利用光致發螢光探討表面電漿子與氮化銦鎵/氮化鎵量子井耦合行為

Abstract

在本研究中，我們在單一量子井（氮化鎵/氮化銦鎵）厚度分別為15奈米及120奈米的氮化鎵覆蓋層上製作三種不同金屬結構，並在室溫與低溫下量測變激發功率光激螢光與時域解析螢光頻譜的衰變時間。此三種結構包括在氮化鎵的覆蓋層上分別鍍上銀薄膜和隨機分佈的銀奈米顆粒，以及沒任何金屬結構的對照組樣品。銀薄膜和銀奈米顆粒兩種結構是為了要區分出表面電漿極化子及侷域表面電漿子和量子井耦合現象的差異。 覆蓋層厚度為120奈米的樣品因為沒有有效的表面電漿子與量子井耦合行為，導致三種金屬結構區域的光激螢光量測結果無明顯變化趨勢。覆蓋層厚度為15奈米的樣品因銀薄膜與銀奈米顆粒和量子井有顯著的表面電漿極化子或侷域表面電漿子的耦合現象，所以無論在室溫或低溫下，其光激螢光的積分強度相較於對照組樣品皆提升。低溫下，對於在銀薄膜與銀奈米顆粒上所量測到光激螢光的積分強度比例隨著變激發功率僅有微弱的改變，主要是因為低溫下的表面電漿子耦合效應較弱，其積分強度的提升主要來自銀與介電質介面的反射而造成量子井處激發強度提高所致。室溫下，銀奈米顆粒所量測光激螢光的積分強度比例隨著變激發功率也只是微弱改變，可知室溫下侷域表面電漿子的耦合現象並不隨著變激發功率而變。然而，室溫下的銀薄膜所量測光激螢光的積分強度便隨著變激發功率而有顯著的增加，由此可知室溫下表面電漿極化子耦合強度隨著量子井中的載子濃度增加而增強。此現象亦可以從銀薄膜所量測到的內部量子效率相較於對照組樣品有較大的增加比例，以及比對照組樣品有較陡峭衰減的光激螢光時間得知。 隨著激發功率增加而增強的表面電漿極化子效應可能是因為量子史塔克效應的載子屏蔽效應，此效應會將量子井的發光波長藍移，使得發光波長更接近表面電漿極化子共振波長，因此有更強的耦合效果。

The pump power dependent, continuous and time-resolved photoluminescence (PL) measurements at 10 and 300 K in the three regions on two single InGaN/GaN quantum well (QW) epitaxial structures with the GaN capping layer thicknesses at 15 and 120 nm are performed. Among the three regions, an Ag film and a random distribution of Ag nanoparticles (NPs) are fabricated on the GaN capping layers to form the Ag-film and Ag-NP regions for inducing surface plasmon polariton (SPP) and localized surface plasmon (LSP) couplings with the QW, respectively, and comparing their behaviors. The region without any metal structure is used as the control condition. Without a significant surface plasmon (SP) coupling effect on the 120-nm capping layer, the PL measurement leads to the results of inconsistent variation trends among the three regions. With significant SPP or LSP coupling on the 15-nm capping layer, the integrated PL intensities in the Ag-film and Ag-NP regions are enhanced with respect to that in the control region at either 10 or 300 K. The weak dependencies of intensity ratio on pump power in the Ag-film and Ag-NP regions at 10 K indicate that at this temperature, the SP coupling effect is weak and the enhancements are mainly caused by the increased reflections at the Ag/GaN interfaces. Also, the weak dependence of intensity ratio on pump power in the Ag-NP region at 300 K implies the essentially fixed LSP coupling strength as pump power increases. However, the increasing trend of intensity ratio with pump power in the Ag-film region at 300 K shows that the SPP coupling strength can increase with the carrier density in the QW. This observation is confirmed by the larger increasing slope of internal quantum efficiency (IQE) and the steeper decrease of PL decay time with pump power in the Ag-film region. The increasing trend of the SPP coupling strength with pump power can be attributed to the screening of the quantum-confined Stark effect, which can blue-shift the QW emission wavelength for the QW to interact with the SPP of a higher density of state.