利用聚焦離子束沉積鉑與矽奈米線接觸之傳輸機制

Abstract

在本論文中，我們利用化學濕蝕刻備製n型一維單晶奈米線，並且利用聚焦離子束的直寫技術將鉑金屬沉積在矽奈米線上以連接金電極。 在第一部分中，我們研究金屬鉑與矽奈米線接觸的電性與接觸電阻，我們發現到利用聚焦離子束沉積的鉑會和矽奈米線在接面形成歐姆接觸，其中特性接觸電阻值大約為10-6 (Ω-cm2)，這個現象和之前的研究有非常大的不同因為，理論上金屬鉑對矽接觸應是蕭基接觸。為了要了解這個現象，我們研究了接觸面的結構特性與電流傳輸機制。從穿隧式顯微鏡的觀察我們可以發現到矽奈線會形成非晶的現象，並且因為電子在非晶的材料上其平均自由路徑很短，所以電流傳輸機制應該用擴散理論來解釋之。根據擴散理論，我們計算出非常低的有效能障，自由載子濃度與遷移率。根據我們的研究，費米能階釘住效應是造成很小的有效能障的主因，進而造成金屬鉑對矽奈米線形成歐姆接觸 在第二部分中，我們研究了電子在利用聚焦離子束沉積的鉑金屬連接的矽奈米線之傳輸機制。在導電率與溫度的關係中，我們可以分成兩個區塊來討論。在高溫區中，電子利用熱激發在奈米線上傳輸。電子在矽奈米線的施體能接經由熱激發到導電帶，其熱活化能為0.045電子伏特。因為聚焦離子束在矽奈米線造成的無序效應，在低溫區中，電子傳輸機制可以用莫特的變程跳躍傳輸解釋之。根據莫特的理論，我們計算了莫特參數如跳躍距離、在費米能階的電子態密度、跳躍能量與定域化長度，其值與莫特的理論相符合。

In this thesis, The galvanic wet etching was adopted to fabricate single-crystalline n-Si nanowires (NWs) at room temperature in HF/AgNO3 solution. The focus-ion-beam-deposited Pt (FIB-Pt) was employed to connect photolithographically prepared Cr/Au (5/60 nm) pads on a Si substrate which was capped with a 600 nm thick SiO2 layer deposited by plasma-enhanced chemical vapor deposition (PECVD). First, we investigate the electrical characteristic of FIB-Pt contact to Si NWs under various Ga+ dosage. We show that FIB-Pt contact to Si NWs forms very low contact resistance compared with present studies, the specific contact resistivity was estimated in the order of 10-6 Ω-cm2. To gain insight on ultra low contact resistance, the morphologic of structure and electrical transport have been studied. The image of TEM demonstrates that the existence of FIB-induced amorphization under Pt contacts, the ultra low effective barrier height was calculated using diffusion theory. The free carrier concentration and mobility has also been estimated. Due to the existence of FIB-induced amorphization, the Fermi level pinning by surface states might be responsible for the barrier height lowering and further lower the specific contact resistivity. Second, we study the transport mechanism of Ga+ doped and undoped of n-Si NWs. The temperature-dependent four-probe measurements indicate that the electrical conductivity of the Si NWs exhibits two regimes. Thermal activated transport dominates at low temperature range. The Mott’s variable range hopping (Mott-VRH) model is applied to the conduction at high temperature range due to FIB-induced disorder. Mott’s parameters of Si NWs, such as hopping energy, hopping distance and density of states at Fermi level have been estimated.