Chap. 2.1 Fabrication of AAO and Mask
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Enough reaction time (Al 100% reaction to Al2O3) Si wafer clean
Barrier layer coat clean
Al layer coater
Al annealing
AAO
AAO tmplate/TiN/Si-sub
Widen the pores diameter
XRD crystalline
PL optics
SE TE
XR crystalline crystalline
HR.SAE
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As-prepared procedure was RCA clean for p-type Si-substrate (100).
For barrier the follow-up thermal treatment, we employed TiN thin film as barrier layer, and deposited it on Si substrate by DC sputter in N2 atmosphere, and in vacuum chamber (1X10-6 Torr). Moreover, the barrier layer provided with conductivity to play a role of sub-electrode. For the prepared AAO template, Al thin film of required thickness was first deposited on the Si wafer by thermal evaporation in vacuum chamber (<5X10-7 Torr). The purity of Al source was 5N (99.999%) ingots of Al.
annealing the Al thin film on TiN/Si-sub at 350-500℃ was in order to release mechanical stress and enhance the crystallization quality. As we knew, the porous morphology of AAO depended on the quality of Al.
The two step anodic procedure, which has been reported in detail elsewhere, was employed to prepare high order porous channel arrays of AAO. Following, the major procedure would list simply. First anodic treatment was carried out in a 0.3M oxalic solution(H2C2O4) at 12℃ and applied a constant polarization voltage for a few time. The produce was a thin nano-porous AAO in the top surface. Then, we removed the thin nano-porous film by wet chemical etching with a mixed solution of H3PO4
and CrO3 at 60℃, and therefore the surface on the Al film was exhibited a relatively ordered indent pattern. The indent pattern was the as-prepared porous pattern and determined the surface morphology of AAO channel
However, the existing porous diameter was not wind enough. The pore diameter of the AAO template was widened in a 5% H3PO4 solution for a widened time.
The mechanism of two step anodic procedure was displayed at ch1-3, and the surface morphology and initial porous diameter were mainly determined by applied polarization voltage and electrolyte. Among of every electrolyte of AAO which were reported in previous literature, the reaction was carried out and based on acid solution, including chromic acid (CrO3), sulfuric acid (H2SO4), oxalic acid (H2C2O4), boric acid(H3BO3), or phosphoric acid(H3PO4). Those acid solutions were controlled to effective reaction by different solution concentration and reaction temperature.
Actuality, we achieved the porous length and diameter that we ordered by controlled the reaction time and widened time, when we fixed the parameter such as applied polarization voltage, space of electrodes, reaction temperature and electrolyte. The structure of result was AAO template on barrier layer of TiN and substrate of Si (AAO/TiN/Si).
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For the preparation of the AAO mask, an Al sheet of about several mm in thickness was employed and replaced the Al thin film on TiN/Si-sub in the above process. The Al sheet was passed through the same process flow as fabrication procedure of AAO, such as annealing, two step anodic treatment, and widened treatment. The structure of result was AAO on Al sheet, and we must remove the unnecessary Al sheet first.
Further, we removed the bottom (barrier layer) of AAO according to chemical etching in H3PO4 for removed time. At least, the product was the AAO mask without bottom.
Si wafer clean
Fill ZnO into the pores
Widen the pores diameter
(Annealing for ZnO with Al2O3)
Remove AAO template Remove ZnO on the AAO surface
High order ZnO NW array
Chap. 2.2 ZnO with AAO
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(Ellipses was listed in chap 2.1.1)
PL
I-V FE
optics SE
TE
XR crystalline crystalline
HR.SAE
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As the references of ch1-2-2, there were some main methods that could synthesis ZnO. Through by considering and testing, we choose the hydrothermal method from others. At present, one of the most serious points to fabrication was the synthesis temperature. The physical limit of material was too high temperature to apply to integration procedures. In other points, we had to consider other factors, including as experiment cost, migration ration of reactant on nano-scale porous, interaction between reactants, experiment environment, restricts of every kinds of machines, etc.
Following list showed the main chemical reactions of ZnO hydrothermal method in aqueous solution:
(CH2)6N4 + 10H2O → NH4OH + 6HCHO ………….[eq. 5]
Zn2+ + 2OH- → ZnO + H2O……….[eq. 6]
Above reactions could take place at relatively low temperature of between 75 and 95 . ℃ ℃ By the way, many details of the hydrothermal procedures were studied and reported, such as optimum reaction PH, effect
into sealed vessel, which contain an aqueous solution (Milli Q, 18.2MΩcm) of zinc nitrate hexahydrate (Zn(NO3) 6H‧ 2O, 0.01M) and diethylenetriamine (HMTA, C6H12N4, 0.01M) for ZnO NWs hydrothermally grown at ~95 for 1hr~2.5hrs.℃
After the hydrothermal procedure, the pores of AAO were full of ZnO.
The surface and cross-section morphology was observed by field emission scanning electron microscopy (FE-SEM). It appeared the flower-liked ZnO on the top surface, and we removed the park by chemical etching in HNO3 solution. Then, we etched the AAO and retained ZnO which were in the AAO pores by different selectivity. It would form a high ordered ZnO array from above processes. Further, we measured and discussed the physical and chemical properties.
Chap. 2.3 Capacitance
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Enough reaction time (Al 100% reaction to Al2O3)
Si wafer clean
Barrier layer coat clean
Al layer coater
Al annealing
AAO/TiN/Si
AAO template/TiN/Si-sub
Widen the pores diameter
Remove ZnO on the AAO surface Fill ZnO into the pores
capacitor
Coat Al film for electrode
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As the flow chat, we defined the electrodes when the ZnO was full on AAO and removed superfluous ones. We forecasted it could provide a large capacitance. This was a simple capacitance structure in nano-scale and provided large surface area to increase capacitance.
Chap. 2.4 Measurement Equipment
The materials and electricity analysis indicated that the several kinds characteristics of the ZnO NWs. Through these analyses, the crystalline, lattice structure, defects of crystalline, chemical composition, morphology, field emission and capacitance could be understood easily.
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Typically, the XRD (X-ray diffraction) method determines the crystal structure and the information of the lattice arrangements. Generally, the theta two-theta method was employed to analysis the intensities of the diffraction peaks. Through the specific peak was sharp or width, it indicated the grain size and the crystal structure. Then, the Full-Width-Half-Maximum (FWHM) indicated that the crystalline and lattice parameter.
The FE-SEM (field emission scanning electron microscopy) relied on the higher accelerate voltage than thermal SEM which offered good resolution to analyze the geometry. Through the accurate focus ability
from the FE-SEM, the morphology of the nanowires was easily measured and calculated their aspect ration.
The HR-TEM (high resolution transmission electron microscopy) had the well transmission ability that could offer many detail information for the selected area to observe. It could indicate the lattice structure, micro region composition, and high resolution images of surface morphology.
The information which observed by the HR-TEM listed as following:
(a) Bright view images: the images revealed the morphology directly.
(b) Dark view images: the images indicated fewer defects such as line defect or lattice mismatch which hided in bright view images.
(c) High resolution lattice images: the clear lattice fringes would indicate the atom arrangement and help to calculate the d-spacing along the c-axis if it was a quality crystalline.
(d) Selected area electron diffraction (SAED) patterns: the possible d-spaceings would be calculated by diffraction rules form SAED patterns.
After compare the lattice parameter, the crystalline of ZnO NWs is easily to identify and discuss which process made.
For TEM, the major challenge was the sample preparation, which could prepare by some methods, such as thin sheet sample by FIB (Focus Ion Beam), nanotube or nanowire by supersonic shake methods, etc.
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The fundamental analyses were used to precious judge the composition and what kinds elements of the ZnO NWs.
The XPS (X-ray Photoelectron Spectroscopy) or ESCA (Electron Spectroscopy for Chemical Analysis) were the key tools in surface analyses, mainly causes were the two features: Quantitative analysis and Information on the chemical nature and state of the detected elements. By absorbing a photon, an atom gains an energy amount equal to hν. It released an electron to regain its original stable energy state. The released electron retains all the energy from the striking photon. It can then escape from the atom, and even further from matter and kinetic energy. We could judge the elements through by detecting the energy of the released electrons.
While used the HR-TEM, the electron diffraction spectra (EDS) detected the elements which the ZnO NWs composed. The spectra were very sensitive that help to simply analysis the atomic weight percentage of each element due to the small focus electron beam and high accelerate voltage.
About the photoluminescence (PL) spectrum, the mechanism was discussed at ch1-2-3. Through by detecting the energy of the released electrons, we could infer the defects of crystalline.
The morphology of the ZnO nanowires was analyzed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4700I, Japan) and high resolution transmission electron microscopy (HRTEM, Philips
tecani-20). The chemical composition of the ZnO nanowires was characterized by Auger electron microprobe (AES, VG Scientific Microlab 350, UK).
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A Keithley 237 source-measure unit was used for measuring the current–voltage (I –V) and field emission characteristics. Field emission measurements were carried out in a vacuum chamber with a pressure of 5x10-6 Torr at room temperature. The measurement equipment was illustrated in FIG-2.4-1.