低應力氮化矽量子點微碟共振腔之製作與光學量測

Abstract

本論文中，我們利用電漿增強式化學氣相沉積系統(PECVD)沉積氮化矽薄膜，並利用此製程可調變薄膜應力的特性，使薄膜在受激發時不會承受過度伸張(tensile)或壓縮(compress)的應力，導致薄膜破裂(crack)，大幅提升氮化矽薄膜對熱耐受性，使其從原本承受 500 kw/cm2 的激發功率密度就會破裂，到承受激發功率密度高達 2168 kw/cm2 仍完好無損，將此薄膜當作共振腔材料，期望微碟元件可承受超出雷射閥值(threshold)的能量，達成雷射(lasing)的目的。 在製程上，先以 PECVD 沉積氮化矽薄膜，再旋塗一層硒化鎘/硫化鋅(CdSe/ZnS)膠狀量子點做為主動層材料，之後再以PECVD 沉積氮化矽薄膜，形成薄膜上下包覆量子點的三明治結構，接著以電子束微影技術定義微碟圖案，再以反應離子蝕刻機(RIE)將圖案轉移至氮化矽上，並且有良好的側壁垂直性，為防止氮化矽薄膜在蝕刻出矽支柱時受損，選用SU-8 作為保護層，以標準黃光製程定義第二道曝光的圖形，並藉著曝後烤固化 SU-8，最後使用對矽有等向性蝕刻的 RIE recipe 蝕刻出微碟結構支柱(pedestal)。 首先我們在量子點的吸收波段進行主動式量測，也就是以微光激發螢光光譜系統測量元件的性能，使用波長位於 532nm 的 Nd:YAG 連續式固態雷射作為激發光源，藉由中性灰度濾鏡(Neutral Density Filter)調整激發功率，以單光儀(monochromator)收光搭配光子計數系統(photon counting system)測量量子點發光波段的頻譜圖。 接下來為了探討光學共振腔本身的特性，我們進行被動式的量測，也就是在遠離量子點吸收波段的 1330nm 附近，藉由寬頻光源超輻射二極體(Super Luminescent Diode, SLD)搭配U 型錐形光纖(U shape taper fiber)與微碟元件耦合進行傳輸頻譜量測，成功觀察到品質因子 Q 為 955 的模態且根據自由光譜區(FSR)的公式推導出其等效折射率為 1.75，比過去實驗室製作元件的等效折射率 1.67 還要高，表示氮化矽薄膜之矽含量確實有提升導致其折射率上升。 我們分析主動式的量測沒有觀察到模態和雷射的原因，氮化矽薄膜的光學吸收係數(optical absorption coefficient)隨著矽含量增加在可見光波段會增加，且作為保護層的 SU-8 由於其折射率比空氣大，會降低共振腔侷限光場的能力，使光在環繞時光場在主動層的比率下降，造成元件增益(device gain)減少，最終導致即使微碟有 WGM 訊號也難以透過量測系統觀察。

In this thesis, we utilized a plasma-enhanced chemical vapor deposition (PECVD) system to deposit silicon nitride films. Leveraging the tunable stress characteristics of this process, the films avoid excessive tensile or compressive stress upon excitation, which prevents cracking and significantly enhances their heat tolerance. Consequently, the capacity of silicon nitride films to withstand excitation power density improved from an initial 500 kW/cm², where they would crack, to enduring up to 2168 kW/cm² without damage. We employed these films as resonator materials, aiming for the microdisk devices to endure energy beyond the laser threshold and achieve lasing. The fabrication process involves first depositing a silicon nitride film using PECVD, followed by spin-coating a layer of Cadmium Selenide/Zinc Sulfide (CdSe/ZnS) colloidal quantum dots as the active layer material. Another layer of silicon nitride film is then deposited by PECVD, forming a sandwich structure with the quantum dots enclosed. Next, we define the microdisk pattern via electron beam lithography. This pattern is transferred to the silicon nitride using reactive ion etching (RIE), ensuring good vertical sidewall profiles. To protect the silicon nitride film from damage during the etching of the lower pedestal, SU-8 is chosen as the protective layer. A second exposure pattern is defined using standard photolithography with SU-8, followed by post-exposure baking to make sure the SU-8 cross linking. Finally, RIE with an isotropic etching recipe for silicon is used to etch the lower pedestal. Initially, in the quantum dot absorption band , we use a micro-photoluminescence (micro-PL) spectroscopy system for active measurements to evaluate the device performance. A 532 nm Nd:YAG continuous wave solid-state laser serves as the excitation source. The excitation power was adjusted using a neutral density filter, and a monochromator coupled with a photon counting system was used to measure the photon counts at different wavelengths, thus generating the spectrum. To investigate the characteristics of the optical resonant cavity itself, passive measurements were conducted. These measurements were performed away from the quantum dot absorption band. Transmission spectrum measurements were performed using a superluminescent diode (SLD) broadband light source near the wavelength of 1330 nm, coupled with the microdisk device through a U-shaped taper fiber. We successfully observed a mode with a Q factor of 955 ,and based on the free spectral range (FSR) formula, deduced an effective refractive index of 1.75. This value is higher than the previous effective refractive index of 1.67 in the lab-made devices, indicating an increased silicon content in the silicon nitride film, resulting in a higher refractive index. Analysis of the active measurements revealed the absence of WGM modes and laser emission. The optical absorption coefficient of the silicon nitride film around visible band increases with higher silicon content. Additionally, the SU-8, acting as a protective layer, reduce confinement factor of microdisk resonator because of its higher optical index than air, decreasing the ratio of the field in the active layer when light surrounds it, resulting in a reduction in device gain. This ultimately makes it difficult to observe the whispering gallery mode (WGM) signals even if present.