不同磷化銦鎵夾層下砷化銦量子點之光學特性

Abstract

我們以溫變的光致螢光光譜(photoluminescence,PL)及光調制反射率光譜(photoreflectance,PR)來研究砷化銦/砷化鎵(InAs/GaAs)自我組成量子點結構的光學性質。我們的樣品(樣品Sm,樣品Sc和樣品St)是以史傳斯基─克拉斯坦諾夫長晶模式(Stranski–Krastanov growth mode)為基礎的氣相源分子束磊晶成長術(gas-source molecular-beam epitaxy)成長在長晶方向在(100)面砷化鎵基板(有n+摻雜)，成為有三種不同晶格常數夾層的堆積量子點的雷射結構。三個樣品的夾層樣品分別為晶格匹配(Sm)、壓縮應力(Sc)和弛張應力(St).我們可以看到後面兩個量子點有較高的螢光光譜能量. 因此我們可以瞭解由不同晶格常數的夾層形成之應力場的影響. 樣品Sm,樣品Sc和樣品St在室溫時的主要發光能量是在0.99eV 1.004eV 以及0.998eV。在光致螢光光譜中，我們觀察到兩個發光的特徵，而且在高能量的部分呈現出很長的延伸。另外，在溫變的光螢光光譜實驗當中，我們接著觀察並討論到總發光強度隨溫度增強的現象。因此，我們了解在抑制量子點發光的機制當中，熱游離與載子的重新分布都扮演重要的角色。同時，為了更確定在量子點中載子的機制，我們也進行了激發光源強度變化的能態載子注入光譜實驗並找出了第二群量子點的形成。 在光調制反射率光譜實驗中，我們明顯地觀察到從量子點所產生的光學躍遷，並且分辨出基態與激發態的光學特徵。同時，從溫變實驗而得到各個能態的能量也相當吻合由晶格受熱擴張與電子晶格交互作用而推衍出的法西尼公式(Varshni’s formula)。由實驗所得到結果可知道出砷化銦/砷化鎵自我組成量子點的密度與均勻性受到夾層的磷化銦鎵材料的影響相當明顯。 為了了解我們從溫變光致螢光光譜及光調制反射率光譜實驗所得到的光學特徵，可以由原子力顯微鏡(Atomic Force Microscopy, AFM)來粗略地估算量子點的大小去進行單一半高金字塔形狀量子點的電子性質的計算。最後我們得到計算的能態與證實了對於能態來說應力夾層磷銦化鎵的影響會小於量子點尺寸的影響。

The optical properties are studied with temperature dependent photoreflectance (PR) and photoluminescence (PL) experiments on InAs/GaAs quantum dot (QD) structures. The samples (Sm, Sc, and St) are grown by the gas-source molecular-beam epitaxy in the Stranski–Krastanov growth mode on an n+-doped GaAs (100) substrate and three multi-QD-stack lasers structure are deposited with different lattices InGaP cladding layer. Three QD samples with lattice-matched (Sm), compressive-strained (Sc) and tensile- strained (St) for the cladding layer were grown, and the later two QDs samples have the higher transition energy of the luminescence. Therefore, we could realize the effect of strain field reducing to different lattices cladding layer. At room temperature, the major peaks in the PL spectra of the QD samples are 0.989eV for Sm, 1.004eV for Sc, and 0.998eV for St, respectively. Two peaks with long-tail on high energy portion are observed in PL spectra and integrated intensity is also discussed. Therefore, the thermionic emission plays important roles in the PL quenching. To determine the mechanism of the carriers in QDs, state filling experiments are carried out and verify the second group of InAs QDs. From the PR experiments, optical transitions originated from the InAs QDs are observed. Transition energies of the ground state and excited states are verified. The temperature dependence transition energies following the Varshni’s semi-empirical formula was attributed to the dilation of lattice and electron-lattice interaction. The results from PL and PR spectra reveal that size and uniformity of InAs/GaAs QDs are affected by InGaP cladding layers. In order to determine the transition energies observed from the temperature dependent PL and PR experiments, the AFM images have been carried out to estimate the diameters and heights roughly of the samples for theoretical calculations of the InAs/GaAs single QD with the truncated pyramidal shape. Consequently, the theoretical calculations of the InAs/GaAs single QD with the truncated pyramidal shape are carried out the ground state energy transition of InAs QD and verify that the effect of strain reducing to the mismatch of the cladding layer is lower than the effect reducing to the size distribution for the transition energy feature.