Development of nanosphere lithography and its applications 2

Chapter Introduction 1.1 Introduction Nanostructures and nanosystems have attracted much attention in modern science and technology due to their unique physical and chemical properties, which results in not only improving performance of current devices and processes but also potentially generating many new applications in physical [1, 2], chemical [3, 4] and bio technologies [5, 6]. Since the introduction of the theory of quantum mechanics in the twenty of last century, it is a well known fact, that objects change their behavior if they approach a certain lower size limit. Below this size limit, certain energies are allowed or forbidden, sharp steps in energy spectra arise and the general physics of objects change as shown in Fig.1.1. This quantum effects not only greatly improve some device performance, such as high speed transistors [7] but also result in some totally new devices like the semiconductor laser [8] or the superconducting nanowires [9]. Fig. 1.1 Quantum effects of matter. So far it has been demonstrated that nanostructures exhibit particularly peculiar and interesting characteristics, for example: quantized excitation [8], Coulomb blockade [10], single-electron tunneling [11], and metal-insulator transition [12]. These phenomena occur in structures small enough for quantum mechanical effects to dominate. Besides the quantum effects, new phenomena result from the nanostructures or nanosystems also occur and applied in physical, chemical and biotechnologies. For example, ballistic movement of an electron in a semiconductor [13], near- and farfield diffraction of visible light [14], diffusion of an active species close to an electrode [15], excitation of collective resonance by light [16]. Fabrication and study of these systems have become active areas of research in physics, material science, chemistry and biology. In addition, photonic crystals are periodic optical nanostructures that affect the motion of photons in much the same way that ionic lattices affect electrons in solids. Combining the quantum dots and the photonic crystals has already attracted much attention [17-20]. However, how to fabricate such periodic nanostructures efficiently is still a big challenge. 1.2 Review of nanofabrication technologies The ability to fabricate structures from the micro- to the nanoscale with high precision in a wide variety of materials is of crucial importance to the advancement of micro- and nanotechnology and the nanosciences. The semiconductor industry has been pushing high-precision nanoscale lithography to manufacture ever-smaller transistors and higher-density integrated circuits (ICs). Critical issues, such as resolution, reliability, speed, and overlay accuracy, all need to be addressed in order to develop new lithography methodologies for such demanding, industrially relevant processes. On the other hand, less stringent conditions are found in many other areas, for example, photonics, micro- and nanofluidics, chip-based sensors, and most biological applications. Beside traditional lithographical technologies, such as photolithography, e-beam lithography, several alternative approaches towards nanostructure fabrication have been exploited in the past 15 years. These techniques include microcontact printing (or soft lithography) [21], nanoimprint lithography (NIL) [22], scanning-probe-based techniques (e.g., atomic force microscope lithography) [23], dip-pen lithography [24], and nanosphere lithography. In the thesis, I will focus on the development of nanosphere lithography (NSL), and its various applications, such as fabricating surface nanostructures, forming templates and growing nanostructures through the templates. Basically, lithography is a chemical process to pattern parts of a thin film or the bulk of a substrate. These patterns can be formed on a mask and be transferred to other thin film which can be used to form various small devices, such as integrated circuit, MEMS and small devices including light emitting diodes. Traditional lithographic techniques include photolithography [25], in which light is used as an energy source to change the photoresist; e-beam lithography [26], in which electrons are used to change the chemical properties of the resist. Recently various lithography techniques, for example, nanoimprinting [22], and nanosphere lithography [27], have been developed to overcome the problems arising from traditional lithographic techniques. 1.2.1 Lithography with photons The well known lithography is photolithography, which uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist" on the substrate. After development, a deposition or etch process is applied to form the pattern on a film or a substrate as shown in Fig. 1.2. Fig. 1.2 Basic outline of optical lithography processes. The diagram shows the optical radiation entering the system, which is then filtered by the chromium mask. The image is then projected on to the resist, and any non-exposed material is removed during developing. Photons have been used for many years to induce chemical reactions in photographic materials or resist polymers. The lithographic technology is an invaluable tool for micro-fabrication in a broad range of applications in science and technology and one of the most widely used and highly developed technologies now practiced [25]. In this process, the mask is placed in physical contact with, or in close proximity to, the resist. Most fabrication in the integrated circuit industry uses such lithography. Photolithography is a fast technique to form patterns due to its large area expose. The minimum feature size that can be obtained by this process is primarily determined by diffraction that occurs as light passes through the gap between the mask and the resist. Even with the use of elaborate vacuum systems to pull the mask and substrate together, it is still difficult in practice to reduce the gap between a conventional rigid mask and a rigid flat substrate to less than ~1 µm over large areas. As a result [28], the resolution of contact mode photolithography is typically 0.5-0.8 µm when UV light (360-460 nm) is used. The resolution of photolithography increases as the wavelength of the light used for exposure decreases. Feature sizes of 250 nm can be obtained when 248 nm UV light is used. However, there are big problem with transparency of optical parts when wavelength of light is further decreased [29]. Although photolithography was demonstrated with soft EUV and X-rays many years ago, to fabricate the masks and optics capable of supporting a robust, economical method still provides significant unsolved challenges [30]. 1.2.2 Lithography with Particles Fig. 1.3 Basic electron optical column in which the beam is formed. The image is formed on the resist, and the deflectors control the position of the beam on the resist. Theoretically, photolithography can get the resolution down to 30 nm at X-Ray wavelengths. However, as finer resolutions are demanded by industry, shorter wavelengths must be used. Gamma rays cannot be used, as the mask nor the resist will absorb them. From the quantum mechanical principle of wave-particle duality [31] and the de Broglie equation , it is found that an electron with an energy of 10 keV has a wavelength of around 12 pm. This obviously represents a huge reduction in wavelength compared to X-Ray radiation, and therefore electron beam lithography has the possibility at a better resolution than any of the electromagnetic methods previously considered. Electron beam lithography replaces the photons with an electron beam, and utilizes a different system with image formation between the source and the resist with no mask in the system (Fig. 1.3). Because of using a beam of electrons, whose direction can be controlled by a magnetic field, there is no need for a mask in the lithographic system. A computer controls the strength of the magnetic field, whilst there is very little diffraction from the electrons, so the patterns produced on the resist are extremely accurate, even though it suffers from scattering in the resist. Less than 10 nm features has been obtained by this technique [32]. However, electron beam lithography still accounts a big problem. The system has a very low throughput due to its series nature of writing, only one point on the resist can be exposed at any given time. 1.2.3 Nanoimprinting The principle of nanoimprinting is very simple. Figure 1.4a shows a schematic of the originally proposed NIL process [33, 34]. A hard mold that contains nanoscale surface-relief features is pressed into a polymeric material cast on a substrate at a controlled temperature and pressure, thereby creating a thickness contrast in the polymeric material. A thin residual layer of polymeric material is intentionally left underneath the mold protrusions, and acts as a soft cushioning layer that prevents direct impact of the hard mold on the substrate and effectively protects the delicate nanoscale features on the mold surface. Fig.1.4 (a) Schematic of the originally proposed NIL process. (b) Scanning electron microscopy (SEM) image of a fabricated mold with a 10 nm diameter array. (c) SEM image of hole arrays imprinted in poly(methyl methacrylate) by using such a mold [34]. Advantages of the NIL are that it demonstrated ultrahigh resolutions soon after its inception. Figures 1.4 b and c show SEM images of a mold with a pillar array (pillar diameter 10 nm) and an imprinted 10 nm hole array in poly(methyl methacrylate) (PMMA) that were obtained almost a decade ago [35]. NIL is inherently high-throughput, because of parallel printing, and it requires only a simple equipment, leading to low-cost processes. A variation of the NIL technique that uses a transparent mold and UV-curable precursor liquid to define the pattern (step-andflash imprint lithography) has been demonstrated [36], allowing the process to be carried out at room temperature and making it attractive for IC semiconductor device manufacturers. However, it still has some challenges in meeting the stringent requirements of various applications, such as mold fabrication, mold surface preparation, NIL resist, and residual layer problem [34]. 1.3 Nanosphere lithography Nanosphere lithography (NSL) is an inexpensive, simple to implement, inherently parallel, high throughput general nanofabrication technique capable of producing an unexpectedly large variety of nanostructures and well-ordered 2D nanostructural arrays. The inception of “natural lithography” dates from the seminal work of Fischer and Zingsheim with the introduction of “naturally”-assembled polystyrene latex nanospheres as a mask for contact imaging with visible light in 1981 [37]. In 1982, Deckman and co-workers greatly extended the scope of Fischer’s approach by demonstrating that a self-assembled nanosphere monolayer could be used as both a material deposition and etch mask. Deckman coined the term “natural lithography” to describe this process. Deckman and co-workers continued to explore various fabrication parameters and possible applications of natural lithography but always employed a single layer (SL) of nanospheres as the mask [38]. The third stage in the evolution of natural lithography, renamed nanosphere lithography (NSL) to be more operationally descriptive, is represented by the work of Van Duyne et al. [39], who extended the SL methodology with (1) the development of a double layer (DL) nanosphere mask, (2) atomic force microscopy (AFM) studies of SL and DL periodic particle arrays (PPAs) of Ag on mica, and (3) fabrication of defect-free SL and DL PPAs of Ag on mica with areas of 10-100 µm2 that were large enough to permit microprobe studies of nanoparticle optical properties [39]. As an efficient nanofabrication technique, NSL is being used in laboratories around the world to study the size-dependent optical, magnetic, electrochemical, thermodynamic, and catalytic properties of materials. Fig. 1.5 (a) side and (b) top-views of self-assembly of nanospheres It has been demonstrated that the self-assembly process to form 2D ordered arrangement of the nanospheres starts from a nucleation [40]. The nucleus formation is governed by attractive capillary forces appearing between spheres partially immersed in a liquid layer. Then, crystal growth occurs through convective particle flux caused by the water evaporation from the already ordered array (Fig. 1.5). In principle, the monolayer of hexagonally close-packed (hcp) spheres can be used as a mask to form nanoparticles by depositing other materials through the holes between the spheres. Actually, nanostructures can also be created on the substrate by dry etching or infiltrating process as seen in Fig. 1.6. Methods to deposit a nanosphere solution onto the desired substrate include spin coating [41], drop coating [42], template-directed growth [43], angled cooling plates and Langmuir-Blodgett techniques [44]. All these deposition methods require that the nanospheres are able to freely diffuse across the substrate, seeking their lowest energy configuration. This is often achieved by chemically modifying the nanosphere surface Fig. 1.6 Nanosphere lithography used to create various nanostructures with a negatively charged functional group, such as carboxylate or sulfate that is electrostatically repelled by the negatively charged surface of a substrate such as mica or glass. Following the self-assembly of the nanosphere mask, a metal or other material is then deposited by thermal evaporation, electron beam deposition (EBD), or pulsed laser deposition from a source normal to the substrate through the nanosphere mask to a controlled mass thickness dm. After the metal deposition, the nanosphere mask is removed by sonicating the entire sample in a solvent, leaving behind the material deposited through the nanosphere mask on the substrate. In the simplest NSL case, only a monolayer of hcp nanospheres with a diameter of D is self-assembled onto the substrate. When one deposits metal through the monolayer mask, the three-fold interstices allow deposited metal to reach the substrate, creating an array of triangular shaped nanoparticles with P6mm symmetry [Figs. 1.7(a) and (b)]. Simple geometric calculations define the relationship between 10 nanostructured surface, and the fabricated Ti/Al n-contact on the n+ GaN region can be clearly seen in Fig. 5.5(c). 5.2.2.2 Results and discussion Figure 5.5(b) shows an optical micrograph of the nanostructures formed at the designed p-GaN surface. The gray contrast represents the nanostructured regions. It is noted that the edges of the nanostructured regions are not straight line, implying the “bad” areas along the edges of photoresist patterns are not straight due to its selfassembling features. Fig. 5.5 (c) shows an optical micrograph of the fabricated GaN LED die. The dark patch contrast represents the nanostructured region on the Ni/Au contact area. The p type contact was formed at the nanostructured region Figure 5.6 shows a SEM image of the formed surface structures. It is clearly observed that the base of the nanostructures is hexagonal-like shape. This phenomenon resulted from the reshaping of the PS spheres. The power used for GaN material ICP dry etching is too high that leads to melting of the PS, then reshapes to hexagonal based particles due to the limitation of hexagonal arrangement of the PS spheres. The cross-section view SEM image (inset in Fig. 5.6) reveals that truncated cone structures are formed. The base diameter of the truncated cone is ~680 nm, the top diameter is ~350 nm, and the height is ~260 nm, respectively. After the fabrication of the nanostructure array on the p-GaN surface, the emission from the sample with/without the nanostructure regions was tested using room temperature photoluminescence (PL) mapping. Fig. 5.7(a) shows the photoluminescence map over the wafer with the peak emission at approximately 495.8 nm. The PL intensity of the sample with the surface nanostructures is three times as compared to the region without the structures as shown in Fig. 5.7(b), a line scan of the wafer along the line shown in Fig. 5.7(a). The yellow patch-like patterns indicate the die regions with the regular nanostructure arrays, giving stronger 139 luminescence than the conventional p-GaN region (indicated in blue). The nanostructures assist the photons with multiple opportunities to escape from the wafer Fig. 5.6 A SEM image (inset, a cross-section view) showing the nanostructures created on the p-GaN surface. surface and redirect the photons which are originally emitted out of the escape cone back into the escape cone. This is similar to the effect of the surface roughening technique adopted for flip-chip LEDs [32] and also for wet-etched GaN template LEDs [33]. Fig. 5.7 (a) The photoluminescence (PL) mapping across the region with and without (blue area) the nanostructures, and (b) line scan of the PL along the line showing in (a). After the device fabrication, the effect of injection current on the electroluminescence (EL) from LEDs with nanostructure arrays and non-patterned p140 GaN was studied. Based on the EL spectra shown in fig. 5.8(a) for the LEDs with the nanostructure patterned p-GaN region, there is a prominent shift in the wavelength from 516 nm at an injection current of mA to 488 nm at 70 mA. A similar phenomenon is also observed for the non-patterned p-GaN LEDs. This is attributed to the incorporation of indium rich nanostructures embedded in the InGaN well layer. At a low injection current, the electrons and holes recombine mainly at the InGaN nanostructures (with higher indium composition). Figure 5.8(b) shows the plots of light output power as a function of the injection current for both types of LED. The light output power from both LEDs increases linearly with an initial injection current up to 50 mA but for the nonpatterned GaN LEDs, it starts to saturate at a higher injection current. However, for the case of LEDs with nanostructure patterned p-GaN surface, the output power continues to increase steadily even after 70 mA. The light output power of such patterned GaN LEDs was about 1.8 times higher than the conventional non-patterned Fig.5.8 (a) Electroluminescence spectra from the LEDs with nanostructures created on the p-GaN surface. (b) Plots of the light output power as a function of injection currents for both LEDs with and without the surface nanostructures. LEDs at the injection current of 70 mA. The use of nanostructures on the p-GaN surface improves the escape probability of photons due to the angular randomization of photons through internal scattering from the nano-cone sidewalls. As expected, this effect leads to an increase in the light extraction efficiency of these LEDs. Based on cathodoluminescence measurements, P´erez-Sol´orzano et al [34] have identified the 141 confinement of emission mainly at the sidewalls of pyramidal structures for InGaN/GaN MQWs emitting between 490 and 510 nm, and this is quite similar to our observations in LEDs with nano-truncated-cone-patterned p-GaN. 5.3 Effect of ordered surface Au nanostructures on LEDs Surface plasmonics is an important physical phenomenon, which attracts more attentions recently due to its unique properties[35-38]. It has been demonstrated that the surface plasmonic can enhance the light emitting properties for LEDs due to the interaction of the surface plasmonic and the active layers[36]. However, for most real LEDs, the top surface where the metal structures can be formed is far away from its active region so that the interaction is weak. In addition, most metals, used in generating the surface plasmon, are not transmittance, which limits its applications in LEDs. So far, there are no such reports regarding metal nanostructures applied in the LEDs. In this section, investigation of effects of Au honeycomb nanostructures created on red LEDs has been carried out. Strong enhancement has been observed. 5.3.1 Experiments Similar that as shown in Fig. 5.9, the monolayer of the PS nanospheres with 520 nm diameter was formed on top of a red LED wafer (emitting wavelength of 650 nm). The wafer then was treated by O2 RIE to reduce the diameters of the nanospheres to 500 nm (wafer I), 460 nm (wafer II), 420 nm (wafer III) and 370 nm (wafer IV), respectively. Then an 100 nm Au film was deposited for all wafers by evaporation under the conditions: basic pressure of 10-6 Torr and deposition rate of 0.05 nm/s. Then, the PS spheres were removed to form a honeycomb-like Au nanostructure. 142 5.3.2 Results and discussion Figures 5.9(b) and (c) shows the optical microscope and SEM images of the Au nanostructures formed by the method, indicating excellent uniformity although there are point and line defects. Fig. 5.10 shows the details of the Au nanostructures formed through 500 nm [Fig. 5.10(a)], 460 nm [Fig. 5.10(b)], 420 nm [Fig. 5.10(c)], and 370 nm [Fig. 5.10(d)] residual spheres, respectively. It is clearly observed that the diameter of the nanoholes is increased with the diameter of the residual spheres. The separation of the nanoholes is kept at the same as the original spheres (~520 nm). However, the surface of the Au films is not flat as observed in the SEM images showing in (e) to (g). A thick circular ring around the hole is formed. Fig. 5.9 (a) Schematic showing the procedure to create Au nanostructures. (b) A micrograph and (c) a SEM image of the Au nanostructures 143 Fig. 5.10 (a)-(d) Top-view of the SEM images showing the Au nanostructures formed through 500, 460, 420 and 370 nm residual spheres etched from 520 nm PS spheres. (e)-(g), Au nanostructures formed from the 370 nm residual spheres viewed at near 45o, 60o, and near 90o, respectively. There are also protruding portions located at the interstitial position of the honeycomb lattice, which are formed due to more materials deposited through the large opening between the three nanoparticles. The formation of thick ring around the holes has probably resulted from the atoms rebounding or reevaporation from the surface and the surface diffusion. The re-evaporated atoms will attach to the surface of the PS spheres near the bottom. Therefore, the surface curvature follows the spherical particles, which has been observed for the top portion of the ring. In addition, wetting properties of the surface of PS polymer and the GaP surface also play an important role for the final shape. Figure 5.11 shows PL results of the wafer with and without the Au nanohoneycomb. As comparison, original LED wafer and the wafer covered by 100 nm Au film are also displayed. It is observed that the PL signal for the wafer covered with 100 nm Au film is weak due to reflection and absorption of the excitation light and PL. However, remarkable PL has been observed for the Au nanohoneycomb structured wafer formed from the 420 nm residual spheres, 144 but the intensity is lower than the reference wafer. When the diameter of the nanoholes increased to ~500 nm, wafer I [fig. 5.10(a)], the PL intensity is significantly increased, more than times enhancement is obtained. Due to the thick GaP top layer (~5 µm), the enhancement of the PL should not come from the interaction between the surface plasmon and the active layer of the LED. Therefore, it is attributed to the enhancement of light extraction from the LED. The Au honeycomb structure acts as 2D photonic crystal which scatters the light out off the plane parallel to the LED surface then couples to the surface plasmon polaritons (SPPs) through the extraordinary optical transmission (EOT) effect and radiates out. In this situation, the surface plasmon resonance has to match the scattering modes to enhance the light extraction. However, both the SPPs and the modes generated by 2D photonic crystals depend on the ratio of lattice a and radius R of the structures. In the case of wafer III, the diameter of the hole changing results in the partial match of the resonance. Therefore, one part of the light is absorbed or reflected by the Au nanohoneycomb, which leads to PL decreasing compared with the original wafer. Electrical excitation of the LED with the Au nanohoneycomb for the wafer I has been tested. Comparing with that without such nanostructures, the emission is much stronger as seen in Fig. 5.12(a), photographical images of the emitting light. Beside the enhancement of light extraction from the LED, the improvement of current distribution also plays an important role for the lighting at the electrical excitation. Actually, the thick top GaP layer is designed to be a current distribution layer to increase the current injection 145 efficiency in the LED structures. However, the resistance of the semiconductor is still much higher than metals. So, the current injection efficiency will be Fig. 5.11 PL spectra of the wafers: original surface (black), covered with 100 nm Au film (red), nanostructured Au film with 420 nm holes (green) and nanostructured Au film with 500 nm holes (blue), respectively. increased much by the nano-scaled honeycomb. In Fig. 5.12(b), we also plot the light output power as a function of injection currents for both the LEDs with (green) and without (black) the surface Au nanostructures. In addition, the result of the wafer with surface nanostructures created on the GaP top layer [Fig. 5.4(b)] is also plotted (red) as comparison. It is clearly observed the light output power of the wafer with the Au nanohoneycomb is much higher than that without such structures, it is higher than 146 that with surface nanostructures fabricated on the GaP top layer as discussed in section 5.2.1. Fig. 5.12 (a) Photographic images showing the light emitting from the wafer with (lift) and without (right) the Au nanohoneycomb structures. (b) Plots of the light output power as a function of injection currents for both LEDs with (blue) and without (black) the surface Au nanostructures. In addition, the result of the wafer with surface nanostructures created on the GaP top layer [Fig. 5.4(b)] is also plotted (red) as comparison. 5.4 Summary In this chapter, NSL has been used to create nanostructures on LEDs. For both GaAs based and GaN based LEDs, the created surface nanostructures significantly enhance the light extraction efficiency due to the 2D photonic crystal or surface roughening effects. ~2.43 times enhancement of light output power has been observed for the GaAs based red LEDs, while it is ~1.9 times for the GaN based blue LEDs. Effects of Au honeycomb nanostructures on the LEDs have been investigated firstly. Primary experimental results show that strong enhancement can be obtained if right parameters are chosen for the honeycomb nanostructures. The structures can also be served as a current distribution layer to improve current injection efficiency. Further investigation is needed to optimize the nanostructures. 147 References M. G. 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An, H.C. Ong, “Surface-plasmon-mediated emission from metalcapped ZnO thin films”, Appl. Phys. Lett 86, 251105 (2005). 149 Chapter Summary and future plan 6.1 Summary In this thesis, an inexpensive, fast and flexible technology - nanosphere lithography, for fabricating nanostructures was systemically investigated. It includes, 1) development of new techniques to extent the capability of the nanosphere lithography to fabricate more uniform and more complex nanostructures, 2) using the nanostructures created by the nanosphere lithography to investigate the nano-growth of semiconductor and the formation of metal nanostructures, and 3) using the nanostructures created by the nanosphere lithography to improve the performance of LEDs. A guided self-assembly process was developed to overcome the problem that a closely-packed single or double layer of nanospheres can be obtained only on small areas (10 μm level). In most cases, single layer formation is accompanied by double layer or multilayer, which cannot be consistently reproduced. In my method, an array of square wells of dimension 400x400 µm2 patterned by photoresist was formed by a standard photolithography on a inch GaAs substrate. This size of the wells is larger than a LED die. Then, the self-assembly of nanospheres was started after spinning the solution of nanosphere of appropriate concentration on the surface of the wells. Experimental results showed that single layered spheres was obtained in almost all the wells over the wafer. Areas with multilayers were found only along the edges of the squares, which can be sacrificed during subsequent device processing. By adjusting the concentration of the colloidal solution or the spin speed, double layer of the nanospheres can be also obtained in the wells. 150 A technique that allows the creation of nanostructures in selected locations or with different feature sizes in one step nanosphere lithography process is demonstrated for the first time. The experiments were carried out on a 100 nm thick SiO2 film deposited on a GaAs substrate. Small squares were formed by standard photolithography and dry etched down to 50 nm, resulting in two areas with different thicknesses on the SiO2 film. After removing the photoresist, a double layer of PS nanospheres with diameter of 300 nm were self-assembled to cover the whole surface of the SiO2 film. Then dry etching of SiO2 was performed in areas between nanospheres, which were used as a mask. An array of nano-hole openings can be formed only in the thinner SiO2 regions and not in the thicker SiO2 layer if dry etching is terminated when the etching front of the holes in the thinner SiO2 areas just reach the GaAs surface. If etchants chosen could etch both the PS spheres and the SiO2 at the same time, then tilted sidewalls of the etched SiO2 holes could be formed. Therefore, when the etching front of the nanoholes in the thicker SiO2 regions reached the GaAs substrate, the size of the nano-openings in the thinner SiO2 layer was enlarged. Therefore, arrays of the nano-openings with two different diameters can be formed in one step on the SiO2 film deposited on the GaAs substrate. The nanostructures created in the SiO2 film described above were used as a template to investigate nano growth of MOCVD of semiconductors. Ordered arrays of InGaAs/GaAs nanobars were successfully grown in selected regions of the GaAs substrate through the holes in the SiO2 template. Dual-sized InGaAs/GaAs nanobars were obtained in a one-step MOCVD for the first time. Some techniques to control the shapes of nanostructures in both the vertical and lateral directions and their arrangement were further developed. A multi-cycle etching technique to fabricate 3D nanostructures from a 2D nanosphere array was invented. By using the technique, several GaAs surface nanostructures with different crosssection profile were created. By adjusting the etch durations and the number of 151 repeated cycles, any shaped nanostructures can be obtained. An array of lens-like structures with diameter of 2µm was demonstrated on a Si substrate. The concept of 3D masks was demonstrated for the first time. A 3D network was formed by infiltrating liquid silica into the space between the bilayered array PS spheres of 300 nm diameter. After solidifying the silica film, a RIE dry etching was used to etch the silica film to expose the top portion of the PS sphere array. The PS spheres were then dissolved chemically to form the silica 3D hollow structure. This 3D nanostructure was used as a mask to create surface nanostructures. By using the mask, more complex nanostructures were created successfully on Si and GaAs substrates. Experimental results also indicated that such mask can be easily modified by: 1) etching the PS spheres before infiltration of the liquid silica, 2) etching the silica film to different depth and then dissolving the PS spheres, and 3) selecting such etchants by which the substrate and the 3D silica mask could be etched at the same time. The effects of RIE etching conditions, such as type of etchants, chamber pressure as well as diameter of spheres in shaping the cross-section profiles of the nanostructures, are systemically investigated. An ordered periodic array of quasi-freestanding nanodisks of SiO2, with disk thickness of 100 nm, diameter of 1400 nm supported by 140 nm Si pillar, was successfully obtained. Nanostructures created by nanosphere lithography were used to investigate nano growth of III-V compound materials by MOCVD. A GaN film and the film with InGaN/GaN multi-quantum wells were successfully grown on an ordered array of nanopillars created on a Si (111) substrate. PL enhancement of ~7 times and times has been observed from the GaN film and the quantum wells, respectively. The nanosphere lithography was also applied to fabricate unique metal nanostructures. Light transmission was systemically investigated through an Au film with subwavelength holes. Extraordinary optical transmission with wide spectra was observed which can be modified by the degree of order of the nanoholes, the period 152 and the hole size. Special 3D Au nanostructures were designed and fabricated to investigate the surface plasmonic effects. An array of glass disks and PS particle aligned vertically was created on a glass sheet by the method of multi-cycle-etching. A ~100 nm Au film was deposited on the 3D structures. For these structures, strong extraordinary optical transmission was observed. The experimental results also showed that the peak position and intensity of the extraordinary optical transmission can also been strongly modified by changing the structural parameters. A guided annealing method to form an ordered array of Ag nanoparticles was proposed. A SiO2 honeycomb nanostructure was formed on a Si substrate through a single layered array of PS spheres. A film of Ag with different thicknesses was deposited on the SiO2 template followed by annealing. Experimental results indicated that, with annealing duration of 30 minutes, the shape and distribution of the Ag nanoparticles depended not only on the annealing temperature, but also on the deposited thickness of the Ag. For the 10 nm Ag film deposition, a dot-like Ag was formed inside the holes of the SiO2 honeycomb. With increase in annealing temperature, the small dots merged to form larger dots. However, several dots were still distributed inside the hole patterns even when the temperature was increased up to 700 0C. For the 30 nm thick Ag film deposition, a film-like Ag was formed over the whole SiO2 patterned Si surface. In this case, one big dot was formed inside the hole when annealed at 700 0C. These results imply that there are two mechanisms for forming the Ag nanoparticles. When the Ag film is 10 nm thick, the annealing is dominated by the diffusion of atoms. For the thicker Ag film, the annealing is dominated mainly by the surface energy of the Ag material and the boundary defined by the SiO2 patterns. For the case of Ag nanoparticles formed on an unpatterned Si substrate, size and position of the Ag nanoparticles are random. In the thesis, the nanosphere lithography was also used to improve the performance of light emitting diodes (LEDs). Surface nanostructures were created on GaAs-based red and GaN-based blue LEDs. ~2.43 times enhancement of light output 153 power was observed for the GaAs-based red LEDs, while the enhancement was ~1.9 times for the GaN-based blue LEDs. Effects of Au honeycomb nanostructures on the LEDs were investigated. Primary experimental results show that strong enhancement can be obtained if right parameters are chosen for the honeycomb nanostructures. The structures can also be served as a current spreading layer to improve current injection efficiency. 6.2 Future plan The physical behaviors of surface plasmonics associated with the nanostructured metals is still unclear and need to be investigated, such as relation of surface plasmons with shapes of the metal nanostructures and coupling of the surface plasmons between the complex nanostructures. For application in LEDs, although strong enhancement of light output of LEDs was obtained by the surface nanostructures, the mechanism of the enhancement is still unclear. Especially, the effects of the nanostructured Au film on the performance of LEDs need to be systemically investigated. Further investigation is also needed to understand the role of the Au nanostructures in improving current spreading. 154 [...]... “Angle-Resolved Nanosphere Lithography: Manipulation of Nanoparticle Size, Shape, and Interparticle Spacing”, J Phys Chem B 106, 1898 (20 02) 18 Chapter 2 Development of nanosphere lithography 2. 1 Introduction Although nanosphere lithography (NSL) has been used to create nanostructures on various substrates [1-6], it still lacks the control of shapes and arrangements of the nanostructures due to the nature of NSL... the limits of lithography , Nature 406, 1 027 (20 00) [30] R Fabian Pease, Y Chou, Lithography and Other Patterning Techniques for Future Electronics” Proceedings of the IEEE 96, 24 8 (20 08) [31] M Alonso and E.J Finn, "Physics", Addison Wesley, Harlow, England, (19 92) [ 32] H.G Craighead, R.E Howard, L.D Jackel, P.M Mankiewich, “10‐nm linewidth electron beam lithography on GaAs”, Appl Phys Lett 42, 38... by one-step lithography 2. 2 Self-assembly of colloidal crystals 2. 2.1 Introduction Self-assembly of the hexagonal closed-packed (hcp) monolayer or bilayer of colloidal spheres, is a basis of nanosphere lithography (NSL) [1- 12] Self-assembly is a type of process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, 20 local interaction... image of the monolayer arrays of 300 nm PS spheres formed at a SiO2 film with micro-wells Figure 2. 7(a) illustrates a schematic of patterns created on a SiO2 surface by photolithography combined with dry etching The thickness of the SiO2 film is 150 nm deposited on a GaAs substrate by PECVD Width and depth of the wells are 8µm and 80 nm with a separation of 5 µm Figure 2. 7(b) shows a SEM image of the... angle between the nanosphere mask and the beam of material being deposited The size and shape of the three-fold interstices of the nanosphere mask change relative to the 12 deposition source as a function of , and accordingly, the deposited nanoparticles’ shape and size are controlled directly by (Figure 1.9) Fig 1.8 Schematic illustration (a) and representative AFM image (b) of nanoring and SL PPA fabrication... (1999) [20 ] G C La Rocca, “Organic photonics: Polariton lasing”, Nature Photonics 4, 343 (20 10) [21 ] Ravi S Kane, Shuichi Takayama, Emanuele Ostuni, Donald E Ingber, George M Whitesides, “Patterning proteins and cells using soft lithography , Biomaterials 20 , 23 63 (1999) [22 ] S Y Chou, P R Krauss, P J Renstrom, “Imprint of sub 25 nm vias and trenches in polymers”, Appl Phys Lett 67, 3114 (1995) [23 ] S... the diameter of hole opening of the nanocavities Fig 2. 12( a) shows a SEM image of the hole openings of the cavity array, where 150 nm silica is etched off When the etch depth reaches the 170 nm, the opening is enlarged as seen in Fig .2. 12( b) Fig 2. 12 Top-view SEM images of the periodic ordered nanocavities To form the nanocavities the top of the silica film was etched down (a) 150 nm and (b) 170 nm... 89, 011908 (20 06) [ 42] C L Haynes, A D McFarland, M T Smith, J C Hulteen, and R P Van Duyne, “Angle-Resolved Nanosphere Lithography: Manipulation of Nanoparticle Size, Shape, and Interparticle Spacing”, J Phys Chem B 106, 1898 (20 02) [43] A van Blaaderen, R Ruel, P Wiltzius, “Template-directed colloidal crystallization”, Nature 385, 321 (1997) [44] S Rakers, L F Chi, H Fuchs, “Influence of the evaporation... surface of the silica and the top of spheres is around 50 nm for the wafer with an array of 600 nm spheres A RIE etching of the silica film was carried out to expose the top of the PS spheres Etchant of CH4 with flow rate of 25 sccm at a chamber pressure of 15 mTor was used The etching duration is varied from 2 to 3 minutes Finally, the PS spheres were removed by toluene to form the array of silica... silica cavities Results and discussion Fig 2. 11 A cross-section view of a SEM image of the silica nanocavities Figure 2. 11 shows a cross section SEM image of the solidified silica film containing 900 nm PS spheres Uniform flat surface of the film is clearly observed 34 The thickness of the film is estimated to be 820 nm, and the thickness from the surface of the film to the top of the spheres is around . (or soft lithography) [21 ], nanoimprint lithography (NIL) [22 ], scanning-probe-based techniques (e.g., atomic force microscope lithography) [23 ], dip-pen lithography [24 ], and nanosphere lithography. . e-beam lithography [26 ], in which electrons are used to change the chemical properties of the resist. Recently various lithography techniques, for example, nanoimprinting [22 ], and nanosphere lithography. parameters and possible applications of natural lithography but always employed a single layer (SL) of nanospheres as the mask [38]. The third stage in the evolution of natural lithography, renamed nanosphere