Won Jong Lee

Versatile Carbon Hybrid Films Composed of Vertical Carbon Nanotubes Grown on Mechanically Compliant Graphene Films

[*] Prof. S. O. Kim, D. H. Lee, J. E. Kim, T. H. Han, J. W. Hwang, Prof. S. W. Jeon, Prof. S. H. Hong, Prof. W. J. Lee Department of Materials Science and Engineering, KAIST Daejeon 305-701 (Republic of Korea) E-mail: sangouk.kim@kaist.ac.kr Dr. S. Y. Choi Convergence Components and Materials Laboratory Electronics and Telecommunication Research Institute (ETRI) Daejoen 305-700 (Republic of Korea)

Flexible room-temperature NO2 gas sensors based on carbon nanotubes/reduced graphene hybrid films

We present a flexible room temperature NO2 gas sensor consisting of vertical carbon nanotubes (CNTs)/reduced graphene hybrid film supported by a polyimide substrate. The reduced graphene film alone showed a negligible sensor response, exhibiting abnormal N–P transitions during the initial NO2 injection. A hybrid film, formed by the growth of a vertically aligned CNT array (with CNTs 20u2002μm in length) on the reduced graphene film surface, exhibited remarkably enhanced sensitivities with weak N–P transitions. The increase in sensitivity was mainly attributed to the high sensitivity of the CNT arrays. The outstanding flexibility of the reduced graphene films ensured stable sensing performances in devices submitted to extreme bending stress.

Development and Evaluation of 3-D SiP with Vertically Interconnected Through Silicon Vias (TSV)

For high density and performance of microelectronic devices, the 3-D system in package (SiP) has been considered as a superb microelectronic packaging system. The development and evaluation of stacked chip type 3-D SiP with vertically interconnected TSV are reported. The process includes; 55 mum-diameter via holes by reactive ion etching (RIE), SiO2 dielectric layer by thermal oxidation, Ta and Cu seed layers by ionized metal plasma (IMP), Cu via filling by electroplating, Cu/Sn bump for multi-chip stacking and finally chip-to-PCB bonding with Sn-3.0Ag-0.5Cu solder and ENIG pad. A prototype 3-D SiP with 10 stacked chips was successfully fabricated. High frequency electrical model of the TSV was proposed and the model parameters were extracted from the measured S-parameters. The proposed model was verified by TDR/TDT (time domain reflectometry/time domain transmission) and eye-diagram measurement. And then, contact resistances of Cu via and bump joint were discussed.

Highly Efficient Vertical Growth of Wall-Number-Selected, N-Doped Carbon Nanotube Arrays

We demonstrate a straightforward approach for rapid growth of wall-number selected, N-doped CNT arrays. Highly uniform nanopatterned iron catalyst arrays were prepared by tilted deposition through block copolymer nanotemplates. PECVD growth of CNTs from the nanopatterned catalysts in an NH(3) environment generated vertical N-doped CNTs with a fine-tunability of their carbon wall numbers. The optimized growth conditions produced 52 microm long N-doped CNTs within 1 min. Owing to N-doping, the wall-number selected CNTs including DWNTs and TWNTs demonstrated enhanced electro-conductivity and chemical functionality. This remarkably fast growth of highly uniform N-doped CNTs, whose material properties and chemical functionalizability are reinforced by N-doping, offers a new area of large-scale nanofabrication, potentially useful for diverse nanodevices.

Flexible Field Emission of Nitrogen-Doped Carbon Nanotubes/Reduced Graphene Hybrid Films

The outstanding flexible field emission properties of carbon hybrid films made of vertically aligned N-doped carbon nanotubes grown on mechanically compliant reduced graphene films are demonstrated. The bottom-reduced graphene film substrate enables the conformal coating of the hybrid film on flexible device geometry and ensures robust mechanical and electrical contact even in a highly deformed state. The field emission properties are precisely examined in terms of the control of the bending radius, the N-doping level, and the length or wall-number of the carbon nanotubes and analyzed with electric field simulations. This high-performance flexible carbon field emitter is potentially useful for diverse, flexible field emission devices.

Effects of oxygen partial pressure on the microstructure and electrical properties of indium tin oxide film prepared by d.c. magnetron sputtering

We prepared ITO films using d.c. magnetron sputtering and investigated effects of oxygen partial pressure on the microstructure and the electrical properties of the films. The ITO films deposited at low oxygen partial pressure showed resistivity of 2 × 10−4Ω cm and optical transmittance of about 90%. The resistivity increased as the oxygen partial pressure increased. The preferred orientation of the film was changed as the oxygen partial pressure was changed. The films deposited at high oxygen partial pressure consisted of relatively large grains, but those deposited at low oxygen partial pressure consisted of two distinctive features: aggregate of small grains and that of long grains. Possible cause of the observations was speculated using the oxygen vacancy concentration.

Investigation of Pt/Ti Bottom Electrodes for Pb(Zr,Ti)O 3 Films

Pt films, as bottom electrodes for PZT capacitors, were deposited on Ti/SiO2/Si substrates using DC magnetron sputtering under various deposition conditions. The effects of post-annealing on surface morphology, element diffusion, microstructure and structural phase of the Pt films were investigated. The structure and electrical properties of PZT films deposited on Pt/Ti/SiO2/Si electrodes were also studied. As the deposition temperature of the Pt film increases and the deposition rate decreases, the film becomes dense so that Ti out-diffusion and film deformation are suppressed. The out-diffused Ti faciliates the formation of nucleation sites for perovskite PZT films. However, excess Ti out-diffusion not only decreases the total capacitance of the PZT films due to formation of an interfacial layer having a low dielectric constant but also degrades the leakage current characteristics of the PZT films due to deformation of the Pt electrode.

Epitaxial growth of sol-gel PLZT thin films

Epitaxial lead lanthanum zirconate titanate [PLZT(9/50/50)] thin films were fabricated on various single crystal substrates using the spin coating of metallo-organic solutions. The films were heat-treated at 700 °C for 1 h using the direct insertion method. The films were epitaxially grown with (100), (100), and (110) being parallel to the SrTiO 3 (100), the MgO(100), and the sapphire (01 1 2) substrates, respectively. The epitaxy of the films was investigated using x-ray diffraction, pole figures, rocking curves, and scanning electron microscopy.

Microstructure and Electrical Properties of Tantalum Oxide Thin Film Prepared by Electron Cyclotron Resonance Plasma-Enhanced Chemical Vapor Deposition

High-quality tantalum oxide thin film was prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) for high-density memory devices. The tantalum oxide film deposited at 205°C showed excellent electrical properties: E bd= 4.4 MV/cm, e (Ta2O5)=25 and J< 1×10-9 A/cm2 at 2.5 V. The deposited film was annealed at various temperatures in an oxygen ambient. The microstructure and the composition of the annealed tantalum oxide film were examined and they were related to the electrical properties of the film. The growth of the interfacial silicon oxide layer was observed by using a high-resolution transmission electron microscope (TEM) and its effects on the electrical properties of the dielectric film were also studied. The electrical properties of the film could not be improved by annealing in an oxygen ambient at high temperatures due to the crystallization of the tantalum oxide film and the growth of the interfacial silicon oxide layer.

Formation of a lead zirconate titanate (PZT)/Pt interfacial layer and structural changes in the Pt/Ti/SiO2/Si substrate during the deposition of PZT thin film by electron cyclotron resonance plasma-enhanced chemical vapor deposition

Lead zirconate titanate (PZT) thin films were fabricated on Pt(70 nm)/Ti(100 nm)/SiO2/Si substrates at 470°C by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD). A Pb-deficient interfacial layer approximately 25 nm thick was found between the PZT film and the Pt substrate by high magnification transmission electron microscopy (TEM) examination. It was concluded that the interfacial layer was produced by the short residence time of Pb oxide molecules on the Pt substrate during the early stage of the PZT film growth. During the deposition of PZT film, the Pt/Ti/SiO2/Si substrate was altered into the Pt/TiO2/Pt/TiO2/TiSi2/SiO2 structure. The structural change was attributed to the Ti out-diffusion into the Pt layer, the oxidation of Ti by the in-diffused oxygen, and the formation of TiO2/Ti-silicide by the reaction between the SiO2 and Ti layers.