一維奈米氮化鋁的製備與光電特性分析

Abstract

利用常壓熱壁式的化學氣相沈積法(atmosphere pressure chemical vapor deposition; APCVD)，我們已成功地在矽基板上成長出氮化鋁(aluminum nitride; AlN)一維奈米晶體（包含奈米針尖- nanotips與奈米柱- nanorod）。同時也藉由場發射掃瞄式電子顯微術(field-emission scanning electron microscopy; FESEM)，穿遂式電子顯微術(transmission electron microscopy; TEM)，選區電子繞射術(selected-area electron diffractometry; SAED)，X光繞射術(X-ray diffraction; XRD)，與拉曼散射(Raman scattering)等量測技術，對各式的氮化鋁樣品之形貌，結構，晶向與組成做了詳細的特性研究。應用方面，本實驗針對一維奈米氮化鋁的光學特性做了深入的研究，以及一維奈米氮化鋁基板於表面增強拉曼(surface enhanced Raman Spectroscopy; SERS)方面的特性分析，同時也針對具有奈米尖端的氮化鋁針尖做了詳盡的場發射特性研究。 論文中詳細描述了一維奈米氮化鋁的成長特性與結構分析。一維奈米氮化鋁材料主要以氣相傳輸與凝結(vapor transport and condensation process; VTCP) 為合成的機制。在化學汽相沈積過程中，金屬鍍層的厚度控制對於一維奈米氮化鋁針尖材料的尺寸控制有很大的影響。此外，不管使用何種金屬鍍層厚度或種類，合成出來的一維奈米氮化鋁針尖具有類似的針尖角度，稱之為自我選擇的針尖角度(self-selective apex angle)。為此，我們提出了成長模型來描述一維奈米氮化鋁針尖的成長特性與機制。 針對不同的合成溫度對於一維奈米氮化鋁材料表面型態的影響也做了深入的探討。發現到了在低溫合成時(950 ℃)，一維奈米氮化鋁會形成針尖狀(nanotip); 然而在高溫合成時(1200 ℃)，一維奈米氮化鋁會長成柱狀 (nanorod)。為了解釋這種現象，我們引進了分子的擴散長度 (diffusion length)以及表面能量(Ehrlich-Schwoebel barrier)的理論來討論一維奈米氮化鋁由低溫針尖狀轉變為高溫柱狀的現象。 一維奈米氮化鋁的光學特性方面，我們使用了陰極螢光(cathodoluminescence), 光激發螢光(photoluminescence), 熱螢光(thermoluminescence), 以及紫外光吸收(UV absorption)等技術來研究分析，並觀察到了一維奈米氮化鋁材料同時具有直接能隙放光的特性以及因為氧及其它摻雜物所造成的缺陷放光特性。因此，我們提出了一維奈米氮化鋁材料的吸收與放光模型。 一維奈米氮化鋁材料於表面增強拉曼的應用上，我們利用離子束濺鍍系統將奈米銀微粒 (5-10 nm)鍍在一維奈米氮化鋁基板上。並使用Rhodamine 6G (10-6 M) 當做測試的樣品。其實驗結果顯示，在拉曼強度的表現上，具有106 的增強效果。 電性方面，一維奈米氮化鋁材料成長於不同摻雜種類及電阻值的矽基板上(p+, p, n+, and n type-Si)，來探討基板對於埸發射特性的影響。針對一維奈米氮化鋁所特有的奈米尖端，其場發射的特性測試結果顯示，當一維奈米氮化鋁針尖(AlNNTs)成長於p+-Si 基板時，此類材料具備最佳的場發射密度 (max. current density ~0.22 A/cm2)，其用於驅動10 □A/cm2電流密度的最低啟動電場(turn-on field)約在6.5 V/um，且具有相當穩定的特性(10小時內僅5% 變化)。反之如果將一維奈米氮化鋁材料成長於n- or n+- Si上的話，就沒有辧法得到任何的場發射電流。論文中也提出了矽-氮化鋁異質接面(Si-AlN heterojunction)的模型來解釋這種現象。

Single crystal hexagonal aluminium nitride nanotips are fabricated using vapor transport, from aluminium powders heated to 950°C in presence of ammonia gas, and condensation process (VTCP) on silicon substrates with or without catalyst layer. The resultant tips have very sharp nanoscale apexes (~ 1 nm) while their base and length up to hundreds of nm and several microns, respectively, are observed. Critical role of the gold catalyst layer thickness in controlling the size of tip has been demonstrated in addition to a catalyst-free growth mode resulting in lesser control over the nanotip morphology. Nevertheless, a remarkably narrow distribution in the apex angle of the nanotips, regardless of the use of catalyst in VTCP, has been obtained. Unlike the commonly observed ridge and pyramid structures, the nanotips produced by VTCP have higher angles (~81°) between the tilted (221) and the basal (001) planes that encase it. A mechanism for this self-selective apex angle in aluminum nitride nanotip growth has been proposed. However, if we take the growth temperature as a variable parameter, but keep other condition fixed. At lower growth temperatures, around 950 °C, AlN nanotips with apex diameters of 10 nm, base diameters of ~100 nm, and length of ~2000 nm were obtained. On the other hand, as the growth temperature approached 1200 °C, we observed a broadening of the tip area, a shortening of the height and a complete transformation to AlN nanorods. Compelling microscopic evidences were obtained to show that stacked AlN platelets of diminishing size formed the building blocks of the nanotips. A reducing Ehrlich- Schwoebel barrier introduced into a diffusion mediated growth model explains the formation of AlN nanorods at increasing growth temperatures. The optical properties of wurtzite AlN nanotips are characterized by cathodoluminescence (CL), photoluminescence (PL), thermoluminescence (TL), and UV absorption. CL measurement shows two defect related transitions around 2.1 and 3.4 eV and a well resolved excitonic feature in the near band-edge transition ~ 6.2 eV. Two broad peaks centered nearly at 2.1 and 3.4 eV were also observed from the PL and TL spectra. Analysis of the excitation spectra of both PL and TL measurements suggested the existence of multifold energy levels within the gap. The optical absorption spectrum shows that the nanotips exhibited three absorption centers located at 6.2, 5.0, and 3.4 eV, respectively. A new substrate for surface enhanced Raman spectroscopy has been developed in the form of AlN nanotips. The surface area of the nanotips is quiet high for the post-metal deposition. Ion beam sputtered silver self assemble on these substrates as nanoparticles of 5-10 nm diameter and these metallic nanoparticles act as surface enhancement centers for Raman spectroscopy. Standard molecule such as Rhodamine 6G of concentrations of 10-6 M has been studied on these substrates and enhancements in the range of 106 was observed. Here, we report the field emission (FE) properties of the quasi-aligned aluminum nitride (AlN) nanotips grown on differently doped (p+, p, n+, and n type) silicon (Si) (100) substrates. The AlN nanotips grown on p+-type Si substrate showed the lowest turn-on field of 6 V/□m (highest current density of 0.22 A/cm2 at a field of 10 V/□m), whereas no significant emission could be obtained using n+- and n- type Si substrates. Band diagrams of the heterojunction between the Si substrate and the AlN nanotips have been used to explain this charge carrier related FE from wide band gap AlN nanotips. The FE from the AlN nanotips prepared on p+- and p- type Si substrates were found to be stable for more than 10 hours while extracting a constant current density of 100μA/cm2 with the applied field varying by only about 5% and 10%, respectively.