Chun-Wei Chang

Effect of Nonsolvent on the Formation of Polymer Nanomaterials in the Nanopores of Anodic Aluminum Oxide Templates

We study the effect of nonsolvent on the formation of polymer nanomaterials in the nanopores of porous templates. Water (nonsolvent) is added into a poly (methyl methacrylate) (PMMA) solution in dimethylformamide (DMF) confined in the nanopores of an anodic aluminum oxide (AAO) template. Water forms a wetting layer on the pore wall and causes the PMMA solution to be isolated in the center of the nanopore, resulting in the formation of PMMA nanospheres or nanorods after the solvent is evaporated. The formation of the polymer nanomaterials induced by nonsolvent is found to be driven by the Rayleigh-instability-type transformation. Without adding the nonsolvent, PMMA chains precipitate on the walls of the nanopores after the solvent is evaporated, and PMMA nanotubes are obtained.

PROPERTIES OF PLASMA-ENHANCED CHEMICAL-VAPOR-DEPOSITED A-SINX-H BY VARIOUS DILUTION GASES

The effects of dilution gases on hydrogenated amorphous silicon nitride (a‐SiNx:H) films were investigated. Silane and ammonia were used as the reactive species, while nitrogen, helium, hydrogen, and argon were used as the dilution gases in a plasma‐enhanced chemical vapor‐deposition system at a substrate temperature of 300 °C. The electrical, physical, and chemical properties of the a‐SiNx:H films were found to be highly sensitive to the various kinds and flow rates of the carrier gases in the deposition. Additionally, the physical properties of growth rate, refractive index, and etching rate were also investigated. The hydrogen bonding configuration was explored by infrared spectroscopy. The total hydrogen concentrations for all a‐SiNx:H films were observed to be smaller than 3.0×1022 cm−3. The electrical properties were characterized by I‐V and C‐V measurements in metal‐insulator‐semiconductor structures. The breakdown strength was determined at the current density of 3 mA/cm2; in addition, the dominan...

The study of emitter thickness effect on the heterostructure emitter bipolar transistors

AlGaAs/GaAs heterostructure emitter bipolar transistors were grown with emitter thicknesses varied from 300 to 900 A and the emitter thickness effects on the current gain and offset voltage were studied. It was found that both the current gain and offset voltage are strongly dependent on the emitter thickness. The current gain decreases with increasing emitter thickness. For an emitter thickness smaller than 500 A, the offset voltage decreases with increasing emitter thickness, but for an emitter thickness larger than 500 A, the offset voltage stays at a nearly constant value. Offset voltage as low as 55 mV was obtained for an emitter thickness of 700 A. This low offset voltage, mostly contributed by the geometric effect, indicates that the base‐emitter junction is a homojunction. From the current gain and offset voltage considerations, the optimal emitter thickness was found to be about 300–500 A.

Microwave-annealing-induced nanowetting: a rapid and facile method for fabrication of one-dimensional polymer nanomaterials

Template wetting methods have been broadly applied to fabricate diverse one-dimensional polymer nanomaterials. The currently used template wetting methods, however, have shortcomings and disadvantages such as long processing times, thermal degradation, and difficulties in controlling the lengths. In this work, we develop a novel microwave-annealing-induced nanowetting (MAIN) method to fabricate one-dimensional polymer nanomaterials using porous anodic aluminum oxide (AAO) templates. Upon microwave annealing, the polymer chains are infiltrated into the nanopores of the AAO templates, and the morphologies of the polymer nanomaterials can be controlled by the annealing conditions. The growth rates of the polymer nanomaterials using the MAIN method are faster than those using the traditional thermal annealing method. This work not only provides a time-saving method to fabricate one-dimensional polymer nanomaterials with controlled morphologies, but also offers a better understanding of the effect of microwave annealing on the wetting behaviors of polymer melts.

ANALYSIS OF DIFFERENTIAL GAIN IN GAAS/ALGAAS QUANTUM-WELL LASERS

The differential gain of a quantum‐well laser is studied theoretically with use of both a parabolic band model and a valence‐band‐mixing model. In the valence‐band‐mixing model, the gain profile is derived from the multiband effective mass theory (k⋅p method) as well as the density matrix formalism. The peak gain including the band‐mixing effect is significantly reduced to 1.5–2 times when compared to the conventional parabolic band model. There is still a larger differential gain using the parabolic band model than using the band‐mixing model. The magnitudes of differential gains for these two models give the order of 10−16–10−15 cm2, which is in agreement with the experimental results. Besides, the quantum‐well thickness also influences the differential gain, which is enhanced by a thinner quantum‐well structure.

Morphology control of three-dimensional nanostructures in porous templates using lamella-forming block copolymers and solvent vapors

The microphase separation behavior of block copolymers confined in cylindrical nanopores has been extensively investigated. Recently, the solvent-annealing-induced nanowetting in templates (SAINT) method has been demonstrated to be a versatile approach for the infiltration of block copolymers into the nanopores of porous templates. The function of the annealing solvents, however, is still not well understood, especially in the morphology control of the fabricated block copolymer nanostructures. In this work, we elucidate the function of the annealing solvents in the SAINT method using a lamella-forming block copolymer, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), and anodic aluminum oxide (AAO) templates. By changing the composition of the annealing solvents, different morphologies such as the concentric lamellar morphology, the winding cylinder morphology, and the irregular hybrid morphology are observed, mainly caused by the annealing-solvent-induced volume change. The morphology of the block copolymer nanostructures can be further confirmed using an HF solution to remove the PDMS domain selectively.

Selective Template Wetting Routes to Hierarchical Polymer Films: Polymer Nanotubes from Phase-Separated Films via Solvent Annealing

We demonstrate a novel wetting method to prepare hierarchical polymer films with polymer nanotubes on selective regions. This strategy is based on the selective wetting abilities of polymer chains, annealed in different solvent vapors, into the nanopores of porous templates. Phase-separated films of polystyrene (PS) and poly(methyl methacrylate) (PMMA), two commonly used polymers, are prepared as a model system. After anodic aluminum oxide (AAO) templates are placed on the films, the samples are annealed in vapors of acetic acid, in which the PMMA chains are swollen and wet the nanopores of the AAO templates selectively. As a result, hierarchical polymer films containing PMMA nanotubes can be obtained after the AAO templates are removed. The distribution of the PMMA nanotubes of the hierarchical polymer films can also be controlled by changing the compositions of the polymer blends. This work not only presents a novel method to fabricate hierarchical polymer films with polymer nanotubes on selective regions, but also gives a deeper understanding in the selective wetting ability of polymer chains in solvent vapors.

Fabrication of Multicomponent Polymer Nanostructures Containing PMMA Shells and Encapsulated PS Nanospheres in the Nanopores of Anodic Aluminum Oxide Templates

Multi-component polymer nanomaterials have attracted great attention because of their applications in areas such as biomedicine, tissue engineering, and organic solar cells. The precise control over the morphologies of multi-component polymer nanomaterials, however, is still a great challenge. In this work, the fabrication of poly(methyl methacrylate)(PMMA)/poly-styrene (PS) nanostructures that contain PMMA shells and encapsulated PS nanospheres is studied. The nanostructures are prepared using a triple solution wetting method with anodic aluminum oxide (AAO) templates. The nanopores of the templates are wetted sequentially by PS solutions in dimethylformamide (DMF), PMMA solutions in acetic acid, and water. The compositions and morphologies of the nanostructures are controlled by the interactions between the polymers, solvents, and AAO walls. This work not only presents a feasible method to prepare multi-component polymer nanomaterials, but also leads to a better understanding of polymer-solvent interactions in confined geometries.

NANOMETER THICK SI/SIGE STRAINED-LAYER SUPERLATTICES GROWN BY AN ULTRAHIGH-VACUUM CHEMICAL-VAPOR-DEPOSITION TECHNIQUE

High quality Si/Si1−xGex superlattices having layers as thin as 1.5 nm have been grown by an ultrahigh vacuum/chemical vapor deposition system. High‐resolution double‐crystal x‐ray diffraction, and conventional and high‐resolution cross‐sectional transmission electron microscopy were used to evaluate the crystalline quality of these superlattices. A dynamical x‐ray simulation program was employed to analyze the experimental rocking curves. Excellent matches between experimental rocking curves and simulated ones were obtained for all superlattices with various periodicity. A cross‐sectional transmission electron micrograph of an 80 period Si(4.2 nm)/Si0.878Ge0.122 (1.5 nm) superlattice, in which each individual layers was clearly resolved, demonstrated the capability of this growth technique for nanometer thick layer deposition.

Characterization of the Si/SiGe heterojunction diode grown by ultrahigh vacuum chemical vapor deposition

The unipolar Si/SiGe heterojunction diode grown by ultrahigh vacuum chemical vapor deposition at 550 °C is demonstrated. The dark current density measured at 77 K is (2.5±0.1)×10−7 A/cm2 for the barrier height of 176±8 meV, at a reverse bias of 1 V. The barrier heights are measured from the activation analysis of the saturation current and compared to the theoretical values. The barrier height decreases as the thickness of the SiGe strained layer exceeds the critical thickness.