熱退火對InAsN/InGaAs量子點結構電子放射之影響

Abstract

本論文主要是藉由光性及電性的量測，包括光激發螢光頻譜(PL)、電容電壓(C-V)、導納頻譜(C-F&G-F)、深層能階暫態頻譜儀(DLTS)的量測，來探討在InAsN/InGaAs這種quantum dot-in-well (DWELL) 結構中，其量子能階，缺陷能階和電子放射機制經由快速熱退火(rapid thermal annealing)後的改變。樣品是InAs量子點成長2.2 ML，並摻入17%的氮，所形成的InAsN量子點 (As-grown)，再予以熱退火700℃ 2分鐘(RTA 700)和800℃ 3分鐘 (RTA 800)作為進一步研究。 由TEM分析上發現摻入氮的樣品其量子點尺寸下降，以至於在PL量測中未得到預期的波長紅移現象，並且可看見InAsN量子點的發光效率降低和半高寬(full width at half maximum, FWHM)變寬的現象，以及出現一個不同於量子點的低能量訊號(localized states)，推測以上現象是因為所摻入的氮含量高達17%所致。 氮分佈的不均勻使得發光效率變低，而且氮本身的群聚效應在量子能階附近產生了localized states ，且經由傅立葉分析(Fast Fourier filtered image)觀察，在量子點周圍發現一些錯位差排缺陷(misfit dislocation defect)，這些現象都將影響電子的放射機制。 樣品經過熱退火過程後，在PL量測中可看到量子訊號增強，localized states訊號減弱，這是因為氮在量子點中分佈變均勻及摻氮所產生的點缺陷減少，降低非輻射再結合的現象，PL強度因而增強；而電性量測中更可發現localized states減少，使背景摻雜濃度恢復，電子放射機制因而改變，同時也觀察到量子穿隧效應。在論文中也使用理論計算模擬電性量測結果，證明熱退火過程確實讓樣品的量子特性恢復。

The effects of post-growth thermal annealing on optical and electrical properties of InAsN /InGaAs dot-in -well structures grown by molecular beam epitaxy on GaAs(100) were studied by using capacitance-voltage (C-V) profiling, bias-dependent deep level transient spectroscopy (DLTS) and photoluminescence (PL) measurements. An unexpected blueshift of the PL emission energy of the InAs QDs with N incorporation is observed. This effect could be induced by a decrease of QDs size with N incorporation. In addition, a broadening of the QDs PL emission and a decrease in PL intensity are seen, which is attributed to the QD size fluctuations, due to a wide energy distribution of the density of states. After annealing, the PL intensity is increased and the PL linewidth is reduced. Furthermore, the low energy tail created by N incorporation is reduced by the annealing process. These results suggest that annealing can reduce compositional fluctuation between the QDs and remove the non-radiative centers associated with the point defects. Also, a significant improvement on the electrical properties of the rapid thermal annealing (RTA) samples can be obtained. After annealing, the C-V spectra show that a C plateau is split into two plateaus, corresponding to the quantum states and localized states. The result suggests that the thermal annealing process improves the quality of QDs. Furthermore, the carrier-depletion effect is reduced and a quantum tunneling effect is observed. In summary, annealing recovers the background concentration due to the decrease of the localized states. It enhances the quantum effect of InAsN /InGaAs dot-in-well structures, which can be seen from the optical and electrical measurements. The result of C-V simulation is consistent with the experiment data.