Browsing by Author "Chen, Shuo-Wei"

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Effect of Strains and V-Shaped Pit Structures on the Performance of GaN-Based Light-Emitting Diodes
(2020-04-01)
Chen, Shuo-Wei
;
Chang, Chia-Jui
;
Lu, Tien-Chang
;
光電工程學系
;
Department of Photonics
Strains and V-shaped pits are essential factors for determining the efficiency of GaN-based light-emitting diodes (LEDs). In this study, we systematically analyzed GaN LED structures on patterned sapphire substrates (PSSs) with two types of growth temperature employed for prestrained layers and three different thickness of n-type GaN layers by using cathodoluminescence (CL), microphotoluminescence (PL), and depth-resolved confocal Raman spectroscopy. The results indicated that V-pits formation situation can be analyzed using CL. From the emission peak intensity ratio of prestrained layers and multiple quantum wells (MQWs) in the CL spectrum, information regarding strain relaxation between prestrained layers and MQWs was determined. Furthermore, micro-PL and depth-resolved confocal Raman spectroscopy were employed to validate the results obtained from CL measurements. The growth conditions of prestrained layers played a dominant role in the determination of LED performance. The benefit of the thick layer of n-GaN was the strain reduction, which was counteracted by an increase in light absorption in thick n-type doped layers. Consequently, the most satisfactory LED performance was observed in a structure with relatively lower growth temperature of prestrained layers that exhibited larger V-pits, leading to higher strain relaxation and thinner n-type GaN layers, which prevent light absorption caused by n-type GaN layers.
Effects of Nanoscale V-Shaped Pits on GaN-Based Light Emitting Diodes
(2017-02-01)
Chen, Shuo-Wei
;
Li, Heng
;
Chang, Chia-Jui
;
Lu, Tien-Chang
;
光電工程學系
This paper reviews the formation of nanoscale V-shaped pits on GaN-based light emitting diodes (LEDs) grown by the metal organic chemical vapor deposition (MOCVD) system and studies the effect of V-shaped pits on quantum efficiency. Since V-pits could provide potential barriers around threading dislocations to lessen non-radiative recombinations in such a high defect environment. In our study, multiple InGaN/GaN quantum well samples with different emission wavelengths of 380, 420, 460, and 500 nm were grown, each with different nanoscale V-shaped pits of three diameters for 150, 200, and 250 nm, respectively. It was found that the multiple quantum well (MQW) sample with larger V-pits had a lower pit density, but a relatively larger total V-pits defected area. The optimum diameter of V-pits showing the highest quantum efficiency from the MQW sample depended on the emission wavelength. MQW samples with wavelengths of 380 and 500 nm exhibited the best internal quantum efficiency (IQE) performance at the smallest V-pits area; however, the best performance for MQW samples with wavelength around 420 and 460 nm occurred when large V-pit areas were presented. Photoluminescence (PL) peak shifts and Raman shifts can provide a relationship between quantum-confined Stark effect (QCSE) and IQE, as well as a comparison between strain and IQE. The results obtained in this phenomenological study shall provide a useful guide line in making high-performance GaN-based LEDs with wide emission spectra.
Effects of three-dimensional strain distribution on the performance of GaN-based light-emitting diodes on patterned sapphire substrates
(2019-06-01)
Chen, Shuo-Wei
;
Li, Heng
;
Chang, Chia-Jui
;
Lu, Tien-Chang
;
光電工程學系
Depth-resolved confocal Raman and micro photoluminescence (PL) are adopted to analyze GaN-based light-emitting diodes (LEDs) with an ex situ sputtered AlN nucleation layer on patterned sapphire substrates (PSSs). The strain on PSSs is measured by Raman spectral mapping, and the results demonstrate that the strain distribution within the LED structures strongly correlates to the pattern array of the PSS. Cone regions with less dislocations have lower strain than plane regions. The array pattern of the PSS's surface can directly affect strain existing in the later crystal growth even at the position of the layer before the p-GaN growth. Moreover, the two-dimensional micro-PL mapping of the GaN-based LED samples on a PSS reveals a close relationship with the strain distribution on the sample surface. It is found that the strain as well as the PL intensity difference can be mitigated by proper pre-strained layers in order to realize high efficiency LEDs. (C) 2019 The Japan Society of Applied Physics
Improved performance of GaN based light emitting diodes with ex-situ sputtered AlN nucleation layers
(2016-04-01)
Chen, Shuo-Wei
;
Li, Heng
;
Lu, Tien-Chang
;
光電工程學系
;
Department of Photonics
The crystal quality, electrical and optical properties of GaN based light emitting diodes (LEDs) with ex-situ sputtered physical vapor deposition (PVD) aluminum nitride (AlN) nucleation layers were investigated. It was found that the crystal quality in terms of defect density and x-ray diffraction linewidth was greatly improved in comparison to LEDs with in-situ low temperature GaN nucleation layer. The light output power was 3.7% increased and the reverse bias voltage of leakage current was twice on LEDs with ex-situ PVD AlN nucleation layers. However, larger compressive strain was discovered in LEDs with ex-situ PVD AlN nucleation layers. The study shows the potential and constrain in applying ex-situ PVD AlN nucleation layers to fabricate high quality GaN crystals in various optoelectronics. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Significant Improvement of GaN Crystal Quality with Ex-situ Sputtered AlN Nucleation Layers
(2016-01-01)
Chen, Shuo-Wei
;
Yang, Young
;
Wen, Wei-Chih
;
Li, Heng
;
Lu, Tien-Chang
Ex-situ sputtered AlN nucleation layer has been demonstrated effective to significantly improve crystal quality and electrical properties of GaN epitaxy layers for GaN based Light-emitting diodes (LEDs). In this report, we have successfully reduced X-ray (102) FWHM from 240 to 110 arcsec, and (002) FWHM from 230 to 101 arcsec. In addition, reverse-bias voltage (Vr) increased around 20% with the sputtered AlN nucleation layer. Furthermore, output power of LEDs grown on sputtered AlN nucleation layer can be improved around 4.0% compared with LEDs which is with conventional GaN nucleation layer on pattern sapphire substrate (PSS).