Yongwoo Lee

Relationship between crystallographic orientation and 3.42 eV emission bands in GaN grown by HVPE on Si substrate

The relationship between the 3.42 eV emission band in GaN and its crystallographic orientation was investigated. The peak position of the band shifts to lower energies with increase of the film thickness. The basal and/or pyramidal plane oriented GaN layers had a lower band intensity than those of prismatic plane oriented GaN films, and the polycrystalline structured GaN film showed both strong and neutral donor bound exciton band intensities. The band in GaN gradually shifted to lower energies with an increase of the luminescence intensity of the excitonic band indicating that the band in GaN is not at a fixed energy level, but depends upon its crystal structural qualities.

Fabrication of freestanding GaN by HVPE and optically pumped stimulated emission at room temperature

Single-crystalline freestanding GaN, having a current maximum size of and a thickness of 350 , was fabricated by hydride vapour phase epitaxy (HVPE) growth of thick-film GaN on a sapphire substrate and subsequent removal of the host sapphire substrate. We also observed the room-temperature optically pumped stimulated emission (SE) from a 1 mm width cleaved cavity prepared from freestanding GaN. At a maximum power density of 2 MW , the peak energy and FWHM of SE were 3.318 eV and 8 meV, respectively. The threshold pumping power density for the one set of SE was 0.37 MW , and the output becomes highly TE-polarized.

Comprehensive evaluation of early retention (fast charge loss within a few seconds) characteristics in tube-type 3-D NAND flash memory

A fast charge loss within a few seconds, which is referred to as early retention, was observed in tube-type 2y word-line stacked 3-D NAND flash memory for the first time, and the origin of the early retention was comprehensively evaluated. Using a fast-response pulse I-V system, the early retention characteristics from microseconds to seconds were thoroughly investigated, and the correlations with various program and erase levels were examined using solid and checkerboard patterns. Our findings indicate that the early retention mainly originates from the lateral charge loss through the shared charge trap layers and suggest that the program and erase levels should be balanced and optimized to reduce the early retention.

Evaluation of interface trap densities and quantum capacitance in carbon nanotube network thin-film transistors.

The interface trap density in single-walled carbon nanotube (SWNT) network thin-film transistors (TFTs) is a fundamental and important parameter for assessing the electronic performance of TFTs. However, the number of studies on the extraction of interface trap densities, particularly in SWNT TFTs, has been insufficient. In this work, we propose an efficient technique for extracting the energy-dependent interface traps in SWNT TFTs. From the measured dispersive, frequency-dependent capacitance-voltage (C-V) characteristics, the dispersive-free, frequency-independent C-V curve was obtained, thus enabling the extraction and analysis of the interface trap density, which was found to be approximately 8.2 × 10(11) eV(-1) cm(-2) at the valence band edge. The frequency-independent C-V curve also allows further extraction of the quantum capacitance in the SWNT network without introducing any additional fitting process or parameters. We found that the extracted value of the quantum capacitance in SWNT networks is lower than the theoretical value in aligned SWNTs due to the cross point of SWNTs on the SWNT network. Therefore, the method proposed in this work indicates that the C-V measurement is a powerful tool for obtaining deep physical insights regarding the electrical performance of SWNT TFTs.

Highly transparent tactile sensor based on a percolated carbon nanotube network

The demand for transparent and flexible electronic devices, which are an emerging technology for the next generation of sensors, continues to grow in both applications and development due to their potential to make a significant commercial impact in a wide variety of areas. Here, we demonstrate a highly transparent tactile sensor with 92% optical transparency in the visible range based on solution-processed 99% metallic CNTs attached on a polydimethylsiloxane (PDMS) film. We efficiently reconstructed the pressed, stimulated spatial location by increasing the injection current (Iinjection) during electrical resistance tomography (ERT) that computed the internal two-dimensional (2-D) resistivity distribution.The demand for transparent and flexible electronic devices, which are an emerging technology for the next generation of sensors, continues to grow in both applications and development due to their potential to make a significant commercial impact in a wide variety of areas. Here, we demonstrate a highly transparent tactile sensor with 92% optical transparency in the visible range based on solution-processed 99% metallic CNTs attached on a polydimethylsiloxane (PDMS) film. We efficiently reconstructed the pressed, stimulated spatial location by increasing the injection current (Iinjection) during electrical resistance tomography (ERT) that computed the internal two-dimensional (2-D) resistivity distribution.

Three-Dimensional Printed Poly(vinyl alcohol) Substrate with Controlled On-Demand Degradation for Transient Electronics

Electronics that degrade after stable operation for a desired operating time, called transient electronics, are of great interest in many fields, including biomedical implants, secure memory devices, and environmental sensors. Thus, the development of transient materials is critical for the advancement of transient electronics and their applications. However, previous reports have mostly relied on achieving transience in aqueous solutions, where the transience time is largely predetermined based on the materials initially selected at the beginning of the fabrication. Therefore, accurate control of the transience time is difficult, thereby limiting their application. In this work, we demonstrate transient electronics based on a water-soluble poly(vinyl alcohol) (PVA) substrate on which carbon nanotube (CNT)-based field-effect transistors were fabricated. We regulated the structural parameters of the PVA substrate using a three-dimensional (3D) printer to accurately control and program the transience time of the PVA substrate in water. The 3D printing technology can produce complex objects directly, thus enabling the efficient fabrication of a transient substrate with a prescribed and controlled transience time. In addition, the 3D printer was used to develop a facile method for the selective and partial destruction of electronics.

Three-Dimensionally Printed Micro-electromechanical Switches

Three-dimensional (3D) printers have attracted considerable attention from both industry and academia and especially in recent years because of their ability to overcome the limitations of two-dimensional (2D) processes and to enable large-scale facile integration techniques. With 3D printing technologies, complex structures can be created using only a computer-aided design file as a reference; consequently, complex shapes can be manufactured in a single step with little dependence on manufacturer technologies. In this work, we provide a first demonstration of the facile and time-saving 3D printing of two-terminal micro-electromechanical (MEM) switches. Two widely used thermoplastic materials were used to form 3D-printed MEM switches; freely suspended and fixed electrodes were printed from conductive polylactic acid, and a water-soluble sacrificial layer for air-gap formation was printed from poly(vinyl alcohol). Our 3D-printed MEM switches exhibit excellent electromechanical properties, with abrupt switching characteristics and an excellent on/off current ratio value exceeding 106. Therefore, we believe that our study makes an innovative contribution with implications for the development of a broader range of 3D printer applications (e.g., the manufacturing of various MEM devices and sensors), and the work highlights a uniquely attractive path toward the realization of 3D-printed electronics.

Semiconducting carbon nanotube network thin-film transistors with enhanced inkjet-printed source and drain contact interfaces

Carbon nanotubes (CNTs) are emerging materials for semiconducting channels in high-performance thin-film transistor (TFT) technology. However, there are concerns regarding the contact resistance (Rcontact) in CNT-TFTs, which limits the ultimate performance, especially the CNT-TFTs with the inkjet-printed source/drain (S/D) electrodes. Thus, the contact interfaces comprising the overlap between CNTs and metal S/D electrodes play a particularly dominant role in determining the performances and degree of variability in the CNT-TFTs with inkjet-printed S/D electrodes. In this work, the CNT-TFTs with improved device performance are demonstrated to enhance contact interfaces by controlling the CNT density at the network channel and underneath the inkjet-printed S/D electrodes during the formation of a CNT network channel. The origin of the improved device performance was systematically investigated by extracting Rcontact in the CNT-TFTs with the enhanced contact interfaces by depositing a high density of CNTs u...