Chap. 1.1 Introduction
Recently, the nanotechnology have been extensively researched, which contains many kinds of different dimensional nanostructures. For example, there are one dimensional (1D) nanostructure, such as nano-tubes1,2,3, nano-wires4 and nano-rods5,6,7; two dimensional (2D) nanostructure, such as nano-sheets8,9, nano-planes and nano-discs; three dimensional (3D) nanostructure, such as nano-dots10 , 11. The above- mentioned are great importance in studying the physical and chemical properties of nano-materials or constructing functional nano-scale devices, which have been investigated in the potential applications of photonic, electro-optical and electronic device. For example, some applications of different kinds of nanostructures were extensively studied, including to nanodot for nonvolatile memory (nano-crystal memory device, IBM, 1995), nanosheet for solar cell, nanowire for field emission12 and photofluorescent device, etc.
About the studies of one dimensional (1D) nanostructure were quite popular, since the first discovery of carbon nanotubes (CNTs) that were fabricated in 199113. Considerable effort of CNTs has been devoted to the synthesis and characterization of nanoscale. Because of their attractive physical properties enable them to play a major role in nanoscale electronic and nanoscale devices, large studied about CNTs was published. Moreover, many materials were researched to replace carbon in order to improve some insufficient effects of CNTs. So, expect for carbon, a novel material,
ZnO, was attractive and utilized for nanostructure.
The subject was focused on the characteristic and fabrication of 1D ZnO nanostructure. The particulars will be discussed at following sections.
Chap. 1.2 ZnO Characteristics and Nanostructures
ZnO is a wide band gap semiconductor (3.37ev) with a large exciton binding energy (60meV) at room temperature, and is one of the most important and versatile semiconductor materials. Previous studies were investigated into its optical and electric properties14, as well as its ferromagnetic, transparency, photoconductivity, piezoelectric, field emission12, , and light-emitting properties. Further, ZnO has attracted much attention since possible applications in phosphors, transparent conducting films for solar cells, ultra-violet (UV) laser devices15 and flat panel displays were suggested16,17,18.
Since the first report of ultraviolet lasing from ZnO nanowires19, nanostructure of ZnO has stimulated intensive research. Indeed, ZnO nanostructures are interesting to study not only the property of UV lasing emission in nanowire form, but also because a wide variety of morphologies have been prepared. Some kinds of the ZnO nanostructures contain nanowire, nanorod, nanosheet, nanoribbon, nanotube, nanoplate, tetrapod, and flower-liked20 structures. Indeed, based on the nano-scale
Furthermore, the ZnO nanowires are believed to be a viable candidate which compare to CNTs for field emission device, due to its excellent thermal and chemical stability. So, about the prominent properties and the synthetic methods of ZnO nanowires we are interested will be discussed in next sections.
ChChaapp.. 11..22..11 ZnZnOO NNaannoowwiirreess
To come back to the subject, the focus is the 1D ZnO nanowires. The nanowire array is the major structure to investigate; contrary, the signal nanowire was no more attraction as nanowire array, and the main causes were fabricated hardly and applied restrictedly.
To continue mention of Chap. 1.2 , the applications of the ZnO nanowires were focused on the properties of photoluminescence and field-emission (the mechanisms would be discussed at Chap. 1.2.3 ), due to its excellent thermal and chemical stability. There are some qualitative and specific factors for measurement and analysis objectively. About factors of metrical, there are work function and square resistor, etc; about factors of morphology is such as aspect ratio, number density and surface roughness.
We will cit some instants and illustrate some applications at following contents:
1. nanowires array
In particular, exploration of the materials for flat panel displays has been a hot topic for the last few decades. So, current trends in
nanotechnology and nanomaterials play an important role in the potential applications. Then, high aspect ratio one-dimensional (1D) nanostructures, such as nanotubes, nanowires and nanobelts, have been extensively studied with a view to use in vacuum microelectronic devices, including field emission displays (FEDs), electron sources, microwave devices and high power rf amplifiers, because of the advantage of the low turn-on electric field and high electron emission efficiency12.
Recently, ZnO nanowire emitters have been reported to exhibit good emission properties with high stability, low threshold electric field, high emission current density, good emission stability and durability. The ZnO NWs have been synthesized by various procedures (we will discuss in next section); however, the main challenge in fabricating the nanowire based FED devices is the high synthesis temperature which retards the integration processes for FED device structures. A hydrothermal method would offer a superior route for FED fabrication because of the catalyst-free growth, low cost, low reaction temperature, large area and uniform production, environment friendliness and process compatibility with VLSI. As above advantages, the hydrothermal method for low temperature was applied in our works.
2. Field emission triode21,22
Basing on nanowire array, the ZnO NW triode shows good and controllable emission properties with the turn-on anode electric field,
density higher at the same turn-on electric field which compared with one without applying gat structure.
However, the main advantages for the nanowire based field emission device (FED) are a large area display, high uniformity, high productivity, high brightness, low cost, low power consumption and reliability. From this point of view, triode-type FED devices with low driving voltage, high resolution and controllable electron emission characteristics are candidates for constituting a new generation of FED devices. Fig. 1.2-1illustrates the fabrication flow of a ZnO NW triode field emission array.
Fig. 1.2-1: (a) It is the fabrication flow of a ZnO NW triode field emission array. (b) the measurement for FE property21.
3. Doped ZnO nanowires18
We can study the variation and effect on the photoluminescence and field-emission properties by doping the different elements into ZnO nanowire. Farther, we can modulate those characteristic to satisfy with application actuality. For example, Norton and co-workers have reported that the P0.02Mg0.1Zn0.9O film showed stable p-type behavior by adding Mg and P together—doping with Mg to increase the band gap and P to introduce the acceptor level23’24.
Moreover, such suitable doped NWs exhibit better field-emission properties including the lower threshold electric field, lower resistance in series, and higher emission current density, in comparison with undoped NWs. The improved field-emission characteristic is attributed to the changes in surface state of the nanostructure at both threshold voltage and F-N field-emission regions. Furthermore, the modulated band gap can be obtained with suitable doping, which may also be able to be applied for the future enanohomojunction optoelectronic and field-emission devices.
ChChaapp.. 11..22..22 SySynntthheettiicc MMetethhooddss ooff ZZnnOO NNaannoowwiirreess
Synthesis methods of ZnO nanostructures can be achieved by various techniques, for example, including as vapor-liquid-solid (VLS) growth12, metal-organic vapor phase epitaxy (MOVPE) procedures, physical
synthesis techniques employed some general semiconductor process machinery equipments, classed like as chemical vapor deposition (CVD), physical vapor deposition (PVD), electrical gun evaporation, thermal evaporation, high temperature chamber, etc. Those synthesis techniques look like similar for results. But, there were some different advantages and handicaps during some procedures. We focused on those handicaps which were fatal; some handicaps of physical property can not be overcome and endured for some applications, which for some materials, such as polymer substrates, will be melted due to high temperature treatment.
Consequently, one of the most serious handicaps to fabricate the nanowire is the high synthesis temperature, which will restrict the selection of substrate materials, even affect integration procedures. Thus, the main challenge in fabricating the NW based FED devices is the high synthesis temperature which retards the integration processes for FED device structures.
Particularity, the hydrothermal method offers a superior route for FED fabrication because of the catalyst-free growth, low cost, low reaction temperature, large area and uniform production, environment friendliness and process compatibility with VLSI.
Farther, following discussion was about the comparison of VLS with hydrothermal method, and listed the difference in their detail process and certain limits.
First, the main deficiency in VLS process was the high synthesis temperature (750-950 )℃ . And, the synthesis temperature of hydrothermal
method was low than 100 .℃ However, the high temperature would provide well aligned array and excellent structure oppositely, such as high quality and high aspect ratio.
Another notable difference of VLS with hydrothermal method was the catalyst. For example, the VLS employ Cu (copper) or Au (gold)12,25 as catalyst for growth ZnO. And, the intrinsic properties of catalyst, such as surface morphology and large different of lattice parameter with reaction elements, would cause to variation different of product. Following the example12, the surface of the Cu catalyst film has many ordered square and hexagonal Cu island structures and the lattice parameter of Cu (a=3.215 Å) is close to that of the ZnO (a=3.253 Å). Consequently, the ZnO nanowires can vertically grow from Cu catalyst layer on the Si substrate with VLS, rather than grow from Au catalyst layer. However, the hydrothermal method was always catalyst-free. Generally, the VLS process was complex than hydrothermal method in many particulars, such as flow ratio of carrier gas would obviously effect the product of VLS process.
However, we identify the VLS process was more complex than hydrothermal method. But, we also can not disregard the advances of VLS process. We could select suitably and search the balance according to our applications.
Following would list in Tab.1.2-1 about the comparison between VLS process and hydrothermal method.
Tab. 1.2-1: It shows details of comparison between VLS process and hydrothermal method.
ChChaapp.. 11..22..33 FiFieelldd EEmmisissisioonn aanndd PPLL CChhaarraacctteerriissttiiccss
◎ Field emission
In brief words, the field emission is a point discharge phenomenon. It was easily occurred at a long and sharp rod liked needle. When we applied a negative voltage on it, the tip would exhibit a large electric field then other parks naturally. If the applied filed is large enough, the potential barrier would narrow and an appreciable number of electrons at the Fermi level can tunnel through. We set a grounded probe or plant in a suitable distance between with the tip to receive would emit electrons from the tip. So we could constitute measurement equipment illustrated at Fig- 2.4-1, and we could measure the current I (a current passed by probe or plant) and calculate the current density (J) that was I divided by A (area of the probe or plant).
However, this point discharge phenomenon we observed was not sure Metho
d
synthesis temperatu
re
catalyst procedure quality of product
compatibili ty with
VLSI
VLS higher requisite More complex excellent no
Hydr o- therm
al
lower no simple acceptable yes
of field emission; we must check some properties to fit some conditions of the field emission mechanism. We could demonstrate by utilizing those discharge properties fitted the Fowler-Nordheim (FN) equation. The simple mode was that making sure the curve which was the relationship between J and E (applied voltage divided by the emission distance) to fit the F-N relationship. the FN relationship is as follows:
※Fowler-Nordheim (FN) equation:
J = (Aβ2E2/Φ) exp(-BΦ3/2 /βE)
A=1.56x10-10 (AV-2eV) ; B=6.83x103 (VeV-3/2μm-1)
Where J is the current density, E is the applied field, Φ is the work function of the ZnO (4.45 eV), β is the field enhancement factor, and A, B are constants, A = 1.56 × 10−10 (AV−2 eV) and B = 6.83 × 103 (V eV−3/2 μm−1)9.
※The field emitting enhancement factor, β:
m=BΦ3/2d/β; m=slop → β= BΦ3/2/m
where the value of β can be calculated from the slope of the F–N plot.
The calculated β value would determine the field emission was good or not.
Thus, large β value represented a well field emission mechanism;
contrariwise small β value meant a poor field emission mechanism. And
◎ Photoluminescence (PL)
The photoluminescence analysis was a simple physical mechanism which was illustrated in Fig. 1.2-2. In brief statement, about the mechanism of photoluminescence was an energy transport. Generally, the excitation source was some kinds of lamps which could emit different wave length. Different lamp which was filled of different gas would manage to provide a limited range of wave length. For example, we could employ two different kinds of lamps to provide a wide range of wave length. As we know, different metrical absorb particular wave length to provide energy for electron. If electron obtained enough energy, it would be transition from low energy level to high energy level; then, it would release energy when it transit to low energy level. The energy could be released by two type of light: one was excitation, another was emission.
And, if the released light was classified according to wave length, it could class as fluorescence and luminescence.
To take a point of ZnO NW array, a photoluminescence analyzer with Xe lamp as an excitation source (~320 nm) was used for optical studies at room temperature. The major photoluminescence peak was the ultraviolet (U.V.) emission which was due to band-edge emission of ZnO, and it could detect different peak. In many studies, those peaks at special wave lengths for ZnO were reported to be caused by the Zn vacancy, oxygen deficiency, deep level, or singly ionized oxygen vacancies, etc. In other words, those
defects of product would cause different energy states excited in the bend-gap. So, those minor peaks would suggest that the produce excited defects.
Fig. 1.2-2: It illustrates the mechanism of the photoluminescence.
hν
e
-e
-e
-hν Conduction band
Valence band e
-Chap. 1.3 Anodic Aluminum Oxide (AAO) & MASK
Anodic Aluminum Oxide (AAO)
◎
The Anodic Aluminum Oxide (AAO) structure would provide some properties for nanotechnology study, which including high ordered density of pores, controllable diameter and length of pores, large area and high uniform, stable chemical property of product, etc. As the result, recently, the AAO applications in nano-scale were studied extensively. Because of the one-dimensional nano-materials exhibit interesting properties, the AAO which could provide controllable one-dimensional pores was an excellent candidate to assistant fabrication.
Generally, the AAO structure was fabricated by a conventional two-step method. From the previous literature29, the two-step was classed as 1.as-perpared pore formation and 2.stable porous growth.
The interaction mechanisms were illustrated at Fig. 1.3-1. Pores grow perpendicular to the surface with the equilibrium of field-enhanced oxide dissolution at the oxide/electrolyte interface and oxide growth at the metal/oxide interface. While the latter is due to the migration of oxygen containing ions (O2-/OH-) from the electrolyte through the oxide layer at the pore bottom, Al3+ ions which simultaneously drift through the oxide layer are ejected into the solution at the oxide/electrolyte interface. The fact that Al3+ ions are lost to the electrolyte has been shown to be a prerequisite for porous oxide growth, whereas Al3+ ions which reach the oxide/electrolyte interface contribute to oxide formation in the case of barrier oxide growth. A possible origin of forces between neighboring
pores is therefore the mechanical stress which is associated with the expansion during oxide formation at the metal/oxide interface. Since the oxidation takes place at the entire pore bottom simultaneously, the material can only expand in the vertical direction, so that the existing pore walls are pushed upwards. Under usual experimental conditions the expansion of aluminum during oxidation leads to less than twice the original volume, since Al ions are mobile in the oxide under the electric field, so partly the oxidized aluminum does not contribute to oxide formation. In experiments, we could observe the relative thickness of the porous alumina layer compared to the consumed aluminum was found to vary with applied voltage and electrolyte concentration.
Following list showed the main chemical reactions of AAO:
Al(s) → Al3+(oxide) + 3e- ...[eq. 1] 3/2H2O(l) → 3H+(aq) + 3/2O2-(oxide) ...[eq. 2] 1/2Al2O3(s) + 3H+(aq) → Al3+(aq) + 3/2H2O(l) ...[eq. 3] 3H+(aq) +3e- → 1/2H2(g) ...[eq. 4]
The AAO cross-section structure after conventional fabrication was illustrated at Fig. 1.3-2. We could control the porous diameters by utilizing chemical etching to widen the pore.
Fig. 1.3-1: It illustrates the expansion of aluminum during anodic oxidation. On the left the level of the un-oxidized metal surface is depicted.
Fig. 1.3-2: The structure of AAO after anodization.
The Fig. 1.3-3 revealed direct proportion between anodic voltage with porous diameter of AAO; when the applied voltage was increase, the porous diameter would increase, and no matter what kinds of electrolyte.
The main cause was the applied voltage formed a field-enhance porous reaction. By the way, we obtain different porous diameter at the same applied voltage in different electrolyte.
Fig. 1.3-3: Relation between anodic voltage with pore diameter of AAO30.
The mechanism details of as-prepare pores were shown in the Fig.
1.3-4. The partial electric field would become more concentration in the Al2O3 surface when the applied voltage was increased gradually. Those partial area would higher reaction ratio (the field-enhance oxide dissolution at the oxide/electrolyte interface and oxide growth at the
The Al3+ and O2+ were combined to form Al2O3 in the surface.
Simultaneously, it would dissolve in the electrolyte due to that the as-prepare pores areas had higher concentrative voltage and higher reaction ratio (those complex ion interactions were discussed at above content). Further, we obtained the Al surface formed the vertical porous array after the second anodic step.
Fig. 1.3-4: The process of the partial electric field in the Al2O3 surface.
Fig. 1.3-5: 1. SEM micrographs of the bottom view of anodic alumina layers.
Anodization was conducted in 0.3 M (1.7 wt %) sulfuric acid at 10 °C at 25 V (a), 0.3 M (2.7 wt %) oxalic acid at 1 °C at 40 V (b), and 10 wt % phosphoric acid at 3 °C at 160 V (c). Pore opening was carried out in 5 wt
% phosphoric acid at 30 °C for 30 min. (a) 35 °C for 30 min. (b) and 45 °C
Fig. 1.3-6: Lower magnification SEM micrographs of porous alumina anodized in sulfuric (a), oxalic (b), or phosphoric acid (c). The anodization conditions are the same as those in Fig. 1.3-5.
◎ MASK
After the two step process, the main structure was AAO on Al sheet or substrate. The mask was fabricated by etching treatment for removing the film under AAO. Removing the redundant Al sheet and barrier layer was by the different selectivity of etching treatments. We would obtain a nano-channel array look like meshed thin film. It could be employed as mask to fabricate nanotube or nanowire if we widened the porous diameter to achieve suitable scale.
However, the as-prepared pore would determine the further porous properties; however, the as-prepared pore would defined by first anodic procedure, electrolyte, and experiment condition. Hence, some methods were reported in order to improve this question. For example, we could utilize FIB lithography or SiC hard mask to parent the required nano-scale porous array. Those methods were illustrated at Fig. 1.3-8. Although it could achieve the high ordered pore, the process became more complex.
Fig. 1.3-7: The high order pore array by FIB lithography.
Fig. 1.3-8: The high order pore array by SiC pattern.
Chap. 1.4 Capacitance
Based on the AAO porous template, one material was deposited on it as electrode. The dielectric was the compound of one metrical with alumina. To control the quality of nano-scale structure was a challenge. It always had large current leakage due to the not well crystalline of alumina.
Based on the AAO porous template, one material was deposited on it as electrode. The dielectric was the compound of one metrical with alumina. To control the quality of nano-scale structure was a challenge. It always had large current leakage due to the not well crystalline of alumina.