YMnO3 films were prepared by pulsed laser deposition (PLD) on epitaxial (111) Pt/(0001) sapphire substrates. A systematic study regarding the deposition conditions revealed that the composition of the YMnO3 epitaxial films changed depending on deposition temperature and period, due to the evaporation of Mn from the target and/or the film surface during the deposition. To overcome this issue, low-temperature growth, improvement in target density and the change in the substrate heating system were attempted. To enhance photonic ablation, the excimer laser for the ablation was changed from KrF (248 nm) to ArF (193 nm). Eventually, the ferroelectricity of the YMnO3 film deposited at 740°C was significantly improved. Furthermore, the YMnO3 film with almost identical ferroelectric property was successfully obtained even at 700°C.

Electro-Optic Effect in Epitaxial ZnO:Mn Thin Films

We have studied the electro-optic effect of ZnO films. The use of ZnO which exhibits ferroelectricity with Li doping is believed to be one of the solutions for enhancing the electro-optic effect. In this paper, the birefringence shift measurement was performed for nondoped ZnO and Mn-doped ZnO films as the first step for evaluating the electro-optic effect of ZnO:Li film. Nondoped ZnO film exhibited poor dielectric property and no electro-optic response has been observed due to its large leakage current. Mn doping to ZnO led to a significant improvement of the large leakage current problem. The sample showed a linear electro-optic response, and we succeeded in measuring the electro-optic effect induced by dipolar polarizability at the ac bias voltage frequency above 10 kHz. Below this frequency, the electro-optic response includes the space charge effect.

Direct Piezoelectric Properties of Mn-Doped ZnO Epitaxial Films

The direct piezoelectric properties of Mn-doped ZnO epitaxial films prepared by pulse laser deposition was investigated. The resistivity of the ZnO films was markedly decreased by Mn doping. The effective transverse piezoelectric coefficient (e31,f) of the ZnO films was measured by substrate bending and collecting developed charges. The dependence of induced charge density on applied strain indicates that the obtained piezoelectric response originates from the spontaneous polarization of the Mn-doped ZnO films. The e31,f values of 3 and 5 at. % Mn-doped ZnO films were determined to be -1.09 and -1.10 C/m2, respectively, which are comparable to the theoretical value.