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This paper was published with a typographical error in Acknowledgments on page 86. The grant number NRF-2019M3C1B909055 should read NRF-2019M3C1B9090559.

Polycrystalline HfO2 dielectrics grown on (−2 0 1) beta-gallium oxide (β-Ga2O3) using high temperature ALD process is investigated. A low capacitance–voltage (C-V) hysteresis of

Notable progress achieved in studying MoS2 based phototransistors reveals the great potential to be applicable in various field of photodetectors, and to further expand it, a durability study of MoS2 phototransistors in harsh environments is highly required. Here, we investigate effects of gamma rays on the characteristics of MoS2 phototransistors and improve its radiation hardness by incorporating CdSe/ZnS quantum dots as an encapsulation layer. A 73.83% decrease in the photoresponsivity was observed after gamma ray irradiation of 400 Gy, and using a CYTOP and CdSe/ZnS quantum dot layer, the photoresponsivity was successfully retained at 75.16% on average after the gamma ray irradiation. Our results indicate that the CdSe/ZnS quantum dots having a high atomic number can be an effective encapsulation method to improve radiation hardness and thus to maintain the performance of the MoS2 phototransistor.

So far many of research on transition metal dichalcogenides (TMDCs) are based on a bottomgate device structure due to difficulty with depositing a dielectric film on top of TMDs channel layer. In this work, we study different effects of various passivation layers on electrical properties of multilayer MoS2 transistors: spin-coated CYTOP, SU-8, and thermal evaporated MoOX. The SU-8 passivation layer alters device performance least significantly, and MoOX induces positive threshold voltage shift of ~8.0 V due to charge depletion at the interface, and the device with CYTOP layer exhibits decreased field-effect mobility by ~50% due to electric dipole field effect of C-F bonds in the end groups. Our results imply that electrical properties of the multilayer MoS2 transistors can be modulated using a passivation layer, and therefore a proper passivation layer should be considered for MoS2 device structures.

We demonstrate a two-dimensional (2D) multilayered molybdenum disulfide (MoS2) transistor with molybdenum (Mo) side and edge contacts, which is deposited using a dc-sputtering method. It exhibits field-effect mobility of 23.9 cm $^{\mathrm {2}}$ /Vs and ON/OFF ratio of $10^{\mathrm { {6}}}$ in a linear region. A current–voltage study under different temperatures (300–393 K) reveals that the Mo–MoS2 transistor shows a band transport characteristics, and a Schottky barrier height of 0.14 eV is estimated using a thermionic emission theory. Finally, the side and edge contacts of Mo–MoS2 are confirmed through the transmission electron microscope analysis. Our results not only show that Mo can be an alternative contact metal to other low work-function metals but also that the edge contact may play an important role in resolving the performance degradation over thickness increase of the MoS2 channel layer.

Compared with conventional piezoelectric transductions, an all-optical high-frequency ultrasound (HFUS) transducer is a promising modality for high-resolution ultrasound imaging. We demonstrate an all-optical HFUS transducer by integrating a carbon nanotube-polydimethylsiloxane composite film with an etalon thin-film structure incorporating SiO2/TiO2 distributed Bragg reflectors and an SU-8 resonator. The optical and acoustic characteristics are investigated for two different configurations (forward and backward operation modes). The maximum amplitude of the pulse echo in backward mode is approximately twofold higher than that of the forward mode. This difference is contributed by the increased reflectance and the absorptive loss of the incident pulsed laser in the forward mode. The pulse echo from the transducer exhibits a broad frequency bandwidth of 27 MHz. Furthermore, the scalability of the 2-D all-optical transducer array is also evaluated by characterizing the optical properties of the etalon across an area of 0.1 × 0.2 mm2. Our experimental results show that the proposed transducer is a promising candidate for high-resolution ultrasound imaging systems.

Microdonut-shaped GaN/Inx Ga1-x N light-emitting diode (LED) microarrays are fabricated for variable-color emitters. The figure shows clearly donut-shaped light emission from all the individual microdonut LEDs. Furthermore, microdonut LEDs exhibit spatially-resolved blue and green EL colors, which can be tuned by either controlling the external bias voltage or changing the size of the microdonut LED.

We present a method of epitaxially growing thermodynamically stable gallium nitride (GaN) nanorods via metal-organic chemical vapor deposition (MOCVD) by invoking a two-step self-limited growth (TSSLG) mechanism. This allows for growth of nanorods with excellent geometrical uniformity with no visible extended defects over a 100 mm sapphire (Al2O3) wafer. An ex-situ study of the growth morphology as a function of growth time for the two self-limiting steps elucidate the growth dynamics, which show that formation of an Ehrlich-Schwoebel barrier and preferential growth in the c-plane direction governs the growth process. This process allows monolithic formation of dimensionally uniform nanowires on templates with varying filling matrix patterns for a variety of novel electronic and optoelectronic applications. A color tunable phosphor-free white light LED with a coaxial architecture is fabricated as a demonstration of the applicability of these nanorods grown by TSSLG.

Color-selective or wavelength-tunable capability is a crucial feature for two-dimensional (2-D) semiconducting material-based image sensor applications. Here, we report on flexible and wavelength-selective molybdenum disulfide (MoS2) phototransistors using monolithically integrated transmission Fabry-Perot (F-P) cavity filters. The fabricated multilayer MoS2 phototransistors on a polyarylate substrate exhibit decent electrical characteristics (μFE > 64.4 cm2/Vs, on/off ratio > 106), and the integrated F-P filters, being able to cover whole visible spectrum, successfully modulate the spectral response characteristics of MoS2 phototransistors from ~495 nm (blue) to ~590 nm (amber). Furthermore, power dependence of both responsivity and specific detectivity shows similar trend with other reports, dominated by the photogating effect. When combined with large-area monolayer MoS2 for optical property enhancement and array processing, our results can be further developed into ultra-thin flexible photodetectors for wearables, conformable image sensor, and other optoelectronic applications.

Color-selective or wavelength-tunable capability is a crucial feature for two-dimensional (2-D) semiconducting material-based image sensor applications. Here, we report on flexible and wavelength-selective molybdenum disulfide (MoS2) phototransistors using monolithically integrated transmission Fabry-Perot (F-P) cavity filters. The fabricated multilayer MoS2 phototransistors on a polyarylate substrate exhibit decent electrical characteristics (μFE > 64.4 cm2/Vs, on/off ratio > 106), and the integrated F-P filters, being able to cover whole visible spectrum, successfully modulate the spectral response characteristics of MoS2 phototransistors from ~495 nm (blue) to ~590 nm (amber). Furthermore, power dependence of both responsivity and specific detectivity shows similar trend with other reports, dominated by the photogating effect. When combined with large-area monolayer MoS2 for optical property enhancement and array processing, our results can be further developed into ultra-thin flexible photodetectors for wearables, conformable image sensor, and other optoelectronic applications.

We propose a new approach called localized photothermal curing (LPC) to fabricate carbon nanotube (CNT)-polydimethylsiloxane (PDMS) composite transmitters for optoacoustic generation of high-frequency, high-amplitude ultrasound. The LPC uses nanosecond-pulsed laser irradiation to cure prepolymer liquid surrounding CNT strands in a composite form. These transmitters can produce peak pressure amplitudes 20-fold higher than a Cr reference film. We expect that the process allows great flexibility in design and fabrication of ultrasound transmitters in terms of shape and dimension.

We fabricated and characterized hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) with a channel length of 10 on both spherical and flat surfaces using maskless laser-write lithography (LWL). In addition to the electrical performance, the threshold voltage shift of the a-Si:H TFT under bias-temperature stress is investigated and discussed in comparison to a device fabricated on a flat surface. The obtained results show that the a-Si:H TFTs fabricated by LWL method on a curved surface are suitable for pixel switches and circuits, which are needed to realize image sensor arrays and/or displays on a nonplanar surface.

We have demonstrated electrically and optically enhanced MoS2 phototransistors on a flexible polyarylate substrate using poly(4-vinylphenol) (PVP) as a gate dielectric as well as an encapsulation layer. With the PVP encapsulation, the field-effect mobility (μFE) increases up to twofold, and the subthreshold slope as well as the on/off ratio (ION/IOFF) improves, which are desirable features for the photoresponsive characteristics. Consequently, both photoresponsivity and detectivity increase up to two orders of magnitude (×100) from visible to infrared spectrum. Such enhancement is associated with the n-doping effect of hydroxyl groups in PVP and reduced recombination centers by filling interface traps and surface states. Our results reveal that the transparent PVP polymer can be a promising encapsulation method in order to enhance the electrical and photoresponsive characteristics of MoS2-based flexible devices.

Hemispherical image sensors are very promising technology for cameras, surveillance systems and artificial vision. We report on the electrical performance of the hydrogenated amorphous silicon thin-film transistor passive pixel image sensor (PPS) circuits fabricated on a hemispherical substrate using maskless laser-write lithography (LWL). The level-to-level registration and alignment over the curved surface with a high accuracy are demonstrated for the LWL in this work. The obtained results clearly show that it is possible to realize active-matrix PPS with a 150 mu m pixel pitch and a dynamic range of about 40 dB that is suitable for hemispherical image sensors.

We report on the design and fabrication methods for a hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) on a non-planar substrate using laser-write lithography (LWL). Level-to-level alignment with a high accuracy is demonstrated using LWL method. The fabricated a-Si:H TFT exhibits a field-effect mobility of 0.27 cm2/V s, threshold voltage of 4.9 V and on/off current ratio of not, vert, similar6 × 106 in a saturation regime. The obtained results demonstrate that it is possible to fabricate the a-Si:H TFTs and complex circuitry on a curved surface, using a well-established a-Si:H TFT technology in combination with the maskless lithography, for hemispherical or non-planar sensor arrays.

We propose the use of nanostructured germanium (Ge) fabricated by simple metalassisted chemical (MAC) etching as a high-frequency optoacoustic ultrasound transmitter. As an acoustic transfer medium, an elastomeric polymer, polydimethylsiloxane (PDMS), is spin-coated on top of Ge nanostructures, which is prepared with three different thicknesses with various MAC etching time in order to compare optoacoustic conversion efficiency. Under pulsed laser excitation, the Ge transmitter generates ultrasound pressure of 7.5 times stronger than that of Cr reference with comparable high frequency spectra (primary: 15 MHz and 6dB roll-off at 27 MHz) to CNT-PDMS composite. Considering its simple fabrication process without substrate limitation, the nanostructured Ge overlaid with PDMS can offer a promising approach for a highly efficient optoacoustic transmitter and toward all-optical high-frequency ultrasound transducers. (C) 2016 Optical Society of America

The responsivities of MoS2 based photodetectors or phototransistors have been improved by integrating additional layers or implementing a hetero-structure. Despite complicated processes, these methods only improve the overall responsivity and are not capable of wavelength selective enhancement. This study reports the wavelength-selective enhancement of photo-responsivity in a multi-layer MoS2 phototransistor by employing a reflective Al metal gate. We have observed that the photo-responsivities are enhanced at a wavelength of ∼500 nm (FWHM = 140 nm) in the Al metal-gated multi-layer MoS2 phototransistors. The finite difference time domain method also confirmed that the wavelength for enhanced responsivity can be adjusted according to the thicknesses of the MoS2 and gate insulating layer. Using this effect, the Al metal-gated multi-layer MoS2 phototransistors can be tuned for specific applications.

In this letter, we study the current-temperature-stress-induced electrical instability of single and multiple hexagonal (HEX) hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) connected in parallel. The influence of the threshold voltage shift of a single HEX TFT on the overall electrical performance of multiple HEX TFTs is discussed. The results indicate that a-Si:H HEX TFTs have an improved electrical stability and a threshold voltage shift linear dependence on a number of connected HEX-TFT units.

Using thin sub-stoichiometric molybdenum trioxide (MoOx, x

Vertically aligned InGaN/GaN nanorod (NR)-based phosphor-free light emitting diodes (LEDs) using SiO2 nanohole patterns are demonstrated. The highly ordered SiO2 nanoholes were realized on a 2 μm-thick n+GaN template by a two-step dry etching process. The use of C4F8/O2/Ar plasma chemistries under the low pressure is found to greatly enlarge the bottom diameter of each hole, exhibiting high aspect ratio (AR ∼ 9) and vertical etch profile (∼89°). SAG technique was used to define the height of the GaN NRs while the width is determined by the trimethylgallium flow rate and growth temperature. An LED structure consisted of three-pairs of InGaN/GaN quantum well and AlGaN electron blocking layer on the sidewall of the nanorod in a core-shell structure. The wavelengths were successfully tuned by controlling pitches of the rods, which was caused by the different growth rate and indium incorporation of conformally overgrown InGaN multiquantum wells. At the operating current density of 1.5 A/cm2 (65 mA), NR-based s...

Various strategies and mechanisms have been suggested for investigating a Schottky contact behavior in molybdenum disulfide (MoS2) thin-film transistor (TFT), which are still in much debate and controversy. As one of promising breakthrough for transparent electronics with a high device performance, we have realized MoS2 TFTs with source/drain electrodes consisting of transparent bi-layers of a conducting oxide over a thin film of low work function metal. Intercalation of a low work function metal layer, such as aluminum, between MoS2 and transparent source/drain electrodes makes it possible to optimize the Schottky contact characteristics, resulting in about 24-fold and 3 orders of magnitude enhancement of the field-effect mobility and on-off current ratio, respectively, as well as transmittance of 87.4 % in the visible wavelength range.

Hysteresis, which is induced by both extrinsic and intrinsic causes, is often observed in molybdenum disulphide (MoS2) field-effect transistors (FETs), and several extrinsic hysteresis effects have been reported in unpassivated bottom-gate MoS2 device structures. In this study, interface-trap-induced hysteresis and other electrical properties are examined. We experimentally investigate thermally activated trap charges near a silicon-dioxide (SiO2)-MoS2 interface that gives rise to hysteresis in a multilayer MoS2 FET in a temperature region of 10–300 K. The threshold voltage (VTH) and field-effect mobility (μFE) decrease with the increase in temperature, regardless of the gate-bias sweep direction. The hysteresis that coincides with the trend of subthreshold swing increases sharply above T = 150 K as the released charges from interface traps become dominant over the fixed charges. Based on a temperature-dependent hysteresis analysis, we discussed the activation energy of interface traps and maximum interfa...

Conventional ɑ-Si:H phototransistors exhibit poor photosensitivity due to low photo-conversion efficiency. To overcome this intrinsic limit, we introduce gate underlap in ɑ-Si:H phototransistors and demonstrate photosensitivity enhancement. We show that photocurrent can be significantly larger than dark current by 4 orders of magnitude, using 1-μm gate underlap at incident optical power density (Pinc) of 3.2 W/cm2. Our 1-μm gate-underlap phototransistor exhibits higher photosensitivity than the device without gate underlap by 64 times with Pinc = 0.2 W/cm2 and a wavelength of 785 nm. Our gate-underlapped phototransistors also show excellent time-resolved photoswitching behaviors, demonstrating the great potential for highly sensitive photodetectors.

Inverted stagger hydrogenated amorphous silicon (a-Si:H) fork-shaped thin-film transistors (TFTs) were fabricated with a five-photomask process used in the processing of the active-matrix liquid crystal displays (AM-LCDs). We investigated the asymmetric electrical characteristics of a fork-shaped a-Si:H TFT under different bias conditions. To extract the electrical device parameters, we developed asymmetric geometrical factors. Current-voltage measurements indicate that the ON-OFF current ratio of fork TFT can be enhanced significantly by choosing the outer electrode as the drain, while the field-effect mobility and threshold voltage have the identical values when different drain bias conditions are used. Finally, we developed concepts of its possible application to AM-LCDs.