表面氧化層對鍺化鎳奈米線表面形貌的影響

Abstract

藉由調變氫氟酸(HF)蒸氣蝕刻的時間，來探討鍺奈米線外圍的原生氧化層對其鍺化鎳奈米線表面形貌的影響。首先將鍺奈米線轉移到Lacey carbon銅網上，再利用HF蒸氣蝕刻鍺奈米線外圍的原生氧化層，接著利用電子束蒸鍍機在樣品上沉積 4 nm厚的鎳，最後在爐管中以300~500 oC退火一個小時，並利用SEM、TEM、EDS對表面形貌進行分析。 在鍺奈米線有原生氧化層的條件下，以400 oC熱退火一個小時後形成的鍺化鎳奈米線維持均勻表面形貌。但在相同退火條件下，改變HF蒸氣蝕刻時間從1~5 分鐘，將導致奈米線表面形貌不均勻，其與以500 oC熱退火一個小時後形成的鍺化鎳奈米線的表面形貌不均勻的結果相似，並且認為此時已有過度反應發生，主要原因我們推測與鍺化鎳本身熱力特性欲形成表面能較低的狀態有關，其形貌欲從一維轉變成零維，而且非晶氧化物的存在也會使鍺化鎳奈米線表面有淺色往內凹陷的現象甚至是取代原本在奈米線內的鍺化鎳，此與鍺基板反應界面不平整與鍺化鎳薄膜的厚度變薄的現象相呼應。因此，我們認為鍺奈米線在反應成鍺化鎳後其外圍的原生氧化層可以維持奈米線的表面形貌的均勻性。

We explored the influences of surface native oxide on the morphology of the nanowires after germanide formation by changing etching time using buffered hydrofluoric acid. Ge nanowires (NWs) was transferred to lacey carbon film covered on copper grids. The buffered hydrofluoric acid steam was used to etch the native oxide or oxide shell on Ge NWs, and a 4 nm thick Ni film was deposited on the samples using electron beam evaporation. The samples were then annealed at 300~500 °C for 1 hr with the ramping rate of 1 °C/s in a furnace. The surface morphology of nickel germanide NWs were investigated by SEM, TEM, and EDS characterizations. With native oxide, the NWs transformed to germanide after annealing at 400 oC for 1 hr and kept the uniform shape. Nevertheless, at the same annealing condition, the NWs surface became rough with etching treatments from 5 to 1 min, which is similar to the result of annealing the NWs at 500 oC for 1hr. We considered that nickel germanide NWs had overreacted at 500 oC. We speculated that the main reason for the changes on the surface roughness was associated with the nature in thermodynamics where the NWs favor low surface energy as transforming 1D to 0D-like shapes. In addition, the amorphous oxide may cause the sunken appearance and even replace the nickel germanide within the NWs. The observations are in agreement with the thin film reactions where we saw rough interface between nickel germanide and Ge(111) substrate and the thickness of nickel germanide is thinner than expected. Therefore, we conclude that the native oxide covering the Ge NWs maintain the uniformity of the NW surface morphology after reactions.