This work was financially supported of by the Ministry of Science and Technology and National Science Council of Republic of China (contract nos. NSC 101-2221-E-027-042 and NSC 101-2622-E-027-003-CC2). D. H. Wei thanks the National Taipei University of Technology (TAIPEI TECH) for the Dr. Shechtman Prize Award.

1. Substrate Preparation and Cleaning
<ol>
	<li>Cut 10 mm x 10 mm silicon substrates from Si(100) wafer.</li>
	<li>Cut 10 mm x 10 mm glass substrates.</li>
	<li>Use ultrasonic cleaner to clean the silicon and glass substrates with acetone for 10 min, alcohol for 10 min, and then isopropanol for 15 min.</li>
	<li>Rinse the substrates with deionized (DI) water three times.</li>
	<li>Blow-dry the substrates with a nitrogen gun.</li>
</ol>
2. DEZn Preparation and Preservation
Note: Diethylzinc (C2H5)2Zn, also called DEZn, is a highly pyrophoric organozinc compound consisting of a zinc center bound to two ethyl groups. Never work alone when using DEZn. DEZn is very toxic and sensitive to the oxygen and water, be sure not to place the DEZn near the water. Always wear protective masks and eye protection; all procedures must be performed in the hood. Most importantly, unused DEZn must be stored in a 5 oC environment.
Note: For the first use of DEZn, follow step 2. If not, start the experiment from step 3.
<ol>
	<li>Use syringe to draw out 30 ml&#160;DEZn from the bottle and then inject into a beaker placed in a steel cylinder.</li>
	<li>Use a galvanized iron pipe to connect the steel cylinder with the reaction chamber.</li>
	<li>Use mechanical pump and ball valve to pump down the steel cylinder in vacuum environment (to 6 Torr). 
	Note: DEZn will severely react with oxygen, it must be maintained in the vacuum environment.</li>
	<li>Store the unused DEZn in a 5 oC environment.</li>
</ol>
3. PECVD Chamber Preparation and Synthesis of ZnO Thin Films
Note: The schematic diagram of plasma enhanced chemical vapor deposition is depicted in Figure 1.
<ol>
	<li>Set the working distance between showerhead electrode and sample stage at 30 mm.</li>
	<li>Place the substrates on the sample stage of reaction chamber in proper location where there is a 3 cm distance from the DEZn inlet.</li>
	<li>Open the rotary pump and gradually open the gate valves and butterfly valve.</li>
	<li>Wait until the background pressure of the reactor chamber is lower than 30 mTorr.</li>
	<li>Close the gate valves and butterfly valve, which connects to the rotary pump.</li>
	<li>Then open the turbo pump and relative gate valves to reach high vacuum of 3 x 10-6 Torr.</li>
	<li>After reaching the necessary&#160;vacuum condition, open the heat controller and heat the sample stage to the synthesis temperature (200, 300, 400, 500, and 600 oC for different experiment parameters).</li>
	<li>When the temperature and the pressure reach the necessary condition, close the turbo pump and then open the gate valves and butterfly valve which connects to the rotary pump simultaneously.</li>
	<li>Next, open the gas inlet valves and turn on the argon gas flow controller simultaneously.</li>
	<li>Flow the argon gas (0.167 ml/sec) into the chamber.</li>
	<li>Set the chamber pressure to 500 mTorr.</li>
	<li>Turn on the RF (13.56 MHz) generator and matching network, then set the RF power at 100 W for purging the samples surface for 15 min.</li>
	<li>After finishing the purge of samples, turn the RF power down to 70 W.</li>
	<li>Next, turn on the carbon dioxide gas controller and gas inlet valve.</li>
	<li>Flow the carbon dioxide (0.5 ml/sec) into the chamber.</li>
	<li>Set the working pressure at 6 Torr.</li>
	<li>After the chamber pressure reaches 6 Torr, flow the high pure argon as carrier gas (0.167 ml/sec) for carrying diethylzinc (DEZn) into the chamber and open ball valve connected to the DEZn simultaneously. At the same time, start the synthesis of ZnO films.</li>
	<li>Continue the plasma synthesis of ZnO films for 5 min.</li>
	<li>After the ZnO films have been synthesized, seriatim turn off the RF generator, ball valve, heat controller and all of gas flow controllers along with gas inlet valves.</li>
	<li>Take out the sample when the sample stage temperature cools down to RT. Note: The cooling rate is approximately 1.8 oC/min.</li>
</ol>
4. Preparation of Interdigitated-like Pattern onto As-synthesized ZnO Thin Film
Note: The schematic of lithography process is depicted in Figure 3.
<ol>
	<li>Use a hot plate to bake the as-synthesized ZnO sample at 150 oC for 10 min.</li>
	<li>Place the sample on the spin coater, and then dispense the liquid solution of photoresist (S1813) with 100 &#181;l&#160;onto the ZnO sample.</li>
	<li>Run the spin coater at 800 rpm for 10 sec and then accelerate to 3,000 rpm for 30 sec to produce a uniformly thin layer.</li>
	<li>Soft bake the photoresist-coated ZnO sample at 105 oC for 90 sec.</li>
	<li>After the soft-baking, use UV light to expose the photoresist-coated sample trough a photomask by mask aligner. The exposure time is 2 sec and the power is 400 W. 
	Note: The pattern of photomask is designed as interdigitated-like, which is 0.03 mm wide and 4 mm long (14 pairs) and has an inter-electrode spacing of 0.15 mm as depicted in Figure 2. It is worth noting that the total photosensitive area is 84.32 mm2 for the detector.</li>
	<li>After the exposure procedure, use tweezers to clip the sample, and then immerse into the diluted developer (mix 50 ml&#160;of developer and 150 ml&#160;of deionized water) through actions of swinging from side to side for 35 s to obtain the developed sample.</li>
	<li>Rinse the developed sample with DI water and dry with nitrogen gas.</li>
	<li>Use the optical microscope to check the pattern intact. If not, use acetone to remove the photoresist and repeat steps 4.2 to 4.7 until the perfect pattern has been obtained.</li>
	<li>Hard bake the sample at 120 oC for 20 min.</li>
</ol>
5. Deposition of Pt Top Electrode and Chemical Lift-off
<ol>
	<li>Use the RF magnetron sputtering system to deposit a thin conductive Pt layer (100 nm) on the top of the developed sample before proceeding to chemical lift-off procedure.</li>
	<li>Set the distance between target and substrate at 13 mm.</li>
	<li>Use the mechanical pump to reach a rough vacuum of 5 mTorr.</li>
	<li>Then, use the turbo pump to obtain a high vacuum of 7 x 10-7 Torr.</li>
	<li>Wait until the chamber reaches the high vacuum, close the turbo pump and open the mechanical pump subsequently.</li>
	<li>Flow the argon gas at 0.3 ml/sec&#160;into the chamber by mas flow controller until the chamber pressure reach the working pressure of 100 mTorr.</li>
	<li>Turn on the direct current (DC) discharge power supply and set the DC power at 15 W for sputtering the Pt thin film electrode onto the sample for 25 min.</li>
	<li>After the Pt electrode layer has been deposited by magnetron sputtering method, take out the sample from the chamber.</li>
	<li>Immerse the sample into the acetone liquid for chemical lift-off process by ultrasonic cleaner to remove the photoresist.</li>
	<li>Set the cleaning time at 1 min to thoroughly remove photoresist, and then obtain the interdigitated-like Pt electrode onto the ZnO thin film.</li>
</ol>
6. RTA process
<ol>
	<li>Place the as-fabricated Pt/ZnO sample into the RTA system.</li>
	<li>Use the mechanical pump and gate valve to pump down the RTA chamber pressure to 20 mTorr.</li>
	<li>Wait until the chamber pressure reaches 20 mTorr, flow argon gas at 0.3 ml/sec into the chamber and set the working pressure of 5 Torr.</li>
	<li>Next, set the heating rate as 100 oC/min.</li>
	<li>Then, anneal the sample at 450 oC for 10 min.</li>
	<li>Once annealed, wait until the sample cools to RT, then take out the sample.</li>
</ol>

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The authors declare that they have no competing financial interests.

Critical steps and modifications
In step 1, the substrates should be thoroughly cleaned and steps 1.3 to 1.5 followed to make sure that there is no grease or organic and inorganic contaminations on the substrates. Any grease or organic and inorganic contaminations on the substrate surface will significantly reduce the adhesion of the film.
Step 2 is the most important procedure before the ZnO film preparation process. DEZn is very toxic and violently reacts with wat...

ZnO is a promising wide-band-gap functional semiconductor material due to its unique properties such as high chemical stability, low cost, non-toxicity, low power threshold for optical pumping, wide direct band gap (3.37 eV) at RT and large exciton binding energy of ~60 meV 1-2. Recently, ZnO thin films have been employed in many application fields including transparent conductive oxide (TCO) films, blue light emitting device, field-effect transistors, and gas sensor 3-6. On the other hand, ZnO is a candidate material to replace indium tin oxide (ITO) owing to indium and tin being rare and expensive. Moreover, ZnO possesses&#160;high optical transmittance in the visible wavelength region and low resistivity compared with ITO films 7-8. Accordingly, fabrication, characterization and application of ZnO has been extensively reported. This present study focuses on synthesizing high c-axis (0002) ZnO thin films by a simple and effectively method and its practical application towards a UV photodetector.
The recent research report findings indicate that the high quality ZnO thin film could be synthesized by various techniques such as sol-gel method, radio frequency magnetron sputtering, metal organic chemical vapor deposition (MOCVD), and so on 9-14. Each technique has its advantages and disadvantages. For example, a principal advantage of sputtering deposition is that target materials with very high melting point are effortlessly sputtered onto the substrate. In contrast, the sputtering process is difficult to combine with a lift-off for structuring the film. In our study, the plasma enhanced chemical vapor deposition (PECVD) system was employed to synthesize high quality c-axis ZnO thin films. Plasma bombardment is a key factor in the synthesizing process that can increase the thin film density and enhance the ion decomposition reaction rate 15. In addition, the high growth rate and large-area uniform deposition are other distinctive advantages for PECVD technique.
Except for the synthesis technique, the good adhesion on the substrate is another considered issue. In many studies, the c-plane sapphire has been widely used as the substrate to synthesize high c-axis ZnO thin films because the ZnO and sapphire have the same hexagonal lattice structure. However, the ZnO was synthesized on sapphire substrate exhibiting rough surface morphology and high residual (defect-related) carrier concentrations due to the large lattice misfits between the ZnO and c-plane sapphire (18%) oriented in the in-plane direction 16. Compared with the sapphire substrate, a Si wafer is another widely used substrate for the ZnO synthesis. Si wafers have been extensively used in semiconductor industry; and thus, growth of high quality ZnO thin films on Si substrates is very important and necessary. Unfortunately, the crystal structure and thermal expansion coefficient between the ZnO and Si are obviously different leading to deterioration of crystal quality. Over past decade, great efforts have been made to improve the quality of ZnO thin films onto Si substrates by using various methods including ZnO buffer layers 17, annealing in various gas atmosphere 18, and passivation of the Si substrate surface 19. The present study successfully offered a simple and effectively method to synthesize high c-axis ZnO thin film onto Si substrates without any buffer layer or pre-treatment. The experiment results indicated that the ZnO thin films synthesized under the optimal growth temperature showed the good crystal and optical qualities. The crystalline structure, RF plasma composition, surface morphology, and optical properties of ZnO thin films were investigated by X-ray diffraction (XRD), optical emission spectroscopy (OES), field emission scanning electron microscopy (FE-SEM), and RT photoluminescence (PL) spectra, respectively. Moreover, the transmittance of ZnO thin films was also confirmed and reported.
The as-synthesized ZnO thin film served as a sensing layer for UV photodetector application was also investigated in this study. The UV photodetector has great potential applications in UV monitoring, optical switch, flame alarm, and missile warming system 20-21. There are many types of photodetectors which have been carried out such as positive intrinsic negative (p-i-n) mode and metal-semiconductor-metal (MSM) structures including Ohmic contact and Schottky contact. Each type has its own advantages and drawbacks. Currently, MSM photodetector structures have attracted intensive interest due to their outstanding performance in responsivity, reliability and response and recovery time 22-24. The results presented here have shown that the MSM Ohmic contact mode was employed to fabricate ZnO thin film based UV photodetector. Such a kind of photodetector typically reveals a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector.

<table><tbody><tr><td>RF power supply</td><td>ADVANCED ENERGY</td><td>RFX-600</td><td></td></tr><tr><td>Butterfly valve</td><td>MKS</td><td>253B-1-40-1</td><td></td></tr><tr><td>Mass flow controller</td><td>PROTEC INSTRUMENTS</td><td>PC-540</td><td></td></tr><tr><td>Pressure controller</td><td>MKS</td><td>600 series&amp;nbsp;</td><td></td></tr><tr><td>Heater</td><td>UPGRADE INSTRUMENT CO.</td><td>UI-TC 3001</td><td></td></tr><tr><td>Sputter gun</td><td>AJA INTERNATIONAL</td><td>A320-HA</td><td></td></tr><tr><td>DEZn 1.5M</td><td>ACROS ORGANIC USA, New Jersey</td><td></td><td>also called Diethylzinc (C2H5)2Zn</td></tr><tr><td>Spin coater&amp;nbsp;</td><td>SWIENCO</td><td>PW - 490</td><td></td></tr><tr><td>I-V measurement</td><td>Keithley</td><td>Model: 2400</td><td></td></tr><tr><td>Photocondutive measurement&amp;nbsp;</td><td>Home-built</td><td></td><td></td></tr><tr><td>UV light sourse</td><td>Panasonic</td><td>ANUJ 6160</td><td></td></tr><tr><td>Mask aligner</td><td>Karl Suss</td><td>MJB4</td><td></td></tr><tr><td>Photoresist</td><td>Shipley a Rohm &amp;amp; Haas company</td><td>S1813</td><td></td></tr><tr><td>Developer</td><td>Shipley a Rohm &amp;amp; Haas company</td><td>MF319</td><td></td></tr><tr><td>Silicon wafer</td><td>E-Light Technology Inc</td><td>12/0801</td><td></td></tr><tr><td>Glass substrate</td><td>CORNING</td><td>1737</td><td>P-type / Boron</td></tr></tbody></table>

synthesis and characterization of high c-axis zno thin film by plasma enhanced chemical vapor deposition system and its uv photodetector application

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto silicon (Si) substrates via different synthesized temperatures by using plasma enhanced chemical vapor deposition (PECVD) system. The effects of different synthesized temperatures on the crystal structure, surface morphologies and optical properties have been investigated. The X-ray diffraction (XRD) patterns indicated that the intensity of (0002) diffraction peak became stronger with increasing synthesized temperature until 400 oC. The diffraction intensity of (0002) peak gradually became weaker accompanying with appearance of (10-10) diffraction peak as the synthesized temperature up to excess of 400 oC. The RT photoluminescence (PL) spectra exhibited a strong near-band-edge (NBE) emission observed at around 375 nm and a negligible deep-level (DL) emission located at around 575 nm under high c-axis ZnO thin films. Field emission scanning electron microscopy (FE-SEM) images revealed the homogeneous surface and with small grain size distribution. The ZnO thin films have also been synthesized onto glass substrates under the same parameters for measuring the transmittance.
For the purpose of ultraviolet (UV) photodetector application, the interdigitated platinum (Pt) thin film (thickness ~100 nm) fabricated via conventional optical lithography process and radio frequency (RF) magnetron sputtering. In order to reach Ohmic contact, the device was annealed in argon circumstances at 450 oC by rapid thermal annealing (RTA) system for 10 min. After the systematic measurements, the current-voltage (I-V) curve of photo and dark current and time-dependent photocurrent response results exhibited a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector application.

The ZnO (0002) thin films with high c-axis preferred orientation have been successfully synthesized onto the Si substrates by using the PECVD system. The carbon dioxide (CO2) and the diethylzinc (DEZn) were used as oxygen and zinc precursors, respectively. The crystal structure of ZnO thin films was characterized by X-ray diffraction (Figure 4), indicating that the ZnO thin film synthesized at 400 oC with the strongest (0002) diffraction peak. When the synthesized temperatu...

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Synthesis and Characterization of High c-axis ZnO Thin Film by Plasma Enhanced Chemical Vapor Deposition System and its UV Photodetector Application

We offered a method to directly synthesize high c-axis (0002) ZnO thin film by plasma enhanced chemical vapor deposition. The as-synthesized ZnO thin film combined with Pt interdigitated electrode was used as sensing layer for ultraviolet photodetector, showing a high performance through a combination of its good responsivity and reliability.

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto ...

synthesis-characterization-high-c-axis-zno-thin-film-plasma-enhanced

Universidad San Sebastian

Research

JoVE Journal

Engineering

15.2K Views.  National Taipei University of Technology (TAIPEI TECH). We offered a method to directly synthesize high c-axis (0002) ZnO thin film by plasma enhanced chemical vapor deposition. The as-synthesized ZnO thin film combined with Pt interdigitated electrode was used as sensing layer for ultraviolet photodetector, showing a high performance through a combination of its good responsivity and reliability.

Video: Synthesis and Characterization of High c-axis ZnO Thin Film by Plasma Enhanced Chemical Vapor Deposition System and its UV Photodetector Application

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 &#181;m were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min&#160;of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Procedures are outlined to prepare segmented and coaxial nanowires via templated electrodeposition in nanopores. As examples, segmented nanowires consisting of Ag and ZnO segments, and coaxial nanowires consisting of a TiO2 shell and a Ag core were made. The nanowires were used in photocatalytic hydrogen formation experiments.

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and ...

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Here we present a protocol for synthesizing Zn1-xMgxO/Cu2O heterojunctions in open-air at low temperature via atmospheric pressure spatial atomic layer deposition (AP-SALD) of Zn1-xMgxO on cuprous oxide. Such high quality conformal metal oxides can be grown on a variety of substrates including plastics by this cheap and scalable method.

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O ...

Chemistry

Synthesis of Hierarchical ZnO/CdSSe Heterostructure Nanotrees

A two-step chemical vapor deposition procedure is here employed to prepare tree-like hierarchical ZnO/CdSSe hetero-nanostructures. The structures are composed of CdSSe branches grown on ZnO nanowires that are vertically aligned on a transparent sapphire substrate. The morphology was measured via scanning electron microscopy. The crystal structure was determined by X-ray powder diffraction analysis. Both the ZnO stem and CdSSe branches have a predominantly wurtzite crystal structure. The mole ratio of S and Se in the CdSSe branches was measured by energy dispersive X-ray spectroscopy. The CdSSe branches result in strong visible light absorption. Photoluminescence (PL) spectroscopy showed that the stem and branches form a type-II heterojunction. PL lifetime measurements showed a decrease in the lifetime of emission from the trees when compared to emission from individual ZnO stems or CdSSe branches and indicate fast charge transfer between CdSSe and ZnO. The vertically aligned ZnO stems provide a direct electron transport pathway to the substrate and allow for efficient charge separation after photoexcitation by visible light. The combination of the abovementioned properties makes ZnO/CdSSe nanotrees promising candidates for applications in solar cells, photocatalysis, and opto-electronic devices.

Here, we prepare and characterize novel tree-like hierarchical ZnO/CdSSe nanostructures, where CdSSe branches are grown on vertically aligned ZnO nanowires. The resulting nanotrees are a potential material for solar energy conversion and other opto-electronic devices.

A two-step chemical vapor deposition procedure is here employed to prepare tree-like hierarchical ZnO/CdSSe hetero-nanostructures. The structures are ...

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 &#176;C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 &#176;C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.

Columnar zinc oxide structures in the form of rods are synthesized via aerosol-assisted chemical vapor deposition without the use of pre-deposited catalyst-seed particles. This method is scalable and compatible with various substrates based on either silicon, quartz or polymers.

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, ...

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule ...

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Attainment of high-quality Schottky contacts is imperative for achieving efficient gate modulation in heterostructure field effect transistors (HFETs). We present the fabrication methodology and characteristics of Schottky diodes on Zn-polar BeMgZnO/ZnO heterostructures with high-density two dimensional electron gas (2DEG), grown by plasma-assisted molecular beam epitaxy on GaN templates.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device ...

The Effect of Anodization Parameters on the Aluminum Oxide Dielectric Layer of Thin-Film Transistors

Aluminum-oxide (Al2O3) is a low cost, easily processable and high dielectric constant insulating material that is particularly appropriate for use as the dielectric layer of thin-film transistors (TFTs). Growth of aluminum-oxide layers from anodization of metallic aluminum films is greatly advantageous when compared to sophisticated processes such as atomic layer deposition (ALD) or deposition methods that demand relatively high temperatures (above 300 &#176;C) such as aqueous combustion or spray-pyrolysis. However, the electrical properties of the transistors are highly dependent on the presence of defects and localized states at the semiconductor/dielectric interface, which are strongly affected by the manufacturing parameters of the anodized dielectric layer. To determine how several fabrication parameters influence the device performance without performing all possible combination of factors, we used a reduced factorial analysis based on a Plackett-Burman design of experiments (DOE). The choice of this DOE permits the use of only 12 experimental runs of combinations of factors (instead of all 256 possibilities) to obtain the optimized device performance. The ranking of the factors by the effect on device responses such as the TFT mobility is possible by applying analysis of variance (ANOVA) to the obtained results.

Anodization parameters for growth of the aluminum-oxide dielectric layer of zinc-oxide thin-film transistors (TFTs) are varied to determine the effects on the electrical parameter responses. Analysis of variance (ANOVA) is applied to a Plackett-Burman design of experiments (DOE) to determine the manufacturing conditions that result in optimized device performance.

Aluminum-oxide (Al2O3) is a low cost, easily processable and high dielectric constant insulating material that is particularly appropriate for use as the ...

Metal Oxide Heterojunction for Photocatalytic Activities

There is a significant global demand for improvements in synthesis techniques and their optimal characteristics, especially for industrial-scale applications. Sol-gel-based solution combustion synthesis (SG-SCS) is a simple method to produce ordered porous materials. In this regard, Pearson&#39;s hard and soft acids and bases theory assists in selecting host-dopant reactivity to form a proper heterojunction.
The formation of a heterojunction also changes the essential properties of the materials, improving photocatalysis via charge transfer or synergistic activities. A calcination temperature of 500 &#176;C is ideal for this process based on the results of the stability assessment via a differential thermogravimetry ratio analysis (DTG).
The nanoscale dimensions of the nanoparticles (NPs) and nanocomposites (NCs) generated were validated using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Furthermore, the scanning electron microscopy micrographs and BET analyses confirmed the porosity nature of the materials. HRTEM, X-ray photoelectron spectroscopy, and energy-dispersive X-ray investigations established the materials composition. The study found that NCs degraded the acid orange 8 (AO8) color more efficiently than bare ZnO.

The development of a heterojunction boosts the photocatalytic activities of solution combustion synthesis, which is a time-/energy-efficient process. Advanced analytical characterization techniques were used in this protocol to evaluate the materials&#39; characteristics, and nanocomposites demonstrated improved acid orange-8 dye degradation.

Synthesis and Characterization of High c-axis ZnO Thin Film by Plasma Enhanced Chemical Vapor Deposition System and its UV Photodetector Application

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