論文主要探討表面電洞聚集效應在二硫化鎢(WS2)層狀半導體單晶光電導特性上的作用。實驗上透過比較具有原始表面(Non-fresh surface)與新撕表面(Fresh surface) WS2單晶的光電導特性差異以瞭解其物理機制。首先在原始表面WS2單晶受到波長638 nm的雷射照射後，可以看到分別存在著快速及慢速段的光電流反應。由功率相依的高時間解析光電導(TRPC)量測指出原始表面WS2單晶快速段的反應時間(載子活期)落在38 ~ 118 μs。進一步透過溫度相依TRPC量測，可證明原始表面WS2的快速反應段是由表面能帶彎曲所主導。另一方面，在新撕表面WS2也可以觀察到有類似的兩段光電流反應，其快速段反應時間(10 ~ 22 μs)快於原始表面樣品，溫度相依TRPC量測也觀察到不一樣的活化能，顯示確實有移除掉表面的電洞聚集，使得表面能帶彎曲變弱。相對於原始表面WS2，新撕表面樣品表現出較接近本質半導體的特性(能帶間非直接能隙復合)。上述的能帶彎曲效應也可以解釋為何在原始表面的慢速反應段(1 ~ 60 s)的載子活期會遠長於新撕表面的WS2。

This thesis primarily investigates the effect of surface hole accumulation (SHA) on photoconduction properties in single-crystal tungsten disulfide (WS2) to explore whether it allows the material to approach its true intrinsic characteristics. The WS2 crystals with two different surface conditions including the pristine old surface (non-fresh surface with SHA) and the newly exfoliated surface (fresh surface without SHA) were compared and investigated. Upon irradiation with a wavelength of 638 nm, the WS2 with non-fresh surface exhibits the photocurrent response comprising a fast and a slow components. Power-dependent time-resolved photoconductivity (TRPC) measurements reveal that the response time (carrier lifetime) of the fast component in the non-fresh surface WS2 are in the range of 38~118 μs. Temperature-dependent TRPC measurements show that the fast response component of the non-fresh surface sample is dominated by the surface band bending (SBB) induced by SHA.
On the other hand, similar two-step photoresponse was also observed in the WS2 with fresh surface. The response time of the fast component in the fresh WS2 at 10~22 μs are higher than that of the non-fresh samples. Temperature-dependent TRPC measurements further reveal a lower activation energy in the fresh samples. These results all indicate the SHA layer or SBB has been successfully removed or diminished. The WS2 with the fresh surface exhibits an nearly intrinsic semiconductor nature. The proposed model based on the SHA effect also explains the distinct photoresponse behaviors of the slow component in fresh and non-fresh WS2.

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