氧化鋅薄膜之雙軸向應變效應與光學增益之研究

Abstract

為了研究氧化鋅薄膜和基板之間的應力，我們成長氧化鋅薄膜在三種不同的基板：藍寶石基板(001)面、矽(111)面與碳化矽(001)面。接著用波長355nm的三倍頻Nd:YVO4雷射激發磊晶品質佳的氧化鋅薄膜，量測其光學增益值。從反射式高能電子繞射圖及X光繞射□□rocking curve掃描中，我們發現成長在藍寶石基板和碳化矽基板上的氧化鋅薄膜品質較佳。我們也針對成長在藍寶石基板和碳化矽基板上的氧化鋅薄膜(202)面，做X光繞射□-scan掃描，清楚的發現其具有六重對稱，確定為磊晶薄膜。至於成長在矽(111)面的氧化鋅薄膜，並沒有發現六重對稱。利用 X光繞射對氧化鋅薄膜 (002) 和 (110)面做□-2□掃描，我們決定三種樣品的氧化鋅薄膜c軸與a軸的晶格常數。配合室溫光激光光譜的分析，分析出氧化鋅薄膜和三種基板間的雙軸向應變效應。除了晶格常數匹配的影響，熱膨脹效應其實也影響著薄膜和基板間的應力。 此外，我們也量測了生長於藍寶石基板的氧化鋅薄膜上在不同激發強度下的光學增益值。發現雷射激發強度在4.585MW/cm2時，氧化鋅薄膜的激發輻射主是來自於激子與激子間的非彈性碰撞。當雷射激發強度達到8.45MW/cm2時，激子的數目會增加，激子間不再有庫倫作用力，而形成電子電洞的電漿態。當雷射激發強度達到25.3MW/cm2，這時候的最大光學增益值是187 cm-1。隨著雷射激發強度的增加，氧化鋅薄膜的激發輻射將會逐漸由電子電洞形成的電漿態產生。

We study the effect of biaxial stress on the ZnO thin films by growing those onto Al2O3 (001), Si (111) and SiC (001). We use a third harmonic of Nd:YVO4 laser (□=355nm) to measure the in-plane optical gain of high-quality ZnO epilayers on Al2O3. From reflection high energy electron diffraction (RHEED) observation and □□rocking curve, we found qualities of ZnO/Al2O3 and ZnO/SiC are better. Phi scans for (202) peaks of three samples indicate six-fold symmetry and epitaxial growth of ZnO thin films onto Al2O3 and SiC except Si (111). We have investigated X-ray □-2□ scan for (002) and (110) reflections to determine c- and a-axis lattice constants of ZnO thin films on different substrates. With room-temperature (RT) PL spectra of three samples, we can derive the stress states in films. The thermal stress can also play a key role in deciding the residual strain in film. Beside we found that the inelastic exciton-exciton (ex-ex) scattering dominantly contributes to the optical gain at excitation densities, 4.585 MW/cm2 and the electron-hole plasma (EHP) state at excitation densities, 8.45MW/cm2. Maximum gain of 187 cm-1 at the excitation density of 25.3 MW/cm2 is obtained. With increasing excitation, density contribution from the EHP state gradually takes over at the lower energy side in the optical gain spectrum.