Yung Chun Tu

Light Output Improvement of GaN-Based Light-Emitting Diodes Using Hydrothermally Grown ZnO Nanotapers

A new two-step hydrothermal growth (HTG) process with a shorter processing time and better growth control is proposed for the synthesis of ZnO nanotapers (NTs). The application of HTG ZnO NTs as surface roughening nanostructures to improve the light output power (Lop) of GaN-based LEDs is demonstrated. Compared with that of ZnO nanowires, the use of ZnO NTs leads to an improvement in Lop by 24.5% at 350 mA, which could be attributed to the fact that tapered ends of ZnO NTs offer more constructive photon scattering to maximize light extraction.

Improving source/drain contact resistance of amorphous indium–gallium–zinc-oxide thin-film transistors using an n+-ZnO buffer layer

To avoid high temperature annealing in improving the source/drain (S/D) resistance (R DS) of amorphous indium–gallium–zinc-oxide (α-IGZO) thin-film transistors (TFTs) for flexible electronics, a simple and efficient technique using a sputtering-deposited n+-ZnO buffer layer (BL) sandwiched between the S/D electrode and the α-IGZO channel is proposed and demonstrated. It shows that the R DS of α-IGZO TFTs with the proposed n+-ZnO BL is reduced to 8.1 × 103 Ω as compared with 6.1 × 104 Ω of the conventional one. The facilitation of carrier tunneling between the S/D electrode and the α-IGZO channel through the use of the n+-ZnO BL to lower the effective barrier height therein is responsible for the R DS reduction. Effects of the chamber pressure on the carrier concentration of the sputtering-deposited n+-ZnO BL and the thickness of the BL on the degree of improvement in the performance of α-IGZO TFTs are analyzed and discussed.

Hydrothermal Growth of Quasi-Monocrystal ZnO Thin Films and Their Application in Ultraviolet Photodetectors

Quasi-monocrystal ZnO film grown using the hydrothermal growth method is used for the fabrication of Cu2O/ZnO heterojunction (HJ) ultraviolet photodetectors (UV-PDs). The HJ was formed via the sputtering deposition of p-type Cu2O onto hydrothermally grown ZnO film (HTG-ZnO-film). The effect of annealing temperature in the nitrogen ambient on the photoluminescence spectra of the synthesized ZnO film was studied. The optoelectronic properties of Cu2O/ZnO film with various Cu2O thicknesses (250–750 nm) under UV light (365 nm; intensity: 3 mW/cm2) were determined. The UV sensitivity of the HTG-ZnO-film-based UV-PDs and the sputtered ZnO-film-based UV-PDs were 55.6-fold () and 8.8-fold (), respectively. The significant gain in sensitivity ( = 630%) of the proposed ZnO-film-based device compared to that for the device based on sputtered film can be attributed to the improved photoelectric properties of quasi-monocrystal ZnO film.

Enhanced light output of vertical GaN-based LEDs with surface roughened by refractive-index-matched Si3N4/GaN nanowire arrays

A surface roughening scheme with Si3N4-coated GaN nanowire (NW) arrays is proposed to improve the light extraction efficiency of GaN-based vertical light-emitting diodes (VLEDs). The scheme allows a graded refractive index that varies from 2.5 (GaN) to 1.9–2.0 (Si3N4) to 1 (air). Experimental results show that the use of 0.8-µm-long GaN NW arrays coated with a 250-nm-thick Si3N4 film enhances the light output power by 28.7% at 350 mA compared with that of regular VLEDs with a KOH-roughened surface. This enhancement is attributed to the Si3N4/GaN NW arrays effectively releasing total internal reflection and minimizing Fresnel loss.

Improving Crystalline Silicon Solar Cell Efficiency Using Graded-Refractive-Index SiON/ZnO Nanostructures

The fabrication of silicon oxynitride (SiON)/ZnO nanotube (NT) arrays and their application in improving the energy conversion efficiency (η) of crystalline Si-based solar cells (SCs) are reported. The SiON/ZnO NT arrays have a graded-refractive-index that varies from 3.5 (Si) to (Si3N4 and ZnO) to (SiON) to 1 (air). Experimental results show that the use of 0.4 μm long ZnO NT arrays coated with a 150 nm thick SiON film increases by 39.2% under AM 1.5 G (100 mW/cm2) illumination as compared to that of regular SCs with a Si3N4/micropyramid surface. This enhancement can be attributed to SiON/ZnO NT arrays effectively releasing surface reflection and minimizing Fresnel loss.

Preparation and Characteristic of Relative-Humidity Sensors Based on Laterally Grown ZnO Nanowires

The use of laterally oriented zinc oxide nanowires (ZnO NWs) grown by a hydrothermal growth (HTG) method for relative-humidity (RH) sensing devices at room temperature (RT) is demonstrated. Sensing response under various RH conditions (12–96%) at RT is presented and discussed. A humidity sensor based on laterally oriented ZnO NWs with a sensing response (R12%/R96%) as high as 2.2 was obtained at RT. The RT-operable humidity-sensing characteristics with comparably good sensitivity of the proposed humidity sensors is attributed to the full utilization of the entire NW surface, because the current path is aligned with the orientation of the bridged lateral ZnO NWs during the humidity sensing application, thus making possible.

Ultraviolet light emitting diodes based on hydrothermal grown crystalline n-ZnO on p-GaN film

The growth of single crystalline ZnO film on p-GaN using the hydrothermal method is proposed and its application on n-ZnO/p-GaN heterojunction light emitting diodes (HJ-LED) is demonstrated. The effect of thermal annealing in the nitrogen ambient on the optical and electrical properties of hydrothermally grown ZnO film (HTG-ZnO film) onto a p-GaN substrate is investigated. The current-voltage (I-V) curves in darkness show that the prepared n-HTG-ZnO film/p-GaN HJ with thermal annealing has good rectifying characteristics and a 150% improvement in leakage current at -4 V has been achieved for the n-ZnO/p-GaN with thermal annealing. Strong ultraviolet lights emission from the annealed n-ZnO/p-GaN HJ-LED at around 375 nm without defect-related emissions in the visible region are observed from electroluminescence (EL) spectra.

Enhanced light output of GaN-based thin-film flip-chip light-emitting diodes by surface texturing using laser ablation and chemical etching

A surface texturing scheme using ablation etching by KrF excimer laser irradiation and chemical wet etching is demonstrated to improve the light-extraction efficiency of GaN-based thin-film flip-chip light-emitting diodes (TFFC-LED). Both simulated and experimental results on the light emission characteristics are presented and discussed. For the TFFC-LED (die size of 1×1 mm2) with the proposed surface roughening scheme, enhancements in light output power (LOP) by 13.08% (12.81%) and wall-plug efficiency (WPE) by 2.87% (2.25%) at 350 mA (700 mA) as compared with that of the TFFC-LED without surface texturing are achieved experimentally.