Abstract
本論文的研究主題為利用準彈性(菊池,Kikuchi)電子繞射和多重能量相位相加法研究Ga/Si(111)r3*r3R30度的表面原子結構.我們利用低能電子繞射裝置和CCD攝影機記錄Ga/Si(111)r3*r3R30度表面的準彈性(菊池,Kikuchi)電子繞射圖型,並從這些繞射圖型中獲取準彈性(菊池,Kikuchi)電子放射能譜.使用多重能量相位相加法(IEPSM)對此電子放射能譜作富利葉轉換(Fourier-transformation),可獲此表面的三度空間原子影像.為了增強來自表面原子的訊號,電子束除了垂直入射表面外,尚以相對於表面垂線26度,50度和80度等角度入射.當電子束垂直或以26度入射時,只能得到bulk的原子影像.當電子束以50度或80度入射時,我們可獲得在Ga/Si(111)r3*r3R30度表面上位於鎵(Ga)下方矽原子的原子影像.但由於以下兩個因素使得鎵(Ga)吸附在 Si(111)表面上的位置無法從我們的研究中決定:(i)因為準彈性(菊池,Kikuchi)電子不能選擇元素,所以矽放射子(emitter)和鎵放射子周圍原子的影像能同時出在我們獲得的影像中.(ii)在T4模型中,原子相對於鎵放射子的位置,不是與bulk中原子相對於矽放射子的位置相似,就是與H3模型中原子相對於鎵放射子的位置相似.The multiple-energy quasi-elastic (Kikuchi) electrondiffraction patterns of the Ga/Si(111) r3*r3 R30 degreessurface have been measured by use of the commercial low energyelectron diffraction (LEED) optics and the charge- coupleddevice (CCD) camera. The integral-energy phase-summing method(IEPSM) is used to Fourier-transform the electron emissionspectra extracted from the measured diffraction patterns. Thethree-dimensional real space atomic images are obtained forthis surface. In addition to the normal incidence, the electronbeams are also directed off-normally to the sample with angles26 degrees, 50 degrees and 80 degrees relative to the surfacenormal in order to enhance the surface structure information.When the electron beams are directed to the surface normally oroff-normally with angle 26 degrees, only the atomic images ofthe silicon bulk are observed and no images induced by theadsorption of the gallium are obtained. As the electron beamsare directed to the surface off-normally with angles, 50degrees and 80 degrees,the atomic images of the silicon atomsbelow the gallium on the Ga/Si(111) r3*r3 R30 degrees surfacecan be reconstructed. However,the adsorption sites of thegallium on the Si(111) surface can not be determined in ourstudies for two reasons: (i) Because the quasi-elastic(Kikuchi) electrons are not chemical-specific, the images ofthe atoms surrounding the silicon and gallium emitters can bothappear in the same reconstructed images. (ii) The relativelocations of the atoms neighbor to gallium in T4 model aresimilar either to those of the atoms neighbor to the bulkemitter, or to those of the atoms neighbor the gallium in H3model.