ICMCTF2015 Session F1: Nanomaterials and Nanofabrication

Aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) semiconductors and their corresponding ternary films, such as InGaN, offer attractive properties, with high breakdown fields and widely tunable direct band gaps. Currently, III-nitrides are primarily deposited with molecular beam epitaxy and chemical vapor deposition. The addition of Atomic Layer Epitaxy (ALE) to the possible growth techniques is driven by the need for even thinner films integrated into complex heterostructures, somthing that is increasingly difficult to achieve by conventional techniques. Furthering the attraction of ALE is the promise of lower growth temperatures that allow the deposition of a winder range of indium containing ternary films.

Here we report on ALE in a plasma-equipped Ultratech/Cambridge Nanotech atomic layer deposition system to grow AlN, GaN, and InN at temperatures significantly lower than needed for molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD).[1] In growing epitaxial materials, the substrates and corresponding surface preparation procedures are important. The substrates include silicon(111), sapphire, and MOCVD gallium nitride on sapphire, as well as graphene.[2]

For InN on a-plane sapphire, the expected wurtzitic hexagonal phase was heteroepitaxially grown for films deposited in the temperature window of 220 to 260 C, well below the typical minimum 450 C temperature used in MOCVD. At an even lower temperature, 183 C, the heteroepitaxial InN on a-plane sapphire was discovered to be cubic phase with a NaCl structure, a phase of InN that had before been unreported.[3] Heteroepitaxial AlN films were grown on GaN/sapphire at a much lower temperature (500 C) than by MOCVD, typically 1100 C or more. Finally, GaN has been included in ALE deposited ternaries in the relatively low temperature window of 250-400 C even as its optimization continues. The ALE grown III-N films have carbon and oxygen contamination that hinders their immediate use in many applications, and improving film purity is a major focus. In addition, initial depositon of indium containing ternaries indicates that more stoichiometries are available by ALE than by MOCVD.

The possibilities for greater use of III-nitrides are apparent even at the early stages of progress in atomic layer epitaxy. Further characterization during and after deposition of the films should lead to materials suitable for use in high electron mobility transistors, as well as optoelectronic devices.

[1] N. Nepal et al., Appl. Phys. Lett. 103 082110 (2013)

[2] N. Nepal et al., Cryst. Growth Des. 13 1485 (2013)

[ 3 ] N. Nepal et al., Appl. Phys. Express 6 061003 (2013)

Wafer equivalent single crystalline silicon thick film is expected as an ideal active layer of the next generation Si solar cells. Fast rate epitaxial deposition process is required for production of such films over a wide area especially to serve giant electronics. Mesoplasma CVD can be one of the potential candidates since one can envision that the plasma in the 0.1–10 Torr range will combine the advantages of both low pressure and thermal plasmas; that is, high processing rates are expected by a direct transport of radicals and/or atoms on a plasma flow while attaining relatively low damage as a result of low electron temperatures. In fact, we have demonstrated deposition of the epitaxial thick films with Hall mobility of ~240 cm²V^-1s^-1 at rates as fast as 700 nm/sec using trichlorosilane (TCS) as a source gas under the mesoplasma condition. Uniqueness of this process also includes that the material yield for Si epitaxy from TCS has reached ~60% while the state-of-the-art SIEMENS process for Si production reaches conventionally ~30%.

In the process of the mesoplasma CVD, source gas is decomposed completely to the atomic state in the plasma flow and is directed toward the substrate. Within the thin thermal boundary layer present ahead of the film growth surface, liquid-like Si clusters are expected to form as deposition precursors during rapid condensation of high temperature Si vapor, as evidenced by the SAXS in-situ detection of formation of Si clusters of which constituent Si atoms are loosely bound [1,2]. Molecular dynamics simulation has also revealed that such a cluster deforms upon impingement on a film growth surface and facilitates instantaneous and spontaneous atom rearrangement to maintain the epitaxial relationship [3]. Owing to its unique cluster characteristics, epitaxial deposition over a wide area of 20 mm × 80 mm on a moving substrate was confirmed even by the injection type CVD with inductively coupled plasma, as far as the substrate temperature is maintained higher than 500 ˚C [4].

Furthermore, non-equilibrium chemical effects of the mesoplasma environment, i.e. freezing-in the high temperature chemistries during rapid condensation and generation of atomic hydrogen, are both found to quite effectively suppress the thermodynamically stable Si-Clx molecule formation at the film deposition region, leading to a significant improvement in the Si production yield [5].

References: [1] Kambara, et al., JAP, 99 (2006) 074901. [2] Diaz, et al., JAP, 104 (2008) 013536. [3] Chen, et al., J. Phys. D, 46 (2013) 425302-1. [4] Wu, et al., Sci. Technol. Adv. Mater.,15 (2014) 035001. [5] Wu, et al., Plasma Chem. Plasma Process, 33, (2013) 433.

Zeolite Y is synthesized in the current study by a simple Sol-Gel technique. We found that the crystal growth was controlled by varying the hydrogel synthesis time and the annealing temperature. The resulting products at various crystallization times and temperatures are studied with X-ray powder diffraction (XRD), Scanning electron microscopy (SEM), High resolution transmission electron microscopy (HRTEM), Polarization/Impedance studies. Microstructure and size in SEM images of the final Zeolite Y annealed at 300^oC revealed the formation of cubic structure. XRD analysis revealed the higher levels of crystallization at varying temperatures. Zeolites Y were dispersed in Poly ethylene glycol (PEG) in the ratio 1:20 and coated on mild steel for the formation of membrane. The membrane consisted of top layer with thickness of 1.3-2.0µm. Crystals in the top layer showed cubic morphology.

Low and high stacking fault energy (SFE) materials such as Cu, Cu alloys (CuAl, CuZn, CuAg and CuNi), Al, and Al alloys (Al5052, Al5456, Al6013, and Al- 5.3 wt.% Mg) were sputtered in order to study nanotwinned (nt) structures. The SFE of the Cu and Cu alloys is relatively low (<45 mJ/m²), while Al has a high SFE of approximately166 mJ/m² and is not expected to yield a nt structure. In order to develop a comprehensive study of the twinning phenomena, control the microstructure, and evaluate a theoretical model in low and high SFE materials, different sputtering conditions, such as high sputtering rates, interrupted sputtering, substrate heating, and substrate chilling have been used. Transmission electron microscopy and focused ion beam microscopy were used to characterize the microstructures. Overall, the Cu-alloys showed highly nt structures, with significantly different twin spacings. The Al alloys were strongly (111) textured and had fully columnar grains, with some systems showing a nt structure.

Low-temperature, low-pressure plasmas play an essential role in the etching and deposition of Si-based materials for the manufacturing of microelectronic devices. As new technologies emerge, recent trends have shifted from Si- to carbon-based materials. In particular, carbon-based nanomaterials such as carbon nanotubes and graphene are of great interest for nanoscale electronic devices. However, carbon is characterized by an enormous range of diversity that creates a significant synthetic challenge. A key challenge is therefore the development of processes that are capable of producing well-defined carbon-based nanomaterials. In this talk, I will present our recent efforts in this area based on our platform technology, atmospheric-pressure microplasmas. I will show that these novel plasma sources are capable of generating size- and compositionally-controlled metal nanoparticles for the catalytic growth of chirally-enriched single-walled carbon nanotubes [1]. Microplasmas can also serve as an atmospheric-pressure source of radicals such as atomic hydrogen to reduce graphene oxide at low temperatures [2]. Recently, we have discovered that hydrocarbon precursors can be dissociated in a microplasma to homogenously nucleate carbon nanoparticles [3]. Careful characterization of the as-synthesized product reveals the presence of nanometer-sized diamond particles (nanodiamonds). I will discuss these results in detail, highlighting the potential advantages of plasma processing for carbon nanomaterial synthesis and modification.

[1] W-H. Chiang and R. M. Sankaran, Nat. Mater.8, 882 (2009).

[2] S. W. Lee and R. M. Sankaran, J. Phys. Chem. Lett. 3, 772 (2012).

[3] A. Kumar et al., Nat. Comm.4, 2618 (2013).

The practical realization of electronic, sensor, solar conversion and other devices made of two-dimensional (2D) materials with semiconductor and dielectric properties critically depends on the availability of suitable synthesis routes. These synthesis routes can allow for reproducible, substrate agnostic, scalable, and cost effective processing technologies. Results from the most recent research in developing physical vapor deposition (PVD) methods to grow few layer thick 2D materials from transition metal dichalcogenides with semiconducting behavior (such as MoS₂ and others) and boron nitride with dielectric behavior are presented. Pulsed DC magnetron sputtering from MoS₂ targets and pulsed laser deposition from BN targets were optimized to produce 2D materials on a variety of substrate materials, such as amorphous silicon oxide and glass, highly oriented sapphire and graphite, as well as flexible polymers at areas of over one square inch. Thermodynamically driven tendency to form islands is overcome by maximizing ad-atom atomic mobility through the control of incident flux ionization state, energies, and densities, while avoiding defect formation (i.e., vacancy creation by sputtering of S atoms). In-situ XPS was used to analyze the film stoichiometry and initial growth stages. Pin-hole and gap free, 2D MoS₂ and BN films of 1-5 nm thickness were produced over 4 cm² areas as confirmed by TEM, conductive AFM, Raman and electrical probe measurements. 2D MoS₂ semiconducting and 2D BN dielectric films characteristics were verified with electrical probe and field effect transistor device studies.

Graphene nanosheet(GNs), consisting of a single layer of sp² carbon atoms arranged in honeycomb structure, with high surface area and high electron mobility values of 15,000 cm²V⁻¹s⁻¹ at room temperature, has attracted tremendous attentions in replacing graphite and to be used in anode active material of lithium-ion batteries(LIBs). α-type MnO₂(α-MnO₂) has attracted great interest as anode materials in LIBs for their high theoretical capacity, environmentally compatible, low cost, and special properties. The prospect of combining α-MnO₂ and reduced graphene oxide(rGO) to obtain a synergistic effect of their respective merits is under consideration as a possible strategy for obtaining improved anode materials for high-power LIB applications. In this study, α-MnO₂/rGO nanocomposites was synthesized via a facile one-step chemical route. Aqueous suspension containing MnCl₂˙4H₂O(α-MnO₂ precursor) with graphene oxide(GO) sheets was reacted at 83 °C for 30 min. The morphology and structure of as-synthesized nanocomposites were analyzed by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. It was found that due to the dissolution-crystallization and oriented attachment mechanisms, which enhanced the chemical interaction between GO and α-MnO₂. In Li-ion batteries operating, rGO played role as an electronic conductive buffer layers to suppress the volume change of α-MnO₂ and enhance the charge-transfer performance, and α-MnO₂ played role as not only a spacer to suppress agglomeration and restacking of rGO but also redox site for enhancement of Li-storage capacity and ionic diffusion rate.

SnS of orthorhombic (OR) and zincblende (ZB) phases were synthesized in a simple and facile colloidal method. The tin precursor was synthesized using tin oxide (SnO) and oleic acid (OA) while the sulfur precursor was prepared using sulfur powder (S) and oleyamine (OLA). The sulfur precursor was then injected quickly into the tin precursor and then the SnS nanocrystals were precipitated at a final reaction temperature of 180°C. The results show that the hexamethyldisilazane (HMDS) can be successfully used as a surfactant to synthesize monodispersed 20 nm zinc blende phase SnS nanoparticles; on the other hand, irregular nanosheet orthorhombic phase SnS were obtained without HMDS.The optical properties of the direct and indirect band gap of orthorhombic SnS film are 1.25eV and 1.24 eV, respectively. Zinc blende phase SnS film has a higher direct and indirect energy band gap with 1.61eV and 1.26eV, respectively. The nanocrystal SnS films were prepared by spin coating on the FTO substrates for photovoltaic measurement. The photocurrent density of the SnS (ZB)/FTO and SnS (OR)/FTO are 57.61 μA/cm² and 191.8 μA/cm², respectively. at -1V applied voltage at 150W from a Xe lamp.

Dodecane (C₁₂H₂₆) is used as a solvent or starting material for manufacturing of many organic syntheses and recently considerable interest was paid to dodecane as a possible alternative to petroleum-based jet fuel such as Jet-A, S-8, and other conventional aviation fuels. Dodecane is highly flammable with low explosive limit so continuous monitoring of its concentration in air is necessary.

Zinc oxide as n-type semiconductor has attracted the interest of many scientists and have been the subject of intensive investigations as gas sensors mainly because of its high electrical conductivity, excellent catalytic activity and chemical stability. In principle, electrical conductivity of n-type semiconductor increases (or decreases) when reducing (or oxidizing) gases are adsorbed on its surface. The performance of the active layers in a number of modern devices, and especially gas sensors, is strongly depended on their specific surface area and could be improved by tailoring of their morphologies.

In this paper, we investigate the influence of sputtering parameters like incidence angle and substrate temperature for obtaining different film morphologies (dense, porous, nano-wires and nano-trees). After a short description of the experimental device used for the deposition stage and the hydrocarbon sensing bench, a first part will be dedicated to the chemical, microstructural and structural characterization (SEM, XRD, …) of the coatings in relation with their deposition parameters. Hall effect measurements will be done to determine the electrical resistivity and the carriers concentration and mobility of the films as a function of their morphology. Finally, the sensing performance of these coatings as dodecane-sensor will be discussed depending on dodecane concentrations and sensitive surface’s temperature.

Keywords: Sputtering - Semiconductor - ZnO nanostructure films - Gas sensor

Southwest Research Institute, with teaming partner Exothermics Inc., have been developing a hybrid processing method to fabricate macroscaled, layered nanocomposite materials that mimic the structure of nacre. Current research in the biomimetic field is pointing towards importance of two governing mechanisms that contribute towards energy dissipation in nacre: 1) platelet interlocking features and 2) strong adhesion of a ductile organic phase to the mineral plates. In our current study, we employed a vacuum roll coating process to prepare nano-engineered alumina inorganic platelets with precisely controlled size, shape and morphology of designed interlocking features through top-down physical vapor deposition of multilayer films on embossed substrates. Then a novel High Power Impulse Plasma Source (HiPIPS) was employed in a roll-to-roll, layer-by-layer method to surface functionalize alumina platelets in order to modify interfacial strength with an organic matrix. Subsequently, a few industrial processing methods including slip casting and tape casting were evaluated to fabricate macroscale layered materials with desired in-plane alignment of platelets, physical packing characteristics and percolation behavior when compounded. The microstructure of all compositions of the fabricated nacre-like materials was examined using scanning electron and optical microscopy, while the mechanical properties were analyzed using three-point bend tests. In this presentation, an overview of hybrid processing method and resulting materials will be given with a specific focus on vacuum roll coating, HiPIPS and areas for further development.

Carbon films in different type of binding configurations lead to their applications in a myriad of fields such as optics, biology, tribology and nanomachinery. The amorphous structured carbon films has shown outstanding mechanical properties, while the sp² hybridized nanocrystallites play an important role on the electrical, thermal, and even magnetic behaviors of carbon films. In order to meet the demand for surface conduction and other potential electronic applications, novel type of carbon film is expected with the combination of good mechanical property from amorphous structure and high conductivity from nanocrystallite structure.