- 學位論文
由穿隧電子所引發之氨吸附基轉變
Transformation of adsorbed-NH2 by tunneling electrons
摘要
氨分子在室溫Si ( 111 )-7 × 7表面產生解離吸附，分裂產物H2N吸附於矽adatom或矽rest atom上。H2N被證實吸附在矽adatom的形成機率是12.1 %；且在樣品負偏壓掃描下，H2N會塞入矽adatom與下層矽原子之間，產生亞穩態，若進一步掃描，亞穩態會解離出一個氫原子。本實驗便是以此為根據，希望藉由非彈性電子穿隧，估計H2N在轉變過程中，是需要幾個電子來達成；同時也仔細觀察亞穩態在兩種偏壓掃描下的變動情形。 由於實驗是在non in-situ下進行，所以先對樣品表面缺陷數量作估計。在高溫熱處理Si ( 111 )- 7 × 7後五小時左右，表面的single defects所佔其半晶胞的比率約是5 %。曝上氨氣後，使用下面的掃描條件觀察轉換事件：正偏壓1.5 V，負偏壓2.0 V，穿隧電流0.05至0.3 nA，掃描時間大約105 s，面積是35 × 35 nm2 ；固定電壓、時間與面積，只改變穿隧電流因素下，分別求出H2N吸附在矽adatom的初態轉變為亞穩態的穿隧電子數目為1個，以及從亞穩態進一步解離出氫原子的末態所需穿隧電子數目亦為1個。從初態經由亞穩態到末態的整個轉變過程是兩個電子事件。 此外在正負偏壓作用下，亞穩態皆有機率回到初態和轉變為末態。原因是不同於初態的電子態密度，僅在費米能階1 eV以下才有大量態密度，亞穩態的電子態密度，在費米能階以上及以下皆存在，同時其能量又比初態及末態都還來得高。基於此兩種情況，解釋了為何初態僅在負偏壓作用才會有反轉產生，而亞穩態卻在正偏壓及負偏壓作用下皆有回到初態及轉變為末態的機會。
並列摘要
An NH3 molecule is dissociatively adsorbed on Si ( 111 )-7 × 7 at room temperature. The dissociated fragment NH2 can be adsorbed on top of either silicon adatom or rest atom. The probability for NH2 to be adsorbed on top of adatom is confirmed to be 12.1 %. Under the scanning of negative sample biases, the atop adsorbed-H2N can be displaced into the backbond of adatom, forming a metastable state. With further scanning of negative sample biases, the metastable state will be transformed into a final and stable state by liberating an H atom. Based on these observations, we want to know how many inelastic tunneling electrons are needed to induce the transformations in our experiments and, in particular, to study the changes of metastable state under the scanning of both polarities of sample bias. Since our experiments were performed non in-situ, therefore, we first estimated the defect density on Si ( 111 )-7 × 7 surface. About five hours later to the annealing treatment of sample, we found the density of single defect (one dark adatom in a half cell) was about 5%. After the dosage of NH3, we used the following scanning conditions to acquire STM images: 1.5 V and 2.0 V of sample bias, 0.05 to 0.3 nA of tunneling current, frame time of about 105 s, and scanning area of 35 × 35 nm2. By varying the tunneling current and fixing the other conditions, we found that the number of tunneling electron to induce both transformations, the initial state (adatom-adsorbed NH2) to metastable state and metastable state to final state, is unit. In other words, the whole transformation from initial state to final state is a two-electron process. On the other hand, we also observed that metastable state has the probability to return back to the initial state or to transform into the final state under the scanning of positive or negative sample bias. This can be interpreted by their potential energies and DOS structures near Fermi level. The initial state only has intense occupied DOS below 1 eV and almost has no unoccupied DOS from 0 to 3 eV. The metastable state has DOS on the both sides of Fermi level, but its potential energy is higher than the other states. These facts explain what we observed in our experiments very well.