寬能隙半導體微共振腔強耦合作用之研究

Abstract

半導體微共振腔中在強耦合作用下，光子與激子會混合形成新的準粒子，我們稱之為極化子(polariton)，這樣的粒子具有玻色子的特性，其有效質量很小並且擁有可調變的色散曲線，因此在研究固態系統的光與物質交互作用中，半導體微共振腔是一項十分有潛力的研究領域。在此論文中，我們主要著重於在強耦合作用下寬能隙材料的微共振腔特性研究，分別為混合型氧化鋅微共振腔以及電激發氮化銦鎵/氮化鎵多量子井微共振腔。 首先，我們研究混合型氧化鋅微共振腔中的高反射率分布式布拉格反射鏡，其結構為四分之一光學厚度的氮化鋁/氮化鎵薄膜組成。在20對氮化鋁/氮化鎵分布式布拉格反射鏡中，反射率為97.2%並且停止帶的寬度為36.6奈米。另外，值得注意的是氮化鋁/氮化鎵分布式布拉格反射鏡其反射率中心波長短於380奈米時，反射頻譜會受到氮化鎵嚴重的吸收使得反射率下降。實驗結果可藉由轉換矩陣計算出在考慮氮化鎵的吸收時反射頻譜的變化。 藉由1.5光學厚度的氧化鋅微共振腔，我們在室溫條件下，分別在反射頻譜和螢光激發頻譜觀察到68 meV的拉比分裂值(Rabi-splitting)。然而在實驗結果中，上極化子由於與激子連續能態的散射，使得在實驗結果中無法清楚地觀察到上極化子分支。另外，為了對上極化子分支的鑑別度做進一步的討論，我們藉由數值模擬的方式對不同物理機制條件下的上極化子分支做討論。模擬結果顯示在氧化鎵與氧化鋅的微共振腔中，其主動層厚度分別在□ 和0.25□的情況下，上極子分支會變的無法分辨，主要的原因是在於寬能隙材料塊材的微共振腔中，具有較大的厚度與吸收係數之間乘積。另外，在共振腔與激子兩者模態的能量差距的選擇上，會偏向於能量差為負值，因為在此條件下，上極化子在較小的波向量會偏向激子特性，較不會發生激子連續能態散射情形。此外，在寬能隙材料當中，上極化子分支的鑑別度也會同時受到非均勻性變寬的影響。而在結構的設計上，量子井結構的微共振腔因為具有較大的激子束縛能以及較小的厚度與吸收係數乘積，所以在結構的考量上，量子井結構的微共振腔相較於塊材結構會較容易觀察到上極化子分支。 在此碩論的最後章節中，我們在電激發氮化銦鎵的微共振腔中觀察到強耦合的情況。藉由變溫電激發頻譜，我們在280k的溫度條件下觀察到6meV的拉比分裂值。另外，我們再藉由變角度電激發頻譜的實驗，在7.4度的收光角度下觀察到7meV的拉比分裂值。最後我們在改變注入電流的情形下，可以發現隨著注入電流的增加拉比分裂值也隨著變小，進一步地證明我們在電激發氮化銦鎵微共振腔元件中觀察到激子與光子的強耦合現象。

In the strong coupling regime, a new quasi-particle termed cavity –polariton with bosonic characteristics is created from the mixing of photons and excitons. It has very small effective mass as well as controllable dispersion curves, so that semiconductor microcavities (MCs) have been widely investigated due to the enhanced control of light-matter interaction in solid-state systems. In the thesis, we focus the strong coupling regime between cavity photon and exciton in two wide bandgap semiconductor microcavities- ZnO based hybrid microcavity and InGaN/GaN multiple-quantum-well microcavity. First of all, we investigate high-reflectivity blue-violet distributed Bragg reflectors (DBRs) based on AlN/GaN quarter-wave layers for the hybrid ZnO-based MCs. The 20-pairs AlN/GaN DBRs achieve peak reflectivity of 97.2% at 440 nm together with a stopband width of 36.6 nm. Furthermore, the growth of 20-pair AlN/GaN DBR will suffer significant influence of GaN absorption when the designed DBR wavelength is shorter than about 380 nm. The experimental reflectivity spectra are modeled by transfer matrix theory including the effect of GaN absorption to compare the experimental and theoretical results. For bulk ZnO-based microcavity with 1.5λ optical thickness, we observed a vacuum Rabi-splitting as large as 68 meV at RT based on the angle-resolved photoluminescence and reflectivity experiments. It is found that the upper polariton branch (UPB) is blurred in both experiments due to the absorption from scattering states. Furthermore, we present in detail the possible physical mechanisms leading to the broadening of UPBs for different designs of MCs by numerical simulations based on GaAs, GaN and ZnO materials. The calculated results show that the UPBs of the GaN- and ZnO-based MCs will become indistinct when the thickness of optical cavity is larger than □ and 0.25□, respectively, mainly attributed to the larger product of the absorption coefficient and the active layer thickness. In wide-bandgap materials, it would be relatively easier to observe the UPB in the case of negative exciton-photon detuning due to the exciton-like UPB and lower absorption of scattering states. In addition, the inhomogeneous broadening would be an important factor causing the invisible UPB in wide-bandgap semiconductor MCs. We demonstrate that in a ZnO/ZnMgO multiple-quantum-well MCs, the UPB could be well-defined due to the large 2D exciton binding energy and the small product of absorption coefficient and active layer thickness. These results show that the UPBs can be properly defined in wide-bandgap semiconductor MCs by appropriate design of MC structures. In the last of the thesis, we report the strong coupling regime in electrically pumped InGaN-based microcavity. Through the temperature-dependent electroluminescence (EL) spectra, there is a Rabi-splitting about 6 meV at a temperature of 280 K. Moreover, the angle-resolved EL spectra at 180 K show the 7-meV Rabi-splitting at 7.4□, and the splitting value between lower polariton and upper polariton decreases with the increase of current density, which further evidence the strong coupling regime in electrically pumped InGaN-based microcavity.