硒化鎘/硫化鋅膠狀量子點光穩定性及其應用於微碟共振腔雷射之研究

Photo-stability of Colloidal CdSe/ZnS Quantum Dots and Their Application to Microdisk Lasers

鄭智怡(Chih-Yi Cheng)

指導教授 ： 毛明華

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2017年)

https://doi.org/10.6342/NTU201701090

摘要

在本論文中，我們以硒化鎘/硫化鋅膠狀量子點作為主動層材料並結合介電質共振腔，有異於過去三五族磊晶成長量子點微碟共振腔雷射之研究，可將微碟雷射的發光波長推進至可見光範圍。膠狀量子點因化學合成的製作方式具有很大的調變性，例如可改變其材料、合成條件、殼厚度等。此外，量子點發光波長也可藉由改變尺寸大小來調變。膠狀量子點相較於磊晶成長量子點具備有成本低廉、容易整合於各式元件及波長涵蓋可見光波段等之優點。然而，因其表面/體積比很高，在空氣中容易氧化使其發光強度很快衰減，在元件應用中是一大挑戰。
為了突破膠狀量子點在空氣中容易氧化的問題，首先，我們製作具三明治結構的二氧化矽埋覆量子點微碟共振腔雷射。三明治結構可以保護量子點減緩氧化，此外，量子點埋覆於二氧化矽中除了可利用二氧化矽在可見光透明的特性，也可增加量子點主動層發光與共振腔光場的重合。我們製作各種不同尺寸的微碟共振腔雷射，皆可於室溫下以連續波操作，其中直徑為10微米的微碟，其雷射閾值約為500 kW/cm2，品質因子可達1900。元件的雷射特性可藉由改變激發功率來觀察，在低於元件雷射閾值時，我們已可觀察到迴音廊模態，模態半寬隨激發功率增加而減小，此一特性證實元件從自發放光到受激放光的雷射產生過程。之後，我們製作氮化矽微碟共振腔作為比較，氮化矽的折射率雖然高於二氧化矽，可增加光場的侷限性，但以相同條件激發氮化矽微碟會產生微碟損壞的情形，此原因可歸咎於氮化矽材料本身的特性，例如薄膜應力與熱膨脹係數等。此結果證實二氧化矽微碟共振腔較適合作為高強度激發/連續波操作的應用如雷射元件。
然而，使用二氧化矽製作三明治結構的微碟雷射其光強度仍隨激發時間迅速衰減，我們認為主要原因是由於分布靠近於微碟側壁的量子點仍暴露於空氣中。因此，為了提升量子點材料與元件的光穩定性，我們研究可成長高均勻性、高品質薄膜的原子層沉積技術應用於量子點微碟雷射製程的可行性。
為了瞭解原子層沉積後量子點發光及穩定性的變化，在整合至元件前，我們首先研究量子點於薄膜上的特性。我們將量子點旋轉塗佈在基板上，以原子層沉積技術成長厚度僅十奈米的氧化鋁薄膜將量子點包覆保護，並比較有無氧化鋁保護的量子點的發光特性。在成長氧化鋁後量子點發光強度為原本的62%。我們藉由不同激發光源、激發功率、環境等變因，探討硒化鎘/硫化鋅膠狀量子點因光激發產生的不同效應如光氧化及光激發螢光增強現象。沒有氧化鋁保護的量子點在空氣中因光氧化使其發光強度不斷降低。相反地，氧化鋁保護的量子點在空氣中其發光穩定性大幅提升且與其在真空中的光穩定性相近。我們以時間解析量測樣品的載子動態行為，觀察到沒有保護的量子點因光氧化的因素使其載子平均生命期隨照射時間增加而縮短，光氧化產生表面缺陷使量子點的發光強度減弱並增加非輻射複合。藉由比較有保護與無保護的量子點樣品之光強度衰減，我們發現光氧化造成的表面缺面不但使量子點的非輻射複合增強且意外地也會降低輻射複合，非輻射複合增強且輻射複合減弱使量子點發光強度變得很弱但是載子生命期卻沒有大幅變短。另一方面，有氧化鋁保護之量子點因不受光氧化影響而能維持其輻射複合。此研究結果顯示出以原子層沉積的氧化鋁薄膜能大幅提升量子點在空氣中的光穩定性，這對量子點發光元件的應用上非常重要。
最後，我們將此方式應用在元件上，以原子層沉積成長氧化鋁將量子點微碟共振腔雷射包覆，成功使量子點微碟雷射能在空氣中穩定操作。我們比較元件包覆前後的特性與光穩定性，發現不同尺寸的微碟元件特性皆較包覆前提升。整合量測與模擬的結果顯示，氧化鋁薄膜包覆元件可以避免量子點光氧化並增加共振腔的光場侷限。此結果展示了膠狀量子點元件包覆的重要性，而此製程方法未來也可應用於製作其他空氣中穩定操作的量子點元件。

關鍵字

量子點 ； 量子點雷射 ； 微共振腔元件

並列摘要

In this dissertation, we used colloidal quantum dots as active medium and combined with dielectric microcavity to fabricate microdisk lasers which different from previous researches of epitaxially grown group III-V quantum-dot microdisk lasers. In this way, we can tune the emission wavelength of microdisk lasers to visible spectral range. Colloidal quantum dots are fabricated by chemical synthesis method which offers a huge variety of their designs including materials, synthesis conditions, and shell thickness. In addition, the emission wavelength is tunable by changing size of quantum dots. Compare to the epitaxial quantum dots, colloidal quantum dots have many advantages such as low cost, easy integrated into devices and emission wavelength in visible spectral range. However, the high surface area to volume ratio of colloidal quantum dots renders them vulnerable to oxidation in air and result in fast photoluminescence intensity decrease. It will degrade the device performance and is a major challenge in application.
In order to solve the problem of quantum-dot photo-oxidation in air, first, we fabricated SiO2 embedded colloidal CdSe/ZnS quantum-dot microdisk with sandwich structure. The sandwich structure can increase the overlap between quantum dot active medium and the optical field. Besides, quantum dots can be sandwiched in SiO2 which is not only transparent for the quantum dot emission in the visible spectral range but also prevents quantum dots from photo-oxidation. We studied the microdisks with different diameters which all can be operated by continuous wave excitation in room temperature. The threshold power density of the 10-μm-diameter microdisk resonators is about 500 kW/cm2 and the quality factor is 1900. Lasing behavior was observed from power dependent emission spectra. At below threshold, small whispering gallery mode peaks were observed. The spectral width narrowing was observed as the excitation power density increases. It confirms that the transition from spontaneous emission to stimulated emission of the laser device. Then, we fabricated Si3N4 microdisks for comparison. Although the refractive index of Si3N4 is higher than SiO2, which can enhance the optical confinement, Si3N4 microdisks were observed a severely damage when it were excited by the same excitation condition of SiO2 microdisk laser. The causes for the damage are attributed to the material properties, such as stress and thermal expansion coefficient. It indicates that the SiO2 microdisks more suitable for high excitation and/or continuous wave applications such as lasers.
However, the photo-stability of microdisk lasers is still degraded with irradiation time even in SiO2 sandwiched structure. It is because the quantum dots around microdisk sidewall are still expose in air. In order to enhance the photo-stability of quantum dot material and devices, we study the application of atomic layer deposition technique to microdisk lasers due to the conformal and high quality film deposition properties of the atomic layer deposition.
We first study the properties of quantum dot film before applied to microdisks for the purpose of understanding the change of quantum dot emission and stability after atomic layer deposition. We spin-coated the colloidal quantum dots on substrate and then deposit a 10 nm thick Al2O3 film by atomic layer deposition to passivate quantum dots. We compared the emission properties of both unpassivated and passivated quantum dots. 62% of the original peak photoluminescence intensity remained after atomic layer deposition. The photo-oxidation and photo-induced fluorescence enhancement effects of both the unpassivated and passivated quantum dots were studied under various conditions, including different excitation sources, power densities, and environment. The unpassivated quantum dots showed rapid photoluminescence degradation due to strong photo-oxidation in air. In contrast, the photo-stability of passivated quantum dots in air is enormously enhanced and is similar to the results in vacuum. Furthermore, recombination dynamics of the unpassivated and passivated quantum dots were investigated by time-resolved measurements. The average lifetime of the unpassivated quantum dots decreases with laser irradiation time due to photo-oxidation. Photo-oxidation creates surface defects which reduces the quantum dot emission intensity and enhances the non-radiative recombination rate. From the comparison of photoluminescence decay profiles of the unpassivated and passivated quantum dots, photo-oxidation-induced surface defect unexpectedly also reduce the radiative recombination rate. These cause the strongly decrease of quantum dot photoluminescence while carrier lifetime is not reduced much. On the other hand, the Al2O3 passivated quantum dots do not suffer from the reduction of radiative recombination rate. Our results demonstrate that Al2O3 passivation of quantum dots using the atomic layer deposition technique can significantly enhance photo-stability in air. This is essential for the applications of colloidal quantum dots in light-emitting devices.
Then we apply this method to fabricate microdisk devices. We encapsulated the microdisks with Al2O3 using atomic layer deposition, and successfully demonstrated the stable operation of devices in air. The device performances were improved with Al2O3 encapsulation for all different size. The measurement and simulation results indicated that the encapsulation by Al2O3 film not only can prevent quantum dots to photo-oxidation but also enhance the optical confinement of microdisks. The results demonstrate important process of quantum-dot based device encapsulation. This technology can also be used to fabricate other air-stable colloidal quantum-dot devices for practical applications.

並列關鍵字

Quantum dots ； Quantum dot lasers ； Microcavity devices

參考文獻

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