Minseok Choi

Native point defects and dangling bonds in α-Al2O3

We performed hybrid functional calculations of native point defects and dangling bonds (DBs) in α-Al2O3 to aid in the identification of charge-trap and fixed-charge centers in Al2O3/III-V metal-oxide-semiconductor structures. We find that Al vacancies (VAl) are deep acceptors with transition levels less than 2.6 eV above the valence band, whereas Al interstitials (Ali) are deep donors with transition levels within ∼2 eV of the conduction band. Oxygen vacancies (VO) introduce donor levels near midgap and an acceptor level at ∼1 eV below the conduction band, while oxygen interstitials (Oi) are deep acceptors, with a transition level near the mid gap. Taking into account the band offset between α-Al2O3 and III-V semiconductors, our results indicate that VO and Al DBs act as charge traps (possibly causing carrier leakage), while VAl, Ali, Oi, and O DBs act as fixed-charge centers in α-Al2O3/III-V metal-oxide-semiconductor structures.

An empirical study on the adoption of information appliances with a focus on interactive TV

Home-style information appliances are expected to increase in number and variety rapidly in the near future. User adoption of new technology for information appliances may be different from adoption of other technologies in that the appliances are mainly used at-home by consumers who have never encountered such technology before.We developed a theoretical model of technology adoption specific to interactive TV, a representative example of information appliances, based on prior research regarding general technology acceptance. We also conducted a large-scale online survey to test the validity of the proposed model. The results from pretest and pilot studies indicated that measures for the proposed model met content validity, reliability and construct validity. Finally, results from LISREL analysis indicated that three factors influencing behavioral intention were attitude, subjective norm and perceived behavioral control. Attitude was influenced by attitudinal belief, which could be measured by perceived usefulness, trialability, result demonstrability, image and enjoyment. Subjective norm was influenced by normative belief, Which could be measured by belief from family and belief from friends. Perceived behavioral control Was influenced by control belief, which could be measured by rapidity of change in technology, cost and ease of use. This paper concludes with statements of implications and limitations of the study results.

Impact of carbon and nitrogen impurities in high-κ dielectrics on metal-oxide-semiconductor devices

We investigate the electronic structure of carbon and nitrogen impurities, which are commonly incorporated during atomic-layer deposition of high-κ oxides such as Al2O3 and HfO2. The impact on metal-oxide-semiconductor devices is assessed by examining formation energies, transition levels, and band alignment between the oxide and semiconductors such as GaN, Si, and III-As. Carbon introduces charge-state transition levels near the semiconductor conduction-band edges, resulting in border traps and/or leakage current. Nitrogen acts as a source of negative fixed charge but may also be effective in alleviating the problem of carrier traps associated with native defects.

Design and fabrication of micro optical film by ultraviolet roll imprinting

With increasing demand for large-scale functional optical films with microstructure in the field of flat panel displays, a technology capable of fabricating large-scale polymeric micro-patterns has received much attention. To fabricate large-area micro-optical films, we designed and constructed an ultraviolet roll imprinting system consisting of a roll stamp, a material dispensing unit, a pair of flattening rollers, a contact roller, and a releasing roller. Two methods for fabricating roll stamps were considered: direct machining of the roll base and wrapping a flat stamp around the roll base. As practical examples of the roll imprinting process, we designed and fabricated a lenticular lens array, a pyramidal pattern, and a microlens array, and measured and analyzed their geometrical and optical properties. Our results suggest that the proposed UV roll imprinting process is a feasible method for mass producing large-scale functional optical films.

Elimination of flux loss by optimizing the groove angle in modified Fresnel lens to increase illuminance uniformity, color uniformity and flux efficiency in LED illumination

A Fresnel lens is an optical component that can be used to create systems more compact, cost-effective, and lightweight than those using conventional continuous surface optics. However, Fresnel lenses can usually cause a loss of flux efficiency and non-uniform distribution of illuminance due to secondary refraction by surface discontinuities, especially along the groove facet. We therefore proposed to modify a groove angle in the Fresnel lens and analyzed interrelation between the groove angle and multiple optical performances, such as flux efficiency and the uniformity of illuminance and color. The groove angle was optimized to maximize the uniformity and efficiency in the target viewing angle considering various weights of merit functions. Specifically, in our study, when the uniformity of illuminance had a little more weight than the flux efficiency (ratio of 0.6:0.4), final optimum groove angles of 24.7 degrees , 29.4 degrees , and 31.3 degrees were obtained at target viewing angles of 20 degrees , 30 degrees , and 40 degrees , respectively. We also fabricated a modified Fresnel lens with a groove angle of 29.4 degrees using UV-imprinting. The real optical performance of the fabricated Fresnel lens was then compared to that of a spherical lens.

Injection Molding of Nanopillars for Perpendicular Patterned Magnetic Media with Metallic Nanostamp

In this work, we focus on two important technical issues that need to be overcome before the injection molding technology can be used in the mass production of patterned media, possibly, of small form factor or non-rotating type. Firstly, we describe a fabrication method of a metallic nanostamp through the use of an ultraviolet (UV)-imprinted polymeric nanomaster. Secondly, we examine a method to use a passive heating of the nanostamp to increase the replication quality and to obtain the desired height of the injection-molded nanopillars. Finally, we show that the magnetic film on each injection-molded nanopillars with passive heating has a single magnetic domain.

Fabrication of Hybrid Microoptics Using UV Imprinting Process with Shrinkage Compensation Method

With increasing demand for compact aspherical optics in the field of imaging and optical data storage, fabrication technology for low cost micro aspherical optics has become a research priority. There are various types of micro aspherical optics, such as injection molded lens, glass molded lens, glass and hybrid lens. Among these types of lens, the hybrid is regarded as one of the most suitable because it combines good optical properties with low cost. The hybrid lens is fabricated by fabricating an aspherical layer on a spherical glass lens. To fabricate the hybrid lens at low cost, a UV imprinting process is preferred for its simplicity. However, in the conventional UV imprinting process volumetric shrinkage of the photopolymer causes various problems such as surface wrinkling and asymmetric local shrinkage. To overcome such limitations of the UV imprinting process, a shrinkage compensation method using an iris diaphragm to control the direction of polymerization is proposed and analyzed experimentally. To evaluate the proposed UV imprinting process, a hybrid lens was designed and fabricated, and its geometrical property was measured and compared with the design value.

A Soft-Decision Adaptation Mode Controller for an Efficient Frequency-Domain Generalized Sidelobe Canceller

In this paper, we propose a new soft-decision adaptation mode controller (SD-AMC) for frequency domain generalized sidelobe canceller (GSC) as a speech enhancement system. Contrarily to conventional systems that update filter coefficients in a hard-decision manner using voice activity detection (VAD), the proposed method flexibly controls the step-sizes of adaptive filters depending on the probability of speech presence in each frequency bin. Therefore, it further improves the system performance for various environments without much consideration on noise type and signal to noise ratio (SNR) of input signal. It also improves the robustness of GSC system by avoiding the miss-classification error by the hard-decision logic. Experimental results with speech recognition systems verify that the SD-AMC shows higher performance than ideally designed hard-decision approaches.

Nanoimprinting of conductive tracks using metal nanopowders

A method for metal nanopowder imprinting is proposed as a patterning process for conductive tracks, which is inexpensive and scalable down to submicrometers. To examine the feasibility of the proposed method, conductive tracks with widths of submicrometers to a few tens of micrometers were formed using a thermal nanoimprinting system, and the effects of processing conditions on the degree of replication and the degree of sintering and the electric conductivity of imprinted conductive tracks were analyzed.

Time-Frequency Domain Impulsive Noise Detection System in Speech Signal

This paper presents a new impulsive noise detection algorithm in speech signal. The proposed method employs the frequency domain characteristic of the impulsive noise to improve the detection accuracy while avoiding the false-alarm problem by the pitch of the speech signal. Furthermore, we proposed time-frequency domain impulsive noise detector that utilizes both the time and frequency domain parameters which minimizes the false-alarm problem by mutually complementing each other. As the result, the proposed time-frequency domain detector shows the best performance with 99.33 % of detection accuracy and 1.49 % of false-alarm rate.