臺灣博碩士論文加值系統

本論文旨在研究一維氧化鎢奈米結構從奈米線轉化成奈米片之演變機制以及
針對其應用在氣體感測器與場發射器之物性、化性及電特性等之探討。於本研究中，
係經由一簡易之熱退火處理製程直接於濺鍍沉積之鎢基薄膜上自組成長氧化鎢奈米
線或奈米片，並藉由FESEM、HRTEM、SAED、XRD 及XPS 等各項物性及化性分
析工具解析所製備奈米材料的成長特性、成分及晶型結構。此外，利用一由Agilent
34970A、E3646A 及HP 4284A LCR 偵測計所構築之量測系統進行氣體感測之量測。
於研究中，我們也利用Keitheley 237 設備及於室溫與5×10-6 torr 高真空度下進行場發
射電特性之量測以及電場增強因子、驅動電流與起始電場等相關參數之萃取。
首先，我們針對氧化鎢奈米結構從奈米線轉化成奈米片的一種可能成長機制進
行研討。本研究中，為探討溫度效應及時間效應對所成長奈米結構的影響，所有試片
係在氮氣(N2)氛圍下分別以750°C/0.5~3 h 及650~950°C/0.5 h 的熱預算進行熱退火處
理製程，直接於濺鍍沉積之鎢薄膜上自組成長氧化鎢奈米線及奈米片。從SEM 分析
結果顯示，於濺鍍沉積之鎢薄膜上自組成長氧化鎢奈米線的最低成長溫度應該介於
500~600°C 之間，此時的熱預算值主要係提供足夠的活化能以形成WOx 奈米晶型以
及W18O49 的成核位址。當試片分別在750°C 溫度下進行0.5 及1 h 熱退火處理後，可
成長稠密且均勻分佈的氧化鎢奈米線，其典型的奈米線直徑、長度與成長密度分別介
於15~25 nm、0.15~0.30 μm 與250~300 μm-2；此自組成長之氧化鎢奈米線主要先藉由
鎢與薄膜內含氧量的化學反應形成WOx 奈米晶型之後，再經由WOx 奈米晶粒邊界所
形成的W18O49 的成核位址開始成長；在熱退火期間，參與反應的氧可能來自於鎢靶
材的不純以及鎢薄膜曝露於大氣環境下所造成。當所提供的熱預算進一步增加，即試
片分別在750°C 溫度下進行>2 h 或在>850°C 溫度下進行0.5 h 的熱退火處理後，試片
的表面形態發生一戲劇性的變化。我們觀察出此階段退火期間所成長奈米片的密度與
尺寸隨著熱預算的增加而增大，然而卻伴隨著奈米線成長密度的降低。經由HRTEM
及SAED 分析，自組成長的氧化鎢奈米線及奈米片已被證實為沿[010]方向成長的
W18O49 單斜晶相。
從我們的實驗與分析結果顯示，原先二根鄰近的奈米線會藉由它們的晶面緊密
結合而形成一個非晶接面，並因熱預算的進一步增加而產生再度結晶。基於這些現象
歸納，我們提出一項由氧化鎢奈米線演變到奈米片的可能轉換機制如下：由於最初所
成長的奈米線不僅稠密且方向雜亂，鄰近的奈米線會沿著它們錯置的[010]方向彼此
連接，並在它們(502) 的晶面間扭轉或旋轉構成一個不穩定的邊界；當熱預算的進一
步增加時，此發散的不穩定的邊界會形成一個暫穩態的非晶接面；一旦所供應的熱預
算足夠高，則將會進一步降低二者(502) 非晶接面的表面能而產生再結晶；最後，所
有的邊界接面消失且形成具有相同晶相的W18O49。此相似的演繹過程亦可應用到從
多根奈米線轉化到奈米束的成長機制探討。
實際上，一維氧化鎢奈米線所展現的物理及化學特性與塊材或薄膜有很大之差
異，因此很適合作為氣體感測器的敏感材料；此外，由於一維氧化鎢奈米線具有較大
的表面積/體積比值(SVR)、巨額數量的表面原子以及電子侷限的維度，因此使用氧化
鎢奈米線氣體感測器來偵測氣體亦會有實質上的改善以及降低元件之工作溫度。故在
本研究同時亦提出一種具氧化鎢奈米線之金屬/二氧化矽/金屬結構氣體感測器：此三
層結構的材料係使用n 型矽基板以及WCx(W:C=70:30 wt%)靶材，製備過程中利用射
頻濺鍍及電漿增強式化學氣相沉積系統分別沉積WCx 電極與SiO2 薄層，底層及頂層
電極厚度分別為60 nm 及120 nm，並由此二電極層之間距來決定中層SiO2 的厚度；
依據先前實驗氧化鎢奈米線成長特性，我們將中層SiO2 的厚度控制在約300 nm，並
藉由活化離子蝕刻方式將元件圖案化，另使用BOE 溼式蝕刻方式進行SiO2 周圍之側
向蝕刻作為上下二電極氧化鎢奈米線成長之空窗，最後再將試片置於爐管內並於氮氣
環境下(60 sccm)，進行750°C 及30 分鐘之熱退火處理，以自組式成長出可橋接上下
兩電極之氧化鎢奈米線，此製程不須使用到昂貴的次微米微影技術。
實驗結果顯示，自組合成之氧化鎢奈米線的典型長度及直徑分別為0.15~0.4
μm 及17~25 nm，並穩固地在上下二電極間形成橋接。另經由XRD 分析證實自組合
成之氧化鎢奈米線的主要晶相仍為單斜W18O49，其與穩定晶相之WO3 相較，單斜
W18O49 晶相因具有氧空缺，因此基本上屬於n 型半導體之氣體感測行為。由本研究
製備之氧化鎢奈米線成長特性估算，單根氧化鎢奈米線的SVR 約比傳統WO3 薄膜氣
體感測器高出7.5~11.6 倍。由於所製備的氧化鎢奈米線應用於氣體吸附時存在著氧空
缺以及較大的SVR，因此當我們將八個氧化鎢奈米線氣體感測器串接並應用在NO2
氣體感測行為分析，呈現出9.3 的高靈敏度及僅約9 秒的極短應達時間等優越測功
能，即使低至2 ppm 濃度的NO2 亦可被偵測到，因此進一步增加串接的數量以改善
氣體感測器的靈敏度是可以被期待的。
基於本研究群先前之研究已成功自組合成具較高長/徑比之氧化鎢奈米線，典
型的低起始電場僅約為1.7 V/μm，所展現出之優越場發射電特性足可與CNTs 相媲
美。為了拓展其在低電壓真空微電子元件之應用，本研究首次提出利用電流熔斷之新
穎技術以製備具奈米間隙尺寸之氧化鎢奈米線側向場發射元件。本研究所提出之側向
場發射元件的製備流程主要區分為四個階段：首先，係以n-Si 作為基板，並利用濕
式氧化成長厚度約為500 nm 左右的SiO2 層以完成SiO2/Si (SOI)基板製備。其次，藉
由傳統微影製程技術及直流濺鍍法於SOI 基板上濺鍍沉積約50 nm 厚度之圖案化鎢
薄膜(最窄通道尺寸為20 μm(L)×5 μm(W))以完成tungsten-on-SOI (W/SOI)結構製備。隨
之，於具圖案化之W/SOI 結構上及大氣環境下通入220~280 mA/μm2 之電流密度以進
行薄膜通道之熔斷作業，此階段的實驗結果發現，可形成最小奈米間隙之臨界電流密
度為220 mA/μm2；當電流密度進一步增加，此時可觀察到所形成的奈米間隙尺寸將
隨著所通入電流密度的增加而變寬。當所驅動的電流密度高於280 mA/μm2 時，將可
能因瞬間過熱而導致鎢薄膜奈米間隙邊緣的毀損；基於我們實驗的結果，最適化的電
熔條件為通入240 mA/μm2 電流密度以及10 sec 通電時間，此時因潛在焦爾熱所形成
之分離奈米間隙約為0.2~0.4 μm。最後，將所有於最適化電熔條件下所熔斷之試片分
批置入爐管內並於N2 氛圍下進行750~850°C 及30 min 之熱退火處理；實驗結果發
現，熔斷元件在經800°C/30 min 退火製程後在分離鎢薄膜間隙邊緣有較均勻及較佳
氧化鎢奈米線成長，其典型之長度及寬度分別為125~220 nm 及17~25 nm，在此階短
段有效的氧化鎢奈米線間之奈米間隙已又更進一步降為約0.18 μm。
另由場發射特性量測可發現，經750°C 及850°C 退火後之元件，其呈現相當
高的起始電壓(在發射電流為1 μA 前題下，分別約為16.5 及12 V)以及較低的電場增
強因子(分別約為56 及340 μm-1)；然而經800°C 退火後之元件則展現出相當優越的場
發射特性，其在發射電流同為1 μA 前題下具有極低之起始電壓(~2.7 V)、較高的電場
增強因子(~660 μm-1)及在操作電壓為81.8 V 時有高達8 mA 的發射電流；此相當低的
起始電壓主要歸諸於氧化鎢奈米線的長/徑比以及側向氧化鎢奈米線的尖端之間所形
成的奈米間隙。

In this thesis, study on the growth mechanisms of the evolution of one-dimensional
tungsten oxide (TO) nanostructures from nanowires to nanosheets and their applications in
gas sensors and field emitters were proposed and investigated. Nanowires and nanobelts
were self-synthesized on sputter-deposited tungsten-based thin film by a simple thermal
annealing process in N2 ambient. Techniques of various analysis including FESEM,
HRTEM, SAED, XRD and XPS were employed to characterize the growing conditions and
properties of nanostructures. In addition, a measurement system consists of Agilent
34970A, E3646A and HP 4284A LCR meter, was used for gas sensing measurement. In
addition, Keitheley 237 was also used to measure the field-emission characteristics, which
was carried out at room temperature in a vacuum chamber with a pressure of 5×10-6 torr.
The dependences of the field enhancement factor (β), driving current, and turn-on voltage
were extracted by current-voltage characteristic based on Fowler-Nordheim equation.
First, a possible mechanism for the transformation of nanomorphology from wires
to sheet is reported. In experiments, a simple thermal annealing process was employed to
self-synthesize TOs nanostructures on sputtering-deposited tungsten films in N2 ambient
with thermal budgets of 750°C for 0.5~3 h and 650~900°C for 0.5 h, respectively. Based
on experimental results, the lower limit of temperature for the self-synthesis of tungsten
oxide nanowires (TONWs) from sputtering-deposited W films might lie in the range of
500~600°C, which could be related to the temperature necessary to provide enough
activation energy for the formation of WOx nanocrystallites and W18O49 nucleation sites.
For samples annealed at 750°C for 0.5 and 1 h, dense and uniformly distributed TONWs
with typical diameters of 15~25 nm, typical lengths of 0.15~0.30 μm, and typical densities
of 250~300 μm-2 were obtained. The self-synthesis of TONWs can be related to the
formation of WOx nanocrystallites caused by chemical reaction of tungsten and oxygen
contained in the film and then commenced from the nucleus sites of W18O49 crystallites at
the grain boundaries of WOx nanocrystallites on the surface of samples during thermal
annealing. Oxygen might be from impurities in the tungsten target and/or the exposure of
tungsten films to air. Further increases the thermal budgets, which samples annealed at
750°C for >2 h or >850°C for 0.5 h, a drastic change in surface morphologies of the
annealed samples with both the density and size of the nanosheets are seen increasing
accompanied with the decrease in the wire density is observed. Both the self-synthesis of
the TO nanowires and nanobelts have been identified that comprise the major
crystallization phase of monoclinic W18O49 (JCPDS No. 36-0101) with a growth direction
of [010].
In addition, based on experimental results that show the formation and
re-crystallization of an amorphous interface layer between two neighboring nanowires
which were linked together by their (502) planes, a possible mechanism for the
transformation of nanomorphology from wires to sheet was proposed. First, neighboring
nanowires might connect to each other with misalignments along their [010] direction
because the nanowires are dense (250~300 μm-2) and random in orientation. Then, a twist
and/or rotation misalignment ent between their (502) planes might be presented. However,
the boundary is not stable, which should give it a higher surface energy. Accordingly, the
divergence of the crystallography was tailored during thermal annealing and led to the
formation of an amorphous interface. Once the thermal budget was high enough, the
amorphous interface re-crystallized to further reduce the surface energy of the two
amorphous/ (502) interfaces. Finally, all interfaces or boundaries disappeared and
nanosheets with the same crystallography of W18O49 was formed. The same evolution
process is also applicable to the formation of nanobundles from more than two neighboring
nanowires, which were also found in samples annealed at a high thermal budget.
Essentially, the use of TONWs as gas sensing materials is very attractive because
they exhibit physical and chemical properties that are very different from those of their
bulk or film counterparts. In addition, the sensitivity of TONW sensors to gases can be
substantially improved and the working temperature can also be reduced because of the
large surface to volume ratio (SVR) as well as the enormous number of surface atoms
and/or dimensional confinement of electrons. Therefore, a three-layer structure
TONW-based gas sensor was fabricated and its sensing performance for NO2 of different
concentrations at different working temperatures was also investigated.
During the fabrication processes of the proposed TONW-based gas sensor. N-type
Si wafers and WCx (W:C=70:30 wt%) target were used. RF sputtering and
plasma-enhanced chemical vapor deposition (PECVD) were employed for the deposition
of WCx (W:C=70:30 wt%) electrodes and the SiO2 layer, respectively. The thicknesses of
the bottom electrodes, middle SiO2 layer and top electrodes were 60, 300 and 120 nm,
respectively. Then, the devices were patterned by reactive ion etching (RIE)and subjected
to buffer oxide wet etching for 90 s to remove the SiO2 layer from the periphery. Finally,
thermal annealing at 750°C in N2 (60 sccm) ambient for 30 min was conducted for the
self-growth of TONWs in the window area created by the side etching of the SiO2 layer.
Experimental results showed that self-growth TONWs have main phase of
W18O49(010) and typical lengths and diameters of 0.15~0.4 μm and 17~25 nm, respectively.
The present sandwich structure allows TONWs to firmly link the top and bottom electrodes
without the use of submicron photolithography. The monoclinic W18O49 behaves as an
n-type semiconductor with oxygen vacancies. Essentially, a single TONW has SVRs of
around (1.6~2.35)×106 cm-1, which was found to be about 7.5~11.6 times higher than those
of the film WO3 sensors prepared at a firing temperature of 700°C. Because of the presence
of oxide vacancies and large SVR of TONWs for gas absorption, good sensor performance
with a sensitivity as high as 9.3, a short response time (about 9 s), and a detectability of 2
ppm of NO2 was achieved using eight sensors in a series connection. It is expected that the
sensitivity of gas sensors can be further increased by increasing the number of TONWs in
each cell in series-connected sensors.
In addition, the self-synthesis of TONWs with a high aspect ratio had been obtained
from our previous studies, which exhibit good electron field emission characteristics with a
typical turn-on field of about 1.7 V/μm were comparable with CNTs. In order to expand its
application into low voltage vacuum operable microelectronics, use of current fusing
method for the fabrication of lateral W18O49 nanowires in a tip-to-tip configuration with a
nanogap is proposed for the first time.
The key fabrication processes of the proposed lateral-type field emission device is
divided into two main stages. Firstly, n-type 0.01 Ω-cm <100> silicon wafers were used
and the 500-nm-thick wet SiO2 layer was grown at 1050°C for 1 h. In this stage,
silicon-on-insulator (SOI) substrates were obtained. Secondary, 50-nm-thick patterned W
films with tunnel length of 20 μm and width of 5 μm were sputter-deposited on the SOI
substrate by photolithography and dc sputtering method. Then, current densities ranging
from 220 to 280 mA/μm2 were applied to the W films on SOI for current fusing in
atmospheric air. It was found that, for 50-nm-thick patterned W films, the critical current
density for the generation of nanogap is about 220 mA/μm2. The nanogaps were observed
widening as the current density was further increased. When the density was driven over
280 mA/μm2, it might cause an instant over-heating and destroy the W thin film on the
edge of the nanogaps. Based on our experimental results, under an optimized current
density of 240 mA/μm2 for 10 s, nanogap in the range of 0.2~0.4 μm could be obtained.
Finally, all fused samples were subjected to thermal annealing from 750 to 850°C
temperature in N2 ambient for 30 min. It was found that the sample annealed at 800°C
exhibited uniform morphology of TONWs on the edges of the nanogap. The typical length,
diameter, and aspect ratio of TONWs grown on the sample annealed at 800°C were
125~220 nm, 17~25 nm, and 5~13, respectively, while the effective nanogap of the lateral
TONWs in tip-to-tip configuration were further reduced to be of about 0.18 μm. A
comparison for the 750°C and 850°C samples, it reveals that the turn-on voltage is quite
high (about 16.5 and 12 V, respectively) at 1 μA and the field enhancement factor is very
low (about 56 and 340 μm-1 respectively). However, the 800°C-annealed sample exhibited
a fairly good field emission characteristics with a low turn-on voltage of 2.7 V at 1 μA and
a high electric field enhancement factor of ~ 660 μm-1. Stable and profound
Fowler-Nordheim characteristics were observed at around 81.8 V with the FE current in
excess of 8 mA. The relatively low turn-on voltage might be attributed to the high aspect
ratio of TONWs and the realization of the nanogap of the lateral TONWs in tip-to-tip
configuration.

Abstract (in Chinese) I
Abstract (in English) V
Acknowledgement (Chinese) X
Contents XI
Table captions XIV
Figure captions XV
Chapter 1 Introduction
1.1 Fundamentals of Nanotechnology 1
1.1.1 Definition of Nanotechnology 1
1.1.2 Significance of Nanostructured Materials 2
1.1.3 Effects of Quantum Confinement on Transport 2
1.1.4 Structural Aspects 4
1.1.5 Electronic Properties of the Nanostructures 5
1.1.6 Synthesis and Fabrication of Nanomaterials 6
1.2 Introduction of Tungsten-based Nanomaterials 8
1.2.1 Structural Properties of Tungsten Oxide 8
1.2.2 The Synthesis of Tungsten Oxide Nanostructures 13
1.3 Applications of Tungsten-based Nanomaterials 15
1.3.1 Application as Electrochromatic Devices 16
1.3.2 Application as Gasochromatic Devices 18
1.3.3 Application as Gas Sensors 19
1.3.4 Application as Field Emitter 21
1.4 Motivations and Thesis Organization 24
Chapter 2 Theory and Experimental Procedures
2.1 Theory of Gas Sensing 27
2.2 Theory of Field Emission 30
2.3 Experimentals for Studying W18O49 Nanostructures from Wires to Sheets 33
2.4 Experiments for Gas Sensors Fabrication Based on W18O49 Nanowires with a W/SiO2/W Structure 36
2.5 Experiments for the Fabrication of Lateral W18O49 Nanowires in a Tip-to-Tip
Configuration with a Nanogap Using Current Fusing Technique 39
Chapter 3 The Evolution of Tungsten Oxide Nanostructures from Nanowires to Nanosheets
3.1 FESEM Images of Tungsten Oxide Nanostructures Annealed Under Various Thermal Budgets 43
3.2 HRTEM Images of A Single Nanowire Obtained from Samples Annealed at 750°C/1 h and at 900°C/0.5 h 45
3.3 HRTEM Image of an Immature Nanosheet Structure Obtained from Samples Annealed at 750°C/2 h and 3 h 46
3.4 A Possible Mechanism Behind the Evolution of TO Nanostructures from Wire toSheet 47
3.5 Summary 49
Chapter 4 Development of Gas Sensors Based on Tungsten Oxide Nanowires in a Metal/SiO2/Metal Structure and Their Sensing Responses to NO2
4.1 SEM Images of the Proposed Device Structure Before and After Thermal Annealing 51
4.2 XRD Analysis of Annealed WCx electrodes 52
4.3 TEM Image and SAED Pattern of WOx Nanowires 53
4.4 Sensor Response versus Working Temperature 55
4.5 Transient Response of Sensors Connected in Series 56
4.6 Transient Response of Serial Connections of Four and Eight Sensors 58
4.7 Summary 59
Chapter 5 Use of Current Fusing Method for the Fabrication of Leteral W18O49 Nanowires in a Tip-to-Tip Configuration with a Nanogaps
5.1 FESEM Images of the Prepared Samples After Current Fusing with Different Current Density in Atmospheric Air 63
5.2 A Relationship Between Resistance and Fusing Time 65
5.3 EDS Analysis at Different Locations of Fused Sample 66
5.4 SEM Images of the Fused Samples by Annealing at 700-850°C in N2 ambient 67
5.5 XRD Analysis of Current-Fused Samples Before and After Thermal Annealing at 750–850°C for 30 min 68
5.6 Field emission characteristics of fused/annealed samples 69
5.7 Summary 70
Chapter 6 Conclusion and Future Work
6.1 Conclusion 71
6.2 Suggestion for Future Work 74
Reference 76
Publication List 90
Vita 94

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