Ching-Hua Chiu

Nanoscale epitaxial lateral overgrowth of GaN-based light-emitting diodes on a SiO2 nanorod-array patterned sapphire template

High efficiency GaN-based light-emitting diodes (LEDs) are demonstrated by a nanoscale epitaxial lateral overgrowth (NELO) method on a SiO2 nanorod-array patterned sapphire substrate (NAPSS). The transmission electron microscopy images suggest that the voids between SiO2 nanorods and the stacking faults introduced during the NELO of GaN can effectively suppress the threading dislocation density. The output power and external quantum efficiency of the fabricated LED were enhanced by 52% and 56%, respectively, compared to those of a conventional LED. The improvements originated from both the enhanced light extraction assisted by the NAPSS and the reduced dislocation densities using the NELO method.

Size-Dependent Strain Relaxation and Optical Characteristics of InGaN/GaN Nanorod LEDs

In this paper, InGaN/GaN nanorod LEDs with various sizes are fabricated using self-assembled Ni nanomasks and inductively coupled plasma-reactive ion etching. Photoluminescence (PL) characteristics exhibit size-dependent, wavelength blue shifts of the emission spectra from the nanorod LEDs. Numerical analyses using a valence force field model and a self-consistent Poisson, Schrodinger, and drift-diffusion solver quantitatively describe the correlation between the wavelength blue shifts and the strain relaxation of multiple quantum wells embedded in nanorods with different averaged sizes. Time-resolved PL studies confirm that the array with a smaller size exhibits a shorter carrier lifetime at low temperature, giving rise to a stronger PL intensity. However, the PL intensity deteriorates at room temperature, compared to that of a larger size, possibly due to an increased number of surface states, which decreases the nonradiative lifetime, and hence reduces the internal quantum efficiency.

High-Performance GaN-Based Vertical-Injection Light-Emitting Diodes With TiO

We have designed and fabricated a new type of GaN-based thin-film vertical-injection light-emitting diode (LED) with TiO2-SiO 2 omnidirectional reflector (ODR) and n-GaN roughness. The associated ODR designed for LED operation wavelength at 455 nm was integrated with patterned conducting channels for the purpose of vertical current spreading. With the help of laser lift-off and photo-electrochemical etching technologies, at a driving current of 350 mA and with chip size of 1 mm times 1 mm, the light-output power and the external quantum efficiency of our thin-film LED with TiO2-SiO2 ODR reached 330 mW and 26.7%. The result demonstrated 18% power enhancement when compared with the results from the thin-film LED with Al reflector replace

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Using a GaN nanorod template in a hydride vapor phase epitaxy (HVPE) system can manufacture a freestanding GaN (FS-GaN) substrate with threading dislocation densities down to ~ 107 cm-2. In this letter, we report InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) grown on this FS-GaN substrate. The defect densities in the homoepitaxially grown LEDs were substantially reduced, leading to improved light emission efficiency. Compared with the LED grown on sapphire, we obtained a lower forward voltage, smaller diode ideality factor, and higher light-output power in the same structure grown on FS-GaN. The external quantum efficiency (EQE) of LEDs grown on FS-GaN were improved especially at high injection current, which brought the efficiency droop phenomenon greatly reduced at high current density.

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We have developed a simple method to fabricate nanoscale masks by using self-assembly Ni clusters formed through a rapid thermal annealing (RTA) process. The density and dimensions of the Ni nano-masks could be precisely controlled. The nano-masks were successfully applied to GaN-based light-emitting diodes (LEDs) with nano-roughened surface, GaN nanorods, and GaN-based nanorod LEDs to enhance light output power or change structure properties. The GaN-based LED with nano-roughened surface by Ni nano-masks and excimer laser etching has increased 55% light output at 20 mA when compared to that without the nano-roughened process. The GaN nanorods fabricated by the Ni nano-masks and ICP-RIE dry etching showed 3.5 times over the as-grown sample in photoluminescence (PL) intensity. The GaN-based nanorod LEDs assisted by photo-enhanced chemical (PEC) wet oxidation process were also demonstrated. The electroluminescence (EL) intensity of the GaN-based nanorod LED with PEC was about 1.76 times that of the as-grown LED. The fabrication, structure properties, physical features, and the optical and electrical properties of the fabricated devices will be discussed.

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In this work, n-GZO/a:amorphous-Si(i:intrinsic)/p( + )-Si photodiodes are fabricated. We employed a nanosphere lithographic technique to obtain nanoscale patterns on either the a-Si(i) or p( + )-Si surface. As compared with the planar n-GZO/p( + )-Si diode, the devices with nanopatterned a-Si(i) and nanopatterned p( + )-Si substrates show a 32% and 36.2% enhancement of photoresponsivity. Furthermore, the acceptance angle measurement reveals that the nanostructured photodiodes have larger acceptance angles than the planar structure. It also shows that the device with the nanocone structure has a higher acceptance angle than that with the nanorod structure.

Omnidirectional Reflector and n-GaN Roughness

In this work, we have grown In0.2Ga0.8N/GaN multiple quantum well (MQWs) epitaxial structure on vicinal sapphire substrates by low pressure metal-organic chemical vapor deposition and investigated the relationship between carrier localization degree and vicinal angles of sapphire substrates. The optical analysis confirmed that the In0.2Ga0.8N/GaN MQWs grown on 0.2°-off sapphire substrate exhibited the smallest carrier localization degree and more ordered In0.2Ga0.8N/GaN MQW structure. In addition, mechanisms for carrier localization in In0.2Ga0.8N/GaN MQWs grown on vicinal substrate were discussed based on the results obtained from the power and temperature dependent photoluminescence measurements. The Raman spectrum showing the in-plane compressive stress of the GaN epitaxial structures grown on vicinal sapphire substrates revealed the relation between the dislocation density and the carrier localization degree in MQWs. From transmission electron microscopy images, the threading dislocation density (TDD)...

Reduction of Efficiency Droop in InGaN Light-Emitting Diode Grown on Self-Separated Freestanding GaN Substrates

Enhancing the Emission Efficiency of In0.2Ga0.8NOGaN MQW Blue LED by Using Appropriately Misoriented Sapphire Substrates Zhen-Yu Li, Wu-Yih Uen, Ming-Hua Lo, Ching-Hua Chiu, Po-Chun Lin, Chih-Tsang Hung, Tien-Chang Lu, Hao-Chung Kuo, Shing-Chung Wang,* and Yen-Chin Huang Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Department of Electronic Engineering, College of Electrical Engineering and Computer Science, Chung Yuan Christian University, Chung-Li 32023, Taiwan

Nano-Processing Techniques Applied in GaN-Based Light-Emitting Devices With Self-Assembly Ni Nano-Masks

AlGaInP-based metal-bonding light-emitting diodes (LEDs) with micro- and nanoscale textured surface were investigated. The device surface with microbowls and nanorods were formed by a chemical wet-etching and dry-etching technique for enhancing light-extraction purpose. The luminous intensity could be enhanced 65.8% under 20-mA current injection as compared with the plane surface LEDs. The maximum wall-plug efficiency was achieved 14.1% at 7.5-mA operation.

Investigation of light absorption properties and acceptance angles of nanopatterned GZO/a-Si/p + -Si photodiodes

A free-standing nanopillar with a diameter of 300 nm, and a height of 2 μm is successfully demonstrated by focused ion beam milling. The measured micro-photoluminescence (μ-PL) from the embedded InGaN/GaN multiple quantum wells shows a blue shift of 68 meV in energy with a broadened full-width at half maximum, ~200meV. Calculations based on the valence force field method suggest that the spatial variation of the strain tensors in the nanopillar results in the observed energy shift and spectrum broadening. Moreover, the power-dependent µ-PL measurement confirms that the strain-relaxed emission region of the nanopillar exhibits a higher radiative recombination rate than that of the as-grown structure, indicating great potential for realizing high-efficiency nano devices in the UV/blue wavelength range.