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GB2469869A - Continuous ZnO films - Google Patents

Continuous ZnO films Download PDF

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GB2469869A
GB2469869A GB0907550A GB0907550A GB2469869A GB 2469869 A GB2469869 A GB 2469869A GB 0907550 A GB0907550 A GB 0907550A GB 0907550 A GB0907550 A GB 0907550A GB 2469869 A GB2469869 A GB 2469869A
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zno
zinc oxide
piezoelectric
substrate
seed layer
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Mohsen Miraftab
Jikui Luo
Manzoor Ahmad
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University of Bolton
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University of Bolton
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    • G01N29/022—Fluid sensors based on microsensors, e.g. quartz crystal-microbalance [QCM], surface acoustic wave [SAW] devices, tuning forks, cantilevers, flexural plate wave [FPW] devices
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Abstract

High quality zinc oxide (ZnO) films are fabricated, which comprise densely packed ZnO nanorods only at the surface (see Fig. 4), or composite/hybrid piezoelectric (PE)-films with ZnO nanostructures 3 embedded in other piezoelectric materials at the surface such as piezoelectric polymer or powders 4. ZnO nanorods with large aspect ratio 3 are synthesized by solution, and hybrid films are fabricated by embedding the nanorods in other piezoelectric materials and sintering to form continuous surface films. The films possess high piezoelectric, electrical and optoelectric properties owing to nanodimensions, quantum confinement and high crystallinity. The film surfaces are smooth and continuous both crystallographically and acoustically, therefore are suitable for fabrication of piezoelectric devices and microsystems such as surface acoustic wave (SAW) devices, film bulk acoustic wave devices, power generators and electronic devices such as solar cells, laser and light emission device and transistors.

Description

Continuous ZnO Films The present invention relates to a high quality zinc oxide (ZnO) film comprising ZnO nanorods, and to composite/hybrid piezoeleciric (PE)-films with embedded ZnO nanostructures that are suitable for acoustic wave and large area electronic and optoelectronic device fabrication, and also to methods for their manufacture.
Acoustic wave devices have been in commercial use for a long time, mainly as, for example, filters and oscillators in electronics and communications, as torque and tyre pressure sensors in automotive sectors, and as chemical and physical sensors (e.g. as humidity, temperature and mass sensors) for monitoring in industrial and environmental sectors.
Recently they have found increasing use in biochemical sensing and microfluidics owing to their extremely high sensitivity, simple device structure and operation, and low fabrication cost. Since the base-mass of a surface acoustic wave (SAW) device, such as the Love-wave SAW can be made extremely small using a thin film waveguiding layer, the SAW sensors have extremely high sensitivity and low detection limitation. SAW devices have been utilized to pump and mix liquids * with volumes from as little as a few picolitres (p1) up to tens of microlitres (.tl) on the * *** *. channel-less, flat-surface of the SAW devices with no moving components. Sensors *: *° and microfluidics are the two main components for integrated biodetection systems or * S..
lab-on-a-chip. * *S * * . *.. .
SAW devices are traditionally made from bulk piezoelectric materials such as LiNbO3, LiTaO3 and quartz. However, these materials are expensive and cannot be integrated with electronics for control and signal processing which is necessary for lab-on-chip and biodetection systems. Piezoelectric thin films, such as PZT (lead zirconate titanate), ALN and ZnO have been developed and used to fabricate SAW devices. However, the PZT film has an extremely high damping effect, and SAW devices on a PZT film suffer from severe signal attenuation.
PE-A1N films are much more difficult to obtain using normal physical vapour deposition methods as the processing time is excessively long. In addition, the piezoelectric properties are relatively poor as it is difficult to obtain the (002) crystal orientation and the surface of the A1N films is too rough for acoustic device fabrication. The PE-ZnO thin film possesses good piezoelectric properties and high electro-mechanical coupling coefficients, and can be easily deposited on various substrates such as silicon and glass. It has therefore been the favourite choice for thin film-based SAW devices.
ZnO is not only useful for piezoelectric applications, but also possesses unique optical and electrical properties suitable for electronic device applications, such as solar cells, light emitting diode (LED), laser and displays etc. Many technologies have *: been developed to deposit ZnO thin films, such as sputtering, pulsed laser deposition, S...
chemical vapour deposition and solution based synthesis, spray and sol-gel deposition L:' * etc. Such techniques are well known to a skilled person. High quality single crystalline ZnO thin films can also be grown by molecular beam epitaxy (MBE) and metal organic chemical vapour deposition (MOCVD) but at extremely high cost, which is prohibitive for mass applications. Spray and sol-gel deposited ZnO thin films need repeated coating and annealing to form a required thickness (typically <0.5.tm) due to large built-in stress. The film quality is poor for PE-applications due to uncontrollable crystal orientations, small grain sizes and inclusion of organics. Laser-assisted ZnO deposition is only suitable for small area deposition, typically just sufficient to provide proof of concept. DC (direct current) or magnetron sputtering deposition has the advantage of being a simple and low temperature process with ZnO films possessing relatively high piezoelectric properties. Sputtering deposited films typically have large grain sizes and a columnar structure with (002) crystal orientation (as shown in Figure 1 hereinbelow), a necessary condition for PE-application. The drawback is that sputtering deposition, like most other deposition techniques discussed above, is a slow process with a typical deposition rate of <1 Onmlminute. It takes up to ten hours to deposit a ZnO film up to a few micrometers thick which is the thickness necessary for SAW devices used in microfluidics and sensors with medium operating frequencies (i.e. from a few MHz to hundreds of MHz). This leads to high fabrication costs and long-time usage of the equipment. Furthermore, the piezoelectric properties of the sputtered ZnO films are not as good as that of bulk PE-materials due to the inherent polycrystalline structure.
ZnO thin films can also be synthesised electrochemically in aqueous solutions *: : : : for electronic and optoelectronic uses. Chemicals such as Zn(N03)2 and ZnCI2 have *::: been used to synthesize the ZnO nanocrystalline films with ZnO grain sizes ranging * from a few nanometres to a few tens of nanometres (hereinunder called ZnO * * nanocrystalline film). Most of the synthesis process involves reduction of nitrate ions L: : to generate hydroxide ions that react with Zn2 to form Zn(OH)2 for the subsequent dehydration into ZnO with the help of an electric field.
The advantages of solution-based synthesis are that it requires a simple and low temperature process with no requirement for sophisticated equipment, and the ZnO films can be deposited on various substrates such as metals, glass and polymers, suitable for large area applications at low cost. Deposition rates of up to about 200 nm/mm were achieved, and the crystal quality can be controlled by adjusting the synthesis conditions (such as temperature, electropotential and chemical concentrations) and the selection of the seed layer such as ZnO, ITO (indium tin oxide) and 1n203.
However, ZnO nanocrystalline films normally have random crystal orientations, and the grain sizes are very small, only up to a few tens of nanometres.
Although electrodeposited ZnO films have been intensively studied for electronics and optoelectronics applications, no satisfactory results have been achieved due to poor crystallinity and low carrier mobilities. Furthermore, no thus manufactured ZnO films have been used for fabrication of acoustic devices, mostly due to the very weak PE-effect of the thin films owing to small grain size and uncontrollable crystal orientations.
Chemical synthesis of ZnO nanostructures has made tremendous progress recently. ZnO with aggregated shapes such as nanorod, nanobelt, na.noneedle and *: : : . nanowire with diameters from a few nanometres to a few hundreds of nanometres has *...
been synthesized. Among them, ZnO nanorods with (002) crystal orientation have * exhibited not only good properties for applications in solar cells, nano-electronics and optoelectronics, but also have an extraordinarily high piezoelectric constant and L: electro-mechanical coupling coefficient, much better than those of bulk PE-materials owing to enhanced effects by nano-dimensional structures and a large surface to bulk ratio. The piezoelectric constant of ZnO nanorods is three times that of ZnO bulk material and at least 5-10 times better than those of ZnO thin films deposited by the aforementioned methods. However, these nanostructures also have their disadvantages.
The major setback of the current processing of ZnO nanorod arrays (NRAs) is the low packing density with a high percentage of porosity and gaps being found between the nanorods. This makes the materials only suitable for applications in nanodevices and some optoelectronics, but not in acoustic devices such as SAW devices and normal large area planar electronic devices such as field effect transistors, solar cells and LEDs etc., for which material continuity in plane is essential.
Therefore, it is desired to devise high quality ZnO thin films with high compactness and/or density of ZnO NRAs, and having a flat and smooth surface suitable to offer high electrical and piezoelectric properties.
According to the present invention, there is provided a zinc oxide film comprising a zinc oxide nanorod array wherein the nanorod array is substantially continuous on a surface of the film. This film is hereinafter denoted as a ZnO nanorod film. It contains a quantity of highly aligned, densely packed ZnO nanorods to form a continuous film with a smooth surface suitable for direct fabrication of surface S...
acoustic wave and electronic devices. S.I
* S5 S There is also provided a zinc oxide film comprising a zinc oxide nanorod array :.: .* embedded in a quantity of a fine grained piezoelectric material sufficiently to create a * substantially smooth surface. This film is hereinafter denoted as a hybrid ZnO film. * .* * S S S.. *
Typically the quality of the fine grained piezoelectric material is sufficient so that the surface is substantially level with the top of the NRAs.
The ZnO which makes up the nanorod array has good piezoelectric properties while the fine grained piezoelectric materials have piezoelectric properties which are less effective than those of the nanorod arrays. This film has a smooth surface suitable for direct fabrication of surface acoustic wave and electronic devices.
According to a further aspect of the invention, there is also provided a method of manufacturing a zinc oxide film comprising a zinc oxide nanorod array embedded in a quantity of a fine grained piezoelectric material sufficiently to create a substantially smooth surface, comprising the steps of: i) providing a substrate material; ii) providing a seed layer material on the substrate; iii) providing a source of zinc oxide to grow the nanorod array on the seed layer; and iv) filling in any gaps between the nanorod arrays with the fine grained piezoelectric material sufficiently to create a substantially smooth surface. I... * S *
According to a further aspect of the invention, there is also provided a method **** .: :* of manufacturing a zinc oxide film comprising a zinc oxide nanorod array wherein the nanorod array is substantially continuous on a surface of the film, comprising the steps of: i) providing a substrate material; ii) providing a seed layer material on the substrate; iii) providing a source of zinc oxide to grow the nanorod array on the seed layer; and iv) applying an electropotential between the substrate and an anode electrode.
Both the hybrid and nanorod ZnO films possess high PE-properties owing to the existence of high quality dense NRAs. The film can be grown (electro-)chemically on low-cost large-scale glass and plastic substrates. It is a low temperature process with a high deposition rate, and requires no expensive facility, leading to much reduced costs.
By "fme grain" or "fme grained" it is meant a material having a particle size of from about 1 nm to up to about lOOnm, more typically from about 1 nm to about 2Onm for the applications discussed herein, so that they can go into the gaps between the ZnO NRA.
By "substantially continuous" it is meant that there are no gaps between the NRAs inside the film, while at the surface, there may be gaps measuring no more than * : ::: tens of nanometres which is typically only treated as the roughness of the film. * ***
According to one embodiment, the substrates may include, but are not limited to, semiconductors such as silicon, GaAs, GaN, and diamond, insulators such as quartz, sapphire, glass and soft materials such as plastics and polymers. * *. * * * *** S
The substrate may first be cleaned, if desired. After an optional cleaning of the substrate, a seed layer is deposited on the said substrate with a thickness typically ranging from a few mn to a few pm. The deposition methods may include, but are not limited to, chemical synthesis, physical deposition methods such as DC or magnetron-sputtering, e-beam deposition, evaporation, MOCVD and MBE etc. Materials which may be used as the seed layers include, but are not limited to, ZnO, indium tin oxide (ITO), 1fl203, Sn02 and metals such as In, Sn, Zn, Cu, Al etc., which can be used for ZnO nanostructure growth directly. Where an electrochemical synthesis of the ZnO films is used, a conductive seed layer is typically used. Suitable materials for the conductive seed layers may include, but are not limited to, those listed above for chemical synthesis, and the above insulating materials when doped, such as doped ZnO, 1n203, Cu02 and Sn02 etc. A two-step synthesis process may be used to first deposit seed layer, and is then switched to ZnO growth. In this case, additional chemicals are used to deposit ZnO nanoparticles on the aforementioned substrates as seeds for the sequential ZnO nanorod growth.
According to the invention, high quality discrete ZnO nanorods are prepared chemically with well aligned (002) crystal orientation on various substrates. The said substrates coated with seed layers are immersed in an aqueous solution with the * : : : surface to be coated oriented facing downwards to avoid deposition of unwanted precipitated particles. The chemical solution could be in a container such as a bottle, **** * :* and the container can be kept in a hot water bath or in an oven where a temperature of * about 40 to about 100°C can be maintained. * ** * * * *** *
The most common chemical used in the art for the growth of ZnO NRAs is zinc nitrate, Zn(N03)2, in addition to other chemical compounds containing hydroxide 01-f ions. These other compounds include, but are not limited to, NI-140H, H202, NaOH, or methenamine (C6H12N4) which can produce Off ions through the following chemical reactions: C6H12N4+H20 4-COH2+4NH3 (1) 4NH3+H20 -*NH4+ Off (2) Additives such as but not limited to HNO3 and KOH are often used for pH value adjustment, which is typically maintained at a value of between about 4 to about 9. The typical chemical reactions which take place are as follows, Zn(N03)2 -* Zn2 + 2N03, (3) N03 + H20 + 2e N02 +20W (4) Zn2 + OW -* Zn(OH)2 (5) *: ::: Zn(OH)2 -p ZnO + H20. (6) I... * I * ***
The synthesis is typically carried out at about 20 to about 100°C, and lasts for *** : about 4 to up to tens of hours. Once the substrates are immersed in the chemical synthesis solution, no further action is needed except to maintain the temperature constant. As the chemicals in the solution are gradually exhausted, the growth rate reduces, and eventually the growth stops. The length, diameter and density of the ZnO nanorods can be controlled by adjusting the process conditions such as the temperature, pH value and concentrations of zinc nitrate and other chemicals. After growth, the substrates with ZnO nanorods will be washed thoroughly in distilled water, dried in air or nitrogen, and baked at a raised temperature, such as at about 100°C, for a few minutes if the electrochemical synthesis is not to be followed immediately. The so-synthesized ZnO nanorod arrays are discrete, separated from each other. Although they could be densely packed, they are not acoustically substantially continuous films.
Although these discrete ZnO nanorods flow possess extraordinary piezoelectric properties, they alone can not be directly used to fabricate acoustic wave devices and large area planar electronic devices, as the nanorods are separated apart from each other. It is from this point that the invention differs and is able to provide the continuous films which are so suitable for acoustic purposes.
According to the present invention, substantially flat, smooth ZnO piezoelectric films can be obtained using composite/hybrid ZnO film with embedded ZnO NRAs, wherein the ZnO NRAs are synthesized by the aforementioned chemical * : : : synthesis method, while the gaps between the individual ZnO NRAs are filled by fine **** grains or nanoparticles or polymers of piezoelectric materials. The said fine grained * * piezoelectric materials may include, but are not limited to, chemically or * electrochemically synthesized fine ZnO grains, piezoelectric sol-gel such as ZnO and PZT. The said piezoelectric polymers may include, but are not limited to polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene (PVDF/TrFE). Other solid piezoelectric nanoparticles can also be used to fill the gaps between the ZnO NRAs, including but not exclusive of, fine grained powders of all piezoelectric materials such as PZT (including those PZT compounds with additives of Mn, Mg, Nb, Co. Sr etc), ZnO, A1N, GaAs, GaN, physically deposited ZnO, AIN, GaAs, GaN, PZT and PZT compounds with additives of Mn, Mg, Nb, Co, Sr etc..
Four back filling methods are described hereunder in order to achieve the hybrid ZnO piezoelectric films with substantially flat, smooth surfaces for acoustic and electronic device applications.
Alternatively, as discussed above, substantially uniform and smooth thin ZnO nanocrystalline films can be electrochemically deposited using the zinc nitrate solution (the same as that used for ZnO NRA chemical synthesis) with no additional hydroxide ions in the solution. Therefore, at the end of the chemical synthesis of the discrete NRAs represented by equations 3-6 (after which hydroxide ions tend to be exhausted), the synthesis process can be switched from a chemical deposition to an electrochemical depositionjy applying an electropotential between the cathode substrate with ZnO NRAs and a zinc metal anode electrode. The electrochemical solution consists of Zn(N03)2 with additives for pH value adjustment with a value of between about 4 and 9. The electrochemical reactions undertaken are as follows: *... * I **,.*
H202+2e---* 20H-(7) I..
4:,. Zn24 + 20FF -f Zn(OH)2 (8) Zn(OH)2 -ZnO + H20. (9) The electrodeposition can be carried out by a two-electrode or a three-electrode configuration. For the two-electrode deposition, the substrate with ZnO NRAs as a cathode is connected to a negative potential while the metal is used as the positive electrode. The potential between the two electrodes is typically from about 0.6 to about 2.0 V, depending upon variable such as the growth rate and the chemical concentration, etc. The depositing current density is typically in the range of a few mAicm2 to a few tens of mA/cm2. The current density and potential drop quickly for the two-electrode configuration, leading to non-uniform growth and a slowing down of the growth. The problem can be solved by using three-electrode configuration. For the three-electrode deposition, another electrode, such as AgIAgCl, is used as the reference with negative potential applied to the sample cathode for film growth. The potential of this configuration is independent of the current density used; therefore a constant uniform growth can be achieved. Zn is typically used as the anode for both configurations as a continuous supply of the Zn ion source for the electrodeposition.
Other metal electrodes such as Pt are often used for electrodeposition as well if sufficient supply of zinc ions is secured. Since the hydroxide ions do not affect the growth of nanocrystalline ZnO film during electrochemical synthesis significantly, the residual hydroxide ions in the ZnO NRA chemical synthesis solution do not affect the quality of the nanocrystalline ZnO thin films, The growth temperature is typically from room temperature to up to about 80°C.
The growth rate, grain size and crystal quality of the ZnO thin films can be L:" controlled by adjusting the temperature, electrical potential and current density. This process can be adjusted to obtain a fast growth rate with fine grain sizes so that they can substantially fill all empty spaces between the ZnO nanorods to form a continuous film with high piezoelectric properties and electric-mechanical coupling coefficient.
The key issues for this process are to suppress anisotropic growth along (002) crystal orientation (the growth direction of the nanorods) and to promote isotropic growth, so that a substantially continuous film with flat surface can be obtained.
ZnO nanocrystalline films with fme grain sizes and no preferential crystal orientations can be achieved with a high nucleation rate at a high current density and low temperature as fast growth at low temperature leads to growth of ZnO with no sufficient time and energy to migrate. A film hence exhibits the random orientations.
This has been disclosed in previous publications for ZnO nanocrystalline film with no gaps and voids. Fine grains can also be obtained using pulsed electrodeposition, a common method used for nanocrystalline material deposition.
Since the resistivity of the chemically-synthesized discrete ZnO nanorods is very high, ZnO film growth by electrochemical deposition typically starts at the root of the ZnO NRAs owing to the relatively low resistance, and gradually buries the ZnO NRAs to form a substantially thin film with no voids. This method has been used to grow ZnO films with mixed large and small crystal grains. Conducting substrates are necessary for electrodeposition. The said substrates coated with ITO or fluorine-S...
* *** doped, tin oxide or ZnO have been widely used for electro synthesis of ZnO nariocrystalline films. These layers were also used as the seed layers for ZnO NRA chemical synthesis. Therefore, substrates with a coated conductive seed layer can be used for the synthesis of hybrid piezoelectric films directly.
According to another embodiment of the present invention, a sd-gel method can be used to produce piezoelectric materials to fill the gaps between the ZnO NRAs.
The sol-gel is a fluid with a relatively low viscosity, and various sol gels have been used to form nanocrystalline thin films on various substrates such as silicon and glass.
The sol gel piezoelectric materials may include, but are not limited to, ZnO, PZT, LiNbO3, LiTaO3, BaTiO3, LiTiO3, KNbO3 and GaN etc. The preparation procedure, taking ZnO so! gel just as one example, may be as follows: after ZnO NRA chemical synthesis, ZnO sol-gel is spin-coated on it, the substrate is then dried at a mild temperature, e.g between about 80-120°C, for a few minutes, and then is annealed at a temperature of between about 300-1100°C for a few minutes, up to about 30 mins, to form nanocrystalline ZnO between the NRAs. This process may be repeated several times to substantially fill the gaps and cover the whole surface, and consequentially form a substantially flat and smooth surface ZnO film which is suitable for acoustic and electronic device fabrication. Other sol gels such as PZT and its compounds are well developed, and can also be used in the same process aforementioned. However, advantageously, ZnO so! gel is used to fill the gaps as the coupling between the ZnO NRAs and nanocrsytalline ZnO film as they have the same material structure and properties.
*: : : According to the present invention, another embodiment is to use piezoelectric polymers such as PVDF and its copolymers PVDF/TrFE as a soft' media to back fill * :* the gaps between the ZnO NRAs to obtain a flat, continuous film for acoustic and electronic device fabrication. PVDF and its copolymers PVDF/TrFE can be diluted . : :* with solvents to obtain low viscosity, and thus can be easily spin coated to fill the gaps between the ZnO nanorods to form a flat and smooth surface. The piezoelectric polymers can be poled to induce the piezoelectric property at an electric field of a few tens of MVm to a few hundreds of MV m1 and a temperature of between about 100- 130°C. This can be implemented using the metal deposited on the hybrid ZnO film before etching to produce top electrodes, e.g. an interdigited transducer electrode for SAW devices, and the conducting seed layer as the other electrode. The poling process is typically as follows: the temperature is raised to between about 100-130°C, an electrical field is applied and maintained to a required value for a duration from one mm to tens of minutes, and the temperature is lowered to room temperature while
the electric field is maintained.
According to one aspect of the invention, any fine grained solid piezoelectric material used to fill the gaps between ZnO NRAs may be nanoparticulate. Such piezoelectric nanoparticulate materials may include, but are not limited to, A1N, ZnO, GaAs, GaN, LiNbO3, LiTaO3, BaTiO3, LiTiO3, KNbO3, PZT and PZT compounds doped with strontium, niobium and magnesium etc. Typically, they may have grain sizes from a few nanometres to a few tens of nanometres, such as from about 5 to about 100 nm, typically from about 10 to about 80 nm. The back filled piezoelectric nanoparticles are typically sintered at a temperature in the range of about 400 to about 1100°C for a duration up to about 30 mins, to form a well coupled film with a ZnO NRA with a substantially flat and smooth surface for acoustic and electronic device I.. * S *
fabrication. S...
In some instances, it may be that the hybrid ZnO films have a rough surface, * . not suitable for direct fabrication of electronic and acoustic wave devices, directly L: :* after its manufacture. Therefore, according to the present invention, the ZnO hybrid films with rough surfaces can be processed to obtain a smooth surface for device fabrication by using chemical mechanical polishing (CMP) techniques which are well known to persons skilled in the art.
According to a further aspect, the zinc oxide film comprising a zinc oxide nanorod array wherein the nanorod array is substantially continuous on a surface of the film may also be manufactured using a mixture of a zinc source and source of OH ions in suitable concentrations.
Under optimal chemical formulation and growth conditions, acoustically continuous ZnO films with a flat and smooth surface which comprises only densely packed ZnO nanorods can be fabricated. Such dense films are suitable for direct application to device fabrication. For example, using a mixture of Zn(N03)z and C6H12N4 at different concentration ratios, continuous ZnO films with a substantially smooth surface have been obtained. The said films consist of only the densely packed ZnO nanorods as shown in Figure 3.
According to the present invention, an exemplary but non-limiting chemical solution for synthesis of the ZnO nanorod films comprises Zn(N03)2 and methenamine with a concentration ratio typically of about 10:1 and a molar concentration typically in the range of about 0.05 to about 0.2M for Zn(N03)2. The growth temperature is typically from about 50 to about 90°C. The growth typically *: : : . lasts for about 1 to about 20 hours, and the film thickness obtained is typically in the range of about 1 toabout6 tm.
:* The relative ratios of concentrations of Zn(N03)2 and methenamine may be * * from about 20:1 to about 1:2, more typically from about 15:1 to 5:1, with the molar * concentration typically in the range of about 0.05 to about 0.2M for Zn(N03)2..
It will be appreciated that combinations of sources of Zn and 0H may be used other than Zn(N03)2 and methenamine, and in similar concentrations, for this synthesis of the ZnO nanorod films.
Also provided in one embodiment of the invention is the use of a zinc oxide film as detailed hereinabove in the manufacture of an acoustic wave device or an electronic device.
The invention will now be described further by way of example with reference to the following examples and figures which are intended to be illustrative only and in no way limiting upon the scope of the invention.
Figure 1 shows an scanning electron microscope (SEM) picture of ZnO film deposited by magnetron sputtering. The film has a columnar structure perpendicular to the substrate.
Figure 2 shows a schematic drawing of a hybrid ZnO film with embedded ZnO nanorod arrays in continuous piezoelectric material in accordance with one embodiment of the invention.
Figure 3 shows SEM pictures of grown dense ZnO nanorods and a ZnO nanorod film with a substantially smooth surface suitable for device fabrication directly without further treatment. *...
: Figure 4 shows a schematic drawing of the film preparation of the continuous * S * ZnO nanorod arrays in accordance with one embodiment of the invention. * ** * . S
Both the hybrid and nanorod ZnO films have a structure which is similar to a . : that shown in Figure 1, but importantly with aligned ZnO nanorods to replace the columnar structure. Both types of ZnO films possess high PE-properties owing to the existence of high quality dense NRAs. The film can be grown (electro-)chemically on low-cost large-scale glass and plastic substrates.
The preparation of the source material and substrate are schematically shown in Figure 2 for the preparation of the hybrid film with ZnO NRA embedded in a continuous PE-flim. Firstly, a seed layer 2 is deposited on a substrate I using any of the deposition techniques outlined above. The substrate 1 with the deposited seed layer 2 is immersed in an aqueous solution with the seed layer facing downwards to avoid deposition of unwanted precipitation particles. The whole system is kept at a slightly elevated temperature, such as between about 40-100°C.
The aqueous solution comprises Zn(N03)2 arid a source of hydroxide ions and the chemical reactions take place as illustrated by equations (3)-(6) above. The ZnO nanorods arrays 3 are grown on the seed layer until the 0H source is exhausted. After the growth of the ZnO nanorod arrays, they are washed with distilled water, dried and baked at about 100°C.
The gaps between the ZnO nanorod arrays are then substantially filled by fine grained piezoelectric materials 4. This provides the flat, smooth surface required for acoustic and electronic applications.
Figure 3 shows SEM pictures of a large area continuous, densely packed ZnO .. : nanorod film having a smooth surface manufactured using Zn(N03)2 and *s* . * * ***.
methenamine directly without further treatment. According to this embodiment of the * ** :.: invention, continuous ZnO nanorod film can be achieved using an appropriate 0$s * :. concentration of Zn(N03)2 and methenamine as detailed above. The specific conditions employed for this continuous nanorod films are: Zn(N03)2: methenamine 0.1:0.O1M Temperature: 75-85°C pH value: 6.7-7.1 Duration: >2hrs The left image in Figure 3 shows a large smooth area (the size bar is 20 nm), and two images to its right are of higher magnifications (the size bars are I jim). The pictures show a continuous film free of gaps between ZnO nanorods.
Figure 4 is similar to Figure 2 but illustrates the manufacture of the ZnO nanorod arrays in accordance with the invention. As with the hybrid ZnO arrays, a seed layer 2 is deposited on a substrate I using any of the deposition techniques outlined above. Again, the substrate 1 with the deposited seed layer 2 is immersed in an aqueous solution with the seed layer facing downwards to avoid deposition of unwanted precipitation particles. The whole system is kept at a slightly elevated temperature, such as between about 40-100°C.
The aqueous solution again comprises Zn(N03)2 and a source of hydroxide ions, and the chemical reactions take place as illustrated by equations (3)-(6) above.
* :: :: This time, once the Off source is exhausted, the process is changed to an *.*.
***.. electrochemical process. An electropotential is applied between the newly grown ZnO * * nanorod arrays 3 and a metal anode electrode. The process then proceeds according to * the chemical equations (7)-(9) detailed above, and the gaps between the ZnO NRAs : are substantially filled. Adjustment of temperature, potential and current density can used to control the growth rate, grain size and quality of the crystals of the ZnO nanorod arrays. * S* ** * S*S a * . *5S* * a. * I * *.* S
S I..
S * I* * 0 0 S.. S

Claims (33)

  1. Claims 1. A zinc oxide film comprising a zinc oxide nanorod array wherein the nanorod array is substantially continuous on a surface of the film.
  2. 2. A zinc oxide film according to claim I wherein the zinc oxide nanorod array is embedded in a quantity of a fine grained piezoelectric material sufficiently to create a substantially smooth surface.
  3. 3. A zinc oxide film according to claim 1 or claim 2, wherein the film further comprises a substrate comprising silicon, GaAs, GaN, sapphire, diamond, quartz, glass, a metal, a plastic or a polymer.
  4. 4. A zinc oxide film according to claim 3, wherein where the substrate is a plastic, the plastic is selected from polyethylene terephthalate, polytetrafluoroethylene, or polyvinylacetate, or wherein where the substrate is a metal, the metal is selected from zinc, copper, iron or steel.
  5. 5. A zinc oxide film according to claim 3 or claim 4, wherein the film further * .1S .: : :* comprises a seed layer on the substrate. S.S * ** 1 e S., *
  6. 6. A zinc oxide film according to claim 5, wherein the seed layer comprises ZnO, ITO, 1n203, Sn02, or doped versions of any thereof, or metals such as zinc, copper or aluminium.
  7. 7. A zinc oxide film according to any of claims 2-6, wherein the fine grained piezoelectric material comprise fine ZnO grains, a piezoelectric sol-gel material, or a piezoelectric polymer.
  8. 8. A zinc oxide film according to claim 7, wherein the piezoelectric sol-gel material is selected from ZnO, PZT, LiNbO3, LiTaO3, BaTiO3, LiTiO3, KNbO3 and GaN.
  9. 9. A zinc oxide film according to claim 7, wherein the piezoelectric polymer is selected from poly(vinylidene fluoride) (PVDF) and its copolymer poly(vinylidene fluoride) trifluoroethylene (PVDF/TrFE).
  10. 10. A zinc oxide film according to any preceding claim, wherein the said ZnO * : nanorods are between about 1-5 jim in length. * . *... * ** * * S S.. *
    * *
  11. 11. A method of manufacturing a zinc oxide film comprising a zinc oxide nanorod array embedded in a quantity of a fine grained piezoeleetric material sufficiently to create a substantially smooth surface, comprising the steps of: i) providing a substrate material; ii) providing a seed layer material on the substrate; iii) providing a source of zinc oxide to grow the nanorod array on the seed layer; and iv) filling in any gaps between the nanorod arrays with the fine grained piezoelectric material sufficiently to create a substantially smooth surface.
  12. 12. A method of manufacturing a zinc oxide film comprising a zinc oxide nanorod array wherein the nanorod array is substantially continuous on a surface of the film, comprising the steps of: i) providing a substrate material; ii) providing a seed layer material on the substrate; iii) providing a source of zinc oxide to grow the nanorod array on the seed layer; and iv) applying an electropotential between the substrate and an anode electrode. * * * * I *1II * IISIS*
  13. 13. A method according to claim 11 or claim 12, wherein the substrate comprises silicon, GaAs, GaN, sapphire, diamond, quartz, glass, a metal, a plastic or a
    Spolymer.
  14. 14. A method according to any of claims 10-13, wherein the seed layer comprises ZnO, ITO, 1n203, Sn02 or doped versions of any thereof, or a metal such as zinc, copper or aluminium.
  15. 15. A method according to any of claims 10-14, wherein the seed layer is deposited on the substrate using a technique selected from chemical synthesis, sputtering, pulsed laser deposition, chemical vapour deposition, sol-gel deposition, spray, e-beam deposition, evaporation, metal organic chemical vapour deposition or molecular beam epitaxy.
  16. 16. A method according to any of claims 10-15, wherein the ZnO nanorods are synthesized chemically in an aqueous solution comprising a zinc source and a source of hydroxide ions.
  17. 17. A method according to claim 16, wherein the zinc source is Zn(N03)2.
  18. 18. A method according to claim 16 or claim 17, wherein the source of hydroxide * : ions is selected from methenamine, N1-L1OH, 11202 and NaOH. S... * . * *.* * .. * * . S.. S
    *.
  19. 19. A method according to claim 17 or claim 18, wherein the source of the 0H ions is methenamine and the mixture of Zn(N03)2 and methenamine has a concentration ratio of Zn(N03)2:methenamine of between about 2:1 to about 10:1 at a molar concentration of about 0.05 to about 0.2M for Zn(N03)2.
  20. 20. A method according to any of claims 10-15, wherein the substrate and seed layer is immersed in the aqueous solution with the seed layer facing downwards.
  21. 21. A method according to any of claims 10-20, wherein the seed layer comprises a conductive material.
  22. 22. A method according to claim 21, wherein the conductive material is selected from indium tin oxide, doped ZnO, doped 1n203, doped Sn02 and Zn, Au, Al.
  23. 23. A method according to any of claims 10 or 12-22, wherein the fme grained piezoelectric material is synthesized electrochemically.
  24. 24. A method according to claim 23, wherein the fine grained piezoelectric material is a sol-gel material. * *. * * S S*. *S S.. * a. * * S * S
  25. 25. A method according to claim 24, wherein the sol-gel material is selected from ZnO, LiNbO3, LiTaO3, BaTiO3, LiTiO3, KNbO3, GaN, or lead zirconate titanate (PZT).
  26. 26. A method according to claim 23, wherein the fine grained piezoelectric material is a piezoelectric polymer that is spin coated on the zinc oxide nanorod arrays to form a substantially continuous film.
  27. 27. A method according to claim 26, wherein the piezoelectric polymers are selected from poly(vinylidene fluoride) arid its copolymer poly(vinylidene fluoride) trifluoroetylene (PVDFITrFE).
  28. 28. A method according to claim 26 or claim 27, wherein the piezoelectric polymers are electrically poled along the zinc oxide nanorod direction.
  29. 29. A method according to any of claims 10 or 12-28, wherein the fine grained piezoelectric material are selected from nanostructure solid crystal or ceramic S...::::. piezoelectric material powders such as AIN, ZnO, LiNbO3, LiTaO3, BaTiO3, 5..* LiTiO3, KNbO3, GaAs, GaN, PZT and its compounds with additives such as Co, tvln, Mg, Nb, or Sr.S * S. * S *5. S
  30. 30. A method according to any of claims 24-29, wherein the fine grained piezoelectric material is dried at and sintered at elevated temperature to form a film.
  31. 31. A method according to any of claims 10-30, further comprising a step of polishing a surface of the ZnO nanorod arrays to obtain a smoother surface.
  32. 32. Use of a zinc oxide film according to any of claims 1-9 in the manufacture of an acoustic wave device or an electronic device.
  33. 33. A zinc oxide film or method substantially as described herein in thedescription and drawings. * * * ** * S.. * * * *v* * .* . U S 550 a a **0S * ** * * S 0S0 S
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