分子束磊晶成長InAs/InGaAs 量子點之銻表面活化效應

Abstract

本論文主要是探討銻元素(Sb)對InAs/GaAs 量子點的影響，以分子束磊晶成 長表面及內部量子點在摻雜不同銻分子等效壓力下所造成的銻表面活化效應。量 子點的形狀、大小和高寬比影響電子結構、光學特性以及發光波長，利用摻雜 Sb 可以調變InAs/GaAs 量子點的密度、形貌(高寬比)與體積大小。成長過程摻 雜銻元素所產生的表面活化效應，會降低材料表面能與原子遷移率，延長2D 成 長抑制3D 成長，造成濕潤層厚度增加降低量子點密度。透過原子力顯微鏡與穿 透式電子顯微鏡影像分析，發現一般量子點密度為3×1010 cm-2，當摻雜Sb 進入量 子點內密度會下降，隨著Sb 含量越多密度下降幅度越大，表面量子點密度最低 達到1×108 cm-2 以下；摻雜Sb 於量子點下方緩衝層形成InAs/GaAsSb 結構，密度 會上升達到5,5×1010 cm-2。量子點密度的改變也伴隨著體積的改變，因為強烈聚 集效應使的量子點密度降低體積變大，相反的聚集效應弱時量子點密度高體積 小。透過橫截面高角度環狀暗場影像與橫截面高解析穿透式電子顯微鏡觀察，結 構InAsSb/GaA 高寬比較小，結構InAs/GaAsSb 高寬比較大。摻雜Sb 在量子點內 部或緩衝層可以調變量子點密度、形貌及尺寸。此外我們成長了三片不同厚度 InAsSb 量子點，主要探討Sb 對量子點成核與應力鬆弛的影響，研究顯示摻雜 Sb 會延遲QDs 的成核與提早產生應力鬆弛現象 摻雜Sb 元素於量子點內部的研究，表面量子點產生強烈聚集現象導致密度 大量下降，高寬比下降，透過橫截面穿透式電子顯微鏡影像分析內部量子點也有 聚集與高寬比下降的現象，特別的是內部量子點聚集後產生類似量子井結構，這 II 是因為量子點高寬比下降加上相鄰量子點底部彼此相連。光激發螢光光譜顯示類 似量子井結構在低溫下會主導整個發光特性，螢光強度比量子點基態訊號強3 倍，會使原本正常的激發態訊號峰值半高寬由正常75 meV(室溫下)增大到105 meV(低溫30 K)，變溫光激發螢光光譜顯示峰值能量藍移由正常~40 meV 增大到 ~94 meV。此外室溫光激發螢光光譜顯示摻雜最多的樣品螢光強度減小3.5、發 光波長產生藍移，是因為量子點密度下降與高度減小所致。內部量子點基態與量 子井能階能量差隨熱退火溫度上升會越來越小，是因為量子點結構熱退火後藍移 量比量子井大。透過橫截面穿透式電子顯微鏡影像，當Sb 摻雜較少時量子點每 顆獨立開來清晰可見，當Sb 摻雜較多時量子點彼此群聚形貌類似量子井。 一般表面量子點光激發螢光光譜強度非常弱，因為表面有很多懸浮鍵產生的 表面態，它會直接捕捉表面量子點內的侷限載子進行非輻射複合，使發光強度減 弱與半高寬變寬，透過摻雜Sb 可以降低載子在表面的複合速度與消失速度，抑 制非輻射複合來提升發光強度，此研究有助於量子點應用在生物感測器發展上。

This work investigates the surfactant effect on exposed and buried InAs quantum dots (QDs) by incorporating Sb into the QD layers with various Sb beam equivalent pressures (BEPs). Secondary ion mass spectroscopy shows the presence of Sb in the exposed and buried QD layers with the Sb intensity in the exposed layer substantially exceeding that in the buried layer. Incorporating Sb can reduce the density of the exposed QDs by more than two orders of magnitude. However, a high Sb BEP yields a surface morphology with a regular periodic structure of ellipsoid terraces. A good room-temperature photoluminescence (PL) at ~1600 nm from the exposed QDs is observed, suggesting that the Sb incorporation probably improves the emission efficiency by reducing the surface recombination velocity at the surface of the exposed QDs. Increasing Sb BEP causes a blueshift of the emission from the exposed QDs due to a reduction in the dot height as suggested by atomic force microscopy. Increasing Sb BEP can also blueshift the ~1300 nm emission from the buried QDs by decreasing the dot height. However, a high Sb BEP yields a quantum well-like PL feature formed by the clustering of the buried QDs into an undulated planar layer. These results indicate a marked Sb surfactant effect that can be used to control the density, shape, and luminescence of the exposed and buried QDs. This study investigates the effects of surfactant and segregation from InAs surface quantum dots (SQDs) by incorporating antimony (Sb) into the QD layers. The Sb surfactant effect extends planar growth and suppresses dot formation. IV Incorporating Sb can reduce the density of SQDs by more than two orders of magnitude. Photoluminescence (PL) reveals enhancement in the optical properties of InAs SQDs as the Sb beam equivalent pressure (BEP) increases. This improvement is caused by the segregation of Sb on the surface of SQDs, which reduces non-radiative recombination and suppresses carrier loss. The dark line at the SQDs surface in the transmission electron microscopic image suggests that the incorporated Sb probably segregates close to the surface of the SQDs. These results indicate a marked Sb segregation effect that can be exploited to improve the surface-sensitive properties of SQDs for biological sensing.