二維鈷原子島在銀/鍺(111)√3×√3表面聚集與成長之研究

Abstract

在超高真空的環境中（<10-10mbar），利用分子束蒸鍍鎗（MBE）成功地在半導體鍺（111）表面成長鈷的磁性薄膜，並藉由掃描穿隧顯微鏡（STM）觀察其成長初期的行為。為了阻隔鈷與鍺形成化合物，我們先在鍺的表面加鍍單一層原子的銀，並加熱使其形成(√3×√3)的穩定重構，再行鍍0.35ML的鈷並加熱至200℃以上，即成功地發現鈷原子在表面上形成具有週期性的二維的原子島，這些二維的原子島的厚度在兩層以下僅相差0.05nm，其後則皆相差0.2nm，而且兩層以下的鈷原子排列較為鬆散，其結構週期相對於基底鍺為(√13×√13)，然兩層以上的鈷原子排列較為緊密，結構週期則變為(2×2)。我們也發現，兩層以下的鈷原子磊晶方向具有特殊的鏡像對稱模式，鏡射面分別平行鍺(111)面上的 、 、和 三個方向，而鈷以一層伴隨一層地成長至第三層之後則會恢復與基底鍺相同的堆積方向，這些足以說明鈷與鍺之間雖然隔著一層銀，但關係仍然密不可分。此外，當加熱溫度達到300℃以上，鈷獲得較多的動能足以逐漸克服基底的影響而較為自由運動，其總體擴散活化能約為2.39eV，並且為了降低整體表面自由能，鈷趨向不斷地聚集並往三維的方向成長，使表面露出更多有銀的部分，也因此出現更大或更高的鈷原子島。

The Co magnetic super thin films on semiconductor Ge(111) surface are grown successfully by in situ depositing with MBE. Their growth behaviors and equilibrium structures are observed by STM and LEED. In order to prevent alloy reactions of cobalt and germanium, the Ag/Ge(111)√3×√3 surface, produced by depositing 1ML Ag onto Ge(111)-c(2×8) and anneal up to 500℃, is chosen as substrate. After depositing 0.35ML cobalt and annealing to 200℃, some structured 2D cobalt islands were started to be found. Raising the annealing temperature to 300℃~500℃,the islands become larger and higher. These 2D Co islands construct two shapes, the lower Shape 1 and the higher Shape 2. The Shape 1 islands (under 2 atomic layers) have the period of (√13×√13) and each layer separates with 0.05nm. Especially, the structure of this shape is of reflection symmetry and the mirror planes are along , , and axes of the Ge(111) surface. The Shape 2 islands (over 3 atomic layers) show another period of (2×2) and return to cobalt own separation of 0.2nm. Those island sizes also depend on annealing temperatures. Besides, by nucleation theory, the total diffusion activation energy E=2.39eV. All of the phenomenon may relate to the surface free energy and interface constrains.