单晶和孪晶的Zn0.96Co0.04O稀磁半导体薄膜的制备与研究

采用分子束外延技术分别在不同晶面的蓝宝石（sapphire Al₂O₃）基片上制备了沿c轴生长的Zn_0.96Co_0.04O稀磁半导体薄膜.发现在Al₂O₃（1120）晶面（a面）上薄膜是二维层状外延生长的高质量单晶薄膜，而在Al₂O₃（0001）晶面（c面）上薄膜却具有有趣的孪晶结构，部分区域相互之间有一个30°的面内转动来减少和基片之间的失配度.在孪晶薄膜中存在的这些相互旋转形成的区域界面上会引起载流子强烈的散射作用，导致载流子迁移率的下降和平均自由程的缩短.利用X射线吸收精细结构技术证明了无论单晶还是孪晶的Zn_0.96Co_0.04O薄膜中所有的Co都以+2价替代进入了ZnO的晶格，而没有形成任何杂相.而对其磁性研究发现，孪晶的薄膜样品比高质量的单晶薄膜样品具有大得多的饱和磁矩.这充分说明孪晶薄膜中的铁磁性来源与缺陷有关.我们还对铁磁性耦合机制进行了探讨. 关键词： Co掺杂ZnO 稀磁半导体 X射线吸收精细结构 单晶和孪晶薄膜

Synthesis and electric, magnetic properties of single crystalline and twin-crystalline Co-doped ZnO thin films

c-oriented Co-doped ZnO (Zn_(0.96)Co_(0.04)O) thin films have been grown on the a-plane and c-plane sapphire substrates using molecular-beam epitaxy, respectively. In contrast to single crystalline Zn_(0.96)Co_(0.04)O film grown on a-plane sapphire, the film grown on c-plane sapphire were found to be twin-crystalline; some domains have a 30° rotation to reduce the large mismatch. The presence of these rotation domains in the twin-crystalline ZnO thin film causes much more carrier scattering at the boundaries, leading to much lower mobility and smaller mean free path of the mobile carriers, as compared with those of the single crystalline one. X-ray absorption fine structure studies revealed that most Co ions were in divalent state and actually substituted into ZnO lattice without formation of any detectable secondary phase. As compared with just a small saturation moment in the single crystalline Zn_(0.96)Co_(0.04)O thin film, the much stronger ferromagnetism in the twin-crystalline one indicates that the ferromagnetic origin should be highly correlated with structural defects in the twin-crystalline film. The possible magnetic mechanisms were discussed.