Research for the deposition of new piezoelectric <Ta_2O_5> single crystal films and their physical properties.

<Ta_2O_5>薄膜已用于形成薄膜电容器，最近，用于光学波导和太阳能电池的抗反射涂层。这些应用基本上是基于这种氧化物的高化学稳定性，特别是其最佳的光学性能，如高折射率和非常低的光吸收系数。在以前的工作中，通过热氧化蒸发到衬底上的钽膜，射频溅射和反应直流溅射获得了<Ta_2O_5>薄膜。在以往的工作中获得的<Ta_2O_5>薄膜都是无定形的，而不是单晶。在本研究中，我们明确了以下三点：(1)用平面磁控管反应直流二极管溅射法制备了<Ta_2O_5>薄膜。通过改变溅射条件可以制备定向、非晶和多晶薄膜。特别是，取向膜是单轴取向膜，其中< β > - <Ta_2O_5>属于点群“m”的x轴垂直于衬底生长。此外，该薄膜还具有压电特性，是一种迄今未见报道的新型压电薄膜。(2)在取向<Ta_2O_5>薄膜中，当hk=1.6时，表面声波的最大机电耦合系数<K^2>为1.54%。使用石英衬底时，声表面波在131 MHz时的传输损耗为13.3 dB/cm。(3)取向<Ta_2O_5>薄膜的一阶TCD为零。熔融石英衬底在hk=1.78时二阶TCD为0.148 ppm/ <C^2>， PYREX衬底在hk=1.27时二阶TCD为0.052ppm/ <C^2>。<Ta_2O_5>薄膜具有良好的化学稳定性，可提供具有良好温度特性的SAW器件。期望制备出性能更优异的<Ta_2O_5>薄膜。

<Ta_2O_5> thin films have been used for forming thin-film capacitors and, more recently, for optical wave-guides and for antireflection coating on solar cells. These application are essentially based on the high chemical stability of this oxide and especially on its optimum optical properties such as a high refractive index and a very low absorption coefficient for light. In previous works, the <Ta_2O_5> thin films were obtained by the thermal oxidation of a tantalum film evaporated onto a substrate, rf sputtering, and reactive dc sputtering. All <Ta_2O_5> thin films obtained in previous work were amorphous, and were not single crystals.In this research, We have clarified next three points that(1) the <Ta_2O_5> thin films are fabricated by the planar-magnetron reactive DC diode sputtering method. These kind of films such as oriented, amorphous and polycrystalline can be fabricated by varying the sputtering condition. Especially, the oriented film is one axis oriented film where the x-axis of <beta> - <Ta_2O_5> belonging to point group " m " is grown perpendicular to the substrate. Furthermore the film shows piezoelectric properties and is a new piezoelectric film which has not been reported thus far.(2) In the oriented <Ta_2O_5> thin film, the maximum electromechanical coupling coefficient <K^2> for the surface acoustic wave is 1.54% at hk=1.6. The propagation loss of the SAW is 13.3 dB/cm at 131 MHz when the fused quartz substrate is used.(3) The first-order TCD of the oriented <Ta_2O_5> thin film is zero. The second-order TCD is 0.148 ppm/ <C^2> at hk=1.78 for the fused quartz substrate and is 0.052ppm/ <C^2> at hk=1.27 for the PYREX substrate, respectively.The <Ta_2O_5> film is chemically stable and may supply SAW device with good temperature characteristics. The fabrication of <Ta_2O_5> thin film with more excellent properties is expected.