低溫生長鉍薄膜之結構與電性的研究

Structures and electrical properties of bismuth thin films grown at low temperature

吳耿碩(Keng-Shuo Wu)

指導教授 ： 陳銘堯

國立臺灣大學/理學院/物理研究所/博士(2008年)

https://doi.org/10.6342/NTU.2008.01015

摘要

我們採用脈衝雷射蒸鍍法在玻璃基板上成長鉍薄膜，並且觀察鉍薄膜在不同基板溫度（110 K 到 473 K）的生長狀況。實驗方法包括用X-ray繞射來分析薄膜的晶體結構，還有利用X-ray反射來測量薄膜厚度及表面粗糙度。另外，我們還使用原子力顯微鏡觀察薄膜的表面。在電性測量上，我們用Van der Pauw 4點量法來測量薄膜的電阻值和霍爾係數。我們發現薄膜的方向、晶粒的大小、表面粗糙度都會隨基板溫度而改變。結果顯示，在低溫下（110 K < T < 130 K）成長的鉍薄膜和高溫下成長的情況有很大的差異。例如：當生長溫度從298 K 降到110 K時，薄膜方向會從（111）轉變成（110）方向。而且在沒有任何額外退火(anneal)的處理下，表面粗糙度也從1.2 nm減少到0.5 nm。由於脈衝雷射蒸鍍法提供給粒子的動能很高，我們相信蒸鍍到基板表面的粒子有足夠的動能可以到達最低能量態。因此我們推論：在低於130 K時，鉍薄膜在熱力學上最穩定的狀態是(110)方向。
在電性方面的表現，低溫成長的鉍薄膜和高溫成長的鉍薄膜也呈現很不一樣的特性。例如：一般鉍薄膜都是p-type，但是低溫成長(110)方向的鉍薄膜卻是n-type。另外我們在10 K的低溫下也測量到低溫成長的鉍薄膜的電阻率會隨著厚度有震盪的變化。我們將這個現象歸因於量子侷限效應（quantum size effect）。更進一步地分析不同厚度下電性和溫度的關係可以發現：當厚度很薄的時候，由於表面的效應，鉍薄膜會有金屬性的傾向（metallic-like）而不是半導體。

關鍵字

鉍薄膜 ； 脈衝雷射蒸鍍 ； 薄膜方向 ； 粗糙度 ； 電性

並列摘要

Bismuth thin films were grown by pulsed laser deposition on glass substrates with the substrate temperature from 110 K to 473 K. The structure of the films was characterized by X-ray diffraction. The surface morphology was studied by atomic force microscopy and X-¬ray reflectivity. The electrical properties of the films were probed by Van der Pauw measurement. We observed the changes in the orientation, grain size and roughness of the bismuth films as a function of the substrate temperature. The results for the Bi films grown at low temperatures (110 K < T < 130 K) were very different from those for the (111)-oriented Bi films grown at higher temperatures. As the temperature was reduced from 298 K to below 130 K, the preferred orientation of the Bi films was changed from (111) into (110), and the surface roughness was reduced from 1.2 nm to 0.5 nm without any annealing. Considering the highly energetic adatoms generated by the short UV laser pulses and hence their capability to escape from the local energy minima, the orientation transformation below 130 K might be an indication that the (110)-oriented Bi thin film on glass is thermodynamically more stable than the (111)-oriented one at low temperatures.
The electronic properties of the Bi thin films, grown by pulsed laser deposition at 110 K on glass substrates in vacuum, were also very different from those grown at higher temperatures. The conduction type changed from p- to n-type, indicated by the negative Hall coefficients of the films measured from 10 to 300 K. Oscillating thickness dependence of the resistivity was observed at 10 K, which is attributed to the quantum size effect. Moreover, the temperature and thickness dependence of the electrical properties shows the trend that when the films are thinner, they become more metallic-like rather than semiconductive due to the surface effect.

並列關鍵字

Bismuth thin films ； pulsed laser deposition ； preferred orientation ； roughness ； electrical properties

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