锗硅量子点的自组织生长和微结构的研究

【作者】 林健晖；

【导师】 蒋最敏；

【作者基本信息】 复旦大学 ， 凝聚态物理， 2009， 博士

【摘要】 以半导体量子点为代表的半导体纳米结构在光电子、微电子和单电子器件领域有重要的应用前景,在硅衬底上自组织生长的GeSi量子点由于与成熟的硅集成电路工艺兼容更具有特殊的意义,是当前的研究热点之一.研究GeSi量子点中的组分和应变分布,有助于理解量子点的结构和物理特性之间的关系,同时也有助于了解量子点的生长机制.我们利用透射电子显微术研究了单个GeSi/Si(001)量子点的组分和应变分布.通过分析GeSi量子点的高分辨透射电子显微镜图像,获得了单个量子点中各处的晶格变化.根据弹性理论和Vergard定律,由晶格常数得到了量子点的组分分布和应变分布.结果显示量子点中心偏顶部区域的Ge组分最高,项部表面的Ge组分较低,底部侧表面的Ge组分最低,其组分分别为0.7,0.6和0.3.量子点中的Si原子是在量子点的生长过程中通过表面扩散而不是界面扩散进入量子点的.量子点中的应变是部分弛豫的,其应变分布为,量子点中间最大,底部比顶部大.量子点中的应变弛豫是通过Si衬底的形变来实现的.在实际器件应用中,Ge/Si量子点往往需要被掩埋在Si材料中.我们研究了初始覆盖Si层的厚度和温度对自组织GeSi/Si(001)量子点形状、大小及其组分和应变的影响.量子点在400℃下覆盖Si层后,形貌均没有变化:在640℃下初始覆盖Si层后,形貌从圆顶演变为由金字塔或棚屋.利用拉曼光谱实验研究量子点在覆盖Si层前后的组分和应变的变化.根据实验结果得到没有覆盖Si层量子点的平均Ge组分和应变分别为0.74和-0.98%;在400℃下覆盖Si层后,量子点里面的组分和应变均没有改变;在640℃下覆盖Si层后,量子点的Ge组分降低和应变增加.覆盖厚度3.2(?)的Si层后,量子点的平均Ge组分和应变分别为0.71和-1.74%;覆盖厚度6.4(?)的Si层后,量子点的平均Ge组分和应变分别为0.69和-1.63%.考虑到体积的变化,量子点总的应变能增大.通过分析,量子点表面能的变化远小于应变能的变化.考虑到量子点下方的衬底应变能的变化,系统(衬底+量子点)的总能量才有可能下降.Si覆盖层在高温下生长,被掩埋量子点的形貌和组分会发生显著变化,从而量子点的物理性质以及周围的应变Si的特性也随着发生明显变化.这不是人们所希望的.降低Si覆盖层的生长温度,可以使得被掩埋量子点的形貌和组分保持不变.然而低温生长的Si覆盖层的晶体质量尤其是随着覆盖层厚度的变化很少被人们关注.这对于利用量子点周围应变Si制作器件而言是一个十分重要的问题.我们利用透射电子显微学研究了GeSi/Si(001)量子点低温Si覆盖层的晶体质量及其随覆盖层厚度的变化.研究结果表明,在温度300℃下,覆盖Si层后量子点的形貌和组分保持不变,然而当Si覆盖层厚度超过一定数值时(20-30 nm),在量子点的正上方产生了层错或者微孪晶等面缺陷,并且这些面缺陷随着覆盖层厚度的增加扩展到样品表面.这一发现对应变Si层的器件应用无疑是值得关注的结果.另外,对于两个靠近的量子点,这种面缺陷在两个量子点的内侧上方优先产生.根据低温生长Si覆盖层中应变能随厚度的变化及其释放定性地解释了这些现象.在量子点上低温生长的Si覆盖层经过高温退火之后,表面会形成与量子点密度和尺寸相近的方坑,通过TEM实验发现每个坑的正下方均有一个量子点.在Si层中,量子点上方区域是应变集中的区域.尽管在低温下覆盖一定厚度的Si层后,在量子点正上方的Si层会形成堆垛层错等面缺陷,但是Si层中依然有较大的残余应变.在高温下,Si原子在应变的驱使下,从应变能高的区域迁移到较低的区域而最终形成了坑.更多还原

【Abstract】 Self-assembled GeSi islands grown via the Stranski-Krastanov mode on Si(OOl) substrates have been widely investigated during the last decades.The interest is mainly driven by their promising applications in a new generation of devices compatible with the existing Si technology and by the understanding of strained layer epitaxy.It is well known that the composition of the islands,together with their shape and size, is a critical parameter in determining their optoelectronic properties.As a consequence,to determine the distribution of the composition and misfit strain in the islands is important to understand their structure-property relationship as well as their growth mechanism.In the first part,the composition and the strain distributions in the GeSi/Si(001) coherent islands have been determined by digital analysis of HRTEM images.The results show high Ge content at the central region of island,whereas a lower Ge content near the outmost shell. The island is partially relaxed by the substrate deformation(tensile strain) and strain concentrated around the edge of islands.When the GeSi islands are put into actual applications in optoelectronic and microelectronic devices,it is necessary to bury islands in Si,i.e.to grow an additional Si layer on the GeSi islands.Previous investigations revealed that the GeSi island shape changes after Si capping at a high temperature,whereas by means of low temperature deposition,GeSi islands are embedded into a Si matrix without appreciably altering their shape and composition.The strain energy and/or surface energy may play a key role in the shape transformation.The knowledge about evolution of composition and stain energy distribution is very important to fully understand the island growth mechanism during Si capping.In the second part,the changes of the Ge content and the strain in the self-assembled islands on Si(001) at initial Si capping are investigated by Raman spectroscopy.Both peaks of Ge-Ge and Ge-Si vibration modes show blueshifts after Si capping at 640℃with a layer thickness of 0.32 and 0.64 nm.According to the peak positions of Ge-Ge and Ge-Si,the strain and the Ge composition in the islands are analyzed. It is found that the strain energy in an island increases remarkably after Si capping.After simple analysis on the surface energy,it is concluded that the strain energy in the substrate in association with an island formation and evolution should be included in accounting for the resulting island shape transition during Si capping.For the application of the microelectronic device,if one wants to use the islands as stressors to introduce a strain,thus to increase the electron mobility in the Si capping layer, the preservation of island morphology is useful.However,little attention has been paid on the quality of the low-temperature grown Si capping layer,although such information is also critical for the final devices.In the third part,planar defects were observed in the Si capping layers by TEM,which were overgrown on the GeSi islands at low temperature of 300℃.The generation of the planar defects benefits the strain energy relaxation in the Si capping layer.In the fourth part,after the quantum dots with a low-temperature-grown Si capping layer annealing at a high temperature of 640℃,pits were observed on the surface.The density of the pits was nearly equal to that of the quantum dots,and it’s found that the pits were exactly over the quantum dots by TEM.Similar to the case of the planar defects,the generation of the pits benefits the strain energy relaxation in the Si capping layer.更多还原