以光致螢光量測研究C面ZnO/Zn0.88Mg0.12O不對稱雙重量子井的耦合效應

Abstract

不對稱雙重量子井結構是由兩種不同厚度的量子井所組成，當其中作為分隔的障壁層減少至特定寬度時，兩井之間會發生耦合效應，使得載子有機率從窄井穿隧至寬井。 我們透過脈衝雷射沉積法在C面的藍寶石基板上成長不對稱的氧化鋅量子井結構，並利用光致螢光量測搭配有限差分法模擬來探討量子井間的耦合效應。在我們的研究中發現寬井對窄井的螢光強度比在耦合的情況下有顯著的增加，另一方面因為耦合效應使得不同量子井間的載子存在複合機率，其中複合對應的能量則因為不同量子井之間載子波函數的交疊減少，使得對應的激子束縛能降低而有所增加。我們也觀察到當障壁層與量子井的寬度同時降低時，在有耦合效應下窄井與寬井的螢光強度與半高全寬相對於沒有耦合的情況有相當程度的變化。透過將變化趨勢與模擬結果比較以及參考前人在相關領域的研究成果，我們認為寬井的半高全寬增加以及窄井的螢光強度降低是歸因於載子從窄井穿隧至寬井所造成的。此外，在有耦合效應下窄井與寬井的激子束縛能大小相對於沒有耦合的情況有反轉的現象發生，這也說明在我們有耦合效應的樣品中存在有載子的穿隧行為。

Asymmetric double quantum well (ADQW) structure is consisted of two quantum wells with different well widths. While the width of barrier between two wells reduce specifically, coupling effect will take place and make carriers probable tunneling from the narrow well to the wide well. We have fabricated ZnO/Zn0.88Mg0.12O ADQW on c-plane sapphire substrate by pulsed laser deposition, photoluminescence (PL) measurement results compared with finite difference simulation are used to investigate the coupling in the ADQW. In our research, we observe that the PL intensity ratio of wide well to narrow one apparently increase under coupling. On the other hand, coupling effect makes carriers in different wells probable recombining, the corresponding exciton transition energy increase since the spatial separation of electrons and holes wave function resulting in reduction of exciton binding energy. While barrier and well width reduce at the same time, the PL emission intensities and full widths at half maximum (FWHM) of wide and narrow wells in the coupled ADQW are significantly different compared to the uncoupled ADQW. Through comparing the experimental and simulate results, we attribute both the FWHM of wide wells increasing and the PL emission intensities of the narrow wells decreasing to the carriers tunnel from narrow well to wide well. In addition, the swap of the narrow and wide well exciton binding energy between coupled and uncoupled ADQW can be deemed to the evidence of coupling between two wells with thin barrier and carriers tunneling from narrow well to wide well is indeed.