JunSeok Lee

p‐Type Conduction Characteristics of Lithium‐Doped ZnO Nanowires

Nanostructured electronic devices are expected to facilitate the continuing miniaturization of electronic devices and enable ultralow power device operation. In particular, 1D nanostructures such as nanowires (NW) and nanotubes have attracted great interest over the past decade because of their specifi c physical properties and their potential as building blocks for next-generation nanoelectronic devices. [ 1 , 2 ] Among various 1D materials, zinc oxide (ZnO), which has a direct and wide bandgap, is a promising candidate for light-emitting diodes, UV and gas sensors, transistor channels, and other devices that can utilize the unique vertical alignment characteristics and highly ordered single crystalline properties of NW structures. [ 3–6 ] However, because undoped ZnO NWs are intrinsically n-type, their use in practical devices has been hindered and much effort has been dedicated toward the development of p-type ZnO NWs. In particular, control and manipulation of the doping process is increasingly becoming a key approach for the realization of p-type ZnO NWs. To realize p-type ZnO NWs, the initial dopant candidates tested included group V elements to substitute for O and group III elements to substitute for Zn, despite the large size mismatches in both cases. Recently, group I species such as Li and Na have been used to synthesize p-type ZnO NWs based on the expectation that these elements would function as shallow acceptors in ZnO host materials. [ 7–9 ] Li has the smallest ionic radius (0.76 Å) of group I species, which is very close to that of Zn (0.74 Å). Furthermore, several reports of excited centers observed using electron paramagnetic resonance spectroscopy have indicated that Li atoms can act as shallow acceptors in substantial forms of Zn sites (Li Zn ). [ 10 , 11 ] In addition, it is well-known that Li has specifi c advantages over other dopant

Memory characteristics of cobalt-silicide nanocrystals embedded in HfO2 gate oxide for nonvolatile nanocrystal flash devices

Cobalt-silicide (CoSi2) nanocrystals (NCs) were investigated for use in charge storage for metal oxide semiconductor (MOS) devices with thin HfO2 tunneling and control oxide layers. CoSi2 NCs were synthesized by exposure of Co∕Si∕HfO2 tunneling oxide/Si stacks to an external UV laser. Observations from transmission electron microscopy and x-ray photoelectron spectroscopy clearly confirm the formation of CoSi2 NCs and the values of Co–Si bonding energies that are shifted 0.3eV from original values, respectively. The CoSi2 NCs in MOS devices exhibited a large memory window of 3.4V as well as efficient programming/erasing speeds, good retention, and endurance times.

In situ compatibilization of PET/PS blends through carbamate-functionalized reactive copolymers

To investigate the effect of reactive compatibilization in the immiscible poly(ethylene terephthalate) (PET)/polystyrene (PS) blend, poly(styrene-co-methacryloyl carbamate) (PSM) was synthesized as a reactive compatibilizer. The interfacial reaction of the carbamate group in PSM with OH/COOH in PET was confirmed by atomic force microscopy. The interfacial roughness developed rapidly with an increase in the methacryloyl carbamate (MAC) content and then leveled off above the optimum content (3.8 wt %). These results were well-reflected in the interfacial adhesion, morphology, and mechanical properties of the PET/PS blends, showing a maximum value at the optimum MAC content. The existence of a maximum value is believed to stem from a reciprocal relationship between the sufficient formation of in situ copolymer and the fast diffusion rate of reactive polymers at the interface.

Photoluminescence Analysis of Energy Level on Li-Doped ZnO Nanowires Grown by a Hydrothermal Method

The optical and structural properties of Li-doped ZnO nanowires grown by a hydrothermal method are reported herein. The low-temperature and temperature-dependent photoluminescence spectra clearly exhibited emission peaks that confirmed the presence of a lithium impurity as an acceptor dopant. Particularly, the acceptor energy level of the Li dopant was estimated to be 121 meV from the PL spectra. This value was also indicated from an Arrhenius plot of the integrated PL intensity of the A°X emission as a function of temperature. These results are in agreement with theoretical and experimental results of previously considered p-type dopants reported in other studies.

Charge trap memory characteristics of AlOx shell-Al core nanoparticles embedded in HfO2 gate oxide matrix

The memory behavior of natively oxidized AlOx shell-Al nanoparticles (NPs) in a metal oxide semiconductor (MOS) structure was investigated. Transmission electron microscopy images clearly demonstrate the formation of an AlOx shell (thicknesses of 1–1.5 nm), surrounding Al (sizes of 5–7 nm) core NPs in the MOS structure. Electrical measurements exhibited a memory window of 3.6 V, together with a promising charge retention time of about 10 years. A possible band model needed for enhanced retention characteristics was given by considering the electron/hole barrier width and the additional interface states through the AlOx shell as a method of tunneling barrier engineering.

Electrostatic force microscopy measurements of charge trapping behavior of Au nanoparticles embedded in metal- insulator- semiconductor structure

Charge trapping properties of electrons and holes in Au nanoparticles embedded in metal-insulator-semiconductor (MIS) on p-type Si (100) substrates were investigated by electrostatic force microscopy (EFM). The Au nanoparticles were prepared with a unique laser irradiation method and charged by applying a bias voltage between EFM tip and sample. The EFM system was used to image charged areas and to determine quantitatively the amount of stored charge in the Au nanoparticle-inserted MIS structure. In addition, charge trapping characteristics of the samples were carried out with electrical measurements, such as capacitance-voltage and current-voltage measurement for memory characteristics. Finally, the comparison of EFM results with the electrically measured data was done to determine the amount of stored charge in the Au nanoparticle-inserted MIS structure, confirming the usefulness of EFM system for the characterization of nanoparticle-based non-volatile devices.

Investigation of transparent conducting oxide/Si junction for the emitter wrap through solar cells

We have investigated properties of ITO/Si with shallow doped emitter for the increasing fill factor of emitter wrap through (EWT) solar cells. The ITO is prepared by DC magnetron sputter on p-type mono-crystalline silicon substrate. The contact resistance of ITO/Si with shallow doped emitter was measured by the transmission length method (TLM). As an experimental result, the contact resistance and the series resistance of ITO/Si with shallow doped emitter were obtained 0.0705 Ωcm2 and 0.1821 Ωcm2, respectively. As a result of analysis by the contact resistance for optimization of ITO layer, the fill factor of EWT solar cells expected to increase above 80%.

Improvements of optical, mechanical, and geometrical properties of DVD-RAM substrate

It is necessary to improve optical, geometrical, and mechanical properties in the optical disk substrates as the information storage devices with high storage density using short wavelength lasers are being developed. However, the conventional injection molding process is no longer appropriate for producing the high-density information storage optical disk substrates with superb optical, geometrical, and mechanical properties. It is, therefore, inevitable to develop new concepts for the molding processes. In the present study, DVD-RAM substrates were fabricated by injection-compression molding, which is regarded as the most suitable process to manufacture optical disk substrates. The effects of various processing conditions on the birefringence distribution, the land-groove structure including the groove depth and the surface roughness of the land, and the radial tilt were examined experimentally. It was found that the birefringence, which is regarded as one of the most important optical properties for optical disk, was very sensitive to the mold wall temperature history. Also, the integrity of the replication, represented by the land-groove structure, and the radial tilt were influenced mostly by the mold temperature and the compression pressure.