"Interface Controlled Heteroepitaxy of Ionic Crystals and Covalent Crystals Using Ionicity Control Layr"

从组成材料的离子性角度研究了不同材料异质外延的界面控制，提出了“离子性控制层”的概念，即通过插入具有中间离子性的材料来调节较大的离子性差异。本课题实验采用GaAs/CaF_2界面，采用ZnSe作为离子控制层。在ZnSe（100）表面生长SrF_2薄膜，采用MBE法测定其外延温度为250℃。以往许多关于氟化物在各种半导体衬底上异质外延的报道都是根据衬底材料的离子性来排序的，包括本实验的结果。结果表明，随着外延膜与衬底之间的离子性差异变小，外延温度降低。利用RHEED和AFM优化了CaF_2（111）表面几nm ZnSe层的生长条件。在此基础上，在ZnSe层上生长GaAs层，并观察了生长的GaAs层结晶度与ZnSe层生长条件之间的关系。结果表明，在ca_2表面沉积1 ~ 5nm厚的ZnSe，并在ca_2表面进行300℃退火后，GaAs层x射线振荡曲线的FWHM比在CaF_2表面直接生长的GaAs降低了50%左右。AFM观察了ZnSe在CaF_2表面生长的初始阶段和GaAs在ZnSe层上生长的过程，结果表明，与CaF_2表面相比，ZnSe表面GaAs的成核密度增加。推测这种高密度成核有助于提高GaAs层的结晶度。

Interface control of heteroepitaxy of very different materials was investigated from the viewpoints of ionicity of the composed materials and a concept of "ionicity control layr" in which large difference of the ionicity was accommodated by inserting a material having intermediated ionicity was proposed. In experiments in this project, GaAs/CaF_2 interface was employed and ZnSe was examined as the ionicity control layr.The epitaxial temperature for SrF_2 film grown on ZnSe (100) was evaluated as 250ﾟC using MBE method. Many previous report concerning heteroepitaxy of fluorides on various semiconductor substrates were ordered in ionicity of substrate materials, including the result of this experiment. It was found that epitaxial temperature was decreased as difference of ionicity between epilayr and substrate became small.Growth condition of a few nm ZnSe layr on CaF_2 (111) surface was optimized using RHEED and AFM.Based on these observation, GaAs layr was grown on the ZnSe layr, and the relation between crystallinity of the overgrown GaAs layr and the growth condition of the underlying ZnSe layr was observed. As a result, FWHM of X-ray rocking curve from the GaAs layr was reduced to about 50% compared with direct growth of GaAs on CaF_2 surface, when 1-5nm thick ZnSe was deposited on CaF_2 surface at 200ﾟC and was post annealed at 300ﾟC prior to the GaAs growth. AFM observation for initial stage of ZnSe growth on CaF_2 surface and GaAs growth on the ZnSe layr revealed that nucleation density of GaAs was increased on ZnSe surface compared with CaF_2 surface. It was speculated that this high density nucleation was contribute to the improvement of crystallinity of GaAs layr.