以氣-液-固生長方法形成氧化鎵鋅奈米線的極化特性

Abstract

利用球面相差修正掃描式穿透電子顯微鏡的環型明場拍攝技術，我們研究在富鋅以及富氧條件下成長於不同極性氮化鎵基板上的氧化鎵鋅奈米線及薄膜極性。我們利用銀奈米顆粒當作生長催化劑經由氣-液-固生長方式成長氧化鎵鋅奈米線，在生長氧化鎵鋅奈米線期間，通過氣-固成長方式同時可在奈米線間隙成長氧化鎵鋅薄膜。雖然薄膜的極性遵循生長基板的極性，但是在富鋅(氧)的成長條件下，生長在不同極性基板的奈米線卻總是鋅(氧)極性的。換句話說，無論生長的基板極性如何，氧化鎵鋅奈米線的極性都是由其富鋅或富氧成長條件所控制。在奈米線生長初期，由於在三相線上所形成的核島之晶格尺寸差異極大，當從多個核島起始的橫向生長形成氧化鎵鋅雙層結構時，會產生高密度的堆疊缺陷。在此情況下，晶體會產生頻繁的極性反轉，因此氧化鎵鋅的極性極不穩定。當奈米線的橫截面積隨生長而變小時，不同位置的核島之晶格變得較均勻，使得堆疊缺陷減少，而極性變成較穩定。在富鋅(氧)的生長條件下，鋅(氧)極性結構中氧化鎵鋅的橫向生長速率因有較多可用的懸掛鍵而較高，所以在奈米線成長中，鋅(氧)極性結構的生長佔優勢，使得奈米線生長最終是鋅(氧)極性，而與生長基板的極性無關。

By using the annular bright field images of scanning transmission electron microscopy observations, the polarities of Ga-doped ZnO (GaZnO) nanowires (NWs) and thin films grown on GaN templates of different polarities under the Zn- and O-rich conditions are studied. The NWs are formed through the vapor-liquid-solid process using Ag nanoparticles as growth catalyst. The thin films are deposited simultaneously through the vapor-solid process between NWs during NW growth. Although the polarity of a thin film follows that of the growth template, the NWs grown on templates of different polarities under the Zn- (O-) rich condition are always Zn (O) polar. In other words, the polarity of a GaZnO NW is controlled by the Zn- or O-rich growth condition, irrespective of the polarity of growth template. During the early stage of NW growth, because the lattice sizes among different nucleation islands (NIs) formed at the triple-phase line are quite different, stacking faults of a high density are produced when the lateral growths from multiple NIs form a GaZnO double-bilayer. In this situation, frequent domain inversions occur and GaZnO polarity is unstable. When the NW cross-sectional size becomes smaller along growth, the more uniform lattices of different NIs lead to fewer stacking faults and hence more stable polarity. Under the Zn- (O-) rich growth condition, because the lateral growth rate of GaZnO in the Zn- (O-) polar structure is higher due to more available dangling bonds, the growth of Zn- (O-) polar structure dominates NW formation such that the NW is Zn (O) polar irrespective of the polarity of growth template.