掃描式電場力顯微術於氮化鎵島狀結構之研究

Abstract

在本論文中，經由掃描式電場力顯微術（EFM）的原理，我們發展出一個模型。利用這個模型，我們可以求出所使用的原子力顯微術探針的表面功函數絕對值，並在不同探針與樣品間距，量測以基頻頻率ω和2倍頻率2ω振盪的受力（分別以Fω和F2ω表示）的映像來計算探針與樣品表面靜電荷之間的庫侖力。經過測定功函數後的探針可以用來量測其他樣品的表面功函數。EFM的實驗結果顯示，在圓盤狀氮化鎵島狀結構的樣品上，Fω和F2ω的映象同樣具有很好的解析表面結構的能力，我們還發現島狀結構區域的Fω 和F2ω受力強度較平坦處小，而且島狀結構高度愈高，受力愈小。我們也分別計算在矽晶圓與氮化鎵島狀結構表面上的靜電荷與探針之間以ω頻率振盪的庫侖力大小，發現其作用模式在矽晶圓上較適合用圓錐體模型來解釋，在氮化鎵島狀結構上的實驗結果用點電荷對圓盤的模型來解釋較佳，我們將這兩種差異歸因於樣品表面靜電荷的區域相對於探針的尺寸大小所造成的電場分佈不一樣所致。在無窮大平板（如本實驗的矽晶圓）上，表面靜電荷所生成的電場是均勻分佈的，作用在探針上是影響探針整體，所以可用圓錐體模型來解釋實驗結果。但在圓盤狀氮化鎵島狀結構上的靜電荷所生成的電場並不均勻且隨高度增加其強度會快速減少，在計算探針與氮化鎵島狀結構上的靜電荷之間以ω頻率振盪的庫侖力時，探針尖端的球體是主要的受力位置，所以可以用點電荷對圓盤的模型來解釋其實驗結果。

We had developed a model based on the equations related to the principle of scanning electric force microscopy (EFM). By this model, we can obtain the surface work function of the tip we used in our atomic force microscopy (AFM) system and calculate the ω-term Coulombic force between the tip and the surface electrostatic charge (ESC) on the sample in the ω-term and 2ω-term force (signed Fω and F2ω, respectively) mappings at different tip-sample distances. The calibrated value of tip work function can be used to characterize the surface work function of other samples. The EFM results show that the Fω and F2ω force mappings on disk-like GaN islands can be well-resolved. The intensities of the electric forces Fω and F2ω on the region of GaN islands are comparatively lower as respect to that of the plane surface. The higher the island is, the lower the value of intensity is. We can also calculate the ω-term Coulombic force between the tip and the surface ESC on Si wafer and that on the disk-like GaN island, respectively. We found that the reactive Coulombic force can be interpreted by the cone model on the Si wafer, and by the charged point-disk model on the disk-like GaN island, respectively. We believed that such discrepancy can be attributed to the difference of electric field distribution resulted from the dimension of the ESC distribution region on the sample surface as compared with the tip. The uniform electric field acting on all part of the tip above the infinite plane (such as the Si wafer) results in using the cone mode to interpret the experimental results. But, in the single disk-like GsN island whose electric field resulted from ESC on surface is not uniform and decreases rapidly with increasing height. The sphere of the tip is the main position suffered the ω-term Coulombic force between the tip and the surface ESC on the single disk-like GaN island. As a result, we can use the charged point-disk model to interpret the ω-term Coulombic force between the tip and the ESC purely on the single disk-like GaN island.