探討高含量的氮摻入InAs 自聚式量子點中含氮成份不均勻之特性影響

Abstract

本論文主要是探討摻入高含量的氮於InAs/InGaAs dot-in-well(DWELL)結構中，氮在量子點中的含量波動效應(composition fluctuation effect)的影響並經由長晶的條件以及氮的特性針對所量測的光性及電性藉由不同熱退火的樣品做探討；不同熱退火條件的樣品有As-grown、熱退火700°C (RTA700) 2 分鐘和800°C (RTA800) 3 分鐘。通入大量的氮於S-K 長晶模式的條件下成長InAsN 量子點，由於通入氮之後延遲了2D 轉3D 的時間，在TEM 下看到了明顯的InAsN wetting layer 形貌，在高的長晶溫度增強了氮的遷移長度(migration length)以及氮喜歡群聚的特性而形成了含氮量不均勻的InAsN 量子點。藉由光性與電性的研究分析配合理論計算可以估計出InAsN 量子點中的氮原子含量分佈，觀察到熱退火後InAsN 量子點的數目減小、濃度分佈趨向均勻。 此外，在LED紅外光(940 nm)的光激發下，當光激發了量子點能階產生電子電洞對時，電子電洞會經由2 種機制產生訊號，分別為複合(recombination)以及放射(emission)貢獻在三種訊號：螢光(Photoluminescence)、光電流(Photocurrent)和光電容(Photocapacitance)。因此，比較了不同熱退火樣品之光電流和光電容變化，以及藉由改變溫度及波長觀察光電流與光電容變化趨勢；當電子與電洞放射速率相差較大時，光電容的訊號變強而光電流的訊號變弱。藉由光電流的量測分析出在InAsN 樣品中，量子點的電子放射機制為熱游離放射，而經熱退火後的樣品中，量子點的電子放射機制則是藉由熱放射至InGaAs 覆蓋層後再穿隧至GaAs 層。而配合光電容的量測結果以及我們提出的理論計算模型，求出電子與電洞的放射速率並確認了在InAsN/GaAs 材料中導帶與價帶能量比例為6 比4。

In this study, the nitrogen (N) composition fluctuation effect of a high nitrogen incorporation into self-assemble InAs quantum dots (QDs) grown on GaAs substrate by molecular beam epitaxy (MBE) on S-K mode is investigated by using current-voltage measurement (I-V), capacitance-voltage (C-V), and photoluminescence measurement. Incorporating nitrogen into InAs extend the 2D critical thickness to 3D growth. We observe a decrease in PL intensity and a broadening of the PL emission with incorporating N, the emission include a nitrogen-contain InAsN wetting layer and a InAsN QDs with nitrogen composition fluctuation. The N distribution of InAsN QDs can be estimated by theory calculation on C-V profiling. After annealing process, the density of InAsN QDs decreases and the nitrogen composition fluctuation is reduced. The excitation of the electron-hole pairs in the QDs by illumination on the QDs is also studied. The electron-hole pairs can lead to the photoluminescence, photocurrent and photocapacitance by mechanisms of carrier recombination and emission. Furthermore, we analyze temperature and energy dependences of photocurrent and photocapacitance on samples of before and after annealing. Comparing with the electron and hole emission rates from the InAsN quantum state, photocapacitance is obviously found as the difference of electron and hole emission rate enhance. On the photocurrent analysis, electron escape process is thermal emission from the quantum state to the GaAs band edge on the sample before anneling, and the electron escape process is the phonon-assisted tunneling from quantum state via the InGaAs capping layer state to the GaAs conduction band edge on the sample after annealing. A theory model by a simple rate equation can explain the mechanisms of illumination on QDs, estimate the hole emission rate, and the conduction-band offset ratio is roughly 60% (6：4) for the InAsN/GaAs material system.