Chang Fu Dee

Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires

Silicon/zinc oxide (Si/ZnO) core-shell nanowires (NWs) were prepared on a p-type Si(111) substrate using a two-step growth process. First, indium seed-coated Si NWs (In/Si NWs) were synthesized using a plasma-assisted hot-wire chemical vapor deposition technique. This was then followed by the growth of a ZnO nanostructure shell layer using a vapor transport and condensation method. By varying the ZnO growth time from 0.5 to 2 h, different morphologies of ZnO nanostructures, such as ZnO nanoparticles, ZnO shell layer, and ZnO nanorods were grown on the In/Si NWs. The In seeds were believed to act as centers to attract the ZnO molecule vapors, further inducing the lateral growth of ZnO nanorods from the Si/ZnO core-shell NWs via a vapor-liquid-solid mechanism. The ZnO nanorods had a tendency to grow in the direction of [0001] as indicated by X-ray diffraction and high resolution transmission electron microscopy analyses. We showed that the Si/ZnO core-shell NWs exhibit a broad visible emission ranging from 400 to 750 nm due to the combination of emissions from oxygen vacancies in ZnO and In2O3 structures and nanocrystallite Si on the Si NWs. The hierarchical growth of straight ZnO nanorods on the core-shell NWs eventually reduced the defect (green) emission and enhanced the near band edge (ultraviolet) emission of the ZnO.

Porous (001)-faceted Zn-doped anatase TiO2 nanowalls and their heterogeneous photocatalytic characterization

The synthesis of a poriferous and high energy (001) faceted anatase Zn-doped TiO2 nanowall (ZnTNW), vertically grown on an indium tin oxide substrate, is presented. The ZnTNW was prepared using a modified liquid phase deposition method using zinc nitrate (Zn(NO3)2·xH2O) as a fluoride scavenger in the presence of hexamethylenetetramine. In a typical procedure, the ZnTNW nanowall with length and thickness of approximately 2 μm and 60 nm, respectively, can be obtained from the reaction during a 5 h growth process. X-ray diffraction analysis shows that the nanowall has an anatase structure with a dominant high energy (001) basal plane. Meanwhile, the X-ray energy dispersive analysis confirms the presence of Zn in the TiO2 nanowall. High resolution transmission electron microscopy analysis results reveal, surprisingly, that the ZnTNW is single crystalline in nature although it has a highly porous (surface and bulk) structure. Photocatalytic properties of the ZnTNW were examined in the degradation of methylene blue. It was found that the ZnTNW exhibits excellent photocatalytic efficiency with kinetic reaction rate, turnover number and turnover frequency as high as 0.004 min−1, 760 and 11 min−1, respectively. The photocatalytic performance of the ZnTW was found to be higher for about 10% and 50% than the pristine TiO2 nanowalls and (001) faceted poriferous TiO2 microtablet, which reflected the effective effect of the Zn doping. The ZnTNW may find potentially use in photocatalytic heterogeneous applications.

Structure deformation of indium oxide from nanoparticles into nanostructured polycrystalline films by in situ thermal radiation treatment

A microstructure deformation of indium oxide (In2O3) nanoparticles by an in situ thermal radiation treatment in nitrous oxide plasma was investigated. The In2O3 nanoparticles were completely transformed into nanostructured In2O3 films upon 10 min of treatment time. The treated In2O3 nanoparticle sample showed improvement in crystallinity while maintaining a large surface area of nanostructure morphology. The direct transition optical absorption at higher photon energy and the electrical conductivity of the In2O3 nanoparticles were significantly enhanced by the treatment.

Threading Dislocation Blocking in Metamorphic InGaAs/GaAs for Growing High-Quality In0.5Ga0.5As and In0.3Ga0.7As on GaAs Substrate by Using Metal Organic Chemical Vapor Deposition

High quality In0.3Ga0.7As and In0.51Ga0.49As epilayers have been successfully grown on the GaAs substrate by MOCVD. A cross-sectional study by transmission electron microscopy showed that the threading dislocations (TDs) have been successfully contained and limited within the buffer layers designed to stop the elongation of TDs into the In0.3Ga0.7As and In0.51Ga0.49As epilayers. A TD density of 1?106 cm-2 in a fully relaxed In0.51Ga0.49As epilayer was achieved. The measurement of lifetimes of n- and p-type In0.51Ga0.49As has been done by using time-resolved photoluminescence. A great reduction in the number of recombination centers in the InGaAs epilayer has been shown.

Hierarchical Si/ZnO trunk-branch nanostructure for photocurrent enhancement

Hierarchical Si/ZnO trunk-branch nanostructures (NSs) have been synthesized by hot wire assisted chemical vapor deposition method for trunk Si nanowires (NWs) on indium tin oxide (ITO) substrate and followed by the vapor transport condensation (VTC) method for zinc oxide (ZnO) nanorods (NRs) which was laterally grown from each Si nanowires (NWs). A spin coating method has been used for zinc oxide (ZnO) seeding. This method is better compared with other group where they used sputtering method for the same process. The sputtering method only results in the growth of ZnO NRs on top of the Si trunk. Our method shows improvement by having the growth evenly distributed on the lateral sides and caps of the Si trunks, resulting in pine-leave-like NSs. Field emission scanning electron microscope image shows the hierarchical nanostructures resembling the shape of the leaves of pine trees. Single crystalline structure for the ZnO branch grown laterally from the crystalline Si trunk has been identified by using a lattice-resolved transmission electron microscope. A preliminary photoelectrochemical (PEC) cell testing has been setup to characterize the photocurrent of sole array of ZnO NR growth by both hydrothermal-grown (HTG) method and VTC method on ITO substrates. VTC-grown ZnO NRs showed greater photocurrent effect due to its better structural properties. The measured photocurrent was also compared with the array of hierarchical Si/ZnO trunk-branch NSs. The cell with the array of Si/ZnO trunk-branch NSs revealed four-fold magnitude enhancement in photocurrent density compared with the sole array of ZnO NRs obtain from VTC processes.

Effect of Graded AlxGa1-xN Layers on the Properties of GaN Grown on Patterned Si Substrates

2.2-µm-thick crack-free GaN films were grown on patterned Si substrates. The crack-free GaN films were obtained by patterning Si substrate and optimizing the graded AlxGa1-xN layers. With the increase of the graded AlxGa1-xN layer thickness, the GaN crystal quality improved as judged from the X-ray diffraction data. By applying multi-AlxGa1-xN layers on the patterned Si substrate, a 31% reduction of tensile stress for the GaN film was obtained as measured by micro-Raman. For the AlGaN/GaN high electron mobility transistor grown on 1×1 cm2 larger patterns, the device exhibits maximum drain current density of 776 mA/mm and maximum transconductance of 101 mS/mm.

A new analytical model for lateral breakdown voltage of double-gate power MOSFETs

A simple analytical model for the surface potential, surface field distribution and lateral breakdown voltage of double-gate power MOSFETs has been proposed. The model is based on the analytical solution for the two-dimensional Poisson equation. From this solution, the reliance of breakdown voltage on the device parameters is also investigated. The validity of this model is demonstrated by comparison with numerical simulation and experimental values.

Single reactor deposition of silicon/tungsten oxide core-shell heterostructure nanowires with controllable structure and optical properties†

We report the controllable growth of silicon/tungsten oxide (Si/WO3) core–shell heterostructure nanowires via a two-step route using a home-built plasma-assisted hot-wire chemical vapour deposition reactor. Uniform coating of WO3 shell indicates a clear preference for growth on the single crystalline Si nanowires. Structure and crystallinity of the WO3 shell are strongly dependent on the filament temperature (Tf). X-ray diffraction patterns and micro-Raman spectra suggested that there was a structure evolution from amorphous into crystallite WO3 monoclinic structure when Tf was increased to 1300 °C and above. The WO3 shell exhibits stoichiometric tungsten trioxide structure as identified by micro-Raman and X-ray photoemission spectra analyses, which showed only W6+–O vibration modes for the former analysis and W6+ energy band for the latter. Microstructure, crystal lattice, interface and growth orientation of the core–shell nanowires were recognized using a high resolution transmission electron microscopy. Our results showed that the core–shell nanowires had preserved the optical transmittance of Si core at a longer wavelength, while showing an additional transmission band edge at a shorter wavelength due to tungsten oxide coating. Their optical absorption increased to 80% and above in visible region, owing to the one-dimensional Si NWs backbone. Optical band gap of the core–shell nanowires showed a variation from 2.4 to 1.8 eV with Tf. This superior visible light absorption core–shell nanowires architecture subsequently enhanced the photocurrent density of the crystalline WO3 nanostructures.

Reliability study of high-κ La2O3/HfO2 and HfO2/La2O3 stacking layers on n-In0.53Ga0.47As metal-oxide-semiconductor capacitor

This study investigates the time-dependent dielectric breakdown (TDDB) characteristics of La2O3/HfO2 and HfO2/La2O3 stacking layers on an n-In0.53Ga0.47As metal–oxide–semiconductor capacitor. Both designs improved the reliability compared with a single layer of HfO2. The TDDB followed the thermochemical E model. The current transportation mechanism changed from thermionic emission to Frenkel–Poole emission because of the traps creation under voltage stress. Both designs resulted in similar lifespans and voltage accelerating factors. However, the La2O3/HfO2 design had a longer lifespan because of the lower interface trap density and insertion of the HfO2 diffusion barrier layer between La2O3 and n-In0.53Ga0.47As. The oxide stacks exhibited excellent reliability and achieved a lifespan of 28.4 years.

Low resistance copper-based ohmic contact for AlGaN/GaN high electron mobility transistors

Ti/Al/Ni/Cu ohmic contact for AlGaN/GaN structure has been fabricated. The Ni layer played an important role in achieving low specific contact resistance (rc), smooth morphology, and excellent edge acuity. With a 50-A Ni layer, a rc of 1.35 × 10−6 Ω-cm2 and a root-mean-square roughness of 7.65 nm have been realized. The characterization results indicated that no evidence of Cu diffusion into the semiconductor layers. The formation of Al-Cu and Ti-Cu alloys might have confined the Cu within the ohmic metal. In the absence of gold, the surface roughening caused by Au-Al alloy in conventional Ti/Al/Ni/Au structure was also prevented.