摻雜釤離子氧化鋅與單層過渡金屬二硫屬化合物薄膜之光譜性質研究

Abstract

我們量測摻雜釤離子氧化鋅薄膜的拉曼散射光譜、穿透光譜及橢圓偏振光譜，研究不同摻雜濃度對氧化鋅薄膜光譜性質的影響。另外，我們量測單層過渡金屬二硫屬化合物薄膜(MoS2、MoSe2、WS2及WSe2)的變溫橢圓偏振光譜，探討單層過渡金屬二硫屬化合物薄膜的光譜性質。 未摻雜氧化鋅薄膜的拉曼散射光譜顯示兩個拉曼特徵峰，頻率位置為99.1 cm-1和437.9 cm-1，分別為E2(low)及E2(high)振動模，隨釤離子濃度上升，E2(low)藍移，E2(high)紅移，強度逐漸下降，並於釤離子濃度3%以上消失。穿透光譜顯示隨著釤離子濃度增加，紫外光區的透光率增加。吸收光譜展現隨著釤離子濃度增加，能隙逐漸藍移，我們分別以柏斯坦-莫斯位移理論(Burstein-Moss effect)及Zn1-xSmxO合金能帶結構解釋低濃度(≤5%)與高濃度(≥8%)摻雜樣品能隙之變化。 藉由分析變溫橢圓偏振光譜數據，我們取得單層過渡金屬二硫屬化合物薄膜之變溫折射率與消光係數能譜圖，隨著溫度上升，整體折射率與消光係數強度逐漸下降，結構紅移。我們觀察到所有樣品於近紅外與可見光區域皆有兩個明顯的吸收峰值，標記為A，B激子，為自由激子於布里淵區K(K´)點之躍遷。緊接在自由激子後的數個結構為電子於布里淵區Λ與M點之躍遷。所有樣品之光學能隙皆隨著溫度上升而紅移，A，B激子紅移，半高寬增寬。A，B激子之能量差為自旋耦合分裂能量，其不隨溫度變化而改變。

We investigated the samarium (Sm) doping effects on the optical properties of ZnO thin films and reported the temperature dependence spectroscopic ellipsometry studies of monolayer transition metal dichalcogenides thin films such as MoS2, MoSe2, WS2, and WSe2.

Room temperature Raman scattering spectrum of undoped ZnO thin film shows two phonon modes at approximately 99.1 cm-1 and 437.9 cm-1, displaying symmetries of E2(low) and E2(high). With increasing Sm doping, E2(low) mode shows a blueshift. By contrast, E2(high) mode shows a redshift. Furthermore, the intensities of these two phonon modes decrease and become completely diminished for the 3% Sm doped sample. The intensities of optical transmission spectra show an increase in the ultraviolet region and the band gap energy shows a blueshift as the Sm contents increases. We attributed this feature to the Burstein-Moss effect for 3% and 5% doped samples and the band characteristics of ternary Zn1-xSmxO alloys for 8% and 10% doped samples.

The temperature dependent refractive index and extinction coefficient spectra of monolayer transition metal dichalcogenides thin films were extracted from the ellipsometry parameters. Room temperature optical absorption spectra of these materials show two excitonic transitions (denoted as A and B excitons). They originate from the spin-split direct gap transitions at the K points of the Brillouin zone. Several high energy absorptions are associated with the electronic transitions at the Λ and M points. With increasing temperature, the intensity of refractive index decreases and the absorption structures show a redshift. Notably, the valence spin-orbit coupling is temperature independent.