矽鍺奈米異質結構之製備與應用研究; Study on SiGe Hetero-epitaxial Nanostructures and Their Application

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Title:	矽鍺奈米異質結構之製備與應用研究;Study on SiGe Hetero-epitaxial Nanostructures and Their Application
Authors:	張宏臺;Chang,Hung-Tai
Contributors:	材料科學與工程研究所
Keywords:	矽鍺;CVD磊晶;熱電;自組裝;量子點;SiGe;CVD Epitaxy;Thermoelectric;Self assembled;Quantum Dot
Date:	2013-07-10
Issue Date:	2013-08-22 11:44:18 (UTC+8)
Publisher:	國立中央大學
Abstract:	於矽基板上成長的應變誘發自組裝三維方向鍺/矽鍺量子點，不論在基礎研究或應用領域上都是非常熱門的主題。鍺/矽(100)材料系統被公認為所有“自組裝量子點”之形成機制及表面形態進化的基礎研究模型。就技術發展而言，鍺/矽(100)材料在未來電子、熱電及光電元件之應用上有極大之潛力。本論文首先介紹藉由原子力顯微鏡搭配選擇性蝕刻可得知鍺量子點經由退火效應後的成份分佈變化。實驗結果證實鍺量子點在成長與原位退火後時會有成份重新分佈。我們同時發現，一旦超巨大半球形鍺量子點產生後，鍺量子點的非對稱分佈(半月形)會轉變成幾乎對稱(十字形)的結構。此外，超巨大半球形鍺經由長時間退火及蝕刻後，會呈現富含矽成份的雙環結構。利用包含表面擴散過程的基本熱力學原理即可解釋這些現象。本論文也提出成長複合式鍺/矽/鍺量子點。藉由嵌入性矽薄膜之作用，可成功製備出高濃度、高品質鍺量子點。嵌入矽薄膜可作為優先成核位置，讓後續鍺原子能有效的沉積於上，並大幅降低聚集形鍺量子點的形成機率。另外在選擇性蝕刻實驗中發現，上層富含鍺部分被帶走後，與之前的研究相較，被蝕刻之複合式鍺/矽/鍺量子點呈現出金字塔形之結構，而傳統式鍺量子點則呈現對稱成分分佈之環形結構。此外，藉由調控嵌入矽薄膜之結構參數可以進一步去控制複合式鍺/矽/鍺量子點之形狀及尺寸。實驗結果同時顯示嵌入矽薄膜之厚度小於2奈米以下，則無法抑制上下層鍺原子的交互擴散。論文之第三部分為有效地製備出高品質薄膜狀之複合式量子點，此複合式量子點之薄膜層可作為熱電材料應用。利用複合式量子點之概念，成功開發出三重式鍺/矽/鍺/矽/鍺之複合量子點結構。選擇性化學式蝕刻實驗顯示，嵌入性矽薄膜之表面擴散以及矽鍺合金的效應在複合式量子點中成長機制中扮演了重要的角色。此薄膜狀複合式量子點材料被證明了，比起傳統式鍺量子點更能有效的降低熱傳導效率（κ）。此原因可能為複合式鍺量子點具有高界面矽/鍺密度及區域性合金效應可增強聲子散射。本論文最後介紹自組裝矽鍺量子點於成長於應變矽鍺磊晶層上之研究。矽鍺量子點成長於應變矽鍺磊晶層上，由於矽鍺磊晶應變層之表面粗糙度上升和應變能的影響，導致量子點密度及均勻性都能提升。而量子點之尺寸提升是因為鍺潤溼層厚度的降低，導致矽原子能輕易跨越鍺潤溼層界面進入矽鍺量子點而提升尺寸。藉由薄膜矽覆蓋於矽鍺磊晶應變層上，可調控矽鍺磊晶應變層之總應變能，進而去調控量子點之尺寸及均勻性。綜合以上實驗結論，矽鍺量子點成長於矽鍺磊晶應變層可作為未來光電及熱電之潛力材料。 Strain-driven self-assembly of three-dimensional Ge or SiGe islands on Si substrates is an active topic of both fundamental and practical interests. The Ge/Si (100) material system has been regarded as prototypical for investigating the basic phenomena, for example, the formation of ‘‘self-assembled quantum dots’’. Technologically, Ge/Si (100) is of interest for possible applications in electronic, thermoelectric and optoelectronic devices. In the first part of this thesis, the annealing effects on the composition distribution of Ge islands on Si (001) were investigated by atomic force microscopy combined with selective wet chemical etching. Experimental results demonstrate that there is a strong composition redistribution occurring during island growth and postgrowth annealing. We observe that, once Ge superdomes appear, the asymmetric composition profile of the Ge domes transforms into an almost symmetric structure. Moreover, the Ge superdomes exhibit a double-ring composition profile of Si after long-time annealing. These phenomena could be explained within a simple thermodynamic model that involves only surface diffusion process. After the discussion on the growth of SiGe QDs and the annealing effect, the composite Ge/Si/Ge islands (CQDs) have also been investigated. After upper Ge-rich parts were removed by selective etching, the etched Ge/Si/Ge composite islands exhibit pyramid structures, which differed from the ring-like isocompositional profiles observed in conventional Ge islands. The shape and size distribution of Ge/Si/Ge composite islands can be further controlled by tuning the island parameters. Experimental results also demonstrate that the inserted Si layer less than 2 nm is disabled from blocking the Ge atoms out-diffusion from sub-dots. In the third part of the thesis, we present an effective approach to grow high-quality thin film of composite quantum dots (CQDs) as a building block for thermoelectric materials, in which 3 times the usual Ge deposition can be incorporated within a 3-fold CQD. Selective chemical etching experiments reveal that a thin Si inserted layer in the CQDs modifies the growth mechanism through surface-mediated diffusion and SiGe alloying. Such thin-film-like CQD materials are demonstrated to exhibit reduced thermal conductivity κ with respect to the conventional QDs, perhaps as a consequence of enhanced diffusive phonon scattering from the high Si/Ge interface density and enhanced local alloying effect. Finally, the growth of self-assembled Ge(Si) islands on a strained Si1-xGex layer is studied. The size and the surface density of islands are found to increase in the strain SiGe layer. The increased surface density and uniformity are related to augmentation of the surface roughness after deposition of the SiGe layer. The increased island size is attributed to enhanced Si intermixing and to a wetting layer thickness reduction. The influence of Si cap on SiGe strain layer is also discussed. In order to control the size and uniformity of Ge QDs, the Si spacer can tune the strain energy from SiGe layer. These results indicate that Ge QDs on SiGe strain layer would be potentially useful as an optoelectronic and thermoelectric material.
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