Nanometer scale domain inversion and application to new devices

LiNbO_3、LiTaO_3等铁电材料的晶体在室温下具有很大的自发极化和180 μ m的畴结构。纳米畴反转的实现将使超高容量存储器等新型光电子器件的开发成为可能，本论文对电子束辐照钽酸锂单晶实现纳米畴反转的基础研究进行了综述。大容量存储器也被研究作为一个应用。通过改进的电子束图案化设备对小于2 μ m正方形的区域的照射产生大至140 μ m的反转域。理论分析表明：（1）库仑作用的影响很强，电荷分布近似为一个具有恒定垂直角的三角锥，称为“沙漏”模型; (2)有两种阈值，畴形核阈值和畴扩展阈值。我们已经成功地在500 μ m厚的晶体中形成了小至670 nm φ的反转畴。基于压电效应，本文还提出了一种新的反演域成像方法--相干检测法。实现了对畴的非破坏性和直接观察。P-E滞后曲线的测量结果表明：（1）Ps值高达51 μ C/cm ^2，曲线呈理想的正方形; (2)畴形核阈值和畴扩展阈值分别为204 kV/cm和120 kV/cm。因此，LiTaO_3单晶的畴反转适合于存储器应用。上述结果为纳米级畴反转技术的建立和器件的应用提供了参考。

The crystal of ferroelectric materials such as LiNbO_3 and LiTaO_3 have large spontaneous polarization and 180ﾟ domain structure at room temperature. Development of new opto-electronic devices such as ultra high capacity memory would be possible by realization of the nanometer scale domain inversion.In this study, basic research of the nanometer scale domain inversion by an irradiation of electron beam to LiTaO_3 single crystal. High capacity memory is also investigated as an application. An irradiation to the area of smaller than 2 mum square by modified electron beam patterning equipment produces an inversion domain as large as 140 mumphi. Theoretical analysis reveal (1) an effect of coulomb Interaction is very strong, and charge distribution is estimated to be a trigonal pyramid with constant vertical angle, named as "sand glass" model. (2) There are two kinds of threshold value, the threshold of domain nucleation and the threshold of domain expansion. We have succeeded to form an inversion domain as small as 670 nm phi in the crystal of 500mum thick. A new method for imaging the inversion domains, named coherent detection, is also proposed based on the piezoelectric effect. Non-destructive and direct observation of the domains is achieved. A measurement of P-E hysteresis curve reveals (1) Ps is as large as *51muC/cm^2, and the curve is ideal square shape. (2) The threshold values are 204kV/cm and 120kV/cm for threshold of domain nucleation and threshold of domain expansion, respectively. Therefore domain inversion of LiTaO_3 single crystal is suitable for memory application. The results shown above would be informative for establishing the nanometer scale domain inversion technique and the device application.