埋覆量子點之介電質微碟共振腔的製作與特性分析

Abstract

利用介電質材料製作微碟共振腔的主要目的是為了避免昂貴又費時的半導體製程系統。此外，改用介電材料也希望能把操作頻率往高能的可見光區轉移，使微碟共振腔的應用更為廣泛，而不僅限於光通訊使用的紅外光頻段。近年來，已成熟的微電子和積體電路製程技術例如:電漿輔助式化學氣體相沉積系統（plasma-enhanced chemical vapor deposition, PECVD）、光蝕刻微影(photo-lithography)，與反應式離子蝕刻系統（reactive ion etch, RIE）相較於分子束磊晶(molecular beam epitaxy, MBE) ，皆能提供較為快速與便宜的製程技術。而且，半導體不僅在製程設備較為昂貴，半導體塊材也比介電材料之塊材成本更高。 二氧化矽(silicon dioxide, SiO2)已被許多研究認為可用在微共振腔的材料例如：微碟、微柱體、微球體等共振腔，甚至結合於光子晶體(photonic crystals)，形成迴音廊模態(whispering gallery mode, WGM)的結構。然而，二氧化矽的折射率限制了結構能侷限的場量，尤其是在共振腔尺寸縮小時。為了避免低侷限係數(confinement factor)，本研究採用氮化矽(silicon nitride, SiNx)因此材料的折射率為 2.05，比二氧化矽高約0.6。氮化矽有許多與二氧化矽材料類似的特性，所以氮化矽可在製程方面代替氧化矽製作出埋覆硒化鎘�硫化鋅（Cadmium Selenide�Zinc Sulfide, CdSe/ZnS）膠狀量子點微碟共振腔。本論文利用電漿輔助化學氣體相沉積、電子束微影(e-beam lithography)、反應式離子蝕刻、和濕式蝕刻，已製作埋覆氮化矽埋覆硒化鎘量子點微碟共振腔直徑約為10 μm。 迴音廊模態可在兩種材料製作出的結構形成，利用光激發螢光量測(micro-photoluminescence)架構，並以532 nm雷射光源激發。雖然Ｑ值(quality factor, Q-factor)不高，但在初步的量測下，利用埋覆式製作的結構表現出高量率的多模態共振。此外，利用同樣方式製作了被動氮化矽微碟共振腔顯示了超過104Ｑ值。這代表應用於微共振腔上，氮化矽是個很適合的介電材料。此論文的主動氮化矽微碟共振腔未能由 far-field 收光方式量出頻譜因發光訊號太弱，很合理的跟理論搭配解釋了高侷限係數的量測必須要由 fiber-coupling 方式量頻譜。

The primary objective in creating a microdisk using dielectric media is to create a disk fabrication procedure that avoids expensive, time-consuming, and high maintenance equipments of semiconductor fabrication systems. And, in doing so, the operable wavelength of the microdisk microcavity is targeted towards the visible wavelength range to cover a wider variety of applications as opposed to optical communication available mostly only to infrared frequencies. This research produces such microdisk microcavity using silicon dioxide and silicon nitride material with embedded CdSe colloidal quantum dots. Silicon dioxide has been focused by many as promising dielectric material for creating microcavities such as micropillars, microspheres, and even recently in photonic crystals to support whispering gallery modes. However, its low refractive index imposes a limit on how well this dielectric material can confine electromagnetic waves especially with decreasing structure dimensions. Silicon nitride is introduced to resolve this need with an increased index of approximately 2.05 compared to the 1.45 of silicon dioxide. With many similar qualities and properties to silicon dioxide, silicon nitride can substitute silicon dioxide in the fabrication of microdisks with embedded (「sandwiched」) colloidal CdSe/ZnS quantum dots if a fabrication process can be successfully developed. In this thesis, using a combination of PECVD, RIE, photo-lithography, and wet etching, silicon nitride microdisks are fabricated with diameters in the proximity of 10 μm and are capable of supporting whispering gallery modes. However, the best observed modes were seen in 10 and 12 μm microdisks. In addition, only the 12 μm microdisks are capable of supporting higher order radial modes. Active silicon nitride microdisks with evident WGM have also been fabricated through a similar fabrication method with the change of wet etching to undercut the microdisk structures. Whispering gallery modes are observed in both silicon dioxide and silicon nitride microdisks through a home-made micro-photoluminescence experimental setup using an excitation source of 532 nm green laser. The effect of sandwiching has shown significant improvements in repeatability in fabricating microdisks that are capable of supporting whispering gallery modes despite relatively low quality factors from our preliminary measurement results. Study on passive silicon nitride microdisks with the same fabrication technique developed here shows very high quality factors larger than 104. This indicates the huge application potentials of silicon nitride as suitable dielectric materials for optical microcavities. Whispering gallery modes of the same active silicon nitride microdisks, however, failed to be detected via far-field detection setup. According to sources listed in this thesis, this concurs with theoretical analysis due to the nature of higher confinement allowing less light to be radiated in such microdisks and therefore making far-field detection of such microdisks less likely.