Jung-Wook Min

III-nitride core–shell nanorod array on quartz substrates

We report the fabrication of near-vertically elongated GaN nanorods on quartz substrates. To control the preferred orientation and length of individual GaN nanorods, we combined molecular beam epitaxy (MBE) with pulsed-mode metal–organic chemical vapor deposition (MOCVD). The MBE-grown buffer layer was composed of GaN nanograins exhibiting an ordered surface and preferred orientation along the surface normal direction. Position-controlled growth of the GaN nanorods was achieved by selective-area growth using MOCVD. Simultaneously, the GaN nanorods were elongated by the pulsed-mode growth. The microstructural and optical properties of both GaN nanorods and InGaN/GaN core–shell nanorods were then investigated. The nanorods were highly crystalline and the core–shell structures exhibited optical emission properties, indicating the feasibility of fabricating III-nitride nano-optoelectronic devices on amorphous substrates.

Optical and structural properties of microcrystalline GaN on an amorphous substrate prepared by a combination of molecular beam epitaxy and metal–organic chemical vapor deposition

Microscale platelet-shaped GaN grains were grown on amorphous substrates by a combined epitaxial growth method of molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD). First, MBE GaN was grown on an amorphous substrate as a pre-orienting layer and its structural properties were investigated. Second, MOCVD grown GaN samples using the different growth techniques of planar and selective area growth (SAG) were comparatively investigated by transmission electron microscopy (TEM), cathodoluminescence (CL), and photoluminescence (PL). In MOCVD planar GaN, strong bound exciton peaks dominated despite the high density of the threading dislocations (TDs). In MOCVD SAG GaN, on the other hand, TDs were clearly reduced with bending, but basal stacking fault (BSF) PL peaks were observed at 3.42 eV. The combined epitaxial method not only provides a deep understanding of the growth behavior but also suggests an alternative approach for the growth of GaN on amorphous substances.

Direct Growth of III-Nitride Nanowire-Based Yellow Light-Emitting Diode on Amorphous Quartz Using Thin Ti Interlayer

Consumer electronics have increasingly relied on ultra-thin glass screen due to its transparency, scalability, and cost. In particular, display technology relies on integrating light-emitting diodes with display panel as a source for backlighting. In this study, we undertook the challenge of integrating light emitters onto amorphous quartz by demonstrating the direct growth and fabrication of a III-nitride nanowire-based light-emitting diode. The proof-of-concept device exhibits a low turn-on voltage of 2.6 V, on an amorphous quartz substrate. We achieved ~ 40% transparency across the visible wavelength while maintaining electrical conductivity by employing a TiN/Ti interlayer on quartz as a translucent conducting layer. The nanowire-on-quartz LED emits a broad linewidth spectrum of light centered at true yellow color (~ 590 nm), an important wavelength bridging the green-gap in solid-state lighting technology, with significantly less strain and dislocations compared to conventional planar quantum well nitride structures. Our endeavor highlighted the feasibility of fabricating III-nitride optoelectronic device on a scalable amorphous substrate through facile growth and fabrication steps. For practical demonstration, we demonstrated tunable correlated color temperature white light, leveraging on the broadly tunable nanowire spectral characteristics across red-amber-yellow color regime.

Water splitting to hydrogen over epitaxially grown InGaN nanowires on a metallic titanium/silicon template: reduced interfacial transfer resistance and improved stability to hydrogen

Water splitting using InGaN-based photocatalysts may make a great contribution to future renewable energy production systems. Among the most important parameters that need to be optimized are those related to substrate lattice-matching compatibility. Here, we directly grow InGaN nanowires (NWs) on a metallic Ti/Si template, for improving the water splitting performance compared to a bare Si substrate. The open circuit potential of the epitaxially grown InGaN NWs on metallic Ti was almost two times higher than when directly grown on the Si substrate. The interfacial transfer resistance was also reduced significantly after introducing the metallic Ti interlayer. An applied-bias-photon-to-current conversion efficiency of 2.2% and almost unity faradaic efficiency for hydrogen generation were achieved using this approach. The InGaN NWs grown on Ti showed improved stability for hydrogen generation under continuous operation conditions, when compared to those grown on Si, emphasizing the role of the semiconductor-on-metal approach in enhancing the overall efficiency of water splitting devices.

Role of quantum-confined stark effect on bias dependent photoluminescence of N-polar GaN/InGaN multi-quantum disk amber light emitting diodes

We study the impact of quantum-confined stark effect (QCSE) on bias dependent micro-photoluminescence emission of the quantum disk (Q-disk) based nanowires light emitting diodes (NWs-LED) exhibiting the amber colored emission. The NWs are found to be nitrogen polar (N-polar) verified using KOH wet chemical etching and valence band spectrum analysis of high-resolution X-ray photoelectron spectroscopy. The crystal structure and quality of the NWs were investigated by high-angle annular dark field - scanning transmission electron microscopy. The LEDs were fabricated to acquire the bias dependent micro-photoluminescence spectra. We observe a redshift and a blueshift of the μPL peak in the forward and reverse bias conditions, respectively, with reference to zero bias, which is in contrast to the metal-polar InGaN well-based LEDs in the literature. Such opposite shifts of μPL peak emission observed for N-polar NWs-LEDs, in our study, are due to the change in the direction of the internal piezoelectric field. The quenching of PL intensity, under the reverse bias conditions, is ascribed to the reduction of electron-hole overlap. Furthermore, the blueshift of μPL emission with increasing excitation power reveals the suppression of QCSE resulting from the photo-generated carriers. Thereby, our study confirms the presence of QCSE for NWs-LEDs from both bias and power dependent μPL measurements. Thus, this study serves to understand the QCSE in N-polar InGaN Q-disk NWs-LEDs and other related wide-bandgap nitride nanowires, in general.We study the impact of quantum-confined stark effect (QCSE) on bias dependent micro-photoluminescence emission of the quantum disk (Q-disk) based nanowires light emitting diodes (NWs-LED) exhibiting the amber colored emission. The NWs are found to be nitrogen polar (N-polar) verified using KOH wet chemical etching and valence band spectrum analysis of high-resolution X-ray photoelectron spectroscopy. The crystal structure and quality of the NWs were investigated by high-angle annular dark field - scanning transmission electron microscopy. The LEDs were fabricated to acquire the bias dependent micro-photoluminescence spectra. We observe a redshift and a blueshift of the μPL peak in the forward and reverse bias conditions, respectively, with reference to zero bias, which is in contrast to the metal-polar InGaN well-based LEDs in the literature. Such opposite shifts of μPL peak emission observed for N-polar NWs-LEDs, in our study, are due to the change in the direction of the internal piezoelectric field. Th...

Evolutionary growth of microscale single crystalline GaN on an amorphous layer by the combination of MBE and MOCVD

To integrate multiple functional devices on a chip, advances in epitaxial growth on heterosubstances are required. We developed a method of combined epitaxial growth using molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD) as one approach to achieve an epitaxial layer on an amorphous substrate. This two-stage combined growth can be used to grow binary gallium nitride (GaN) on any thermally durable substance. The first MBE growth step provided effective nucleation with uniform morphology. Meanwhile, the second MOCVD growth step resulted in improved crystalline quality. Detailed analysis of grain-to-grain and layer-to-layer interfaces was performed with high-resolution transmission electron microscopy (TEM) characterization. This study gives a deep understanding of the growth behavior, thereby supporting the generation of perfect single crystalline GaN, enabling the realization of optoelectronic GaN-based devices on an amorphous layer.

Improved Light Output Power of Chemically Transferred InGaN/GaN Light-Emitting Diodes for Flexible Optoelectronic Applications

Recent needs of semiconductor lighting sources have pursued diverse functionalities such as flexibility and transparency under high quantum efficiency. Inorganic/organic hybrid light-emitting diodes (LEDs) are one way to meet these requirements. Here, we report on flexible III-nitride-based LEDs and the improvement of their electrical and optical properties. To realize high light emission power and stable current operation, high-quality epitaxy and elaborate chip processing were performed. The fabricated flexible LEDs showed over threefold optical output power compared to normal LEDs on Si and had comparable forward voltage and series resistances.

Contact resistance reduction of ZnO thin film transistors (TFTs) with saw-shaped electrode

We report on a saw-shaped electrode architecture ZnO thin film transistor (TFT), which effectively increases the channel width. The contact line of the saw-shaped electrode is almost twice as long at the contact metal/ZnO channel junction. We experimentally observed an enhancement in the output drive current by 50% and a reduction in the contact resistance by over 50%, when compared to a typically shaped electrode ZnO TFT consuming the same chip area. This performance enhancement is attributed to the extension of the channel width. This technique can contribute to device performance enhancement, and in particular reduce the contact resistance, which is a serious challenge.

Unleashing the potential of molecular beam epitaxy grown AlGaN-based ultraviolet-spectrum nanowires devices

Abstract. There have been recent research advances in AlGaN-based self-assembled nanowires (NWs) as building blocks for ultraviolet (UV) optoelectronics grown by plasma-assisted molecular beam epitaxy. We review the basic growth kinetics on various foundry-compatible-metal/silicon-based substrates and the epistructure design for UV devices. We highlight the use of diffusion-barrier-metal thin film on silicon substrate as a solution to enhance device performance. NWs offer the opportunity to mitigate the detrimental quantum-confined Stark effect (QCSE), which lowers the recombination rate thereby reducing the device efficiency. On the other hand, the polarization-induced doping from the graded composition along NWs can be advantageous for eluding the inefficient doping in AlGaN-based UV devices. Sidewall surface states and the associate passivation treatment, as well as the use of ultrafast electron-microscopy characterization, are crucial investigations in shedding light on device performance under the influence of surface dangling bonds. For investigating the electrical performance of individual NWs and NWs light-emitting diode as a single entity, recent reports based on conductive atomic force microscopy measurements provide fast-prototyping in-process pass-fail evaluation and a means of improving growth for high-performance devices. Stress tests of NWs devices, crucial for reliable operation, are also discussed. Beyond applications in LEDs, an AlGaN-based NWs solar-blind photodetector demonstrated leveraging on the dislocation-free active region, reduced QCSE, enhanced light absorption, and tunable-composition features. The review opens pathways and offers insights for practical realization of AlGaN-based axial NWs devices on scalable and low-cost silicon substrates.

Enhanced photoelectrochemical performance of InGaN-based nanowire photoanodes by optimizing the ionized dopant concentration

InGaN-based nanowires (NWs) have been extensively studied for photoelectrochemical (PEC) water splitting devices owing to their tunable bandgap and good chemical stability. Here, we further investigated the influence of Si doping on the PEC performance of InGaN-based NW photoanodes. The Si dopant concentration was controlled by tuning the Si effusion cell temperature (TSi) during plasma-assisted molecular beam epitaxy growth and further estimated by Mott-Schottky electrochemical measurements. The highest Si dopant concentration of 2.1 × 1018 cm−3 was achieved at TSi = 1120 °C, and the concentration decreased with further increases in TSi. The flat-band potential was calculated and used to estimate the conduction and valence band edge potentials of the Si-doped InGaN-based NWs. The band edge potentials were found to seamlessly straddle the redox potentials of water splitting. The linear scan voltammetry results were consistent with the estimated carrier concentration. The InGaN-based NWs doped with Si at TSi = 1120 °C exhibited almost 9 times higher current density than that of the undoped sample and a stoichiometric evolution of hydrogen and oxygen gases. Our systematic findings suggest that the PEC performance can be significantly improved by optimizing the Si doping level of InGaN-based NW photoanodes.InGaN-based nanowires (NWs) have been extensively studied for photoelectrochemical (PEC) water splitting devices owing to their tunable bandgap and good chemical stability. Here, we further investigated the influence of Si doping on the PEC performance of InGaN-based NW photoanodes. The Si dopant concentration was controlled by tuning the Si effusion cell temperature (TSi) during plasma-assisted molecular beam epitaxy growth and further estimated by Mott-Schottky electrochemical measurements. The highest Si dopant concentration of 2.1 × 1018 cm−3 was achieved at TSi = 1120 °C, and the concentration decreased with further increases in TSi. The flat-band potential was calculated and used to estimate the conduction and valence band edge potentials of the Si-doped InGaN-based NWs. The band edge potentials were found to seamlessly straddle the redox potentials of water splitting. The linear scan voltammetry results were consistent with the estimated carrier concentration. The InGaN-based NWs doped with Si at T...