AVS2007 Session SE-TuM: Glancing Angle Deposition

Tuesday, October 16, 2007 8:00 AM in Room 617

Tuesday Morning

Start	Invited?	Item
8:00 AM		SE-TuM-1 Temperature Effect on the Glancing Angle Deposition of Si Nanostructures C. Patzig, B. Rauschenbach (Leibniz Institute of Surface Modification, Leipzig, Germany) It is well-known that combining a glancing angle deposition (GLAD) process with suitable substrate rotation offers the ability to grow nanostructures with various shapes, including spirals, screws or vertical posts. When depositing on bare substrates, different growth phenomena are often encountered: Starting of as single fibers at the substrate, adjacent structures will merge together at a certain stage of growth. Also, dying out of structures due to a competitive growth of neighboured structures, and a broadening of the structures diameter with increasing structure height is often observed. However, for most applications, well-aligned, non-merged structures with comparable diameters are needed. Therefore, it is important to understand the influence of the deposition parameters on the nano-scaled structures in order to be able to control their growth. Here, the effect of the substrate temperature T_S on the growth of different Si nanostructures is studied. An ion-beam induced GLAD process was used to grow Si nanostructures at different substrate rotational velocities and at different substrate temperatures T_S ranging from room temperature (RT) up to 360°C, while all other deposition parameters where held constant. Due to the different rotational velocities, spirals, screws and vertical posts could be deposited. Analyzing the structures by means of scanning electron microscopy , it is found that T_S strongly influences the morphology of the grown structures. For the spirals and the screws, T_S effects the critical structure height h_crit at which the single fibers start merging together. From RT to T_S = 300°C, h_crit is increased with increasing T_S from h_crit(RT) = 150nm to h_crit(300°C) = 350nm (for the spiral structures), thus giving the possibility to grow spirals consisting of single fibers without merging together over a larger thickness range. However, it was found that increasing the temperature over T_S = 300°C results in a sudden drop of the critical height h_crit (360°C) = 130nm. Moreover, the total structure height was found to be dependent on T_S as well, indicating a change of the film density. For the posts, it was found that T_S influences the total number of posts and the inter-post-distance as well as the total structure height , showing a change of the film density as found for the spirals and screws.
8:20 AM		SE-TuM-2 Gold Nanorod Arrays for Surface Enhanced Raman Scattering Imaging of Micro-objects M. Suzuki, K. Nakajima, K. Kimura (Kyoto University, Japan); T. Fukuoka (JST Kyoto Pref. CREATE, Japan); Y. Mori (Doshisha University, Japan) Recently, we have demonstrated the direct formation of Au nanorods with a quasi-parallel major axis on a template layer of SiO₂ having a strongly anisotropic surface morphology. Those Au nanorods show excellent surface enhanced Raman scattering (SERS) properties.¹ Since the number density of nanorods is considerably high, we tried to apply our nanorod arrays to SERS imaging of micro-objects. Template layers of SiO₂ were prepared by the serial bideposition technique (SBD) on a glass substrate. During the SBD, the deposition angle measured from the surface normal was fixed at an angle of 79°, while the azimuthal angle was changed rapidly by 180° with each deposition of a 10-nm-thick layer. After repeating 30 cycles of the serial bideposition, Au was evaporated at a deposition angle of 73° onto the fabricated template layer. Owing to the self-shadowing, Au nanorods aligned in such a way where their major axes are parallel with each other. On the Au nanorod arrays, surface-modified polystyrene (PS) beads (5 µm in diameter), which have pyridyldithio group on their surface, were distributed. Raman spectra were measured by scanning the laser (λ=785 nm) with 1 µm step in a 40x40 µm² region. Raman peaks those originate from pyridyldithio group were detected only at the points where the PS beads and Au nanorods coexist. By assigning the Raman intensity levels to colors, SERS images were obtained. The PS beads were successfully resolved. Therefore, Au nanorod arrays are useful for the imaging of micro-objects such as cells and dusts without any labeling by other chemicals. ¹ M. Suzuki, K. Nakajima, K. Kimura, T. Fukuoka, and Y. Mori, Analytical Sciences (2007) in press.
8:40 AM	Invited	SE-TuM-3 Applications of Porous Thin Films Fabricated by Glancing Angle Deposition M.J. Brett (University of Alberta, Canada) Considerable efforts have been made by a number of research groups to advance the Glancing Angle Deposition (GLAD) technique in order to achieve precisely engineered and porous thin film structures. Carefully controlled substrate motion and glancing incidence evaporative flux enable fabrication of regular and random arrays of metals, semiconductors and insulators in architectural shapes such as posts, chevrons and helices. After a brief overview of the GLAD process and some advanced fabrication algorithms, this presentation will highlight some recent advances in applications of these coatings, in particular their use in sensor devices. Porous nanostructures of insulator materials have been deposited over planarized interdigitated electrodes and demonstrate fast (100 ms) response in conjunction with capacitive detection. Optical filters have been fabricated with the GLAD process to produce spectral hole or narrow bandpass characteristics ideal for optical detection through shift of the transmission peak. These filters have also been chemically functionalized to alter their sensitivity. Ag nanostructures will be presented that utilize surface plasmon resonance absorption for optically based sensing of biochemical compounds. For these and other applications, GLAD provides the advantages of broad material choice and precise control of microstructure shape and degree of porosity.
9:20 AM		SE-TuM-5 Periodic Nanostructures by Glancing Angle Deposition C.M. Zhou, D. Gall (Rensselaer Polytechnic Institute) Periodic Ta nanopillar arrays were grown by glancing angle deposition onto patterned substrates. Both the effects of pattern size and surface diffusion on morphological evolution were studied by varying the pattern length-scale and by growing at temperatures T_s ranging form 200 to 900 °C. The surface patterning experiments show a direct scaling, indicating that the overall nanopillar morphologies are determined by geometric shadowing and are for T_s = 20 °C independent of material parameters such as the characteristic length-scale for surface diffusion. However, at high growth temperatures, the increased adatom diffusion length causes Ta nanopillars to grow in a competitive growth mode, which in turn results in the breakdown of the regular array morphology. Glancing angle deposition has also been extended to fabricate novel Ta/Si two component nanostructures onto self-assembled close-packed silica nanosphere arrays. The two component nanostructures are shaped into zigzags or nanopillars by adjusting the deposition angle and/or the substrate rotation. By manipulating the sequence of the deposition, that is, by sequential or simultaneous deposition from two sources, complex nanostructures are formed where the two components are stacked vertically, laterally, or in a checker board arrangement. Scanning electron microscopy, back scattered imaging, and transmission electron microscopy provide clear compositional and microstructural contrast and show sharp vertical and horizontal interfaces.
9:40 AM		SE-TuM-6 Electrically Actuated Alq₃ Nanospring Arrays G.D. Dice, M.J. Brett, D. Wang, J.M. Buriak (University of Alberta, Canada) We report the fabrication and characterization of an electrically variable Fabry-Perot interferometer constructed from a nanospring array placed between partially reflective mirrors. Electrostatic compression of the nanosprings provides peak transmission wavelength control. Previous work studied the compression of SiO₂ nanosprings,¹ and individual Si nanospring actuation through electric current applied via a contact mode atomic force microscope (AFM).² High quality nanostructures have recently been created using the organic material tris (8-hydroxyquinoline) aluminum (Alq₃), which has a significantly smaller Young's modulus than inorganic films.³ Glancing Angle Deposition (GLAD) is a single step physical vapour deposition (PVD) technique utilizing an oblique deposition angle to create porous thin film helical structures.¹ Our device is constructed from three films deposited on a conductive indium tin oxide electrode. A 25 nm thick aluminum film forms both a partially reflective mirror and one parallel capacitor plate. A two turn helical Alq₃ nanospring film (300 nm pitch)is then deposited via GLAD PVD. A patterned 25 nm thick layer of aluminum forms the final mirror layer and top capacitor plate. Spring compression as a function of applied voltage was measured through a conducting contact mode AFM. The measured compression varies from 0 nm at 0 V, to ~ 1.2 nm at 6 V. We calculate the Young's modulus of the deposited Alq₃ to be ~ 0.93 GPa. A shift in the peak transmission wavelength from 582.4 nm to 580.8 nm was measured utilizing a fiber-coupled white light source and spectrometer as the applied voltage was raised from 0 V to 10 V. This 1.6 nm wavelength shift corresponds to a physical spring compression of 1.73 nm for the nanospring structure which has an effective refractive index of 1.42. ¹ M. W. Seto, K. Robbie, D. Vick, and M. J. Brett, "Mechanical response of thin films with helical microstructure," J. Vac. Sci. Techol. B, vol. 17, pp. 2172-2177, Sep. 1999. ² J. P. Singh, D. L. Liu. D. X. Ye, R. C. Picu, T. M. Lu, and G. C. Wang, "Metal-coated Si springs: Nanoelectromechanical actuators," Appl. Phys. Lett, vol. 84, pp. 3657-3659, Apr. 2004. ³ P. C. P. Hrudey, K. L. Westra, and M. J. Brett, "Highly ordered organic Alq3 chiral luminescent thin films fabricated by glancing-angle deposition,"Adv. Mater., vol. 18, pp. 224-228, Jan. 2006.
10:40 AM		SE-TuM-9 Effect of Thermal Oxidation and Annealing on the Structure and Morphology of Glancing Angle Deposited TiO_x Films W.J. Kiether, C.R. Guarnieri, H.T. Nagle (North Carolina State University) The typical Glancing Angle Deposited (GLAD) structure is an isolated, self-similar construct on a nanometer scale. Therefore, each structure can react to external influences independently of the other structures, as a separate, albeit similar system. This small scale system independence allows the nanostructures to exhibit different material characteristics than conventional thin films. As with other nanoscale structures, the large surface-volume ratio enhances surface effects, but the isolated GLAD structures are also effectively pinned on the substrate suppressing cross-structure grain growth and other microstructure propogation phenomena. Therefore, they provide an excellent surface engineering template to investigate the effects of annealing and thermal oxidation on thin film structure and morphology. Oxidation of titanium, with the existence of the metastable anatase and stable rutile polymorphs within the titania system, is significantly influenced by the microstructure obtained by GLAD films. Simple GLAD structures (pillars and chevrons) were deposited using reactive e-beam evaporation with a titanium source at various partial pressures of oxygen and argon. Additional process variables included deposition rate, flux angle of incidence, rotation speed, and substrate. Post deposition anneals were conducted in oxidizing, inert, and reducing atmospheres from 150-1200 °C. XRD patterns and SEM images served as the primary forms of film characterization. The combination of reactive evaporation at higher oxygen partial pressures and lower temperature anneals formed films with the highest percentage of the anatase phase. Depositions at lower oxygen partial pressures yielded films with a higher percentage of the rutile phase, which supports the role of oxygen vacancies as nucleation centers for the anatase to rutile transformation. Higher temperature annealing produced rutile films as expected from the thermodynamics of the titanium dioxode system, however the thermal stability of anatase phase structures was significantly enhanced by the GLAD microstructure. The titanium films also exhibit highly ordered crystallographic textures dependent on the flux angle of incidence for both ss-deposited films and after subsequent thermal oxidation. Finally, SEM images provide an interesting perspective on the dynamics of oxidation, grain growth, and sintering for a surface reaction dominated oxidation process.
11:00 AM		SE-TuM-10 Glancing Angle Deposition of Organic Molecules M.D. Fleischauer, G.D. Dice, S. Tsoi (University of Alberta, Canada); B. Szeto, M.J. Brett (University of Alberta / NRC National Institute for Nanotechnology, Canada) Glancing Angle Deposition (GLAD) has developed in to a widely-used nanostructuring technique because of the wide range of possible structures and material choices. Efforts to understand the fundamental mechanisms central to GLAD, including self-shadowing and limited surface diffusion, have largely focused on metals and oxides. Hrudey et al.¹ recently demonstrated that the GLAD technique can also be used for organometallics such as the luminescent material tris (8-hydroxyquinoline) aluminum (Alq₃). Unlike their inorganic counterparts, GLAD-fabricated Alq₃ nanostructures are smooth, show a self-ordered periodicity, and do not broaden or bifurcate. A solid wetting layer was also observed to form below the Alq₃ structures whose thickness varies with deposition conditions. Alq₃ molecules differ from metal atoms in that they are larger (1 nm dia.), heavier (ca. 460 a.m.u.), approximately planar, and possess an electronic quadrupole moment. A better understanding of how these properties lead to wetting layer formation, self-ordering, and smooth feature morphology, as a function of deposition conditions, is critical to the future realization of GLAD-nanostructured optical and optoelectronic organic devices. Here, we will present our investigations of the growth of Alq₃ nanostructures during GLAD. Methods to control film morphology (including wetting layer thickness) via deposition conditions and substrate preparation will be presented. Special attention will be paid to the initial stages of Alq₃ film growth during GLAD, especially compared to initial growth mechanisms observed at normal incidence by Brinkmann et al.,^2,3 with an eye towards predicting the behaviour of other organic and organometallic materials in the glancing angle regime. ¹P.C.P. Hrudey, K.L. Westra, and M.J. Brett, Adv. Mater. 18, 224 (2006). ²M. Brinkmann, S. Graff, and F. Biscarini, Phys. Rev. B 66, 165430 (2002). ³M. Brinkmann, F. Biscarini, C. Taliani, I. Aiello, and M. Ghedini, Phys. Rev. B, 61, R16339 (2000).
11:40 AM		SE-TuM-12 Tailoring the Wettability of Vertically Aligned Si Nanorod Arrays by Glancing Angle Deposition J.G. Fan, A. Collins, Y.P. Zhao (University of Georgia) We report a facile method to tailor the wettability of vertically aligned Si nanorod arrays by glancing angle deposition. As-prepared Si nanorod array surface is hydrophilic, while after coating a fluorocarbon layer, it becomes hydrophobic. For vertically aligned nanorod arrays, when increasing the nanorod height from tens of nanometers to several microns, the as-prepared samples become more hydrophilic while fluorocarbon coated samples become more hydrophobic. A wetting transition from a rough surface to a composite surface is observed at the same critical nanorod height (~ 150 nm for deposition angle of 86°) for both hydrophilic and hydrophobic surfaces. This critical height decreases when increasing the sample deposition angle, i.e., reducing the surface coverage of the nanorods. With a deposition angle of 88°, both superhydrophilic (contact angle = 0) and superhydrophobic (contact angle = 170°, and contact angle hysteresis < 10°) surfaces are obtained. For tilted nanorod arrays, strong anisotropic wetting behavior is observed in the plane formed by the vapor incident direction and the substrate normal. After hydrophobization, the difference between the left and right (nanorod tilting direction) contact angles of a water droplet could be as large as 30°. Such a droplet is able to move along the nanorod tilting direction under external disturbances. This tilting nanorod array provides a new morphological manipulation method for liquid delivery in microfluidic or nanofluidic devices.
12:00 PM		SE-TuM-13 Chemical Modification of Nanocomposite Si-SiOx Films Obtained by Oblique Deposition I.Z. Indutnyy, V.A. Dan’ko, I.Yu. Maidanchuk, V.I. Min’ko, P.E. Shepeliavyi (V. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine) Nanocrystalline silicon (nc-Si) embedded in SiOx matrix recently have attracted much interest due to perspective of application in optoelectronics and photonics. One of the methods that allow forming nanocomposite Si-SiOx films is vacuum deposition of amorphous SiOx with further anneal in vacuum or inert atmosphere. It leads to formation of nc-Si with diameter about 3-5 nm. Photoluminescence (PL) at room temperature is observed in such structures, position of PL band is in range of 700-950 nm. In recent papers we offered new method that allow to control nc-Si size and spectral range of PL by forming porous SiOx film using oblique deposition of silicon monoxide. Film obtained by oblique deposition has columnar structure (diameter of columns 10-50 nm). Annealing of such film leads to formation of nc-Si in volume limited by column size, that is nc-Si of smaller size are forming. Film obtained by oblique deposition is porous (porosity up to 57%). In this work we investigated the influence of chemical treatment in ammonia and acetone vapor on PL spectra of porous nc-Si-SiOx structures. Oblique deposited films were treated by saturated vapour of acetone or ammonia before anneal. This chemical treatment leads to the considerable changes in PL spectra effecting as on the band shape as on intensity. A new intensive PL band (with peak position near 590 nm) appears after annealing in samples treated in ammonia, and with peak near 600-610 nm in samples treated in acetone vapor. IR transmission spectra of treated and annealed films demonstrates appearance of silicon nitrides bands (for samples treated in ammonia) and bands connected with carbonization (for samples treated in acetone). It is assumed that the changes in PL spectra is caused by modification of nc-Si-SiOx interface with N or C atoms. Replacement of oxygen in Si-SiOx interface by N or C modifies electronic structure of excitons involved in light emitting process. Shortwave band (590-610 nm) that appears after chemical treatment is blueshifted for 0,6 eV in comparison with nontreated samples. The value of 0,6 eV is in good agreement with other works where oxide matrix with embedded nc-Si was replaced by nitride or carbon matrix. Thus chemical treatments in ammonia and acetone vapor are efficient methods that allow to increase PL intensity of the silicon nanocrystals embedded in the oxide matrix and vary PL peak position in wide range from 560 to 950 nm.