- 學位論文
使用有機化學氣相沉積系統於矽(111)基板上成長三五族異質結構奈米線及晶體結構控制之研究
Growth and Crystal Structure Control of III-V Heterostructure Nanowires on Si (111) Substrate by MOCVD
摘要
由於獨特的材料和物理特性，三五族奈米線(III-V nanowires)作為未來電子和光電應用上深具廣泛的吸引力。在此論文中，使用金屬有機化學氣相沉積法在矽基板上合成無催化劑(catalyst free)的垂直InAs，InGaAs和InSb奈米線將被研究及討論。掃描電子顯微鏡，高分辨率透射電子顯微鏡和能量色散X射線光譜和拉曼光譜皆用於研究奈米線的形態，晶體質量和元素組成。 使用兩階段成長方式磊晶成長高密度(80/µm2)及較小直徑(28nm)的InAs 奈米線:第一階段證實在特定銦莫耳分率(molar fraction: 1.710^5 moles/min)下的臨界成核溫度是減小InAs原子核及奈米線直徑的關鍵因素；第二階段成長中發現使用特定五三比將大幅的增加InAs奈米線的密度(45 µm2 to 80 µm2)並可維持相同的奈米線直徑。InAs奈米線呈現纖鋅礦晶體結構且無觀察到任何缺陷(stacking faults)。 由於表面活性劑的影響，要直接在矽基板上成長InSb是相當複雜。本論文利用兩階段成長方式的概念，將上述的InAs奈米線當核心，在矽基板上生長無外來催化劑(catalyst free)的垂直InAs / InSb異質結構奈米線。在此部分研究必須小心控制InSb磊晶參數，當成長過程在富銦地帶(indium rich region)，將有利於InAs奈米線上成長InSb奈米線。具方向性InSb奈米線的形態和生長速率皆受生長溫度和生長時間的控制。生長時間少於10分鐘產生軸向InAs / InSb異質結構，當成長時間延長到10分鐘以上，同軸橫向生長將促進InSb包裹的InAs / InSb core-shell異質結構。對於這種生長條件，InSb的晶體結構受限於閃鋅礦。 本論文亦研究了V / III比對自催化(self-catalyst)InSb奈米線晶體結構的影響。為了以防止自催化劑被耗盡，成長過程中有效的V / III比將扮演重要角色，在研究過程中發現V / III比可將InSb 奈米線晶體結構從閃鋅礦改變為纖鋅礦。藉由仔細改變五族莫耳流量(molar flow)來研究InSb奈米線纖鋅礦晶體結構之成長。使用現有的磊晶生長模型，可以解釋為觀察到的晶相轉變歸因於液相In-Sb合金的表面能低於純銦的表面能，這使得InSb奈米線結構轉變為纖鋅礦而不是鋅閃鋅礦結構。對於其他莫耳流量，InSb奈米線表現出閃鋅礦晶體結構。 InSb生長延伸到InGaAs奈米線上，並首次獲得成功的InGaAs / InSb異質結構。 為了實現高密度集成，在矽基板上藉由選擇性區域外延結合InAs和InGaAs奈米線陣列是一個重要主題，研究發現在V / III比為264時可獲得高縱橫比(high aspect ratio)的InAs和InGaAs。通過保持固定的V / III比率264來減少磊晶生長前體的總莫耳流量，我們發現WZ主要晶體結構發生於高莫耳流量，而對於低莫耳流量，奈米線表現出纖鋅礦/鋅閃鋅礦混合的晶體結構(2~3monolayers)。電學特性證明，纖鋅礦主導奈米線的電阻率比polytype奈米線低1.6倍。
並列摘要
Owing to the unique material and physical properties, III-V nanowires have been attractive extensively as a promising candidate for realizing future electronic and optoelectronic applications. In this thesis, synthesis and characterization of external catalyst free vertical InAs, InGaAs and InSb nanowires on silicon substrate by metal organic chemical vapor deposition are studied. Scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy and Raman spectroscopy are deployed to investigate nanowire morphology, crystal quality and element composition of the nanowire. Using two step growth process, high density (80/µm2) InAs nanowires with smaller diameter of ~28nm is achieved. (i) critical nucleation temperature for a specific In molar fraction (approximately 1.710^5 moles/min) is the key factor to reduce the size of the nuclei and hence the diameter of the InAs NWs, and (ii) critical V/III ratio during the 2nd step growth will greatly increase the density of the InAs NWs (from 45 µm2 to 80 µm2) by maintaining the small diameter. The InAs nanowire exhibits a Wurtzite crystal structure without any stacking faults. Due to surfactant effect, direct nucleation of InSb on silicon substrate is highly complex. By utilizing the concept of two step growth process, using InAs stem, foreign catalyst free vertical InAs/InSb heterostructure nanowire is grown on silicon substrate. Careful selection of InSb growth parameters leads to indium rich region, which facilitates indium self-catalyzed InSb nanowire growth on InAs stem. Axial InSb nanowire morphology and growth rate are controlled by growth temperature and growth time. Growth time less than 10 min yields axial InAs/InSb heterostructure and in above 10 min, co-axial lateral growth facilitates InSb wrapped InAs/InSb core-shell heterostructure. The crystal structure of the InSb is limited with zincblende for this growth condition. The influence of V/III ratio on self-catalyzed InSb nanowire crystal structure is investigated. The effective balance of V/III ratio is highly required to prevent from self-catalyst to be consumed. This balanced V/III ratio changes the crystal structure from zincblende to wurtzite. The exact growth window for wurtzite crystal structure is investigated by carefully varying the group-V molar flow. Using existing growth model, it is interpreted that the observed crystal phase transition is attributed to a lower surface energy of the liquid In-Sb alloy droplet than the pure indium droplet, which makes triple phase line nucleation (wurtzite) rather than center nucleation (zincblende). For other molar flows, InSb nanowire exhibits zincblende crystal structure. The InSb growth is extended on InGaAs stem and obtained successful InGaAs/InSb heterostructure for the first time. To make high density of integration, it is motivated to integrate array of InAs and InGaAs nanowires on silicon substrate by selective area epitaxy. It is found that high aspect ratio of InAs and InGaAs is obtained at V/III ratio of 264. It is interested to see that as scaling down the total molar flow of the growth precursors by keeping constant V/III ratio of 264, we found WZ dominant crystal structure for high molar flow and whereas for low molar flow, nanowire exhibits wurtzite/zincblende mixed crystal structure for every 2~3 monolayers. The electrical characteristics prove that wurtzite dominant nanowire exhibits 1.6 times lower resistivity than polytype nanowires.