Browsing by Author "Chiang, Chih-Yung"

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InGaN-Based Light-Emitting Diodes Grown on a Micro/Nanoscale Hybrid Patterned Sapphire Substrate
(2016-12-21)
Ke, Wen-Cheng
;
Lee, Fang-Wei
;
Chiang, Chih-Yung
;
Liang, Zhong-Yi
;
Chen, Wei-Kuo
A hybrid patterned sapphire substrate (hybrid-PSS) was prepared using an anodic aluminum oxide etching mask to transfer nanopatterns onto a conventional patterned sapphire substrate with microscale patterns (bare-PSS). The threading dislocation (TD) suppression of light-emitting diodes (LEDs) grown on a hybrid-PSS (HP-LED) exhibits a smaller reverse leakage current compared with that of LEDs grown on a bare-PSS (BP-LED). The strain-free GaN buffer layer and fully strained InGaN active layer were evidenced by cross-sectional Raman spectra and reciprocal space mapping of the X-ray diffraction intensity for both samples. The calculated piezoelectric fields for both samples are close, implying that the quantum-confined Stark effect was not a dominant mechanism influencing the electroluminescence (EL) peak wavelength under a high injection current. The bandgap shrinkage effect of the InGaN well layer was considered to explain the large red-shifted EL peak wavelength under high injection currents. The estimated LED chip temperatures rise from room temperature to 150 degrees C and 75 degrees C for BP-LED and HP-LED, respectively, at a 600-mA injection current. This smaller temperature rise of the LED chip is attributed to the increased contact area between the sapphire and the LED structural layer because of the embedded nanopattern. Although the chip generates more heat at high injection currents, the accumulated heat can be removed to outside the chip effectively. The high diffuse reflection (DR) rate of hybrid-PSS increases the escape probability of photons, resulting in an increase in the viewing angle of the LEDs from 130 degrees to 145 degrees. The efficiency droop was reduced from 46% to 35%, effects which can be attributed to the elimination of TDs and strain relaxation by embedded nanopatterns. In addition, the light output power of HP-LED at 360-mA injection currents exhibits a similar to 22.3% enhancement, demonstrating that hybrid-PSSs are beneficial to apply in high-power LEDs.
InGaN-based light-emitting diodes grown on various aspect ratios of concave nanopattern sapphire substrate
(2018-10-31)
Ke, Wen-Cheng
;
Chiang, Chih-Yung
;
Son, Widi
;
Lee, Fang-Wei
;
電子物理學系
This study presented the optoelectronic properties of InGaN-based light-emitting diodes (LEDs) grown on nanoscale-patterned sapphire substrates (NPSSs) with various aspect ratio (AR) concave nanopatterns. Concave nanopatterns with low ARs of similar to 0.5 (LAR-NPSS) and high ARs of 2, 3 and 4 (HAR-NPSS) were achieved using an anodic aluminum oxide template as the dry etching mask. The diffuse transmission (DT) rate of the incident light wavelength at 450 nm increased linearly, from 2.2% for LAR-NPSS to 12.7% for HAR-NPSS with an AR of 4. However, the diffuse reflection (DR) rate for HAR-NPSS did not increase linearly with the increasing AR values. The limit of the DR rate for HAR-NPSS could be attributed to the guiding effect when the incident light entered into the high AR concave nanopattern; most of the incident light could not be reflected (returned to the air), therefore there was no significant contribution to the DR rate. The luminous intensities of LEDs grown on bare sapphire (B-LED), HAR-NPSS (HAR-LED) and LAR-NPSS (LAR-LED) were 39, 38 and 74 mcd, respectively. Interestingly, the lowest brightness of any LED was the HAR-LED, which had higher DT and DR rates. This significant discrepancy could be attributed to a large number of voids occurring at the initial growth stage of the LED epitaxial layer on the HAR-NPSS that caused the emitted photons to remain trapped inside the LED. In addition, increasing the LED epilayer coverage area (i.e. 1.6-fold) with the inclined facets of low AR concave nanopatterns effectively scattered the emitted photons, leading to an increased light emitting intensity of the LED. The experimental results indicated that the specific AR concave nanopattern of an NPSS could increase the LED epitaxial layer coverage area and prevent void formation, which plays a crucial role in achieving high-brightness in InGaN-based LEDs.