Light-Controlled Self-Assembly of Ge/Si Quantum Dots for Optoelectronic Devices

自聚性鍺/矽量子點(self-assembled Ge/Si quantum dots)的研究在1990年晚期才開始進行。當量子點小於10奈米(nm)時，量子點內量子侷限(quantum confinement)的效應會讓量子點能隙(band gap)吸收波段(absorption wavelength)改變。因此，透過控制鍺/矽量子點的大小，鍺/矽量子點應用在光度感應器(photodetector)、LED與太陽能電池(solar cell)這幾種光電元件上有著頗有潛力的發展。 在我們的研究中，我們用脈衝雷射沉積(pulsed laser deposition, PLD)的方式，在攝氏400度的矽基板(100)上成長2A的鍺。在此厚度下，矽與鍺之間4.2%晶格大小不匹配(lattice mismatch)所造成的應力(strain)不夠大，無法以斯特蘭斯基─克拉斯坦諾夫成長(Stranski–Krastanow growth,S-K growth)的方式形成量子點，因此我們在此厚度(2A)下沒有看到量子點。然而，在此樣品上，使用波長為355奈米的脈衝雷射光照射後，我們觀察到了量子點的形成，我們觀察到最小的量子點大小為平均直徑13.3奈米，同時，也達成了密度為1.6x10^11cm^(-2)的量子點。 實驗結果指出，透過改變鍺的厚度跟打在基板上雷射的能量，能夠控制鍺量子點在矽基板上形成的大小。在製造更佳效能的光電元件上有著不錯的潛力。It was only in the late 1990s did the research of germanium on silicon self-assembled quantum dots (QDs) begin. Due to the effect of quantum confinement in the quantum dots with dot size less than 10 nm, the absorption wavelength of the energy bandgap shifts. Therefore, by controlling the size distribution of Ge/Si QDs, there is big potential for the application in optoelectronic devices such as photodetectors, LEDs, and solar cells. In our experiments, thin film of germanium with thickness of 2 A was grown on silicon (100) by pulsed laser deposition (PLD) with 400°C substrate temperature. With such small thickness (2 A), the strain due to 4.2% lattice mismatch between germanium and silicon is not enough for the Stranski–Krastanow(S-K) growth of QDs. As a result, no QDs were observed. However, after the irradiation of the sample with 355-nm pulsed laser beam, the formation of QDs was observed. The smallest average quantum dot diameter observed was about 17 nm and the density of dots was 1.6x10^11cm^(-2). The results indicate that by varying different parameters such as the thickness of germanium thin film and the fluence of the laser beam irradiated on the sample, one can control the size of the Ge QDs formed on Si substrate. This technique provides control over QDs size, which in result provides a new route for the fabrication of the optoelectronic devices with better performances.口試委員會審定書i 誌謝ii 摘要iii Abstract iv 1 Inroduction 1 1.1 Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Growth of the Ge/Si(100) Quantum Dots . . . . . . . . . . . . . . . . . . 2 1.3 Applications of Ge/Si Quantum Dots . . . . . . . . . . . . . . . . . . . . 3 1.3.1 Quantum Dot Photodiodes . . . . . . . . . . . . . . . . . . . . . 5 1.3.2 Quantum Dot LEDs . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.3 Quantum Dot Solar Cells . . . . . . . . . . . . . . . . . . . . . . 7 1.4 Control of the Growth of Ge/Si Quantum Dots . . . . . . . . . . . . . . . 8 1.4.1 Growth Through Nanostencil . . . . . . . . . . . . . . . . . . . 8 1.4.2 Pre-Patterning . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4.3 Laser Induced Periodic Surface Structure on Silicon . . . . . . . 10 1.4.4 Laser Irradiation on Germanium Quantum Dots . . . . . . . . . . 11 1.5 Motivation and Gaols . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2 Materials and Methods 14 2.1 Pulsed Laser Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.1.1 Introduction to Pulsed Laser Deposition . . . . . . . . . . . . . . 14 2.1.2 The Mechanism of Pulsed Laser Deposition . . . . . . . . . . . . 15 2.1.3 Dependence of Laser Fluence . . . . . . . . . . . . . . . . . . . 17 2.2 Experimental Methods of Pulsed Laser Deposition . . . . . . . . . . . . 17 2.2.1 Light Source of Ablation Beam in PLD . . . . . . . . . . . . . . 17 2.2.2 Experimental Setup of PLD . . . . . . . . . . . . . . . . . . . . 18 2.2.3 Issue of the Air Pressure in PLD Chamber . . . . . . . . . . . . . 21 2.2.4 Details of the Target and Substrate . . . . . . . . . . . . . . . . . 22 2.2.5 Preparation of the silicon substrates . . . . . . . . . . . . . . . . 22 2.2.6 Determination of the Fluence of Ablation Beam . . . . . . . . . . 24 2.3 Experimental Methods of Control Beam . . . . . . . . . . . . . . . . . . 25 2.3.1 Light Source of Control Beam . . . . . . . . . . . . . . . . . . . 25 2.3.2 Experimental Setup of Control Beam . . . . . . . . . . . . . . . 26 2.3.3 Determination of Wavelength and Fluence of Control Beam . . . 27 2.4 Diagnostic Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.4.1 Surface Profiler . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.4.2 Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . 29 2.4.3 Field Emission Scanning Electronic Microscope . . . . . . . . . 31 2.5 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 32 3 Experimental Results and Discussion 33 3.1 Growth of Germanium Quantum Dots . . . . . . . . . . . . . . . . . . . 33 3.1.1 Films of Various Substrate Temperature . . . . . . . . . . . . . . 33 3.1.2 Films of Various Effective Thickness . . . . . . . . . . . . . . . 34 3.2 Laser-Induced Formation of Germanium Quantum Dots . . . . . . . . . . 35 3.2.1 Effect of Control Beam Fluence . . . . . . . . . . . . . . . . . . 37 3.2.2 Films of Various Shot Numbers of Control Beam . . . . . . . . . 39 3.2.3 Films of Various Effective Thickness . . . . . . . . . . . . . . . 41 4 Summary and Future Prospective 44 References 4