Chi-Chung Kei

Highly efficient flexible inverted organic solar cells using atomic layer deposited ZnO as electron selective layer

With atomic-layer-deposition grown zinc oxide as the electron selective layer, we developed plastic substrate compatible processing for organic photovoltaic devices and demonstrated flexible inverted organic solar cells on poly(ethylene naphthalate) with a power conversion efficiency of 4.18%.

Fabrication of Ag-loaded multi-walled TiO2 nanotube arrays and their photocatalytic activity

TiO2 multi-walled nanotube arrays (MWNTAs) were synthesized by atomic layer deposition (ALD) using anodic aluminum oxide (AAO) as the template. TiO2 and Al2O3 layers were alternately deposited into the AAO pores by ALD. The tube thickness and the gap span were controlled by the ALD cycle numbers of TiO2 and Al2O3, respectively. A MWNT composed of two or three concentric tubes with different diameters was formed after removal of Al2O3. Silver nanoparticles were then deposited on both inner and outer surfaces of the tubes by photochemical reduction. Degradation of methylene blue was carried out to evaluate the photocatalytic activity of bare and silver-loaded TiO2 MWNTAs. It was found that the amount of TiO2, the reaction surface area, the crystalline phase, and silver modification were the factors to determine the photocatalytic activity. The triple-walled MWNTAs showed the best performance, and Ag loading further enhanced the activity.

Characteristics of Atomic-Layer-Deposited Al2O3 High-k Dielectric Films Grown on Ge Substrates

This paper describes the structural and electrical properties of Al 2 O 3 thin films grown through atomic layer deposition onto Ge substrates over a wide deposition temperature range (50-300°C). From grazing-incidence X-ray reflectivity and X-ray photoelectron spectroscopy, we found that increasing the deposition temperature improved the Al 2 O 3 film density and its dielectric stoichiometry; nevertheless, dielectric intermixing between main Al 2 O 3 and interfacial GeO 2 appeared at temperatures above 200°C, along with degradation of the GeO 2 /Ge interface. Accordingly, a relatively large gate leakage current (J g ) and a high density of interfacial states D it (>10 13 cm -2 eV -1 ) were observed as a result of deterioration of the entire Al 2 O 3 /Ge structure at higher deposition temperatures. In addition, although subsequent high-temperature processing at 600°C in a N 2 ambient could relieve the oxygen-excessive behavior further, i.e., to provide a more stoichiometric film, the accompanying GeO x volatilization close to the dielectric interface caused greater damage to the electrical performance. Only forming gas annealing (H 2 /N 2 , 1:10) at low temperature (300°C) improved the capacitance-voltage characteristics of the Pt/Al 2 O 3 /Ge structure, in terms of providing a lower value of D it (ca. 6 X 10 11 cm -2 eV -1 ), a lower value of J g , and a reduced hysteresis width.

Deposition of platinum on oxygen plasma treated carbon nanotubes by atomic layer deposition

Platinum nanoparticles were deposited on oxygen plasma treated carbon nanotubes (CNTs) by atomic layer deposition (ALD). The treatment time with oxygen plasma generated by microwaves under a power of 600 W varied from 5 to 20 s. The number of ALD cycles was controlled at 5-125. X-ray photoelectron spectroscopic analysis indicated that oxygen plasma can graft oxygen-containing functional groups to the CNT surface to act as nucleation sites for growth of Pt nanoparticles. Formation of very uniform and well distributed Pt nanoparticles of a size of 1.60-4.80 nm was achieved. The growth rate of Pt nanoparticles could be controlled by the number of ALD cycles and oxygen plasma treatment time. This offers a dry process to deposit well-dispersed metallic nanoparticles on selected support materials.

Atomic Layer Deposition of Zinc Oxide on Multiwalled Carbon Nanotubes for UV Photodetector Applications

A novel process for the fabrication of ZnO-carbon nanotubes (CNT) nanocomposites with high uniformity by atomic layer deposition (ALD) of ZnO on multiwalled carbon nanotubes was reported, and their applications in UV photodetectors were investigated. Two types of photodetectors, p- and n-type, were developed by alternating the ALD reaction cycles. In addition, a schematic model was proposed to explain the p- to n-type conversion of the ZnO-CNTs, which accounts for the amount and surface coverage of ZnO on CNTs.

Fabrication of TiO2 nanotubes by atomic layer deposition and their photocatalytic and photoelectrochemical applications.

The formation of TiO(2) nanotubes was conducted by atomic layer deposition (ALD) with tris-(8-hydroxyquinoline) gallium (GaQ(3)) nanowires as a template at different substrate temperatures, 50, 100, and 200 °C. TiO(2) nanotubes were formed only at 50 and 100 °C. Although a higher growth rate at 50 °C was observed, nanotubes with better uniformity, conformality, and less residual chloride were obtained at 100 °C because of a different formation mechanism. A photocatalysis test of TiO(2) nanotubes prepared by different cycle numbers at 100 °C was conducted. It showed that TiO(2) nanotubes prepared by 400 cycles of ALD and treated at 700 °C for 1 h to form anatase phase had the best photocatalytic performance. Compared with P-25, the nanotubes showed higher photocatalytic degradation of rhodamine B and water splitting efficiency.

Deposition of uniform Pt nanoparticles with controllable size on TiO2-based nanowires by atomic layer deposition and their photocatalytic properties.

TiO2-based nanowires were prepared by a hydrothermal method, and Pt nanoparticles were deposited on the nanowires by atomic layer deposition (ALD). The size and loading of Pt nanoparticles with very good uniformity could be controlled by the ALD cycle number, along with acid or water treatment of the nanowires. The lower growth rate and the loading of Pt nanoparticles were obtained with water treatment. With acid treatment, the growth rate and loading were increased, and this increase was attributed to the formation of higher density of hydroxide groups on the nanowires. The photocatalytic activities of Pt-deposited nanowires were investigated. The deposition of Pt on the nanowires resulted in both enhanced light absorption in the visible region and electron–hole separation efficiency. The water-treated nanowires exhibited the highest degradation rate of rhodamine B and the highest hydrogen evolution rate. A maximum amount of hydrogen evolution, 13.8 mmol g−1 in 6 h, was achieved.

Carbon nanotube-supported Cu3N nanocrystals as a highly active catalyst for oxygen reduction reaction

A controllable synthesis of a hybrid electrocatalyst consisting of copper(I) nitride nanoparticles (Cu3N) grown on carbon nanotubes (CNTs) by plasma enhanced atomic layer deposition (ALD) is presented. The island growth mechanism during ALD led to the formation of uniformly distributed Cu3N nanoparticles on the surface of CNTs. The size of copper nitride particles strongly influenced the electrocatalytic properties, and it could be precisely tuned by controlling the cycle number of ALD. The Pt-free non-precious nanocrystals coupling with CNTs exhibited pronounced electrocatalytic activity for oxygen reduction reaction (ORR). Koutecky–Levich analysis on the ORR current densities indicated that the Cu3N@CNT electrodes in alkaline media follow a mixed two- and four-electron transfer ORR pathway, whose mass activities are comparable to that of a typical Pt/C electrode. This report reveals a dry process to fabricate a well-dispersed metal nitride on a selected support material as an ORR catalyst that could enhance the catalytic activity by synergistic chemical coupling effects.

Junction and Device Characteristics of Gate-Last Ge p- and n-MOSFETs With ALD-

In this paper, we investigated the characteristics of Ge junction diodes and gate-last p- and n-metal-oxide-semiconductor field-effect transistors with the atomic-layer-deposited- Al₂O₃ gate dielectrics. The magnitudes of the rectifying ratios for the Ge p⁺-n and n⁺-p junctions exceeded three and four orders of magnitude (in the voltage range of plusmn1 V), respectively, with accompanying reverse leakages of ca. 10^-2 and 10^-4 A ldr cm^-2, respectively. The site of the primary leakage path, at either the surface periphery or junction area, was determined by the following conditions: 1) the thermal budget during dopant activation, and 2) whether forming gas annealing (FGA) was employed or not. In addition, performing FGA at 300degC boosted the device on-current, decreased the Al₂O₃/Ge interface states to 8 times 10¹¹ cm^-2 ldr eV^-1, and improved the reliability of bias temperature instability. The peak mobility and on/off ratio reached as high as 225 cm² ldr V^-1 ldr s^-1 and > 10³, respectively, for the p-FET (<i>W</i>/<i>L</i> = 100 mum/4 mum), while these values were less than 100 cm² ldr V^-1 ldr s^-1 and ca. 10³, respectively, for the n-FET (<i>W</i>/<i>L</i> = 100 mum/9 mum). The relatively inferior n-FET performance resulted from the larger source/drain contact resistance, higher surface states scattering, and lower substrate-doping concentration.

\hbox{Al}_{2}\hbox{O}_{3}

In this study we investigated the interfacial chemistry occurring between an atomic-layer-deposited Al2O3 high-k film and a GaAs substrate and the impact of sulfidization and thermal annealing on the properties of the resultant capacitor. We observed that sulfide passivation of the Al2O3∕GaAs structure improved the effect of Fermi level pinning on the electrical characteristics, thereby providing a higher oxide capacitance, smaller frequency dispersion, and reduced surface states, as well as decreased interfacial charge trapping and gate leakage currents. Photoemission analysis indicated that the (NH4)2S-treated GaAs improved the quality of the as-deposited Al2O3 thin film and preserved the stoichiometry of the dielectric during subsequent high-temperature annealing. This behavior was closely correlated to the diminution of GaAs native oxides and elemental arsenic defects and their unwanted diffusion. In addition, thermal processing under an O2 atmosphere, relative to that under N2, decreased the thicknes...