- 學位論文
矽/鍺材料的再吸收與鈍化現象
Reabsorption and Passivation Effects on Si/Ge Materials
摘要
為了遵循摩爾定律，持續微縮電晶體尺寸是半導體界努力的方向，由於多功能型平板和智慧型手機的普及，此需求更是重要。因為高載子遷移率、可與當今矽製程結合、具有1550 nm光通訊波段可利用於通訊和遠紅外光應用，所以矽/鍺材料逐漸扮演電子與光電元件的重要角色。在本篇論文中，著重研究矽/鍺再吸收與鈍化現象的元件物理以及光電特性，此研究對於電子與光電元件有重要的影響。該論文研究成果若結合於發光二極體和太陽能電池等光電元件可與現今製程相容並應用於生物醫療、光通訊傳輸、遠紅外光光源等，可增加半導體晶片的價值。 在論文的第一部分，主要研究再吸收效應對於光激發光和發光二極體的影響。鍺材料有直接能隙(0.8 eV)與非直接能隙(0.66 eV)的材料特性，其中直接能隙主要可應用於光通訊與遠紅外光光源等應用。有鑑於此，為了提升該能隙的放光效率，在光激發光的實驗中，發現晶圓的厚度和表面復合效率對於直接能隙的放光影響，由於直接能隙大於非直接能隙約140 meV，所以在光激發光中，直接能隙放光容易被非直接能隙吸收而降低放光效率，為了降低此現象，將晶圓厚度縮減可增強直接能隙放光結果。而當中透過光激發光強度的變化、厚度的變化和表面復合效率的模擬可以將晶圓的等效生命週期得出。其中，比較磊晶鍺與晶圓鍺的光激發光結果也是本部份的另一重點，缺陷會導致非發光復合增強而使得直接能隙放光增加。根據光激發光的實驗結果得知再吸收效應對於直接能隙有明顯的影響。在應用上，發光二極體的結構也會影響直接能隙的放光效率。垂直金屬結構的pn發光二極體有較弱的直接能隙放光現象，若改成水平金屬結構則提升直接能隙放光強度，主要因為載子分佈在表面所致。由光激發光實驗結果比較p型和n型鍺直接能隙放光效率，在相同厚度和濃度下，p型鍺有較高的直接能隙放光強度，由模擬中分析p型鍺的直接能隙與非直接能隙的能量差小於n型鍺，能隙微縮現象較明顯，此外，濃度較高者，載子生命週期短，造成載子主要分布於表面而降低直接能隙被吸收的可能。對於應用於光電元件可以使用p型鍺材料在直接能隙有較佳的表面。 為了提升元件效率，高載子遷移率的材料已成為主要研究方向，如鍺通道電晶體，而太陽能電池更需要表面鈍化以增加效率，所以表面鈍化研究成為重要的一環，其中氧化鍺為主要鈍化鍺表面的材料，可用於光電與電子元件中。利用光學和電學特性分析氧化鍺材料特性，如表面復合效率、電容電壓曲線和光激發光頻譜等等，可以了解氧化鍺鈍化鍺表面的原因，而薄膜中的固定電荷可以建立電場而降低載子復合，降低表面復合效率。對於未來元件開發應用有重要的地位。 第二部分著重研究鈍化對於光激發光、表面復合效率、電容電壓曲線和太陽電池效率的影響。鈍化主要目的是降低表面缺陷數目防止載子復合而降低發光效率或是太陽能電池效率。另外一方面，電漿浸沒式離子佈植技術可應用於小尺寸與大尺寸的太陽電池中，鈍化表面與邊界的缺陷而有效提升效率，未來可應用於工業界。
並列摘要
More-than-Moore is used to replace Moore’s law due to the physical limitation during the CMOS scaling down. SiGe/Ge materials as light source, modulator, and detector become popular for further integration with current CMOS industry. In this dissertation, the reabsorption and passivation effects on Si and Ge materials are investigated and discussed. In the first part of the dissertation, reabsorption effects on bulk Ge and epitaxial Ge are investigated. The higher photon energy of direct emission than Ge bandgap causes significant reabsorption by Ge itself. The reabsorption depends on the excessive carrier distribution in the Ge. The main factors to determine the distribution are carrier lifetime, surface recombination velocity (SRV), wafer thickness and pumping power. For diffusion length larger than Ge wafer thickness, the larger rear SRV not only decreases the integrated photoluminescence intensity but also decreases the reabsorption effect because the carrier concentration near the rear surface is reduced by the surface recombination. The intensity ratio of direct band emission to indirect band emission increases with decreasing Ge thickness. The bulk lifetime can be extracted by fitting the intensity ratio of direct emission to indirect emission as a function of wafer thickness. The effect of rear SRV on reabsorption was also studied. The reabsorption effects on light emitting diodes (LEDs) are observed. Since the direct bandgap has higher energy than the indirect bandgap, photons emitted by direct bandgap recombination will be strongly absorbed in the LED. This photon reabsorption can be reduced by changing the geometry of the LED from a vertical geometry, where carriers diffuse deeply into the LED and photons are emitted deep within the LED structure, to lateral LED geometry, where photons are emitted closer to the LED surface, and reabsorption is reduced, resulting in enhanced photon emission. The increase of the relative strength of the direct bandgap emission from p-Ge with increasing acceptor concentration is proposedly due to the decreasing bandgap difference between direct and indirect bandgap. The energy difference determines the relative electron population of direct and indirect conduction valleys, which reflects the respective strength of photoluminescence. For the same carrier concentration, p-Ge has the stronger direct bandgap emission than n-Ge. The stronger bandgap narrowing of the direct valley than indirect valleys due to acceptor concentrations are confirmed by the extraction of the bandgap difference from the photoluminescence spectra. Next, the surface recombination of Ge is reduced by GeO2 passivation due to both the reduction of interface state density and the field effect of the oxide charge. The increase of photoluminescence intensity with increasing GeO2 thickness is mainly caused by the reduction of interface state density, since the fixed charge density remains similar. The p-Ge lifetime is extracted by quasi-steady-state photoconductance on Ge with different thicknesses. For Si solar cell, surface passivation is an important key for high efficiency. Plasma immersion ion implantation of hydrogen with low kinetic energy (1 keV) is used to passivate the solar cells to gain extra efficiency. The plasma sheath encloses the wafer cell and hydrogen ions can be implanted into all the cell surfaces. The passivation is omnidirectional, and is particularly effective at the edge surface of the solar cell. The large diffusion length of Si cell is susceptible to edge defects, since the photo-generated carriers can diffuse to the edge surfaces and recombine to degrade the efficiency. Moreover, the dielectric/silicon interface can be effectively passivated due to energetic hydrogen from plasma.